A Solvent-Mediated Excited-State Intermolecular Proton Transfer Fluorescent Probe for Fe3+ Sensing and Cell Imaging

Constructing excited-state intermolecular proton transfer (ESIPT-e) fluorophores represents significant challenges due to the harsh requirement of bearing a proton donor-acceptor (D-A) system and their matching proton donating-accepting ability in the same molecule. Herein, we synthesized a new-type ESIPT-e fluorophor (2-APC) using the “four-component one-pot” reaction. By the installing of a cyano-group on pyridine scaffold, the proton donating ability of -NH2 was greatly enhanced, enabling 2-APC to undergo ESIPT-e process. Surprisingly, 2-APC exhibited dual-emissions in protic solvents ethanol and normal fluorescence in aprotic solvents, which is vastly different from that of conventional ESIPT-a dyes. The ESIPT emission can be obviously suppressed by Fe3+ due to the coordination reaction of Fe3+ with the A-D system in 2-APC. From this basis, a highly sensitive and selective method was established using 2-APC as a fluorescent probe, which offers the sensitive detection of Fe3+ ranging from 0 to 13 μM with the detection limit of 7.5 nM. The recovery study of spiked Fe3+ measured by the probe showed satisfactory results (97.2103.4%) with the reasonable RSD ranging from 3.1 to 3.8%. Moreover, 2-APC can also exhibit aggregation-induced effect in poor solvent or solid-state, eliciting strong red fluorescence. 2-APC was also applied to cell-imaging, exhibiting good cell-permeability, biocompatibility and color rendering. This multi-mode emission of 2-APC is significant departure from that of conventional extended p-conjugated systems and ESIPT dyes based on a flat and rigid molecular design. The “one-pot synthesis” strategy for the construction of ESIPT molecules pioneered a new route to achieve tricolor-emissive fluorophores.

Electronic state of a fluoranthene-urea compound and the kinetics of its emissive tautomer state in the presence of acetate anions

In order to investigate the electronic states of emissive tautomers (T*) of aromatic-urea compounds formed via excited-state intermolecular proton transfer (ESPT) with acetate anion (Ac), a new aromatic-urea compound (3FU)...

4-Azidocinnoline—Cinnoline-4-amine Pair as a New Fluorogenic and Fluorochromic Environment-Sensitive Probe

A new type of fluorogenic and fluorochromic probe based on the reduction of weakly fluorescent 4-azido-6-(4-cyanophenyl)cinnoline to the corresponding fluorescent cinnoline-4-amine was developed. We found that the fluorescence of 6-(4-cyanophenyl)cinnoline-4-amine is strongly affected by the nature of the solvent. The fluorogenic effect for the amine was detected in polar solvents with the strongest fluorescence increase in water. The environment-sensitive fluorogenic properties of cinnoline-4-amine in water were explained as a combination of two types of fluorescence mechanisms: aggregation-induced emission (AIE) and excited state intermolecular proton transfer (ESPT). The suitability of an azide–amine pair as a fluorogenic probe was tested using a HepG2 hepatic cancer cell line with detection by fluorescent microscopy, flow cytometry, and HPLC analysis of cells lysates. The results obtained confirm the possibility of the transformation of the azide to amine in cells and the potential applicability of the discovered fluorogenic and fluorochromic probe for different analytical and biological applications in aqueous medium.

Quantum Surprises from the Watson-Crick and Hoogsteen G·C Nucleobase Pairs: A Comprehensive QM/QTAIM Investigation

In this study at the MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) level of theory in the isolated state it was revealed 14 novel physico-chemical mechanisms of the tautomerization of the G·C nucleotide base pairs in the Watson-Crick G·C(WC) / G*·C*(WC), reverse Watson-Crick G*·C*(rWC) / G·C*O2(rWC), Hoogsteen G*t·C*(H) / G*N7·C(H) or reverse Hoogsteen G*t·C*(rH) / G*tN7·C(rH) configurations into the wobble (wWC, wH) and reverse wobble (rwWC, rwН) base pairs: 1. G·C(WC)↔G·C*(rwWC), 2./3. G*·C*(WC)↔G·C*(rwWC)/G*N2·C*(rwWC), 4. G*·C*(rWC)↔G*·C(wWC), 5. G·C*O2(rWC)↔G·C*(wWC); 6./7./8./9. G*t·C*(H)↔G*t·C(rwН)/G*t·C*O2(wH)/G*t·C*O2(rwН)/G*tN7·C*(rwН)↔G*t·C*O2(rwН), 10. G*N7·C(H)↔G*t·C(wH) amino, 11./12. G*t·C*(rH)↔G*N7·C*(wН)/G*t·C(wН), 13. G*tN7·C(rH)↔G*tN7·C*(wН)↔G*t·C(wН) and 14. G*N7·C*(rwH)↔G*N7·C*(rwH) perp↔G-·C+(wH)↔G*t·C(rwН) reaction pathways. It was established that the presence in the base pair of the two anti-parallel neighboring H-bonds is a necessary and sufficient condition for the implementation of such transformations, since it enables intermolecular proton transfer between the bases inside the base pair. It was found out that these tautomeric transitions are controlled by the TSs with quasi-orthogonal structure, which are tight G+·C-/G-·C+ ion pairs, joined by at least two parallel intermolecular H-bonds, connected on a common negatively charged endocyclic N-/C- atoms – proton acceptor. All reaction pathways have been reliably confirmed. These transitions are accompanied by the changing of the mutual cis-orientation of the N9H and N1H glycosidic bonds of the bases on the trans-orientation and vice versa. These data complement the reported earlier mechanisms of the tautomerisations of the classical A·T and G·C DNA base pairs. Experimental verification of the novel G·C nucleobase pairs is looking as an attractive task for the future research.

Efficient Colorimetric Fluoride Anion Sensor Based on π-Conjugated Carbazole Small Molecule

The ability to detect fluoride anions with high selectivity and sensitivity by using the naked eye is crucial yet challenging. In this study, a novel, simple conjugated organic dye, N-tert-butyldimethylsilyl-3,6-diiodocarbazole (CA-TBMDS) was developed and used for the first time as a colorimetric sensor for fluoride. CA-TBMDS was found to be a highly sensitive fluoride chemosensor, with a detection limit as low as 3 × 10−5 M. The reaction of CA-TBMDS with fluoride anions in a tetrahydrofuran solution resulted in a color change from colorless to yellow under ambient light, which can be discriminated by the naked eye. The sensor operated via intermolecular proton transfer between the amide units and the fluoride anion, as confirmed by proton nuclear magnetic resonance titration. CA-TBMDS is not only highly sensitive to fluoride anions, but also exhibits high sensitivity in the presence of various ions. This work demonstrates that N-butyldimethylchlorosilane-based organic dyes have prospective utility as a type of fluoride anion chemosensor.