Influence of steric hindrance on luminescence anisotropy of resonance dipole–dipole transfer of electronic excitation energy

Abstract

An orientation factor is calculated and the effect of steric hindrance in the donor and acceptor molecules is found for the efficiency of resonance dipole-dipole transfer of electronic energy. General expressions for the acceptor luminescence anisotropy under such conditions in isotropic media (gases and liquids) are obtained taking into account orientational relaxation in ensembles of donors and acceptors. It is shown that the luminescence anisotropy of an acceptor with significant steric hindrance can increase by more than an order of magnitude compared with the case with no hindrance.Introduction. Resonance dipole-dipole transfer is known to be one of the most efficient pathways via which molecules transfer excitation energy [1,2]. A very important characteristic of this process is the anisotropy of the transfer, which is determined by an orientational factor that depends on the mutual configuration of the donor and acceptor. The anisotropy of energy transfer is exceedingly important in photochemistry and photobiology. Therefore, studies specially dedicated to calculations of the orientational factor for a donor and acceptor conjugated to a macromolecule have been published [3][4][5][6]. Also, the theory of the anisotropy of time-resolved luminescence that takes into account the orientational factor and internal rotations of the chromophores with intramolecular resonance energy transfer in biomolecules has been developed [7,8].This type of transfer will also be effective for a donor enclosed in inclusion complexes where contact exchange mechanisms are substantially limited and shielded by the molecule's own shell or that of the host molecule and in various nanostructures. It has been shown convincingly using energy transfer between chlorophyll a and pheophytin a encapsulated in micelles of the surfactant Triton X-100 [9] that this transfer has an induced-resonance dipoledipole nature.Intermolecular energy exchange through a dipole×dipole mechanism has been studied rather well in the condensed state for viscous solutions and for molecular diffusion [2]. The role of the orientational factor in concentrational depolarization of luminescence in solutions has been studied theoretically [10,11]. Very many versions of the theory of concentrational depolarization have been published [2]. However, the polarization characteristics of acceptor luminescence that are associated with this type of transfer have not been studied in the gas phase. Also, the influence of steric hindrance on the polarization has also not been studied theoretically. In addition, a series of experimental studies on the transfer of electron excitation energy in the gas phase from benzene derivatives to various acceptors showed that they could affect the ratio of the dipole×dipole and exchange transfer mechanisms [1,12] and, therefore, the anisotropy of the transfer and its manifestation in the luminescence polarization of the acceptors. Substantial shielding of the quenching process was observed during studies in the gas phase of the quenching effi...