Squaraine chemistry: effects of solvent and temperature on the fluorescence emission of squaraines

Abstract

The squaraines — a class of donor-acceptor-donor molecules — were shown to exhibit intense multiple fluorescence in the visible region. The three emission bands, designated α, β and γ 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 relaxed, twisted excited state respectively. In this work, the results of an investigation of the effects of the solvent and temeperature on the absorption and fluorescence emission of bis(4-dibutylaminophenyl)squaraine (1) are reported. The data confirm that squaraine forms solute-solvent complexes in solvents. Evidence is provided that the solute-solvent interactions is short range, and that polar solvents usually exert a bathochromic effect on the absorption maximum wavelength λmax. Along with the bathochromic shift, a gradual change in the composition of the emission band, from being dominated by the α emission in ether to being dominated by the β emission in polar solvent, is observed. Both λmax and Keq (the equilibrium constant for complexation) are shown to correlate well with the solvent parameter π*. The results indicate that the solute-solvent complexation process is responsible for the bathromic shift in λmax and the composition change in the emission spectra. This is responsible for the bathochromic shift in λmax and the composition change in the emission spectra. This conclusion is supported by variable and low temperature spectral data. While the lifetime of excited 1 is shown to be independent of the solvent (2.3±0.1 ns), the lifetimes of the excited solute—solvent complex and the relaxed, twisted excited state are solvent sensitive, varying from 0.6 to 3.5 ns respectively. From the fluorescence lifetime and the radiative decay rate, the rate of the radiationless decay, which involves rotation of the CC bond between the phenyl ring and the four-membered ring of squaraine, can be calculated. The rotation rate is shown to increase rapidly as the twisting of the squaraine chromophore increases. Finally, this work also shows that the intensity of the γ emission not only by lowering the temperature but also by a rigid matrix at room temperature. The results support the postulation that a twisting motion is required for the generation of the relaxed excited state.