Van Der Waals Form Research Articles

Heteroatom (B, N, P, and S)-Doped Cyclodextrin as a Hydroxyurea (HU) Drug Nanocarrier: A Computational Approach.

Theoretical examination of hydroxyurea adsorption capabilities toward the cyclodextrin surface for proper drug delivery systems was carried out utilizing DFT simulations. The study aims to assess the efficacy of doped cyclodextrin (doped with boron, nitrogen, phosphorus, and sulfur atoms) in increasing its stability and efficiency in intermolecular interactions, hence facilitating optimal drug delivery. The adsorption energies were found to follow a decreasing order of B@ACD-HU>N@ACD-HU>P@ACD-HU>S@ACD-HU with energies of -0.046, -0.0326, -0.015, and 0.944 kcal/mol, respectively. The S@ACD-HU complex, unlike previous systems, had a physical adsorption energy. The N@ACD-HU and B@ACD-HU complexes had the shortest bond lengths of 1.42 Å (N122-C15) and 1.54 Å (B126-C15), respectively. The HOMO and LUMO values were also high in identical systems, -6.367 and -2.918 eV (B@ACD-HU) and -6.278 and -1.736 eV (N@ACD-HU), respectively, confirming no chemical interaction. The N@ACD-HU has the largest energy gap of 4.54 eV. For the QTAIM analysis and plots, the maximum electron density and ellipticity index were detected in B@ACD-HU, 0.600 au (H70-N129) and 0.8685 au (H70-N129), respectively, but N@ACD-HU exhibited a high Laplacian energy of 0.7524 a.u (H133-N122). The fragments' TDOS, OPDOS, and PDOS exhibited a strong bond interaction of greater than 1, and they had different Fermi levels, with the highest value of -8.16 eV in the N@ACD-HU complex. Finally, the NCI analysis revealed that the complexes were noncovalent. According to the literature, the van der Waals form of interactions is used in the intermolecular forces of cyclodextrin cavities. The B@ACD-HU and N@ACD-HU systems were more greenish in color with no spatial interaction. These two systems have outperformed other complexes in intermolecular interactions, resulting in more efficient drug delivery. They had the highest negative adsorption energies, the shortest bond length, the highest HOMO/LUMO energies, the highest energy gap, the highest stabilization energy, the strongest bonding effect, the highest electron density, the highest ellipticity index, and a strong van der Waals interaction that binds the drug and the surface together.

First-principles investigation on the interfacial interaction and electronic structure of BiVO4/WO3 heterostructure semiconductor material

The BiVO4/WO3 heterostructure material is promising photoanode architecture in photoelectrocatalysis hydrogen generation system. However, most studies focused on the relationship between BiVO4/WO3 heterostructure material preparation method, morphology design, heteroatom doping and efficiency. Its internal mechanism and interface engineering have not been investigated in theory. In this work, these questions are answered by exploring the structure, electronic and optical properties of the system, as well as investigating the band arrangement and charge transfer when the interfacial interaction goes from van der Waals form to binding form. The binding form occurs when WO3 slab and BiVO4 slab are close enough to form a coherent boundary. It is found that the combination of WO3 and BiVO4 can form a heterostructure of type-II energy band arrangement and the formation of built-in electric field at the interface, which allows for better photogenerated charge carrier separation. When the interface binding form appears, the interface channel effect makes the maximum effective electron accumulation more than 4 times that of van der Waals form. Our work not only provides a perspicacious understanding of the photoexcited carrier separation mechanism for BiVO4/WO3 heterostructure, but also sheds light on exploring interfacial interaction in other heterostructure semiconductor materials.

Photoassociative spectroscopy of a halo molecule in Sr86

We present two-photon photoassociation to the least-bound vibrational level of the X$^1\Sigma_g^+$ electronic ground state of the $^{86}$Sr$_2$ dimer and measure a binding energy of $E_b=-83.00(7)(20)$\,kHz. Because of the very small binding energy, this is a halo state corresponding to the scattering resonance for two $^{86}$Sr atoms at low temperature. The measured binding energy, combined with universal theory for a very weakly bound state on a potential that asymptotes to a van der Waals form, is used to determine an $s$-wave scattering length $a=810.6(12)$\,$a_0$, which is consistent with, but substantially more accurate than the previously determined $a=798(12)\,a_0$ found from mass-scaling and precision spectroscopy of other Sr isotopes. For the intermediate state, we use a bound level on the metastable $^1S_0-{^3P_1}$ potential. Large sensitivity of the dimer binding energy to light near-resonant with the bound-bound transition to the intermediate state suggests that $^{86}$Sr has great promise for manipulating atom interactions optically and probing naturally occurring Efimov states.

