Electrochemistry and Photophysics of Donor-Substituted Triarylboranes: Symmetry Breaking in Ground and Excited State

• Published in: Chemistry : a European journal 2006-03, Vol.12 (8), p.2358-2370
• Main Authors: Stahl, Rainer, Lambert, Christoph, Kaiser, Conrad, Wortmann, Rüdiger, Jakober, Ruth
• Format: Article
• Language: English
• Subjects: boranes; charge transfer; fluorescence spectroscopy; solvatochromism; symmetry breaking; UV/Vis spectroscopy
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200500948

We synthesized a series of amino substituted triarylboranes (TABs) 1–3 by copper(I)‐catalyzed cross‐coupling reactions. The title compounds were investigated by means of cyclic voltammetry (CV) and UV‐visible absorption and fluorescence spectroscopy. Electrochemical oxidation of tris(4‐carbazolyl‐2,6‐dimethylphenyl)borane (3) leads to the formation of an electroactive polymer film on the electrode surface. The charge‐transfer (CT) absorption band of all three TABs shows a pronounced negative solvatochromism, while the emission is positively solvatochromic. By combining Jortner's theory, AM1 computations, and electrooptical absorption measurements (EOAM), this unexpected behavior was shown to be due to a dipole inversion upon S0→S1 excitation. Furthermore, polarized steady‐state fluorescence spectroscopy and EOAM prove that the ground‐state geometry of 3 is of lower symmetry than D3 and that the excitation energy can be transferred from one subchromophore to another within the lifetime of the excited state. Exciton‐coupling theory was used to quantitatively analyze this excitation transfer. A star‐shaped, carbazole‐substituted triarylborane (see picture) and its 1D model compounds are studied by cyclic voltammetry, UV/Vis absorption and fluorescence spectroscopy, and electro‐optical absorption measurements. The results prove that the ground‐state symmetry of the parent borane is lower than D3.