Ke-Li Han

Hydrogen Bonding in the Electronic Excited State

Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated. Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtosecond time-resolved vibrational spectroscopy is used to directly monitor the ultrafast dynamical behavior of hydrogen bonds in the electronic excited state. It is important to note that the excited-state hydrogen-bonding dynamics are coupled to the electronic excitation. Fortunately, the combination of femtosecond time-resolved spectroscopy and accurate quantum chemistry calculations of excited states resolves this issue in laser experiments. Through a comparison of the hydrogen-bonded complex to the separated hydrogen donor or acceptor in ground and electronic excited states, the excited-state hydrogen-bonding structure and dynamics have been obtained. Moreover, we have also demonstrated the importance of hydrogen bonding in many photophysical processes and photochemical reactions. In this Account, we review our recent advances in electronic excited-state hydrogen-bonding dynamics and the significant role of electronic excited-state hydrogen bonding on internal conversion (IC), electronic spectral shifts (ESS), photoinduced electron transfer (PET), fluorescence quenching (FQ), intramolecular charge transfer (ICT), and metal-to-ligand charge transfer (MLCT). The combination of various spectroscopic experiments with theoretical calculations has led to tremendous progress in excited-state hydrogen-bonding research. We first demonstrated that the intermolecular hydrogen bond in the electronic excited state is greatly strengthened for coumarin chromophores and weakened for thiocarbonyl chromophores. We have also clarified that the intermolecular hydrogen-bond strengthening and weakening correspond to red-shifts and blue-shifts, respectively, in the electronic spectra. Moreover, radiationless deactivations (via IC, PET, ICT, MLCT, and so on) can be dramatically influenced through the regulation of electronic states by hydrogen-bonding interactions. Consequently, the fluorescence of chromophores in hydrogen-bonded surroundings is quenched or enhanced by hydrogen bonds. Our research expands our understanding of the nature of hydrogen bonding by delineating the interaction between hydrogen bonds and photons, thereby providing a basis for excited-state hydrogen bonding studies in photophysics, photochemistry, and photobiology.

The time-dependent quantum wave packet approach to the electronically nonadiabatic processes in chemical reactions

The time-dependent quantum wave packet approach has been improved and formulated to treat the multiple surface problems and thus provided a new simple, yet a clear quantum picture for interpreting the reaction mechanism underlying the nonadiabatic dynamical processes. The method keeps the salient feature of the original quantum wave packet theory developed for single surface problems, i.e. the introduction of the absorbing potential and the grid basis including the discrete variable representation and the fast Fourier transformation, which makes the present methodology a very efficient implement for the nonadiabatic quantum scattering calculations. Here, we review the theoretical basis of this approach and its applications to the fundamental triatomic chemical reactions, the latter include the nonadiabatic dynamics calculations on the F + H2, F + HD, F + D2, O(1D) + N2, O(3P, 1D) + H2, D+ + H2, and H+ + D2 reactions. We also present a thorough historical overview of the theoretically nonadiabatic dynamical investigations particularly on the triatomic systems, and show how the time-dependent wave packet approach complements the time-independent quantum scattering theory. Contents PAGE 1.  Introduction 202 2.  Historical overview 203 3.  Time-dependent quantum wave packet approach for A + BC reaction 206  3.1. Propagation of the wave function 206  3.2.  Preparation of the initial wave function 208  3.3.  Analysis of the final wave function 209 4.  Examples 210  4.1.  Nonadiabatic effects on the reaction mechanism of F(2P3/2,2P1/2) + H2 97 210  4.2.  The reactivity of the ground and the excited spin state F(2P3/2,2P1/2) atoms with D2 98 212  4.3.  Nonadiabatic investigation on the F(2P3/2,2P1/2) + HD reaction 99,100 213  4.4.  Electronic quenching process in the O(1D) + N2 → O(3P) + N2 reaction 101 218  4.5.  The intersystem crossing effects in the O(3P,1D) + H2 reaction 102 221  4.6.  Nonadiabatic quantum calculations on the D+ + H2 reaction 103 225  4.7.  Nonadiabatic investigation on the H+ + D2 reaction 104 229 5.  Conclusions 231 Acknowledgments 233 References 233

A Near-IR Reversible Fluorescent Probe Modulated by Selenium for Monitoring Peroxynitrite and Imaging in Living Cells

We have developed a near-IR reversible fluorescent probe containing an organoselenium functional group that can be used for the highly sensitive and selective monitoring of peroxynitrite oxidation and reduction events under physiological conditions. The probe effectively avoids the influence of autofluorescence in biological systems and gave positive results when tested in both aqueous solution and living cells. Real-time images of cellular peroxynitrite were successfully acquired.

