The interplay of intersystem crossing and internal conversion in quadrupolar tetraarylpyrrolo[3,2- b]pyrroles

Foundation for Polish Science (TEAM POIR.04.04.00-00-3CF4/16-00); National Science Centre, Poland, under QuantERA programme, project 2017/25/Z/ST2/03038 and OPUS 2020/37/B/ST4/00017; European Union's Horizon 2020 research and innovation programme under the Marie Skłodowka-Curie grant agreement no. 101007804; JSPS KAKENHI Grant Number 21H01887; Ministry of Education and Science of Ukraine for support (Project No. 0121U107533); Academy of Finland through project 346369; National Academic Infrastructure for Supercomputing in Sweden (NAISS 2023/5-77) at the National Supercomputer Centre (NSC) at Linköping University partially funded by the Swedish Research Council through grant agreement no. 2022-06725; Swedish Research Council through starting grant no. 2020-04600; Olle Engkvists Stiftelse (Sweden), project number 212-0136.

Adding nitro groups to aromatic compounds usually quenches their fluorescence via intersystem crossing (ISC) or internal conversion (IC). Herein, we investigated centrosymmetric 1,4-dihydropyrrolo[3,2-b]pyrroles linked to variously substituted nitro-heteroaryls. A 1,4-orientation of the nitro substituent versus the electron rich 1,4-dihydropyrrolo[3,2-b]pyrrole core invokes a strong fluorescence in non-polar solvents and intense two-photon absorption while a 1,3-orientation of push–pull substituents results in a dramatic hypsochromic shift of absorption, weak, bathochromically shifted emission and weak two-photon absorption. The combined experimental and computational study indicates that the primary responsible factors are: (1) the difference in electron density distribution in the LUMO; (2) the difference in μ10. IC is a dominant mechanism of non-radiative dissipation of energy in all these dyes but as long as the distribution of electron density within the HOMO and LUMO is delocalized on the 1,4-dihydropyrrolo[3,2-b]pyrrole core as well as on the nitroaromatic moieties its rate is slower than the fluorescence rate in non-polar solvents.