The influence of tetraphenylethylene moieties on the emissive properties of dipyrrolonaphthyridinediones

Abstract

Despite being highly emissive in solution, aggregation of dipyrrolonaphthyridinedione (DPND) molecules typically results in the quenching of fluorescence. DPNDs can be efficiently converted into π-extended derivatives containing rotatable aryl rings via a direct arylation methodology. The presence of phenyl substituents at positions 3 and 9 of the DPND core is sufficient to cause moderate fluorescence in the solid state. When tetraphenylethylene moieties, typical aggregation induced emitters, are coupled through biaryl linkages in these same positions, a 50 nm shift in absorption and almost 120 nm shift in fluorescence compared to the parent DPND is observed. The radiative S₁ → S₀ transitions have large oscillator strengths regardless of whether phenyls or tetraphenylethylene groups are coupled to DPND and strong orange or red emission in solution is observed. Vibrations involving these substituents play an important role in the dissipation of the electronic excitation energy. X-ray crystallographic studies revealed that although the distance between DPND cores is strikingly larger in bis(TPE)₂DPND compared to that in diphenylDPND, their photophysical properties in the solid state are very similar. Computational studies have also shown that, in contrast to our experimental results, neither of these DPNDs should be particularly emissive in the solid state, due to the low oscillator strengths calculated for model dimers derived from their X-ray crystal structures.