Conjugated Triarylboryl Donor-Acceptor Systems Supported by 2,2′-Bipyridine: Metal Chelation Impact on Intraligand Charger Transfer Emission, Electron Accepting Ability, and "Turn-on" Fluoride Sensing

To investigate the impact of metal-chelation on intraligand charge transfer emission involving a triarylboron group, three 2,2'-bipyridine derivative molecules, 5,5'-bis(BMeS2)-2,2'-bipy (B2bpy), 5-(BMeS2)-5'-(NPh2)-2,2'-bipy (BNbpy), and 5,5'-bis(NPh2)-2,2'-bipy (N2bpy) have been synthesized, which can be described as donor-only, donor-acceptor, and acceptor-only systems. Each of these molecules displays distinctive electrochemical and photophysical properties with BNbpy and N2bpy being bright emitters and B2bpy being a strong electron acceptor. In addition, BNbpy displays a "turn-on" fluorescent response while B2bpy has a "turn-off" response upon binding with fluoride ions. These molecules can readily chelate to a PtPh2 Or a PtCl2 group, producing square planar complexes Pt(B2bpy)Ph-2 (Pt-1), Pt(B2bpy)Cl-2 (Pt-1a), Pt(BNbpy)Ph-2 (Pt-2), Pt(BNbPY)Cl-2 (Pt-2a), and Pt(N2bpy)Ph-2 (Pt-3) that have significantly altered electrochemical and photophysical properties from those of the free ligands. Metal chelation has been found to greatly enhance the electron accepting ability of the three ligands, especially B2bpy and BNbpy. The Ph and Cl auxiliary ligands have also been found to have a significant impact on the electrochemical and photophysical properties of the complexes. B2bpy complexes Pt-1 and Pt-1a are not luminescent at ambient temperature while BNbpy complexes Pt-2 and Pt-2a display room temperature phosphorescence in solution under air that has a similar "turn-on" response toward fluoride ions as the free BNbpy does but with a much more dramatic color switch (orange or red to blue-green). The persistent intraligand N -> B charge transfer transition in the BNbpy complexes is believed to play a key role in their unique phosphorescent response toward fluorides. The complex Pt-3 displays a bright blue-green phosphorescence in solution at ambient temperature. Density functional theory computations established that the lowest electronic transition in the Pt(II) complex is from the Pt(II) d orbital and the auxiliary ligand to the pi* orbital of the 2,2'-bipy derivative ligand.