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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]

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]

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 internally contracted multi-reference configuration interaction method (MRCI) with Davidson modification and the Douglas–Kroll scalar relativistic correction has been used to calculate the BSe molecule at the level of aug-cc-pV5Z basis set. The calculated electronic states, including 9 doublet and 6 quartet Λ-S states, are correlated to the dissociation limit of B( 2 P u ) + Se( 3 P g ) and B( 2 P u ) + Se( 1 D g ). The Spin-orbit coupling (SOC) interaction is taken into account via the state interaction approach with the full Breit-Pauli Hamiltonian operator, which causes the entire 15 Λ-S states to split into 32 Ω states. This is the first time that the spin-orbit coupling calculation has been carried out on BSe. The potential energy curves of the Λ-S and Ω electronic states are depicted with the aid of the avoided crossing rule between electronic states of the same symmetry. The spectroscopic constants of the bound Λ-S and Ω states were determined, which are in good agreement with the experimental data. The transition dipole moments (TDMs) and the Franck-Condon factors (FCs) of the transitions from the low-lying bound Ω states A 2 Π(I) 3/2 , B 2 Π(I) 1/2 and C 2 Δ(I) 3/2 to the ground state X 2 Σ + 1/2 have also been presented. Based on the previous calculations, the radiative lifetimes of the A 2 Π(I) 3/2 , B 2 Π(I) 1/2 and C 2 Δ(I) 3/2 were evaluated. [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 The sensing mechanism of the ratiometric fluorescence chemosensor 2-((6-(diethylamino)quinolin-2-yl)methylene)malononitrile (3A) for the cyanide anion (CN−) has been investigated theoretically. The calculated results show that the small reaction barrier (8.58 kcal/mol) of 3A implies a rapid response speed to CN−, and that the large interaction energy (25.75 kcal/mol) between 3A and CN− indicates a high selectivity to CN−. The results of condensed dual descriptor calculation confirm that CN− attacks the C 2 site of 3A rather than other sites. The nucleophilic addition reaction of CN− breaks the original conjugation structure of 3A and results in the redistribution of the charge. The frontier molecular orbitals (MOs) and the Hirshfeld population analysis demonstrate that the long-rang electronic transition between the diethylamino moiety and the dicyanovinyl group in 3A is cut off after the addition of CN− and a local electronic transition between the quinoline and diethylamino moiety is formed. These changes lead to a blue shift (63 nm) of the fluorescence emission in the nucleophilic addition product (P) compare to 3A. Graphical abstract Unlabelled Image Highlights • 3A has an excellent selectivity to CN-. • The C 2 site in 3A is the reaction site. • The different charge transfer results in the spectral changes. [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]

Abstract Asymmetric substitution has not been termed as an essential factor in studying photo-induced ultrafast dynamics of molecular system. Asymmetric 4-hydroxybut-3-en-2-one (HEO), together with symmetric malonaldehyde (MA) and acetylacetone (AA), have been provided as target sample to study the nonradiative decay (ND) processes of β-diketones. An effective ND pathway of the three molecules is presented that their excited second (S 2) states transfer to first (S 1) state by nonadiabatic surface hopping, and then transfer to triplet (T 1) state by crossing minimum energy crossing point (MECP), after which decay to ground (S 0) state through MECP. More importantly, the asymmetric substitution of HEO induces the proton transfer in the S 1 state and generates a proton-transferred conformer with lowest energy, which does not occur for MA and AA. This change exploits a new ND pathway that the S 1 state decays to the proton transferred T 1 state and then undergoes reverse proton transfer to S 0 state through the MECPs between the three states. The two pathways of HEO give detailed energy and geometric information on surface hopping of S 2 /S 1 and MECPs of S 1 /T 1 /S 0 , and interpret the reason of the ND pathway while not spectra emission. This result is significantly different from the previous reported ND pathway of photoisomerization or conical intersection between different states. This work shows that asymmetric substitution changes the molecular structure and then changes their spectra behaviors. Graphical Abstract Unlabelled Image Highlights • Asymmetric substitution is termed as essential factor in nonradiative decay progress. • An effective nonradiative decay pathway of symmetric β-diketones is provided in detail. • Asymmetric substitution exploits a new nonradiative decay pathway with proton transfer. • Excited state molecular dynamics and MECP searching is both used in this work. [ABSTRACT FROM AUTHOR]

