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Excited state intramolecular proton transfer (ESIPT) of organic molecules has been drawing continuously considerable interests. It is not only one of the most basic processes in life but also exists in wide of application fields. In this work, we theoretically make a thorough inquiry about molecular excited state trends and ESIPT procedures for the novel highly miscible 2,5‐bis (benzoxazol‐2‐yl)thiophene‐3,4‐diol‐ethyl (BBTE) molecule. Based on DFT and TDDFT means, we first examine and certify hydrogen bond O1–H2···N3 role in BBTE. Probing into molecular structure and infrared (IR) vibrational behaviors, we validate O1–H2···N3 of BBTE is enhanced in S1 state via photo‐induced excitation. Accessorial negative electronic densities over N3 atom facilitate attracting hydrogen proton, which caters to the truth of strengthening hydrogen bond in S1. In addition, frontier orbital gap indicates the solvent polarity plays vital roles in affecting excited state courses for BBTE. By means of potential energy curves (PECs) in three kinds of solvents, we propose the ultrafast ESIPT mechanism for BBTE with explaining previous experimental characteristics. Along the way of ESIPT, we present the regulation mechanism for BBTE via solvent polarities. The enhanced hydrogen bond and charge redistribution facilitate excited state intramolecular proton transfer (ESIPT) reaction for 2,5‐bis (benzoxazol‐2‐yl)thiophene‐3,4‐diol‐ethyl (BBTE) system. Frontier orbital gap reveals that solvent polarity could play vital roles in controlling excited state dynamical behaviors for BBTE compound. Solvent polarity could regulate and control the ultrafast ESIPT behavior for BBTE molecule. [ABSTRACT FROM AUTHOR]

The mechanisms of 2-(Benzo[d]thiazol-2-yl)phenol-based bifunctional probe (HBT-FS) for detecting fluoride (F−) and sulfite (SO32–) based on excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) have been theoretically studied. Laplacian bond order of HBT-FS indicates that the F− ion cleaves the Si-O bond and then forms Compound 2 possessing a six-membered ring with a hydrogen bond. Potential energy curves and dynamic simulations confirm that ESIPT in Compound 2 occurs along with this hydrogen bond and forms a keto structure with an emission at 623 nm, which agrees with the observed experimental value (634 nm) after adding F−. Therefore, the fluorescence red-shift (from 498 to 634 nm) of HBT-FS observed in experiment after adding F− is caused by ESIPT. The SO32– ion is added to the C5 site of HBT-FS, which is confirmed by orbital-weighted dual descriptor, and then forms Compound 3 with fluorescence located at 404 nm. The experimentally measured fluorescence at 371 nm after adding SO32– is assigned to Compound 3. Charge transfer analyses indicate that the ICT extent of Compound 3 is relatively weak compared with that of HBT-FS because of the destruction of the conjugated structure by the addition reaction of SO32–, which induces the blue-shift of the fluorescence of HBT-FS from 498 to 371 nm. The different fluorescence responses make HBT-FS a fluorescent probe to discriminatorily detect F− and SO32–. [ABSTRACT FROM AUTHOR]

The 1,8-Dihydroxydibenzo[a,c]phenazine (DHBP) is chosen to investigate the relationship among the extension of π-conjugation framework, excited-state proton transfer and the Stokes shift. The IR spectra, bond and angle analyses show that the two intramolecular hydrogen bonds in DHBP molecule are both significantly strengthened in S1 state. The proton donor methanol molecule can inhibits the hydrogen bond strengthening in S1 state. The increment of π-conjugation framework effectively decreases the Stokes shift and promotes the proton transfer. The potential energy surfaces of DHBP indicate that the reverse double-proton transfer can spontaneously happen in S0 state, and the double proton can easily transfer for its relative low potential energy barrier in S1 state. [ABSTRACT FROM AUTHOR]

