Triplet and reversed triplet mechanism CIDEP studied by quenching experiments

Abstract

Quenching experiments are shown to provide a convenient tool to check for the presence of triplet mechanism (TM) spin polarisation in time-resolved EPR spectra following laser flash photolysis. The effect of the triplet quenchers, trans-l,3-pentadiene, fumaronitrile, azo-tert-butane and azo-n-butane upon the spectra following laser photolysis of acetone/propan-2-ol and benzophenone/propan-2-ol photosystems show that no TM polarisation is present in the former system but emissive TM is present in the latter. Use of 2,2'-azo-bis[isobutryronitrile] produces an anomalous emissive polarisation upon quenching, which is tentatively attributed to a reversed TM in the tfiplet sensitised azo-compound.

The Excited Triplet State of Azoalkanes: Electron Spin Polarization and Magnetic Field Effects During Triplet-Sensitized Photolysis of trans-Azocumene in Solution

“…a cleavage into radicals competing with a fast state selective ISC 3 AC→AC to the ground state, as it has been observed previously also for the triplet state of AIBN [13,26,60]. If this interpretation is correct, then the cumyl radical yield must depend on the magnetic field strength…”

Magnetic field dependence of the deactivation rates of triplet azocumene in solution

“…Second, at 355 nm the absorption coecients of 1-NN and AC are 3200 M ¡1 cm ¡1 and 40 M ¡1 cm ¡1 , respectively. Thus, nearly all the light is absorbed primarily by 1-NN, which is known to undergo fast ISC within about 10 ps to its triplet state 3 (1-NN) [16]. Quenching of 3 (1-NN) by AC then leads to 3 AC, which partially cleaves into cumyl radicals and N 2 .…”

“…Thus, nearly all the light is absorbed primarily by 1-NN, which is known to undergo fast ISC within about 10 ps to its triplet state 3 (1-NN) [16]. Quenching of 3 (1-NN) by AC then leads to 3 AC, which partially cleaves into cumyl radicals and N 2 . Azoalkanes are known to be e cient triplet quenchers [1,17,18], and the triplet energies of (1-NN) might be spin polarized by the p-type TM and transfer this polarization via 3 AC to the radicals.…”

“…When populated via triplet sensitization it deactivates rapidly by two competing processes, radiationless return to the singlet ground state (sometimes accompanied by some trans±cis isomerization) and cleavage into nitrogen and two alkyl radicals, according to scheme 1, where 3 S is the triplet sensitizer. Not much is known about that triplet state, its structure and its lifetime.…”

“…Recently, we have studied by time-resolved EPR (TREPR) spectroscopy the triplet quenching of acetone [3] and benzophenone [4] by AIBN (2,2 0 -azobis[isobutyronitrile]) in solution. It was found that during the cleavage process of triplet AIBN into nitrogen and 2-cyano-2-propy l radicals the electron spin system of the radicals becomes polarized in emission.…”