Fast excited-state deactivation in N(5)-ethyl-4a-hydroxyflavin pseudobase

We present a study of the excited-state behavior of N(5)-ethyl-4a- hydroxyflavin (Et-FlOH), a model compound for bacterial bioluminescence. Using femtosecond pump-probe spectroscopy, we found that the Et-FlOH excited state exhibits multiexponential dynamics, with the dominant decay component having a 0.5 ps lifetime. Several possible mechanisms for fast excited-state decay in Et-FlOH were considered: (i) excited-state deprotonation of the -OH proton, (ii) thermal deactivation via ¹n,π* → ¹π, π* conical intersection, and (iii) excited-state release of OH ^- ion. These mechanisms were excluded based on transient absorption studies of two model compounds (N(5)-ethyl-4a-methoxyflavin, Et-FlOMe, and N(5)-ethyl-flavinium ion, Et-Fl⁺) and based on the results of time-dependent density functional theory (TD-DFT) calculations of Et-FlOH excited-states. Instead, we propose that the fast decay in Et-FlOH is caused by S₁ → S₀ internal conversion, initiated by the excited-state nitrogen planarization (sp³ → sp² hybridization change at the N(5)-atom of Et-FlOH S₁ state) coupled with out-of-plane distortion of the pyrimidine moiety of flavin.