J. MATER. CHEM., 1994, 4( 12), 1851-1854 2-Amino-5-nitropyridinium Acetophosphonate: A Engineered Non-linear Optical Crystal J. Pecaut and R. Masse* Laboratoire de Cristallographie, associe a I'llniversite Joseph Fourier, CNRS, Cedex 09, France Deliberately B.P. 766, 38042 Grenoble The crystal structure of 2-amino-5-nitropyridinium acetophosphonate confirms the possibility to design at request non- centrosymmetric structures based on the 2-amino-5-nitropyridinium polar chromophore. The non-centrosymmetry in this class of materials is mainly dependent on the structure of the associated counter-anion. Layered anionic aggregates always favour non-centrosymmetrical organization of non-linear cations as evidenced by many crystal structure investigations. During the past 15 years a great variety of organic crystalline and polymeric materials with enhanced quadratic non-linear optical properties have been proposed for three-wave-mixing optical devices.* Their design was guided both by the measure- ment of non-linear efficiency at the molecular level based on EFISH by the shape of materials resulting from the geometry of non-linear chromophores and by mechanical, thermal and chemical requirements. The chromo- phores used in this engineering were mainly derivatives of nitroanilines, stilbenes and polyenes, modified through adequate chemical substitutions with a view to optimizing the tensorial components Bijk (zijk)and the transparency band- width.The enhancement of the macroscopic second-order susceptibility coefficients in uniaxial or biaxial crystals built up from one- or two-dimensional molecular units (nitroanil- ines) and related phase-matching configurations are dependent on the orientation6 of the optimal chromophore in the crystal.Such ideal packings are extremely difficult to engineer: the crystal structure of N-(4-nitrophenyl)-~-prolinolillustrates an exceptional arrangement of chromophores7 in the point-group 2. A remarkable family of molecular crystals exhibiting large quadratic macroscopic susceptibilities designed from 2-amino- 5-nitropyridine has been intensely studied: 2-adamantyl-amino-Snitropyridine (AANP),8,9 2-cyclooctylamino-5-nitro-pyridine (COANP),lo-l' 2 [(S)-phenylethyl] amino-5-nitro- pyridine ( MBANP),I2-l4(4-nitro-2-pyridinyl)-(S)-phenylalani-no1 (NPPA)'5,16 and N-(5-nitro-2-pyridy1)leucinol (NPLO)." These crystals exhibit structural organization of the non-linear chromophores in herringbone motifs with the slipped conformation, which favours large second-order optical non- linearities, as established by Di Bella et A crystal engineer- ing strategy based on the encapsulation of the 2-amino-5- nitropyridinium cations in inorganic and organic layered host matrices has been developed with a view to obtaining crystals with a large transparency range (IR+UV), high packing cohesion of chromophores and improved mechanical and thermal resistances compared to molecular crystals built up with the 2-amino-5-nitropyridine as chromophore." This approach, which combines the cohesion of ionic polymeric inorganic (organic) lattices with the enhanced polarizability and flexibility of organic molecules, has been proposed by several authors and clearly illustrated with (H,PO,-),, (H,AsO,-), polyanions.20-22 Encouraged by the first physical investigation^,^^ it was fascinating to determine the parameters which control the building of acentric frameworks in the special case of crystals including 2-amino-5-nitropyridinium cations (2A5NP + ).Furthermore, some centric and acentric crystals structures combining 2ASNP+ cations with inorganic anions of various shapes and charges have been solved, showing the role played by the layered anionic mat rice^.