Ultrafast Nuclear Rearrangement Governs Excited-State Radial π-Conjugation and Electron Transfer Dynamics in Organic Nanoring.

Radial π-conjugation provides a unique platform for extending electronic delocalization in cyclic molecular architectures. Although radial π-conjugation is relatively robust against static disorders in the ground state, both theoretical and experimental observations indicate that excited states undergo localization through exciton self-trapping once nanorings exceed a critical size. However, direct observation of the ultrafast structural dynamics underlying exciton localization remains a major challenge. Here, we employ ultrafast time-resolved impulsive stimulated Raman spectroscopy (TR-ISRS) to track the radial π-conjugation during excited-state structural relaxation. Using [n]cycloparaphenylene ([n]CPP) as a model nanoring system, we show that intrinsic structural metrics-ring strain and curvature-dictate the ultrafast nuclear dynamics. Comparative TR-ISRS analysis along two distinct nonadiabatic transition pathways provides direct vibrational evidence for exciton self-trapping, a process previously inferred only by ultrafast electronic spectroscopies. Furthermore, we demonstrate that the electron-transfer (ET) dynamics of photoexcited [n]CPP are governed by the character of the resulting excitonic state: delocalized excitons promote ultrafast, near-quantitative ET, whereas self-trapped excitons substantially suppress the ET rate. These findings establish a direct mechanistic link among radial π-conjugation, ultrafast structural dynamics, and electron-transfer processes, and offer design principles of functional cyclic π-conjugated materials.