Search

The time-dependent density functional theory method was performed to investigate the electronically excited states of the hydrogen-bonded complex formed by coumarin 102 (C102) chromophore and the hydrogen-donating aniline solvent. At the same time, the electronic excited-state hydrogen-bonding dynamics for the photoexcited C102 chromophore in solution was also reconsidered. We demonstrated that the intermolecular hydrogen bond CO...H-N between C102 and aniline molecules is significantly strengthened in the electronically excited-state upon photoexcitation, since the calculated hydrogen bond energy increases from 25.96 kJ/mol in the ground state to 37.27 kJ/mol in the electronically excited state. Furthermore, the infrared spectra of the hydrogen-bonded C102-aniline complex in both the ground state and the electronically excited state were also calculated. The hydrogen bond strengthening in the electronically excited-state was confirmed for the first time by monitoring the spectral shift of the stretching vibrational mode of the hydrogen-bonded N-H group in different electronic states. Therefore, we believed that the dispute about the intermolecular hydrogen bond cleavage or strengthening in the electronically excited-state of coumarin 102 chromophore in hydrogen donating solvents has been clarified by our studies.

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 conversion of cholesterol to pregnenolone is a physiologically essential process which initiates with two sequential hydroxylation processes catalyzed by cytochrome P450 side-chain cleavage enzyme (P450). Extensive efforts have been exerted; however, the mechanistic details remain obscure. In this work, we employed the dispersion-corrected density functional theoretical (DFT-D) calculations to investigate the mechanistic details of such hydroxylation processes. Calculated results reveal that the active intermediate Compound I (CpdI) of P450 hydroxylates cholesterol efficiently, which coincides with previous spectrometric observations. The hydrogen bond effect of water molecule within the active site lowers the energy barrier significantly. Intriguingly, the adjacent hydrogen bond (H-bond) between the hydroxyl group of the substrate and the oxo group of CpdI in the second hydroxylation affects the H-abstraction significantly. Such H-bond was weakened during the C-H bond activation process, increasing the energy barriers by approximately 2 kcal/mol, which is different to the intermolecular H-bond effect of water found by Shaik et al. that decreases the barrier by about 4 kcal/mol. Such adjacent H-bond also affects the transition state by bending the alignment of the C-H-O moiety, and consequently lowering the kinetic isotope effect values. Besides, a series of DFT-D calculations (Grimme's D2, D3-zero, and D3-BJ methods) were performed and accessed to find out an appropriate protocol for H-bond containing hydroxylation process. Our results show that DFT-D single-point energies (SPE) based on geometries optimized with non-dispersion-corrected DFT varies drastically and sometime presents unreasonable results. DFT-D SPE calculations on DFT-D optimized geometries present stable and reasonable results. [ABSTRACT FROM AUTHOR]

