Decoupling Redox Hopping and Catalysis in Metal-Organic Frameworks -based Electrocatalytic CO2 Reduction.

Traditional MOF e-CRR, constructed from catalytic linkers, manifest a kinetic bottleneck during their multi-electron activation. Decoupling catalysis and charge transport can address such issues. Here, we build two MOF/e-CRR systems, CoPc@NU-1000 and TPP(Co)@NU-1000, by installing cobalt metalated phthalocyanine and tetraphenylporphyrin electrocatalysts within the redox active NU-1000 MOF. For CoPc@NU-1000, the e-CRR responsive CoI/0 potential is close to that of NU-1000 reduction compared to the TPP(Co)@NU-1000. Efficient charge delivery, defined by a higher diffusion (Dhop =4.1×10-12  cm2 s-1 ) and low charge-transport resistance ( R C T M O F ${{R}_{{\rm C}{\rm T}}^{{\rm M}{\rm O}{\rm F}}}$ =59.5 Ω) in CoPC@NU-1000 led FECO =80 %. In contrast, TPP(Co)@NU-1000 fared a poor FECO =24 % (Dhop =1.4×10-12  cm2 s-1 and R C T M O F ${{R}_{{\rm C}{\rm T}}^{{\rm M}{\rm O}{\rm F}}}$ =91.4 Ω). For such a decoupling strategy, careful choice of the host framework is critical in pairing up with the underlying electrochemical properties of the catalysts to facilitate the charge delivery for its activation.

Is the origin of green fluorescence in unsymmetrical four-ring bent-core liquid crystals single or double proton transfer?

The origin of green fluorescence in unsymmetrical four-ring bent-core liquid crystals (BCLCs) is not understood clearly. There is no analysis of the photo-physical properties, in particular, the excited state dynamical behaviour, of these molecules. Because of the availability of two proton transfer sites in these systems, there is no clear explanation of the involvement of single or double proton transfer reactions in the origin of the large Stokes shifted green fluorescence band. Therefore, we employ the femtosecond transient absorption spectroscopy technique to recognize the formation of transient species in the excited state and its associated dynamics in the femto-picosecond time domain. In order to validate the experimental photo-physical properties, the time-dependent density functional theory (TDDFT) calculations have been performed. Our results indicate that the four-ring bent-core system is an excellent example of systems exhibiting two proton transfer reactions in a sequential process. Further, these two proton transfer sites are not electronically coupled to each other; therefore, monoketo and diketo tautomers exhibit very close absorption and emission positions. The large Stokes shifted green emission in these systems is mainly contributed by the monoketo tautomer (MK-C*). The linking ester functional group of both sites plays a significant role in controlling the rate of proton transfer reactions. A good correlation is observed between theoretical and experimental results.

Origin of green photoluminescence in four-ring bent-core molecules with ESIPT, selective sensing of zinc ions by turn-on emission and their liquid crystal properties.

Fluorescent four-ring symmetrical/unsymmetrical molecules containing alkyl chains of a varied number of -CH2- groups with a bent-core have been synthesized to explore their liquid crystalline (LC) and photophysical properties. Some of these molecules depending upon their alkyl chain length were found to exhibit B1 and B7 liquid crystalline phases and are characterized by various analytical techniques such as FT-IR, 1H NMR, mass, POM, DSC, single crystal XRD, etc. Crystal structure determination reveals the hydrogen bonded enol form of these molecules with non-planar bent-molecular geometry. Intramolecular hydrogen bonding was found to play an important role in the stabilization of these molecules and in the origin of their green photoluminescence (GPL). The photo-physical experimental results through various control experiments clearly demonstrate that the origin of the large Stokes shifted GPL of these molecules can be attributed to the excited state intramolecular proton transfer (ESIPT) process. The formation of various types of anionic species and their stability were explored through steady-state and time-resolved fluorescence measurements. These compounds are found to be good turn-on PL probes in the selective detection of zinc ions at the micromolar level. Upon binding of zinc ions with the bent-core molecule, the structural changes have been investigated through NMR spectroscopy.

Molecular origin of photoluminescence of carbon dots: aggregation-induced orange-red emission.

