Study of electron transfer interaction between hypocrellin and N,N-diethylaniline by UV-visible, fluorescence, electron spin resonance spectra and time-resolved transient absorption spectra

Abstract

Hypocrellin A (HA) and hypocrellin B (HB) extracted from Hypocrella bambusae (B. et. Br) Sacc are perylene quinoid pigments. Quenching of HA or HB fluorescence by N,N-diethylaniline (DEA) results in Stern-Volmer plots; the quenching rate constants in CH3CN are 2.28 × 1010 M−1s−1 for HA and 2.34 × 1010 M−1s−1 for HB and are in agreement with those calculated from the semiclassical Marcus theory. The values of the free energy change ΔG for electron transfer between HA∗ or HB∗ and DEA calculated from Weller's equation are −1.21 eV for HA and −1.24 eV for HB. Electron spin resonance (ESR) signals of semiquinone radical anions of HA or HB have been detected on illumination (λ = 550 nm) of HA or HB and DEA in anaerobic dimethylsulphoxide and CH3CN solutions, indicating that an electron transfer has occurred from the ground state of DEA to excited states of HA or HB. The UV-visible spectra of HA in the presence of DEA in dimethylformamide and the time-resolved transient absorption spectra of the interaction of HA and DEA in different concentrations of HA and DEA in CH3CN are examined. The observation of transient absorption of the semiquinone radical anion of HA (λmax = 620 nm) is also evidence of this electron transfer process. The interaction of HA and N-ethylaniline, aniline is also studied by UV-visible, ESR and fluorescence spectra.