Extension of molecules with an inverted singlet-triplet gap with conjugated branches to alter the oscillator strength

Molecules that violate Hund's rule and possess negative singlet-triplet gaps (ΔE_ST) have been actively studied for their potential usage in organic light emitting diodes without the need for thermal activation. However, the weak oscillator strength from the symmetry of such molecules has been recognized as their shortcoming for their application in optoelectronic devices. A group of molecules with a common structural motif involving the original molecule with an inverted gap having branches consisting of conjugated molecules of varied structures and extent of conjugation have been predicted to have desirable oscillator strength, but only few detailed and comprehensive studies regarding the form of excited states and the reason behind the improved oscillator strength have been carried out. We show in this work a series of analyses that suggest that the increase of oscillator strength is correlated with the nature of the excited state changing from a localized excitation to a delocalized excitation involving the central molecule and the branches. The resulting oscillator strength thus depends on the energetic matching of the branching molecule and the central molecule, rather than solely the oscillator strength of the central molecule. From the ΔE_ST inversion point of view, the static correlation with low-lying doubly excited configurations, the key mechanism behind the inversion in the localized excited state, weakens as the excited states delocalize. As a consequence, the dynamic correlation has a more decisive effect in determining the singlet-triplet gap.