Deciphering and quantifying linear light upconversion in molecular erbium complexes

Single-center light upconversion corresponds to the pilling up of low-energy photons via successive linear absorptions: a phenomenon commonly observed in lanthanide-doped low-phonon ionic solids or nanoparticles. Its ultimate miniaturization in molecular complexes opens challenging perspectives in term of improved reproducibility, chemical control and optical programming. However, highenergy vibrations inherent to coordination complexes severely limit the efficiency of successive excited-state absorptions (ESA) responsible for the gain in photon energy. By carefully wrapping three polyaromatic ligand strands around trivalent erbium, we managed to induce low-power room temperature near infrared (exc = 801 nm or 966 nm) to visible green (em = 522 nm and 545 nm) light upconversion within mononuclear coordination complexes [Er(Lk)3]3+ operating either in the solid state or in non-deuterated solution. The calculated upconversion quantum yields set the zero-level of an elemental erbium-centered molecular ESA mechanism, a values which favorably compares with cooperative upconversion (CU) previously implemented in sophisticated multisite Yb2Tb supramolecular assemblies. The various dependences of the upconverted emission on the incident excitation power imply different mechanisms, which can be tuned by molecular design

Red-Emitting Fluorophores as Local Water-Sensing Probes

Fluorescent probes are known for their ability to sense changes in their direct environment. We introduce here the idea that common red-emitting fluorophores recommended for biological labeling and typically used for simple visualization of biomolecules can also act as reporters of the water content in their first solvent sphere by a simple measurement of their fluorescence lifetime. Using fluorescence spectroscopy, we investigated the excited-state dynamics of seven commercially available fluorophores emitting between 650 and 800 nm that are efficiently quenched by H2O. The amount of H2O in their direct surrounding was modulated in homogeneous H2O–D2O mixtures or, in heterogeneous systems, by confining them into reverse micelles, by encapsulating them into host–guest complexes with cyclodextrins, or by attaching them to peptides and proteins. We found that their fluorescence properties can be rationalized in terms of the amount of H2O in their direct surroundings, which provides a general mechanism for protein-induced fluorescence enhancements of red-emitting dyes and opens perspectives for directly counting water molecules in key biological environments or in polymers

Erbium complexes as pioneers for implementing linear light-upconversion in molecules

Since the non-linear optical (NLO) response of matter to incident excitation light does not require long-lived intermediate excited states working as relays, the conversion of low-energy photons into high energy light beams using second-harmonic generation (second-order NLO process) or two-photon absorption (third-order NLO process) can be implemented either in low-phonon macroscopic solids or in molecules containing high-energy vibrations. However, harnessing the very weak non-linear absorption coefficients requires (very) intense excitation sources, typically lasers, for getting reasonable emitted intensities. In contrast, the piling of successive near-infrared photons to get visible emission using linear optics, i.e. upconversion, is much more efficient, but it depends on the existence of intermediate excited states possessing long residence lifetimes. Therefore, upconversion usually occurs in low-phonon ionic solids or nanoparticles doped with pertinent activators. The recent recognition that trivalent erbium coordination complexes possessing high-frequency oscillators may act as dual visible/near-infrared activators, which implies the existence of at least one long-lived intermediate excited state in these complexes, paved the way for the implementation of the first upconversion processes within isolated molecules. Beyond a justification for using trivalent lanthanides, and especially erbium, for the manipulation of the energy of photons in molecules using linear optics, this tutorial review summarizes the current level of developments in the field of molecular-based upconversion and discusses some forthcoming challenges

Monitoring Fe(II) Spin-State Equilibria via Eu(III) Luminescence in Molecular Complexes: Dream or Reality?

