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The electronically excited singlet states of nitroaromatic compounds are often presumed to be essentially non-fluorescent. Nonetheless, a growing number of reports in the literature have demonstrated that certain structural types of nitroaromatics can indeed fluoresce, and often quite efficiently. Consideration of the mechanisms responsible for the typical fast or ultrafast non-radiative deactivation of the excited singlet states of nitroaromatics points to several general principles for their design that combine the strong electron-withdrawing properties of the nitro group with reasonable fluorescence quantum yields. An overview of published examples of fluorescent nitroaromatics emphasizes these concepts in the context of the importance of chromophore architecture and conformation and the defining roles of excited state charge transfer and solvent polarity in modulating the non-radiative decay channels that compete with fluorescence. Overcoming the stigma that nitroaromatics are intrinsically destined to be non-fluorescent thus paves the way for incorporating the strongly electron-withdrawing nitro group into the existing toolbox for the development of new nitro-substituted fluorophores and chromophores tuned to specific applications. 

Abstract Anthocyanins, which are responsible for most of the red, blue and purple colors of fruits and flowers, are very efficient at absorbing and dissipating light energy via excited state proton transfer or charge-transfer mediated internal conversion without appreciable excited triplet state formation. During the maturation of red wines, grape anthocyanins are slowly transformed into pyranoanthocyanins, which have a much more chemically stable pyranoflavylium cation chromophore. Development of straightforward synthetic routes to mono- and disubstituted derivatives of the pyranoflavylium cation chromophore has stimulated theoretical and experimental studies that highlight the interesting absorption and emission properties and redox properties of pyranoflavylium cations. Thus, p-methoxyphenyl substitution enhances the fluorescence quantum yield, while a p-dimethylaminophenyl substituent results in fast decay via a twisted intramolecular charge-transfer (TICT) state. Unlike anthocyanins and their synthetic analogs (flavylium cations), a variety of pyranoflavylium cations form readily detectable excited triplet states that sensitize singlet oxygen formation in solution and exhibit appreciable two-photon absorption cross sections for near-infrared light, suggesting a potential for applications in photodynamic therapy. These excited triplet states have microsecond lifetimes in solution and excited state reduction potentials of at least 1.3 V vs. SCE, features that are clearly desirable in a triplet photoredox catalyst.