The time-dependent density functional theory method was performed to investigate the excited-state hydrogen-bonding dynamics of N-(2-hydroxyethyl)-1,8-naphthalimide (2a) and N-(3-hydroxyethyl)-1,8-naphthalimide (3a) in methanol (meoh) solution. The ground and excited-state geometry optimizations, electronic excitation energies, and corresponding oscillation strengths of the low-lying electronically excited states for the complexes 2a + 2meoh and 3a + 2meoh as well as their monomers 2a and 3a were calculated by density functional theory and time-dependent density functional theory methods, respectively. We demonstrated that the three intermolecular hydrogen bonds of 2a + 2meoh and 3a + 2meoh are strengthened after excitation to the S1 state, and thus induce electronic spectral redshift. Moreover, the electronic excitation energies of the hydrogen-bonded complexes in S1 state are correspondingly decreased compared with those of their corresponding monomer 2a and 3a. In addition, the intramolecular charge transfer of the S1 state for complexes 2a + 2meoh and 3a + 2meoh were theoretically investigated by analysis of molecular orbital. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The geometrical and spectral properties of the hydrogen-bonded clusters formed by 6-aminocoumarin (6AC) with solvents of different hydrogen-bonding abilities have been investigated at the CPCM-PBE0/6-311++G(d, p) level of theory. Upon photoexcitation, A type hydrogen bonds will be weakened whereas hydrogen bonds of B and C types should be strengthened. The weakening of hydrogen bond A is responsible for the blue-shifts of the absorption spectra in HFIP and TFE while strengthening of hydrogen bonds B and B are the reasons for the red-shifts of the absorption spectra in DMSO. The absorption spectra of cluster 6AC-(HO) is in better agreement with the experimental result than 6AC-(HO), which dose not support the conjecture of E. Krystkowiak. Moreover, the stabilization effects of the different types of intermolecular hydrogen bonding on the absorption spectra properties of the hydrogen-bonded 6AC clusters are discussed in detail. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The time-dependent density functional theory (TDDFT) method has been carried out to study the influences of hydrogen bonding and solvent polarity on the spectral properties of 4-aminophthalimide (4AP) clusters formed with hydrogen-accepting solvents triethylamine (TEA) and dimethyl sulfoxide (DMSO). The ground- and S-state geometry structure optimizations, hydrogen bond energies, absorption and emission spectra for both the 4AP monomer and its two triply hydrogen-bonded clusters 4AP + (TEA) and 4AP + (DMSO) have been calculated using DFT and TDDFT methods respectively with the hybrid exchange correlation functional PBE1PBE and split-valence basis set 6-311++G(d,p). It has been demonstrated that the two hydrogen bonds I and II formed with the amine group of 4AP are significantly strengthened while the hydrogen bond III formed with the imide group is slightly weakened due to the intramolecular charge transfer from the amine group to the two carbonyl groups of the 4AP molecule upon photoexcitation. In addition, the hydrogen bonds formed by 4AP with DMSO are stronger than those formed with TEA, which together with its strong polarity, should be the main reasons for the more redshifts of both the absorption and the fluorescence spectra of 4AP in solvent DMSO than those in TEA. [ABSTRACT FROM AUTHOR]

In this work, the time-dependent density functional theory (TD-DFT) method was used to study the electronic excited-state dynamics of the hydrogen-bonded p-Cresol-NH-HO complex. The intermolecular hydrogen bonds O-H···N and C-O···H were demonstrated by the optimized geometric structure of the hydrogen-bonded p-Cresol-NH-HO complex. The infrared spectra (IR spectra) of the hydrogen-bonded p-Cresol-NH-HO complex in the ground and excited states were also calculated by using the density functional theory (DFT) and TD-DFT methods. It is demonstrated that hydrogen bond O-H···N can be strengthened while hydrogen bond C-O···H is weakened upon photoexcitation to the S state. The significant changes of the hydrogen bond from the calculated bond lengths in different electronic states can be observed. In addition, the spectral shifts of the stretching vibrational mode of the hydrogen-bonded O-H group in different electronic states are accounted for the hydrogen bond changes in the S state too. [ABSTRACT FROM AUTHOR]

Abstract: Significant intermolecular hydrogen bonding strengthening in the excited state is proposed to facilitate the photoinduced electron transfer process from the electron donating solvents aniline (AN) and N-methylaniline (MAN) to the coumarin dye 337 (C337). Time-dependent density functional theory (TD-DFT) method has been applied to investigate the ground and the two lowest singlet excited states of the hydrogen-bonded C337–AN and C337–MAN compounds, with a view to monitoring their geometries and photochemical properties. The present theoretical study indicates that the PET process which occurs in the lowest singlet excited state S1 after the full relaxation of the hydrogen-bonded complex in the S2 state should be coupled with and facilitated by the significant strengthening of the intermolecular hydrogen bonding. [ABSTRACT FROM AUTHOR]

Abstract: The time-dependent density functional theory (TDDFT) method has been applied to investigate the excited states of 2′-deoxyguanosine (2′-dG) and its monohydrated complex (2′-dG-W1). The intramolecular hydrogen bonding N3⋯H2–O2 of the keto conformer is weakened in the S1 state, while that of the enol conformers is strengthened. The two intermolecular hydrogen bondings of 2′-dG-W1 are weakened in the S1 state, while N3⋯H2–O2 gets more strengthened. Moreover, the fluorescence of 2′-dG-W1 is shifted to higher frequencies compared with that of monomer 2′-deoxyguanosine. Therefore, it is concluded that the weakening of intermolecular hydrogen bonding in the S1 state induces fluorescence strengthening of 2′-deoxyguanosine. [Copyright &y& Elsevier]

