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Synthesis and Second-Order Nonlinear Optical Properties of New Copper(II), Nickel(II), and Zinc(II) Schiff-Base Complexes. Toward a Role of Inorganic Chromophores for Second Harmonic Generation Pascal G. Lacroix* Laboratoire de Chimie Inorganique, Universite´ Paris-Sud, CNRS URA 420, 91405 Orsay, France, and Laboratoire de Chimie de Coordination, CNRS, 205 route de Narbonne, 31077 Toulouse cedex, France
Santo Di Bella Dipartimento di Scienze Chimiche, Universita` di Catania, 95125 Catania, Italy
Isabelle Ledoux Laboratoire de Bagneux, CNET, 196 Avenue Henri Ravera, 92220 Bagneux, France Received September 12, 1995. Revised Manuscript Received November 20, 1995X
A new Schiff-base ligand based on the condensation of diaminomaleonitrile and 4-(diethylamino)salicylaldehyde is reported with its copper, nickel, and zinc complexes. Their secondorder nonlinear optical properties are investigated by electric field induced second harmonic (EFISH) and ZINDO quantum-chemical calculations to probe the role of the metal center in the nonlinearity. All the complexes exhibit a second-order nonlinear response that is larger than that of the ligand with an hyperpolarizability (β) value of 400 ((100) 10-30 cm5 esu-1 for the zinc derivative at 1.34 µm. Theoretical calculations indicate that the two-level model is inadequate to describe the nonlinearity in such systems.
Introduction Currently, second-order nonlinear optical (NLO) properties of molecular materials are widely investigated for their potential applications in the newly emerging optoelectronic and optical signal processing. To date, organic molecules have focused the most intense activity.1,2 Beyond the first experimental studies and the * To whom correspondence should be addressed at Laboratoire de Chimie de Coordination, CNRS. X Abstract published in Advance ACS Abstracts, January 1, 1996. (1) (a) Molecular Nonlinear Optics; Zyss, J., Ed.; Academic: New York, 1994. (b) Molecular Nonlinear Optics: Materials, Physics and Devices; Zyss, J., Ed.; Academic Press: Boston, 1993. (c) Nonlinear Optical Properties of Organic Materials V; Williams, D. J., Ed.; SPIE Proc. 1992, 1775. (d) Introduction to Nonlinear Optical Effects in Molecules and Polymers; Prasad, N. P., Williams, D. J., Eds.; Wiley: New York, 1991. (e) Nonlinear Optical Properties of Organic Materials IV; Singer, K. D., Ed.; SPIE Proc. 1991, 1560. (f) Organic Molecules for Nonlinear Optics and Photonics; Messier, J., Kajzar, F., Prasad, P., Eds.; Kluwer Academic Publishers: Dordrecht, 1991. (g) Materials for Nonlinear Optics: Chemical Perspectives; Marder, S. R., Sohn, J. E., Stucky, G. D., Eds.; ACS Symposium Series 455; ACS: Washington, DC, 1991. (h) Organic Materials for Nonlinear Optics II; Hann, R. A., Bloor, D., Eds.; Royal Society of Chemistry: London, 1991. (i) Nonlinear Optical Properties of Organic Materials III; Khanarian, G., Ed.; SPIE Proc. 1990, 1337. (j) Nonlinear Optical Effects in Organic Polymers; Messier, J., Kajzar, F., Prasad, P., Ulrich, D., Eds.; Kluwer Academic Publishers: Dordrecht, 1989. (k) Nonlinear Optical Properties of Organic Materials; Khanarian, G., Ed.; SPIE Proc. 1989, 971. (l) Nonlinear Optical Properties of Organic Materials II; Khanarian, G., Ed.; SPIE Proc. 1989, 1147. (m) Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D. S., Zyss, J., Eds.; Academic Press: Orlando, 1987; Vols. 1 and 2. (2) For recent reviews see: (a) Benning, R. G. J. Mater. Chem. 1995, 5, 365. (b) Dalton, L. R.; Harper, A. W.; Ghosn, R.; Steir, W. H.; Ziari, M.; Fetterman, H.; Shi, Y.; Mustacich, R. V.; Jen, A. K.-Y.; Shea, K. J. Chem. Mater. 1995, 7, 1060. (c) Optical Nonlinearities in Chemistry; Burland, D. M., Ed.; Chem. Rev. 1994, 94, No 1. (d) Eaton, D. F. Science 1991, 253, 281.
