Michel Sliwa

A promising fast, negative, red-absorbing photochromic system based on an intraprotein intermolecular charge transfer (CT) complex between a flavin cofactor and a substrate-analogue inhibitor MXA (methylthioacetate or methylselenoacetate; X = S, Se) within monomeric sarcosine oxidase (MSOX) displays MXA-independent, barrierless, high-yield femtosecond photoactivation associated with CT-interaction disruption coupled to MXA isomerization. Assessing the origin of the known MXA-dependence of the nanosecond thermal recovery requires a very precise determination of activation energy barriers. Here, a newly developed visible pump–probe setup employing two Ti:sapphire femtosecond lasers spanning the picosecond to microsecond time scale was used. Whereas the room temperature thermal recovery time for MSeA is more than double that for MTA (7.5 ns vs 2.9 ns), the dissociation rate, the quantum yield (0.85), and recombination activation enthalpy (21 kJ·mol–1) are MXA-independent. Quantum mechanical calculations rationalize this value as the intrinsic enthalpic barrier for MXA isomerization. The observed variation in thermal recovery is due to the activation entropy, which is negative (on the order of −15 J·mol–1·K–1) and MXA-dependent. This dependence is attributed to a differently ordered transition state. Implications for the design and application of this class of photochromic systems are discussed.

Real-space evaluation of exciton migration in a nanodomain potentially provides substantial information in advanced material design, that is, however, still challenging as optical excitation and detection are diffraction limited to submicrometer resolutions. To explore the real-space evaluation, we used rare-earth core-shell upconverting nanoparticles, with thulium Tm ³⁺ as emitters and Yb ³⁺ as sensitizers (TmUCNP), for nanometric photoexcitation of conducting polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). The TmUCNPs in MEH-PPV solid were photoexcited with a near-infrared laser. The visible emission of the Tm ions allowed for the production of excited states of MEH-PPV through excitation energy transfer. The emission of MEH-PPV spectrally separable from Tm ions permitted individual imaging of their emission spots and single-particle emission dynamics measurement. The luminescence images of the TmUCNPs and MEH-PPV film, analyzed using the localization method, provided two-dimensional (2D) nanoscale distributions of the emissive sites in MEH-PPV surrounding the TmUCNPs. From the 2D map of the emissive sites, we determined the length of excitation energy transfer in the MEH-PPV solid to be ≦55 nanometers.

Achieving optical gain in colloidal quantum dots (QDs) requires band-edge state filling beyond optical transparency, typically requiring multiexciton populations that are nevertheless rapidly depleted by Auger recombination. Electron charging offers a route to reduce this threshold by prefilling the lowest electron states of QDs; however, conventional photochemical charging approaches are not readily applicable to solid-state gain media because they rely on solution-phase redox chemistry. Here, we demonstrate that ZnO–QD mixed nanoparticle assemblies enable tunable electron charging and reduce the optical gain threshold. We study these assemblies in solutions and in the solid state, demonstrating a three-order-of-magnitude increase in charging efficiency in ZnO–QD films. This discrepancy originates from proximity-dependent electron transfer between ZnO nanoparticles and QDs, which we capture using a unified charging model that combines particle ratio and spatial proximity. The resulting proximity-controlled charging enables subexcitonic optical gain and amplified spontaneous emission in ZnO–QD assemblies, offering a route toward low-threshold QD gain media.

Cyanobacteria have produced Earth’s oxygen for 2.4 billion years by adapting to fluctuating irradiance. This adaptation relies on orange carotenoid protein (OCP), which mediates light-intensity– dependent photoprotective energy dissipation using a unique two-photon absorption mechanism. Photon absorption by ground-state OCP (OCP ^O ) generates a metastable intermediate (OCP ^1hν ) that either relaxes thermally or, upon absorption of a second photon within ∼1 s, converts to the active photoprotective state (OCP ^R ). By integrating static and time-resolved crystallography, cryo-EM, computation, spectroscopy and biochemistry, we assign the structure of OCP ^1hν , establish its functional relevance and capture structural snapshots along the OCP ^O →OCP ^1hν and OCP ^1hν →OCP ^R photochemical pathways. We elucidate the molecular mechanism of OCP, which serves as a unique biological circuit breaker protecting the photosynthetic machinery from high light flux.

