Confirmation of K-Momentum Dark Exciton Vibronic Sidebands Using ¹³C-labeled, Highly Enriched (6,5) Single-walled Carbon Nanotubes

A detailed knowledge of the manifold of both bright and dark excitons in single-walled carbon nanotubes (SWCNTs) is critical to understanding radiative and nonradiative recombination processes. Exciton–phonon coupling opens up additional absorption and emission channels, some of which may “brighten” the sidebands of optically forbidden (dark) excitonic transitions in optical spectra. In this report, we compare ¹²C and ¹³C-labeled SWCNTs that are highly enriched in the (6,5) species to identify both absorptive and emissive vibronic transitions. We find two vibronic sidebands near the bright ¹E₁₁ singlet exciton, one absorptive sideband ∼200 meV above, and one emissive sideband ∼140 meV below, the bright singlet exciton. Both sidebands demonstrate a ∼50 cm^–1 isotope-induced shift, which is commensurate with exciton–phonon coupling involving phonons of A₁^′ symmetry (D band, ω ∼ 1330 cm^–1). Independent analysis of each sideband indicates that both sidebands arise from the same dark exciton level, which lies at an energy approximately 25 meV above the bright singlet exciton. Our observations support the recent prediction of, and mounting experimental evidence for, the dark K-momentum singlet exciton lying ∼25 meV (for the (6,5) SWCNT) above the bright Γ-momentum singlet. This study represents the first use of ¹³C-labeled SWCNTs highly enriched in a single nanotube species to unequivocally confirm these sidebands as vibronic sidebands of the dark K-momentum singlet exciton

Charge Separation in P3HT:SWCNT Blends Studied by EPR: Spin Signature of the Photoinduced Charged State in SWCNT

Single-wall carbon nanotubes (SWCNTs) could be employed in organic photovoltaic (OPV) devices as a replacement or additive for currently used fullerene derivatives, but significant research remains to explain fundamental aspects of charge generation. Electron paramagnetic resonance (EPR) spectroscopy, which is sensitive only to unpaired electrons, was applied to explore charge separation in P3HT:SWCNT blends. The EPR signal of the P3HT positive polaron increases as the concentration of SWCNT acceptors in a photoexcited P3HT:SWCNT blend is increased, demonstrating long-lived charge separation induced by electron transfer from P3HT to SWCNTs. An EPR signal from reduced SWCNTs was not identified in blends due to the free and fast-relaxing nature of unpaired SWCNT electrons as well as spectral overlap of this EPR signal with the signal from positive P3HT polarons. However, a weak EPR signal was observed in chemically reduced SWNTs, and the <i>g</i> values of this signal are close to those of C₇₀-PCBM anion radical. The anisotropic line shape indicates that these unpaired electrons are not free but instead localized

Unraveling the ¹³C NMR Chemical Shifts in Single-Walled Carbon Nanotubes: Dependence on Diameter and Electronic Structure

The atomic specificity afforded by nuclear magnetic resonance (NMR) spectroscopy could enable detailed mechanistic information about single-walled carbon nanotube (SWCNT) functionalization as well as the noncovalent molecular interactions that dictate ground-state charge transfer and separation by electronic structure and diameter. However, to date, the polydispersity present in as-synthesized SWCNT populations has obscured the dependence of the SWCNT ¹³C chemical shift on intrinsic parameters such as diameter and electronic structure, meaning that no information is gleaned for specific SWCNTs with unique chiral indices. In this article, we utilize a combination of ¹³C labeling and density gradient ultracentrifugation (DGU) to produce an array of ¹³C-labeled SWCNT populations with varying diameter, electronic structure, and chiral angle. We find that the SWCNT isotropic ¹³C chemical shift decreases systematically with increasing diameter for semiconducting SWCNTs, in agreement with recent theoretical predictions that have heretofore gone unaddressed. Furthermore, we find that the ¹³C chemical shifts for small diameter metallic and semiconducting SWCNTs differ significantly, and that the full-width of the isotropic peak for metallic SWCNTs is much larger than that of semiconducting nanotubes, irrespective of diameter

Effect of Solvent Polarity and Electrophilicity on Quantum Yields and Solvatochromic Shifts of Single-Walled Carbon Nanotube Photoluminescence

In this work, we investigate the impact of the solvation environment on single-walled carbon nanotube (SWCNT) photoluminescence quantum yield and optical transition energies (<i>E_ii</i>) using a highly charged aryleneethynylene polymer. This novel surfactant produces dispersions in a variety of polar solvents having a wide range of dielectric constants (methanol, dimethyl sulfoxide, aqueous dimethylformamide, and deuterium oxide). Because a common surfactant can be used while maintaining a constant SWCNT–surfactant morphology, we are able to straightforwardly evaluate the impact of the solvation environment upon SWCNT optical properties. We find that (i) the SWCNT quantum yield is strongly dependent on both the polarity and electrophilicity of the solvent and (ii) solvatochromic shifts correlate with the extent of SWCNT solvation. These findings provide a deeper understanding of the environmental dependence of SWCNT excitonic properties and underscore that the solvent provides a tool with which to modulate SWCNT electronic and optical properties

Photoinduced Energy and Charge Transfer in P3HT:SWNT Composites

Using steady-state photoluminescence and transient microwave conductivity (TRMC) spectroscopies, photoinduced energy and charge transfer from poly(3-hexylthiophene) (P3HT) to single-walled carbon nanotubes (SWNTs) are reported. Long-lived charge carriers are observed for excitons generated in the polymer due to interfacial electron transfer, while excitation of the SWNTs results in short-lived carriers confined to the nanotubes. The TRMC-measured mobility of electrons injected into the SWNTs exhibits a surprisingly small lower limit of 0.057 cm²/(V s), which we attribute to carrier scattering within the nanotube that inhibits resonance of the microwave electric field with the confined carriers. The observation of charge transfer and the lifetime of the separated carriers suggest that the primary photoinduced carrier generation process does not limit the performance of organic photovoltaic (OPV) devices based on P3HT:SWNT composites. With optimization, blends of P3HT with semiconducting SWNTs (s-SWNTs) may exhibit promise as an OPV active layer and could provide good solar photoconversion power efficiencies