The thermodynamics of proton-coupled electron transfer (PCET) in weakly coupled organic pseudobases was investigated using 2,7-dimethyl-9-hydroxy-9-phenyl-10-tolyl-9,10-dihydroacridine (AcrOH) and 6-phenylphenanthridinol (PheOH) as model compounds. Pourbaix diagrams for two model compounds were constructed using the oxidation potentials and the pK_a values obtained, respectively, from cyclic voltammetry and photometric titrations. Our comparative study reveals the importance of having the redox active –N center closer to –OH functionality on the thermodynamics of PCET process: PheOH exhibits a wider range of pH values (pH = 2.8 to 13.3) in which both the alcohol and the corresponding alkoxy radical are expected to coexist in solution. This result indicates that a concerted mechanism is more likely to be discovered in pseudobases analogous to PheOH. The thermochemical data also indicate that the concerted PCET mechanism cannot be achieved if water is used as the proton acceptor: assuming the pK_a of hydronium ions as −1.7, the PCET involving PheOH or AcrOH as proton/electron donors and water as the proton acceptor is expected to follow the stepwise ET/PT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.

The photophysics of flavins is highly dependent on their environment. For example, 4a-hydroxy flavins display weak fluorescence in solution, but exhibit strong fluorescence when bound to a protein. To understand this behavior, we performed temperature-dependent fluorescent studies on an N(5)-alkylated 4a-hydroxy flavin: the putative bacterial luciferase fluorophore. We find an increase in fluorescence quantum yield upon reaching the glass transition temperature of the solvent. We then employ multiconfigurational quantum chemical methods to map the excited-state deactivation path of the system. The result reveals a shallow but barrierless excited state deactivation path that leads to a conical intersection displaying an orthogonal out-of-plane distortion of the terminal pyrimidine ring. The intersection structure readily explains the observed spectroscopic behavior in terms of an excited-state barrier imposed by the rigid glass cavity.

The photophysics of flavins is highly dependent on their environment. For example, 4a-hydroxy flavins display weak fluorescence in solution, but exhibit strong fluorescence when bound to a protein. To understand this behavior, we performed temperature-dependent fluorescent studies on an N(5)-alkylated 4a-hydroxy flavin: the putative bacterial luciferase fluorophore. We find an increase in fluorescence quantum yield upon reaching the glass transition temperature of the solvent. We then employ multiconfigurational quantum chemical methods to map the excited-state deactivation path of the system. The result reveals a shallow but barrierless excited state deactivation path that leads to a conical intersection displaying an orthogonal out-of-plane distortion of the terminal pyrimidine ring. The intersection structure readily explains the observed spectroscopic behavior in terms of an excited-state barrier imposed by the rigid glass cavity.

The thermal and light-induced O − O bond breaking of 2-ethyl-4-nitro-1(2H)-isoquinolinium hydroperoxide (IQOOH) were studied using ¹H NMR, steady-state UV/vis spectroscopy, femtosecond UV/vis transient absorption (fs TA) and time-dependent density functional theory (TD DFT) calculations. Thermal O − O bond breaking occurs at room temperature to generate water and the corresponding amide. The rate of this reaction, k = 5.4 · 10⁻⁶ s⁻¹, is higher than the analogous rates of simple alkyl and aryl hydroperoxides; however, the rate significantly decreases in the presence of small amounts of methanol. The calculated structure of the transition state suggests that the thermolysis is facilitated by a 1,2 proton shift. The photochemical process yields the same products, as confirmed using NMR and UV/vis spectroscopy. However, the quantum yield for the photolysis is low (Φ = 0.7%). Fs TA studies provide additional detail of the photochemical process and suggest that the S₁ state of IQOOH undergoes fast internal conversion to the ground state, and this process competes with the excited-state O − O bond breaking. This result was supported by the fact that the model compound IQOH exhibits similar excited-state decay lifetimes as IQOOH, which is assigned to the S₁  S₀ internal conversion. Copyright © 2013 John Wiley & Sons, Ltd.