0897-4756/96/2808-0541$12.00/0
first modelization schemes, based on the simple twolevel frequency dispersion model,3 more elaborated chemically oriented quantum chemical methods have been required in order to obtain more accurate description of the electronic factors responsible for the NLO response of these molecules. These methods are very helpful to refine the “molecular engineering” for material optimization at the microscopic scale and to select appropriate chromophores among numerous potential candidates. More recently, these investigations have been extended to organometallic molecules.4-13 In many cases the agreement between theory and experiment has been (3) Oudar, J. L. J. Chem. Phys. 1977, 67, 446. (4) For a recent review see: Long, N. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 21. (5) (a) Kanis, D. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 8203. (b) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Chem. Mater. 1991, 3, 19. (c) Kanis, D. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 10339. (d) Kanis, D. R.; Lacroix, P. G.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10089. (6) Lacroix, P. G.; Lin, W.; Wong, G. K. Chem. Mater. 1995, 7, 1293. (7) Houlton, A.; Jasim, N.; Roberts, R. M. G.; Silver, J.; Cunningham, D.; Mcardle, P.; Higgins, T. J. Chem. Soc., Dalton Trans. 1992, 14, 2235. (8) Loucif-Saı¨bi, R.; Delaire, J. A.; Bonazzola, L.; Doisneau, G.; Balavoine, G.; Fillebeen-Khan, T.; Ledoux, I.; Puccetti, G. Chem. Phys. 1992, 167, 369. (9) (a) Wright, M. E.; Toplikar, E. G.; Kubin, R. F.; Setzer, M. D. Macromolecules 1992, 25, 1838. (b) Wright, M. E.; Toplikar, E. G. Macromolecules 1992, 25, 6050. (c) Wright, M. E.; Sigman, M. S. Macromolecules 1992, 25, 6055. (d) Wright, M. E.; Toplikar, E. G.; Lackritz, H. S.; Kerney, J. T. Macromolecules 1994, 27, 3016. (10) (a) Calabrese, J. C.; Cheng, L. T.; Green, J. C.; Marder, S. R.; Tam, W. J. Am. Chem. Soc. 1991, 113, 7227. (b) Cheng, L. T.; Tam, W.; Eaton, D. F. Organometallics 1990, 9, 2856. (c) Cheng, L. T.; Tam, W.; Meredith, G. R.; Marder, S. R. Mol. Cryst. Liq. Cryst. 1990, 189, 137.
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Chem. Mater., Vol. 8, No. 2, 1996 Chart 1
acceptable.5 On the other hand, only a few studies were undertaken on inorganic molecules.14-20 Moreover, to the best of our knowledge, no comprehensive experimental and/or theoretical studies have been carried out to fully understand the second-order NLO response upon the electronic properties of the coordinated metal center. In a previous paper, we started to investigate the molecular second-order NLO properties of three homologous unsubstituted cobalt(II), nickel(II), and copper(II) Schiff-base complexes,21 whose NLO response strongly depends upon the metallic electronic configuration. We report here the synthesis and second-order NLO experimental and theoretical study of three new copper(II), nickel(II), and zinc(II) donor-acceptor Schiffbase complexes (ML) and the related 2,3-bis[[(2-hydroxy-4-(diethylamino)phenyl)(methylene)]amino] 2-butenedinitrile ligand H2L (Chart 1), all of them exhibiting quite large second-order nonlinearities. By comparing the NLO properties of the ligand with those of the metal complexes, a more rational synthetic approach can be reached for the elaboration of transition metal-based chromophores suitable for NLO applications. Experimental Section Materials and Equipment. Diaminomaleonitrile and 4-(diethylamino)salicylaldehyde were purchased from Janssen Chimica and were used without further