Dynamics and Transient Absorption Spectral Signatures of the Single-Wall Carbon Nanotube Electronically Excited Triplet State

We utilize femtosecond-to-microsecond time domain pump–probe transient absorption spectroscopy to interrogate for the first time the electronically excited triplet state of individualized single-wall carbon nanotubes (SWNTs). These studies exploit (6,5) chirality-enriched SWNT samples and poly[2,6-{1,5-bis(3-propoxysulfonic acid sodium salt)}naphthylene]ethynylene (PNES), which helically wraps the nanotube surface with periodic and constant morphology (pitch length = 10 ± 2 nm), providing a self-assembled superstructure that maintains structural homogeneity in multiple solvents. Spectroscopic interrogation of such PNES-SWNT samples in aqueous and DMSO solvents using E₂₂ excitation and a white-light continuum probe enables E₁₁ and E₂₂ spectral evolution to be monitored concomitantly. Such experiments not only reveal classic SWNT singlet exciton relaxation dynamics and transient absorption signatures but also demonstrate spectral evolution consistent with formation of a triplet exciton state. Transient dynamical studies evince that (6,5) SWNTs exhibit rapid S₁→T₁ intersystem crossing (ISC) (τ_ISC ∼20 ps), a sharp T₁→T_n transient absorption signal (λ_max(T₁→T_n) = 1150 nm; full width at half-maximum ≈ 350 cm^–1), and a substantial T₁ excited-state lifetime (τ_es ≈ 15 μs). Consistent with expectations for a triplet exciton state, T₁-state spectral signatures and T₁-state formation and decay dynamics for PNES-SWNTs in aqueous and DMSO solvents, as well as those determined for benchmark sodium cholate suspensions of (6,5) SWNTs, are similar; likewise, studies that probe the ³[(6,5) SWNT]* state in air-saturated solutions demonstrate ³O₂ quenching dynamics reminiscent of those determined for conjugated aromatic hydrocarbon excited triplet states.