Synthesis, crystal structure and photoluminescence of [Rh(III)(phpy)₂(dithiooxamide)]Cl (phpy = 2-(2-pyridyl)phenyl)

3.1.1 Absorption

The absorption spectrum of [Rh(phpy)₂(dtoxa)]Cl (Fig. 2) is solvent independent and shows its longest-wavelength band at λ_max=380 nm with ε=4400 mol⁻¹cm⁻¹. In analogy to other Rh(III)(phpy) complexes [13] it is assigned to a π-π* IL (intraligand) transition of the phpy ligand.

Fig. 2:

Absorption (abs) and excitation (exc) spectrum of a 0.5×10⁻³m solution of [Rh(phpy)₂(dtoxa)]Cl in CH₂Cl₂ in a 1-cm cell. vertical axis: absorption and excitation (intensity in arbitrary units).

The longest-wavelength band of the free dtoxa molecule appears near 500 nm, but its intensity is very low (ε<20). It is of the nπ* type. In the case of [Rh(phpy)₂(dtoxa)]Cl, the dtoxa IL triplet absorption may be obscured by the long-wavelength tail of the intense phpy IL absorption. However, the excitation band (Fig. 2) may reveal the position of the dtoxa IL triplet absorption.

3.1.2 Emission

At r.t. the free dtoxa molecule is not luminescent, neither in solution nor in the solid state. However, in an ethanol glass at T=77 K (Fig. 3) it exhibits an emission at λ_max=580 nm, and two further features at approximately 615 and 655 nm.

Fig. 3:

Emission spectrum of dtoxa in EtOH at T=77 K, in a quartz tube (3 mm inner diameter) placed in a dewar with a quartz extension at the bottom.

This emission is suggested to originate from the lowest-energy n-π* triplet of dtoxa [14]. This emission resembles that of dmdtoxa (N,N′-dimethyldithioxamide), which undergoes a blue shift, owing to the alkylation of the amide groups. In both cases, the vibrational progression amounts to approximately 1000 cm⁻¹, which corresponds to the C=S stretching mode [14]. The salt [Rh(phpy)₂(dtoxa)]Cl is luminescent in the solid state as well as in solution at r. t. In CH₂Cl₂ it displays a relatively intense orange luminescence (Fig. 4) with λ_max=607 nm.

Fig. 4:

Emission spectrum of [Rh(phpy)₂(dtoxa)]Cl in CH₂Cl₂ in a 1-cm cell, excitation at 420 nm, intensity (Int) in arbitrary units.

In addition, a pronounced shoulder at 580 nm and a weak inflection at ≈655 nm appear. Again, these features are separated by approximately 1000 cm⁻¹ (see Fig. 3). This emission is thus suggested to originate from the lowest-energy IL (dtoxa) triplet in agreement with a conclusion drawn for the emission of Rh(phpy)₂(quinoline-8-thiolate) [13). The integrated emission intensity of [Rh(phpy)₂(dtoxa)]⁺ was compared with that of [Ru(2,2′-bipy)₃]²⁺. The quantum yield of the rhodium complex amounts to ca. 10⁻³ to be compared with that of [Ru(2,2′-bipy)₃]²⁺ (Φ=0.04) [1]. The lifetime of the dtoxa IL triplet of [Rh(phpy)₂(dtoxa)]⁺ is very short (τ=3.74 ns). However, it has to be taken into account that the emission of dmdtoxa at T=77 K is already very short-lived (τ=3×10⁻⁵ s) [14]. In the case of the rhodium complex an increased spin-orbit coupling (heavy-atom effect) should be additionally in operation yielding this exceptionally short triplet lifetime. This also explains the observation that the emission is not quenched by oxygen because a diffusion-controlled process would be too slow for an efficient luminescence quenching. Moreover, in view of the absence of a fluorescence from the free dtoxa, it is even less probable that it occurs in the case of the rhodium complex. In addition, the emission of [Rh(phpy)₂(dtox)]⁺ (Fig. 4) does not overlap with the absorption or excitation band (Figs. 2 and 4), as it should be observed for a fluorescence. In the solid state, the rhodium complex displays its emission at λ_max=638 nm and a concomitant excitation with λ_max=500 nm. The conclusion, that the emission of [Rh(phpy)₂(dtoxa)]Cl is a dtoxa IL phosphorescence agrees with the previous interpretation, that complexes of the type [Rh(III)(phpy)₂(L-S)] are luminescent in solution at r. t. only, if the emissive excited state is an IL triplet and rather low in energy [13].