Molecular Packing Dependent Solid State Fluorescence Response of

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Molecular Packing Dependent Solid State Fluorescence Response of Supramolecular Metal-Organic Frameworks: Phenoxo-bridged Trinuclear Zn (II) Centered Schiff Base Complexes With Halides and Pseudohalides Nidhi Dwivedi, Sailaja S. Sunkari, Abhineet Verma, and Satyen Saha Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00948 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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Crystal Growth & Design

Molecular

Packing

Dependent

Solid

State

Fluorescence Response of Supramolecular MetalOrganic Frameworks: Phenoxo-bridged Trinuclear Zn (II) Centered Schiff Base Complexes With Halides and Pseudohalides. †

Nidhi Dwivedi,a,b Sailaja S. Sunkari,a* †Abhineet Vermab and Satyen Sahab*

a.

Dept. of Chemistry, Mahila Maha Vidyalaya, Banaras Hindu University, Varanasi

221005, India. Email: [email protected]. b.

Dept. of Chemistry, Institute of Science, Banaras Hindu University, Varanasi 221005,

India. Email: [email protected] (†ND and AV have equal contribution)

KEYWORDS Trinuclear-Zn Schiff base complexes, crystallographic studies, photophysical studies.

ACS Paragon Plus Environment

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ABSTRACT: Molecular packing plays a key role in determining the photophysical properties in the aggregated state, but fine controlling of molecular packing is a great challenge. Here we report a series of new phenoxo-bridged homometallic trinuclear Zn (II) metal complexes (BN3, BNCS, BN(CN)2, BCl, BBr and BI [Complexes 1-6]) having different anions with an aim to vary the photophysical properties of the complexes, where B stands for the ligand, N,N'-bis(3ethoxysalicylidenimino)-1,4-diaminobutane. All the complexes are structurally characterized by Single Crystal X-ray Diffraction (SCXRD) technique. The crystallographic investigation indicates that the complexes contain two types of Zn (II) centers: distorted square pyramidal and octahedral. The crystal structures are stabilized by intermolecular hydrogen bonding and C-H---π interactions leading to distinct supramolecular frameworks. The ground state and excited state electronic properties of these metal complexes have been investigated. In solution state, all the metal complexes are found to be moderately fluorescent and emit at similar wavelengths with no significant effect of anions on fluorescence emission. However, interestingly, solid state emission properties have been found to be significantly dependent on molecular assembly. In this series, dicyanamide [2] and thiocyanate [3] complexes exhibit significantly redshifted high fluorescence response in the solid state which is attributed to their distinct packing in comparison to others. To the best of our knowledge, these complexes are the first example of Schiff base complexes demonstrating intriguing molecular packing – dependent fluorescence emission.


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INTRODUCTION Luminescent compounds are attracting current research interest because of their vast applications in emitting materials for solid state organic light emitting diodes, light harvesting materials for photocatalysis, lasers, fluorescent sensors for organic or inorganic analyses and as biological materials with explosive mimics. [1-4] Zinc Schiff bases complexes have been extensively studied for their interesting photophysical properties. [3, 5] Spectroscopic experiments suggest that coordination to a Zn (II) center would influence the molecular aggregation via non-covalent interactions [6] such as electrostatic interactions, stacking and hydrogen bonding. By modifying these interactions, assembly of new materials with exciting properties may be generated. [7-9] For example, the use of ‘salen’ style Schiff base ligands can lead to a variety of supramolecular assemblies that can determine their spectroscopic and photophysical properties in solution in relation to the nature of molecular aggregation. The d10 configuration of Zn (II) allows architecting flexible coordination environments thus forming metal complexes with tetrahedral to octahedral geometries, which is an essential criterion for designing a wide variety of metalorganic frameworks that may have interesting photophysical properties. Controlling the molecular interactions of aggregates by tuning the steric hindrance of the ligands can also enhance the solid state emission. [10, 11] Thus molecular packing plays a vital role in determining the spectroscopic properties that can be used to modulate their properties accordingly.

