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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.

The frictional behaviors of methyl oleate (MO), methyl palmitate (MP), methyl laurate (ML), and methyl stearate (MSt) as additives in hexadecane have been examined in a boundary lubrication test regime using steel contacts. It was found that the transient attributes of coefficient of friction (COF)-time spectra are a sensitive measure of adsorption equilibria. Critical additive concentrations were defined and used to perform novel and simple Langmuir analyses that provide an order of adsorption energies: MSt > MP > MO ≥ ML. Application of Langmuir, Temkin, and Frumkin-Fowler-Guggenheim adsorption models via nonlinear fitting of a general cooperative model demonstrates the necessary inclusion of cooperative effects in the applied model. In agreement with the qualitative features of steady-state COF-concentration plots, MSt modeling requires minimal cooperative interaction terms. However, MO, MP, and ML data require large attractive interaction terms to be adequately fitted. Primary adsorption energies calculated via the cooperative model are necessarily decreased, whereas total adsorption energies correlate well with values obtained via critical concentration analyses. These results and comparisons with previous adsorption studies of MO and MSt suggest that primary (ester-surface) and secondary (alkyl-surface) adsorbate-adsorbent, adsorbateadsorbate, and (free-additive) adsorpt-adsorpt interactions collectively determine both the calculated primary and the cooperative interaction energies.Paper no. J11008 in JAOCS 82, 293-299 (April 2005).

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