Excited-State Proton Transfer and Decay in Hydrogen-Bonded Oxazole System： MS-CASPT2//CASSCF Study

Herein we have employed high-level multi-reference CASSCF and MS-CASPT2 electronic structure methods to systematically study the photochemical mechanism of intramolecularly hydrogen-bonded 2-（2＇-hydroxyphenyl）-4-methyloxazole. At the CASSCF level, we have optimized minima, conical intersections, minimum-energy reaction paths relevant to the excited-state intramolecular proton transfer （ESIPT）, rotation, photoisomerization, and the excited-state deactivation pathways. The energies of all structures and paths are refined by the MS-CASPT2 method. On the basis of the present results, we found that the ESIPT process in a conformer with the OH... N hydrogen bond is essentially barrierless process; whereas, the ESIPT process is inhibited in the other conformer with the OH... O hydrogen bond. The central single-bond rotation of the S1 enol species is energetically unfavorable due to a large barrier. In addition, the excited-state deactivation of the S1 keto species, as a result of the ultrafast ESIPT, is very efficient because of the existence of two easily-approached keto S1/S0 conical intersections. In stark contrast to the S1 keto species, the decay of the S1 enol species is almostly blocked. The present theoretical study contributes valuable knowledge to the understanding of photochemistry of similar intramolecularly hydrogen-bonded molecular and biological systems.

Experimental and Theoretical Investigation on Excited State Intramolecular Proton Transfer Coupled Charge Transfer Reaction of Baicalein

The excited state intramolecular proton transfer （ESIPT） coupled charge transfer of baicalein has been investigated using steady-state spectroscopic experiment and quantum chemistry calculations. The absence of the absorption peak from S1 excited state both in the experi-mental and calculated absorption spectra indicates that S1 is a dark state. The dark excited state S1 results in the very weak fluorescence of solid baicalein in the experiment. The fron- tier molecular orbital and the charge difference densities of baicalein show clearly that the S1 state is a charge-transfer state whereas the S2 state is a locally excited state. The only one stationary point on the potential energy profile of excited state suggests that the ESIPT reaction of baicalein is a barrierless process.

Mechanism of Excited State Double Proton Transfer in 2-Amino-3-Methoxypyridine and Acetic Acid Complex

The excited-state double-proton transfer （ESDPT） mechanism of 2-amino-3-methoxypyridine and acetic acid com- plex is studied by the density functional theory （DFT） and time-dependent DFT with CAM-B3LYP functional. The complex is connected through two different types of inter-molecular hydrogen bonds. After photo-excitation, both hydrogen bonds get strengthened, which can facilitate the ESDPT reaction. The scanned potential energy curve along the proton transfer coordinate indicates that the ESDPT reaction proceeds in a stepwise pattern.

Theoretical investigation on the excited state intramolecular proton transfer in Me_(2)N substituted flavonoid by the time-dependent density functional theory method

Time-dependent density functional theory(TDDFT)method is used to investigate the details of the excited state intramolecular proton transfer(ESIPT)process and the mechanism for temperature effect on the Enol^(*)/Keto^(*)emission ratio for the Me_(2)N-substited flavonoid(MNF)compound.The geometric structures of the S_(0) and S_(1) states are denoted as the Enol,Enol^(*),and Keto*.In addition,the absorption and fluorescence peaks are also calculated.It is noted that the calculated large Stokes shift is in good agreement with the experimental result.Furthermore,our results confirm that the ESIPT process happens upon photoexcitation,which is distinctly monitored by the formation and disappearance of the characteristic peaks of infrared(IR)spectra involved in the proton transfer and in the potential energy curves.Besides,the calculations of highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)reveal that the electronegativity change of proton acceptor due to the intramolecular charge redistribution in the S_(1) state induces the ESIPT.Moreover,the thermodynamic calculation for the MNF shows that the Enol^(*)/Keto^(*)emission ratio decreasing with temperature increasing arises from the barrier lowering of ESIPT.

Theoretical study on the mechanism for the excited-state double proton transfer process of an asymmetric Schiff base ligand

Excited-state double proton transfer(ESDPT)in the 1-[(2-hydroxy-3-methoxy-benzylidene)-hydrazonomethyl]-naphthalen-2-ol(HYDRAVH_(2))ligand was studied by the density functional theory and time-dependent density functional theory method.The analysis of frontier molecular orbitals,infrared spectra,and non-covalent interactions have crossvalidated that the asymmetric structure has an influence on the proton transfer,which makes the proton transfer ability of the two hydrogen protons different.The potential energy surfaces in both S_(0)and S_1 states were scanned with varying O-H bond lengths.The results of potential energy surface analysis adequately proved that the HYDRAVH_(2)can undergo the ESDPT process in the S_1 state and the double proton transfer process is a stepwise proton transfer mechanism.Our work can pave the way towards the design and synthesis of new molecules.

