Diradical and Ionic Characters of Open-Shell Singlet Molecular Systems.

Adopting density functional theory and a hybrid exchange-correlation functional, the relationship between the second hyperpolarizability (gamma) and the diradical character has been investigated for diphenalenyl-based compounds containing different heterocyclic five-membered central rings (C(4)H(4)X, where X = NH, PH, O, S, CH(2), SiH(2), BH, GaH, C=O, C=S, and C=Se) or substituted by donor (NH(2))/acceptor(NO(2)) groups. It turns out that these structural modifications can tune the diradical character from 0.0 to 0.968 and lead to variations of gamma over more than 1 order of magnitude, demonstrating the controllability of gamma in this family of compounds. In particular, when the central ring is strongly aromatic, the diradical character is larger than 0.7, which is associated with pretty large gamma values except for almost the pure diradical case (y approximately 1). On the other hand, when the aromaticity decreases--or the antiaromaticity increases--the diradical character and the second hyperpolarizability get smaller. These relationships are correlated to structural (bond length alternation) and charge distribution (charge transfer between the phenalenyl rings and the central ring) properties, which account for the relative importance of the resonance diradical, zwitterionic, and quinoid forms. Therefore, the diradical character and the second hyperpolarizability can be controlled by the aromaticity of the ring while the paradigm of the enhancement of gamma for intermediate diradical character is globally verified. Then, upon introducing donor groups, the zwitterionic character increases, leading to closed-shell species and small second hyperpolarizabilities. In the case of substitution by acceptor groups, the charge transfer is reduced but the diradical character and the second hyperpolarizability hardly changes.

In general, the electronic structures of a molecular system is characterized by using the “diradical character”, which is well defined in quantum chemistry and implies a chemical index of a bond nature. In this chapter, we present analytical expressions for electronic energies and wavefunctions of the ground- and excited states as well as for the excitation energies and transition properties based on symmetric two-site diradical models with different diradical characters using the valence configuration interaction method.KeywordsSymmetric diradical systemDiradical characterExcitation energyTransition momentValence configuration interaction

The diradical state is an important electronic state for understanding molecular functions and should be elucidated for the in silico design of functional molecules and their application to molecular devices. The density functional theory calculation with plane-wave basis and correction of the on-site Coulomb parameter U (DFT+U/plane-wave calculation) is a good candidate of high-throughput calculations of diradical-band interactions. However, it has not been investigated in detail to what extent the DFT+U/plane-wave calculation can be used to calculate organic diradicals with a high degree of accuracy. In the present study, using typical organic diradical molecules (bisphenalenyl molecules) as model systems, the discrepancy in the optimum U values between the two electronic states (open-shell singlet and triplet) that compose the diradical state is detected. The calculated results show that the reason for this U value discrepancy is the difference in electronic delocalisation due to π-conjugation between the open-shell singlet and triplet states, and that the effect of U discrepancy becomes large as diradical character decreases. This indicates that it is necessary to investigate the U value discrepancy with reference to the calculated results by more accurate methods or to experimental values when calculating organic diradicals with low diradical character. For this investigation, the local magnetic moments, unpaired beta electron numbers, and effective magnetic exchange integral values can be used as reference values. For the effective magnetic exchange integral values, the effects of U discrepancy are partially cancelled out. However, because the effects may not be completely offset, care should be taken when using the effective magnetic exchange integral value as a reference. Furthermore, a comparison of DFT+U and hybrid-DFT calculations shows that the DFT+U underestimates the HOMO-LUMO gap of bisphenalenyls, although a qualitative discussion of the gap is possible.

