Breaking the Kasha Rule for More Efficient Photochemistry

Abstract

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.

Excitation‐Wavelength‐Dependent Emission and Delayed Fluorescence in a Proton‐Transfer System

“…Our experimental observations (including the excitation‐wavelength‐dependent emission and the dependence of the excitation spectra on the emission wavelength) may indicate a possible violation of Kasha's rule in the zinc(II) complex. Compounds breaking this rule use high energy excited states ( n >1) for emission or photochemical reactions . Our hypothesis is supported by the results of DFT calculations according to which Zn 2+ ions facilitate the S 2 →T 2 →T 1 and S 2 →T 1 intersystem crossing.…”

“…Compoundsb reaking this rule use high energy excited states (n > 1) for emission or photochemical reactions. [35][36][37] Our hypothesis is supported by the resultso fD FT calculations according to which Zn 2 + ions facilitate the S 2 ! T 2 !T 1 and S 2 !T 1 intersystem crossing.…”

Lanthanide-Based Coordination Polymers with a 4,5-Dichlorophthalate Ligand Exhibiting Highly Tunable Luminescence: Toward Luminescent Bar Codes

“…52 However, recent improvements of time-resolved spectroscopies have revealed much more complex optical deexcitation schemes in lot of cases where higher excited states than the first singlet and triplet excited states are involved. [53][54] Scheme 4 . Schematic Jablonski diagram of the different energy transfers.…”

Fluorescence Properties of Flavin Semiquinone Radicals in Nitronate Monooxygenase

“…bound to NMO violates the Kasha–Vavilov rule, which assumes that internal conversions between excited states are faster than any competing excited‐state processes . Some known exceptions to the Kasha–Vavilov rule, such as the blue shifts of emission spectra when excited to higher excited states, occur when the rates of emission decay from high excited states are comparable or faster than internal conversion to the lowest excited state …”