Abstract: Four low-lying conformers (Ia, Ib, IIa, IIb) of 2-allylphenol are studied by density function theory (B3LYP) with the aug-cc-pVDZ basis set. The conformer Ia having the intramolecular hydrogen bond OH---π is the most stable. The photoelectron spectrum (PES) of 2-allylphenol recorded by Kowshi et al. (2001) are reassigned on the basis of the present electron propagator theory calculations. The results show that the strong ionization band at 10.01eV is contributed from the most stable conformer Ia, and the weak ionization band at 9.72eV is from the next stable conformer IIb. [Copyright &y& Elsevier]

Abstract: The combined density functional theory (DFT) and time-dependent density functional theory (TDDFT) method was used to study the electronic spectral properties of different deprotonated forms of esculetin. By comparing the experimental absorption and fluorescence bands with the calculated electronic spectra, it is evidently demonstrated that the minor absorption and fluorescence bands observed at slightly longer wavelengths than the principal bands in experiments are predominantly from the de-H3 form of the esculetin monomer. Furthermore, we clarified the relationship between electronic spectral shifts and electronic excited-state intramolecular hydrogen bonding changes: the strengthening of intramolecular hydrogen bond can induce an electronic spectral blueshift while the intramolecular hydrogen bond weakening can result in an electronic spectral redshift. [Copyright &y& Elsevier]

Abstract: The time-dependent density functional theory (TDDFT) method was carried out to investigate the excited state intramolecular proton transfer (ESIPT) process of 3-hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one (1a). 1a has two tautomeric forms: one is 1a(O), which is induced by intramolecular hydrogen bond O–H⋯O=C, and the other one is 1a(N), which is caused by intramolecular hydrogen bond O–H⋯N. From excited state to tautomer excited state coming from ESIPT, the hydroxyl hydrogen breaks away and the dissociated hydrogen adsorbed on pyridinic nitrogen or carbonyl oxygen formed new intramolecular HB and the corresponding bond length and bond angle varied greatly. In comparison, a similar process of proton transfer for 1a(N)H+ protonated 1a(N) from ground state to excited state was obtained. This detailed proton transfer mechanism was provided by molecular orbitals analysis and it may be applied to molecular switch and organic Lewis acid/base. We investigated the excited state proton transfer mechanism of the four molecules through the theoretical method for the first time and gave unambiguous geometry of excited state. [Copyright &y& Elsevier]

Abstract: In the present work, the electronic excited-state hydrogen bonding dynamics of coumarin chromophore in alcohols is revisited. The time-dependent density functional theory (TDDFT) method has been performed to investigate the intermolecular hydrogen bonding between Coumarin 151 (C151) and methanol (MeOH) solvent in the electronic excited state. Three types of intermolecular hydrogen bonds can be formed in the hydrogen-bonded C151–(MeOH)3 complex. We have demonstrated again that intermolecular hydrogen bonds between C151 and methanol molecules can be significantly strengthened upon photoexcitation to the electronically excited state of C151 chromophore. Our results are consistent with the intermolecular hydrogen bond strengthening in the electronically excited state of Coumarin 102 in alcoholic solvents, which has been demonstrated for the first time by Zhao et al. At the same time, the electronic excited-state hydrogen bond cleavage mechanism of photoexcited coumarin chromophores in alcohols proposed in some other studies about the hydrogen bonding dynamics is undoubtedly excluded. Hence, we believe that the two contrary dynamic mechanisms for intermolecular hydrogen bonding in electronically excited states of coumarin chromophores in alcohols are clarified here. [Copyright &y& Elsevier]

