Exploring the Relationship between Intermolecular Interactions and

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Exploring the Relationship between Intermolecular Interactions and Solid-state Photophysical Properties of Organic Co-crystals Rohit Bhowal, Suprakash Biswas, Athulbabu Thumbarathil, Apurba Lal Koner, and Deepak Chopra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10643 • Publication Date (Web): 14 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018

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The Journal of Physical Chemistry

Exploring the Relationship between Intermolecular Interactions and Solid-State Photophysical Properties of Organic Co-crystals Rohit Bhowal, a Suprakash Biswas,a Athulbabu Thumbarathil,a Apurba L.Koner, a* Deepak Chopra*[a] Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462066.

ABSTRACT:Five new binary co-crystals have been prepared through molecular selfassembly ofπ-electron-rich molecules like Phenanthrene (PHEN), Benzo[c]cinnoline (BCC) and Phenazine (PHNZ) in the presence ofπ-electron-deficient molecules like Tetrafluoro-1,4-benzoquinone (TFQ), Tetrachloro-1,4-benzoquinone (TCQ) and 1,2,4,5Tetracyanobenzene (TCNB), taken in an equimolar ratio. Crystal structure analysis revealed that in three binary co-crystals the constituent molecules were alternately

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sandwiched in a cofacial arrangement through π···π interactions, while in the remaining two binary co-crystals the constituent molecules were aligned in a unique edge-to-face manner through lp···π interactions. Co-crystals with π-stacking arrangement were fluorescent, whereas almost complete quenching of luminescence was observed in those having edge-to-face alignment of molecules. The photophysical observations of these co-crystals have been demonstrated via energetic quantification of intermolecular interaction topology, which provides a molecular level understanding of factors controlling their solid-state absorption and luminescence behavior.

INTRODUCTION Non-covalent intermolecular interactions in the solid-state have the potential to direct luminescence which is of great significance in crystal engineering.1–5 Organic molecular materials for electronic applications are principally developed based on π–π stacking interactions, the length of the π-conjugation and also on the presence of heteroatoms. Fluorescent properties of organic compounds rely not only on the structure of fluorophore but also on the architecture of crystal packing and simultaneously on the


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nature and strength of intermolecular interactions.6 Organic compounds with molecular packing directed luminescence7–8 have gained significant popularity and attention due to their remarkable properties and promising optoelectronic applications in the fields of lasers,9 sensors,10 organic light-emitting diodes,11and optical data storage.12 Numerous strategies have been employed to acquire tunable luminescent materials,13 since traditional

structure

tailoring

techniques

on

single

molecular

materials

have

accomplished restricted success. A practical alternative method of tuning these properties is by combining materials as blends or co-crystals.14 Supramolecular organic co-crystal chemistry primarily focuses on various non-covalent interactions such as hydrogen bonds, C–H···π, π···π,and Charge-Transfer (CT) interactions.15–26 In addition to these, lp···π intermolecular interactions27-29 have become significantly important due to their role in the stability of supramolecular and biomolecular entities.30 A viable cocrystal synthesis strategy31–32 has the potency to induce some of the superior properties of the individual components by synergistic effects33–34 and hence co-crystal preparation can modulate emission colors and luminescence efficiency.35-38


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It has been generally found that π-conjugated organic molecules consisting of electron donating or withdrawing substituents have the usual propensity to co-crystallize through π–π stacking interactions in two different packing motifs namely, segregated stacking39–40 and mixed stacking.41 In segregated stacking mode the π-electron-rich (A) and π-electron deficient (B) molecules form separate π-stacked columns (A–A–A–A, B– B–B–B), whereas in mixed stacking mode the constituent molecules are alternately arranged over each other along the π-stacking column (A–B–A–B). The crystal structure of

Co-crystals

1

(Phenanthrene+Tetrafluoro-1,4-benzoquinone),

(Benzo[c]cinnoline+1,2,4,5-Tetracyanobenzene)

and

3

2

(Phenazine+1,2,4,5-

Tetracyanobenzene) have been found to adopt mixed stacking arrangement, wherein Co-crystal 1 the constituent molecules are cofacially placed over each other, while in Co-crystal 2 and 3 the constituent molecules are parallelly displaced along the long molecular axis.42 But Co-crystals 4 (Benzo[c]cinnoline+Tetrafluoro-1,4-benzoquinone) and 5 (Benzo[c]cinnoline+Tetrachloro-1,4-benzoquinone) displays a unique T-shaped edge-to-face stacking pattern, in which Benzo[c]cinnoline (BCC) molecule orthogonally directs the nitrogen heteroatoms over the planar aromatic face of the tetrahalogenated-


