Charge Transfer vs. Arene-Perfluoroarene Interactions in Modulation

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C: Energy Conversion and Storage; Energy and Charge Transport

Charge Transfer vs. Arene-Perfluoroarene Interactions in Modulation of Optical and Conductivity Properties in Cocrystals of 2,7-di-tert-Butylpyrene Arkalekha Mandal, Anwesha Choudhury, Parameswar Krishnan Iyer, and Prasenjit Mal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03827 • Publication Date (Web): 09 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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The Journal of Physical Chemistry

Charge Transfer vs. Arene-Perfluoroarene Interactions in Modulation of Optical and Conductivity Properties in Cocrystals of 2,7-di-tert-Butylpyrene Arkalekha Mandal,a* Anwesha Choudhury,b Parameswar Krishnan Iyer,b,c* Prasenjit Mala*

aSchool

of Chemical Sciences, National Institute of Science Education and Research (NISER),

HBNI, Bhubaneswar, PO Bhimpur-Padanpur, Via Jatni, District Khurda, Odisha 752050, India bCentre of Nanotechnology, Indian Institute of Technology Guwahati, Guwahati, Assam, India,

781039. cDepartment

of Chemistry, Indian Institute of Technology Guwahati, Guwahati, Assam, India,

781039. Corresponding Authors Email: [email protected] (PM); [email protected] (PKI); [email protected] (AM)

ABSTRACT Herein we have demonstrated switching of n-type semiconductor to insulator nature by utilizing π∙∙∙π stacking interactions viz. charge transfer (CT) and arene-perfluoarene (A-AP) in cocrystals of π-donor 2,7-di-tert-butylpyrene. The binary (1:1) co-crystals were obtained with π-acceptors fluoranil and octafluoronaphthalene (OFN) via CT and A-AP interactions, respectively. Hirshfeld surface, natural bond orbital (NBO) and energy decomposition analyses (EDA) confirmed that the CT interaction is stronger than the A-AP interaction. The difference between two π∙∙∙π interactions was further ascertained by spectroscopic studies including UVVis and fluorescence in solid state, EPR and IR. The CT cocrystal showed n-type semiconductor behavior and prominent fluorescence quenching while the A-AP interaction led to small fluorescence quenching with insulator property. 1 ACS Paragon Plus Environment

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INTRODUCTION In recent years significant interests has been drifted towards controlling chemical behavior of a system by the use of non-covalent or weak interactions.1 Similarly physical properties including luminescence, electrical conductivity, thermal behavior in solid state is governed by weak non-covalent interactions like hydrogen bonding2,3 and halogen bond,4,5 π∙∙∙π stacking,6,7 anion∙∙π,8,9 cation∙∙π interaction, etc.10,11 Among them, the effect of hydrogen bonding in mediating physical properties is most prominent in chemical and biological systems,12-14 followed by π∙∙∙π stacking interaction.15-18 The strong face to face π∙∙∙π stacking19,20 is often a consequence of forming electron donor acceptor (EDA) complexes leading to charge transfer21 or assembling of two aromatics bearing complementary quadrupole moment.22 Charge transfer (CT) interactions between π-electron rich donor (D) and π-electron deficient acceptor (A) moieties have been reportedly applied to prepare organic charge transfer cocrystals with tunable molecular functionality owing to their partial ionic nature (Dρ+Aρ-, 0 ˂ρ˂ 1).23,24 Since the last decade, CT cocrystals are gaining importance as they have found application in organic photovoltaic (OPVs)25 as well as photoconductivity devices,26 organic field effect transistors (OFETs),27-31 organic metal32 and ferroelectrics.33,34 A number of organic charge transfer cocrystals have been identified with novel optical properties including aggregation induced emission enhancement (AIEE)35 and optical non-linearity.36 Nevertheless, the scope of charge transfer cocrystals is limited due to inadequate number of available donor / acceptor pairs. A closely similar face to face π∙∙∙π stacking interaction is observed in a number of cocrystals which consists of perfluorinated aromatic systems as acceptors.37,38 This interaction formally called as arene-perfluoroarene (A-AP) interaction, in general, forms 1:1 π-donor: π-acceptor complexes with nearly parallel alternative π-stacks because of complementary charge on the carbon atoms of the stacked rings.39 This interaction owing to its strong π-stacked directional