The novel spectral behaviors of Aloesaponarin 1 (AS1) are investigated by studying the dynamics process of excited state intramolecular proton transfer (ESIPT). Two intramolecular hydrogen bonds (HB1 and HB2) are formed between hydroxyl and carbonyl groups of AS1. The calculated potential energy curves of AS1 demonstrate that the ESIPT process along HB1 is energy favorable while not along HB2. The analysis of potential energy curves describes clearly the dynamic behaviors of the proton transfer process from hydroxyl group to carbonyl group along HB1. The infrared spectra of AS1 confirm that the stretching absorption peak of hydroxyl group in HB1 disappears and that a new peak corresponding to hydroxyl group appears in the first excited state, which depicts the ESIPT process indirectly. The fluorescence peaks of AS1 (636 nm), AS2 (Aloesaponarin 1 3-O-methyl ether, 629 nm) and AS3 (Aloesaponarin 1 8-O-methyl ether, 522 nm) demonstrate that the fluorescence behavior of AS1 is primarily effected by HB1 rather than HB2. The large Stokes shifts of AS1 (206 nm) indicate that the absorbed energy is partly transferred to non-harmful long fluorescence through ESIPT process, which plays important role in the explanation for the UV protection property of AS1. The inducement and influence factors of ESIPT process of AS1 are illustrated by analyzing electrostatic potential, molecular orbital and natural bond orbital. [ABSTRACT FROM AUTHOR]

The excited state intramolecular proton transfer (ESIPT) dynamics and photophysical properties of 4-( N , N -dimethylamino)-3-hydroxyflavone (DMAF) in acetonitrile (ACN) and ethanol (EtOH) were investigated by density-functional theory (DFT) and time-dependent density-functional theory (TDDFT) methods at B3LYP/6-31G (d,p) level. The calculation results show that hydrogen bond in the first excited state (S 1 ) is strengthened, which can facilitate the process of proton transfer. The absorption and emission spectra obtained through the vertical excitation energies agree well with the experiment, which confirm the rationality of the choice of theoretical methods. In addition, the frontier molecular orbitals (MOs) analysis indicates the intramolecular charge transfer, which can reveal the tendency of ESIPT reaction. In order to explore the mechanism of the ESIPT process, the potential energy curves of the ground state (S 0 ) and the S 1 state have been constructed. Based on the energy potential barrier of 3.48 kcal/mol for DMAF molecule and 5.48 kcal/mol for 4-( N , N -dimethylamino)-3-hydroxyflavone/ethanol (DMAF-EtOH) complex in the S 1 state, we can find that the proton transfer occurs more easily in acetonitrile solvent. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fluorescence quenching of the probe MIPY-DNP is caused by PET. • Strong fluorescence of product MIPY-OH is ascribed to ESIPT. • Single fluorescence observed in the experiment corresponds to the keto form of MIPY-OH. • Sensing mechanism for the detection of PhSH is proposed. The fluorescent turn-on response mechanism of an imidazo [1,5-α]pyridine-based probe MIPY-DNP for the detection of thiophenols (PhSH) has been theoretically studied. The first excited singlet (S 1) state of probe MIPY-DNP is a dark state accompanying with complete charge transfer from the imidazo [1,5-α]pyridine group to the 2, 4-dinitrophenyl moiety, which implies the occurrence of the nonradiative photo-induced electron transfer (PET) process and explains the fluorescence quenching. The injection of PhSH induces the thiolysis of 2,4-dinitrophenyl ether group in the probe MIPY-DNP, and then the enol form of product MIPY-OH is formed. Due to lower potential barrier (6.72 kcal/mol) obtained by potential energy curves (PECs) in S 1 state, the enol form of product MIPY-OH is converted into the keto form through the excited-state intramolecular proton transfer (ESIPT) process. Subsequently, the keto form of MIPY-OH emits strong fluorescence at 484 nm, which is consistent with the fluorescence observed in the experiment (478 nm). The frontier molecular orbitals (FMOs) and hole-electron analysis indicate that the electron transfer and distribution properties of the excited state, which further provides a reasonable explanation for the fluorescence quenching of probe MIPY-DNP and strong fluorescence of product MIPY-OH. Theoretical calculations gives a detailed detection mechanism of the probe MIPY-DNP towards PhSH based on the multi-mechanisms of PET and ESIPT. [ABSTRACT FROM AUTHOR]

