Research on the mechanism of proton transfer in regulating the fluorescence properties of organic molecules

Different competition mechanism between ESPT and TICT process regulated by protic and aprotic solvent in DHP

Unveiling the effect of solvent polarity on the excited state intramolecular proton transfer mechanism of new 3-hydroxy-4-pyridylisoquinoline compound.

The molecule with excited-state intramolecular proton transfer (ESIPT) has wide applications in fluorescent probe, biology imaging, light-emitting materials, etc. Biologically active oxygen hypochlorite (HClO) exists widely in the biological and chemical environment, which can pose a great threat to human health. Design of HClO-sensitive molecules in solvents is very important. Recently, Wu et al. [Wu L L, Yang Q Y, Liu L Y, et al. <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1039/C8CC03717E">2018 <i>Chem. Commun.</i> <b>54</b> 8522</ext-link>] designed an ESIPT-based HBT-OMe probe molecule, which can detect HClO due to its methoxy-hydroxy-benzothiazole. They found that the fluorescence intensity of the system gradually increases with HClO increasing. However, the microscopic mechanism of this highly efficient fluorescent probe is not well understood. Therefore, in this work, we theoretically investigate the ESIPT mechanism of the HBT-Ome and its product molecule by using density functional theory and time-dependent density functional theory. Based on polarizable continuum model (PCM) with the integral equation formalism variant (IEFPCM) and Becke’s three-parameter hybrid exchange function with the Lee-Yang-Parr gradient-corrected functional (B3LYP) as well as the TZVP basis, the optimized structures are obtained. The structures show that the HBT-Ome product molecules tend to undergo proton transfer in the excited state but HBT-OMe molecules cannot undergo the proton transfer process. The analysis of frontier molecular orbitals not only explains the reason why the fluorescence of the HBT-Ome product is enhanced, but also demonstrates that the HBT-Ome fluorescence intensity is diminished owing to twisted intramolecular charge transfer in the excited state. It is twisted intramolecular charge transfer that leads smaller charge density to be overlapped and the fluorescence intensity of HBT-OMe molecule to be further weakened. Infrared vibrational spectrum shows the enhancement of intramolecular hydrogen bond of O—H, which indicates the tendency of proton transfer. The molecular covalent interaction analysis shows that the intramolecular interactions of HBT-OMe remain largely unchanged clearly. The intramolecular O—H bonding interaction is weakened, and the N—H bonding interaction is increased for HBT-OMe product molecule. The enhancement of intramolecular hydrogen bond of N—H further illustrates the trend of proton transfer. The calculated potential energy curve provides direct evidence for the occurrence of ESIPT in the HBT-Ome product molecule. Our work is of great significance in designing and synthesizing the HClO fluorescent probes based on ESIPT molecules.

Density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods are adopted to explore the ground‐state and excited‐state intramolecular double hydrogen bonding interactions as well as the excited state intramolecular proton transfer (ESIPT) mechanism for 5,5′‐(9,9‐dihexyl‐9H‐fluorene‐2,7‐diyl)bis(2‐benzo[d]thiazol‐2‐yl)phenol) (abbreviated as Ia) system. The simulated electronic spectra of the Ia system (ie, absorption and fluorescence spectra) are reappeared by experimental results, which reveals the reasonability and correctness of the calculated theory adopted in this work. We firstly verify that the dual intramolecular hydrogen bonds of Ia should be enhanced in the first excited state via geometrical parameters (ie, bond lengths and bond angles) and infrared (IR) vibrational spectra. Then, insights into the photo‐excitation aspects, we confirm that the charge redistribution facilitates the ESIPT tendency. Particularly, the increased electronic densities around proton acceptors could play important roles in attracting proton H2 and H5 atoms for the Ia system. At last, via constructing potential energy surfaces (PESs), we clearly clarify the excited state intramolecular single proton transfer mechanism for the Ia system. This work not only clarifies the detailed ESIPT mechanism for the novel Ia system and makes up for the deficiencies in previous experiment but also promotes a deeper understanding about the excited state behaviors involved in multiple hydrogen bonding interactions.

