Hydrogen-bonded Molecular Systems Research Articles

The Interplay between ESIPT and TADF for the 2,2'-Bipyridine-3,3'-diol: A Theoretical Reconsideration.

Organic molecules with excited-state intramolecular proton transfer (ESIPT) and thermally activated delayed fluorescence (TADF) properties have great potential for realizing efficient organic light-emitting diodes (OLEDs). Furthermore, 2,2'-bipyridine-3,3'-diol (BP(OH)2) is a typical molecule with ESIPT and TADF properties. Previously, the double ESIPT state was proved to be a luminescent state, and the T2 state plays a dominant role in TADF for the molecule. Nevertheless, whether BP(OH)2 undergoes a double or single ESIPT process is controversial. Since different ESIPT channels will bring different TADF mechanisms, the previously proposed TADF mechanism based on the double ESIPT structure for BP(OH)2 needs to be reconsidered. Herein, reduced density gradient, potential energy surface, IR spectra and exited-state hydrogen-bond dynamics computations confirm that BP(OH)2 undergoes the barrierless single ESIPT process rather than the double ESIPT process with a barrier. Moreover, based on the single ESIPT structure, we calculated spin-orbit coupling matrix elements, nonradiative rates and electron-hole distributions. These results disclose that the T3 state plays a predominant role in TADF. Our investigation provides a better understanding on the TADF mechanism in hydrogen-bonded molecular systems and the interaction between ESIPT and TADF, which further provides a reference for developing efficient OLEDs.

Significance Of Nuclear Quantum Effects In Hydrogen Bonded Molecular Chains.

In hydrogen-bonded systems, nuclear quantum effects such as zero-point motion and tunneling can significantly affect their material properties through underlying physical and chemical processes. Presently, direct observation of the influence of nuclear quantum effects on the strength of hydrogen bonds with resulting structural and electronic implications remains elusive, leaving opportunities for deeper understanding to harness their fascinating properties. We studied hydrogen-bonded one-dimensional quinonediimine molecular networks which may adopt two isomeric electronic configurations via proton transfer. Herein, we demonstrate that concerted proton transfer promotes a delocalization of π-electrons along the molecular chain, which enhances the cohesive energy between molecular units, increasing the mechanical stability of the chain and giving rise to distinctive electronic in-gap states localized at the ends. These findings demonstrate the identification of a class of isomeric hydrogen-bonded molecular systems where nuclear quantum effects play a dominant role in establishing their chemical and physical properties. This identification is a step toward the control of mechanical and electronic properties of low-dimensional molecular materials via concerted proton tunneling.

Artificial Neural Networks as Mappings between Proton Potentials, Wave Functions, Densities, and Energy Levels.

Artificial neural networks (ANNs) have become important in quantum chemistry. Herein, applications to nuclear quantum effects, such as zero-point energy, vibrationally excited states, and hydrogen tunneling, are explored. ANNs are used to solve the time-independent Schrödinger equation for single- and double-well potentials representing hydrogen-bonded molecular systems capable of proton transfer. ANN mappings are trained to predict the lowest five proton vibrational energies, wave functions, and densities from the proton potentials and to predict the excited state proton vibrational energies and densities from the proton ground state density. For the inverse problem, ANN mappings are trained to predict the proton potential from the proton vibrational energy levels or the proton ground state density. This latter mapping is theoretically justified by the first Hohenberg-Kohn theorem establishing a one-to-one correspondence between the external potential and the ground state density. ANNs for two- and three-dimensional systems are also presented to illustrate the straightforward extension to higher dimensions.

The nature of three-body interactions in DFT: Exchange and polarization effects.

