Strong optical second-harmonic generation in a chiral diaminodicyanoquinodimethane system M. Ravi," D. Narayana Rao,b Shmuel Cohen,c Israel Agranatd and T. P. Radhakrishnan"" "School of Chemistry, University of Hyderabad, Hyderabad-500 046, India bSchool of Physics, University of Hyderabad, Hyderabad-500 046, India 'Department of Inorganic Chemistry, The Hebrew University of Jerusalem, Jerusalem-91 904, Israel dDepartmentof Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem-91 904, Israel A series of amine donor-substituted dicyanoquinodimethane molecules with large calculated hyperpolarizabilities has been investigated. Molecular and crystal structure details of a prototypical system are presented. Moderate to strong phase-matched powder second-harmonic generation is observed in chiral amine-substituted compounds. The synthesis of molecules with large hyperpolarizability values, jl,and the fabrication of non-centrosymmetric crystals for quadratic non-linear optical (NLO) applications like second-harmonic generation (SHG),electro-optic modulation and optical parametric oscillation are fascinating problems in molecular materials chemistry.' Although a number of mol- ecules have been developed in recent years, very few with large p values form suitable non-centrosymmetric crystal lattices having large quadratic electric susceptibilities. Transparency in the visible range, thermal and chemical stability and phase- matchability of the SHG, which are desirable for device applications, are also not obtained in many cases.Although push-pull quinonoid systems are potential candi- dates for NLO materials, we are not aware of any demon- stration of strong SHG in crystalline materials based on such molecules. Amine-substituted dicyanoquinodimethane mol-ecules, first synthesised by a du Pont group,, are an attractive class of substrates towards this goal.3 The prototypical mol- ecule, 2-(4-dicyanomethylenecyclohexa-2,5-dienylidine)imida-zolidine [R,, R,=NH(CH,),NH in Fig. 13 was reported4 to have a very large p value of (-240+60) x esu at an excitation energy of 1.17 eV. Some related systems (unpub- Ncm;:NC nR' -N WNH H* R' = -N , R2= -N-CH-Naph3 I methylpyrrolidine was added and the solution was stirred at CH3 50 "C for 10 min.After standing for 5 h at 30 "C, the solution was cooled to 10°C and filtered to obtain 90% yield of the 6 light green compound 6,which was recrystallized from aceto- nitrile as pale yellow crystals. The relevant IR vibrational frequencies, elemental analytical data and crystal structure or mass spectral data for compounds Fig. 1 Molecules considered in this study 1-6 are provided below. J. Muter. Chem., 1996,6(7),1119-1122 1119 lished) have been mentioned in Appendix I of ref. 1. Tricyanoquinodimethane zwitterionic structures deposited as Z-type Langmuir-Blodgett films have been shown to produce strong SHG.' The crystal structures of the zwitterionic chromo- phores and new synthetic procedures for their preparation have also been reported.6 Semi-empirical quantum chemical calculations indicated that amine-substituted dicyanoquinodimethanes have large molecular hyperpolarizabilities. The calculations also indicated zwitterionic ground states with appreciable dipole moments.We have synthesised substituted dicyanoquinodimethanes (Fig. 1) with a variety of amine donors; R1=R, =pyrrolidinyl (1), and piperazinyl (2); R, =pyrrolidinyl with R,=S-a-methylbenzylamino (3), R-a-methylbenzylamino (4), S-naph-thylethylamino (5) and S-2-methoxymethylpyrrolidinyl(6).We have also prepared derivatives with R, =R, =piperidinyl, mor- pholinyl, p-toludinyl, etc., but these will not be discussed in this paper. We have investigated the powder SHG efficiency of these materials and report here the observation of moderate activity in 3 and 4 and strong activity in 