Ground-state conformational equilibrium and photochemical behavior of syn and anti N,N'-dimethyl-N,N'-di-1-naphthylurea protophanes.

The structure, spectroscopy, and photochemistry of N,N'-dimethyl-N,N'-di-1-naphthylurea have been investigated and compared to the properties of the corresponding secondary diarylurea N,N'-di-1-naphthylurea and the tertiary mono arylurea N,N,N'-trimethyl-N'-1-naphthylurea. The crystal structures and solution NMR spectra of the tertiary and secondary dinaphthylureas establish that they adopt folded (E,E) and extended (Z,Z) structures, respectively, both in the solid state and in solution. In solution, the tertiary E,E-dinaphthylurea exists as a mixture of syn and anti conformations separated by a barrier of ca. 14 kcal/mol, as determined by variable-temperature (1)H NMR spectroscopy. Computational exploration of the ground-state potential energy surface suggests that the lowest energy pathway for interconversion of the syn and anti conformers requires concurrent rotation about both the nitrogen-naphthalene and the nitrogen-carbonyl single bonds. The tertiary dinaphthylurea exhibits blue-shifted absorption and red-shifted emission attributed to excitonic interactions between the naphthalene rings. The secondary dinaphthylureas and mono naphthylurea have typical naphthalene-like monomer absorption and fluorescence spectra. Dual exponential fluorescence decay is assigned to the two conformers of the tertiary dinaphthylurea. Nonlinear fitting of the fluorescence decay times provides activation parameters for singlet decay of the two conformers. The decay process is attributed to nonsynchronous naphthalene-naphthalene bonding which, in the case of the syn conformer, results in the formation of a [2+2] intramolecular adduct. The preferred E,E conformation and moderate barrier to conformational isomerization make the tertiary dinaphthylurea an attractive building block for larger self-organizing pi-stacked aromatic arrays.