Photobleaching-Assisted Near-Field Absorption Spectroscopy: Its Application to Single Tubular J-Aggregates

We propose photobleaching-assisted near-field optical spectroscopy to obtain the absorption properties of nanomaterials. We used bis(3-sulfopropyl)-5,5′,6,6′-tetrachloro-11′-dioctylbenzimidacarbocyanine (C8S3) double-wall tubular J-aggregates for demonstrating this method. We photobleached the J-aggregate by photoirradiation. We obtained the absorption spectrum of the J-aggregates by subtracting the transmission spectra observed before and after the photobleaching. The spectrum showed an intense asymmetric peak near 595 nm, which was assigned to the J-bands. These features were well reproduced by theoretical model calculations. We estimated the oscillator strength per molecule from the observed absorbance, and we found that the determined oscillator strength is comparable to that of the monomer. These results indicate that the proposed method is applicable for obtaining near-field absorption spectra of nanomaterials in a semi-quantitative manner.