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Investigation of photochromic and acidochromic behaviors of a set of pyridyl- and pyrimidylethynylated mono- and bis-benzopyrans reveals an intriguing influence of the N -heteroaryl ring on spectrokinetic properties of the photogenerated o -quinonoid colored reactive intermediates. While the absorption maxima of the pyridylethynylated bis-benzopyran and its photogenerated o -quinonoid colored species undergo bathochromic shifts by ca. 40 and 22 nm, respectively, in the presence of an acid (e.g., trifluoroacetic acid (TFA)), the same remain unaffected for the analogous pyrimidylethynylated bis-benzopyran and its photogenerated o -quinonoid colored species under similar conditions. Modification of the photochromic behavior of these benzopyrans and, hence, spectrokinetic properties of their photogenerated o -quinonoid species in the presence of H + is a consequence of relative proton affinities of N -heteroaryl rings, i.e., pyridyl/pyrimidyl, and the resonance effects relayed through the ethynyl spacers in a push-pull π-delocalized-type skeleton; the mesomeric effects operate in a contrasting manner depending on the N -heteroaryl ring in the absence and in the presence of an acid. These molecular systems offer a unique opportunity to modulate both photochromic and acidochromic properties of benzopyrans and their photogenerated colored o -quinonoid intermediates by leveraging N -heteroaromatic rings., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

A water-stable In-MOF, constructed based on a conformationally-flexible tetraacid linker, i.e., 2,7-bis(3,5-dicarboxyphenyl)-9,9'-diphenyl-9H-fluorene, i.e., H4DPF, is shown to exhibit a significantly enhanced solid-state fluorescence quantum yield (φf) of 23% in comparison with that of the linker (φfca. 4%) as a consequence of rigidification of the latter by metalation. Application of external stimulus in the form of grinding of the In-MOF leads to a drastic enhancement by 29%, φf from 23 to 52%. Solid-state absorption and emission spectra show that the absorption in the region of 368-550 nm gets diminished with a concomitant change in the emission maximum with a blue shift upon grinding. Fluorescence enhancement with grinding is correlated with a gradual reduction in the size of the particles, as established by SEM analysis. MOF particle aggregation has been invoked to account for the observed fluorescence enhancement in addition to a subtle conformational change in the structure of the linker upon grinding. Intriguingly, the ground MOF particles exhibit aggregation behaviour in the DMF-water solvent system with the emission further increasing up to 75% for the increase in the water fraction (fw) from 0 to 60%; hydrophobic aggregation of particles evidently leads to a change in the conformation of the linker and particle aggregation-enhanced emission (AEE). De-aggregation of particles ensues for fw = 70-90%, as reflected by a gradual decrease in the emission intensity. It is shown that the suspension of ground In-MOF particles in water permits sensing of metal ions, in particular Al3+ ions, by fluorescence quenching with detection at a sub-ppb level. The observed results comprise first demonstration of both mechanoluminescence and AEE of MOF particles.

2D metal-organic nanosheets (MONs), akin to graphene, have aroused immense contemporary interest. In our quest to develop functional 2D MONs based on organic linkers designed de novo, we reasoned that benzene-tetrabenzoic acid, which has been exploited tremendously in the construction of pillared metal-organic frameworks (MOFs), could be maneuvered readily to access redox-active MONs based on the benzoquinone/hydroquinone redox couple. Herein, we show that the self-assembly of 2,3,5,6-tetrakis(p-carboxyphenyl)hydroquinone H 4 BTA with Zn(NO 3 ) 2 does lead to 2D metal-organic nanosheets that stack down the y axis, affording a layered Zn MOF. Although the crystals of the latter do not exhibit a discernible chemically induced redox switching behavior, the 2D MONs accessed by ultrasound-induced liquid-phase exfoliation (UILPE) lend themselves to a facile redox switching behavior. Treatment of a dispersion of the 2D MONs in methanol with phenyliodine(III) diacetate (PIDA) results in the oxidation of the hydroquinone core to benzoquinone. Remarkably, the latter can be reverted to the former by treatment with ascorbic acid as a reducing agent; indeed, the redox process can be made out by the naked eye. The results constitute the first example of chemically induced redox switching of 2D MONs. In view of emergent applications of 2D materials in general and MONs in particular, for example, improvement of the performance of membranes in separations by doping with MONs, the redox-switchable property may lead to the development of unique materials with heretofore unexplored potential., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