Artful molecular design through varying molecular backbone substituent groups and effective πconjugation lengths or impacting the intra/intermolecular interactions to achieve high emission in the solid state is the most common method for generating supramolecular assemblies. [12-16]


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Organic salts have advantages for the design of solid state emitters because both their cation and anion constituents [17-24] could be changed to tune the photophysical properties. Polynuclear transition metal complexes with salen-type ligands have also been extensively studied as luminescent materials [25] besides organic salts. Salen-type ligands have been proven to be very effective in constructing supramolecular architectures with copper, zinc and other d-and f-block metal ions. [26-28] The anions used, either as terminal or as bridging ligands alter the chemistry of the complex to a great extent and affect their supramolecular assembly in the solid state. [29] The cationic fluorogens are usually designed with complex and flexible structures to alter the photophysical properties against the strong dipole-dipole interactions, which open the nonradiative decay channels quenching the solid state luminescence. The changes of the anions would also greatly influence the emission in aggregation of organic salts as well as with metal complexes, because the strength of electrostatic attraction between cationic and anionic species and steric hindrance may deeply affect the molecular packing in aggregation. [30-35] The pseudohalides can coordinate the metal ions through both terminal and bridging modes and affect the electronic structure of the metal ions and dependent properties.

In this paper, synthesis, structural and photophysical studies of a new series having six trinuclear Zn (II) complexes with halidies and pseudohalides are presented. Pseudohalide ions act as the terminal ligand in the presence of a potentially hexadentate Schiff base. Further, the photophysical properties of these Zn (II) complexes in solution and solid state have been thoroughly investigated. The complexes exhibit anion dependent solid state fluorescence response, which is explained on the basis of differences in their molecular packing. Among all the six complexes, [2] and [3] exhibit high and considerable redshifted fluorescence response in


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the solid state. The title complexes are the first example of Schiff base complexes demonstrating intriguing molecular packing - dependent emission which is the main attraction of this paper.

EXPERIMENTAL SECTION Material and Methods. 3-ethoxysalicyaldehyde, 1,4-diaminobutane, Zn(CH3COO)2·2H2O, NaN3, KSCN, NaN(CN)2, KCl, KBr and KI were purchased from Sigma Aldrich and were used without further purification. Infrared spectra were recorded on a Perkin Elmer Spectrum-2 FTIR spectrometer using KBr pellets in the region 400 - 4000 cm-1. TGA were performed using a Perkin-Elmer STA 6000 instrument under a nitrogen atmosphere at a heating rate of 10 °C min-1. Electronic spectra were recorded on a CARY 100 BIO UV–Vis spectrophotometer which has a DRA reflectance accessory attachment for solid sample measurement. Fluorescence spectra were recorded on a Horiba spectrofluorometer (Fluoromax). Crystal data for complexes [1], [2], [4] and [6] were collected on an Oxford Diffraction X Caliber EoS diffractometer using graphite monochromatized Mo-Kα radiation at 293 K and data for complexes [3] and [5] were collected on a Bruker Kappa APEXII using D8 X-ray generator, Cryoflex (Model Tr - 60) Mo-Kα radiation at 200 K. Time-resolved fluorescence intensity decays were collected using a commercial TCSPC setup (Life Spec II, Edinburgh Instruments, U.K.). All samples were excited at 377 nm and the full width at half-maxima of the instrument response function was about 100 ps. For lifetime measurements, peak counts of 3000 were collected with the emission polarizer oriented at magic-angle polarization and decays were collected at 455 nm. The decay curves were analyzed by deconvoluting the observed decays with the instrument response function (IRF) to obtain the intensity decay function, manifested as a sum of two exponentials for the present study.


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Preparation of ligand (B). Scheme 1 depicts the synthetic procedure of the ligand (B). To an ethanolic solution (50 mL) of 3-ethoxysalicyaldehyde (2.0 mmol) was added an ethanolic solution (50 mL) of 1,4-diaminobutane (1.0 mmol) with stirring. The mixture was stirred for 3 h under reflux yielding a yellow precipitate, which was separated by filtration and washed with cold ethanol to obtain pure product.

Scheme 1. Synthetic route for the preparation of the ligand (B).

Preparation of trinuclear complexes [1-6]. All the complexes were synthesized in a similar manner, except for the change in anion. A typical detailed procedure is given for complex [1] (BN3) and for all the remaining complexes the analytical details are only reported. Scheme 2 shows the general reaction scheme followed for synthesis of the complexes.


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Scheme 2. The synthetic route for the preparation of complexes [1-6].

