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

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journal contribution
posted on 2016-02-22, 17:58 authored by Jaehong Park, Pravas Deria, Michael J. Therien
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) (τISC ∼20 ps), a sharp T1→Tn transient absorption signal (λmax(T1→Tn) = 1150 nm; full width at half-maximum ≈ 350 cm–1), and a substantial T1 excited-state lifetime (τes ≈ 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.