Deciphering the unusual fluorescence in weakly coupled bis-nitro-pyrrolo[3,2-b]pyrroles

Valentine I. Vullev,Eli M. Espinoza,Yevgen M. Poronik,Daniel T. Gryko,Hans Ågren,Glib V. Baryshnikov,John A. Clark,Irena Deperasińska

doi:10.1038/s42004-020-00434-6

Abstract

Electron-deficient π-conjugated functional dyes lie at the heart of organic optoelectronics. Adding nitro groups to aromatic compounds usually quenches their fluorescence via inter-system crossing (ISC) or internal conversion (IC). While strong electronic coupling of the nitro groups with the dyes ensures the benefits from these electron-withdrawing substituents, it also leads to 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 large 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.

Highlights

Electron-deficient π-conjugated functional dyes lie at the heart of organic optoelectronics
Placing the nitro groups at para, meta, or ortho positions (Fig. 1a) makes it possible to examine the effects of donoracceptor coupling on the excited-state dynamics
While the coupling should be strong for both, 1o and 1p, the steric hindrance of the nitro group when it is at the ortho position twists the conformation and decreases the electronic coupling between the nitrobenzene moiety and the pyrrolopyrrole core

Summary

Introduction

Electron-deficient π-conjugated functional dyes lie at the heart of organic optoelectronics. While strong electronic coupling of the nitro groups with the dyes ensures the benefits from these electron-withdrawing substituents, it leads to fluorescence quenching. While balanced donor-acceptor coupling ensures fast radiative deactivation and slow ISC essential for large 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. The rotation around the carbon-nitrogen bond, linking the nitro group with the aromatic ring, can lead to conical intersections between the excited and the ground states providing non-radiative pathways for efficient internal conversion (IC)[8]. The same CT character of singlet excited states, lowers their energy levels bringing them close to the ground state that can lead to the formation of conical intersections (CIs) providing pathways for efficient IC24

Methods

Results

Conclusion