Luminescence of extended and folded N,N'-diarylureas

The relationship between molecular structure and luminescence of a series of secondary and tertiary N,N'-diarylureas (aryl = phenyl, 2-naphthyl, 2-anthryl, and pyren-1-yl) has been investigated at 77 K and at room temperature in the glass-forming solvent methyltetrahydrofuran. The secondary diarylureas possess planar extended structures, whereas the tertiary diarylureas possess folded structures in which the aryl groups are face-to-face. The secondary and tertiary diphenylureas display broad, weak fluorescence attributed to an n,pi* singlet state at low temperature and are non-fluorescent at room temperature. The other secondary diarylureas are strongly fluorescent both at 77 K and at room temperature. Their fluorescence resembles that of the parent arene and is assigned to a lowest pi,pi* singlet state. The fluorescence of the other tertiary diarylureas is similar to that of the secondary diarylureas at 77 K, but is broad and red-shifted in fluid solution, as a consequence of intramolecular excimer formation. The properties of these novel intramolecular excimers are compared with those of 1,3-diarylalkanes and paracyclophanes.