PUB - Publikationen an der Universität Bielefeld

A series of diphenylacetylenes (17-22) with one 1,3,2-benzodiazaborolyl end group and a second end group X were synthesized and fully characterised. These borylated systems show intense blue luminescence in non-polar solvents, whereas green luminescence was observed in polar solvents. The LUMOs in all the molecules under study are mainly located on the diphenylacetylene bridge, while the HOMOs are mainly located on the diazaborole part. Unlike their absorption maxima, their emission maxima are characterized by large solvatochromic shifts in solvents of increasing polarity, which is indicative of significant transition dipole moment upon excitation. Using the Lippert-Mataga approximation changes of the dipole moment between the ground-state and the excited-state of ca. 19 D for these substances were estimated. A series of novel organic donor-π-acceptor systems (40, 41, 43, 44, 48, 49 and 101)containing benzodiazaborolyl groups as π-donors and dimesitylboryl groups as π-acceptors was synthesized. The compounds are all fluorescent with large Stokes shifts up to 9800 cm-1. While the electron-accepting property of the dimesitylboron group is known to be between that of the cyano and nitro group, the photophysical studies and computations show that the electron-donating effect of the benzodiazaborolyl group is between that of dimethylamino and methoxy groups. The excited-state (S1) geometries show the borolyl group 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 π-systems containing benzodiazaborolyl groups as terminal substituents. In the course of a program on synthesis, structure and properties of 1,3,2-diazaboroles we were also interested in the origin of the donor character of benzodiazaboroles, even though they contain a three-coordinate boron atom. Thus we synthesized a variety of compounds, with a dimesitylboryl group as π-acceptor and an additional functional group, which is closely related to a benzodiazaborole (104, 106, 111, 112, 114 and 115). Thereby we have found significant changes in the photophysical properties as a consequence of the introduction of a diazaborole unit instead of a "common" donor substituent. A benzodiazaborole unit leads to species with high extinction coefficients, transition dipole moments, fluorescence quantum yields and large Stokes shifts. In conclusion it is obvious, that the benzodiazaborole unit confers unique photophysical properties to extended π-systems.