Hydrogen Transfer vs Proton Transfer in 7-Hydroxy-quinoline·(NH3)3: A CASSCF/CASPT2 Study

Protein-activated and FRET enhanced excited-state intermolecular proton transfer fluorescent probes for high-resolution imaging of cilia and tunneling nanotubes in live cells

Excited-state intermolecular proton transfer (inter-ESPT) fluorescent probes responsive to specific bioactive molecules should be greatly promising for protein sensing, DNA mutation simulating and cellular process regulating. However, the inter-ESPT molecules are recessive ESPT fluorophores, which need the assistance of other molecules with both hydrogen-bond accepting and donating abilities to turn on the tautomeric fluorescence. Valid design strategies to create powerful inter-ESPT fluorescent probes are poorly developed, particularly for proteins as targets. We recently reported a unique supramolecular strategy to trigger the inter-ESPT process based on the probe-protein recognition by H-bonding and to image protein-based subcellular structures in live cells. Herein, we found that our inter-ESPT probes (inter-ESPT-01) bearing a 2-amino-3-cyanopyridine scaffold can anchor proteins and light up the "invisible" ESPT state, so as to image the proteins or the protein-based subcellular organelles. More importantly, the inter-ESPT emission of inter-ESPT-01 can be significantly enhanced by the FRET process between amino and imino tautomers, endowing the inter-ESPT-01 probes with super-bright tautomeric fluorescence. The expressed proteins Ecallantide and MarTX were selected as the models to light up the inter-ESPT fluorescence of the probes and revealed that the inter-ESPT process can be triggered by the specific probe-protein recognition events. In the use of the super-bright inter-ESPT fluorescence, not only the proteins, but also the protein-based cilia and tunneling nanotubes (TNTs) can be specifically visualized in living cancer cells. Furthermore, such recognition-driven strategy allows us to construct a unique inter-ESPT probe to track and image specific endogenous proteins in live cells, highlighting the potential of inter-ESPT fluorogens as novel intelligent biomaterials.

Heavy-Atom Quantum Tunnelling in Spin Crossovers of Nitrenes

We simulate two recent matrix-isolation experiments at cryogenic temperatures, in which a nitrene undergoes spin crossover from its triplet state to a singlet state via quantum tunnelling. We detail the failure of the commonly applied weak-coupling method (based on a linear approximation of the potentials) in describing these deep-tunnelling reactions. The more rigorous approach of semiclassical golden-rule instanton theory in conjunction with double-hybrid density-functional theory and multireference perturbation theory does, however, provide rate constants and kinetic isotope effects in good agreement with experiment. In addition, these calculations locate the optimal tunnelling pathways, which provide a molecular picture of the reaction mechanism. The reactions involve substantial heavy-atom quantum tunnelling of carbon, nitrogen and oxygen atoms, which unexpectedly even continues to play a role at room temperature.

A Solvent-Mediated Excited-State Intermolecular Proton Transfer Fluorescent Probe for Fe3+ Sensing and Cell Imaging

Constructing excited-state intermolecular proton transfer (ESIPT-e) fluorophores represents significant challenges due to the harsh requirement of bearing a proton donor-acceptor (D-A) system and their matching proton donating-accepting ability in the same molecule. Herein, we synthesized a new-type ESIPT-e fluorophor (2-APC) using the "four-component one-pot" reaction. By the installing of a cyano-group on pyridine scaffold, the proton donating ability of -NH2 was greatly enhanced, enabling 2-APC to undergo ESIPT-e process. Surprisingly, 2-APC exhibited dual-emissions in protic solvents ethanol and normal fluorescence in aprotic solvents, which is vastly different from that of conventional ESIPT-a dyes. The ESIPT emission can be obviously suppressed by Fe3+ due to the coordination reaction of Fe3+ with the A-D system in 2-APC. From this basis, a highly sensitive and selective method was established using 2-APC as a fluorescent probe, which offers the sensitive detection of Fe3+ ranging from 0 to 13 μM with the detection limit of 7.5 nM. The recovery study of spiked Fe3+ measured by the probe showed satisfactory results (97.2103.4%) with the reasonable RSD ranging from 3.1 to 3.8%. Moreover, 2-APC can also exhibit aggregation-induced effect in poor solvent or solid-state, eliciting strong red fluorescence. 2-APC was also applied to cell-imaging, exhibiting good cell-permeability, biocompatibility and color rendering. This multi-mode emission of 2-APC is significant departure from that of conventional extended p-conjugated systems and ESIPT dyes based on a flat and rigid molecular design. The "one-pot synthesis" strategy for the construction of ESIPT molecules pioneered a new route to achieve tricolor-emissive fluorophores.

