Electron Spin Dynamics of the Intersystem Crossing in Aminoanthraquinone Derivatives: The Spectral Telltale of Short Triplet Excited States

Abstract

We studied the excited state dynamics of two bisamino substituted anthraquinone (AQ) derivatives, with absorption in the visible spectral region, which results from the attachment of a electron-donating group to the electron-deficient AQ chromophore. Femtosecond transient absorption spectra show that intersystem crossing (ISC) takes place in 190−320 ps, and nanosecond transient absorption spectra demonstrated an unusually short triplet state lifetime (2.06−5.43 μs) for the two AQ derivatives. Pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show an inversion of the electron spin polarization (ESP) phase pattern of the triplet state at a longer delay time after laser flash. Spectral simulations show faster decay of the T y sublevel than the other two sublevels (τ x = 15.0 μs, τ y = 1.50 μs, τ z = 15.0 μs); theoretical computation predicts initial overpopulation of the T y sublevel, and rationalizes the short T 1 state lifetime and the ESP inversion. Theoretical computations taking into account the electron-vibrational coupling, i.e., the Herzberg−Teller effect, successfully rationalize the TREPR experimental observations.