Coordination-Driven Fluorescent J-Aggregates in a ... - ACS Publications

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Coordination-Driven Fluorescent J‑Aggregates in a Perylenetetracarboxylate-Based MOF: Permanent Porosity and Proton Conductivity Nivedita Sikdar,† Dipak Dutta,‡ Ritesh Haldar,§ Turjya Ray,† Arpan Hazra,† Aninda Jiban Bhattacharyya,*,‡ and Tapas Kumar Maji*,†,§ †

Molecular Materials Laboratory, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), ‡Solid State and Structural Chemistry Unit, Indian Institute of Science (IISc), and §New Chemistry Unit, JNCASR, Bangalore 560064, India S Supporting Information *

ABSTRACT: Here we report the synthesis and structural characterizations of a new 3D functional metal−organic framework {[K8(PTC)2(H2O)1.5]·4H2O}n formed by the self-assembly of KI and chromophoric linker perylenetetracarboxylate (PTC). The structure determination shows a 3D pillared-layer framework, where perylene cores are arranged in an unusual end-to-end off-slipped and zigzag arrangement directed by KI−carboxylate bonding. Photophysical studies revealed a broad absorption band with λmax of 531 nm and bathochromically shifted red emission centered at 655 nm. This characteristic emission has been assigned due to Jcoupling of the PTC linkers in the solid state. The framework contains 1D water-filled channel, and the desolvated framework shows permanent porosity, “as realized by type-I CO2 adsorption profile at 195 K. Interestingly, the guest and coordinated water molecules in the framework are connected via H-bonding and based on these characteristics, the framework was further exploited for proton conduction. It shows remarkable conductivity of 1 × 10−3 S cm−1 under ambient conditions (98% RH) with low activation energy.

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INTRODUCTION Organic chromophores with polyaromatic cores have attracted great attention due to their potential applications in lightemitting diode (LED) and photovoltaics (PV) and as bio/ chemo sensors and molecular-conducting materials;1−5 however, such chromophoric molecules with remarkable photostability and high quantum yield preferably form sandwich-type H-aggregates, which exhibit sufficiently quenched fluorescence due to delocalized excitons or excimer formation.6−11 Thus, various optoelectronic materials based on J-aggregation of the chromophores have been designed and developed.12,13 The slipped arrangement of the chromophores, that is, J-type aggregation, leads to a bathochromic shift of the absorption band and also higher exciton mobility, which is important for light-harvesting application.7 To date, fluorescent J-aggregates have been mainly studied on organic supramolecular assembly of several polyaromatic chromophores (e.g., substituted perylene/naphthalenebisimide, cyanine, squaraine dyes) extended via various noncovalent interactions;6−11 however, such studies on inorganic−organic hybrid assemblies like in metal− organic frameworks (MOFs) are underexplored.14 We envisage that the coordination-driven slipped arrangement of chromophoric linker could be another approach for designing fluorescent MOF based on J-aggregation. Such metal-directed © XXXX American Chemical Society

J-aggregation of the chromophores is observed in natural lightharvesting pigment chlorophyll.8−11 In addition, coordinationdriven spatial organization of chromophores appears to be advantageous because such hybrid assemblies would exhibit interesting optoelectronic properties coupled to enhanced thermal and mechanical properties.15 Furthermore, the recent upsurge in design and synthesis of proton conductive MOFs stemmed from their potential use in electrochemical devices such as fuel cells, sensors, and so on.16−23 In particular, MOF materials require proton carriers like H3O+ and H+ from different acids, counterions like NH4+, imidazolium cations, sulfonate and phosphonate, as well as proper H-bonded proton-conducting pathway.24−27 Hydrogenbonded (H-bonded) water-channel-assisted proton conduction under ambient condition has been described recently where conductivity can be modulated dynamically by changing the humidity, which is paramount for sensing applications;28,29 however, the stability of the framework under high humid condition and lack of definite H-bonding pathway are the limiting factors that need to be addressed. In this present work Received: April 29, 2016 Revised: June 4, 2016

