Electronic Spectroscopy of Chloro(terpyridine)platinum(II)

Abstract

The electronic spectrum of [Pt(tpy)Cl]^+ (tpy = 2,2':6',2"-terpyridine) is influenced dramatically by intermolecular stacking interactions in solution and in the solid state. The crystal structure of [Pt(tpy)Cl]ClO_4 (monoclinic, P2_1/c (No. 14); a = 7.085(2), b = 17.064(5), c = 26.905(8) Å; β = 90.0(1) °; Z = 8) consists of discrete Pt_2 units (Pt-Pt = 3.269(1) Å) arranged along an infinite tpy-π stack (spacing ~ 3.35 Å). Variable-temperature and concentration studies of the absorption and emission spectra of [Pt(tpy)Cl]^+ suggest that similar metal-metal and ligand-ligand interactions persist in the solution phase. The high concentration, low-temperature emission spectrum (5:5:1 ethanol:methanol:DMF) reveals a 740-nm band indicative of M-M oligomerization, a 650-nm band attributable to tpy π-π interactions, and a 470-nm band characteristic of mononuclear [Pt(tpy)Cl]^+ π-π* emission. Concentration-dependent absorption spectra were fit to a "two-dimer" model, yielding equilibrium constants for the formation of Pt-Pt-, and tpy-tpy-bound dimers of 1.3(1) x 10^3 and 1.0(1) x 10^3 M^(-1), respectively, in 0.1 M aqueous NaCl. The low temperature solid-state luminescence of [Pt(tpy)Cl]^+ is assigned to a ^3(MMLCT) (MMLCT = metal-metal-to-ligand charge transfer) transition. The energy of this band is highly dependent on the counterion (PF_6^-, ClO_4^- , C1^-, CF_3SO_3^-), in line with the different colors of these various salts. In contrast, the room-temperature solid-state emission spectra are more difficult to interpret. While the red perchlorate salt exhibits a relatively narrow emission band at 725 nm (red-shifted from the 77-K maximum at 695 nm), consistent with a 3(MMLCT) transition, the orange (Cl^-, ClO_4^-, CF_3SO_3^-) and yellow (PF6^-) salts have extremely broad room-temperature emission bands that all appear at nearly the same energy (λ_(max) ~ 640 nm). We assign this luminescence to an eximeric intraligand transition resulting from π- π interactions and propose that the temperature dependent emissions from the orange and yellow solid materials originate from multiple electronic states.