Universality in the photophysics of π-conjugated polymers and single-walled carbon nanotubes

Semiconducting carbon-based organssic π-conjugated systems have been intensely investigated over the past several decades. In particular, their photophysics has been and continues to be of strong interest because of fundamental curiosity as well as current and promising technological applications. From a fundamental perspective, interest in carbon-based π-conjugated systems originates from their remarkable differences from the conventional inorganic band semiconductors. In contrast to the latter, strong, short-range, repulsive Coulomb interactions occur among the π-electrons in the organics, and these interactions contribute to a significant fraction of the optical gap. Theoretical understanding of π-conjugated systems therefore necessarily requires going beyond traditional band theory. Experimentally, even as exciton formation is found to be common in these materials, the standard technique of comparing the thresholds of linear absorption and photoconductivity for the determination of the exciton binding energy fails in noncrystalline organic materials because of the existence of disorder and inhomogeneity in these systems. Nonlinear optical spectroscopy and, in particular, ultrafast modulation spectroscopy have played valuable roles in elucidating the underlying electronic structures and photophysics of π-conjugated systems.