J. MATER. CHEM., 1991, 1(6), 1057-1059 Studies of Non-linear Optical Organic Materials: Crystal and Molecular Structure of 2-Dicyanomethylene-l,3-dioxolane Parthasarathi Dastidar, Tayur N. Guru ROW*and Kailasam Venkatesan Department of Organic Chemistry, Indian Institute of Science, Bangalore-560 012, India The structure of 2-dicyanomethylene-1 ,Sdioxolane has been determined from X-ray diffraction data. The compound crystallizes in the chiral space group Cc and has two molecules in the asymmetric unit. The correlation between the molecular packing and the second-harmonic generation (SHG) is discussed. Keywords: X-Ray Diffraction; Non-linear optical material; Second-harmonic generation The search for new organic materials with non-linear efficiency surpassing those existing has been pursued extensively in recent years.' As part of a research programme on developing good non-linear optical materials and to gain insight into the relation between the macroscopic non-linear susceptibility x(2) and the molecular packing in the unit ell,^-^ the crystal structure of the title compound has been determined.The necessary conditions to be satisfied for showing SHG are: (i) the space group must be non-centrosymmetric, (ii) the differ- ence between the ground-state dipole moment and the excited dipole moment should be large and (iii) the molecule should have loosely bound electrons that can be displaced by an optical field. Experimental The title compound was prepared by the reaction of tetra- cyanoethene and ethylene glycol in the presence of urea.5 A pale-yellow, needle-shaped crystal formed by slow evapor- ation of an alcohol-water solution, with dimensions 1.0 mm x 0.5 mm x 0.5 mm was used for X-ray diffraction measurements. Preliminary Weissenberg photographs showed it to be a C-centred monoclinic system.The lattice parameters were refined by using 25 reflections with 20<8/"<40 from graphite-monochromated Cu-Ka radiation (A= 1.5418 A) on a Nonius CAD-4 diffractometer. Intensity data of the 1252 reflections were collected at 293 K in the 01/28 scan mode with 8 between 0 and 60". Three standard reflections (1 T 4, T 3 2, 2 0 6) were monitored after every 100 reflections and showed no significant decomposition or movement of the crystal. Corrections were made for Lorentz effects and polariz- ation but not for absorption effects.The structure was solved by direct methods (SHELX86).6 The best E-map gave all non- hydrogen atoms of both the molecules in the asymmetric unit. Full matrix least-squares refinements (SHELX76)7 were per- formed for positional parameters and anisotropic thermal parameters. All the hydrogen atoms were located by a differ- ence Fourier map. However, they were not refined but their contributions to the structure-factor calculation were con- sidered. The final R and R, were 0.037 and 0.045, respectively, for 1105 significant reflections [IF,[ 2 34 lFol)];the weighting scheme was w = 1.0/[a2(lFoI)+0.0017 lF,12]. The atomic scat- tering factors were taken from ref.8. Calculations were performed on a DEC1090 computer. Results and Discussions The C(2)=C(3) and C(2')=C(3') distances [1.369(4) and 1.356(4) A, respectively] are longer than that of the C=C bond in ethene [1.336(2) A].9 There is a corresponding reduction in the length of donor C,,2-0 and acceptor Csp2-CSp bonds: O(1)-C(3), 0(2)-C(3), O(l')-C(3'), 0(2')-C(3') distances are 1.299(3), 1.309(4), 1.309(4) and 1.313(4) A,respectively, and are shorter than the distance [1.354(16) A] reported for the C,,Z-O bond." On the acceptor side, the distances C( 1)-C(2), C(6)-C(2), C(1')-C(2') and C(6')-C(2') are 1.406(5), 1.409(6), 1.420(4) and 1.404(5) A, shorter than the distance of 1.431(14) A reported for Csp2-CSp bond." These values show that delocalization of IT electrons occurs in the title molecule.The contribution of the microscopic hyperpolarizability to macro- scopic non-linear susceptibility f2) depends on the packing of the molecules in the crystalline state. From the nature of the approximate molecular symmetry, namely mm2 with the two-fold axis coinciding with the ethenic C(2)=C(3) bond, it is reasonable to assume that the charge-transfer axis coincides with this ethenic bond. The angle between the crystallographic b axis and the charge-transfer axis for both the molecules in the asymmetric unit is 28.4" and differs significantly from the theoretically