The molecular origin of the photoluminescence of carbon dots (CDs) is not known. This restricts the design of CDs with desired optical properties. We have synthesized CDs starting from carbohydrates by employing a simple synthesis method. We were able to demonstrate that the CDs are composed of aggregated hydroxymethylfurfural (HMF) derivatives. The optical properties of these CDs are quite unique. These CDs exhibit an excitation-independent PL emission maximum in the orange-red region (λ ∼ 590 nm). These CDs also exhibit excitation as well as monitoring wavelength-independent single exponential PL decay. These observations indicate that only one type of chromophore (HMF derivative) is present within the CDs. Several HMF derivatives are aggregated within the CDs; therefore, the aggregated structure cause a large Stokes shift (∼150 nm). By several control experiments, we showed that the same aggregated chromophore unit (HMF derivative), and not the individual fluorophores, is the fluorescing unit. The emission maximum and the single exponential PL lifetime are independent of the polarity of the medium. The existence of a low-lying trap state could be reduced quite significantly. A model has been proposed to explain the interesting steady state and dynamical photoluminescence behaviour of the CDs. As the molecular origin of their photoluminescence is known, CDs with desired optical properties can be designed.

Strong electron donation induced differential nonradiative decay pathways for para and meta GFP chromophore analogues.

Z-E Isomerisation because of rotation around the exocyclic double bond (known as the τ-twist) and not any other internal conversion has been reported to be the major nonradiative decay channel for non-hydroxylic unconstrained para and meta GFP chromophore analogues. The equation Φf + 2ΦZE = 1 has been shown to hold well for both para and meta GFP chromophore analogues. If the above equation holds true, then upon reducing the extent of Z-E isomerisation (ΦZE), the fluorescence quantum yield (Φf) should increase. To probe the above proposition two sets of non-hydroxylic unconstrained para and meta GFP chromophore analogues were synthesized. Quite interestingly by introducing the strongly electron donating -NEt2 group to the benzenic moiety these para and meta GFP chromophore analogues were shown to exhibit differential optical behaviour w.r.t. the extent of the solvatochromic shift, Φf, ΦZE, and τf. For the first time it has been shown that the well accepted equation Φf + 2ΦZE = 1 does not hold at all for these non-hydroxylic unconstrained meta analogues. Although ΦZE has been shown to be <10%, Φf is much lower than the expected near unity value for these meta analogues. After detailed investigation into the nonradiative excited state decay channel, contrary to literature reports, energy gap law governed internal conversion and not Z-E isomerisation was shown to be the major nonradiative decay channel for these meta analogues. Two models are put forward to understand the differential optical behaviour of these para and meta GFP chromophore analogues. Support from X-ray crystal structures, NMR experiments, and computational calculations has also been provided.

Synthesis of hydrophobic photoluminescent carbon nanodots by using L-tyrosine and citric acid through a thermal oxidation route.

Hydrophobic photoluminescent carbon nanodots (CNDs) were fabricated by using citric acid and L-tyrosine precursor molecules through a simple, facile thermal oxidation process in air. These CNDs (less than 4 nm in size) exhibited a characteristic excitation wavelength dependent emission and upconversion emission properties and are insoluble in water, but soluble in organic solvents. FTIR and (1)H NMR analyses showed a selective participation of L-tyrosine molecule during the carbonization process at 220 °C without a disturbance of its benzylic protons and aromatic phenyl ring bearing hydroxy group. TEM and XRD studies revealed a quasi-spherical morphology and poor-crystalline nature of CNDs. Because the presence of the hydroxy group of L-tyrosine is dominating at the surface, these CNDs are also soluble in water under basic conditions. The effects of base and silver nanoparticles on the luminescence properties of CNDs were studied and a quenching of fluorescence was observed. These tyrosine-passivated CNDs are applicable for both biologically and commercially.

A new family of four-ring bent-core nematic liquid crystals with highly polar transverse and end groups.

Non-symmetrically substituted four-ring achiral bent-core compounds with polar substituents, i.e.., chloro in the bent or transverse direction in the central core and cyano in the lateral direction at one terminal end of the molecule, are designed and synthesized. These molecules possess an alkoxy chain attached at only one end of the bent-core molecule. The molecular structure characterization is consistent with data from elemental and spectroscopic analysis. The materials thermal behaviour and phase characterization have been investigated by differential scanning calorimetry and polarizing microscopy. All the compounds exhibit a wide-ranging monotropic nematic phase.