Intramolecular excimer fluorescence from folded ground state rotamers of N,N′dimethyl-N,N′-dipyrenylurea protophanes

The molecular structure, absorption, and fluorescence spectra of N,N′-dimethyl-N,N′-di-1-pyrenylurea and N,N′ -dimethyl-N,N′-di-2-pyrenylurea have been investigated and compared to the properties of N,N,N′-trimethyl-N′-pyrenylurea model compounds. Di-1-pyrenylurea exists as a mixture of folded (E,E) syn- and anti-rotamers that interconvert via flipping of one of the pyrene rings to an unfolded (E,Z) rotamer geometry. The symmetric di-2-pyrenylurea exists as a single folded (E,E) conformation which is in equilibrium with a less-stable, unfolded (E,Z) rotamer. The absorption and fluorescence spectra of the dipyrenylureas at 77 K in a rigid glass are similar to those of monopyrenylurea model compounds. However, in fluid solution, the dipyrenylureas exhibit excimer fluorescence and very weak monomer fluorescence which have identical decay times. This behavior is attributed to fast rotational equilibrium between folded rotamers which exhibit excimer fluorescence and unfolded rotamers which exhibit monomer fluorescence. The behavior of the dipyrenylureas is compared with that of other systems that form intramolecular pyrene excimers and diarylureas.