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The singlet fission (SF) dynamics of pentacene linear aggregate models are investigated using the quantum master equation method by focusing on the Frenkel excitonic (FE) coupling effects on the SF rate and double triplet (TT) yield as well as on their aggregate size dependences. It is found that for the dimer model, unrealistically large FE couplings are needed to provide significant effects on the SF dynamics, while for the larger aggregate models a realistic FE coupling causes significant variations in the SF dynamics: as increasing the aggregate size, the SF rate rapidly increases, attains the maximum at 8‐mer (~3 times enhancement as compared to the non‐FE‐coupling case) and then decreases, approaching a stationary value after 12‐mer, although the stationary TT yield at 20‐mer remains slightly smaller than that in the non‐FE‐coupling case. These features are explained based on the relative relaxation factors between the adiabatic exciton states. The present results contribute to constructing the design guidelines for highly efficient SF aggregates. © 2018 Wiley Periodicals, Inc. The singlet fission (SF) dynamics of pentacene linear aggregates are investigated using the quantum master equation method by focusing on the Frenkel excitonic (FE) coupling effects on the SF rate and double triplet yield as well as on their aggregate size dependences. It is found that a realistic FE coupling causes significant size‐dependent variations in the SF rate, which are explained by the relative relaxation factor analysis. [ABSTRACT FROM AUTHOR]

Two‐fold C−C cross‐coupling of N‐heterocyclic carbenes [NHCs; SIPr=C(NArCH2)2, 1; IPr=C(NArCH)2, 2; Me‐IPr=C(NArCMe)2, 3; Ar=2,6‐iPr2C6H3] with 4,4′′‐diiodo‐p‐terphenyl under Ni catalysis furnished [(SIPr)(C6H4)3(SIPr)](I)2 (4), [(IPr)(C6H4)3(IPr)](I)2 (5), and [(Me‐IPr)(C6H4)3(Me‐IPr)](I)2 (6). Two‐electron reduction of 4–6 with KC8 readily afforded NHC analogues of Müller's hydrocarbon (MH), [(SIPr)(C6H4)3(SIPr)] (7), [(IPr)(C6H4)3(IPr)] (8), and [(Me‐IPr)(C6H4)3(Me‐IPr)] (9), respectively, as highly colored crystalline solids. Quantum chemical calculations suggested that the singlet ground state for 7–9 possesses a vertical singlet–triplet energy gap ΔES‐T of −7.24 to −7.60 kcal mol−1, which is significantly lower compared to that of the NHC analogues of Thiele's (TH) and Chichibabin's (CH) (18–38 kcal mol−1) hydrocarbons. Importantly, the calculated diradical character (y) of 7–9 (y≈0.6) is considerably higher compared to that of the related TH and CH (y=0.1–0.4), suggesting the open‐shell singlet character of 7–9. Radical taming by NHCs. Crystalline N‐heterocyclic carbene (NHC) analogues of the highly reactive Müller's hydrocarbon (A), [(NHC)(C6H4)3(NHC)] (B) are reported [NHC=C(NArCH2)2, SIPr; C(NArCH)2, IPr; C(NArCMe)2, Me‐IPr; Ar=2,6‐iPr2C6H3]. They feature a singlet ground state with small singlet–triplet energy gaps (ΔES‐T≈7.4 kcal mol−1) along with an intermediate diradical character (y=65 %). [ABSTRACT FROM AUTHOR]

We developed the quantal cumulant mechanics for treating multidimensional quantum many-body clusters including two-body interaction such as the Morse potential. To evaluate an effective potential appearing in the actual calculation, a Gaussian fitting method was adopted to approximate the Morse potential. The number of the Gaussians that are required to reproduce the total energy of a classical three-dimensional (3D) Morse 3 (M3) cluster is 31, where the error is 10−6. We compared structures of the classical M3 cluster with those of quantum counterpart and found that the quantum structure have broad distribution due to zero point vibration effects. The symmetry of the cluster becomes lower from D3h to D2h by applying the diagonal approximation to the cumulant matrices. Conversely, the original and spherical approximation holds the symmetry constraint. We also perform the same analyses on 2D M4 cluster with two different stable structures, where one has D3h symmetry and the other D2h symmetry. In the latter case, the diagonal approximation accidentally gives the same results as the original one, when two of three Cartesian axes coincide with the cluster symmetric axes. When the cluster rotates with respect to these axes, the results of the diagonal approximation deviate from those by the original one and the artificial symmetry breaking is also found. We also evaluate the optimized structures of the highly symmetric small Morse clusters M n ranging from n = 4-7 and compare with those with the corresponding classical ones. We found that the errors of both the diagonal and spherical approximations in total energy decrease with the number of particles. This fact indicates that these approximations will be useful to investigate static and dynamical properties of many-particle quantum clusters with low computational cost. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The anionic derivative of the pharmaceutical compound 3-methyl-1-phenyl-2-pyrazolin-5-one (MCI-186) is known to deactivate the free radicals in biological systems by donating its excess electron. For the purpose to investigate the mechanism that the MCI-186 anion exhibits the antioxidative activity in aqueous solutions, we have performed a series of quantum mechanical/molecular mechanical (QM/MM) simulations based on the density functional theory in combination with the theory of energy representation (QM/MM-ER). For the computational convenience, MCI-186 has been modeled by 1,3-dimethyl-2-pyrazolin-5-one (DMP) by substituting the phenyl group in MCI-186 by a methyl one. We have applied the QM/MM-ER approach to compute free energy change δ G associated with the electron detachment from DMP anion in water. To make comparisons, we have also performed the simulations for a natural radical scavenger, ascorbic acid modeled by 3,4-dihydroxy-furan-2-one (DHF). In the QM/MM-ER approach, the distribution functions of the solute-solvent interaction energy, which serve as fundamental variables to describe the free energy, are to be constructed with the QM/MM force field. The free energy changes δ G were computed as 99.1, 108.3 kcal/mol for DMP and DHF anions, respectively, whereas that for hydroxyl anion OH taken as a reference system was evaluated as 138.9 kcal/mol. Thus, it was revealed that the free energies δ G for DMP and DHF anions are much smaller than that for hydroxyl anion, which directly indicates the high activities of the molecules as electron donors. We found that the small δ G values can be mainly attributed to the small values in the absolutes of the solvation free energies. The fractional charge analyses provided the evidence that the delocalization of the excess charge on the solute gives rise to the destabilization of these radical scavengers in the solute-solvent interaction. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