[Zn3(L)2(N3)2(H2O)] ([1] = BN3): 20 ml methanolic solution of ligand (0.38 g, 1.0 mmol), was added to 15 ml methanolic solution of Zn(CH3COO)2·2H2O (0.43 g, 2.0 mmol) while stirring, followed by drop-wise addition of 15 mL methanolic solution of NaN3 (0.13 g, 2.0 mmol). The mixture was allowed to stir for 3 h at room temperature (25 ºC). The light yellow colored precipitate formed was filtered and washed with cold methanol followed by acetonitrile and dried in vacuum. By a slow layer diffusion of hexane into a CH2Cl2 solution of the complex at room temperature (25 ºC), light yellow colored plate-shaped single crystals of [1], suitable for SCXRD structure determination were obtained after 2 days. The yield and characterization data of the ligand and all the complexes are given in the ESI (Table S1 and Figure S2). Complexes [2-6] (BNCS, BN(CN)2, BCl, BBr and BI) were synthesized in a similar manner as for BN3, except that KSCN (0.19 g, 2.0 mmol), NaN(CN)2 (0.17 g, 2.0 mmol), KCl (0.149 g, 2 mmol), KBr (0.23 g, 2.0 mmol) and KI (0.33 g, 2.0 mmol) respectively were used instead of NaN3. Single crystals were also obtained in a similar manner with variations in days needed for the growth of suitable single crystal.


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Crystallographic Data Collection and Refinement. Crystal data for complexes [1, 2, 4 and 6] are collected on an Oxford Diffraction X Caliber EoS diffractometer using graphite monochromatized Mo - Kα radiation at 298 K (λ = 0.71073 Å) whereas for complexes [3 and 5] data are collected at 200 K on a Bruker Kappa APEX II diffractometer having the same source of radiation. The structures are solved by direct methods and refined by full matrix least squares on F2 using SHELX-2016. [36] The non-hydrogen atoms are refined with anisotropic thermal parameters. Disordered carbon atom of the ethoxy group (C19) of complexes [1-6] is modelled by splitting into two halves with positions refined by FVAR refinement and thermal parameters by anisotropic refinement. All the hydrogen atoms are geometrically fixed and allowed to refine using a riding model. The refinement converged to final values R1 = 0.0552; 0.0643; 0.0437; 0.0446; 0.0406; 0.0371 and wR2 = 0.1234; 0.1319; 0.1067; 0.1194; 0.1060; 0.0847 respectively for complexes [1-6]. Important crystal data for all complexes are presented in Table 1. Drawings are made using ORTEP-III and MERCURY. [37]


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Table 1. Crystal data and details of structure refinements for complexes [1-6].

Compound

[1] BN3

[2] BNCS

[3] BN(CN)2

Empirical formula C44H54N10O9Zn3 C46H52N6O8S2Zn3 C48H52N10O8Zn3 CCDC No. 982136 981654 1844913 Formula weight 1077.06 1077.16 1093.13 Space group C2/c C2/c P21/n a (Å) 15.6568(11) 9.3972(10) 9.721(5) b (Å) 15.7534(10) 24.763(4) 24.430(5) c (Å) 19.2668(14) 20.520(2) 20.634(5) 90 90 90 α (º) β (º) 100.607(7) 101.509(11) 99.240(2) 90 90 90 γ (º) 3 V (Å ) 4670.9(6) 4679.1(10) 4760(4) T 293(2) 293(2) 200(2) Z 4 4 4 Crystal system monoclinic monoclinic monoclinic Dcalc(mg m-3) 1.532 1.529 1.527 -1 µ (mm ) 1.596 1.674 1.565 2 GOF on F 1.107 1.035 1.080 2 a R (Fo ) 0.0552 0.0643 0.0437 2 b Rw (Fo ) 0.1234 0.1319 0.1067 a b = ∑| | − | | ∑ | | , = [∑ (| | − | |) / ∑ | | ]/ .

[4] BCl

[5] BBr

[6] BI

C44H54Cl2N4O9Zn3 1501280 1063.97 C2/c 15.5708(12) 15.8607(13) 19.854(3) 90 101.905(9) 90 4797.7(9) 293(2) 4 monoclinic 1.473 1.657 1.051 0.0446 0.1194

C44H58Br2N4O11Zn3 1849608 1168.87 C2/c 15.443(8) 15.835(8) 19.953(10) 90 102.703(2) 90 4759.7(4) 200(2) 4 monoclinic 1.651 3.240 1.071 0.0406 0.1060

C44H54I2N4O9Zn3 1501281 1246.87 C2/c 15.983(5) 15.831(5) 19.742(5) 90 103.457(5) 90 4858(2) 293(2) 4 monoclinic 1.683 2.795 1.027 0.0371 0.0847


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RESULT AND DISCUSSION Synthesis and General Characterization. A series of homometallic trinuclear Zn (II) complexes have been synthesized in-situ in a step-wise manner without the isolation of mononuclear complexes. The reaction of stoichiometric amounts of ligand, namely, N,N-bis(3ethoxysalicylidene)-1,4-diaminobutane

