Transient Phenomena of Polyphenyls in the Higher Triplet Excited State

Transient phenomena of polyphenyls (PP), such as biphenyl (BP), p-terphenyl (TP), and 1,3,5-triphenylbenzene (TPB) in the higher triplet excited states (T n , n ≥ 2), were investigated with the nanosecond-nanosecond (ns-ns) two-color two-laser excitation method. PP(T n ) was generated by the excitation of PP in the lowest triplet excited state (PP(T 1 )) at the wavelength tuned to the absorption of PP(T,). Bleaching of the transient absorption of BP(T,) and TP(T,) was observed during the second laser irradiation in the presence of chloroalkanes (RC1) such as carbon tetrachloride (CCl 4 ), dichloromethane (CH 2 Cl 2 ), or 1,2-dichloroethane (DCE). In the case of TPB, clear bleaching of TPB(T 1 ) was observed in the presence of CCl 4 , while no bleaching was observed in the presence of CH 2 Cl 2 and DCE. No formation of PP + was detected during the quenching of PP(T n ) by RCl. Therefore, the bleaching of PP(T 1 ) is attributed to triplet energy transfer quenching of PP(T n ) by RCl followed by the C-Cl bond cleavage. The lifetimes (τ T n ) of PP(T n ) were estimated from the dependence of the quenching efficiency on the RCl concentration. It was found that the τ T n value decreased with the increasing number of benzene rings, BP(T n ) (950 ps) > TP(T n ) (620 ps) > TPB(T n ) (360 ps). The quenching mechanism of PP(T n ) by RCl was discussed.