Photophysics of the platinum(II) terpyridyl terpyridylacetylide platform and the influence of Fe(II) and Zn(II) coordination

The synthesis, structural characterization, and photoluminescence (PL) properties of the square-planar terpyridylplatinum(II) complex [ ( t )Bu 3tpyPtCCtpy] (+) ( 1) and the octahedral trinuclear Fe (II) and Zn (II) analogues [Fe( ( t )Bu 3tpyPtCCtpy) 2] (4+) ( 2) and [Zn( ( t )Bu 3tpyPtCCtpy) 2] (4+) ( 3) are described. The photophysical properties of the mononuclear Pt (II) complex 1 are consistent with a charge-transfer excited-state parentage producing a large Stokes shift with a concomitant broad, structureless emission profile. The Fe-based ligand-field states in 2 provide an efficient nonradiative deactivation pathway for excited-state decay, resulting in a nonemissive compound at room temperature. Interestingly, upon chelation of 1 with Zn (II), a higher energy charge-transfer emission with a low-energy shoulder and a 215 ns excited-state lifetime is produced in 3. A spectroscopically identical species relative to 3 was produced in control experiments when 1 was reacted with excess protons (HClO 4) as ascertained by UV-vis and static PL spectra measured at room temperature and 77 K. Therefore, the chelation of Zn (II) to 1 is acid-base in nature, and its Lewis acidity renders the highest occupied molecular orbital level in 1 much less electron-rich, which induces a blue shift in both the absorption and emission spectra. At 77 K, complexes 1, 3, and protonated 1 display at least one prevalent vibronic component in the emission profile (1360 cm (-1)) resembling PL emanating from a ligand-localized excited-state, indicating that these emitting states are inverted relative to room temperature. These results are qualitatively confirmed by the application of time-dependent theory using only the 1360 cm (-1) mode to reproduce the low-temperature emission spectra.