Modulation of the optical properties of soluble N-alkylated 4-pyridyl diketopyrrolopyrrole derivatives

Abstract

The absorption and fluorescence properties of N-alkylated 3,6-di (pyridin-4-yl)-2,5-dihydropyrrolo [3,4-c]pyrrole-1,4-dione (PyrDPP NH) derivatives in solution and the solid state are reported, along with the modulation of these properties under varying acidity or basicity. The compounds with hexyl (PyrDPP N-Hex) and methoxy ethoxy ethyl (PyrDPP N-MEE) chains were synthesised from PyrDPP NH giving highly soluble dyes. In addition, the mono-substituted derivatives were also isolated. A range of solvents was explored to provide understanding regarding their potential use in applications ranging from organic electronics to sensors. Optically the dyes are highly fluorescent and, as a result of N-alkylation breaking the hydrogen bond network, emissive in the solid state with large Stokes shifts. The nature of the chain dictates the π interactions and packing of the materials, resulting in modulation of the solid-state absorption spectra and theoretical mobility predicted from the crystal structures. The methoxy ethoxy ethyl chain twists out of the plane and bends to allow considerable molecular overlap and small interchromophore displacement with strikingly similar π interactions to the parent PyrDPP NH, resulting in a contribution of H-type aggregation to the absorption band and an order of magnitude improvement in theoretical mobility compared with the hexyl analogue. In contrast, PyrDPP N-Hex possesses a distorted CH-π interaction between pyridyl and lactam units, with greater molecular displacement manifesting in dominant J -aggregation contribution to absorption band and moderate mobility. The results emphasise the potential for sidechain modification and show structural evidence for the potential benefits of the flexible polar chains. The nitrogen lone pair of the pyridyl ring can be protonated in acidic environment to produce a photometric response suggesting potential as a proton sensor. The mono-substituted compounds are responsive to both base and acid, making them interesting for molecular logic applications.