Conformational Heterogeneity in Large Macrocyclic Thiophenes

For conjugated macrocycles, conformational disorder plays a key role in determining whether the unique form of excitons that are fully delocalized over the cyclic framework (cyclic excitons) is formed by photoexcitation. We have investigated the ring size dependence of conformations and photophysical properties of macrocyclic thiophenes of varying ring sizes (C-5NT_NV) by using single-molecule fluorescence spectroscopy. We measured modulation depth, M, values and fluorescence intensities. As the ring size increases, the correlation plots of the two parameters show bimodal distributions, revealing that larger macrocycles exhibit extremely congested linear structures. The size dependence of structural changes in macrocyclic thiophenes have been clearly confirmed by molecular dynamics simulation. The number of torsional defects from simulated structures, in conjunction with survival times from fluorescence intensity trajectories and photon coincidence measurements, demonstrated the existence of multiple acyclic chromophores in the larger macrocycles from the ground state due to complete deformation of circular structures.