Adjusting the Layer Charges of Host Phyllosilicates To Prevent

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Adjusting the Layer Charges of Host Phyllosilicates to Prevent Luminescence Quenching of Fluorescence Dyes Limei Wu, Guocheng Lv, Meng Liu, Zhaohui Li, Libing Liao, and Caofeng Pan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07243 • Publication Date (Web): 14 Sep 2015 Downloaded from http://pubs.acs.org on September 14, 2015

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The Journal of Physical Chemistry

THE JOURNAL OF PHYSICAL CHEMISTRY C

Adjusting the Layer Charges of Host Phyllosilicates to Prevent Luminescence Quenching of Fluorescence Dyes Limei Wu, † Guocheng Lv,*,† Meng Liu, † Zhaohui Li, †,‡,§ Libing Liao, *,† Caofeng Pan*,¶ †

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing100083, PR China ‡ Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan 70101 § Geosciences Department, University of Wisconsin – Parkside, Kenosha, WI 53144, U.S.A. ¶ Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Science, Beijing100083, PR China

ABSTRACT: Luminous properties of organic fluorescent dyes could be effectively enhanced by adjusting the layer charge of the inorganic host. In this study, we constructed a novel inorganic/organic composite whose layer charge is tunable. This composite material greatly enhanced the luminous efficiency of lucigenin. Compared to the crystalline lucigenin, the fluorescence lifetime of lucigenin increased by 26 folds and luminous intensity was 14 times higher after being intercalated into the interlayer of the saponite host. In addition, the thermal stability was greatly improved. The tunable layer charge can provide a static confinement to the fluorescence dyes and better distribute the lucigenin molecules to prevent fluorescence quenching due to aggregation. The inhibition of fluorescence quenching by intercalating the fluorescence dye into the interlayer of the host inorganic substrate reveals a theoretical significance for the development of high performance organic luminous materials.

KEYWORDS: Inorganic/organic composite; Adjustable layer charge; Fluorescence quenching; Light-emitting efficiency

route enhanced physical stability and compatibility, but non-

INTRODUCTION When fabricating luminescent materials, small organic

uniformity and aggregation were still inevitable for the

molecules, which have wide sources and types, are easier to be

resulting film materials. Fabricating luminescent films with

chemical modified and purified to have a wider coverage of

desirable architecture, high flexibility, and stability remains as a

emission spectrum. The type of materials has received much

challenge.

attention over the past decades because of their unique size-

Saponite (SAP) is a 2:1 type trioctahedral phyllosilicate of

dependent optical properties1. However, stabilization and

the smectite group of clay minerals. The SAP structure is

enhancement of their luminous intensity are critical when

composed of a central octahedral sheet with essentially a

applied in solid devices. Although several approaches for the

brucite [Mg3(OH)6] structure, in which four out of six OH–

assembly of flexible film materials involving in using small

groups are replaced by oxygen atoms. This octahedral sheet is

organic

been

sandwiched in between two tetrahedral sheets. The ideal

developed , they usually suffer from phase separation,

structural formula of SAP can be presented as Mx+[Mg3][Si4–

molecules

and

organic

polymers

have

2,3

aggregation of small organic molecules, and/or decreased

xAlx]O10(OH)2•nH2O,

where M is the exchangeable interlayer

fluorescence quantum yield4,5. Using a casting process, Tetsuka

cation. Generally, the isomorphous substitution of Al3+ for Si4+

et al. reported an alternative method to stabilize small organic

in the tetrahedral sheets is dominant and SAP is negatively

molecules

by

charged. The net negative charges are compensated by

incorporating some inorganic clays6. Such an inorganic-organic

exchangeable interlayer cations. In synthesis of SAP, charge

of

luminescence

in

organic

polymers

density of tetrahedral sheets could be adjusted by changing the

|1

ACS Paragon Plus Environment


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ratio of Si/Al, so that the strength of the electric field and thus,

continuous stir until a uniform gel was eventually obtained. The gel

the interaction forces between sheets and interlayer cations

was transferred to a polytetrafluoroethylene-lined autoclave and

would be different. SAP has a rigid structure, which can

heated at 200 °C for 24 h.