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nature has often been applied in the syntheses of luminescent cocrystals40,41 as well as dictating a robust supramolecular assembly in ferromagnetic cocrystals42 and hydrogels.43

a) Donor Di-t-BuPy Fluorescent F

O F

F

F

F

F

F F

b) Acceptor F O

Fluoranil

F F

F OFN

c) 1:1 Donor-Acceptor Cocrystals 1 (Di-t-BuPy: Fluoranil) CT Interaction Good n-Type Semiconductor More fluorescence quenching

2 (Di-t-BuPy: OFN) A-AP Interaction Poor n-Type Semiconductor Less fluorescence quenching

Scheme 1 Coformers and cocrystals used for the study. Two closely related face to face π∙∙∙π stacking interactions often lead to discernible optical and electronic properties which give the impetus to recognize their origin and nature. Herein, we have explored supramolecular, optical and electronic features of two cocrystals comprising of 2,7-di-tert-butylpyrene as the π-donor with fluoranil and octafluoronaphthalene (OFN) as πacceptors. The donor di-tert-butylpyrene endorses strong face to face π∙∙∙π stacking as a result of possessing large tert-butyl substituent.44 The emission profile of unsubstituted pyrene is broad and unstructured due to excimer formation45,46 while quantum yield in solid state is 31%. Introduction of bulky substituent to reduce the possibility of excimer formation and thereby increase quantum yield was cited earlier.47 Indeed 2,7-di-tert-butylpyrene exhibits strong fluorescence in solid state with quantum yield of 38% and characterized with a structured emission band. Charge transfer cocrystals of fluoranil are renowned for showing semiconductivity48,49 or application in host guest cocrystallization.50 On the other hand, octafluoronaphthalene has been used as a coformer in several luminescent cocrystals assembled 3 ACS Paragon Plus Environment


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via A-AP interaction51-53 as well as cocrystal formation induced photocyclization reaction. The cooperative effect of charge transfer and arene-perfluoroarene interactions in cocrystals to harvest white light emitting system has been reported recently.54 In contrast, comprehensive quantitative analysis on difference in nature of charge transfer and arene-perfluoroarene interactions is still unavailable. Therefore, quantitative analyses on crystal packing and optoelectronic properties of two cocrystals would pave understanding of supramolecular facets of charge transfer and arene-perfluoroarene interactions and their potential application for designing materials with tunable supramolecular functionality. RESULTS AND DISCUSSION The cocrystals were synthesized by liquid assisted mechanical grinding method.55 Immediate change of color from yellow to brown upon addition of single drop of methanol was observed during formation of cocrystal 1 but no change of color was witnessed for cocrystal 2 (Fig. 1a). UV-Vis spectrum of mixed solution of di-t-BuPy and fluoranil (1:1) in methanol did not show any new peak or peak shift indicative of charge transfer (Fig. S4, see ESI†). In contrast, UVVis spectrum of cocrystal 1 in solid state exhibited a broad charge transfer band around 620 nm while no distinguishable charge transfer band was observed in cocrystal 2 (Fig. 1c). Cocrystal 1 exhibits spurious EPR signal (Fig. 1d) with g ≈ 2.002 corresponding to an unpaired electron demonstrating the occurrence charge transfer28,30 whereas cocrystal 2 is completely EPR silent. Molecular electrostatic potential (MEP) diagram exhibits the presence of strong πhole on both fluoranil and OFN core, thereby predicting strong π∙∙∙π stacking with electron rich di-t-BuPy (Fig. S5, see ESI†). An inspection on the energy levels of frontier molecular orbitals of the coformers reveals small difference in energy (1 eV) between HOMO of di-t-BuPy and LUMO of fluoranil (Fig. 1b). The energy difference between HOMO of di-t-BuPy and LUMO of OFN is quite high being 3.3 eV. These values indicate that charge transfer from donor HOMO to acceptor LUMO is viable in cocrystal 1 but not in 2 due to the considerable energy 4 ACS Paragon Plus Environment

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difference between donor HOMO and acceptor LUMO (Fig. 1b).29,30 To verify the calculated HOMO-LUMO energy difference in coformers, cyclic voltammetry in acetonitrile solution were performed for the coformers using Ag/AgCl reference. The energy of di-t-BuPy HOMO is -5.09 eV while LUMO energies of fluoranil and OFN are respectively -4.65 eV and -3.27 eV (Fig. S1-S3, see ESI†). The experimental HOMO/ LUMO energies do not match exactly with theoretically calculated values but follows the similar trend. The molecular quadrupole moments of di-t-BuPy, fluoranil and OFN are respectively -5.5×10-3, +1.64×10-3 and +7.66×10-3 D-Å calculated at B3LYP/6-311G(d,p) level of theory. Thus, attractive interaction between opposite molecular quadrupoles on di-t-BuPy and OFN moieties will aid in strong π∙∙∙π stacking.

