Solvent Switching between Charge Transfer and Intraligand Excited States in a Multichromophoric Platinum(II) Complex

Abstract

The photophysical properties of a new multichromophoric platinum(II) complex, Pt(dbbpy)(C≡C−nap)2 (1) (dbbpy = 4,4‘-(di-tert-butyl)-2,2‘-bipyridine; C≡C−nap = 1-ethynylnaphthalene), is reported in four different solvent systems of varying polarity: toluene, 2-methyltetrahydrofuran, dichloromethane, and acetonitrile. Since the low-energy dπ Pt → π* dbbpy metal-to-ligand charge transfer (MLCT) transitions in this chromophore are negative solvatochromic in nature, increasing solvent polarity raises the energy of the MLCT state. At the same time, the low-lying, metal-perturbed π,π* triplet intraligand (3IL) state present within the molecule on the −C≡C−nap fragments are largely invariant to solvent polarity as evidenced by the absorption and emission properties of the model chromophore lacking the low-energy charge transfer state, Pt(dppe)(C≡C−nap)2 (2) (dppe = 1,2-bis(diphenylphosphino)ethane). In solvents of low polarity, the static and dynamic absorption and luminescence spectroscopy of 1 are largely consistent with a dπ Pt → π* dbbpy MLCT assignment. As the solvent polarity is increased, the excited-state absorption and emission properties of 1 are significantly altered, eventually resembling the spectral features seen in 2. However, the excited-state decay kinetics for the structured emission measured in 2 is well beyond that observed for 1. The discrepancy in lifetime indicates that the excited state of 1 in high polarity solvents is not composed of pure 3IL character. In fact it is very likely that a significant electronic interaction exists between the 3MLCT and 3IL states in this molecule at ambient temperature. To further probe this possibility, static and dynamic luminescence experiments were performed on 1 and Pt(dbbpy)(C≡C−Ph)2 (3), C≡C−Ph is phenylacetylene, at 77 K as a function of matrix polarity. In all glass matrixes at low temperature, the lowest-lying state in 1 is 3IL, whose energy is nearly independent of the matrix. The reference molecule 3 possesses a low-lying 3MLCT state, and we believe it reasonably models the 3MLCT energy in 1 at low temperature. In essence, variation in the solvent glass changes the 3MLCT−3IL energy gap which modifies the observed excited-state lifetimes at 77 K, decreasing as the apparent energy gap decreases. The experimental results suggest stronger mixing of the two triplet states at room temperature relative to 77 K, but we cannot definitively rule out contributions from thermal equilibrium. Unfortunately, the relative energy separation of the two states in 1 does not permit a quantitative evaluation of potential configuration mixing of the two states. However, the close energetic proximity of the two triplets along with the solvent-dependent nature of the charge transfer state permits a wide variety of photophysical properties to be displayed by a single molecular system.