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

Abstract

ABSTRACT 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.