Single-Phase White-Light-Emitting and Photoluminescent Color

Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

pubs.acs.org/CR

Single-Phase White-Light-Emitting and Photoluminescent ColorTuning Coordination Assemblies Mei Pan, Wei-Ming Liao, Shao-Yun Yin, Si-Si Sun, and Cheng-Yong Su*

Downloaded via UNIV OF NEW ENGLAND on August 22, 2018 at 01:21:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Ministry of Education (MOE) Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China ABSTRACT: Metal−organic complexes assembled from coordinative interactions are known to be able to display a wide range of photoluminescent behaviors benefiting from an extensive number of metal ions, organic linkers, and inclusion guests, depending on the multifaceted nature of their chemical structures and photophysical properties. In the past two decades, the white-light-emitting (WLE) and photoluminescent color-tuning (PLCT) materials based on the single-phase metal−organic coordination assemblies have merited particular attention and gained substantial advances. In this review, we give an overview of recent progress in this field, placing emphasis on the WLE and PLCT properties realized in the single-phase materials, which covers the origin, generation, and manipulation of different types of photoluminescence (PL) derived from ligand-centered (LC), metal/cluster-centered (MC or CC), excimer/exciplex-based (EX), metal-to-ligand or ligand-to-metal charge-transfer-based (MLCT or LMCT), or guest-included emissions. The coordination assemblies in this topic can be generally classified into three categories [(1) mono/ homometallic coordination assemblies based on main group (s,p-block), transition (d-block), or lanthanide (f-block) metal centers, (2) s/p−f-, d−f-, or f−f-type heterometallic coordination assemblies, and (3) guest-included coordination assemblies] for which WLE and PLCT properties can be achieved by virtue of either a wide-band/overlapped emission covering the whole visible spectrum from a single emitting center or a combination of complementary color emissions from multiple emitting centers/origins. Some state-of-the-art assembly methods and successful design models relevant to the above three categories are elaborated to demonstrate how to achieve efficient and controllable white-light emission in a single-phase material through a tunable PL approach. Potential applications in the fields of lighting and displaying, sensing and detecting, and barcoding and patterning are surveyed, and at the end, possible prospects and challenges for future development along this line are proposed.

CONTENTS 1. Introduction 2. Photoluminescence Origins Relevant to SinglePhase WLE/PLCT Coordination Assemblies 2.1. Ligand-Centered (LC) Photoluminescence 2.2. Excimer or Exciplex (EX)-Based Photoluminescence 2.3. Metal-Centered (MC) Photoluminescence 2.4. Charge Transfer (CT) State Based Photoluminescence 2.5. Single-Phase WLE and PLCT Strategies 3. Assembly Methods for Single-Phase WLE/PLCT Coordination Assemblies 3.1. One-Pot Assembly Applying Heterotopic Ligands 3.2. Stepwise Assembly Applying Metalloligands 3.3. Statistic Replacement Method 3.4. Epitaxial Growth Method 3.5. Copolymerization Method 3.6. Postencapsulation Method 4. Design Models for Single-Phase WLE/PLCT Coordination Assemblies 4.1. Single-Phase Mono/Homometallic Coordination Assemblies © XXXX American Chemical Society

4.1.1. Main-Group-Metal (s,p-Block) Centered Complexes 4.1.2. Transition-Metal (d-Block) Centered Complexes 4.1.3. Lanthanide (f-Block) Based Complexes 4.2. Single-Phase Heterometallic Coordination Assemblies 4.2.1. Heteronuclear d−f Complexes 4.2.2. Heteronuclear s/p−f Complexes 4.2.3. Isomorphous f−f Solid-Solution Complexes 4.3. Single-Phase Guest-Included Coordination Assemblies 4.3.1. Organic-Dye-Included Complexes 4.3.2. Ln3+-Included Complexes 4.3.3. Other Metal-Compound-Included Complexes 5. Potential Applications for Single-Phase WLE/ PLCT Coordination Assemblies 5.1. White-Light-Emitting Diodes (WLEDs) 5.2. Sensing and Detecting 5.3. Barcoding and Patterning

F F G H H I I K K K M N O P P

Q S W X X Y Y AB AB AB AC AD AD AG AI

Received: April 5, 2018

Q A

DOI: 10.1021/acs.chemrev.8b00222 Chem. Rev. XXXX, XXX, XXX−XXX

ppy, bpy, TATPT ppy, bpy, TATPT

2,5-bis((S)-2-hydroxypropylthio)terephthalic acid

f-block @MOF d-block Ir3+

s,p-block Pb2+

Eu3+,Tb3+@1 [Ir(ppy)2(bpy)]+@1

PbL2

B

Htzib HMq BTB, OA H2bpda, H2bda, phen 1,4-NDCH2, dppe TPPE 3-(2-thienyl) pyrazole

s,p-block Pb2+

s,p-block Pb2+

s,p-block In3+

s,p-block Ga/In3+

s,p-block Mg2+

d-block Pt2+ d-block Au+

d-block Au+

[Pb(NO3)(tzib)]n, [Pb(tzib)2]n

[Pb(Mq)2]2·3H2O, [Pb4(Mq)6]·(ClO4)2 In3+-MOF (SMOF-1)

Ga1/In1

Mg-CP

Pt(II) metallacages [{Au(L)}3]

AuIII complexes 4-cyanobenzoate butane-1,4-diylbis (pyridine-3-carboxylate benzo-18-crown-6 L(N4,N4’-di(pyridin-4-yl) biphenyl-4,4′dicarboxamide), H3BTC 4,4′-oxidibenzoic acid

d-block Ag+

d-block Ag+

d-block Cu+

d-block Zn2+

d-block Cd2+

[AgL]n·nH2O

[Ag(L)(NO3)]∞

{[K(benzo-18- crown6)]2(Cu2I4)}n Zn(L)(HBTC) (H2O)2

[(Me2NH2)[RbCd4 (OBA)5]·H2O

substituted 2-phenylpyridine

H2pydc

s,p-block Bi3+, Pb2+

pyridine 2,6-dicarboxylic acid

Im pyridine-2,6-dicarboxylate

Bi/Pb-MOF

LnMOF@PVA

[Ba(Im)2] white@HKUST-1

mixed f-block Eu/ Tb, Eu/Gd, Tb/Gd Eu2+/Ba2+ mixed f-block Eu/ Tb/Gd mixed f-block Eu/ Tb/Gd

[LnnLn′1−n(TTP)2 ·H2O]Cl3

solution

solution

solution

hydrothermal

hydrothermal

hydrothermal

layer

hydrothermal

hydrothermal

hydrothermal

postencapsulation postencapsulation

solid solution

codoping LPE pump

solid solution

metalloligand metalloligand

H2pydbdc 3-TPyMNTB TTP

solid solution

H3imdc, Him

mixed f-block Eu/ Tb/La d−f d−f

[H(H2O)8][LnZn4 (imdc)4(Him)4] Ir2Eu Ag−Eu-MOF

solution

assembly strategy

TETP

ligand(s)

f-block Dy3+

type of metal center(s)

Dy-MOF

complex

Table 1. List of Representative Single-Phase WLE and PLCT Coordination Assemblies

homometallic MOF homometallic linear chain 1D linear polymer homometallic MOF 3D framework

mononuclear

homometallic MOF homometallic 2D polymer homometallic 3D network cage trinuclear

2D layer, homometallic mononuclear

MOF, homometallic

3D net, homometallic

guest@MOF guest@MOF

mixed-Ln-MOF

framework mixed-Ln-MOF

mixed-Ln-MOF

trinuclear pcu-net

homometallic utp-MOF mixed-Ln-MOF

structure features

dual, 427, 566

dual, 424, 539

broad, 465, 632

broad, 470, 514

dual, 427, 566

dual

dual

dual, 410, 580

broad

broad

multi, 475, 615

multi, 430, 460, 480, 556 441, 470, 520, 563 broad, 380, 595

multi, 459, 515, 600

multi, RGB dual, 425, 530

multi, RGB

broad multi, RGB

multi, RG/GB/RB

dual, blue, red dual, blue, red

multi, RGB

dual, blue, yellow

representive emissions/nm

ref

ILCT, MLCT

(0.30, 0.33)

96

95

π−π*, stacking

91, 92 93

90

82 89

81

80

79

78

94

(0.33, 0.38)

(0.29, 0.33) to (0.32, 0.40) (0.31, 0.33)

(0.333, 0.333) (0.28, 0.29)

(0.27, 0.30), (0.28, 0.35) (0.30, 0.30)

(0.29, 0.29), (0.29, 0.30)

ILCT, MLCT, LLCT XMCT, 3CC

LC(AIE), MLCT monomeric, excimeric 3 ILCT perturbed by metal center π−π*, MLCT

LLCT

LLCT

LC, LMCT

LMCT, MC

77

75

74 74

68

62 65

60

56 57

53

42

LMCT, π → π*

WLED

LED WLED

nanofiber

potential applications

76

(0.35, 0.40)

(0.27, 0.30), (0.25, 0.29), (0.24, 0.28)

(0.41, 0.48), (0.44, 0.46), (0.50, 0.42) (0.36, 0.32) (0.31, 0.33)

(0.417, 0.556) (0.37, 0.38)

(0.42, 0.32), (0.45, 0.37)

(0.33, 0.33)

(0.33, 0.35)

CIE coordinate of WLE

LMCT, intraligand π → π*, n → π*

guest, host MLCT (host), MLCT or LLCT (guest) LC, LMCT, MC

LC, MC

5d−4f LC, MC

LC, MC

MLCT, MC LC, MC

LC, MC

LC, MC

luminescence origins

Chemical Reviews Review


C

H2PI and H3PI2C CR1 macrocycle-appended naphthalimide 3-(1,8-naphthalimido) propanoic acid H3sfpip TMPBPO IP, H2dpdc MBDC, STP

d-block Zn2+

f-block Eu3+ f-block Eu3+

f-block Eu3+

f-block Eu3+ f-block Pr3+

f-block Sm3+

f-block La3+

mixed f-block Eu/ Sm/Gd d−f d−f s/p−f

s/p−f

mixed f-block Eu/ Tb/La mixed f-block Eu/ Tb/La mixed f-block Eu/ Tb/La mixed f-block Eu/ Tb/Gd mixed f-block Eu/ Tb/Gd mixed f−d-blockEu/Tb

{[Zn6(HPI)4(HPI2C)4]·solvent

CR1-Eu Eu3+ complex

{[Ln(L2)3(H2O)] ·H2O}n

Eu-MOF LIFM-17(Pr)

Sm3+ complex

[La(MBDC)(STP)]

[Ln(TPIA)(H2O)3]·5.5H2O

Pb2EuL2

{[Ln2(L)2] ·(H2O)3·(Me2NH2)2}n

EuxTb1‑xL

lanthanide 4-(dipyridin-2-yl)aminobenzoate {[LnOH(H2O)6] [Zn2Ln4(4Htbca)2 (4-tbca)8 (H2O)12]}n· 6nH2O Eu@Tb-BTB

[H2NMe2]3[Gd1−x−yEuxTby(L)3]

Ln2(PDA)3(H2O)5

Ln-BTPCA

mixed f-block Eu/ Tb mixed f-block Eu/ Tb

bipo•−

d-block Zn2+

[Zn(bipo•−)(L)]n

Ir3+/Ln3+ dyad Zn2Eu Al3Eu2

4,4′-trimethylene dipyridine 2-hydroxyl (phosphono)acetate

d-block Zn2+ d-block Zn2+

1,3-bis(4-carboxyphenyl)

BTB

Htbca

4-(dipyridin-2-yl)aminobenzoate

pyridine-2,6-dicarboxylic acid

PDA

5-(3,5-dicarboxybenzyloxy)-isophthalic acid BTPCA

triCB-NTB

LpPH 8-hydroxylquinolinate 2-methyl-8-hydroxyquinolate

TPIA

H6TTHA, Hpt

ligand(s)

d-block Cd2+


[Cd5(HTTHA)2 (Hpt)4(H2O)]·4H2O NTHU-4W zinc phosphonate

complex

Table 1. continued

solid solution

solid solution

solid solution

solid solution

solid solution

solid solution

solid solution

solid solution

heterotopic ligand

metalloligand metalloligand metalloligand

hydrothermal

solution

assembly strategy

mixed-Ln-MOF

mixed-Ln-MOF

mixed-Ln-MOF

mixed-Ln-MOF

mixed-Ln-MOF

mixed-Ln-MOF

mixed-Ln-MOF

mixed-Ln-MOF

trinuclear trinuclear penta- heteronuclear heteronuclear MOF

homometallic 1D chain 3D homometallic MOF homometallic MOF homometallic MOF 2D network

homometallic 2D network 3D network homometallic 3D framework homometallic 1D MOF homometallic 1D chain mononuclear mononuclear

structure features

multi, RGB

multi, RGB

multi, RGB

multi, RGB

multi, RGB

multi, RGB

multi, RGB

multi, 448, 542, 618

multi, blue, red

dual, blue, red dual, blue, red dual, blue, red

multi, RGY

dual, 430, 550

multi

dual multi

triple

triple, 460, 550, 612 triple

broad, 456, 514, 594, 640 408, 460, 555

dual, 433, 550 dual, 442, 614

single, broad


LC, MC

LC, MC

LC, MC

LC, MC

LC, MC

LC, MC

LC, MC

LC, MC

LC, LMCT, MC

MLCT, MC ILCT, MC ILCT, MC

LC, MC

ILCT

LC, MC

ILCT, MC LC, MC

LC, MC LC (monomer, excimer), MC LC, MC

ESIPT, aggregate

π → π*, LMCT

defects in lattice

LMCT


(0.32, 0.36), (0.32, 0.34), (0.31, 0.33), (0.34, 0.32)

(0.33, 0.33)

(0.32, 0.34)

(0.33, 0.31), (0.31, 0.33) (0.33, 0.34)

(0.33, 0.33)

(0.41, 0.33), (0.40, 0.32), (0.35, 0.34)

(0.27, 0.36)

(0.26, 0.30), (0.33, 0.33) (0.34, 0.32)

(0.31, 0.35) (0.36, 0.35)

(0.33, 0.24)

(0.28, 0.28)

(0.34, 0.36)

(0.34, 0.36)


barcode


ref

135

133

132

130

129

128

127

126

125

119 120 124

118

117

116

114 115

113

111 112

103

101

98 99

97



D

d-block Ir3+ d-block Pt2+

d-block Pt2+

d-block Pt2+

d-block Pt2+

N966 FPt

Pt-4

PtL26Cl

PtL30Cl

dyes@Zr-MOF dyes@MOF Acf@MOF EuTb@IFP-1 Ln 3+@In1

[Ir(ppy)2(bpy)]+@ NENU-524 Alq3@NENU-521 iridium(III) complex

mixed f-block Eu/ Tb mixed f-block Eu/ Tb/Nd Sm/Yb mixed f-block Eu/ Tb/Gd

mixed f-block Eu/ Tb mixed f-block Eu/ Tb, Eu/Gd, mixed f-block Dy/Eu/Gd, Sm/Dy/Gd mixed f-block Dy/Eu/Gd, Sm/Dy/Gd mixed f-block Eu/ Tb mixed f-block Eu/ Tb/Gd Eu/Tb/ Tm mixed f-block Eu/ Tb/Gd mixed f-block Eu/ Tb, Sm/Dy, Nd/Yb mixed f-block La/ Ce/Pr/Sm/Eu


d-block Zr4+ d-block Zn4+ d-block Zn4+ f-block @MOF mixed f-block Eu/ Dy d-block Ir3+ s,p-block Al3+ d-block Ir3+

[Ln2(TATB)2 (DMSO) 6]·DMF·DMSO·H2O [LnL(glu)]n·2nH2O, [LnL(glu)(H2O)]n [Ln2(TDA)3(bipy)2 (H2O)2]·bipy·2H2O EuxTbyL, EuxTbyGd1−x−yL

{[Ln2(BDPO)1.5 (DMA)3 (H2O)]·5H2O}n SiO2@Ln-dpa

[Ln2(L)2 (DMAC)2]·nH2O

tdt[Ln(EDTA)]3

LnL

DyxEuyGd1−x−yL, SmxDyyGd1−x−yL

[Gd(3-SBA)(IP)OH (H2O)]·H2O

2∞[Ln2Cl6(bipy)3] ·(bipy)2

complex

Table 1. continued

hydrothermal postencapsulation

ppy, bpy, H2btca 2-NH2−H2bdc 8-hydroxyquinoline H3TPA, H2TDA 5-(9-carbazolyl)-2- phenyl-1,2,3-benzotriazole 1-methyl-2-phenyl imidazole 2-(4′,6′-difluoro phenyl) pyridinato

L30

L26

di(2-pyridinyl) benzene

postencapsulation postencapsulation postencapsulation postencapsulation solid solution

2,6-naphthalene dicarboxylic acid 1,2-Bis(4′-pyridyl methylamino)ethane Adenine benzene-1,3-dicarboxylate 2-methylimidazolate- 4-amide-5-imidate FDA, HFDA