6.Crystal structure determinations indicated centrosymmetric lattices in 1 and 2 and non-centrosymmetric lattices in 3 and 6.Molecular and crystal-structure characterisation of 3 and preliminary struc- tural data on 6 are presented. We believe this to be the first case of crystalline quinonoid systems which satisfy several of the requirements for useful device materials noted above.Experimental Syntheses Compounds 1 and 2were prepared using the general procedure reported in ref. 2 and 3-6 were prepared using an extension of this procedure. The following details of the synthesis of 6 provide an illustrative example. 7-P yrrolidino-7,8,8-tricyanoquinodime t hane was prepared as described in ref.2. To a warm solution of 0.10 g of this compound in 20 ml tetrahydrofuran, 0.04 ml S-( +)-2-methoxy- 7,7-Dipyrrolidino-8,&dicyanoquinodimethane, 1. Ref 2 reported the preparation of this compound FTIR (KBr pellet) v/cm-l 2175, 2130 (conjugated nitnle), 1595, no N-H vibration Elemental analysis Found (Cak for C18H2oN4) C, 74 35 (73 94), H, 6 82 (6 89), N, 18 72 (19 17) Molecular and crystal structure detFrmined space group P21/c, a= 16 473, b=8481, c=25383A, a=900, p=l052, y=899", R=O046, Rw=0081 7,7-Dipiperazino-8,8-dicyanoquinodimethane,2. FTIR (KBr pellet) v/cm-' 3150 (N-H stretch), 2180, 2140 (conjugated nitnle stretch), 1595 Elemental analysis Found (Calc for C18H22N6) C, 66 31 (67 08), H, 6 82 (6 83), N, 25 39 (26 08) Molecular and crystal structureodetermined space group Pi, u=13372, b=15614, c=8756A7 a=993, p=1056, y=971", R=O034, Rw=0061 7-Pyrrolidino-7-[(S)-a-methylbenzylamino]-8,8-dicyano-quinodimethane (MBPDQ), 3.MS(E1) m/z 342(4), 327(4), 105(76), 91(76), 77(30), 70(100) FTIR (KBr pellet) v/cmP1 3387 (N-H stretch), 2174, 2128 (conjugated nitnle stretch), 1601cm-' Optical rotation, [a]k5-395 (c 004, MeOH) Compound 3 was recrystallized from dichloromethane for elemental analysis Found (Calc for C22H22N4 H20) C, 73 13 (73 33), H, 6 22 (6 66), N, 15 26 (15 55) Crystal structure details are provided in the next section 7-Pyrrolidino-7-[ (R)-a-met hylbenzylamino]-8,8-dicyano-quinodimethane, 4.MS(E1) m/z 342( 14), 327( 17), 105(52), 91(61), 77(23), 70(100) FTIR (KBr pellet) v/cm-' 3390 (N-H stretch), 2173, 2128 (conjugated nitrile stretch), 1601 Optical rotation, [a]h5 +240 (c 0 05, MeOH) Elemental analy- sis Found (Calc for C22H2,N4) C, 77 60 (77 19), H, 6 85 (6 43), N, 15 54 (16 37) 7-P yrr olidino-7-[(S)-naph th yle thylamino] -8,&dicyano- quinodimethane, 5. MS(E1) m/z 392( 16), 377( 16), 222(8), 155(64), 141(81), 97(14), 70(100) FTIR (KBr pellet) v/cm-' 3350 (N-H stretch), 2172, 2131 (conjugated nitnle stretch), 1597 Optical rotation, Calk5+69 (c 0 05, MeOH) Elemental analysis Found (Calc for C26H24N4) C, 79 60 (79 59), H, 6 42 (6 12), N, 13 85 (1429) 7-grITolidb7- [(S)-2-methoxymet hylpyrrolidino]-8,&yano-quinodimethane (PMPDQ), 6.MS(E1) m/z 336( loo), 222(8), 181(8), 167(13), 71(24) FTIR (KBr pellet) v/cm-l 1448 (CH, -0-CH, stretch), 2170, 2129 (conjugated nitnle stretch), 1599 Optical rotation, [a];' +1687 (c 0 08, CHC1,) Elemental analysis Found (Calc for C20H24N40) C, 71 34 (71 43), H, 741 (7 14), N, 16 65 (16 67) Crystal structure details are provided in the next section Techniques The IR and UV-VIS spectra were recorded on a Jasco-5300 FTIR spectrometer and a Jasco-7800 spectrophotometer, respectively Powder SHG measurements were carried out using the Kurtz-Perry powder techniqueg with the fundamental wave- length (1064 nm) of a Q-switched Nd YAG laser The powders were graded using standard sieves and packed between glass plates The sample thickness was maintained constant by means of uniform 02mm thick Teflon sheets inserted as spacers between the glass plates Study of the SHG intensity as a function of particle size indicated that 3-6 were all phase- matchable