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The functionalization of silicon electrodes with π-conjugated chromophores opens new avenues to engineer hybrid semiconducting interfaces relevant to information storage and processing. Notably, molecularly dissolved π-conjugated units, such as ferrocene derivatives, are traditionally exploited as building blocks to construct well-defined interfaces that establish electrochemically addressable platforms with which to investigate electron transfer properties and charge storage capabilities. In contrast, planar π-conjugated building blocks such as naphthalene diimide (NDI) cores enable the formation of solvated aggregates equipped with emergent electronic structures not manifested by the parent, molecularly dissolved building blocks. To interrogate the extent to which the aggregated states of π-conjugated chromophores can be leveraged to regulate the n-type semiconducting properties of functionalized electrodes, we have devised an amphiphilic rylene core (NDI) that demonstrates a non-negligible degree of aggregation in an aqueous medium. Characterization of the electronic structures of the NDI-derived aggregates using a combination of electrochemistry, reductive titration experiments, and spectroelectrochemistry unveils the existence of π-anion stacks, the formation of which is contingent on the initial concentration of NDI building blocks. We show that grafting n-doped NDI aggregates on silicon electrode precursors equipped with a high density of anchoring groups by means of "click" reaction enables the formation of the hybrid Si-NDI electrode (Si-NDI-15@1) that facilitates electron injection by more than 400 mV when compared to Si interfaces constructed from molecularly dissolved NDI units. Furthermore, the engineering of a Si precursor surface characterized by a low density of anchoring groups provides additional proof to highlight that the potentiometric properties recorded for Si-NDI-15@1 originate from NDI units, evidencing a non-negligible degree of aggregation. The present work delivers tools to manipulate the potentiometric properties of functionalized electrodes by leveraging on the electronic structures of aggregated, π-conjugated precursors.

The molecular engineering of chromophores that enables the controlled and reliable formation of hierarchical solid-state architectures is at the forefront of developing hybrid semiconducting materi...

The surface coverage and molecular composition of redox-active molecules anchored on conductive surfaces regulate the kinetic and thermodynamic parameters of charge transfer reactions, providing a means to tune the electrochemical properties of hybrid materials. Herein, anchoring strategies and structural properties of redox-active probes, derived from ferrocene (Fc) and naphthalene diimide (NDI), are shown to regulate the electrochemical properties of functionalized p-doped Si(111) surfaces. Covalent functionalization of hydrogen-terminated Si(111) surfaces with Fc and NDI affords redox-active hybrid interfaces characterized through microscopy, spectroscopy, and voltammetry methods. Molecular design and synthetic grafting strategies modulate the electrochemical properties of the Fc-functionalized Si surfaces with a much higher (ca. 25 times) surface coverage (1.25 × 10-10 mol cm-2) for one-step photografting compared to divergent synthetic routes. Interestingly, the thermal grafting of an alkadiyne followed by "click" reaction with ferrocenyl-azide leads to one of the highest surface coverages (9.97 × 10-10 mol cm-2) of organo-iron reported and a significant anodic shift of the half-potential (>350 mV) compared to photografting methods. Similar experiments with NDI units exhibited electrochemical properties that diverge from those recorded for NDI in solution. The results presented herein offer access to novel redox-active Si interfaces that evidence tunable electrochemical properties of potential interest for microelectronic applications.

Two regioisomeric pairs of heptahelical mono- and biscoumarins that are differentiated by "inward" and "outward" disposition of the pyran-2-one moiety have been synthesized and investigated to understand the influence of helicity on excited-state and chiroptical properties. A slight variation in the helicities is found to manifest in contrasting excited-state properties of coumarin-annelated heptahelicenes; in addition to the intramolecular charge transfer, structural relaxation in the excited state is shown from theoretical calculations to cause decrease in the fluorescence quantum yield for a system with higher helicity. The optically pure enantiomers of heptahelical coumarins exhibit helicity-dependent chiroptical properties, namely, specific rotations, molar ellipticities, Cotton effects, and anisotropic dissymmetry factors. Theoretical calculations point to factors that are not readily explicable.

