One- and two-photon time-resolved fluorescence study of neurotransmitter amino acid–5,6-benzocoumarin conjugates

The time-resolved photophysical characterisation of a set of fluorescent conjugates of neurotransmitter amino acids (glutamic acid, tyrosine and β-alanine) based on a methoxylated 5,6-benzocoumarin fluorophore was performed. Both one- and two-photon excitation were employed to excite the fluorophore and the cross-section with wavelength for the two-photon process determined. The photophysical characterisation was supported using a molecular modelling study, which helped elucidate the molecular transitions (by a ZINDO calculation) and differences in the electron density between the HOMO and LUMO in geometry (PM3) optimised structures. The effect of prolonged irradiation of the conjugates was monitored via fluorimetry and differences in the kinetics of the process observed were found to depend on the amino acid employed.