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

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 S upon populating "bright" higher singlet excited states. The S-S energy decreases with the increase in CPP size, and hence, [9]CPP exhibits a faster S → S internal conversion decay. Higher triplet states act as receiver states for the intersystem crossing happening either via S or S. The wavepacket evolving on the receiver triplet state would decay to lower states via multiple conical intersections and reach T. The estimated size-dependent fluorescence and emission energies are in good accord with the experiment.