2,6-di(2′,4′-dicarboxyl phenyl)pyridine

mononuclear

mononuclear

mononuclear

mononuclear mononuclear

guest@MOF guest@MOF mononuclear

guest@MOF guest@MOF guest@MOF guest@MOF 3D framework

3D framework

3D framework

hydrothermal

2D framework 2D topology

hydrothermal

TATB

core−shell

3D framework

hydrothermal

solid solution

dpa

L(2-(2-sulfophenyl) imidazo-(4,5-f) (1,10)- phenanthroline), glu TDA, bipy

hydrothermal

BDPO, DMA

3D framework

mixed-Ln-MOF

solid solution hydrothermal

mixed-Ln-MOF

solid solution

L, DMAC

mixed-Ln-MOF

solid solution

N-phenyl-N′-phenyl bicyclo[2,2,2]-oct-7ene-2,3,5,6-tetracarbox diimide tetracarboxylic acid L(4,4′-((2-((4-carboxyphenoxy)methyl)2-methylpropane-1,3-diyl)bis(oxy))dibenzoic acid) tdt, EDTA

mixed-Ln-MOF

mixed-Ln-MOF

structure features

solid solution

solid solution

assembly strategy

3-SBA, IP

bipy

ligand(s)

dual, blue-green, red

dual, blue-green, red

dual, blue, orange

wide, 440−800 dual, blue, orange

dual, 445, 570 broad wide

multi, RGB multi, RGB dual, 410, 510 multi, RGB multi, RGB

multi, RGY

multi

multi, RGY

multi, RGB

multi, RGY

multi, RGB

multi, RGB

multi, RGB

multi, RB

multi, RGB

multi, RB

multi, RG


(0.281, 0.360)

MLCT, π → π* monomeric, excimeric monomeric, excimeric monomeric, excimeric monomeric, excimeric

(0.36, 0.37), (0.37, 0.39)

(0.34, 0.35)

(0.33, 0.36)

(0.30, 0.34) (0.29, 0.33) (0.52, 0.47)

(0.33, 0.33)

(0.36, 0.34), (0.32, 0.33), (0.29, 0.32) (0.32, 0.34) (0.33, 0.33) (0.31, 0.35)

(0.40, 0.35)

(0.323, 0.339)

(0.3323, 0.3349)

(0.34, 0.37)

(0.329, 0.33), (0.331, 0.33)

(0.33, 0.34)

(0.33, 0.33), (0.33, 0.32)

(0.33, 0.33)


host, guest host, guest MLCT

host, guest guest LMCT (host) guest, host guest, host

LC, MC

LC, MC

LC, MC

LC, MC

LC, MC

LC, MC

LC, MC

LC, MC

LC, MC

LC, MC

LC, MC

LC, MC


WOLED

WOLED

WOLED

WOLED WOLED

OLED


ref

168

167

166

163 164

158 159 162

148 149 150 156 157

147

146

145

144

143

142

141

140

139

138

137

136



E

Zn

mixed f-block Eu/ Tb/Gd

Zn-MOF

[H2NMe2]3[Ln(dipic)3]

Eu3+/Cu2+@UiO-66-(COOH)2

mixed f-block Eu/ Tb mixed f-block Eu/ Tb d−f

[Eu1.35Tb1.65Zn6 (bipy)2(Hmimda)7]n 2-Eu/Tb

hydrothermal

dipic

H2hpi2cf

1,2,4,5-benzenetetracarboxylic acid

solid solution

solvothermal

hydrothermal

hydrothermal

PM, TP Hbpt H3imdc, imidazole

f-block Dy3+ f-block Dy3+ d−f

Dy(PM)3(TP)2 Dy compound Dy−Zn-MOF

hydrothermal hydrothermal hydrothermal

H2thTPDC

POP, dppb 2-NDCH 1,10-phen 1,4-NDCH2 1,10-phen H4ntc

d-block Cu+ s,p-block Mg2+ s,p-block Mg2+ s-block @MOF

[Cu(POP)(dppb)] BF4 Mg compound Mg-CP Sr-MOF

hydrothermal

phenylmethylimidazole

d-block Pd2+

Pd3O3

assembly strategy

bipy, Hminda

bzimb phenylmethylimidazole

d-block Pt2+ d-block Pt2+

Pt2+ complex 8 Pt2O2

Hppol

ligand(s)

d-block Pt2+


Pt2+ complex 5

complex

Table 1. continued

heteronuclear MOF coordination polymer mixed-Ln-MOF

mixed-Ln-MOF

mononuclear tetranuclear heteronuclear MOF mixed-Ln-MOF

mononuclear 0D 3D framework

mononuclear

mononuclear mononuclear

mononuclear

structure features

multi, RGB

blue-cyan

dual, 393, 615

multi, RGB

multi, RGB

broad, 486, 574, 664

dual, blue, yellow

single, broad, 494 dual, 360, 460 dual, 380, 540 multi

dual

dual dual

dual


LC, MC

LC (E*, K*)

LC, MC

LC, MC

LC, MC

LC, MC

monomeric, excimeric 3 IL, 3MLCT monomeric, excimeric monomeric, excimeric MLCT LC, LLCT LC, LLCT ILCT, LLCT, MLCT LC, MC


(0.31, 0.33)

(0.35, 0.40) (0.28, 0.29) (0.28, 0.33), (0.39, 0.44) (0.331, 0.342)

(0.33, 0.37)

(0.34, 0.47)

(0.35, 0.39) (0.46, 0.47)

(0.33, 0.42)


barcode, patterning

EL EL

LED

OLED

OLED

PHOLEDs OLED

WOLED


ref

188

187

184

183

182

177 178 181

174 175 175 176

173

170 172

169



Chemical Reviews 6. Conclusions and Prospects Author Information Corresponding Author ORCID Notes Biographies Acknowledgments Abbreviations References

Review

near-infrared (NIR) region. All these attributes greatly expand the emitting range and tuning modes compared to those of traditional organic or inorganic photoluminescent materials, thereby promoting substantial exploration of luminescent coordination assemblies for applications in such fields as photophysical color-tuning, optoelectronic devices, photoresponsive sensors/detectors, and so on. Especially, by the combination and modulation of luminescence from different origins generated in coordination assemblies, single-phase color-tuning and white-light-emitting materials can be readily designed and achieved by taking advantage of the facile coordination assembly procedure, which is easily controlled and predicted. It is worth noting that the latest research progress on the luminescence properties and applications of metal−organic coordination complexes has been intensively summarized in some earlier review papers,20−27 while WLE materials including small molecules and polymers for organic lightemitting diodes (OLEDs), inorganic semiconductors, and hybrid composites3,8,14,16,28,29 have also been well outlined. Nevertheless, little attention has been paid to a survey of photoluminescent metal−organic coordination complexes focusing on the WLE property realized in a single-phase material, for which emission color-tuning is inevitably involved in most cases, which itself is of importance in many fields such as sensing, labeling, imaging, and anticounterfeiting methodologies. The scope of this review is limited to photoluminescent coordination assemblies targeting SPWLE materials, specifically focusing on how to design and integrate various chromophore centers of distinct emissive nature into a singlephase material for the WLE property via photoluminescent color-tuning (PLCT) approaches. In the following sections, we will first give a brief description of the PL origins relevant to WLE/PLCT strategies and state-of-the-art assembly methods for single-phase WLE/PLCT coordination assemblies (see Table 1 for a representative list of WLE and PLCT coordination assemblies). Then the fruitful design models reported so far will be introduced by classification into three major categories (mono/homometallic, heterometallic, and guest-included systems, Scheme 1). Finally, the potential applications in such fields as WLEDs, sensing, detecting, barcoding, and patterning will be presented.

AM AN AN AN AN AN AO AO AP

1. INTRODUCTION The design and construction of color-tunable and white-lightemitting diodes (WLEDs) has received great attention due to their wide application in full-color flat-panel displays and solidstate lighting for the benefit of energy conservation and equipment miniaturization.1−4 Traditionally, white-light-emitting diodes (LEDs) are produced by mixing a blue LED with a yellow phosphor, or via blending discrete LEDs with red, green, and blue (RGB) primary colors.5−7 This may raise difficulties in the integration process and bring some intrinsic problems such as readsorption, phase separation, color variation, high cost, and complicated technology. To solve these problems, an alternative choice has been proposed in recent years to pursue single-phase white-light-emitting (SPWLE) materials with both finer color-controlling properties and higher luminescent efficiency. For example, by incorporating RGB chromophores into a single polymer chain, single-component white polymer light-emitting diodes (WPLEDs) have been fabricated successfully.8−10 Hybrid II− VI semiconductor bulk materials or nanocrystals exhibiting broad-band emission that covers most of the visible light spectrum can also bring direct and bright white light.11−14 In some studies, white light is achieved in lanthanide-metal-doped zeolites, organic compounds, or SiO2 glasses.15−17 On the other hand, white light can also be generated through an upconversion process by incorporating several lanthanide phosphors into one single inorganic matrix either in thin films or in oxide powders.18,19 All these materials provide novel candidates for the preparation of WLEDs or other kinds of solid-state lighting systems. In the past two decades, the enthusiastic studies in diverse metal−organic complexes assembled from metal ions and organic ligands via coordination driving forces offer new opportunities for developing white-light-emitting (WLE) materials by way of a single-phase strategy. The combination of organic and inorganic moieties into one coordination system can provide multiple emitting centers simultaneously. Furthermore, the encapsulation capability of some hollow coordination assemblies, especially porous metal−organic frameworks (MOFs) or metal−organic cages (MOCs), provides an excellent platform for doping multichromophores into the cavities of the single-phase coordination matrix to achieve tunable colors and white light emissions. According to the emitting chromophore and mechanism, the photoluminescence (PL) of metal−organic complexes can be generated from different origins: ligand-centered (LC) or excimer/exciplex (EX)-based emission, metal-centered (MC) or cluster-centered (CC) emission, ligand-to-metal charge transfer (LMCT) or metal-to-ligand charge transfer (MLCT) emission, guest-induced emission, and the combination of multiple emissive origins. The emitting band can cover from the ultraviolet (UV) region to the visible region and to the

2. PHOTOLUMINESCENCE ORIGINS RELEVANT TO SINGLE-PHASE WLE/PLCT COORDINATION ASSEMBLIES Photoluminescence is defined as the emission of light under stimulation of energy absorption typically in the form of photons. There are mainly two types of photoluminescence: (1) fluorescence generated by the radiative transition from singlet excited states (most often the lowest energy state S1) to the singlet ground state (S0) with the decay lifetime usually on the time scale of nanoseconds, (2) phosphorescence produced by the radiative transition from triplet excited states (usually T1) to the S0 state with longer decay dynamics on microsecond or even second time scales. Given the diversified structures and compositions of coordination assemblies, their PL behaviors can arise from different building components: organic ligand, excimer/exciplex, metal/cluster center, charge transfer state between metal and ligand, and guest chromophore introduced into the inner cavity or onto the surface. In the following, we will sort out and introduce briefly the various PL origins applicable to WLE/PLCT strategies. F


Chemical Reviews

Review

phosphorescence, generating ligand-centered (LC) emission in the coordination assemblies. Usually, after the coordination with metal/cluster centers, the stability and rigidity of organic ligands are enhanced; therefore, the nonradiative decay processes can be retarded, and the luminescent intensity and decay lifetime may be improved compared with those of free organic molecules. Moreover, the close packing of chromophores might lead to ligand-to-ligand charge transfer (LLCT), resulting in a bathochromic shift or blue shift of the emitting wavelength, as well as broadening of the emission band. Furthermore, the LC emission in coordination assemblies is also subject to environmental influence deriving from intraand intermolecular interactions, such as perturbation from the metal centers, orientation of the ligands, coordination geometries, solvents, guests, etc. Therefore, the reasonable control of these factors can be applicable and decisive in the tuning of emission colors and other photophysical properties in coordination assemblies. The relevant literature is abundant, so we only present two examples here. In a coordination framework assembled from pyridine-2,4,6-tricarboxylate and Zn2+, the photoluminescence is largely red-shifted to 467 nm from the original 415 nm in free ligand owing to metal binding.30 In the second example, the two-dimensional and three-dimensional Zn2+ frameworks from 4,4′-stilbenedicarboxylic acid and Zn2+ with the same building components but different coordination environments give rise to distinct PL behaviors, emitting green or blue light, respectively.31 We specified a unique kind of organic ligand characteristic of the excited-state intramolecular proton transfer (ESIPT) process. In such an ESIPT-type ligand, the proton donor can coexist with the acceptor, and reversible proton transfer can occur between two sites, thereby resulting in abrupt luminescent changes. For example, in an ESIPT ligand showing enol−keto tautomerism, the enol form constitutes the stable ground state (E). Upon UV radiation, the molecule transits from the ground enol state (E) to the excited enol state (E*), and then a fast ESIPT process happens, leading to the excited keto state (K*). When the K* state returns to the ground keto state (K), so-called keto emission is generated; while the emission generated during transition from E* to E is called enol emission (Scheme 2). The keto emission usually shows a larger Stokes shift, in comparison to enol emission, and its emitting wavelength and intensity are easily affected by environmental factors such as solvents. Therefore, ESIPTtype ligands can provide promising potentials in fabrication of color-tuning and photoresponsive coordination assemblies.32−34

Scheme 1. Three Types of Design Strategies for SinglePhase WLE/PLCT Coordination Assembliesa

a

Type I = wide-band WLE from monomer or excimer/exciplex emission in monometallic complexes and a combination of multicolor emissions from LC (ligand-centered), MC (metal-centered), and MLCT/LMCT states in homometallic complexes, type II = combination of multicolor emissions in heterometallic complexes, and type III = guest-included complexes (see more detail in section 2.5).

Scheme 2. Energy Absorption, Migration, and Emission Processes in ESIPT-Type Ligands

2.1. Ligand-Centered (LC) Photoluminescence

Organic molecules containing conjugated π luminophores are widely applied as ligands in the fabrication of photoluminescent coordination assemblies in either discrete or infinite structural character. Upon absorption of photons with appropriate energy, a series of photophysical processes may happen within the organic ligands, including inner conversion, vibrational relaxation, intersystem crossing, and fluorescence or G


Chemical Reviews

Review

As a demonstrating example, assembly of a Mg2+-MOF with ESIPT-type ligand DHT (2,5-dihydroxyterephthalate) gives rise to distinct solvo-fluorochromism determined by the ESIPT character of the ligand. In the protonic solvent ethanol (EtOH), the Mg2+-MOF shows emission peaks at 404 and 429 nm, while in aprotic DMSO, the emission is substantially redshifted to 508 nm; even in water, the emission maximum is further shifted to 532 nm (Figure 1).35

Figure 2. Zn2+-4,4′-bipy MOFs possess exciplex emissions, showing (a) edge-to-face and (b) face-to-face packing with different emission spectra. Reprinted from ref 36. Copyright 2007 American Chemical Society.

2.3. Metal-Centered (MC) Photoluminescence

Metal-centered (MC) photoluminescence in coordination assemblies can be generated by the f → f transitions (sometimes including d → f transitions wherein inner-shell d orbitals are involved) of lanthanide ions, d → d transitions of transition-metal ions, and s → p transitions of some maingroup-metal ions, as well as so-called cluster-centered luminescence encountered in coordination assemblies containing multinuclear metal-cluster building units. Basically, the f → f transitions of Ln3+ ions are the most frequently applied, featuring sharp emission peaks and long decay lifetimes (up to the microsecond or millisecond time scale) of specific colors. For example, Eu3+, Tb3+, Sm3+, and Tm3+ can emit typically visible luminescence of red, green, orange, and blue colors, respectively, while Yb3+, Nd3+, and Er3+ emit in the nearinfrared region, depending on the energy gap between the emitting and ground levels of different Ln3+ ions (Scheme 3).37,38 Due to very low efficiency in absorbing photons by parity forbidden f → f transitions of Ln3+ ions, organic ligands are

Figure 1. Normalized emission spectra of Mg2+-MOF with ESIPT ligand in different solvents and corresponding photographs of the solutions under UV irradiation: (a) ethanol, (b) DMSO, (c) water, and (d) DMF. Reprinted with permission from ref 35. Copyright 2010 Royal Society of Chemistry.

2.2. Excimer or Exciplex (EX)-Based Photoluminescence

An excimer is usually defined as a dimer in an excited state, while an exciplex refers to a heterodimeric case (or more than two species) in an excited state. The formation of an excimer or exciplex in coordination assemblies can be induced by the orientation of two chromophores especially in the aggregated assemblies upon absorption of a photon, which is more or less associated with the π−π stacking states. The resulting emission is often red-shifted compared with that of the monomeric molecule. In an example reported by McManus and coworkers,36 two Zn2+-4,4′-bipy MOFs having 1D ladder and 2D square grid topologies both possess the characteristic exciplex emission. The exciplex emission in the 1D complex with faceto-face packing states shows a longer wavelength and a shorter decay lifetime compared with the 2D structure, in which the exciplex energy is associated with edge-to-face stacking (Figure 2). The excimer or exciplex emissions are very common and important in the assembly of WLE complexes and devices, such as the Pt2+-complex-based organic light-emitting diodes (OLEDs) and other homo- or heteronuclear coordination assemblies, which will be further discussed in the following sections.

