Three-Step Spin State Transition and Hysteretic Proton Transfer in the Crystal of an Iron(II) Hydrazone Complex

A proton–electron coupling system, exhibiting unique bistability or multistability of the protonated state, is an attractive target for developing new switchable materials based on proton dynamics. Herein, we present an iron(II) hydrazone crystalline compound, which displays the stepwise transition and bistability of proton transfer at the crystal level. These phenomena are realized through the coupling with spin transition. Although the multi-step transition with hysteresis has been observed in various systems, the corresponding behavior of proton transfer has not been reported in crystalline systems; thus, the described iron(II) complex is the first example. Furthermore, because proton transfer occurs only in one of the two ligands and π electrons redistribute in it, the dipole moment of the iron(II) complexes changes with the proton transfer, wherein the total dipole moment in the crystal was canceled out owing to the antiferroelectric-like arrangement.     

Energy Flow in a Purpose‐Built Cascade Molecule Bearing Three Distinct Chromophores Attached to the Terminal Acceptor

A multicomponent cluster has been synthesised in which four disparate chromophores have been covalently linked through a logical arrangement that favours efficient photon collection and migration to a terminal emitter. The primary energy acceptor is a boron dipyrromethene (Bodipy) dye and different polycyclic aryl hydrocarbons have been substituted in place of the regular fluorine atoms attached to the boron centre. The first such unit is perylene, linked to boron through a 1,4‐diethynylphenyl unit, which collects photons in the 320–490 nm region. The other photon collector is pyrene, also connected to the boron centre by a 1,4‐diethynylphenyl spacer and absorbing strongly in the 280–420 nm region, which itself is equipped with an ethynylfluorene residue that absorbs in the UV region. Illumination into any of the polycyclic aryl hydrocarbons results in emission from the Bodipy unit. The rates of intramolecular electronic energy transfer have been determined from time‐correlated, single‐photon counting studies and compared with the rates for Coulombic interactions computed from the Förster expression. It has been necessary to allow for i) a more complex screening potential, ii) multipole–multipole coupling, iii) an extended transition dipole moment vector and iv) bridge‐mediated energy transfer. The bridge‐mediated energy transfer includes both modulation of the donor transition dipole vector by bridge states and Dexter‐type electron exchange. The latter is a consequence of the excellent electronic coupling properties of the 1,4‐diethynylphenyl spacer unit. The net result is a large antenna effect that localises the photon density at the primary acceptor without detracting from its highly favourable photophysical properties.     

Photoconductive Core–Shell Liquid‐Crystals of a Perylene Bisimide J‐Aggregate Donor–Acceptor Dyad

A novel core–shell structured columnar liquid crystal composed of a donor‐acceptor dyad of tetraphenoxy perylene bisimide (PBI), decorated with four bithiophene units on the periphery, was synthesized. This molecule self‐assembles in solution into helical J‐aggregates guided by π–π interactions and hydrogen bonds which organize into a liquid‐crystalline (LC) columnar hexagonal domain in the solid state. Donor and acceptor moieties exhibit contrasting exciton coupling behavior with the PBIs’ (J‐type) transition dipole moment parallel and the bithiophene side arms’ (H‐type) perpendicular to the columnar axis. The dyad shows efficient energy and electron transfer in solution as well as in the solid state. The synergy of photoinduced electron transfer (PET) and charge transport along the narcissistically self‐assembled core–shell structure enables the implementation of the dye in two‐contact photoconductivity devices giving rise to a 20‐fold increased photoresponse compared to a reference dye without bithiophene donor moieties.     

Formation of a metastable phase induced by a liquid crystalline phase in a novel chloropoly(aryl ether ketone)

The phase transition behavior of a thermotropic liquid crystalline poly(aryl ether ketone) synthesized by nucleophilic substitution reactions of 4,4′‐biphenol (BP), and chlorohydroquinone (CH) with 1,4‐bis(4‐fluorobenzoyl)benzene (BF) has been investigated by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). The copolymer exhibits multiple first order phase transitions, which are associated with crystal‐to‐smectic liquid crystal transition and smectic liquid crystal‐to‐isotropic transition. When the cooling rate is low (< 10°C/min), only stable crystal form I is formed. With the cooling rate being high (>20°C/min), the metastable crystal form II is formed, which always coexists with form I. The liquid crystalline phase plays an important role in the formation of metastable phase form II.     

