Bidirectional Solvatofluorochromism of a Pyrrolo[3,2‑b]pyrrole–Diketopyrrolopyrrole Hybrid

Hybridization of electron donors and acceptors provides routes to long-wavelength absorbing and fluorescing dyes. Varying the coupling of low-lying charge-transfer (CT) states with the ground and different locally excited states profoundly affects the photophysics of such donor–acceptor conjugates. Herein, we hybridize an electron-deficient diketopyrrolopyrrole (DPP) moiety with an electron-rich pyrrolopyrrole (PP) that is symmetrically N-arylated with 4-nitrophenyl substituents. The lowest Franck–Condon state is located on the DPP ring structure and dominates the photophysics of the hybrid. Similar to the DPP moiety, the hybrid exhibits optical absorption that is invariant to the solvent media. The PP donor considerably modulates its fluorescence by undergoing electron transfer to the locally excited DPP to form a CT state. For nonpolar media, an increase in solvent polarity causes a bathochromic shift of the fluorescence reaching the longest wavelengths for chloroform and DCM. A further increase in the medium polarity moves the fluorescence maximum hypsochromically back to where it is for alkane solvents. This bidirectional solvatofluorochromism accompanies a polarity-induced increase in the nonradiative decay rates leading to a decrease in the emission quantum yield. The solvent dependence of the energy level of the CT state is responsible for the observed polarity-induced fluorescence behavior of the hybrid. This emission behavior, along with the solvent invariance of the absorption, results in multimodal sensitivity to the solvation environment.