Fig. 1 (a) Color change during formation of cocrystal 1 but not for 2, (b) HOMO/ LUMO distribution of di-t-BuPy, fluoranil and OFN calculated at B3LYP/6-311G(d,p) level of theory, (c) UV-Vis spectra in solid state show distinctive charge transfer band for cocrystal 1 but not for 2, (d) EPR spectrum of cocrystal 1 shows very weak signal due to charge transfer.



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Detailed structural analyses of cocrystal 1 and 2 were carried out to explore the difference in nature of the charge transfer (CT) and arene-perfluoroarene (A-AP) interactions. The molecular planes of di-t-BuPy and fluoranil are aligned perfectly parallel in cocrystal 1 while molecular planes of OFN and di-t-BuPy are aligned at an angle 2.32º. The centroid∙∙∙centroid (Cg∙∙∙Cg) distance is 3.271(2) Å in cocrystal 1 and 3.471(2) Å in cocrystal 2 (Fig. 2a). The values indicate charge transfer interaction is marginally stronger than arene-perfluoroarene interaction. The trend is also reflected by binding energies of π-stacked D−A dimers in cocrystal 1 (-13.4 kcal/mol) and 2 (-11.9 kcal/mol) calculated at M06-2X/6-311G(d,p) level of theory in gas phase with basis set superposition (BSSE) correction. It is relevant to mention that centroid∙∙∙centroid (Cg∙∙∙Cg) distance between donor and acceptor moieties in pyrene:fluoranil and pyrene:OFN cocrystal are 3.530 Å and 3.512 Å respectively,56 hence stronger π∙∙∙π interaction in cocrystal 1 and 2 is a consequence of introducing bulky tert-butyl substituent.44 For comprehensive understanding of two π∙∙∙π stacking interaction, Hirshfeld surfaces of two cocrystals were analyzed. The curvedness plots of cocrystal 1 exhibit broad green regions indicating flat surface separated by small blue ridges of positive curvature (Fig. 2c). The upper and lower views of cocrystal 1 showcase similar flat green regions of self-complementarities diagnostic of π∙∙∙π stacking interaction.57,58 Likewise, the upper and lower views of shape index surfaces of cocrystal 1 are characterized with red and blue triangular regions which can be super-imposed on each other through translation. These triangular patterns are indicative of C∙∙∙C or π∙∙∙π interaction (Fig. S12, see ESI†).57 It is pertinent to mention that the donor and acceptor moieties of cocrystal 1 individually show curvedness plots with large flat green regions demonstrating π∙∙∙π stacking interaction (Fig. S13, see ESI†). The upper and lower faces of curvedness plots of cocrystal 2 exhibit large flat green regions corresponding to π∙∙∙π stacking (Fig. 2c). 6 ACS Paragon Plus Environment

