Emissive Substoichiometric Covalent Organic Frameworks for Water Sensing and Harvesting

Emissive covalent organic frameworks (COFs) have recently emerged as next-generation porous materials with attractive properties such as tunable topology, porosity, and inherent photoluminescence. Among the different types of COFs, substoichiometric frameworks (so-called Type III COFs) are especially attractive due to the possibility of not only generating unusual topology and complex pore architectures but also facilitating the introduction of well-defined functional groups at precise locations for desired functions. Herein, the first example of a highly emissive (PLQY 6.8%) substoichiometric 2D-COF (COF-SMU-1) featuring free uncondensed aldehyde groups is reported. In particular, COF-SMU-1 features a dual-pore architecture with an overall bex net topology, tunable emission in various organic solvents, and distinct colorimetric changes in the presence of water. To gain further insights into its photoluminescence properties, the charge transfer, excimer emission, and excited state exciton dynamics of COF-SMU-1 are investigated using femtosecond transient absorption spectroscopy in different organic solvents. Additionally, highly enhanced atmospheric water-harvesting properties of COF-SMU-1 are revealed using FT-IR and water sorption studies.The findings will not only lead to in-depth understanding of structure-property relationships in emissive COFs but also open new opportunities for designing COFs for potential applications in solid-state lighting and water harvesting.

Combining machine learning and high-throughput experimentation to discover photocatalytically active organic molecules

Light-absorbing organic molecules are useful components in photocatalysts, but it is difficult to formulate reliable structure–property design rules. More than 100 million unique chemical compounds are documented in the PubChem database, and a significant sub-set of these are π-conjugated, light-absorbing molecules that might in principle act as photocatalysts. Nature has used natural selection to evolve photosynthetic assemblies; by contrast, our ability to navigate the enormous potential search space of organic photocatalysts in the laboratory is limited. Here, we integrate experiment, computation, and machine learning to address this challenge. A library of 572 aromatic organic molecules was assembled with diverse compositions and structures, selected on the basis of availability in our laboratory, rather than more sophisticated criteria. This training library was then assessed experimentally for sacrificial photocatalytic hydrogen evolution using a high-throughput, automated method. Quantum chemical calculations and machine learning were used to visualise, interpret, and ultimately to predict the photocatalytic activities of these molecules, covering a much broader chemical space than for previous polymer photocatalyst libraries. By applying unsupervised learning to the molecular structures, we identified structural features that were common in molecules with high catalytic activity. Further analysis using calculated molecular descriptors within a suite of supervised classification algorithms revealed that light absorption, exciton electron affinity, electron affinity, exciton binding energy, and singlet–triplet energy gap had correlations with the photocatalytic performance. These trained predictive models can be used in future studies as filters to deprioritise or discard would-be low-activity candidate molecules from experiments, and to prioritize more favourable candidates. As a demonstration, we used virtual in silico experiments to show that it was possible to halve the experimental cost of finding 50% of the most active photocatalysts by using the machine learning model as an experimental advisor. We further showed that the ML advisor trained on the 572-molecule library could be used to make predictions for an unseen set of 96 molecules, achieving equivalent predictive accuracies to those in the initial training set. This marks a step toward the machine-learning assisted discovery of molecular organic photocatalysts and the approach might also be applied to problems beyond photocatalytic hydrogen evolution, such as CO₂ reduction and photoredox chemistry.

We developed models to predict the photoactivity of organic molecules for photocatalytic hydrogen evolution by integrating experiment, computation, and machine learning. This marks a step toward the data-driven discovery of molecular photocatalysts.