The development of supramolecular tools to modulate the excitonic properties of non-covalent assemblies paves the way to engineer new classes of semicondcuting materials relevant to flexible electronics. While controlling the assembly pathways of organic chromophores enables the formation of J-like and H-like aggregates, strategies to tailor the excitonic properties of pre-assembled aggregates through post-modification are scarce. In the present contribution, we combine supramolecular chemistry with redox chemistry to modulate the excitonic properties and solid-state morphologies of aggregates built from stacks of water-soluble perylene diimide building blocks. The n-doping of initially formed aggregates in an aqueous medium is shown to produce π-anion stacks for which spectroscopic properties unveil a non-negligible degree of electron-electron interactions. Oxidation of the n-doped intermediates produces metastable aggregates where free exciton bandwidths (ExBW) increase as a function of time. Kinetic data analysis reveals that the dynamic increase of free exciton bandwidth is associated with the formation of superstructures constructed by means of a nucleation-growth mechanism. By designing different redox-assisted assembly pathways, we highlight that the sacrificial electron donor plays a non-innocent role in regulating the structure-function properties of the final superstructures. Furthermore, supramolecular architectures formed via a nucleation-growth mechanism evolve into ribbon-like and fiber-like materials in the solid-state, as characterized by SEM and HRTEM. Through a combination of ground-state electronic absorption spectroscopy, electrochemistry, spectroelectrochemistry, microscopy, and modeling, we show that redox-assisted assembly provides a means to reprogram the structure-function properties of pre-assembled aggregates.

We show that n-doping supramolecular assemblies built from perylene diimide units provides a means to modulate the structure-function properties of these materials. In addition to highlighting the design principles, a combination of spectroscopic and microscopic characterization correlates an increase in the free-exciton bandwidth with the formation of mesoscale hierarchical superstructures.

A zinc metal–organic framework, i.e., Zn-MOF (Zn-DBC), with ca. 27% solvent-accessible void volume was synthesized from a rationally designed tetraacid based on sterically insulated dibenzo[g,p]chr...

Summary Exfoliation of layered porous metal-organic frameworks (MOFs) in a top-down fashion constitutes a successful approach to 2D metal-organic nanosheets (MONs), the analogs of graphene. The planned synthesis of layered MOFs is a challenging crystal-engineering exercise. Development of photochromic MONs that respond to light is a much bigger challenge; indeed, photochromic MONs are heretofore unknown. Here, we demonstrate engineering of layered MOFs and access to photochromic MONs. The strategy entails the use of programmed photochromic organic linkers characterized by a bulged core and metal-mediated self-assembly that occurs only in two dimensions, leading to layered MOFs. MONs exfoliated from such MOFs exhibit brilliant photochromism perceptible to the naked eye. We show that the MONs can be entrapped in an ormosil matrix for the development of polymer composites that display uncomplicated thermal bleaching kinetics. Thus, immobilization of photochromic 2D MONs containing void spaces in a polymer matrix for ophthalmic lens and other applications is compellingly demonstrated.

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 H6TPA based on the 1,3,5-triphenylbenzene core to access layered MOFs by metal-assisted self-assembly. Treatment of H6TPA with In3+ 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.H6TPA, 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.

MediaChromes are molecular systems characterized by a rigid aromatic backbone bearing electron-donating (D) and accepting (A) groups conjugated in the so-called push-pull D–π–A fashion. Electronic excitation of such systems results in increase of the dipole moment in the excited state due to intramolecular charge transfer (ICT), leading to remarkable excited-state properties. We have investigated fluorosolvatochromic and halochromic (acidochromic) behavior of carbazolo-phenazine (CP), a new bipolar molecular system characterized by conjoined donor carbazole and acceptor phenazine heterocycles. CP is shown to exhibit exceptional fluoromediachromic behavior due to ICT in the excited state; the difference in the dipole moments of ground- and singlet-excited states of CP is ca. 3.6 D units. ICT occurs also in the ground state upon protonation, which manifests in brilliant naked-eye visible violet coloration, namely, acidochromism.

Access to novel bipolar carbazolo-phenazines is demonstrated by one-pot IBX-initiated multistep cascade, which involves oxidation of 2-naphthols to 1,2-naphthoquinones, Diels-Alder cycloaddition, aerial dehydroaromatization and cyclocondensation. With unprotected dienic indolylacrylates, N H⋯O hydrogen bonding in the transition state controls the product selectivity almost exclusively in favor of Diels-Alder cycloaddition over the competing Michael addition. The utility of one of the carbazolo-phenzines as applied to selective naked-eye sensing of fluoride is demonstrated.