(B)

(1.0

mmol)

in

methanol

solvent,

with

Zn(CH3COO)2·2H2O (2.0 mmol) followed by the addition of anions like N3-, N(CN)2-, NCS-, Cl-, Br- and I- (2.0 mmol), resulted in the formation of desired product, as shown in Scheme 2. The ligand and all the complexes are characterized by FT-IR (spectra are given in ESI S2), NMR (1H &

13

C) and elemental analysis in addition to structure determination by single crystal X-ray

diffraction. To study the crystallinity and pure phase, powder X-ray diffraction pattern (PXRD) have also been performed and patterns are presented in ESI S3. In FT-IR (Figure ESI S2), all six complexes exhibit a sharp peak due to the azomethine (C=N) group of the Schiff base in the range 1624 - 1630 cm-1. [21] The phenolic C–O stretching bands at 1249 cm-1 in the spectra of (B) was shifted to 1217 cm-1 in case of complexes, supporting the deprotonation and coordination of the phenolic oxygen donors to the metal center. These anions show their characteristic vibrational band in FT-IR spectra. The appearance of a strong and sharp vibrational band at 2077 cm-1 is due to the presence of N3- group in BN3, 2083 cm-1 for NCS- in BSCN, 2170 cm-1 (symmetric stretch), 2287 cm-1 (asymmetric stretch) for N(CN)2- in complex BN(CN)2. [38] Sharp bands appearing at nearly 526 and 421 cm-1 correspond to the Zn-N and ZnO stretching frequencies, respectively. The rest of the spectral patterns and band positions of all the complexes are very similar. The thermal stability of all the complexes have been studied by thermogravimetric analysis (TGA) as shown in Figure ESI S4. All the complexes are stable upto


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minimum of 250 °C. However, their thermal decomposition profile found to depends on the anion. Single Crystal X-ray Diffraction of Complexes. Crystal structures for a series of homometallic trinuclear Zinc complexes [1-6] are determined by single crystal X-ray diffraction analysis. Crystallographic data for the complexes are presented in Table 1. Complexes [1, 2 and 4-6] are isostructural, crystallizing in the monoclinic space group C2/c while complex [3], BN(CN)2 crystallizes in monoclinic P21/n space group. In [1, 2, 4-6], there are two Zn (II) centers for one ligand and one anion in the asymmetric unit. The outer Zn (II) is five coordinated with distorted square pyramidal geometry; two phenoxy oxygen and two amino nitrogens occupy the equatorial sites and anions occupy the axial pyramidal position. The central Zn (II) is six coordinated with distorted octahedral geometry; four phenoxo oxygens coordinate through the equatorial positions and two ethoxy oxygens occupy the axial position of the octahedron. The other half of the asymmetric unit is generated by inversion symmetry as shown in Figure. 2. In [3], there are three crystallographically independent Zn (II) centers, two ligands and two anions in the asymmetric unit. The coordination environment of the terminal and central Zn (II) atoms are same as for others as shown in Fig. 1. Addison parameter of 0.66 for [1], 0.66 for [2], 0.72 for [3] and 0.63 for [4], [5] and [6] justify the distorted geometries of the metal ions. Table 2 lists the angles significantly deviating from 90° and 180°, presumably due to restriction induced by chelation of ligand.


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[1] [2] Figure 1. Coordination environment of Zn1 and Zn2 in complex [1] and [2] respectively.


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[1]

[2]

[3]

[4]

[5]

[6]

Figure 2. Structures of Complexes [1-6] obtained from SCXRD studies, showing atoms as 40 % probability ellipsoids. Atom numbering is shown for crystallographically independent unit, the other part is inversion generated, except for [3] where three crystallographically independent Zn (II) centers, two ligands and two anions are present in the asymmetric unit.


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Table 2: Comparative angles (°) and bond lengths (Å) in crystal lattice for the complexes [1-6].

