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

Trimethylsilyl-protected dialkynes incorporating azobenzene linker groups, MeSiC≡CRC≡CSiMe (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(EtP)PtCl] and the deprotected dialkynes in a 2:1 ratio in PrNH/CHCl gives the platinum(II) diynes trans-[Ph(EtP)PtC≡CRC≡CPt(PEt)Ph], while the dehydrohalogenation polycondensation reaction between trans-[(BuP)PtCl] and the dialkynes in a 1:1 molar ratio under similar reaction conditions affords the platinum(II) polyynes, [-Pt(PBu)-C≡CRC≡C-]. 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.