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DOI: 10.1021/acs.jpcc.6b04347 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C we aim to address these issues. Here for the first time we have shown coordination-driven J-aggregation in a perylenetetracarboxylate (PTC)-based MOF {[K8(PTC)2(H2O)1.5]·4H2O}n [PTC = 3,4,9,10-perylenetetracarboxylate] (1)]. The use of such tetratopic linker30−32 led to a 3D pillared layer framework where PTC linkers are arranged in an end-to-end off-slipped stacking that resulted in J-aggregation. The red-shifted brightred emission of 1 compared with PTC linker has been correlated to the J-aggregation. Compound 1 shows permanent porosity, as realized by CO2 adsorption profile at 195 K. This 3D porous structure contains a large number of guest and coordinated water molecules that are connected through Hbonding throughout the structure. Hence, 1 was further exploited for proton conduction, and it exhibits remarkable conductivity of 1 × 10−3 S cm−1 under ambient condition (98% relative humidity (RH)) with low activation energy.

Table 1. Crystal Data and Structure Refinement Parameters of {[K8(PTC)2(H2O)1.5]·4H2O}n (1)

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EXPERIMENTAL SECTION Materials and Methods. All of the reagents employed here were of analytical grade and used as provided without further purification. Potassium chloride (KCl) and potassium hydroxide (KOH) were obtained from Spectrochem (PVT). 3,4,9,10Perylenetetracaboxylicdianhydride was obtained from TCI chemicals. 3,4,9,10-Perylenetetracarboxylic acid is obtained by hydrolyzing the corresponding anhydride under basic conditions.33 Elemental analysis was carried out using a Thermo Fischer Flash 2000 Elemental Analyzer. IR spectra were recorded on a Bruker IFS 66v/S spectrophotometer using KBr pellets in the region 4000−400 cm−1. Powder X-ray diffraction (PXRD) patterns were recorded by using Cu−Kα radiation (Bruker D8 Discover; 40 kV, 30 mA). The results from the PXRD patterns are in close agreement with that calculated from the single-crystal structure determination. Thermogravimetric analysis (TGA) was carried out (Mettler Toledo) in a nitrogen atmosphere (flow rate = 50 mL min−1) in the temperature range 30−800 °C (heating rate = 3 °C min−1). X-ray Crystallography. X-ray single-crystal structural data of 1 were collected on a Bruker D8 VENTURE equipped with a microfocus X-ray source with graphite monochromated Mo− Kα radiation (λ = 0.71073 Å) operating at 50 kV and 30 mA. A crystal of suitable size was chosen and mounted on a thin glass fiber with commercially available superglue. The data were collected at 210 K. The program SAINT33 was used for integration of diffraction profiles, and absorption correction was made with SADABS program.34 The crystal structure was solved by SIR 9235 and refined by full matrix least-squares method using SHELXL-97.36 All hydrogen atoms were fixed by HFIX and placed in ideal positions. Potential solvent-accessible area or void space was calculated using the PLATON multipurpose crystallographic software. All crystallographic and structure refinement data of 1 are summarized in Table 1. All calculations were carried out using SHELXL 97, PLATON,37 and WinGX system,38 ver 1.70.01.39 The potassium centers are highly disordered, and it has been resolved after assigning the disordered atoms. K4 and K9 are in special positions, and hence the total number of total potassium centers is nine. The guest water molecules are disordered and could not be properly located. No satisfactory disorder model could be achieved, and hence the presence of solvent molecules is calculated from TGA, FT-IR, and elemental analysis. The details of bond distances and bond angles are summarized in Tables S1 and S2. CCDC-1469106 contains the supplementary

a

parameters

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empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T (K) μ (mm−1) Dcalcd (g/cm3) F (000) reflections [I > 2σ(I)] total reflections unique reflections λ(Mo−Kα) Rint GOF on F2 R1[I > 2σ(I)]a Rw[all data]b

C48H17.5K8O18.25 1198 monoclinic P21/c 14.700(5) 14.543(5) 22.914(6) 90 100.884(5) 90 4811(3) 4 210 0.795 1.655 2414 7391 83984 8513 0.71073 0.038 1.02 0.0726 0.2115

R = Σ∥F0| − |Fc∥/Σ|F0|. bRw = [Σ{w(F02 − Fc2)2}/Σ{w(F02)2}]1/2.