expected value of 54.7" for optimal molecular orientation and phase-matching configuration for the point group m." Furthermore, the charge-transfer axes for both the molecules in the asymmetric unit make an angle of 6 1.7"with the crystallographic mirror plane.As this value is relatively close to 90" it is not beneficial for constructive addition of the molecular hyperpolarizability (p) to the macroscopic second-order electronic susceptibility f2) because reflection about the mirror plane would cancel the molecular hyperpola- rizability. We observe that the molecular packing is unfavour- able for SHG efficiency. The second-harmonic generation for one sample of a powdered specimen of the title compound is only twice that of urea,I2 although with a different powdered sample the value was as low as half that of urea.13 This highlights the limitation of the powder SHG method.The low SHG efficiency could be partly due to the strength of the donor and acceptor groups at the vicinal carbons of the ethenic bond being not so good as discussed earlier. It is obvious that although in both the molecules the observed C=C bond length is significantly larger than that of ethene, these distances are not as large as in other push-pull ethenes with -NMe2 and -C02Me as donor and acceptor groups substituted at the vicinal carbon atom^.'^.'^ However, we have no knowledge of the value of the difference between the ground-state dipole moment and excited-state dipole moment of the title molecule upon which the magnitude of the second- order molecular hyperpolarizability depends.'' It seems clear from the above discussions that the low SHG efficiency arises from the unfavourable molecular packing and perhaps to a lesser degree on the poor donor and acceptor strength. Note that the occurrence of two molecules in the asymmetric unit in this crystal is by no means conducive for achieving good SHG. An interesting question is whether the SHG efficiency would be larger if there were only one molecule in the asymmetric unit. It is reasonable to expect the SHG to be larger with one molecule in the asymmetric unit, but the practical realization of achieving it presents the problem of crystal engineering, central to the design of good non-linear optical materials. Crystal Data C6H4N202, M = 136.0, monoclinic, a =5.288( 1) A, b = 15.044( 1) A, c = 16.356( 1) A, fl= 99.23( 1)’.I/= 1284.4(2) A3 space group Cc, 2=8, pc= 1.406 g cm -3, p(Cu-Ka, A = 1.54 18 A)=8.33 cm-’. The final atomic coordinates with their estimated standard deviations are given in Table 1. The numbering scheme is shown in Fig. 1 and the packing of the molecules viewed Fig. 1 ORTEP plot of a single molecule of title compound with numbering scheme. The other molecule in the asymmetric unit is numbered in the same way Table 1 The fractional atomic coordinates (x lo4) of non-hydrogen atoms of the title compound with their e.s.d.s in parentheses atom X Y z 2248 5788(1) 3527 -229(5) 6443(1) 4302(1) 11 l(7) 41 32(2) 3716(2) -595( 7) 4896(2) 4121(2) 477(6) 5701(2) 3988(2) 2935(8) 6723(2) 3508(3) 1234(8) 7 I 7 l(2) 4007(2) -2476(8) 4827(2) 4638(2)631(9) 3523(2) 3356(2) -4020(9) 4761(3) 5053(2) 1399l(5) 6712(1) 701 l(1) I0734(5) 6056(1) 6231(1) 11670(8) 8 362( 2) 6826( 2) 1053 7( 7) 7596(2) 641 l(2) 11741(6) 6802(2) 6549(2) 14707(7) 5778( 2) 7024( 3) 12476(8) 5322(2) 6528(2) 8 159( 8) 7672(2) 5893(2) 12555(9) 8976(2) 7 172(2) 6 194(8) 7742(3) 5486(2) J.MATER. CHEM., 1991, VOL. 1 7 Fig. 2 Packing of the molecule of the title compound in the unit cell viewed down the a axis Table 2 Bond distances and angles involving non-hydrogen atoms of the title compound with their e.s.d.s in parentheses atoms distance/A atoms distance/A molecule I molecule I1 O(1)-C(3) 1.299(3) O(1’)-C( 3’) 1.309(4) 0(1)-C(4) 1.455(3) O(I ’)-C(4‘) 1.456(3) 0(2)-C(3) 1.308(4) O(2’)- C( 3’) 1.313(4)w-C(5) 1.466(4) O(2’)-C(5’) I .469(4) C(1)-C(2) 1.406(4) C( l‘)-C(2’) 1.420(5) C(11-N 1) 1.146(5) C( 1’)-N(1‘) 1.144(5) CW-C(3) I .369(4) C( 2’) -C( 3’) 1.356(4) C(2)-C(6) I .409(5) C(2’)-C(6‘) 1.404(5) C(4)- (35) 1.471(6) C(4)-C(5’) 1.488(5) C(6)--N(2) 1.147(6) C(6)- N( 2’) 1.146(6) atoms angle/” atoms angle/” molecule I molecule I1 C(3)-O( I)-C(4) 108.4(2) C(3’)-O( l’)-C(4) 108.5(2) C( 3)-O( 2) -C(5) 108.1(2) C( 3’)- O(2’)-C(5’) 108.6(2) C(2)-C(l)-N(l) 177.2(4) C(2’)-C(l’)-N(l’) 178.6(4) C(I)-C(2)-C(3) 120.0(3) C( lF)-C(2‘)-C(3’) 