provide a confined microenvironment to luminescent molecules

For BNMA intercalation into the SAP interlayer, a desired

at the nanoscale. Interactions between the guest molecules and

amount of BNMA was dissolved in 100 mL of distilled water

host SAP sheet in photoactive molecule/SAP material are

followed by addition of 0.5 g of the synthetic SAP. The mixtures

beneficial to improve luminous efficiency of photoactive

were stirred at 60 °C for 6 h, then centrifuged at 10000 rpm for 20

molecules. Adjusting charge density would generate different

min. The supernatants were filtered through 0.22 µm syringe filters

orientations of the guest interlayer molecules, thus achieving

before being analyzed for the equilibrium BNMA concentrations.

microscopic control of the distance between molecules, leading

The final solid products were washed six times with deionized water

to uniform distribution. This process can effectively restrict

and dried at 80 °C.

quenching of photoactive molecules caused by molecule

Elemental analysis was conducted on a Rigaku RIX 2000 X-

accumulation, obtaining luminescence without redshift and

ray fluorescence spectrometer (XRF). Loss-on-ignition was obtained

broadening.

by weight loss of the sample ignited in a furnace at 900 °C for 2 h

This research was mainly concentrated on intercalation and

and allowed to cool in a desiccator to minimize moisture absorption.

self-assembly of positively charged luminous organic dye bis-

Powder XRD analyses were performed on a Rigaku D/max-

N-methylacridinium nitrate BNMA, also called lucigenin) and a

III diffractometer (Tokyo, Japan) with a Ni-filtered CuKα radiation

layered material SAP, forming inorganic/organic composite.

at 30 kV and 20 mA. Orientated samples were scanned from 3° to

The interaction force between the SAP sheets and BNMA was

15°at 2°/min with a scanning step of 0.01°. For un-oriented samples,

modified by adjusting the layer charges of SAP. This composite

powder samples were packed in horizontally held trays. The changes

is beneficial to orientation effects of luminous organic

in the XRD peak positions reflect the intercalation of BNMA into

molecules in the gallery, to achieve microscopic control on

layered silicates. The Bragg equation was applied to calculate the

distances of luminous organic molecules, effectively restricting

basal spacing of SAP. The gallery heights in intercalated hybrids

quenching of photoactive molecules by accumulation and

were deduced from XRD peak positions of the (001) reflection of the

obtaining luminescence without redshift and broadening.

hybrids.

Meanwhile, the composite would enhance the physical and

Cation exchange capacity (CEC) and exchangeable cations

chemical stability of luminous organics and is expected to solve

were determined using 60 mg of each sample dried at 105°C.

low stability and short lifetime of common luminous organic

Exchangeable cations were replaced by ammonium by washing the

devices. Moreover, by combining suitable luminescent building

samples five times with 25 mL of a 1 mol/L ammonium acetate

blocks, the BNMA-SAP system could be applied to other

solution (pH=7) for 12 h. Excess ammonium was removed by

multicolor

as

washing six times with ethanol. All supernatant liquids were

luminescent sensors, molecular thermometers, and fluorescent

collected and prepared for atomic absorption spectroscopy to

antiforgery devices.

determine exchangeable cations.

systems, which

can be potentially

used

EXPERIMENT AND METHODS

Both

SAP with different charge densities was prepared by controlling the added ratio of Si, Al, and Mg. A buffer solution was prepared by dissolving 9.00 g of NaOH and 16 g of NaHCO3 in 250

27

Al MAS NMR and 29Si cross-polarization (CP) MAS

NMR measurements were carried out on a Bruker AVANCE III 400 WB spectrometer. The resonance frequency of 29

29

Si was 79.5 MHz.

