Second-harmonic generation from Langmuir–Blodgett films of centrosymmetric squaraines with heterocyclic donor groups

Abstract

Langmuir–Blodgett (LB) monolayers of squaraine dyes of general formula D–(C₄O₂)–D, where D is a heterocyclic donor, exhibit second-harmonic generation (SHG) with, for example, χ_zzz⁽²⁾=100–150 pm V^–1 at 1.064 µm for 2,4-bis[(3,3-dimethylindolin-2-ylidene)methyl]squaraine. The behaviour is not an inherent property of the molecule and the second-order susceptibility is too high to arise from polarisation of the molecular film at the interface. Instead, the SHG is attributed to non-centrosymmetric aggregation with intermolecular charge transfer between the donor (heterocycle) and acceptor (C₄O₂) groups of adjacent molecules playing a significant role and contributing to the susceptibility. The nonlinear optical properties are discussed with reference to films of 1,4-bis(pyrrol-2-yl)squaraine derivatives which show either very weak or no SHG. The behaviour is dependent upon the type of aggregation and the extent of the donor–acceptor interaction.

References

1. G. J. Ashwell, G. Jefferies, D. G. Hamilton, D. E. Lynch, M. P. S. Roberts, G. S. Bahra and C. R. Brown, Nature, 1995, 375, 385; G. J. Ashwell, M. P. S. Roberts, N. D. Rees, N. J. Calos and C. H. L. Kennard, Langmuir, submitted for publication.
2. G. J. Ashwell, D. G. Bucknall, G. M. S. Wong, G. S. Bahra and C. R. Brown, Langmuir, 1997, 13, 1629.
3. G. J. Ashwell, P. Leeson, G. S. Bahra and C. R. Brown, J. Opt. Soc. Am. B, in press.
4. G. J. Ashwell, G. Jefferies, N. D. Rees, P. C. Williamson, G. S. Bahra and C. R. Brown, Langmuir, submitted for publication.
5. G. J. Ashwell, G. S. Bahra, C. R. Brown, D. G. Hamilton, D. E. Lynch and C. H. L. Kennard, J. Mater. Chem., 1996, 6, 23.
6. M. Tristani-Kendra, C. J. Eckhardt, J. Bernstein and E. Goldstein, Chem. Phys. Lett., 1983, 98, 57; M. Tristani-Kendra and C. J. Eckhardt, J. Chem. Phys., 1984, 81, 1160; J. Bernstein and E. Goldstein, Mol. Cryst. Liq. Cryst., 1988, 164, 213.
7. C. W. Dirk, W. C. Herndon, F. Cervantes-Lee, H. Selnau, S. Martinez, P. Kalmegham, A. Tan, G. Campos, M. Velez, J. Zyss, I. Ledoux and L. Cheng, J. Am. Chem. Soc., 1995, 117, 2214.
8. G. J. Ashwell, Adv. Mater., 1996, 8, 248.
9. K. Y. Law and C. C. Chen, J. Phys. Chem., 1989, 91, 5184; C. C. Chen, K. Y. Law and D. G. Whitten, J. Phys. Chem., 1996, 100, 5949.
10. G. J. Ashwell, J. Mater. Chem., preceding paper.
11. J.-L. Brédas and E. Brouyère, personal communication.
12. D. E. Lynch, U. Geissler, I. R. Peterson, M. Floersheimer, R. Terbrack, L. F. Chi, H. Fuchs, N. J. Calos, B. Wood, C. H. L. Kennard and G. J. Langley, J. Chem. Soc., Perkin Trans. 2, 1997, 827.
13. N. Kuramoto, K. Natsukawa and K. Asao, Dyes and Pigments, 1989, 11, 21.
14. A. Treibs and K. Jacob, Angew. Chem., Int. Ed. Engl., 1965, 4, 694.
15. I. R. Girling, N. A. Cade, P. V. Kolinski, J. D. Earls and G. H. Cross, Thin Solid Films, 1985, 132, 101.
16. C. Bosshard, K. Sutter, P. Prêtre, J. Hulliger, M. Flörsheimer, P. Kaatz and P. Günter, Organic Nonlinear Optical Materials, Advances in Optics, Vol. 1, ed. A. F. Garito and F. Kajzar, Gordon and Breach, Basel, 1995.
17. Y. Kobayashi, M. Goto and M. Kurahashi, Bull. Chem. Soc. Jpn., 1986, 59, 311.
18. J. Bernstein, M. Tristani-Kendra and C. J. Eckhardt, J. Phys. Chem., 1986, 90, 1069.
19. G. J. Ashwell and P. Leeson, in Electrical and related Properties of Organic Solids ed. R. W. Munn, A. Miniewicz and B. Kuchta, NATO ASI Series, 1997, 24, 297, Kluwer, Dordrecht.
20. G. J. Ashwell, P. D. Jackson, G. Jefferies, I. R. Gentle and C. H. L. Kennard, J. Mater. Chem., 1996, 6, 137.
21. G. J. Ashwell, D. Lochun, G. S. Bahra, C. R. Brown, I. R. Girling and C. H. L. Kennard, Supramol. Sci., 1995, 2, 131.
22. G. J. Ashwell, T. Handa, G. Jefferies, T. W. Walker, M. R. Bryce and A. M. Grainger, Supramol. Sci., 1997, 4, 219.
23. A. Triebs and K. Jacob, Liebigs Ann. Chem., 1966, 699, 153.
24. L. P. Yu, M. Chen, L. R. Dalton, X. F. Cao, J. P. Jiang and R. W. Hellwarth, Mater. Res. Soc. Symp. Proc., 1990, 173, 607.