Structure−Optical Property Relationships in Organometallic Sydnones

As part of an effort to develop a spectroscopic structure−property relationship in platinum acetylide oligomers,
we have prepared a series of mesoionic bidentate Pt(PBu3)2L2 compounds containing sydnone groups. The
ligand is the series o-Syd-(C6H4−C⋮C)n-H, where n = 1−3, designated as Syd-PEn-H. The terminal oligomer
unit consists of a sydnone group ortho to the acetylene carbon. We synthesized the platinum complex (Syd-PEn-Pt), the unmodified ligands (PEn-H), and the unmodified platinum complexes (PEn-Pt). The compounds
were characterized by various methods, including X-ray diffraction, 13C NMR, ground-state absorption,
fluorescence, phosphorescence, and laser flash photolysis. From solving the structure of Syd-PE1-Pt, we find
the angle between the sydnone group and the phenyl group is 45°. By comparison of the 13C NMR spectra
of the sydnone-containing ligands, the sydnone complexes with the corresponding unmodified ligands and
complexes not containing the sydnone group, the sydnone group is shown to polarize the nearest acetylenes
and have a charge-transfer interaction with the platinum center. Ground-state absorption spectra of the
complexes in various solvents give evidence that the Syd-PE1-Pt complex has an excited state less polar than
the ground state, while the PE1-Pt complex has an excited state more polar than the ground state. In all the
higher complexes the excited state is more polar than the ground state. The phosphorescence spectrum of the
Syd-PE1-Pt complex has an intense vibronic progression distinctly different from the PE1-Pt complex. The
sydnone effect is small in Syd-PE2-Pt and negligible in Syd-PE3-Pt. From absorption and emission spectra,
we measured the singlet-state energy ES, the triplet-state energy ET, and the singlet−triplet splitting ΔEST. By
comparison with energies obtained from the unmodified complexes, attachment of the sydnone lowers ES by
∼0.1 eV and raises ET by ∼0.1 eV. As a result, the sydnone group lowers ΔEST by ∼0.2 eV. The trends
suggest one of the triplet-state singly occupied molecular orbitals (SOMOs) is localized on the sydnone group,
while the other SOMO resides on the rest of the ligand.