J. MATER. CHEM., 1994, 4( lo), 1571-1577 Synthesis and Second-harmonic Generation Properties of 2-(4=Nitroanilino)=1,3,5=Triazine Derivatives Hisatomo Yonehara,* Wen-Bing Kang, Tatsuo Kawara and Chyongjin Pac Ka wamura Institute of Chemical Research, 637 Sakado, Sakura, Chiba 285, Japan The synthesis and non-linear optical properties of a series of 2-(4-nitroanilino)-l,3,5-triazine compounds are degscribed. The triazines show various activities in powder second-harmonic generation (SHG) depending on the structures and have absorption maxima at d350 nm, shorter by 30-50 nm than those of the parent nitroanilines. 2-(4-Nitroanilino)-4,6-diphenyl-l,3,5-triazine affords different crystalline materials depending on the recrystallization solvent. A crystal formed by recrystallization from toluene reveals a high powder SHG activity comparable with that of 2-methyl- 4-nitroaniline, while recrystallization from N,N-dimethylformamide gives an SHG-inactive crystal in which solvent molecules are incorporated in a 1: 1 ratio by hydrogen bonding.The crystal structure of the latter was determined. Conjugated n-electron systems with intramolecular charge transfer have been recognized as potential candidates for second-harmonic generation (SHG) arising from the creation of large optical dipole moments. 4-Nitroaniline (4-NA) and 4-dimethylamino-4'-nitrostilbene (DANS) are typical com-pounds of high second-order hyperpolarizability (b).However, their net SHG effects in the crystalline state are negligible as a consequence of their centrosymmetric crystal structures.Intense experimental and theoretical effort has been devoted to chemical modifications of the basic molecular structures of 4-NA and DANS in order to develop non-centrosymmetric crystals without a significant decrease in fl.' For instance, 2-methyl-4-nitroaniline (MNA) and 3-methyl-4-methoxy-4'- nitrostilbene (MMONS)2 exhibit high SHG effects in the crystalline state because of the creation of non-centrosymmetric crystal structures by the introduction of the additional methyl substituent. However, these compounds have the serious disadvantages of relatively low melting points and long cut-off wavelengths, which would prevent their application in practical SHG devices for the generation of blue-green light.SHG-active organic compounds should meet the additional requirements of short cut-off absorption wave-lengths and high melting points. From this viewpoint, we have performed semiempirical molecular orbital calculations (MOPAC and CNDOjS-CI) for molecular design of key compounds. Consequently, it is predicted that the modifi- cation of the 4-NA and 4-amino-4'-nitro-trans-stilbene (4-ANS) chromophores with triazine groups may produce compounds with short cut-off wavelengths while retaining large non-linear activities. Another important benefit of the modification with triazine groups would come from the expectation of high melting points associated with thermal stabilities, since triazine compounds have been used as possible intermediates for the preparation of thermally stable organic polymer^.^ This paper deals with the preparation and SHG properties of a series of 2-(4-nitroanilino)-1,3,5-triazines and (4'-nitrostilbenylamino)-1,3,5-triazines.A parti- cularly interesting finding is that recrystallization of 2-(4-nitroanilino)-4,6-diphenyl-1,3,5-triazinefrom toluene gives a crystalline compound possessing a high powder SHG activity comparable with that of 2-methyl-4-nitroaniline, a high melting point (268 "C) and a relatively short absorption maximum.Experimental General The structure determination of the compounds was carried out by IR spectra (KBr disks) on a Jasco A-202 IR spec-trometer, 'H NMR spectra on a JEOL JNM-GSX-400 spec- trometer for CDCl, or ['H6] dimethyl sulfoxide (DM SO-d,) solution with tetramethylsilane (TMS) as the internal stan- dard, FD mass spectra on a Shimadzu 9100-MK mass spectrometer and UV-VIS spectra on a Hitachi U-3500 spectrophotometer.Reflectance spectra of powder hamples were recorded using an integrating-sphere attachment. Melting points were measured on a Yanagimoto micro melting-point apparatus and are uncorrected. Measurement of Second-harmonic Generation The SHG properties of the compounds were measured by a powder method4 using a Q-switched Nd : YAG laser SL803, SPECRTON Laser Systems Ltd.) operating at 1.064 iim fun- damental wavelength with 10 ns pulse duration and 10 Hz repetition freq~ency.~ Theoretical second-order hyperpc )lariza- bilities of these compounds were calculated by MIOPAC (PM3, ver 5.0),6,7and the calculations of absorption maxima of some triazine derivatives were carried out using CNDO/S-CI.6,7 Preparation of Triazine Compounds The triazine compounds were prepared according to eqn.( 1)-( 6). Commercially available 2,4,6-trichloro-1,3,5-1 riazine (C1,-TRAZ), 4-nitroaniline, 2-methyl-4-nitroanilirie, 2-chloro-4-nitroaniline, 3-nitroaniline, 4-nitrophenol and 4-nitrothiophenol were used as received. 4-ANS,' 2-chloro-4,6-diphenyl- 1,3,5-triazine (ClPh,-TRAZ)9 and 2-chloro-4,6-dimethoxy-1,3,5-triazine[Cl( MeO),-TNAZ] lo were prepared according to the literature methods (TRAZ, 1,3,5-triazine group). 2,4-Dichloro-6-arylamino-1,3,5-triazinesand 2,4-Dichlor0-6-phenoxy (or 6-thiophenoxy)-l,3,5-triazine(la-f, 6a) C1,-TRAZ (1.84 g, 10mmol) was added to a solution of 4-NA, MNA, 2-chloro-4-nitroaniline, 4-nitrothiophenol or %nitro- aniline (10mmol) in acetone (20 ml) with stirring at 0 "C,to a solution of 4-nitrophenol (10 mmol) in acetone (20 ml) at room temperature, and to a solution of 4-ANS (10 mmol) in benzene (20ml) under reflux.Continuous stirring of the solutions for 0.5-2 h resulted in the gradual formatiion of crystals. After addition of 5 wt.% aqueous potassium hydro- gencarbonate (20 g) to the reaction mixture, the resultant crystals were collected and washed with water and then with methanol to give la (%!YO), lb (86%), lc (56%), Id (81%), le (86%), If (goo/,) and 6a (62%).J. MATER. CHEM., 1994, VOL. 4 CI I d 1 a-f NO2 R X la &NO2 H NH lb &NO2 CH3 NH IC &NO2 CI NH Id &NO2 H 0 1. &NO2 H S 11 %NO2 H NH CI CI Nu I I NAN + Nu NAN CI ANANH9N02 NuANA NHpN02 R' d 1 2 3 l;R Nu 2a H MeO 2b H Et2N 2c H MeCOCHC02Et 2d CHB MeO 38-MeO 3b -OH CI CI (3) MeANA NHeNO, 20 OMe OMe I 1.-4a 2-Chloro-4-methoxy (or 2,4-dimethoxy) -6-arylamino-1,3,5- triazines (2a, 2d,3a) and 2,4-Dimethoxy-6-[4-( 4'-nitrophenyl- ethenyl)anilino]- 1,3,5-triazine (6c) For the preparation of 2a, 2d and 3a, la or lb (5 mmol) was treated with a 28 wt.% methanolic solution of sodium methox- ide (5 mmol in the cases of 2a and 2d; 10 mmol in the case of 3a) at room temperature for 3 h.Similarly, 6c was obtained from the reaction of 6a (5 mmol) with a 28 wt.% methanol solution of sodium methoxide (10 mmol) at room temperature for 8 h. The resultant precipitates were filtered off and washed with methanol to give 2a (ca. loo%), 2d (79%), 3a (900/) and 6c (62%). 2-Chloro-4-diethylamino-6-(4-nitroani1ino)- 1,3,5-triazine (2b) A solution of diethylamine (0.365 g, 5 mmol) in 10ml toluene was added to a suspension of la (1.43 g, 5 mmol) in 50 ml toluene, and the mixture was refluxed under stirring for 3 h and then cooled to room temperature. To the reaction mixture 4b was added 5 wt.% aqueous potassium hydrogencarbonate (20g) and the resultant crystals were collected and washed with water and then with toluene.White crystals of 2b were obtained in 62% yield. 