For more than 30 years, nonlinear optical (NLO) properties of molecular systems have been actively studied both theoretically and experimentally due to their potential applications in photonics and ...optoelectronics. Most of the NLO molecular systems are closed‐shell species, while recently open‐shell molecular species have been theoretically proposed as a new class of NLO systems, which exhibit larger NLO properties than the traditional closed‐shell NLO systems. In particular, the third‐order NLO property, the second hyperpolarizability γ, was found to be strongly correlated to the diradical character y, which is a quantum‐chemically defined index of effective bond weakness or of electron correlation: the γ values are enhanced in the intermediate y region as compared to the closed‐shell (y = 0) and pure open‐shell (y = 1) domains. This principle has been exemplified by accurate quantum‐chemical calculations for polycyclic hydrocarbons including graphene nanoflakes, multinuclear transition‐metal complexes, main group compounds, and so on. Subsequently, some of these predictions have been substantiated by experiments, including two‐photon absorption. The fundamental mechanism of the y–γ correlation has been explained by using a simple two‐site model and the valence configuration interaction method. On the basis of this y–γ principle, several molecular design guidelines for controlling γ have been proposed. They consist in tuning the diradical characters through chemical modifications of realistic open‐shell singlet molecules. These results open a new path toward understanding the structure—NLO property relationships and toward realizing a new class of highly efficient NLO materials. WIREs Comput Mol Sci 2016, 6:198–210. doi: 10.1002/wcms.1242
This article is categorized under:
Structure and Mechanism > Molecular Structures
Structure and Mechanism > Computational Materials Science
Electronic Structure Theory > Ab Initio Electronic Structure Methods
The quantum treatment of electrons at the Kohn‐Sham Density Functional Theory (DFT) level of approximation is used in order to optimize the structure of four organic and organo‐metallic crystalline ...systems, starting from experimentally determined structures, allowing subsequent quantum‐level theoretical analysis, the accent here being put on crystals having nonlinear optical (NLO) properties. By employing DFT, one must select an exchange‐correlation functional (XCF), that is, a mathematical description of the electron interactions. Among most of the XCFs, a well‐known general discrepancy is the lack of London‐type interactions description. One of the most popular correction schemes consists of adding a posteriori the contribution of these interactions by using empirical expressions. Here, the approach relies on the simple D* scheme, a variation of the D2 correction originally elaborated by Grimme for molecular clusters, where the D2 correction is scaled to account for the interactions in the crystal solid state. The performance of this simple D* scheme is demonstrated and the most suitable scaling factors for four prototypical organic and organo‐metallic NLO crystals are determined for four representative XCFs: zero (no correction) for PBESol, PBESol0, and ωB97X, and 0.7 for B3LYP.
The scaling factor of the D* London dispersion correction scheme, based on the Grimme D2 approach, has been tuned for four exchange‐correlation functionals to determine the structures of organic and organometallic nonlinear optical materials in close agreement with experimental data. This leads to a very simple yet efficient approach, enacted at the density functional theory level of theory using periodic boundary conditions.
The electronic and optical properties of metallo-phthalocyanine derivatives have been calculated by using density functional theory. Starting from a reference Zn(II) phthalocyanine the structure has ...been varied by changing the nature of the transition metal, by adding carboxylic functions, as well as by considering the extension of the aromatic rings with fused phenyl moieties, with the perspective of optimizing their performance as sensitizers in solar cells. Calculations demonstrate that the transition metal (Ni, Mn, Fe, Co, and Cu versus Zn) has a small impact on the free energies of the primary processes (injection, recombination, and regeneration), that adding several carboxylic acid functions has a cooperative role to optimize the injection process, and that the addition of fused phenyl rings increases the light harvesting efficiency and the free energy of injection.
•DFT is efficient for designing metallo-phthalocyanines with optimized substituents•The transition metal has a small impact on the free energies of the primary processes•carboxylic acid functions have a cooperative role to optimize the injection process•Fused phenyl rings increase the light harvesting efficiency and the free energy of injection
The rational design of a geometrically constrained boron Lewis superacid featuring exceptional structure and reactivity is disclosed. It enabled the formation of non‐classical electron deficient ...B−H−B type of bonding, which was supported by spectroscopic and structural parameters as well as computational studies. Taming the pyramidal Lewis acid electrophilicity through weak coordinating anion dissociation enabled a series of highly challenging chemical transformations, such as Csp2−H and Csp3−H activation under a frustrated Lewis pair regime and the cleavage of Csp3−Si bonds. The demonstration of such rich chemical behaviour and flexibility on a single molecular compound makes it a unique mediator of chemical transformations generally restricted to transition metals.