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In addition, shape index surface of upper and lower of cocrystal 2 exhibit complementary blue and red triangular regions related by translation (Fig. S15, see ESI†). The same trend is followed when donor and acceptor moieties of cocrystal 2 are taken into consideration individually (Fig. S16, see ESI†). The dnorm surface of cocrystal 1 exhibits multiple small red spots as well as white region corresponding to π∙∙∙π stacking interaction demonstrating moderately strong nature of interaction (Fig. 2b).57,58 In contrast, π∙∙∙π stacking interaction observed in cocrystal 2 is characterized primarily with white region along with single weak red spot indicating interaction primarily at van der Waals radii in dnorm surface (Fig. 2b).57,58 As discerning the nature of CT and A-AP interactions from visualization of Hirshfeld surfaces can be counterintuitive, we have opted for additional energy decomposition and natural bond orbital analyses to avoid any trivialities. Energy splitting analyses (EDA)59 carried out on πstacked D−A dimers in CrystalExplorer at B3LYP/D2 level of theory demonstrates A-AP interaction lowers the electrostatic contributions compared to CT interaction (Table 1). Similarly, polarization contribution of A-AP interaction is negligible compared to CT interaction while London dispersion contribution is comparable for the duo. Higher electrostatic and polarization contribution are consistent with dipolar attractive nature of CT interaction.60-63 In contrast, similar values for exchange-repulsion term for both interactions are the consequence of electronegative substituents which withdraw electron density from aromatic system rendering less repulsion. The higher London dispersion contribution associated with A-AP interaction is indicative of relatively pronounced non-polar nature of the interaction compared to its CT counterpart.63 In addition, natural bond orbital analyses (NBO) were performed on crystal geometry of the cocrystals to compare relative strength of CT and A-AP interactions (Fig. 2d). The second order perturbation energy E(2) values demonstrate that CT interaction leads to stronger π∙∙∙π stacking compared to A-AP interaction which is in accordance with favorable electron density transfer. We have analyzed the distribution of 7 ACS Paragon Plus Environment


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frontier molecular orbitals and molecular charge employing DFT calculations to further differentiate between CT and A-AP interactions. The HOMO and LUMO in cocrystal 1 are localized on the donor and acceptor moieties respectively with considerable molecular orbital offset confirming charge transfer in ground state (Fig. 2e). In contrast, HOMO in 2 is localized on donor moiety and LUMO is delocalized over the whole system with larger contribution from acceptor moiety, thus MO offset is not properly maintained (Fig. 2e). Cocrystal 1 is also characterized with prominent Mulliken charge separation between the donor and acceptor moieties confirming dipolar attractive nature of CT interaction. No such prominent Mulliken charge separation is observed in cocrystal 2 which is consistent with quadrupole-quadrupole interactive nature of arene-perfluoroarene (A-AP) interaction (Fig. S20, see ESI†).

Fig. 2 (a) Comparison of π∙∙∙π stacking in cocrystals 1 and 2, (b) dnorm plotted at Hirshfeld surfaces of cocrystal 1 and 2, red, white and blue regions indicate interactions respectively at 8 ACS Paragon Plus Environment

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less than van der Waals radii, van der Waals radii and more than van der Waals radii, (c) Curvedness plots of cocrystal 1 and 2 showing both upper and lower views, (d) NBO analyses for π∙∙∙π stacking with E(2) energies in kcal/mol, and (e) MO distribution in cocrystals.

Table 1. Energetic of charge transfer (CT) and arene-perfluoroarene (A-AP) interactions

Cocrystal

Eelectrostatic (kcal/mol)

Epolarization (kcal/mol)

Edispersion (kcal/mol)

Erepulsion (kcal/mol)

Etotal (kcal/mol)

1

-70.8

-36.6

-79.3

42.9

-132.6

Binding energy in gas phase (kcal/ mol) -13.4

2

-16.7

-9.9

-87.1

46.0

-67.7

-11.9

*Etotal = Eelectric+Epolarization+Edispersion+Erepulsion Additional stabilization in crystal packing of 1 and 2 was achieved by weak C−H∙∙∙F hydrogen bonds (Fig. S8 and S9, see ESI†).64 The C−H∙∙∙F hydrogen bonds are characterized with multiple large red spots on Hirshfeld surface of 1 indicating interaction at less than van der Waals radii (Fig. S11d, see ESI†). However, C−H∙∙∙F interaction in cocrystal 2 is characterized with white region corresponding to interaction at van der Waals radii (Fig. S14a, see ESI†). Weak F∙∙∙F intermolecular contact in cocrystal 2 can be termed as type I interhalogen interaction65 which imparts negligible stabilization energy of 0.7 kcal/mol. Energy decomposition analyses reveal both C−H∙∙∙F hydrogen bond and F∙∙∙F interhalogen interaction have major dispersive contribution (Table S2 and S3, see ESI†). We have explored the absorption characteristics associated with charge transfer and areneperfluoroarene interactions using TD-DFT calculations at CAM-B3LYP/6-311G(d,p) level. The charge transfer band observed in cocrystal 1 is comprised of CT0→CT1 and CT0→CT2 transitions (Table 2). The CT0→CT1 transition (calculated at 600 nm) possesses primarily HOMO-1→LUMO character, and the HOMO-1 and LUMO orbitals are localized respectively on di-t-BuPy and fluoranil moieties. The CT0→CT2 transition (calculated at 446 nm) has sole contribution from HOMO-2→LUMO transfer while the HOMO-2 orbital is spread over donor 9 ACS Paragon Plus Environment


