Benzophenones in the higher triplet excited states

Abstract

Transient phenomena of benzophenone (BP) in the higher triplet excited state (Tn) have been investigated by the two-colour two-laser excitation method. Triplet energy transfer from BP(Tn) to quenchers (Q) occurred within the duration of a laser pulse (5 ns) to give Q(T1) with higher triplet energy than that of BP(T1). The quantum yield of the triplet energy-transfer quenching of BP(Tn) by CCl4 was found to be 0.0023 +/- 0.0002 from the bleaching of the transient absorption of BP(T1) and the absorbed photon number. It appears that internal conversion from BP(Tn) to BP(T1) is the predominant process. The lifetimes (tauTn) of BP(Tn) and several substituted benzophenones (BPs) in the higher triplet excited state [BPs(Tn)] were estimated from the dependence of the Q concentration on the efficiency of the triplet energy-transfer quenching of BP(Tn) by Q, and found to be 110-450 ps, depending on the nature of the substituents on the BPs. The effect of the substituents on tauTn may be explained by the energy gap between the Tn and T1 states, because the main deactivation pathway for BPs(Tn) is the internal conversion process. In contrast, the substituent effect on the lifetimes of BPs(T1) cannot be explained by the energy gap law. The transient behaviour of Q(T1) depends on the properties of the quencher. Sequential triplet energy transfer from Q(T1) to BP occurred for p-dichlorobenzene and tert-butylbenzene as quenchers, while Q(T1) reacted partly with Q to form triplet excimers (3Q2*) for benzene, chlorobenzene, and o-dichlorobenzene as quenchers. When CCl4 was used as the quencher, the homolytic cleavage of a C-Cl bond of CCl4(T1) occurred to give Cl* and Cl3C* radicals.

Higher Triplet Excited States of Benzophenones and Bimolecular Triplet Energy Transfer Measured by Using Nanosecond–Picosecond Two‐Color/Two‐Laser Flash Photolysis

“…[11] Recently, we reported the TET from BP(T n ) to various quenchers (Q) to give Q(T 1 ), and the sequential TET from Q(T 1 ) to BP. [13,16] It was found that in the case of p-dichlorobenzene (DCB) as Q, the sequential TET from DCB(T 1 ) to BP occurred with a yield of 100 %; however, the yield for Bz as Q was not 100 %, because of the competitive reactions from Bz(T 1 ). [11,13,16] No sequential TET process was observed for carbon tetrachloride (CCl 4 ) as Q, because of the rapid C À Cl bond cleavage of CCl 4 (T 1 ) to give CCl 3 C and ClC.…”

“…[13,16] It was found that in the case of p-dichlorobenzene (DCB) as Q, the sequential TET from DCB(T 1 ) to BP occurred with a yield of 100 %; however, the yield for Bz as Q was not 100 %, because of the competitive reactions from Bz(T 1 ). [11,13,16] No sequential TET process was observed for carbon tetrachloride (CCl 4 ) as Q, because of the rapid C À Cl bond cleavage of CCl 4 (T 1 ) to give CCl 3 C and ClC. The lifetimes of BP(T n ) (t Tn ) and several BP derivatives in the T n states have been estimated from the dependence of the quenching efficiency on the concentration of Q ([Q]).…”

“…The lifetimes of BP(T n ) (t Tn ) and several BP derivatives in the T n states have been estimated from the dependence of the quenching efficiency on the concentration of Q ([Q]). [13] However, without the direct measurements of t Tn , the reaction mechanism from BP(T n ) to Q cannot be fully clarified. Here, we report the direct measurements of the t Tn value and bimolecular TET rate constant (k TET ) from BP(T n ) to Q by using the nanosecond-picosecond (ns-ps) two-color/two-laser flash photolysis method.…”

“…This tendency was predicted from results of our previous work based on the bimolecular quenching of BPs(T n ). [13] TET from BP(T n ) to Q: Previously, the TET from BP(T n ) to Q, such as DCB, Bz, and CCl 4 , was investigated by using ns-ns two-color/two-laser flash photolysis. [13,16] The complete recovery of BP(T 1 ), due to the TET from Q(T 1 ) to BP, was observed in the case of DCB, but incomplete recovery and no recovery were observed in the cases of Bz and CCl 4 , respectively.…”

“…[13] TET from BP(T n ) to Q: Previously, the TET from BP(T n ) to Q, such as DCB, Bz, and CCl 4 , was investigated by using ns-ns two-color/two-laser flash photolysis. [13,16] The complete recovery of BP(T 1 ), due to the TET from Q(T 1 ) to BP, was observed in the case of DCB, but incomplete recovery and no recovery were observed in the cases of Bz and CCl 4 , respectively. To clarify the TET process from BP(T n ) to Q, the ns-ps two-color/two-laser flash photolysis of BP in CCl 4 and Bz was performed.…”