Excited State Intramolecular Single Proton Research Articles

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

To expound specific excited state processes of the novel excitation wavelength dependent emission BH-BA fluorophore for better subsequent applications, this wok mainly focus on exploring photo-induced hydrogen bonding geometrical changes, excited state intramolecular proton transfer (ESIPT) mechanism and related regulated behavior via solvent polarity. The differences of structural parameters, infrared (IR) vibrational spectra, core-valence bifurcation (CVB) index as well as electronic densities ρ(r) between S0 and S1 states related to dual hydrogen bonds (O1-H2···N3 and O4-H5···N6) reveal S1-state hydrogen bonding strength facilitate ESIPT behaviors for BH-BA system. Of particular note, O4-H5···N6 plays a more dominant role. Photo-induced intramolecular charge transfer (ICT) and variations of Hirshfled and NPA charges over atoms related to hydrogen bonding moieties promote the ESIPT tendency for BH-BA. Combined potential energy surfaces (PESs), transition state (TS) and intrinsic reaction coordinate (IRC) paths, we illustrate the excited state intramolecular single proton transfer (ESISPT) mechanism of BH-BA should occur along with O4-H5···N6 hydrogen bonding wire, which could be adjusted by surrounding solvent polarity.

Excited-state intramolecular double proton transfer mechanism associated with solvent polarity for 9,9-dimethyl-3,6-dihydroxy-2,7-bis(4,5-dihydro-4,4-dimethyl-2-oxazolyl)fluorene compound

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.

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

In this work, we mainly focus on exploring the excited state hydrogen bonding dynamics and excited state intramolecular proton transfer (ESIPT) mechanism for a SAA derivative with para-position electron-withdrawing cyano group (CN-SAA). Insight into the geometrical changes, infrared (IR) spectra, and atomic charge populations, we infer dual intramolecular hydrogen bonds of CN-SAA can be enhanced in S1 state. Adopting three manners to estimate hydrogen bonding energies, we further confirm S1-state hydrogen bonding strengthening, which triggers ESIPT reaction. In view of photo-induced excitation, we find charge reorganization plays important roles in promoting ESIPT behavior for CN-SAA. Combining core-valence bifurcation (CVB) index and energy gap between HOMO and LUMO orbitals in solvents with different polarities, we predict nonpolar solvents contribute to facilitate ESIPT process. To clarify detailed ESIPT mechanism, we construct potential energy surface (PES), search transition state (TS) form, perform intrinsic reaction coordinate (IRC) energetic profile calculations and Born-Oppenheimer molecular dynamics simulations. We not only clarify the excited state intramolecular single proton transfer (ESISPT) reaction mechanism for CN-SAA compound, but also present the ESISPT regulating mechanism via solvent polarity.

Unraveling photo-excited behaviors and proton transfer mechanisms for coexisting 5-methoxy-salicylaldhyde azine isomers

In view of the paramount importance of excited state relaxation for organic fluorophores, in this work, we mainly focus on deciphering the ground-state and excited state dynamical behaviors for the novel 5-methoxy-salicylaldhyde azine compound. Based on constructing potential energy surface (PES) and calculating Boltzmann distribution ratio, we firstly present three isomers of 5-methoxy-salicylaldhyde azine isomers (i.e., anti, anti-syn and syn) could coexist in S0 state via twisting methyl dihedral angles (Φ1 and Φ2). Upon photoexcitation, we find the intramolecular dual hydrogen bonds of anti, anti-syn and syn compounds are enhanced in S1 state. Given asymmetry of anti-syn structure, we present the O1-H2···N3 should be largely affected, which plays more significant roles in excited state process than the O4-H5···N6 one. Charge redistribution around hydrogen bonding moieties deriving from photoexcitation illustrates the nature of strengthening dual hydrogen bonds and promotes the excited state proton transfer (ESPT) behaviors for these three isomers. Insights into excited state PESs along with proton transfer paths, we confirm the excited state intramolecular single proton transfer (ESISPT) mechanism are supported kinetic and thermodynamic for anti, anti-syn and syn compounds. Especially for anti-syn form, the ESISPT process is inclined to proceed along with O1-H2···N3 rather than the O4-H5···N6 one. This work illustrates the interconversion and coexistence of three isomers in S0 state, based on which the specific ESISPT mechanisms of them are elaborated theoretically. We hope this work could pave the way for designing and developing novel applications based on 5-methoxy-salicylaldhyde azine fluorophore.

Solvent polarity dependent excited state hydrogen bond effects and intramolecular double proton transfer mechanism for 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d’]bisimidazole system

In this work, we probe into the photo-induced excited state hydrogen bonding interactions and excited state proton transfer (ESPT) behaviors for a representative benzo[1,2-d:4,5-d’]bisimidazole derivative (i.e., 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d’]bisimidazole (HPBB)) compound. In view of aprotic solvents with different polarities, cyclohexane (CYH), dichloromethane (DCM) and acetonitrile (MeCN) solvents are considered. Analyzing hydrogen-bond geometrical parameters, infrared (IR) vibrational spectra, Mayer bond order and predicting hydrogen bonding energy (E(HB)), we verify dual hydrogen bonds of HPBB are strengthened in S1 state. Particularly, in nonpolar solvent, the enhanced excited state hydrogen bonds become more obvious. The intriguing charge redistribution and frontier molecular orbitals (MOs) reveal hydrogen bonding acceptance ability of acceptor moieties becomes stronger, which plays a crucial role in capturing hydroxyl proton via photoexcitation. To check and explore ESIPT mechanism, we present the solvent polarity dependent asynchronous excited state intramolecular double proton transfer (ESIDPT) mechanism. That is, nonpolar solvent promotes excited state intramolecular single proton transfer (ESISPT) process for HPBB, while polar solvent contributes to ESIDPT behavior with the primary single proton-transfer product in S1 state. This work not only makes a rational attribution to experimental phenomena, but also clarifies detailed excited state behaviors for HPBB and presents regulating ESIPT mechanism via solvent polarity.

