Data-driven design of organic electronic materials

Singlet fission (SF) is a multiexciton-generating process whereby an excited state singlet (S1) is converted into two lower-energy triplets (T1). The inclusion of a SF-capable material into a photovoltaic device offers the potential for the absorption of photons above the bandgap of traditional solar cell materials, thus increasing the theoretical limit of power conversion efficiency. However, the number of reported SF-capable materials remains remarkably low, and trends pointing towards new molecules have not yet been established. Intramolecular SF (iSF) enables the tuning of bi-chromophoric systems and offers an appealing alternative to the intermolecular SF process, which relies heavily on geometric factors. We show that the building-block approach to conjugated donor-acceptor (D-A) polymer synthesis provides fertile ground to formulate simple and robust design rules for iSF based on the energy (E(S1) = 2 × E(T1)) and character of the S1 and T1 states. The suitability of the proposed guidelines, which are evaluated using descriptors extracted from time-dependent density functional theory (TDDFT), is validated by the correct identification of some of the few polymers that have previously been shown to exhibit SF experimentally. When used in concert with statistical models, these guidelines provide a cost-effective screening protocol for the identification of promising D-A units for iSF from a curated database of 117K reported crystal structures.This dataset is then used to address the (almost) inverse challenge: the search for molecules with inversion of the S1 and T1 states (E(S1) < E(T1)). This property unlocks high-efficiency organic light-emitting diode (OLED) emitters in which fluorescence from S1 is no longer impeded by population transfer to the thermodynamically-favored T1 state. Such inversions have been identified in only one class of molecules to date. Following a pre-screening based on TDDFT, approximate second-order and equations-of-motion coupled-cluster methods are employed to refine the excited state energies. Compounds with known inversions are recovered, confirming the validity of the screening methodology, and a new molecular class with this behavior is identified. This class consists of non-alternant hydrocarbons whose pentalene-like cores are aromatically stabilized in a D2h symmetry by their peripheral polycyclic connectivities, which imbues them with the inverted character of the normally unstable and antiaromatic D2h-pentalene. Put together, these virtual screening methodologies significantly enrich the palette of both SF and OLED emitter materials. The building blocks discovered show promise for the next generation of efficiency improvements in devices.