Anisotropy of dielectric relaxation in crystal form II of poly(vinylidene fluoride)

The anisotropy of the crystalline relaxation (α relaxation) in oriented poly(vinylidene fluoride) in crystal form II has been studied. The dielectric increment Δε is analyzed on the basis of the two-site model. A linear relation between Δε/χξ and cos²θ is obtained, where χ is the degree of crystallinity, ξ is the ratio of the internal field to the applied field, and θ is the angle between the applied electric field and the molecular axis. The dipole moment changes direction only along the molecular axis in the relaxation in crystal form II; the molecular motion cannot be explained by chain rotation around the molecular axis. Possible models for the α relaxation are proposed: change in conformation with internal rotation can occur in the crystalline chains, and defects in the crystalline regions play an important role in the α relaxation.     

Through‐Space Conjugated Molecular Wire Comprising Three π‐Electron Systems

A [2.2]paracyclophane‐based through‐space conjugated oligomer comprising three π‐electron systems was designed and synthesized. The arrangement of three π‐conjugated systems in an appropriate order according to the energy band gap resulted in efficient unidirectional photoexcited energy transfer by the Förster mechanism. The energy transfer efficiency and rate constants were estimated to be >0.999 and >10¹² s^?1, respectively. The key point for the efficient energy transfer is the orientation of the transition dipole moments. The time‐dependent density functional theory (TD‐DFT) studies revealed the transition dipole moments of each stacked π‐electron system; each dipole moment was located on the long axis of each stacked π‐electron system. This alignment of the dipole moments is favorable for fluorescence resonance energy transfer (FRET).     

Structures of heterogeneous proton-bond dimers with a high dipole moment monomer: covalent vs electrostatic interactions

A number of calculated structures of heterogeneous proton-bound dimers containing monomers such as acetonitrile, cyanamide, vinylene carbonate, and propiolactone, which have high dipole moments, are presented. These proton-bound dimers are predicted to have a structural anomaly pertaining to the bond distances between the central proton and the basic sites on each of the monomers. The monomers with the high dipole moments also have the larger proton affinity and, on the basis of difference in proton affinities, it would be expected that the proton would be closer to this monomer than the one with the lower proton affinity. However, the proton is found to lie substantially closer to the monomer with the lower proton affinity in most cases, unless the difference in proton affinity is too large. Simply stated, the difference in proton affinities is smaller than the difference in the affinity to form an ion-dipole complex for the two monomers and it is the larger affinity for the high dipole moment monomer (which also has the higher proton affinity) to form an ion-dipole complex that is responsible for the proton lying closer to the low proton affinity monomer. The bond distances between the central proton and the monomers are found to be related to the difference in proton affinity. It is found, though, that the proton-bound dimers can be grouped into two separate groups, one where the proton-bound dimer contains a high dipole moment monomer and one group where the proton-bound dimer does not contain a high dipole moment monomer. From these plots it has been determined that a high dipole moment monomer is one that has a dipole moment greater than 2.9 D.     

Measurement of the electronic transition dipole moment by Autler-Townes splitting: Comparison of three- and four-level excitation schemes for the Na2 A 1Sigma(u)+ - X 1Sigma(g)+ system

We present a fundamentally new approach for measuring the transition dipole moment of molecular transitions, which combines the benefits of quantum interference effects, such as the Autler-Townes splitting, with the familiar R-centroid approximation. This method is superior to other experimental methods for determining the absolute value of the R-dependent electronic transition dipole moment function mu(e)(R), since it requires only an accurate measurement of the coupling laser electric field amplitude and the determination of the Rabi frequency from an Autler-Townes split fluorescence spectral line. We illustrate this method by measuring the transition dipole moment matrix element for the Na2 A 1Sigma(u)+ (v' = 25, J' = 20e)-X 1Sigma(g)+ (v" = 38, J" = 21e) rovibronic transition and compare our experimental results with our ab initio calculations. We have compared the three-level (cascade) and four-level (extended Lambda) excitation schemes and found that the latter is preferable in this case for two reasons. First, this excitation scheme takes advantage of the fact that the coupling field lower level is outside the thermal population range. As a result vibrational levels with larger wave function amplitudes at the outer turning point of vibration lead to larger transition dipole moment matrix elements and Rabi frequencies than those accessible from the equilibrium internuclear distance of the thermal population distribution. Second, the coupling laser can be "tuned" to different rovibronic transitions in order to determine the internuclear distance dependence of the electronic transition dipole moment function in the region of the R-centroid of each coupling laser transition. Thus the internuclear distance dependence of the transition moment function mu(e)(R) can be determined at several very different values of the R centroid. The measured transition dipole moment matrix element for the Na2 A 1Sigma(u)+ (v' = 25, J' = 20e)-X 1Sigma(g)+ (v" = 38, J" = 21e) transition is 5.5+/-0.2 D compared to our ab initio value of 5.9 D. By using the R-centroid approximation for this transition the corresponding experimental electronic transition dipole moment is 9.72 D at Rc = 4.81 A, in good agreement with our ab initio value of 10.55 D.     

