Modulation of Intersystem Crossing Rate by Minor Ligand Modifications in Cyclometalated Platinum(II) Complexes

Photophysical properties
of four new platinum­(II) complexes comprising extended ppy (Hppy =
2-phenylpyridine) and thpy (Hthpy = 2-(2′-thienyl)­pyridine)
cyclometalated ligands and acetylacetonate (acac) are reported. Substitution
of the benzene ring of Pt–ppy complexes 1 and 2 with a more electron-rich thiophene of Pt–thpy complexes 3 and 4 leads to narrowing of the HOMO–LUMO
gap and thus to a red shift of the lowest energy absorption band and
phosphorescence band, as expected for low-energy excited states of
the intraligand/metal-to-ligand charge transfer character. However,
in addition to these conventional spectral shifts, another, at first
unexpected, substitution effect occurs. Pt–thpy complexes 3 and 4 are dual emissive showing fluorescence
about 6000 cm–1 (∼0.75 eV) higher in energy
relative to the phosphorescence band, while for Pt–ppy complexes 1 and 2 only phosphorescence is observed. For
dual-emissive complexes 3 and 4, ISC rates kISC are estimated to be in order of 109–1010 s–1, while kISC of Pt–ppy complexes 1 and 2 is much faster amounting to 1012 s–1 or more. The relative intensities of the fluorescence and phosphorescence
signals of Pt–thpy complexes 3 and 4 depend on the excitation wavelength, showing that hyper-intersystem
crossing (HISC) in these complexes is observably significant.