Scheme 3. Partial Energy Diagrams for the Ln3+ Ionsa

a

The main emitting levels are drawn in red, and the ground level is indicated in blue. Reprinted with permission from ref 37. Copyright 2005 Royal Society of Chemistry.

H


Chemical Reviews

Review

profiles and shorter decay lifetimes on the nanosecond time scale. Especially, bright d → f blue emission has been reported in some Ce3+ complexes,46 which meets the urgent need of strong blue-light-emitting chromophores for the mixture of white light. Some Ln3+ ions of di- or tetravalence (Eu2+, Tb4+, Sm2+, Yb2+) are also highly luminescent with d → f emissions.48 Compared with the characteristically sharp f → f emission of Ln3+ ions, MC emissions from transition-metal or main-groupmetal complexes are usually very broad, covering hundreds of nanometers in many cases. This is due to the fact that the excited states of transition-metal or main-group-metal ions differ greatly from their ground states, leading to a wide emitting band and large Stokes shift, similar to the d → f emission of Ln3+ ions (e.g., Ce3+ and Eu2+). Usually, d → d transition undergoes radiationless deactivation, and emission is only seldom observed for the first series of transition metals, e.g., in Cr3+ complexes.49 Unfortunately, this kind of emission is very weak, and only observable in robust matrixes under low temperature. Comparatively, relatively strong s → p emission can be achieved in the main-group-metal complexes. Especially, in semidirectionally coordinated Pb2+ and Bi3+ complexes, the existence of lone pair electrons on the metal s orbitals results in strong s → p emission with a long emitting wavelength, benefiting construction of coordination assemblies for WLE.50

utilized to form coordination assemblies for the sake of light adsorption. Taking advantage of the so-called “antenna effect”, the ligand absorbs the photon energy, transiting from the singlet ground state 1S0 to excited state 1S1 or 1S2. Intersystem crossing (ISC) then follows, leading to triplet state 1T, which can pass energy via an energy transfer (ET) process to the accepting levels of Ln3+ ions to emit the characteristic f → f luminescence. Generally, the energy gap between the triplet 1 T1 state of the ligand and the accepting f-level of Ln3+ will determine the ET efficiency. Under certain circumstances, the energy migrates from the ligand to the Ln3+ ion almost in total to result in high sensitization of the Ln3+ luminescence, and the ligand emission is completely quenched. In the cases where energy migration takes place partly and the ligand emission is reserved, the remaining emission (often in blue color) of the ligand can be combined with the red emission of Eu3+ and green emission of Tb3+ to result in white light. This kind of emission combination has been widely applied in SPWLE lanthanide materials and will be further illustrated in the following sections. Besides the above most commonly accepted 1T → Ln3+ ET route, there are also some indications that the energy can be directly transferred from the singlet 1S1 state to the f-levels of Ln3+ ions, called singlet sensitization.39,40 Moreover, the ET process has been found to be able to take place directly from or via the bridge of ILCT (intraligand charge transfer) states (Scheme 4).41−43

2.4. Charge Transfer (CT) State Based Photoluminescence

Charge transfer (CT) emission is produced in an electron donor−acceptor system during the transition from excited CT states to the ground state, among which LMCT and MLCT are the two most typically observed CT states in coordination assemblies. LMCT refers to the partial electron transfer from ligand-localized orbitals to metal-localized orbitals, which occurs easily in coordination complexes containing maingroup-metal or transition-metal cations with high oxidation states, especially those with d0, s0, or p0 vacant orbitals (such as Pb2+ and Bi3+). In some coordination complexes with the same ligand but different kinds of metal centers, the tendency to undergo the LMCT process is directly relevant to the electronaccepting ability of the metal ions. Higher oxidation potency of the metal ions usually leads to a smaller transition energy gap, and LMCT states are therefore easier to achieve. On the contrary, the MLCT process means partial electron transfer from metal-localized orbitals to ligand-localized orbitals, which is frequently observed in the second or third series of transition-metal complexes. In these complexes, the metal centers usually have d6, d8, or d10 configurations, and the ligands have vacant π* orbitals with low energy levels. For some valence-variable metal ions residing in the low valence state (such as Ru2+ and Ir3+), the MLCT process can easily happen, in which the electrons are partially transferred from the d orbitals of the metal centers to vacant π* orbitals of the ligands. In general, both LMCT and MLCT emissions will show energy shifts compared with the pure ligand-centered emission (Scheme 5).51

Scheme 4. Energy Absorption, Migration, Emission (Plain Arrows), and Dissipation (Dotted Arrows) Processes in Ln3+ Complexes Involving an ILCT-Type Liganda

a

Abbreviations: LC, ligand-centered; MC, metal-centered; ILCT, intraligand charge transfer; S, singlet state; T, triplet state; A, absorption; F, fluorescence; P, phosphorescence; nr, nonradiative; ISC, intersystem crossing; ET, energy transfer.

In some d−f or p−f heteronuclear coordination assemblies, MLCT or LMCT states can also transfer energy to the Ln3+ ions.44 Furthermore, in some multi-Ln3+ codoping complexes, energy transfer can happen between Ln3+ ions with different energy levels. For example, ET can occur from Tb3+ to Eu3+, leading to intensified red emission of Eu3+.45 Last but not least, d → f transition can result in bright luminescence for lanthanide complexes. Different from normal f → f emission, the parity-allowed electric-dipole d−f transitions of some Ln3+ ions can lead to much higher light output with wider emission

2.5. Single-Phase WLE and PLCT Strategies

The WLE and PLCT properties can be easily achieved through a single-phase strategy by fully applying the designability and controllability of metal−organic complexes in the form of discrete structures of Werner-type complexes, polynuclear complexes, MOCs, or infinite structures of coordination polymers (CPs) or MOFs, and even guest-included complexes. I


Chemical Reviews

Review

white light emission, which incorporate mixed Ln3+ ions into a single-phase multilanthanide isomorph. For example, red and green luminescence is characteristic of Eu3+ and Tb3+ ions, respectively, while blue luminescence could be obtained from the appropriate coordination environment of the Ce3+ ion from 5d → 4f transitions. Therefore, a solid solution comprising these three lanthanide emitting centers can, in principle, generate tunable PL and white light emission through growth of isomorphous crystals. Nevertheless, the efficient blue emission of Ce3+ is not common in coordination complexes, though sometimes it can be achieved depending on the ligand field and crystal field splitting.46,47 In terms of practical effect, the blue emission is more often supplied by a Gd3+- or La3+coordinated organic ligand itself (LC emission), instead of the uncertain blue emission from the d−f transition of the Ce3+ ion compared with the steady red and green emissions from the f− f transitions of Eu3+ and Tb3+ ions. Type III is based on coordination assemblies capable of introducing additional emitting centers through guest inclusion. Such a type of system is characteristic of porous structures such as MOFs or MOCs, the host and the included guests of which can serve as different color-emitting chromophores. To achieve designable and controllable PL color-tuning and white light emission in single-phase coordination assemblies, the following considerations and practices might be instructive when applying the above three strategies. First, from the viewpoint of ligand design, apart from the coordination ability, the organic chromophore itself can donate certain color (e.g., blue emission) as a compensation for what is usually lacking in MC emission of Ln complexes. Simultaneously, the efficient energy transfer from ligand to metal is usually essential for the luminescence efficiency of the metal chromophore (so-called antenna effect). Therefore, a balance between the ET part to the metal and the residual PL part from the ligand itself should be elaborated, so as to meet the efficient WLE purpose. Normally, proper energy gaps between the excited states of the donating ligand and the accepting metal are decisive. Meanwhile, besides the typical ET pathway from the T1 state of the ligand to metal center, more extended energy levels are available in predesigned ligands with alternative excited states, such as ILCT, ESIPT, etc., which are helpful for the broad and efficient sensitization of the whole series of Ln3+ ions.35,42 This offers more choices of chromophoric ligands, and along this line of design and modification of organic ligands, a total emitting efficiency from various chromophores can be tuned, balanced, and enhanced. Second, from the viewpoint of metal centers, some universal principles in relation with the singlephase WLE and PLCT strategies might be generalized: (1) For main-group-metal (s,p-block) complexes, ILCT and LMCT emissions are mostly involved. Moreover, the lone electron pair effects of metal ions such as Pb2+ can be utilized to provide additional MC emission. (2) For transition-metal (d-block) complexes, PL color-tuning and white light emission are usually associated with the combination of π → π* or MLCT transitions with higher energy levels, in comparison with excimer/exciplex (EX) contributors with lower energy levels. (3) For lanthanide (f-block) complexes of homonuclear, heteronuclear, or guest-included hybrid compositions, colortuning and white light emission can be achieved by incorporating variously luminescent Ln3+ ions, chromophoric ligands, and/or additional organic/inorganic emitters into one single-phase coordination assembly, the different kinds of chromophores of which can be located in either the host or the

Scheme 5. Schematic Representation of Different Types of Charge Transfer in MLn Coordination Complexesa

a

Reprinted with permission from ref 51. Copyright 2013 Royal Society of Chemistry.

Especially for the multivariate or multifunctional coordination assemblies, multiple inorganic (metal-centered) and organic (ligand-centered) chromophores can be allocated rationally into single-phase coordination assemblies. However, to reach the aim of efficient and controllable PL color-tuning and white light emission, both chemical and physical effects must be taken into consideration. First, the effective structural design and modification is essential to make metals/clusters, ligands, and inclusion guests adaptable to each other, including the structural configurations which determine the coordination preferences, and the electronic states which define the absorption, energy transfer, and emission properties. Second, from a photophysical view, PLCT and especially WLE can be achieved with different approaches by putting different numbers of chromophores into a single-phase coordination assembly. For example, the monochromatic approach utilizes a single emitting chromophore with wide-band emission covering the whole visible spectrum, while the dichromatic approach applies two complementary emitting chromophores (such as blue and yellow/orange), and the tri- or multichromatic approaches comprise three primary RGB (red, green, and blue) or more emitting chromophores. On the basis of the above considerations, we sort out three major strategies for the design and construction of single-phase WLE and PLCT coordination assemblies as shown in Scheme 1. Type I is based on coordination assemblies containing only one kind of metal center (mono- or homometallic) with either discrete or infinite structures. In this case, the emitting origins can come from MC (including s/p-, d-, and f-block metals or metal clusters), LC, EX, or LMCT/MLCT states, which are able to generate a single wide-band emission or overlapped multiple emissions covering the whole visible spectrum. As a result, tunable color and white light emission can be obtained in a single phase by control of the environmental conditions such as the solution, concentration, temperature, excitation wavelength, and so on, if necessary. Type II is based on heterometallic coordination assemblies, in which the cooperation of Ln3+ centers with another kind of metal center is the most frequently used approach to construct d−f, s−f, or p−f heteronuclear coordination assemblies as the single-phase WLE and PLCT materials. In these structures, the Ln3+ center usually serves as one source of luminescence, and the other type of metal center or metal-binding ligand supplies complementary luminescence. Alternatively, the construction of so-called solid-solution-type f−f isomorphous structures can provide another approach to achieve facile PL color-tuning and J


Chemical Reviews

Review

guest positions. Third, from the viewpoint of synergetic efficiency and application capacity in practical materials and devices, the spatial allocation, separation, and concentration of different chromophores combined in one single-phase coordination assembly, as well as the morphology (physical form) of materials, have to be delicately controlled. For instance, appropriate spatial separation among different Ln3+ ions by the organic linkers in f−f heteronuclear complexes can effectively prevent both the aggregated quenching effect and unwanted energy transfer among each other, so as to achieve a strictly linear PL color-tuning behavior. On the other hand, the spatial segregation of different chromophores in one singlephase coordination assembly can be smartly controlled by such methods as epitaxial growth or copolymerization, offering opportunities for fabrication of fine color-tuning materials or miniaturized optoelectronic devices. In the following sections, more detailed elaboration involving different single-phase WLE and PLCT strategies will be introduced with selected examples.

Scheme 6. Assembly Methods for Single-Phase WLE/PLCT Heterometallic or Guest-Included Coordination Assemblies

reaction, and tunable luminescence from yellow to white was obtained through the doping of Eu and Tb ions in this La−Zn heteronuclear framework. The Ln3+ ion is eight-coordinated in a triangular dodecahedron environment with eight O atoms from four imdc3− anions, and the Zn2+ ion is five-coordinated in distorted trigonal-bipyramid geometry with one N atom from im− and two N and two O atoms from imdc3− anions (Figure 3). The O- and N-coordinating ends on the imdc3−

3. ASSEMBLY METHODS FOR SINGLE-PHASE WLE/PLCT COORDINATION ASSEMBLIES Owing to versatile coordination assembly strategies based on plentiful metal ions of specific photophysical nature and diversified organic ligands easy to tailor and modify, the combination and modulation of photoluminescence with different origins can be readily implemented in coordination assemblies to achieve single-phase WLE and PLCT properties. As described above, three major categories of single-phase WLE and PLCT coordination assemblies can be classified on the basis of the emitting metal centers involved in luminescence, namely, mono/homometallic (type I), heterometallic (type II), and guest-included (type III) coordination assemblies. For the type I complexes, the assembly methods, despite versatility in view of structural diversity and complexity, have been relatively straightforward and well outlined in earlier reviews.20,52 Therefore, we will not discuss their construction strategies in detail herein. Instead, we will focus on some stateof-the-art assembly methods of single-phase WLE/PLCT coordination assemblies with type II and type III compositions, including the heterotopic ligand method, metalloligand method, statistic replacement method, epitaxial growth method, and copolymerization method for the assembly of single-phase heterometallic coordination assemblies, and postencapsulation processes to incorporate functionalized guests into the host structures for the aim of color-tuning and white light emission (Scheme 6).

Figure 3. Coordination environment of Ln 3+ and Zn 2+ in [H(H2O)8][LnZn4(imdc)4(Him)4]. Reprinted from ref 53. Copyright 2012 American Chemical Society.

anion show an obviously distinct preference between Ln3+ and Zn2+ ions.53 It should be noted that, due to the fact that preferential coordination of O and N donors toward many transition-metal ions is not sufficiently distinctive with regard to thermodynamic and kinetic considerations, this one-pot assembly method is not always applicable, and in some cases, mixed O- and N-coordinating assemblies of homometallic structure rather than heterometallic structure will be formed.

