Electronic structure of BN-aromatics: Choice of reliable computational tools.

The importance of having reliable calculation tools to interpret and predict the electronic properties of BN-aromatics is directly linked to the growing interest for these very promising new systems in the field of materials science, biomedical research, or energy sustainability. Ionization energy (IE) is one of the most important parameters to approach the electronic structure of molecules. It can be theoretically estimated, but in order to evaluate their persistence and propose the most reliable tools for the evaluation of different electronic properties of existent or only imagined BN-containing compounds, we took as reference experimental values of ionization energies provided by ultra-violet photoelectron spectroscopy (UV-PES) in gas phase-the only technique giving access to the energy levels of filled molecular orbitals. Thus, a set of 21 aromatic molecules containing B-N bonds and B-N-B patterns has been merged for a comparison between experimental IEs obtained by UV-PES and various theoretical approaches for their estimation. Time-Dependent Density Functional Theory (TD-DFT) methods using B3LYP and long-range corrected CAM-B3LYP functionals are used, combined with the ΔSCF approach, and compared with electron propagator theory such as outer valence Green's function (OVGF, P3) and symmetry adapted cluster-configuration interaction ab initio methods. Direct Kohn-Sham estimation and "corrected" Kohn-Sham estimation are also given. The deviation between experimental and theoretical values is computed for each molecule, and a statistical study is performed over the average and the root mean square for the whole set and sub-sets of molecules. It is shown that (i) ΔSCF+TDDFT(CAM-B3LYP), OVGF, and P3 are the most efficient way for a good agreement with UV-PES values, (ii) a CAM-B3LYP range-separated hybrid functional is significantly better than B3LYP for the purpose, especially for extended conjugated systems, and (iii) the "corrected" Kohn-Sham result is a fast and simple way to predict IEs.

Two BN isosteres of anthracene: synthesis and characterization.

The synthesis of two parental BN anthracenes, 1 and 2, was developed, and their electronic structure and reactivity behavior were characterized in direct comparison with all-carbon anthracene. Gas-phase UV-photoelecton spectroscopy studies revealed the following HOMO energy trend: anthracene, -7.4 eV; BN anthracene 1, -7.7 eV; bis-BN anthracene 2, -8.0 eV. The λmax of the lower energy band in the UV-vis absorption spectrum is as follows: anthracene, 356 nm; BN anthracene 1, 359 nm; bis-BN anthracene 2, 357 nm. Thus, although the HOMO is stabilized with increasing BN incorporation, the HOMO-LUMO band gap remains unchanged across the anthracene series. The emission λmax values for the three investigated anthracene compounds are at 403 nm. The pKa values of the N-H proton for BN anthracene 1 and bis-BN anthracene 2 were determined to be approximately 26. BN anthracenes 1 and 2 do not undergo heat- or light-induced cycloaddition reactions or Friedel-Crafts acylations. Electrophilic bromination of BN anthracene 1 with Br2, however, occurs regioselectively at the 9-position. The reactivity behavior and regioselectivity of bromination of BN anthracenes are consistent with the electronic structure of these compounds; i.e., (1) the lower HOMO energy levels for BN anthracenes stabilize the molecules against cycloaddition and Friedel-Crafts reactions, and (2) the HOMO orbital coefficients are consistent with the observed bromination regioselectivity. Overall, this work demonstrates that BN/CC isosterism can be used as a molecular design strategy to stabilize the HOMO of acene-type structures while the optical band gap is maintained.

UV-photoelectron spectroscopy of BN indoles: experimental and computational electronic structure analysis.