A universal modified van der Waals equation of state. Part I: Polymer and mineral glass formers.

PVT data of glass formers (minerals and polymers) published in the literature are re-analyzed. All the polymer glass formers (PS, PVAc, PVME, PMMA, POMS, PBMA, PVC, PE, PP, PMPS, PMTS, PPG) present two main properties which have never been noted: a) the isobars P(V) have a fan structure characterized by the two parameters T* and V*; b) the isotherms verify the principle of temperature-pressure superposition for P < P*. From these properties we show that the Equation Of State (EOS) can be put on a modified van der Waals form (VW-EOS), (V - V*) = (V0 - V*)P*/(P + P*). The characteristic pressure P* and the covolume V* are T and P independent. In polymer glass formers P* and V* have same values in the α (melt) and β (glass) domains. The characteristic temperatures T* deduced from the Fan Structure of the Isobar (FSIb) above and below T(g) are different. The characteristic temperature T*(α) of the melt state is found near the Vogel temperature T0 for linear polymers and more than 100 K below T0 for atactic polymers (with pendent groups). This difference in atactic polymers (and in some low molecular weight compounds) is explained by the importance of the β motions due to the pendent groups. The independence of T0 on P is discussed. A modified VFT equation (analogous to the compensation law and Meyer-Neldel rule) giving the relaxation time τ of the α motions as a function of P and T is proposed. The fan structure of the isotherm logτ versus P is explained. It is shown that organic non-polymeric liquids (C6H12, C6H14, DHIQ, OTP, Glycerol, Salol, PDE, DGEBA), mineral glass (SiO2, Se, GeSe4, GeSe2, GeO2, As2O3 and two metallic glasses (LaCe and CaAl alloys) verify this VW-EOS with similar accuracy. The relation P* = B 0/γ B among the characteristic pressure P*, the zero-pressure modulus B0 and the Slatter-Grüneisen anharmonicity parameter γ(B0 deduced from the VW-EOS, is observed in all the glass formers.

Field-theoretic description of charge regulation interaction.

In order to find the exact form of the electrostatic interaction between two proteins with dissociable charge groups in aqueous solution, we have studied a model system composed of two macroscopic surfaces with charge dissociation sites immersed in a counterion-only ionic solution. Field-theoretic representation of the grand canonical partition function is derived and evaluated within the mean-field approximation, giving the Poisson-Boltzmann theory with the Ninham-Parsegian boundary condition. Gaussian fluctuations around the mean field are then analyzed in the lowest-order correction that we calculate analytically and exactly, using the path integral representation for the partition function of a harmonic oscillator with time-dependent frequency. The first-order (one loop) free-energy correction gives the interaction free energy that reduces to the zero-frequency van der Waals form in the appropriate limit but in general gives rise to a monopolar fluctuation term due to charge fluctuation at the dissociation sites. Our formulation opens up the possibility to investigate the Kirkwood-Shumaker interaction in more general contexts where their original derivation fails.

Contrasting the wide Feshbach resonances inLi6andLi7

We compare and contrast the wide Fehbach resonances and the corresponding weakly bound states in the lowest scattering channels of ultracold $^6$Li and $^7$Li. We use high-precision measurements of binding energies and scattering properties to determine new interaction potentials, incorporating non-Born-Oppenheimer terms to account for the failure of mass scaling between $^6$Li and $^7$Li. Correction terms are needed for both the singlet and the triplet potential curves. The universal formula relating binding energy to scattering length is not accurate for either system. The $^6$Li resonance is open-channel-dominated and the van der Waals formula of Gao [J. Phys. B 37, 4273 (2004)] gives accurate results for the binding energies across much of the resonance width. The $^7$Li resonance, by contrast, is weakly closed-channel-dominated and a coupled-channels treatment of the binding energies is required. Plotting the binding energies in universal van der Waals form helps illustrate subtle differences between the experimental results and different theoretical forms near the resonance pole.

