Anti-Kasha Fluorescence in Molecular Entities: The Central Role of the Electron-Vibrational Coupling

According to Kasha’s rule, emission of a photon in a molecular system always comes from the lowest excited state. A corollary of this rule, i.e., the Kasha-Vavilov rule, states that the emission spectra are independent of the excitation wavelength. Although these rules apply for most of the molecular systems, violations of these rules are often reported for molecular systems. The prototypical case of a Kasha’s rule violation is the fluorescence observed from S2 in azulene. Thanks to the advances in both theoretical and experimental research, other types of anomalous fluorescence arising from higher-lying excited states (e.g., excitation energy transfer (EET)-based dual emissions, thermally-activated fluorescence etc.), and which mechanistically differ from the azulene-like anomalous fluorescence, are more recurrently reported in the literature. However, the underlying mechanisms leading to these anomalous emissions can be numerous and they are not yet well understood. In order to shed some light into the above phenomena, this account provides a comprehensive review on this topic. We herein report quantum chemical investigations in target molecular systems breaking Kasha’s rule. The latter molecules were chosen as they were unambiguously reported to display anti-Kasha fluorescence. Our studies highlight three different types of anti-Kasha scenarios. Specifically, i) the strong electronic-, weak vibrational-nonadiabatic coupling (NAC) regime (here named as Type I case, i.e., azulene-like); ii) the strong electronic-, strong vibrational-NAC regime (Type II case, i.e., thermally-activated S2 fluorescence); and the iii) very weak electronic NAC regime (Type III case, i.e., EET dyads). In addition, by combining state-of-the-art quantum chemical calculations with excited-state decay rate theories and appropriate excited-state kinetic models, we provide semi-quantitative estimations of photoluminescence quantum yields for the most rigid molecular entities. Finally, we propose the use of simple theoretical descriptors relying on calculations of the excited-state density difference and the electron-vibrational coupling to classify anomalous emissions according to their coupling scenario. Besides the fundamental interest of the above investigations, the herein developed computational protocols and descriptors will be useful for the tailored design of dyes with tunable and unconventional fluorescence properties and their exploitation in a wide range of areas, i.e., from organic-light emitting diodes (OLEDs) to bioimaging, small molecule fluorescent probes and photocatalysis. Finally, our theoretical framework enables attaining a holistic understanding of the interconversion processes between excited states, where the electron-vibrational coupling is shown to play a central role in determining the efficacy.