Picosecond relaxation processes in excited state quinoxaline

Abstract

Excited state absorption spectra of quinoxaline in benzene, isooctane, and cis-1,3-pentadiene at 7 and 130 ps after excitation are reported. Picosecond absorption kinetics at 422 and 533 nm are also reported. In the nonaromatic solvents, bands at ∼425 nm (Sn←S1 and Tn←T1) and ∼ 530 nm (mostly Sn←S1) are observed in the transient spectra. Decay of the ∼530-nm band gives k−1isc?23 ps in the nonaromatic solvents. In benzene solvent, the excited state spectra are more complex, the decay kinetics are longer, and quinoxaline–benzene exciplex formation is suggested.

Intramolecular rotational relaxation of compounds which form "twisted intramolecular charge transfer" (TICT) excited states

“…The symmetry axis of the nitrogen lone pair (the relevant donor part of the TICT forming molecule (6) may be rotated by up to around 40°i n a chair/chair conformational change without actually rotating the whole ring skeleton, and with the ß-C atoms essentially unmoved. Thus, the initial part of the intramolecular twisting relaxation, away from the Franck-Condon geometry, will be very fast (small factor r in eq 3), until the piperidine ring gets quasirigid at an "effective" Franck-Condon twist angle 0'po Due to steric interaction, the molecule is somewhat twisted in ground and initial Franck-Condon excited states (<^FC * 0), thus the effective ' = |0'FC -0cr| may be very small as compared to tjiat of 5 and lead to a strong enhancement factor /cr for fe(6) (eq 3), although the quasirigid piperidine ring will constitute a slower rotator than the pyrrolidine ring of 5 because of its asymmetric rotation axis (see Figure 8). Considering the flexibility of 2, and its increased conformational possibilities and effective form factor , the relative_order for k may be predicted as k(f>) > k(2) and fc(6) > k(5\.…”