Impact of Polymer Matrix on Polymer Mechanochromism from Excited State Intramolecular Proton Transfer

Mechanochromic polymers based on non-covalent changes have attracted much attention recently.Herein,we report the impact of inter/intramolecular hydrogen bonds on polymer mechanochromism from the excited state intramolecular proton transfer (ESIPT) process.PhMz-NH2-OH and PhMz=2A are designed and obtained by simple and high-yield synthesis,and are connected into polyurethane and poly(methyl acrylate-co-2-ethylhexyl acrylate),respectively.In the initial state,the PhMz-NH2-OH@PU sample shows blue fluorescence from the excited enol form (E*) excitons,owing to intermolecular hydrogen bonds that interrupt the ESIPT reactions but the PhMz=2A@PMA-2-EA sample expresses cyan fluorescence belonging to the excited keto form (K*) emission,implying that the intramolecular hydrogen bonds matter.Furthermore,under stretching,external force can tune the emission of the PhMz=2A@PMA-2-EA sample from K* to E* state.Though external force can putatively still promote a bond rotation,ESIPT reactions remain equivalently interrupted in both the relaxed and stressed states in a hydrogen-bond donating environment.DFT calculation confirms the force-induced increase in dihedral angle for the transition of ESIPT-on/off.Thus,PhMz-NH2-OH@PU and PhMz=2A@PMA-2-EA showed disparate initial ESIPT states and further different responses/sensitivity to force.This study reports a novel and efficient strategy for enriching mechanochromic investigation and extending the applications of ESIPT reactions.

A Versatile BenzimidazoleSubstituted Spirolactam Mechanophore:When Excited State Intramolecular Proton Transfer Couples with Rhodamine

Exploration of multicolor mechanochromic bulk polymers based on a single mechanophore is a big challenge to date.Herein,we report a versatile benzimidazole-substituted spirolactam mechanophore where excited state intramolecular proton transfer(ESIPT)coupled with rhodamine.The mechanophore was facilely synthesized and then covalently linked to polyurethane(PU)chains.The PU film containing the mechanophore(1@PU)showed cooperative photochromism upon irradiation involving simultaneous enhancement of normalized enol and rhodol emissions based on a cooperative ESIPT process and the ring-opening reaction of spirolactam.Moreover,the film exhibited dual-mode multicolor mechanochromism upon stretching and compression.The normalized intensity of enol emission increased and the fluorescence turned from light green to cyan after stretching,then red coloration appeared from colorless after compressing.Control experiments and density functional theory calculations confirmed that the stretch-induced increase of enol emission was attributed to torsion of the dihedral angle between xanthene and benzimidazole in the mechanophore via force-induced disaggregation and direct force action on the isolated mechanophore.Torsion of the dihedral angle and the ring-opening reaction of spirolactam in a single mechanophore occurred sequentially during compression,resulting in an observed red coloration.This study might provide a glimpse into the design of novel multicolor mechanochromic mechanophores.

Relationship between ESIPT properties and antioxidant activities of 5-hydroxyflavone derivates

It is of great significance to study the relationship between the excited state intramolecular proton transfer(ESIPT)properties and antioxidant activities of compounds in the field of life sciences.In this work,two novel compounds 5HF-OMe and 5HF-NH2 are designed through introducing a methoxy-and amino-group into the structure of 5-hydroxyflavone(5HF)respectively.The relationship between the ESIPT reaction and antioxidant activities of the three compounds is studied via the density functional theory(DFT)and time-dependent DFT(TD-DFT)methods.The calculated potential energy curves suggest that the rate of ESIPT reaction will gradually slow down from 5HF to 5HF-OMe and 5HF-NH2.In addition,the antioxidant activities of the three compounds gradually enhance from 5HF to 5HF-OMe and 5HF-NH2,which can be seen from the calculated energy gaps and ionization potential values.Interestingly,the above results imply that the rate of ESIPT reaction has a negative relationship with the antioxidant activities of the compounds,i.e.,the slower rate of ESIPT reaction will reflect the higher antioxidant activity of the compound,which will provide valuable reference for detecting the antioxidant activity of compound via the photophysical method.