119.0(3) C(l)-C(2)--C(6) 119.3(3) C( l’)-C(2’)-C(6) 119.4(3) C( 3)-C(2) -C(6) 120.7( 3) C(3’)-C(2’)- C(6’) 12 1.6( 3) O(I)-C(3)-0(2) 114.7(3) O(l’)-C(3’)-0(2’) 114.4(3) O(1)-C(3)-C(2) 122.5(3) O(I’)-C(3‘)-C(2’) 123.1(3)O(2)- C( 3)- C( 2) 122.8(3) O(2’)-C( 3’)-C(2’) 122.5( 3) O(1)-C(4)-C( 5) 104.7( 3) O(1’)-C(4)-C(5’) 104.8( 3) 0(2)-C( 5)-C(4) 104.0(3) O(2’)- C( 5’)-C(4) 10333) C( 2)- C( 6)-N( 2) 179.1 (4) C( 2’) -C( 6’) -N( 2) 178.4(4) down the a axis is shown in Fig.2. Bond distances and angles appear in Table 2.7- There are two independent molecules in the asymmetric unit. Both the molecules have similar conformations. The five- membered ring in both the molecules is in an envelope form. The deviation of C(5) in molecule I from the plane containing C(3), 0(1), O(2) and C(4) is 0.035(3) A and that of C(5’) in molecule I1 from the corresponding plane is 0.053(3) A.The deviation of C(2) [0.016(3) A] from a plane defined by C(1), C(3), C(6) and that of C(2’) [0.016(3) A] from a plane defined by C( 1’), C(3’), C(6’) indicates slight pyramidality. Deviations of atoms N(l) and N(2) from the least-squares plane defined by C(1), C(2), C(3) and C(6) are 0.071(3) and 0.025(3) A, respectively, and those of N(1’) and N(2’) from the plane defined by C(l’), C(2’), C(3’) and C(6’) are equal [0.045(3)A]. We also observe non-linearity in C-CN groups (Table 2).169” There are intermolecular short contacts between N and C involving both the independent molecules: C(4) N(l)= 3.089 8, and C(4) -.* N( 1’) =3.090 A, but C-H .-.N hydrogen bonds ?Supplementary data available from the Cambridge Crystallo- graphic Data Centre: see Information for Authors, J.Muter. Chem., 1991, Issue 1 or 4. J. MATER. CHEM., 1991, VOL. 1 1059 are not observed. If we accept 3.4 A as the van der Waals distances for C N with no acid-base character,I8 then the observed C N contacts indicate a fairly strong acid-base interaction. It is found that angles 0(1)-C(4) ... N(1) and O(1’)-C(4’) -.. N(1’) are ca. 166” whereas C(5)-C(4) N(l)and C(5’)-C(4’) ... N(1’) are ca. 89”. Furthermore, the devi- 4 5 6 7 D. Kanagapushpam, K. Venkatesan and T. S. Cameron, Acta Crystallogr., Sect C, 1988, 44,337. W. J. Middleton and V. A. Englehardt, J. Am. Chem. SOC., 1958, 80, 2788. G. M. Sheldrick, SHELX86, program for crystal structure solu- tion, Gottingen University, Germany. G. M. Sheldrick, SHELX76, program for crystal structure deter- ations of N(l) and N(1’)from the planes C(5), C(4), O(1) and C(5’), C(4’), O(1’) are 0.27 and 0.24 A.The calculated values indicate that the observed short C N contacts could be due to intermolecular interaction between the nitrogen lone pair and the C-0 (o*)antibonding orbital. 8 9 10 mination, University of Cambridge. International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, 1974, vol. IV, pp. 202-207. L. S. Bartell, E. A. Roth, C. D. Hollowell, K. Kuchitsu and J. E. Young Jr., J. Chem. Phys., 1965,4, 2683. F. H. Allen, 0. Kennard, D. G. Watson, L. Brammer, A. G. Orpen and R. Taylor, J. Chem. SOC., Perkin Trans. 2, 1987, 1. We thank Drs. D. F. Eaton and Y. Wang, Du Pont, Willming- ton, for SHG measurements and the University Grant com- 11 12 J.Zyss and J. L. Oudar, Phys. Rev. A, 1982,4, 2028. J. F. Nicoud and R. J. Twieg, in Nonlinear Optical Properties of Organic Molecules and Crystals, ed. D. S. Chemla and J. Zyss, mission and Council of Scientific and Industrial Research, India for financial support. 13 14 Academic Press, New York, 1987, vol. 2. D. F. Eaton and Y. Wang, personal communication. D. Kanagapushpam and K. Venkatesan, Acta Crystallogr., Sect. C, 1988, 44,337. References 15 D. Adhikesavalu, Nirupa U. Kamath and K. Venkatesan., Proc. Indian Acad. Sci. (Chem. Sci.), 1983, 92, 449. 16 N. Ramasubbu, J. Rajaram and K. Venkatesan, Acta Crystallogr., 1 Non-linear Optical Properties of Organic and Polymeric Materials, Sect. B, 1982, 38, 196. ed. D. J. Williams, ACS Symp. Ser., American Chemical Society, 17 D. A. Mathews, J. Swanson, M. H. Mueller and G. D. Stucky, Washington D.C., 1983. J. Am. Chem. SOC., 1971,93, 5945. 2 D. Kanagapushpam and K. Venkatesan, Acta Crystallogr., Sect. 18 J. R. Witt, D. Britton and C. Mahon, Acta Crystallogr., Sect. B, C, 1987,43, 1597. 1972, 28, 950. 3 D. Kanagapushpam, K. Padmanabhan and K. Venkatesan, Acta Crystallogr., Sect. C, 1987, 43, 1717. Paper 1/03268B; Received 1st July, 1991