Effect of Thiophene substitution on the Photophysical and biological properties of Diketopyrrolopyrrole derivatives.

This study reports the design, synthesis, and comprehensive characterization of three thiophene-substituted diketopyrrolopyrrole derivatives (EHDPP-T, EHDPP-BT, and EHDPP-PT). DFT and TD-DFT revealed a systematic narrowing of the HOMO-LUMO energy gap and enhanced donor-acceptor interactions with increasing π-conjugation, which was consistent with experimental photophysical data showing pronounced bathochromic shifts in absorption and emission across the EHDPP-T → EHDPP-BT→EHDPP-PT series. Photophysical investigations manifested strong fluorescence, with quantum yields ranging from 56 to 67 % in chloroform. Increasing conjugation length resulted in a steady reduction of emission efficiency due to enhanced non-radiative deactivation pathways. Solvatochromic studies confirmed high emission efficiencies across various solvents, while low temperature measurements highlighted the greater rigidity and stability of the less conjugated derivatives. Cytotoxicity assays showed that all compounds are non-toxic under conditions relevant for fluorescence microscopy. Live-cell imaging confirmed efficient intracellular uptake and bright emission, with EHDPP-T exhibiting preferential lysosomal localization and partial mitochondrial accumulation.