IT’S GOOD TO BE FLEXIBLE: ENERGY TRANSPORT FACILITATED BY CONFORMATIONAL FLUCTUATIONS IN LIGHT-HARVESTING POLYMERS

We investigate the mechanism of energy transfer between Ruthenium (II) (Ru) and Osmium (II) (Os) polypyridyl complexes affixed to a polyfluorene backbone (PF-RuOs) using a combination of time-resolved emission spectroscopy and coarse-grained molecular dynamics (CG MD). Photoexcitation of a Ru chromophore initiates Dexter-style energy hopping along isoenergetic complexes followed by sensitization of a lower energy Os trap. While we can determine the total energy transfer rate within an ensemble of solvated PF-RuOs from time-dependent Os* emission spectra, heterogeneity of the system and inherent polymer flexibility give rise to highly multi-exponential kinetics. We developed a three-part computational kinetic model to supplement our spectroscopic results: 1) CG MD model of PF-RuOs that simulates molecular motions out to 700 ns and reduces computational time by ~400x compared to all atom MD, 2) Energy transfer kinetic simulations in CG MD PF-RuOs that produce time-resolved Ru and Os excited state populations and 3) Computational experiments that interrogate the mechanisms by which motion aids energy transfer. Good agreement between simulated and experimental emission transients reveals our kinetic model accurately simulates the molecular motion of PF-RuOs during energy transfer. Simulated results indicate that pendant flexibility allows 81% of the excited state to sensitize an Os trap compared to a 48% occupation when we treat pendants statically. Our computational experiments show how static pendants are only able to engage in local energy transfer. The excited state equilibrates across a domain of complexes proximal to the initial excitation and becomes trapped within that unique, frozen locality. Flexibility enables pendants to swing in and out of the original domain spreading the excited state out to ± 30 away from the initial excitation. This work was made possible through extensive collaborations with the groups of Dr. John Reynolds and Dr. Kirk Schanze.Doctor of Philosoph