Excited-state Hydrogen Bond Research Articles

Nonadiabatic dynamics simulation of photoisomerization mechanism of the second stablest isomer of N-salicilydenemethylfurylamine

The photoisomerization processes of the second stablest isomer in the aromatic Schiff base, N-salicilydenemethylfurylamine, in the gas phase have been studied by static electronic structure calculations and surface-hopping dynamics simulations based on the Zhu-Nakamura theory. Various stable structures are obtained in the optimization because of different orientations of methyl-furyl part with respect to the salicylaldimine part and different orientations of hydroxy group with respect to the benzene ring. Upon photoexcitation into the first excited state, bond isomerization in the salicylaldimine part is completely suppressed until the strong excited-state hydrogen bond is broken. The decay pathway involves two excited-state minima, one in cis-enol form and the other in cis-keto form. After the excited-state proton transfer, twists of bonds lead to a conical intersection between the ground and excited states. After internal conversion around a conical intersection, the molecule is stabilized in cis- or trans-keto form. If the reverse hydrogen transfer process occurs in the ground state, the molecule will finally end up in the cis-enol region. The cis-keto and trans-keto isomers are observed as photoproducts. According to our full-dimensional nonadiabatic dynamics simulations, we find the excited-state intramolecular proton transfer and torsions of three single bonds in the chain to be responsible for photoisomerization of the second stablest isomer of N-salicilydenemethylfurylamine.

Insight into the excited-state intramolecular double-proton transfer of the 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol: one-step or stepwise mechanism?

The excited-state double-proton transfer mechanism of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol has been theoretically investigated based on the methods of density functional theory and time-dependent density functional theory. Geometric structure comparison and infrared vibrational spectra analysis confirm that the strengthening of the intramolecular hydrogen bond in the first excited state (S1) has facilitated the proton transfer process. The frontier molecular orbitals analysis illustrates that the nature of the hydrogen bond enhancement lies in the charge redistribution upon photo-excitation, which is further confirmed by NBO charge analysis quantitatively. The reduced dimensionality (two-dimensional) potential energy surfaces have been scanned for both the ground state (S0) and the first electronic excited state to reveal the mechanism and pathway of the two protons being transferred. Compared with the concerted path of proton transfer, the stepwise path with one proton being transferred first and then a second one followed exhibits a relatively mild energy barrier and seems to be more rational and favorable.

Orientation hydrogen-bonding effect on vibronic spectra of isoquinoline in water solvent: Franck-Condon simulation and interpretation.

The excited-state orientation hydrogen-bonding dynamics, and vibronic spectra of isoquinoline (IQ) and its cationic form IQc in water have been investigated at the time-dependent density functional theory quantum chemistry level plus Franck-Condon simulation and interpretation. The excited-state orientation hydrogen bond strengthening has been found in IQ:H2O complex due to the charge redistribution upon excitation; this is interpreted by simulated 1:1 mixed absorption spectra of free IQ and IQ:H2O complex having best agreement with experimental results. Conversely, the orientation hydrogen bond in IQc:H2O complex would be strongly weakening in the S1 state and this is interpreted by simulated absorption spectra of free IQc having best agreement with experimental results. By performing Franck-Condon simulation, it reveals that several important vibrational normal modes with frequencies about 1250 cm-1 involving the wagging motion of the hydrogen atoms are very sensitive to the formation of the orientation hydrogen bond for the IQ/IQc:H2O complex and this is confirmed by damped Franck-Condon simulation with free IQ/IQc in water. However, the emission spectra of the IQ and IQc in water have been found differently. Upon the excitation, the simulated fluorescence of IQ in water is dominated by the IQ:H2O complex; thus hydrogen bond between IQ and H2O is much easier to form in the S1 state. While the weakened hydrogen bond in IQc:H2O complex is probably cleaved upon the laser pulse because the simulated emission spectrum of the free IQc is in better agreement with the experimental results.

