Ultrafast excited state intramolecular proton transfer (ESIPT) mechanism for 2,6-bis(benzothiazolyl-2-yl)phenol: A theoretical investigation

Tactfully regulating the ESIPT and TICT mechanism in the AIE-active multifunctional triphenylamine Schiff-base compound (TPASB) by methyl substitution

The triphenylamine Schiff-base (TPASB) with dual proton transfer sites (N1…H1-O1 [R1] and N2…H2-O2 [R2]), which is crucial in the field of optoelectronic materials. Herein, a novel molecular design strategy for preparing of TPASB-1 and TPASB-2 via the selective methylation of the hydroxyl group at the R2 or R1 position was proposed. The analysis of electronic structures and potential energy surfaces revealed that a single excited state intramolecular proton transfer (ESIPT) process of TPASB occurs only at R1. Nevertheless, the ESIPT process of TPASB-2 was successfully turned on at R2. More noteworthy is that compared to TPASB, the methylation of hydroxyl group at the R2 position triggers the TICT process of TPASB-1, effectively reducing the potential barrier of ESIPT at the R1 position. This theoretical study explains the role of the substituent effect in regulating ESIPT behaviour, and provides valuable guidance for synthesising efficacious ESIPT-active compounds.

Modulating the ESIPT Mechanism and Luminescence Characteristics of Two Reversible Fluorescent Probes by Solvent Polarity: A Novel Perspective

As reversible fluorescent probes, HTP-1 and HTP-2 have favourable applications for the detection of Zn2+ and H2S. Herein, the impact of solvent on the excited-state intramolecular proton transfer (ESIPT) of HTP-1 and HTP-2 was comprehensively investigated. The obtained geometric parameters and infrared (IR) vibrational analysis associated with the intramolecular hydrogen bond (IHB) indicated that the strength of IHB for HTP-1 was weakened in the excited state. Moreover, structural torsion and almost no ICT behaviour indicated that the ESIPT process did not occur in HTP-1. Nevertheless, when the 7-nitro-1,2,3-benzoxadiazole (NBD) group replaced the H atom, the IHB strength of HTP-2 was enhanced after photoexcitation, which inhibited the twisting of tetraphenylethylene, thereby opening the ESIPT channel. Notably, hole-electron analysis and frontier molecular orbitals revealed that the charge decoupling effect was the reason for the fluorescence quenching of HTP-2. Furthermore, the potential energy curves (PECs) revealed that HTP-2 was more inclined to the ESIPT process in polar solvents than in nonpolar solvents. With a decrease in solvent polarity, it was more conducive to the ESIPT process. Our study systematically presents the ESIPT process and different detection mechanisms of the two reversible probe molecules regulated by solvent polarity, providing new insights into the design and development of novel fluorescent probes.

Exploring the effect of nitrile substituent position on fluorescence quantum yield of ESIPT-based oxazoline derivatives: A TDDFT investigation

We explore the mechanism specifically on quantum yields difference of 2-(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)-3-hydroxybenzonitrile (1-CN) and 4-(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)-3-hydroxybenzonitrile (3-CN) by density functional theory and time-dependent density functional theory within the Tamm-Dancoff approximation. The structures optimization and the potential energy curves scanning of singlet excited state directly prove that the excited state intramolecular proton transfer (ESIPT) can take place in 1-CN and 3-CN molecules. The calculated spectra show that the fluorescence peaks of two molecules come from the emission of keto* configuration. The non-covalent interaction and the atomic dipole moment corrected Hirshfeld charge are also analyzed. Through the comparison of emission oscillator strength between 1-CN and 3-CN molecules suggests that the radiative transition process is not the main reason for the difference on quantum yields. Internal conversion process is also excluded on account of the large energy gap between S₀ and S₁. Considering the interaction between singlet and triplet states, both molecules can undergo intersystem crossing. The prominent difference is that, compared with 3-CN, the larger spin-orbit coupling constant and smaller energy level difference promote the intersystem crossing process of 1-CN. This provides direct evidence for the fluorescence quantum yield of 1-CN is lower than that of 3-CN. We envision that the present work can provide help for the synthesis and application of ESIPT compounds with high quantum yields.

Solvent polarity dependent ESIPT behavior for the novel flavonoid-based solvatofluorochromic chemosensors

In this work, we explore the excited-state intramolecular proton transfer (ESIPT) mechanisms and relative solvent effects for three novel 3-hydroxylflavone derivatives (i.e., HOF, SHOF, and NSHOF) in acetonitrile, dichloromethane, and toluene solvents. Through calculations, we optimize the structures of HOF, SHOF, and NSHOF. Through the analysis of a series of structural parameters related to hydrogen bonding interactions, it could be found that the hydrogen bonds of the three derivatives are all enhanced in the S₁ state, and more importantly, the excited-state hydrogen bonds of HOF are stronger than those of SHOF and NSHOF. In order to explore the effects of solvent polarity, we analyze the core-valence bifurcation (CVB) index, infrared (IR) vibration spectrum, and the potential energy curves. We find that for HOF, SHOF, and NSHOF, the strength of the excited-state hydrogen bonds increases as the solvent polarity decreases. The solvent polarity dependent ESIPT mechanisms pave the way for further designing novel flavonoid-based solvatofluorochromic probes in future.

Effects of azole rings with different chalcogen atoms on ESIPT behavior for benzochalcogenazolyl-substituted hydroxyfluorenes

ESIPT behavior has attracted a lot of eyes of researchers in recent years because of its unique optical properties. Due to its large Stokes shift and double emission fluorescence, white light can be generated in the fluorophore based on the excited state intramolecular proton transfer (ESIPT) principle. The excited state proton transfer behavior of hydroxylated benzoxazole (BO-OH), benzothiazole (BS-OH) and benzoselenazole (BSe-OH) have been investigated in heptane, chloroform and DMF solvents. By comparing the infrared vibration spectra and the variation of bond parameters from the S₀ to S₁ states, and analyzing the frontier molecular orbitals, the influence of hydrogen bond dynamics, the solvent polarity, charge redistribution and the effects of different proton acceptors on proton transfer were observed. The only structural difference among the three substituted hydroxyfluorenes is the heteroatom in the azole ring (oxygen, sulfur and selenium, respectively). We have scanned the potential energy curve of the ESIPT process, and compared the potential barrier, it is found that the heavier chalcogen atoms are more favorable for proton transfer. At the same time, the potential application of changing heteroatoms in the azole ring by walking down the chalcogenic group in crystal luminescence color regulation is also discussed.

Ultrafast nonadiabatic excited-state intramolecular proton transfer in 3-hydroxychromone: A surface hopping approach

We employ the ab initio molecular dynamics within the surface hopping method to explore the excited-state intramolecular proton transfer taking place on the coupled "bright" S₁ (ππ*) and "dark" S₂ (nπ*) states of 3-hydroxychromone. The nonadiabatic population transfer between these states via an accessible conical intersection would open up multiple proton transfer pathways. Our findings reveal the keto tautomer formation via S₁ on a timescale similar to the O-H in-plane vibrational period (<100 fs). Structural analysis indicates that a few parameters of the five-membered proton transfer geometry that constitute the donor (hydroxyl) and acceptor (carbonyl) groups would be adequate to drive the enol to keto transformation. We also investigate the role of O-H in-plane and out-of-plane vibrational motions in the excited-state dynamics of 3-hydroxychromone.

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.