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, 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.