Reaction of Oxygen with the Singlet Excited State of [n]Cycloparaphenylenes (n = 9, 12, and 15): A Time-Resolved Transient Absorption Study Seamlessly Covering Time Ranges from Subnanoseconds to Microseconds by the Randomly-Interleaved-Pulse-Train Method.

Abstract

Reaction of 3O2 with singlet excited state (S1) of highly luminescent cycloparaphenylenes (CPPs), i.e., [n]CPP where n = 9, 12, and 15 in solution has been studied by transient absorption (TA) measurements seamless for the time range from subnanosecond to microsecond based on the randomly-interleaved-pulse-train (RIPT) method recently developed by our group. We found efficient quenching of S1 by 3O2 through observation of Sn ← S1 transient absorption and the steady state fluorescence measurements. Concomitantly, we have become aware of the acceleration of the rate of intersystem crossing (ISC) from S1 to the triplet excited state (T1) through the observation of the evident enhancement of Tn ← T1 absorption intensity. We have established the analysis procedure to evaluate the rate constant of ISC (kISC0) in the absence of O2 and the bimolecular rate constant of ISC induced by 3O2 (kISCO2) only by using TA decay data in the presence of O2. On the basis of these analyses, we further succeeded in determining the quantum yield of T1 (ΦT) with and without O2. In addition, the absorption coefficient of T1 (εT1) and S1 (εS1) could be estimated with reference to that of T1 of C60. These photophysical parameters are largely dependent on the ring size, where the lifetime of S1 (τS) in the absence and presence of O2 dominates ΦT as well as the quantum yield of fluorescence (ΦF).