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.

We theoretically show that diaza (N2)-substitution to s-indacene with 4n π-electrons, by which the number of π-electrons in N2-s-indacene amounts to 4n+2, is a new strategy to design efficient singlet fission (SF) molecules. By N2-substitution, the diradical character and the exchange integral are found to be tuned moderately, leading to satisfying the excitation energy level matching condition for SF with a large triplet excitation energy. On the basis of the effective electronic coupling related to the SF rate, we explore the optimal slip-stack dimer packings for fast SF. Their underlying mechanisms are well understood from the odd-electron density, resonance structure, and frontier orbital distribution, as the functions of the N2-substituted positions. Furthermore, aromaticities of N2-s-indacenes are evaluated explicitly on the basis of the magnetically induced current. Although N2-s-indacenes display strengths of aromaticities similar to that of anthracene, a local decrease in aromaticity is found to correlate to the spatial feature of diradical character, i.e., odd-electron density. The present findings not only newly propose N2-s-indacenes as feasible SF molecules but also contribute to comprehending the interplay between aromaticity and diradical electronic structures contributing to SF.

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Crack propagation tests of lead-free solder were conducted using center-notched plates under strain-controlled conditions of fast-fast (pp), slow-slow (cc), slow-fast (cp), and fast-slow (pc) strain waveforms. A method to estimate creep J-integral and fatigue J-integral from load-displacement relations was proposed, and those integrals were used to correlate the crack propagation rate. For the case of pp waveform, the crack propagation rate was expressed as a power function of the fatigue J-integral. For the other cases, the creep component greatly accelerates the crack propagation rate when compared at the same values of the fatigue J-integral. The creep crack propagation rate was expressed as a power function of the creep J-integral for each case of cp, pc and cc waveforms. The crack propagation rate for cp and pc waveform is higher than that for cc waveform. In fatigue loading under pp waveform, the path of crack propagation was macroscopically straight, perpendicular to the maximum principal stress direction. The introduction of creep components by slow strain rates promoted shear-mode crack propagation. The predominant feature of fracture surfaces observed by scanning electron microscopy was striations for pp waveform, and grain boundary fracture for cp and pc waveforms. Grain fragmentation was evident on the fracture surface made under cc waveform.

The effect of applying a static electric field on the singlet fission dynamics was investigated using the quantum master equation method. The singlet fission dynamics of pentacene dimer models was greatly accelerated by applying a static electric field.