Fluorogenic in-situ Labelling of Gelatin Polymer in Aqueous Solution and Hydrogel

Gelatin polymers made from partially degraded collagen are important biomaterials, but their in-situ analysis suffers from uncontrollable covalent labelling and poor spatial-temporal imaging resolution. Herein, three tetrazolate-tagged tetraphenylethylene fluorophores (TPE-TAs) are introduced for practical fluorogenic labelling of gelatin in aqueous phase and hydrogels. These probes with aggregation-induced emission characteristics offer negligible background and elicit turn-on fluorescence by simply mixing with the gelatin in aqueous phase, giving a detection limit of 0.15 mg/L over a linear dynamic range up to 100 mg/L. This method does not work for collagens and causes minimal interference with gelatin properties. Mechanistic studies reveal a key role for multivalent electrostatic interactions between the abundant basic residues in gelatin (e. g., lysine, hydroxylysine, arginine) and anionic tetrazolate moieties of the lipophilic fluorophore synergistically in spatially rigid macromolecular encapsulation to achieve fluorogenic labelling. The AIE strategy by forming non-covalent fluorophore-gelatin complexes was developed for novel hydrogels that exhibited reversible fluorescence in response to dynamic microstructural changes in the hydrogel scaffold upon salting-in/out treatments, and enabled high spatial-temporal imaging of the fiber network in lyophilized samples. This work may open up avenues for in-situ imaging analysis and evaluation of gelatin-based biomaterials during processes such as in vivo degradation and mineralization.

Ultrafast photophysics of an orange-red thermally activated delayed fluorescence emitter: the role of external structural restraint

The application of thermally activated delay fluorescence (TADF) emitters in the orange-red regime usually suffers from the fast non-radiative decay of emissive singlet states (kSNR), leading to low emitting efficiency in corresponding organic light-emitting diode (OLED) devices. Although kSNR has been quantitatively described by energy gap law, how ultrafast molecular motions are associated with the kSNR of TADF emitters remains largely unknown, which limits the development of new strategies for improving the emitting efficiency of corresponding OLED devices. In this work, we employed two commercial TADF emitters (TDBA-Ac and PzTDBA) as a model system and attempted to clarify the relationship between ultrafast excited-state structural relaxation (ES-SR) and kSNR. Spectroscopic and theoretical investigations indicated that S1/S0 ES-SR is directly associated with promoting vibrational modes, which are considerably involved in electronic-vibrational coupling through the Huang-Rhys factor, while kSNR is largely affected by the reorganization energy of the promoting modes. By restraining S1/S0 ES-SR in doping films, the kSNR of TADF emitters can be greatly reduced, resulting in high emitting efficiency. Therefore, by establishing the connection among S1/S0 ES-SR, promoting modes and kSNR of TADF emitters, our work clarified the key role of external structural restraint for achieving high emitting efficiency in TADF-based OLED devices.

From Stable Radicals to Thermally Robust High-Spin Diradicals and Triradicals

Stable radicals and thermally robust high-spin di- and triradicals have emerged as important organic materials due to their promising applications in diverse fields. New fundamental properties, such as SOMO/HOMO inversion of orbital energies, are explored for the design of new stable radicals, including highly luminescent ones with good photostability. A relation with the singlet-triplet energy gap in the corresponding diradicals is proposed. Thermally robust high-spin di- and triradicals, with energy gaps that are comparable to or greater than a thermal energy at room temperature, are more challenging to synthesize but more rewarding. We summarize a number of high-spin di- and triradicals, based on nitronyl nitroxides that provide a relation between the experimental pairwise exchange coupling constant J/k in the high-spin species vs experimental hyperfine coupling constants in the corresponding monoradicals. This relation allows us to identify outliers, which may correspond to radicals where J/k is not measured with sufficient accuracy. Double helical high-spin diradicals, in which spin density is delocalized over the chiral π-system, have been barely explored, with the sole example of such high-spin diradical possessing alternant π-system with Kekulé resonance form. Finally, we discuss a high-spin diradical with electrical conductivity and derivatives of triangulene diradicals.

