Dual-Functional Manganese-Doped ZnO-MOF Hybrid Material with Enhanced Hydrolytic Stability: A Fluorescent Photoinduced Electron Transfer Sensor for the Ultraselective Detection of Acetic Acid and Chromium (VI).

In recent years, the synthesis of metal-organic framework (MOF)─nanocomposites has received wide attention from the scientific fraternity due to the presence of a tunable hierarchical architecture and invasive versatility in applications. The present work focuses on the solvothermal synthesis of a novel hybrid MOF-nanocomposite through the impregnation of Mn-doped ZnO nanoparticles onto the matrix of a pioneer metal-organic framework that is composed of zinc metal connected with terephthalic acid linkers (MOF-5). The hierarchical arrangements of the prepared material were further assessed by Fourier-transform infrared spectroscopy (FTIR), powder X-ray diffraction (PXRD), high-resolution transmission electron microscopy (HR-TEM), UV-visible, photoluminescence (PL), and dynamic light scattering (DLS) measurements. The porosity analysis via nitrogen sorption measurements at 77 K showed that the material is porous with hierarchical micro-, wide micro-, and mesopores. The SAED pattern confirms the polycrystallinity of the material, which is in good agreement with the data obtained from PXRD analysis. Effective integration of Mn-doped ZnO onto the MOF structure was confirmed by XPS analysis, and the study further identified the oxidation state of the elements present. The synthesized analyte is an efficient fluorescent chemosensor for the detection of acetic acid, which can find further potential applications in intracellular imaging. Interestingly, the same compound also selectively detects the presence of Cr(VI) ions, thereby acting as a dual sensor, which finds applications in the sensing and removal of environmental contaminants. The material showed a sharp and intense emission at 569 nm at an excitation wavelength of 320 nm, and it exhibits high quenching efficiencies of 99.87 and 71.43% toward the sensing of µM level concentration of acetic acid and Cr2O72-, respectively. The highly efficient fluorescent sensing of pollutants, even at a shorter linear range, discarded the possibility of sensing the pollutants at higher concentration ranges. The Ksv value for the detection of acetic acid and Cr(VI) is found to be 3.7017 × 106 and 11.0324 × 106 M-1, respectively, which further confirms the higher sensing ability of the synthesized fluorophore. The mechanistic studies and density functional theory calculations of Mn-doped ZnO@MOF-5 reveal that photoinduced electron transfer plays a significant role in the turn-off response toward acetic acid and Cr2O72- ions. In the case of acetic acid, in addition to photoinduced electron transfer, hydrogen bonding interactions may also lead to fluorescence quenching. To the best of our knowledge, no precedent work has been reported for the sensing of acetic acid in the solution state. All other fluorescent sensing reports put forward the sensing and adsorption of acetic acid in the gaseous state, which makes this material a pioneer among others for the detection of acetic acid in the solution phase.

Supramolecular Interactions-Assisted Polymorphism and Unique Mechanofluorochromism in 9,10-Bis((E)-4-(trifluoromethyl)styryl)anthracene.

A fluorinated distyrylanthracene (DSA) derivative, 9, 10-bis((E)-4-(trifluoromethyl)styryl)anthracene abbreviated as 4FDSA with two crystalline polymorphs (4FDSA-G (green emission) and 4FDSA-O (orange emission)), showing remarkable aggregation-induced enhanced emission and mechanofluorochromic characteristics, was developed. One of the polymorphs in its crystalline arrangement exhibits the hardly seen F…F interactions. It questions the conventional belief of the non-polarisable nature of fluorine atoms in forming the halogen bond. The twisted molecular conformation facilitated by the various supramolecular interactions resulted in the formation of another intensely emissive bluer nanocrystal (4FDSA-NC) at aggregating conditions. Even though, the both polymorphs show distinct tricolor luminescence switching on the action of mechanical force, fumigation of ground crystals with solvent vapor resulted in the formation of a more thermodynamically favorable 4FDSA-NC form. The work demonstrates the effect of supramolecular interactions assisted conformational changes in tuning the unique mechanofluorochromic characteristics of the polymorphic crystals.

