The effects of molecular aggregation and isomerization on the fluorescence of "push-pull" hyperpolarizable chromophores

Hyperpolarizable organic molecules are attracting interest for use in nonlinear optical devices. Some zwitterionic merocyanine chromophores with exceptionally high first hyperpolarizabilities have been synthesized that possess an electron donor moiety coupled to an electron acceptor through a conjugated double bond system. Aligned arrays of these molecules tethered to a polyurethane backbone and spun onto a substrate to form a thin film, respond to the application of external electric fields with changes to their refractive indices, i.e. the electro-optic effect. This behavior can be capitalized upon to fabricate optical switches and modulators. High concentrations, or loadings, of these nonlinear optical molecules in thin films are required to produce a usable electro-optic effect, ca. 0.1-0.5 mol L-1, and this promotes molecular aggregation which alters their polarizabilities and also has implications for their photostabilities. The fluorescence spectra of these molecules in polymer films at ambient temperature and in solution in a 9:1 ethanol-water mixture over a range of temperatures down to 80 K show a substantial blue shift as the temperature is lowered. This is attributed to a reduction in the solvating power of the solvent as it becomes increasingly more viscous at lower temperatures. At the lowest temperatures studied a shoulder on the lower energy side of the fluorescence spectral distribution is apparent which is ascribed to the formation of antiparallel dimers/aggregates; a consequence of the highly dipolar character of these molecules. In addition, dual fluorescence is observed in high viscosity environments suggesting the involvement of twisting about the bridging conjugated bond system that links the electron donor to the electron acceptor. (c) 2006 Elsevier B.V. All rights reserved.