Reversible Near-Infrared Fluorescent Probe Introducing Tellurium to Mimetic Glutathione Peroxidase for Monitoring the Redox Cycles between Peroxynitrite and Glutathione in Vivo

The redox homeostasis between peroxynitrite and glutathione is closely associated with the physiological and pathological processes, e.g. vascular tissue prolonged relaxation and smooth muscle preparations, attenuation hepatic necrosis, and activation matrix metalloproteinase-2. We report a near-infrared fluorescent probe based on heptamethine cyanine, which integrates with telluroenzyme mimics for monitoring the changes of ONOO(-)/GSH levels in cells and in vivo. The probe can reversibly respond to ONOO(-) and GSH and exhibits high selectivity, sensitivity, and mitochondrial target. It is successfully applied to visualize the changes of redox cycles during the outbreak of ONOO(-) and the antioxidant GSH repair in cells and animal. The probe would provide a significant advance on the redox events involved in the cellular redox regulation.

Effect of location of energy barrier on the product alignment of reaction A+BC

The trajectory calculations of heavy heavy–light, light light–light, heavy light–light, and light heavy–light mass combination reactions on attractive and repulsive potential surfaces have been carried out to study the dependence of the product rotational alignment on collision energies. The calculated results for heavy heavy–light mass combination reaction are compared with the predictions from the constrained product orbital angular momentum model. The final rotational angular momentum was found to be perpendicularly polarized with respect to the reagents’ relative velocity vector on either attractive or repulsive potential surface. There is similar behavior of the product rotational alignments as a function of collision energies for heavy heavy–light and heavy light–light reactions, i.e., the more anisotropic the distribution of the product rotational angular momentum vector is, the higher the collision energies are, whether the potential surface is attractive or repulsive. However, the calculations fo...

Time-dependent density functional theory study on hydrogen-bonded intramolecular charge-transfer excited state of 4-dimethylamino-benzonitrile in methanol

The time‐dependent density functional theory (TDDFT) method was carried out to investigate the hydrogen‐bonded intramolecular charge‐transfer (ICT) excited state of 4‐dimethylaminobenzonitrile (DMABN) in methanol (MeOH) solvent. We demonstrated that the intermolecular hydrogen bond C≡N···HO formed between DMABN and MeOH can induce the C≡N stretching mode shift to the blue in both the ground state and the twisted intramolecular charge‐transfer (TICT) state of DMABN. Therefore, the two components at 2091 and 2109 cm−1 observed in the time‐resolved infrared (TRIR) absorption spectra of DMABN in MeOH solvent were reassigned in this work. The hydrogen‐bonded TICT state should correspond to the blue‐side component at 2109 cm−1, whereas not the red‐side component at 2091 cm−1 designated in the previous study. It was also demonstrated that the intermolecular hydrogen bond C≡N···HO is significantly strengthened in the TICT state. The intermolecular hydrogen bond strengthening in the TICT state can facilitate the deactivation of the excited state via internal conversion (IC), and thus account for the fluorescence quenching of DMABN in protic solvents. Furthermore, the dynamic equilibrium of these electronically excited states is explained by the hydrogen bond strengthening in the TICT state.

Role of Intramolecular and Intermolecular Hydrogen Bonding in Both Singlet and Triplet Excited States of Aminofluorenones on Internal Conversion, Intersystem Crossing, and Twisted Intramolecular Charge Transfer