The role of hydrogen bonding in the fluorescence quenching of 2,6-bis((E)-2-(benzoxazol-2-yl)vinyl)naphthalene (BBVN) in methanol

The excited state hydrogen bonding dynamics of BBVN in hydrogen donating methanol solvent was explored at the TD-BMK/cc-pVDZ level of theory with accounting for the bulk environment effects at the polarizable continuum model (PCM). The heteroatoms of the BBVN laser dye form hydrogen bonds with four methanol molecules. In the formed BBVN-(MeOH)4 complex, the A-type hydrogen bond (N…HO), of an average strength of 25kJmol−1, is twofold stronger than the B-type (O…HO) one. Upon photon absorption, the total HB binding energy increases from 78.5kJmol−1 in the ground state to 82.6kJmol−1 in the first singlet (S1) excited state. In consequence of the hydrogen bonding interaction, the absorption band maximum of the BBVN-(MeOH)4 complex, which was anticipated at 398nm (exp. 397), is redshifted by 5nm relative to that of the free dye in methanol. The spectral shift of the stretching vibrational mode for the hydrogen bonded hydroxyl groups (with a maximum shift of 285cm−1) from that of the free methanol indicated the elevated strengthening of hydrogen bonds in the excited state. The vibrational modes associated with hydrogen bonding provide effective accepting modes for the dissipation of the excitation energy, thus, decreasing the fluorescence quantum yield of BBVN in alcohols as compared to that in the polar aprotic solvents. Since there is no sign of photochemistry or phosphorescence, it seems reasonable in view of the outcomes of this study to assign the major decay process of the excited singlet (S1) of BBVN in alcohols to vibronically induced internal conversion (IC) facilitated by hydrogen bonding.

Unveiling the three-center hydrogen bond dynamic behavior in ground and excited states

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.

Excited-state hydrogen bond strengthening of coumarin 153 in ethanol solvent: a TDDFT study

Excited-state hydrogen bond strengthening of coumarin 153 in ethanol solvent: a TDDFT study

TDDFT Study on Excited-State Hydrogen Bonding of 2′-Deoxyguanosine in H2O Solution

The mono and dihydrated complexes of 2′-deoxyguanosine have been used to elucidate the importance of the 2′-hydroxy group in the hydration. Density functional theory and time-dependent density functional theory methods were performed to investigate the ground-and excited-state hydrogen bonding properties of 2′-deoxyguanosine-water (2′-dG-W) and 2′-deoxyguanosine-2water (2′-dG-2W). Infrared spectra, geometric optimizations, frontier molecular orbitals and Mulliken charges have also been studied. The results demonstrated that the excited-state intramolecular hydrogen bonding dynamics of complexes 2′-dG-W and 2′-dG-2W behaves differently upon photoexcitation, while their intermolecular hydrogen bonding dynamics behaves similarly. Moreover, the significant weakening of the intermolecular hydrogen bond O4⋯H1–N1 and the formation of the new strong hydrogen bond O4⋯H3–N2 in the 2′-dG-2W upon photoexcitation were due to the geometric structure bending of guanine and the rigidity of related molecules. In addition, the charge transfer properties were theoretically investigated by analysis of molecular orbital.

Excited state hydrogen bonding fluorescent probe: Role of structure and environment

An environment sensitive fluorescent probe, 11-benzoyl-dibenzo[a,c]phenazine (BDBPZ), has been synthesized and characterized that acts via excited state hydrogen bonding (ESHB). On interaction with hydrogen bond donating solvents the fluorescence intensity of BDBPZ increases abruptly with a concomitant bathochromic shift. The extent of fluorescence increment and the red-shift of λmax depend on hydrogen bond donating ability of the solvent associated. ESHB restricts the free rotation of the benzoyl group and hence blocks the non-radiative deactivation pathway. BDBPZ forms an exciplex with organic amine in nonpolar medium that readily disappears on increasing the polarity of the solvent. In polar environment the fluorescence of both the free molecule and excited state hydrogen bonded species are quenched on addition of amine unlike its parent dibenzo[a,c]phenazine (DBPZ), that remains very much inaccessible towards the solvent as well as quencher molecules due to its structure. This newly synthesized derivative BDBPZ is much more interactive due to the benzoyl group that is flanked outside the skeletal aromatic rings of DBPZ, which helps to sense the environment properly and thus shows better ESHB capacity than DBPZ.

