Fluence-Dependent Photoinduced\nCharge Transfer Dynamics\nin Polymer-Wrapped Semiconducting Single-Walled Carbon Nanotubes

Because an individual single-walled carbon nanotube (SWNT)\ncan\nabsorb multiple photons, the exciton density within a single tube\ndepends upon excitation conditions. In SWNT-based energy conversion\nsystems, interactions between excitons and charges make it possible\nfor multiple types of charge transfer reactions. We exploit a SWNT-molecular\ndonor–acceptor hybrid system (<i>R</i>-PBN(b)-Ph<sub>6</sub>-PDI-[(6,5) SWNT]) that fixes spatial organization and stoichiometry\nof perylene diimide (PDI) electron acceptors on the nanotube surface,\nto elucidate how excitation fluence affects ultrafast charge separation\n(CS) and the nature of charge recombination (CR) dynamics triggered\nupon SWNT near-infrared excitation. Pump–probe data characterizing\nthese photoinduced CS and thermal CR reactions were acquired over\nexcitation fluences that produce ∼5–125 excitons per\n700 nm long nanotube. These experiments show that optical excitation\ngives rise to CS states in which PDI radical anions (PDI<sup>–•</sup>) and SWNT hole polarons (SWNT<sup>•+</sup>) have geminate\nand nongeminate spatial relationships. Under low excitation fluences,\nthe observed dynamics reflect CR reactions of these geminate and nongeminate\nCS states. As excitation fluence increases, persistent excitons, which\nhave not undergone CS, undergo reaction with ([SWNT(<sup>•+</sup>)<i><sup>n</sup></i>]-(PDI<sup>–•</sup>)<i><sub>n</sub></i>) CS states to produce lower-energy CS states\nthat are characterized by hole (SWNT<sup>•+</sup>) and electron\n(SWNT<sup>•–</sup>) polarons. When nongeminate SWNT<sup>•+</sup> and SWNT<sup>•–</sup> charge carriers\nare generated, CR dynamics depend on the time scale required for these\noppositely charged solvated SWNT polarons to encounter each other.\nBecause SWNT excitons have substantial excited-state reduction (<sup>1</sup><i>E</i><sup>–/*</sup>) and excited-state\noxidation (<sup>1</sup><i>E</i><sup>*/+</sup>) potentials,\nthey can drive additional charge transfer reactions involving initially\nprepared CS states under experimental conditions where excess excitons\nare present.