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di-t-BuPy moiety (Fig. 3a). No broad charge transfer band in visible region was observed in cocrystal 2 (Fig. 1c). The calculated absorption band at 402 nm with oscillator strength f = 0.005, has contributions from both HOMO→LUMO and HOMO-1→LUMO transitions. The HOMO is localized on donor di-t-BuPy, in contrast, HOMO-1 and LUMO+1 are delocalized on the entire system with major contribution from donor and acceptor moieties respectively. The absorption bands calculated at 350 nm and 339 nm (experimental absorptions at 340 and 325 nm) have originated from multiple electronic transitions involving donor based orbital → acceptor based orbital; as well as transitions between a donor (acceptor based) based orbital to another donor (acceptor based) based orbital. All the transitions are characterized with significant transition dipole moment (Fig. 3b and Fig. S25, see ESI). Thus the absorption bands at 350 and 339 nm do not have sole origin from donor → acceptor electronic transitions and possesses no charge transfer characteristic. In contrast, the weak absorption band at 402 nm has considerable charge transfer nature (Figure 3b). This finding is consistent with energy decomposition analysis which indicates less prominent charge transfer character of A-AP interaction. The C=O stretching frequencies in cocrystal 1 and neutral fluoranil are respectively 1691 and 1697 cm-1; the small shift C=O stretching frequency indicates predominant neutral nature of cocrystal 1. The ionicity ρ can be calculated from C=O stretching frequency using the following equation: ρ = 2Δν/ν0(1-ν12/ν02) while Δν = ν0-νCC, in addition ν0, νCC and ν1 are respectively C=O stretching frequency values of neutral fluoranil, cocrystal 1 and fluoranil radical anion.66 The C=O stretching frequency value of fluoranil radical anion is 1556 cm-).67 The ionicity ρ in cocrystal 1 is 0.0444 calculated by this method, such lower value of ρ is not favorable for high value of electrical conductivity.


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Table 2. TD-DFT calculation of cocrystals 1 and 2 at CAM-B3LYP/6-311G(d,p) level of theory, calculated wavelength, oscillator strength and orbital contributions Cocrystal

Calculated wavelength λ (nm)

Oscillator strength f

Transition electric moment (D)

Transition energy (eV)

Orbital contributions

599

0.0620

1.105

2.07

HOMO-1→LUMO, 100%

446

0.0017

0.161

2.78

HOMO-2→LUMO, 100%

402

0.0052

0.323

3.09

HOMO→LUMO, 67% HOMO-1→LUMO, 23%

3.55

HOMO-1→LUMO+3, 12% HOMO→LUMO, 16% HOMO→LUMO+1, 38% HOMO→LUMO+2, 36%

3.66

HOMO-1→LUMO, 11% HOMO-1→LUMO+3, 10% HOMO→LUMO, 10% HOMO→LUMO+1, 29% HOMO→LUMO+3, 41%

1

2

349

339

0.1049

0.1237

1.107

1.174

The emission maxima λmax of di-t-BuPy, cocrystals 1 and 2 in solid state are respectively 459, 456 and 399 nm (Fig. 3c) while the excitation wavelength (λ = 265 nm) is same for all. Small blue shift of λmax in cocrystal 2 can be recognized due to the electron withdrawing effect by the fluorine atoms which reduces the electron density on di-t-BuPy. Additionally, incorporation of OFN molecules isolates the di-t-Bupy moieties in crystal and in turn restricts aggregation of fluorophores. The restriction on aggregation of fluorophoric units upon cocrystallization is also responsible for more structured fluorescence band in cocrystal 2 compared to pristine di-tBupy. Fluorescence spectrum of cocrystal 1 shows remarkable blue shift by 60 nm owing to disordered arrangement of fluoranil molecules in crystal lattice. Such disordered crystal packing reportedly leads to second harmonic generation (HSG) effect and notable blue shift.52,53 In addition to blue shift of emission maxima, considerable quenching of fluorescence intensity and lifetime is observed in cocrystals. The average fluorescence excited state lifetimes 11 ACS Paragon Plus Environment