Theoretical investigation of fluorescence changes caused by methanol bridge based on ESIPT reaction

The different fluorescence behavior caused by the excited state proton transfer in 3-hydroxy-4-pyridylisoquinoline(2a)compound has been theoretically investigated.Our calculation results illustrate that the 2a monomer in tetrahydrofuran solvent would not occur proton transfer spontaneously,while the 2a complex in methanol(MeOH)solvent can undergo an asynchronous excited state intramolecular proton transfer(ESIPT)process.The result was confirmed by analyzing the related structural parameters,infrared vibration spectrum and reduced density gradient isosurfaces.Moreover,the potential curves revealed that with the bridging of single MeOH molecular the energy barrier of ESIPT was modulated effectively.It was distinctly reduced to 4.80 kcal/mol in 2a-MeOH complex from 25.01 kcal/mol in 2a monomer.Accordingly,the ESIPT process induced a fluorochromic phenomenon with the assistant of proton-bridge.The elucidation of the mechanism of solvent discoloration will contribute to the design and synthesis of fluorogenic dyes as environment-sensitive probes.

Influence of intramolecular hydrogen bond formation sites on fluorescence mechanism

The fluorescence mechanism of HBT-HBZ is investigated in this work. A fluorescent probe is used to detect HClO content in living cells and tap water, and its structure after oxidation by HCl O(HBT-ClO) is discussed based on the density functional theory(DFT) and time-dependent density functional theory(TDDFT). At the same time, the influence of the probe conformation and the proton transfer site within the excited state molecule on the fluorescence mechanism are revealed. Combined with infrared vibrational spectra and atoms-in-molecules theory, the strength of intramolecular hydrogen bonds in HBT-HBZ and HBT-ClO and their isomers are demonstrated qualitatively. The relationship between the strength of intramolecular hydrogen bonds and dipole moments is discussed. The potential energy curves demonstrate the feasibility of intramolecular proton transfer. The weak fluorescence phenomenon of HBT-HBZ in solution is quantitatively explained by analyzing the frontier molecular orbital and hole electron caused by charge separation. Moreover, when strong cyan fluorescence occurs in solution, the corresponding molecular structure should be HBT-ClO(T). The influence of the intramolecular hydrogen bond formation site on the molecule as a whole is also investigated by electrostatic potential analysis.

Effect of intramolecular and intermolecular hydrogen bonding on the ESIPT process in DEAHB molecule

Density functional theory（DFT） and time-dependent density functional theory（TDDFT） methods are used to investigate the influences of intramolecular and intermolecular hydrogen bonding on excited-state intramolecular proton transfer（ESIPT） for the 4-N,N-（diethylamino）-2-hydroxybenzaldehyde（DEAHB）. The structures of DEAHB and its hydrogenbonded complex in the ground-state and the excited-state are optimized. In addition, the detailed descriptions of frontier molecular orbitals of the DEAHB monomer and DEAHB-DMSO complex are presented. Moreover, the transition density matrix is worked out to gain deeper insight into the orbitals change. It is hoped that the present work not only elaborates different influence mechanisms between intramolecular and intermolecular hydrogen bonding interactions on the ESIPT process for DEAHB, but also may be helpful to design and develop new materials and applications involved DEAHB systems in the future.

Theoretical insights into photochemical ESITP process for novel DMP-HBT-py compound

We execute the density functional theory(DFT) and time-dependent density functional theory(TDDFT) approaches to make a detailed exploration about excited state luminescent properties as well as excited state intramolecular proton transfer(ESIPT) mechanism for the novel 2,6-dimethyl phenyl(DMP-HBT-py) system. Firstly, we check and confirm the formation and stabilization of hydrogen bonding interaction for DMP-HBT-py. Via optimized geometrical parameters of primary chemical bond and infrared(IR) spectra, we find O–H··· N hydrogen bond of DMP-HBT-py should be strengthened in S1 state. Insights into frontier molecular orbitals(MOs) analyses, we infer charge redistribution and charge transfer(ICT)phenomena motivate ESIPT trend. Via probing into potential energy curves(PECs) in related electronic states, we come up with the ultrafast ESIPT behavior due to low potential barrier. Furthermore, we search the reaction transition state(TS)structure, the ultrafast ESIPT behavior and mechanism of DMP-HBT-py compound can be re-confirmed. We sincerely wish this work could play roles in further developing novel applications based on DMP-HBT-py compound and in promoting efficient solid emitters in OLEDs in future.

