- Gideon Odonkor: Department of Chemistry, University of Nevada, Reno, Nevada, USA.
- Samuel O Odoh: Department of Chemistry, University of Nevada Reno, Reno, Nevada, USA. ORCID
Computational investigations of Inverted Singlet-Triplet (INVEST) emitters often rely on ADC(2) and TD-DFT excitation energies (EEs) obtained with the vertical approximation. Here, we first considered several cyclazine derivatives and examine the sensitivity of vertical EEs (VEEs) as well as singlet-triplet gaps, ��E gaps, to the level at which the ground state (S) structure was optimized. For cyclazine, VEEs and vertical gaps from ADC(2) or TD-DFT are spread over a narrow range (<���0.064���eV) whether the S structure is optimized with various DFT, CCSD, and RI-MP2 methods. However, for asymmetric cyclazines, depending on the protocol for optimizing S structures, not only are VEEs spread over a substantially wider range (up to 0.75���eV) but so are vertical ��E gaps (up to 0.30���eV), leading to cases where, with different S structures, one obtains positive vertical ��E gaps or significantly negative gaps. We relate this behavior to the introduction of significant asymmetry and bond-length variations in the cyclazine derivatives, formed by ligand functionalization or modification of the cyclazine core. On a more positive note, adiabatic EEs (AEEs) and adiabatic ��E gaps display significantly lower sensitivity (7-30�� less) to the geometry optimization protocols than their vertical analogs. Crucially, for cyclazine, the M06-HF functional with 100% non-local exchange provides the closest S geometry to available CCSD(T) data. We show that this effect exists also for other frameworks (e.g., azulene, pentaazaphenalene, and non-alternant polycyclic hydrocarbons) that have been considered for the INVEST property, with VEEs spread over a broader range of up to 1.19���eV and vertical ��E gaps over a range of 0.62���eV. For INVEST emitters, it is therefore extremely important to judiciously choose the computational protocol for optimizing ground state geometries, in computing VEEs and vertical ��E gaps.