3.1. One-Pot Assembly Applying Heterotopic Ligands

3.2. Stepwise Assembly Applying Metalloligands

Organic ligands with heterotopic dative sites showing different coordination preferencesfor example, O-containing groups preferring relatively “hard” metal ions and N-containing groups tending to somewhat “soft” metal ions according to the wellknown hard and soft acids and bases theory (HSAB)have been substantially applied for the assembly of heteronuclear coordination complexes. The assembly procedure is typically a “one-pot” reaction, in which the heterotopic ligand and two different kinds of metal salts are mixed together in solution, and the heteronulear complexes are formed via an in situ selfsorting process. As an example, 3D d−f heterometallic lanthanide−zincMOFs [H(H2O)8][LnZn4(imdc)4(Him)4] (Ln = La, Pr, Eu, Gd, Tb; H3imdc = 4,5-imidazoledicarboxylic acid; Him = imidazole) were synthesized via an in situ hydrothermal

To avoid mismatch between different types of metal ions and coordination sites, the assembly of heteronuclear complexes is often accomplished via a “stepwise method”, in which one kind of metal center is incorporated with a predefined coordination set first, and then further linked with another kind of metal center into a heteronuclear structure. This method imparts a starting building block with only one kind of free coordination donor, which benefits the convergence of the assembly process in the sense that it represents a “clusterlike” node and meanwhile provides better control over the construction of heteronuclear structures.54 Such a stepwise method is also called the “metalloligand method” in the literature, the preorganized coordination precursor of which is called the metalloligand. The metalloligand offers structural rigidity that places the auxiliary donors in a predefined conformation, and K


Chemical Reviews

Review

facilitates coordination with the secondary metal ions into heteronuclear structures.55 In view of the thermodynamic and kinetic attributes, the metalloligand is usually stable enough to be “inert” to resist dissociation between the ligand and the first metal ion during assembly with the second metal ions, while the coordination interactions between the metalloligands and the secondary metal ions are more labile and kinetically favored, avoiding coordination competition between the first and the second metal ions and destruction of the predesigned metalloligand. Up to now, different metalloligands based on pblock, d-block, or f-block metal ions have been widely applied for the assembly of single-phase WLE and PLCT coordination assemblies. Scheme 7 shows a typical WLE d−f heteronuclear complex assembled from the Ir3+ metalloligand. The Ir3+ metalloligand

Scheme 8. Stepwise Assembly of a 4d−4f Heteronuclear MOF Using the 3-TPyMNTB Ligand57

donor set and the monodentate pyridyl arms on the ligands exhibit discriminable coordination tendency during the stepwise assembly route, although both are with Ncoordinating ends. The reason is that the preassembled [Ln(3-TPyMNTB)2]2+ metalloligand has sufficient thermodynamic stability owing to the chelating effect of the tetradentate tripodal benzimidazolyl N-donor set, which remains intact during secondary assembly with the Ag+ ions because of the labile nature of the Ag−N bonds. Importantly, the combination of Ag+-sensitized ligand emission with characteristic Eu3+ emission results in direct white light in this heteronuclear Eu− Ag-MOF, which will be further discussed in the following section.57 Liu and co-workers58 applied another kind of Ln metalloligand (O-ending) to synthesize photoluminescent d−f heteronuclear complexes. The Ln metalloligand was prepared by reaction of Ln(ClO4)3·6H2O with H2ODA (oxydiacetic acid), and further coordinated with Cd2+ to afford heteronuclear Ln−Cd complexes (assigned as 2 in Figure 4). Control experiments using a “one-pot” method to synthesize 2 only resulted in a very low yield (20%, while for low contents of europium doping, the distribution of the luminescence centers was almost ideal without macroscopic phase segregation. In doped complex [Ba1−xEux(Im)2], which is obtained from the partial replacement of Ba2+ by Eu2+ (x < 0.2), the d−f transitions of Eu2+ are shifted toward a yellow emission compared to the green emission in similar doped complex [Sr1−xEux(Im)2], due to the difference of the host lattice ion.62

Figure 6. Illustration of the epitaxial growth method for producing layer-by-layer WLE Ln-MOF materials and optical microscopy of RMOF, B-MOF, and G-MOF under UV excitation. Reprinted with permission from ref 63. Copyright 2014 Royal Society of Chemistry.

seeds during 3 days. Importantly, although the microscopy pictures displayed transmetalation regions in the epitaxially grown crystals, lifetime values gave evidence that the ET among the different layers is not significant. As a result, pure white-light emissions were obtained in the layer-by-layer architectures with CIE (International Commission on Illumination) coordinates of (0.337, 0.336), (0.339, 0.330), (0.338, 0.337), and (0.333, 0.336) for the RGB-MOF, RBGMOF, BRG-MOF, and BGR-MOF, respectively. Recently, our group reported a series of bimetallic and trimetallic hierarchical Ln-MOF single crystals with core−shell or striped models through the anisotropic epitaxial growth method, which gave potential applications to miniaturized multicolor displaying, barcoding, anticounterfeiting, and white light illuminating, etc. (Figure 7). The hierarchical crystals were attained from a two-step epitaxial process: First, monometallic crystals (e.g., Eu3+) were produced from the self-assembly of Ln3+ ions and TMPBPO (1,1′-(2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene)dipyridinium-4-olate) ligand, which were employed as seeds. Then the seed crystals were immersed into the solution of mixed acetone/H2O (v/v = 3/1) including another metal ion (e.g., Tb3+) and TMPBPO. Slow diffusion of acetone promoted successful epitaxial growth of second layers outside the seed crystals into hierarchical single crystals, which showed core−shell-like or striped-like configurations. Both of these core−shell or striped epitaxially grown crystals were investigated by a micro-Raman spectrometer, and multicolor to white-light emission can be read out by changing the positions and orientations.64 Gu and co-workers65 reported a modified liquid-phase epitaxial (LPE) pump method to achieve a single-phase Ln3+compound-encapsulated MOF film with white-light emission (Figure 8). Through this approach, Ln(pdc)3@HKUST-1 (Ln = Eu, Tb, or Gd) films were obtained easily, which exhibit red, green, and blue color when excited at 365 nm. The hybrid Ln(pdc)3 mixed by Eu(pdc)3, Tb(pdc)3, and Gd(pdc)3 was encapsulated into HKUST-1 for homogeneous composite films. A white@HKUST-1 was achieved, and the component ratio of Gd3+/Eu3+/Tb3+ was determined to be 0.0131/0.157/

3.4. Epitaxial Growth Method

Epitaxial growth of a coordination structure on the surface of another one represents an interesting alternative method for the production of single-phase multicolor-emitting materials. Compared with the random distribution of color centers in doping methods, epitaxial growth endows the ordered engineering of multiemitting layers based on the continuity between single-crystal interfaces. Each layer can still act as an independent crystal and maintain its intact optical properties, which permits better control of ET processes without intramolecular interference, and therefore enables the effective construction of WLE and PLCT materials with desirable production of color tones. Rodrigues and co-workers63 reported the first example of visible WLE and color-tunable Ln-MOF using distinct layer-by-layer epitaxial growth (Figure 6). First, phase-pure and single-color-emitting [Ln2(Mell)(H2O)8] (Mell = mellitate anion; Ln = Eu3+, Tb3+, and Gd3+ corresponding to R-MOF, G-MOF, and B-MOF, respectively) were grown into relatively large crystals with dimensions varying from 1.39−1.95 × 1.39−1.91 × 0.62−1.90 mm. These crystals were used as seeds (cores) to further grow into multilayered core−shell structures, in which the seeding crystals were supported on the hypodermic needles and immersed in the protective needle caps containing unreacted solution of a different kind of Ln3+ ion and ligand precursor. The subsequent layers can be grown outside the original crystal N


Chemical Reviews

Review

Figure 8. Modified LPE pump method and photographs for growth of Ln(pdc)3@HKUST-1 thin films on functionalized substrates. Reprinted from ref 65. Copyright 2015 American Chemical Society.

the triplet energy state of the applied NTB-type ligand is slightly elevated, which becomes more efficient for ET to Tb3+ than Eu3+, and balances the ratio between green and red emissions from these two kinds of metal centers. As a result, tunable color to white-light emission with an overall luminous efficiency of >5% was achieved in the Eu/Tb copolymerized single-phase materials, providing promise for application in WPLEDs.66 A similar approach was also applied to obtain pure WLE Eu/Gd co-metallopolymers.67 Alternatively, color-tunable solid-solution-type Ln complexes can be incorporated into PVA (poly(vinyl alcohol)) for the preparation of single-phase polymeric nanofibers using electrospinning method. As shown in Figure 9, for the LnMOF@PVA materials containing ([Ln(DPA)(HDPA)] (H2DPA = pyridine-2,6-dicarboxylic acid) Ln compounds, the emission spectra of Tb0.95Eu0.05MOF@PVA, Tb0.8Eu0.2MOF@PVA, and Tb 0.5 Eu0.5 MOF@PVA show intensity Figure 7. Representative demonstration of anisotropic epitaxial growth of bimetallic and trimetallic hetero-Ln-MOFs from monometallic crystal seed and photographs of bimetallic and trimetallic hierarchical single crystals showing visually distinguishable colors. Reprinted with permission from ref 64. Copyright 2017 John Wiley & Sons.

0.830 by ICP technology. The corresponding CIE coordinates of this WLE coordination assembly film were (0.37, 0.38), with CRI (color rendering index) and CCT (correlated color temperature) being 82.8 and 5880 K, respectively. 3.5. Copolymerization Method

Recently, the copolymerization method incorporating multiple Ln3+ chromophores into one single-phase metallopolymer material provides an effective approach to construct WLE and PLCT coordination assemblies. As an example, we successfully fabricated a series of homo- or hetero-Ln-metallopolymers, through incorporation of different Ln3+ coordination monomers onto a PMMA polymer backbone, to achieve multicolored PL and white-light emission. After copolymerization,

Figure 9. CIE diagram and images of EuMOF@PVA, TbMOF@PVA, Tb0.95Eu0.05MOF@PVA, Tb0.8Eu0.2MOF@ PVA, and Tb0.5Eu0.5MOF@PVA deposited on the substrates upon excitation at 254 nm. Reprinted with permission from ref 68. Copyright 2013 Royal Society of Chemistry. O


Chemical Reviews

Review

changes between the typical Eu3+ and Tb3+ red/green emissions, thereby resulting in the emitting colors changing from green-yellow (0.41, 0.48) to yellow (0.44, 0.46) to orange (0.50, 0.42).68 Venkatesan and co-workers69 synthesized a new class of cyclometalated pyridine N-heterocyclic carbene (NHC)−Pt2+ complexes, doping in poly(methyl methacrylate) (PMMA) films to result in single-phase WLE material. These Pt2+ complexes show tunable monomeric high-energy triplet emission and concentration-dependent low-energy phosphorescence. The combination results in excellent white-light emission with CIE-1931 coordinates (0.31, 0.33). The alkyne ligands play the role of a deep blue chromophore, and the square planar geometry of Pt2+ provides the possibility of intermolecular interactions, resulting in aggregation and leading to lower energy emission. By effectively controlling the weight percentage of Pt2+ complexes in PMMA films from 1 to 80 wt %, an overall phosphorescence emission covering the entire visible spectrum will be produced, realizing a singlemolecule white-light triplet emitter. At low concentration, the origin of the monomeric emission is assigned to the excited state of an admixture of 1LLCT, 3LLCT, 1MLCT, and 3MLCT, while with the increasing weight percentage, the low-energy excimer emission appeared, and a maximum emission QY was obtained at 20 wt %.

biphenyldicarboxylic acid) with etb topology, which possesses the largest 1D hexagonal nanotube-like channels of 24.5 × 27.9 Å ever reported. Guest-included JUC-48·Rh6G can be obtained by immersing JUC-48 for 36 h in ethanol solutions of Rh6G with different concentrations. Interestingly, the emission peaks exhibit a blue shift (574, 567, and 555 nm) when the concentration of the Rh6G solution used to prepare JUC-48·Rh6G is 10−3, 10−4, and 10−5 M−1, respectively, manifesting that the Rh6G dye in the crystals is present as free monomers and shows fine fluorescence properties. The emission spectra of JUC-48·Rh6G are temperature-dependent, for which the emitting intensity at 563 nm was enhanced linearly when the temperature was lowered from 298 to 77 K. Therefore, the postencapsulation material can serve as a potential candidate for applications in temperature-sensing devices. Wang and co-workers73 reported the incorporation of a laser dye, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), into a stilbene-based MOF and naphthalene-based MOF (IRMOF-8) under ultrasonication. Besides the blue emission peaks of pure MOFs at 405 and 410 nm, new red emission bands originated from DCM guest molecules appeared at 590 nm in both DCM@stilbene-MOF and DCM@IRMOF-8, exhibiting two-color-tunable luminescence (Figure 11). Both experimental and theoretical calculations show that there is ET from the host to the guests. Furthermore, after treatment with different volatile organic solvents, the relative intensity ratios between the blue and red emissions show selective changes, which can be applied for ratiometric luminescent sensors. Li and co-workers74 successfully encapsulated a yellowemitting iridium complex [Ir(ppy)2(bpy)]+ or Eu/Tb3+ into a mesoporous blue-emitting Cd-MOF, [(CH 3 ) 2 NH 2 ] 15 [(Cd2Cl)3(TATPT)4]·12DMF·18H2O (H6TATPT = 2,4,6tris(2,5-dicarboxylphenylamino)-1,3,5-triazine), containing a giant cavity and window size (15.5 × 15.5 Å2) to result in bright-white-light emission. For the pocket of [Ir(ppy)2(bpy)]+ guest molecules with a size of ∼10 × 11 Å2, the powder samples of Cd-MOF were immersed into DMF solutions of the Ir complex for a couple of days. An obvious color change from colorless to light yellow manifests the successful encapsulation of [Ir(ppy)2(bpy)]+ into the matrix of Cd-MOF. With different concentrations of encapsulated guests, the observed PL color changed from pure blue of the original Cd-MOF to white (3.5 wt % guest) and yellow (8.8 wt % guest), respectively (Figure 12). Soaking Cd-MOF in DMF solutions containing nitrate salts of Eu3+ and Tb3+ in a molar ratio of 1:2 also leads to a WLE compound.

3.6. Postencapsulation Method

MOFs have highly ordered and pore-size-designable hollow structures, which can accommodate emitting guests such as Ln3+ ions and fluorescent dyes to tune the photophysical properties.70 Petoud, Rosi, and co-workers developed a new class of porous metal adeninate materials named “bio-MOFs” with formula [Zn8(ad)4(BPDC)6O·2Me2NH2·8DMF·11H2O] (ad = adeninate; BPDC = biphenyldicarboxylate) as rigid and permanently porous (∼1700 m2/g) structures. The samples were soaked in DMF solutions of nitrate salts of Tb3+, Sm3+, Eu3+, or Yb3+ to introduce Ln3+ ions into the pores. EDX spectroscopy and elemental analysis (EA) manifest the successful incorporation of the Ln3+ cation into the matrix to afford Tb3+@bio-MOF-1, Sm3+@bio-MOF-1, Eu3+@bioMOF-1, and Yb3+@bio-MOF-1, which can retain the original crystallinity as confirmed by powder XRD. As shown in Figure 10, pink, green, and red emissions can be obtained in these

4. DESIGN MODELS FOR SINGLE-PHASE WLE/PLCT COORDINATION ASSEMBLIES Applying various preparation methods, including but not limited to those described in the above section, a variety of single-phase color-tunable and WLE coordination assemblies have been designed and constructed in the past few years. Generally, the metal ions in these complexes not only contribute to the robustness and integrity of the coordination skeletons, but also serve as one or multiple emitting center(s) in most cases, playing important roles in the modulation of WLE and PLCT properties. Therefore, in the following section, we introduce the design models for the single-phase WLE/PLCT coordination assemblies based on the classification of emitting metal centers. This will give an insight into

Figure 10. Bio-MOF encapsulation and sensitization of Ln3+ cations. Reprinted from ref 71. Copyright 2011 American Chemical Society.

postencapsulated MOFs, which exhibit reasonably high quantum yields even in a water environment, manifesting the efficient protection of luminescent Ln3+ ions within the pores, and enhanced ET from the sensitizer embedded in the MOF to the Ln3+ centers.71 Zhu, Qiu, and co-workers72 prepared an MOF structure of [Cd3(bpdc)3(dmf)]·5dmf·18H2O (JUC-48; H2bpdc = 4,4′P


Chemical Reviews

Review

applying the above-introduced WLE and PLCT strategies, i.e., mono/homometallic systems containing emissive main-groupmetal (s, p-block), transition-metal (d-block), and lanthanidemetal (f-block) ions, heterometallic systems containing mixed metal (s/p−f, d−f, f−f) centers, and guest-included systems containing additional emissive metal centers through guest encapsulation. The structural models and photoluminescence properties in these three types of coordination assemblies will be concisely surveyed. 4.1. Single-Phase Mono/Homometallic Coordination Assemblies

4.1.1. Main-Group-Metal (s,p-Block) Centered Complexes. Coordination assemblies based on s, p-block metals are less reported compared with the abundant d- and f-block metal complexes, and those possessing WLE and color-tunable properties are even scarce. Actually, as stated in section 2, the lone electron pair effect in some main-group-metal (such as Pb2+ and Bi3+) complexes can result in strong s−p emission and/or LMCT emission with broad luminescent peaks to lead to white-light emission. As an example, using two modified 1,4benzenedicarboxylic acid (bdc) ligands, L1 and L2, two Pb2+MOFs with similar 3D net structures were obtained. In PbL1 (1), the CH3S− groups are not coordinated, while in PbL2 (2), the hydroxyl groups participate in coordination (Figure 13). This slight change in structure significantly impacts the luminescent properties of the two complexes. PbL1 emits yellow-green light, while PbL2 displays bright white emission. The broad emission in 2 is comprised of LC emission with the maximum at 459 nm, LMCT emission centered around 515 nm, and tailing emission beyond 600 nm which can be

Figure 11. (Upper) Incorporation of DCM guests into the nanochannels of MOFs: molecular structure of DCM (A); host structures of stilbene-MOF (B) and IRMOF-8 (C); schematic host− guest structures of DCM@stilbene-MOF (D) and DCM@IRMOF-8 (E). (Lower) Photographs of crystals of pure stilbene-MOF (a, b) and IRMOF-8 (e, f), DCM@stilbene-MOF (c, d), and DCM@IRMOF-8 (g, h) under daylight (a, c, e, g) and UV irradiation (b, d, f, h) observed using a fluorescence microscope; fluorescence spectra of stilbene-MOF (i) and IRMOF-8 (j) samples before and after modification with DCM. Reprinted with permission from ref 73. Copyright 2014 Nature Publishing Group.

Figure 12. Scheme of the encapsulation of [Ir(ppy)2(bpy)]+ in CdMOF and the emitting color change with different ratios of guests. Reprinted with permission from ref 74. Copyright 2013 Nature Publishing Group.

