Transient spectroscopic insights into nitroindole's T₁ state: Elucidating its intermediates and unique photochemical...

Indoles are notable for their distinct photophysical and photochemical properties, making them useful indicators in biological systems and promising candidates for a variety of pharmaceutical applications. While some indoles exhibit room temperature phosphorescence, such a phenomenon has not been observed in nitroindoles. Typically, adding of a nitro group into aromatic compounds promotes ultrafast intersystem crossing and increases the formation quantum yield of the lowest excited triplet (T₁). Therefore, understanding the reactivity of nitroindoles' T₁ states is imperative. This study investigated the physical properties and chemical reactivities of the T₁ state of 6-nitroindole (³HN-6NO₂) in both polar aprotic and protic solvents, using transient absorption spectroscopy. Our results demonstrate the basicity and acidity of ³HN-6NO₂, emphasizing its potential for protonation and dissociation in mildly acidic and basic conditions, respectively. Furthermore, ³HN-6NO₂ has a high oxidizing capacity, participating in electron transfer reactions and proton-coupled electron transfer to produce radicals. Interestingly, in protic solvents like alcohols, ³HN-6NO₂ dissociates at the -NH group and forms N-H…O hydrogen-bonded complexes with the nitro group. By identifying transient absorption spectra of intermediates and quantifying kinetic reaction rate constants, we illuminate the unique properties of the T₁ state nitroindoles, enriching our understanding of their photophysical and photochemical behaviors. The results of this study have significant implications for their potential application in both biological systems and materials science.