matenals The materials showed no sign of decomposition even on prolonged irradiation with a laser power of 1 GW cmP2 (6 ns, 10 Hz) Crystal structure data were collected on an Enraf-Nonius CAD4 computer-controlled diffractometer Cu-Ka (A= 1120 J Muter Chem, 1996,6(7), 1119-1122 1 54178 A) radiation with a graphite crystal monochromator in the incident beam was used The standard CAD4 centnng, indexing and data collection programs were used The unit- cell dimensions were obtained by a least-squares fit of 24 centred reflections in the range 23 d8d 31 Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambndge Crystallographic Data Centre (CCDC) See Information for Authors, J Muter Chem, 1996, Issue 1 Any request to the CCDC for this material should quote the full literature citation and the reference number 1145/2 Results and Discussion To assess the potential of the diaminodicyanoquinodimethane systems for quadratic NLO applications, we obtained theoreti- cal estimates of their molecular hyperpolanzabilities Molecular geometries were optimised using the AM1 semi-empirical quantum chemical procedure The calculated geo- metnes were in reasonable agreement with the molecular structures obtained in the single-crystal studies descnbed below In particular, the calculations indicated strong out-of- plane twisting of the N-C-N donor groups The p values were evaluated using a sum-over-states method"? with all single and pair excitations within a manifold of twelve molecu- lar orbitals included in the configuration interaction scheme Table 1 gives the calculated ground-state dipole moments (pg),the changes in dipole moment for the lowest excitation states (Ap) and the ~,,,(O) values of the molecules 1-6 The pg values are high and indicate a strongly zwitterionic ground state Several excited states involve reverse charge-transfer leading to large, negative Ap values as seen in the case of the first excited state Stokes' shift studies based on our recently reported procedure" revealed negative solvatochromism of these molecules in support of this mechanism pveC(O)are the projections of the hyperpolarizability tensor on the major dipole axis, calculated for Jzm=O eV These static p values reported in Table 1 for 1-6 are quite large in view of the short conjugation lengths [for cornpanson, j3,,(0) of p-nitroani1inel0 is ca -9 x loP3' esu] This may arise from the large Ap contnbutions A detailed theoretical analysis12 carried out on the parent system 7,7-diamino-8,8-dicyanoquinodimethane indicates that the contribution of the first excited state to p is approximately 50% and the rest is made up of contributions from a large number of excited states, the usual p2 level approxi-mation is not quite applicable to these quinonoid molecules The Amx values of the electronic absorption bands of compounds 1-6 are all close to 400nm (Table 1) and the crystals appear light yellow or nearly colourless Therefore these materials are potentially useful for applications in the visible range where resonant absorption effects will be minimal t p values calculated using this program agree well with expenmental results, eg the calculated value of the molecule in ref 4 is -243 x 10 30 esu (ho=117 eV) Table 1 Calculated ground-state dipole moments, changes in dipole moment for the lowest excitation state and hyperpolanzabilities, expenmental absorption maxima in acetonitnle solution, melting points and powder SHG efficiencies (phase-matchable) relative to urea (ca 150 pm particles) of molecules 1-6 molecule pg/D Ap/D p,,(Op Lax/nm mp/"C SHG/U 1 141 -94 61 375 305 00 2 122 -151 60 415 300 00 3 125 -112 31 368 245 30 4 125 -112 31 368 245 32 5 132 -121 39 370 270 05 6 143 -183 66 398 290 275 "In units of esu, Calculated at fio=0 Melting points are all >245 'C (Table l), about twice those of