Tailoring interchromophore interactions for triplet pair production and separation in molecular assemblies*

We report a library of molecules coupled to each other with tunable strengths and geometries to investigate the fundamental properties of spin-entangled triplet pairs, and further to evaluate the evolution toward independent triplets or spin-polarized quintet states. A system composed of perylene oligomers leverages the flexibility of a covalent phenylakynyl bridge to effectively "gate" multi-chromophore delocalization in the singlet excited state and localization in the triplet state to produce long-lived independent triplets with relatively high efficiency. Transient spectroscopy and calculations support the notion of a delocalized excitation in the planar excited state and subsequent picosecond-scale torsional dynamics that isolates triplets. The triplet yield depends strongly on the number of chromophores, rising from a few percent for the dimer to > 30% for the tetramer. The additional accessible isolated triplet configurations enable the formation of product states despite relatively large singlet fission endothermicity.

By contrast, a second architecture involves a rigid norbornyl bridge between tetracene and pentacene chromophores. This rigidity enforces an essentially static geometry in the excited state that is thus amenable to a deep theoretical analysis, both in terms of triplet pair production and spin polarization. In particular, prediction of electron paramagnetic resonance (EPR) spectra are compared with experimental data to develop and validate appropriate spin Hamiltonian models for various situations. These concepts are extended to "dimer-like" pairs of molecules in crystals that were designed to minimize long-range intermolecular interactions that foster diffusion but maintain local geometries that promote formation of a long-lived, strongly exchanged-coupled triplet pair.