Figure 13. (a) Local coordination environment around the Pb2+ ion and white-light LED based on complex PbL2. (b) CIE coordinates for emissions of 1 (λex = 365 nm) and 2. Reprinted from ref 75. Copyright 2012 American Chemical Society.

how to achieve color tunability and white-light emission in a single-phase material by selecting different metal ions and Q


Chemical Reviews

Review

assigned to MC transitions involving the s and p orbitals of Pb2+ clusters. Combination of these emissions results in white CIE coordinates at (0.27, 0.30), (0.25, 0.29), and (0.24 0.28), by the excitation of 300, 350, and 400 nm, respectively. WLED was fabricated from 2 by dipping a commercial 360 nm UVLED lamp into the slurry of the complex.75 zur Loye and co-workers76 also reported two singlecomponent WLE Bi3+- and Pb2+-MOFs. At the excitation of 380 nm, the Bi3+-MOF Bi3(μ3-O)2(pydc)2(Hpydc)(H2O)2 (H2pydc = pyridine-2,5-dicarboxylic acid) emits white light with a broad emission spectrum covering 400−600 nm, showing three distinct maxima at 430, 460, and 480 nm and a shoulder around 556 nm, which is attributed to an LMCT transition and a change in the intraligand π → π* and/or n → π* transitions. Similarly, the Pb2+-MOF Pb(pydc)(H2O) exhibits slightly “whiter” photoluminescence, with a distinct emission maximum at 441 nm and three broad shoulders around 470, 520, and 563 nm. The broad emissions in these two p-block metal complexes make them attractive candidates for WOLED materials. Zheng, Guo, and co-workers77 synthesized two new Pb2+ CPs featuring 2D layer structures, [Pb(NO3)(tzib)]n and [Pb(tzib)2]n, from the reaction of a rigid ligand, 1-tetrazole-4imidazole-benzene (Htzib) with lead(2+) nitrate in different solvents. Both polymers display broad emission throughout the full visible spectrum, and are tunable to near white light by varying the excitation wavelengths. Recently, we designed four Pb2+ coordination assemblies applying the ligands 2-methyl-8-hydroxyquinoline (HMq), 4,4′,4″-(2,2′,2′′-nitrilotris(methylene)tris(1H-benzo[d]imidazole-2,1-diyl)tris(methylene))tribenzonitrile (triBZNTB), and 4,4′,4″-(2,2′,2′′-nitrilotris(methylene)tris(1Hbenzo[d]imidazole-2,1-diyl)tris(methylene))tribenzoic acid (H3triCB-NTB). The coordination of Pb2+ with the simple bidentate HMq ligand in the pH range 7−8 gave mononuclear complex 1, [Pb(Mq)2]2·3H2O, while in the pH range 5−6 afforded tetranuclear complex 2, [Pb4(Mq)6]·(ClO4)2. The coordination spheres in these two complexes are both semidirected. In comparison, coordination of Pb2+ with the two tripodal ligands triBZ-NTB and H3triCB-NTB resulted in mononuclear complex 3 and Pb−Zn heteronuclear complex 4, in which the coordination spheres around Pb2+ centers are holodirected. Distinctly, the combination of LMCT and MC s → p emissions in the semidirected Pb2+ complexes leads to single-component white-light luminescence: the emission from LMCT (475−480 nm) and the emission from MC transition (615−616 nm). Density functional theory (DFT) calculations of complexes 1 and 2 clearly manifest the contribution from π orbitals of Mq− ligands, d orbitals of Pb2+ ions, and 6s2 lone pair electrons on the Pb2+ to the HOMO components, as well as the π* orbitals of Mq− ligands, and vacant p orbitals of Pb2+ to the LUMO components, which might suggest strong LMCT and MC transition tendencies in these two complexes and support the observation of white-light photoluminescence due to the combination of two kinds of emissions.78 Nenoff and co-workers reported a WLE In3+-MOF, In(BTB)2/3(OA)(DEF)3/2 (BTB = 1,3,5-tris(4-carboxyphenyl) benzene; OA = oxalic acid; DEF = N,N-diethylformamide), also named SMOF-1, which features a porous (3,4)-connected topology with corrugation and interpenetration. As shown in Figure 14, at the excitation from 350 to 380 nm, the complex emits a broad-band emission which can be assigned to the LMCT of the BTB → In3+ transition. This broad emission

Figure 14. Molecular building blocks and topological representation of complex SMOF-1 and emission spectra of SMOF-1 excited between 330 and 380 nm. Reprinted from ref 79. Copyright 2012 American Chemical Society.

results in direct white light with favorable CRI values within intended ranges (81−85), but very high CCT values (21642− 33290 K). By the doping of red-emitting Eu3+ ions into the pore structure of SMOF-1, the CRI and CCT values can be tuned to better fit the set target of CRI ≈ 90 and CCT ≈ 3200 K.79 Two 2D Ga3+- or In3+-based coordination polymers, {M[(2,2′-bpda)(1,4-bda)0.5 (phen)]·0.5H2 O} n (M = Ga (Ga1), In (In1); 2,2′-H2bpda = 2,2′-biphenyldicarboxylic acid; 1,4-H2bda = 1,4-benzenedicarboxylic acid; phen = 1,10phenanthroline), were prepared by the hydrothermal method. The complexes exhibit tunable fluorescence from blue to green, white, and yellow light by varying the temperature and solvents. Theoretical calculations manifest that the electron densities of the HOMO states are mainly located in 2,2′bpda2− units, whereas the densities for the LUMO are distributed on the phen units, which suggests that the emission observed may be originated exclusively from LLCT transition between phen and 2,2′-bpda2−, while Ga3+/In3+ and 1,4-bda2− contribute little.80 As a low-cost, nontoxic, and abundant s-block metal ion, the Mg2+ ion was chosen to construct three CPs by using the auxiliary ligand 4,4′-dipyridyl (bpy), 1,2-di-4-pyridylethene (dpe), or 1,3-di(4-pyridyl)propane (dppe), together with 1,4naphthalenedicarboxylic acid (1,4-NDCH2), resulting in [Mg3(OH)2(1,4-NDC)2(bpy)(H2O)]·0.5H2O (1), [Mg3(OH)2(1,4-NDC)2(dpe)(CH3OH)2]·H2O (2), and [Mg3(OH)2(1,4-NDC)2(dppe)(H2O)] (3), respectively. Similar 3D networks are formed to contain [Mg3(OH)2]n chains linked by the 1,4-NDC ligands (Figure 15). Tunable and white-light R


Chemical Reviews

Review

lacages, 1 and 2, were assembled from TPE-based ligands possessing a cofacial arrangement (Figure 16). Different from

Figure 16. Synthesis (a), AIE (b), and solvent (c) effect on the lightemitting properties of Pt2+ metallocages. Reprinted with permission from ref 82. Copyright 2015 Nature Publishing Group.

the normal ACQ (aggregation-caused quenching) and AIE molecules, the two MOCs not only are highly emissive in dilute solutions, but also exhibit tunable color emissions upon molecular aggregation, and as an extension of AIE behavior, color-tunable fluorescence can be induced by simply varying the volume ratio of the mixed poor/good solvents in which different aggregation states of the Pt2+ cages are formed. Furthermore, the MOCs exhibit variable-wavelength visiblelight emission when dispersed in different solvents, including white-light emission in THF. This can be attributed to two effects, solubility and polarity difference, and the latter effect will impact the nature of the MLCT processes inside the metallacages and decrease the emission efficiency. Au+ complexes have a strong tendency to form excimers due to the presence of AuI···AuI (aurophilicity) interactions.84 Some important developments have been achieved in trinuclear gold complex systems. Fackler and Burini et al. investigated the interaction between trinuclear gold and organic molecules in formed acid−base adducts, showing potential application in the optoelectronic field.85 They recently reported the first gold(I) trinuclear cluster coordinating with different N,N and C,C donors from mixed ligands.86 The Balch group studied the intermolecular interactions in trinuclear gold polymorphs, and they found that the hexagonal form is crucial to the solvoluminescence.87 Lintang et al. found that the permeation of Ag+ ions could change the emission color of the confined gold(I)−pyrazolate complex [Au3Pz3] in nanoscopic channels of mesoporous silica.88 The Au+ clusters linked by aurophilic interactions can usually emit bright phosphorescence in the long-wavelength region. However, the lack of short-wavelength emission in these Au+ cluster complexes failed to achieve white-light

Figure 15. WLE Mg-CPs featuring LLCT character. Reprinted with permission from ref 81. Copyright 2015 Royal Society of Chemistry.

emission can be achieved in Mg-CPs 1−3. Especially, for 3, two obvious emission maxima at 410 and 580 nm located in the blue- and yellow-light regions can be detected, upon varying the excitation wavelengths from 355 to 380 nm. The CIE coordinate is located at (0.30, 0.30) when irradiated at 370 nm, and the quantum yields are 2.42%, 2.35%, and 2.36% at 360, 370, and 380 nm excitation, respectively. To probe the origin of the luminescence, DFT calculations were conducted. The highest HOMO is mainly associated with the π-bonding orbitals from the auxiliary bpy, bpe, or bppe ligand, whereas the LUMO is on the π*-antibonding orbitals of the naphthalene rings of the NDC ligand. Therefore, the origin of the long-wave emission is mainly attributed to the intraligand π*−π transitions, featuring LLCT character from the auxiliary ligands to the NDC ligand.81 4.1.2. Transition-Metal (d-Block) Centered Complexes. The d-block complexes, especially those based on Ir3+/Pt2+ centers with traditional Werner-type simple coordination motifs, usually involve MLCT plus excimer/exciplex (EX) emissions as SPWLE materials widely applied for WLE devices (see more examples in section 5.1). Herein, we will mainly introduce the assembly models of d-block-containing WLE coordination assemblies featuring MOC or MOF configurations with supramolecular sense. Huang, Stang, and co-workers82,83 reported an interesting 2+ Pt metallocage with white-light emission in certain kinds of solvents (THF, tetrahydrofuran) based on the aggregationinduced emission (AIE) effect. Two tetragonal Pt2+ metalS


Chemical Reviews

Review

emission. Alternatively, Li and co-workers reported a trinuclear [{Au(L)}3] cluster from 3-(2-thienyl)pyrazole (HL) as shown in Scheme 9.89 At the excitation of 340 nm, there are both Scheme 9. Structural Formula of the [{Au(L)}3] Cluster (Left) and the Chair-Stacking Pattern Forming the Excimer (Right)89

short-wavelength emission at 408 nm originating from the [{Au(L)}3] monomer and broad emission covering 500−600 nm which can be attributed to the triplet excimer formed from the chair stacking of the [{Au(L)}3] monomers. This results in a CIE coordinate of (0.28, 0.29), falling into the white-light region with a quantum yield of 0.259. Furthermore, the relative intensity of the two emissions can be tuned by different solvent concentrations and excitation wavelengths, showing the character of aggregation, which is regulated by AuI···AuI aurophilic interactions. Also, WLE could be achieved from tuning the singlet- and triplet-derived emissions. Recently, the Venkatesan group90 reported such a monomolecular gold(III) complex covering the entire visible spectrum (QY = 28%), and CIE coordinates from (0.29, 0.33) to (0.32, 0.40) were obtained. A direct WLE MOF, [AgL]n·nH2O (L = 4-cyanobenzoate), with tunable yellow to white luminescence by variation of the excitation light was reported by Guo and co-workers.91,92 The MOF displays maximum emissions at around 427 and 566 nm when excited at 355 and 330 nm, respectively. As we can see in Figure 17, at the excitation of 330 nm, the emission intensity at 427 nm is very weak, and the emission intensities at 513, 566, and 617 nm are strong, generating yellow luminescence, while at the excitation of 350 nm, the intensities of the two peaks at 427 and 566 nm are comparable, leading to direct white light to the naked eye. The CIE chromaticity coordinates of the white-light emissions excited at 350 and 349 nm are (0.31, 0.33) and (0.33, 0.34), respectively, comparable to those of pure white light. The two emissions at high energy and low energy can be assigned to intraligand π−π* transition and MLCT emission, respectively. White-light emission was also obtained in another Ag+ complex with a structure of [Ag(L)(NO3)]∞ {L = butane-1,4-diylbis(pyridine-3-carboxylate)}.93 Zhu and co-workers94 synthesized a new 1D CP with Cu2I2 alternating cluster units linked by benzo-18-crown-6. Treating a suspension of CuI/KI in toluene with quantitative benzo-18crown-6 at 120 °C for 24 h afforded the corresponding complex (C1). As shown in Figure 18, two potassium complexes of benzo-18-crown-6 units assembled into a

Figure 17. (a) 3D packing diagram and (b) solid-state PL spectra of [Ag(4-cyanobenzoate)]n·nH2O by variation of excitation light under the same metrical conditions. The inset shows the PL images of a sample excited by 350 and 330 nm light, respectively. Reprinted from ref 91. Copyright 2009 American Chemical Society.

Figure 18. Crystal structure and emission spectra of cuprous halide cluster compounds. Reprinted with permission from ref 94. Copyright 2014 Royal Society of Chemistry.

sandwich through two arms between one peripheral O and the centered K atoms are linked by a rhombus Cu2I2 cluster and two I− into a 1D linear CP {[K(benzo-18-crown6)]2(Cu2I4)}n. Irradiated by UV light (282 nm) at room temperature, C1 exhibits a very broad emission covering 360− 800 nm, with two maximum peaks centered at 465 and 632 nm. The emissions can be attributed to halide-to-metal charge transfer (XMCT) and Cu2I2 cluster-centered (3CC) transitions, respectively, and afford strong white luminescence as a whole. Upon lowering the temperature to 77 K, the longwavelength emission of C1 is red-shifted, while the shortwavelength one disappears, due to the changes of Cu−Cu interactions at low temperature. T


Chemical Reviews

Review

Several Zn/Cd2+-based CPs have been reported to have white-light emission, mainly due to the metal-perturbed ligand luminescence. Guo and co-workers designed five Zn2+-MOF structural isomorphs featuring 1D hexagonal channels functionalized by different organic shutters (Figure 19). For

Figure 20. (a) 3D connectivity and (b) solid-state emission spectra of complex (Me2NH2)[RbCd4(OBA)5]·H2O. Reprinted with permission from ref 96. Copyright 2013 Royal Society of Chemistry.

A temperature-dependent white-light emission was observed in the complex [Cd5(HTTHA)2(Hpt)4(H2O)]·4H2O (1; H6TTHA = 1,3,5-triazine-2,4,6-triaminehexaacetic acid; Hpt = pyridinium-4-thiolate) upon cooling. The complex displays an intense green emission centered at 520 nm by the excitation of 368 nm at room temperature, with an absolute quantum yield of 1.17% (Figure 21), while upon lowering the

Figure 19. (a) Structural isomorphs showing 1D hexagonal channel in Zn2+-MOFs functionalized by different organic shutters. (b) Solidstate PL spectra and CIE coordinates of Zn2+-MOF with −COOH decorating units (polymer 5). Reprinted with permission from ref 95. Copyright 2012 Royal Society of Chemistry.

Zn(L)(HBTC)(H2O)2 (polymer 5; L = N4,N4′-bis(pyridin-4yl)biphenyl-4,4′-dicarboxamide; H3BTC = 1,3,5-benzenetricarboxylic acid) with −COOH decorating units, tunable PL properties from yellow to white and then to blue can be achieved by means of the variation of excitation light. At the excitation of 370 nm, besides the blue emission at 424 nm which is related to the ligand π → π* transition, there is another emission band at 539 nm. The later emission may be associated with the stacking states between pyridyl rings linked by π−π interactions. The two emissions are comparable in intensity and lead to direct white light.95 Dual emission was also observed in an inorganic−organic composite framework with a 3D inorganic Cd−O−M (M = Cd, Rb) connectivity, namely, (Me2NH2)[RbCd4(OBA)5]· H2O (H2OBA = 4,4′-oxydibenzoic acid). The MOF structure features a unique {Cd7Rb}n helix and nanoscale 20-membered {Cd16Rb4} and {Cd18Rb2} rings. Furthermore, the MOF shows a wavelength-dependent photochromic response. At appropriate excitation, a direct white-light emission can be observed, contributed by both ILCT and MLCT transitions. As seen in Figure 20, the complex shows double-plateau emissions at 427 and 566 nm during the excitation wavelength from 306 to 342 nm. The first emission is contributed to the interligand π−π* transition, and the second peak is originated from the MLCT transition. At the excitation of 312 nm, the two emissions have approximately identical intensity, and result in almost pure white light with CIE coordinates of (0.30, 0.33).96

Figure 21. Normalized emission (red) and excitation (green) spectra of 1 collected in the solid state (a) at room temperature and (b) 10 K. (c) CIE-1931 chromaticity diagram showing the white fluorescence of 1 at 10 K. Inset: fluorescent image of the “white” luminescing 1 at 10 K. Reprinted from ref 97. Copyright 2011 American Chemical Society.

temperature to 10 K, a broad emission band nearly covering the entire visible spectrum from 415 to 750 nm appeared with CIE coordinates of (0.34, 0.36), falling into the white-light region. In addition, the quantum yield significantly increases to 16.02% at the excitation wavelength of 360 nm. The emission bears the nature of LMCT originating from the thiolate group to Cd2+.97 On the contrary, a porous Zn2+−gallophosphate complex shows white-light emission upon heating. As seen in Figure 22, the light-brown pallet of NTHU-4Y, {(H2tmdp)2[Zn3Ga6OU


Chemical Reviews

Review

changed by shifting the wavelength of excited light, the luminescent color of 1-250 can be tuned among yellow-green, purple-blue, bright white, and blue-green, respectively.99 Another zinc(2+) 4-(5H-tetrazolyl)benzoic CP also shows white-light emission after heat treatment from 420 to 430 °C, but PXRD indicates small alteration of the crystal structure topology and local distortion in the heated sample.100 Different from the above excitation or temperature-dependent WLE Zn/Cd2+ complexes, an invariably WLE Zn2+-MOF was reported by Yong and co-workers.101 Starting from a zwitterionic radical ligand (bipo•−), isostructural 1D MOFs [M(bipo•−)(L)]n [M = Zn; L = HCOO− (1), SCN− (1a), N3− (1b)] were obtained (Figure 23). According to the different Figure 22. (a) Photographs of NTHU-4Y (left) and NTHU-4W (right) under daylight and exposure to a 365 nm UV beam. (b, left) NTHU-4Y invariably emits at 550 nm with excitation by 365 nm (cyan), 400 nm (brown), 465 nm (green), and 500 nm (red); (right) NTHU-4W emits tunable white-to yellow luminescence with the blue emission peaked at 433 nm. Nearly perfect white light would result by excitation at 390 nm. Reprinted from ref 98. Copyright 2005 American Chemical Society.

