Computational Investigation on the Spectroscopic Properties of Thiophene Based Europium β-Diketonate Complexes

Design and Mechanism of Rare-Earth Singlet Oxygen Sensing: An Experimental and Quantum Chemical Approach

The sensitive detection of singlet oxygen (¹O₂) is one key issue in various photochemical analyses, reactions, and processes; it is indispensable for designing catalysts for photodynamic therapies. Corresponding fluorescence-based organic ¹O₂ monitor luminophores may be equipped with rare-earth complexes with several intrinsic advantages. The design of the necessary ligands being a tedious, time-consuming effort, often involving empirical guesswork, we decided to support our experimental work with quantum chemical calculations. Hence, next to the experimental core, this paper suggests the additional use of time-dependent density functional theory (TDDFT) on suitable, free β-diketonate ligands to devise corresponding Eu³⁺ complexes as ¹O₂ probes eventually; the free ligand calculations obviously allow profoundly reduced computational efforts. Novel β-diketonate-substituted dimethyl anthracene complexes of Eu³⁺, Tb³⁺, and Gd³⁺ and their endoperoxidized descendants were thus synthesized, compared to known related complexes and analyzed with regard to their electronic characteristics; in addition, spectroscopy of a Eu³⁺ complex with ancillary epoxiphenanthroline for subsequent attachment to biological substrates featuring -NH₂ or -SH groups was included. The spectroscopic determination of the decisive lowest triplet (T₁) states of the Gd complexes could be matched by the Tamm-Dancoff approximation (TDA)/TDDFT calculations on the free ligands satisfactorily if suitable functionals were applied. Most significantly, the results suffice to describe the luminescence "switch-on" mechanism of this complex in the presence of ¹O₂.

Accessing lanthanide-based, in situ illuminated optical turn-on probes by modulation of the antenna triplet state energy

Luminescent lanthanides possess ideal properties for biological imaging, including long luminescent lifetimes and emission within the optical window. Here, we report a novel approach to responsive luminescent Tb(iii) probes that involves direct modulation of the antenna excited triplet state energy. If the triplet energy lies too close to the 5D4 Tb(iii) excited state (20 500 cm-1), energy transfer to 5D4 competes with back energy transfer processes and limits lanthanide-based emission. To validate this approach, a series of pyridyl-functionalized, macrocyclic lanthanide complexes were designed, and the corresponding lowest energy triplet states were calculated using density functional theory (DFT). Subsequently, three novel constructs L3 (nitro-pyridyl), L4 (amino-pyridyl) and L5 (fluoro-pyridyl) were synthesized. Photophysical characterization of the corresponding Gd(iii) complexes revealed antenna triplet energies between 25 800 and 30 400 cm-1 and a 500-fold increase in quantum yield upon conversion of Tb(L3) to Tb(L4) using the biologically relevant analyte H2S. The corresponding turn-on reaction can be monitored using conventional, small-animal optical imaging equipment in presence of a Cherenkov radiation emitting isotope as an in situ excitation source, demonstrating that antenna triplet state energy modulation represents a viable approach to biocompatible, Tb-based optical turn-on probes.

New 1,3-Disubstituted Benzo[h]Isoquinoline Cyclen-Based Ligand Platform: Synthesis, Eu3+ Multiphoton Sensitization and Imaging Applications

The development of lanthanide-based luminescent probes with a long emission lifetime has the potential to revolutionize imaging-based diagnostic techniques. By a rational design strategy taking advantage of computational predictions, a novel, water-soluble Eu³⁺ complex from a cyclen-based ligand bearing 1,3-disubstituted benzo[h]isoquinoline arms was realized. The ligand has been obtained overcoming the lack of reactivity of position 3 of the isoquinoline moiety. Notably, steric hindrance of the heteroaromatic chromophore allowed selective and stoichiometry-controlled insertion of two or three antennas on the cyclen platform without any protection strategy. The complex bears a fourth heptanoic arm for easy conjugation to biomolecules. This new chromophore allowed the sensitization of the metal center either with one or two photons excitation. The suitability as a luminescent bioprobe was validated by imaging BMI1 oncomarker in lung carcinoma cells following an established immunofluorescence approach. The use of a conventional epifluorescence microscope equipped with a linear structured illumination module disclosed a simple and inexpensive way to image confocally Ln-bioprobes by single photon excitation in the 350–400 nm window, where ordinary confocal systems have no excitation sources.

Antenna triplet DFT calculations to drive the design of luminescent Ln3+ complexes

Density functional theory-based methods have been exploited to look into the structural, vibrational and electronic properties of antenna ligands, all of them being crucial factors for the reliable design of customized luminescent lanthanide (Ln3+) complexes. The X-ray structures, UV-Vis absorption spectra and triplet (T1) energies of three novel β-diketone ligands with a thienyl group and naphthyl (L1), phenanthryl (L2), and pyrenyl (L3) polycyclic aromatic hydrocarbons as substituents have been modelled. Vibronic progressions provide a strong contribution to the L1 and L2 absorption spectra, while the L3 absorption spectrum needs the assumption of different conformational isomers in solution. T1 energies have been estimated either through the vertical- or the adiabatic-transition approach. The comparison with the phosphorescence spectra of Gd3+ complexes allowed us to infer that the latter approach is the most suitable one, in particular when sizable ligands are involved. Results obtained for the isolated antennas can be directly compared with those of the corresponding Ln3+ complexes, due to the unanimously accepted assumption that the excitation is ligand-centred.

Unveiling the Relationship between Energy Transfer and the Triplet Energy Level by Tuning Diarylethene within Europium(III) Complexes

Luminescence performance and photoisomerization control of sensitized energy transfer in a series of Eu(acac)₃De complexes that contain photochromic diarylethene (De) as the ligand are studied by theoretical methods. Both the open-ring and closed-ring isomers exhibit a consistent coordination mode between the Eu^III ion and De. An unneglected weak interaction originating from electrostatic attraction is found in the region of the coordinate bond Eu-N. The open-ring isomer has higher triplet energy levels than ⁵D₁ and ⁵D₀ of the Eu^III ion, which facilitates forward energy transfer from De to the Eu^III ion. The closed-ring isomer, for the extended conjugated system formed in cyclization, has a much lower triplet energy level than ⁵D₀ of the Eu^III ion. The energy-gap deficit makes energy transfer unavailable. By utilization of this phenomenon, regulation of energy transfer and reversible on/off luminescence switching of the europium(III) complex can be achieved. The forward and backward energy-transfer rates in different channels are also calculated for the series of complexes. A statistics diagram is obtained to exhibit the change trend of energy-transfer rates in the forward and backward directions as a function of the triplet energy level, which indicates the contribution of different channels to energy transfer in each level region and figures out that the optimal triplet energy level should be in the range of 21740-19532 cm^-1.

Ab initio calculation of energy levels of trivalent lanthanide ions

A fully ab initio computational scheme employing CASSCF/XMCQDPT2/SO-CASSCF for the absorption and emission spectra of trivalent lanthanide complexes is presented.

Mechanistic aspects of the gas-phase coupling of thioanisole and chlorobenzene to dibenzothiophene catalyzed by atomic Ho(+)

Mechanistic aspects of the novel gas-phase generation of dibenzothiophene via coupling of thioanisole and chlorobenzene, employing atomic Ho(+) as a catalyst, have been investigated using Fourier-transform ion cyclotron resonance mass spectrometry in conjunction with density functional theory (DFT) calculations.