Complex

[1] BN3

[2] BNCS

Axial Sites (Atoms)

Bond length with Zn1 (Å)

N2

2.119(3)

O2

2.121(3)

N1

2.131(4)

O3

2.107(3)

[3]

N1

2.103(5)

BN(CN)2

O3

2.098(3)

[4] BCl

[5] BBr

[6] BI

Cl1

2.2566(11)

N1

2.074(3)

Br1

2.4042(12)

N1

2.072(6)

I1

2.6193(10)

N1

2.057(3)

Angle at Zn1

Equatorial Bond length Sites with Zn1 (Å) (Atoms)

163.79(12)

163.53(15)

165.30(2)

120.63(9)

119.30(15)

117.00(10)

Trimeric unit (Atoms)

Distance With Zn2 (Å)

Angle at central Zn2 O2-Zn2O2-Zn2O3 O3# (°) (°)

79.50

158.78

77.29

99.62

78.51

149.55

79.56

158.19

79.17

159.82

78.79

159.90

N1

2.045(3)

O3

1.994(2)

N3

1.989(4)

O2

2.043(3)

O3

2.005(3)

O4

2.326(3)

N2

2.022(5)

O3

2.042(3)

N3

1.993(4)

O2

2.034(3)

O2

2.024(3)

O1

2.339(3)

N2

2.012(5)

O3

2.027(4)

N3

1.998(6)

O2

2.002(3)

O2

1.999(4)

O1

2.344(4)

N2

2.128(3)

O3

1.990(2)

O2

2.113(2)

O2

2.033(2)

O3

2.037(2)

O4

2.346(3)

N2

2.111(5)

O3

1.992(4)

O2

2.109(4)

O2

2.024(4)

O3

2.031(4)

O4

2.343(5)

N2

2.118(3)

O3

1.997(2)

O2

2.101(3)

O2

2.027(3)

O3

2.025(3)

O4

2.324(3)


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The crystal structure of [1] is stabilized by intermolecular C-H----N hydrogen bonding (C10H10A----N3 = 2.724 Å; C10-H10A----N4 = 2.746 Å) between hydrogen of butyl group (H10A) and nitrogen atoms (N3, N4) of azide moieties of the neighboring trimeric unit (Fig. 3a). Also, the crystal structure is further stabilized by weak C-H----π interactions (C11-H11B----π = 3.708 Å) between the centroid of the ethoxy-substituted phenyl ring and hydrogen (H11B) of butyl group of neighboring molecules (Fig. 3b). The crystal structure of the complex [2] is stabilized by intermolecular C-H----S hydrogen bonding (C12-H12----S1 = 2.986 Å; C20B-H20D----S1 = 2.853 Å) between hydrogen of imine group (H10A), ethoxy (H20D) and sulphur atom (S1) of neighboring thiocyanate moieties and C-H----O hydrogen bonding (C21-H21B----O4 = 2.599Å) between hydrogen (H14) of ethoxy group and ethoxy oxygen atom (O1) of neighboring complex. In addition, the crystal structure is further stabilized by C-H----π interactions (C21-H21B----π = 3.543 Å) between the centroid of ethoxy-substituted phenyl ring and hydrogen (H21B) of an ethoxy group of neighboring molecules (Fig. 3c, 3d, and 3e). A closer look at the packing pattern reveals that complexes [1, 4-6] adopt Type I packing while [2] and [3] adopt Type II packing as shown in Figure 4. In type I packing, the ligands are oriented in a zig – zag fashion, with the metal ions sitting in the grooves and the metal bound anions projecting outwards from the grooves. While in Type II packing ligands pack in layers sandwiched between the peripheries made by anions and Zn (II) centers projecting outwards from anionic chains. Such packing arrangement also reveals that the distance between the phenyl rings of the ligands within the trinuclear unit is dependent on the angle at the central Zn2 as depicted in Figure 4c. In


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Figure 3. View of (a) C-H----N and (b) C-H----π intermolecular interactions between neighboring molecules in complex [1]; (c) Intermolecular C-H----S interactions; (d) Intermolecular C-H----O interactions and (e) C-H----π interactions between neighboring molecules in complex [2].

case of [2] and [3] the phenyl rings are separated by ~3.8 Å with an angle of 151° – 153° at the central Zn2. However for the remaining complexes the ring separation increases (~ 4.7 Å) with a


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simultaneous decrease in the angle (~139° – 142°). These distance and angle dependent separation affect the steady state fluorescence and lifetime of the excited species as discussed in solid state photophysical section.

(a)

(b) Angle (o)

Concentric distance (Å)

[1] BN3

142.6

4.6

[2] BNCN

153.4

3.7

[3] BN(CN)2

151.5

3.8

[4] BCl

142.7

4.7

[5] BBr

141.4

4.7

[6] BI

139.7

4.7

Complex

(c) Figure 4. (a) Packing pattern for complexes [1, 4-6] (termed as Type I), (b) Packing pattern for complexes [2 and 3] (termed as Type II). (c) Variation in phenyl ring distance with angle at central Zn2.

