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Polymorphism of 2,5-Diphenyl-1,4-distyrylbenzene withTwo cis Double Bonds: The Essential Role of AromaticCH/Hydrogen Bonds

Zengqi Xie, Linlin Liu, Bing Yang, Guangdi Yang, Ling Ye, Min Li, andYuguang Ma*,

Key Lab for Supramolecular Structure and Materials of Ministry of Education, and

Department of Materials Science and Engineering, Jilin University, 2699 Qianjin Avenue,Changchun 130012, P. R. China

Received June 1, 2005; Revised Manuscript Received June 21, 2005

ABSTRACT: Three polymorphs of 2,5-diphenyl-1,4-distyrylbenzene with two cis double bonds (cis-DPDSB) havebeen prepared by altering the crystal growth conditions. The crystal data reveal that there are in total fourcrystallographically independent conformations of the molecule in the three polymorphs and the backbone of cis-DPDSB largely deviates from a plane with large torsion angles in each conformation. For the three-dimensionalconformations of the molecules, there is no face-to-face stacking in all polymorphs. The aromatic CH/hydrogenbonds, which are identified as the main driving force for the crystal packing with interaction distances about 2.7-3.1 , are the origin of the variable conformations and polymorphic formations during the crystal growth processes.Temperature-dependent IR spectra have shown that the intermolecular aromatic CH/hydrogen bonds influencethe aromatic C-H vibrations, and the interactions become slightly decoupled when the temperature is elevated.

Introduction

Polymorphism, which can be defined as the existenceof a particular compound in more than one crystalstructure, is a widespread phenomenon,1 and it hasattracted more attention because several packing ar-rangements or various conformations can result insignificant changes of the physical properties of com-pounds.2 Thus, a common strategy for the optimizationof the physical properties of crystalline organic materi-als is to alter the supramolecular arrangement at themolecular level. Although most attention has focusedon crystal polymorphism of pharmaceutical compounds,3

this phenomenon is of great significance to aromatic-conjugated photoelectronic materials.4 Important prop-erties of organic semiconductors, such as charge-transfermobility, conductivity, on/off ratio, and luminescenceefficiency, are all affected by the change in crystal latticeand morphology. For example, in the case ofR-quater-thiophene (R-4T),5 R-sexithiophene (R-6T),6 and penta-cene,7 polymorphism is responsible for the varyingphysical properties. Generally, the more closed contactsbetween adjacent molecules, the higher charge mobilityin a single crystal. In some luminescent materialsystems, such as trans-distyrylbenzene and p-terphenyl,theoretical studies reveal that the solid luminescence

efficiency is dominated by molecular packing modes, andas an expectation the J-aggregation and the verticalmolecules with vertical dipoles will have higher lumi-nescence efficiencies.8 For the urgent need of highcharge mobility in organic field-effect transistors (OFET)and high solid luminescence efficiency in organic light-

emitting devices (OLED), the study of the packinghabits of these materials becomes more and moreimportant.

The careful selection and modification of a supramo-lecular synthons directional and nondirectional forces,such as hydrogen bonding, electrostatic forces, -interactions, and CH/ hydrogen bonds, have becomeimportant tools in crystal engineering.9 Among theseinteractions, the CH/hydrogen bond is a weak attrac-tion between the C-H bond and the -system, whichexists widely in organic aromatic compounds.10 It hasbeen reported that the CH/ hydrogen bond plays acrucial role as a driving force of the crystal packing and

is important in terms of the conformation of organiccompounds.11 2,5-Diphenyl-1,4-distyrylbenzene with twocis double bonds (cis-DPDSB) is a model compound forphenyl-substituted poly(p-phenylene vinylene)s (P-PPVand DP-PPV), one of the most important conjugatedpolymers for light-emitting diodes.12 Our previous studyreveals that cis-DPDSB shows strong fluorescence in thecrystal despite torsion molecular conformation. Thesupramolecular interactions reduce the instability ofthis molecule at the excited state.13 In our continuedeffort to examine the packing mode of cis-DPDSB, werecently observed that this compound tends to adoptseveral packing modes in crystals. Herein we report thepolymorphism of cis-DPDSB and the role of aromaticCH/ hydrogen bonds on the crystal packing and thedifference among the crystallographically independentmolecular conformations. The influence of aromaticCH/ hydrogen bonds on the vibrations of aromaticC-H is also discussed.