Laser control of double proton transfer in porphycenes: towards an ultrafast switch for photonic molecular wires

Electronic excitation energy transfer along a molecular wire depends on the relative orientation of the electronic transition dipole moments of neighboring chromophores. In porphycenes, this orientation is changed upon double proton transfer in the electronic ground state. We explore the possibility to trigger such a double proton transfer reaction by means of an infrared pump-dump laser control scheme. To this end, a quantum chemical characterization of an asymmetrically substituted porphycene is performed using density functional theory. Ground state geometries, the topology of the potential energy surface for double proton transfer, and \(\hbox{S}_0\rightarrow\hbox{S}_1\) transition energies are compared with the parent compound porphycene and a symmetric derivative. Employing a simple two-dimensional model for the double proton transfer, which incorporates sequential and concerted motions, quantum dynamics simulations of the laser-driven dynamics are performed which demonstrate tautomerization control. Based on the orientation of the transition dipole moments, this tautomerization may lead to an estimated change in the Förster transfer coupling of about 60%.     

Splitting of carbonyl stretching frequencies from transition dipole coupling

The observed splitting of the carbonyl stretching frequencies of hydrogen bonded carboxylic acid dimers has been explained by other authors in terms of transition dipole coupling. In this paper it is shown that the transition dipole interaction for carbonyl groups gives very small splittings when reasonable dipole moment changes are used.     

Theoretical study of the mechanism of proton transfer in tautomeric systems: Alloxan

Semiempirical SCF-MO studies of tautomerism in alloxan preclude the possibility of direct proton transfer in the gas phase due to the strain in the four-centred transition state, in which the proton being transferred is forced to come close to the positively charged carbon atom at the opposite corner of the four-membered ring. However, in aqueous solution, the activation barrier reduces appreciably, not only due to reduction in strain, but also due to charge separation in the transition state, which is stabilized due to ionic resonance. The N-H bond is almost broken, while the O-H bond is only partially formed in the transition state. The other stabilizing effect in aqueous solution is due to bulk solvent dielectric effects, which stabilize the transition state to a greater extent due to its higher dipole moment. Although the transition states for proton transfer to the neighbouring oxygen atoms on either side have comparable energies, as the mechanisms of proton transfer leading to the formation of the 2-hydroxy and 4-hydroxy tautomers are similar, bulk solvent effects are larger in the latter due to the higher dipole moment of the transition state. The reason is the almost complete separation of the two entities, i.e. the alloxan anion and the hydronium ion in the latter case, indicating that in this case a dissociative mechanism of the kind encountered in acid-base equilibria is operating.     

Cyano Analogues of 7‐Azaindole: Probing Excited‐State Charge‐Coupled Proton Transfer Reactions in Protic Solvents

The interplay between excited‐state charge and proton transfer reactions in protic solvents is investigated in a series of 7‐azaindole (7AI) derivatives: 3‐cyano‐7‐azaindole (3CNAI), 5‐cyano‐7‐azaindole (5CNAI), 3,5‐dicyano‐7‐azaindole (3,5CNAI) and dicyanoethenyl‐7‐azaindole (DiCNAI). Similar to 7AI, 3CNAI and 3,5CNAI undergo methanol catalyzed excited‐state double proton transfer (ESDPT), resulting in dual (normal and proton transfer) emission. Conversely, ESDPT is prohibited for 5CNAI and DiCNAI in methanol, as supported by a unique normal emission with high quantum efficiency. Instead, the normal emission undergoes prominent solvatochromism. Detailed relaxation dynamics and temperature dependent studies are carried out. The results conclude that significant excited‐state charge transfer (ESCT) takes place for both 5CNAI and DiCNAI. The charge‐transfer specie possesses a different dipole moment from that of the proton‐transfer tautomer species. Upon reaching the equilibrium polarization, there exists a solvent‐polarity induced barrier during the proton‐transfer tautomerization, and ESDPT is prohibited for 5CNAI and DiCNAI during the excited‐state lifespan. The result is remarkably different from 7AI, which is also unique among most excited‐state charge/proton transfer coupled systems studied to date.     

