Energy transfer to the low-energy triplet states of 1,3-dicarbonylazomethine dyes: The role of unique geometries and nonadiabatic behavior

The observed kinetics of energy transfer to PhCO-(C=NAr)-CONHPh (1), PhCO-(C=NAr)-COPh (2), and PhCONH-(C=NAr)-CONHPh (3) (Ar = p-NMe2C6H4) is consistent with formation of two triplet states for each dye: T-1 and T-2. A Balzani analysis of the data observed for 1 affords triplet energies of 23.4 +/- 1.6 and 38.5 +/- 1.7 kcal/mol for T-1 and T-2, respectively, and requires that T-1 be formed in reactions having substantial nonadiabatic character. A similar result is found for 2. For 3, T-1 and T-2 are estimated at 25.0 and 38.5 kcal/mol, respectively, but formation of 3-T-1 is found to be largely adiabatic. To rationalize these observations, the dyes have been studied by DFT methods (B3LYP/6-31G*). In the ground state (I-S-0) 1 is computed to have a geometry in which the azomethine double bond is coplanar with the anilide fragment but is perpendicular to the carbonyl of the ketone. Similarly, 2-S-0 is calculated to have a twisted 1,3-dicarbonyl arrangement. 3-S-0, on the other hand, exhibits an intramolecular H-bond between the proton of one anilide and the carbonyl of another, and has a large degree of planarity in the 1,3-dicarbonyl moiety. For all compounds, the lowest triplet state (T-1) is calculated to have a geometry in which the azomethine bond is twisted by 70 degrees -80 degrees relative to the ground state, and to have a near-planar arrangement between the azomethine and the two carbonyl groups. The calculated lowest triplet energies are tin kcal/mol): 23.0 and 27.5 for two isomers of 1, 25.2 for 2, and 23.4 for 3. The large geometrical differences between the ground and the lowest triplet states are proposed to provide an efficient mechanism for triplet state stabilization, and to stand at the origin of the observed nonadiabatic energy transfer in 1 and 2. The presence of planarized equilibrium concentrations of 3 due to intramolecular H-bonding can explain the greater rates of energy transfer to 3 compared to 1 and 2.