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of di-t-BuPy, cocrystal 1 and 2 are respectively 111, 94 and 64 ns (Fig. S26, see ESI†). The quenching of fluorescence is observed due to charge transfer interaction.68 The effect of fluorescence quenching in cocrystal 1 is more prominent than 2 which is due to prominent charge transfer in 1.68

Fig. 3 (a) MOs taking part in electronic absorption in cocrystal 1 demonstrate significant localization and consequent charge transfer nature of transition, (b) MOs involved in S0→S1 absorption in 2 show less offset and insignificant charge transfer character of transition, and (c) Fluorescence spectra in solid state demonstrate maximum quenching in cocrystal 1.

Cocrystal 1 is expected to exhibit semiconductor behavior which stems from mixed stack arrangement in charge transfer cocrystals.27-32 In contrast, cocrystal 2 assembled via A-AP interaction and should be deemed as an insulator due to no partial charge separation between π-donor and π-acceptor moieties disfavoring transfer of charge carriers. To fathom the origin of n-type semiconductor nature of cocrystal 1 we have calculated transfer integrals for two charge transfer pathways viz. super-exchange and direct path.69,70 The transfer integrals for super-exchange pathway, tesuper and thsuper values have corresponding values 75.8 and 8.5 meV 12 ACS Paragon Plus Environment

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(Fig. 4b) following ‘energy splitting’ method of Zhu et al.27 In contrast, electron (tedirect, 0.8 meV) and hole (thdirect, 2 meV) transfer integral values are negligible for direct paths (Fig. S27, see ESI†). As the super-exchange along the π∙∙∙π stacking direction was reported to be more effective for charge carrier transport,27-30 cocrystal 1 is expected to behave as n-type semiconductor. Electron transfer between adjacent acceptor LUMOs in a charge transfer cocrystal can take place via both bridging donor HOMO and donor HOMO-1 orbital whereas hole transfer between adjacent donor HOMOs occur only via bridging acceptor LUMO. According to perturbation theory, transfer integral t is related to super-exchange integral by tesuper ≈ t2/ΔE (ΔE is the energy difference of donor HOMO or HOMO-1 orbital and acceptor LUMO).69-72 The values of transfer integral t for electron transfer via donor HOMO and HOMO-1 orbital are respectively 276 and 358 meV. It is pertinent to specify that the acceptor LUMO and donor HOMO-1 orbital both exhibit vertical periodicity (ungerade symmetry) while donor HOMO possess horizontal periodicity (gerade symmetry). As a result, electron transfer via bridging donor HOMO-1 is favored over bridging donor HOMO owing to symmetry matching while hole transfer is disfavored for symmetry mismatch (Fig. 4c).71,72 The weak n-type semiconductor nature of cocrystal 1 is the manifestation of comparatively larger energy difference of 1.7 eV between donor HOMO-1 and acceptor LUMO resulting in less favored electron transfer.



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Fig. 4 (a) Electron transfer integrals in cocrystal 1 calculated from ADA triad, (b) Hole transfer integral in 1 obtained from DAD triad, and (c) Donor HOMO-1, HOMO, and acceptor LUMO in 1 .

Fig. 5 (a) Schematic representation of the device configuration used for conductivity Measurement using two contact probe device; (b) The current-voltage (I-V) graphs of cocrystal 1 and cocrystal 2. Aluminum electrodes of width (W) 800µm were deposited and a channel length (L) of 40µm was used for two terminal conductivity measurement. Solution of cocrystal 1 and cocrystal 2 was made in chlorobenzene with the concentration of 25 mg/mL. Each solution was dropcasted on the device in between two electrodes and dried at room temperature. It can be