Hydrogen-bonding dynamics of photoexcited coumarin 138 and 339 in protic methanol solution: Time-dependent density functional theory study

The ground and electronically excited-state intermolecular hydrogen bonds between coumarin 138 (C138)/coumarin 339 (C339) and the protic methanol solvent are investigated by using time-dependent density functional theory method. The methanol solvent can act as a hydrogen-donating or hydrogen-accepting moiety at the proper site of organic chromophore. Our theoretical investigation explores the formation of one and multiple hydrogen bonds and demonstrates the strengthening of the intermolecular hydrogen bonds in the excited state. According to our calculation, upon photoexcitation the isolated coumarins and hydrogen-bonded clusters are initially excited to the first excited state and the intermolecular hydrogen bonds are strengthened in the excited state. The obvious red shifts, as large as 21nm, of the steady-state absorption spectra are observed. In addition, the infrared spectra in the ground and excited states are calculated to explore the hydrogen bonding dynamics. The calculated CO, NH, OH stretching modes are red shifted induced by the electronic excitation and intermolecular hydrogen bond interaction. The strengthening of the intermolecular hydrogen bonds is also confirmed by the geometric parameters in the ground and excited states with TDDFT method.

The Charge Transfer Phenomenon in Benzene–Pyrene–Sulfoxide/Methanol System: Role of the Intermolecular Hydrogen Bond in Excited States

Sulfoxide is an ideal functional group containing S=O for exploring excited-state hydrogen bond dynamics. Benzene–pyrene–sulfoxide (BPS) molecule, as one of the important fluorescent chemosensors, was selected to complete the hydrogen bond dynamics of sulfoxides functional group connecting to methanol (MeOH). The ground-state and excited-state geometric structures were investigated based on density functional theory and the time-dependent density functional theory methods, respectively. The calculated absorption and emission spectra of BPS chemosensor agreed well with the experimental results, demonstrating the theory we adopted is reasonable and effective. The phenomenon of variable-length S=O and H–O bands in the first excited state (S1) as well as variable-short hydrogen bond S=O···H–O demonstrated that the intermolecular hydrogen bond were strengthened. Bathochromic shift stretching vibrational modes of both S=O and H–O regions in the S1 state manifested hydrogen bond were strengthened authentically. In addition, the frontier molecular orbitals (MOs), depicting the nature of the electronically excited states, supported that the S1 state of BPS–MeOH was a local excited state with a π–π* transition, whereas the second excited state was the charge transfer state.

TDDFT study on the excited-state proton transfer of 8-hydroxyquinoline: Key role of the excited-state hydrogen-bond strengthening

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been employed to study the excited-state intramolecular proton transfer (ESIPT) reaction of 8-hydroxyquinoline (8HQ). Infrared spectra of 8HQ in both the ground and the lowest singlet excited states have been calculated, revealing a red-shift of the hydroxyl group (–OH) stretching band in the excited state. Hence, the intramolecular hydrogen bond (O–H···N) in 8HQ would be significantly strengthened upon photo-excitation to the S1 state. As the intramolecular proton-transfer reaction occurs through hydrogen bonding, the ESIPT reaction of 8HQ is effectively facilitated by strengthening of the electronic excited-state hydrogen bond (O–H···N). As a result, the intramolecular proton-transfer reaction would occur on an ultrafast timescale with a negligible barrier in the calculated potential energy curve for the ESIPT reaction. Therefore, although the intramolecular proton-transfer reaction is not favorable in the ground state, the ESIPT process is feasible in the excited state. Finally, we have identified that radiationless deactivation via internal conversion (IC) becomes the main dissipative channel for 8HQ by analyzing the energy gaps between the S1 and S0 states for the enol and keto forms.