purification. Elemental analysis were performed by the Service de Microanalyses du C. N. R. S., in Gif sur Yvette, France. Infrared spectra were recorded on a Perkin Elmer 883 spectrophotometer. UV-vis spectra were recorded in chloroform on a Varian 2300 spectrophotometer, and 1H NMR spectra on a Varian XL 400 spectrometer. Synthesis of H2L Ligand. A modified literature preparation was used.22 Diaminomaleonitrile (108 mg, 10-3 mol) and (11) Whittall, I. R.; Humphrey, M. G.; Hoekless, D. C. R.; Skelton, B. W.; White A. H. Organometallics 1995, 14, 3970. (12) Bunting, H. E.; Green, M. L. H.; Marder, S. R.; Thompson, M. E.; Bloor, D.; Kolinsky, P. V.; Jones, R. J. Polyhedron 1992, 11, 1489. (13) For a review of early NLO studies involving organometallic and coordination complexes see: Nalwa H. S. Appl. Organomet. Chem. 1991, 5, 349. (14) Di Bella, S.; Fragala`, I.; Ledoux, I.; Diaz-Garcia, M. A.; Lacroix, P. G.; Marks, T. J. Chem. Mater. 1994, 6, 881. (15) Bougault, M.; Mountassir, C.; Le Bozec, H.; Ledoux, I.; Pucetti, G.; Zyss, J. J. Chem. Soc., Chem. Commun. 1993, 1623. (16) Zyss, J.; Dhenaut, C.; Chauvan, T.; Ledoux, I. Chem. Phys. Lett. 1993, 206, 409. (17) Calabrese, J. C.; Tam, W. Chem. Phys. Lett. 1987, 133. (18) Thami, T.; Bassoul, P.; Petit, M. A.; Simon, J.; Fort, A.; Barzoukas, M.; Villaeys, A. J. Am. Chem. Soc. 1992, 114, 915. (19) (a) Lequan, M.; Branger, C.; Simon, J.; Thami, T.; Chauchard, E.; Persoons, A. Adv. Mater. 1994, 6, 851. (b) Laidlaw, W. M.; Denning, R. G.; Verbiest, T.; Chauchard, E.; Persoons A. Nature 1993, 363, 58. (20) Coe, B. J.; Foulon, J. D.; Hamor, T. A.; Jones, C. J.; McCleverty, J. A.; Bloor, D.; Cross, G. H.; Axon, T. L. J. Chem. Soc., Dalton Trans. 1994, 3427. (21) Di Bella, S.; Fragala`, I.; Ledoux, I.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 9481. (22) (a) Takahashi, M.; Iwamoto, T. J. Inorg. Nucl. Chem. 1981, 43, 253. (b) Wo¨hrle, D. Makromol. Chem. 1983, 184, 763. (c) Wo¨hrle, D. Polym. Bull. (Berlin) 1985, 13, 57.
Lacroix et al. 4-(diethylamino)salicylaldehyde (386 mg, 2 × 10-3 mol) were stirred for 2 days at room temperature in 100 mL of absolute ethanol containing one drop of sulfuric acid as a catalyst. A dark green precipitate was filtered and washed with ethanol. An impurity of the yellow monoimine was removed by chromatography on silica, using CH2Cl2 as eluent (80% yield). Anal. Calcd (found) for C26H30N6O2: C, 68.10 (67.91); H, 6.59 (6.87); N, 18.34 (18.17). 1H NMR (in CDCl3) 1.269 (m, 12H), 3.471 (m, 8H), 6.310 (s, 2H), 6.324 (d, 2H), 7.208 (d, 2H), 8.534 (s, 2H), 12.934 (s, 2H). Synthesis of ML Complexes. NiL: NiCl2‚6H2O (951 mg, 4 × 10-3 mol) and diaminomaleonitrile (432 mg, 4 × 10-3 mol) were dissolved in 75 mL of hot absolute ethanol. 4-(Diethylamino)salicylaldehyde (1.546 g, 8 × 10-3 mol) in 50 mL of hot absolute ethanol was added, and the red solution was allowed to stand overnight at room temperature, providing gold yellow crystals, which were filtered and washed with ethanol (90% yield). Anal. Calcd (found) for C26H28N6NiO2: C, 60.61 (60.32); H, 5.47 (5.77); N, 16.32 (16.23); Ni, 11.39 (9.01). 