The Impact of the Interplay between Steric and Electronic Effects on the Synthesis and Optical Properties of Diketopyrrolopyrroles Bearing Pyridine Moieties

Fluorescent styryl pyridine-N-oxide probes for imaging lipid droplets

Lipid droplets (LDs) have emerged as major regulators of cellular metabolism, encompassing lipid storage, membrane synthesis, viral replication, and protein degradation. Exclusive studies have suggested a direct link between LDs and cancer, as a notable abundance of LDs is found in cancerous cells. Therefore, monitoring the location, distribution, and movements of LDs is of paramount importance for understanding their involvement in biological processes. To target LDs, we designed and synthesized fluorophores with a styryl scaffold bearing electron-donating amino groups and pyridine-N-oxide, a zwitterionic acceptor moiety. We explored their photophysical properties in various solvents and conducted systematic DFT calculations on the synthesized fluorescent molecules, comparing them with neutral pyridine and cationic pyridinium styryl dyes. The results demonstrate that diphenylaminostyryl pyridine-N-oxide (TNO) shows excellent imaging of LDs, in contrast to the behavior of cationic styrylpyridinium (TNC), which primarily localizes within the mitochondria. Notably, pyridine N-oxide offers several benefits: an increased dipole moment facilitating charge separation between donors and acceptors, substantial HOMO and LUMO stabilization, improved water solubility, favorable redox properties, and bathochromic-shifted absorption/emission spectra, showing promise as a fluorescent tool for probing the cellular-biological realm.

Direct C(3)5-H Polyfluoroarylation of 2-Amino/alkoxy Pyridines Enabled by a Transient and Electron-deficient Palladium Intermediate

Herein, we present an unprecedented azine-limited C5-H polyfluoroarylation of 2-aminopyridines enabled by a transient and electron-deficient perfluoroaryl-Pd species via C-H/C-H coupling. The protocol further allows C3(5)-H polyfluoroarylation of 2-alkoxypyridines guided by sterics and electronics for the first time. The late-stage C-H functionalization of drugs, drug derivatives, and natural product derivatives and synthesis of C5-aryl drug derivatives further demonstrated the method's utility. The preliminary mechanistic studies reveal that the synergistic combination of the bulky yet electrophilic perfluoroaryl-Pd species and the partial nucleophilicity of the C5-position of 2-amino/alkoxy-pyridines is the origin of reactivity and selectivity. Importantly, the first experimental evidence for the role of diisopropyl sulfide is provided.

Design, synthesis and evaluation of enzyme-responsive fluorogenic probes based on pyridine-flanked diketopyrrolopyrrole dyes

The ever-growing demand for fluorogenic dyes usable in the rapid construction of analyte-responsive fluorescent probes, has recently contributed to a revival of interest in the chemistry of diketopyrrolopyrrole (DPP) pigments. In this context, we have explored the potential of symmetrical and unsymmetrical DPP derivatives bearing two or one 4-pyridyl substituents acting as optically tunable group(s). The unique fluorogenic behavior of these molecules, closely linked to N-substitution/charge state of their pyridine unit (i.e., neutral pyridine or cationic pyridinium), has been used to design DPP-based fluorescent probes for detection of hypoxia-related redox enzymes and penicillin G acylase (PGA). In this paper, we describe synthesis, spectral characterization and bioanalytical validations of these probes. Dramatic differences in terms of aqueous stability and enzymatic fluorescence activation were observed. This systematic study enables to delineate the scope of application of pyridine-flanked DPP fluorophores in the field of enzyme biosensing.

How To Make Nitroaromatic Compounds Glow: Next-Generation Large X-Shaped, Centrosymmetric Diketopyrrolopyrroles

Red‐emissive π‐expanded diketopyrrolopyrroles (DPPs) with fluorescence reaching λ=750 nm can be easily synthesized by a three‐step strategy involving the preparation of diketopyrrolopyrrole followed by N‐arylation and subsequent intramolecular palladium‐catalyzed direct arylation. Comprehensive spectroscopic assays combined with first‐principles calculations corroborated that both N‐arylated and fused DPPs reach a locally excited (S₁) state after excitation, followed by internal conversion to states with solvent and structural relaxation, before eventually undergoing intersystem crossing. Only the structurally relaxed state is fluorescent, with lifetimes in the range of several nanoseconds and tens of picoseconds in nonpolar and polar solvents, respectively. The lifetimes correlate with the fluorescence quantum yields, which range from 6 % to 88 % in nonpolar solvents and from 0.4 % and 3.2 % in polar solvents. A very inefficient (T₁) population is responsible for fluorescence quantum yields as high as 88 % for the fully fused DPP in polar solvents.

aza-BODIPY synthesis towards vis/NIR functional chromophores based on a Schiff base forming reaction protocol using lactams and heteroaromatic amines

The Schiff base forming reaction of lactams and heteroaromatic amines led to creation of a new class of aza-BODIPY analogues as visible and near infrared functional chromophores.

Pyrrolopyrrole aza boron dipyrromethene based two-photon fluorescent probes for subcellular imaging

A series of two-photon-absorbing pyrrolopyrrole aza boron dipyrromethenes have been prepared which can serve as fluorescent probes for subcellular imaging.