Heterogeneous singlet fission in a covalently linked pentacene dimer

Molecular dimers are generally regarded as essential tools for probing structure-property relationships in condensed-phase systems, revealing complexities where structural tuning is challenging. Traditionally treated as "static,"with properties defined by their optimized geometry, we argue that dimers are "dynamic,"exhibiting considerable conformational heterogeneity over time, which significantly influences interchromophore coupling strengths. Illustrating this, we explore the singlet fission dynamics of a pentacene dimer linked by phenyl-diketopyrrolopyrrole and acetylene bridges. The unrestricted rotations yield a myriad of rotational conformers, each altering the singlet fission processes, evident through excitation-energy-dependent transient absorption and electron paramagnetic resonance spectroscopy. This necessitates considering not only broad distribution of rate constants but also multidimensional potential-energy surfaces with multiple sub-ensembles, leading to "heterogeneous singlet fission."Consequently, these findings challenge the prevailing static approach to molecular dimer photophysics, suggesting that individual steps in excited-state relaxation pathways cannot be delineated by unique rate constants and yields.