Experimental Section

cis-DPDSB was synthesized, purified, and characterized asdescribed before.13 The single crystal of polymorph-r wasprepared by vaporizing mixed solvents of chloroform andethanol (1:2) slowly at room temperature. Single crystals of

* To whom correspondence should be addressed. Fax: (+86) 431-5168480. E-mail: ygma@jlu.edu.cn.

Key Lab for Supramolecular Structure and Materials of Ministryof Education.

Department of Materials Science and Engineering.

CRYSTAL

GROWTH

&DESIGN

2005VOL.5,NO.51959-1964

10.1021/cg0502441 CCC: $30.25 2005 American Chemical SocietyPublished on Web 07/23/2005

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polymorph- and polymorph- were obtained in a flask at onetime by vaporizing mixed solvents of chloroform and methanol(1:2) slowly at room temperature, and they were successfullyseparated manually for single-crystal X-ray diffraction experi-ments according to their different morphologies (polymorph-is in block form and polymorph- is in thin slice form). All thecrystal growth progresses were performed under rigorousexclusion of light.

X-ray Crystallography. The diffraction experiments werecarried out on a Rigaku R-AXIS RAPID diffractometer equippedwith a Mo KR and Control Software using the RAPID AUTOat 293 ((2) C. Empirical absorption corrections were appliedautomatically. The structures were solved with direct methodsand refined with a full-matrix least-squares technique usingthe SHELXS v. 5.1 programs,14 respectively. The space groupswere determined from the systematic absences, and their

correctness was confined by successful solution and refinementof structures. Anisotropic thermal parameters were refined forall the non-hydrogen atoms. The hydrogen atoms were locatedfrom difference maps and refined with isotropic displacement.

The powder diffraction was detected with a Rigaku X-raydiffractometer (D/max r A, using Cu KR radiation of wave-length 1.542 ). IR spectra were measured on a Bruker IFS-66V FT-IR spectrometer equipped with a liquid nitrogen-cooledMCT detector. The spectra were collected at 4 cm-1 resolution.To measure the IR spectra at elevated temperatures, a KBrplate mixed with the title compound was inserted into asample holder in the copper block, which had a heater within.The temperature controller was homemade and allowed atemperature stability of better than 1 C when the sample washeated to each scheduled temperature, which was kept at leastfor 5 min prior to IR measurements to establish thermal

equilibrium. The temperature was monitored with a thermo-couple connected with the sample holder.

Results and Discussion

cis-DPDSB is an oligophenylenevinylene (OPV) trimerwith two cis double bonds (Chart 1). We prepared thecrystals in different mixed solvents and obtained threepolymorphs. The single crystal of polymorph-r wasprepared by vaporizing mixed solvents of chloroform andethanol (1:2) slowly at room temperature under rigorousexclusion of light. However, the recrystallization of cis-DPDSB from a chloroform/methanol (1:2) solution yieldedtwo other concomitant conformational polymorphs (pol-

ymorphs- and -). All of the three polymorphs arecolorless but have different morphologies. The growthhabits are blocks for polymorph-r and - and prisms(thin slices) for polymorph-. The concomitant confor-mational polymorphs- and - were successfully sepa-rated manually for single-crystal X-ray diffraction ex-periments according to their different morphologies(Figure 1). The crystal data and structure refinementsof three polymorphs are tabulated in Table 1.

Crystal Structure of Polymorph-r. The crystalstructure of polymorph-r was found to be monoclinicwith the space group of P2(1)/c. Investigations of thecrystal structure reveal that weak intermolecular in-teractions are responsible for the observed self-assembly

in the lattice. As shown in Figure 2a, an aromatic CH/hydrogen bond I is formed between H14 and the centralphenyl ring of its adjacent molecule (the distance fromH14 to the central point of the phenyl ring is 3.00 and the angle of the C-H...ring is 154.6). Everymolecule connects with four adjacent molecules throughfour aromatic CH/hydrogen bonds, and as a result, atwo-dimensional plane is formed (Figure 2b). The pack-ing arrangement in a single crystal is shown in Figure3. It must be noticed that the end phenyl ring thatconnects to the double bond (for example ring A shown

in Figure 3) is inserted between the two adjacent rings(shown as ring B and C) of the other molecules, and thedistance between phenyl ring A and ring B (or ring C)is 3.61 , which means a weak - interaction existsand some intramolecular motions c.a. twisting of doublebond and twisting of the end phenyl ring that connectsto the double bond are not free. Both the aromatic CH/hydrogen bond and the weak -interaction make themolecule more rigid and stable in the crystal lattice.

