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

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(Tn, n ≥ 2)) initially generated by the excitation of AH(T1) at the wavelength tuned to the absorption of AH(T1). AH(Tn) decays to a AH(T2) with the longest lifetime among AH(Tn) 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(Tn) 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).