Mechanistic studies of photoinduced intramolecular and intermolecular electron transfer processes in RuPt-centred photo-hydrogen-evolving molecular devices.

The photoinduced electron transfer properties of two photo-hydrogen-evolving molecular devices (PHEMDs) [(bpy)2Ru(II)(phen-NHCO-bpy-R)Pt(II)Cl2](2+) (i.e., condensation products of [Ru(bpy)2(5-amino-phen)](2+) and (4-carboxy-4'-R-bpy)PtCl2; bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline; RuPt-COOH for R = COOH and RuPt-CN for R = CN) were investigated. RuPt-CN demonstrates higher photocatalytic performance relative to RuPt-COOH arising from a larger driving force for the intramolecular photoinduced electron transfer (PET) associated with a stronger electron-withdrawing effect of R (ΔGPET = −0.43 eV for RuPt-CN and −0.16 eV for RuPt-COOH). This is the first study on PET events using ultrafast spectroscopy. Dramatic enhancement is achieved in the rate of PET in RuPt-CN (1.78 × 1010 s(−1)) relative to RuPt-COOH (3.1 × 109 s(−1)). For each system, the presence of three different conformers giving rise to three different PET rates is evidenced, which are also discussed with the DFT results. Formation of a charge-separated (CS) state [(bpy)2Ru(III)(phen-NHCO-bpy(−˙)-R)Pt(II)Cl2](2+) in the sub-picosecond time regime and recombination in the picosecond time regime are characterized spectrophotometrically. The CS-state formation was found to compete with reductive quenching of the triplet excited state by EDTA whose dianionic form ion-pairs with dicationic RuPt-COOH. Thus, a key intermediate [(bpy)2Ru(II)(phen-NHCO-bpy(−˙)-R)Pt(II)Cl2](+) (i.e., the one-electron-reduced species) prior to the H2 formation was found to be formed either via reduction of the CS state by EDTA or via formation of [(bpy)2Ru(II)(phen(−˙)-NHCO-bpy-R)Pt(II)Cl2](+) by reductive quenching of the triplet excited state. More importantly, it is also shown that some of the conformers in solution possess a CS lifetime sufficiently long to drive hydrogen evolution from water.

Generation of triarylamine radical cations through reaction of triarylamines with Cu(II) in acetonitrile. A kinetic investigation of the electron-transfer reaction.

Triphenylamine derivatives react with Cu(2+) in acetonitrile to give radical cations, which subsequently undergo dimerization to provide tetraphenylbenzidine derivatives. Kinetic aspects of radical cation formation were examined by stopped-flow spectrophotometry. A broad range of triphenylamine derivatives were studied, and the driving force for the electron-transfer reaction ranged from +3.67 to -8.56 kcal M(-1) with rate constants varying from 1.09 x 10(2) to 2.15 x 10(5) M(-1) s(-1) for these systems. Reorganization energy for the electron-transfer reaction was estimated using experimentally determined activation parameters. Fitting of the rate data to the Marcus equation using different values of the electronic coupling matrix element H(el) provided a good fit with H(el) = 100 cm(-1) suggesting that electron transfer in the TPA/Cu(2+) system conforms to the Marcus-type electron transfer. Furthermore, the high reorganization obtained from these studies is consistent with significant bond cleavage in the transition state, and a mechanism consistent with the experimental data is proposed.

Cu(II)-mediated generation of triarylamine radical cations and their dimerization. an easy route to tetraarylbenzidines.

Triphenylamine (TPA) derivatives react with Cu2+ in acetonitrile to give TPA radical cations which undergo dimerization and deprotonation reactions to yield tetraphenylbenzidines (TPB). Synthetic utility of this reaction is demonstrated using several triphenylamine derivatives, and yields in excess of 80% are obtained in most cases. Involvement of the amine radical cations in these reactions was confirmed by ESR and absorption spectroscopic studies. A mechanism consistent with all observations is proposed. This study also revealed a very good correlation between the free energy change for radical cation formation and product yields.