title: Fast excited-state deactivation in N(5)-ethyl-4a-hydroxyflavin pseudobase.
abstract: 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 (1)n,π* → (1)π,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 S(1) → S(0) internal conversion, initiated by the excited-state nitrogen planarization (sp(3) → sp(2) hybridization change at the N(5)-atom of Et-FlOH S(1) state) coupled with out-of-plane distortion of the pyrimidine moiety of flavin.
authors.affiliation: Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, United States.
authors.forename: Dapeng
authors.initials: D
authors.lastname: Zhou
authors.forename: Ekaterina
authors.initials: E
authors.lastname: Mirzakulova
authors.forename: Renat
authors.initials: R
authors.lastname: Khatmullin
authors.forename: Igor
authors.initials: I
authors.lastname: Schapiro
authors.forename: Massimo
authors.initials: M
authors.lastname: Olivucci
authors.forename: Ksenija D
authors.initials: KD
authors.lastname: Glusac
mesh_terms: D005415:Flavins / Q000138:chemical synthesis / Q000737:chemistry*; D015394:Molecular Structure; D011789:Quantum Theory; D013237:Stereoisomerism
chemical_list: D005415:Flavins