Locking-in the conformation of supramolecular assemblies provides a new avenue to regulate the (opto)electronic properties of robust nanoscale objects. In the present contribution, we show that the covalent tethering of a perylene bisimide (PBI)-derived supramolecular polymer with a molecular locker enables the formation of a locked superstructure equipped with emergent structure-function relationships. Experiments that exploit variable-temperature ground-state electronic absorption spectroscopy unambiguously demonstrate that the excitonic coupling between nearest neighboring units in the tethered superstructure is preserved at a temperature (371 K) where the pristine, non-covalent assembly exists exclusively in a molecularly dissolved state. A close examination of the solid-state morphologies reveals that the locked superstructure engenders the formation of hierarchical 1D materials which are not achievable by unlocked assemblies. To complement these structural attributes, we further demonstrate that covalently tethering a supramolecular polymer built from PBI subunits enables the emergence of electronic properties not evidenced in non-covalent assemblies. Using cyclic voltammetry experiments, the elucidation of the potentiometric properties of the locked superstructure reveals a 100-mV stabilization of the conduction band energy when compared to that recorded for the non-covalent assembly.

Trigonal 3-connecting imidazole-annulated triptycene triacid (H3TPA) is a molecular module that is programmed for orthogonal self-assembly. Upon treatment with metal salts such as CoCl2, Mn(NO3)2, Zn(NO3)2 and Cd(NO3)2, highly porous isostructural MOFs are obtained in which the TPA linker undergoes metal–ligand coordination and hydrogen bonding through carboxylate groups and imidazole moieties, respectively. The MOFs are constructed by hydrogen bond-mediated offset stacking of porous (3,3) honeycomb layers formed by the self-assembly of TPA with 3-connecting triangular bimetallic SBUs. We show that the interlayer hydrogen bonds can be disrupted by solvents such as DMSO, leading to solvent-induced delamination of 2D metal–organic nanosheets. The delamination is signaled by turn-on of fluorescence, which is suppressed in the bulk state. Indeed, the extent of exfoliation with different solvents – as reflected from fluorescence quantum yields as well as solvent-induced shifts in emission maxima – can be nicely correlated with Gutmann's solvent DN numbers, which are a measure of the ability of solvents to accept hydrogens in hydrogen bonds. The results demonstrate how the bulk materials with layered structures can be (i) engineered in a ‘bottom-up’ approach by orthogonal self-assembly of an organic linker created by de novo design and (ii) in turn be exfoliated in a ‘top-down’ approach by solvent-induced ultrasonication. As bulk materials, the hydrogen-bonded MOFs lend themselves to selective as well as high adsorption of CO2 under ambient conditions as a result of the nitrogenous environment of pores conferred by the benzimidazole moieties.

Diarylbenzo- and naphthopyrans are one of the important and well-known classses of organic photochromic systems. Their photolysis leads to highly colored o -quinonoid intermediates. The mechanistic details and spectrokinectic properties of the photogenerated colored intermediates have been extensively studied. Accordingly, stereoelectronic effects have been shown to play a crucial role in controlling properties such as spectrokinetics, fatigue resistance and colorability. Much research continues unabated to develop photochromic materials with properties that surpass the existing ones for application in ophthalmic lenses as well as optical data storage devices. In the realm of materials chemistry, the diarylpyran unit is presently exploited as an essential design element to produce functional materials. Given the facile synthetic access to molecular systems that respond to light as an external stimulus leading to widely distinct physicochemical properties, we believe that more exciting research based on diarylpyran photochemistry will germinate in materials chemistry and biology in the near future. This review with a coverage of the phenomenon since its inception to the present state of application for functional materials–through synthesis and mechanistic details–should serve as an invaluable resource to an uninhibited chemist to exploit the photochromism of diarylpyrans for diverse functions/applications.

Biginelli reaction involves acid-catalyzed one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones (DHPMs) using easily-accessible starting materials, namely, aldehyde, active methylene compound and (thio)urea. DHPMs have stimulated resurgence of interest in the past two decades due to their wide ranging pharmacological activities and presence of diverse natural products. Presently, green approaches to asymmetric Biginelli reaction are being explored for bioactive chiral DHPMs. In materials chemistry, DHPMs are increasingly finding applications in the development of materials such as polymers, adhesives, fabric dyes, etc. In view of the ease with which the Biginelli reaction is conducted, many exciting prospects await for its exploitation in various fields.

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 H4 BTA with Zn(NO3 )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.