PHOTOPHYSICAL STUDIES Solution and Solid State UV-Visible Spectroscopy. The UV-Vis absorption spectra of ligand (B) and its Zn-complexes [1-6] in solution (DMSO as solvent) and in solid state (diffuse reflectance spectra) are presented in Fig. 5 (a and b respectively). As usual for Schiff base based ligand, two high energy bands are observed, at 278 and 330 nm, and are attributed to the π-π* transitions. The band at 424 nm, common with ligands having azomethine functional group is due to n-π* transition in azomethine group. [39] On coordination with metal ions, the lowest


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energy band is found to be blue shifted as can be seen in Figure 5 (a). Such blue shifting of lowest energy band on complexation can also be found in the literature. [23] All complexes display two characteristic bands, at 280 nm and 375 nm, due to π-π* {(intra-ligand charge transfer / ILCT) and (ligand to metal ion charge transfer / LMCT respectively)}. As can be seen from Figure 5 (a) that there is absolutely no effect of variation of anions in the solution phase UV-visible absorption spectra of the complexes. However, the molar extinction coefficient (ε) for complexes [2] and [3] are much higher (minimum of 55 % higher) than compared to the rest as can be seen in Table 4. This indicates that light-absorbing capacity is much more in these two complexes which might be due to close packing in [2] and [3] as mentioned in preceding section. The LMCT band appears at 375 nm for all complexes [1-6]. However, interestingly changes in reflectance, though small, could be observed in solid state reflectance spectra (Fig. 5b). The reflectance data for complexes [2] and [3] shows different behavior compared to others.

(a)

(b) o

Figure 5. (a) UV-visible absorption spectra of B (grey line) and complexes [1-6] in DMSO (~ 2 X 10--5 M) at 25 C, (b) Solid state diffuse reflectance spectra of B (grey line) and complexes [1-6].


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Steady-State Fluorescence Spectroscopic Studies in Solution and Solid Phase. Since this kind of Schiff bases show chrematistic fluorescence emission, solution and solid state fluorescence spectroscopic studies have been performed for ligand and all the six complexes. While the solution phase emission, recorded in DMSO is shown in Figure 6, the same in the solid state is depicted in Figure 7. All the complexes are excited at 375 nm (i.e., at their absorption peak maxima). Emission features are exactly same in solution; a broad band having emission peak maxima at around 478 nm. The emission band which is due to ligand to metal charge transfer (LMCT) is slightly blue shifted compared to ligand alone. Blue shifting of emission band on complexation of similar system is well reported. [23] Since the emission bands depicted in Figure 6 are taken by exciting at 478 nm of samples having exactly same optical density (0.25), the relative intensities seen in the figure reflects the quantum yield of different complexes. However, quantum yields are also determined taking Coumarin 153 (Φ=0.539 in DMSO at 25 °C) as standard and the values are given in Table 3.


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Figure 6: Fluorescence spectra of complexes [1-6] in DMSO (~1 × 10 excitation wavelength are kept same for all.

Page 20 of 32

−5

M). Optical densities of solutions at the

It can be noted from Table 3 that on complexation, quantum yield enhances which is presumably due to the greater rigidity of the ligand upon complexation. [24-26] Further, it is to be noted that no drastic differences in emission in peak position or intensity can be observed on variation of anion (Figure 6 and Table 3). Table 3. Quantum Yield (Φ) of metal complexes [1-6]. These quantum yields are measured with reference to Coumarin 153 in DMSO solvent at 25°C. Quantum yield (Φ )

Compound

0.060 0.113 0.113 0.112 0.111 0.111 0.110

B [1] [2] [3] [4] [5] [6]


20

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On the contrary, interesting variation in fluorescence responses among complexes on variation of anions can be observed clearly in solid state. The solid state fluorescence spectra of complexes [1-6] are presented in Figure 7. All the spectra are blue shifted compared to the ligand, similar to that in solution state. Though all the complexes show broad band (400 - 600 nm) similar to that in solution, the fluorescence peak maxima are not the same for all complexes when excited at the same wavelength, i.e., at absorption peak maxima (375 nm). More interestingly, all the complexes can be classified into two groups based on their fluorescence peak maxima, one group emits at ~ 450 nm and another ~ 510 nm. It is to be mentioned here that no excitation wavelength dependent fluorescence emission is detected (Figure S5). Figure 7 also shows that complexes [2] and [3] emit green fluorescence (see also optical image given in the Figure S6) while rest emit blue emission, indicating a more than 50 nm shift in emission peak maxima. Therefore, while in solution, fluorescence is mainly ligand centered, in solid, it is packing dependent. To get a better understanding on emitting species, time-resolved fluorescence spectroscopic studies have been undertaken and described below. Time-resolved Fluorescence Spectroscopic studies in Solution and Solid State. The fluorescence lifetimes of all the complexes have been measured in solution and in solid state. The fluorescence decay curves along with their fits using equation; I() = ∑ exp (−/ ) have been shown in Figure 8. This figure and Table 4 shows that in the solution state, all complexes show more or less similar fluorescence lifetimes.