Uncovering photo-induced hydrogen bonding interaction and proton transfer mechanism for the novel salicylaldehyde azine derivative with para-position electrophilic cyano group

Time-resolved fluorescence spectroscopy for studying stepwise excited-state intramolecular double proton transfer

Owing to the importance of excited state dynamical relaxation, the excited state intramolecular proton transfer (ESIPT) mechanism for a novel compound containing dual hydrogen bond (abbreviated as “1-enol”) is studied in this work. Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) method, the experimental electronic spectra can be reproduced for 1-enol compound. We first verify the formation of dual intramolecular hydrogen bonds, and then confirm that the dual hydrogen bond should be strengthened in the first excited state. The photo-excitation process is analyzed by using frontier molecular orbital (HOMO and LUMO) for 1-enol compound. The obvious intramolecular charge transfer (ICT) provides the driving force to effectively facilitate the ESIPT process in the S1 state. Exploration of the constructed S0-state and S1-state potential energy surface (PES) reveals that only the excited state intramolecular single proton transfer occurs for 1-enol system, which makes up for the deficiencies in previous experiment.

Unraveling photo-induced proton transfer mechanism and proposing solvent regulation manner for the two intramolecular proton-transfer-site BH-BA fluorophore

Switching the ESIPT and TICT process of DP-HPPI via intermolecular hydrogen bonding

Excited-state intramolecular proton transfer (ESIPT) has been well investigated in recent years, whereas we investigated the system containing double hydrogen bonds. We explore the influence of solvent polarity on the proton transfer mechanism for 9,9-dimethyl-3,6-dihydroxy-2,7-bis(4,5-dihydro-4,4-dimethyl-2-oxazolyl)fluorene compound ((Oxa-OH)2). We consider three solvents with different polarities: cyclohexane, chloroform and acetonitrile. Various analytical methods indicate that hydrogen bonding is enhanced in the first excited state, which facilitates the proton transfer within the excited-state molecule. The frontier molecular orbital theory is employed to demonstrate that the system is subjected to photoexcitation to generate charge density redistribution. Three-dimensional potential energy surfaces are also constructed along the direction of hydrogen bond elongated, and their corresponding projection in the X-axis and Y-axis planes are plotted. The comparison of the potential barriers and the transition state structures of each proton transfer path reveals the excited-state intramolecular single proton transfer mechanism of the system. By comparing the potential barriers in different solvents, we also propose a mechanism that can modulate the excited-state proton transfer by changing the solvent polarity.

Coupled intramolecular proton and charge transfer reactions play an important role in many biological and chemical reactions. In this article, we report the relaxation dynamics of the excited states of a donor substituted 1,3-diketone (DMADK, ) using steady state and ultrafast transient absorption and fluorescence spectroscopic techniques. Dramatic dependence of the fluorescence quantum yield, nonradiative rate as well as the excited state relaxation pathways on solvent polarity reveals the solvent controlled excited state intramolecular proton transfer (ESIPT) directed intramolecular charge transfer (ICT) dynamics. The molecules in the ground state coexist in two possible cis-enol (Enol-A and Enol-B) forms. Time-dependent density functional theory (TDDFT) calculations reveal solvent polarity controlled thermodynamic stabilization of one of the tautomeric structures in the S1 state, dictating the direction of proton transfer and subsequent structural relaxation. In low and medium polarity solvents, the S1 state of Enol-B (Enol-B*) undergoes ultrafast ESIPT leading to the population of Enol-A*, followed by the ICT process. In polar solvents, the ESIPT process is reversed to populate Enol-B*, which undergoes a twisted intramolecular charge transfer (TICT) process via twisting of the N,N-dimethylaniline group. This work demonstrates that the strength of electronic coupling between the donor and acceptor group determines the structure of the ICT state.

