HOMO–LUMO energy gap control in platinum(ii) biphenyl complexes containing 2,2′-bipyridine ligands

Abstract

A series of platinum(ii) biphenyl 2,2'-bipyridine complexes containing electron-donating and electron-withdrawing moieties on the 4 and 4' positions of the bipyridine ligand exhibit emission from excited states in the 600 nm region of the spectrum upon excitation in the metal-to-ligand charge transfer transition located near 450 nm. These complexes are distorted from planarity based on both single crystal structure determinations and density functional theory (DFT) calculations of isolated molecules in acetonitrile. The DFT also reveals the geometry of the lowest-lying triplet state (LLTS) of each complex that is important for emission behavior. The LLTS are assigned based on the electron spin density distributions and correlated with the singlet excited states to understand the mechanism of electronic excitation and relaxation. Time-dependent DFT calculations are performed to compute the singlet excited state energies of these complexes so as to help interpret their UV-Vis absorption spectra. Computational and experimental results, including absorption and emission energy maxima, electrochemical reduction potentials, LLTS, singlet excited states, and LUMO and HOMO energies, exhibit linear correlations with the Hammett constants for para-substituents σp. These correlations are employed to screen complexes that have not yet been synthesized. The correlation analysis indicates that the electronic structure and the HOMO-LUMO energy gap in Pt(ii) complexes can be effectively controlled using electron-donating and electron-withdrawing moieties covalently bonded to the ligands. The information presented in this paper provides a better understanding of the fundamental electronic and thermodynamic behavior of these complexes and could be used to design systems with specific applications.

Diethyl 2,2′-bipyridine-4,4′-dicarboxylate

“…The bond lengths and bond angles of the title free ligand are basically the same as those observed for the coordinated ligand in Pt(bph)(4,4 0 -diethoxycarbonyl-2,2 0 -bipyridine) (Rillema et al, 2015). Upon coordination to transition metals, the two pyridine rings have the pyridine N atoms cis to one another, resulting in -delocalization over the whole ligand.…”

“…In both molecules, the two pyridine rings are arranged such that the pyridine N atoms are trans to one another. Molecule A is more planar than molecule B, with the ethyl carboxylate group [C-C-O-C( O)] being inclined to the pyridine ring by 2.11 (15) in A, and 5.69 (15) in B.The bond lengths and bond angles of the title free ligand are basically the same as those observed for the coordinated ligand in Pt(bph)(4,4 0 -diethoxycarbonyl-2,2 0 -bipyridine) (Rillema et al, 2015). Upon coordination to transition metals, the two pyridine rings have the pyridine N atoms cis to one another, resulting in -delocalization over the whole ligand.…”

“…Ru 2+ , Pt 2+ , and Re 1+ , for use in solar energy conversion studies due to their excellent electronic properties (Cruz et al, 2010;Rillema et al, 2015;Villegas et al, 2005). Upon photoexcitation, electrons are channeled from the metal center to the diimmine ligand on its pathway to the ground state.…”

Luminescent metal complexes featuring photophysically innocent boron cluster ligands

“…This electrochemical behavior is consistent with other square planar Pt(II) species undergoing a reversible ligand-centered reduction and irreversible metal-centered oxidation. 2–4,12a,12c,15a Further, DFT calculations support these data ( vide infra ).…”

“…2,3 Furthermore, in prototypical mixed ligand complexes, (L)(L′)Pt(II), the HOMO is generally both metal- and ligand-based (Pt(II) and L) while the LUMO is located on the remaining ligand (L′). 4 This ultimately complicates the predictability of ligand effects on emission properties. A class of compounds that could kinetically stabilize the Pt(II) square planar framework without participating in electronic transitions would prove very useful for creating improved phosphorescent emitters for the next generation of OLED devices.…”