crystallographic data for this paper. This data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/deposit/. Adsorption Study. The adsorption isotherms of N2 (77 K) and CO2 (195 K) for compound 1 were measured by using QUANTACHROME QUADRASORB SI analyzer. The adsorbent sample (∼100−150 mg) was placed in the glass sample cell maintained at 180 °C under a 0.1 Pa vacuum for ∼12 h prior to measurement of the isotherm. Helium gas at a certain pressure was introduced in the sample cell and allowed to diffuse by opening the valve. The change in pressure allowed an accurate determination of the volume of the total gas phase. The amount of gas adsorbed was calculated readily from pressure difference (Pcal − Pe), where Pcal is the calculated pressure with no guest adsorption and Pe is the observed equilibrium pressure. The water vapor adsorption isotherm is measured at 298 K by using BELSORP-aqua-3 analyzer. The activated sample of 1 (1′) was prepared as previously mentioned. The solvent vapor is degassed fully by repeated evacuation. Dead volume was measured with helium gas. The adsorbate was placed into the sample tube, the change of the pressure was monitored, and the degree of adsorption was determined by the decrease in pressure at the equilibrium state. All operations were computercontrolled and automatic. Synthesis of Compound {[K8(PTC)2(H2O)1.5]·4H2O}n (1). In a typical synthetic procedure, KCl (0.058 g, 0.8 mmol) and KOH (1.12 g, 2 mmol) are added to 6 mL of water and sonicated for getting homogeneous solution. In addition to that, Zn(NO3)2 is added to the solution as a seeding agent. Afterward, PTC (0.042 g, 0.1 mmol) is added and sonicated until it dissolves completely. 6 mL of N,N′-dimethylformamide (DMF) is added dropwise to the mixture and shaken B


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Figure 1. (a) Coordination environment of potassium centers in 1 (purple: potassium, red: carboxylate oxygens, and green: coordinated water molecules). View of dense 2D K−O layer (b) along bc and (c) along ac plane. (d) 3D pillared layer framework along the b direction.

among which O3w and O4w are bridging water molecules (Figure 1a). All metal centers are highly interconnected to each other through several bridging oxygen atoms from the carboxylate groups of PTC and water molecules and thus form a dense KI−O−KI 2D layer along the bc plane (Figure 1b,c). This 2D layer is further connected via PTC linkers along a direction and forms a 3D pillared layer porous framework (Figure 1d). Closer inspection of the structure shows that in the 2D layer carboxylate oxygens and coordinated water molecules are H-bonded within a range of 2.5 to 2.9 Å (D−H··· A distance) along the c direction. We were also able to find one guest water molecule (with an occupancy of 0.75) in the channel that is H-bonded to coordinated O2w and O1w along the a direction. The pore is filled with additional 3.25 guest water molecules (total four water molecules), as confirmed from TGA and CHN analysis. Thermal Stability and Porous Property. Thermogravimetric analysis (TGA) of 1 shows an initial weight loss in the temperature range of 30−125 °C (exptl, 6.5 wt %; calcd, 6.4 wt %) corresponding to loosely bound four guest water molecules (Figure S1a). The next step exhibits release of 1.5 coordinated water molecules from the framework (exptl, 2.5 wt %; calcd, 2.3 wt %) in the temperature range of 125−250 °C. The desolvated 1′ is thermally stable up to 300 °C. The good correspondence of the simulated PXRD pattern with 1 indicates high purity of the sample (Figure S1b). Upon removal of guest and coordinated water molecules (1′), the powder X-ray diffraction pattern remains the same, demonstrating that structural integrity and rigidity are maintained even after desolvation, but the difference in relative intensity of Bragg’s diffractions indicates the effect of heat treatment on overall crystallinity.

periodically. On complete addition of DMF, the mixture was kept at 90 °C in oil bath. Dark-red crystals are obtained after 5 days, which were washed with water for 10−12 times to remove extra unreacted linker and metal ions; then, the crystals were dried overnight at room temperature. Compound 1 has been characterized by elemental analysis, TGA, PXRD, Fourier transform infrared (FTIR) spectroscopy, and single-crystal Xray diffraction study. Yield: 60% with respect to metal. Elemental analysis (%) for 1 calculated for C48H27K8O21.5: C: 45.67; H: 2.22; Found C: 46.03; H: 2.17. FT-IR (4000−400 cm−1): 3429 (s), 1594 (s), 1562 (s), 1412 (m) cm−1.