2-Chloro-4-( l-ethoxybutane-1,3-dione-2-yl)-6-(4-nitroani1ino)-1,3,5-triazine (2c) To a suspension of NaH (0.44 g, 11mmol) in DMF (30 ml) was added ethyl acetoacetate (1.4 g, 11 mmol) in DMF (5 ml) under stirring at room temperature. After evolution of hydro- gen gas had ceased, la (2.86 g, 10 mmol) was added to the reaction mixture. After stirring the mixture at room tempera- ture for 3 h, usual work-up procedures gave 2c (38%). 2-Chloro-4-methyl-6-( 4-nitroanilino)- 1,3,5-tria:ine (2e) C1,Me-TRAZ was prepared from a reaction of C1,-TRAZ with methyl magnesium iodide in ether. A mixture of C1,Me-TRAZ (0.33 g, 2 mmol) and 4-nitroaniline (0.27 g, 2 mmol) in J.MATER. CHEM., 1994, VOL. 4 5a 4-ANS 25 ml acetone was refluxed for 4 h and then cooled to room temperature. After addition of 0.67 wt.Yo aqueous potassium hydrogencarbonate (30 ml) to the reaction mixture, the result- ant precipitates were collected and washed with water to give 2e (94%). 2,4-Dihydroxy-6-(aryZamino)-1,3,5-triazines (3b, 6b) A solution of la or 6a (2mmol) in a mixed solvent of DMSO (10 ml) and H20 (2 ml) was stirred at 50 "C for 2 h in the case of la or at 100"C for 5 min in the case of 6a, and then aqueous potassium hydrogencarbonate was added to the solution. The resultant crystals were collected and washed with water to give 3b as pale-yellow crystals in almost quantitative yield or were subjected to column chromatogra- phy (Kieselgel 60, methylene dichloride as eluent) to give 6b in 62% yield.2,4-Dimethoxy-6-(4-formylphenoxy)-l,3,5-triazine (4a) and 2,4- dimetho.vy-6-( 4-dicyanovinylphenox y)- 1,3,5-triazine (4b) A 20 ml solution of potassium hydroxide (10 mol) and 4-hydroxybenzaldehyde (10mmol) in methanol was added to a solution of C1( MeO),-TRAZ" ( 10 mmol) in 30 ml methanol and the mixture was stirred at room temperature for 2.5 h. The resultant white precipitates were collected and washed with methanol to give 4a (77%). A mixture of 4a (3.8 mmol), malononitrile (1.2mmol) and a few drops of piperidine in 40 ml methanol was stirred at room temperature for 2.5 h. The resultant white precipitates were collected and washed with methanol to give 4b (50%).2-(4-Nitroanilino)-4,6-diphenyE-1,3,5triazine (5a) A mixture of ClPh,-TRAZ (55 mmol) and 4-NA (165 mmol) in 150 ml DMF was heated at 120 "C for 8 h and then cooled to room temperature. After addition of 200ml ethanol and 100 ml water to the reaction mixture, the resultant precipitates were collected, washed with ethanol and then recrystallized from toluene to give 5a-1 (900/) as pale-yellow crystals or from DMF to give 5a-2 (90%) as colourless crystals. 2,4- Dimethyl-6- [4-( 4'-nitrophen ylet heny 1)anilinol- 1,3,5-triazine (6d) To a mixture of 0.33 ml diethyl ether solution of methyl magnesium iodide (1.1 mmol) and 20 ml DMF was added 6a (0.52 mmol), and the mixture was stirred at room temperature 6a 6b-f Y Z 6b OH OH 6c Me0 Me0 6d Me Me be ANS CI 6f 2-Me-4-N02-C,H3 CI for 10 min.After the usual work-up procedures, column chromatography of the product (Kieselgel 60, methylene dichloride as eluent) gave 6d in 85% yield. 2-Chloro-4,6-his[4-( 4'-nitrophen ylethenyl )aniline]-1,3,S-triazine (6e) and 2-Chloro-4-( 2-methyl-4-nitroanilino)-6-[4-(4'-nitrophenylethenyl)anilino]-l,3,5-triazine(6f) To a solution of 4-ANS or MNA (0.54 mmol) in 25 rnl DMF at -40 "C was added a 0.4 ml hexane solution of 15% n-butyllithium (0.60 mmol) under stirring. After 1 h, 6a (0.52 mmol) was added to the DMF solution. The mixture was stirred at -40°C for 1h, gradually warmed to room temperature and then poured into ice-water. The resultant precipitates were filtered and purified by column chromatogra- phy (Kieselgel 60, methylene dichloride as eluent) to give 6e (45%) or 6f (60%).X-Ray Structural Investigation of 2-( 4-Nitroanilino)-4,0- diphenyl-1,3,5-triazine (5a) Samples of (5a) used