Surface Photochemistry: 3,3′-Dialkylthia and Selenocarbocyanine Dyes Adsorbed onto Microcrystalline Cellulose

In this work, thia and selenocarbocyanines with n-alkyl chains of different length, namely with methyl, ethyl, propyl, hexyl and decyl substituents, were studied in homogeneous and heterogeneous media for comparison purposes. For both carbocyanine dyes adsorbed onto microcrystalline cellulose, a remarkable increase in the fluorescence quantum yields and lifetimes were detected, when compared with solution. Contrary to the solution behaviour, where the increase in the n-alkyl chains length increases to a certain extent the fluorescence emission Φ(F) and τ(F), on powdered solid samples a decrease of Φ(F) and τ(F) was observed. The use of an integrating sphere enabled us to obtain absolute Φ(F)'s for all the powdered samples. The main difference for liquid homogeneous samples is that the increase of the alkyl chain strongly decreases the Φ(F) values, both for thiacarbocyanines and selenocarbocyanines. A lifetime distribution analysis for the fluorescence of these dyes adsorbed onto microcrystalline cellulose, evidenced location on the ordered and crystalline part of the substrate, as well as on the more disordered region where the lifetime is smaller. The increase of the n-alkyl chains length decreases the photoisomer emission for the dyes adsorbed onto microcrystalline cellulose, as detected for high fluences of the laser excitation, for most samples.