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

Abstract

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.

How fast is optically induced electron transfer in organic mixed valence systems?

“…Furthermore, the electronic coupling also has no strong influence on the IC rate as is obvious when looking at the coupling in Table 1 which covers a factor of six on going from of 1 + to 3 + . Such a thermalisation may be simulated by the Sulzer-Wieland formalism 53-55 as has recently been done in case of verdazyl radicals by Voehringer et al 56 However, this model is based on a number of assumptions concerning the potential surfaces involved which prompted us to calculate the cooling of spectra explicitly based on the ground and excited state potentials as given in Fig. 0.3-0.5 ps (MeCN and DCM) to ca.…”

Data analysis in transient electronic spectroscopy – an experimentalist's view

“…By doing a transient absorption experiment on Verdazyl, exciting the D 1 ← D 0 transition using 800 nm light, Weinert et al observed that after a c. 500 fs induction period (which they deduce to possibly provide an upper limit on the D 1 lifetime), the transient absorption spectra decay display a time-dependent peak position which shifts to higher energy with time, and that the time traces display non-exponential relaxation back to zero. [76] They thus interpreted this as the signature of a rapid internal conversion back to the ground state, producing a vibrationally hot ground state. Thus, they used the Sulzer-Wieland formalism (equations ??…”

“…[73] Nonetheless, the paper presents a remarkably detailed investigation of the relaxation of the Verdazyl chromophore. [76] but with the D 1 ← D 0 transition scaled, as was needed for direct comparison. Time delays are 500 fs (brown), 1 ps (red), 2 ps (orange), 3 ps (green), 5 ps (light blue), and 10 ps (dark blue).…”

“…Time delays are 500 fs (brown), 1 ps (red), 2 ps (orange), 3 ps (green), 5 ps (light blue), and 10 ps (dark blue). Reproduced from reference [76] with permission from the PCCP Owner Societies.…”

Ultrafast Photogeneration of a Tetrazolinyl Radical

“…This assignment is corroborated by the transient absorption profile, which resembles the absorption of the triphenylverdazyl radical. [20,21] The latter compound is an open-shell system with an additional carbon compared to TTC, giving rise to a six-membered ring and a non-planar and non-aromatic NNCNN sequence with a delocalized unpaired electron, as also present in ro-TTCC. Several studies have pointed out the close relation of these radicals.…”