Experimental and Theoretical Studies on Organic D-[pi]-A Systems Containing Three-Coordinate Boron Moieties as both [pi]-Donor and [pi]-Acceptor

Four linear π-conjugated systems with 1,3-diethyl-1,3,2-benzodiazaborolyl [C6H4(NEt)2B] as a π-donor at one end and dimesitylboryl (BMes2) as a π-acceptor at the other end were synthesized. These unusual push-pull systems contain phenylene (1,4-C6H4; 1), biphenylene (4,4'-(1,1'-C6H4)2; 2), thiophene (2,5-C4H2S; 3), and dithiophene (5,5'-(2,2'-C4H2S)2; 4) as π-conjugated bridges and different types of three-coordinate boron moieties serving as both π-donor and π-acceptor. Molecular structures of 2, 3, and 4 were determined by single-crystal X-ray diffraction. Photophysical studies on these systems reveal blue-green fluorescence in all compounds. The Stokes shifts for 1, 2, and 3 are notably large at 7820-9760cm-1 in THF and 5430-6210cm-1 in cyclohexane, whereas the Stokes shift for 4 is significantly smaller at 5510cm-1 in THF and 2450cm-1 in cyclohexane. Calculations on model systems 1'-4' show the HOMO to be mainly diazaborolyl in character and the LUMO to be dominated by the empty p orbital at the boron atom of the BMes2 group. However, there are considerable dithiophene bridge contributions to both orbitals in 4'. From the experimental data and MO calculations, the π-electron-donating strength of the 1,3-diethyl-1,3,2-benzodiazaborolyl group was found to lie between that of methoxy and dimethylamino groups. TD-DFT calculations on 1'-4', using B3LYP and CAM-B3LYP functionals, provide insight into the absorption and emission processes. B3LYP predicts that both the absorption and emission processes have strong charge-transfer character. CAM-B3LYP which, unlike B3LYP, contains the physics necessary to describe charge-transfer excitations, predicts only a limited amount of charge transfer upon absorption, but somewhat more upon emission. The excited-state (S1) geometries show the borolyl group to be significantly altered compared to the ground-state (S0) geometries. This borolyl group reorganization in the excited state is believed to be responsible for the large Stokes shifts in organic systems containing benzodiazaborolyl groups in these and related compounds.