in XRD studies were prepared by recrystallization from DMF or toluene. Powder difkaction patterns were obtained with a Rigaku RAD-I1 diffract ometer. Scans were taken with a 28 step size of 0.02", using Cu-Ka radiation. A single crystal of 5a-2 was obtained by slow evaporation of the solvent from a DMF solution of 5a at room temperature. A colourless single crystal of C21H15N502-C3H7N0having approximate crystal dimensions of 0.5 mm x 0.2 mm x 0.5 mm was mounted on glass fibre. All measurements weice made on a Rigaku AFC5R diffractometer with gsaphite-monochromated Mo-Ka radiation.Experimental details for the crystal-structure determination were essentially the same as those reported previo~sly.~,~ The final cycle of full-matrix least-squares refinement was based on 2075 observed reflec- tions [I >3.0 a(l)]and 355 variable parameters and converged with unweighted and weighted agreement factors of R' =0.052 and R, =0.057 [w=4FO2/o2(Fo2)].? t Tables of atomic coordinates, bond lengths and bond angles have been deposited with the Cambridge Crystallographic Data Centre; see Information for Authors, in issue 1. Table 1 Calculated values of p and the longest absorption maxima (icalc)of some selected triazine compounds compared with those of 4-NA, MNA and 4-ANS flcalc/10-30esu compound PM3 CNDO/S-CI" ;*talc/nmb la 7.1 - - Ib 5.7 - - 3a 7.9 13.4 369.0 3b 4.6 11.2 353.2 5a 10.2 23.2 270.2 6a 20.7 ~ - 4-NA 6.3 15.8 393.0 MNA 6.11 17.7 393.9 4-ANS 37.6 - - "Ref.6 and 7. hicalccalculated by CNDO/S-CI. Results and Discussion Molecular-orbital (MO) Calculation of Hyperporlarizability (p) of Triazine Derivatives Table 1 lists the calculated /3 values (Pcalc)and the calculated longest absorption maxima (Acalc) of several triazine derivatives compared with those of 4-NA and MNA. The CNDO/S-C16,7 calculations of Pcalcand Acalc were performed for 3a, 3b and 5a, which have no chlorine substituents, because no param- eters for chlorine atom are given in the semiempirical MO method.The Pcalc values (PM3) of la, 3a and 5a are comparable with or greater than those of 4-NA and MNA, suggesting that the intrinsic hyperpolarizability of the 4-NA chromophore might be kept in the derivatization with the triazine groups. Similarly, the Pcalcvalues of lb, 3b and 6a are relatively high, again predicting that the triazine derivatives of 4-NA, MNA and 4-ANS would be potentially active in SHG. Another interesting result in the theoretical calculations is that the Acalc values are shorter by more than 24 nm than those of 4-NA and MNA. These results imply that the triazine derivatization of the chromophores should lead to significant blue shifts of the absorption maxima which are certainly beneficial to the construction of SHG-active materials for the generation of blue-green light.In particular, note that 5a shows much shorter icalcand considerably greater Pcalcthan MNA does. Synthesis and Second-harmonic Generation of Triazine Derivatives The dichlorotriazine compounds la-f and 6a were con-veniently synthesized by condensation reactions of C1,-TRAZ with the corresponding nitroarylamines, 4-nitrophenol and 4-nitrothiophenol [eqn. (l)] and were further converted to J. MATER. CHEM., 1994, VOL. 4 2a-d, 3a-b and 6b-f with ease [eqn. (2) and (6)]. Similarly, the monomethyl compound 2e was obtained by the derivatiz- ation of 4-NA with C1,Me-TRAZ, which had been prepared by the methylation of C1,-TRAZ with methyl magnesium iodide [eqn.