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RESULTS AND DISCUSSION Crystal Structure Description of 1. Compound 1 crystallizes in the monoclinic crystal system of P21/c space group (Table 1). There are nine potassium centers with a total positive charge of eight, and these are coordinated to carboxylate oxygens of two PTC linkers (total negative charge of eight) and water molecules. Interestingly, all potassium centers have unique coordination environment, where the coordination number varies from 4 to 7. In the structure, K1, K3, and K6 are in distorted pentagonal bipyramidal geometry with a coordination number of seven. K2, K4, K7, and K9 have coordination number of six, where K4 and K9 are in special position with perfect octahedral arrangement. K5 and K8 have coordination number of five and four, respectively. K2, K5, K7, and K8 have positional disorders, and those are assigned as K2A, K5A, K7A, K8A, and K8B, respectively. KI−O bond distances are in the range of 2.433(7)−3.03(3) Å (Table S1).40,41 Here K2, K7, K5, and K8 are coordinated to water molecules in addition to carboxylate oxygens of PTC linkers, C


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Figure 2. (a) CO2 adsorption−desorption isotherm at 195 K (green) and N2 adsorption−desorption isotherm (black) at 77 K of desolvated 1′. (b) Water vapor adsorption isotherm of 1′ at 298 K (P0 = 3.169 kPa).

Figure 3. (a) Solid-state packing of PTC linkers along the bc plane showing end-to-end off-slipped/zigzag π···π interactions; carboxylate groups are removed for clarity. (b) Absorbance spectra for PTC linker (black, 10−6 M solution) and 1 (red) in dispersed state in MeOH; inset shows optical image of the one single crystal and bulk compound in day light. (c) Solution-state emission spectrum for PTC linker (black, 10−6 M solution) and solid-state emission spectrum for 1 (red) (λex = 485 nm); inset shows the red emission of the compound on UV light irradiation.

Photophysical Study. A closer look at the structure of 1 reveals that the PTC linkers are stacked in an alternate offslipped and zigzag column with an intermolecular distance of 3.4 to 4.0 Å along the bc plane (Figure 3a). The slipping angle within the off-slipped column is 26°, and the end-to-end π···π distance between two successive PTC chromophores is 3.4 Å. This off-slipped stacking of PTC linkers resembles J-type aggregation in the solid state. Moreover, the PTC linkers in the zigzag column show strong edge-to-face C−H···π interaction with the next lying off-slipped column in the range of 3.1 Å. Such different types of packing arrangement of PTC chromophores motivated us to study the photophysical properties of 1. Compound 1 shows a broad absorption

To check the permanent porosity of the framework 1′, we have measured N2 (kinetic diameter 3.64 Å) adsorption isotherm at 77 K up to 1 bar. It reveals a type-II profile with a total uptake of 5 mL g−1, emphasizing only surface adsorption (Figure 2a);42,43 however, CO2 (kinetic diameter 3.3 Å) adsorption measurement at 195 K up to 1 bar exhibits typical type-I profile, suggesting microporous nature of the framework (Figure 2a). The total CO2 uptake is 43 mL g−1 corresponding to Langmuir surface area of ∼156 m2 g−1. CO2 uptake can be attributed to the high quadrupole moment of CO2 molecules, which induces a strong interaction with the polar pore surface decorated with aromatic π-electron cloud, unsaturated metal sites along with polar carboxylate oxygens.42,43 D


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Figure 4. (a,b) Nyquist plots of 1 in the temperature range of (21−35) °C under 75 and 98% RH, respectively. (c) Arrhenius plots of 1 under different RH conditions (filled and empty symbols represent heating and cooling cycles, respectively). (d) View of H-bonding pathway along the ac plane.

