Tatu Kumpulainen

Dual-component Fmoc-Phe- and Fmoc-Leu-based hydrogels exhibit tunable mechanical properties and thermoresponsiveness. The antibacterial activity against Gram-positive and Gram-negative pathogens can be tuned by varying the building-block ratio.

In photoactive proteins, coupling between the chromophore and protein matrix is exquisitely tuned. Proton transfer reactions can mediate this coupling, as in proton-coupled electron transfer and excited-state proton transfer. Additional mechanisms involving proton dislocations may exist but remain undiscovered. Here, we present a femtosecond crystallographic movie of the phytochrome from Deinococcus radiodurans . The structures reveal a space-conserving mechanism for rotation of the D-ring in the excited state. We observe rearrangement of a conserved hydrogen bond network within 300 fs, which precedes the isomerization reaction of the chromophore. Aided by molecular modeling and independently confirmed by femtosecond infrared spectroscopy, we attribute these changes to a protonation shift of the strictly conserved histidine-260. Although this histidine lies close to the photoexcited π-orbitals of the chromophore, it is not directly part of them. We propose that this “remote-controlled” proton transfer relays photoexcitation near-instantaneously to the protein matrix. This mechanism may be widely used to transduce cofactor signals to their hosting enzymes.

Excited-state proton transfer (ESPT) in aprotic organic solvents has received limited attention due to their inability to accept protons. However, bimolecular ESPT from a photoacid to an organic base in such media enables systematic studies on the influence of macroscopic solvent parameters on the ESPT as demonstrated in this work. The full photocycle starting from initial deprotonation in a hydrogen-bonded donor–acceptor complex to full dissociation in the excited state followed by slow recombination in the ground state was characterized by various spectroscopic methods in solvent mixtures of varying polarity. The initial deprotonation producing contact ion pairs is ultrafast (sub-100 fs) and requires minimal solvent reorganization. The contact ion pairs dissociate via a distinct intermediate, the so-called solvent-separated ion pair, preceding the fully dissociated free ion pairs. The time scale of the ion pair dynamics is dominated by viscosity whereas the yield is determined by the polarity. In low polarity solvents (ε_r < 10), the population is trapped as solvent-separated ion pairs and full dissociation becomes operative only at intermediate polarity. Ground-state recombination of the intermediate ion pair species is fast and thus a significant population of fully dissociated ground-state ions is produced only above intermediate polarities.

To date, there is still a lack of suitable methods for chemical modification of graphene surfaces in situ. Photocleavable compounds have great potential for in situ modification of chemical species with visible light, leaving desired chemical functionalities on the surface. Green light‐sensitive boron dipyrromethene (BODIPY) derivatives can release diverse cargos, including biologically important amines. In this study, green light‐sensitive photocleavable BODIPY derivatives with different aromatic anchors for surface binding were designed and synthesized with the ultimate goal of selective graphene surface patterning. The effects of different aromatic anchor groups on photocleaving capability, their structural and photophysical properties and photocleavage in different conditions were studied. Photocleavage was significantly faster, and more controllable photoreaction was achieved in inert conditions than in the presence of oxygen (ambient air). In addition, pyrene as an anchor lowered fluorescence quantum yield and intensities of absorption and emission maxima of the entire BODIPY derivative, making it a poorer fluorophore than the derivatives with the naphthalene and phenyl anchors.

Restricting intramolecular movements by increasing viscosity, encapsulation or intercalation leads to aggregation-induced fluorescence emission in semicroconaines, non-fluorescent analogs of squaraines, making them excellent turn-on probes.

Time‐resolved fluorescence techniques are essential for studying the excited‐state dynamics of molecules and materials, as well as their interactions with the surrounding media. Fluorescence upconversion provides femtosecond time resolution, but its limited spectral coverage often restricts its application. Broadband variants of fluorescence upconversion spectroscopy (FLUPS) offer improved spectral coverage but require balancing spectral bandwidth against signal strength. In this paper, we introduce a three‐angle method (3‐AM) for FLUPS designed to overcome these limitations. This method enhances spectral coverage, photometric accuracy, and signal‐to‐noise ratio by recording and averaging spectrally resolved upconversion signals at three distinct crystal angles. Unlike other techniques that rely on spectral reconstruction or continuous crystal rotation during acquisition, the 3‐AM relies on fixed crystal positions, allowing for robust post‐acquisition corrections. We validate the performance and reproducibility of the 3‐AM by comparing it with the conventional fixed‐angle approach, as well as through blind testing in two independent laboratories. Despite variations in experimental configuration, the 3‐AM consistently produces reproducible spectral line shapes across the two setups. The enhanced performance and reliability demonstrate its practicality for laboratories with broadband FLUPS capabilities. Thus, the 3‐AM is a valuable tool for investigating ultrafast radiative processes in previously inaccessible molecular and material systems.

The 32nd edition of the Jyväskylä Summer School (JSS32) was in high demand for the ultrafast spectroscopy chapter. On August 7–11 2023 nearly 40 students gathered to Jyväskylä to tune in for the JSS32 course entitled UltrafastSpectroscopy: phenomena, experiments and data analysis methods. The course was coordinated by Senior Lecturer Tatu Kumpulainen and featured lectures each morning from two invited speakers, Prof. Eric Vauthey, University of Geneva, Switzerland, and Assoc. Prof. Jens Uhlig, Lund University, Sweden. In addition to the lectures, the course featured four workshops in the afternoons aimed at introducing the participants to the practical aspects of steady-state and time-resolved spectroscopies. Besides the scientific content, JSS32 offered several social events where participants could network with other participants of the whole summer school.

Understanding the dynamics of Förster resonance energy transfer (FRET) in fluorophore-functionalized nanomaterials is critical for developing and utilizing such materials in biomedical imaging and optical sensing applications. However, structural dynamics of noncovalently bound systems have a significant effect on the FRET properties affecting their applications in solutions. Here, we study the dynamics of the FRET in atomistic detail by disclosing the structural dynamics of the noncovalently bound azadioxotriangulenium dye (KU) and atomically precise gold nanocluster (Au₂₅(p-MBA)₁₈, p-MBA = para-mercaptobenzoic acid) with a combination of experimental and computational methods. Two distinct subpopulations involved in the energy transfer process between the KU dye and the Au₂₅(p-MBA)₁₈ nanoclusters were resolved by time-resolved fluorescence experiments. Molecular dynamics simulations revealed that KU is bound to the surface of Au₂₅(p-MBA)₁₈ by interacting with the p-MBA ligands as a monomer and as a π-π stacked dimer where the center-to-center distance of the monomers to Au₂₅(p-MBA)₁₈ is separated by ∼0.2 nm, thus explaining the experimental observations. The ratio of the observed energy transfer rates was in reasonably good agreement with the well-known 1/R⁶ distance dependence for FRET. This work discloses the structural dynamics of the noncovalently bound nanocluster-based system in water solution, providing new insight into the dynamics and energy transfer mechanism of the fluorophore-functionalized gold nanocluster at an atomistic level.

Experiment and theory reveal similarities and differences between porphycenes substituted at the meso position with amino and nitro groups.