Polarisation resolved ultrafast spectroscopic investigation of photoinduced structural dynamics in DCM dye

This thesis reports on the results of steady-state, polarisation and time-resolved spectroscopic investigations of the excited-state configurational dynamics of the well-known donor-acceptor dye molecule DCM. The aim of this study was to investigate the structural configuration of the excited-state or DCM and add to the discussion about the proposed, but controversial, existence of a twisted intramolecular charge transfer (TICT) state in this molecule. Using a suite of purpose built laser spectroscopy experiments, the spectral dynamics of DCM and a planar, conformationally-restricted analogue molecule were probed. Steady-state, time-resolved fluorescence and pump-supercontinuumprobe measurements were made on the molecules in a range of solvents. Further information was obtained by analysing the polarisation-dependence of these signals in order to determine the time-dependent anisotropy r(t) - a parameter that has features and dynamics determined by the transition dipole moment of the probe molecule and the rotational diffusion of the molecules in solution. By performing a number of different solvent dependent measurements focussing on various solvent parameters, the dynamics and the features seen were attributed to the viscosity, dielectric constant and dipole moment of the solvent. The de-convoluted dynamics were then compared with that of the planar twistrestricted molecule, DCJ. Analysis of the data, produced strong evidence to support the formation of an intramolecular charge transfer (ICT) state after photoexcitation. However, the results indicate that the ICT state of DCM does not involve a twisting conformation about the dimethylamino bond, despite this being the twist position most often cited in the literature with regards to a TICT in this molecule.