Structure and piezochromism of pyrene-1-carbaldehyde at high pressure

The crystal structure of pyrene-1-carbaldehyde (PA), a model polyaromatic hydrocarbon, highly luminescent in the solid state and crystallizing in the triclinic system, has been re-determined at several pressures ranging from atmospheric up to 3 GPa using a diamond anvil cell. A `multi-crystal' approach was used in crystal structure determination, significantly improving completeness of X-ray diffraction data attainable for such a low-symmetry system. The crystal structure consists of infinite π-stacks of PA molecules with discernible dimers, which resemble aggregates formed by pyrene derivatives in solution as well as in the solid state. A series of measurements showed that the average inter-planar distance between individual molecules within π-stacks decreases with pressure in the investigated range. This results in piezochromic properties of PA: a significant sample color change as well as a red-shift of fluorescence with pressure, as studied with UV-vis spectroscopy. Periodic DFT calculations allowed us to relate the variations in the crystal structure with pressure to the changes in the electronic structure of this material.

Pressure-Dependent Structural and Luminescence Properties of 1-(Pyren-1-yl)but-2-yn-1-one

The crystal structure of 1-(pyren-1-yl)but-2-yn-1-one (1a, a polynuclear aromatic hydrocarbon displaying enhanced luminescence in the solid state, has been re-determined at several pressures ranging from atmospheric up to 3 GPa using a Diamond Anvil Cell (DAC). These experiments were augmented by periodic DFT calculations at pressures up to 4.4 GPa. UV-Vis fluorescence of 1a at non-ambient pressures has also been investigated. The crystal structure consists of infinite π-stacks of anti-parallel 1a molecules with discernible dimers, which may exemplify aggregates formed by pyrene derivatives in solution and thin films, and is predominantly stabilized by dispersion. The average inter-planar distance between individual molecules within π-stacks decreases with pressure in the investigated range. This results in piezochromic properties of 1a: a red-shift of sample color, as well as a bathochromic shift of fluorescence with pressure (by ca. 100 nm at 3.5 GPa). Two-component fluorescence spectra support the hypothesis that at least two types of excimers are involved in the electronic excitation processes in crystalline 1a.

Enhanced Resonance Energy Transfer and White-Light Emission from Organic Fluorophores and Lanthanides in Dendron-based Hybrid Hydrogel

In this paper, we have investigated the use of poly(aryl ether) dendron-based gel as a medium for resonance energy transfer (RET) from organic donors (phenanthrene, naphthalene, and pyrene) to lanthanide [Eu(III) and Tb(III)] ions. The gel has been prepared through self-assembly of glucose-cored poly(aryl ether) dendrons in a dimethyl sulfoxide/water mixture (1:9 v/v). The efficiency of RET was calculated by metal-centered emission quantum yield measurements in the gel medium. While there was no resonance energy transfer observed between the donor-acceptor pairs in solution, efficient RET has been observed in the gel medium. The metal-centered quantum yield values were 11.9% for phenanthrene-Eu(III), 3.9% for naphthalene-Eu(III), and 3.6% for pyrene-Eu(III) systems. Partial RET in the system has been utilized to generate white-light emission from the gel by incorporating an additional lanthanide ion, Tb(III), along with the organic donors and Eu(III). The CIE (Commission Internationale d'Eclairage) coordinates obtained for gels formed by phenanthrene-Tb(III)-Eu(III) (PTE), naphthalene-Tb(III)-Eu(III) (NTE), and pyrene-Tb(III)-Eu(III) (PyTE) were (0.33, 0.32) for PTE, (0.35, 0.37) for NTE, and (0.35, 0.33) for PyTE. The correlated color temperatures (CCT) for white-light-emitting gels were calculated, and the values (5520 K for PTE, 4886 K for NTE, and 4722 K for PyTE) suggest that the system generates cool white light.

Additional insights into luminescence process of polycyclic aromatic hydrocarbons with carbonyl groups: photophysical properties of secondary N-alkyl and tertiary n,n-dialkyl carboxamides of naphthalene, anthracene, and pyrene

