Impact of Cyclic Strain on the Structural Relaxation Dynamics of Macrocyclic Thiophenes

The delocalized exciton on conjugated polymers plays a momentous role in efficient charge generation and transport processes. Because the exciton is delocalized over the conjugated backbone, the exciton delocalization is highly susceptible to structural properties, such as conformational disorder and torsional relaxation. Here, we investigated impact of cyclic strain on the structural relaxation with a series of cyclic oligothiophenes, C-10T2V-C-40T8V, as a simplified and controlled conjugated systems with macrocyclic geometry. The excitation energy dependent transient absorption experiments revealed the conformational heterogeneity of all conjugated macrocycles and the structural relaxation rate are largely affected by their cyclic strain. In particular, through the comparative analysis with transient absorption and anisotropy measurements, we found that C-15T3V-C-40T8V basically undergo the similar extent of torsional relaxation energetically and structurally with lowest energy excitation regardless of ring size whereas the structural relaxation is decelerated by large cyclic strain in the smaller ring. Collectively, our findings provide a deeper understanding for the exciton delocalization in conjunction with cyclic structure and a new insight into structural engineering for optimizing organic-base devices.