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1,4-benzoquinone molecules leading to the formation of strong N(lp)···π interactions. The first report on lp···π interactions was published by K. Hiraokaet al., in which the interactions of different halogen anions with the π-system of hexafluorobenzene was discussed.43 The Cambridge Structural Database (CSD)44 search reveals that all the reported co-crystals containing Tetrafluoro-1,4-benzoquinone (TFQ) or Tetrachloro-1,4benzoquinone (TCQ) molecules as one its components, does not involve lp···π interactions between the constituent molecules. Hence this is the first report wherein the presence of such intermolecular interactions in their respective co-crystal structures with Benzo[c]cinnoline (BCC) has been experimentally realized. The presence of strong

lp···π interactions in Co-crystals 4 and 5 has a profound influence on their solid-state photoluminescence (PL) spectra,45 which distinguishes them from Co-crystals 1, 2 and 3. It has also been perceived that the existence of hydrogen bonding interactions which are electrostatic in nature, horizontal or diagonal to the π-stacked columns in Cocrystals 2 and 3 are responsible for the hypsochromic shifts from the absorption maxima of their respective electron-rich component in the solid-state absorption spectra. The role of hydrogen bonds in determining the extent of hypsochromic shift in the solid-state


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absorption spectrum has been investigated by determining the alignment of dipole moments in the asymmetric unit of the co-crystals, since the dipole moment vectors always prefer to be aligned along with electrostatic dominated interactions.46 Herein, a quantitative assessment of nature and energetics associated with these interactions and their subsequent consequences on absorption and luminescence behavior of the co-crystals has been explored in this investigation. The study is supported by the energy framework analysis which enables us to gain knowledge about the nature and strength of intermolecular interactions. Natural Bond Orbital (NBO) analyses have been performed to provide insight into the non-covalent interactions between various parts of molecules. EXPERIMENTAL SECTION Materials.Phenanthrene,

Tetrafluoro-1,4-benzoquinone

andTetrachloro-1,4-

benzoquinone were purchased from Sigma Aldrich; Benzo[c]cinnoline, Phenazine and 1,2,4,5-Tetracyanobenzene were purchased from TCI Chemicals and all HPLC grade solvents were purchased from Finar and were used directly without any further purification.


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Co-crystal Preparation.Co-crystal 1. Equimolar amounts of Phenanthrene and Tetrafluoro-1,4-benzoquinone were grinded for 75 minutes in a mortar using a pestle with addition of methanol drops at regular intervals of 15 minutes. The color changed instantly and permanently from light yellow to intense red instantly after a few minutes of mechanical grinding. The resulting co-crystal powder mixture was dissolved in a 1:1 diethyl ether and chloroformsolvent mixture and left for recrystallization by slow evaporation method at 4˚C for nearly a week to obtain red co-crystals (Figure S1a). Co-crystal

2.

Equimolar

quantities

of

Benzo[c]cinnoline

and

1,2,4,5-

Tetracyanobenzene were grinded for 75 minutes in a mortar using a pestle with addition of methanol drops at regular intervals of 15 minutes. The resulting co-crystal powder mixture was dissolved in dichloromethane and hexane solvent mixture and left for recrystallization by slow evaporation method at 4˚C for a few days to obtain yellow cocrystals (Figure S1b). Co-crystal 3. Equimolar amounts of Phenazine and 1,2,4,5-Tetracyanobenzene were grinded for 75 minutes in a mortar using a pestle with addition of methanol drops at regular intervals of 15 minutes. The resulting co-crystal powder mixture was dissolved in


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dichloromethane and acetonitrile solvent mixture and left for crystallization by slow evaporation method at 4˚C for a few days to obtain yellow co-crystals (Figure S1c). Co-crystal 4. Equimolar ratios of Benzo[c]cinnoline and Tetrafluoro-1,4-benzoquinone were grinded for 75 minutes in a mortar using a pestle with addition of methanol drops at regular intervals of 15 minutes. The resulting co-crystal powder mixture was dissolved in chloroform solvent and left for crystallization by slow evaporation method at 4˚C for nearly a week to obtain yellow co-crystals (Figure S1d). Co-crystal 5. Equimolar quantities of Benzo[c]cinnoline and Tetrachloro-1,4benzoquinone were grinded for 75 minutes in a mortar using a pestle with addition of methanol drops at regular intervals of 15 minutes. The resulting co-crystal powder mixture was dissolved in dichloromethane solvent and left for crystallization by slow evaporation method at 4˚C for a few days to obtain yellow co-crystals (Figure S1e). Powder X-Ray Diffraction (PXRD). The experimental Powder X-ray diffraction patterns of all the co-crystals were recorded on PANalytical Empyrean X-ray Diffractometer with a Cu Kα radiation (λ= 1.5418 Å). The bulk powder of each sample was placed in a silica