Here we report the substitution effects of N-alkyl and N,N-dialkyl carboxamide groups on the fluorescence properties of polycyclic aromatic hydrocarbon chromophores, so as to control their fluorescence properties. The fluorescence properties of compounds obtained using solvents with different polarities showed very little change, indicating that the modified compounds do not form charge transfer states. TD-DFT calculations and measurements performed at low temperature (78 K) and in viscous solvents revealed that the N-alkyl and N,N-dialkyl carboxamide groups tend to reduce the contributions from intersystem crossing and increase those from internal conversion. Considering that the fluorescence mechanism of low-fluorescence carbonyl compounds such as aldehyde and ketone is dominated by intersystem crossing and that of high-luminescence carbonyl compounds such as carboxylic acid and ester is dominated by a radiative process, it can be said that the photophysical process of N-alkyl and N,N-dialkyl carboxamides is novel. In addition, the calculation results for excited states indicated that such contributions can be controlled by selecting the appropriate polycyclic aromatic hydrocarbon or amide structure, in addition to solvent viscosity and temperature.

Cavitand octa acid forms a nonpolar capsuleplex dependent on the molecular size and hydrophobicity of the guest

We have been exploring the use of a deep cavity cavitand known by the trivial name 'octa acid' as a photochemical reaction cavity for manipulating photochemical and photophysical properties of organic molecules. In the current study, we have monitored the micropolarity of the interior of the cavitand by recording the fluorescence of five different organic probes. They all indicate that the interior of octa acid capsuleplex (2:1, H/G complex) is nonpolar and does not contain water molecules in spite of the complex being present in water. The nature of the octa acid-probe complex in each case has been characterized by 1H NMR data to be a 2:1 capsuleplex. Photophysical and 1H NMR experiments were employed to probe the factors that control the structure of the complex, 2:2, 2:1, and 1:1. The data we have on hand suggest that the structure of the host/guest complex depends on the size and hydrophobicity of the guest molecule.

Environment-Sensitive Fluorophore Emitting in Protic Environments

The unusual fluorescence properties of 8-methoxy-4-methyl-2H-benzo[g]chromen-2-one (1) are described. The fluorophore 1 is almost nonfluorescent in aprotic solvent (e.g., fluorescence quantum yield Phi(f) < 0.0003 in n-hexane), whereas it strongly fluoresces at long wavelengths (>450 nm) in protic solvent (e.g., Phi(f) = 0.21 in methanol). The fluorophore 1 also shows good applicability in developing a new fluorogenic (fluorescent "off-on") sensor. [structure: see text]

Intelligent fluorescent nucleoside in sensing cytosine base: importance of hydrophobic nature of perylene fluorophore

Fluorescence response upon hybridization of perylene labeled oligonucleotide probes depends on the microenvironment experienced by the perylene fluorophore. In mismatched duplex ((Per)U-C), enhanced fluorescence was observed while in matched duplex ((Per)U-A) fluorescence intensity decreased considerably. This observation will be a promising research effort in giving rise to a new powerful tool in detection of SNP.

Pyrene-Labeled Base-Discriminating Fluorescent DNA Probes for Homogeneous SNP Typing

This paper describes the design of novel base-discriminating fluorescent (BDF) nucleobases and their application to single nucleotide polymorphism (SNP) typing. We devised novel BDF nucleosides, (Py)U and (Py)C, which contain a pyrenecarboxamide chromophore connected by a propargyl linker. The fluorescence spectrum of the duplex containing a (Py)U/A base pair showed a strong emission at 397 nm on 327 nm excitation. In contrast, the fluorescence of duplexes containing (Py)U/N base pairs (N = C, G, or T) was considerably weaker. The proposed structure of the duplex containing a matched (Py)U/A base pair suggests that the high polarity near the pyrenecarboxamide group is responsible for the strong A-selective fluorescence emission. Moreover, the fluorescence of the duplex containing a (Py)U/A base pair was not quenched by a flanking C/G base pair. The fluorescence properties are quite different from previous BDF nucleobases, where fluorescence is quenchable by flanking C/G base pairs. The duplex containing the C derivative, (Py)C, selectively emitted fluorescence when the base opposite (Py)C was G. The drastic change of fluorescence intensity by the nature of the complementary base is extremely useful for SNP typing. (Py)U- and (Py)C-containing oligodeoxynucleotides acted as effective reporter probes for homogeneous SNP typing of DNA samples containing c-Ha-ras and BRCA2 SNP sites.

