Temperature-Dependent Exciton Properties of Two Cylindrical J-Aggregates

The temperature dependence of the steady-state excitonic absorption and emission spectral features of the J-aggregates of the amphiphilic cyanine dye 3,3′-bis(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3) was examined over the temperature range from 77 to 298 K. Two C8S3 J-aggregate structures were investigated: well-separated, double-walled nanotubes and bundles of agglomerated nanotubes that spontaneously assemble over long periods of storage. Absorption and emission spectral line broadening and Stokes shift are presented, and the responses of both aggregates are evaluated as a function of temperature. Both J-aggregates exhibit two fluorescence bands. We found that, across the measured temperature range, the ratio of the nanotube’s emission bands is well described with Boltzmann statistics, while that of the bundles is not. Additionally, the relative quantum yield of the nanotubes increased dramatically upon cooling, while the bundles’ quantum yield exhibited a significantly smaller increase over the same temperature range—an observation we attribute to the bundles’ greater absolute quantum yield.