Time-dependent density functional theory method was performed to investigate the intramolecular and intermolecular hydrogen bonding in both the singlet and triplet electronic excited states of aminofluorenones AF, MAF, and DMAF in alcoholic solutions as well as their important roles on the excited-state photophysical processes of these aminofluorenones, such as internal conversion, intersystem crossing (ISC), twisted intramolecular charge transfer (TICT), and so forth. The intramolecular hydrogen bond C=O...H-N can be formed between the carbonyl group and amino group for the isolated AF and MAF. However, no intramolecular hydrogen bond for DMAF can be formed. At the same time, the most stable conformation of DMAF is out-of-plane structure, where the two dihedral angles formed between dimethyl groups and fluorenone plane are 163.1 degrees and 41.74 degrees, respectively. The formation of intramolecular hydrogen bond for AF and MAF is tightly associated with the intersystem crossing of these aminofluorenones. Furthermore, the ISC process can be dominantly determined by the change of intramolecular hydrogen bond between S(1) and T(1) states of aminofluorenones. Since the change of hydrogen bond between S(1) and T(1) states of AF is stronger than that of MAF, the rate of ISC process for AF is faster than that for MAF. Moreover, the rate constant of the ISC process of DMAF is nearly close to zero because of the absence of intramolecular hydrogen bond. On the other hand, the intermolecular hydrogen bond C=O...H-O can be also formed between all aminofluorenones and alcoholic solvents. The internal conversion process from S(1) to S(0) state of these aminofluorenones is facilitated by the intermolecular hydrogen bond strengthening in the electronic excited state of aminofluorenones because of the decrease of energy gap between S(1) and S(0) states. At the same time, the change of intermolecular hydrogen bond between S(1) and T(1) states for AF is much stronger than that for MAF, which may also contribute to the faster ISC process for AF than that for MAF in the same solvents. The TICT process plays an important role in the deactivation of the photoexcited DMAF, since the TICT process along the twisted dihedral angle is nearly barrierless in the S(1) state of DMAF. However, the TICT cannot take place for AF and MAF because of the presence of the intramolecular hydrogen bond.

Lithium-Doped Conjugated Microporous Polymers for Reversible Hydrogen Storage†

Hydrogen storage is of great interest as environmentally clean and efficient fuels are required for future energy applications. Several pioneering strategies have been developed and significant performances have been achieved for hydrogen storage, including chemisorption of dihydrogen in the form of light metal hydrides, metal nitrides and imides, physisorption of dihydrogen onto carbon, clathrate hydrates, and porous network materials such as carbon nanotubes (CNTs), zeolites, and metal–organic framework (MOF) materials. However, hydrogen storage in these systems requires either high pressure or very low temperature, or both, thus severely limiting the applicability for mobile applications, which require working conditions of 1– 20 bar and ambient temperature. The synthesis of functional materials with high hydrogen uptake and delivery under safe and ambient conditions remains a key challenge for establishing hydrogen economy. It has been reported that atomically dispersed alkalimetal ions (e.g., Li andNa) are capable of clustering several H2 molecules bound through electrostatic charge–quadrupole and charge-induced dipole interactions. Thus ab initio simulations showed that Li-doped pillared graphene can bind reversibly up to 6.5 mass% of H2 at 20 bar at room temperature. In addition, ab initio simulations showed that doping of MOFs with atomically dispersed alkali-metal cations can reversibly achieve up to 5.5 mass% of H2 at 100 bar at room temperature. These results suggest that the high electron affinity of the sp carbon framework can essentially separate the charge from the Li center, thus providing strong stabilization of the molecular H2 and dramatically improving the hydrogen uptake value compared to that of undoped systems. Recently, experimental investigations also showed that H2 uptake of the MOFs can be remarkably improved by introduction of Li ions into MOF systems. For instance, an Li-doped MOF, which was prepared by reaction of lithium diisopropylamide (LDA) with theMOF MIL-53(Al), was reported to exhibit nearly double the hydrogen uptake compared with an undoped MOF. The doping of Li into the MOF has also been reported to remarkably enhance the isosteric heats of H2 adsorption compared to those of the undoped MOF. To date, no material that consists of an active Li dopant and has ultrahigh hydrogen storage capacity has been reported. The difficulty in demonstrating this concept relates to whether agglomeration of the Li atoms occurs during synthesis. Recently, conjugated microporous polymers (CMPs) have received considerable research interest for hydrogen storage because of their finely tunable microporosity, large surface areas, and high stability. Herein, we report the first experimental evidence that Li ion dopants dramatically enhance hydrogen storage in a CMP matrix. The hydrogen storage amount can reach up to 6.1 wt% at 1 bar and 77 K, which is among the best reported to date for physisorption hydrogen storage materials including MOFs and CNTs. The CMP we selected was produced from 1,3,5-triethynylbenzene, which has active sites (C C bonds) for binding of metallic ions, large BET surface areas with microporous character, and good chemical (totally insoluble in all organic solvents), and thermal stability (thermogravimetric analysis (TGA) shows that the thermal decomposition temperature of the CMP is greater than 300 8C). These physicochemical properties suggest that the selected CMP is appropriate as a host for Li doping. Also, this material contains only three kinds of light elements (C, H, and Li), which is a great advantage for gravimetric adsorption. We synthesized the CMP by Pd/Cu-catalyzed homocoupling polymerization. To dope the CMP with Li, we immersed the CMP in a solution of the naphthalene anion radical salt (LiC10H8C ) in THF. The mixture was stirred for several hours under an inert atmosphere to allow thorough penetration of Li ions into the CMP network. The mixture was filtered and the solid product was washed with dry THF several times followed by removal of the solvent at room temperature and subsequent removal of the naphthalene under vacuum at 120 8C. The field emission scanning electron microscopy (FE-SEM) images (Figure 1a,b) show that the CMP and Li-CMP consist of agglomerated microgel particles and have porous features. TGA shows that the CMP have good thermal stability (Figure 1c, thermal decomposition temperature> 300 8C). In the case of the CMP treated with LiC10H8C (0.5 wt% Li), an obvious weight loss (ca. 10%) was observed in the temperature range 100–150 8C. This feature suggests the removal of the residual naphthalene absorbed in the CMP matrix, and is consistent with a previous report. Figure 1d shows the high-resolution [*] K.-L. Han, Prof. W.-Q. Deng State Key Lab of Molecular Reaction Dynamics Dalian Institute of Chemical Physics, Chinese Academy of Sciences Dalian 116023 (China) E-mail: dengwq@dicp.ac.cn Homepage: http://www.nmce.dicp.ac.cn/

