Direct observation of triplet-triplet intermediates in the photocyclization reaction of triphenylamine using transient absorption spectroscopy

Triphenylamine (TPA) molecules are extensively employed in optoelectronic materials and photochemical reactions. The present research examines the ultrafast dynamics of photocyclization reaction utilizing transient absorption (TA) spectroscopy. In the femtosecond TA spectrum, the excited state absorption (ESA) signal for TPA at 627 nm diminishes within 2.5 ns, while a triplet-triplet absorption (TTA) signal emerges at 524 nm, gradually decaying over 25.2 ns in the subsequent nanosecond TA spectrum. As the TTA signal diminishes, a new positive TTA signal corresponding to the photocyclization intermediate, N-phenyl-4a,4b-dihydrocarbazole (NPDHC), appears at 426 nm. Following the decay of the TTA signal of NPDHC over 494.5 ns, another positive absorption signal is observed at 612 nm, which is associated with the ESA signal from both NPDHC and N-phenylcarbazole. In contrast, Tetra-N-phenylbenzidine (TPB) and N,N '-Bis(3-methylphenyl)-N,N '-bis(phenyl)benzidine (TPB-CH3) exhibit only a positive ESA signal and a negative stimulated emission (SE) signal, with no detectable TTA signal or cyclization product signal. Theoretical simulations reveal that the spin-orbit coupling (SOC) constant of TPA (0.26 cm-1) is higher than that of the two other molecules (0.09 cm-1). This research presents direct observations of the triplet-triplet intermediates in the photocyclization reaction of TPA in real-time.