^^-^^ A quasi-polar alignment of 2A5NP' cations has been observed in 2-amino-5-nitropyridinium L-monohydrogen-tartrate structure,27 evidencing the role of the two-dimensional hydrogen-bonded aggregate (C4H506-),. The use of (C4H,06 -), in crystal engineering has been clearly e Kplained and systematized by Aekeroy et Analogues of tartrate, dihydrogenmonophosphate and dihydrogenmonoarsenate layered host matrices are capable of inducing stable acentric structures with 2A5NP' as guests if the requirement pH<2 is respected and a short 2D hydrogen-bonded anionic network is formed. The acetophosphonic acid has been selected because the first acidic function of the phosphonate group is relatively strong and allows proton transfer towards the 2-amino-5- nitropyridine (pH 2, 20 "C) following eqn.( 1): C5H5N302+H+*(P03H)-CH2-C02H-The acetophosphonate anion has two hydrogen-donor groups, -P-OH and -C02H, and sufficient hydrogen acceptors to frame a 2D anionic network based on two strong hydrogen bonds 0-He. .O; such a layer is capable of favouring herring- bone assemblies as a consequence of avoiding the formation of local inversion centres in the structure. The observed crystal morphology (point group 222), the positive signal in second- harmonic generation (Nd3+: YAG laser, 1.06 pm) md the crystal structure investigation of 2-amino-5-nitropyridinium acetophosphonate confirm our assumption. Experimental Crystals of 2-amino-5-nitropyridinium acetophosphonate were prepared by dissolving 0.001 mol of purified 2-amino-5- nitropyridine (2A5NP) in 20 ml water containing 0.002 mol acetophosphonic acid at 30°C.Slow evaporation of the solution yielded transparent crystalline needles up to 4mm x 2 mm x 2 mm in size. The chemical formula was estab- lished via the crystal-structure investigation. The cell param- eters and space group mentioned in Table 1 were determined by traditional X-ray methods using four-circle diffractometer data. The P2,2121 space group was confirmed both by the last reliability factor (Table 1)and a positive second-harmonic generation2' powder test (2.5 x urea) from an Nd3+: YAG laser fundamental beam (1.06 pm). The crystal structure was solved by direct methods using Multan 7730 and difference Fourier syntheses (Table 2).Full-matrix least-squares refine- ments were performed on F, using a unitary weighting scheme. Scattering factors for neutral atoms andf,f' have been taken from International Tables for X-ray Crystallography ,31 The J. MATER. CHEM., 1994, VOL. 4 Table 1 Crystal data, intensity measurements and structural refine- Table 3 Interatomic distances (A), bond angles (') and their esds ment parameters observed in the cation-anion packing C5H6N302+C2H405P-hydrogen bonds connecting the cations to the anionic framework formula wt. 279.15 and between them space group p2 12 12, cell parameters 5.003(1)A; b=8.090(1) A, H(5)--0( 1) 1.90( 3) ~=27.65( 1) A H(5)--N(1) 0.78(3) 0(1)-H(5)-N( 1) 160( 3) N(1)--0(1) 2.655( 2) diffractometer CAD4 Nonius radiation, monochr.Ag-Ka, graphite scan mode o scan H(6)-C(3) 0.94(3) data collection limits 2"<0<30"; O<h<7; O<k<12; H(6)--0(2) 2.48( 3) O(2)-H (6)- C( 3) 134( 2) 0<1<41 H(6)--0(3) 3.00( 3) 0(3)-H(6)-C( 3) 117(2) number of reflections total =3872, independent =2372, C(3)--0(2) 3.212( 3) with 12341) C(3)--0(3) 3.543( 3) number of variables, R, RM. 203, 0.035, 0.038 p in final AF synthesis/e A-3 pmax=0.55; pmin= -0.39 W7)-C(5) 0.87(4) Z 4 H(7)--0(7) 2.53(4) C(S)-H(7)-0( 7) 156( 3) D,/g cmp3 1.656 C(5)--0(7) 3.3 52 (4) F(000) 576 T/K 293 H( 10)-0(4) 2.08(3) crystal size/mm 0.19 x 0.19 x 0.32 H( 10)-N(2) 0.80(3) O(4)-H( 10)-N(2) 154(3) p/cm-' 1.586 N(2)--0(4) 2.833( 3) H(9)--0(2) 1.94(3) H(9)-"2) 0.92( 3) 0(2)-H(9) -NI 2) 166(2) Tableo2 Positional parameters, B,,/A2 