This chapter theoretically explains the electronic structures of open-shell singlet systems with a wide range of open-shell (diradical) characters. The definition of diradical character and its ...correlation to the excitation energies, transition properties, and dipole moment differences are described based on the valence configuration interaction scheme using a two-site model with two electrons in two active orbitals. The linear and nonlinear optical properties for various polycyclic aromatic hydrocarbons with open-shell character are also discussed as a function of diradical character.
The thermal decomposition of silver acetate (CH3COOAg) was investigated to reveal the factors controlling the formation of Ag nanoparticles (NPs). The overall kinetic behavior was interpreted as ...partially overlapping two reaction steps using systematic kinetic and morphological analyses. Although the apparent activation energies were comparable (approximately 75 kJ mol–1), the initial reaction step was regulated by the first order law because of the consumption of reactive sites on the end surfaces of columnar crystals, whereas the subsequent reaction step advanced by shrinkage of the side surfaces of the crystals with an accelerating linear shrinkage rate, resulting in slimming of the crystals. A large surface area of the reactant crystals was exposed to the reaction atmosphere during the course of the reaction by the self-induced migration of the Ag product to the surfaces of the Ag-NP aggregates formed at certain parts of the reactant surfaces. As a result, the atmospheric water vapor affected the kinetic behavior by significantly lowering the reaction temperature. As a possible explanation for these phenomena, a physical mechanism involving evaporation of the reactant and simultaneous condensation of the product is proposed herein.
We theoretically investigate the role of charge-transfer (CT) states being stabilized by surrounding environments on singlet fission (SF) dynamics in a pentacene crystal. Using a polarizable force ...field to calculate induction energy by the surrounding molecules, the stabilization of the CT dimer energy is assessed for its convergence in massively large crystalline cells. The behavior of the convergence and the converged energies are found to depend on the local dimer configuration, that is, parallel slip-stack or tilted face-to-edge packings. Using the resultant crystalline-effective CT energies, the SF dynamics simulation based on the quantum master equation is performed for both dimers and multimers in the crystal. It is found for the multimer that the populations of the Frenkel excitons and CT excitons exhibit recurrence motions between neighboring sites and that double-triplet excitons are then gradually generated in an ultrafast timescale (<100 fs). From the analysis on relative relaxation factors (RRFs) between the adiabatic exciton states, the mechanism of the rapid SF rate in the crystal is revealed, where the intrusions of the diabatic CT configurations into all the adiabatic states effectively accelerate the SF process. Furthermore, the RRF is used to predict the double-triplet exciton yield at the extreme of the extended large size cell. The present findings will provide a clue to construct design guidelines for efficient SF based on the control of CT energy by environmental engineering.
First isolation and characterization of biphenalenylidenes, which have long been unidentified reactive intermediates on the decomposition pathway of phenalenyl radical, were accomplished. ...Photoinduced electrocyclic ring-opening reaction of anti-dihydroperopyrene resulted in a successful conversion to E-biphenalenylidene, which enabled a detailed investigation of the electronic structure of E-biphenalenylidene by means of spectroscopic techniques. A stereoisomer, Z-biphenalenylidene, was also observed by suppressing a facile E–Z isomerization to E-biphenalenylidene in a rigid matrix. Furthermore, Z-biphenalenylidene demonstrated a thermal ring-closure in conrotatory process, which is not conforming to the Woodward–Hoffmann rule. These unusual reactivities of biphenalenylidene are ascribed to the ground states destabilized by its singlet biradical character, which was fully supported by theoretical calculations. The presence of E-biphenalenylidene on the decomposition pathway of phenalenyl was confirmed experimentally, leading to the full understanding of the decomposition mechanism of phenalenyl.
The characteristics of the edge state, which is a peculiar magnetic state in zigzag-edged graphene nanoribbons (ZGNRs) that originates from electron–electron correlation in an edge-localized π-state, ...are investigated by preparing and characterizing quarteranthene molecules. The molecular geometry that was determined from the X-ray analysis is consistent with a zigzag-edge-localized structure of unpaired electrons. The localized electrons are responsible for the peculiar magnetic (room-temperature ferromagnetic correlation), optical (the lowest-lying doubly excited state), and chemical (peroxide bond formation) behaviors. On the basis of these distinguishing properties and a careful consideration of the valence bonding, insight into the edge state of ZGNRs can be gained.
Singlet fission (SF) occurs as a result of complex excited state relaxation dynamics in molecular aggregates, where a singlet exciton (FE) state is converted into a double-triplet exciton (TT) state ...through the interactions with several other degrees of freedom, such as nuclear motions. In this study, we combined quantum dynamics simulation based on the quantum master equation approach with all-atom-based classical molecular mechanics/molecular dynamics to examine the thermal structural fluctuation (i.e., static disorder) effects of intermolecular configuration on SF in pentacene crystal models. In particular, we considered two types of static-disordered models, in which excited states are assumed to interact with nuclear motions of intermolecular modes in the classical mechanical/statistical manner. We found that the introduction of static disorder effects leads to a faster decay of coherence between the FE and charge transfer (CT) states in the early stage of SF, contributing to the accelerations of several FE → TT relaxation pathways. Such acceleration in these models is shown to be attributed to fluctuations in the energies and electronic coupling of the CT states based on relative relaxation factor analysis. The present study is expected to contribute to further development of bottom-up materials design for efficient SF in condensed phases where the exitonic system interacts with nuclear motions in various coupling strengths.