21


Page 22 of 32

Figure 7. Solid Fluorescence spectra of ligand and complexes [1-6] (λexc = 375 nm). Shown beside is the actual appearance of solid samples when seen under UV-irradiation conditions.

As seen in Figure 8, there is no drastic influence of anions on the lifetime in solution state, as all complexes have a lifetime of 4 to 10 ns. However in solid state the two complexes [2] and [3] shows much higher lifetime in the range of 15 to 30 ns, the remaining having a lifetime of 4 to 10 ns only.


22

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Figure 8. Fluorescence decay curves for all the six complexes both in solution as well as in solid state. The lifetime of the complexes have been evaluated by fitting these decay curves with standard biexponential decay equation and found to fit very well.

In solution, the complexes shows two different decay times, one in the range of 1.5 ns having maximum contribution (>60%) while another varies between 10 - 40 ns with less than 40% contributions in amplitude. There is no definite trend observed in solution state fluorescence lifetimes. However, interestingly in solid state, the decay profiles of complexes [2] and [3] are seen to be quite different from the rest (Figure 8). These two complexes shows considerably higher fluorescence lifetimes (8.83 and 3.99 ns having amplitude of 94.7 and 99.4%) compared to other complexes whose lifetimes remain between 1.51 to 3.23 ns having more than 97% contribution in amplitude.


23

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 24 of 32

Table 4. Data related to absorption (steady state and time-resolved) and fluorescence emission of all the complexes studied. All fluorescence decays are fitted with biexponential equation which is found to be of best fitting. τ1 (ns) and τ2 (ns) are the fluorescence lifetimes while A1 (%) and A2 (%) are the corresponding amplitudes. Complex

Solution state Absorption

λmax (nm)

ε

Steady state

Steady state

Fluorescence lifetime

fluorescence

fluorescence

λmax (nm)

λmax (nm)

(L mol1

Solid State

τ 1 (ns)

τ 2 (ns)

A1 (%)

A2 (%)

Fluorescence lifetime

τ 1 (ns)

τ 2 (ns)

A1 (%)

A2(%)

cm-1)

[1] BN3

376

681

472

1.48

11.08

77.8

22.2

458

3.23

14.70

97.2

2.8

[2] BNCS

376

1169

483

1.47

11.01

78.0

22.0

511

8.83

21.17

94.7

5.3

[3] BN(CN)2

376

1264

475

1.70

27.02

69.4

30.6

512

3.99

21.25

99.4

0.6

376

703

478

1.23

12.47

86.3

13.7

450

2.64

8.87

99.9

0.1

376

689

482

1.39

7.52

68.4

31.6

442

1.51

4.67

97.3

2.3

376

719

482

1.64

39.33

61.08

39.0

451

1.51

5.61

97.2

2.8

[4] BCl [5] BBr [6] BI


24

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This data indicates that while in solution, two decaying species are present, but in solid state primarily only one fluorescent species is present. Therefore, the higher lifetime decay response correlates with one type of packing and lower lifetime decay with another type of packing. Initially it may appear that this change is due to change in anion, but as it is seen only in solid state, that possibility is ruled out. After a critical analysis of molecular packing in solid state, we observed very interesting similarities between complexes [2] and [3] from the rest: i) The dimensions of crystallographic axis of the unit cell (a, b, c) are very similar for [2] and [3] and differs considerably from others which on the other hand have very similar values (vide Figure ESI S7), ii) the separation of phenyl rings in crystal packing is also systematically different in [2] and [3] (~3.8 Å, with an angle of ~152°) than the rest complexes (~ 4.7 Å, ~ 140°). In addition, packing of [1, 4-6] is quite different from that of for [2] and [3] with reference to orientation of ligand moiety. Based on these packing diagrams, these six complexes are divided into two groups as Type I and Type II packing (Figure 4). In Type I, referring to complexes [1, 4-6], the Zn (II) metal is dispersed homogeneously about the packing encapsulated by the ligand and anions moieties; whereas Type II referring to packing in complexes [2] and [3] shows a layered packing in which ligand moieties form layers embedded in the anionic peripheries and Zn (II) centers are projecting outward in an alternative fashion. In Figure ESI S8 it is also clearly seen that only complexes [2] and [3] have overlapping anion moieties whereas in all the other four complexes they are just side-by-side. We therefore attribute the difference in fluorescence behavior to the variation in the packing pattern in solid state. To the best of our knowledge, we have not come across any report where Schiff base ligand based complexes shows such packing dependent fluorescence emission in solid state.