Crystal Structure of Polymorph-. The crystalstructure of polymorph- is found to be triclinic withthe space group ofP1h. Unlike in polymorph-r, only thearomatic CH/ hydrogen bond becomes the main su-pramolecular interaction in the crystal lattice. The C7

Chart 1. Molecular Structure ofcis-DPDSB

Figure 1. Photographs of polymorph- and polymorph-under natural light.

Table 1. Crystal Data and Structure Refinements ofThree Polymorphs

identification code polymorph-r polymorph- polymorph-

empirical formula C34 H26 C34 H26 C34 H26formula weight 434.55 434.55 434.55T/K 293(2) 293(2) 293(2)crystal system monoclinic triclinic triclinicspace group P2(1)/c P1h P1ha/ 10.078(4) 7.5099(7) 5.4867(3)b/ 13.220(10) 9.1405(11) 12.864(8)

c/ 9.756(8) 10.2177(13) 17.893(9)R/ 90 72.744(3) 97.670(6)/ 111.36(3) 72.976(3) 90.915(1)/ 90 74.679(5) 96.004(3)V/3 1210.6(14) 628.27(12) 1244.2(1)Z 2 1 2density/Mg/m3 1.192 1.149 1.160 (Mo KR) /mm-1 0.067 0.065 0.065 range/ 2.66-25.21 2.38-27.48 1.15-27.48reflns collected 3110 2641 5280independent reflns 1956 2641 5280reflns observed 623 1857 1828R(int) 0.0998 0.0000 0.0000GOF 0.789 1.055 0.971R1 [I> 2(I)] 0.0538 0.0506 0.0646wR2 [I> 2(I)] 0.0603 0.1508 0.0887R1 (all data) 0.1907 0.0731 0.2091

wR2 (all data) 0.0804 0.1679 0.1086final diffFmax(e -3)

0.166, -0.195 0.220, -0.246 0.143, -0.181

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(and C7A) of every molecule acts as a proton donor andthe end phenyl ring along the p-terphenyl direction actsas a proton acceptor to form an aromatic CH/hydrogen

bond II (Figure 4a). The distance from proton to theacceptor phenyl ring is 2.71 , which means the at-tracting force of the aromatic CH/ hydrogen bond IImay be helpful to limit the torsion vibration of the endphenyl ring along the distyrylbenzene direction andmake the molecule rigid in the crystal. One moleculeconnects with two adjacent molecules through fouraromatic CH/ hydrogen bonds, and then a one-dimensional supramolecular synthon is formed alongthe b-axis (as shown in Figure 4b). On the basis of the

Figure 2. (a) The schematic of the aromatic CH/hydrogenbond I in polymorph-r. (b) The two-dimensional supramolecu-lar plane formed in polymorph-r through aromatic CH/hydrogen bonds (dotted lines). The molecules are drawn indifferent colors for clarity.

Figure 3. Crystal packing image of polymorph-r (viewedalong the a-axis). The hydrogen atoms have been omitted forclarity. Ring A is inserted between the two adjacent rings (Band C) of other molecules and then some intramolecularmotions c.a. twisting of double bond and twisting of the endphenyl ring that connect to double bond are not free.

Figure 4. (a) The schematic of the aromatic CH/hydrogenbond II in polymorph-. (b) The one-dimensional supramo-lecular synthon formed in polymorph- through aromatic CH/hydrogen bonds (dotted lines).

Figure 5. (a) The schematic of the aromatic CH/hydrogen

bonds III and IV in polymorph-. (b) The one-dimensionalsupramolecular synthon formed in polymorph- through aro-matic CH/hydrogen bonds. The molecule Ais drawn in blackand molecule B is in gray.

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inspection of the crystal structure of polymorph-, thereis no - interaction in the crystal packing.

