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.