Charge transfer vs. proton transfer in the excited-state dynamics of biomimetic pyranoflavylium cations

Journal of Photochemistry and Photobiology

James B. Derr¹  John A. Clark²  Mimi K. Billones³  Valentine I. Vullev⁴  Maryann Morales⁴  Eli M. Espinoza⁴  Gustavo Thalmer de Medeiros Silva⁵  Frank H. Quina⁵  Cassio Pacheco da. Silva⁵ 

[1] Department of Biochemistry, University of California, Riverside, CA 92521, USA;Department of Bioengineering, University of California, Riverside, CA 92521, USA;Department of Biology, University of California, Riverside, CA 92521, USA;Department of Chemistry, University of California, Riverside, CA 92521, USA;Instituto de Química, Universidade de São Paulo, Avenida Lineu Prestes 748, Cidade Universitária, São Paulo 05508-900, Brazil;

关键词: Pyranoflavylium cations;    Pyranoanthocyanins;    Charge transfer;    Proton transfer;    Pump-probe spectroscopy;   

DOI  :  

来源: DOAJ

【 摘 要 】

Pyranoflavylium cations are synthetic analogues of pyranoanthocyanin, colored pigments, formed from grape anthocyanins during the maturation of red wines. Studies of a series of monosubstituted pyranoflavylium cations, ranging from methoxy (PF+-OMe) to cyano (PF+-CN), have shown that they display fluorescence and form triplet states that sensitize singlet oxygen formation in acidified acetonitrile. In alcohol-water mixtures, they behave as photoacids, undergoing adiabatic excited state proton transfer (ESPT) to water on a picosecond timescale, as confirmed in this report by femtosecond pump-probe spectroscopy. In contrast, the corresponding dimethylamino substituted pyranoflavylium cation (PF+-NMe2) is virtually non-fluorescent under the same conditions and exhibits a long-wavelength absorption band that has been attributed to a charge-transfer (CT) transition. Indeed, pump-probe spectroscopy of PF+-NMe2 in acidified acetonitrile shows ultrafast (<1 ps) formation of a CT state that decays back to the ground state with a 12–13 ps lifetime. In acidified methanol, the initial Franck-Condon CT state (ca. 3 ps lifetime) converts to a 13 ps lifetime CT state analogous to that in acetonitrile. In 50:50 ethanol:water and 30:70 methanol:water mixtures, PF+-NMe2 exhibits a short-lived (3–8 ps) initial CT state, an intermediate lifetime (30 ps) CT state and a much longer lived (130 ps) species attributed to a twisted intramolecular CT state. Thus, in addition to demonstrating that the photophysics of PF+-NMe2 is dominated by CT rather than ESPT, pump-probe spectroscopy provides details of the solvent-dependent dynamics of the CT decay pathways.

【 授权许可】

Unknown