2D metal-organic framework nanosheets (MONs) lie at the heart of contemporary research on metal-organic materials. We have rationally designed and synthesized a fluorescent 6-connecting hexaacid linker H 6 TPA based on the 1,3,5-triphenylbenzene core to access layered MOFs by metal-assisted self-assembly. Treatment of H 6 TPA with In 3+ salt does indeed lead to a layered porous MOF, i.e.In-TPA. It is shown that the ultrasonication-induced liquid phase of exfoliation (UILPE) of the latter in a top-down fashion affords few-layer 2D metal-organic nanosheets (MONs). The delamination of 2D MONs in ethanol occurs with 'turn-on' fluorescence, which is otherwise suppressed in the bulk material. The exfoliated MONs in ethanol exhibit blue fluorescence with a respectable quantum yield of 0.15 and serve as efficient sensory materials for 'turn-off' fluorescence detection of explosive nitroaromatic compounds (NACs). While all nitroaromatics are found to quench the fluorescence of MONs, the most electron-deficient trinitrotoluene (TNT) exhibited the highest efficiency; at 2 mM concentration of TNT, the fluorescence of 2D MONs was found to be quenched with an efficiency of 80% (permitting the detection of TNT at ca. 11 ppm level), while that of the unmetallated linker, i.e.H 6 TPA, was quenched with only 5% efficiency. The unique attributes of MONs, namely the rigidity of the linker upon metallation and porosity that facilitates guest confinement within the pores, for efficient sensing of nitroaromatics are thus demonstrated for the first time.

A comprehensive investigation of the photophysics of a cofacially oriented bis-coumarin based on naphthalene, i.e., Cou-Nap, designed and synthesized to examine the influence of π-electronic communication between the two fluorophores, reveals exceptional excited-state properties. While the anticipated [2 + 2] photocycloaddition is not observed despite the fact that the two reactive coumarin units are at a distance of 3.8 Å, the fluorescence quantum yields and singlet lifetimes in different solvents are found to be remarkably higher when compared to those of the parent coumarin and a mono-coumarin model system, i.e., Cou-Dur. In addition to large solvent-induced Stokes shifts, Cou-Nap displays intriguing temperature-dependent emission in a nonpolar solvent such as cyclohexane. The observed photophysical properties are reconciled based on the so-called 'phane effect' that is operative in cyclophanes. In the latter, an effective π-π interaction between the aromatic rings modifies the attributes of the chromophore in such a manner that the observed properties cannot be associated with the individual aromatic rings. The temperature-dependent emission is proposed to arise as a consequence of thermally activated ISC from the singlet-excited state to one of the higher energy triplet states. The results constitute, for the first time, the demonstration of modification of the excited-state properties of a fluorophore in a non-cyclophane system by 'phane effect'.

The fluoride-mediated desilylation reaction has been exploited, for the first time, to trigger ring-opening of photochromic diarylbenzo-/naphthopyrans into highly colored anionic merocyanine dyes with high molar absorptivities to permit naked-eye sensing. The absorption spectral shifts, i.e., differences in the absorption maxima of colorless and colored forms, observed for a rationally designed set of silyloxy-substituted diarylpyrans subsequent to fluoride-induced ring opening are remarkably high (330-480 nm), and are unknown for any colorimetric probe. In particular, the disilyloxy-substituted diphenylnaphthopyran and its analog, in which the diphenyl groups are fused in the form of fluorene, allows "naked-eye" detection of fluoride in subppm levels (<1.0 ppm) in THF as well as in DMSO-H2O. The sensing is specific for fluoride among various other anions. This approach for colorimetric sensing of fluoride by ring-opening of the otherwise photochromic benzo-/naphthopyrans is heretofore unprecedented.