1H NMR (in CDCl3) 1.234 (t, 12H), 3.415 (q, 8H), 6.310 (s, 2H), 6.322 (d, 2H), 7.090 (d, 2H), 7.505 (s, 2H). CuL: CuCl2‚2H2O (85.2 mg, 5 × 10-4 mol) in 100 mL of absolute ethanol was added to a solution of 193.2 mg (10-3 mol) of 4-(diethylamino)salicylaldehyde and 54 mg (5 × 10-4 mol) of diaminomaleonitrile in 100 mL of absolute ethanol. Gold yellow crystals were kept growing for a few days, filtered, and washed with ethanol (65% yield). Anal. Calcd (found) for C26H28N6CuO2: C, 60.04 (59.78); H, 5.43 (5.72); N, 16.16 (15.74). ZnL: A mixture of 386.5 mg (2 × 10-3 mol) of 4-(diethylamino)salicylaldehyde and 108 mg (10-3 mol) of diaminomaleonitrile in 50 mL of hot absolute ethanol was added to 219.5 mg (10-3 mol) of Zn(CH3COO)2‚2H2O in 150 mL of hot absolute ethanol and stirred for 2 days at 70 °C. The resulting red mixture was filtered hot and cooled in a freezer, which provided the appearance of dark blue crystals (50% yield). Anal. Calcd (found) for C26H28N6ZnO2‚C2H5OH: C, 59.21 (58.34); H, 6.03 (5.76); N, 14.80 (15.48); Zn, 11.51 (11.73). 1H NMR (in CDCl3) 1.140 (t, 12H), 3.257 (q, 8H), 5.607 (s, 2H), 5.991 (d, 2H), 6.805 (d, 2H), 8.041 (s, 2H); (ethanol) 1.232 (t, 3H), 3.420 (q, 2H). EFISH Measurements. The principle of EFISH technique is reported elsewhere.3,23 A Q-switched Nd3+:YAG laser operates at 1.34 µm, delivering 60 ns pulses focused onto a liquid cell and synchronized with a dc field applied to a solution containing the chromophore. By translation of the cell perpendicularly to the incident beam, the variation of the propagation length in the solution creates Maker fringes, whose amplitude and periodicity are related to the nonlinearity of the solution. The experiment was performed for each compounds, by using solutions of increasing concentrations (x ) 10-4 to 5 × 10-3 mol/L) in chloroform, the solvent being chosen to maximize the molecular solubility. The dipole moment was measured independently by a classical method based on the Guggenheim theory.24 Further details of the experimental methodology and data analysis are reported elsewhere.23 Calculation of NLO Response. The all-valence INDO/S (intermediate neglect of differential overlap) method,25 in connection with the sum-over excited-hole states (SOS) formalism,26 was employed. Details of the computationally efficient ZINDO-SOS-based method for describing second-order molecular optical nonlinearities have been reported elsewhere.27 The (23) (a) Ledoux, I.; Zyss, J. Chem. Phys. 1982, 73, 203. (b) Barzoukas, M.; Josse, D.; Fremaux, P.; Zyss, J.; Nicoud, J. F.; Morley, J. O. J. Opt. Soc. Am. B 1987, 4, 977. (24) Guggenheim, E. A. Trans. Faraday Soc. 1949, 45, 203. (25) (a) Zerner, M.; Loew, G.; Kirchner, R.; Mueller-Westerhoff, U. J. Am. Chem. Soc. 1980, 102, 589. (b) Anderson, W. P.; Edwards, D.; Zerner, M. C. Inorg. Chem. 1986, 25, 2728. (c) Bacon, A. D.; Zerner, M. C. Theor. Chim. Acta (Berlin) 1979, 53, 21. (d) Ridley, J.; Zerner, M. C. Theor. Chim. Acta (Berlin) 1973, 32, 111. (26) Ward, J. F. Rev. Mod. Phys. 1965, 37, 1. (27) (a) Kanis, D. R.; Ratner, M. A.; Marks, T. J. J. Chem. Rev. 1994, 94, 195. (b) Kanis, D. R.; Marks, T. J.; Ratner, M. A. Int. J. Quantum Chem. 1992, 43, 61.
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monoexcited configuration interaction (MECI) approximation was employed to describe the excited states. In all calculations, the lowest 160 energy transitions between SCF and MECI electronic configurations were chosen to undergo CI mixing and were included in the SOS. Metrical parameters used for the calculations of NiL were taken from crystal structure data of the related bis(salicylaldiminato)nickel(II) complex28 assuming a C2v symmetry. Standard27b bond distances and bond angles were assumed for the donor and the acceptor groups. Calculations were performed by replacing the diethylamino donor groups with simpler dimethylamino groups. Calculations on H2L were performed assuming an idealized cis planar structure (C2v symmetry).