The fluorescence properties of 4′-methoxy-3-hydroxyflavone (M3HF) dye in different solvents were investigated through experimental (Phys. Chem. Chem. Phys. , 2018, 20 , 7885) and theoretical (Org. Chem. Front. , 2019, 6 , 218) methods. However, the intermolecular hydrogen bonds between M3HF and solvents were ignored. In this work, we investigated the effect of methanol (MeOH) and N , N -dimethylformamide (DMF) solvents on the excited-state intramolecular proton transfer (ESIPT) of M3HF fluorescent dye. In excited state (S 1), the intramolecular hydrogen bonds are significantly strengthened, which can facilitate the ESIPT processes. The calculated absorption and fluorescence spectra agree well with the experimental date. The fluorescence spectra of M3HF and ESIPT tautomers (T⁎) were found to be sensitive to the solvent polarity. Upon photo-excitation, the electron density of the M3HF molecular is redistributed, which can provide driving force for the ESIPT. The polar solvents MeOH (hydrogen bond donor) and DMF (hydrogen bond acceptor) can form different types of intermolecular hydrogen bonds with M3HF. The two different bonding modes of intermolecular hydrogen bonds are expected to weaken the intramolecular hydrogen bond of M3HF to varying degrees. The analysis of the potential energy curves indicate that the ESIPT processes of M3HF can be hindered by the intermolecular hydrogen bonds. The intermolecular hydrogen bond of M3HF-DMF complex is weaker than that of M3HF-MeOH complex, while the potential barrier of the ESIPT process in DMF solvent is higher than that of in the MeOH solvent. This is principally because, in DMF solvent, the hydroxyl group H 1 atom of M3HF can be captured by the O 3 atom of DMF and form O 3 H 1 bond with O 3 atom in the intermediate process of ESIPT. There appears an energy barrier hopping point on the potential energy curve of M3HF in DMF solvent but does not appear in MeOH solvent. Unlabelled Image • The fluorescence spectra of M3HF and ESIPT tautomers (T⁎) are sensitive to the solvent polarity. • The polar solvents MeOH and DMF can form different types of intermolecular hydrogen bonds with M3HF. • The intermolecular hydrogen bonds are expected to weaken the intramolecular hydrogen bond of M3HF. • The ESIPT processes of M3HF can be hindered by the intermolecular hydrogen bonds. • There appears an energy barrier hopping point on the potential energy curve in DMF but does not appear in MeOH solvent. [ABSTRACT FROM AUTHOR]

Abstract In this work, the influence of cyanide group substituted on the excited-state intramolecular proton transfer of 2-(2-hydroxyphenyl)benzothiazole (HBT) has been theoretical studied. It is found that the intramolecular hydrogen bonds of HBT and its cyanide substituted derivatives are significantly strengthened in the first excited (S 1) state, while this strengthening tends to decrease with the substitution of cyano group. The natural population analysis (NPA) is performed to interpret this phenomenon and the calculated results show that the introduction of cyano groups induces intramolecular charge transfer, leading to the decrease of electrostatic interaction of intramolecular hydrogen bond in the S 1 state compared to isolated HBT. Therefore, proton transfer of HBT and its derivatives become difficult with the introduction of cyano groups, which is consistent with the calculated potential barriers (1.95, 2.71 and 2.81 kcal/mol respectively) of the three molecules in the S 1 state. This work interprets the hindrance of cyanide group on the proton transfer of HBT and its derivatives in the S 1 state, which will bring new insight into the study of analogous molecular system. Graphical Abstract Unlabelled Image Highlights • The strengths of hydrogen bond are decreases with the substituted of cyano group. • The redistribution of charge will promote the ESIPT process. • The introduction of cyanide group in HBT can hinder the proton transfer process to some extent. [ABSTRACT FROM AUTHOR]