for non-hydrogen atoms" and N(2)--0(2) 2.847( 2) Bi,,/A2 for hydrogen atoms refined isotropically (estimated standard deviations in parentheses) hydrogen bonds in the acetophosphonate layer ensuring the anionic aggregation atom Y 4' Z B/A2 H(l)-C(1) 0.91( 3) 0.06391 (9) -0.01 137( 6) 0.42399( 2) 1.791( 6) H(1)--0(4) 2.42( 3) C(1)-H( 1)-O(3) 152(2)0.1110(3) 0.1705(2) 0.4 1467 (5) 2.43(2) C( 1)--0(4) 3.255( 3) -0.1994( 3) -0.0767(2) 0.40645 (6) 2.52(2) 0.2868( 3) -0.1182( 2) 0.39905( 6) 2.64( 3) H(2)--C(1) 0.93( 3) 0.1105( 4) -0.0510(2) 0.48763(8) 2.20( 3) H(2)--0(4) 2.57( 3) C( l)-H(2)-0(4) 152( 2) -0.0886( 4) 0.0318( 2) 0.5201 5( 7) 2.05(3) C(1 )--0(4) 3.429( 3) -0.2917( 3) -0.03 59 (2) 0.53384( 6) 3.21 (3) -0.0218(3) 0.1826( 2) 0.53257(6) 2.87( 3) H(4)--0(2) 2.21 (3) 0.275 (7) -0.010(4) 0.495( 1) 2.1 (6)" H(4)--0(3) 0.40(3) 0(2)-H(4)-0( 3) 160(5)0.098 (6) -0.166( 3) 0.491 3( 9) 1.3(6)* 0(2)--0(3) 2.600( 2) 0.895(7) 0.228 (4) 0.554( 1) 2.8(8)* 0.361 (7) 0.893 (4) 0.404( 1) 3.5( 8)* H(3)-0(1) 1.86(3)0.5960(5) 0.2088 (3) 0.33472(8) 2.80( 4) H(3)--0(5) 0.80(3) O(1)-H(3)-0( 5) 159( 3) 0.3967(5) 0.2653( 3) 0.30624(8) 3.01(4) 0(1)--0(5) 2.630( 2) 0.3285(5) 0.4331(4) 0.30597( 9) 3.24(4) 0.4658( 5) 0.5395(3) 0.33451(8) 2.96( 4) 0.6795(4) 0.479S( 3) 0.36355( 7) 2.29(3) 0.7327(4) 0.3159( 2) 0.36247( 7) 2.49 (3) cohesion through a multiple hydrogen-bond network.Several 0.8240( 4) 0.5753( 3) 0.39182( 7) 2.97(3) types of hydrogen bonds occur in the anion aggregation 0.2481(5) 0.1472(4) 0.27716( 9) 4.59( 5) 0.3072( 6) O.OOlO( 3) 0.28007(8) 6.27(6) (Table 3).Two reladively short O-H---0 contacts [H---O= 0.0777(5) 0.2010( 4) 0.2502( 1 ) 8.38(7) 1.86(3), 2.21(3) A] a,nd a long C(l)-H(2)---0(4) 0.8 15( 7) 0.269( 4) 0.382( 1) 2.2(7)* [H(2)---0(4)=2.57(3) A] ensure the cohesion between the 0.643( 6) 0.096( 3) 0.3380( 9) 1.3 (6)* anions, building an infinite double chain in the a direction. 0.220(8) 0.478(5) 0.285(1) 3.9(9)* These chains are assembled in the b direction throuFh 0.418(6) 0.656(4) 0.33S( 1) 1.8(6)* C(1)-H( 1)---0(4) bonds [H( 1)---0(4)=2.42(3) A].0.808(6) 0.688(3) 0.391 3( 9) 1.2(6)* 0.920( 7) 0.533(4) 0.412(1) 3.3(8)* Taking into account the main works on the structural evidence of C-H---0 bonds in molecular ~rystals,~'-~' we assume a B,,=4/3 XiCj pij a, aj.that the existence of such interactions in this crystal is due to the observed hydrogen-bond geometry (Table 3). Furthermore, in this case the polar nature of the anion Enraf-Nonius SDP program32 operating on a micro-Vax I1 increases the possibility of multiple hydrogen bonds between computer was applied for all the calculations. The structure the anion and its neighbours. Each cation is anchored onto was drawn using the Molview program.33 the acetophosphonate layer through three N---H ---0 bonds and the three-centre hydrogen bond C(3) -H(3)---0(2), Results O(3). The twisting angle between the planes of the NO, group and the pyridinium ring is 3.6" due to the C(5)-H(7)---0(7) The 2-amino-5-nitropyridinium acetophosphonate contact interconnecting the cations.The interatomic distances, (2A5NPAP) crystal structure is built with 2A5NP+ cations bond angles and anisotropic thermal parameters observed in and acetophosphonate anions interconnected through a three- the cation and anion structures have been deposited with the dimensional hydrogen-bonding network (Fig. 1). Half-Cambridge Crystallographic Data Centre. The second-har- herringbone motifs of cations organized in a double layer monk generation signal29 from a powder sample of 2A5NPAP are sandwiched between acetophosphonate aggregates. The at 530 nm is weak: 2.5 x urea (Nd3+: YAG laser, 1.06 pm). anionic layer parallel to the (ab) plane (Fig.2) gains its This low