(3)]. For comparison with the nitro compound 3a, the (dicyan0)ethenylphenoxytriazine4b was also prepared according to eqn. (4). Another important compound is the diphenyltriazine 5a, which was synthesized by the phenylation of C1,-TRAZ with phenyl magnesium bromide followed by the condensation reaction with 4-NA [eqn. (5)].Tables 2-6 summarize the observed SHG activities in powder, absorption maxima (,Imax)and melting points of the triazine compounds together with 'H NMR and IR data. The powder SHG activity of la is relatively high, as expected from the theoretical calculations. A1though the /Icalc value of la is comparable with that of MNA, the observed activity in the powder SHG is lower by a factor of 2.8 than that of MNA, presumably implying a non-centrosymmetric crystalline structure of la with partially cancelled molecular dipole moments.While the Pcalc values of 3a and 3b are significantly high, 3a and 3b are SHG-inactive. Therefore, it can be predicted that the dipole moments of 3a and 3b would be cancelled in centrosymmetric crystal structures. These discrepancies between Dcalc values and observed powder SHG activities have frequently been observed, probably because of the limitation of the prediction using simple MO calculation^.^,^ It is of significance to the SHG properties of the triazine compounds to note that the dichlorotriazine derivative of 4-NA (la) is the most active material of the dichlorotriazines (la-f and 6a), while no net powder SHG activity was observed with lb, the dichlorotriazine of MNA.This is in sharp contrast to the well known fact that 4-NA is totally inactive in powder SHG while MNA is active. Presumably, the dichlorotriazine derivatization of 4-NA should cause a change from the centrosymmetric crystal structure of 4-NA to a non-centrosymmetric one in la. On the other hand, an opposite change in crystal structure should occur in the triazine derivat- ization of MNA. Similarly, lc, which has a chlorine substituent at the 2 position of the 4-NA chromophore, shows a relatively low SHG activity, comparable with that of urea. In the case of the 3-NA derivative (If), the SHG activity is half that of la, as is the relationship of the reported values between 4-NA'l and 3-NA.12 The 4-nitrophenoxy dichlorotriazine (Id) is four times more active in powder SHG than urea, while the thiophenol analogue (le) is much less active.Although the chloromethoxytriazine of 4-NA (2c)shows an SHG activity ca. 10 times greater than urea, other similar compounds (2a, b, d, e, 3a and 3b) are almost inactive in powder SHG. Presumably, the formation of intermolecular hydrogen bonding would stabilize centrosymmetric crystal structures. Another interesting compound is 4b, which pos- Table 2 Physical and SHG properties of the triazine compounds, la-f &ax powder SHG (ethanol) mp 'H NMR compound efficiency" /nm 1°C (DMSO-d,) la 20 323 310 7.92 [2 H, d, arom.], 8.28 [2 H, d, arom.], 11.63 Ib lc 7 10-3 1 304 309 239 200 2.37 [3 H, s, CH,], 7.70 [2 H, d, arom.], 8.11 -c1 H, s, NHI [2 H, d, arom.], 10.97 [1 H, s, NH] Id le If 4.1 0.8 10 267 255 268 185 191 118 7.92 [2 H, d, arom.], 8.62 [2 H, d, arom.] 7.47-7.81 [l H, m, arom.], 7.98 [2 H, d, arom.],7.72 [2 H, d, arom.], 8.08 [2 H, d, arom.] 8.57 [l H, s, arom.], 11.53 [l H, s, NH] 'Values relative to urea (standard) determined by the powder method. 3350 (NH) 3350 (NH), 1540, 1318 (NO,) 3330 (NH), 1340 (NO,) 1520, 1350, (NO,), 1205 (PhO) 1510, 1350 (NO,) 3290 (NH), 1570, 1340 (NO,) J.MATER. CHEM., 1994, VOL. 4 Table 3 Physical and SHG properties of the triazine compounds, 2a-d and 3a )-,ax IR (KBr) powder SHG (ethanol) mp 'H NMR Vmax compound efficiency" /nm /"c 6, (DMSO-d6) /cm-' 2a 0.37 327 261 4.0 [3 H, s, OCH,], 7.97 [2 H, d, arom.], 8.25 3370 (NH), 1560, 1330 (NO,) [2 H, d, arom.], 11.2 [I H, s, NH] 2b 0.02 337 194 0.9-1.33 [6 H, m, 2CH,], 3.0-3.8 [4 H, m, 2 3370 (NH), 1520, 1320 (NO,) CH,], 7.9 [2 H, d, arom.], 8.15 [2 H, d, arom.], 10.57 [1 H, s, NH] 2c 9.4 326 246 1.27 [3 H, t, CH,], 2.43 [3 H, S, C=OCH,], 3360 (NH), 1720 (C=O), 4.17 [l H, s, CHI, 