the well known organic NLO materials.'7l3 This probably arises from the strongly zwitterionic nature of these molecules and the resultant electrostatic crystal forces.The thermal stability is relevant for poling processes and in enhancing the damage thresholds. Table 1 also provides the SHG activities of 1-6 relative to urea powder with particle sizes of ca.150 pm, which are discussed below. The dipyrrolidinyl (1)7 and dipiperazinyl (2)* derivatives gave centrosymmetric crystals (space groups P2Jc and Pi, respectively) and showed no SHG, as did the other achiral derivatives we prepared. The large pg values appear to encour- age centrosymmetric crystal-lattice formation. The chiral derivatives 3-6 were prepared with the intention of fabricating non-centrosymmetric crystal lattices. Single-crystal X-ray structure analysis of the transparent plate-like crystals of MBPDQ (3) obtained from acetonitrile solution indicated an acentric space group, P2', with two MBPDQ molecules and two solvent molecules in the unit cell. The crystallographic data are provided in Table 2.The molecu- lar structure and the unit cell view along the b axis are shown in Fig. 2. The N9-C7-N14 plane on the donor side is twisted out of the quinonoid ring plane by ca. 49.8'. This general feature is observed in all bis(dialky1amine)-substituted dicyano-quinodimethanes we have characterised structurally, and it arises from the steric repulsion between the ortho-H atoms on the quinonoid ring and the H atoms on the C atom attached to the N in the donor moiety. The bond lengths (Table 3) in the conjugation unit indicate a strongly benzenoid character resulting from the intramolecular charge transfer which is accentuated by the out-of-plane twisting of the donor group. The molecular alignments in the crystal are nearly head-to- tail, the non-centricity arising from the presence of the chiral carbon atoms alone.Powders of MBPDQ with particles of size 3150 pm showed a moderate SHG, ca. 3 times that of urea (Table 1). The particle-size dependence of the SHG inten- sity (Fig. 3) indicates that MBPDQ is a phase-matchable material. In support of this, the 3U SHG intensity is obtained in the crystals as well. The stereoisomer of 3 (i.e. 4) showed very similar linear and non-linear optical properties (Table 1); the specific choice of the configuration (R or S) therefore does not appear to be crucial. The naphthyl derivative, 5, was found to have a low SHG (Table 1). Owing to the presence of the chiral centres these systems are also expected to show non-centrosymmetric crystal lattices.However, we did not investigate their crystal structures, since their NLO properties did not show any improvement over those of 3. Since the chiral centres in 3, 4 and 5 are on a side chain, non-centrosymmetry may be attained by small bond rotations, rather than by reorientation of the molecular dipoles. Thus, deviation from a centric lattice would be small. To overcome this problem, we prepared the (S)-2- methoxymethylpyrrolidine derivative 6 (PMPDQ) where the Table 2 Crystallographic data for 3 molecular formula C22H22N4* CH3CN sp$ce group p2144 12.715( 3)bl+ 8.068 (2) CIA 10.422( 2) P/dFgrees 91.03(2) VIA3 1068.9(5) Z 2 Pcalclg cm -1.19 p (Cu-Ka)/cm-' 5.36 no. of unique reflections 2182 no.of reflections with 1220(Z) 2063 R 0.050 Rw 0.076 Fig. 2 (a)Molecular structure of 3 from single-crystal X-ray analysis; H atom of only the stereogenic centre is shown. (b)Stereoview of the unit cell of 3 along the b axis. Table3 Significant bond lengths in 3 from single-crystal X-ray analysis; the atom labellings are as shown in Fig. 2(a) bond bond length/A 1.393 1.383 1.400 1.406 1.378 1.398 1.467 1.447 1.308 1.343 1.402 1.403 1.146 1.153 chiral centre is part of one of the five-membered rings that form the donor moiety. Crystals of