Intermolecular hydrogen bonding of N-methylformamide in aqueous environment: A theoretical study

The intermolecular hydrogen bonding of N-methylformamide (NMF) in water is investigated. Ground-state geometry optimizations and binding energies were calculated with density functional theory (DFT). Electronic transition energies, and corresponding oscillator strengths of low-lying electronically excited states of free NMF monomers and hydrogen-bonded NMF–(H2O)2,3 complexes were calculated with time-dependent density functional theory (TDDFT). Electronic absorption spectra red-shift occurs due to the formation of the intermolecular hydrogen bonds ONMF⋯HOwater and NH⋯Owater. Larger electronic absorption spectrum red-shift in trans-NMF–(H2O)2,3 than in cis-NMF–(H2O)2,3 can be attributed to stronger excited state hydrogen bond strengthening in trans-NMF–(H2O)n complexes.

Sensing mechanism for biothiols chemosensor DCO: roles of excited-state hydrogen-bonding and intramolecular charge transfer.

The biothiols sensing mechanism of (E)-7-(diethylamino)-3-(2-nitrovinyl)-2H-chromen-2-one (DCO) has been investigated using the density functional theory (DFT) and time-dependent DFT methods. The theoretical results indicate that the excited-state intermolecular hydrogen bonding (H-B) plays an important role for the biothiols sensing mechanism of the fluorescence sensor DCO. Multiple H-B interaction sites exist in DCO and in its Michael addition product DCOT, which then induce the formation of the H-B complexes with water molecules, DCOH2 and DCOTH4. In the first excited state, the intermolecular H-Bs between water molecule and DCO in DCOH2 are cooperatively and generally strengthened and thus induced the weak fluorescence emission of DCO, while the cooperative H-Bs between water molecule and DCOT in DCOTH4 are overall weakened and thus responsible for the enhanced fluorescence emission of DCOT. Moreover, the theoretical results suggest that the blue shift of the UV-Vis absorption spectrum of DCOT can be attributed to the relatively weak excited-state intramolecular charge transfer in DCOT compared to DCO.

Intermolecular hydrogen-bonding effects on photophysics and photochemistry

Intermolecular hydrogen bonding, as an important site-specific interaction between hydrogen donor and acceptor molecules both in the gas phase and in solution, plays a significant role and has a remarkable influence on the photophysics and photochemistry of chromophores in the hydrogen-bonding surroundings. In recent years, the excited-state structures and dynamics of the intermolecular hydrogen bonding have been widely investigated by using both the experimental and theoretical methods. This review article focuses on the recent research progress of the important intermolecular hydrogen-bonding effects on the non-adiabatic photophysical processes and photochemical reactions. Firstly, the corresponding relationship between electronic spectral red-shift or blue-shift and excited-state intermolecular hydrogen bond strengthening or weakening has been clarified. A dynamic equilibrium induced by the intermolecular hydrogen bond strengthening in the electronically excited state of fluorenone chromophore was used to explain the steady-state spectral features. The stepwise mechanism of excited-state double proton transfer reaction for the 2-aminopyridine/acid systems was demonstrated. The role of intermolecular hydrogen bonding on the excited-state proton transfer in the sensing mechanism of fluorescent probes has also been discussed. Moreover, a dihydrogen-bonded complex formed by borane-trimethylamine and phenol showed interesting geometric structures and infared spectrum in electronically excited state.

Multiple hydrogen bonding in excited states of aminopyrazine in methanol solution: Time-dependent density functional theory study

Aminopyrazine (AP) and AP-methanol complexes have been theoretically studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The excited-state hydrogen bonds are discussed in detail. In the ground state the intermolecular multiple hydrogen bonds can be formed between AP molecule and protic solvents. The AP monomer and hydrogen-bonded complex of AP with one methanol are photoexcited initially to the S2 state, and then transferred to the S1 state via internal conversion. However the complex of AP with two methanol molecules is directly excited to the S1 state. From the calculated electronic excited energies and simulated absorption spectra, we find that the intermolecular hydrogen bonds are strengthened in the electronic excited states. The strengthening is confirmed by the optimized excited-state geometries. The photochemical processes in the electronic excited states are significantly influenced by the excited-state hydrogen bond strengthening.