We investigate the singlet fission (SF) dynamics of a slip-stack-like pentacene ring-shaped aggregate model, which is constructed by rotating each pentacene unit around its longitudinal axis in an ...H-aggregate ring. The aggregate size (N) and rotation angle (α) dependences of SF rates and double triplet (TT) yields are clarified using the quantum master equation method. It is found that there exist optimal ranges of the rotation angle α for each N, yielding efficient SF with high SF rates and TT yields. For example, in an 8-mer model, SF rates at α = 23 and 43° are 18.9 and 38.6 times as high as that at α = 30°, respectively, and the TT yields are as high as 0.871, 0.988, and 0.882 at α = 23, 30, and 43°, respectively. Analysis of the relative relaxation factors shows that the many-to-many relaxation paths from adiabatic Frenkel exciton (FE)-like states to TT-like states are opened by tuning α at relevant aggregate sizes, causing fast and high-TT-yield SF, and efficient SF occurs at α = 40° for medium N (7 ≤ N ≤ 10) or at α = 30° for large N (>10). This mechanism is interpreted by the second-order perturbation theory for electronic couplings. Namely, the inequality in the energies of charge-transfer states CA and AC states, where the cation (C) and anion (A) are located at two neighboring sites in anticlockwise and clockwise directions, respectively and the change in the amplitude and sign of the couplings between the FE, CT, and TT states are found to cause quantum superposition of the FE and TT states, which contribute to the high TT yield and SF rate. The present results contribute to a deeper understanding of SF dynamics in ring-shaped aggregates as well as to the development of their new design guidelines.
Vibronic coupling plays a crucial role in singlet fission whereby a singlet exciton splits into two triplet excitons. In order to reveal the physicochemical origin of the vibronic coupling associated ...with singlet fission as well as to clarify its relationship with chemical structure, we evaluate relevant vibronic couplings from the viewpoint of their spatial contributions described by vibronic coupling density. From the analysis using a model tetracene dimer, a typical singlet fission system, the frequency dependence of vibronic couplings in each electronic state is found to be significantly different from that of another depending on the nature of the electronic structure (intra/intermolecular excitation) and the related vibrational motion. These findings contribute not only to the fundamental understanding of the singlet fission mechanism from the viewpoint of vibronic couplings but also to opening a new path to designing highly efficient singlet fission materials through phonon–bath engineering.
Open‐shell character, e. g., diradical character, is a quantum chemically well‐defined quantity in ground‐state molecular systems, which is not an observable but can quantify the degree of effective ...bond weakness in the chemical sense or electron correlation strength in the physical sense. Because this quantity also correlates to specific excited states, physicochemical properties concerned with those states are expected to strongly correlate to the open‐shell character. This feature enables us to open a new path to revealing the mechanism of these properties as well as to realizing new design principles for efficient functional molecular systems. This account explains the open‐shell‐character‐based molecular design principles and introduces their applications to the rational design of highly efficient nonlinear optical and singlet fission molecular systems.
The ground‐ and excited–state electronic structures of open‐shell molecular systems are clearly described from the viewpoint of their open‐shell character. On the basis of these results, diradical‐character‐based design principles of open‐shell singlet molecules for a new class of nonlinear optics (NLO) and singlet fission (SF) systems are presented together with computational results on several real and model NLO and SF molecular systems.
Some organic radicals violate the Aufbau principle and have unique electronic structures in which the energy level of the singly occupied molecular orbital (SOMO) is formally lower than that of the ...highest occupied molecular orbital (HOMO). We synthesized a novel organic radical with a SOMO–HOMO converted electronic structure, TPA-R•, a novel donor–acceptor hybrid of triphenylamine (an electron donor), and a stable polychlorinated diphenyl(4-pyridyl)methyl radical (an electron acceptor). TPA-R• exhibited fluorescence in the near-infrared region (λmax = 910 nm) in cyclohexane from a polar intramolecular charge-transfer excited state. Cyclic voltammetry, absorption spectroscopy, and density functional theory calculation revealed the inversion of the SOMO and HOMO levels in the electronic structure of TPA-R•. Addition of trifluoromethanesulfonic acid to TPA-R• caused a two-step change. Protonation initially occurred on the diphenylpyridylmethyl radical moiety to form TPA-RH•+. This protonation induced large modulation in the frontier orbitals, while the SOMO–HOMO converted electronic structure was conserved. Further addition of the acid caused unprecedented intramolecular electron transfer from the triphenylamine moiety to the protonated radical moiety, generating TPA•+-RH. TPA-RH•+ and TPA•+-RH could be switched by changing the acidity of the solution. These results constitute the first example of the multistep switching behavior stimulated by a single external stimulus in the SOMO–HOMO converted non-Aufbau electronic structure and demonstrate its great potential for realizing unique molecular photonic and electronic functions.
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