Minh-Huong Ha-Thi

Sensitized emission of luminescent lanthanide complexes based on a phosphane oxide derivative.

The photophysical properties of a complex based on diphenylphosphanoethane (DPPE) fluorescent ligands linked to a europium ion have been investigated by different spectroscopic methods. Upon complexation with europium, the interaction of the phosphane oxide group with europium leads to a red shift of the absorption spectrum and a strong quenching of the ligand emission. The typical sensitized emission of Eu(3+) is observed upon excitation of the ligand with a fluorescence quantum yield of 1%. Time-resolved absorption and emission experiments have been performed in order to investigate the photophysical mechanism involved in this complex. Photophysical studies show that an energy-transfer mechanism occurs from both the first excited singlet and triplet states of the ligand, and the population of the europium ion to the (5)D(1) state takes place, from which the (5)D(0) state is populated. Additionally, electron transfer from the excited singlet state of the ligand to the europium ion appears as a very efficient process.

An Efficient Indirect Mechanism for the Ultrafast Intersystem Crossing in Copper Porphyrins

The ultrafast dynamics of copper tetraphenylporphyrin (CuTPP), copper octaethylporphyrin (CuOEP), and of the free base tetraphenylporphyrin (H2TPP), excited in the S2 state have been investigated in the gas phase by femtosecond pump/probe experiments. The porphyrins were excited in the Soret band at 400 nm. Strikingly, the S2-S1 internal conversion in H2TPP is very rapid (110 fs), as compared to that of ZnTPP (600 fs), previously observed. In turn, CuTPP and CuOEP, excited in S2, follow an efficient and different relaxation pathway from that of other open-shell metalloporphyrins. These two molecules exhibit a sequential four-step decay ending on a slow evolution in the nanosecond range (2)S2 → (2)CT → (2)T → (2)Ground State. This latter evolution is linked to the formation of the (2)T, tripdoublet state in CuTPP, observed in the condensed phase. It is shown that an intermediate charge transfer state plays a crucial role in linking the porphyrin centered (1)ππ* and (3)ππ* configurations. A simple model is presented that allows a rapid evolution between these two configurations, via coupling of the porphyrin π system with the free d electron on the copper. The mechanism obviates the need for the spin orbit coupling within the porphyrin. The result is that these copper porphyrins can exhibit an ultrafast apparent intersystem crossing, unprecedented for organic molecules.

Calixarene-Based Fluorescent Sensors for Cesium Cations Containing BODIPY Fluorophore

New fluorescent molecular sensors based on a calix[4]arene biscrown-6 ether as coordination site and BODIPY derivative as signaling unit were synthesized, and their photophysical properties were characterized. The complexation properties of these sensors with potassium and cesium cations were investigated using both steady-state and time-resolved fluorescence methods. The studies show that the sensitivity with cations depends upon the position of substituted coordination site on the BODIPY core. The complexation with cations does not have much effect on the absorption and emission wavelength when the coordination site (calix[4]arene biscrown-6 ether) is introduced at the meso position of the BODIPY core. In contrast, the same calix[4]arene biscrown-6 ether attached via a styryl linker to the α-position of BODIPY core leads to a sensitive sensor for alkali cations thanks to the better conjugation between the coordination site and the BODIPY core. The complexation of cations induces a hypsochromic shift of the absorption and emission maximums due to the diminution of donor character of the oxygen atoms in the coordination site. The stability constants of complexes with potassium and cesium ion were measured.

A Highly Selective Potassium Sensor for the Detection of Potassium in Living Tissues

The development of highly selective sensors for potassium is of great interest in biology. Two new hydrosoluble potassium sensors (Calix-COU-Alkyne and Calix-COU-Am) based on a calix[4]arene bis(crown-6) and an extended coumarin were synthesized and characterized. The photophysical properties and complexation studies of these compounds have been investigated and show high molar extinction coefficients and high fluorescence quantum yields. Upon complexation with potassium in the millimolar concentration range, an increase of one- and two-photon fluorescence emission is detected. A twofold fluorescence enhancement is observed upon excitation at λ=405 nm. The ligands present excellent selectivity for potassium in the presence of various competitive cations in water and in a physiological medium. The photophysical properties are not affected by the presence of a large amount of competing cations (Na+ , Ca2+ , Mg2+ , etc.). Ex vivo measurements on mouse hippocampal slices show that Calix-COU-Alkyne accumulates extracellularly and does not alter the neuronal activity. Furthermore, the sensor can be utilized to monitor slow extracellular K+ increase induced by inhibition of K+ entry into the cells.