Results and Discussion Structure of the Compounds. Any comparison between experimental and calculated NLO properties should require a precise knowledge of the molecular structure of the chromophore. The synthesis of salicylaldiminato Schiff base complexes is well documented, and more than 500 crystal structures are reported in the Cambridge Structural Database System.29 Complexes based on diaminomaleonitrile and salicylaldehydes have been also reported,22 but no crystal structures have been solved to date. On the other hand, all our attempts to grow single crystals suitable for crystal structure resolutions were unsuccessful. Elemental analysis and NMR data are consistent with the expected structures. Similar Cu(II) and Ni(II) complexes with N,N′-bis(salicylidene)-1,2-phenylenediamine have been reported to be planar.30 Therefore, it can be reasonably inferred that present NiL and CuL complexes are square planar as well. The structure of the zinc derivative cannot be defined as precisely; therefore no calculations were performed on this molecule. Many salicylaldimine zinc complexes have been described with an idealized tetrahedric structure.31 The nature of the present ligand likely forces a square-pyramidal arrangement of zinc, with an apical molecule of solvent as it has been observed in several related structures.32-34 This is consistent with the elemental analysis and the presence of one molecule of ethanol clearly evidenced by NMR. Spectroscopic Properties. It is well-known that the degree of charge transfer in the ground state of molecules containing CN groups can be determined from the infrared absorption frequency of the CN stretch mode, when the withdrawing effect of the nitrile group is involved in a molecular system containing a donor counterpart.35-37 However, very similar frequencies (28) Manfredotti, G.; Guastini, C. Acta Crystallogr. 1983, C39, 863. (29) Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB21EZ, U.K. (30) (a) Montgomery, H.; Morosin, B. Acta Crystallogr. 1961, 14, 551. (b) Cassoux, P.; Gleizes, A. Inorg. Chem. 1980, 19, 665. (31) Holm, R. H.; Everett, G. W.; Chakravorty, A. Prog. Inorg. Chem. 1966, 7, 83. (32) (a) Hall, D.; Moore, F. H. Proc. Chem. Soc. 1960, 256. (b) Hall, D.; Moore, F. H. J. Chem. Soc. A 1966, 1822. (33) Van Veggel, F. C. J. M.; Harkema, S.; Bos, M.; Verboom, W.; Van Straveren, C. J.; Gerritsma, G. J.; Reinhoudt, D. N. Inorg. Chem. 1989, 28, 1133. (34) Flassbeck, C.; Wieghardt, K.; Bill, E.; Butzlaff, C.; Trautwein, A. X.; Nuber, B.; Weiss, J. Inorg. Chem. 1992, 31, 21. (35) Inoue, M.; Inoue, M. B. Inorg. Chem. 1986, 25, 37. (36) Bandrauk, A. D.; Truong, K. D.; Carlone, C.; Jandl, S. J. Phys. Chem. 1985, 89, 434. (37) Robles-Martinez, J. G.; Salmeron-Valverde, A.; Alonso, E.; Soriano, C. Inorg. Chim. Acta. 1991, 179, 149.
Figure 1. UV-visible optical absorption spectra of H2L (dotted line) and NiL recorded in chloroform at the same concentration (c ) 4 × 10-5 mol L-1). Table 1. Experimental and ZINDO Calculated Linear Optical and Nonlinear Optical Properties of the H2L and ML Complexesa calculated data
experimental data
λmax µ βvec, (10-30 β0, (10-30 λmax compd (nm) (D) cm5 esu-1) cm5 esu-1) (nm) H2L NiL CuL ZnL
415 6.0 447 2.9
40 76
25 41
570 584 574 586
µ (D)
βvec (10-30 cm5 esu-1)
11 6.2 6.8 6.5
109 ( 10 235 ( 40 200 ( 30 400 ( 100
a Experimental and calculated β data refer to a 1.34 µm incident laser radiation.
