Mechanism for Highly Efficient Non-Radiative Deactivation of Electronic Excitation in Rutin

Steady-state and pulsed spectroscopic methods are used to study the spectroscopic and photophysical properties of the biologically important plant pigment rutin at room temperature and 77 K in organic solvents and a buffer solution at pH 7.0. The large dipole moment μ e = 13.3 D of the rutin molecule in a Franck–Condon excited state indicates that rutin is dipolar in this excited state. The nonstationary S 1 → Sn induced absorption spectra are characterized by a short-wavelength band at λ abs max = 460 nm and low-intensity absorption in the 500–750 range which clearly belongs to associates of rutin. No residual induced absorption which might be related to triplet-triplet T 1 →T k transitions in rutin was observed over the entire spectral range for times >50 ns. S 1 → S 0 fluorescence with a quantum yield Φ fl ~ 10 –4 was also observed at room temperature. The fluorescence and fluorescence excitation spectra manifest a weak dependence on the excitation and detection wavelengths, which may be related to the presence of conformers in the solution owing to rotation of the phenol B ring around a single 1′–2 bond. Lowering the temperature of a glassy frozen solution of rutin in ethanol to 77 K raises Φ fl by a factor of 750. A rate constant k ic = 3.7·1011 s –1 for internal conversion from the S 1 state at room temperature is calculated from the spectral-luminescence data. It is found that the main channel for exchange of electronic excitation energy in the rutin molecule at room temperature is S 1 (π,π * ) ~~> S 0 -internal conversion induced by the charge-transfer state.