The photophysical properties and excited state intramolecular proton transfer (ESIPT) of salicylideneaniline (1a) and its derivatives (1b - 1e) with different substituents have been investigated using the DFT and TD-DFT methods. The calculated results indicate that the introduction of electron-withdrawing group CN weakens the intramolecular hydrogen bond (H···N). However, the introduction of electron-donating group N(CH 3) 2 strengthens it. When the CN and N(CH 3) 2 groups are introduced simultaneously, the intramolecular hydrogen bond (H···N) is weakened. In addition, swapping the CN and N(CH 3) 2 group positions can enhance the intramolecular hydrogen bond (H···N). Compared to 1a , the absorption and emission spectra of compounds 1b - 1e are red-shifted. Frontier molecular orbital analyses prove that the more intense intramolecular charge transfer characters caused by CN and N(CH 3) 2 substituents is responsible for the red shift of spectra. Potential energy curves indicate that ESIPT in salicylideneaniline (1a) and the CN substituted derivative (1b) is a non-barrier process, while in the N(CH 3) 2 substituted derivative (1c) and the CN and N(CH 3) 2 co-substituted derivative (1d), ESIPT needs to overcome the energy barriers of 2.32 kcal/mol and 3.38 kcal/mol, respectively. Exchanging the positions of CN and N(CH 3) 2 groups (1e) makes the ESIPT into a barrier-free process. Therefore, the substitution and position of CN and N(CH 3) 2 groups can affect the ESIPT process and induce different photophysical properties. Unlabelled Image • CN substituent weakens the intramolecular hydrogen bond (H···N) in the S 0 state. • N(CH 3) 2 substituent strengthens the H···N bond in the S 0 state. • CN and N(CH 3) 2 substituents make the absorption and emission spectra red-shifted. • The energy barriers of the S 1 state are affected by CN and N(CH 3) 2 groups. [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]

The time-dependent density functional theory (TDDFT) is used to investigate the effects of solvent polarity and hydrogen bond on coumarin 500 (C500). Both the absorption maximum and the hydrogen bond strength of C500 hydrogen bonded complex are confirmed to have a linear relationship with the solvent polarity function Δf. The type B (at carbonyl group position) complex whose absorption spectrum is extremely agreement with the experimental results is demonstrated to be preponderant among all the hydrogen bonded complexes. The investigation of C500-phenol complex (type B) shows that the strengthening of hydrogen bond in the S 1 state induces the fluorescence intensity of C500 weakened. The solvent polarity is the key influencing factor on intermolecular hydrogen bond by comprising the influence of solvent polarity and substituent on C500. [ABSTRACT FROM AUTHOR]

Highlights: [•] From FPe to FPe–3MeOH, hydrogen bonds are strengthened and increases dipole moment. [•] In S1, hydrogen bonds are further strengthened and leads to larger dipole moments. [•] Hydrogen-bond strengthening plays important role in the charge transfer process. [•] Hydrogen-bond strengthening lead to red-shifts of electronic and vibrational spectra. [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]

Researchers have proposed different effective mechanisms of hydrogen bonding (HB) on the fluorescence of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) and its derivatives. Herein, excited state transition and dynamics analysis confirm that the fluorescence of PD (a derivative of PRODAN with ethyl replaced by 3-hydroxy-2,2-dimethylpropan) emits from the planar intramolecular charge transfer (PICT) state rather than twist ICT (TICT) state, because the fluorescence emission and surface hopping from the TICT state to the twist ground (T-S 0 ) state is energy forbidden. Nevertheless, the strengthening of intramolecular-HB (intra-HB) and intermolecular-HB (inter-HB) of PD-(methanol) 2 smooth the pathway of surface hopping from TICT to T-S 0 state and the external conversion going to planar ground state by decreasing the energy difference of the two states. This smoothing changes the fluorescence state of PD-(methanol) 2 to the TICT state in which fluorescence emission does not occur but surface hopping, leading to the partial fluorescence quenching of PD in methanol solvent. This conclusion is different from previous related reports. Moreover, the inter-HB strengthening of PD-methanol in PICT state induces the cleavage of intra-HB and a fluorescence red-shift of 54 nm compared to PD. This red-shift increases to 66 nm for PD-(methanol) 2 for the strengthening of the one intra-HB and two inter-HBs. The dipole moments of PD-methanol and PD-(methanol) 2 respectively increase about 10.3D and 8.1D in PICT state compared to PD. The synergistic effect of intra-HB and inter-HB induces partial quenching of PD in methanol solvent by turning on the TICT state and fluorescence red-shift. This work gives a reasonable description on the fluorescence red-shift and partial quenching of PD in methanol solvent, which will bring insight into the study of spectroscopic properties of molecules owning better spectral characteristics. [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]