We present a comprehensive electronic structure analysis of two BN isosteres of indole using a combined UV-photoelectron spectroscopy (UV-PES)/computational chemistry approach. Gas-phase He I photoelectron spectra of external BN indole I and fused BN indole II have been recorded, assessed by density functional theory calculations, and compared with natural indole. The first ionization energies of these indoles are natural indole (7.9 eV), external BN indole I (7.9 eV), and fused BN indole II (8.05 eV). The computationally determined molecular dipole moments are in the order: natural indole (2.177 D) > fused BN indole II (1.512 D) > external BN indole I (0.543 D). The λmax in the UV-vis absorption spectra are in the order: fused BN indole II (292 nm) > external BN indole I (282 nm) > natural indole (270 nm). The observed relative electrophilic aromatic substitution reactivity of the investigated indoles with dimethyliminium chloride as the electrophile is as follows: fused BN indole II > natural indole > external BN indole I, and this trend correlates with the π-orbital coefficient at the 3-position. Nucleus-independent chemical shifts calculations show that the introduction of boron into an aromatic 6π-electron system leads to a reduction in aromaticity, presumably due to a stronger bond localization. Trends and conclusions from BN isosteres of simple monocyclic aromatic systems such as benzene and toluene are not necessarily translated to the bicyclic indole core. Thus, electronic structure consequences resulting from BN/CC isosterism will need to be evaluated individually from system to system.

Syntheses of rod-shaped fluorescent 1,3,2-benzodiazaboroles with phosphonium, and phosphane chalcogenide acceptor functions.

A series of 1,4-phenylenes X-C(6)H(4)-BDB with a 1,3,2-benzodiazaborolyl (BDB) and a phosphorus based end group [X = PPh(2) (2), P(O)Ph(2) (3), P(S)Ph(2) (4), P(Se)Ph(2) (5), P(AuCl)Ph(2) (6) and P(Me)Ph(2) (7)] as well as 2-(2')thienyl-1,3,2-benzodiazaboroles with a second end group X [X = PPh(2) (8), P(S)Ph(2) (9), P(Se)Ph(2) (10) and P(Me)Ph(2) (11)] in the 5' position were synthesised using established methodologies. Molecular structures of 2-9 and 11 were determined by X-ray diffraction. Compounds 3, 4, 6, 7, 9 and 11 show intense blue luminescence in cyclohexane, toluene, chloroform, dichloromethane and tetrahydrofuran with pronounced solvatochromism. Thereby Stokes shifts in the range of 8950-10,440 cm(-1) and quantum yields up to 0.70 were observed in dichloromethane solutions. In contrast to this, for the selenides 5 and 10 quantum yields are small (<0.1). The absorption maxima (298-340 nm) are well reproduced by TD-DFT computations (B3LYB/G-311G(d,p)) and arise from strong HOMO-LUMO transitions. With the exception of 5 and 10 the HOMOs of the molecules under study are mainly located on the benzodiazaborole group. In 5 and 10 the HOMOs are on the selenium atoms. The LUMOs of all new neutral molecules are mainly represented by the phenylene or thiophene bridge. In the phosphonium cations the LUMOs have additional contributions from the phosphonium unit.

UV-photoelectron spectroscopy of 1,2- and 1,3-azaborines: a combined experimental and computational electronic structure analysis.

We present a comprehensive electronic structure analysis of structurally simple BN heterocycles using a combined UV-photoelectron spectroscopy (UV-PES)/computational chemistry approach. Gas-phase He I photoelectron spectra of 1,2-dihydro-1,2-azaborine 1, N-Me-1,2-BN-toluene 2, and N-Me-1,3-BN-toluene 3 have been recorded, assessed by density functional theory calculations, and compared with their corresponding carbonaceous analogues benzene and toluene. The first ionization energies of these BN heterocycles are in the order N-Me-1,3-BN-toluene 3 (8.0 eV) < N-Me-1,2-BN-toluene 2 (8.45 eV) < 1,2-dihydro-1,2-azaborine 1 (8.6 eV) < toluene (8.83 eV) < benzene (9.25 eV). The computationally determined molecular dipole moments are in the order 3 (4.577 D) > 2 (2.209 D) > 1 (2.154 D) > toluene (0.349 D) > benzene (0 D) and are consistent with experimental observations. The λ(max) in the UV-vis absorption spectra are in the order 3 (297 nm) > 2 (278 nm) > 1 (269 nm) > toluene (262 nm) > benzene (255 nm). We also establish that the measured anodic peak potentials and electrophilic aromatic substitution (EAS) reactivity of BN heterocycles 1-3 are consistent with the electronic structure description determined by the combined UV-PES/computational chemistry approach.