The intramolecular proton transfer reaction of the 2-amino-3-(2′-benzoxazolyl)-quinoline (ABO) and 2-amino-3-(2′-benzothiazolyl)-quinoline (ABT) molecules in both S 0 and S 1 states at B3LYP/6–311 ++G(d,p) level in ethanol solvent have been studied to reveal the deactivation mechanism of the tautomers of the two molecules from the S 1 state to the S 0 state. The results show that the tautomers of ABO and ABT molecules may return to the S 0 state by emitting fluorescence. In addition, the bond lengths, angles and infrared spectra are analyzed to confirm the hydrogen bonds strengthened upon photoexcitation, which can facilitate the proton transfer process. The frontier molecular orbitals (MOs) and natural bond orbital (NBO) are also calculated to indicate the intramolecular charge transfer which can be used to explore the tendency of ESIPT reaction. The potential energy surfaces of the ABO and ABT molecules in the S 0 and S 1 states have been constructed. According to the energy potential barrier of 9.12 kcal/mol for ABO molecule and 5.96 kcal/mol for ABT molecule, it can be indicated that the proton transfer may occur in the S 1 state. [ABSTRACT FROM AUTHOR]

The non-fused five-membered ring structure of 2-(1 H -pyrrol-2-yl)pyridine (5-HB) is more flexible than the fused six-membered ring structure of 10,11,12,13-tetrahydro-9 H -quinolino[8,7- a ]carbazole (6-HB) to form the intramolecular hydrogen bond between pyrrole and pyridine, while the six-membered ring structure possesses more favorable distance and orientation between pyrrole and pyridine. In the present work, we carried out the electronic structure calculations and nonadiabatic dynamics simulations to gain insight into the effect of different structure feature on the excited-state intramolecular proton transfer (ESIPT) processes of 5-HB and 6-HB. The geometric parameters, IR vibrational spectra, bond critical point (BCP) parameters and reduced density gradient (RDG) show that the intramolecular hydrogen bond of 6-HB is much stronger than that of 5-HB. The 6-HB has a lower kinetic stability and a higher chemical activity than 5-HB through the analysis of the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The pyridine N1 atom of 6-HB shows much stronger ability than that of 5-HB to attract the hydrogen proton based on the natural population analysis (NPA). The analysis of the potential energy curves and time evolution of selected bond distances indicate that the ESIPT process of fused six-membered ring structure is easier and faster than the non-fused five-membered ring structure. Our findings might be useful for the design of relevant ESIPT systems. [ABSTRACT FROM AUTHOR]

2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol (BBT) is a newly synthesized water-soluble fluorophore dye with two intramolecular hydrogen bonds. It was demonstrated that BBT exhibits the excited-state intramolecular single and double proton transfer emissions in recent experiment. In order to explain the phenomenon, the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) are used to study the ground states and excited states of BBT and its tautomers. The theoretical results confirm that the hydrogen bonds are strengthened upon photoexcitation. To further reveal the proton transfer mechanism, potential energy surfaces of the S 0 and S 1 states have been constructed. According to the energy potential barrier of 13.69 kcal/mol, it was indicated that the double proton transfer may occur in stepwise rather than simultaneously in the S 1 state unless the excited molecule can overcome the barrier. [ABSTRACT FROM AUTHOR]