Diarylbenzopyrans and their annulated derivatives are a class of photochromic compounds, which have been extensively investigated for photochromism. In fact, some naphthopyrans are employed industrially in ophthalmic lens applications. Photoirradiation of 2,2-diphenylbenzopyran leads to the formation of colored o-quinonoid intermediates. The latter revert too rapidly to allow their observation only at low temperatures (173–263 K). Annulation and other strategies are exploited to observe the colored o-quinonoid intermediates that persist for a few minutes at room temperature. We have examined photochromism of a set of silyloxy-substituted 2,2-diphenylbenzo- and naphthopyrans to explore how the mesomeric effects transmitted by the strongly e-donating silyloxy group substituted in the ring and at phenyl groups manifest in the spectrokinetic properties of their photogenerated colored o-quinonoid intermediates. We show that silyloxy substitution in the benzopyran ring leads to remarkable stabilization of the colored intermediates to enable their persistence at 283 K for a few minutes. In contrast, similar substitution in the C2-phenyl rings destabilizes the colored intermediates.

Phenyl/arylethynylation of 2,2-diphenylbenzopyrans (i.e., chromenes) permits modulation of the spectral properties of photogenerated colored o-quinonoid intermediates, and brings about remarkable stabilization of the latter, rendering the phenomenon of photochromism observable readily at room temperature. Modification of the spectrokinetic properties of the o-quinonoid intermediates are a consequence of resonance effects, with the substituent effects being relayed nicely from one end of the molecule to the other. Functionalization at the 6-position of the chromene nucleus leads to more redshifted absorptions as well as to increased lifetimes of the colored species when compared with functionalization at the 7-position. Given that photochromism is observable in the parent chromene at only low temperatures, phenyl/arylethynylation, which precludes steric inhibition of resonance, is demonstrated to render bleaching kinetics of the colored intermediates that approach those reported for industrially relevant diarylnaphthopyrans.

Anionic merocyanine colored dyes based on (4-nitro/cyanophenyl)-substituted thiazol-2-hydrazides (THAs), which are donor-π-acceptor type push-pull molecular systems and characterized with a strong intramolecular charge transfer (ICT) in the ground state, have been examined as reverse solvatochromic systems. THAs are shown to exhibit both positive and negative solvatochromism with a reversal of the latter occurring for solvents of ET(30) value of ca. 45. The observed behavior is traceable to relative contributions of benzenoid and quinonoid resonance forms of the THAs and their stabilization by solvents to different degrees. Solvatochromic studies in binary mixtures of polar aprotic and protic solvents reveal that the latter are always preferred in the solvation microsphere of THAs. Multiparametric treatment of the ET(dye) parameters by Catalan and Kamlet-Taft linear solvation energy relationships (LSERs) compellingly bears out the remarkable influence of hydrogen-bond donating (HBD) acidity, dipolarity and polarizability of the media on the spectral properties of the anionic dyes; hydrogen-bond accepting (HBA) basicity of the solvents is found to influence the least.

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 (

A highly luminescent and water-stable homochiral Zn-MOF, i.e., Zn-PLA, developed based on pyrene-tetraacetic, selectively senses histidine amongst all other amino acids and also differentiates d and l isomers, as revealed by quenching of the fluorescence of its aqueous suspension., A highly luminescent and water-stable homochiral Zn-MOF, i.e., Zn-PLA, has been developed based on a pyrene-tetralactic acid, which inherently features concave shapes for guest inclusion, to explore sensing of amino acids by fluorescence quenching; the solid-state fluorescence quantum yield of the MOF was found to be 46%. The fluorescence of an aqueous suspension of Zn-PLA was shown to be quenched specifically by histidine amongst all the other amino acids. Selective sensing of histidine is of prime importance due to its relevance in a variety of biological functions. The lack of quenching of fluorescence of Zn-PLA by all the amino acids other than histidine has been rationalized based on the exchange of the cationic dimethylammonium species in the MOF crystals with histidine that is protonated in water; the latter is envisaged to quench the fluorescence via charge transfer in the excited state. Furthermore, the homochiral crystals of Zn-PLA were found to permit enantiodiscrimination in the quenching by the d- and l-forms such that the ratio of enantioselectivity, i.e., Kd/Kl, is 1.8, as determined by Stern–Volmer quenching plots. The highly selective as well as enantiodifferentiating sensing of amino acids by MOFs is unprecedented for any sensor type.