Tuning Low-Lying Excited States and Optical Properties in IndenoFluorene Diradicaloids and Longitudinally Extended Derivatives: A Computational Perspective

In this work, we have considered the family of indenofluorene (IF) and its longitudinally elongated variants fluorenofluorene and diindenoanthracene and investigated their low-lying excited states and optical properties via quantum-chemical studies at the density functional theory (DFT) level. Singlet ground-state diradicals exhibit distinct optical properties due to the presence of a low-lying state dominated by a doubly excited configuration (DE state), often below the lowest allowed singly excited state (SE state). IFs and their elongated derivatives, with tunable diradical character and both symmetric and nonsymmetric structures, provide an ideal platform for exploring DE state energy modulation and spectroscopic behavior. The study shows that absorption spectra simulated using time-dependent (TD) calculations based on unrestricted broken-symmetry antiparallel-spin reference configuration (TDUDFT) closely match the available experimental data. Additionally, it reveals distinct spectral behavior for symmetric and nonsymmetric derivatives, highlighting the role of lowest-lying weakly allowed excited states potentially promoting non-radiative deactivation pathways.