Photophysical properties of halo-derivatives of angelicins

The properties of the lowest excited singlet and triplet state of four halo-angelicins (HA) have been investigated by steady-state and time-resolved spectrometric techniques. The study has been performed in solvents with different polarity and proticity: cyclohexane, dioxane, acetonitrile, ethanol and 2,2,2-trifluoroethanol. Absorption and emission spectra showed that the nature of lowest singlet state is mainly pi,pi* and the transitions are allowed or partially allowed. Absorption coefficients, fluorescence quantum efficiency and lifetimes are also presented. Flash photolysis investigations have indicated the presence of a unique transient assignable to the lowest triplet state T-1. The triplet state has been characterized in terms of absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yields. The racing of both intersystem crossing and fluorescence quantum yields going from a non-polar to a highly polar-protic solvent indicates the presence of a S-2 state, n,pi* in nature, close lying to the pi,pi* one. The quantum yields of the singlet decay pathways (fluorescence, intersystem crossing and internal conversion) depends markedly on the energy gap between S-1 and S-2 states in agreement with the manifestation of the "proximity effect". In fact in cyclohexane HA decay mainly through S-1 -> S-0 internal conversion, while in trifluoroethanol their fluorescence and intersystem crossing increase significantly. Singlet-oxygen quantum yields have been also determined in order to understand the possible application of the investigated drugs in photodynamic therapy. The absence of photochemical pathways allowed the indirect evaluation of the internal conversion decay bringing to the achievement of a complete decay pathways picture