Photophysics and spectroscopy of 1,2-Benzazulene

Neutral vs Charged Luminescent Radicals: Anti-Kasha Emission and the Impact of Molecular Surrounding

Organic luminescent materials attract growing interest as an elegant solution for sustainable and inexpensive light-emitting devices. Most of them are neutral-emitting molecules with an implicit restriction of 25% internal quantum efficiency due to a spin-forbidden nature of the T1 → S0 transition. Utilizing organic radicals allows one to overcome such limits by theoretically boosting quantum yield up to 100%. Recently, different light-emitting radicals based on carbonyl- and carboxyl-substituted benzenes were synthesized and stabilized in different polymer matrices or ionic liquids. While some of them were proved to be suitable luminescent materials, the exact theoretical explanation of the nature of their emission is missing. There are two main hypotheses proposed in the literature. The first one suggests that the origin of luminescence is D2 → D0 anti-Kasha emission from anion radicals, while the second theory is based on D1 → D0 Kasha emission from neutral protonated radicals. In this work, we investigate both hypotheses and compare their derivatives with the available experimental data. We used density functional theory and complete-active space perturbation theory to investigate the absorption and emission properties in various aromatic carbonyl radicals. We found that both emission mechanisms can coexist simultaneously, with a dominant emission contribution made by anion radicals because of better agreement between oscillator strengths and radiative rate constants. Our numerical simulations agree with the experimental data and provide theoretical foundations for the fabrication of next-generation light-emitting devices based on luminescent radicals.

Prospects for useful fission from singlet states higher than S1 in aggregated organic chromophores

The few known reports and the likely prospects of finding new efficient routes to exciton fission from higher excited singlet states, Sn (n > 1), are reviewed. Aggregates of molecules that have large S2-S1 electronic energy spacings and/or emit measurable "contra-Kasha" emission may offer further opportunities. Among these, electronically excited molecular systems that exhibit known efficient (T1 + T1) triplet-triplet annihilation producing S2 could exhibit efficient singlet fission in aggregates when appropriately substituted to meet the necessary energy requirements. The potential problem of loss of triplet excitons via 2T1 → Tn>1 + S0 triplet-triplet annihilation following (S2 + S0) singlet fission is addressed. Aggregates of substituted azulenes and aliphatic thiones and dithiones are particularly attractive and are discussed in detail.

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

According to Kasha's rule, the 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. 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 (e.g., excitation energy transfer (EET)-based dual emissions, thermally activated fluorescence, etc.) are more recurrently reported in the literature. Sometimes, these anomalous processes involve higher-lying excited states but are mechanistically different from the azulene-like anomalous fluorescence. However, the underlying mechanisms leading to these anomalous emissions can be numerous and are not yet well understood.In order to shed some light on the above phenomena, this Account provides a comprehensive review of this topic. We herein report quantum chemical investigations in target molecular systems breaking Kasha's rule. The latter molecules were chosen because 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 the 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 semiquantitative 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 the attainment of 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.

Synthesis, Reactivity, and Properties of Benz[a]azulenes via the [8 + 2] Cycloaddition of 2H-Cyclohepta[b]furan-2-ones with an Enamine

Starting with the reaction of 2H-cyclohepta[b]furan-2-ones with an enamine, which was prepared from 4-tert-butylcyclohexanone and pyrrolidine, benz[a]azulenes having both formyl and tert-butyl groups were obtained in the three-step sequence. Subsequently, both the formyl and tert-butyl groups were eliminated by heating the benz[a]azulene derivatives in 100% H₃PO₄ to give benz[a]azulenes without these substituents in high yields. In terms of product yield, this method is the best one ever reported for the synthesis of the parent benz[a]azulene so far. The conversion of the benz[a]azulene derivatives with a formyl group into cyclohept[a]acenaphthylen-3-one derivatives was also investigated via Knoevenagel condensation with dimethyl malonate, followed by Brønsted acid-mediated intramolecular cyclization. The structural features including the bond alternation in the benz[a]azulene derivatives were revealed by NMR studies, NICS calculations, and a single-crystal X-ray structural analysis. The optical and electrochemical properties of a series of benz[a]azulene derivatives were evaluated by UV/Vis, fluorescence spectroscopy, and voltammetry experiments. As a result, we found that some benz[a]azulene derivatives showed remarkable luminescence in acidic media. In addition, the benz[a]azulene derivatives with the electron-withdrawing group and cyclohept[a]acenaphthylen-3-one derivative displayed good reversibility in the spectral changes under the electrochemical redox conditions.