Shining light on the excited state energy cascade in kinetically inert Ln(iii) complexes of a coumarin-appended DO3A ligand.

Our understanding of the solution structure of the complexes between a coumarin-appended 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate (DO3A) ligand and Eu(iii), Gd(iii), Tb(iii) and Y(iii) ions allows us to investigate the excited state energy transfer cascade that result in sensitised lanthanide centred luminescence, and we can do so in unprecedented detail. Spectroscopic data from solutions with ambient oxygen levels, without oxygen and solid solutions at 77 K unequivocally show that the simple model for the antenna principle is not valid. We find that the Tb(iii) complex photophysics is determined by the non-emissive 5D3 state, not the emissive 5D4 state as usually assumed. This scrutiny of the energy transfer cascade in these antenna appended lanthanide complexes, show that detailed knowledge of solution structure and experimental photophysics are needed to rationalise the properties of lanthanide based dyes.

π-Expanded Thioxanthones-Engineering the Triplet Level of Thioxanthone Sensitizers for Lanthanide-Based Luminescent Probes with Visible Excitation.

Invited for this month's cover are the collaborating groups of Dr. Thomas Just Sørensen at the University of Copenhagen, Denmark and Dr. Robert Pal at Durham University, United Kingdom. The front cover shows the clouds parting for a cell imaged using a thioxanthone-appended EuIII complex. This work shows that lanthanide luminescence can be used in optical bioimaging with microscopes equipped with the common blue laser line. Read the full text of the article at 10.1002/cplu.201900309.

π-Expanded Thioxanthones - Engineering the Triplet Level of Thioxanthone Sensitizers for Lanthanide-Based Luminescent Probes with Visible Excitation.

Bright lanthanide based probes for optical bioimaging must rely on the antenna principle, where the lanthanide-centred excited state is formed by a complex sensitization process. Efficient sensitization of lanthanide-centred emission occurs via triplet states centred on the sensitizing chromophore. Here, the triplet state of thioxanthone chromophores is modulated by extending the π-system. Three thioxanthone chromophores-thioxanthone, benzo[c]thioxanthone, and naphtho[2,3-c]thioxanthone were synthesised and characterised. The triplet state energies and lifetimes is found to change as expected, and two dyes are found to be suitable sensitizers for europium(iii) luminescence. Reactive derivatives of thioxanthone and benzo[c]thioxanthone were prepared and coupled to a 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) lanthanide binding pocket. The photophysics and the performance in optical bioimaging of the resulting europium(iii) complexes were investigated. It is concluded that while the energetics favour efficient sensitization, the solution structure does not. While it was found that the complexes are too lipophilic to be efficient luminescent probes for optical bioimaging, we successfully demonstrated bioimaging using europium(iii) luminescence following 405 nm excitation.

Investigating subtle 4f vs. 5f coordination differences using kinetically inert Eu(iii), Tb(iii), and Cm(iii) complexes of a coumarin-appended 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate (DO3A) ligand.

In order to reveal subtle differences between the solution chemistries of trivalent 4f and 5f elements, the physicochemical and photophysical properties of europium(iii), terbium(iii) and curium(iii) complexes formed with a 7-methoxy-coumarin appended 1,4,7,10-tetraazadodecane-1,4,7-triacid (DO3A) ligand were studied. All three complexes were found to be kinetically inert and exhibit stability constants similar to their 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) equivalents. The Cm(iii) and Eu(iii) complexes feature strong sensitised emission, while the triplet energy of the coumarin prohibits efficient sensitisation of the Tb(iii) analogue. The data presented here indicate significant differences in perturbation of the sensitising chromophore photophysics between the 4f and 5f elements. In contrast, the size of the metal center appears to not be a determining factor for the physicochemical properties of these kinetically inert Eu(iii), Tb(iii), and Cm(iii) complexes.

Composed in the f-block: solution structure and function of kinetically inert lanthanide(iii) complexes.