Thermally Ultrarobust S = 1/2 Tetrazolinyl Radicals: Synthesis, Electronic Structure, Magnetism, and Nanoneedle Assemblies on Silicon Surface

Open-shell organic molecules, including S = 1/2 radicals, may provide enhanced properties for several emerging technologies; however, relatively few synthesized to date possess robust thermal stability and processability. We report the synthesis of S = 1/2 biphenylene-fused tetrazolinyl radicals 1 and 2. Both radicals possess near-perfect planar structures based on their X-ray structures and density-functional theory (DFT) computations. Radical 1 possesses outstanding thermal stability as indicated by the onset of decomposition at 269 °C, based on thermogravimetric analysis (TGA) data. Both radicals possess very low oxidation potentials <0 V (vs. SCE) and their electrochemical energy gaps, Ecell ≈ 0.9 eV, are rather low. Magnetic properties of polycrystalline 1 are characterized by superconducting quantum interference device (SQUID) magnetometry revealing a one-dimensional S = 1/2 antiferromagnetic Heisenberg chain with exchange coupling constant J'/k ≈ -22.0 K. Radical 1 in toluene glass possesses a long electron spin coherence time, Tm ≈ 7 μs in the 40-80 K temperature range, a property advantageous for potential applications as a molecular spin qubit. Radical 1 is evaporated under ultrahigh vacuum (UHV) forming assemblies of intact radicals on a silicon substrate, as confirmed by high-resolution X-ray photoelectron spectroscopy (XPS). Scanning electron microscope (SEM) images indicate that the radical molecules form nanoneedles on the substrate. The nanoneedles are stable for at least 64 hours under air as monitored by using X-ray photoelectron spectroscopy. Electron paramagnetic resonance (EPR) studies of the thicker assemblies, prepared by UHV evaporation, indicate radical decay according to first-order kinetics with a long half-life of 50 ± 4 days at ambient conditions.

Synthesis of stable class 5 mesoionic benzo[c]tetrazolo[2,3-a]cinnolinium thiolate, dicyanomethylide, and amides

Although class 5 mesoionic compounds show interesting electrical behaviour, they are generally unstable, readily undergoing ring-opening reactions. We designed and synthesized a bridged mesoionic derivative, benzo[c]tetrazolo[2,3-a]cinolinium (BTC), as a stable class 5 mesoionic compound, which was further transformed to the corresponding thiolate, cicyanomethylide, and amide. The intramolecular bridging imparted stability to the BTC thiolates and amides: the BTC thiolates were unsusceptible to ring-opening at high temperatures, and the BTC amides were stable in the absence of electron-withdrawing groups on the amide nitrogen. The properties of the BTC thiolate were compared with those of 2,3-diphenyltetrazolium derivatives based on UV-Vis absorption spectroscopy, single-crystal X-ray diffraction and quantum calculations.

Ultrafast transient absorption and solvation of a super-photoacid in acetoneous environments

The phenomenon of photoacidity, i.e., an increase in acidity by several orders of magnitude upon electronic excitation, is frequently encountered in aromatic alcohols capable of transferring a proton to a suitable acceptor. A promising new class of neutral super-photoacids based on pyranine derivatives has been shown to exhibit pronounced solvatochromic effects. To disclose the underlying mechanisms contributing to excited-state proton transfer (ESPT) and the temporal characteristics of solvation and ESPT, we scrutinize the associated ultrafast dynamics of the strongest photoacid of this class, namely tris(1,1,1,3,3,3-hexafluoropropan-2-yl)8-hydroxypyrene-1,3,6-trisulfonate, in acetoneous environment, thereby finding experimental evidence for ESPT even under these adverse conditions for proton transfer. Juxtaposing results from time-correlated single-photon counting and femtosecond transient absorption measurements combined with a complete decomposition of all signal components, i.e., absorption of ground and excited states as well as stimulated emission, we disclose dynamics of solvation, rotational diffusion, and radiative relaxation processes in acetone and identify the relevant steps of ESPT along with the associated time scales.

Monitoring the photochemistry of a formazan over 15 orders of magnitude in time

2,3,5-triphenyltetrazolium chloride (TTC) may convert into phenyl-benzo[c]tetrazolocinnolium chloride (PTC) and 1,3,5-triphenylformazan (TPF) under irradiation with light. The latter reaction, albeit enzymatically rather than photochemically, is used in so-called TTC assays indicating cellular respiration and cell growth. In this paper, we address the photochemistry of TPF with time-resolved spectroscopy on various time scales. TPF is stabilized by an intramolecular hydrogen bond and switches photochemically via an E-Z isomerization around an N=N double bond into another TPF-stereoisomer, from which further isomerizations around the C=N double bond of the phenylhydrazone group are possible. We investigate the underlying processes by time-resolved spectroscopy in dependence on excitation wavelength and solvent environment, resolving several intermediates over a temporal range spanning 15 orders of magnitude (hundreds of femtoseconds to hundreds of seconds) along the reaction path. In a quantum-chemical analysis, we identify 16 stable ground-state isomers and discuss which ones are identified in the experimental data. We derive a detailed scheme how these species are thermally and photochemically interconnected and conclude that proton transfer processes are involved.