7.93 [2 H, d, arom.], 8.23 1580, 1340 (NO,)[2 H, d, arom.], 11.37 [I H, s, NH] 2d 3 x 10-3 314 200 2.38 [3 H, s, CH,], 3.93 [3 H, s, CH,], 7.78 3350 (NH), 1570, 1340 (NO,) [l H, d, arom.], 8.10 [2 H, d, arom.], 11.2 [1 H, s, NHI 3a 0.1 331 223 3.97 [6 H, s, 20CH,], 7.97 [2 H, d, arom.], -8.18 [2 H, d, arom.], 10.67 [l H, s, NH] 3b 0.04 328 >320 7.97 [2 H, d, arom], 8.20 [2 H, d, arom.] "Values relative to urea (standard) determined by the powder method.Table 4 Physical and SHG properties of the triazine compounds, 4a-b Amax I R (KBr) powder SHG (ethanol) mP 'H NMR "ma, compound efficiency" /nm /"c bH (DMSO-d6) /cm -4a 0.06 -149.5 3.95 [6 H, s, 20CH,], 7.52 [2 H, d, arom.], 8.0 -[2 H, d, arom.], 10.07 [1 H, s, CHO] 4b 4 x 10-4 --3.88 [6 H, s, 20CH,], 7.53 [2 H, d, arom.], -8.05 [2 H, d, arom.], 8.55 [l H, s, CHI "Values relative to urea (standard) determined by the powder method. Table 5 Physical and SHG properties of the 2-(nitroanilino)-4,6-diphenyl-1,3,5-triazinetriazinecompounds, 5a-b Amax IR (KBr) powder SHG (ethanol) mp 'H NMR "max compound efficiency" /nm /"c 6, (DMSO-d6) /cm-5a-1 104 342 268 5a-2' 0.2 342 268 7.5-7.65 [6 H, m, arom.], 7.81 [1 H, s, NH], 3330 (NH), 1530, 1321:) (NO,) 8.0 [2 H, d, arom.], 8.34 [2 H, d, arom.] "Values relative to urea (standard) determined by the powder method.bObtained by recrystallization from toluene. 'Obtained by recrysta,llization from DMF. Found: C, 64.8; H, 4.7; N, 19.2%. Calc. for C,,H,,N,O, (]:I adduct of 5a with DMF); C, 65.2; H, 5.0; N, 19.0. Mass s,pectrum m/z: 369 (Mi). imax(ethanol)/nm (&/dm3 mol-' cm-'); 265 (6.05 x lo4), 342 (3.63 x lo4). Table 6 Physical and SHG properties of the triazine compounds, 6a-f Lax IR (KBr) powder SHG (ethanol) mp 'H NMR Vmax compound efficiency" /nm 1°C 6, (DMSO-ds) /cm-' 6a 11 3 70 >300 7.40 [l H, d, CH=], 7.57-7.93 [7 H, m, =CH 1560, 1335 (NO,) and arom.], 8.20 [2 H, d, arom.], 11.23 [l H, s, NHI 6b 0.5 368 >300 7.43 [I H, d, CH=], 7.63-7.83 [7 H, m, =CH 3000-3200 (OH), 1554, 1355 and arom.], 8.23 [2 H, d, arom.], 9.27 [1 H, (NO,) s, NH], 10.77 [2 H, s, OH]6c 0.6 376 >300 3.88 [6 H, S, 20CH3], 7.37 [l H, d, CH=], 1550, 1345 (NO,) 7.47-7.97 [7 H, m, =CH and arom.], 8.17 [2 H, d, arom.], 10.02 [1 H, s, NH]6d 8.9 379 >300 3.33 [6 H, S, 2CH3], 7.40 [I H, d, CH=], 1540, 1340 (NO,) 7.53-8.00 [7 H, m, =CH and arom.], 8.27 [2 H, d, arom.], 10.20 [1 H, s, NH] 6e 6.5 393 >300 7.37 [2 H, d, CH=], 7.50-7.97 [14 H, m, =CH 1540, 1345(NO,) and arom.], 8.22 [4 H, d, arom.], 10.17 [2 H, s, NHI 6f 0 380 >300 3.34 [3 H,S, CH,], 7.37 [I H, d, CH=], 1540, 1345 (NO,) 7.50-8.00 [S H, m, =CH and arom.], 8.34 [4 H, m, arom.], 10.17 [2 H, s, NH] "Values relative to urea (standard) determined by the powder method.sesses the dicyanovinyl group,13 which is a strongly electron- withdrawing group. However, this compound is SHG-inactive. These are typical examples underlying a general understanding that the crystal structures cannot be designed only from pertinent molecular structure^.^^^ For the 4-ANS derivatives, their SHG activities reveal behaviour similar to that of the 4-NA derivatives. Both dichlorotriazine derivatives of 4-NA and 4-ANS (la and 6a) show high SHG activities while the monochlorotriazine derivatives (3a, b, d and 6b-d) show almost no net SHG activities. The dimethyltriazine of 4-ANS (6d) shows an SHG activity ca.nine times greater than that of urea. 6e appears to be of interest related with 'lambda (A)-shape' molecular which are proposed to be effective for the construction of non-centrosymmetric crystal structures. Although 6e reveals an SHG activity 6.5 times greater than that of urea, the similar A-shaped compound (6f) with the 4-ANS and MNA substituents shows no net SHG activity. A particularly interesting observation