Lowest Electronic Excited States of Platinum(II) Diimine Complexes

Abstract

Absorption and emission spectra of Pt(diimine)L2 complexes (diimine = 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (dmbpy); L = pyrazolate (pz-), 3,5-dimethylpyrazolate (dmpz-), or 3,4,5-trimethylpyrazolate (tmpz-)) have been measured. Solvent-sensitive absorption bands (370-440 nm) are attributed to spin-allowed metal-to-ligand charge-transfer (1MLCT) transitions. As solids and in 77 K glassy solution, Pt(bpy)(pz)2 and Pt(dmbpy)(pz)2 exhibit highly structured emission systems (lambda max approximately 494 nm) similar to those of the diprotonated forms of these complexes. The highly structured bands (spacings 1000-1400 cm-1) indicate that the transition originates in a diimine-centered 3(pi-->pi*) (3LL) excited state. The intense solid-state and 77 K glassy solution emissions from 3MLCT[d(Pt)-->pi*(bpy)] excited states of complexes with dmpz- and tmpz- ligands occur at longer wavelengths (lambda max = 500-610 nm), with much broader vibronic structure. These findings are consistent with increasing electron donation of the pyrazolate ligands, leading to a distinct crossover from a lowest 3LL to a 3MLCT excited state.

A Theoretical Investigation of Substituent Effects on the Absorption and Emission Properties of a Series of Terpyridylplatinum(II) Acetylide Complexes

“…This result is in accordance with the electronic-structure calculation results since introduction of an electron-donating group leads to the LUMO being lifted and the energy gap (ΔE HOMO-LUMO ) being increased, which causes the absorption to be blue-shifted. This is consistent with the literature, [24] where the blue shift arising from methyl substitution of the bipyridyl ligand is attributable to destabilization of the π* level by electron-donating methyl substituents. The introduction of an electron-withdrawing group (CN) makes the absorption red-shift (from 452 nm to 487 nm for the first absorption).…”

Structures, Redox and Spectroscopic Properties of PdII and PtII Complexes Containing an Azo Functionality

“…Such luminescence, which usually occurs from metal-to-ligand charge transfer (MLCT) or intraligand πǞπ* excited states, has instigated several structure-spectroscopic correlation studies to clarify their behavior. [7,8] In yet another field of research, Pt II complexes have found use as catalysts and catalyst precursors for C-H bond activation. [9][10][11] Indeed, one of the first studies of C-H bond activation by transition metal complexes was reported for complexes of Pt II and Pd II with azobenzene.…”

Structural, Photophysical, and Electrophosphorescent Properties of Platinum(II) Complexes Supported by Tetradentate N2O2 Chelates

“…[11,20] Finally, it is noteworthy that the above assignment is in essence equivalent to the S(p)/Pt(d) 3 p*(diimine) (i.e. 'charge-transfer-to-diimine') assignment designated by Eisenberg for the lowest energy absorption band in a related family of Pt II diimine derivatives containing aromatic dithiolate ligands.…”