2,2′-Disilylazobenzenes featuring double intramolecular nitrogen?silicon coordination: a photoisomerizable fluorophore?

Dalton Transactions Pub Date: 2015-07-24 DOI: 10.1039/C5DT02038G

Abstract

(E)-4,4′-Dimethyl-2,2′-disilylazobenzenes were synthesized. Double intramolecular N?Si coordination in the bis(fluorodimethylsilyl) and bis(trifluorosilyl) derivatives was confirmed using X-ray crystallographic analysis and 29Si NMR spectroscopy. In the absorption bands, due to the π,π* transitions, introduction of silyl groups was found to cause a bathochromic shift. In contrast to most azobenzenes, which do not fluoresce at all, the (E)-2,2′-bis(trifluorosilyl)azobenzene derivative with the N?Si coordination fluoresced a yellow-green colour at room temperature. Methyl and trifluorosilyl groups lowered the n and π* orbitals, as revealed by DFT calculations. As a result, the lowest singlet excitation energy state is found to be the allowed π,π* transition, different from the forbidden n,π* transition in general azobenzenes, as revealed by TD-DFT calculations. The allowed transition character of the lowest singlet excited state and moderately rigid conformation of the azo moiety, provided by the double N?Si coordination, account for the fluorescence emission. Nevertheless, the N?Si coordination is weak enough to be cleaved upon photoexcitation, and thus the (E)-2,2′-disilylazobenzenes undergo photoisomerization to the (Z)-isomers. Both the photoisomerization and fluorescence emission properties of the azobenzene moiety have been achieved for the first time. After photoisomerization of the (E)-2,2′-disilylazobenzenes to the corresponding (Z)-isomer, they do not fluoresce. This change in the fluorescence intensity upon photoisomerization is useful for the regulation of fluorescence properties. Therefore, this compound can be recognized as a unique photoisomerizable fluorophore to regulate the fluorescence intensity using a single light source.

Graphical abstract: 2,2′-Disilylazobenzenes featuring double intramolecular nitrogen?silicon coordination: a photoisomerizable fluorophore
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