Synthesis, Crystal Structure, High-Resolution Optical Spectroscopy, and Extended Hückel Calculations on Cyclometalated [Re(CO)₄(ppy)] (ppy = 2-Phenylpyridine)

Abstract

The cyclometalated complex [Re(CO) 4 (ppy)], where ppy -) 2-phenylpyridine, was synthesized. Its crystal and molecular structure was determined by X-ray diffraction. The lowest energy electronic excitations were studied by high-resolution optical spectroscopy at cryogenic temperature. The first excited state in [Re(CO) 4 (ppy)] is nominally a triplet ligand-centered state, with 1.8% metal-to-ligand charge transfer character mixed in through spin-orbit coupling. This induces a shortened lifetime (89 µs at 10 K) and the occurrence of metal-ligand vibrations in both absorption and luminescence spectra. The transition moment of the first electronic excitation lies in the molecular plane with a tilt toward the pyridine part of the ppy -ligand. This is derived from the polarized absorption measurements and confirmed by an extended Hückel calculation. Splittings in the IR absorption bands are resulting from the presence of two crystallographically equivalent complexes in the unit cell which are tilted with respect to each other.

Cyclometalation Using d-Block Transition Metals: Fundamental Aspects and Recent Trends

“…213 The excited state properties were investigated by high-resolution optical spectroscopy in a variety of C,N-rhenacycles. 214,215 The first excited state has been identified as a ligand-centered 3 π-π* state with only little 1 MLCT character. A CH 3 group on the ortho position of the pyridine ring has only little impact on the excited state properties.…”

Cyclometallated Organoiridium Complexes as Emitters in Electrophosphorescent Devices

“…This work has established that luminescence from these complexes originates from a lowest excited state that is best described as a ligand centered triplet ( 3 LC) with singlet and triplet metal -to -ligand -charge -transfer ( 1,3 MLCT) character mixed in through the spin -orbit coupling [69 -71] . A schematic representation of the energy level mixing in 4d 6 and 5d 6 complexes is shown in Scheme 3.2 [65,69] .…”

“…where Ψ T 1 is the wavefunction of the lowest excited state and α is a coeffi cient that gives an estimate of the degree of singlet character mixed into the unperturbed triplet state ( 3 LC) [69] . The value of α can be approximated with the formula…”

“…where 〈 3 LC| H so | 1 MLCT 〉 is the spin -orbital coupling matrix element, characterizing the strength of spin -orbital coupling between 3 LC and 1 MLCT, and ∆ E is the energy difference between the 3 LC and 1 MLCT transitions [69] . Note that the value of α in Eq.…”

“…Equations (3.1) and (3.2) have been used to correlate α values with the luminescent properties of diimine and cyclometalated Rh(III) and Ir(III) complexes [64] . The mixing of 1 MLCT character into what is principally a 3 LC state has dramatic effects on the optical properties of those complexes [68,69] . The oscillator strength and the consequent radiative decay rate of the S 0 -T 1 transition in luminescent metal complexes are signifi cantly increased when a small amount of 1 MLCT character is mixed into the lowest excited state.…”