How the nitro group position determines the emission properties of π-expanded diketopyrrolopyrroles

Two complex π-expanded diketopyrrolopyrroles (EDPPs) have been prepared following a multistep but straightforward strategy. We discovered that the fate of these molecules in the excited state can be controlled by subtle differences in their structure. When NO2 groups are located at a distant position, the quadrupolar, centrosymmetric dye exhibits strong red emission across the solvents’ polarity scale. However, when NO2 groups are adjacent to the lactam moiety, the EDPPs exhibit negligible emission even in non-polar solvents. Density functional theory (DFT) calculations indicate that the primary distinction between the two molecules lies in the structural planarity. The molecule with NO2 groups adjacent to the lactam moiety exhibits a loss of planarity due to the coulombic repulsion between these groups. The calculations also suggest that the nitro group does not participate in the S0 → S1 excitation. Furthermore, for both compounds, the first two excited states (one bright and one dark) are found to be very close in energy. The change in molecular geometry affects the non-radiative deactivation of excited states, leading to the two distinct emission behaviors. Experiments in a glassy solvent at low temperatures reveal that at 77 K the photophysics of both dyes becomes the same, which proves that thermal activation is the key mechanism for the non-radiative decay of the excited state for non-emissive EDPPs.