Quantum-classical mechanics: Luminescence spectra in polymethine dyes and J-aggregates. Nature of the small Stokes shift

Abstract

Abstract It can be shown that the joint and full-fledged (beyond the adiabatic approximation) motion of light electrons and very heavy nuclei in molecular and chemical physics in the framework of quantum mechanics can be only singular. This singularity is damped in a new theoretical approach called dozy-chaos mechanics or quantum-classical mechanics. Earlier, the effectiveness of the damping procedure for the above singularity was demonstrated in the simplest case of quantum-classical mechanics (of quantum-classical transitions), namely, by quantum-classical mechanics of elementary electron-charge transfers in condensed media and its applications to the shape of optical absorption bands in polymethine dyes and their aggregates and to other problems. In this article, on the basis of the quantum-classical mechanics of elementary electron-charge transfers, we find out the nature of the luminescence band shape and the small Stokes shift in polymethine dyes and J-aggregates. The very small experimental value of the Stokes shift (1 nm in the J-band) is explained by two factors, both of which are associated with a high degree of self-organization of molecular quantum transitions in J-aggregates. The first of these is related to the stimulation of the electronic transition in the J-chromophore by the chaotic dynamics (weak dozy chaos) of the reorganization of the nuclear environment, which leads to the well-known narrow and red-shifted absorption J-band. The second factor is associated with the spontaneous loosening of the excited electronic state immediately before the act of the photon’s creation by the electron in spontaneous emission. For polymethine dyes and J-aggregates, the universal effect of spontaneous dynamic loosening turns out to be abnormally strong, it must be supposed, due to the very extended π -electron systems undergoing the quantum-classical transitions there. A quantitative explanation is also given for the position and shape of the optical bands in J-aggregates for luminescence and optical absorption in the case of an admixture of monomers of a polymethine dye in an aqueous solution. The experimental fact of the absence of the optical band of the monomers on the shortwave wing of the J-band of luminescence with its apparent presence on the shortwave wing of the J-band of optical absorption is explained by a significant increase in the dynamic self-organization of quantum-classical transitions in J-aggregates as compared to just a small increase in the dynamic self-organization of these transitions in monomers upon the change from optical absorption to luminescence.