Mechanics of rolling of nanoribbon on tube and sphere

The configuration of graphene nano-ribbon (GNR) assembly on carbon nanotube (CNT) and sphere is studied through theoretical modeling and molecular simulation. The GNR can spontaneously wind onto the CNT due to van der Waals (vdW) interaction and form two basic configurations: helix and scroll. The final configuration arises from the competition among three energy terms: the bending energy of the GNR, the vdW interaction between GNR and CNT, the vdW between the GNR itself. We derive analytical solutions by accounting for the three energy parts, with which we draw phase diagrams and predict the final configuration (helix or scroll) based on the selected parameters. The molecular simulations are conducted to verify the model with the results agree well with the model predicted. Our work can be used to actively control and transfer the tube-like nanoparticles and viruses as well as to assemble ribbon-like nanomaterials.

Virtual Resonance and Frequency Difference Generation by van der Waals Interaction

The ability to explore the interior of materials for the presence of inhomogeneities was recently demonstrated by mode synthesizing atomic force microscopy [L. Tetard, A. Passian, and T. Thundat, Nature Nanotech. 5, 105 (2009).]. Proposing a semiempirical nonlinear force, we show that difference frequency ω_ generation, regarded as the simplest synthesized mode, occurs optimally when the force is tuned to van der Waals form. From a parametric study of the probe-sample excitation, we show that the predicted ω_ oscillation agrees well with experiments. We then introduce the concept of virtual resonance to show that probe oscillations at ω_ can efficiently be enhanced.

The ground X Σ1g+ electronic state of the cesium dimer: Application of a direct potential fitting procedure

A collection of 16,544 fluorescence series spectroscopic line positions involving the A (1)Sigma(u)(+)-->X (1)Sigma(g)(+) transition in Cs(2) has been analyzed by a modern direct potential fitting procedure to generate the first fully analytical potential energy function for the ground electronic state, and precise energy term values for the excited A (1)Sigma(u)(+) state. The potential function yields an accurate representation of spectroscopic data that span 99.24% of the well depth and the number of fitted parameters is significantly less than half the number of parameters determined in conventional Dunham analyses. A novel variant of the Morse/long-range potential model has been employed in the representation of the ground state potential, and a critical comparison has been made with an extended modified Lennard-Jones potential model. Proper account has been taken of the known long-range van der Waals form of the potential, and our final potential function is determined with constrained literature values of the C(8) and C(10) dispersion energy coefficients, along with a fitted value of C(6)=3.31(5) x 10(7) cm(-1) A(6)=6870(100) a.u. The fitted dissociation energy (D(e)) is compared with the precisely known value based on the recent analysis of data from a two-photon transfer process (STIRAP) in ultracold Cs atoms. It is concluded that hyperfine effects in the X (1)Sigma(g)(+) state are not negligible, and that the estimate of D(e)=3649.84(7) cm(-1) obtained in this work represents an effective dissociation limit lying between the two lowest hyperfine limits. Precise rotational and centrifugal distortion constants for the ground electronic state have also been calculated through conventional perturbation theory. These estimates are fully consistent with the derived potential function and the experimental spectroscopic information.

Synthesis and structural characteristics of lithocholate triads: steroid-type channels occupied by spacer fragments

Reported in this paper are the syntheses and X-ray investigations of C(2) symmetrical molecular A-B-A triads consisting of two steroid units (lithocholic acid or its methyl ester) joined together by linkers derived from bifunctional molecules such as terephthalic acid or N,N'-dicarboxypiperazine. Unlike their monomeric analogues, some of these compounds form inclusion complexes. All steroidal triads form crystals that are highly pseudo-centrosymmetric, in which the constituting molecules are held together either exclusively by van der Waals forces or form lattice inclusion complexes, with guest molecules hydrogen bonded to the host. The presence of carboxyl groups promotes the inclusion of pyridine molecules and the formation of the well known carboxylic acid...pyridine hydrogen bonds. Combined with pairwise face-to-face pi-stacking between pyridine rings, these hydrogen-bond interactions lead to the formation of extended supramolecular tapes, analogous to polymers. The co-crystals of pyridine and a lithocholic acid triad undergo a symmetry-lowering phase transition from a P1 cell with Z = 1 to a P1 cell with Z = 2. The two structures are virtually the same, the two independent molecules in the larger cell being related by pseudo-translation. Changes in the type of spacer between two methyl lithocholate units from planar aromatic (terephthalic acid) to highly puckered aliphatic six-membered ring (N,N'-dicarboxypiperazine) bring about inclusion properties and changes in side-chain conformation in a crystal. Although the efficient packing of these highly elongated molecules is hindered, as indicated by low values of crystal density, ranging from 1.16 to 1.19 g cm(-3), several very short C...O and H...H contacts are present in the crystals.

