This paper provides a systematic review and analysis of different phenomena that violate a basic principle, Kasha's rule, when applied to photochemical reactions. In contrast to the classical route of ultrafast transition to the lowest energy excited state and photochemical reaction starting therein, in some cases, these reactions proceed directly from high-energy excited states. Nowadays, this phenomenon can be observed for a number of major types of excited-state reactions: harvesting product via intersystem crossing; photoisomerizations; bond-breaking; and electron, proton, and energy transfers. We show that specific conditions for their observation are determined by kinetic factors. They should be among the fastest reactions in studied systems, competing with vibrational relaxation and radiative or nonradiative processes occurring in upper excited states. The anti-Kasha effects, which provide an important element that sheds light on the mechanisms of excited-state transformations, open new possibilities of selective control of these reactions for a variety of practical applications. Efficient utilization of excess electronic energy should enhance performance in the systems of artificial photosynthesis and photovoltaic devices. The modulation of the reporting signal by the energy of excitation of light should lead to new technologies in optical sensing and imaging.

No abstract

It is well known in spectroscopy of complex organic molecules that the large width of their absorption and emission bands is largely due to the existence of a continuous set of vibrational sublevels in each electronic state. The spectroscopic properties of dye molecules in solution are in addition influenced by the surrounding medium.(1-3) For several decades it had been believed that because of the fast energy exchange between the vibrational sublevels the fluorescence spectrum of organic dye in solution was independent of the frequency of the exciting light. In 1970 it was shown (4) for the first time that apart from molecular vibrations there is another cause of the substantial broadening of electronic spectra of organic molecules in solution, namely, the fluctuations of the structure of the solvation shell surrounding the molecule. The variation of the local electric field caused by the fluctuation of the shell structure leads to a statistical distribution of the frequencies of the electronic transitions of the molecules and, therefore, to inhomogeneous broadening of the dye spectrum.This broadening was experimentally demonstrated (4)(5)(6) from the dependence of the fluorescence spectrum of a dye solution on the exciting radiation frequency at 77°K. Later, Personov et al.(7) observed inhomogeneous broadening of electronic spectra from frozen solutions of complex molecules at lower (liquid helium) temperatures. It was shown that at liquid helium temperatures discrete fluorescence spectra of complex molecules with

No abstract

No abstract