Photoinduced energy transfer across non-covalent bonds in the nanoscale: cyclodextrin hosts with enhanced luminescent properties for guest communication

Abstract

Two photoactive cyclodextrin hosts have been prepared and studied in participation on photophysical processes, one with a ruthenium tris(bipyridyl) core which forms a trimeric host and the other with an appended anthracene unit. An acetylide functionalized bipyridyl cyclodextrin, 1, was prepared by palladium coupling of mono-6-propargyl permethylated β-CD and 4-bromo-2,2′-bipyridine. The ruthenium complex, [Ru1₃]Cl₂, shows luminescence from the ³MLCT centered at 650 nm with a quantum yield of 0.044 and a lifetime at room temperature of 1.1 μs in aerated aqueous solution. The enhanced luminescence properties have been attributed to extra delocalisation afforded by the three propargyl units appended to the complex and the stabilization of the MLCT state. An anthracene functionalised cyclodextrin 2 was prepared from mono-6-O-p-toluenesulfonyl permethylated β-cyclodextrin and 9-anthracene methanol. It shows fluorescence at room temperature with a maximum at 400 nm with a lifetime of 7 ns. Transient absorption spectroscopy has been used to elucidate the excited state properties of [Ru1₃]Cl₂ and 2hosts. A broad band centered around 600 nm in the [Ru1₃]Cl₂ transient spectrum corresponds to the absorption of the π-radical anion of the ethynyl fragment of ligand 1 and in 2 a signal at 595 nm was observed, corresponding to the absorption of the anthracene excited singlet state. We used a bisadamantyl guest to assemble the two hosts and studied the energy transfer from the ruthenium core to the anthracene unit by steady state emission spectroscopy. The photoinduced energy transfer process in assemblies of host 2 with ruthenium or osmium metalloguests, [Ru(biptpy)(tpy)](NO₃)₂ and [Os(biptpy)(tpy)](NO₃)₂, was studied by femtosecond transient spectroscopy and steady state emission spectroscopy. The singlet state of the anthracene pendant unit acted as a donor to the MLCT of the Ru(II) or Os(II) metalloguests. An energy transfer rate from the singlet anthracene excited state to the ¹MLCT of the ruthenium centre of 4.8 × 10⁸ s⁻¹ was estimated in the case of the ruthenium guest. The photoinduced triplet energy transfer (∼6 × 10⁶ s⁻¹) from a longer lived luminescent guest Rubph to the anthracene unit was studied by transient absorption spectroscopy.