Manuel Tropiano

Reversible Luminescence Switching of a Redox-Active Ferrocene–Europium Dyad

The copper-catalyzed cycloaddition reaction between a propargyl-appended europium complex and azidomethylferrocene yields a d-f dyad whose photophysical properties can be reversibly switched by varying the oxidation state of the ferrocene chromophore.

A family of 13 tetranuclear zinc(II)-lanthanide(III) complexes of a [3 + 3] Schiff-base macrocycle derived from 1,4-diformyl-2,3-dihydroxybenzene

A family of thirteen tetranuclear heterometallic zinc(II)-lanthanide(III) complexes of the hexa-imine macrocycle (L(Pr))(6-), with general formula Zn(II)(3)Ln(III)(L(Pr))(NO(3))(3)·xsolvents (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm or Yb), were prepared in a one-pot synthesis using a 3:1:3:3 reaction of zinc(II) acetate, the appropriate lanthanide(III) nitrate, the dialdehyde 1,4-diformyl-2,3-dihydroxybenzene (H(2)L(1)) and 1,3-diaminopropane. A hexanuclear homometallic zinc(II) macrocyclic complex [Zn(6)(L(Pr))(OAc)(5)(OH)(H(2)O)]·3H(2)O was obtained using a 2:0:1:1 ratio of the same reagents. A control experiment using a 1:0:1:1 ratio failed to generate the lanthanide-free [Zn(3)(L(Pr))] macrocyclic complex. The reaction of H(2)L(1) and zinc(II) acetate in a 1:1 ratio yielded the pentanuclear homometallic complex of the dialdehyde H(2)L(1), [Zn(5)(L(1))(5)(H(2)O)(6)]·3H(2)O. An X-ray crystal structure determination revealed [Zn(3)(II)Pr(III)(L(Pr))(NO(3))(2)(DMF)(3)](NO(3))·0.9DMF has the large ten-coordinate lanthanide(III) ion bound in the central O(6) site with two bidentate nitrate anions completing the O(10) coordination sphere. The three square pyramidal zinc(II) ions are in the outer N(2)O(2) sites with a fifth donor from DMF. Measurement of the magnetic properties of [Zn(II)(3)Dy(III)(L(Pr))(NO(3))(3)(MeOH)(3)]·4H(2)O with a weak external dc field showed that it has a frequency-dependent out-of-phase component of ac susceptibility, indicative of slow relaxation of the magnetization (SMM behaviour). Likewise, the Er and Yb analogues are field-induced SMMs; the latter is only the second example of a Yb-based SMM. The neodymium, ytterbium and erbium complexes are luminescent in the solid phase, but only the ytterbium and neodymium complexes show strong lanthanide-centred luminescence in DMF solution.

Thermodynamics of Core Metal Replacement and Self-Assembly of Ca2+ 15-Metallacrown-5†

The equilibria for core Ca(2+) replacement by Ln(3+) in copper(II) 15-MC-5 complexes have been investigated using a series of visible spectrophotometric titrations of calcium(II) metallacrowns ({Ca(II)[15-MC(Cu(II)(N)(L))-5]}(2+)) with Ln(3+) ions (H(2)L = pheha, (S)-alpha-phenylalaninehydroxamic acid, or trpha, (S)-alpha-tryptophanhydroxamic acid). These studies allowed the determination of the equilibrium constants for the reaction {Ca(II)[15-MC(Cu(II)(N)(L))-5]}(2+) + Ln(3+) --> {Ln(III)[15-MC(Cu(II)(N)(L))-5]}(3+) + Ca(2+) in methanol/water 9:1 (Ln(3+) = La(3+), Gd(3+), Dy(3+), Er(3+)) or 99:1 (Ln(3+) = La(3+), Nd(3+), Gd(3+), Dy(3+), Er(3+), Yb(3+)), respectively. The log K for these reactions decreases with increasing atomic number of the lanthanide(III), ranging from 6.1 to 3.91 in methanol/water 9:1. The same behavior is observed in methanol/water 99:1, although the constants are uniformly lower (log K = 4.09-2.52). A significant thermodynamic selectivity was observed for the later lanthanides (Gd(3+)-Yb(3+)) while a smaller selectivity is present throughout the beginning of the series (La(3+)-Gd(3+)). This observation has been interpreted on the basis of the size correspondence between the metal ions and the metallacrown cavity. The overall stability of the {Ca(II)[15-MC(Cu(II)(N)(L))-5]}(2+) in methanol/water 9:1 has been determined by pH-spectrophotometric titrations with HCl. The resulting log K values are 63.46(12) and 65.05(13) for pheha and trpha, respectively (Ca(2+) + 5Cu(2+) + 5HL(-) = {Ca(II)[15-MC(Cu(II)(N)(L))-5]}(2+) +5H(+)). The stability of both the La(3+) and Ca(2+) 15-metallacrown-5 complexes in the presence of high Na(+) concentrations has also been demonstrated by spectophotometric studies. Based upon these observations, the preference of the 15-MC-5 for Ca(2+) complexation compared to crown ethers has been quantitatively demonstrated for the first time.

