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 quantum yields, vibronic borrowing accounts for medium dependent IC via back CT. These mechanistic paradigms set important design principles for molecular photonics and electronics.