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observed from that cocrystal 1 shows Schottky behavior.73 Cocrystal 1 demonstrates weak semiconducting behavior while in comparison cocrystal 2 shows insulating behavior. The conductance was calculated from the current-voltage (I-V) graph and values are 2.8 nS and 0.5 nS for cocrystal 1 and 2 respectively. The small conductivity value of cocrystal 1 (3.5 μS/m) is in accordance with the lower degree of charge transfer as predicted from IR stretching frequency value. To further verify ability of the cocrystals to transport electrons, OFETs were made with the cocrystals acting as the active layer. On cleaned glass substrates ≈ 100 nm thick aluminum gate was deposited by thermal evaporation followed by spin coating and annealing with PVA solution in DI water at 100 ºC for 30 min to form a layer of 1 µm. On this layer cocrystal 1 and 2 in chlorobenzene solution (concentration of 25 mg/mL) were spin coated on separate substrates to form a layer of thickness ~100 nm. The Al source and drain contacts of thickness ~70nm were deposited on the active layer through a shadow mask with a channel length and width of 40 µm and 800 µm, respectively. The device architecture is shown in Fig. 6a.

Fig. 6 (a) Schematic representation of OFET used for conductivity measurement; (b) Electron transfer characteristic of cocrystal 1; (c) Electron transfer characteristic of cocrystal 2.

The transfer characteristic of the FET with cocrystal 1 and 2 is shown in Fig. 6(a) and (b) 𝐿

∂𝐼𝐷

respectively. The field-eﬀect mobility (µ) is calculated using the equation in, μ =𝑊𝐶𝑖𝑉𝐷𝑆∂𝑉𝐺 15 ACS Paragon Plus Environment


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where, W and L respectively denote channel width and length, Ci (14.3 nF/cm2 as extracted from Capacitance-Voltage measurement) is the capacitance per unit area of the gate dielectric layer, and 𝑉𝐺, 𝑉𝑡h and 𝑉𝐷𝑆 are the gate, threshold and source-drain voltages, respectively;

∂𝐼𝐷 ∂𝑉𝐺

is extracted from the slope of the transfer characteristic curve. The electron mobility was calculated for cocrystal 1 as 7×10-3 cm2V-1s-1 whereas cocrystal 2 exhibited electron transfer mobility 10-4 cm2V-1s-1 which is a order less, inferring that cocrystal 1 promotes better transport of electrons in comparison to cocrystal 2. Very weak n-type semiconductor property of cocrystal 2 is a consequence of small charge transfer i.e., electrostatic nature of areneperfluoarene interaction.

CONCLUSIONS Herein we report the difference between two closely similar face to face π∙∙∙π stacking interaction viz. charge transfer (CT) and arene-perfluoroarene (A-AP) in structurally analogous 1:1 binary cocrystal systems employing 2,7-di-tert-butylpyrene as π-donor and fluoranil, octafluoronaphthalene as π-acceptors. Comprehensive comparison of two face to face π∙∙∙π stacking interactions is hitherto unreported and our study unravels the disparity of these interactions. Quantitative discrimination of two interactions via Hirshfeld surface, DFT and natural bond orbital (NBO) analyses reveal that CT interaction has predominant electrostatic contribution while A-AP interaction is primarily dispersive in nature. The dipole attractive nature of CT interaction imparts partial ionic nature in di-t-BuPy: fluoranil cocrystal leading to weak n-type semiconductor behavior. Contrastingly A-AP interaction in di-t-BuPy: OFN cocrystal results in insulator property owing to its quadrupole attractive nature. In addition, charge transfer interaction directs to sizeable quenching of fluorescence of pristine donor while the A-AP interaction does not induce substantial aggregation induced quenching. Therefore, 16 ACS Paragon Plus Environment

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discernible optical and electronic properties afforded by these interactions can be utilized towards the development of new optoelectronic materials with tunable functionality.

Conflicts of interest There are no conflicts to declare.

Supporting Information Syntheses and characterization techniques, DFT and Hirshfeld surface analyses methods, crystal refinement and hydrogen bonding interaction tables, ORTEP and crystal packing diagrams, Hirshfeld and natural bond orbital analyses for hydrogen bonds, calculations for UV-Vis and fluorescence spectra, PXRD and thermogravimetric (TGA) analyses. This information is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENT AM is thankful to DST India (SERB) for fellowship (NPDF/SERB/CH000511) and N. Preeyanka (NISER Bhubaneswar), Shuvendu Ghosh (IIT Guwahati) for help in spectral measurements. We are thankful to Dr. Debaprasad Mondal (Department of Chemistry, IIT Ropar) for crystal refinement.

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TOC Graphic


Charge Transfer vs. Arene-Perfluoroarene Interactions in Modulation

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