A TD-DFT molecular screening for fluorescence probe based on excited-state intramolecular proton transfer of 2’-hydroxychalcone derivatives

TDDFT study on a fluorescent probe for distinguishing analogous thiols based on smiles rearrangement

The complete excited-state sensing mechanism of a fluorescent probe capable of distinguishing cysteine/homocysteine and glutathione from analogous biological thiols has been investigated. Using a TDDFT method, the nature of the fluorescence differences in the detection of thiols by the probe has been explained at the molecular level. Calculation results imply that the probe undergoes photoinduced electron transfer (PET) from the fluorophore to the nitrobenzooxadiazole (NBD)-based acceptor in the excited state. In the presence of a thiol, the NBD moiety is cleaved and the red fluorescence emission of the fluorophore is enhanced through inhibition of the PET process. The sulfur-substituted NBD-thiol product is predisposed to undergo excited-state torsion, leading to fluorescence quenching. However, for cysteine and homocysteine, their appropriate distances lead to Smiles rearrangements with relatively low activation energies (26.60 kJ/mol and 42.94 kJ/mol, respectively) and the emission of a distinct green fluorescence at ambient temperature. It has been theoretically confirmed that the distance between two reactive sites, such as sulfhydryl and amino moieties, can be used to distinguish different thiols, thus providing rational support for the control of fluorescence activity and the design of probe molecules.

An aggregation induced emission chalcone fluorescent probe with large Stokes shift for biothiols detection

The homeostasis of biothiols is closely related to the health of organisms. In view of the important role of biothiols, a fluorescent probe (7HIN-D) for the detection of intracellular biothiols was developed based on a simple chalcone fluorophore 7HIN with "ESIPT + AIE" characteristics. The probe 7HIN-D was obtained by introducing a biothiols specific DNBS (2,4-dinitrobenzenesulfonyl) unit as a fluorescence quencher to the fluorophore 7HIN. The nucleophilic substitution reaction between biothiols and probe 7HIN-D will release the DNBS unit and the fluorophore 7HIN, which exhibits a "turn on" AIE fluorescence with a large Stokes shift of 113 nm. The probe 7HIN-D displays high sensitivity and good selectivity to biothiols, and the detection limits value of probe 7HIN-D for GSH, Cys and Hcy were 0.384 μmol/L, 0.471 μmol/L and 0.638 μmol/L, respectively. In addition, the probe has been successfully used for fluorescence detection of endogenous biothiols in living cells due to its excellent performance, good biocompatibility and low cytotoxicity.

Theoretical Investigation on the ESIPT Process and Detection Mechanism for Dual-Proton Type Fluorescent Probe

Recently, a new fluorescent probe AE-Phoswas reported to detect the activity of alkaline phosphatases (ALP) in different living cell lines. Here, we present an in-depth computational analysis of the mechanism and source of the fluorescence of the AE-Phos probe. There is an intermediate product (AE-OH-Phos) in the experiment as well as a different configuration of products that may emit fluorescence. It is essential to investigate the origin of fluorescence and the detection mechanism of the probe, which could help us eliminate the interference of other substances (including an intermediate product and possible isomers) on fluorescence during the experiment. According to the change of geometric parameters and Infrared spectra, we deduce that the dual intramolecular hydrogen bonds of salicylaldehyde azine (SA) were enhanced at the excited state, while AE-OH-Phos was attenuated. Considering the complex ESIPT behavior of the dual proton-type probe, the potential energy surfaces were further discussed. It can be concluded that the single proton transfer structure of SA (SA-SPT) is the most stable form. Both the concerted double proton transfer process and stepwise single proton transfer process of SA were forbidden. The fluorescence for SA was 438 nm, while that of SA-SPT was 521 nm, which agrees with the experimentally measured fluorescence wavelength (536 nm). The conclusion that single proton transfer occurs in SA is once again verified. In addition, the distribution of electron-hole and relative index was analyzed to investigate the intrinsic mechanism for the fluorescence quenching of the probe and the intermediate product. The identification of the origin of fluorescence sheds light on the design and use of dual-proton type fluorescent probes in the future.

Efficient Discovery of Visible Light-Activated Azoarene Photoswitches with Long Half-Lives Using Active Search

Photoswitches are molecules that undergo a reversible, structural isomerization after exposure to certain wavelengths of light. The dynamic control offered by molecular photoswitches is favorable for materials chemistry, photopharmacology, and catalysis applications. Ideal photoswitches absorb visible light and have long-lived metastable isomers. We used high-throughput virtual screening to predict the absorption maxima (λ_max) of the E-isomer and half-life (t_1/2) of the Z-isomer. However, computing the photophysical and kinetic stabilities with density functional theory of each entry of a virtual molecular library containing thousands or millions of molecules is prohibitively time-consuming. We applied active search, a machine-learning technique, to intelligently search a chemical search space of 255 991 photoswitches based on 29 known azoarenes and their derivatives. We iteratively trained the active search algorithm on whether a candidate absorbed visible light (λ_max > 450 nm). Active search was found to triple the discovery rate compared to random search. Further, we projected 1962 photoswitches to 2D using the Uniform Manifold Approximation and Projection algorithm and found that λ_max depends on the core, which is tunable by substituents. We then incorporated a second stage of screening to predict the stabilities of the Z-isomers for the top candidates of each core. We identified four ideal photoswitches that concurrently satisfy the following criteria: λ_max > 450 nm and t_1/2 > 2 h.These candidates had λ_max and t_1/2 range from 465 to 531 nm and hours to days, respectively.

Molecular design of amino-type hydrogen-bonding molecules for excited-state intramolecular proton transfer (ESIPT)-based fluorescent probe using the TD-DFT approach

A molecular screening of a new series of NH-type molecules for ESIPT-based fluorescent probes has been carried out using time-dependent density. functional theory.