Angelw: near infrared quadrupole chromophore composed of super donor and acceptor based on three coordinated boron atom

Luminescent materials are widely used in lighting, telecommunications and biological imaging Due to the limitation of the wavelength of light that the human eye can receive, researchers mainly explore the molecules whose absorption and emission wavelength are in the range of visible light However, the rapid development of biology and optical fiber industry has stimulated the research of NIR chromophore Near infrared absorbing materials usually have large conjugated systems and / or contain metal ions, among which tri coordinated organic boranes have been widely used in the development of electronic and optical materials Recently, two new quadrupole a-π-d-π-a chromophores have been designed and synthesized by Professor Holger Braunschweig, Professor Todd B Marder and their collaborators from the University of welzburg, Germany They have strong electron donor dibornene core, strong electron acceptor bis (trimethylphenyl) boron group (BMES 2) and bis (2,4,6-trifluoromethyl) phenyl (bfmes 2) terminal group Relevant achievements were published on angelw Chem Int ed (DOI: 10.1002/anie.201900889) under the title of "near infrared quadrupolar chromophores combining three coordinated born based supermarket and acceptor units" Triplex boron atoms are generally considered electron deficient, but there are also examples of them acting as electron donors For example, Weber, fox, and Marder et al Used 1,3,2-benzodiazoboxyheterocyclopentane group as donor in donor receptor conjugation system (Figure 1) Therefore, combining with the known strong electron donor and acceptor, we have designed a quadrupole molecule with excellent photoelectric properties (source: angelw Chem Int ed.) first, we used the coordination strategy of 1,3-diisopropylimidazol-2-subunit (I PR) with BBR 2 fragment and KC 8 reduction couple to form diborane 5A and 5b (Figure 2) Compound 5A is dark green crystal and compound 5b is dark purple The crystal structure of compounds 5A and 5b is shown in Figure 3 (source: angelw Chem Int ed.) (source: angelw Chem Int ed.) Later, the author compared the crystal parameters with the known dibornene with an electrically neutral group (sime 3) and an electron donor group (NPH 2) (Table 1) The thiophene group and B = B of compound 5A are not in the same plane (c1-b1-b2-s1 = 23.5 (3) ° respectively), while the whole b4c8 central unit of 5b is basically coplanar The dihedral angle of BC3 plane and thiophene plane (5A: 13.9 °; 5B: 12.1, 16.5 °) indicates that the homotrimethylene group on the external boron atom allows the empty boron P Z orbit to be arranged in an overlapping manner with the conjugated system, which is consistent with the previous report It is predicted that the increase of π electron receptivity of substituents on thiophene linked groups will lead to the increase of bond lengths of B1-B2, C1-C2 and C3-C4, and the shortening of bond lengths of b1-c1, C2-C3 and c4-b3 In this system, the bond length increases or decreases regularly with the change of 5-substituents (NPH 2 > sime 3 > BMES 2 > bfmes 2), which indicates that the compounds with boron group have strong conjugation in the ground state The B = B bond of 5A (1.607 (3) ޹) and 5b (1.618 (6) ޹) is up to the upper limit of the dibornene compound, which indicates that the delocalization of B = B electrons runs through the whole π system (source: angelw Chem Int ed.) in order to obtain more structural features of 5A and 5b, the author carried out density functional theory (DFT) calculation (Figure 4) The electron absorbing group CF 3 on the aromatic ring of triarylborane in 5B makes the frontier orbital of unoccupied electrons more stable than that already occupied, which makes it have a homo LUMO energy gap smaller than 5A (5A: 1.95 EV; 5B: 1.52 EV) In addition, the LUMO of 5b is more inclined to triarylborane, which has a significant delocalization effect on the f MES group, which greatly affects its photophysical properties (source: angelw Chem Int ed.) the author further studied the optical properties of 5A and 5b Due to the small energy gap of HOMO-LUMO and the energy isolation of two front-line orbits, 5A shows very low maximum energy absorption value and very high extinction coefficient at 840 nm and 828 nm respectively in THF and toluene, which is consistent with the Homo (π) - LUMO (π *) feature calculated by TD-DFT (Figure 5) (source: angelw Chem Int ed.) in addition, 5A showed near-infrared fluorescence at 884 nm and 863 nm respectively in THF and toluene solutions at room temperature, but its weak intensity affected the determination of fluorescence lifetime and quantum yield (Figure 6) The electron absorption group CF 3 on the 5B aryl not only shifts the absorption red to the near infrared region (1092 nm in THF) (Figure 5), but also enhances the non radiation attenuation in the solution, which makes 5B unable to be observed at room temperature In the 77K 2-methf, 5b shows weak emission, and the emission peak is at 1234 nm (Figure 6) (source: angelw Chem Int ed.) in a word, two new near-infrared quadrupole a-π-d-π-a chromophores have been constructed based on the super donor and receptor units of three coordinated boron atoms By connecting π donor and π acceptor with thiophene group, the effective conjugation and small energy gap of HOMO-LUMO were obtained, and the absorption and emission wavelengths were significantly red shifted Next, the author will focus on changing the structure of the compounds and improving their tolerance to the environment.