We have utilized an upconverting nanoparticle (UCNP) with thulium (Tm) ions as emitters and Ytterbium (Yb) ions as sensitizers (TmUCNP) for nanometric photoexcitation of a film of conjugated polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). The TmUCNPs surrounded by a thin layer of MEH-PPV were photoexcited at 976 nm using a continuous-wave (CW) near-infrared laser. The NIR light was selectively absorbed by the sensitizer of the TmUCNPs, leading to the formation of submillisecond-lifetime excited states of Tm ions in the UCNPs through stepwise multiphoton energy transfer between Yb and Tm. The visible emission from the Tm ions, coupled with their long excited-state lifetime, allowed for the production of locally excited states of the MEH-PPV through the excitation energy transfer (EET) from the individual TmUCNPs. This EET was confirmed by spectrally separated wide-field single-particle imaging and single-particle emission dynamics measurement with a hyperspectral confocal microscope. The spectrally separated wide-field images of the TmUCNPs and the MEH-PPV film, analyzed using a 2D localization method, provided a 2D spatial distribution of the emissive trap sites in the MEH-PPV surrounding individual TmUCNPs. From the 2D map of the emissive sites, the length of EET in the MEH-PPV solid was determined to be in the range from several to 50 nm. Thus, nanometric local photoexcitation using single UCNPs enabled direct evaluation of EET length in nano-space.

Recently a novel class of reversible protein photoswitches has been discovered that is based on a charge transfer (CT) complex composed of the flavin cofactor and a substrate‐analogue inhibitor molecule in the family of sarcosine oxidase flavoproteins. Here, excitation of the CT band results in barrierless dissociation of the CT complex on the femtosecond timescale followed by its thermally activated reformation, on the timescale of a few nanoseconds at ambient temperature. The photoreaction is thought to involve a well‐defined isomerization of the inhibitor without its dissociation from the protein. This reaction occurs with an unusually high quantum yield (~80 %), is initiated by absorption in the red part of the visible absorption spectrum, and leads to a photoproduct absorbing in the blue spectral region (negative photochromism). Therefore, this class of photoswitches can be considered a promising template for developing a new class of fast negative photochromic compounds for Life Science applications provided the lifetime of the photoproducts (‘light state’) can be prolonged. Potential future developments will be discussed.

Orange carotenoid protein (OCP) is a photoactive protein that mediates photoprotection in cyanobacteria. OCP binds different ketocarotenoid chromophores such as echinenone (ECN), 3'- hydroxyechinenone (hECN), and canthaxanthin (CAN). In the dark, OCP is in an inactive orange form known as OCP^O; upon illumination, a red active state is formed, referred to as OCP^R, that can interact with the phycobilisome. Large gaps still exist in the mechanistic understanding of the events between photon absorption and formation of the OCP^R state. Recent studies suggested that more than one photon may be absorbed during the photocycle. Using a two-pulse excitation setup with variable time delays between the pulses, we demonstrate that canthaxanthin-functionalized OCP^O forms the OCP^R signature after absorption of a single photon. By contrast, OCP^O complexed with hECN or ECN does not photoconvert to OCP^R upon single photon absorption. Instead, OCP^R is formed only upon absorption of a second photon arriving roughly one second after the first one, implying the existence of a metastable light-sensitive OCP^1hv intermediate. To the best of our knowledge, a sequential 2-photon absorption mechanism in a single biological photoreceptor chromophore is unique. It results in a nonlinear response function with respect to light intensity, effectively generating a threshold switch. In the case of OCP, this prevents down regulation of photosynthesis at low light irradiance.

Photocyclization reactivity of inverse-type diarylethenes strongly depends on the dihedral angle in aryl groups. Especially, the photocyclization reaction in crystal requires a dihedral angle between the thiophene and phenyl rings greater than 81°.

We present iridium(iii) probes for bioorthogonal metabolic labeling, enabling versatile bioimaging applications including super-resolution X-ray fluorescence nanoscopy and photoluminescence lifetime imaging microscopy to advance sialylation studies.