Excited-state Intramolecular Single Proton Transfer 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.

Uncovering photo-excited intramolecular charge transfer and ESIPT mechanism for 5,5′-(9,9-dioctyl-9H-fluorene-2,7-diyl) bis(2-benzo[d]thiazol-2-yl) phenol compound

In view of their facile synthetic routes and remarkable photo-induced stabilities, excited state intramolecular proton-transfer (ESIPT)-vibrant luminous materials have drawn more and more attention. In this work, we mainly focus on the photo-induced excitation behaviours, including excited state hydrogen bonding interactions and ESIPT reaction of a new 5,5′-(9,9-dioctyl-9H-fluorene-2,7-diyl)-bis(2-benzo[d]thiazol-2-yl) phenol molecule theoretically. To be consistent with the previous experiment, we name this molecule as IIa. Using molecular electrostatic potential (MEP), we explored the dual intramolecular hydrogen bonding interactions in IIa form. Then on the level of chemical geometries and infrared (IR) spectra, we confirmed that the double hydrogen bonds should be enhanced upon photoexcitation. Probing into charge reorganisation, we found that hydrogen proton does prefer to be attracted by enhanced electronic densities around N3 and N6. Combining potential energy surfaces (PESs) and electronic spectra, the excited state intramolecular single proton-transfer (ESISPT) behaviour could be verified for IIa although it owns two hydrogen bonds.

Aggregation Promotes Excited-State Intramolecular Proton Transfer for Benzothiazole-Substituted Tetraphenylethylene Compound.

Excited-state intramolecular proton transfer (ESIPT) coupled to aggregation-induced enhanced emission (AIEE) offers a better route to design more efficient highly emissive materials. These materials usually present potential abilities in biochemical sensing fields. In this work, we mainly focus on ESIPT and AIEE mechanism for the benzothiazole-substituted tetraphenylethylene compound (BTZ-TPE-1a) in liquid and solid phases. It is worth mentioning that BTZ-TPE-1a is biocompatible in nature and plays important roles in cell imaging and cell viability. Given its nontoxicity and biocompatiblility properties, BTZ-TPE-1a is an excellent imaging agent in cancer biomedical imaging. We first explore hydrogen-bonding interactions for the BTZ-TPE-1a system. The strengthening dual hydrogen bonds and intramolecular charge transfer (ICT) resulting from photoexcitation reveals an ESIPT tendency. Simulated electronic spectra and potential energy surfaces (PESs) indicate the excited-state intramolecular single-proton-transfer (ESISPT) mechanism for BTZ-TPE-1a molecule. Related experimental reports confirm our results. By performing Born-Oppenheimer molecular dynamics (BOMD) simulations starting from a transition state (TS) structure, we can further verify the ESISPT mechanism. In the solid phase, quantum mechanics and molecular mechanics (QM/MM) simulation is realized in the ONIOM model, on the basis of which the AIEE mechanism of BTZ-TPE-1a is elaborated. We not only illustrate the specific ESISPT mechanism for BTZ-TPE-1a and compensate for the inadequacy of the experiment but also present direct insights into ESIPT and AIEE in the aggregation state (i.e., aggregation promotes ESIPT for BTZ-TPE-1a). We hope this work promotes further development of benzothiazole-substituted tetraphenylethylene compounds in biomedical and optoelectronic applications.

Ultrafast Excited-State Intermolecular Proton Transfer in Indigo Oligomer.

It has been a long-lasting debate whether indigo undergoes excited-state proton transfer and how this contributes to its photostability. A prevailing point of view is that a sub-picosecond excited-state intramolecular single proton transfer occurs; however, it has been studied mostly under dilute solution conditions. In this work, excited-state structural dynamics of indigo oligomers formed at millimolar concentration in dimethyl sulfoxide is investigated using femtosecond visible pump spectroscopy, infrared and visible probe spectroscopies, and steady-state infrared and fluorescence spectroscopies. Experimental evidence indicates the presence of transient intermolecular electronic excited-state proton transfer, which is supported by quantum-chemistry computations. The formed enol species disappears with a time constant of 200-300 fs, followed by a relatively slow nonradiative relaxation to the electronic ground state. Our results reveal new photochemistry of indigo particularly in its oligomeric state.

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.

Electronic structure calculations and nonadiabatic dynamics simulations of excited-state relaxation of Pigment Yellow 101.

Pigment Yellow 101 (PY101) is widely used as a typical pigment due to its excellent excited-state properties. However, the origin of its photostability is still elusive. In this work, we have systematically investigated the photodynamics of PY101 by performing combined electronic structure calculations and trajectory-based nonadiabatic dynamics simulations. On the basis of the results, we have found that upon photoexcitation to the S1 state, PY101 undergoes an essentially barrierless excited-state intramolecular single proton transfer generating an S1 keto species. In the keto region, there is an energetically accessible S1/S0 conical intersection that funnels the system to the S0 state quickly. In the S0 state, the keto species either goes back to its trans-enol species through a ground-state reverse hydrogen transfer or arrives at the cis-keto region. In addition, we have found an additional excited-state decay channel for the S1 enol species, which is directly linked to an S1/S0 conical intersection located in the enol region. This mechanism has also been confirmed by our dynamics simulations, in which about 54% of the trajectories decay to the S0 state via the enol S1/S0 conical intersection; while the remaining ones employ the keto S1/S0 conical intersection. The gained mechanistic information helps us understand the photostability of the PY101 chromophore and its variants with the same molecular scaffold.

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).

Excited-state intramolecular single and double proton transfer emission of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol

In this work, excited-state intramolecular single proton transfer emission (S-PTE) and double proton transfer emission (D-PTE) are presented by employing 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol (1) as a prototype. 1 has been strategically designed and synthesized in an aim to explore single proton transfer and double proton transfer in the excited state. In solution 1 exhibits the lowest lying S0 → S1 absorption at ∼395 nm. Upon excitation, two large Stokes shifted emission bands maximized at 475 nm and 550 nm are resolved, which are ascribed to the tautomer emission resulting from single proton transfer and double proton transfer isomers, respectively. It is found that solvents have greatly influenced on excited-state intramolecular proton transfer emission.