We propose a physically motivated decomposition of density functional theory (DFT) 3-body nonadditive interaction energies into the exchange and density-deformation (polarization) components. The exchange component represents the effect of the Pauli exclusion in the wave function of the trimer and is found to be challenging for density functional approximations (DFAs). The remaining density-deformation nonadditivity is less dependent upon the DFAs. Numerical demonstration is carried out for rare gas atom trimers, Ar2-HX (X = F, Cl) complexes, and small hydrogen-bonded and van der Waals molecular systems. None of the tested semilocal, hybrid, and range-separated DFAs properly accounts for the nonadditive exchange in dispersion-bonded trimers. By contrast, for hydrogen-bonded systems, range-separated DFAs achieve a qualitative agreement to within 20% of the reference exchange energy. A reliable performance for all systems is obtained only when the monomers interact through the Hartree-Fock potential in the dispersion-free Pauli blockade scheme. Additionally, we identify the nonadditive second-order exchange-dispersion energy as an important but overlooked contribution in force-field-like dispersion corrections. Our results suggest that range-separated functionals do not include this component, although semilocal and global hybrid DFAs appear to imitate it in the short range.

Unprecedented Ferroelectric-Antiferroelectric-Paraelectric Phase Transitions Discovered in an Organic-Inorganic Hybrid Perovskite.

As a promising candidate for energy storage capacitors, antiferroelectric (AFE) materials have attracted great concern due to their congenital advantages of large energy storage ability from double polarization versus electric field (P-E) hysteresis characteristics in contrast to ferroelectrics and linear dielectrics. However, antiferroelectricity has only been discovered in inorganic oxides and some hydrogen-bonded molecular systems. In view of the structural diversity and unique physical properties of organic-inorganic hybrid system, it remains a great opportunity to introduce antiferroelectricity into organic-inorganic hybrid perovskites. Here, we report that polarizable antiparallel dipole arrays can be realized in an organic-inorganic hybrid perovskite, (3-pyrrolinium)CdBr3, which not only exhibits an excellent ferroelectric property (with a high spontaneous polarization of 7.0 μC/cm2), but also presents a striking AFE characteristic revealed by clear double P-E hysteresis loops. To the best of our knowledge, it is the first time that such successive ferroelectric-antiferroelectric-paraelectric phase transitions have been discovered in organic-inorganic perovskites. Besides, a giant dielectric constant of 1600 even at high frequency of 1000 kHz and a bulk electrocaloric effect with entropy change of 1.18 J K-1 kg-1 under 7.41 kV/cm are also observed during the phase transition. Apparently, the combined striking AFE characteristic and giant dielectric constant make (3-pyrrolinium)CdBr3 a promising candidate for next generation high-energy-storage capacitors.

Excited-State Proton Transfer and Decay in Hydrogen-Bonded Oxazole System: MS-CASPT2//CASSCF Study

Herein we have employed high-level multi-reference CASSCF and MS-CASPT2 electronic structure methods to systematically study the photochemical mechanism of intramolecularly hydrogen-bonded 2-(2′-hydroxyphenyl)-4-methyloxazole. At the CASSCF level, we have optimized minima, conical intersections, minimum-energy reaction paths relevant to the excited-state intramolecular proton transfer (ESIPT), rotation, photoisomerization, and the excited-state deactivation pathways. The energies of all structures and paths are refined by the MS-CASPT2 method. On the basis of the present results, we found that the ESIPT process in a conformer with the OH···N hydrogen bond is essentially barrierless process; whereas, the ESIPT process is inhibited in the other conformer with the OH···O hydrogen bond. The central single-bond rotation of the S1 enol species is energetically unfavorable due to a large barrier. In addition, the excited-state deactivation of the S1 keto species, as a result of the ultrafast ESIPT, is very efficient because of the existence of two easily-approached keto S1/S0 conical intersections. In stark contrast to the S1 keto species, the decay of the S1 enol species is almostly blocked. The present theoretical study contributes valuable knowledge to the understanding of photochemistry of similar intramolecularly hydrogen-bonded molecular and biological systems.

