Fluence-Dependent Photoinduced Charge Transfer Dynamics in Polymer-Wrapped Semiconducting Single-Walled Carbon Nanotubes.

Abstract

Because an individual single-walled carbon nanotube (SWNT) can absorb multiple photons, the exciton density within a single tube depends upon excitation conditions. In SWNT-based energy conversion systems, interactions between excitons and charges make it possible for multiple types of charge transfer reactions. We exploit a SWNT-molecular donor-acceptor hybrid system (R-PBN(b)-Ph₆-PDI-[(6,5) SWNT]) that fixes spatial organization and stoichiometry of perylene diimide (PDI) electron acceptors on the nanotube surface, to elucidate how excitation fluence affects ultrafast charge separation (CS) and the nature of charge recombination (CR) dynamics triggered upon SWNT near-infrared excitation. Pump-probe data characterizing these photoinduced CS and thermal CR reactions were acquired over excitation fluences that produce ∼5-125 excitons per 700 nm long nanotube. These experiments show that optical excitation gives rise to CS states in which PDI radical anions (PDI^-•) and SWNT hole polarons (SWNT^•+) have geminate and nongeminate spatial relationships. Under low excitation fluences, the observed dynamics reflect CR reactions of these geminate and nongeminate CS states. As excitation fluence increases, persistent excitons, which have not undergone CS, undergo reaction with ([SWNT(^•+)ⁿ]-(PDI^-•)_n) CS states to produce lower-energy CS states that are characterized by hole (SWNT^•+) and electron (SWNT^•-) polarons. When nongeminate SWNT^•+ and SWNT^•- charge carriers are generated, CR dynamics depend on the time scale required for these oppositely charged solvated SWNT polarons to encounter each other. Because SWNT excitons have substantial excited-state reduction (¹E^-/*) and excited-state oxidation (¹E^*/+) potentials, they can drive additional charge transfer reactions involving initially prepared CS states under experimental conditions where excess excitons are present.