Size-Dependent Relaxation Processes of Photoexcited [n]Cycloparaphenylenes (n=5-12): Significant Contribution of Internal Conversion in Smaller Rings

[n]Cycloparaphenylenes ([n]CPPs; n, number of phenyl rings) have gained considerable attention because they exhibit interesting properties owing to their highly strained structure and radially oriented p orbitals. Recently, [n]CPPs with n >= 5 have been synthesized, but the ring-size dependence of the deactivation processes of the excited states has not been explained particularly for smaller [n]CPPs (n <= 7). In the present study, we characterized the deactivation processes of [n]CPPs (5 <= n <= 12) using transient absorption spectroscopy at sub-pico-, sub-nano-, nano-, and microsecond time scales. Although the fluorescence quantum yield increased with the ring size, the longest S-1 -state lifetime was observed with [8]CPP, and both the decrease and increase of the ring size resulted in the decrease of the lifetime. Characterization of the intersystem crossing and internal conversion processes explained unique ring-size dependence of the deactivation processes of [n]CPPs, i.e., the enhanced radiation rate of the larger CPP and the fast internal conversion rate of smaller CPP dominate their S-1-state lifetimes.