Toward an Accurate and Efficient Theory of Physisorption. I. Development of an Augmented Density-Functional Theory Model

Currently available density functionals cannot describe the dispersion component of the interaction energy present in weakly bound complexes. Moreover, the exchange energy as obtained from the density-functional theory is often incorrect. Examples of problematic cases include clusters of van der Waals-bound rare-gas atoms and most hydrogen-bonded molecular systems. Thus, accurate ab initio methods to treat intermolecular forces should be used in such systems. These methods are, however, too slow to be applicable to the large systems needed to model adsorption. This is why DFT continues to be used, where, in addition, a quite common compensation of errors sometimes produces some sort of agreement with the corresponding experimental data. In this paper, we analyze in detail the inadequacy of standard DFT for describing the weak binding present in a few rare gas-rare gas, metal atom-rare gas, and metal atom-metal atom dimers.Inspired by the success of the Hartree-Fock plus (damped) dispersion (HFD) method, we test the use of an improved hybrid model in which to a density-functional interaction energy (with corrected exchange and avoidance of double-counting of dispersion), a (damped) dispersion expansion is added in the usual way.Comparisons with accurate theoretical or experimental benchmarks show that our DFdD method using the revPBEx or revPBEx+VWNc functionals and accurate dispersion coefficients is found to recover the interaction energy curves very well for many of the tested systems. The sec and paper in this series will describe the use of the DFdD method for physisorption for the previously well-studied (but not solved) case of Xe/Cu(111).

Mobility and conductivity of ionic and bonded defects in hydrogen-bonded chains with nonlinear interactions

The proton conductivity and the mobility arising from motions of the ionic and bonded defects, in hydrogen-bonded molecular systems are investigated by means of the quantum mechanical method. Our two component model goes beyond the usual classical harmonic interaction by inclusion of a quartic interaction potential between the nearest-neighbor protons. Among the rich variety of soliton patterns obtained in this model, we focus our attention to compact kink (kinkon) solutions to calculate analytically, the mobility of the kinkon-antikinkon pair and the specific electrical-conductivity of the protons transfer in the hydrogen-bonded systems under an externally applied electrical-field through the dynamic equation of the kinkon-antikinkon pair. For ice, the mobility and the electrical conductivity of the proton transfer obtained are about 5.307×10-7 m2 V-1 s-1 and 6.11×10-4 Ω-1 m-1, respectively. The results obtained are in qualitative agreement with experimental data.

Photophysics of inter- and intra-molecularly hydrogen-bonded systems: Computational studies on the pyrrole–pyridine complex and 2(2′-pyridyl)pyrrole

The role of electron and proton transfer processes in the photophysics of hydrogen-bonded molecular systems has been investigated with ab initio electronic-structure calculations. We discuss generic mechanisms of the photophysics of a hydrogen-bonded aromatic pair (pyrrole–pyridine), as well as an intra-molecularly hydrogen-bonded π system composed of the same molecular sub-units (2(2′-pyridyl)pyrrole). The reaction mechanisms are discussed in terms of excited-state minimum-energy paths, conical intersections and the properties of frontier orbitals. A common feature of the photochemistry of these systems is the electron-driven proton transfer (EDPT) mechanism. In the hydrogen-bonded complex, a highly polar charge transfer state of 1ππ ∗ character drives the proton transfer, which leads to a conical intersection of the S 1 and S 0 surfaces and thus ultrafast internal conversion. In 2(2′-pyridyl)pyrrole, out-of-plane torsion is additionally needed for barrierless access to the S 1–S 0 conical intersection. It is pointed out that the EDPT process plays an essential role in the fluorescence quenching in hydrogen-bonded aromatic complexes, the function of organic photostabilizers, and the photostability of biological molecules.

Computational Studies of the Photophysics of Hydrogen-Bonded Molecular Systems

The role of electron- and proton-transfer processes in the photophysics of hydrogen-bonded molecular systems has been investigated with ab initio electronic-structure calculations. Adopting indole, pyridine, and ammonia as molecular building blocks, we discuss generic mechanisms of the photophysics of isolated aromatic chromophores (indole), complexes of pi systems with solvent molecules (indole-ammonia, pyridine-ammonia), hydrogen-bonded aromatic pairs (indole-pyridine), and intramolecularly hydrogen-bonded pi systems (7-(2'-pyridyl)indole). The reaction mechanisms are discussed in terms of excited-state minimum-energy paths, conical intersections, and the properties of frontier orbitals. A common feature of the photochemistry of the various systems is the electron-driven proton-transfer (EDPT) mechanism: highly polar charge-transfer states of 1pipi*, 1npi*, or 1pisigma* character drive the proton transfer, which leads, in most cases, to a conical intersection of the S1 and S0 surfaces and thus ultrafast internal conversion. In intramolecularly hydrogen-bonded aromatic systems, out-of-plane torsion is additionally needed for barrierless access to the S1-S0 conical intersection. The EDPT process plays an essential role in diverse photophysical phenomena, such as fluorescence quenching in protic solvents, the function of organic photostabilizers, and the photostability of biological molecules.

