Quantifying the Polarization of Exciton Transitions in Double-Walled Nanotubular J‑Aggregates

Abstract

A fully consistent model for the exciton band structure of double-walled 3,3′-bis­(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3) J-aggregates was developed using reduced linear dichroism (LD^r) spectroscopy on flow aligned samples. Chemical oxidation was utilized to “turn off”outer wall optical absorption and produce stable aggregate samples with a simplified absorption profile associated only with the nanotube inner wall. The oxidized aggregates were aligned in a flow cell to collect LD^r spectra; these spectra reveal a series of both polarized and isotropic transitions. Four spectral transitions, assigned to be purely parallel or perpendicular to the aggregate long axis, that fit both the experimental LD^r and isotropic spectra were used create a model for oxidized J-aggregate excitonic absorption. The LD^r spectral study was repeated using pristine J-aggregates, and the spectrum for the full double-walled J-aggregates could be fit using six total transitions: four from the oxidized fit and two additional transitions distinct to the outer wall. A quantitative model that agrees with experimental absorption and emission spectral results and aligns with current theory was constructed wherein the energies and polarizations of excitonic transitions remained consistent for both the unperturbed and chemically oxidized C8S3 J-aggregates. The polarization studies also reveal, in contrast to the strongly polarized transitions that comprise the low-energy region of the excitonic aggregate spectrum, that the high-energy absorption is unpolarized and attributed to highly localized exciton transitions that arise due to disorder