Crystal Structure of Polymorph-. The crystalsystem of polymorph- is also triclinic with the samespace group ofP1h as polymorph-. The main structuralfeature of the packing is that there are two crystallo-graphically independent conformational molecules (Aand B, Z ) 2) in the crystal lattice. There are twodifferent aromatic CH/ hydrogen bonds (III and IVshown in Figure 5a) formed between the two indepen-dent conformational molecules. Interaction III is formedbetween the H7 of molecule Aand the end phenyl ringof molecule B along the distyrylbenzene direction witha distance of 3.09 . When the C16 of molecule B actsas proton donor and the end phenyl ring of molecule Aalong the p-terphenyl direction, the aromatic CH/hydrogen bond IV is formed effectively. The aromaticCH/hydrogen bonds play a role as a driving force ofthe crystal packing and cause the different molecularconformations in the same crystal. Every molecule A(or B) connects with two adjacent B (or A) moleculesthrough aromatic CH/hydrogen bonds, and similar topolymorph-, a one-dimensional supramolecular syn-thon is formed as shown in Figure 5b. Also, there is no-interaction between the adjacent molecules in thecrystal.

As discussed above, the aromatic CH/ hydrogenbonds exist in all three prepared polymorphs, and theyare summarized in Table 2. From the analysis of the

different structures of the polymorphs, it is obvious thatthe aromatic CH/hydrogen bonds play an importantrole in the crystal formation as a driving force tostabilize the crystals. Because of the weakness of thearomatic CH/ hydrogen bonds, polymorphism easilyoccurs during the crystal growth process. For theconcomitant conformational polymorphs- and - (with-out conventional hydrogen bonds and -interactions,mainly CH/ hydrogen bonds in the crystals), thedensity and enthalpy differences should be small. Afterthe nucleus forms, kinetic factors play an important rolein the crystal growth. So different polymorphs can existconcomitantly. From Table 2, we note that all of thedistances from the protons to the center of the acceptor

rings are in the range of 2.7-3.1 , and all the anglesof the C-H...ring center are over 140. On the basis ofthe distances and angles, we conclude that the aromaticCH/hydrogen bond II is the strongest one of the fourinteractions.

Role of Aromatic CH/ Hydrogen Bonds onMolecular Conformations. There are in total fourcrystallographically independent molecular conforma-tions in polymorph-r, polymorph-, and polymorph-.In every conformation, there are large torsions bothalong the distyrylbenzene direction and the p-terphenyldirection, which means the backbone of cis-DPDSBlargely deviates from a plane and cofacial stackingbecomes impossible. The selected dihedral and torsion

angles are tabulated in Table 3. When comparing theconformations of four cis-DPDSB molecules in the threepolymorphs, some differences can be found in terms ofthe bond lengths and torsion angles. As we can see theangles are affected more or less by different packingmodes, and when the crystal structures are inspected,the conformational difference is obviously correlatedwith the aromatic CH/hydrogen bonds. For example,the attractive force of the aromatic CH/hydrogen bondII in polymorph- reduces the torsion angle of C4-C5-C6-C11 (2 ) 27.5) relative to the same angles in otherpolymorphs (the largest torsion angle is 44.2 in molec-ular B of polymorph-). The differences in the indepen-dent molecules caused by such weak supramolecularinteractions indicate the transformable conformationsof cis-DPDSB under different conditions. We also canconclude that the aromatic CH/hydrogen bonds playan important role in the conformation ofcis-DPDSB indifferent polymorphs.

Table 2. Aromatic CH/Hydrogen Bonds in Three Polymorphsa

aromatic CH/hydrogen bond I (in polymorph-r) II (in polymorph-) III (in polymorph-) IV (in polymorph-)

H...ring 3.00 2.71 3.09 3.04C...ring 3.86 3.67 3.99 3.89angle of C-H...ring 154.6 161.7 163.1 145.0

aDistances are reported in angstroms, and the angles are reported in degrees.

Table 3. Selected Dihedral and Torsion Angles (deg) inDifferent Conformational Molecules of Three

Polymorphs

polymorph-r polymorph- polymorph- A polymorph- B

1 43.1 45.8 36.7 40.52 38.4 27.5 43.5 44.23 54.8 55.9 49.6 52.1

Figure 6. Powder X-ray diffractometric patterns of thecalculated data from the unit cell structure of the threepolymorphs (top three traces) and the experimental data ofthe microcrystals deposited in methanol (bottom trace). Theexperimental data are consistent with the calculated data ofpolymorph-, which indicates the microcrystals are purepolymorph-.

