Intramolecular Excimer Emissions of <i>syn-</i> and <i>anti-</i>[3.3](3,9)Carbazolophanes in Solutions

Abstract

The photophysical properties of carbazole excimers were investigated using syn- and anti-[3.3](3,9)carbazolophanes: the syn-carbazolophane is a model compound for a fully overlapped carbazole excimer and the anti-carbazolophane for a partially overlapped carbazole excimer the structures of which have been determined by X-ray analysis. 1H NMR spectra showed that two carbazole moieties in the syn- and anti-[3.3](3,9)carbazolophanes did not flip even in solution owing to short [3.3](3,9)linkage: geometrical alignments of two carbazole moieties remained the same as those determined by the X-ray analysis. Absorption bands of the carbazolophanes were broadened and blue- or red-shifted compared with those of 1,3-bis(3-methyl-N-carbazolyl)propane used as a reference compound. These spectral features were explained by Kasha's molecular exciton theory. Fluorescence spectra of both carbazolophanes were red-shifted, broad, and structureless bands. These broad emissions were ascribed to intramolecular excimer fluorescence of the carbazolophanes, which was insensitive to solvent polarity. The dipole moment in the excited state was estimated to be ∼0 D for the anti-carbazolophane and to be 4.8 D for the syn-carbazolophane from Lippert−Mataga plots. The radiative rate of excimer emission was 3.4 × 106 s-1 for the anti-carbazolophane and 7.2 × 105 s-1 for the syn-carbazolophane. Our [3.3](3,9)linkage carbazolophanes clearly showed that these different photophysical properties of carbazole excimers result from the geometries of their carbazole moieties.