Self-modelling analysis applied to nanosecond transient absorption spectroscopy of flavone: an aid to elucidate and characterise reaction intermediates

A nanosecond transient absorption spectroscopy study of flavone performed with a 248 nm pump radiation has been investigated with the support of a chemometric treatment: SIMPLISMA. The experimental spectra obtained in various solvent with a pump-probe delay lower than about 2 micros are in quite good concordance with those already presented in the literature. Nevertheless after about 10 micros, the spectrum pattern significantly evolves as a function of time particularly for the methanolic solution. A qualitative analysis together with a SIMPLISMA chemometric treatment of the experimental data allowed to elucidate and characterise two interdependent transient species in the alcoholic medium: the lowest T1 triplet state of flavone and the ketyl radical forming by H-abstraction reaction from the solvent. In cyclohexane and acetonitrile, the same species seem to be produced in the studied time-scale but the radical form is generated with variable quantum yield depending on the solvent polarity. The pure spectrum and the photochemical kinetics of each reaction intermediate could have been determined with the help of the second derivative SIMPLISMA calculation procedure.

Physicochemical Studies on the Adsorption Properties of Asbestos

Toxic asbestos fibers are known to be carriers of carcinogenic polyaromatic hydrocarbons (PAH) when ingested by human lungs. To clarify the asbestos-PAH interactions, the adsorption properties of four different asbestos, namely amosite, anthofillite, crocidolite, and chrysotile (termed AMOS, ANTHO, CROX, and CHRYS, respectively), toward pyrene (py) were investigated by electron paramagnetic resonance (EPR) and fluorescence spectroscopy, both steady state and time resolved. Pyrene was labeled with a nitroxide TEMPO radical (py-T) for the EPR study. EPR provided information on the adsorption ability of the surface toward py-T and on the interactions between py-T and surface sites, mainly cooperative in nature. Pyrene molecules organize on AMOS samples in a packed layer; the radical works as a separator and prevents the attaching of the py-T molecules at the surface, but the N-O groups of the few adsorbed molecules directly interact with the surface. The interacting sites at the ANTHO surface are quite far from each other; therefore py-T instead of py may form a monolayer at the asbestos surface. The surface of CROX strongly adsorbs both excimers of py and monomers of py-T, orienting the nitroxide groups toward the polar sites at the surface. The surface of CHRYS is the most adsorbing toward py and py-T, but the nitroxide group does not directly interact with the surface. By increasing concentration, py-T saturates the CROX and the CHRYS surface interacting sites, and then forms a disordered liquid-like multilayer. The close average distance calculated for py-T adsorbed at the CROX surface (12 Å) suggests that the py-T molecules intercalate in an "up-and-down" conformation. The chemical composition of the different asbestos and their different structural properties affect the distribution of the polar sites at the external surface, modifying the adsorption properties. This study confirms the synergetic effect between the toxicity of the asbestos fibers and the adsorption of polyaromatic hydro carbons at their surface, which mainly depends on the surface properties of the asbestos. Copyright 2001 Academic Press.

Hydrophobic, Highly Conductive Ambient-Temperature Molten Salts

New, hydrophobic ionic liquids with low melting points (<-30 degrees C to ambient temperature) have been synthesized and investigated, based on 1,3-dialkyl imidazolium cations and hydrophobic anions. Other imidazolium molten salts with hydrophilic anions and thus water-soluble are also described. The molten salts were characterized by NMR and elemental analysis. Their density, melting point, viscosity, conductivity, refractive index, electrochemical window, thermal stability, and miscibility with water and organic solvents were determined. The influence of the alkyl substituents in 1, 2, 3, and 4(5)-positions on these properties was scrutinized. Viscosities as low as 35 cP (for 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide (bis(triflyl)amide) and trifluoroacetate) and conductivities as high as 9.6 mS/cm were obtained. Photophysical probe studies were carried out to establish more precisely the solvent properties of 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide). The hydrophobic molten salts are promising solvents for electrochemical, photovoltaic, and synthetic applications.