Ultrafast intramolecular electronic energy-transfer dynamics in a bichromophoric molecule

Abstract

Intramolecular electronic energy-transfer (intra-EET) dynamics has been investigated in 2-(9-anthryl)-1H-imidazo [4,5-f] [1,10]-phenanthroline (AIP), a newly synthesized bichromophoric molecule, using the steady-state and time-resolved absorption and fluorescence spectroscopic techniques. In AIP, anthracene (AN) and 1H-imidazo [4,5-f] [1,10]-phenanthroline (IP) molecules are directly linked to each other through a C-C σ bond and without any intervening molecular bridge. Two constituent chromophoric moieties of this bichromophoric molecule interact relatively weakly in the ground state. In the excited singlet state, however, the AN moiety transfers its excitation energy quantitatively (the efficiency of energy transfer, φ_EET, is near unity) and rapidly (the rate of energy transfer, k_EET, is 1.8×10¹¹ s^-1 in methanol) to the unexcited IP moiety. k_EET decreases linearly with increase in viscosity of the solvents, and the process is significantly retarded in rigid glass matrixes. These observations suggest that, for an efficient EET process, the molecule needs to attain a conformational geometry, which is different from that of the ground state, by undergoing a conformational relaxation process following photoexcitation. The theoretically calculated energy-transfer rate (5.1×10⁹ s^-1) due to the Förster dipole-dipole-induced resonance-interaction mechanism is about 2 orders of magnitude smaller than the experimentally determined energy-transfer rate. Hence, the Dexter through-space exchange-interaction mechanism, which becomes predominant at shorter interchromophoric separation (R ~6.3 Å in AIP) and requires specific conformation for efficient orbital overlap, should have the major contribution to the intra-EET process in AIP. Viscosity dependence of k_EET suggests that we possibly measure the rate of the conformational relaxation process using the intra-EET process as the probe.