25


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CONCLUSION In summary, six new phenoxo-bridged homometallic trinuclear zinc complexes with Schiff base ligand and different anions are synthesized, characterized and their fluorescence properties are studied. All the six complexes are fully investigated by SCXRD. Analysis of SCXRD data of complexes provides details of structural features; the existence of various weak interactions such as C-H----O, C-H----S and C-H----π which are found to contribute significantly to attain particular geometry and stabilize the crystal packing. Photophysical studies of ligand and complexes indicate that while in solution, all complexes show similar absorption and fluorescence behavior, significant differences can be observed in the solid state. Particularly, solid state fluorescence emission is quite interesting as fluorescence emission response is predominantly based on the type of packing arrangement at molecular level. This gives an opportunity to recognize the importance of anion guided molecular packing in solid state emission property. These results indicate that tuning and switching of solid state fluorescence by altering the mode of molecular packing, is a promising concept for designing tunable solid state fluorescent materials. Our results also indicate that it also possible to predict the differences in the type of molecular arrangement of a complex by measuring the fluorescence decay profiles for series of complexes which is much less complicated and less uncertain than obtaining suitable single crystal and getting good diffraction data. ASSOCIATED CONTENT Supporting Information is available on: full characterization data, FTIR spectra, powder X-ray spectra, TGA curves, of ligand and all complexes. In addition, different excitation wavelength


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fluorescence spectra, optical images under UV-light and normal day light, comparison of crystal dimensions, crystal packing figures showing different packing environment for anions are also given in supporting information. This material is available free of charge via the internet at http://pubs.acs.org.

ACKNOWLEDGEMENT We thank Prof. Pratik Sen, and his research scholar Mr. Navin Subba, Department of Chemistry, IIT- Kanpur, for fluorescence lifetime measurement data. The project funding from DST, New Delhi is gratefully acknowledged. Authors are also grateful to Department of Chemistry, Institute of Science, and Department of Chemistry, Mahila Maha Vidyalaya, Banaras Hindu University for providing instrumental and infrastructure facilities. Reference: 1. Ergun, E.; Ergun, Ü.; Đleri, Ö.; Küçükmüzevir, M. An investigation of some Schiff base derivatives as chemosensors for Zn(II): The performance characteristics and potential applications; Spectrochim Acta A Mol. Biomol. Spectrosc. 2018, 203, 273-286. 2. Wang, X.; Yimang, D. S.; Shanshan, X.; Feng, Y.; Yachen, Z.; Lingling, W.; Tian, H. J.; Yu Lin Pu, X. Reaction of Zn(II) with a BINOL-amino-acid Schiff base: An unusual OffOn-Off fluorescence response. Tetrahedron Lett. 2018, 59, 2332-2334. 3. Roy, S.; Bhattacharya, S.; Mohanta, S. Syntheses, Crystal structures and photophysical aspects of discrete and polymeric azido-bridged Zinc(II) and Cadmium(II) complexes: Sensing properties and structural resemblance. ChemistrySelect. 2017, 2, 11091-11099. 4. Nishal, V.; Singh, D.; Kumar, A.; Tanwar, V.; Singh, I.; Srivastava, R.; Kadyan, P. A new

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For Table of Contents Use Only

Molecular Packing Dependent Solid State Fluorescence Response of Supramolecular Metal-Organic Frameworks: Phenoxo-bridged Trinuclear Zn (II) Centered Schiff Base Complexes With Halides and Pseudohalides. Nidhi Dwivedi,a,b Sailaja S. Sunkari,a* Abhineet Verma,b and Satyen Sahab,* (ND and AV have equal contribution)

Six new phenoxo-bridged homometallic trinuclear zinc complexes with Schiff base ligand and different anions are reported. The crystal structures are stabilized by intermolecular hydrogen bonding and C-H----π interactions leading to fascinating supramolecular frameworks. Unique molecular packing dependent solid state fluorescence responses have been observed. These complexes are the first example of Schiff base complexes demonstrating intriguing molecular packing – dependent fluorescence emission.


32

Molecular Packing Dependent Solid State Fluorescence Response of

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