SQUARAINE CHEMISTRY - EFFECTS OF SOLVENT AND TEMPERATURE ON THE FLUORESCENCE EMISSION OF SQUARAINES

The squaraines - a class of donor-acceptor-donor molecules - were show n to exhibit intense multiple fluorescence in the visible region. The three emission bands, designated alpha, beta and gamma in the order of decreasing energy, were found to be emissions from the excited state of squaraine, the excited state of the solute-solvent complex and a re laxed, twisted excited state respectively. In this work, the results o f an investigation of the effects of the solvent and temperature on th e absorption and fluorescence emission of bis(4-dibutylaminophenyl)squ araine (1) are reported. The data confirm that squaraine forms solute- solvent complexes in solvents. Evidence is provided that the solute-so lvent interaction is short range, and that polar solvents usually exer t a bathochromic effect on the absorption maximum wavelength lambda(ma x). Along with the bathochromic shift, a gradual change in the composi tion of the emission band, from being dominated by the alpha emission in ether to being dominated by the beta emission in polar solvents, is observed. Both lambda(max) and K(eq) (the equilibrium constant for co mplexation) are shown to correlate well with the solvent parameter pi . The results indicate that the solute-solvent complexation process is responsible for the bathochromic shift in lambda(max) and the composi tion change in the emission spectra. This conclusion is supported by v ariable and low temperature spectral data. While the lifetime of excit ed 1 is shown to be independent of the solvent (2.3 +/- 0.1 ns), the l ifetimes of the excited solute-solvent complex and the relaxed, twiste d excited state are solvent sensitive, varying from 0.6 to 3.5 ns and 0.7 to 2.9 ns respectively. From the fluorescence lifetime and the rad iative decay rate, the rate of the radiationless decay, which involves rotation of the C-C bond between the phenyl ring and the four-membere d ring of squaraine, can be calculated. The rotation rate is shown to increase rapidly as the twisting of the squaraine chromophore increase s. Finally, this work also shows that the intensity of the gamma emiss ion can be inhibited not only by lowering the temperature but also by a rigid polymer matrix at room temperature. The results support the po stulation that a twisting motion is required for the generation of the relaxed excited state.