(HPO4)(PO4)8]·5H2O; tmdp = 4,4′-trimethylenedipyridine} emits yellow luminescence centered at 550 nm by the excitation from 365 to 500 nm at room temperature. However, after being heated at 280 °C for 4 h, the compound turned to deep brown in color (named NTHU-4W), with the structure remaining almost the same as that of NTHU-4Y, except for a small difference in the disordered MO4 tetrahedron (M denotes mixed Zn and Ga) comprised in the network. Interestingly, NTHU-4W has tunable yellow-to-white luminescence by the variation of the excitation energy. If the excitation wavelength is longer than 420 nm, single-band yellow (550 nm) emission will result, while dual emissions in both the blue (433 nm) and yellow (550 nm) regions will be produced at excitation wavelengths shorter than 420 nm. At the excitation of 390 nm, the intensity of the two emissions is identical, and results in nearly perfect white light.98 Heating-induced white-light emission was also observed in a zinc phosphonate, [(C5H14N2)0.5Zn5(HO3PCH(OH)CO2)(O3PCH(OH)CO2)3·2H2O]·2H2O (1). The complex features a 3D framework comprised of [ZnO6] octahedral and [ZnO5] square-pyramidal subunits, which are bridged by 2-hydroxyl(phosphono)acetate (HO3PCH(OH)CO2H) with tri-, tetra-, and pentadentate coordination modes. 2-Methylpiperazine (C5H14N2) serves as a structure-directing agent, encapsulated into the framework by electrostatic interactions and hydrogen bonding. Under room temperature, complex 1 exhibits purple emission centered at 425 nm, which may originate from the intraligand emission of protonated 2-methylpiperazine. It is interesting that, after heat treatment under different temperatures, the emissions of 1 become significantly red-shifted and broadened due to the additional fluorescent contribution from the 2-hydroxyl(phosphono)acetate moiety, which is quenched in the as-prepared 1 sample. For the post-treatment samples 1200 and 1-300, there exist much broadened emitting bands with maxima at 442 and 614 nm, resulting in yellow and orange-red light, respectively, while for sample 1-250, bright white-light emission is produced when the sample is excited at 365 nm, which can be detected by the naked eye. Furthermore, since the relative intensity of the two emissions from 2hydroxyl(phosphono)acetate and 2-methylpiperazine can be

Figure 23. (a) Syntheses of isostructural metal−anion radical CPs 1, 1a, and 1b from a zwitterionic radical. (b) Solid-state PL spectra of 1a by variation of excitation light under the same metrical conditions. Inset: PL image of a sample excited by 360 nm. Reprinted with permission from ref 101. Copyright 2011 Royal Society of Chemistry.

terminal anions, the three complexes show diverse emissions, among which complex 1a with SCN− terminal anions displays a broad emission band covering 425−750 nm, with peaks at 456 nm (weak, blue PL), 514 nm (strong, green PL), 594 nm (strong), and 640 nm (shoulder, red PL), respectively. The two high-energy emissions can be attributed to π−π* or n−π* transitions from the ligand, and the two low-energy emissions are assigned to LMCT transitions. This broad-band emission results in white-light CIE coordinates of (0.34, 0.36), and it is noteworthy that the WLE will not be affected by alteration in the excitation wavelength. A similar WLE phenomenon was further reported by the authors in a Cd2+-MOF.102 Recently, we reported a solvent- and temperature-responsive white-light-emitting Zn2+ CP from organic ligands possessing ESIPT properties.103 The assembly of two ESIPT ligands, 2(4,5-diphenyl-1H-imidazol-2-yl)phenol (H2PI) and 5-[2-(2hydroxyphenyl)-4,5-diphenyl-1H-imidazol-1-yl]isophthalic acid (H3PI2C), with Zn2+ ions affords a 1D ribbon-like CP {[Zn6(HPI)4(HPI2C)4]·(DMF)5(H2O)3}n (LIFM-22). The ESIPT process in LIFM-22 manifests both enol and keto emission, and aggregate emission is also presented in the solid state at room temperature. Therefore, single-component white light is achieved with CIE coordinates at (0.28, 0.28). V


Chemical Reviews

Review

emission with the CIE coordinates exactly positioned at (0.33, 0.33).93 Similarly, a Eu3+ complex from macrocycle-appended naphthalimide derivative was found to be able to show triple luminescence from naphthalimide monomers (blue), aggregated naphthalimide excimers (green), and Eu3+ centers (red), the balance intensities of which make white-light emission obtainable from a single molecule (Figure 24).112 A 1D chain

Furthermore, the emissions can be mediated by grinding the complex in different solvent systems or changing the temperature, which results in the emitting color switching from blue to green-white. This example affords a new approach to obtain tunable and single-component white-light emission in coordination complexes by applying the ESIPT attribute of the ligand, which also provides potential applications for multistimulus-responsive fluorescent sensors and LEDs. 4.1.3. Lanthanide (f-Block) Based Complexes. Lanthanide ions are universally applied in the fabrication of luminescent complexes, due to their unique, plentiful, and efficient emission of different colors. A number of related works of luminescent lanthanide complexes have been summarized in several reviews for synthesis, visible and NIR luminescence, probes, and biolabeling.104−110 Specifically, the red and green emissions of Eu3+ and Tb3+ ions are most frequently utilized in achieving single-phase WLE and PLCT materials. However, blue color emission is usually difficult to obtain directly from Ln3+ ions. As mentioned above, although Ce3+ could potentially afford wide and strong d → f emission in the blue region with the appropriate coordination environment, it is not easy to achieve. Therefore, the blue emission from the ligand-based component is often necessitated as a supplementary lighting source, and the achievements of SPWLE lanthanide complexes have mainly been reported through the following strategies: (1) homometallic lanthanide complexes, which usually comprise Ln3+ ions with multiple emitting peaks, such as Sm3+, Pr3+, or Dy3+, or, wisely, by the delicate design of homolanthanide complexes incorporating appropriate organic components for compensating emitting light; (2) d−f or p−f heteronuclear complexes, in which the d- or p-block metals or metal-sensitized ligands can provide blue emission to combine with the emission of Ln3+; (3) isomorphous solid-solution-type f−f heterometallic coordination complexes, in which the red and green emissions of Eu3+ and Tb3+ are usually compensated by the blue emission from the La3+/Gd3+-coordinated ligand; (4) multiple Ln3+ emitting centers doped into a single MOF matrix. In general, lanthanide complexes possess certain kinds of emitting color (most frequently red or green from the characteristic f → f transitions of Eu3+ and Tb3+); therefore, most of the color-tunable and WLE assemblies are heteronuclear structures to combine the emissions from different emissive Ln3+ centers. However, there are also some rare examples that the tunable color and white-light emission are achieved by homometallic lanthanide complexes. For instance, Duan and co-workers reported a Eu3+ complex (CR1Eu) which exhibits white light by combining the red emission from Eu3+ and blue and green emission from the organic ligand. The WLE Eu3+ complex is assembled from a coumarin (blue-emitting)−rhodamine 6G (green-emitting) combined organic ligand (CR1) with the red-emitting [Eu(tta)3] (tta = 1,1,1-trifluoro-3-(2-thenoyl)acetone) coordination unit. Due to the multiple chromophores incorporated into this single molecule, the acetonitrile solution (50 μM) of CR1-Eu exhibits characteristic blue emission of the coumarin at 460 nm, green emission of rhodamine 6G at 550 nm, and red emission of Eu3+ at 612 nm, when excited at 415, 525, and 360 nm, respectively. The direct white light with CIE coordinates of (0.33, 0.24) can be obtained by the excitation of 388 nm. Furthermore, the quality of white emission could be improved by increasing the concentration of CR1-Eu in acetonitrile solution to 0.2 mM, which gives a better-balanced pure-white

Figure 24. Controllable three-component luminescence from a 1,8naphthalimide−Eu3+ complex: white-light emission from a single molecule. Reprinted with permission from ref 112. Copyright 2012 Royal Society of Chemistry.

CP, {[Ln(L2)3(H2O)]·H2O}n (HL2 = 3-(1,8-naphthalimido)propanoic acid), was also reported to comprise the ligandbased blue/green luminescence and Eu3+-based red color from partial ET sensitization to result in white-light emission.113 By introducing oxalic acid as an ancillary ligand, Wu and coworkers constructed another Eu3+-MOF with a long πconjugated organic ligand, 2-(2,4-disulfophenyl)imidazo(4,5f)(1,10)-phenanthroline (H3sfpip), and obtained the target white-light emission.114 Dy3+ ions possess multiple f → f transitions capable of emitting in the blue (480 nm), yellow (573 nm), and red (661 nm) regions, in which the yellow color usually dominates over the other two. Therefore, the delicate design of a Dy3+ complex incorporating Dy3+-based yellow emission and ligand-based blue emission simultaneously might afford an SPWLE material through a BY dichromatic strategy with balanced blue (B) and yellow (Y) emissions, which will provide an alternative way to achieve WLE phosphors. To manifest this strategy, we prepared a multiple-color photoluminescent Dy-MOF from the tripodal TETP (1,1′,1″-((2,4,6-triethylbenzene-1,3,5-triyl)tris(methylene))tris(pyridin-4(1H)-one) ligand and dysprosium(3+) nitrate in a water−acetone solvent system. In [Dy(TETP)(NO3)3]·4H2O (Dy-MOF), the Dy3+ centers are nine-coordinated by three TETP ligands through a terminal O and three NO3− anions in a tricapped trigonal prismatic geometry, and each tripodal TETP connects three different Dy3+ ions, thus generating an intricate 3D framework utp topology (Figure 25).41 For the PL property in this Dy-MOF, the TETP ligand plays dual functions: as an organic antenna, it can efficiently sensitize Dy3+ centers to produce yellow light, and simultaneously, TETP is highly blue emissive by itself. Therefore, the balanced yellow and blue emissions will generate white-light emission directly. Furthermore, upon changing the excitation wavelength, the intensity ratios between the yellow and blue W


Chemical Reviews

Review

gradually increases, the blue emission intensity of the ligand is enhanced, while the red emission intensity of Sm3+ is decreased. As a result, pure white light can be obtained at the excitation wavelength of 358 nm.118 4.2. Single-Phase Heterometallic Coordination Assemblies

4.2.1. Heteronuclear d−f Complexes. As mentioned in section 3.2, de Cola and co-workers reported the first singlecomponent d−f WLE complex, Ir2Eu (Scheme 7). In this case, the cyclometalated [IrC2N4] chromophore has dual functions. First, it can sensitize the luminescence of Eu3+ in this heterotrimetallic complex with ηIr→Eu = 38%. Second, the residual broad blue emission of the [IrC2N4] chromophore combines with the Eu3+-centered red emission to give global white-light emission in solution.56 The extent of Ir → Eu ET and WLE is further studied in a series of Ir3+/Ln3+ dyads. It is found that the intercomponent Ir···Eu distance and the conformational flexibility will determine the Ir → Eu ET. If the ET efficiency is up to 90%, the Ir-based blue emission will be quenched and only the Eu-centered red emission appears. While in case the Ir → Eu ET is incomplete, the Ir3+/Ln3+ dyads exhibit dual emission from both Ir and Eu moieties, leading to white-light emission with CIE coordinates at (0.34, 0.32).119 Later on, an SPWLE d−f heteronuclear MOF was reported by our group, in which a 4d−4f Ag−Eu heterometallic MOF was synthesized through a stepwise metalloligand approach (Scheme 8).57 There are two kinds of emitting components in this structure: one is the red emission from the Eu3+-centered f → f transition, which follows the ET mechanism from the ligand to Eu3+ by the antenna effect, and the other is Ag+sensitized LC blue emission featuring 1(EE)* excimer character. The combination of these two emissions results in white-light emission with CIE coordinates from (0.42, 0.32) to (0.45, 0.37) upon changing the excitation wavelength from 310 to 370 nm (Figure 26). Therefore, the ligand in this WLE complex is bifunctional, serving as both an energy donor to Eu3+ and a self-emitting center. Meanwhile, the Ag+ ions also play important roles in achieving white light in this heteronuclear complex: apart from assembling the luminescent Eu−organic precursor into a rigid Ag−Eu-MOF to reduce the nonradiative deactivation processes, it also perturbs the excited 1 ππ* singlet and 3ππ* triplet states of the ligand to facilitate both intersystem crossing S1 → T1 and antenna T1 → f energy transfer, and therefore, it can resensitize the 1(EE)* excimer emission that has been quenched in monomeric Eu3+ species. A heterotrinuclear Zn2Eu- cluster complex with the formula [(Znq2)2](μ-CH3COO){Ln(hfac)2} (q = 8-hydroxylquinolinate; hfac = hexafluoroacetylacetonate) was reported to show dual emission originating from both Znq2-based emitting centers and Eu3+ centers. The relatively balanced intensity between Znq2-based cyan emission and Eu3+-centered red luminescence leads to white-light emission.120 Another Zn−Ln heteronuclear cluster complex with white-light emission was reported by Gao and co-workers,121 in which [Zn2L2EuxDyy(hfac)6] (x + y = 2) was prepared from a flexible linker (H2L = bis(salicylidene)-3,6-dioxa-1,8-diaminooctane) and chelated ligand hfac (hexafluoroacetylacetonate) by codoping a small amount of Eu3+ ion into the Dy3+ complex. The undoping [Zn2L2Dy2(hfac)6] exhibits maximum emission at 482 nm when excited at 310 nm, which should be ascribed to the π−π transition from the ligand. No emission from Dy3+ indicates that there is no ET from Zn2L2 to the Dy center. In the doped

Figure 25. Crystal structure (a) and color-tunable emission (b) of Dy3+-MOF. Reprinted with permission from ref 41. Copyright 2014 Royal Society of Chemistry.

emissions will be varied, resulting in yellow-to-blue PL colortuning in the Dy-MOF. As shown in Figure 25, yellow, white, cyan, and blue emissions with CIE coordinates at (0.39, 0.44), (0.33, 0.35), (0.28, 0.29), and (0.21, 0.20) can be obtained by the excitations of 290, 365, 338, and 373 nm, respectively. Using bipodal zwitterionic ligand TMPBPO (1,1′-(2,3,5,6tetramethyl-1,4-phenylene)bis(methylene)dipyridinium-4olate), we obtained a Pr-MOF (LIFM-17(Pr)) with the formula [Pr(TMPBPO)2(NO3)3]·C3H6O·H2O. LIFM-17(Pr) shows full luminescence in both the visible and near-infrared (NIR) regions, due to the 3P0 → 3HI (I = 4−6), 3P0 → 3FJ (J = 2−4), 1D2 →3F2, 3F4, and 1G4 transitions of Pr3+. Meanwhile, the remaining ligand emission can also reside upon longwavelength excitation. Therefore, orange, white, and green emissions with CIE coordinates at (0.55, 0.37), (0.36, 0.35), and (0.27, 0.46) are achieved by the excitations of 295, 335, and 365 nm, respectively.115 Single-component white-light emission was also observed in a Sm3+ complex, which combines the broad blue-to-greenemitting band of the ligand, as well as the characteristic green-, orange-, and red-emitting peaks of Sm3+ ions.116 In addition, recently, a homometallic La-MOF with the formula [La(MBDC)(STP)] (H2MBDC = isophthalic acid; STP− = 4(2,2′:6′,2″-terpyridin-4′-yl)benzenesulfonate) and 3D polymeric structure was reported to be able to emit white-light emission in the powder sample upon excitation at 370 nm. The WLE is contributed by the combination of two emitting peaks around 430 and 550 nm, respectively. The high-energy emission is due to ILCT of STP, and the low-energy emission is relative to π−π attractions between the parallel ligand STP.117 In another study, Zheng and co-workers used 5-(4(tetrazol-5-yl)phenoxy)isophthalic acid (H3TPIA) to synthesize [Sm(TPIA)(H2O)3]·5.5H2O (1), Eu(TPIA)(H2O)3]· 5.5H2O (2), and [Gd(TPIA)(H2O)3]·5.5H2O (3) complexes, among which the Sm3+ complex emits orange color at the excitation wavelength 340 nm. When the excitation wavelength X