Lanthanide appended rotaxanes respond to changing chloride concentration

Lanthanide appended rotaxanes have been prepared by the CuAAC ‘click’ reaction between an azide appended rotaxane and lanthanide complexes of propargyl DO3A. The resulting complexes are luminescent, and exhibit chloride responsive luminescence behavior consistent with the existence of two independent halide binding pockets, one in the rotaxane cavity and one on the ninth (axial) coordination site of the lanthanide. Strong halide binding to europium gives rise to changes in the relative intensity of the hypersensitive ΔJ = 2 transition compared to the rest of the europium emission spectrum, combined with quenching of the overall intensity of emission as a consequence of non-radiative quenching by the bound halide. The weaker interaction with the rotaxane pocket mediates a subsequent recovery of intensity of the europium centered luminescence despite the considerable separation between the lanthanide and the rotaxane binding pocket.

Luminescence and upconversion from thulium(III) species in solution.

Thulium salts and complexes are shown to be emissive from three states in the excited state manifold of Tm(3+). Formation of the (1)D(2) state can result in luminescence, or in energy transfer to the lower energy (1)G(4) and (3)H(4) emissive states. Where chromophores are present in the ligand structure, emission is restricted to thulium centred emissive states that are lower in energy than the chromophore centred donor state. We have also observed direct multi-photon excitation of the thulium excited state manifold. Furthermore, additional transitions are observed in the multi-photon excitation spectra that are consistent with upconversion as a consequence of sequential single photon absorption and relaxation processes within the thulium excited state manifold.

Time-resolved confocal microscopy using lanthanide centred near-IR emission

A method to uniquely identify signals originating from probes with different emission decay times in luminescence imaging has been developed. By using scanning confocal microscopy in combination with time-correlated single photon counting (TCSPC), Photon Arrival Time Imaging (PArTI) has been realised through off-line plotting of images using the photon arrival times. PArTI is the time-equivalent to spectrally resolved imaging, replacing the energy axis with a photon arrival time axis. Here, lanthanide probes were used to demonstrate the key advantages of the method. PArTI uses TCSPC data, involves no fitting, uses a single pulsed laser line for multicolour imaging, and can be used with a 100 millisecond dwell time per pixel.

Reversible Recruitment and Emission of DO3A-Derived Lanthanide Complexes at Ligating Molecular Films on Gold

The recruitment of DO3A-derived lanthanide complexes by ligation to isophthalic acid and catechol-modified gold surfaces, and their resulting sensitization, is reported herein. Predictably pH-dependent surface recruitment is associated with the expected fingerprint europium and terbium emission characteristics. The intensity of the lanthanide luminescence scales exponentially with spacer length, indicating a strong quenching interaction between the lanthanide and the gold surface. The switchable catechol oxidation state provides a means of electrochemically triggering the release of prior ligated complexes.

Kinetically Stable Lanthanide Complexes Displaying Exceptionally High Quantum Yields upon Long-Wavelength Excitation: Synthesis, Photophysical Properties, and Solution Speciation.

We demonstrate how highly emissive, kinetically stable complexes can be prepared using the macrocyclic scaffold of DO3A bearing coordinating aryl ketones as highly effective sensitizing chromophores. In the europium complexes, high quantum yields (up to 18% in water) can be combined with long-wavelength excitation (370 nm). The behavior in solution upon variation of pH, studied by means of UV-vis absorption, emission, and NMR spectroscopies, reveals that the nature of the chromophore can give rise to pH-dependent behavior as a consequence of deprotonation adjacent to the carbonyl group. Knowledge of the molecular speciation in solution is therefore critical when assessing the luminescence properties of such complexes.

Exploring energy transfer between pyrene complexes and europium ions – potential routes to oxygen sensors

Pyrene chromophores are shown to exhibit reversible energy transfer to europium ions, resulting in oxygen dependent lanthanide luminescence. Two different pathways can give rise to oxygen dependence: rapid reversible energy transfer between the T1 state and the emissive state, or slow T1–5D0 energy transfer on the timescale of triplet state quenching.

Lanthanide Complexes of Azidophenacyl‐DO3A as New Synthons for Click Chemistry and the Synthesis of Heterometallic Lanthanide Arrays

Lanthanide complexes of azidophenacyl DO3A are effective substrates for click reactions with ethyne derivatives, giving rise to aryl triazole appended lanthanide complexes, in which the aryl triazole acts as an effective sensitising chromophore for lanthanide luminescence. They also undergo click chemistry with propargylDO3A derivatives, giving rise to heterometallic complexes.