Lowest Electronic Excited States of Platinum(II) Diimine Complexes

Absorption and emission spectra of Pt(diimine)L_2 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 (λ_(max)≈ 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(π→π*) (^3LL) excited state. The intense solid-state and 77 K glassy solution emissions from ^3MLCT[d(Pt)→π*(bpy)] excited states of complexes with dmpz- and tmpz- ligands occur at longer wavelengths (λ_(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.