Theoretical Investigation of Excited-State Intramolecular Proton Transfer and Photoisomerization of 2-(Iminomethyl)phenol

Chemical bond reorganization in intramolecular proton transfer revealed by ultrafast X-ray photoelectron spectroscopy

Time-resolved X-ray photoelectron spectroscopy (TR-XPS) is used in a simulation study to monitor the excited state intramolecular proton transfer between oxygen and nitrogen atoms in 2-(iminomethyl)phenol. Real-time monitoring of the chemical bond breaking and forming processes is obtained through the time evolution of excited-state chemical shifts. By employing individual atomic probes of the proton donor and acceptor atoms, we predict distinct signals with opposite chemical shifts of the donor and acceptor groups during proton transfer. Details of the ultrafast bond breaking and forming dynamics are revealed by extending the classical electron spectroscopy chemical analysis to real time. Through a comparison with simulated time-resolved photoelectron spectroscopy at the valence level, the distinct advantage of TR-XPS is demonstrated thanks to its atom specificity.

Fluorescent adenine analogues with ESPT characteristic utilized for real-time detecting DNA adduct

The 8-oxo-7,8-dihydro-2'-deoxyguanine (8-oxoG) is the representative damaged nucleoside that may increase the risk of developing diseases. Accordingly, the selective detection of 8-oxoG in DNA with minimal disturbance to the native structure is important to have an in-depth understanding of the formation mechanism and becomes an attractive tool for genomic research. To identify the DNA adduct in real-time efficiently, a series of quasi-intrinsic optical probes are performed based on the natural adenine, which has preference to form a stable base pair with 8-oxoG in the syn conformation. The calculations revealed that the A-analogues in solution could bring red-shifted absorption spectra and bright photoluminescence arisen from the additional π-conjugation by means of fluorophore modification and the ring expansion. Especially, A1 possesses large Stokes shifts and the highest fluorescence intensity in emission, which is proposed as the biosensor to monitor the optical changes in the presence and absence of the considered 8-oxoG. It is found that the fluorescence is insensitive to base pairing with thymine, while the excited state intermolecular proton transfer (ESPT) induced efficient fluorescence quenching is observed upon pairing with the 8-oxoG. To evaluate the direct usefulness of the bright adenine analogues in biological environment, we further examined the influences of linking deoxyribose on the absorption and emission, which are consistent with the experimental data.

Sensing mechanism of fluorescent sensor to Cu2+ based on inhibiting ultra-fast intramolecular proton transfer process

A novel and efficient chemosensor 1 for detecting Cu²⁺ has recently been developed. However, the photophysical properties of chemosensor 1 and its response mechanism to Cu²⁺ are still unclear. Herein, the density functional theory and the time-dependent density functional theory approaches are implemented to investigate the excited state behavior of chemosensor 1 and its sensing mechanism for Cu²⁺ is revealed. Through constructing the potential energy curve with the dihedral angle of hydroxide radical as a variable, the irreversibility of the adjustment of the hydrogen proton direction is determined. This feature provides a favorable geometric configuration condition for the formation of intramolecular hydrogen bond. Moreover, the reduced density gradient analysis and topological analysis are performed to visualize the hydrogen bond strength, it is found that the hydrogen bond is enhanced in first singlet excited state (S₁) compared with that in ground state (S₀). The chemosensor 1 has only a low potential barrier in the S₁ state, indicating that it could undergo an ultra-fast excited state intramolecular proton transfer (ESIPT) process. Furthermore, the reaction sites of chemosensor 1 and Cu²⁺ is theoretically predicted by the electrostatic potential analysis and the coordination mode of 1 + Cu²⁺-H⁺ is confirmed. Thus, we verify that the deprotonation inhibits the ESIPT behavior and leads to fluorescence quenching to achieve the recognition of chemosensor 1 to Cu²⁺. In addition, the binding energy of Cu²⁺ with chemosensor 1 is greater than that of Mg²⁺ and Zn²⁺, the high selectivity of chemosensor 1 to Cu²⁺ is illustrated. Our investigation clarifies the sensing mechanism of chemosensor 1 to Cu²⁺ based on inhibiting ultra-fast ESIPT process, which provides a theoretical basis for the development of new metal ion sensors.