Negative Charge and Solvent Effects on Electronic Excited-State Hydrogen Bonding of 2′-Deoxycytidine 5′-Monophosphate (dCMP) in Aqueous Solution

Abstract Time-dependent density functional theory (TDDFT) was applied to study the influence of negative charge and solvent effects on the hydrogen-bonding dynamics of 2′-deoxycytidine 5′-monophosphate (dCMP) in electronically excited states. The intramolecular hydrogen bond O5–H2···O3 would be weakened with the strengthening of the hydration. Moreover, the greater the negative charge, the stronger the intramolecular hydrogen bond O5–H2···O3 is in [dCMP-H]−/2−–nH2O (n = 1, 2, 3) complexes. We concluded that the negative charge site of the phosphate would be the preferable site of hydration. In comparison with hydrogen-bonded [dCMP-H]−–nH2O (n = 0, 1, 2, 3) complexes, the excitation energies of complexes [dCMP-H]2−–nH2O (n = 0, 1, 2, 3) are lower except for individual excited states, that is to say, the increase of negative charge can make the related electronic spectra shift to red and hydrogen bonds strengthen. According to the results of calculated absorption spectra, the solvent effects by the polarizible continuum model (PCM) can lead to the electronic spectra blue shift.

Fluorescence Quenching of Hydrogen-Bonded Coumarin 102-Phenol Complex: Effect of Excited-State Hydrogen Bonding Strength

The fate of intermolecular hydrogen bond (H-bond) upon electronic excitation of a H-bonded complex has been debated in literature. For a model H-bonded complex, coumarin 102 (C102)-phenol in a noninteracting solvent ethylene tetrachloride, time-resolved infrared spectroscopy experiment of Nibbering and coworkers suggests that the H-bond between the C102 and phenol ruptures upon electronic excitation (C. Chudoba et al. J. Phys. Chem. A1999, 103, 5625-5628). On the contrary, Zhao and Han have demonstrated for the first time that the intermolecular hydrogen bond is significantly strengthened, while not disrupted, in the electronically excited states of the hydrogen-bonded complexes upon electronic excitation using the time-dependent density functional theory method (G. J. Zhao and K. L. Han J. Phys. Chem. A2007, 111, 2469-2474). The two excited-state hydrogen bonding dynamics mechanisms have widely different predictions of the emission or electronic relaxation of the excited H-bonded complex. The excited-state hydrogen-bond strengthening mechanism proposed by Zhao and Han anticipates a stronger intermolecular interaction, while the H-bond breaking mechanism speculates no interaction between C102 and phenol. The speculation has been tested here on the same system (H-bonded C102-phenol complex) in another noninteracting solvent cyclohexane. We found a strong quenching of the C102 emission in the H-bonded complex. Selectively excited (λex = 405 nm) H-bonded complex relaxes on a fast time scale of 400-600 ps and may be attributed to the conversion of the locally excited (LE) state to a nonfluorescent charge transfer (CT) state assisted by the strong excited-state H-bond formation. A minor component (∼10%) of 2.5 to 1.8 ns is ascribed to the LE complex without a H-bond. The findings are in accordance with the new fluorescence quenching mechanism that the excited-state intermolecular hydrogen bond strengthening facilitates CT from phenol to coumarin in the excited state (G. J. Zhao et al. J. Phys. Chem. B2007, 111, 8940-8945). Fluorescence quenching was absent for anisole, where H-bond formation is not possible and was more pronounced for p-Cl-phenol, where even stronger H-bonding is expected.