Phototriggered structures:Latest advances in biomedical applications

Non-invasive control of the drug molecules accessibility is a key issue in improving diagnostic and therapeutic procedures.Some studies have explored the spatiotemporal control by light as a peripheral stimulus.Phototriggered drug delivery systems(PTDDSs) have received interest in the past decade among biological researchers due to their capability the control drug release.To this end,a wide range of phototrigger molecular structures participated in the DDSs to serve additional efficiency and a high-conversion release of active fragments under light irradiation.Up to now,several categories of PTDDSs have been extended to upgrade the performance of controlled delivery of therapeutic agents based on well-known phototrigger molecular structures like o-nitrobenzyl,coumarinyl,anthracenyl,quinolinyl,o-hydroxycinnamate and hydroxyphenacyl,where either of one endows an exclusive feature and distinct mechanistic approach.This review conveys the design,photochemical properties and essential mechanism of the most important phototriggered structures for the release of single and dual(similar or different) active molecules that have the ability to quickly reason of the large variety of dynamic biological phenomena for biomedical applications like photo-regulated drug release,synergistic outcomes,real-time monitoring,and biocompatibility potential.

An ESIPT-based NIR-fluorescent probe for exosome labeling and in situ imaging

Exosomes play significant roles in physiological and tumorigenic processes and it is desirable to visualize and track the exosomes.Herein,a novel amphiphilic fluorescent probe HBT-Exo based on excited-state intramolecular proton transfer(ESIPT)mechanism is reported for exosome-labeling.Its ESIPT characteristics were confirmed by both theory calculation and experimental observation,which enable the probe to show a large Stokes shift as well as near-infrared(NIR)keto-form emission.HBT-Exo displayed excellent biocompatibility and remarkable efficiency for exosome-labeling in gastric cancer cells.Furthermore,the labeled exosomes were successfully applied for the real-time in situ imaging in mouse models.

The exquisite integration of ESIPT, PET and AIE for constructing fluorescent probe for Hg(II) detection and poisoning

Excessive mercury ions(Hg^(2+)) in the environment can accumulate in human body along with the food chain to cause serious physiological reactions.The fluorescence probes were considered as convenient tool with great potential for Hg^(2+) detection.Most existing probes suffer from aggregation-induced quenching(ACQ) effects and insufficient sensitivity.Herein,a novel type of fluorophore was developed by combining the aggregation-induced emission(AIE) and excited state intramolecular proton transfer(ESIPT) characteristics.Subsequently,a phenyl thioformate group with photoinduced electron transfer(PET)effect was connected to give an efficient "turn-on" probe(HTM),which exhibited good selectivity toward Hg^(2+),short response time(30 min),coupled with extremely low detection limit(LOD=1.68 nmol/L).In addition,HTM was used successfully in real samples,cells and drug evaluation,underlying the superiority of HTM to detect Hg^(2+) in practical applications.

Acid-base Vapor Sensing Enabled by ESIPT-attributed Cd(II) Coordination Polymer with Switchable Luminescence

Fluorescence materials based on excited state intramolecular proton transfer(ESIPT)have attracted great attentiori due to the unique four-level energy states.Herein,we report the assembly of a Cd-LF coordination polymer from purposely designed LF(H2hpi2cf)ligand,which can present switchable luminescence behavior by gain or loss protons originated in uncoordinated ESIPT sites and serve as acid-base vapor sensors.Fabricated into in-situ grown film or transparent ink by simple methods,Cd-LF presents facile and portable amine sensor for food spoilage detection and fluorescent anti-counterfeiting ink applications.

Azomethine ylide-formation from N-phthaloylglycine by photoinduced decarboxylation:A theoretical study

In this work, we report the first CASPT2//CASSCF study of the mechanism of the photodecarboxylation of N-phthaloylglycine. The charge transfer excited state S CT （ 1∏∏＊） is initially populated upon irradiation at 266 nm. As a result of a fast internal conversion to the lowest excited singlet state S CT-N （ 1∏∏＊）, this state becomes a favorable precursor state for proton transfer, which triggers decarboxylation. Actually, the excited state intramolecular proton transfer （ESIPT） and decarboxylation processes proceed in an asynchronous concerted way. The ESIPT process is accomplished in the S CT-N （ 1∏∏＊） state, but the CO 2 molecule is finally formed in the ground state via the S CT /S 0 conical intersection. Azomethine ylide is formed in the ground state as a complex with CO 2 . A barrier of ~15 kcal/mol indicates that azomethine ylide is stable in the ground state, which is consistent with the experimental findings. This work provides mechanistic details about the formation of azomethine ylide by photoreaction of N-phthaloylglycine.