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

We studied the excited state dynamics of two bis-amino 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_<i>y</i> sublevel than the other two sublevels (τ_<i>x</i> = 15.0 μs, τ_<i>y</i> = 1.50 μs, τ_<i>z</i> = 15.0 μs); theoretical computation predicts initial overpopulation of the T_<i>y</i> sublevel, and rationalizes the short T₁ 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.