Proton Conductivity. For efficient proton conduction, an extended H-bonded network is imperative. Single-crystal X-ray data reveal the presence of hydrogen bonding among coordinated and guest water molecules. This can be also supported by the FT-IR spectrum of 1, which shows a broad and intense band in the region of 3600−3000 cm−1 centered at 3440 cm−1, again suggesting the presence of H-bonded guest and coordinated water molecules (Figure S5). This motivated us to examine its intrinsic proton conduction behavior under ambient conditions.19−29 Proton conductivity of 1 is measured using the ac impedance spectroscopy by scanning the sample in the frequency range of 10−2 to 106 Hz (signal amplitude: 0.01 V).32 A pellet (of 1) of 10 mm diameter (thickness = 1.2 mm) is sandwiched between two stainless-steel electrodes in a homemade cell under ambient condition (25 °C, 40% RH). The proton conductivity value of the as-synthesized sample under these conditions is 1.16 × 10−6 S cm−1 (Figure S6). Following evacuation of the sample (pressure ≈ 10−1 Pa) for 5 h in a chamber and subsequently filling the same with Ar gas, the conductivity value of 1 decreases to 7.2 × 10−11 S cm−1 at 25 °C (Table S3, Figure S7); however, on re-exposure of the pellet to open atmosphere, the sample regains the conductivity value of 1.08 × 10−6 S cm−1 (Figure S8). Nearly five-order increment in conductivity under ambient condition compared with the measurement carried out under inert condition clearly indicates the role of guest and coordinated water molecules present in the framework. In 1, as confirmed from X-ray crystallographic studies and FT-IR, the guest and coordinated water molecules are in continuous intermolecular H-bonded throughout the framework, and this facilitated the proton conduction in 1; however, upon activation the pellet under an inert atmosphere removes the loosely bound water molecules, and the structure does not contain extended H-bonding, which

spectrum (375−575 nm) with a maximum centered at 531 nm along with one more sideband at 492 nm (Figure 3b). To compare, we have investigated the UV−Vis absorption spectra for dilute aqueous solution of PTC (in 10−6 M KOH solution) (Figure 3b, Figure S2). The monomer PTC chromophore has three well-defined characteristic vibronic progressions with maxima centered at 412, 435, and 465 nm.12,13 This red-shifted band in 1 compared with the monomer PTC linker can be attributed to the solid-state J-aggregation of PTC linkers, as observed in the crystal structure Figure 3a. Solid-state reflectance spectrum was fitted into the Kubelka−Munk equation, and the band gap is calculated to be 2.53 eV, suggesting 1 to be semiconducting in nature (Figure S3). Significant broadening of the UV band can be attributed to the different types of intermolecular packing among PTC linkers along with off-slipped arrangement.40 The solid-state Jaggregation of PTC chromophores is also reflected in the emission spectrum. The monomer PTC shows green emission at λmax of 480 nm in water with a very little Stokes shift of 15 nm (Figure 3c, Figure S2),44−46 but in comparison with the monomer, 1 shows significantly red-shifted characteristic red emission centered at 655 nm (λex = 485 nm) (Figure 3c).40 The excitation spectrum recorded at λmax of 675 nm also shows a band at 490 nm, indicating J-aggregation-based fluorescence in 1 (Figure S4a). The noteworthy decrease in the fluorescence lifetime from 4.6 ns (PTC monomer in 10−6 M KOH solution) to 0.4 ns (compound 1) is also supporting J-aggregation-based (Figure S4b,c) emission in 1.7,8 The dark-red color under visible light and red emission under UV light of 1 are attributed to solid-state packing of PTC chromophores and can be compared with the different organic supramolecular Jaggregation like red pigment perylenediimides.12,13 E


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demonstrated through conductivity measurements on keeping the sample under vacuum (∼10−1 Pa), followed by measurements done in an inert atmosphere. Evacuation leads to partial dehydration of 1, resulting in drastic decrease in conductivity by about five orders of magnitude. This rules out transport of KI ions alongside protons. Application of such high vacuum does not change the structure of 1 (positions of KI ions remain the same) (Figure S16); however, it may disturb the pre-existing water channels.