It has been more than 15 years since the last authoritative report on the solution structure of lanthanide complexes made from cylcen derived polydentate ligands. The field has progressed and diversified, and tools have been developed that should enable a step-change in the field in the imminent future. This will only happen if the tools are used, and the results communicated in a form that is consistent within the field and readily accesible to scientists outside the field. In this perspective, the fundamental tools for designing and investigating kinetically inert lanthanide complexes in solution will be covered. The fundamentals of this type of complexes will be laid out. The conformations of lanthanide complexes from cyclen derived ligands and the rate of exchange between conformations will be linked to their 1H NMR and luminescence spectra. The information rich ligand- and metal centred emission spectra will be discussed, and the time-resolved luminescence decay lifetimes are shown to be directly related to the solution structure. The aim is to provide the reader with the information needed to become excited by lanthanide coordination chemistry.

Shining light on the antenna chromophore in lanthanide based dyes.

Lanthanide based dyes and assays exploit the antenna effect, where a sensitiser-chromophore is used as a light harvesting antenna and subsequent excited state energy transfer populates the emitting lanthanide centred excited state. A rudimentary understanding of the design criteria for designing efficient dyes and assays based on the antenna effect is in place. By preparing kinetically inert lanthanide complexes based on the DO3A scaffold, we are able to study the excited state energy transfer from a 7-methoxy-coumarin antenna chromophore to europium(iii) and terbium(iii) centred excited states. By contrasting the photophysical properties of complexes of metal centres with and without accessible excited states, we are able to separate the contributions from the heavy atom effect, photoinduced electron transfer quenching, excited state energy transfer and molecular conformations. Furthermore, by studying the photophysical properties of the antenna chromophore, we can directly monitor the solution structure and are able to conclude that excited state energy transfer from the chromophore singlet state to the lanthanide centre does occur.

Rationalizing substituent effects in 1-azathioxanthone photophysics.

The influence of an electron donating substituent on the photophysical properties of 1-azathioxanthone dyes has been investigated using optical spectroscopy and theoretical models. The motivation behind the study is based on the fact that thioxanthones are efficient triplet sensitizers, and thus promising sensitizers for lanthanide centered emission. By adding an aza group to one of the phenyl ring systems, direct coordination to a lanthanide center becomes possible, which makes azathoixanthones great candidates as antenna chromophores in lanthanide(III) based dyes. Here, three 1-azathioxanthone derivatives have been synthesized targeting efficient triplet formation following absorption in the visible range of the spectrum. This is achieved by adding methoxy groups to the 1-azathioxanthone core. The derivatives were characterized using absorption, emission, and time-gated emission spectroscopy, where fluorescent quantum yields, singlet and triplet excited states lifetimes were determined. The experimentally determined photophysical properties of the three 1-azathioxanthone compounds are contrasted to those of the parent thioxanthone and is rationalized using the Strickler-Berg equation, Hückel MO theory, and Dewar's rules in combination with computational chemistry. We find that the transition energies follow predictions, but that the overall photophysical properties are determined by the relative energies as well as the nature of the involved states in both the singlet and the triplet excited state manifolds.

Kinetically Inert Lanthanide Complexes as Reporter Groups for Binding of Potassium by 18-crown-6.

The barcode-like spectrum of lanthanide-centered emission has been used in imaging and to make responsive luminescent reporters. The intensities and the shapes of each line in the luminescence spectrum can also report on the coordination environment of the lanthanide ion. Here, we used lanthanide-centered emission to report on the binding of potassium in an 18-crown-6 binding pocket. The responsive systems were made by linking a crown ether to a kinetically inert lanthanide binding pocket using a molecular building block approach. Specifically, an alkyne-appended Ln.DO3A was used as a building block in a copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) "click" reaction with azide-functionalized crown ethers. The resulting complexes were investigated using NMR and optical methods. Titrations with potassium chloride in methanol observing the sensititzed europium- and terbium-centered emissions were used to investigate the response of the systems. The molecular reporters based on aliphatic crown ethers were found to have strongly inhibited binding of potassium, while the benzo-18-crown-6 derived systems had essentially the same association constants as the native crown ethers. The shape of the lanthanide emission spectra was shown to be unperturbed by the binding of potassium, while the binding was reported by an overall increased intensity of the lanthanide-centered emission. This observation was contrasted to the change in spectral shape between propargyl-Ln.DO3A and the triazolyl-Ln.DO3A complexes. The solution structure of the lanthanide complexes was found to be determining for the observed physical chemical properties of these systems.