Intramolecular Triplet Diffusion Facilitates Triplet Dissociation in a Pentacene Hexamer

Abstract

Triplet dynamics in singlet fission depend strongly on the strength of the electronic coupling. Covalent systems in solution offer precise control over such couplings. Nonetheless, efficient free triplet generation remains elusive in most systems, as the intermediate triplet pair 1(T1T1) is prone to triplet‐triplet annihilation due to its spatial confinement. In the solid state, entropically driven triplet diffusion assists in the spatial separation of triplets, resulting in higher yields of free triplets. Control over electronic coupling in the solid state is, however, challenging given its sensitivity to molecular packing. We have thus developed a hexameric system (HexPnc) to enable solid‐state‐like triplet diffusion at the molecular scale. This system is realized by covalently tethering three pentacene dimers to a central subphthalocyanine scaffold. Transient absorption spectroscopy, complemented by theoretical structural optimizations and steady‐state spectroscopy, reveals that triplet diffusion is indeed facilitated due to intramolecular aggregation. The yield of free triplets in HexPnc is increased by a factor of up to 14 compared to the corresponding dimeric reference (DiPnc). Thus, HexPnc establishes crucial design aspects for achieving efficient triplet dissociation in strongly coupled systems by providing avenues for diffusive separation of 1(T1T1), while, concomitantly, retaining strong interchromophore coupling without compromising rapid 1(T1T1) formation.

Controlling Interchromophore Coupling in Diamantane-Linked Pentacene Dimers To Create a “Binary” Pair

“…We were, however, unable to confirm analogous behavior for 1,6-dimer on the longer time scales. We believe that the interchromophoric interactions in 1,6-dimer are so weak that a spin-mixed M (T 1 T 1 ), rather than the spin-pure 1 (T 1 T 1 ), is formed even in the absence of an external magnetic field. − In the weak coupling regime of 1,6-dimer , where the exchange interactions are less than the dipolar couplings, (i.e., J < D ), spatial separation and/or conformational flexibility, which are needed to modulate the exchange interactions, are not supported. − Applying magnetic fields of up to a maximum of 2000 mT in our experiments only alters the singlet character of the triplet pair manifold sublevels but will not promote the formation of spin-pure states via level crossings, in general, and 5 (T 1 T 1 ), in particular. As such, the subsequent dissociation into (T 1 + T 1 ) cannot be manipulated by an external magnetic field.…”