Chemical Reviews

Review

Figure 27. (a) Crystal structure of [Zn2L2Ln2(hfac)6] (Ln = Eu, Tb, Dy). (b) Emission spectrum of the doped complex Dy0.958Eu0.042 excited at 300 nm. Reprinted with permission from ref 121. Copyright 2016 the Royal Society of Chemistry.

is known to emit green to blue luminescence; therefore, the coassembly of such units and red-emitting Eu3+ is able to give direct white light. In the complex [Al3(Mq)4(HMq)(μ3OH)2(μ-OH)2{Eu(hfac)2}] (shortened to Al3Eu2; Mq = 2methyl-8-hydroxyquinolate; hfac = hexafluoroacetylacetonate; Figure 29), upon excitation at 350−420 nm, red emission of Eu3+ can be successfully achieved by sensitization with the Al(Mq)2 light-harvesting chromophore. Together with the complementary cyan luminescence from the Al(Mq)2 unit itself, bright white-light emission is obtained in both the solid state and solution. The quantum yield of the white-light emission excited at 348 nm is ca. 6.1%, and the chromaticity coordinates are (0.27, 0.36). Interestingly, the addition of [Bu4N]F can affect the emission. The quantum yield is increased to 11.6% upon the addition of 0.6 equiv of [Bu4N]F, and the CIE coordinates are shifted to (0.34, 0.33). The reason is the formation of strong F···H−O hydrogen bonds upon the addition of [Bu4N]F, which greatly weakens the O−H vibrations and minimizes the nonradiative deactivation processes, and thereby intensifies the luminescence of Eu3+.124 We assembled two series of Pb−Ln heteronuclear complexes from a tripodal ligand (4,4′,4″-(2,2′,2″-nitrilotris(methylene)tris(1H-benzo[d]imidazole-2,1-diyl)tris(methylene))tribenzoic acid (triCB-NTB)). Different from the PbLn2L2 series, the Pb2LnL2 series of complexes contain Pb−Ln−Pb clusters formed by the linkage of carboxyl groups on triCB-NTB ligands to Pb2+ and Ln3+ simultaneously, which leads to more distinct perturbation to the excited states of the ligand. Therefore, a more effective LMCT process is observed, which increases the ET efficiency to the accepting levels of Ln3+ ions. By the combination of multiple emissions from LC, LMCT, and MC luminophores, the Pb2EuL2 complex emits singlecomponent white-light emission in the solid state.125 4.2.3. Isomorphous f−f Solid-Solution Complexes. As stated before, the similar ionic radii among different Ln3+ ions

Figure 26. Crystal structure (a), PL spectra (b), and dual-emission pathways (c) for WLE generation in single-phase d−f heteronuclear Ag−Eu-MOF. Reprinted from ref 57. Copyright 2012 American Chemical Society.

sample with a molar ratio of 95.8:4.2 for Dy/Eu, both Zn2L2based broad emission and Eu3+-based characteristic emission are detected, resulting in white-light emission with CIE coordinates of (0.335, 0.333) when the sample is excited at 300 nm (Figure 27). Chorazy and co-workers122 synthesized a Dy3+−Co3+ layered framework, namely, {[Dy(4-OHpy)2(H2O)3][Co(CN)6]}·0.5H2O (Dy−Co-MOF; 4-OHpy = 4-hydroxypyridine) by the solution method (Figure 28), which not only showed color-tunable PL from yellow to green-blue, but also exhibited slow magnetic relaxation. A series of different colors could be obtained by the combination of Dy3+- and 4-OHpybased emissions through changing the excitation wavelength. The color-changing trend of emission is explained by different ET pathways. At a low-energy wavelength of 405 nm, only 4OHpy-based green-blue emission appears. At a high-energy wavelength of 270 nm, only Dy3+-based yellow emission is detectable, due to ET from the Co3+ T2g state to the Dy3+ center. A similar system with a WLE attribute was reported later, in which {[Dy(3-OHpy)2(H2O)4][Co(CN)6]}·H2O shows Dy3+-based white-light emission sensitized by ET from 3-OHpy and Co3+ when excited at 312 nm.123 4.2.2. Heteronuclear s/p−f Complexes. The coordination unit of 8-hydroxyquinolate or its analogues with Al3+ ions Y


Chemical Reviews

Review

white light. The combination of RGB emissions is the most common approach. Usually, the blue emission is obtained from the organic ligand often via the coordination enhancement of La3+ or Gd3+. For example, through doping of Eu3+ and Tb3+ ions into a La3+ complex {[La2(L)2]·(H2O)3·(Me2NH2)2}n (H4L = 5-(3,5-dicarboxybenzyloxy)isophthalic acid), whitelight emission was obtained. As shown in Figure 30, the

Figure 30. (a) PL emission spectra of the Eu/Tb-doped La complex (λex = 367 nm, solid samples). (b) CIE chromaticity diagram for Eu0.17Tb0.18La0.65L and Eu0.16Tb0.19La0.65L. Reprinted with permission from ref 126. Copyright 2013 Royal Society of Chemistry.

complexes with compositions of Eu0.17Tb0.18La0.65L and Eu0.16Tb0.19La0.65L show emission peaks at 448, 542, and 618 nm, originating from the ligand, Tb3+, and Eu3+ respectively. These emissions are comparable in intensity, thereby leading to direct white light. The incomplete La3+-hosted ligand to Eu/ Tb ET is significant to realize the white-light emission in the codoped system.119 Similar results were also obtained by doping Eu3+ and Tb3+ ions into La3+-containing complexes, such as [H(H2O)8][LaZn4(imdc)4(Him)4] (Him = imidazole; H3imdc = 4,5-imidazole-dicarboxylic acid),53 La-BTPCA (H3BTPCA = 1,1′,1″-(benzene-1,3,5-triyl)tripiperidine-4-carboxylic acid),127 and La2(PDA)3(H2O)5 (PDA = pyridine-2,6dicarboxylate),128 with different ligand designs and structures. In multicomponent Gd3+/Eu3+/Tb3+ complexes [H2NMe2]3[Gd1−x−yEuxTby(L)3] (H2L = pyridine-2,6-dicarboxylic acid), the ligand serves both as an energy absorption antenna for the red Eu3+ and green Tb3+ emissions and as a blue emission source. It is noted that, to obtain competent blue emission, the composition percentage of Gd3+ has to be much higher than that of the other two components. At the molar ratio of Gd:Eu:Tb = 0.9365:0.0370:0.0265, strong white light is achieved with a high quantum yield up to 62%.129 In lanthanide 4-(dipyridin-2-yl)aminobenzoate CPs containing mixed Ln3+ ions, by changing the concentration profiles of Ln3+ stoichiometrically and the excitation wavelengths, various emission colors and white-light emission are also successfully obtained.130 Our group applied two zwitterionic-type ligands, 1,1′(2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene)dipyridinium-4-olate (TMPBPO) and 1-dodecylpyridin4(1H)-one (DOPO), featuring ππ* and ILCT excited states to assemble a series of Ln3+ solid-solution complexes.131 Due to the wide-band triplet states and additional ILCT states extending into the lower energy levels of the ligands, visible to near-infrared (NIR) PL of f → f transitions from almost the whole series of Ln3+ ions can be broadly and intensively sensitized. On the basis of the isostructural feature of the Ln complexes, color-tunable and white-light emission is achieved in the solid-solution trimetallic complexes LIFM-19 (Gd0.85-

Figure 28. (a) Crystal structure of Dy−Co-MOF showing a single cyanido-bridged layer. (b) Emission colors shown on the CIE 1931 chromaticity diagram. (c) Related schematic energy level diagram. Reprinted with permission from ref 122. Copyright 2016 Royal Society of Chemistry.

Figure 29. (a) Al3Ln2 heteropentanuclear complexes [Al3(Mq)4(HMq)(μ3-OH)2(μ-OH)3{Ln(hfac)3}2]. (b) Emission spectra of the precursor Al(Mq)2(OC6H4CN-4) (cyan), Al3Eu2 complex (black), and Eu(hfac)3(H2O)2 (red) and luminescence images in dichloromethane upon irradiation at 365 nm. Reprinted with permission from ref 124. Copyright 2009 Royal Society of Chemistry.

offer a good chance to form isomorphous crystal structures. This makes it easy to codope different Ln3+ ions, i.e., mixed color emitting centers, into a uniform phase of the coordination complex (so-called solid solution) to achieve Z


Chemical Reviews

Review

Eu, Gd, Tb, Dy, Tm, Yb, Y; H2L = 1,3-bis(4-carboxyphenyl)). The emission spectra of the codoped EuxTb1−xL (x = 0.1−0.9) with different doping concentrations exhibit characteristic transitions of both Eu3+ and Tb3+ ions. With an increase of the Eu3+ concentration, the green luminescence intensity of the Tb3+ ion gradually decreases, while the red luminescence intensity of the Eu3+ ion increases. The luminescence colors of the codoped MOFs can be steadily tuned from green to greenyellow, yellow, orange, red-orange, and red, with corresponding CIE coordinates changing from (0.309, 0.583) to (0.596, 0.313). Furthermore, tuning the excitation wavelength of codoped MOF of Eu0.25Tb0.75L composition from 330 to 395 nm, the emission is found to travel across the yellow to greenyellow and white regions. Actually, white-light emission is afforded by the excitations of 380, 385, 390, and 395 nm, with corresponding CIE coordinates at (0.323, 0.358), (0.323, 0.337), (0.314, 0.327), and (0.339, 0.323), respectively, for the Eu0.25Tb0.75L complex. In addition, a similar color trajectory is also realized in other codoped MOFs with Eu3+ concentrations of x = 0.2, 0.28, and 0.3. On the other hand, linearly temperature-dependent luminescent behavior is also found for the Eu0.2Tb0.8L complex over a wide temperature range, from 40 to 300 K, allowing for the design of a thermometer with an excellent response to temperature.135 The linearly changing trajectory of CIE coordinates of codoped Ln complexes was also observed in another binuclear lanthanide complex series based on the bipy ligand.136 In another example, simply doping a small amount of Eu3+ into [Gd(3-SBA)(IP)OH(H2O)]·H2O (3-SBA = 3-sulfobenzoate; IP = 1H-imidazo[4,5-f ][1,10]phenanthroline) also results in white-light emission, in which the Gd3+-coordinated matrix gives very broad LC emission from 400 to 580 nm covering the blue to green region. By slight addition of the redemitting part from Eu3+ doping with a concentration of 1.03− 3.69%, pure white-light emission with CIE coordinates very close to the ideal (0.333, 0.333) is obtained.137 WLE is also achieved in DyxEuyGd1−x−yL and SmxDyyGd1−x−yL (L = Nphenyl-N′-phenylbicyclo[2,2,2]-oct-7-ene-2,3,5,6-tetracarboxydiimide tetracarboxylic acid) solid-solution-type Ln complexes.138 In another series of heteronuclear Ln complexes, LnL (Figure 32), the blue and green light was provided by Dy3+, and by the codoping of red-emitting Eu3+ or Sm3+, white light was achieved in Dy/Eu/Gd or Dy/Sm/Gd systems. It is noted that the Gd3+ coordination matrix here only provides the doping and dilution platform, and no contribution from the ligand emission is involved in the white light.139 In recent years, more WLE and PLCT examples from f−f heteronuclear mixed-Ln complexes were reported.140−147 Li and co-workers synthesized isostructural 3D single-Ln MOFs [Ln2(L)2(DMAC)2]·nH2O [Ln = La (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7), and Tm (8); L = 5-(bis(4carboxybenzyl)amino)isophthalic acid; DMAC = N,N′-dimethylacetamide]. Specially, two f−f heteronuclear MOFs [Ln = La0.93Eu0.02Tb0.05 (9) and Tm0.47Eu0.18Tb0.35 (10)], which have similar isomorphous structures, can emit pure white light. In addition, Eu-MOF (5) and Nd-MOF (3) can be used for TNP sensing.141 Zhang and co-workers synthesized EuxTbyLa1−x−yTATB (TATB3− = 4,4′,4″-s-triazine-2,4,6-triyl tribenzoate). When the content of Eu3+ and Tb3+ gradually decreases, and the ratio of La3+ increases accordingly, the emitting color changes from pale yellow to pale yellow-green, near white, white, and pale blue-green, gradually. The optimal ratio of Ln3+ ions is 73 mol % La3+, 24 mol % Eu3+, and 3 mol % Tb3+ for

Eu0.06Tb0.09), LIFM-20 (La0.8Eu0.1Tb0.1), and LIFM-20 (La0.6Dy0.2Sm0.2). For LIFM-19 (Gd0.85Eu0.06Tb0.09), orange, yellow, white, green, and red emissions are obtained by the excitations of 320, 350, 365, 370, and 395 nm, corresponding to CIE coordinates of (0.54, 0.41), (0.48, 0.40), (0.35, 0.35), (0.30, 0.41), and (0.62, 0.30). Especially, for the white-light emission by the excitation of 365 nm, the obtained CCT and CRI values are 4833 K and 73. Similar results are found for LIFM-20 (La0.8Eu0.1Tb0.1), while for LIFM-20 (La0.6Dy0.2Sm0.2), yellow, white, and cyan emissions are obtained by the excitations of 290, 335, and 360 nm, including the white-light emission with CIE coordinates of (0.33, 0.32), CCT of 5226 K, and CRI of 97. In other cases, the residual blue emission from the ligand does not rely on the coordination of La3+ or Gd3+, but is achieved by other metal-coordinated (such as Zn2+) d−f heteronuclear complexes, or directly in bimetallic Eu−Tb complexes. As an example, in two Ln-Zn heteronuclear isomers {[LnOH(H 2 O) 6 ][Zn 2 Ln 4 (4-Htbca) 2 (4-tbca) 8 (H 2 O) 12 ]} n · 6nH2O (Ln = Eu or Tb; H2tbca = 4-(1H-tetrazol-5yl)biphenyl-3-carboxylic acid), Eu−Zn only emits characteristic red luminescence of Eu3+, while Tb−Zn has both the green emission of Tb3+ and the LC blue emission, due to the incomplete ET from the ligand to the Tb3+ center. Therefore, by doping of Eu3+ into the Tb−Zn heterometallic MOF, tunable RGB primary colors toward white-light emission are obtained. With a 0.5% Eu3+ doping ratio, nearly ideal white light with CIE coordinates of (0.331, 0.328), a CRI value of 81.7, and a CCT magnitude of 5562 K is achieved,132 while in a CP of Tb-BTB (BTB = 4,4′,4″-benzene-1,3,5-triyl tribenzonate), white light is achieved with the green Tb3+ emission and residual ligand blue emission by appropriate Eu3+ doping.133 Alternatively, the blue emission for the RGB mixture was proposed to be generated from codoping Ce3+ in the solid-solution-type heteronuclear complex (Figure 31).134 Unfortunately, the actualization of such conception might be difficult due to the easy luminescence quenching of Ce3+ by organic ligands. Codoped Ln-MOFs EuxTb1−xL were obtained from the isomorphous nature of {[Ln(L)2(COO)(H2O)2]·H2O}n (Ln =

Figure 31. Photographs of the luminescence of the complexes with general formula [(Ce2−x−yEuxTby)(C8H4O4)3(H2O)4]∞ with x + y ≤ 2 under UV irradiation. Reprinted from ref 134. Copyright 2009 American Chemical Society. AA


Chemical Reviews

Review

Figure 33. Emission spectra of NR/C153@Zr-NDC in a diethyl ether suspension. Reprinted with permission from ref 148. Copyright 2015 Royal Society of Chemistry.