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and non-invasive monitoring of magnetic information storage. In order to approach this goal at the molecular level, three segmental ligand strands L4-L6 were reacted with stoichiometric mixtures of divalent d-block cations (M(II) = Fe(II) or Zn(II)) and trivalent lanthanides (Ln(III) = La(III), Eu(III)) in acetonitrile to give C3-symmetrical dinuclear triplestranded helical [LnM(Lk)3]5+ cations, which can be crystallized with non-coordinating counteranions. The divalent metal M(II) is six-coordinate in the pseudo-octahedral sites produced by the facial wrapping of the three didentate binding units, the ligand field of which induces variable Fe(II) spin-state properties in [LnFe(L4)3]5+ (strictly high-spin), [LnFe(L5)3]5+ (spin-crossover (SCO) around room temperature) and [LnFe(L6)3]5+ (SCO at very low temperature). The introduction of the photophysically active Eu(III) probe in [EuFe(Lk)3]5+ results in europium-centered luminescence modulated by variable intramolecular Eu(III)Fe(II) energy transfer processes. The kinetic analysis implies Eu(III)Fe(II) quenching efficiencies close to 100% for the low-spin configuration and larger than 95% for the high-spin-state. Consequently, the sensitivity of indirect luminescence detection of Fe(II) spin-crossover is limited by the resulting weak Eu(III)-centered emission intensities, but the dependence of the luminescence on the temperature unambiguously demonstrates the potential of indirect lanthanide-based spin-state monitoring at the molecular scale

Metal-Based Linear Light Upconversion Implemented in Molecular Complexes: Challenges and Perspectives

The piling up of low-energy photons to produce light beams of higher energies while exploiting the nonlinear optical response of matter was conceived theoretically around 1930 and demonstrated 30 years later with the help of the first coherent ruby lasers. The vanishingly small efficacy of the associated light- upconversion process was rapidly overcome by the implementation of powerful successive absorptions of two photons using linear optics in materials that possess real intermediate excited states working as relays. In these systems, the key point requires a favorable competition between the rate constant of the excited-state absorption (ESA) and the relaxation rate of the intermediate excited state, the lifetime of which should be thus maximized. Chemists and physicists therefore selected long-lived intermediate excited states found (i) in trivalent lanthanide cations doped into ionic solids or into nanoparticles (2S+1LJ spectroscopic levels) or (ii) in polyaromatic molecules (triplet states) as the logical activators for designing light upconverters using linear optics. Their global efficiency has been stepwise optimized during the past five decades by using indirect intermolecular sensitization mechanisms (energy transfer upconversion = ETU) combined with large absorption cross sections. The induction of light-upconversion operating in a single discrete entity at the molecular level is limited to metal-based units and remained a challenge for a long time because coordination complexes possess high-frequency oscillators incompatible with the existence of (i) scales of accessible excited relays with long lifetimes and (ii) final high-energy emissive levels with noticeable intrinsic quantum yields. In contrast to intermolecular energy transfer processes operating in metal-based doped solids, which require statistical models, the combination of sensitizers and activators within the same molecule limits energy transfers to easily tunable intramolecular processes with first-order kinetic rate constants. Their successful programming in a trinuclear CrErCr complex in 2011 led to the first detectable near-infrared to green light upconversion induced in a molecular unit under reasonable excitation intensity. The subsequent progress in the modeling and understanding of the key factors controlling metal-based light upconversion operating in molecular complexes led to a burst of various designs exploiting different mechanisms, excited-state absorption (ESA), energy transfer upconversion (ETU), cooperative luminescence (CL), and cooperative upconversion (CU), which are discussed in this Account

Universal quenching of common fluorescent probes by water and alcohols

Although biological imaging is mostly performed in aqueous media, it is hardly ever considered that water acts as a classic fluorescence quencher for organic fluorophores. By investigating the fluorescence properties of 42 common organic fluorophores recommended for biological labelling, we demonstrate that H2O reduces their fluorescence quantum yield and lifetime by up to threefold and uncover the underlying fluorescence quenching mechanism. We show that the quenching efficiency is significantly larger for red-emitting probes and follows an energy gap law. The fluorescence quenching finds its origin in high-energy vibrations of the solvent (OH groups), as methanol and other linear alcohols are also found to quench the emission, whereas it is restored in deuterated solvents. Our observations are consistent with a mechanism by which the electronic excitation of the fluorophore is resonantly transferred to overtones and combination transitions of high-frequency vibrational stretching modes of the solvent through space and not through hydrogen bonds. Insight into this solvent-assisted quenching mechanism opens the door to the rational design of brighter fluorescent probes by offering a justification for protecting organic fluorophores from the solvent via encapsulation