The control of internal molecular motions by outside stimuli is a decisive task in the construction of functional molecules and molecular machines. Light-induced intramolecular rotations of photoswitches have attracted increasing research interests because of the high stability and high reversibility of photoswitches. Recently, Henry et al. reported an unprecedented two-dimensional controlled photoswitch, the hemithioindigo (HTI) derivative Z1, whose single bond rotation in dimethyl sulphoxide (DMSO) solvent and double bond rotation in cyclohexane solvent can be induced by visible light (J. Am. Chem. Soc. 2016, 138, 12,219). Here we investigate the intramolecular rotations of the HTI and Z1 in different polar solvents by time-dependent density functional theory (TDDFT) and Nonadiabatic dynamic simulations. Due to the steric hindrance between methyl and thioindigo fragment, the rotations of Z1 in the excited state are obstructed. Interestingly, the HTI exhibits two distinct rotation paths in DMSO and cyclohexane solvents at about 50 fs. The intermolecular hydrogen bonds between HTI and DMSO play an important role in the rotation of HTI in DMSO solvent. Therefore, the HTI is a more promising two-dimensional photoswitch compared with the Z1. Our finding is thus of fundamental importance to understand the mechanisms of this class of photoswitches and design complex molecular behavior. [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]

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]

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]

Abstract: The potential energy curves (PECs) of the X3Π, a1Σ+, b1Π, A3Σ+, B3Σ−, c1Δ, D3Π, 15Π, 31Σ+, 33Π, 21Π, 23Σ+, 13Δ, 15Σ+, 43Π, 23Σ− and 15Σ− electronic states of the BN molecule are calculated using an ab initio quantum chemical method. The PEC calculations have been made for internuclear separations from 0.06 to 1.20nm using the complete active space self-consistent field (CASSCF) method, which is followed by the valence internally contracted multireference configuration interaction (MRCI) approach in combination with a correlation-consistent aug-cc-pV5Z basis set. To improve the quality of PECs, core-valence correlation and relativistic corrections are included. Relativistic correction calculations are carried out using the third-order Douglas–Kroll Hamiltonian (DKH3) approximation. Core–valence correlation corrections are included using a cc-pCVQZ basis set. Relativistic corrections are calculated at the level of a cc-pVQZ basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are corrected for size-extensivity errors by means of the Davidson modification (MRCI+Q). These PECs are extrapolated to the complete basis set limit by the total-energy extrapolation scheme. The spectroscopic parameters are determined by fitting the vibrational levels, which are calculated in a direct forward manner from the analytic potential by solving the ro-vibrational Schrödinger equation using Numerov''s method. The spectroscopic results have been compared in detail with those reported in the literature. Excellent agreement has been found between the present spectroscopic results and the experimental ones. Using the Breit-Pauli operator, the spin–orbit coupling effect on the spectroscopic parameters is included in the X3Π and D3Π electronic states. The vibrational level, inertial rotation and centrifugal distortion constants are calculated for each vibrational state of each electronic state. And those of the first 20 vibrational states of each electronic state are reported when the rotational quantum number equals zero. Comparison with the measurements shows that the present results are accurate. The spectroscopic parameters of the c1Δ, 15Π, 31Σ+, 33Π, 21Π, 23Σ+, 13Δ, 15Σ+, 43Π and 15Σ− electronic states and the vibrational manifolds of all the electronic states obtained here are expected to be reliable predicted results. [Copyright &y& Elsevier]

[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]