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 (tm pz(-))) have been measured. Solvent-sensitive absorption bands (370-440 nm) are attributed to spin-allowed metal-to-ligand charge-transfer ((MLCT)-M-1 ) transitions. As solids and in 77 K glassy solution, Pt(bpy)(pz)2 and Pt(d mbpy)(pz)(2) exhibit highly structured emission systems (lambda(max) approx imate to 494 nm) similar to those of the diprotonated forms of these comple xes. The highly structured bands (spacings 1000-1400 cm(-1)) indicate that the transition originates in a diimine-centered (3)(pi-->pi*) ((LL)-L-3) ex cited state. The intense solid-state and 77 K glassy solution emissions fro m (MLCT)-M-3[d(Pt)-->pi*(bpy)] excited states of complexes with dmpz(-) and tmpz(-) ligands occur at longer wavelengths (lambda(max) = 500-610 nm), wi th much broader vibronic structure. These findings are consistent with incr easing electron donation of the pyrazolate ligands, leading to a distinct c rossover from a lowest (LL)-L-3 to a (MLCT)-M-3 excited state.