We theoretically investigate the excited state intramolecular proton transfer (ESIPT) mechanism about a novel doxorubicin (DOX) system based on density functional theory (DFT) and time-dependent DFT methods. We mainly focus on the double proton transfer process regarding the stepwise versus synchronous dual proton transfer mechanism. The changes in our calculated primary bond lengths and bond angles show that the intramolecular hydrogen bonds (O1–H2···O3 and O4–H5···O6) are both strengthened in the S1 state, which provides the possibility of the ESIPT process. Though our computational simulations, the DOX indicates the lowest absorption at around 490 nm. After excitation, we find three emission bands maximized at 656, 663 and 687 nm, which are ascribed to the tautomer. To further elucidate the ESIPT mechanism, we construct the potential energy surfaces (PESs) of both S0 and S1 states. Clearly, three kinds of stable structures could be located on the S1-state PES. We establish a new mechanism of DOX that excludes synchronous double proton transfer, while the stepwise double proton transfer mechanism is confirmed theoretically. We believe that the systematic investigation for photo-physical and photochemical properties of the antineoplastic drug DOX might be significant to obtain fundamental insight into the transport mechanism of DOX inside cultured cells.

In this work, a new excited state intramolecular proton transfer (ESIPT) mechanism of bis (salicylidene)‐1,5‐diaminonaphthalene (BSD) is proposed based on density functional theory (DFT) and time‐dependent DFT methods. Given the intramolecular dual hydrogen bonds of BSD, we explore the excited state hydrogen bonding interactions for BSD and verify that the dual intramolecular hydrogen bonds of BSD molecule should be strengthened in the first excited state. Further, in the analyses about charge distribution upon excitation, we confirm that the charge redistribution around hydrogen bonding moieties plays important roles in enhancing dual hydrogen bonds and in facilitating ESIPT process. To reveal the detailed ESIPT mechanism, we construct the potential energy surfaces (PESs) in both S0 and S1 states along with hydrogen bonds. Via comparing potential energy barriers among the stable structures on the S1‐state PES, we present the excited state intramolecular single proton transfer mechanism for BSD system. Moreover, the phenomenon and properties of fluorescence for BSD in experiment could be explained reasonably based on the ESIPT mechanism presented in this work.

Studies of excited state intramolecular proton transfer (ESIPT) coupled with charge transfer (ESICT) are presented here. As for a strategic design, we synthesized a series of derivatives based on the famous ESIPT molecules 2-(2’-hydroxy) benzoxazole (HBO) or 2-(2’-hydroxy) benzothiazole (HBT). In HBO and HBT, the lone pair electrons at the nitrogen atom have been incorporated into the resonance, i.e. the construction of aromaticity, such that its electron donating property, comparing with those of alkyl amines, is negligibly small. Upon Franck-Condon excitation, one can thus perceive its lack of ESICT. Instead, ESIPT takes place, forming a proton transfer tautomer, in which the central nitrogen atom acts as the secondary alkyl amine. If the para-position of the nitrogen is anchored by a strong electron withdrawing group, ESICT should proceed simultaneously at every moment ESIPT takes place. In other words, one can envisage such types of reaction as the photon induced ESICT associated with a nuclear (proton) motion, rendering an ideal system to study the role of solvent polarity for the ESICT/ESIPT coupled reaction, and hence a paradigm to test the “Marcus theory (modified by Hynes et al.) based on the proton transfer reaction. Although di-CN group is an electron acceptor anchored on the para position of the center nitrogen atom in diCN-HBO and diCN-HBT, the distance between di-CN group and hydroxy group is long enough to prevent affection in the ground state. Started by Franck-Condon excitation, the ESIPT takes place first to create a chance for electron delocalization. As electron motion is faster than nuclear motion, ESICT is initiated and finished during ESIPT. For ESIPT and ESICT is correlated, the combination rate is solvent polarity dependent. The enol form decay rate of diCN-HBO decreases with increasing solvent polarity. The phenomena provides us an ideal model different from our preceding proton transfer compounds such as 4’-(Dialkylamino)-3-hydroxyflavone, p-N,N-Ditolylaminosalicylaldehydes,and HABT, etc.

Electronic Structure Characteristics of ESIPT and TICT Fluorescence Emissions and Calculations of Emitting Energies