Development of a TDDFT-Based Protocol with Local Hybrid Functionals for the Screening of Potential Singlet Fission Chromophores

Chromophores suitable for singlet fission need to meet specific requirements regarding the relative energies of their S-0 S-1, and T-1 (and T-2) electronic states. Accurate quantum chemical computations of the corresponding energy differences are thus highly desirable for materials design. Methods based on density functional theory (DFT) have the advantage of being applicable to larger, often more relevant systems compared to more sophisticated post-Hartree-Fock methods. However, most exchange correlation functionals do not provide the needed accuracy, in particular, due to an insufficient description of the T-1 state. Here we use a recent singlet fission chromophore test set (Wen, J.; Havlas, Z.; Michl, J. J. Am. Chem. Soc. 2015, 137, 165-172) to evaluate a wide range of DFT-based methods, with an emphasis on local hybrid functionals with a position-dependent exact-exchange admixture. New reference vertical CC2/CBS benchmark excitation energies for the test set have been generated, which exhibit somewhat more uniform accuracy than the previous CASPT2-based data. These CC2 reference data have been used to evaluate a wide range of functionals, comparing full linear-response TDDFT, the Tamm-Dancoff approximation (TDA), and Delta SCF calculations. Two simple two-parameter local hybrid functionals and the more empirical M06-2X global meta-GGA hybrid provide the overall best accuracy. Due to its lower empiricism and wide applicability, the Lh12ct-SsifPW92 local hybrid is suggested as the main ingredient of an efficient computational protocol for prediction of the relevant excitation energies in singlet fission chromophores. Full TDDFT for the S-1, S-2, and T-2 excitations is combined with Delta SCF for the T-1 excitations. Making use also of some error compensation with suitable DFT-optimized structures, even the most critical T-1 excitations can be brought close to the target accuracy of 0.20 eV, while the other excitation energies are obtained even more accurately. This fully DFT-based protocol should become a useful tool in the field of singlet fission.