Van der Waals forces in presence of free charges: An exact derivation from equilibrium quantum correlations

We study interatomic forces in a fluid consisting of a mixture of free charges and neutral atoms in the framework of the quantum many-body problem at nonzero temperature and nonzero density. Of central interest is the interplay between van der Waals forces and screening effects due to free charges. The analysis is carried out in a partially recombined hydrogen plasma in the Saha regime. The effective potentials in the medium between two atoms, or an atom and a charge, or two charges, are determined from the large-distance behavior of equilibrium proton-proton correlations. We show, in a proper low-temperature and low-density scaling limit, that those potentials all decay as r(-6) at large distance r, while the corresponding amplitudes are calculated exactly. In particular, the presence of free charges only causes a partial (nonexponential) screening of the atomic potential, and it does not modify its typical r(-6) decay. That potential reduces to the standard van der Waals form for two atoms in vacuum when the temperature is driven to zero. The analysis is based on first principles: it does not assume preformed atoms and takes into account in a coherent way all effects, quantum mechanical binding, ionization, and collective screening, which originate from the Coulomb potential. Our method relies on the path integral representation of the quantum Coulomb gas.

Microscopic treatment of a barrel drop on fibers and nanofibers

The microscopic approach of Berim and Ruckenstein (J. Phys. Chem. B 108 (2004) 19330, 19339) regarding the shape and stability of a liquid drop on a planar bare solid surface is extended to a liquid barrel drop on the bare surface of a solid cylinder (fiber) of arbitrary radius. Assuming the interaction potentials of the liquid molecules between themselves and with the molecules of the solid of the London–van der Waals form, the potential energy of a liquid molecule with an infinitely long fiber was calculated analytically. A differential equation for the drop profile was derived by the variational minimization of the total potential energy of the drop by taking into account the structuring of the liquid near the fiber. This equation was solved in quadrature and the shape and stability of the barrel drop were analyzed as functions of the radius of the fiber and the microscopic contact angle θ 0 which the drop profile makes with the surface of the fiber. The latter angle is dependent on the fiber radius and on the microscopic parameters of the model (strength of the intermolecular interactions, densities of the liquid and solid phases, hard core radii, etc.). Expressions for the evaluation of the microcontact angle from experimentally measurable characteristics of the drop profile (height, length, volume, location of inflection point) are obtained. All drop characteristics, such as stability, shape, are functions of θ 0 and a certain parameter a which depends on the model parameters. In particular, the range of drop stability consists of three domains in the plane θ 0 – a, separated by two critical curves a = a c ( θ 0 ) and a = a c 1 ( θ 0 ) [ a c ( θ 0 ) < a c 1 ( θ 0 ) ]. In the first domain, below the curve a = a c ( θ 0 ) , the drop can have any height at its apex. In the second and third domains the values of h m are limited; in the second domain (above the curve a = a c 1 ( θ 0 ) ) there is an upper limit of h, h m 1 , and the drops with h m > h m 1 cannot exist, whereas in the third domain (between those curves) the drop can have values of h m either smaller than h m 1 or larger than h m 2 , where h m 2 > h m 1 is a second critical height. For sufficiently large fiber radii, R f ⩾ 1 μm , the critical curves almost coincide and only two domains, the first and the second, remain. The smaller the radius, the larger is the difference between the critical curves and the larger is the second domain of drop stability. The shape of the drop depends on whether the point ( θ 0 , a ) on the θ 0 – a plane is far from the critical curve or near it. In the first case the drop profile has generally a large circular part, while in the second case the shape is either almost planar or contains a long manchon that is similar to a film on the fiber.