The behavior of three-centered hydrogen bond (THB) in excited state is unveiled firstly. The notion proposed in experiment that the THB configuration can be stable in solution has been verified. And one more stable configuration has been found which has two intramolecular hydrogen bonds compared with THB configuration. The analysis of optimization geometrical configurations shows that the THB weakens upon photoexcitation and is apparent weaker than the two-centered hydrogen bond. The reduced electron density gradient (RDG) maps indicate that the weakening of THB compared with two-centered hydrogen bond is due to the steric repulsion of hydrogen bond donors and acceptors. In addition, fluoresce spectra indicate that the 3-position substituent of coumarin has a large blue-shift on maximum fluorescence wavelength compared with the 7-aminocoumarin C500. [ABSTRACT FROM AUTHOR]

The density functional theory (DFT) and time-dependent density functional theory (TDDFT) method were carried out to investigate the ground and excited states of indigo and its derivative molecules. The results demonstrate that the intramolecular hydrogen bond I is weakened and the intramolecular hydrogen bond II is strengthened upon photo-excitation to the S 1 state. In the absorption spectra, the substitution at R 4 R 4 , of indigo causes a significant redshift. In addition, the halogen substitution obviously increases the electron transfer mobility of indigo. It is proved that the halogen substitution may be a new method to design high performance organic semiconductors. [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]

[Display omitted] • The ESIPT of monomer PTI induces keto → enol transfer and the red-shift of fluorescent spectra. • The strengthening of intermolecular hydrogen bond hinders the occurrence of ESIPT of PTI. • The large Stokes shift of PTI is induced by hydrogen bond strengthening while not ESIPT. The fluorescent detection mechanism of 2-(4-nitro-1,3-dioxoisoindolin-2-yl) acetic acid (CORM3-green) on CO-Releasing Molecule-3 (CORM-3) is theoretically studied. Upon reaction with CORM-3, the non-fluorescent CORM3-green is transferred to the keto form of 2-(4-amino-1,3-dioxoisoindolin-2-yl)acetic acid (PTI) to produce strong fluorescence peak located at 423 nm. This peak red-shifts to 489 nm, which is induced by the strengthening of intermolecular hydrogen bond (HB) between PTI and water molecules and attributed to the experimentally observed fluorescence emission at 503 nm. This result is dramatically different from previous reports that the experimental fluorescence corresponds to the proton transferred enol form of PTI. To illustrate this confusion, the calculated fluorescence peak of PTI-Enol is located at 689 nm, which is much larger than that of experimental result. This result excludes the occurrence of excited state intramolecular proton transfer (ESIPT). It is concluded that intermolecular HBs hinders the formation of intramolecular HB and the ESIPT of the keto form of PTI. This conclusion confirms that experimental Stokes shift of 113 nm is mainly caused by the intermolecular hydrogen bonding rather than by ESIPT process. This work proposes a reasonable explanation for the detection mechanism of CORM3-green and experimental fluorescence phenomenon. [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 fluorescent behaviors and reaction mechanism of 10-hydroxybenzo[h]quinolone (HBQ) on the detection of phenylboronic acid (PBA) have been theoretical studied. The potential curves scan of HBQ in the first single (S 1) excited state indicates that the enol form transfers to keto form without any energy barrier. This enol→keto transfer is induced by excited state intramolecular proton transfer (ESIPT) in the S 1 state, which occurs in 50 fs with fluorescence emission located at 615 nm. The fluorescence peak of hydrogen bonded HBQ-PBA blue-shifts about 25 nm compared to monomer HBQ dues to the formation of intermolecular hydrogen bond. Thereafter the intermolecular hydrogen bond interrupts the ESIPT and forms reaction products of benzo[h]quinolin-10-yl hydrogen phenylboronate (BQHP) and water. The result of nonadiabatic dynamics indicates that the internal conversion of BQHP complex is forbidden and the fluorescence emission is the main decay pathway. The BQHP exists a fluorescence emission located at 564 nm (S 1), which blue-shifts about 50 nm compared with that of intermediate of HBQ-PBA. This work provides a reasonable mechanism of HBQ on the detection of phenylboronic acid. Image 1 • The proton transfer of HBQ induces the main fluorescence emission in the S 1 state. • Intermolecular hydrogen bond induce the blue-shift of fluorescent peaks of HBQ. • PBA interrupts the ESIPT and promotes the fluorescence emission of BQHP. [ABSTRACT FROM AUTHOR]