Laser induced fluorescence of jet-cooled non-conjugated bichromophores: bis-phenoxymethane and bis-2,6-dimethylphenoxymethane Original Research Article

The electronic spectroscopy of bichromophoric molecules linked by a OCH2O chain, such as bis-phenoxymethane (1), and bis-2,6-dimethylphenoxymethane (3), has been studied in a supersonic free jet by laser induced fluorescence. The experimental results have been compared to calculations resting on the perturbative CIPSI method, in which the di- and tri-excited configurations involving the π as well the v orbitals have been taken into account. The bis-phenoxymethane (1) molecule shows 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-plane 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 cycles which is very weak. The single transition experimentally observed 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 structure the cycles are equivalent. The study of van der Waals complexes of (1) with usual solvents confirms this interpretation and shows that the equivalence of the cycles is removed by complexation. This contrasts with the bichromophore (3) whose 0—0 transition is blue-shifted by only 25 cm−1 relative to the 2,6-dimethylanisole subunit. Calculations have shown that in this case, the monomeric subunit as well as the bichromophore are in an out-of-plane conformation due to the steric hindrance introduced by the methyl groups. Moreover the excited state of (3) behaves as a weakly fluorescent exciplex whereas the emission resulting from the excitation of (1) is resonant. For the sake of comparison, the fluorescence excitation spectrum of the van der Waals dimer of anisole has also been studied and exhibits a red-shift with respect to bare anisole. The equilibrium geometry and the exciton coupling have also been calculated for the anisole van der Waals dimer, by means of the exchange perturbation theory.