Role of the Electronically Excited-State Hydrogen Bonding and Water Clusters in the Luminescent Metal–Organic Framework

The electronically excited state and luminescence property of metal-organic framework Zn(3-tzba)(2,2'-bipy)(H2O)·nH2O have been investigated using the density functional theory (DFT) and time-dependent DFT (TDDFT). The calculated geometry and infrared spectra in the ground state are consistent with the experimental results. The frontier molecular orbitals and electronic configuration indicated that the origin of luminescence is attributed to a ligand-to-ligand charge transfer (LLCT). We theoretically demonstrated that the hydrogen bond H47···O5═C is weakened in the excited state S1; the weakening of the excited-state hydrogen bonding should be beneficial to the luminescence. To explore the effect of the water clusters on the luminescence, we studied four complexes Zn(3-tzba)(2,2'-bipy)(H2O)·3H2O, Zn(3-tzba)(2,2'-bipy)(H2O)·2H2O, Zn(3-tzba)(2,2'-bipy)(H2O)·H2O, and Zn(3-tzba)(2,2'-bipy)(H2O). The results reveal that the presence of water should play an important role in the emission characteristics of the MOF. Also, the UV-vis absorption and emission spectra of Zn(3-tzba)(2,2'-bipy)(H2O)·3H2O are in good agreement with the experimental results.

Studies on the photophysical properties of 1,8-naphthalimide derivative and aggregation induced emission recognition for casein

A novel water-soluble 1,8-naphthalimide derivative 1, bearing two acetic carboxylic groups, exhibited fluorescent turn-on recognition for casein micelle based on the aggregation induced emission (AIE) character. The photophysical properties of 1 consisting of donor and acceptor units were investigated by the solvation effect. Changing from polar to non-polar solvent increased the solvent interaction; both the excitation and emission spectra were shifted to shorter wavelength and intensity decreased through taking advantage of twisted intramolecular charge transfer (TICT) and self-association fluorescence emission. Moreover, the red-shift and quenching in protic solvent were caused by the excited-state hydrogen bond strengthening effect. The density functional theory (DFT) and time dependent density functional theory (TDDFT) were used to obtain the most stable structure, electronic excitation energy, dipole moments and charge distribution. The AIE mechanism of 1 with casein micelle was due to 1 docked in the hydrophobic cavity between sub-micelles and bound with amino acid residues, resulting in the aggregation of 1 on the casein micelle surface and emission enhancement, based on which, a novel casein assay method was developed. The proposed method exhibited a good linear range from 0.1 to 10.5μgmL−1, with the detection limit of 3.0ngmL−1. Satisfactory reproducibility, reversibility and a short response time were realized. This method was applied for the determination of casein in milk powder samples, avoiding the interferences from other components and illegal additives in milk.

Heteroatom Effects on Electronic Excited State Hydrogen Bonding of Fluorenone-Based Molecular Materials

In this work, the geometry optimizations in the ground state and electronic excitation energies and corresponding oscillation strengths of the low-lying electronically excited states for the isolated fluorenone (FN) and FN-based molecular monomers, the relatively hydrogen-bonded dimers, and doubly hydrogen-bonded trimers, are calculated by the density functional theory and time-dependent density functional theory methods, respectively. We find the intermolecular hydrogen bond CO···HO is strengthened in some of the electronically excited states of the hydrogen-bonded dimers and doubly hydrogen-bonded trimers, because the excitation energy in a related excited state decrease and electronic spectral redshift are induced. Similarly, the hydrogen bond CO···HO is weakened in other excited states. On this basis, owing to the important difference of electronegativity, heteroatoms S, Se, and Te that substitute for the O atom in the carbonyl group of the FN molecule have a significant effect on the strength of the hydrogen bond and the spectral shift. It is observed that the hydrogen bond CTe···HO is too weak to be formed. When the CS and CSe substitute for CO, the strength of the hydrogen bonds and electronic spectra frequency shift are significantly changed in the electronic excited state due to the electron transition type transformation from the ππ* feature to σπ* feature. © 2013 Wiley Periodicals, Inc. Heteroatom Chem 24:153–162, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.21075