Effects of solvent polarity on excited state behaviors for the two intramolecular proton-transfer-site 4,4’-(hydrazine-1,2-diylidene-bis(methanylylidene))-bis(3-hydroxybenzoic acid) compound

Excited state proton transfer (ESPT) behavior of organic fluorophores has been drawing continuously considerable interests, since it owns enormous potential in wide of application fields. Given the paramount importance of excited state relaxations, in this work, we focus on deciphering excited state behaviors for the novel two proton-transfer-site 4,4’-(hydrazine-1,2-diylidene-bis(methanylylidene))-bis(3-hydroxybenzoic acid) (HDBB) system. In aprotic solvents with different polarities, we confirm the dual hydrogen bonds of HDBB should be enhanced in S1 state. It is a remarkable fact that nonpolar solvents play a more vital role in enhancing dual hydrogen bonds in S1 state. Probing into photo-induced excitation, we verify charge reorganization around proton acceptor and donor triggers ESPT tendency. Comparing energy gaps of frontier molecular orbitals (MOs), we further predict the ESPT process of HDBB could be facilitated by nonpolar solvents. To clarify the specific excited state behaviors, potential energy surfaces (PESs) in different solvents are constructed along with dual hydrogen bonds to present the overall ESPT processes. Searching transition state (TS) along reaction path, we confirm the excited state intramolecular single proton transfer (ESISPT) mechanism for HDBB. Further, we successfully present the excited state regulation mechanism via solvent polarity, namely, nonpolar solvents contribute to ESISPT reaction for HDBB compound.

Unraveling photoexcitation dynamical behavior for bis (salicylidene)‐1,5‐diaminonaphthalene (BSD) system

AbstractIn 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.

Ab initio investigation of excited state dual hydrogen bonding interactions and proton transfer mechanism for novel oxazoline compound**Project supported by the National Natural Science Foundation of China (Grant No. 11404112), the Funding Scheme for Young Teachers in Colleges and Universities in Henan Province, China (Grant No. 2017GGJS077), and the Key Scientific Research Project of Colleges and Universities of Henan Province,

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.

A TD‐DFT investigation of the photo‐induced excited state intramolecular proton transfer dynamics for the novel 5,5′‐(9,9‐dihexyl‐9H‐fluorene‐2,7‐diyl)bis(2‐benzo[d]thiazol‐2‐yl)phenol) system

AbstractDensity 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.

Excited state intramolecular single proton transfer mechanism of pigment yellow 101 in solid state: Experiment and DFT calculation.

To investigate fluorescence mechanism of Pigment Yellow 101 (P. Y. 101) in solid state, three aromatic aldehyde azines (1-3) including P. Y. 101 have been synthesized and compared with each other. Results indicated that P. Y. 101 prepared by solvothermal method is actually the mixture of two polymorphs, whose molecular packing mode can be transformed into each other by recrystallizing or external stimuli such as pressure and grinding. The ESIPT properties of 1-3 were investigated by DFT/TD-DFT calculations and time-correlated single photon counting (TCSPC) technique. Both experimental and theoretical results revealed that the dual fluorescence properties of P. Y. 101 in solid state are ascribed to the excited-state intramolecular single proton transfer fluorescence emissions of two structurally different polymorphs rather than the results of the sequential or concerted excited-state intramolecular double proton transfers, which provide a potential valuable tool for developing multistimuli-responsive luminescent materials.

Theoretical insights into excited-state intramolecular proton transfer in 1,8-dihydroxydibenzo[a,h]phenazine

ABSTRACT1,8-Dihydroxydibenzo[a,h]phenazine (DHBP) is a new synthetic compound possessing two intramolecular hydrogen bonds; however, it has been found to exhibit the excited-state intramolecular single proton transfer (ESSPT) behaviour, in recent experiment. To explain the phenomenon reasonably, two combined methods of CASSCF/CASPT2 and DFT/TD-DFT have been employed to investigate the structural and spectral properties of its three tautomers, corresponding to the non-proton-transferred (E), the single-proton-transferred (SK) and the double-proton-transferred (DK) forms. These studies suggest that the E form is the global minimum in the S0 state, while the SK form is the most stable in the S1 state, both of which are responsible for the experimental absorption peak at 2.54 eV and emission band at 1.64 eV, respectively. Because of the relatively high energy barrier, the DK form will play no important role in the fluorescence emission of DHBP. The present results lend a good support to the experimental finding of single proton transfer (SPT).

Crystal structure and feasibility of intramolecular proton transfer reaction of salicylaldazine

The work describes characterization of salicylaldazine by IR, NMR and single crystal X-ray diffraction studies. The compound crystallizes in the monoclinic crystal system with the space group P2 1/ n, a=8.554(3), b=6.338(2), c=11.864(5) Å, and β=107.89(2)°. There are two strong intramolecular hydrogen bonds of the type O–H⋯N and two weak hydrogen bridges of the type C–H⋯O which help the molecules to pack in a layered structure. Semi-empirical calculations have been performed to predict theoretically the feasibility of intramolecular ground and/or excited state proton transfer reaction/s. Potential energy curves have been generated in the ground (S 0) and the lowest excited (S 1) singlet states to judge the feasibility of intramolecular single and double proton transfer reactions. Semi-empirical (AM1) calculations suggest that only the excited state intramolecular single proton transfer process is favored both thermodynamically and kinetically. No other prototropic processes are theoretically feasible.