Si CPMAS NMR spectra were recorded with a contact time of 4.5

mL of deionized water. Then, desired amounts of sodium

ms and a recycle delay of 2 s, using tetramethylsilane as an external

metasilicate (Na2SiO3⋅9H2O) solution were added to the buffer

reference. The corresponding resonance frequency of 27Al was 104.3

solution under vigorous agitation. The obtained solution was noted

MHz. Single pulse magic angle spinning spectra were acquired using

as Solution 1. Solution 2 was prepared by dissolving desired

a high power 0.5 µs pulse, corresponding to a tip angle of 18° and a

amounts of AlCl3⋅6H2O and MgCl2⋅6H2O in 25 mL of deionized

recycle delay of 0.5 s.

water. Then, solution 2 was slowly added into solution 1 with


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Photoluminescence excitation (PLE) and emission (PL)

their corresponding CEC values increased slightly and then

spectra were characterized on fluorescence spectrophotometer

decreased progressively (Table 2). The CEC of A~F is

(HITACHI, F4600) with a photomultiplier tube operating at 400 V,

measured at room temperature. It reveals that sample F has the

and a 150 W xenon lamp was used as the excitation source. The

highest charge density and strongest interlayer electric field

lifetime was recorded on a spectro-fluorometer (HORIBA, JOBIN

strength, while sample A has the lowest and weakest.

YVON FL3-21), and the 370 nm pulse laser radiation (nano-LED) was used as the excitation source.

Solid state NMR is a principle characterization method to study magnetic nuclear structure environment in minerals. The

Molecular simulation was performed under the module

information about Al occupancy in the synthetic SAP can be 27

‘CASTEP’ of Materials Studio 6.0 software to investigate the

obtained from the

sorption sites of BNMA in the interlayer of SAP. The primitive

samples,

unit cell of SAP was optimized with the generalized gradient

corresponding to Al(IV), is much stronger than that at around 9

approximation (GGA) for the exchange-correlation potential

ppm, corresponding to Al(VI)15. This study suggests that the

(PW91) that is appropriate for the relatively weak interactions

preferred sites for Al ions are tetrahedral sites instead of

present in the models studied. The resulting primitive unit cell

octahedral ones. This further confirms the trioctahedral nature

was characterized by the parameters a=15.540 Å, b=17.940 Å,

of the synthesized samples. In addition, as the Si/Al ratio

c=12.56 Å, α=γ=90°, and β=99°. Based on the primitive unit

decreased, the content of Al in SAP increased, the peak

cell, a series of (3×2×1) supercells were built with the interlayer

intensity at 9 ppm enhanced gradually, indicating an increased

spacing determined by the XRD results. Based on the structure

of Al(VI) occupation in octahedra. It is in accordance with XRF

of the preferential adsorption model of BNMA in the layer of

results that Al content in crystal structure increased as Si/Al

SAP predicted by Monte Carlo calculation,

ratio reducing (Table 1).

GGA-PW91 was used to optimize the structure again and to

The

the

29

Al MAS NMR spectra (Figure 3a). For all

resonance

Si

CPMAS

at

approximately

NMR

spectra

also

65

ppm,

provided 29

predict the interaction energy between BNMA and SAP layers to a

complementary evidences for Al occupancy (Figure 3b).

greater accuracy. All of the GGA-PW91 calculations were

signals were recorded at approximately -95, -91, and -86 ppm,

performed using a double numerical plus polarization function (DNP) corresponding

to

Q1Si(0Al),

Q2Si(1Al),

and

Si

Q3Si(2Al),

as basis set and DFT-D correction.

respectively16. As the ratio of Si/Al reduced, signal intensity of

RESULTS AND DISCUSSION

Q1Si(0Al) decreased gradually while that of Q2Si(1Al)

Morphology of the synthetic SAPs. BNMA has a pKa1 value of 3.3 and a pKa2 value of 5.1 (Figure 1), because the equilibrium solution pH value was 3~5, most of the BNMA will exist as monovalent cation. The intercalation process of BNMA into SAP was illustrated in Figure 1.

