Chanakarn Phansa, Miles I. Collins, Amir Asadpoordarvish, Sarah J. Baker, Elango Kumarasamy, Samuel N. Sanders, Andrew B. Pun, Dane R. McCamey, Luis M. Campos, Matthew Y. Sfeir, Akshay Rao, Murad J. Y. Tayebjee
Singlet fission enables the generation of spin-entangled triplet pairs and has recently emerged as a promising route to generate quintet multiexcitons for quantum technologies. Despite this promise, a key challenge remains: the reliable control of quintet spin-sublevel populations, which is crucial for quantum information and sensing applications. With the aim of addressing this, we study intramolecular singlet fission in two oligomers, with a pair of tetracene and pentacene derivatives respectively, bridged by an anthracene unit. The tetracene oligomer has closer HOMO-HOMO and LUMO-LUMO energy alignment between chromophore and bridging unit compared to the pentacene oligomer, a property known as "bridge resonance", which leads to stronger intertriplet exchange coupling, and hence different quintet formation mechanisms. We employ continuous-microwave and pulsed electron spin resonance spectroscopy to probe quintet spin-sublevel populations in these oligomers. Our findings confirm that the tetracene oligomer undergoes predominantly strong-exchange quintet formation, yielding well-defined, reproducible spin-sublevel populations. On the other hand, the pentacene oligomer's weak-exchange quintet formation result in more disordered and less-predictable sublevel populations. These results demonstrate that the tuning of bridge resonance offers a strategy for controlling spin dynamics in singlet fission systems, paving the way for engineered multiexciton states tailored for quantum computing and spintronic devices.