Triplet Population Dynamics of Single Conjugated Polymer Molecules and Nanoscale Assemblies
David J. Walwark Jr., Benjamin D. Datko, and John K. Grey
The formation of long-lived triplet excited electronic states has important ramifications for conjugated organic materials used in optoelectronic devices. In the case of polymers, unravelling various structural factors mediating triplet processes is difficult because of heterogeneity effects due to intrinsic molecular weight polydispersity and large conformational degrees of freedom. Conformation-dependent electronic coupling between chromophore segments also modulates relaxation branching ratios that may vary substantially from molecule to molecule. However, ensemble-level spectroscopy experiments typically average over distinct responses, which disguises important qualities of the overall material photophysical landscape. Suppression of heterogeneity by diluting polymers into inert solid hosts permits single molecule level investigations of conformation-dependent triplet dynamics thereby avoiding the most insidious consequences of ensemble averaging. Interestingly, the multichromophoric nature of polymers can lead to significant likelihoods of multiple coexisting triplets, where population dynamics are revealed from fluorescence quenching dynamics on time scales comparable to triplet lifetimes (i.e., μs to ms). Stochastic photodynamic models are then used to extract key kinetic constants, including bimolecular triplet–triplet annihilation, that tend to exhibit pronounced dependences on polymer conformational ordering. Furthermore, simple processing strategies to selectively control chain conformation and packing order in hierarchical polymer assemblies can be combined with experiment and modeling to uncover the evolution in triplet processes from single molecule to bulk material levels. We posit that molecular-level control can be harnessed to more accurately manage triplet yields and interactions over a large range of time scales, which has potential uses in multiexciton harvesting schemes, such as singlet fission.
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