TEMPERATURE-DEPENDENCE OF PROTON CONDUCTIVITY IN HYDROGEN-BONDED MOLECULAR SYSTEMS

The temperature-dependence of proton electric conductivity in hydrogen-bonded molecular systems with damping effect was studied. The time-dependent velocity of proton and its mobility are determined from the Hamiltonian of a model system. The calculated mobility of (3.57–3.76) × 10-6m2/ Vs for uniform ice is in agreement with the experimental value of (1 - 10) × 10-2m2/ Vs . When the temperature and damping effects of the medium are considered, the mobility is found to depend on the temperature for various electric field values in the system, i.e. the mobility increases initially and reaches a maximum at about 191 K, but decreases subsequently to a minimum at approximately 241 K, and increases again in the range of 150–270 K. This behavior agrees with experimental data of ice.

Influences of Structure Disorder and Temperature on Properties ofProton Conductivity in Hydrogen-Bond MolecularSystems

The dynamic properties of proton conductivity along hydrogen-bondedmolecular systems, for example, ice crystal, with structure disorder ordamping and finite temperatures exposed in an externally appliedelectric-field have been numerically studied by Runge–Kutta way in oursoliton model. The results obtained show that the proton-soliton is veryrobust against the structure disorder including the fluctuation of the forceconstant and disorder in the sequence of masses and thermal perturbation anddamping of medium, the velocity of its conductivity increases withincreasing of the externally applied electric-field and decreasing of thedamping coefficient of medium, but the proton-soliton disperses for quitegreat fluctuation of the force constant and damping coefficient. In thenumerical simulation we find that the proton-soliton in our model isthermally stable in a large region of temperature of T⩽273 K underinfluences of damping and externally applied electric-field in ice crystal.This shows that our model is available and appropriate to ice.

Covalent ferroelectricity in hydrogen-bonded organic molecular systems

We have performed the first-principles calculation on a ferroelectrics phenazine-chloranil acid $(\mathrm{Phz}\text{\ensuremath{-}}{\mathrm{H}}_{2}\mathrm{ca})$, which includes an asymmetric hydrogen bond O-H\ensuremath{\cdots}N. These hydrogen bonds play the role of building bridges for electronic wave functions between molecules instead of proton tunneling. The charge density in this intermolecular region strongly enhances the electric polarization by the shift of the Wannier function centers, which is described by the Berry phase. It is found that the small molecular distortion enhances the covalency between molecules. This extended nature of electronic wave functions is crucial to the hydrogen-bonded ferroelectrics, which has been ignored in the conventional picture.

PROTON CONDUCTIVITY AND THERMODYNAMIC FEATURES IN THE HYDROGEN-BONDED MOLECULAR SYSTEMS

The proton conductivity and thermodynamic features, arising from motions of the ionic and bonded defects, in hydrogen-bonded molecular systems have been investigated by the quantum-mechanical method and the transfer integral way in our model, in which the collective effect and the mutual correlation between the protonic and heavy ionic sublattices are specially considered. We first derived the equations of motion and its soliton solutions from the model Hamiltonian. The results obtained show that this model can simultaneously support motions of the ionic and bonded defects which are due to competition of the double-well potential and non-linearly coupled interaction between the protons and heavy ions. Thus we find out the mobility of the kink-antikink pair and electrical-conductivity of the proton transfer in the hydrogen-bonded systems exposed in an externally applied electrical-field through the dynamic equation of the kink-antikink pair and its solution in this model. For ice, the mobility and electrical conductivity of the proton transfer obtained are about (6.5 - 6.9)×10-6 m 2/ V · s and (7.6 - 8.1)×10-3(Ω · m )-1, respectively, which are in the domain of semiconductors and are basically consistent with experimental values for the crystal. Finally we calculate the free energy and specific heat of the systems with finite temperature by the model Hamiltonian and transfer integral way. The specific heat is also consistent with experimental data. This is a very interesting result.