is that a crystal form (5a-1) obtained by recrystallization of 2-(4-nitroanilino)-4,6-diphenyl- 1,3,5-triazine (5a)from toluene shows the highest SHG activity of all the triazine compounds investigated, being 104 times more active than urea.Similarly, recrystallization of 5a from tetrahydrofuran gives a crystalline material of SHG-active form. In contrast, recrystallization of 5a from DMF gives another crystal form (5a-2), which shows no net SHG activity. In the latter, solvent molecules are incorporated in a 1 : 1 ratio by hydrogen bonding (vide infra). Similarly, 5a gives an SHG-inactive material upon recrystallization from aprotic polar solvents such as N-methylpyrrolidone, 1,3-dimethyl-2-imidazolidinone,ethyl acetate, acetone and 1,4-dioxane. With regard to the cut-off wavelength of optical absorption, note that the dichlorotriazine modification of 4-NA leads to a 47 nm blue shift of the optical absorption without a signifi- cant loss of net SHG activity.Similarly, all the triazine compounds containing the 4-NA chromophore show absorp- tion maxima that are shorter by 33-66nm than the parent 4-NA. In particular, the absorption maximum of If is shorter by ca. 100 nm than that of 4-NA, while maintaining a high SHG activity in the powder. For the 4-ANS chromophore, triazine derivatization also results in 10-35 nm blue shifts with moderately high SHG activities. Although absorption maxima in solution may be often different from those in the solid state, the reflectance spectrum of 5a-1 showed that the cut-off wavelength in powder is 425 nm, shorter by 52-53 nm than that of 4-NA (478 nm) or MNA (477nm). This is certainly beneficial for the generation of blue-green light. Another benefit in the triazine derivatization might be given by the relatively high melting points.For instance, the triazine compounds containing the 4-NA chromophore melt at tem- peratures higher by 47-163°C than 4-NA. Similarly, the melting points of 6a-f are higher by >55"C than that of 4-ANS. The high melting points of the triazine compounds are advantageous for the design of thermally stable SHG materials. Polymorphism and X-Ray Crystal Structure of 244-Nitroanilino)-4,6-diphenyl-l,3,5-triazine(5a) The triazine compound (5a) gave two different types of crystals depending on recrystallization solvents, in which one (5a-1 from toluene) is SHG-active but the other (5a-2 from DMF) is inactive. Table 5 lists the observed SHG intensities of 5a-1 and 5a-2 relative to that of urea.Moreover, note that the formation of the different crystals is not due to simple polymorphism but arises from the incorporation of solvent molecules (DMF) in a 1 :1 ratio in 5a-2, as confirmed by J. MATER. CHEM., 1994, VOL. 4 Fig. 1 Crystalline structure of 5a-2 elemental analysis, 'H NMR spectra for DM SO-d, solutions and thermogravimetric measurements. On the other hand, 5a-1 does not contain any solvent molecules. Annealing 5a-2 at 110°C for 3 h under vacuum gave another solid material (5a-3) containing no solvent molecules, which shows no SHG activity. We obtained a single crystal of SHG-inactive 5a-2, which was of good quality for X-ray crystallographic analysis. The molecular packing in a unit cell is shown in Fig.1. The molecular packing clearly shows that the two phenyl groups and the 4-nitroanilino moiety are almost coplanar with the triazine ring and that DMF molecules are incorporated in a 1 :1 ratio by forming hydrogen bonds between N(4)-H Ff 5a and the carbonyl oxygen of DMF with a distance of 1.95 A. The crystal data show that 5a-2 belongs to the space group P2,,, of a centrosymmetric structure (Table 7). As a conse- quence, the components of the molecular dipole moments along the c axis cancel. These results well agree with the lack of SHG