Reaction of Oxygen with the Singlet Excited State of []Cycloparaphenylenes ( = 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.

Reaction of O with singlet excited state (S) of highly luminescent cycloparaphenylenes (CPPs), i.e., []CPP where = 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 S by O through observation of S ← S transient absorption and the steady state fluorescence measurements. Concomitantly, we have become aware of the acceleration of the rate of intersystem crossing (ISC) from S to the triplet excited state (T) through the observation of the evident enhancement of T ← T absorption intensity. We have established the analysis procedure to evaluate the rate constant of ISC () in the absence of O and the bimolecular rate constant of ISC induced by O () only by using TA decay data in the presence of O. On the basis of these analyses, we further succeeded in determining the quantum yield of T (Φ) with and without O. In addition, the absorption coefficient of T (ε) and S (ε) could be estimated with reference to that of T of C. These photophysical parameters are largely dependent on the ring size, where the lifetime of S (τ) in the absence and presence of O dominates Φ as well as the quantum yield of fluorescence (Φ).