A Theoretical Study on the Photochromic Mechanism of 1-Phenyl-3-methyl-4-（6-hydro-4-amino-5-sulfo-2,3-pyrazine）-pyrazole-5-one

The photochromic mechanism of 1-phenyl-3-methyl-4-（6-hydro-4-amino-5-sulfo-2,3- pyrazine）-pyrazole-5-one has been investigated using the density functional theory（DFT）. The solvent effect is simulated using the polarizable continuum model（PCM） of the self-consistent reaction field theory. According to the crystal structure of the title compound, an intramolecular proton transfer mechanism from enol to keto form was proposed to interpret its photochromism. Bader＇s atom-in-molecule（AIM） theory is used to investigate the nature of hydrogen bonds and ring structures. Time-dependent density functional theory（TDDFT） calculation results show that the photochromic process from enol to keto form is reasonable. The conformation and molecular orbital analysis of enol and keto forms explain why only intramolecular proton transfer is possible. The results from analyzing the energy and dipole moments of enol form, transition state and keto form in the gas phase and in different solvents have been used to assess the stability of the title compound.     

General calculation of 4f-5d transition rates for rare-earth ions using many-body perturbation theory

The 4f-5d transition rates for rare-earth ions in crystals can be calculated with an effective transition operator acting between model 4f(N) and 4f(N-1)5d states calculated with effective Hamiltonian, such as semiempirical crystal Hamiltonian. The difference of the effective transition operator from the original transition operator is the corrections due to mixing in transition initial and final states of excited configurations from both the center ion and the ligand ions. These corrections are calculated using many-body perturbation theory. For free ions, there are important one-body and two-body corrections. The one-body correction is proportional to the original electric dipole operator with magnitude of approximately 40% of the uncorrected electric dipole moment. Its effect is equivalent to scaling down the radial integral (5d/r/4f) to about 60% of the uncorrected HF value. The two-body correction has magnitude of approximately 25% relative to the uncorrected electric dipole moment. For ions in crystals, there is an additional one-body correction due to ligand polarization, whose magnitude is shown to be about 10% of the uncorrected electric dipole moment.     

吩噻嗪给体受体衍生物激发态的电荷转移

A series of N-bonded donor-acceptor derivatives of phenothiazine containing phenyl （PHPZ）, anisyl （ANPZ）, pyridyl （PYPZ）, naphthyl （NAPZ）, acetylphenyl （APPZ）, and cyanophenyl （CPPZ） as an electron acceptor have been synthesized. Their photophysical properties were investigated in solvents of different polarities by absorption and emission techniques. These studies clearly revealed the existence of an intramolecular charge transfer （ICT） excited state in the latter four compounds. The solvent dependent Stokes shift values were analyzed by the modified Lippert-Mataga equation to obtain the excited state dipole moment values. The large excited state dipole moment suggests that the full （or nearly full） electron transfer take place in the A-D systems. In the system of A-D phenothiazine derivatives, the transition dipole moments Mflu were determined mainly by direct interactions between the solvent-equilibrated fluorescence ^1CT state and ground state because of their lack of significant change with increase of the solvent polarity. The electron structure and molecular conformation of phenothiazine derivatives will be significantly changed with the increase of the electron affinity of the N-10 substituent.     

Synthesis and Spectral Properties of 4‐(2,5‐Dimethoxyphenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO)

An interesting flourophore, 4‐(2,5‐dimethoxyphenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO) was synthesized by mixing an equivalent molar quantity of hippuric acid and 2,5‐dimethoxybenzaldehyde in acetic anhydride in the presence of anhydrous sodium acetate. The absorption and fluorescence characteristics of 4‐(2,5‐dimethoxy‐phenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO) were investigated in different solvents. DMPO dye exhibits red shift in both absorption and emission spectra as solvent polarity increases, indicating change in the dipole moment of molecules upon excitation due to an intramolecular charge transfer interaction. The fluorescence quantum yield depends strongly on the properties of the solvents, which was attributed to positive and negative solvatokinetic effects. A crystalline solid of DMPO gave strong excimer like emission at 630 nm due to the excitation of molecular aggregates. This is expected from the idealized crystal structure of the dye that belongs to the B‐type class of Steven's Classification. DMPO displayed fluorescence quenching by triethylamine via nonemissive exciplex formation.     

