Optical and NMR spectra along with atomic coordinates of the title compounds for: Deciphering the Enigma of Unusual Fluorescence in Weakly Coupled Bis-nitro-pyrrolo[3,2-b]pyrroles

Adding nitro groups to aromatic compounds usually quenches their fluorescence via inter-system crossing (ISC) or internal conversion (IC). The electronic coupling of the nitro groups with the dyes should be strong enough to ensure the electron-withdrawing benefits, and weak enough to prevent fluorescence quenching. Here, we demonstrate how such electronic coupling affects the photophysics of acceptor-donor-acceptor fluorescent dyes, with nitrophenyl acceptors and a pyrrolo[3,2-b]pyrrole donor. The position of the nitro groups and the donor-acceptor distance strongly affect the fluorescence properties of the bis-nitrotetraphenylpyrrolopyrroles. Concurrently, increasing solvent polarity quenches the emission that recovers upon solidifying the media. Intramolecular charge transfer (CT) and molecular dynamics, therefore, govern the fluorescence of these nitro-aromatics. While balanced donor-acceptor coupling ensures fast radiative deactivation and slow ISC essential for high fluorescence qu...