Absorption Spectra and Dynamics of Some Excited and Ionic Dicarbazolyl Compounds with Specific Geometrical Structures

Absorption spectra of 7V-ethylcarbazole, 1,2-trans-di-N-carbazolylcyclobutane, 1,3-di-.N-carbazolylpropane, rac-(dd,ll)-2,4-di-N-carbazolylpentane, and meio(i//)-2,4-di-N-carbazolylpentane in the excited singlet, triplet, cationic, and anionic states were measured with nanosecond and picosecond laser photolysis methods. The spectrum of the triplet state and anion radical is common to all these compounds, while the spectrum of the excited singlet state and cation radical is sensitive to their geometrical structure and shows the formation of different excimer and dimer cations. In the excited singlet state the sandwich structure gives an absorption spectrum that is different from the monomer Sn ← S1 spectrum, while the absorption spectral shape of the cation changes from that of the reference monomer and extent of the change increases in the order of sandwich structure < open form < partial overlap one. These results were compared with those for intramolecular naphthalene and phenanthrene systems. On the basis of the relative Tn ←T1 absorbance, the sandwich excimer formation was confirmed to increase the triplet yield. This was abscribed to the longer lifetime compared to that of the monomer fluorescent state and not to the increase of the rate constant of the corresponding intersystem crossing process. The intersystem crossing to the ground state is sensitive to the geometrical structure and enhanced in the partial overlap one. These dicarbazolyl compounds are noted to be good model systems for the study on carbazole aggregates.