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sample holder and measured by a continuous scan between 5˚–60˚ in 2θ with a step size of 0.013103˚. Single Crystal X-Ray Diffraction (SCXRD). The Single Crystal X-ray Diffraction measurements for all the obtained co-crystals were carried out in a Bruker AXS Kappa APEXII Single crystal diffractometer using MoKα radiation (λ= 0.71073 Å) at 100 K. Bruker APEX II Software47 was used for performing unit cell measurement, data collection, data integration, scaling and absorption corrections for all co-crystals. Multi– scan absorption corrections were applied using SADABS.48 The co-crystal structures were solved by direct methods using SIR 201449 and refined with full–matrix least– squares method using SHELXL–201650 present in the program suite WinGX.51 All non– hydrogen atoms were refined anisotropically, and all hydrogen atoms were located from the difference Fourier Map. The PLATON52 and Mercury3.1053 programs were used for structure analysis and molecular and crystal structure drawings. Differential Scanning Calorimetry (DSC). The DSC traces of all the co-crystals were recorded with a PerkinElmer DSC 6000 instrument where approximately 1.0 mg of each


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co-crystal were successively placed in hermetically sealed aluminum pan in vacuum and subsequently scanned at a rate of 5°C/min under a dry nitrogen purge (20mL/min). Infrared Spectroscopy (IR). Infrared Spectra of the co-crystals were obtained from Shimadzu IRAffinity-1S FTIR spectrophotometer operated via IR solution FT-IR control software. About 1mg of the dry co-crystal powder was mixed and pelletized with KBr successively and the percentage transmittances were recorded over the range of 400– 4000 cm–1. UV-vis absorption spectroscopy.Diffuse reflectance UV-Vis.-NIR spectra of solid powdered co-crystals and their π-electron rich components were collected under ambient conditions on a Cary 5000 UV-Vis.-NIR (Agilent) equipped with diffuse reflectance accessory by using reflectance standard disk and BaSO4 as a standard. Emission Spectroscopy. The Photoluminescence emission spectra of the co-crystals and their respective individual precursors were collected using HORIBA-JOBINYVON spectrofluorometer equipped with a 450 W Xenon CW lamp as the excitation source. Fluorescence Microscopy Images.Fluorescence microscopy imaging of all the cocrystals were performed in OLYMPUS IX-83-inverted fluorescence microscope using


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OLYMPUS cellsens dimension 1.1 software. For Co-crystal 1, we have taken the fluorescence image by using TRITC channel with an emission wavelength of 580 nm and exposure time of 30 ms. For Co-crystal 2, fluorescence microscopy images were captured using FITC channel with an emission wavelength of 518 nm and exposure times of 39 ms. In case of Co-crystal 3, fluorescence microscopy images were taken using DAPI channel with an emission wavelength of 465 nm and exposure time of 40ms. Energy Framework Calculations. The energy framework analysis has been performed using CrystalExplorer17.554 to visualize the intermolecular interaction topology in all the binary co-crystals. The “energy framework” was constructed based on the crystal symmetry, and the energies are estimated from B3LYP/ 6-31G(d,p) molecular wave functions calculated at the crystal geometry, summing up the electrostatic, polarization, dispersion, and exchange-repulsion terms based on a scaling factor of 1.057, 0.740, 0.871, and 0.618, respectively. The interactions energies below a certain energy threshold (4 kJ mol-1) are omitted for clarity, and cylinder thickness (80) is proportional to the intermolecular interaction energies.


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RESULTS AND DISCUSSIONS Firstly, we have discussed the formation of five binary co-crystals via, solvent-assisted mechanochemical grinding, which affords Co-crystal 1 from Phenanthrene and Tetrafluoro-1,4-benzoquinone; Co-crystal 2 from Benzo[c]cinnoline and 1,2,4,5Tetracyanobenzene; Co-crystal 3 from Phenazine and 1,2,4,5-Tetracyanobenzene; Cocrystal 4 from Benzo[c]cinnoline and Tetrafluoro-1,4-benzoquinone and Co-crystal 5 from Benzo[c]cinnoline and Tetrachloro-1,4-benzoquinone (Scheme 1). These cocrystals were characterized by Single-crystal X-ray diffraction (SCXRD), Powder X-Ray Diffraction (PXRD), Differential Scanning Calorimetry (DSC) and Infrared Spectroscopic techniques. Thereafter, to gain an understanding of the photophysical properties of the co-crystals, solid-state UV-vis. and fluorescence spectroscopic measurements were performed and fluorescence microscopy images were collected.


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Scheme 1. Chemical structure of π-electron-rich and π-electron-deficient molecules used for co-crystal synthesis. Co-crystal structures of (a) 1, (b) 2, (c) 3, (d) 4 and (e) 5.

SCXRD data allowed the identification of intermolecular interactions and selfassembly fashion in the crystal lattice. The entire crystallographic data is documented in Table

S1.

