Aggregation behavior in naphthalene-appended diketopyrrolopyrrole derivatives and its gas adsorption impact on surface potential

Diketopyrrolopyrrole derivatives containing phenyl and thiophene units adorned with alkoxynaphthalene (Naph-PDPP and Naph-TDPP) were synthesized by a Suzuki cross-coupling reaction. The effect of the phenyl/thiophene units on the aggregation behavior and detailed photophysical properties were investigated by UV-visible, steady-state, and time-resolved fluorescence spectroscopy. The absorption and fluorescence spectra of Naph-PDPP and Naph-TDPP in the solid-state exhibit red-shifted spectral patterns due to strong intermolecular interactions. The concentration-dependent photophysical properties reveal the formation of J-type aggregates at higher concentrations and in the solid state. The extent of aggregate formation is higher for Naph-TDPP. DFT and TD-DFT studies showed that Naph-TDPP containing a thiophene ring in the backbone adopts a more planar geometry than Naph-PDPP and undergoes strong pi-pi stacking interactions that favor the formation of J-aggregates. Scanning Kelvin probe measurements on the thin films of Naph-PDPP and Naph-TDPP were performed (both in the dark and under visible light) upon exposure to different volatile organic vapors (ethanol and triethylamine). The study reveals that under visible light illumination, the Naph-PDPP thin film has significant gas adsorption towards ethanol vapors and alters its sign of response.