Quenching processes of aromatic hydrocarbons in the higher triplet excited states-energy transfer vs. electron transfer

Abstract

Quenching processes of several aromatic hydrocarbons (AH) such as naphthalene (NAP), dibenz[a,h]anthracene (DBA), and chrysene (CHR) in the higher triplet excited states (T₂) by different quenchers (Q) such as p-dichlorobenzene, o-dicyanobenzene aromatic compounds, and chloroalkanes (RCl), have been investigated by the two-color two-laser excitation method. AH in the higher triplet excited states (AH(T_n, n ≥ 2)) initially generated by the excitation of AH(T₁) at the wavelength tuned to the absorption of AH(T₁). AH(T_n) decays to a AH(T₂) with the longest lifetime among AH(T_n) through the fast internal conversion. In the presence of Q, the competition of triplet energy transfer (TENT) and electron transfer (ELT) reactions between AH(T₂) and Q are expected. However, no AH radical cation was observed, especially when the quenchers were chloroalkanes such as carbon tetrachloride (CCl₄), methylene dichloride (CH₂Cl₂), 1,2-dichloroethane, which are good electron acceptors. It is suggested that the TENT is important during the quenching of AH(T_n) by Q. The lifetimes of NAP(T₂), DBA(T₂), and CHR(T₂) were calculated from the TENT quenching experiments. It was found that the lifetimes of AH(T₂) increase in the order of NAP(T₂) (4.5 ps) < DBA(T₂) (16 ps) < CHR(T₂) (60 ps), which is consistent very well with the energy gap law for the transition from AH(T₂) to AH(T₁).