Co-crystal

1

crystallizes

in

the

monoclinic

crystal

system

with


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centrosymmetric space group P21/c and comprises of one PHEN and one TFQ molecule in the asymmetric unit. The molecules form cofacial mixed π-stacking arrangement through Charge-Transfer interactions which can be verified with (dπ–π) ranging between 3.260 to 3.351 Å (Figure S2a) and close centroid-centroid distance (3.706 and 3.595 Å) when viewed along the b axis (Figure 1). Co-crystal 2 crystallizes in the triclinic crystal system with the centrosymmetric space group P-1 containing one BCC and one TCNB molecule in the asymmetric unit. The molecules are stacked alternately in a parallelly displaced manner through π···π interactionsalongthea axis. The molecules in the stacking columns are laterally connected to molecules of neighboring columns through C–H···π(N≡C) contacts between TCNB molecules and bifurcated C–H···N contacts between BCC and TCNB molecules when viewed down the bc plane (Figure 2). Co-crystal 3 crystallizes in the triclinic crystal system with the centrosymmetric space group P-1 and the asymmetric unit consists of half PHNZ and half TCNB molecule. Crystal structure analysis reveals the presence of uniform slipping arrangement of both the molecules through mixed π-stacked columns along a axis. The molecules of successive columns are arranged similar to a chessboard layout


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interlinked through C–H···π(N≡C) interactions (Figure 3). Co-crystal 4 crystallizes in the triclinic crystal system with the centrosymmetric space group P-1 and the asymmetric unit consists of one BCC and half TFQ molecule. TFQ molecules are sandwiched between two perpendicular BCC molecules when viewed along the c axis through the formation of strong N(lp)···π interactions with distances ranging from 2.938 to 3.228 Å (Table S2, Co-crystal 4), all of which are lesser than the sum of the van der Waals atomic radius of nitrogen and carbon of 3.25 Å.55 Two BCC and two TFQ molecules are interlinked to each other forming tetramers through C–H···O interactions along the b axis(Figure 4). Co-crystal 5 crystallizes in the monoclinic crystal system with centrosymmetric space group P21/n containing one BCC molecule and half TCQ molecule in the asymmetric unit. TCQ molecules are sandwiched between two perpendicular BCC molecules, subsequently bridging them via strong N(lp)···π interactions with distances ranging from 2.873 to 3.129 Å (Table S2, Co-crystal 5) and all these interactions are lesser than the sum of van der Waals atomic radius of nitrogen and carbon. These three molecules are aligned in almost perpendicular directions with respect to BCC molecules and the adjacent orthogonal molecular planes are interlinked


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through C–H···π, C–H···O and C–H···Cl interactions, while adjacent molecular planes of similar orientation are connected through π···π interactions between BCC molecules (Figure 5).

Figure 1.Zigzag arrangement of molecules along adjacent stacking columns in Cocrystal 1 with alternate centroid to centroid of 3.706 Å and 3.595 Å.


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Figure 2. Molecular mixed stacking columns of Co-crystal 2 with alternate centroid to centroid distance of 4.927 Å and 3.604 Å, along with C–H···π(N≡C) and bifurcated C– H···N contacts bridging them.

Figure 3. Columnar view of Co-crystal 3 along a axis with a uniform centroid to centroid distance of 3.673 Å along with C–H···π(N≡C) linking the adjacent columns similar to a chessboard layout.


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Figure 4.TFQ molecules sandwiched between two perpendicularly oriented BCC molecules through N(lp)···π contacts represented by 2.795 Å distance between N–N bond centroid in BCC and centroid of TFQ in Co-crystal 4.

Figure 5. TCQ molecules aligned perpendicularly between two BCC molecules having two distinct conformations through N(lp)···π contactsrepresented by 2.777 Å distance between N–N bond centroid in BCC and centroid of TCQ in Co-crystal 5.

To obtain an insight into the electrostatic complementarity of stacking interactions, the molecular electrostatic surface potential (MESP) for all the molecules in the five cocrystals have been mapped onto their crystal geometry using CrystalExplorer17.5. Every molecule in the asymmetric unit of a given crystal structure will have a unique Hirshfeld surface and hence is especially helpful for a direct comparison between same


