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

Abstract

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.

Charge Instability in Quadrupolar Chromophores: Symmetry Breaking and Solvatochromism

“…18,38,43,44,47,49,75 Specifically, the excited-state symmetry breaking predicted by our model for chromophores of class I is in line with recent results reported in Ref. 18 , where TD-DFT calculations predict the appearance of broken-symmetry solutions for the relaxed excited state of quadrupolar and octupolar chromophores.…”

“…However for many of these chromophores, experimental data suggest the existence of polar excited states. For example, strong fluorescence solvatochromism has been observed for quadrupolar and/or octupolar systems, 10,13,17,18,36,42,48,49 revealing the existence of highly J. AM.…”

The Synthesis and One‐ and Two‐Photon Optical Properties of Dipolar, Quadrupolar and Octupolar Donor–Acceptor Molecules Containing Dimesitylboryl Groups

“…[31] Such superimposition is more pronounced for compound 3 owing to the blue shift with respect to the absorption band of compound 6. On the other hand, the emission bands show a marked blue shift showing that the emitting excited state of the V-shaped compounds must be significantly different from that of their dipolar analogues.…”

“…In fact, as already seen from the linear optical properties, one-photon absorption spectra (see Supporting Information) extend over the whole investigated spectral region explored for TPA and consist of a superimposition of several transitions including local carbazole transitions. [31] For chromophores of C 3 symmetry, it is well known that the first excited state which is doubly degenerate is mainly one-photon allowed while the third excited state is one-photon forbidden and shows a much larger TPA activity. [25d,31] This is clearly visible for compounds 4 and 7 from Figure 3 and Table 2.…”

Side-chain boron difluoride formazanate polymers via ring-opening metathesis polymerization

“…1 Main-chain 2 (e.g., 1, 2) and side-chain 3 (e.g., 3, 4) polymers incorporating boron are an intriguing class of polymers, although their development has been relatively slow compared to those based on other main group elements (e.g., S, Si, and P). 4 Formazanate ligands 5, 5,6 derived from formazans 6, 7 are nitrogen-rich analogs of -diketiminates 7.…”