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Impact of the Alkyne Substitution Pattern and Metalation on the Photoisomerization of Azobenzene-Based Platinum(II) Diynes and Polyynes

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posted on 18.10.2016, 16:25 by Rayya A. Al-Balushi, Ashanul Haque, Maharaja Jayapal, Mohammed K. Al-Suti, John Husband, Muhammad S. Khan, Jonathan M. Skelton, Kieran C. Molloy, Paul R. Raithby
Trimethylsilyl-protected dialkynes incorporating azobenzene linker groups, Me3SiCCRCCSiMe3 (R = azobenzene-3,3′-diyl, azobenzene-4,4′-diyl, 2,5-dioctylazobenzene-4,4′-diyl), and the corresponding terminal dialkynes, HCCRCCH, have been synthesized and characterized. The CuI-catalyzed dehydrohalogenation reaction between trans-[Ph­(Et3P)2PtCl] and the deprotected dialkynes in a 2:1 ratio in iPr2NH/CH2Cl2 gives the platinum­(II) diynes trans-[Ph­(Et3P)2PtCCRCCPt­(PEt3)2Ph], while the dehydrohalogenation polycondensation reaction between trans-[(nBu3P)2PtCl2] and the dialkynes in a 1:1 molar ratio under similar reaction conditions affords the platinum­(II) polyynes, [−Pt­(PnBu3)2–CCRCC−]n. The materials have been characterized spectroscopically, with the diynes also studied using single-crystal X-ray diffraction. The platinum­(II) diynes and polyynes are all soluble in common organic solvents. Optical-absorption measurements show that the compounds incorporating the para-alkynylazobenzene spacers have a higher degree of electronic delocalisation than their meta-alkynylazobenzene counterparts. Reversible photoisomerization in solution was observed spectroscopically for the alkynyl-functionalized azobenzene ligands and, to a lesser extent, for the platinum­(II) complexes. Complementary quantum-chemical modeling was also used to analyze the optical properties and isomerization energetics.