Tuning Quintet Multiexciton Sublevel Selection via Bridge Resonance in Singlet Fission

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.