Taming the Lewis superacidity of a pyramidal borane by weakly coordinated anion dissociation enabled the activation of inert C−H bonds and resulted in an unusual coordination mode at boron.
Density functional theory calculations with the M06-2X exchange–correlation functional have been performed to explore the Diels–Alder reaction between 2,5-DMF and ethylene as well as to compare the ...uncatalyzed reaction to the one catalyzed by the AlCl
3
Lewis acid. The uncatalyzed reaction corresponds to a normal electron-demand (NED) mechanism where ethylene is an electron acceptor and 2,5-DMF plays the role of electron donor. This reaction presents a low polar character, its kinetics is little impacted by the solvent dielectric constant, and the formation of the two new σ bonds occurs through a one-step synchronous process. When the LA interacts with ethylene, forming a π-complex, it enhances its acceptor character, further favoring the NED mechanism, which is accompanied by a reduction of the free energy of the transition state. On the other hand, when AlCl
3
is complexed by 2,5-DMF, the inverse electron-demand (IED) mechanism is favored, with ethylene playing the role of the donor. Within both NED and IED mechanism, the LA-catalyzed reaction takes place via a one-step asynchronous process. In addition, it is highly polar, so that the activation barrier decreases with the solvent polarity. Moreover, the calculations have evidenced that the LA forms stable complexes with any of the reactants so that the gain on the activation barrier amounts to 9–12 kcal mol
−1
for the NED mechanism and to 3–9 kcal mol
−1
for the IED one and that the formation of Al
2
Cl
6
dimers impacts the different equilibria. Finally, the decrease of the activation barrier goes in pair with the reduction of the HOMO–LUMO gap, with the greatest decrease recorded when the LA interacts with ethylene according to the NED mechanism.
The frustrated Lewis pair‐catalyzed hydrogenation of unactivated alkenes with H2 is reported. The weak Lewis basicity and high steric hindrance of ortho‐substituted 9‐phosphatriptycene derivatives is ...taken as an advantage to catalyze this reaction in combination with tris(pentafluorophenyl)borane. A scope of unsaturated substrates is described. The thermodynamics and kinetics of hydrogenation investigated by density functional theory show that the protonation of the olefin by the strongly acidic phosphonium cation is the critical step of the reaction.
Main Group catalysis: Ortho‐substituted 9‐phosphatriptycene derivatives are combined with tris(pentafluorophenyl)borane to perform the first FLP‐catalyzed hydrogenation of unactivated alkenes. The choice of the catalyst and the reaction conditions are optimized and a scope of substrates is performed experimentally while a DFT study investigates the thermodynamics and kinetics of the reaction.
A highly bent triarylborane, 9‐boratriptycene, was generated in solution by selective protodeboronation of the corresponding tetra‐aryl boron ate complex with the strong Brønsted acid HNTf2. The ...iptycene core confers enhanced Lewis acidity to 9‐boratriptycene, making it unique in terms of structure and reactivity. We studied the stereoelectronic properties of 9‐boratriptycene by quantifying its association with small N‐ and O‐centered Lewis bases, as well as with sterically hindered phosphines. The resultant Lewis adducts exhibited unique structural, spectroscopic, and photophysical properties. Beyond the high pyramidalization of the 9‐boratriptycene scaffold and its low reorganization energy upon Lewis base coordination, quantum chemical calculations revealed that the absence of π donation from the triptycene aryl rings to the boron vacant pz orbital is one of the main reasons for its high Lewis acidity.
Breaking B–π conjugation: 9‐Boratriptycene is a bent and non‐conjugated triarylborane with an unprecedented geometrical shape, which confers a low reorganization energy and high Lewis acidity power.
βof helicenes built from fused benzenes and thiophenes result from subtle effects.Without substituents the first hyperpolarizabilities (β) are very small.βis enhanced when acceptor or donor groups ...are placed at terminal positions.When these are NO2 groups β is enhanced when helicenes contain few benzene rings whereas in the case of NMe2 groups the enhancement can be damped by three effects.These calculations suggest to replace thiophene by furan rings to achieve even larger β.
Using quantum chemistry methods, structure⿿NLO property relationships are investigated for families of 7-ring helicenes built from thiophene and benzene rings. In absence of donor or acceptor substituents, the first hyperpolarizability (β) is small. On the other hand, with NO2 groups in terminal positions β increases, especially for helicenes containing few benzene rings. For NMe2 substituents the enhancement is damped by the less polarizable benzene rings, the cross-conjugated all-thiophene segments, and the auxiliary acceptor character of the thiophenes. From this analysis, we selected three helicenes and conjectured larger β upon replacing the thiophenes by furans. This hypothesis has been verified.
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