was adjusted by changing the ratio of Si/Al. The result of XRF analysis showed that, as the ratio of Si/Al increased, the relative content of Si in synthesized samples increased and Al content decreased, in agreement with added ratios (Table 1). The content and occupation of Mg in octahedral sheets are critical to discriminate montmorillonite (MMT) from SAP in the smectite group. In MMT octahedral sites are mostly occupied by Al, while those in SAP are mostly occupied by Mg. Formula of different types of SAP can be calculated from result of XRF The

resulting

charge

amount of Al linked to Si in tetrahedral sites increased from 0.3 in A to 1.0 in F with the decrease in Si/Al ratio (Table 1). The amount of Al in tetrahedral sites determined the charge density of SAP. It is changed from 4.19% to 16.23% (Table 1). Correspondingly, electric charge increased from -0.30 to -0.80

In the synthesis of SAP, the charge density of the sheets

analyses.

increased, suggesting an increase in tetrahedral Al content. The

densities

increased

systematically as the Si/Al ratio decreased (Table 2). However,

charge per (Si, Al)4O10 (Table 2). Thus, the layer charge density was controlled precisely by the adjustable Al content in tetrahedra. And, a higher charge density dictated a higher electric field strength of layers17, which determined the arrangement of interlayer BNMA.

Intercalation effect and property. BNMA is easy to accumulate by interaction forces, forming aggregates, causing quenching, thus, lowering its luminous efficiency18. Therefore, BNMA was intercalated into synthesized SAP samples by ion exchange to prevent aggregation. The strength of the electric field in the vicinity of SAP can be very high, which has a profound effect on



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intercalation19. The intercalated amounts of BNMA to SAP were

BNMA intercalation into SAP. Meanwhile, the thermal stability was

0.28 (A), 0.35 (B), 0.22 (C), 0.20 (D), 0.16 (E), 0.15 (F) mmol/g for

enhanced as well. BNMA, in the gallery space of SAP, is under the

samples A through F, respectively (Table 2), decreasing as the

protection of SAP sheets held by electrostatic attraction, making it

charge density of SAP increased, in accordance with the change of

better adaptive to changing ambient temperature.

CEC. XRD patterns showed decreases in interlayer spacing of

SAP has a supramolecular structure, shows a good photo-

BNMA-intercalated SAP as the charge density increased (Figure 2b

thermal stability and an excellent UV barrier. It also affects greatly

and Table 2).

on expanding lifetime and aging resistance of photo-active

To SAP, the higher the charge density of sheet, the

molecule27. Therefore, SAP can effectively restrict degradation and

stronger of the interaction forces between the sheets and the

ionization process, which would reduce the activity of photo-active

interlayer cations, making the exchange of interlayer cations

molecules, and improve the photo-thermal stability. This study

more difficult, thus, the amount of intercalated BNMA will

provided preliminary evidences to solve problems, such as low

20

decrease . However, the interaction force between the sheets

stability and short lifetime, in the application of organic luminous

and intercalated BNMA is enhanced as the charge density

molecular devices.

increases. In addition, higher charge density of sheets will lead

Mechanism analysis of fluorescence enhancement.

to well-distributed intercalation, resulting in symmetric and

The O 2p, Si, Mg/Al 3s, and H 1s orbits for SAP were all

sharp diffraction peaks, indicating increased stacking order and

higher than the top of the valence band and lower than the

stability of the organic/inorganic composite material21,22.

bottom of the conduction band, resulting in no energy

The fluorescence intensity of the composite material at 550

transition, thus, no luminescence. So the top of the valence

nm showed an increasing trend as the charge density of SAP

band and bottom of the conduction band of the composite

increased (or as the amount of BNMA intercalation decreased)

material come from 2p(π) and 2p(π∗ ) orbit of C in main chain

(Figure 4a). The intensity of emission spectra of crystalline

of BNMA28-33. The valence electron of BNMA is trapped in

BNMA is weak, with a broad peak and poor luminescence. The

quantum well formed by SAP sheets, and the electric field of

images of BNMA and BNMA-SAP under white light and

SAP gallery improved luminescent property of BNMA19,34. In

fluorescent light can be seen in Figure 5. There is no redshift

summary, gallery region of SAP stabilized valence electron of

and broadening in fluorescence emission spectra of the

BNMA, promoting to develop light stable and efficient

composite material, demonstrating few molecular aggregates

luminescent organic molecules.

