The transient species in the trans ⇌ cis photoisomerization of the perinaphto–thioindigoid
dye were identified by picosecond laser spectroscopy in the time domain
10 ps–5 ns. An excitation of the trans isomer with a 6 ps, 528 nm pulse to the singlet
state S1 (the peak of S′n ← S1 absorption at 700 nm) results in the intersystem crossing
S1 ⇝ T1 (the peak of Tn ← T1 absorption at 730 nm) with the quantum yield of 0.7 and rate constant (1.4 ± 0.3)· 109s−1. An excitation of the cis isomer results also in
the intersystem crossing (the peak of Tn> ← T1 absorption at 670 nm) with the rate
constant (3.7 ± 1.2)· 1010s−1. The triplet mechanism of the photoisomerization in the
both directions is proposed.
