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. 2023 Dec 4;62(48):19446-19456.
doi: 10.1021/acs.inorgchem.3c02352. Epub 2023 Nov 20.

Polycationic Ru(II) Luminophores: Syntheses, Photophysics, and Application in Electrostatically Driven Sensitization of Lanthanide Luminescence

Richard C Knighton  1 , Joseph M Beames  2 , Simon J A Pope  1
Affiliations

Polycationic Ru(II) Luminophores: Syntheses, Photophysics, and Application in Electrostatically Driven Sensitization of Lanthanide Luminescence

Richard C Knighton et al. Inorg Chem. .

Abstract

A series of photoluminescent Ru(II) polypyridine complexes have been synthesized in a manner that varies the extent of the cationic charge. Two ligand systems (L1 and L2), based upon 2,2'-bipyridine (bipy) mono- or difunctionalized at the 5- or 5,5'-positions using N-methylimidazolium groups, were utilized. The resulting Ru(II) species therefore carried +3, +4, +6, and +8 complex moieties based on a [Ru(bipy)3]2+ core. Tetra-cationic [Ru(bipy)2(L2)][PF6]4 was characterized using XRD, revealing H-bonding interactions between two of the counteranions and the cationic unit. The ground-state features of the complexes were found to closely resemble those of the parent unfunctionalized [Ru(bipy)3]2+ complex. In contrast, the excited state properties produce a variation in emission maxima, including a bathochromic 44 nm shift of the 3MLCT band for the tetra-cationic complex; interestingly, further increases in overall charge to +6 and +8 produced a hypsochromic shift in the 3MLCT band. Supporting DFT calculations suggest that the trend in emission behavior may, in part, be due to the precise nature of the LUMO and its localization. The utility of a photoactive polycationic Ru(II) complex was then demonstrated through the sensitization of a polyanionic Yb(III) complex in free solution. The study shows that electrostatically driven ion pairing is sufficient to facilitate energy transfer between the 3MLCT donor state of the Ru(II) complex and the accepting 2F5/2 excited state of Yb(III).

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Synthetic Routes to the Family of Homologous Polycationic ruthenium(II) Bipyridine Complexes
Scheme 2
Scheme 2. Attempted Synthesis of the Intermediate Species [Ru(L2)2Cl2]Cl4
Figure 1
Figure 1
Assignment of 1H NMR spectra (500 MHz, 298 K, CD3CN) for the family of polycationic Ru(II) complexes.
Figure 2
Figure 2
X-ray crystal structure of the [Ru(bipy)2(L2)][PF6]4 complex (ellipsoids are plotted at the 50% probability level; non-H-bonding hydrogen atoms and counterions omitted for clarity; CCDC no. 2250505).
Figure 3
Figure 3
UV–vis absorption spectra for the family of polycationic ruthenium complexes (MeCN).
Figure 4
Figure 4
Normalized emission spectra for the family of polycationic ruthenium complexes (293 K, aerated MeCN, 10–5 M).
Figure 5
Figure 5
Calculated Kohn–Sham LUMOs for (left to right) [Ru(bipy)2(L1)]3+, [Ru(bipy)2(L2)]4+, [Ru(bipy)(L2)2]6+, and [Ru(L2)3]6+.
Figure 6
Figure 6
Comparison of the NIR emission spectra of [Ru(bipy)3]2+ (orange) and [Ru(bipy)2(L2)]4+ (blue) solutions in the presence of 2 eq [Yb(dpa)3]3– (293 K, aerated D2O, 10–5 M Ru(II) complex, λex = 450 nm). Fitted decay (residual errors shown) for [Ru(bipy)2(L2)]4+ in the presence of 2 eq [Yb(dpa)3]3– (293 K, aerated D2O, 10–5 M, λex = 355 nm, λem = 980 nm). The obtained lifetimes were 0.3 μs (31% relative weighting) and 21.9 μs (69%). The proposed photophysical pathways are shown inset.
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