LASER-INDUCED FLUORESCENCE OF JET-COOLED NONCONJUGATED BICHROMOPHORES- BIS-PHENOXYMETHANE AND BIS-2,6-DIMETHYLPHENOXYMETHANE

The electronic spectroscopy of bichromophoric molecules linked by a OC H2O chain, such as bis-phenoxymethane (1), and bis-2,6-dimethylphenoxy methane (3), has been studied in a supersonic free jet by laser induce d fluorescence. The experimental results have been compared to calcula tions resting on the perturbative CIPSI method, in which the di- and t ri-excited configurations involving the pi as well the sigma orbitals have been taken into account. The bis-phenoxymethane (1) molecule show s a single 0-0 transition which is blue-shifted relative to anisole by more than 400 cm(-1). This blue-shift has been theoretically related to the conformation of the bichromophore which displays an out-of-plan e distorsion of the ether chain relative to anisole. The calculations clearly show that the observed blue-shift of the transition is related to this distorsion, and not to any electronic coupling between both c ycles which is very weak. The single transition experimentally observe d corresponds to the most stable structure of (1) which is in a gauche -gauche conformation relative to the C-O bonds of the chain. In this s tructure the cycles are equivalent. The study of van der Waals complex es of (1) with usual solvents confirms this interpretation and shows t hat the equivalence of the cycles is removed by complexation. This con trasts with the bichromophore (3) whose 0-0 transition is blue-shifted by only 25 cm(-1) relative to the 2,6-dimethylanisole subunit. Calcul ations have shown that in this case, the monomeric subunit as well as the bichromophore are in an out-of-plane conformation due to the steri c hindrance introduced by the methyl groups. Moreover the excited stat e of (3) behaves as a weakly fluorescent exciplex whereas the emission resulting from the excitation of (1) is resonant. For the sake of com parison, the fluorescence excitation spectrum of the van der Waals dim er of anisole has also been studied and exhibits a red-shift with resp ect to bare anisole, The equilibrium geometry and the exciton coupling have also been calculated for the anisole van der Waals dimer, by mea ns of the exchange perturbation theory.