The fundamental relationship between structure and diradical character is important for the development of open-shell diradicaloid-based materials. In this work, we synthesized two structural isomers bearing a 2,6-naphthoquinodimethane or a 1,5-naphthoquinodimethane bridge and demonstrated that their diradical characters and chemical reactivity are quite different. The mesityl-or pentafluorophenyl-substituted octazethrene derivatives OZ-M/OZ-F and their isomer OZI-M (with mesityl substituents) were synthesized via an intramolecular Friedel-Crafts alkylation followed by oxidative dehydrogenation strategy from the key building blocks 4 and 11. Our detailed experimental and theoretical studies showed that both isomers have an open-shell singlet ground state with a remarkable diradical character (y0 = 0.35 and 0.34 for OZ-M and OZ-F, and y0 = 0.58 for OZI-M). Compounds OZ-M and OZ-F have good stability in an ambient environment, while OZI-M has high reactivity and can be easily oxidized to a dioxo product 15, which can be correlated to their different diradical characters. Additionally, we investigated the physical properties of OZ-M, OZ-F, and 15.

The quantum states of an electron-hole pair in one-dimensional semiconductors under a static electric field are theoretically analyzed using a two-band model with on-site Coulomb interaction. In the absence of static field, the electron and hole are always bound, forming an exciton regardless of the Coulomb interaction strength, in contrast to what occurs in higher-dimensional semiconductors. The static field modifies the wave function of the electron-hole pair, turning bound states into continuum states. However, at low static fields, the linear optical spectra resemble those of the unbiased semiconductor, exhibiting a quadratic redshift of the main exciton absorption line as the field increases. When the static field exceeds a critical threshold, the exciton dissociates and the linear optical spectra exhibit signatures of the Wannier-Stark ladder with squally spaced peaks, making them a valuable tool for experimentally probing exciton dissociation.

A ‘modulated potential model’ is presented to describe the potential of diatomic molecules in static electric fields, and investigate the static electric effect on the multiphoton process. The multiphoton transition of HF in static electric and intense laser fields has been investigated using the Lie algebra approach. Multiphoton transition information is obtained by solving the time-evolution operator. This method makes the solving of the Schrödinger equation easier and makes the computation time shorter.

Using spin-unrestricted density functional theory, the longitudinal static second hyperpolarizability γ has been investigated for an asymmetric azulene-like open-shell condensed-ring molecule (1) as well as for its symmetric almost closed-shell and open-shell analogues (pentacene (2) and its dicyclopenta- (3) and dicyclohepta-fused (4) analogues). It is found that the symmetric systems, 3 and 4, having intermediate diradical characters show a 6–9 times enhancement of γ as compared to 2, but that this enhancement is even larger (by a factor of 30) for the asymmetric diradicaloid, 1. Geometrical, electronic, and magnetic descriptors have shown that in 1 the terminal seven- and five-membered rings display a cooperative push–pull effect, leading to a large dipole moment, that seems to reinforce its aromaticity as well as its ionic character, whereas 3 and 4 are mostly neutral diradicaloids and are less aromatic. These features have been rationalized by resorting to an asymmetric two-site model within the vale...

Organic diradicals play an important role in many fields of chemistry, biochemistry, and materials science. In this work, by means of high-level theoretical calculations, we have investigated the effect of representative chemical substituents in p-quinodimethane (pQDM) and Thiele's hydrocarbons with respect to the singlet-triplet energy gap, a feature characterizing their diradical character. We show how the nature of the substituents has a very important effect in controlling the singlet-triplet energy gap so that several compounds show diradical features in their ground electronic state. Importantly, steric effects appear to play the most determinant role for pQDM analogues, with minor effects of the substituents in the central ring. For Thiele like compounds, we found that electron-withdrawing groups in the central ring favor the quinoidal form with a low or almost null diradical character, whereas electron-donating group substituents favor the aromatic-diradical form if the electron donation does not exceed 6-π electrons. In this case, if there is an excess of electron donation, the diradical character is reduced. The electronic spectrum of these compounds is also calculated, and we predict that the most intense bands occur in the visible region, although in some cases characteristic electronic transition in the near-IR region may appear.