Abstract: We propose a novel dynamic exciton expression based on the quantum master equation approach using the ab initio molecular orbital (MO) singly excited configuration interaction (CI) method developed in our previous paper [M. Nakano, M. Takahata, S. Yamada, R. Kishi, T. Nitta, K. Yamaguchi, J. Chem. Phys. 120 (2004) 2359]. This expression is derived from the partition of polarization density in the configuration basis into the electron and hole contributions, and can describe both the coherent and incoherent dynamics of electron and hole density distributions, e.g., dynamic electric polarization, exciton recurrence and exciton migration. [Copyright &y& Elsevier]

We report a study on the static second hyperpolarizabilities (γ) of zethrenes using the UBHandHLYP hybrid density functional theory method. Though zethrenes were originally considered as closed-shell systems, the values of their diradical characters, calculated from the occupation numbers of spin-unrestricted Hartree-Fock natural orbitals (UNOs), demonstrate an open-shell diradical nature, which increases with the size of the middle ring region because of the recovery of the aromaticity in that region. As references, we examine also the γ values of tetrabenzopolyacenes, which are regarded as the corresponding closed-shell systems due to their negligible diradical characters. It is found that the longitudinal γ values for zethrenes are much larger than those of the corresponding reference closed-shell systems, whereas their ratio, γ(zethrene)/γ(reference), is reduced when increasing the length of middle ring regions, i.e., when increasing the diradical character. [ABSTRACT FROM AUTHOR]

In a previous paper we found that the cofacial intermolecular π–π orbital interaction in stacking dimers significantly changes the longitudinal second hyperpolarizability (γ) of the isolated monomer. On the basis of this result, we investigate the longitudinal γ values of π‐conjugated main chains (CnHn+2, 6 ≤ n ≤ 16) interacting in both‐end regions with two small‐size cationic perturbing π‐conjugated molecules, that is, allyl cations (C3H+5). These interacting model systems exhibit remarkable enhancement of γ values as compared with those of isolated main chains in the whole chain‐length region. The γ density analysis reveals that this enhancement is described by the virtual charge transfer between both‐end perturbing molecules via the main chain. The analysis of orbital correlation diagram between the perturbing molecules and main chain molecule also clarifies that such feature of γ density distribution originates in the “weak intermolecular antibondinglike coupling” between the (lowest unoccupied molecular orbital [LUMO], LUMO+1) of cationic perturbing molecules and (highest unoccupied molecular orbital [HOMO], HOMO−1) of main chain molecule. The current result suggests the possibility of novel nano‐size control of nonlinear optical (NLO) properties by adjusting the intermolecular orbital interactions between the main molecule and perturbing molecules. A possible control scheme of longitudinal γ for novel intermolecular interacting NLO systems using modified DNA wires is also proposed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [ABSTRACT FROM AUTHOR]

Metal dithiolene complexes—M(dmit)2—are key building blocks for magnetic, conducting, and optical molecular materials, with singular electronic structures resulting from the mixing of the metal and dmit ligand orbitals. Their use in the design of magnetic and conducting materials is linked to the control of the unpaired electrons and their localized/delocalized nature. It has been recently found that UV–Vis light can control the spin distribution of some [Cu(dmit)2]−2 salts in a direct and reversible way. In this work, we study the optical response of these salts and the origin of the differences observed in the EPR spectra under UV–Vis irradiation by means of wave function-based quantum chemistry methods. The low-lying states of the complex have been characterized and the electronic transitions with a non-negligible oscillator strength have been identified. The population of the corresponding excited states promoted by the UV–Vis absorption produces significant changes in the spin distribution, and could explain the changes observed in the system upon illumination. The interaction between neighbor [Cu(dmit)2]−2 complexes is weakly ferromagnetic, consistent with the relative orientation of the magnetic orbitals and the crystal packing, but in disagreement with previous assignments. Our results put in evidence the complex electronic structure of the [Cu(dmit)2]−2 radical and the relevance of a multideterminantal approach for an adequate analysis of their properties. [ABSTRACT FROM AUTHOR]

Abstract: We investigate a singlet diradical molecule (2-borapropane-1,3-diyl) involving a B atom in an electron donor solvent, i.e., water, from the viewpoint of controlling the (hyper)polarizability through tuning the diradical character. The quantum chemical calculations for a model system reveal that a donation from the O lone pair electrons of a water molecule to the vacant p orbital of the B atom increases the diradical character, leading to a large variation in the (hyper)polarizability of the B-containing compound. The effect of the bulk solvent determined within the PCM scheme is also shown to increase the degree of enhancement of the second hyperpolarizability in the intermediate diradical regime of the B-containing compound. [Copyright &y& Elsevier]