Experimental and theoretical investigation of long-wavelength fluorescence emission in push–pull benzazoles: intramolecular proton transfer or charge transfer in the excited state?

ESIPT is disfavoured and charge-transfer emission, prior to ESIPT, seems to be responsible for long-emission wavelengths.

Stepwise vs concerted excited state tautomerization of 2-hydroxypyridine: Ammonia dimer wire mediated hydrogen/proton transfer

The stepwise and concerted excited state intermolecular proton transfer (PT) and hydrogen transfer (HT) reactions in 2-hydroxypyridine-(NH3)2 complex in the gas phase under Cs symmetry constraint and without any symmetry constraints were performed using quantum chemical calculations. It shows that upon excitation, the hydrogen bonded in 2HP-(NH3)2 cluster facilitates the releasing of both hydrogen and proton transfer reactions along ammonia wire leading to the formation of the 2-pyridone tautomer. For the stepwise mechanism, it has been found that the proton and the hydrogen may transfer consecutively. These processes are distinguished from each other through charge translocation analysis and the coupling between the motion of the proton and the electron density distribution along ammonia wire. For the complex under Cs symmetry, the excited state HT occurs on the A″((1)πσ*) and A'((1)nσ*) states over two accessible energy barriers along reaction coordinates, and excited state PT proceeds mainly through the A'((1)ππ*) and A″((1)nπ*) potential energy surfaces. For the unconstrained complex, potential energy profiles show two (1)ππ*-(1)πσ* conical intersections along enol → keto reaction path indicating that proton and H atom are localized, respectively, on the first and second ammonia of the wire. Moreover, the concerted excited state PT is competitive to take place with the stepwise process, because it proceeds over low barriers of 0.14 eV and 0.11 eV with respect to the Franck-Condon excitation of enol tautomer, respectively, under Cs symmetry and without any symmetry constraints. These barriers can be probably overcome through tunneling effect.

Competition between excited state proton and OH− transport via a short water wire: solvent effects open the gate

We investigate the acid–base proton exchange reaction in a microsolvated bifunctional chromophore by means of quantum chemical calculations.

Wagging motion of hydrogen-bonded wire in the excited-state multiple proton transfer process of 7-hydroxyquinoline·(NH3)3 cluster

In this work, the dynamics of hydrogen bonds (as well as the hydrogen-bonded wire) in excited-state tautomerization of 7-hydroxyquinoline·(NH3)3 (7HQ·(NH3)3) cluster has been investigated by using time-dependent density functional theory (TDDFT). It shows that upon an excitation, the hydrogen bond between -OH group in 7-hydroxyquinoline (7HQ) and NH3 moiety would extremely strengthened in S1 state, which could effectively facilitate the releasing of the proton from the phenolic group of 7HQ moiety to the hydrogen-bonded wire and the forming an Eigen-like cationic wire (NH3···NH4(+)···NH3) in the cluster. To fulfill the different optimal angles of NH4(+) in the wire, a wagging motion of hydrogen-bonded wire would occur in excited state. Moreover, the wagging motion of the hydrogen-bonded wire would effectively promote excited-state proton transfer reaction. As the results, an excited-state multiple proton transfer (ESMPT) mechanism containing two concerted and asymmetrical processes has been proposed for the proton transfer dynamics of 7HQ·(NH3)3 cluster.

Chiroptical, linear, and second-order nonlinear optical properties of binaphthol derivatives

We have investigated the chiroptical, linear, and second-order nonlinear optical (NLO) properties of seven binaphthol derivatives and elucidated structure-property relationships from the micromechanism for the first time. The excitation energies, oscillator strengths, and rotational strengths of the 150 lowest energy electron excitations for the most stable conformers have been calculated at TDB3LYP/cc-pVDZ level of theory. The experimental UV-vis absorption energies were reproduced well by our calculations. The simulated circular dichroism (CD) spectra and calculated optical rotation (OR) values are in reasonable agreement with experimental ones. These results demonstrate that TDDFT calculations can not only describe the electron transition property but also can be used to assign the absolute configurations (ACs) of binaphthol derivatives with high confidence. Whereas OR values are more sensitive to the molecular structures than CD spectra. The electron transition property and chiroptical origin have been assigned and analyzed. These derivatives possess remarkably large molecular first hyperpolarizabilities, especially compound 7 which has a value of 241.65 × 10(-30) esu. This value is about 60 times as large as that of highly π-delocalized phenyliminomethyl ferrocene complex. Moreover, compound 6 exhibits pronounced different second-order NLO response values from neutral state to the two cationic states (6(2+) and 6(4+)), which indicates that this compound could act as a potential NLO switch material. The cooperativity of intramolecular charge transfer of the studied compounds was also discussed.