Probing the Electronic Structure of Platinum(II) Chromophores: Crystal Structures, NMR Structures, and Photophysical Properties of Six New Bis- and Di- Phenolate/Thiolate Pt(II)Diimine Chromophores

A general route for synthesis of six structurally similar Pt(II) diimine thiolate/phenolates chromophores possessing
bulky phenolate or thiolate ligands is reported. The Pt chromophores were characterized using an array of techniques
including 1H, 13C, and 195Pt NMR, absorption, emission, (spectro)electrochemistry, and EPR spectroscopy. Systematic
variation of the electronic structure of the Pt(II) chromophores studied was achieved by (i) changing solvent polarity;
(ii) substituting oxygen for sulfur in the donor ligand; (iii) alternating donor ligands from bis- to di-coordination; and
(iv) changing the electron donating/withdrawing properties of the ligand(s). The lowest excited state in these new
chromophores was assigned to a [charge-transfer-to-diimine] transition from the HOMO of mixed Pt/S (or Pt/O)
character on the basis of absorption and emission spectroscopy, UV/vis (spectro)electrochemistry, and EPR
spectroscopy. One of the chromophores, Pt(dpphen)(3,5-di-tert-butyl-catecholate) represents an example of a Pt(II) diimine phenolate chromophore that possesses a reversible oxidation centered predominantly on the donor
ligand. Results from EPR spectroscopy indicate participation of the Pt(II) orbitals in the HOMO. There is a dramatic
difference in the photophysical properties of carborane complexes compared to other mixed-ligand Pt(II) compounds,
which includes room-temperature emission and photostability. The charge-transfer character of the lowest excited
state in this series of chromophores is maintained throughout. Moreover, the absorption and emission energies
and the redox properties of the excited state can be significantly tuned.