Picosecond Laser Study of Thioindigoid Dyes Photoisomerization

The transient species in the trans ⇌ cis photoisomerization of the perinaphto–thioindigoiddye were identified by picosecond laser spectroscopy in the time domain10 ps–5 ns. An excitation of the trans isomer with a 6 ps, 528 nm pulse to the singletstate S 1 (the peak ofS ′ n← S 1 absorption at 700 nm) results in the intersystem crossingS 1 ⇝ T 1 (the peak ofT n← T 1 absorption at 730 nm) with the quantum yield of 0.7 and rate constant (1.4 ± 0.3)· 10 9 s − 1 . An excitation of the cis isomer results also inthe intersystem crossing (the peak ofT n > ← T 1 absorption at 670 nm) with the rateconstant (3.7 ± 1.2)· 10 10 s − 1 . The triplet mechanism of the photoisomerization in theboth directions is proposed.