Intermolecular interactions in conjugated oligothiophenes. 1. Optical spectra of terthiophene and substituted terthiophenes recorded in various environments

The absorption and fluorescence spectra of terthiophene (TT) as well as thr ee substituted terthiophene derivatives, namely 3,3"-dimethoxy-2,2':5',2"-t erthiophene (DMOTT), 3,3"-dimethyl-2,2':5',2"-terthiophene (DMTT), and 3',4 '-dihexyl-2,2':5',2"-terthiophene (DI-ITT), isolated in an alkane matrix an d in their aggregated forms are reported. The thermochromic properties of s ubstituted oligothiophenes are reported for the first time. The optical cha nges observed for the isolated molecules in the alkane matrix are interpret ed in terms of conformational changes caused by the packing effect. After a ggregation, TT exhibits a splitting in the excitation spectrum, which is pr ovoked by an excitonic effect. On the other hand, it is observed that the c hanges in the optical properties of substituted terthiophenes are close to those found in the alkane matrix. This strongly suggests that the aggregati on process does not produce any significant optical change, other than thos e provoked by conformational changes, in the excitation spectra of these mo lecules. It is suggested that the presence of the side chains weaken the in termolecular interactions, which are responsible for the excitonic effect. Finally, the fluorescence spectra of all the molecules investigated show a new red-shifted band, when aggregated. The new emission cannot be explained by conformational changes and has its origin in the aggregation process it self.