Synthesis and optical properties of chlorophyll dimers bridged by cis- and trans-forms of Pt(II) bis-acetylide complexes

Two isomeric zinc–chlorophyll (Zn–Chl) dimers, cis- and trans-forms 1 and 2 connected by a platinum(II) bis-acetylide complex were synthesized by copper(I)-catalyzed coupling of a monomeric Zn–Chl derivative possessing an ethynyl group at the C3 position 5 with the cis- and trans-isomers of Pt(II) bisphosphine complexes, respectively. The corresponding cis- and trans-forms of Zn–Chl monomers 3 and 4 bonding a phenyl group through Pt(II) bis-acetylide were also prepared. These synthetic complexes were fully characterized by 1H NMR, IR and MS spectroscopy. Visible absorption and circular dichroism spectra of 1–5 were measured in organic solvents, showing that the redmost (Qy) bands of 1 and 2 were shifted to a longer wavelength than those of 3 and 4, respectively. The red shifts are ascribable to the extension of π-conjugation in the chlorin rings through a Pt(II) connecter unit along a molecular y-axis. The Qy maxima of trans-dimer 2 and monomer 4 moved bathochromically, compared with the corresponding cis-isomers 1 and 3. The bathochromic shifts are due to a larger π-conjugation in trans Pt-complexes than in cis isomers.