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molecules in different crystalline environments. Positive regions (blue) of the electrostatic potential preferably interact with complementary negative regions (red) on neighboring molecules. The intermolecular interactions in the co-crystals can be rationalized using the electrostatic potential mapped on the Hirshfeld surface. In Cocrystal 1, the electronegative π-surface of PHEN (Vs,min = -62.6 kJmol-1) interacts with the positive π-surface of TFQ (Vs,max =196.9 kJ mol-1), resulting in the formation of a Charge-Transfer complex through cofacialπ···π stacking interactions (Figure 6a). In Cocrystals 2 and 3 the π-surface of BCC (Vs,min = -29.2 kJ mol-1) and PHNZ (Vs,min = -34.5 kJ mol-1) are less electronegative in comparison to PHEN due to the presence of nitrogen heteroatoms, while TCNB also has lesser electropositive π-surface (Vs,max = 161.4 kJ mol-1 in Co-crystal 2; 162.2 kJ mol-1 in Co-crystal 3) due to lesser electron withdrawing ability of cyano (–C≡N) groups in comparison to the highly electronegative fluorine atoms of TFQ (Figure 6b,c). Consequently, the formation of Charge-Transfer complexes are not possible in Co-crystals 2 and 3 with the help of weak π-electron donors like BCC and PHNZ in the presence of weaker π-electron acceptor TCNB and thus the constituent molecules in both the co-crystals assemble via parallelly displaced


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mixed π-stacked columns. Interestingly in Co-crystals 4 and 5, TFQ and TCQ have highly electropositive π-surface (Vs,max= 235.2 kJ mol-1 in TFQ; 213.7 kJ mol-1 in TCQ) thus making them much stronger π-electron acceptor in comparison to TCNB which in turn favours the formation of strong N(lp)···π interactions with nitrogen heteroatoms of BCC (Vs,min= -257.3 kJ mol-1 in Co-crystal 4; -250.7 kJ mol-1 in Co-crystal 5) (Figure 6d,e). Hence MESP plotted on Hirshfeld surface yields a comprehensive understanding to perceive the rationale behind the development of Charge-Transfer complex in Cocrystal 1 but not in Co-crystals 2 and 3, and also justifies the reason for BCC to assemble via mixed π-stacked columns in the presence of TCNB in Co-crystal 2, while BCC in the presence of TFQ and TCQ in Co-crystals 4 and 5 respectively are oriented in T-shaped edge-to-face stacking pattern through N(lp)···π interactions. In order to analyze the diversity of intermolecular interactions between the molecular components of co-crystals, infrared spectroscopic measurements have been performed. Investigation of the IR spectra unveils the distinct shifts in stretching frequencies of principal peaks in all the co-crystals when compared to their respective parent components.56–58 The position of νC≡N peak (2247 cm-1) of free TCNB molecule is similar


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in Co-crystals 2 and 3, while νC–H peaks (3047, 3113 cm-1) in TCNB undergoes minor blue shift (νC–H=3049, 3115 cm-1) upon formation of Co-crystal 2 and a minor red shift (νC–H=3044, 3111cm-1) upon formation of Co-crystal 3 [Figure S5a]. The νC=O peaks (1674, 1697 cm-1) of independent TFQ molecule experiences red shift (νC=O=1666, 1692 cm-1) in Co-crystal 1 and a minor blue shift (νC=O=1676, 1699 cm-1) in Co-crystal 4, while the νC–F (1333 cm-1) of TFQ experiences red shifts of 13 and 21 cm-1 upon formation of Co-crystal 1 and 4 respectively [Figure S5b] indicating completely different molecular orientations in both the co-crystals. The νC=O peak (1689 cm-1) in independent TCQ molecule is similar to the corresponding νC=O of Co-crystal 5, while the νC–Cl (712, 752 cm-1) of TCQ undergoes a major blue shift (νC–Cl=734, 758 cm-1) due to the presence of C–H···Cl–C hydrogen bonding interactions upon the formation of Co-crystal 5 (Table S2, Co-crystal 5) which is absent in the crystal structure of TCQ [Figure S5c].


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Figure 6. Variation of the MESPs mapped over the Hirshfeld surfaces for both the πelectron rich and π-electron deficient molecules of (a) Co-crystal 1, (b) Co-crystal 2, (c)


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Co-crystal 3, (d) Co-crystal 4 and (e) Co-crystal 5. Values of the electrostatic surface potential is plotted from -52.6 kJ mol-1 (red) to 0.00 kJ mol-1 (white) to +52.6 kJ mol-1 (blue).

Figure 7. Normalized solid-state absorption spectra of (a) Phenanthrene and Co-crystal 1, (b) Benzo[c]crinoline and Co-crystal 2, (c) Phenazine and Co-crystal 3. (d) Normalized emission spectra of Co-crystals 1, 2, 3 and their respective starting materials.

The solid-state absorption spectra provide an adequate indication for the formation of co-crystals. The absorption spectra corroborate considerably with the orientation of molecules in the crystal packing of the respective co-crystals. Co-crystal 1 displays a


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significantly red-shifted broad absorption spectrum (λmax=575 nm) with respect to its donor (Figure 7a) validating the formation of a Charge-Transfer co-crystal.59 This wasalso initially speculated by a visible color change from light yellow to intense red powder during the mechanical grinding process [Figure S6]. These photophysical observations can be explained from energy framework analysis60–63 using Crystal Explorer17.5, which enables us to visualize the nature and strength of molecular stacking interaction topology.64 The relative strengths of intermolecular interaction energies (IE) are depicted by the thickness of tubes connecting the molecules, while different colors indicate the nature of IEs.