Two-Component Solitary Waves in Hydrogen-Bonded Molecular Systems

The dynamics of two-component solitary waves in hydrogen-bonded chains in an external force and damping is investigated. The influence of the motion and the optical mode of the heavy ion sublattice on the portion sublattice is discussed. It will increase the soliton width and decrease the soliton mobility. The general expression for the kink soliton soliton is obtained. The velocity, the mobility and conductivity of the kink soliton are calculated. The results are in good agreement with the experimental data.

Mobility and conductivity of the proton transfer in hydrogen-bonded molecular systems

We calculate the mobility and conductivity of the proton transfer in hydrogen-bonded systems by quantum mechanics. We first give the equation of change of velocity of the kink–antikink pair resulting from proton transfer, from the Hamiltonian in our model. Utilizing the values of the physical parameters we finally find out the mobility of kink–antikink pairs and the electrical conductivity due to the proton transfer in an externally applied electric-field in the systems. These are about (6.5–6.9)×10 −6 m 2/ V s and (7.6–8.1)×10 −3/( Ω m) for ice, respectively, which are basically consistent with experimental data.

Properties of Proton Transfer in Hydrogen-Bonded Systems at Finite Temperature**The project supported by National Natural Science Foundation of China under Grant No. 19974034

The properties of proton transfer along hydrogen-bonded molecular systems are studied at finite temperature. The dynamic equations of the proton transport along the systems are obtained by using a completely quantum mechanics method. From the dynamic equations and its soliton solutions we find out specific heat arising from the motion of solitons in the systems with finite temperature and the critical temperature of the soliton in the protein molecules, which is about 318 K. This shows that we can continuously study some biological phenomena in the living systems by this model.

Comment on `Dynamic properties of proton transferin hydrogen-bonded molecular systems'

A recent model for proton transfer in hydrogen-bonded chains given byPang and Müller-Kirsten (Pang X F and Müller-Kirsten H J W2000 J. Phys.: Condens. Matter 12 885) is criticallyreconsidered. The model violates a basic symmetry of the system.The meaning of the model parameters is overinterpreted. Themodel can be applied only to describe the motion of ionicdefects. The kink solutions corresponding to bonding defectsobtained in this work by Pang and Müller-Kirsten are proven to be incorrect.

Quantum-mechanical properties of proton transport in the hydrogen-bonded molecular systems

The dynamic equations of the proton transport along the hydrogen bonded molecular systems have been obtained by using completely quantum-mechanical method to be based on new Hamiltonian and model we proposed. Some quantum-mechanical features of the proton-solitons have also been given in such a case. The alternate motion of two defects resulting from proton transfer occurred in the systems can be explained by the results. The results obtained show that the proton-soliton has corpuscle feature and obey classical equations of motion, while the free soliton moves in uniform velocity along the hydrogen bonded chains.

Dynamic properties of proton transfer in hydrogen-bonded molecular systems

The mechanism of the formation of ionic and bonding defects and the interaction between protons and heavy ions in hydrogen-bonded molecular systems is investigated and a new model is constructed. This model considers two coupled sublattices corresponding to those of protons and heavy ions in order to study the dynamic properties resulting from deformation and local fluctuation of the heavy ion sublattice due to the protonic displacements. As compared with other models, the model emphasizes, in particular, the collective effect of the motion and the mutual correlation between the two sublattices. The equations of motion admit analytic soliton solutions of different types corresponding to two different defects. The combined effect of the two defects and the proton transfer in the system are described by means of these solutions, and the appropriate conditions associated with actual physical processes. Finally, we discuss in detail the characteristics of the proton transfer and the advantages of the model.