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Influence on the Aromatic C-H Vibration ofCH/ Hydrogen Bonds in Polymorph-. As theclassic hydrogen bond, measurement of the C-H stretch-ing frequency can suggest the existence of C-H groupsas the hydrogen donors and the -group as the hydrogenbond acceptor.15 Usually lower frequency shifts will befound when CH/hydrogen bonds are formed, but theincreased frequency is also observed when there is anortho substituent at a phenyl group for steric interac-

tions.16

The vibrational intensity of hydrogen bonds areall sensitive to the temperature,17 and then it is feasibleto detect the influence of aromatic CH/hydrogen bondson the aromatic C-H vibrations by temperature-de-pendent IR spectra.

When we dropped the saturated THF solution of thetitle compound into stirred methanol, microcrystals ofpure polymorph- were obtained easily, which is con-firmed by wide-angle X-ray diffraction as shown inFigure 6. We recorded the vibrational spectra of poly-morph- by FT-IR at different temperatures to detectthe influence of CH/hydrogen bonds on the aromaticC-H vibrations. First, we elevated the temperaturefrom room temperature to 160 C, which is below the

melting point (192.5 C). Then the temperature wasdecreased step by step until 50 C. The sample was keptconstant at least 5 min prior to IR measurements foreach scheduled temperature to establish thermal equi-librium. Figure 7 shows the spectral changes of severalabsorption bands that correspond to the aromatic C-Hvibrations. The characteristic absorptions of the aro-matic C-H vibrations are at 3022, 1073, and 764 cm-1,which are assigned to the stretching vibration, the

deformation vibration, and out-of-plane bending vibra-tions of aromatic C-H, respectively.18,19 Intensity of thestretching mode increases with the reduced tempera-ture, and the absorption of deformation, and out-of-plane bending modes shift more or less to the higherfrequency with increased intensity. All the changesexhibit the classic characteristic of the formation of ahydrogen bond.20 That means when the temperature isdecreased the aromatic CH/hydrogen bonds becomestronger and stronger. Furthermore, the changes in IRabsorption bands in the reverse direction are observedas sample is heated again, reflecting a reversible tem-perature dependence of CH/hydrogen bond formation.

Conclusion

cis-DPDSB is a nonplanar conjugated molecule, whichdoes not allow cofacial packing. In this case, therelatively weak aromatic CH/ hydrogen bonds areimportant in crystal formation as a driving force tostabilize the crystal. For the variable conformation ofcis-DPDSB and the weakness of aromatic CH/hydro-gen bonds, the energy difference between differentpolymorphs may be ignored, which induces the multiplepacking modes ofcis-DPDSB. In every polymorph, thearomatic CH/hydrogen bonds have different effects onthe molecular conformations, which confirms the sig-nificance of such weak supramolecular interactions in

organic aromatic compounds. Also the intermoleculararomatic CH/hydrogen bonds influence the aromaticC-H vibrations, and the interactions become slightlydecoupled when the temperature is elevated. The casethat only weak aromatic CH/ hydrogen bonds exist,without strong - interactions in the crystal, may beuseful to enhance the solid luminescence efficiency andproduce better mobility properties.

Acknowledgment. We are grateful for financialsupport from National Science Foundation of China(Grant No. 20474024, 20125421, 90101026, 50473001)and by Ministry of Science and Technology of China(Grant No. 2002CB6134003) and PCSIRT.

Supporting Information Available: X-ray crystallo-graphic information files (CIF files) for three polymorphs ofcis-DPDSB are available free of charge via the Internet athttp://pubs.acs.org.

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Spectroscopy; Plenum Press: New York, 1964; pp 260-277.(19) The IR spectra of cis-DPDSB are simulated using the

Gaussian98w program package for windows on PC. Both

geometry optimization and actual frequency calculation areperformed with a DFT/B3LYP method at the level of6-31G** basis set. The calculated frequencies are correctedby a scale factor of 0.9613, which agrees well with the exper-imental results. In addition, we use the HyperChem 7.1 soft-ware to analyze vibrational spectra, and all normal modesare identified and confirmed. The absorption band at 3015cm-1 in cis-DPDSB is due to the stretching vibration of C-Hin cis-vinylene groups, and the absorption band at 3022 cm-1

is due to the stretching vibration of aromatic C-H.(20) Wu, Y. Q.; Hao, Y. Q.; Li, M.; Guo, C. W.; Ozaki, Y. Appl.

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CG0502441

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