Effects of site-specific substitution to hexacene and its effect towards singlet fission

Monosilicon Derivatives of Phenanthrene and Pyrene as Potential Singlet Fission Materials for High-Performance Solar Cells

Singlet fission (SF) is a process in which the energy of a singlet-excited molecule is divided into two triplet excitations. This is a special case of an internal conversion that is spin-allowed and extremely fast. Ideally, this process utilizes one photon to produce two electron-hole pairs. In tandem with a layer of singlet fission material, conventional solar cells can achieve improved efficiency by utilizing higher-energy photons. This density functional theory study provides information about additional efficient SF chromophores that were theoretically modeled by functionalizing phenanthrene and pyrene via site-specific monosilicon substitutions. The SF capabilities of the derivatives were evaluated by calculating the SF thermodynamic driving force (ΔESF) and the excited state's molecular planarity. The most promising monosilicon derivatives with SF capabilities are 3-silaphenanthrene and 1-silapyrene for each family, respectively. All phenanthrene and pyrene monosilicon derivatives are strong closed-shell species, because their multiple diradical characteristics are close to zero. Based on these results, 3-silaphenanthrene and 1-silapyrene were selected for electron excitation analysis, which further demonstrated that the monosilicon functionalization of phenanthrene and pyrene led to a transfer from local excitation characters to hybridized local and charge-transfer characters of the excited states, resulting in a significant change from endoergic to exoergic in the SF chromophores.

An Exhaustive Quantum-Classical Study of C6F6+ Using the Newly Formulated Parallel TDDVR Method

We recently implemented our parallelized quantum-classical dynamical approach, known as the Time-Dependent Discrete Variable Representation (TDDVR) method, which is applied to the spectroscopically important hexafluorobenzene (HFBz) radical cation, where several conical intersections exist in their seven lowest excited electronic states (S11B2u, S21E1g, S31B1u, S41E1u, and S51A2u) considering degeneracy among potential energy surfaces (PESs), to demonstrate their various dynamical aspects. This new parallel version shows almost linear scalability with increasing number of computing processors. To get photoelectron (PE) spectra, Mass-Analyzed Threshold Ionization (MATI) spectra, population dynamics, and many other dynamical observables, the first-principles dynamics is applied at the state-of-the-art level to the corresponding Hamiltonian, where the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) type interactions are involved in those coupled seven electronic states. The quantum-classical method is used to thoroughly analyze the effects of these couplings on the nuclear dynamics of the involved electronic states, and the findings are compared with those observables obtained from experiments. Intrinsic dynamical properties are explained using the reduced densities of the wave packet (WP) in a coupled electronic manifold. The PE and MATI spectra of HFBz computed using TDDVR are found to be in good agreement with earlier experimental data and other theoretically simulated spectra.

Computational Discovery of Intermolecular Singlet Fission Materials Using Many-Body Perturbation Theory

Intermolecular singlet fission (SF) is the conversion of a photogenerated singlet exciton into two triplet excitons residing on different molecules. SF has the potential to enhance the conversion efficiency of solar cells by harvesting two charge carriers from one high-energy photon, whose surplus energy would otherwise be lost to heat. The development of commercial SF-augmented modules is hindered by the limited selection of molecular crystals that exhibit intermolecular SF in the solid state. Computational exploration may accelerate the discovery of new SF materials. The GW approximation and Bethe-Salpeter equation (GW+BSE) within the framework of many-body perturbation theory is the current state-of-the-art method for calculating the excited-state properties of molecular crystals with periodic boundary conditions. In this Review, we discuss the usage of GW+BSE to assess candidate SF materials as well as its combination with low-cost physical or machine learned models in materials discovery workflows. We demonstrate three successful strategies for the discovery of new SF materials: (i) functionalization of known materials to tune their properties, (ii) finding potential polymorphs with improved crystal packing, and (iii) exploring new classes of materials. In addition, three new candidate SF materials are proposed here, which have not been published previously.

Environmental effects on the singlet fission phenomenon: a model Hamiltonian-based study

In the screening of compounds for singlet fission, the relative energies of the constitutive units are decisive to fulfil the thermodynamic rules. From a model Hamiltonian constructed on the local spin states of an active chromophore and its environment, it is suggested that embedding greatly influences the energy differences of the active monomer spin states. Even in the absence of charge transfer, the field generated by a singlet environment produces an increase of the [E(S₁) - E(S₀)]/[E(T₁) - E(S₀)] critical ratio by up to 6% as compared to the one of a free chromophore. Besides, variations are observed when the intimate electronic structure of the singlet environment is modified. This propensity towards singlet fission is even more pronounced (10%) when the environment is switched to the triplet state. Finally, the embedding is likely to reverse the spin state ordering in the limit of vanishing atomic orbital overlaps. Despite its simplicity, the model stresses the importance of the environment spin nature in the quest for singlet fission candidates, and more generally in spectroscopy analysis.

Luminescent excited-state intramolecular proton-transfer dyes based on 4-functionalized 6,6'-dimethyl-3,3'-dihydroxy-2,2'-bipyridine (BP(OH)2-Rs); DFT simulation study

The 4-functionalized 6,6'-dimethyl-3,3'-dihydroxy-2,2'-bipyridine dyes (BP(OH)₂-Rs) have exhibited dienol and diketo emissions. The optimum geometrical structures for ground, singlet and triplet excited states are computed by DFT/B3LYP/6-31++G that showed the planarity of BP(OH)₂-Rs structure. The emission spectra of the molecules are determined in the gas-phase at singlet and triplet excited states using CIS/6-31++G. The theoretical calculations are carried out for BP(OH)₂-Rs to understand the impact of different substituents (R = -H (I), -Br (II), -TMS (III), -C2H (IV), -terpyridine (V) and -bodipy (diazaboraindacene) (VI)) on excited-state intramolecular proton transfer (ESIPT) in singlet and triplet excited states. Based on the calculations, the concerted diproton transfer proceeds in the triplet excited state, in which nπ* state has a significant participation in ESIPT. The spectral variation at ESIPT emission of BP(OH)₂-Rs is influenced by the electron-acceptor ability of the substituents. The compound V revealed a higher spectral intensity compared to the others. From the comparison with the experimental data, the molecule V is almost planar agreed with the X-ray structure and trend variation of wavelengths. The molecule VI contains bodipy chromophore that excitation energy transfers completely from BP(OH)₂ core to a bodipy substituent, leading to emission from the lowest-lying bodipy substituent, and consequently, ESIPT does not occur for this dye.