during the intercalation process23,24. XRD data further indicate

Interaction forces on BNMA in gallery region of SAP are

that the composite material has a highly ordered periodic

mainly electrostatic attraction and Van de Waals. The total

layered structure. Meanwhile, the intensity of XRD diffraction

energy of the six SAP systems with different charges densities

peaks also increased as the charge density increases. The same

was -284.1, -438.6, -659.6, -684.1, -761.8, and -889.2 ev (Table

tendency appears in the lifetime of the composite material

3). The total energy of the system decreased as the charge

(Figure 4b). Liquid lifetime of BNMA is only 0.02 µs (Table

density of SAP sheet increased, indicating enhanced forces

2), while after intercalated into SAP interlayer, its lifetime is

between the SAP sheet and BNMA in gallery region. In

expanded to 0.515 µs, a 26-fold increase.

addition, among energies of the system, electrostatic energy is

The luminescence of the composite material changes

more than Van de Waals interaction35,36, demonstrating that the

significantly under different temperature, which means that the

driving force for BNMA intercalation into SAP is mainly

thermal stability of the materials is a principal factor affecting its

electrostatic interaction. Sheets could provide a confined

application25,26. The luminous intensity of pure BNMA decreases as

microenvironment of electric field, which leads to increases in

environment temperature increased (Figure 4c), disappearing at

charge density as well as amount and efficiency of carriers37.

about 75°C. Despite luminous intensity of BNMA-SAP weakened as

This environment enhanced energy carrying capacity of

temperature increased, it did not disappear until about 300°C (Figure

excitations simultaneously, therefore resulting in an increased

4d). Therefore, compared to pure BNMA, both the luminous

luminous intensity38.

intensity and lifetime of BNMA-SAP were improved greatly after


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Arrangement of BNMA in gallery region of SAP will

smaller tilting angle, less aggregation, and more stable

directly affect interlayer spacing (Figure 6), having a key role

structure. In conclusion, through the modification of layer

39-41

. We

charges, an inorganic/organic luminescent composite material

define orientation angle as the dihedral angle between

with different luminous densities and stabilities can be

conjugate plane of BNMA cation and SAP sheet, to describe

obtained.

arrangement of intercalated BNMA. The surface coverage also

ACKNOWLEDGMENTS

in interpreting system structure and interaction force

plays a significant role, but we just studied the BNMA of interlayer. When the amounts of intercalation are the same, orientation angle are 42, 35, 30, 25, 20, and 0° (almost parallel), with increasing charge density. This effect is a consequence of electrostatic forces, whereby precedence is given to the alkali ions, which have a greater charge density per volume relative to the more bulky organic group17. A lower interlayer density allows the BNMA to assume a higher percentage of energetically favourable anti conformations since more lateral interlayer space is available. A higher interlayer density enforces a higher fraction of gauche conformations because the BNMA pack more densely and assume energetically less favourable chain rotations37. BNMA-intercalated SAP tends to form tilt arrangement. As the charge density increased, the electrostatic force increased, leading to lowering the tilt angle. BNMA in the gallery space of SAP tends to arrange in monolayer parallel to the sheet and the intermolecular force of between BNMA becomes weaker, which could effectively avoid concentration quenching and improve luminous intensity, lifetime, and stability notably due to stronger interaction forces between the SAP sheet and the BNMA molecule42-44.

This research was jointly funded by Beijing Higher Education Young Elite Teacher Project (YETP0634), Beijing Natural Science Foundation (2153041), and the Fundamental Research Funds for the Central Universities (2-9-2015-314).

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Saponite with different Si/Al ratio was synthesized in this study, to obtain gallery fields of different electric field strengths by adjusting sheet structure of saponite. Through analyses of XRF, XRD, and solid state NMR, structures of saponite after different adjustments were characterized. Al has different occupation preference in tetrahedral and octahedral sites, leading to changes in charge density of the sheet. Intercalating BNMA into the gallery region of saponite with different charge densities was achieved through cation exchange, obtaining a composite layered structure of inorganic/organic material. This material greatly enhanced the light stability and efficiency of BNMA. Molecular dynamic simulation indicates that the higher the electric strength is, the greater electrostatic attraction between saponite sheet and BNMA will be minimizing the

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Figure 1. Schematic view of BNMA intercalation into the interlayer of saponite (BNMA-SAP).

