Universal properties of quasi-one-dimensional excitons in semiconducting single-walled carbon nanotubes and ∏-conjugated polymers

The nature of the primary photoexcitations in semiconducting single-walled carbon nanotubes (S-SWCNTs) is of strong current interest. We have studied the emission spectra of S-SWCNTs and two different rr-conjugated polymers in solutions and films, and have also performed ultrafast pump-probe spectroscopy on these systems. The emission spectra relative to the absorption bands are very similar in S-SWCNTs and polymers, with redshifted photoluminescence in films showing exciton migration. The transient photoinduced absorptions (PAs) in SWCNTs and ∏-conjugated polymers are also remarkably similar, with a low energy PA1 and a higher energy PA2 in all cases. Theoretical calculations of excited state absorptions within a correlated rr-electron Hamiltonian find the same excitonic energy spectrum for S-SWCNTs and rr-conjugated polymers, illustrating the universal features of quasi-one-dimensional excitons in carbon-based rr-conjugated systems. In both cases PA1 is an excited state absorption from the optically allowed exciton to a two-photon exciton that occurs below the continuum band threshold. PA1 therefore gives the lower limit of the binding energy of the lowest optical exciton. The binding energy of lowest exciton belonging to the widest S-SWCNTs with diameters ≥ 1 nm in films is 0.3-0.4 eV, as determined by both experimental and theoretical methods.