Luminescent Charge-Transfer Platinum(II) Metallacycle

The photophysical and electrochemical properties of a platinum(II) diimine complex bearing the bidentate diacetylide
ligand tolan-2,2‘-diacetylide (tda), Pt(dbbpy)(tda) [dbbpy = 4,4‘-di-tert-butyl-2,2‘-bipyridine] (1), are compared with
two reference compounds, Pt(dbbpy)(C⋮CPh)2 (2) and Pt(dppp)tda [dppp = 1,3-bis(diphenylphosphino)propane]
(3), respectively. The X-ray crystal structure of 1 is reported, which illustrates the nearly perfect square planarity
exhibited by this metallacycle. Chromophore 2 possesses low-lying charge-transfer excited states analogous to 1,
whereas structure 3 lacks such excited states but features a low-lying platinum-perturbed tda intraligand triplet
manifold. In CH2Cl2, 1 exhibits a broad emission centered at 562 nm at ambient temperature, similar to 2, but with
a higher photoluminescence quantum yield and longer excited-state lifetime. In both instances, the photoluminescence
is consistent with triplet-charge-transfer excited-state parentage. The rigidity imposed by the cyclic diacetylide ligand
in 1 leads to a reduction in nonradiative decay, which enhances its room-temperature photophysical properties. By
comparison, 3 radiates highly structured tda-localized triplet-state phosphorescence at room temperature. The 77
K emission spectrum of 1 in 4:1 EtOH/MeOH becomes structured and is quantitatively similar to that measured for
3 under the same conditions. Because the 77 K spectra are nearly identical, the emissions are assigned as 3tda
in nature, implying that the charge-transfer states are raised in energy, relative to the 3tda levels in 1 in the
low-temperature glass. Nanosecond transient absorption spectrometry and ultrafast difference spectra were determined
for 1−3 in CH2Cl2 and DMF at ambient temperature. In 1 and 2, the major absorption transients are consistent with
the one-electron reduced complexes, corroborated by reductive spectroelectrochemical measurements performed
at room temperature. As 3 does not possess any charge-transfer character, excitation into the ππ* transitions of
the tda ligand generated transient absorptions in the relaxed excited state assigned to the ligand-localized triplet
state. In all three cases, the excited-state lifetimes measured by transient absorption are similar to those measured
by time-resolved photoluminescence, suggesting that the same excited states giving rise to the photoluminescence
are responsible for the absorption transients. ESR spectroscopy of the anions 1- and 2- and reductive
spectroelectrochemistry of 1 and 2 revealed a LUMO based largely on the π* orbital of the dbbpy ligand. Time-dependent density functional theory calculations performed on 1−3 both in vacuum and in a CH2Cl2 continuum
revealed the molecular orbitals, energies, dipole moments, and oscillator strengths for the various electronic transitions
in these molecules. A ΔSCF-method-derived shift applied to the calculated transition energies in the solvent continuum
yielded good agreement between theory and experiment for each molecule in this study.