Solid–liquid equilibria for solutions of binary globular‐protein mixtures

AbstractA theory of the van der Waals form is used to establish solid–fluid phase diagrams for aqueous solutions containing two kinds of globular proteins. Theory is based on a one‐fluid model that uses a composition‐dependent hard‐sphere diameter and a composition‐dependent solvent‐mediated effective temperature to represent protein solutions. The hard‐sphere reference equation of state is based on the model of Young that correlates computer simulations of fluid–solid coexistence curves for binary hard‐sphere mixtures. The attractive perturbation term uses the inverse‐power potential with variable exponent n. Based on the correlation of George and Wilson and Rosenbaum et al., the optimum range of effective temperature is determined for protein crystallization. For binary protein systems, as the size difference increases, the mutual solubility of unlike proteins declines sharply in the precipitated solid phase. The theoretical results obtained here may be useful for design of protein‐separation processes by crystallization.

Correlations for Direct Calculation of Vapor Pressures from Cubic Equations of State

Vapor pressures computed from nine cubic equations of state (EOS's) of the van der Waals form are correlated in corresponding states form using the linear Pitzer principle and a simple temperature function anchored at the critical point. The results strongly suggest that cubic equations of state do not conform to the usually assumed linear dependence of fluid properties on acentric factor. The failure of three-parameter cubic equations of state to predict the exact vapor pressure consistent with a specified acentric factor is analyzed, and new correlations are presented for the characteristic parameter in Soave's cohesion function that minimize this intrinsic error over a wider range of acentric factors. Soave's direct procedure for calculation of vapor pressures from EOS is found to be more accurate than all other approximate methods by at least 1 order of magnitude. However, the leading coefficient in Soave's equations is shown to be inconsistent with the limiting slope of the vapor pressure curve predicted by the equations of state. New direct expressions are developed that eliminate this inconsistency and give good representation of vapor pressures with fewer adjustable coefficients.

Potential energy surface for and pure rotational spectra of isotopomeric Cl2–Ar van der Waals complexes

Pure rotational spectra have been calculated for the three isotopomers of the Cl2–Ar van der Waals complex formed by Cl2 in its ground electronic state. The potential energy surface involved is a sum of pairwise Morse atom–atom interactions, which are merged asymptotically into an anisotropic van der Waals form. The Morse atom–atom interactions have been refined by adjusting their parameters to obtain agreement with both excitation spectra and recent microwave transitions observed for the 35Cl2–Ar van der Waals complex, whereas the anisotropic long-range part of the potential surface has been taken from ab initio results [Mol. Phys. 80, 533 (1993)]. The present model potential surface predicts a T-shaped structure for the complex, in agreement with experiment. Excellent agreement has been found between the calculated and experimental microwave transition frequencies for the 35Cl37Cl–Ar complex. Good agreement has also been found between the experimentally determined bond energy, bond length, and average ‘‘bond angle’’ governing the overall geometry of the complex and the corresponding quantities determined from dynamical calculations based upon the present potential surface. The new potential surface has also been utilized to predict the microwave spectrum of the 37Cl2–Ar isotopomer.

Phase transitions in a vortex gas

It has been shown recently that the motion of solitons at couplings around a critical coupling can be reduced to the dynamics of particles (the zeros of the Higgs field) on a curved manifold with potential. The curvature gives a velocity-dependent force, and the magnitude of the potential is proportional to the distance from a critical coupling. In this paper we apply this approximation to determining the equation of state of a gas of vortices in the abelian Higgs model. We derive a virial expansion using certain known integrals of the metric, and the second virial coefficient is calculated, determining the behaviour of the gas at low densities. A formula for determining higher-order coefficients is given. At low densities and temperatures T ⪢ λ the equation of state is of the Van der Waals form ( P + bN 2/ A 2) ( A - aN) = NT with a = 4 π and b = -4.89 πλ where λ is a measure of the distance from critical coupling. It is found that there is no phase transition in a low-density type-I1 gas, but there is a transition in the type-I case between a condensed and gaseous state. We conclude with a discussion of the relation of our results to vortex behaviour in superconductors.

Multiproperty determination of a new N2–Ar intermolecular interaction potential energy surface

A new multiproperty potential energy surface for the N2–Ar intermolecular interaction is reported. The present determination is based upon molecular beam total differential and integral scattering data, taken together with the temperature dependence of the interaction second virial coefficient, transport properties, transport property field effects, and relaxation phenomena, such as pressure broadening of the depolarized Rayleigh line and longitudinal nuclear spin relaxation. The primary fit has been made to the beam scattering and virial data, and refinements to the potential parameters thus determined have been made by employing the data available for the gas phase transport and relaxation phenomena. The potential energy surface employed is an empirical Morse–Morse–spline–van der Waals form, in which the potential parameters depend upon the angle between the N2 figure axis and the line joining the centers of mass of N2 and Ar. No N2 stretching dependence has been included in the present determination. Comparison is made between the present potential energy surface and two other previously published N2–Ar potential energy surfaces. The present potential energy surface provides the best overall agreement for all available gas phase data for N2–Ar mixtures, and can thus be recommended for calculations of all properties of such mixtures that depend upon the intermolecular interaction.