Time-dependent density functional theory (TD-DFT) method at B3LYP/6-31G(d,p) theoretical level was employed to investigate the excited-state intramolecular proton transfer (ESIPT) reaction of 6-amino-2-(2′-hydroxyphenyl)benzoxazole in dichloromethane and methanol solvents. Upon photo-excitation, the intramolecular hydrogen bond between the hydroxyl and neighboring N atom is significantly strengthened, which provides a driving force in facilitates the ESIPT reaction. The calculated steady-state absorption and fluorescence spectra agree well with the experimental results. Especially, after the photo-excitation from HOMO (π) to LUMO (π*), the rearrangement of electronic density distribution of frontier molecular orbitals (MOs) is a very important positive factor for the ESIPT process. The potential energy curves confirm that, after photo-excitation, the EXIPT reaction occurs with the H atom of the hydroxyl group remove to the neighboring N atom. By contrast, the potential barrier of the 6A-HBO-MeOH complex in the S 1 state falls ca. 3.57 kcal/mol, lower than the isolated 6A-HBO. It is likely that the ESIPT reaction occurs more easily for the 6A HBO-MeOH complex due to the influence of the intermolecular hydrogen bonding. [ABSTRACT FROM AUTHOR]

We propose the double interaction mechanism between Al3+ ions and OCN or the isomer structure OCN-T. The absorption spectra of the sensors (OCN and OCN-T) and complexes with Al3+ (OCN-Al and OCN-T-Al) calculated by our theory agree well with the experimental values. For the fluorescence spectra, the OCN structure is at 429 nm, OCN-T emits a near-infrared fluorescence at 1380 nm, OCN-AL at 460 nm, and OCN-T-AL has no fluorescence. It shows that OCN and its isomer OCN-T are involved in the absorption, and the detected spectral signal is emitted from the OCN-AL complex. [Display omitted] • The double interaction mechanism between Al3+ ions and OCN or the isomer structure OCN-T is proposed. • It is found that the OCN-T molecule directly undergoes excited state proton transfer. • The hydrogen bonding of the OCN molecule is enhanced in the excited state. A new action mechanism for the fluorescent detection on the Al3+ ion of the sensitive 1-((2-hydroxynaphthalen-1-yl)methylene)urea(OCN) is theoretically studied. The extensive theoretical calculations on the OCN and the isomer structure OCN-T are performed. The emission and absorption spectra consistent with the experiment value. The absorption spectra peaks (362 nm and 326 nm) of OCN and OCN-T molecules are attributed to the experimentally observed absorption spectra at 356 nm and 314 nm, respectively. The calculated fluorescence value of the OCN-AL structure is 460 nm, while the OCN-T-AL structure has no fluorescence. These results better explain that OCN and its isomers OCN-T are involved in the absorption, and the detection spectrum signal is emitted from the OCN-AL complex. The OCN and OCN-T molecules are obvious hydrogen bonding systems. The excited state intramolecular proton transfer photochemical behaviors and detecting Al3+ ion photophysical changes were explained for the first time at the molecular level. As driving force of excited state intramolecular proton transfer (ESIPT) reaction, the bond parameters and vibrational frequencies of intramolecular hydrogen bond were analyzed by optimizing structures and calculating infrared spectra, analysis of frontier molecular orbitals. To further elucidate the proton transfer reactive paths, the scanned the potential energy curves (PECs) of OCN and OCN-T chemosensor in different electronic states are plotted. This work proposes a reasonable explanation for the detection mechanism of the OCN sensor. [ABSTRACT FROM AUTHOR]

[Display omitted] • The intermolecular PT of DHC induces the main fluorescence emission in the S 1 state. • Intramolecular hydrogen bond hinders the intermolecular PT. • The dihedral angle torsion of DHC-OH is hindered by the intramolecular hydrogen bond. The fluorescence behavior of firefly luciferin analogues 2-(4-(6-hydroxybenzo[d]thiazol-2-yl)phenyl)-4,5-dihydrothiazole-4-carboxylic acid (DHC) has been theoretically studied. DHC and water molecules undergo intermolecular proton transfer (PT) and form the anion configuration in the excited state. The fluorescence of this anion form (603 nm) is consistent with the experimental result (595 nm). The hydroxyl substitution DHC-OH forms an intramolecular hydrogen bond O1–H1•••N2 that hinders the intermolecular PT and induces the intramolecular PT. The fluorescence peak of DHC-OH-PT at 528 nm was consistent with the experimental results (518 nm), which confirmed that DHC-OH mainly undergoes intramolecular PT. [ABSTRACT FROM AUTHOR]