efficiency should be interpreted through the evalu- J. MATER. CHEM., 1994, VOL. 4 Fig. 1 Projection of 2A5NPAP crystal structure along the a axis. Hydrogen bonds are indicated in dotted lines. Fig.2 Layer of acetophosphonate anions parallel to the (ab)plane. The anionic aggregation is based on 0-H---0 and C-H---0 bonds. The P0,C tetrahedra are hatched. ation of tensorial coefficients and the density of chromophores in the crystal structure: (a) the cations probably do not have the optimal orientation required in the point group 222. The assumption of a simplified one-dimensional description of the molecular non-linear tensor6 with by,, along the molecular axis C(4)-C(7) cannot be applied. The optimal values of angular parameters in this model are a=54.74", $=45".In the case of 2A5NPAP we have calculated a= [b,C(4)C(7)]= 50.7", $=42". i,$ is the angle between the c axis and the projection of the C(4)C(7) vector on the (ac) plane. Although these angles are close to the optimal values, the SHG signal is low, implying that the one-dimensional model is not appro- priate for explaining the weak non-linear optical efficiency of this crystal. The assumption of a two-dimensional description of the molecular tensor would be more appropriate in account- ing for the direction of the ground-state dipole moment which is not along C(4)C(7) in the case of the 2-amino-5-nitropyridi- nium cation and the local-field factors in this ionic structure which can considerably modify the contribution of the Pijk coefficients to the macroscopic susceptibilities.(b) Another important parameter is the density of chromophores (or oscillators) per unit volume of matter. The ratio z= Zl$A5NP+ indicates the proportion of the cell yolume occupied by the chromophores. < T/2A5NP+ > =146.6 A3 is an average volume defined in a previous Various values of z observed in molecular and ionic crystals containing 2A5NP or 2A5NPf entities are compared in Table4. This ratio is a parameter which can explain the enhancement of the macroscopic susceptibilities x( 2) when the required optimal orientation of chromophores in the structure is already reached or the situation is close to optimal. In the structures of 2A5NPC1 and 2A5NPDP the cation axis 0(2)N-C---C-NH2 is tilted at 31" towards the anionic network (Cl-),, or (H,PO,-),,.The non-optimal orientation of chromophores is balanced in 2A5NPCl by the high value of z, explaining why the SHG response is higher than that observed in 2A5NPDP. Conclusion If we wish to design new non-linear optical crystals including the 2-amino-5-nitropyridinium cation as the chromophore, two conditions are presently required: (a) a pH specification allowing the formation of cation and ensuring its stability in the crystal structure, (b)the formation of a two-dimensional anionic layer that is cap%ble of removing the cations to distances dNHzPNHz>4.702 A (Table 5), thus favouring the organization of herringbone assemblies that are inconsistent with the introduction of local inversion centres in the structure.The formation of an anionic layer can be realized from anions Table 4 Occupancy ratio of non-linear optical cations (oscillators) in molecular and ionic structures containing the same geometrical entity, 2A5NP or 2A5NP' crystal 7 (Yo) ref. COANP 43.9 10,11 AANP NPPA 42.2 42.4 8,9 15,16 MBANP 47.6 12-14 2A5N PA P 52.3 this work 2A5NPDP 64.7 22 2ASNPBr 75.7 38 2A5NPC1 79.5 38 1854 J. MATER. CHEM.. 1994, VOL. 4 Table 5 Main intercationic distances in relation with their centric or 5 J. L. Oudar, J. Chem. Phys., 1977,67,446. acentric packing in structures designed with 2A5NP+ non-linear optical cations [t is a distance corresponding to a cell parameter translation] 6 7 8 J.Zyss and J. L. Oudar, Phys. Rec. A, 1982,26,2028. J. Zyss, J. F. Nicoud and M. Coquillay, J. Chem. Phys., 1984, 81,4160. J. F. Nicoud, Mol. Cryst. Liq. Cryst., 1988, 156,257. anions dNn,-w,IA dNH2-NH21A ref. 9 S. Tomaru, T. Kurihava, H. Suzuki, N. Ooba, T. Kaino and 3.454 25 10 S. Matsumoto, Appl. Phys. Lett., 1991,58, 2583. P. Gunter, C. Bosshard, K. Sutter, H. Arend, G. Chapuis, R. J. Twieg and D. Dobrowolski, Appl. Phys. Letr., 1987,50,486. 