Snapshots of Light Induced Accumulation of Two Charges on Methylviologen using a Sequential Nanosecond Pump–Pump Photoexcitation

Methylviologen (MV2+) is perhaps the most used component as a reversible electron acceptor in photophysical studies. While MV2+ is most commonly implicated as a reversible one-electron mediator, its electrochemical properties clearly evidence two successive one-electron reduction processes. In this report, we have investigated on the light driven two-charge accumulation on MV2+ using a multicomponent system composed of the prototypical molecular photosensitizer [Ru(bpy)3]2+ and MV2+ in the presence of ascorbate as reversible electron donor. The sequential addition of two electrons on the methylviologen was tracked upon sequential excitation of the [Ru(bpy)3]2+ at optimized concentration of the electron acceptor. The charge accumulated state carries an energy of 0.9 eV above the ground state and has a lifetime of ca. 50 μs. We have reached a fairly good global yield of approximately 9% for the two-charge accumulation. This result clearly demonstrates the potential of this simple approach for applications in artificial photosynthesis.

New sensitive and selective calixarene-based fluorescent sensors for the detection of Cs+ in an organoaqueous medium

Herein, new fluorescent sensors based on calix[4]arene-biscrown-6 containing extended coumarin as a fluorophore were synthetized and their photophysical properties were characterized. These compounds display intense absorption and emission spectra in the visible region due to extension of the coumarin system. Moreover, complexation properties of these ligands were reported, and the Calix-COU-Benz-CN ligand was able to selectively detect cesium ions in an organoaqueous solvent. Upon the addition of cesium, a blue-shift in the absorption spectra and an enhancement of the emission spectra were observed. This ligand was incorporated in a microfluidic device for the detection of Cs+ ions, and a detection limit of 1.4 μM was achieved for these ions.

Observation in the gas phase of the ligation of 1-Methylimidazole to hemoprotein mimics

Hemoprotein mimics, cobalt picket fence porphyrins have been prepared in the gas phase as neutral molecules for the first time. Their ligation properties have been studied with 1-methylimidazole and compared with those of other cobalt porphyrins, tetraphenyl porphyrin, and cobalt protoporphyrin IX chloride, in view of studying the sterical properties of the ligation. It is shown that the cobalt picket fence porphyrin can only accept one 1-methylimidazole ligand in contrast to less sterically crowded porphyrins like cobalt tetraphenylporphyrin that present two accessible ligation sites. The femtosecond dynamics of these ligated systems have been studied after excitation at 400 nm, in comparison with the unligated ones. The observed transients are formed in much shorter times, 30 fs for the ligated species, as compared to free species (100 fs), supporting the porphyrin to metal charge transfer nature of these transients. The similar decays of the ligated transients <1 ps reveal the absence of photodissociation of the cobalt-1-methylimidazole bond at this step of evolution.

Ultrafast Electronic Relaxation of Excited State of Biomimetic Metalloporphyrins in the Gas Phase

Biomimetic molecules are systems that provide a direct and simpler reproduction of a molecular process essential to the living world, i.e. they mimic a function or a structure of a biomolecular edifice. The idea is to isolate this process and allow a selective study of a specific function by physicochemical methods, in a reductionist way. In that respect, gas phase studies of biomolecules are ideal, since they allow the application of a variety of highly sensitive and selective tools to the study of these model systems: mass spectrometry, photoelectron spectroscopy, for example, provided that the molecular system has been vaporised. In addition, the system is studied in absence of solvent, which greatly simplifies the interpretation of the fundamental effects and consequently allows to infer the effects of the medium since, in the gas phase solvents can be introduced in a gradual manner via clusters. Also gas phase measurements allow a direct comparison with results from quantum chemistry calculations. Great progresses have been fuelled in this direction by the developments of mass spectrometry. The studies in the gas phase have been mostly dedicated to the elucidation of the basic structure of the building blocks of proteins, i.e. small peptides (Simons, 2004). In turn, other biological functions have been less studied by the gas phase biomimetic approach. Hematoproteins hosting hemoporphyrins (the heme, see fig 1) are ideal systems to examine in this way, since they are at the heart of the oxygen and small molecules transport within blood and that their active site is localised very precisely on the metal atom embedded in their central porphyrin. Thus, if one studies the active porphyrinic site of myoglobin at short time scales (in the subpicosecond time domain), the interaction with the environing protein (globin myoglobin) can be minimised. In hemoproteins, the attachment of oxygen to the iron of the heme is a complex process involving a potential barrier. This barrier results from the crossing of two surfaces of