• Excited state intramolecular proton transfer (ESIPT) and vibration relaxation (VR) processes of 1, 8-DHAQ in ethanol were studied by femtosecond transient absorption spectroscopy combined with theoretical calculation. • ESIPT process is supported by optimized geometries of the S 0 and S 1 state, as well as the experimental and calculated dual fluorescence. • The ultrafast processes of ESIPT and VR are sufficiently analyzed, and the lifetimes are 96.8 fs and 4.1 ps, respectively. Photoinduced dynamics of excited-state intramolecular proton transfer (ESIPT) and vibration relaxation (VR) of 1,8-DHAQ in ethanol are investigated by time-resolved transient absorption spectroscopy combined with the quantum chemical calculations. Experimental and calculated results show dual fluorescence is obtained from the S 1 state of the normal form of 9,10-quinone and the tautomer form of 1,10-quinone. Three characteristic bands of excited state absorption and stimulated emission are observed in transient absorption spectroscopy upon excitation with ultrafast laser pulses at 390 nm. ESIPT process happens within 96.8 fs from the normal S 1 state to the tautomer S 1 state, and the VR process is observed in the time constant of 4.1 ps. Ultimately, the tautomer S 1 state decays within 269.1 ps for a deactivation pathway of emitting fluorescence. The mechanism of ESIPT dynamics and ultrafast excited-state dynamics in 1,8-DHAQ in ethanol is concluded by a schematic diagram of four electronic states and vibrational energy levels. [ABSTRACT FROM AUTHOR]

• The hydrogen bond of 6AI trimer in the excited state were discussed. • The reason of hydrogen bond strength was found that is charges transfer. • We got the detailed proton transfer mechanism for 6AI trimer. • The absorption spectra observed in the experiment has been reclassified. The proton transfer mechanism of 6AI trimer in cyclohexane has been theoretical study by time-dependent density functional theoretical methods. After three 6AI monomers are connected to form trimer by hydrogen bonds, the calculated absorption spectra confirm that the experimental absorption peaks of 6AI monomer (about 280 nm) and 6AI trimer (310 nm) are attributed to the 6AI trimer and TPT tautomer respectively. Through the potential energy curves and the potential energy surface, it can be concluded that the 6AI trimer reaches the TPT tautomer in the S1 state through the concerted mechanism rather than the stepwise mechanism. [ABSTRACT FROM AUTHOR]

The sensing mechanism of a salicylideneaniline-based fluorescence chemosensor (SB1) for the cyanide anion (CN−) has been investigated by nonadiabatic dynamics and transition state (TS) theory. The intramolecular hydrogen bonds in SB1 and the product SB1-CN are strengthened after photoexcitation, which induce the intramolecular proton transfer in 114 fs and 167 fs by the results of nonadiabatic dynamics. The fluorescence (626 nm) of SB1 emits from the proton-transfer tautomer rather than the excited state intramolecular charge transfer (ICT) state. The new appeared emission peak (523 nm) after the addition of CN− is attributed to the proton-transfer tautomer of SB1-CN, which has a large blue shift (103 nm) compared to that of SB1. The TS calculations demonstrate the occurrence of reaction from SB1to SB1-CN after overcoming an energy barrier of 10.52 kcal/mol and there is a large interaction energy (95.92 kJ/mol) between SB1 and CN−, both of which confirm that SB1 has high sensitivity and selectivity to CN−. The nucleophilic addition reaction of CN− interrupts the π-conjugation structure of SB1, resulting in an inefficient ICT, which in turn destroys the excited state intramolecular charge/proton transfer (ESICT-ESIPT) coupled system and causes the blue shift of the spectral peak. The work gives a complete explanation about ESICT-ESIPT coupling switching behavior and the sensing mechanism of SB1 for CN−. • Calculated reaction barrier and interaction energy indicate that SB1 has high sensitivity and selectivity to CN−. • Nonadiabatic dynamics confirm that the proton transfer processes of SB1 and SB1-CN occur in the S 1 state. • The emission peaks measured in the experiment are both attributed to the proton-transfer tautomers. • The destruction of the π-conjugation structure of SB1 by CN− causes the blue shift of the spectral peaks. [ABSTRACT FROM AUTHOR]

