Authors: Hanjie Jiang
Source: Deep Blue (University of Michigan) (article, 2022-01-01)
Field: Materials Science — Materials Chemistry

Configuration Interaction Methods to Predictions on Photophysical Properties of Organic Materials Electronic structure methods are routinely applied to molecular systems to provide chemical insights. Accessing accurate models, however, still requires careful tuning and sizable computational resources. This work focuses on the development of methods and strategies that enable the calculations of various excited state properties on larger systems than ever before. The first three chapters focus on the spin-flip configuration interaction family of methods. Chapter II introduces the new implementation of the spin-flip method that can predict charge-transfer state properties with mid- to large-size systems, with a challenging example of a theoretically designed molecular system to harvest multiple electrons from strongly correlated multiexciton states on a single chromophore, quinoidal bithiophene, which has shown from prior studies to be a good intramolecular singlet fission candidate. Using the newly implemented tool, the study implies that electron transfer between the two molecules occurs from the multiexciton state into an excited charge transfer state. This indicates that two-electron transfer may happen sequentially, thus increasing the electron transfer yield of the singlet fission process. The spin-flip methodology is then extended to enable spin-orbit coupling calculations in Chapter III, where benchmark models and specific applications to organic triplet emitters will be given. A detailed analysis of a system that combines a purely organic triplet emitter with the excited-state intramolecular proton transfer (ESIPT) mechanism shows large spin-orbit coupling values between singlet and triplet excited states. This creates viable intersystem crossing transition channels to populate the triplet state. Similarly, the study also shows that the ESIPT triplet emitter, with a large spin-orbit coupling value between the ground and the lowest triplet excited state, shows fast phosphorescence rate. These large intersystem crossing and phosphorescence rates thus make the ESIPT triplet emitter system one of the brightest phosphors. To further explore challenges of computing excited state properties, Chapter IV provide more insight into the solvent effect to the molecular systems, where an implementation of polarizable continuum model is described with an additional benchmark system, perylenediimide (PDI) dimer. The inclusion of the solvent effect in two geometries of PDI dimer suggests that dielectric stabilization creates excitonic trap states in one geometry by breaking symmetry and localizing charge, which outcompetes the multiexciton state and results in consequent relaxation back to the ground state. Therefore, the study, with the implemented solvation description, will show that a promising route to improving efficiency of singlet fission in PDI dimer systems is to find dimer geometries in which single-directional charge transfer is inhibited in the solvent, which may be achieved by suppressing broken symmetry and avoiding polar solvent environments. In general, implementations and calculations on charge transfer state analysis in solar cells, spin-orbit couplings for predicting intersystem crossing pathways and solvent effects of organic material systems that newly implemented in this work can all provide evidence to explain the mechanisms of solar cell and semiconductor materials from a theoretical perspective. While there is still a lot to be done before the computational methods could be applied to a wide variety of large systems, the various methods investigated and extended here show significant promise, especially with the benchmark and actual experimental molecular systems that presented in the chapters, as these systems are indeed applicable to any configuration interaction methods. Chemistry ; Materials science ; Chemical physics ; Nanotechnology Machine Learning in Materials Science