Intersystem crossing pathways in [5]-, [7]-, and [9]cycloparaphenylenes

Abstract

We analyze the energetics and internal conversion dynamics of singlet and triplet manifolds to identify the possible intersystem crossing pathways in odd-numbered [n]cycloparaphenylenes ([n]CPPs, n = 5, 7, and 9). Quantum wavepacket propagation calculations within the linear vibronic coupling framework suggest that both [5]- and [7]CPPs rapidly relax to S2 upon populating “bright” higher singlet excited states. The S2–S1 energy decreases with the increase in CPP size, and hence, [9]CPP exhibits a faster S2 → S1 internal conversion decay. Higher triplet states act as receiver states for the intersystem crossing happening either via S1 or S2. The wavepacket evolving on the receiver triplet state would decay to lower states via multiple conical intersections and reach T1. The estimated size-dependent fluorescence and emission energies are in good accord with the experiment.

Steric Hindrance Governs the Photoinduced Structural Planarization of Cycloparaphenylene Materials

“…Given the great promise in the photophysics of [ n ]­CPPs and their derivatives, extensive studies have been performed to rationalize the associated experimental observations, ,− but uncertainties remain with these materials, importantly including the role of intrinsic steric hindrance in affecting the excited-state molecular structures and the relationships between the electron/hole distribution and structural features upon photoexcitation. Therefore, a systematic study revealing the excited-state characteristics that regulate their optical properties is highly demanded to fully exploit their potential.…”