Oleg S. Vasyutinskii
The talk presents the results of theoretical and experimental studies of polarized fluorescence in flurophores excited by two-photon two-color (2P2C) laser pulses. Quantum mechanical expressions describing the fluorescence polarization have been derived under the condition of isotropic rotation diffusion for arbitrary polarization of each of the three photons involved in the photoprocess . The expressions for the TPEF intensity are based upon the spherical tensor approach and valid for any symmetric, or asymmetric top molecule polarization. The expressions are written in terms of the molecular parameters that have clear tensor notation and contain all molecular information that can be extracted from the experiments .
The experiment has been carried out on p-terphenyl  and DMQ  dissolved in cyclohexane/ paraffin and on indol  and tryptophan dissolved in propandiol. The analysis of the obtained experimental data was based upon the ab initio calculations of the vertical excitation energies and transition dipole moments and allowed for determination of the complete structure of the two-photon absorption tensor, fluorescence lifetimes, and rotational correlation times. The results obtained gave important information on possible two-photon excitation channels and interaction of flurophores with surrounding solute molecules.
The developed technique is important for two-photon fluorescence spectroscopy of fluorophores residues in biomolecules, in particular intracellular investigation of tryptophans in proteins can provide unique information on the protein structure, dynamics, and solvation. Another important application of the obtained results is the possibility of excitation of selective vibration modes in fluorophores by one- and two-photon laser transitions opening a way for the photochemical reaction control of biologically relevant molecules.
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