were observed in the case of H2L (2210 cm-1), NiL (2212 cm-1), CuL (2211 cm-1), and ZnL (2210 cm-1). This suggests that the metals modify very slightly the charge distribution of the π-electronic structure versus that of the free ligand. Actually, the main features on the infrared spectra of the three complexes are the same, except at low energy. In fact, the different frequencies observed (434, 347, and 325 cm-1 for NiL, CuL, and ZnL, respectively) are likely related to the differences in the metal-ligand bond, since no absorption is observed at such frequencies in the H2L free ligand. The UV-visible spectra recorded in chloroform for NiL and H2L were found to be rather similar (Figure 1), which strongly indicates that the electronic properties of these systems are dominated by the donoracceptor-substituted organic chromophore. The absorption maxima of the lowest optical transition were observed at 570 nm for H2L (calculated at 415 nm) and 584 nm for NiL (calculated at 447 nm). There is a significant difference (about 150 nm) between calculations and experiment, but both clearly show a red shift upon complexation, also observed for CuL (574 nm), and for ZnL (586 nm; Table 1). An additional absorption band is observed for NiL in the range 200-300 nm, probably due to a metal-ligand charge transfer transition. Second-Order NLO Properties. This work reports an attempt to relate the second-order NLO response with the electronic properties of the coordinated metal center in a series of four chromophores, including the acentric Schiff-base free ligand. The µ and βvec (vector hyperpolarizability component along the dipole moment (x) direction) values of the four chromophores are reported in Table 1. It is well-known that β strongly depends on the laser frequency (eq 1 indicates the two-
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Table 2. Energies (λmax in nm), Oscillator Strengths (f), Dipole Moment Changes between Ground and Excited States (∆µ in D), and Composition of the Dominant Excited States Involved in the Nonlinearity of H2L and NiL transition
λmax
f
∆µ
state (%)a
compositionb of CI expansion
L
1f2 1f3
415 317
1.14 0.6
3.4 3.3
S2 (34) S3 (20)
NiL
1f2 1f3
447 390
0.89 0.37
4.3 7.0
S2 (21) S3 (30)
0.25 χ75f78 + 0.96 χ76f77 0.26 χ72f78 + 0.34 χ73f77 - 0.48 χ75f77 -0.24 χ75f81 - 0.55 χ76f78 - 0.29 χ76f80 0.29χ77f81 - 0.35χ79f82 - 0.86χ80f81 0.22χ74f81 + 0.20χ77f82 + 0.39χ78f81 - 0.84χ79f81
compd
a S ) singlet excited state. Values in parentheses refer to percent contribution of the excited state to β . b Orbital 76 is the HOMO vec and 77 is the LUMO in H2L. Orbital 80 is the HOMO, and 81 the LUMO in NiL.
level dispersion of β):
βxxx )
3e2p2fδµ W4 2mW3 (W2 - (2pω)2)(W2 - (pω)2)
(1)
In this equation, W ) pωeg is the energy of the chargetransfer transition, ω the fundamental frequency of the laser beam,3 and βxxx is the principal tensor component along the dipolar axis. Equation 1 indicates that when 2ω is close to the absorption maxima, β is strongly enhanced. This effect accounts for the fact that the experimental βvec values are significantly larger than the calculated ones, the experimental λmax (570-586 nm) being much closer to the 670 nm second harmonic wavelength than the calculated values (415-447 nm). In this frequency range, it can be calculated that a displacement of 100 nm in the wavelength of the electronic transition roughly doubles the β value. The 150 nm difference between the calculated and observed transitions can readily account for the difference between the measured and theoretical βvec values. The most striking feature in Table 1 is the significant increases of the βvec value upon metal complexation, the Zn atom displaying the highest βvec value (up to 400 × 10-30 cm5 esu-1, respectively). The role of the nickel will be discussed first with a detailed comparative analysis of the calculated data on H2L and NiL. Composition of mixing coefficients of the CI expansion of the dominant excited states involved in the nonlinearity of H2L and NiL are reported in Table 2. First, it should be pointed out that the origin of the nonlinearity is similar for both H2L and NiL molecules, and can be related to the first two low-lying optical transitions (namely, 1 f 2 and 1 f 3), principally involving the SHOMO, HOMO f LUMO transitions. The first two orbitals are mostly localized on the aromatic rings and the donor groups, while the LUMO is localized on the imine and nitrile groups. The metal moderately modifies the charge distribution on these orbitals and hence, λmax, ∆µ, and f. It is very important to point out that this slight modification could not have been predicted with the intuition of a synthetic chemist as is usually the case for most organic chromophores. In fact, even if most attempts to design organometallic systems for second-order nonlinear optics have mainly been done using metal atoms as replacement of donor or acceptor side groups,38 essentially following the same strategy as in paranitroaniline, this former route based on a simple push-pull two-level model has already been questioned for organometallics5c,d and octupolar systems.39 We have recently investigated three unsubstituted cobalt(II), copper(II), and nickel(II) bissalicylaldiminato (38) Green, M. L. H.; Marder, S. R.; Thompson, M. E.; Bandy, J. A.; Bloor, D.; Kolinsky, P. V.; Jones, R. J. Nature 1987, 330, 360.