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Figure 8. Energy frameworks for co-crystal structures of (a) 1, (b) 2, (c) 3, (d) 4 and (e) 5 representing the total interaction energy (blue) partitioned into electrostatic (red) and dispersive (green) components.

The energy framework of Co-crystal 1 showsa columnar interaction topology, where the π–πstacking IE varies alternately from -42.6 kJ mol-1 to -41.4 kJ mol-1, (highlighted by orange circle, Figure 8a, Figure S7a), which can also be anticipated due to alternate centroid to centroid distance (3.706 Å/ 3.595 Å) along the π–stacking column (Figure 1). The horizontal tubes between PHEN and TFQ molecules (highlighted by pink circle, Figure 8a) represent very weakC–H···F and C–H···O interactions with IEs of -6.5 kJ mol1and

-6.3 kJmol-1(Figure S7a) between the vertical columns. Hence, energy framework

analysis of Co-crystal 1 demonstrates the dominance of π–π stacking interactions over other interactions, which clearly satisfies the condition for development of ChargeTransfer complex through π–π stacking interactions between the π-electron donor and acceptor molecules. Co-crystal 2 has a columnar interaction topology, with successive molecules being translated non-uniformly along the long molecular axis causing the IE


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of π-stacking to vary alternately from -28.0 kJ mol-1 to -49.2 kJ mol-1(Figure S7b) because ofalternate centroid-centroid distance (3.604 Å/ 4.927 Å) (Figure 2).

The

stacked columns are also interconnected by tubes along diagonal directions, represented by C–H···π(N≡C) interactions between TCNB molecules (marked by red circles, Figure 8b) with IE values of -24.2 kJ mol-1 and -24.9 kJ mol-1 (Figure S7b) when viewed down the ac plane, as well as C–H···N interactions65between BCC and TCNB molecules (marked by pink circles, Figure 8b) having an IE value of -34.1 kJmol-1(Figure S7b) when viewed along the b axis. These hydrogen bonding interactions are highly electrostatic in nature, as visible from the larger diameter of the cylinders representing electrostatic contribution in comparison to its dispersive counterpart and their presence exert a significant effect on the solid-state absorption spectra.66UV-Vis. spectrum of Cocrystal 2 undergoes hypsochromic shift with a broad absorption band at around

λmax=380 nm, compared to BCC which absorbs at λmax=425 nm and the reason for this blue shift can be attributed to the formation of electrostatic C–H···N hydrogen bonding interactions (-34.1 kJ mol-1) between nitrogen heteroatoms of BCC and protons of TCNB molecules lateral to the π-stacked columns(highlighted by pink circles, Figure 8b).


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These moderately strong C–H···N hydrogen bonds stabilize the ground state n (HOMO) to a greater extent than the excited state π* (LUMO), thereby increasing the gap between HOMO and LUMO, which justifies the blue shift in absorption spectrum.67 This is also evident from the direction of the dipole moment vector diagonal to the stacking axis in the asymmetric unit (Figure S4b). In case of Co-crystal 3, the solid-state absorption spectrumis quite similar to the spectrum of its constituent PHNZ molecule, where the former with an absorption band at λmax=403 nm exhibits a minor blue shift from λmax=414 nm of the later (Figure 7c). The occurrence of this minor blue shift in the absorption spectrum can be explained after analyzing the energetics of intermolecular interactions. Here it was found that successive molecules have vertical columnar stacking with an IE value of 47.0 kJmol-1 (Figure 8c, Figure S7c) due to uniformity in centroid to centroid distance of 3.673 Å (Figure 3). The vertical columns are also connected by horizontal columns (marked by red circles, Figure 8c), which represents C–H···π(N≡C) interactions between TCNB molecules with IE values of -27.1 kJmol-1 and -25.3 kJmol-1 (Figure S7c), along the b andc crystallographic axes respectively. Furthermore, the diagonal tubes between PHNZ and TCNB molecules represent C–H···π(N≡C) interactions having much weaker


