On the theory of frequency-shifted secondary emission of light-harvesting molecular systems

The expressions are obtained for the intensity of the frequency-shifted secondary emission of a chromophore playing the role of a reaction center in the simplest model three-chromophore molecular “lightharvesting” antenna, which is constructed and oriented in space so that the incident photons coherently excite two of its chromophore pigments. The quantum-field formalism was used, which takes into account the generalized (quantum-electrodynamic) dipole-dipole, as well as radiative and nonradiative dissipative interactions between pigments and the reaction center of the antenna. The special features of the excitation spectrum of the Raman scattering line and the frequency-shifted fluorescence spectrum of the reaction center of the molecular antenna under study are discussed. A comparison of the expressions obtained for the excitation and fluorescence spectra and with the corresponding expressions obtained for a bichromophore molecular system, which differs from a three-chromophore antenna by the absence of one of the pigments, revealed the properties of the mechanism of action of light-harvesting molecular antennas that have not been found earlier. In particular, it is shown that “the light-harvesting” caused by the collective dissipative interactions of pigments with the reaction center of the antenna can substantially exceed a sum of contributions from separate pigments.