activity of 5a-2 in microcrystalline powders. On the other hand, all attempts to obtain a single crystal of 5a-1 of sufficient quality for X-ray crystallographic analysis failed, although recrystallization from toluene gives 5a-1 as fine needles.Table 8 lists the selected d-spacing data and the Table 7 Structure details for 2-(4-nitroanilino)-4.6-diphenyl-1,3,5-triazine (5a-2) recrystallized from DMF chemical formula C2IH,,N,O2formula weight 369.38 colour colourless crystal system monoclinic sp!ce group p2,m44 14.464 (5)bib 10.659 (3) CIA 15.456 (3) Bldqgees 105.42 (2) V/A3 2297.3 (9) z 4 Dcalclg cm -1.068 p( Mo-Ka)/cm-0.67 ;./A 0.7 1069 reflections measured 5767 reflections used 1075 [l >3.00(1)] residuals R 0.052 Rw 0.057 b=4Fo/a2!Fo2)1crystal dimensions/mm 0.50 x 0.20 x 0.50 J. MATER. CHEM., 1994, VOL. 4 Table 8 Selected XRD data (d-spacing and relative intensities), for different crystalline samples of 2-(4-nitroanilino)-4,6-diphenyl-1,3,5-triazine (5a) 5a-1' 5a-2" 5a-3" d ,'A 1/10 d/A I/Io d/A III" 3.21 19 3.28 55 3.50 69 3.69 12 3.38 26 3.51 65 4.58 11 3.94 100 3.62 38 4.84 78 4.04 24 3.67 42 8.08 32 4.36 79 3.71 42 9.26 36 4.85 37 4.21 37 16.36 100 5.23 32 4.91 36 6.04 97 5.08 72 7.06 22 5.14 48 7.57 35 5.34 57 8.91 25 6.93 41 9.22 36 9.01 48 12.24 61 9.49 99 10.21 57 20.7 1 100 "5a-1 and 5a-2 indicate the samples recrystallized from toluene and DMF, respectively, and 5a-3 was obtained by annealing 5a-2 at 110 "C for 3 h under vacuum.relative intensities in XRD analysis of 5a-1-3. The d-spacing data of the crystals are substantially different from each other, indicating that they have different crystal structures. Presumably, the crystalline structure of SHG-active 5a-1 should be non-centrosymmetric. It is suggested that thermal liberation of solvent molecules from 5a-2 would cause a crystal change while retaining a centrosymmetric structure, but does not give 5a-1.Conclusions A series of 2-(4-nitroanilino)-l,3,5-triazinecompounds were prepared in order to investigate structure-activity relation-ships in SHG. The triazine compounds show various activities in powder SHG depending on the structures and have absorp- tion maxima at <350 nm, which are shorter by 30-50 nm than those of the parent nitroanilines. The melting points of these compounds are higher by 47-1 63 "C than 4-nitroaniline.2-(4-Nitroanilino)-4,6-diphenyl-1,3,5-triazine recrystallized from toluene (5a-1) is ca. 100 times more active in SHG than urea, an activity comparable with that of 2-methyl-4-nitro- aniline. Moreover, the absorption edge of this compound is <410 nm, much shorter than that of the parent compound, 4-nitroaniline. This is clearly advantageous for efficient gener- ation of 'blue light'. Another prominent property of this material is its high melting point (269 "C), implying ihermal stability towards laser-induced degradation of the crq'stalline structure by local heating. Recrystallization of 5a from DMF (5a-2) gives an SHG-inactive crystal, in which solvent mol- ecules are incorporated in a l : l ratio by hydrogen bonding.The crystal structure of 5a-2 was determined. We non intend to achieve the growth of a single crystal of 5a-1 large enough for preparation of an SHG device. Finally, note that the chlorotriazine compounds (la-f, 2a-e and 6a, e and f ) have reactive chlorine substituents which would be potentially accessible to further polymer derivatization of the t riazine-substituted chromophores. We gratefully acknowledge Miss Mieko Shimizu (KICR) for her assistance in measuring SHG powder intensities of the compounds. Thanks are due to Mr. Hiroshi Maki (DIC Central Research Laboratories) for carrying out thth X-ray crystallographic analysis of 5a-2. 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