Effects Of Ligand Substitution On The Optical And Electrochemical Properties Of (Pyridinedipyrrolide)Zirconium Photosensitizers

A series of seven bis(pyridinedipyrrolide)zirconium complexes, Zr((PDPR2)-P-R1)(2), where [(PDPR2)-P-R1](2-) is the doubly deprotonated form of [2,6-bis(5-R-1-3-R-2-1H-pyrrol-2-yl)pyridine], were prepared and characterized in solution by NMR, UV/vis absorption, and emission spectroscopy and cyclic voltammetry. The molecular structures were determined by single-crystal X-ray crystallography. All complexes exhibit remarkably long emission lifetimes (tau = 190-576 mu s) with high quantum efficiencies (Phi(PL) = 0.10-0.38) upon excitation with visible light in a benzene solution. The substituents on the pyrrolide rings were shown to have significant effects on the photoluminescence and electrochemical properties of these compounds. The R-2 substituents (R-2 = H, Me, Ph, or C6F5) show only limited effects on the absorption and emission profiles of the complexes but allow systematic tuning of the ground- and excited-state redox potentials over a range of almost 600 mV. The R-1 substituents (R-1 = H, Me, Ph, or 2,4,6-Me3Ph) influence both the optical and electrochemical properties through electronic effects. Additionally, the R-1 substituents have profound consequences for the structural flexibility and overall stability of the compounds. Distortions of the Zr(PDP)(2) core from idealized D-2d symmetry in the solid state can be traced to the steric profiles of the R-1 substituents and correlate with the observed Stokes shifts for each compound. The complex with the smallest ligand system, Zr((PDPH)-P-H)(2), coordinates two additional solvent molecules in a tetrahydrofuran (THF) solution, which allowed the isolation of photoluminescent, eight-coordinate Zr((PDPH)-P-H)(2) (THF)(2). The photoredox catalytic dehalogenation of aryl iodides and aryl chlorides using the most reducing derivative, Zr((PDPMe)-P-Me)(2), highlights the potential of Zr(PDP)(2) photosensitizers to promote challenging reductive transformations under mild conditions upon excitation with green light.