respectively. When DCM, C6, and CBS-127 are coincluded in HSB-W1, WLE materials are obtained by adjusting the contents and relevant intensities of their emissions. Upon excitation at 365 nm, the emission spectrum of HSBW1⊃DCM/C6/CBS-127 (0.003 wt % DCM, 0.002 wt % C6, and 0.0005 wt % CBS-127) covers almost the whole visible spectral region, giving CIE chromaticity coordinates at (0.33, 0.33). Li’s group reported a new Zn2+ MOF as a host−guest chemopalette for white-light emission.150 When the excitation wavelength is less than 330 nm, the maximum emission wavelength of ZnBDCA ([Zn4O(adenine)4(benzene-1,3-dicarboxylate)4Zn2]) is located at about 410 nm due to the LMCT around the Zn4O(ade)4(COO)4. ZnBDCA has a high quantum yield (50%) and nanochannels that can encapsulate acriflavine molecules with bright yellow-green emission to build a host−guest chemopalette, Acf@ZnBDCA. By changing the excitation wavelength and modulating the amounts of acriflavine, Acf@ZnBDCA can readily produce white-light emission. When the concentration of acriflavine in the Acf@ ZnBDCA system is 0.12 wt % and the excitation wavelength is 320 nm, WLE with corresponding CIE coordinates of (0.313, 0.347) is obtained. Alternatively, Nitschke and co-workers synthesized a series of M4L6 tetrahedral cages via the subcomponent self-assembly method (Figure 34). The skeletons of the MOCs are photoluminescent by themselves, due to the incorporation of BODIPY and pyrene fluorophores with red and green emissions onto the ligands. Meanwhile, the cages’ inner cavities are capable of guest inclusion performance. By addition of different amounts of perylene guests with blue emission into the cages, interesting tunable PL properties are observed. Initially, a small amount of perylene in the cage increases both the red and blue emissions, while further on, after the cage reaches its maximum guest-inclusion capability, further addition of perylene only leads to an increase of the blue emission. As a result, when the perylene:cage ratio reaches 3:1, the host−guest system attains comparable blue-, green-, and red-emitting intensities, which lead to direct white light with a quantum yield of 11% and CIE coordinates of (0.30, 0.36).151 4.3.2. Ln3+-Included Complexes. The introduction of Ln3+ ions with appropriate ratios into MOFs has been widely applied to obtain SPWLE materials. Different from the

Figure 32. (a) Structure of the ligand H3L and the complexes LnL. (b) PL emission spectra of the Dy/Eu-doped GdL complexes (λex = 290 nm). (c) PL emission spectra of the Dy/Sm-doped GdL complexes (λex = 290 nm). Reprinted with permission from ref 139. Copyright 2012 Royal Society of Chemistry.

pure white light, corresponding to CIE coordinates of (0.3323, 0.3349).143 4.3. Single-Phase Guest-Included Coordination Assemblies

4.3.1. Organic-Dye-Included Complexes. Postincorporation of organic dyes into the cavity of the MOF matrix presents an effective assembly approach to obtain SPWLE, as introduced in section 3.6. In a recent study, by trapping different dyes into a Zr-based porous MOF, Douhal and coworkers obtained some host−guest composite materials, which produced tunable PL colors as well as white-light emission.148 The NR/C153@Zr-NDC (NR = Nile red; C153 = coumarin 153; Zr-NDC = zirconium naphthalenedicarboxylic acid) complex in a diethyl ether suspension emits bright white light upon excitation of 350 nm, which shows three welldifferentiated peaks (Figure 33). The blue emission (I) arises from Zr-NDC, the green band (II) originates from C153, and the red one (III) is assigned to NR. The combination displays a white light with CIE coordinates of (0.32, 0.34), very close to those of pure white light (0.33, 0.33). Wu and Zhu’s group also reported a strategy to introduce blue-, green-, and red-emitting dyes into the MOF matrix to obtain WLE materials.149 In this work, the neutral MOF HSBW1 (HSB = hydrogenated Schiff base) was used as the host matrix. A large number of screening tests show that the encapsulation process of various dyes into HSB-W1 is guestsize-dependent. By introducing one dye into HSB-W1, the obtained systems HSB-W1⊃DCM, HSB-W1⊃C6, and HSBW1⊃CBS-127 display red, green, and blue emissions, AB


Chemical Reviews

Review

Figure 34. Dye-inclusion MOCs with WLE properties. Reprinted with permission from ref 151. Copyright 2014 Royal Society of Chemistry.

Figure 35. Representation of the assembly process and WLE generation mechanism of Eu3+@Al-MIL-53-COOH. Reprinted from ref 152. Copyright 2014 American Chemical Society.

isomorphous Ln-doping complexes, the MOF matrix here represents a porous structure without an emissive Ln3+ center and only serves as a platform for Ln3+ inclusion into the microor mesopores existing in the MOF. As an example, Ln3+ cations were successfully encapsulated into the channels of Al-MIL-53COOH nanocrystals to obtain WLE, in which the Al3+ framework plays dual functions as both a host to emit blue light and an antenna for protecting and sensitizing the luminescence of Ln3+ cations. As shown in Figure 35, the white-light emission obtained in Eu3+@Al-MIL-53-COOH is based on dual luminescent centers: the characteristic f → f emission of Eu3+ sensitized by the ligand and the LC emission from the framework. Changes in Eu/Tb3+-doping concentration and excitation wavelength show a strong impact on the relative luminescence intensity and emitting colors of the MOF nanocrystals. The Eu3+@Al-MIL-53-COOH material was further prepared into a thin film by a CSD (chemical solution deposition) process from its metastabilized colloidal solution, which shows whitelight emission under 350 nm excitation.152 Similar WLE results are also obtained in the Ln3+-incorporated microporous MOF materials MIL-121 (Al(OH)(H2btec)·H2O; H4btec = pyromellitic acid)153 and JUC-113 ([Zn3(TCPB)2(H2O)2]·2H2O· 4DMF; H3TCPB = 1,3,5-tris(4-carboxyphenoxy)benzene).154 A luminescent Bi−Cd−organic framework with formula Bi2Cd(2,6-pdc)4(H2O)2·H2O (2,6-H2pdc = pyridine-2,6-dicarboxylate) was synthesized by using bismuth(3+) oxides and Cd2+ salts as metal sources under hydrothermal conditions. Single-component white-light emission is obtained by doping Dy3+ into this framework, due to the mixture of blue (479 nm) and yellow (571 nm) emissions of Dy3+ ions, regardless of the contribution of luminescence from the ligand-based framework component.155 In another example, the formation of a WLE MOF is achieved by simultaneous codoping of IFP-1 (3∝[Zn(2methylimidazolate-4-amide-5-imidate)]) with Tb3+ and Eu3+ during the in situ formation of the ligand and the MOF, which

affords the doped luminescent MOFs with porosity almost identical to that of the nondoped framework. The triple emissions with blue, green, and red colors from the IFP-1 backbone, Tb3+, and Eu3+ are therefore combined in one emitter, and lead to effective white-light emission at the excitation of 360 nm. The PL is related to LMCT and MLCT transitions; therefore, a cold white light with a higher blue content is observed at 77 K, and a warmer white light at room temperature (RT) is produced due to the reduction of the organic emission, which can be potentially used for spectroscopic temperature sensing. Total luminescence spectral analysis manifests that the Ln3+ ions do not take the place of Zn2+ as connectivity centers, but are encapsulated into the pore of the MOF matrix. This represents the first example of a WLE coordination complex with porosity in combination with clearly proven luminescence centers inside the pore system of the MOF.156 Fan and co-workers157 reported Ln@In-MOFs by incorporating Ln3+ into the In1 framework with uncoordinated carboxylate groups from the H2FDA ligand (H2FDA = furan2,5-dicarboxylic acid; Figure 36). When Dy3+ and Eu3+ are integrated into the In1 matrix in different ratios, the obtained emission intensities and colors are tunable. The relationship of the luminescence intensity with the Eu3+ content could be linearly expressed with y = 34890x + 537.7, and the relationship between the Eu3+ content and the CIE-x values could be expressed by y = 1.91x − 0.5. With the metal content of Dy0.87Eu0.13, a WLE material is obtained with CIE coordinates of (0.33, 0.33) at the excited wavelength 315 nm. 4.3.3. Other Metal-Compound-Included Complexes. Like the example introduced in section 3.6, in which the Ir3+ complex chromophore is encapsulated in a mesoporous blueemitting MOF to result in direct white light,74 Lan’s group reported another inclusion system, namely, [Ir(ppy)2(bpy)]+@ NENU-524 (NENU-524 = [(CH3)2NH2]2[Zn8(btca)6(2NH2-bdc)3]·8DMF; Figure 37).158 It was prepared by an AC


Chemical Reviews

Review

Figure 37. (a) Structure of NENU-524. (b) WLED assembled using 3.86 wt % [Ir(ppy)2(bpy)]+@NENU-524 coated onto an LED (left) and with the white light when turned on (middle) and turned on continuously for one month (right). Reprinted with permission from ref 158. Copyright 2017 Royal Society of Chemistry. Figure 36. White-light emission of Ln@In-MOFs. Reprinted from ref 157. Copyright 2017 American Chemical Society.

acid) displays bright blue emission with a peak at 435 nm. Furthermore, NENU-521 is used as a host to encapsulate Alq3 with strong green emission in 520−530 nm. As a result, the inclusion complex Alq3@NENU-521 exhibits tunable luminescence from blue to green-yellow. At an optimal concentration of Alq3 (4.14 wt %), the spectrum exhibits a clear broadened emission with CIE coordinates of (0.291, 0.327) in the white region.

ion-exchange process between [Ir(ppy)2(bpy)][PF6] and the platform of anionic blue-emitting NENU-524. In NENU-524, {Zn8(btca)6(2-NH2-bdc)3} units form a microporous cage, and propagate to result in a 3D noninterpenetrating anionic framework. The 1D open hexagonal channels (ca. 13.18 × 13.18 Å2 parallel to the c-axis and ca. 12.27 × 14.63 Å2 along the b-axis) are quite sufficient for entry of [Ir(ppy)2(bpy)]+ (ca. 10 × 11 Å2). Therefore, inclusion compound [Ir(ppy)2(bpy)]+@NENU-524 is obtained. With the excitation of 370 nm, two emission peaks around 445 and 570 nm are produced in the solid luminescent spectrum. The blue emission comes from NENU-524, whereas the yellow emission is derived from [Ir(ppy)2(bpy)]+. The intensity of the yellow emission increases with increasing amounts of encapsulated [Ir(ppy)2(bpy)]+, while the blue emission remains almost the same. When the amount of [Ir(ppy)2(bpy)]+ cation is 3.86 wt %, an optimum white light is obtained with CIE coordinates at (0.300, 0.336). The SPWLE complex shows quite high stability, so it is coated onto a commercial UV-LED to make a simple WLED, for which the bright white light can be sustained in air for up to one month. Compared with the Ir 3 + complex, Alq 3 (tris(8hydroxyquinoline)aluminum), which almost meets all requirements of light-emitting materials, is much cheaper. Lan’s group reported another Alq3@MOF single-phase coordination assembly for white-light emission.159 The nanotubular NENU-521 ([(Zn 4 O) 3 (TPA) 4 (TDA) 3 (H 2 O) 6 ][(Zn 4 O)(TPA)2]2·12DMF; TPA = deprotonated 4,4′,4″-nitrilotribenzoic acid; TDA = deprotonated thiophene-2,5-dicarboxylic

5. POTENTIAL APPLICATIONS FOR SINGLE-PHASE WLE/PLCT COORDINATION ASSEMBLIES 5.1. White-Light-Emitting Diodes (WLEDs)

One of the major objectives to obtain SPWLE coordination assemblies is to facilitate the fabrication of WLEDs. As a matter of fact, abundant work has been done for this aim. Especially, Ir3+ complexes represent promising phosphorescent emitters due to their excellent PL efficiencies, relatively short (millisecond range) PL lifetimes, and versatile color-tuning abilities via ligand control. Typically, Ir3+ complexes can emit blue to red phosphorescence upon opto- or electrostimulation and are widely applied in such fields as light emitting, biological imaging, and PL sensing.160 However, most of the white-light emission in Ir3+-based electroluminescent devices are obtained from the combination of multiple emitting molecules, including one or more kinds of Ir3+ complexes with certain emitting colors ranging from blue to green, yellow, orange, and red.161,162 Instead, as an example of a pure single-component WLE Ir3+ complex, an excited state of mixed character was observed in a simple mononuclear cyclometalated Ir 3 + complex, (acetylacetonato)bis(1-methyl-2-phenylimidazole)iridium(3+) AD


Chemical Reviews

Review

an obvious blue shift (about 40 nm) compared with the free guest molecules. With an increase of the guest concentration, the intensity of the longer wavelength emission at 530 nm is steadily increased, leading to an emitting color change from blue to white, green, and yellow. Especially, at the guest loading weight of 3.5%, the encapsulated MOF can generate bright white light by the excitation of 370 nm (quantum yield 20.4%). The CIE coordinates, color rendering index (CRI), and correlated color temperature (CCT) values are (0.31, 0.33), 80, and 5900 K, respectively. WLEDs were fabricated using this material in two methods: the first one employs a commercially available ultraviolet LED and an [Ir(ppy)2(bpy)]+@1 sample (Ir complex: 3.5 wt %) as a phosphor, resulting in bright white light at an applied voltage of 3.8 V. The second one is made applying an InGaAsN chip (370 nm) and an [Ir(ppy)2(bpy)]+@1 sample (Ir complex: 3.8 wt %), yielding a WLED with CIE, CCT, and CRI values of (0.30,0.35), 84.5, and 5409 K.74 Organoplatinum complexes are excellent emissive candidates for OLEDs due to their tunable and highly efficient phosphorescence, as well as easy manipulation in device doping and fabricating. Uniquely, some Pt2+ complexes have a strong tendency to form bimolecular (BM) excited states (excimer or exciplex), which can emit red-shifted and broader phosphorescence than that from a monomer molecule. This offers a route to achieve white-light emission from a singlephase Pt2+ complex emitting layer (EML). By the combination of short-wavelength emission from excited-state monomer molecules and the long-wavelength emission from the excimer/exciplex states, and furthermore, by adjusting the relative intensity of the two emissions, the color of the EML emission can be tuned to white. This greatly simplifies the device fabrication process, and the OLEDs featuring such excimer/exciplex emissions have been named EXLEDs (excimer or exciplex OLEDs).164−171 According to the working mechanism of such EXLEDs to achieve white light, two structural features are required in the design of WLE Pt2+ complexes: (1) an efficient organic chromophore to ensure a high emitting quantum yield in the monomeric complex and (2) an appropriate Pt2+ coordination configuration (usually square-planar) having little steric hindrance to facilitate the formation of the excimer. Some examples are listed in Scheme 10. According to this principle, a square-planar Pt complex, FPt (Scheme 10), was explored by Jabbour and co-workers, who succeeded in fabricating single-doped white OLEDs.164 FPt can generate blue monomeric emission and orange excimeric

(N966; Figure 38). The wide-band emission covering 440− 800 nm in this complex is cooperatively contributed by MLCT

Figure 38. (a) Absorption (left dotted line) and emission (right full line) spectra of N966 in dichloromethane. The inset shows the chemical structure of N966. The photo shows the white-light emission of N966 obtained upon excitation using 355 nm laser light. (b) Electroluminescence spectrum of WOLED: ITO| PEDOT:PSS|TCTA:N966|TPBI|Ba|Ag. The inset shows the luminance vs voltage. Reprinted with permission from ref 163. Copyright 2009 Royal Society of Chemistry.

and π−π* transitions, as confirmed by DFT calculations. It is noted that no evidence of dual emission for N966 is detected, and only a single emitting excited state of mixed character seems to be the origin of the broad emission. A WOLED is also fabricated from N966 with the device structure of ITO| PEDOT:PSS|TCTA:N966|TPBI|Ba|Ag. The electroluminescence (EL) spectrum of the N966-containing OLED is very broad with a maximum around 570 nm, an fwhm of 165 nm, and CIE coordinates at (0.281, 0.360), residing in the whitelight region. The efficient WOLED reaches a luminance value of more than 1000 cd m−2 at a driving voltage of 9 V.163 Since the application of similar Werner-type simple Ir3+ complexes in WLEDs has been surveyed in other review papers, we will not elaborate more herein. Alternatively, Ir3+ complexes are also widely applied as guest molecules to be introduced into the pores of MOFs to yield interesting photoluminescent properties and white-light emission. As the example already shown in Figure 17 (section 3.6), the postencapsulated MOF [Ir(ppy)2(bpy)]+@1, (1 = [(CH3)2NH2]15[(Cd2Cl)3(TATPT)4]·12DMF·18H2O) manifests tunable luminescence upon different concentrations of the guest Ir complex. Originally, the complex [Ir(ppy)2(bpy)]+ emits yellow light with broad and structureless emission at 570 nm at the excitation of 370 nm, featuring predominant 3MLCT or 3LLCT character. After encapsulation, the PL spectra of the samples exhibit two emission peaks at 425 and 530 nm upon irradiation of 370 nm, with the former emission contributed by the MLCT transition of the host MOF (1), and the latter emission originated from the guest [Ir(ppy)2(bpy)]+, showing

Scheme 10. Some Pt2+ Complexes Applied for WLEDs

AE


Chemical Reviews

Review

PtL26Cl can reach high brightness up to L = 10000 cd m−2, with extremely high EQE up to 20% at low current densities (

Single-Phase White-Light-Emitting and Photoluminescent Color

Recommend Documents