Monte Carlo simulations on mesophase formation using dipolar Gay-Berne model

We report the results of a Monte Carlo simulation of polar particles interacting via the Gay-Berne potential combining dipole-dipole interactions. Simulations were carried out on a system of 256 particles with either a zero dipole moment or longitudinal dipole moment located at the centre of the molecule. The system was found to spontaneously form nematic, smectic and crystal phases from an isotropic phase with a random configuration as temperature was decreased, irrespective of values of the dipole moment. The results do not give any indication of a net polarization even in the system with a strong dipole moment (μ* = 2.00). The transition temperature from the isotropic to nematic phase is not sensitive to the value of the dipole moment within the limits of statistical error, while the transition from the nematic to smectic phase depends on the strength of dipole moment. At lower temperatures forming the smectic or the crystal phase, the translational order along the director increases with increasing dipole moment. The dipolar interactions contribute to the long range ordering.     

Toward assessment of density functionals for vibronic coupling in two‐photon absorption: A case study of 4‐nitroaniline

In this study, we predict vibronic two‐photon absorption (TPA) spectra for 4‐nitroaniline in vacuo. The simulations are performed using density functional theory and the approximate second‐order coupled‐cluster singles and doubles model CC2. Thereby we also demonstrate the possibility of simulations of vibronic TPA spectra with ab initio wavefunction methods that include electron correlation for medium‐sized systems. A special focus is put on the geometric derivatives of the second‐order transition moment and the dipole moment difference between the charge‐transfer excited state and the ground state. The results of CC2 calculations bring new insight into the vibronic coupling mechanism in TPA spectra of 4‐nitroniline and demonstrate that the mixed term is quite large and that it also exhibits a negative interference with the Franck‐Condon contribution. © 2015 Wiley Periodicals, Inc.     

Environmental effects on hydrogen bonded systems: A quantum chemical study of proton relay model systems

In this paper an application of a reaction field theory of solvent effects has been made to study proton transfer mechanisms in hydrogen bonded systems coupled to an environment. The latter is simulated with reaction fields having variable strength and direction (defined with respect to the supermolecule's total dipole moment direction), together with superposed uniform external electric fields. Changes in proton potential curves and some other properties of a model water dimer and a water trimer are reported. The results are discussed in relation to relevant phenomena in biology and biochemistry, namely proton relay systems in enzymatic catalysis.     

The tropolone–isobutylamine complex: a hydrogen‐bonded troponoid without dominant π–π interactions

Tropolone long has served as a model system for unraveling the ubiquitous phenomena of proton transfer and hydrogen bonding. This molecule, which juxtaposes ketonic, hydroxylic, and aromatic functionalities in a framework of minimal complexity, also has provided a versatile platform for investigating the synergism among competing intermolecular forces, including those generated by hydrogen bonding and aryl coupling. Small members of the troponoid family typically produce crystals that are stabilized strongly by pervasive π–π, C—H…π, or ion–π interactions. The organic salt (TrOH·iBA) formed by a facile proton‐transfer reaction between tropolone (TrOH) and isobutylamine (iBA), namely isobutylammonium 7‐oxocyclohepta‐1,3,5‐trien‐1‐olate, C₄H₁₂N⁺·C₇H₅O₂⁻, has been investigated by X‐ray crystallography, with complementary quantum‐chemical and statistical‐database analyses serving to elucidate the nature of attendant intermolecular interactions and their synergistic effects upon lattice‐packing phenomena. The crystal structure deduced from low‐temperature diffraction measurements displays extensive hydrogen‐bonding networks, yet shows little evidence of the aryl forces (viz. π–π, C—H…π, and ion–π interactions) that typically dominate this class of compounds. Density functional calculations performed with and without the imposition of periodic boundary conditions (the latter entailing isolated subunits) documented the specificity and directionality of noncovalent interactions occurring between the proton‐donating and proton‐accepting sites of TrOH and iBA, as well as the absence of aromatic coupling mediated by the seven‐membered ring of TrOH. A statistical comparison of the structural parameters extracted for key hydrogen‐bond linkages to those reported for 44 previously known crystals that support similar binding motifs revealed TrOH·iBA to possess the shortest donor–acceptor distances of any troponoid‐based complex, combined with unambiguous signatures of enhanced proton‐delocalization processes that putatively stabilize the corresponding crystalline lattice and facilitate its surprisingly rapid formation under ambient conditions.