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IE value of -8.6 kJmol-1, which reveals that nitrogen heteroatoms of PHNZ molecule does not participate in hydrogen bonding with TCNB molecule and is only involved in dispersive π···π interactions. This justifies the similarity and minor blue shift in absorption spectrum of Co-crystal 3 with respect to PHNZ. This phenomenon has been further approved by the direction of the dipole moment vector along the π-stacking axis unlike the case of Co-crystal 2 (Figure S4c) which further approves the absence of hydrogen bonding interactions involving nitrogen heteroatoms of PHNZ in Co-crystal 3. In case of Co-crystals 4 and 5 the absorption spectra show a broad structureless band accompanied by a minor blue shift from the absorption maxima of BCC (Figure S8). Thereafter, PL spectroscopic measurements were performed to testify the emission behavior of the assembled Co-crystals 1, 2 and 3 which gave luminescence peaks at around 675 nm, 518 nm and 462 nm, when excited at 575 nm, 380 nm and 405 nm respectively [Figure 7d]. Interestingly, there has been almost complete quenching of luminescence in Co-crystals 4 and 5 (Figure S9, Table S4) and the reason is dedicated to the presence of strongelectrostatic N(lp)···π interactions (Figure 8d, e). These N(lp)···π interactions are favored by the unique edge-to-face orientation of constituent


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molecules in the crystal lattice of these two co-crystals. The alignment of the dipole moment vector passing midway between the nitrogen heteroatoms of BCC and through the center of halogenated-1,4-benzoquinones of the asymmetric unit further substantiates the existence of electrostatic N(lp)···π interactions in Co-crystals 4 and 5 (Figure S4d, e). Fluorescence microscopy images of Co-crystals 1, 2 and 3 were obtained as shown in Figure 9. Co-crystals 1, 2 and 3 exhibits intense red, green and blue luminescence respectively. Additionally, the color coordinates, which correspond with the total luminescence spectra, were calculated by CIE1931 Chromaticity Coordinate Calculation, as shown in Figure 9. The as-obtained Co-crystals 1, 2 and 3 possess λem(max) at around 675, 520 and 463 nm and the corresponding color coordinates are (0.69, 0.30), (0.23, 0.54) and (0.22, 0.28) respectively.


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Figure 9.Fluorescence microscopy images of Co-crystals (a) 1, (b) 2, (c) 3 and (d) CIE colour coordinate of Co-crystals 1, 2 and 3.

In order to unravel the nature of stacking interactions in all the co-crystals, Natural Bond Orbital (NBO) calculations were performed on all of them (Table S5) and the results confirm the presence of strong N(lp)···π interactions, responsible for almost complete quenching of fluorescence in Co-crystals 4 and 5. (Figure 10) clearly shows the occurrence of n → π* orbital interactions.68The magnitude of second-order perturbation energy E(2) for charge-transfer from the lone pairs of nitrogen atoms (n) in BCC to the C=O antibonding orbitals (π*) of TFQ and TCQ were calculated to be 7.12 kJ mol-1 (Figure 10a) and 10.89 kJmol-1 (Figure 10d) in the case of Co-crystal 4, while 7.87 kJ mol-1 (Figure 10g) and 9.63 kJmol-1 (Figure 10j) in the case of Co-crystal 5, which further upholds the role of these N(lp)···π interactions in the solid-state.


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Figure 10.The n → π* orbital interaction in case of (a)-(f) Co-crystal 4 and (g)-(l) Cocrystal 5.

To appraise the thermal properties of these co-crystals, Differential Scanning Calorimetry (DSC) was undertaken. The appearance of a smooth baseline along with a single sharp melting peak indicates that there is no phase transition occurring in the crystalline phase for all the five binary co-crystals (Figure 11). The appearance of


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endothermic melting peaks in between the melting points of individual components of these binary co-crystals (Table S3) indicates formation of significantly new stacking patterns and intermolecular interactions after co-crystallization with the lone exception of Co-crystal 4 whose melting point is lower than both the components.

Figure 11. DSC profile of Co-crystals (1-5).

CONCLUSIONS In summary, we have successfully assembled five new binary co-crystals, and the relationship between the nature and energetics of intermolecular interactions in the crystal geometrywith the solid-state optical properties have been discussed elaborately. Energy framework and MESP analysis of Co-crystal 1 rationalizes thecofacial mixed


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π···π stacking arrangement of the constituent molecules (PHEN and TFQ) in the crystal structure, forming a Charge-Transfer complexwhich is proved by a large bathochromic shift from the absorption maxima of its donor PHEN in the solid-state absorption spectrum.MESP analysis also explains the reason for Co-crystals 2 (BCC and TCNB) and 3 (PHNZ and TCNB) to assemblein the crystal lattice via parallelly displaced mixedπ-stackingarrangement, whereas IE topological analysisreveals that electrostatic dominated C–H···N and C–H···π(N≡C) hydrogen bonding interactions horizontal and diagonal to the π-stacked columnsof Co-crystals 2 and 3 are responsible for the hypsochromic shifts from the absorption maxima of their respective π-electron-rich component.69 However,Co-crystals 4 (BCC and TFQ) and 5 (BCC and TCQ) haveassembled in a unique edge-to-face stacking pattern through N(lp)···π interactions along with some other weak hydrogen bonding interactions. Co-crystals 1, 2 and 3 flauntsred, green and blue fluorescence emission colorsrespectively in the visible region, while Co-crystals 4 and 5 are almost completely non-fluorescent due to the presence of strong N(lp)···π interactions between constituent molecules which have been verified by a thorough NBO analysis of all the co-crystals. The thermal analysis


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indicates greater thermal stability of all the co-crystals than that of their corresponding

π-electron rich constituents with the sole exception of Co-crystal 4. Finally, we can conclude that the variations of molecular orientation in crystal packing leads to a vast diversity intothe nature and energetics of intermolecular interactions,which in turn has a pronounced effect on the tunability of their luminescence behavior and this understanding will help us to develop novel optical materials through co-crystal synthesis. ASSOCIATED CONTENT

Supporting Information.

