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Effects of the number of monomers (N) on the two-photon absorption (TPA) properties of π-stacked multimer models consisting of phenalenyl radicals were investigated theoretically. We conducted spectral simulations for the π-stacked N-mer models (N = 2, 4, and 6) with different stacking distances (d1) and their alternation patterns (d2/d1). Excitation energies and transition dipole moments were calculated at the extended multi-configurational quasi-degenerate second-order perturbation theory (XMC-QDPT2) level based on the complete active space self-consistent field (CASSCF) wavefunctions with the active space orbitals constructed from the singly occupied molecular orbitals (SOMOs) of monomers. The TPA cross-section value per dimer unit at the first peak, originating from the electronic transition along the stacking direction, was predicted to increase significantly as the d2/d1 approaches one, as the d1 decreases, and as the N increases from 2 to 6. These tendencies are similar to the calculation results for the static hyperpolarizabilities. [ABSTRACT FROM AUTHOR]

Diradical character is one of the characteristic quantities of functional open-shell molecules. Prof. Nakano devotedly studied the relationship between diradical character and material properties of open-shell molecules; now, we can use the diradical character as a powerful tool for molecular material designs. It is still unclear how the open-shell molecules are affected by the interaction with the surface although the molecules have been immobilised for device applications. In the present study, the adsorptions of model diradical molecules with s-electrons on the MgO (001) and BaO (001) surfaces are investigated using approximate spin projected density functional theory with plane-wave basis (AP-DFT/plane-wave) to provide a systematic discussion of surface–diradical interactions. The accuracy of AP-DFT/plane-wave was verified by comparisons with the calculated results by NEVPT2. The computational error introduced by DFT calculations on the diradical state (spin contamination error) is reduced by the surface–diradical interaction. In addition, it is shown that (1) the diradical character is amplified by the orbital polarisation effects of oxide ions, and (2) the character decreases when the magnetic orbitals become electron-rich due to electron donation from the surfaces. The two effects are competing; the former is pronounced in Au systems, whereas the latter is pronounced in Ag systems. [ABSTRACT FROM AUTHOR]

Singlet fission (SF) is a photophysical process where one singlet exciton splits into two triplet excitons. To construct design guidelines for engineering directional triplet exciton migration, we investigated the SF dynamics in symmetric linear heterotrimer systems consisting of different unsubstituted or 6,13-disubstituted pentacene derivatives denoted as X/Y (X, Y: terminal and center monomer species). Time-dependent density functional theory (TDDFT) calculations clarified that the induction effects of the substituents, represented as Hammett's para-substitution coefficients σ p , correlated with both the excitation energies of S1 and T1 states, in addition to the energies of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). Electronic coupling calculations and quantum dynamics simulations revealed that the selectivity of spatially separated TT states for heterotrimers increased over 70%, superior to that in the homotrimer: an optimal region of the difference in σ p between the substituents of X and Y for the increase in SF rate was found. The origin of the rise in SF rate is explained by considering the quantum interference effect: reduction in structural symmetry opens new interaction paths, allowing the S1-TT mixing, which contributes to accelerating the hetero-fission between the terminal and center molecules. [ABSTRACT FROM AUTHOR]

Herein, the electron conductivities of [18]annulene and its derivatives are theoretically examined as a molecular parallel circuit model consisting of two linear polyenes. Their electron conductivities are estimated by elastic scattering Green's function (ESGF) theory and density functional theory (DFT) methods. The calculated conductivity of the [18]annulene does not follow the classical conductivity, i.e., Ohm's law, suggesting the importance of a quantum interference effect in single molecules. By introducing electron-withdrawing groups into the annulene framework, on the other hand, a spin-polarized electronic structure appears, and the quantum interference effect is significantly suppressed. In addition, the total current is affected by the spin polarization because of the asymmetry in the coupling constant between the molecule and electrodes. From these results, it is suggested that the electron conductivity as well as the quantum interference effect of π-conjugated molecular systems can be designed using their open-shell nature, which is chemically controlled by the substituents. [ABSTRACT FROM AUTHOR]

Substances containing two π‐congested aromatic systems are attractive targets in synthetic studies as well as efforts designed to explore the unique properties that originate from π‐congestion. Since the time of the computational studies by Schleyer and Warner, the concept of stacked aromaticity created by the encounter of two antiaromatic rings has received much attention. A structure containing π‐congestion between two antiaromatic norcorrole rings was prepared by Shinokubo et al. In contrast, questions about what happens when two aromatic rings are closely stacked have remained unanswered. Specifically, the electronic consequences of stacking of two aromatic π‐conjugated systems have not been fully evaluated. To the best of our knowledge, only one computational study has been performed by Herges, the results of which suggest that [2.2]paracyclophane, possessing short distance (<3.00 Å) between two aromatic ring planes, has a paratropic ring current between two the benzene rings. This observation is consistent with the conclusion that stacked antiaromaticity exists within the π‐congested space between aromatic rings. Herein, we investigated the stacked antiaromaticity of highly π‐congested anthracene dimers using anthracenophane, which possesses a short interplane carbon–carbon distance of ca. 2.80 Å. The absorption spectrum of this substance contains a weak broad band from 450 to 550 nm that is attributed to the HOMO–LUMO transition. These properties exist in planar cyclooctatetraene derivatives that have 8π‐electron antiaromaticity. The results of nucleus‐independent chemical shift and anisotropy of the induced current density calculations indicate that a relatively weak antiaromatic character exists between the central six‐membered rings of the two anthracene moieties in anthracenophane. In addition, an attempt to enhance the stacked antiaromaticity of anthracenophane using pressure to enhance π‐congestion was unsuccessful. [ABSTRACT FROM AUTHOR]