How to Make Nitroaromatics Glow: Next Generation Large, χ-Shaped, Centrosymmetric Diketopyrrolopyrroles.

This article describes a synthetic approach to a new structurally diverse family of π-expanded diketopyrrolopyrroles (DPPs). These red-emissive dyes, based on a previously unknown skeleton, can be easily synthesized via a three-step strategy involving the preparation of diketopyrrolopyrrole followed by N-arylation and subsequent intramolecular palladium-catalyzed direct arylation. A careful selection of the aryl substituents on DPP gives access to a π-extended DPP derivative with intense fluorescence reaching 750 nm. The fact that NO2 groups do not engage in the excited states is responsible for the lack of fluorescence quenching when this functional group is present. Comprehensive spectroscopic assays combined with first principle calculations corroborated that both N-arylated and fused DPPs reach a locally excited (S1) state after excitation, followed by internal conversion to states with solvent and structural relaxation, before eventually undergoing intersystem crossing. Only the structurally relaxed state is fluorescent with lifetimes in the range of several nanoseconds and tens of picoseconds in non-polar and polar solvents, respectively. The aforementioned lifetimes correlate with fluorescence quantum yields, which range from 6% to 88% in non-polar solvents and from 0.4% and 3.2% in polar solvents. For the fully fused DPP a very inefficient (T1) population is responsible for fluorescence quantum yields as high as 88% in polar solvents, which is among the uppermost ever recorded for nitro-aromatics. These findings demonstrate the immense importance of the ability to design, build and fine-tune the structure of large, π-expanded functional dyes so that the strong polarization of centrosymmetric structures is achieved.