Modification of the Kumar-Starling five-parameter cubic equation of state and extension to mixtures

Abstract Chu, J.-Z., Zuo, Y.-X. and Guo, T.-M., 1992. Modification of the Kumar-Starling five-parameter cubic equation of state and extension to mixtures. Fluid Phase Equilibria 77: 181-216. For extending the five-parameter cubic equation of state (KS EOS) proposed by Kumar and Starling (1982) (Ind. Eng. Chem. Fundam, 21:255-262) to polar substances, and improving the phase behavior description of heavy hydrocarbons, the generalized correlations of parameters A2 and A5 were modified, and new orientation parameters were determined. In the near critical region, transition functions for parameters A22, A5 and A5 were introduced, in which the experimental critical compressibility factor was incorporated. The parameters of the modified Kumar-Starling equation of state (MKS EOS) were determined by fitting the vapor pressure and saturated liquid density data simultaneously. The proposed MKS EOS was tested on 127 pure non-polar and polar substances. Extensive comparisons with nine other well-known cubic EOS indicate that the modified version of the KS EOS significantly improved the representation of the phase behavior of pure substances in various regions. The MKS EOS was further extended to mixtures by rearranging its form into conventional van der Waals (VDW) form, and the simple VDW one-fluid mixing rules were applied to the new EOS parameters. The test results on 97 binary and multicomponent mixtures indicate that, as compared with other equations of state, the MKS EOS predicts comparable bubble point pressures, but much more accurate saturated liquid densities.

Vibrational predissociation of the Ar⋅⋅⋅Cl2 van der Waals complex: The small molecule limit for intramolecular vibrational redistribution

A converged three-dimensional quantum treatment of vibrational predissociation in the Ar⋅⋅⋅Cl2(BΠ0u+3,υ′) van der Waals complex is presented. The potential energy surface used is a sum of pairwise Morse atom–atom interactions adjusted asymptotically to a C6/R6+C8/R8 anisotropic van der Waals form. Calculations have been performed in the energy region of Ar⋅⋅⋅Cl2(B,υ′=6, 10, and 11) excited levels. In agreement with the experimental findings, the final rotational distribution of Cl2 is found to be strongly dependent on the initial υ′ state being excited, as well as on the number of vibrational quanta lost in the vibrational predissociation process. The role of intramolecular vibrational redistribution for υ′=10 and 11 for which the Δυ=−1 channel is closed is also studied. It is found that the vibrational predissociation (VP) dynamics are dominated by the coupling of the zero-order ‘‘bright’’ state with a single ‘‘dark’’ state from the υ′−1 manifold of van der Waals vibrationally excited states which then decays to the continuum, and that the product state distribution is determined by the dissociation of the dark state. This is characteristic of the sparse limit for intramolecular vibrational redistribution. It also implies that the dissociation rate is not governed by a simple function of the initial quantum numbers such as the one given by the energy gap law. The golden rule approximation gives surprisingly accurate results for Ar⋅⋅⋅Cl2 dynamics. This will be very useful for fitting a potential energy surface to experimental results.

Dual fluorescence of the jet-cooled anthracene-dialkylaniline complexes

Fluorescence-excitation and fluorescence spectra, as well as fluorescence decay times, are recorded for anthracene-N,N-dimethylaniline (DMA) and anthracene-N,N-diethylaniline (DEA) complexes excited in their 0 0 0 and 0 0 0+385 cm −1 spectral regions. Their excitation spectra are composed of overlapping narrow-band and broad-band systems tentatively assigned to two isomeric forms, with the lowest excited state of van der Waals (A *D) and ionic (A −D +) character. The dual fluorescence (A *D→AD) and (A −D +→AD) with intensity ratio strongly varying from level to level was observed upon the selective excitation of the van der Waals form of the DEA complex. Possible mechanisms of isomerization are discussed.