Figure S1. Photographs of co-crystals taken under optical microscope showing morphological features Co-crystals (a) 1, (b) 2, (c) 3, (d) 4 and (e) 5. Figure S2a. ORTEP of Co-crystal 1 with one PHEN and one TFQ molecule in the asymmetric unit (50% ellipsoidal probability). Figure S2b. ORTEP of Co-crystal 2 with one BCC and one TCNB molecule in the asymmetric unit (50% ellipsoidal probability). Figure S2c. ORTEP of Co-crystal 3 with half PHNZ and half TCNB molecule (part of the molecules with


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green colored Carbon atoms) in the asymmetric unit (50% ellipsoidal probability). Figure S2d. ORTEP of Co-crystal 4 with one BCC molecule and half TFQ molecule (part of the molecule with violet colored Carbon atoms) in the asymmetric unit (50% ellipsoidal probability). Figure S2e. ORTEP of Co-crystal 5 with one BCC molecule and half TCQ molecule (part of the molecule with violet colored Carbon atoms) in the asymmetric unit (50% ellipsoidal probability). Table S1. Crystal Data and Structure Refinement. Table S2. Intermolecular Hydrogen Bonds and Other Interactions in Co-crystals. Figure S3. PXRD patterns of the co-crystals with experimental (red) and simulated (blue). Table S3. Melting points of Starting materials and their corresponding Co-crystals. Figure S4. Alignment of dipole moment vectors in the asymmetric unit of Co-crystal (a) 1, (b) 2, (c) 3, (d) 4 and (e) 5. Figure S5. FTIR Spectra of (a) Co-crystals 2 and 3 compared with TCNB, (b) Co-crystals 1 and 4 compared with TFQ and (c) Co-crystal 5 compared with TCQ. Figure S6. (a) White colored Phenanthrene and (b) yellow colored Tetrafluoro-1,4benzoquinone transforms into (c) intense red colored Co-crystal 1 powder upon mechanical grinding. Figure S7.Illustration of intermolecular interaction energies (IEs) of (a) Co-crystal 1, (b) Co-crystal 2, (c) Co-crystal 3, (d) Co-crystal 4 and (e) Co-crystal 5. Figure S8.


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Normalized solid-state absorption spectra of (a) Benzo[c]cinnolene and Co-crystal 4 and (b) Benzo[c]cinnolene and Co-crystal 5. Figure S9. Solid-state emission spectra of equimolar amounts of (a) BCC and Co-crystal 4 and (b) BCC and Co-crystal 5. Table S4. Solid-state Quantum Yield Values. Table S5. Second Order Perturbation Theory Analysis of Fock Matrix in NBO Basis.

Accession Codes CCDC 1834494–1834498 contain the supplementary crystallographic data for this paper.

These

data

can

be

obtained

free

of

charge

via

www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected] *Email: [email protected] Fax: +91-755-6692392.


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Fax: +91-755-6692392.

ORCID Deepak Chopra: 0000-0002-0018-6007

Apurba L. Koner: 0000-0002-8891-416X

Notes The authors declare no conflict of interest.

ACKNOWLEDGMENT

The authors thank IISER Bhopal for providing infrastructure and research facilities. RB and SB respectively thank IISERB and UGC for their doctoral fellowship. The authors also thank Ajmal Roshan Unniram Parambil and Ramesh Adakkattil for assisting in the design of the journal cover page.

ABBREVIATIONS


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PHEN, Phenanthrene; BCC, Benzo[c]cinnoline; PHNZ, Phenazine; TFQ, Tetrafluoro1,4-benzoquinone;

TCQ,

Tetrachloro-1,4-benzoquinone;

TCNB,

1,2,4,5-

Tetracyanobenzene; PL, Photoluminescence; NBO, Natural Bond Orbital.

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TABLE OF CONTENTS:


53


SYNOPSIS

Co-crystals

with

constituent

molecules

Page 54 of 54

assembling

through

π···π

interactions were fluorescent, while co-crystals with molecules assembling via lp···π interactions were non-fluorescent.


54

Exploring the Relationship between Intermolecular Interactions and

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