pH-Modulated photoinduced electron transfer in a {[ruthenium-adamantyl] · [β-cyclodextrin-methylviologen]} inclusion complex

A novel photoactive complex, [Ru-LH]2+, comprising a ruthenium bisbipyridyl centre coordinated to 2-(4-adamantylphenyl)imidazo[4,5-f] [1,10]phenanthroline (LH) was prepared from the parent complex, [Ru(bpy)2(CAIP)]2+, where CAIP is 2-(4-carboxyphenyl)imidazo[4,5-f] [1,10]phenanthroline. The complex exhibits an intense luminescence and both its absorbance and emission properties are pH dependent. This pH dependence is associated with two ionisable sites at the imidazole on the heteroligand, for which pKas of 1.66 and 8.5 were determined from UV–Vis spectroscopy. The photophysics and resonance Raman spectroscopy of the complex as a function of pH are presented. Whereas the energy of the metal to ligand charge transfer transition is strongly influenced by pH, the pKa∗ suggests the lowest energy emitting state remains on the bipyridyl moiety regardless of pH. The self-assembly of the ruthenium complex with both native β-cyclodextrin and β-cyclodextrin covalently linked to a viologen acceptor, mono-6-(1-alkyl-4,4′-bipyridino)-β-cyclodextrin, CD-MV2+, was studied using 1H NMR spectroscopy. Assembly is driven by a hydrophobic host–guest interaction between the adamantyl pendant and the cyclodextrin. The association constant for self-assembly with β-cyclodextrin was determined to be 2.9 (±0.5) × 104 M−1, independent of the pH of the media. However, association with CD-MV2+ was found to be pH dependent, with Kassoc in neutral media determined to be 8.8 (±0.2) × 103 M−1 compared with 2.24 (±0.2) × 104 M−1 at pH 10. The photophysics of the assembly indicated that photoinduced electron transfer (PET) between the ruthenium centre and the pendant methyl viologen terminus is strongly and reversibly modulated by pH. Between pH 4 and 7, photoinduced electron transfer is, at best, very inefficient. On the other hand, at pHs exceeding 8.5, corresponding to complete deprotonation of the benzimidazole, an efficient electron transfer occurs and the resulting MV+ radical was identified in both transient absorbance studies and steady state photolysis. The origins of this pH-modulated electron transfer process are discussed.

Graphical abstractA novel photoactive supramolecular assembly is described wherein a ruthenium bisbipyridyl centre with an adamantyl pendant is included in the β-cyclodextrin functionalised with a viologen acceptor. Photo induced electron transfer from the ruthenium to viologen is demonstrated to occur. The rate of this process is by modulated by protonation/deprotonation of the benzimidazole bridge.Figure optionsDownload full-size imageDownload as PowerPoint slide