Electron Spin Dynamics\nof the Intersystem Crossing\nin Aminoanthraquinone Derivatives: The Spectral Telltale of Short\nTriplet Excited States

We studied the excited state dynamics of two bis-amino\nsubstituted\nanthraquinone (AQ) derivatives, with absorption in the visible spectral\nregion, which results from the attachment of a electron-donating group\nto the electron-deficient AQ chromophore. Femtosecond transient absorption\nspectra show that intersystem crossing (ISC) takes place in 190–320\nps, and nanosecond transient absorption spectra demonstrated an unusually\nshort triplet state lifetime (2.06–5.43 μs) for the two\nAQ derivatives. Pulsed laser-excited time-resolved electron paramagnetic\nresonance (TREPR) spectra show an inversion of the electron spin polarization\n(ESP) phase pattern of the triplet state at a longer delay time after\nlaser flash. Spectral simulations show faster decay of the T<sub><i>y</i></sub> sublevel than the other two sublevels (τ<sub><i>x</i></sub> = 15.0 μs, τ<sub><i>y</i></sub> = 1.50 μs, τ<sub><i>z</i></sub> = 15.0\nμs); theoretical computation predicts initial overpopulation\nof the T<sub><i>y</i></sub> sublevel, and rationalizes the\nshort T<sub>1</sub> state lifetime and the ESP inversion. Theoretical\ncomputations taking into account the electron-vibrational coupling,\ni.e., the Herzberg–Teller effect, successfully rationalize\nthe TREPR experimental observations.