4.029 6.390 38 11 C. Bosshard, K. Sutter, P. Gunter and G. Chapuis, J. Opt. Soc. Am. B, 1989,6,721. 3.896 5.189(t) 26 12 T. Kondo, N. Ogasawara, R. Ito, K. Ishida, T. Tanase, T. Murata and M. Hidai, Acta. Crystallogr.Sect. C, 1988,44. 102. 4.702 4.8 13(t) 38 13 R. T. Bailey, F. R. Cruickshank, S. M. G. Gurthrie, B. J. McArdle, H. Morrison, D. Pugh, E. A. Shepherd, J. N. Sherwood, 4.807 4.949 (t) 38 C. S. Yoon, R. Kashyap, B. K. Nayar and K. I. White, Opt. Commun., 1988,65,229. 6.717 5.675(t) 22 14 T. Kondo, R. Morita, N. Ogasawara, S. Umegaki and R. Ito, Jpn. J. Appl. Phys., 1989,28, 1622. 6.941 5.814(t) 25 15 T. Uemiya, N. Uenishi, Y. Shimizu, T. Yoneyama and K. Nakatsu, Mol. Cryst. Liq. Cryst., 1990, 182A. 51. 16 K. Sutter, G. Knopfle, N. Saupper, J. Hulliger, P. Gunter and 5.05(t) this work 7.07 17 W. Peter, J. Opt. SOC.Am. B, 1991, 8, 1483. T. Sugimaya, T. Shigemoto, H. Komatsu, Y. Sakagushi and 8.248(t) 7.611(t) 27 18 T.Ukachi, Mol. Cryst. Liq. Cryst., 1993,224,45. S. Di Bella, M. A. Ratner and T. J. Marks, J. .4m. Chem. SOC., 1992,114,5842. 19 R. Masse, M. Bagieu-Beucher, J. Pecaut, J. P. Levy and J. Zyss, that can aggregate through two short hydrogen bonds as in the phosphate salt. Three hydrogen bonds occur in the anion aggregation process of 2-amino-5-nitropyridinium L-tartrate. In the acetophosphonate salt we were waiting for the forma- tion of two short O-H---0 bonds, as was effectively observed; however, two further C-H ---0 bonds appear, 20 21 22 23 Nonlinear Opt, 1993,5413. C. B. Aekeroy, P. B. Hitchcock, B. D. Moyle and K. R. Seddon, J. Chem. Soc., Chem. Commun., 1989,23,1856. R. Masse and A. Durif, 2.Kristallogr., 1990, 190. 19. R. Masse and J. Zyss, Mol.Eng., 1991, 1, 141. Z. Kotler, R. Hierle, D. Josse, J. Zyss and R. Masse, J. Opt. Soc. Am. B, 1992,9,534. building the anionic layer along the u and 6 directions. The structures of 2-amino-5-nitropyridinium chloride and bromide (P2,) reveal that anionic aggregation in layers without the occurence of hydrogen bonds is also possible. Until now our engineering has been based on using numerous crystal struc- ture investigations and chemical observations to predict a 24 25 26 27 M. Bagieu-Beucher, R. Masse and D. Tranqui, 2. Anorg. Allg. Chem., 1991,606,59. J. Pecaut, Y. Lefur and R. Masse, Acta Crystallogr., Sect. B, 1993, 49, 535. J. Pecaut and R. Masse, Acta Crystallogr., Sect. B,1993,49,277. J. Zyss, R. Masse, M. Bagieu-Beucher and J. P. Levy, Adv. Muter., 1993,5, 120; 0.Watanabe, T.Noritake, Y. Hirose, A. Okada and non-centrosymmetric framework in the acetophosphonate salt. At this stage, the use of molecular simulation programs could be very fruitful for designing new acentric structures containing the 2-amino-5-nitropyridinium chromophores and increasing this large family of stable non-linear optical crystals. 28 29 30 T. Kurauchi, J. Muter. Chem., 1993,3, 1053. C. B. Aekeroy and P. B. Hitchcock, J. Muter. Chem., 1993,3,1129. S. K. Kurtz and T. T. Perry, J. Appl. Phys., 1968,39, 3798. P. Main, L. Lessinger, M. M. Woolfson, G. Germain and J. P. Declercq, MULTAN 77, User guide, University of York, England, and Louvain La Neuve, Belgium, 1977. 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Levy and R. Masse, J. Muter. Chom., 1993,3, 999. 18,410. Paper 4/04052J; Received 4th July, 1994