Singlet Fission System Research Articles

Density Analysis of Intra- and Intermolecular Vibronic Couplings toward Bath Engineering for Singlet Fission.

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.

Solution-processable singlet fission photovoltaic devices.

We demonstrate the successful incorporation of a solution-processable singlet fission material, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), into photovoltaic devices. TIPS-pentacene rapidly converts high-energy singlet excitons into pairs of triplet excitons via singlet fission, potentially doubling the photocurrent from high-energy photons. Low-energy photons are captured by small-bandgap electron-accepting lead chalcogenide nanocrystals. This is the first solution-processable singlet fission system that performs with substantial efficiency with maximum power conversion efficiencies exceeding 4.8%, and external quantum efficiencies of up to 60% in the TIPS-pentacene absorption range. With PbSe nanocrystal of suitable bandgap, its internal quantum efficiency reaches 170 ± 30%.

What we can learn from the norms of one-particle density matrices, and what we can't: some results for interstate properties in model singlet fission systems.

The utility of the norms of one-particle density matrices, ∥γ∥, for understanding the trends in electronic properties is discussed. Using several model systems that are relevant in the context of singlet fission (butadiene, octatetraene, and ethylene dimer), the dependence of interstate properties (such as transition dipole moments and nonadiabatic couplings, NACs) on molecular geometries is investigated. ∥γ∥ contains the principal information about the changes in electronic states involved, such as varying degree of one-electron character of the transition; thus, it captures leading trends in one-electron interstate properties (i.e., when ∥γ∥ is small, the respective interstate matrix elements are also small). However, finer variations in properties that arise due to the dependence of the matrix elements of the respective operators may not be reproduced. Analysis of NACs in ethylene dimer reveals that intermolecular components of NACs follow the trends in ∥γ∥ well, as they are determined primarily by the characters of the two wave functions; however, intramolecular components depend on the relative orientation of the two moieties via the dependence in the derivative of the electron-nuclear Coulomb operator. Therefore, intramolecular NACs may exhibit large variations even when the changes in ∥γ∥ are small. We observe large NACs at perfectly stacked geometry; however, larger values (by a factor of 1.6) are observed at slip-stacked (along the long axis) geometries. Larger values of NACs at slip-stacked configurations are due to the breaking of symmetry of the local environment of the heavy atoms and not due to the wave function composition. We found that the variations in ∥γ∥ for ethylene dimer are due to a varying admixture of the charge-resonance configurations in the S1 state, whereas the (1)ME state retains its pure multiexciton character.