Sensitivity of Isomerization Kinetics of 1,3,5-Triphenylformazan on Cosolvents Added to Toluene

Formazan molecules exhibit photochromism because isomerization processes following excitation may occur in both the azo group and the hydrazone group; thus, each formazan may be present in various forms with different colors. The ratio of these forms depends on the illumination conditions and the environment of the formazan with a most incisive sensibility of the thermal anti-syn relaxation of the C═N toward slight traces of impurities in toluene solutions, as reported most prominently for 1,3,5-triphenylformazan. Here, we study the latter compound with transient absorption spectroscopy to investigate the role of these traces by adding small amounts of both protic and aprotic cosolvents. Whereas the activation barrier decreases if the binary solvent mixture has a higher polarity, the role of hydrogen bonding can have a reverse impact on the thermal isomerization rate. Both the addition of an aprotic cosolvent and the addition of a protic cosolvent can slow the reaction due to their hydrogen-bond accepting and hydrogen-bond donating properties, respectively. In the case of methanol as a cosolvent, this effect outweighed that of the polarity increase for small concentrations, which was not observed for the fluorinated alcohol hexafluoroisopropanol. The results are explained in the context of a competition between solute-cosolvent and cosolvent-cosolvent hydrogen bonding.

Bridge-Mediated Charge Separation in Isomeric N-Annulated Perylene Diimide Dimers

The possibility and rate of charge separation (CS) in donor-bridge-acceptor molecules mainly depend on two factors: electronic coupling and solvent effects. The question of how CS occurred in two identical chromophores is fundamental, as it is particularly interesting for potential molecular electronics applications and the photosynthetic reaction centers (RCs). Conjugated bridge definitely plays a crucial role in electronic coupling. To determine the bridge-mediated charge separation dynamics between the two identical chromophores, the isomeric N-annulated perylene diimide dimers (para-BDNP and meta-BDNP) with different conjugated bridge structures have been comparatively investigated in different solvents using femtosecond transient absorption spectra (fs-TA). It is found that the charge separation is disfavored in weak polar solvent, whereas direct spectroscopic signatures of radicals are observed in polar solvents, and the rate of charge separation increases as the solvent polarity increasing. To our surprise, the rate of charge separation in m-BDNP is more than an order of magnitude slower than that in p-BDNP, although there is a larger negative ΔG_CS in m-BDNP. The slow CS rate that occurred in m-BDNP mainly results from the intrinsic destructive interference of the wave function through the meta-substituted bridge. The roles of solvent effects in free energy and electronic coupling for charge separation are further identified with quantum calculations.

How a linear triazene photoisomerizes in a volume-conserving fashion

Understanding deactivation mechanisms of functional groups is a key step to design novel photo-active devices and molecular imaging agents. Here, we elucidate the photochemistry of linear triazenes, an extended analogue of the photo-switchable azo group, exemplarily for the widely used DNA-minor-groove binder berenil. Combining ultrafast spectroscopy and ab initio calculations unveils that the E-azo,s-trans structure of berenil predominates in the gas phase and in aqueous solution, and ADC(2) intrinsic reaction coordinate calculations disclose that the excited-state relaxation to the S1 minima/conical intersections follows a two-step mechanism: N[double bond, length as m-dash]N bond stretching followed by a bicycle-pedal rotation in the triazene bridge. Furthermore, studying the ground-state pathways shows that a fraction of the molecules relaxes back to the E-azo,s-trans isomer while the other part photoisomerizes to the Z-azo,s-trans via a hula-twist motion, as evidenced by experimental quantum yields of Φ ≈ 0.5 found for berenil in water, ethylene glycol, or bound to β-trypsin. Moreover, our studies show that while the excited-state relaxation is insensitive to the environment, the ground-state dynamics depend on biomolecular binding partners.