Intermolecular interactions in conjugated oligothiophenes. 3. Optical and photophysical properties of quaterthiophene and substituted quaterthiophenes in various environments

A detailed analysis of the optical and photophysical properties of 2,2':5': 2 ":5 ",2'''-quaterthiophene (QT), 3,3"'-dimethoxy-2,2':5':2 ":5 ",2"'-quat erthiophene (DMOQT), 3,3"'-dimethyl-2,2':5':2 ":5 ",2'''-quaterthiophene (D MQT) and 3',4 "-didecyl-2,2':5':2 ":5 ",2"'-quaterthiophene (DDQT) in vario us environments is reported. In solution at room temperature, the optical p roperties of the free molecules rue obtained and discussed in terms of the effect of the substitution on the conformation adopted by each derivative i n the electronic ground and first excited states. In a tetradecane matrix a t 77 K, the optical properties are obtained for the isolated molecules in t his rigid medium where the oligothiophenes adopt conformations similar to t hose found in the solid state. The optical properties of the quaterthiophen e derivatives in their aggregated forms and in the solid state are also rep orted and discussed in terms of the substitutional effect on the intermolec ular interactions, which affect the spectral and photophysical propel ties of the isolated molecules. For the first time, a beta,beta'-disubstituted o ligothiophene (DMQT) showing an excitonic splitting similar to that obtaine d for QT is reported. All other substituted oligothiophenes presented show a conformational change, following the aggregation process. This difference is explained by more disordered crystalline forms for DMOQT and DDQT. Theo retical calculations using the ZINDO/S semiempirical method are also perfor med on the crystalline structure of each derivative in an attempt to correl ate the optical properties of these molecules in their aggregated forms and in the solid state with the molecular arrangement found in the crystal.