We investigate the effects of a static electric field on the third-order nonlinear optical (NLO) properties - static second hyperpolarizabilities γ - of symmetric singlet diradical molecules using the valence configuration interaction scheme. It is found that the application of the field strongly (∼ two - three order) enhances the component of γ along the axis joining the two radical sites of symmetric diradicals having intermediate diradical characters as compared to those of closed-shell and pure diradical molecules in absence of a field. These results are expected to open a new path to realize an electric field control NLO switch based on open-shell singlet molecular systems.

The external static electric field (SEF) of man-made origin brings to the substantially increased SEF background in a human environment the biological activity of which is a moot question. The paper reports on rats blood plasma/serum proteome modifications by means of 1D polyacrilamide gel electrophoresis and clotting process alterations after the short- and long-term SEF exposures of 200 kV/m. The results indicate decrease of fast α1 and α2 globular proteins in plasma coinciding with clotting acceleration after the short-term SEF, and attenuation of clotting-dependent proteome modifications reflected with incomplete coagulation after the long-term SEF exposure. Increased lysozyme activity in serum unlike plasma was observed after both SEF exposures. Applied model of the high-voltage SEF environment indicates dependence of biological systems functioning on the external SEF.

Open-shell singlet diradicaloids display unique electronic, nonlinear optical, and magnetic activity and could become novel molecular materials for organic electronics, photonics, and spintronics. However, design and synthesis of diradicaloids with a significant polyradical character is a challenging task for chemists. In this Article, we report our efforts toward a tetraradicaloid system. A series of potential tetraradicaloids by fusion of two p-quinodimethane (p-QDM) units with naphthalene or benzene rings in different modes were synthesized. Their model compounds containing one p-QDM moiety were also prepared and compared. Their ground-state structures, physical properties, and chemical reactivity were systematically investigated by various experimental methods such as steady-state and transient absorption, two-photon absorption, X-ray crystallographic analysis, electron spin resonance, superconducting quantum interference device, and electrochemistry, assisted by density functional theory calculations. It was found that their diradical and tetraradical characters show a clear dependence on the fusion mode. Upon the introduction of more five-membered rings, the diradical characters greatly decrease. This difference can be explained by the pro-aromaticity/antiaromaticity of the molecules as well as the intramolecular charge transfer. Our comprehensive studies provide a guideline for the design and synthesis of stable open-shell singlet polycyclic hydrocarbons with significant polyradical characters.

We have prepared a series of strongly fluorescent N2O2-bis (phenolate) aza-dipyrromethene dyes incorporating boron (aza-BODIPY), aluminum (aza-ALDIPY), gallium (aza-GADIPY), and indium (aza-INDIPY). These compounds were oxidized both chemically and electrochemically to give diradical molecules 4b-7b with an open-shell singlet ground state and a thermally accessible triplet excited state. The ground state behavior of these diradical molecules was confirmed by a wide array of magnetic spectroscopies, including variable-temperature 1H NMR, EPR, and SQUID. X-ray crystallography confirmed the N2O2 coordination sphere in these systems. DFT and TDDFT quantum chemical calculations were used to determine their electronic structures, nature of the excited states, diradical character (y0), and spin density distributions for both open-shell singlet and triplet states. Spectroelectrochemical studies on the deprotonated dianions 4a2- to 7a2- showed the formation of mixed-valent radical anion species with intense absorption and broad intervalence charge transfer bands in the near-infrared region. Upon further oxidation of the mixed-valent radical anion species, diradical molecules 4b-7b were generated.

Water flow through a carbon nanotube: A comparison between static and terahertz electric fields

Electro growth of Al[sbnd]Cu eutectic alloy

The model of the generalized effective potential of the quadrupole field is considered. The model is based on the stability limit of the charged particles' oscillations in the composition of the high-frequency and static quadrupole electric fields. The representation of the polyharmonic oscillations in the variable fields by harmonic oscillations in the field of the generalized effective potential is approximate. Its accuracy increases at the stability limits. With the use of the model the problem of ion oscillations' excitation under the superposition on the quadrupole RF field of the static quadrupole and homogeneous electric fields is solved.