PMPDQ were grown from acetonitrile solution as prisms. Preliminary reflection data were indexed to the non- centric orthorhombic space group, P212121. However, structure refinement only converged to an R value of 0.1 1 (R, =0.15), though data from several crystals were tried.Our SHG studies corroborate the non-centric space group. Large intensities of ca. 25-30 U (Table 1)were obtained for powders with particle sizes 3300 pm and for crystals; the saturation of the SHG intensity at large particle sizes again indicated phase-match- ability (Fig. 3). The large SHG indicates improved alignment of the molecular dipoles; the unit-cell structure at the present state of refinement indicates near-neighbour molecules roughly J. Muter. Chem., 1996, 6(7), 1119-1122 1121 A 0 100 200 300 400 average particle size/pm Fig. 3 Powder SHG intensity (relative to urea particles of ca 150 pm size) at different particle sizes of 3 (0)and 6 (A) orthogonal to each other Note that the molecular twisting in PMPDQ (58 2") is higher than in MBPDQ The large R factor appears to result from disorders or single-bond rotations in the methoxymethyl side chain Conclusion The chiral amine donor-substituted quinonoid molecules reported in this paper open up a new class of easily synthesized compounds with moderate to strong solid-state SHG activity We note that although powder SHGs of up to 100OU have been reported in strongly coloured materials, and thin films capable of efficient SHG have been fabricated, among colour- less or lightly coloured crystalline organic compounds there are only a few which surpass the large SHG we find in this class of push-pull quinonoid systems The phase-matchability observed in these materials is particularly relevant and their thermal stability is superior to the well known crystalline materials showing strong SHG These quinonoid systems, which are mostly colourless or light yellow, are suitable candidates for the development of materials for visible-range applications Finally, the potential derivatives are innumerable and incorporation as pendant groups in polymer chains is possible We are pursuing these possibilities, as well as inclus- ion of these molecules in a variety of matrices for poling experiments We thank Dr J Chandrasekhar for providing the subroutine for hyperpolarizability calculations Financial support from the CSIR and the DST, New Delhi are gratefully acknowledged by M R and T P R respectively References Nonlinear Optical Properties of Organrc Molecules and Crystals, ed D S Chemla and J Zyss, Academic Press, New York, 1987 L R Hertler, H D Hartzler, D S Acker and R E Benson, J Am Chem SOC,1962,84,3387 M Ravi, D N Rao, S Cohen, I Agranat and T P Radhaknshnan, Curr Scz (India), 1995,68,1119 S J Lalama, K D Singer, A F Ganto and K N Desai, Appl Phys Lett, 1981,39,940 G J Ashwell, E J C Dawnay, A P Kuczynski, M Szablewski, I M Sandy, M R Bryce, A M Grainger and M Hasan, J Chem SOC Furaday Trans, 1990, 86, 11 17, G J Ashwell, G Jefferies, E J C Dawnay, A P Kuczynski, D E Lynch, Y Gongda and D G Bucknall, J Muter Chem ,1995,5,975 6 R M Metzger, N E Heimer and G J Ashwell, Mol Cryst Lzq Cryst, 1984,107, 133, J C Cole, J A K Howard, G H Cross and M Szablewski, Acta Crystallogr Sect C, 1995, 51, 715, M Szablewski, J Org Chem ,1994,59,954 7 M Raw, D N Rao, S Cohen, I Agranat and T P Radhakrishnan, unpublished results 8 M Ram, S Cohen, I Agranat and T P Radhakrishnan, Struct Chem , 1966,7,225 9 S K Kurtz and T T Perry, J Appl Phys, 1968,39,3798 10 T Clark and J Chandrasekhar, Isr J Chem B, 1993,33,435 11 M Ravi, A Samanta and T P Radhakrishnan, J Phys Chem, 1994,98,9133 12 M Ravi and T P Radhakrishnan, J Phys Chem ,1995,99,17 624 13 H L Bhat, Bull Muter Sci ,1994, 17, 1233 Paper 5/061671, Received 19th September, 1995 1122 J Muter Chern, 1996,6(7), 1119-1122