Detailed Picosecond Kerr-Gated Time-Resolved Resonance Raman Spectroscopy and Time-Resolved Emission Studies of Merocyanine 540 in Various Solvents

By using the unique fluorescence rejection method of Kerr-gating, detailed picosecond time-resolved resonance Raman experiments have been performed on the highly fluorescent photodynamic therapy dye, Merocyanine 540 (MC540). This has enabled collection of the first-singlet trans excited resonance Raman spectra of this dye in a range of protic and aprotic solvents of varying viscosity and polarity, as well as an organized reverse micelle. The detailed vibrational spectra support the idea that protic solvents form a H-bonded cluster around the oxygen groups of the thiobarbiturate group, which lock the group in position and hinder its rotation. In the reverse micelle containing hexane/AOT and a water pool (ω o = 32) the dye is orientated to permit the thiobarbiturate group to interact with water pool molecules. Using the Kerr-gate setup, time-resolved emission spectra of MC540 were also recorded in various solvents. The dye undergoes fast vibrational cooling (2−10 ps), which can be related to the solvent's thermal diffusivity. A second slower process (20−100 ps) also occurs, which is viscosity dependent and is associated with structural relaxation of the polymethine unit within the dye.