Rational Design of d-PeT Phenylethynylated-Carbazole Monoboronic Acid Fluorescent Sensors for the Selective Detection of α-Hydroxyl Carboxylic Acids and Monosaccharides

We have synthesized three new phenylethynylated carbazole boronic acid sensors, which were predicted to display novel d-PeT fluorescence transduction (PeT, photoinduced electron transfer; fluorophore as the electron donor of the electron transfer, ET) by DFT/TDDFT calculations. The d-PeT effect is characterized by a lower background fluorescence at acidic pH than at neutral pH, which is in stark contrast to the normal a-PeT effect (fluorophore as the electron acceptor of the ET) that shows a strong and undesired background fluorescence at acidic pH. Our experimental results confirmed the theoretical predictions and d-PeT was observed for two of the sensors (with p-dimethylaminophenylethynyl substitution at 6- position of the carbazole core). For the third sensor (with phenylethynyl substitution at 6- position of the carbazole core), however, not d-PeT but rather the normal a-PeT was observed. The discrepancy between the DFT/TDDFT calculations and the experimental observations can be rationalized using free energy changes (Rehm-Weller equations) and the rate constants for the ET (k(ET), Marcus equation). These new d-PeT boronic acid sensors show improved photophysical properties compared to the known d-PeT sensor reported previously by us. In particular, the fluorescence transduction efficiency of the new sensors was improved 8-fold when compared to the known d-PeT boronic acid sensors. Novel fluorescence enhancement/reduction was observed for one of the sensors upon binding with mandelic acid or tartaric acid at pH 5.6. The effect of pH as well as the bonding with analytes on the emission of the sensors were rationalized using DFT/TDDFT calculations. We believe that rational sensor design aided by DFT/TDDFT calculations as well as using free energy changes and electron transfer rate constants to study the emission properties of PeT sensors will become an essential tool in the design of new fluorophores or fluorescent sensors with predetermined photophysical properties.

First-Principles Investigation of Anistropic Hole Mobilities in Organic Semiconductors

We report a simple first-principles-based simulation model (combining quantum mechanics with Marcus-Hush theory) that provides the quantitative structural relationships between angular resolution anisotropic hole mobility and molecular structures and packing. We validate that this model correctly predicts the anisotropic hole mobilities of ruberene, pentacene, tetracene, 5,11-dichlorotetracene (DCT), and hexathiapentacene (HTP), leading to results in good agreement with experiment.