Chiral diketopyrrolopyrrole dyes showing light emission in solid and aggregate states

Abstract

Due to their excellent emissive properties in solution, diketopyrrolo[3,4-c]pyrrole (DPP) derivatives are expected to show interesting features in emission in the solid state by suppressing aggregation-caused quenching. In this work, aggregation-enhanced emission (AEE) and circularly polarised light (CPL) emission in the aggregated state have been studied on achiral and chiral DPPs appended with tetraarylethylene units. Both thiophene and phenyl DPPs (2 and 5) show classical aggregation-caused quenching (ACQ) of the emission behaviour in the solid state, whereas the presence of triphenylethylene units (3 and 6) does not completely quench the emission in the solution state. Nevertheless, enhancement of the emission from the solution to solid state films from 25 to 36% is observed with phenyl DPP (3), whereas the less distorted thiophene analogue 6 still shows a classical ACQ behaviour. When the photoluminescence behaviour in the aggregated states was studied in a series of THF/water mixtures, phenyl DPP 3 showed a 2-fold enhancement in fluorescence intensity when the water fraction was increased from 0% up to 70%, accompanied by an increase in the quantum yield from 17% up to 28%, and the achiral compound 8 showed a 3.5-fold enhancement in the fluorescence intensity when the water fraction was increased from 0% to 50%, accompanied by an increase in the quantum yield from 20% up to 58%. Alkylation of DPPs by the Mitsunobu reaction with N-sec-phenethyl groups has the advantage of enhancing the chiroptical properties in solution due to the proximity of the stereogenic motif. Thus, compound 3, dissolved at a concentration of 10⁻⁶ M in CHCl₃, emitted detectable CPL under excitation at 365 nm, with |g_lum| around 2 × 10⁻⁴, with positive values for the SS enantiomer and negative values for the RR enantiomer. In this work, we show the importance of the position of the stereogenic motif within the chiral unit and the steric demand of the tetraarylethylene motifs needed to induce light emission in the solid state and CPL emission of chiroptical materials based on DPPs.