| The triarylborane-based organic optoelectronic materials have attracted increasing owing to the unique structure and optoelectronic properties. They have been widely studied in a broad range of fields, such as, oganic light emitting diodes (OLED), organic field-effect transistors (OFET), nonlinear optical materials, organic photovoltaic cells (OPVs), organic light-emitting solid and sensor materials, etc.Due to the electron-withdrawing ability of boryl, the triaryboranes usually displays intramolecular charge-transfer (ICT) characteristics when an electron-donating group, such as an amino group, is present. Very recently, we have disclosed o,o’-substituted biphenyl system,o,o’-NMe2, in which an elctron-withdrawing dimesityboryl group and an electron-donating dimethylamino group were introduced at the o,o’-positions of the biphenyl framework. It was revealed that this compound shows very unique photophysical properties. It is highly emissive in solution and solid-state. More over, it shows a long fluorescence emission wavelength and particularly large Stokes shift. The emission wavelength of o,o’-NMe2 is longer not only than its regioisomers, but also even than stillb and diphenylacetylene and stillbene derivatives. Encoraged by the interesting photophysical properties of o,o’-NMe2, we carried out the following works.1. First, we designed and synthesized two isomers of dibenzylamino substituted biphenyls,o,o’-NBn2 and p,p’-NBn2, to further study the changes of amino groups on the optical properties. It was found that when amino and boryl substituent are in o,o’-position of the biphenyl, the change of amino can cause a signifacant change in the optical properties. In cyclohexane, the maximum fluorescence wavelength exhibits has a blue shift of 71 nm from o,o’-NMe2(521 nm) to o,o’-NBn2(450 nm). However, when amino and boryl substituent are in p,p’-position, p,p’-NBn2 and p,p’-NMe2 have the similar absorption and emission, suggesting that the NBn2 and NMe2 have close electron-donating ability. And thus, the significant blue shift of the emission form o,o’-NMe2 to o,o’-NBn2 should be ascribed to the change of conformation as the result of the different steric hindrance of the anino. The crystal structure shows that, the amino and boryl subsituents are in the same side of biphenyl in o,o’-NMe2 with a very short distance (3.59 A) between B and N. In contrat, the amino and boryl subsituents are in the opposite sides of biphenyl in o,o’-NBn2. Theoretical calculations show that, the conformation of o,o’-NMe2 is favorable to stabilization of LUMO in the excited state. These results shows that when amino and boryl substituent are in o,o’-positions, the photophysical properties are tunable via tuning the steric hindrance of amino and thus the stero structure of the biphenyls.2. Promoted by the unique properties of o,o’-NMe2, its structure was further modified to investigate the structure-property relatioships and explore applications. We found that strong electro-donation group ((4-N,N-dimethyl amino) phenyl) into the para positon of amino in o,o’-NMe2, only cause a significant redshift of the emission while the absorption spectrum remains unchanged. However, the absorpton and emission are both signigacntly red-shift when the strong electro-withdrawing group (4-(2,2-cyano-vinyl) phenyl) is introduced into the para positon of amino in o,o’-NMe2. In addition, they both have high fluorescence quantum efficiency in solid-state (film state:ΦF= 0.58, ((4-N,N-dimethyl amino) phenyl);ΦF= 0.32, (4-(2,2-cyano-vinyl) phenyl)). It was nothed the compound containing dicyano vinyl group has two nucleophilic centers, the boron atom and the a-carbon atoms of dicyano vinyl. The fluoried ion will preferentially attack the boron atom and then attack the a-carbon atoms of dicyano vinyl, while the cyanide ions attack them in the opposite order. Consequently, the absorption and emission change in differenct trends when fluoride and cyanide are present and this compound could potentially behave as a bifunction sensor to distinguish fluoride and cyanide ions.3. In addition, we used the binaphthyl istead of biphenyl, to study the effect of the conjugated main chain’s structure. The study found that in the dimethyl amino and dimesitylboryl substituted o,o’-binaphthyl structure, there has a strong π-π interaction between the one mesityl and the naphthyl, which significantly limits the freedom rotation of the mesityl. So, the spiro chirality of the trivalent boron center can be achieved by the chiral induction. Which might provide an effective way to obtain the stable chirality of boron center. |
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