Tuning Interactions to Control Molecular Down Conversion in [2.2]Paracyclophane Bridged Oligo‐Tetracenes

Abstract

AbstractIn tetracene, the energies of the lowest singlet excited state and twice that of the lowest triplet excited state are similar, allowing both down‐conversion (i.e., singlet fission, SF) and up‐conversion (i.e., triplet‐triplet annihilation up‐conversion, TTA‐UC) processes. Through‐space and through‐bond contributions to the inter‐tetracene coupling in purposefully designed oligomers play a crucial role in determining which of the two processes dominates. In this work, the focus is exclusively on SF in newly synthesized oligo‐tetracenes linked by conjugated [2.2]paracyclophane (PCP) building blocks. By choosing different PCP substitution patterns and by varying the degree of substitution the inter‐tetracene couplings are addressed. An independent variable is connecting the tetracences to the PCP at different positions to alter the through‐bond and through‐space coupling of the resulting oligo‐tetracenes. The novel oligo‐tetracenes are investigated by means of steady‐state and time‐resolved absorption and fluorescence spectroscopies with respect to the initial events of SF, that is, the transformation of a singlet excited state into a correlated triplet pair state. Briefly, through‐space couplings are profoundly weaker than through‐bond couplings that enable the correlated triplet pair state formation. If interactions are through‐space, correlated triplet pair state formation is turned off, while it is turned on if through‐bond interactions are operative.