leads to the poor conductivity. This observation encouraged us to measure the proton conductivity of 1 under humid condition by exposing the sample pellet under different RHs (up to 98%). The sample pellet was humidified to a particular RH for sufficient time (∼24 h) prior to the impedance measurement. No noticeable change in the PXRD patterns (Figure S9) of 1 before and after exposing the same pellet to different RH conditions indicates high hydrolytic stability of the sample. It is interesting to observe that with an increase in RH proton conductivity of 1 increases drastically and the observed values are 1.2 × 10−6 (52% RH), 7.9 × 10−6 (75% RH), and 1.0 × 10−3 S cm−1 (98% RH) (Table S3, Figures S10 and S11, Figure 4a,b). This room-temperature (25 °C) proton conductivity value of 1.0 × 10−3 S cm−1 under 98% RH (solely water molecules) is comparable to some of the highest reported proton-conducting MOF systems, like different postsynthetically modified (e.g., acid- or guest-impregnated MOFs) or different linker-mediated (e.g., sulfonated, phosphonated, imidazolium, triazole-based linker; −SH- and −NH-mediated backbone) MOFs (Table S4).32 To obtain a clear perception of the mechanism of proton conduction in 1, we also perform temperature-dependent conductivity measurements (both heating and cooling process) in the temperature range of 21−41 °C under ambient condition (40%, 25 °C) (Figure S12); however, measurements under humid conditions (52, 75, and 98%) are restricted up to a temperature of 35 °C only (Figure 4a−c, Figure S10) because humidity under the present set of experimental conditions would vary with further increase in temperature. We have calculated the activation energy values (Ea) for the proton conductivity under various RH conditions from the least-squares fitting to Arrhenius plots (Tables S3, Figure S13, and S14 and Figure 4c). Measurements at different humidity levels show that on increasing the temperature (at a particular humidity) the conductivity increases linearly, and the trend is reversible while decreasing the temperature at the same rate as the heating (Figure 4c). The activation energies calculated for proton conduction in 1 are 0.47, 0.45, 0.39, and 0.23 eV at 40 (ambient condition), 52, 75, and 98% RH, respectively. The activation energies are well below the energy threshold (0.5 eV) to consider the proton conduction in 1 by the well-known Grotthuss (hopping) mechanism (at least within the range of temperature under investigation).28 The decrease in Ea with rise in RH indicates that at higher RH proton conduction is more facile. It is evident that humidification to different extents eventually establishes the role of water as a proton carrier. We believe that the 2D layers are interconnected in an optimum way through the additional guest water molecules, and the overall framework turns out to be a highly dense interconnected H-bonded network, which leads to the drastic rise in conductivity value (Figure 4d). The water adsorption data for 1′ indicates that 5.8 water molecules per formula unit can be incorporated inside the framework at 1 atm, and surprisingly this order of water adsorption is closely matching with the total number of water molecules (per formula unit) present in the framework (Figure 2b). To account for this fact, a few crystals of 1 are humidified at 98% RH for 3 days and their TGA (Figure S15) is recorded. As expected, up to 60 °C, there is the loss of six weakly bound water molecules (9.73 wt %), indicating coherent contribution of these extra guest water molecules to the proton conduction by forming an interconnected extended H-bonded network throughout the framework. The KI ion transport in 1 along with proton transport is ruled out. It has been previously

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CONCLUSIONS We have synthesized a 3D pillared layer porous framework of KI using a chromophoric PTC linker. Coordination-driven unusual end-to-end off-slipped arrangement of PTC linkers leads to a J-aggregation that resulted in bathochromically shifted red emission in 1. The as-synthesized framework shows high proton conductivity under ambient condition (1.16 × 10−6 S cm−1), which is increased to 1.0 × 10−3 S cm−1 at 98% RH. Finally, our results pave the way for designing novel fluorescent MOF materials based on J- aggregation of the chromophoric linkers, which is underexplored. Additionally, by adopting novel synthetic techniques, choice of suitable organic linker, and their binding mode, multifunctional MOFs can also be synthesized.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04347. Tables of bond lengths and angles for 1, TGA and PXRD plots of 1 at different stages, UV-PL spectra, Nyquist and Arrhenius plots, table for Ea and comparison table of proton conduction values of other componds (PDF) Crystallographic data (CIF) checkCIF/PLATON report (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*T.K.M.: E-mail: [email protected]. *A.J.B.: E-mail: [email protected]. Notes

The authors declare no competing financial interest. CCDC-1469106 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http:// www.ccdc.cam.ac.uk/deposit/.

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ACKNOWLEDGMENTS N.S. acknowledges CSIR (Govt. of India) for fellowship. T.K.M. acknowledges CSIR (Govt. of India) for financial support (Project No. MR-2015/001019). T.K.M. also acknowledges Sheikh Saqr laboratory. N.S. acknowledges Ankit Jain for discussion and Suman Kuila for lifetime measurements. D.D. acknowledges ‘Nano Mission’ for Research Associateship. A.J.B. and D.D. acknowledge ‘Nano Mission’ under the Department of Science and Technology (DST), India (Project No. SR/ NM/NS-88/2010).

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REFERENCES

(1) Katz, H. E.; Bao, Z.; Gilat, S. L. Synthetic Chemistry for Ultrapure, Processable, and High-Mobility Organic Transistor Semiconductors. Acc. Chem. Res. 2001, 34, 359−369.

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