Figure 2. Difference in electronic populations between the ground state and the two excited states involved in β (twolevel terms) for H2L (left), and NiL (right). The black contribution is indicative of a decrease in electron density in the chargetransfer process.
Schiff base complexes whose hyperpolarizability (βvec) values were found to be surprisingly different,21 the Cu(II) and Co(II) complexes exhibiting βvec values ∼3 and ∼8 times larger, respectively, than that of the nickel derivative. This result was explained in terms of a different nature of excited states. This trend toward a better efficiency as the electronic configuration of the metal center changes is observed here also, especially for the zinc derivative (d10 configuration) where βvec is found to be 2 times larger than that of its nickel homologue. The difference in electronic populations between the ground state and the two excited states involved in βxxx,2 two-level terms (transition 1 f 2 and 1 f 3) is shown in Figure 2 in the case of H2L and NiL. The pictures clearly show the importance in the 1 f 3 transition in the NLO response of the complex versus that of the metal. In this transition, the metal acts as a electron donor. Therefore, it could be inferred that on passing from d8 to d9 and d10 metal configuration (39) (a) Zyss, J.; Ledoux, I. Chem. Rev. 1994, 94, 77. (b) Zyss, J. Chem. Phys. 1993, 98, 6583. (c) Zyss, J.; Ledoux, I.; Nicoud, J. F. In Molecular Nonlinear Optics: Materials, Physics and Devices; Zyss, J., Ed.; Academic Press: Boston, 1993.
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NiL, but the most stricking feature is the increase of the intensity of the band around 550 nm at higher concentrations, not observed for NiL. This might imply a tendency for aggregation in solution making the comparison between NiL and ZnL irrelevant, despite the fact that the hyperpolarizability was observed to remain constant upon changing the concentration of ZnL in the EFISH experiments.41 On the other hand, large secondorder nonlinearities have been already reported in zinc complexes15 in a roughly tetrahedric geometry. In light of these observations, such metal probably deserves more careful investigations. Figure 3. UV-visible optical absorption spectrum of ZnL in chloroform. The concentrations are 6.0 × 10-5 (a), 4.0 × 10-5 (b), 1.6 × 10-5 (c), and 6.4 × 10-6 (d) mol/L.
would decrease the NLO response, according with the energetic stabilization of metal 3d subshells along the series. Nevertheless, a comparison of the NLO response of an homologous series is of value only for isostructural compounds, such as the case of presumably planar NiL and CuL complexes. In that case, the similar observed second-order nonlinearity is in agreement with the above findings, but contrast with the trend observed in unsubstituted M(salophen) complexes.21 This suggests that in the present case the donor-acceptor substituents dominate the NLO response, while the coordination of the metal favor the conjugation of the donor-acceptor chromophore and, hence, allows a larger charge-transfer in the excited states. In the zinc(II) derivative, a different structure is expected (vide infra) and different low-lying excited states are probably involved in the NLO response. For instance, it has been reported that a ligand-to-ligand charge transfer could be observed through a zinc(II) atom acting as a bridge.40 Such an explanation would imply that the molecule of ethanol is involved in the nonlinearity as a donor additional substituent. The UV-visible spectra of ZnL were recorded in chloroform at different concentration (Figure 3). Several differences can be observed in these spectra versus that of (40) Koester, V. J. Chem. Phys. Lett. 1975, 32, 575.
Conclusion For about two decades a simple and very helpful twolevel model has allowed organic chemists to select compounds expected to exhibit large second harmonic generation. Recent studies have shown that such a simple model was outmoded especially for organometallic and inorganic chromophores. In addition, metal complexes offer a large variety of new NLO structures that allow to locate the metal atom in a more strategic position at the center of the charge-transfer system, making a better use of the metal-d orbital hybridization schemes in an organic ligand environment. In our study, we have shown that a family of new Schiff-base complexes exhibits a second harmonic response which in some cases is several times larger than that of the free ligand. It should be pointed out that such result could have been hardly predicted without a theoretical calculation, a pure chemical intuition being not appropriate. These results confirm the ability of adequate computational models to predict the nonlinear optical response of metal complexes, and open the route to efficient screening methods for selecting efficient inorganic NLO materials. CM950426Q (41) However, note that EFISH measurements were necessarily performed in an higher range of concentrations (>10-4 mol L-1) than that of the UV-visible spectra. This might imply that at these concentrations the ZnL is already aggregated, thus accounting for the unobserved βvec variation.