The excited-state molecular dynamics of 7-hydroxyquinoline-8-carboxylic acid (HCA) were studied to observe its fluorescent properties and proton transfer process. The two intramolecular hydrogen bonds (O-H⋯O and O-H⋯N) of the Enol form were both strengthened after photoexcitation to the first excited (S 1) state. The experimentally observed fluorescence emission (465 nm) was attributed to the theoretical Keto form (470 nm), which implies the occurrence of an excited-state intramolecular double proton transfer (ESIDPT) process. The nonadiabatic dynamics results demonstrate that a single proton transfer from the carboxylic to the nitrogen atom (process-A) occurs in 55 fs, which excludes the ESIDPT concert pathway. The potential energy surface results indicate that process-A (0.08 kcal/mol) induces the occurrence of a second proton transfer from the phenol to the oxygen atom (process-B). Compared to the stepwise ESIDPT reaction that begins with process-B (1.53 kcal/mol), process-A is energy favorable in the S 1 state. Therefore, we propose a reaction path of the following: Enol in the ground state (S 0) → Enol in the S 1 state → proton transferred Keto in the S 1 state (stepwise pathway begins with process-A) → Keto in the S 0 state (fluorescence emission at 470 nm) → Enol in the S 0 state (reversed proton transfer). This process confirms what was predicted by Chou (J. Phys. Chem. Lett. 2011, 2, 3063). Unlabelled Image • The Enol form changes to Keto form through the ESIDPT progress. • The ESIDPT progress significantly affects the fluorescence properties. • The stepwise double proton transfer process is energy favorable in the excited state. [ABSTRACT FROM AUTHOR]

The fluorescent behaviors and nonadiabatic dynamics of salicylideneanilines and its two derivatives have been theoretical studied. The enol form transfers to cis -keto after photoexcitation to the first single (S 1) excited state without any energy barrier, and the derivatives with substituent of chlorine and hydroxyl have similar dynamics progress. The excited state intramolecular proton transfer occurs in 46 fs by the results of nonadiabatic dynamics together with photoexcitation progress, which is also demonstrated by the results of geometric structure and infrared spectra. The potential curves scan indicates the energy difference before and after the proton transfer reaction of Enol (11.12 kcal/mol) is larger than that of Enol-CL (8.97 kcal/mol), and the Stokes shift of Enol (257 nm) is larger than that of Enol-CL (200 nm) and Enol-OH (198 nm). This comparison confirms the substitute of chlorine and hydroxyl hinder the decrease in aromaticity after photoexcitation and energy difference between the ground (S 0) and the S 1 state, thereafter induce smaller Stokes shift. The energy barrier between Enol and Cis -keto (5.96 kcal/mol) and energy difference between Cis -keto and Trans -keto (10.27 kcal/mol) are so small that there is a decay pathway of Enol (S 0)→proton transferred Cis -keto (S 1)→fluorescent Cis -keto (S 0)→Enol (S 0) or Cis -keto (S 1) → Trans -keto (S 1)→fluorescence of Trans -keto (S 0)→Enol (S 0). Image 1 •. The proton transfer of SA is demonstrated by steady and transient configuration. •. Substituent induces smaller Stokes shift by hindering the decrease in aromaticity. •. Two main decay pathways of SA are proposed with their fluorescent dynamics. [ABSTRACT FROM AUTHOR]