Driving Force Dependent Photoinduced Charge Transfer Dynamics in Polymer-Wrapped Semiconducting Single-Walled Carbon Nanotubes

We investigate the thermodynamic driving-force dependences of photoinduced charge separation (CS) and subsequent charge transfer dynamics in single-walled carbon nanotube (SWNT)-perylenediimide (PDI) donor-acceptor (D-A) superstructures. Pump-probe spectroscopy reveals that [SWNT^{(•+)<i>n</i>}]-(PDI^-•)_<i>n</i> CS states form on an ∼100 fs time scale following photoexcitation; these dynamics are invariant across an ∼400 mV driving force range, indicating that SWNT hole polaron formation time scales are determined by nanotube lattice and solvent relaxation. These CS states feature SWNT hole polarons adjacent to (geminate) and nearby (nongeminate) PDI radical anions. Analysis of the free energy dependence for charge recombination (CR) of [SWNT^•+]^geminate-(PDI^-•) CS states highlights an ∼2 meV value for D-A electronic coupling (H_AB) and ∼0.93 eV for the total reorganization energy (λ_T). A corresponding driving force dependence of the CR dynamics for [SWNT^•+]^nongeminate-(PDI^-•) CS states indicates a diminished H_AB value (∼0.6 meV) and a larger λ_T (∼1.1 eV), consistent with larger transfer distances. SWNT excitons that persist following photoinduced CS drive photooxidation of PDI^-• components of [SWNT^{(•+)<i>n</i>}]-(PDI^-•)_<i>n</i> CS states (¹SWNT* + PDI^•- → PDI + SWNT^•-); this reaction manifests a significantly reduced λ_T value (∼0.67 eV) as the initially prepared SWNT reduced state bears the character of a conduction band injected electron ([SWNT^•-]^CB). This reaction thus gives rise to relaxed, nongeminate SWNT electron and hole polarons on the same nanotube; these polarons react on a 10² ps time scale independent of the electronic structure of these SWNT-PDI superstructures.