Photoinduced Dynamics in Carbon Nanotube Aggregates Steered by Dark Excitons.

Only optically active excitons can be identified by transient absorption spectroscopy, and the actual mechanisms of exciton relaxation in nanoscale systems remain unknown as dipole-forbidden transitions and charge-transfer states are not accounted for. Focusing on interacting (6,4) and (8,4) carbon nanotubes (CNTs), we show that dark excitons largely determine the relaxation pathways for photogenerated excitons in CNT bundles. New channels appear involving asymmetric electron-hole excitations within the same CNT and charge-transfer states, in which the electron and hole are confined to separate CNTs. The energy and charge transfers are facilitated by coupling to both low- and high-frequency phonons. Radial breathing modes are particularly important because they distort the CNT geometry, induce crossings of electronic states, and modulate coupling between CNTs. The time domain simulations reported herein uncover the quantum states and phonon modes that contribute to exciton relaxation in a CNT cluster, elucidating the complete relaxation mechanism. The established role of optically dark states pertains to nonequilibrium dynamics in nanoscale materials in general.

Tailored electronic structure and optical properties of conjugated systems through aggregates and dipole-dipole interactions.

A series of PPVO (p-phenylene vinylene oligomer) derivatives with functional groups of varying electronegativity were synthesized via the Horner-Wadsworth-Emmons reaction. Subtle changes in the end group functionality significantly impact the molecular electronic and optical properties of the PPVOs, resulting in broadly tunable and efficient UV absorption and photoluminescence spectra. Of particular interest is the NO2-substituted PPVO which exhibits photoluminescence color ranging from the blue to the red, thus encompassing the entire visible spectrum. Our experimental study and electronic structure calculations suggest that the formation of aggregates and strong dipole-dipole solute-solvent interactions are responsible for the observed strong solvatochromism. Experimental and theoretical results for the NH2-, H-, and NO2-substituted PPVOs suggest that the stabilization of ground or excited state dipoles leads to the blue or red shift of the optical spectra. The electroluminescence (EL) spectra of H-, COOH-, and NO2-PPVO have maxima at 487, 518, and 587 nm, respectively, in the OLED device. This trend in the EL spectra is in excellent agreement with the end group-dependent PL spectra of the PPVO thin-films.