Polyunsaturated dicarboxylate tethers connecting dimolybdenum redox and chromophoric centers:: Absorption spectra and electronic structures

The absorption spectra of a series of compounds of the type [Mo-2(DAniF)(3)](O2CXCO2)[Mo-2(DAniF)(3)] (DAniF = N,N'-di-p-anisylformamidinate) have been measured and revealed a strong dependence of the electronic transitions and, therefore, the colors upon the chemical nature of the dicarboxylate linker. The more intense colors and lower energy absorptions are observed with those compounds having unsaturated dicarboxylate linkers. Static and time-dependent DFT calculations were undertaken to identify the electronic excitations responsible for the observed colors. For those compounds with chemically unsaturated and fully conjugated dicarboxylate linkers (oxalate, 6; fumarate, 8; acetylene dicarboxylate, 9; cis,cis-muconate, 11; trans,trans-muconate, 12; tamuate, 13; texate, 14; terephthalate, 15), the lowest energy absorptions are Mo-2(4+) delta --> dicarboxylate pi* metal-to-ligand charge transfer transitions. Those compounds with chemically saturated linkers (succinate, 20) have delta --> delta* transitions as their lowest energy absorptions with essentially independent and noninteracting Mo-2(4+) chromophores.