Photophysical Study on the Rigid Pt(II) Complex [Pt(naphen)(Cl)] (Hnaphen = Naphtho[1,2- b][1,10]Phenanthroline and Derivatives

Abstract

The electrochemistry and photophysics of the Pt(II) complexes [Pt(naphen)(X)] (Hnaphen = naphtho[1,2-b][1,10]phenanthroline, X = Cl or C≡CPh) containing the rigid tridentate C^N^N-coordinating pericyclic naphen ligand was studied alongside the complexes of the tetrahydro-derivative [Pt(thnaphen)(X)] (Hthnaphen = 5,6,8,9-tetrahydro-naphtho[1,2-b][1,10]phenanthroline) and the N^C^N-coordinated complex [Pt(bdq)(Cl)] (Hbdq = benzo[1,2-h:5,4-h']diquinoline. The cyclic voltammetry showed reversible reductions for the C^N^N complexes, with markedly fewer negative potentials (around -1.6 V vs. ferrocene) for the complexes containing the naphen ligand compared with the thnaphen derivatives (around -1.9 V). With irreversible oxidations at around +0.3 V for all of the complexes, the naphen made a difference in the electrochemical gap of about 0.3 eV (1.9 vs. 2.2 eV) compared with thnaphen. The bdq complex was completely different, with an irreversible reduction at around -2 V caused by the N^C^N coordination pattern, which lacked a good electron acceptor such as the phenanthroline unit in the C^N^N ligand naphen. Long-wavelength UV-Vis absorption bands were found around 520 to 530 nm for the C^N^N complexes with the C≡CPh coligand and were red-shifted when compared with the Cl derivatives. The N^C^N-coordinated bdq complex was markedly blue-shifted (493 nm). The steady-state photoluminescence spectra showed poorly structured emission bands peaking at around 630 nm for the two naphen complexes and 570 nm for the thnaphen derivatives. The bdq complex showed a pronounced vibrational structure and an emission maximum at 586 nm. Assuming mixed ³LC/³MLCT excited states, the vibronic progression for the N^C^N bdq complex indicated a higher LC character than assumed for the C^N^N-coordinated naphen and thnaphen complexes. The blue-shift was a result of the different N^C^N vs. C^N^N coordination. The photoluminescence lifetimes and quantum yields Φ_L massively increased from solutions at 298 K (0.06 to 0.24) to glassy frozen matrices at 77 K (0.80 to 0.95). The nanosecond time-resolved study on [Pt(naphen)(Cl)] showed a phosphorescence emission signal originating from the mixed ³LC/³MLCT with an emission lifetime of around 3 µs.

Keywords: cyclometalation; pericyclic ligands; photoluminescence; platinum; time-resolved.

Scheme 1

Schematic representation of the studied complexes and related work. A. [Pt(dba)(dmso)] [31]; B. [Pt(bdq)(Cl)] 3 [48]; C. [Pt(naphen)(X)] 1; D. [Pt(thnaphen)X] 2.

Scheme 2

Schematic representation of the protoligands (protonated ligand precursors).

Figure 1

Cyclic voltammograms of 1b (A), 2b (B), and 3 (C) in 0.1 M n-Bu₄NPF₆/THF.

Figure 2

UV-Vis absorption spectra (molar absorption coefficients as a function of wavelength) of 1a, 2a, and 3 in CH₂Cl₂. Validity range: c = 1 × 10⁻⁶ M to 1 × 10⁻⁴ M in CH₂Cl₂ at 298 K.

Figure 3

Left: Normalised photoluminescence spectra of [Pt(naphen)(Cl)] (1a) and [Pt(naphen)(C≡CPh)] (1b) in fluid Ar-purged CH₂Cl₂ solution at 298 K and in a frozen glassy matrix (CH₂Cl₂:MeOH 1:1) at 77 K. Right: Normalized photoluminescence spectra of 1a and 1b in fluid Ar-purged CH₂Cl₂ solution at 298 K and in a frozen glassy matrix (CH₂Cl₂:MeOH 1:1) at 77 K (λ_ex = 350 nm). In all cases, c = 1 × 10⁻⁵ M.

Figure 4

Nanosecond time-resolved emission spectra (A) and transient absorption spectra (B) of [Pt(naphen)(Cl)] (1a) at selected time points. The black lines represent the steady-state emission spectrum (A) and the inverted steady-state emission spectrum (B). In panel (B), the grey-shaded spectrum indicates the inverted ground-state spectrum of the complex.