Ultrafast primary processes of the stable neutral organic radical, 1,3,5-triphenylverdazyl, in liquid solution

Femtosecond spectroscopy with hyperspectral white-light detection was used to elucidate the ultrafast primary processes of the thermodynamically stable organic radical, 1,3,5-triphenylverdazyl, in liquid acetonitrile solution at room temperature. The radical was excited with optical pulses having a duration of 39 fs and a center wavelength of 800 nm thereby accessing its energetically lowest electronically excited state (D1). The apparent spectrotemporal response is understood in terms of an ultrafast primary D1-to-D0 internal conversion that generates the electronic ground state of the radical in a highly vibrationally excited fashion within a few hundred femtoseconds. The replenished electronic ground state subsequently undergoes vibrational cooling on a time scale of a few picoseconds. The instantaneous absorption spectra of the radical derived from the femtosecond pump-probe data are analyzed within the Sulzer-Wieland formalism for calculating the electronic spectra of "hot" polyatomic molecules. The pump-probe spectra together with transient anisotropy data in the region of the D0 → D1 ground-state bleach gives evidence for an additional transient absorption that arises from a dark excited state, which gains oscillator strength with increasing vibrational excitation of the radical by virtue of vibronic coupling.