Figure 2. XRD patterns of synthetic saponite (a) and BNMA-SAP (b) with different Si/Al molar ratios.


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Figure 3.

27

Al MAS NMR (a) and 29Si CPMAS NMR (b) spectra of synthetic saponite.

Figure 4. Fluorescence spectra (a) and fluorescence decay curves (b) of the BNMA and BNMA-SAP, temperature-dependent spectra of the BNMA (c), and BNMA-SAP (d).



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Figure 5. Images of the BNMA and BNMA-SAP. Under sun light (a), and ultraviolet light (b).


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Figure 6. Molecular dynamic simulation of intercalation of BNMA into saponite at different angle as a result of charge density and BNMA loading difference. For all species, C = gray, N = blue, H = white, O = red, Si = yellow, Al = pink, and Mg = green.



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TABLE 1: Chemical composition and chemical formula of SAP. A

Chemical composition (%) SiO2

B

C

D

E

F

60.94

59.30

58.25

57.67

54.35

48.05

Al2O3

4.19

5.22

5.84

7.14

10.88

16.23

MgO

32.66

32.61

32.64

33.03

33.86

33.08

Na2O

2.64

3.08

3.16

2.09

1.67

1.96

Chemical formula Na0.3Mg3.0(Si3.7Al0 Na0.37Mg3.0(Si3.63A Na0.37Mg3.02(Si3.59 (Na0.24Mg0.1 .3

O10)(OH)2

l0.37)O10(OH)2

Al0.41)O10(OH)2

)(Mg2.92Al0.08)[(Si3

(Na0.19Mg0.18)(Mg2 (Na0.24Mg0.28)(Mg2. .89

Al0.11)

Al )O10](OH)2 [(Si3.34Al0.66)O10]( .48 0.52

8

Al0.2)[Si3.0Al1.0O10]

(OH)2

OH)2

Composition of Solution 2

0.26: 3.0

0.3: 3.0

0.45: 3.0

0.62: 3.0

0.73: 3.0

1.2: 3.0

13.19

13.09

12.89

12.91

12.94

13.31

(AlCl3):(MgCl2)

Interlayer spacing (Å)


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THE JOURNAL OF PHYSICAL CHEMISTRY C TABLE 2: The physico-chemical and luminescence properties of the SAP prepared under different Si/Al/Mg ratios. A

B

C

D

E

F

Si/Al/Mg

28.8/2.0/3.0

24.7/2.0/3.0

15.8/2.0/3.0

10.9/2.0/3.0

9.0/2.0/3.0

4.7/2.0/3.0

Layer charge (charge/(Si, Al)4O10))

-0.30

-0.37

-0.41

-0.44

-0.55

-0.80

CEC (mmol/g)

0.92

1.04

0.96

0.88

0.82

0.77

Amount of BNMA intercalation (mmol/g)

0.28

0.35

0.22

0.20

0.16

0.15

Interlayer spacing (Å)

16.55

16.21

15.14

15.04

14.87

14.87

Τ (µs)

0.201

0.216

0.252

0.255

0.323

0.515

TABLE 3: The d001-spacing of saponite before and after BNMA intercalation and interaction energies of BNMA intercalation into saponite of different charge densities. Model

| 13

d

001

(Å)

△E (ev)

A

16.86

-284.1

B

15.61

-438.6

C

15.14

-659.6

D

15.04

-684.1

E

14.87

-761.8

F

14.87

-889.2



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The table of contents graphic

The interaction force between layers and BNMA was modified by adjusting layer charge of the host mineral saponite. This adjustable inorganic/organic composite material greatly enhanced the light stability and efficiency of BNMA.


Adjusting the Layer Charges of Host Phyllosilicates To Prevent

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