Unambiguous Diagnosis of Photoinduced Charge Carrier Signatures in a Stoichiometrically Controlled Semiconducting Polymer-Wrapped Carbon Nanotube Assembly.

Abstract

Single-walled carbon nanotube (SWNT)-based nanohybrid compositions based on (6,5) chirality-enriched SWNTs ([(6,5) SWNTs]) and a chiral n-type polymer (S-PBN(b)-Ph4 PDI) that exploits a perylenediimide (PDI)-containing repeat unit are reported; S-PBN(b)-Ph4 PDI-[(6,5) SWNT] superstructures feature a PDI electron acceptor unit positioned at 3 nm intervals along the nanotube surface, thus controlling rigorously SWNT-electron acceptor stoichiometry and organization. Potentiometric studies and redox-titration experiments determine driving forces for photoinduced charge separation (CS) and thermal charge recombination (CR) reactions, as well as spectroscopic signatures of SWNT hole polaron and PDI radical anion (PDI(-.) ) states. Time-resolved pump-probe spectroscopic studies demonstrate that S-PBN(b)-Ph4 PDI-[(6,5) SWNT] electronic excitation generates PDI(-.) via a photoinduced CS reaction (τCS ≈0.4 ps, ΦCS ≈0.97). These experiments highlight the concomitant rise and decay of transient absorption spectroscopic signatures characteristic of the SWNT hole polaron and PDI(-.) states. Multiwavelength global analysis of these data provide two charge-recombination time constants (τCR ≈31.8 and 250 ps) that likely reflect CR dynamics involving both an intimately associated SWNT hole polaron and PDI(-.) charge-separated state, and a related charge-separated state involving PDI(-.) and a hole polaron site produced via hole migration along the SWNT backbone that occurs over this timescale.