A theoretical comprehension of photophysical processes in Cu2+ sensing by 1,7-di(2-pyridyl)bispyrazolo[3,4-b:4′,3′-e]pyridines

Abstract

Due to its simplicity and sensitivity, metal ion sensing by fluorescent probes has a high biological and ecological impact, and several preliminary applications for Cu²⁺ have been found. However, the poor understanding of photophysical phenomena by which probes work has led to the growth of unhelpful literature. In this way, 4-aryl-1,7-di(pyridin-2-yl)bispyrazolo[3,4-b:4′,3′-e]pyridines Py₂BP₂a-c were studied as tridentate ligands in developing the probe Py₂BP₂a (Ar = Ph, LODCu²⁺ = 26 nM); thus, this previous work is completed herein by DFT/TD-DFT studies to understand the sensing process. The basal and first excited state of Py₂BP₂a-c (Ar: Ph, 4-An, 4-Py) and the parent l,4,7-triphenylbispyrazolo[3,4-b:3′,4′-e]pyridine Ph₃BP₂ were optimised. Results suggest that the probes' fluorescence is due to a twisted intramolecular charge transfer (TICT) and ICT processes around the 4-aryl and 1,7-dipyridin-2-yl groups; likewise, the fluorescence turn-off in the presence of Cu²⁺ by probe Py₂BP₂b is due to a photoinduced electron transfer (PET) process, favouring a ligand-to-metal charge transfer (LMCT). These findings enhance our understanding of the sensing process and open new possibilities for its applications in various fields.