Bio-Inspired Fluorescent Calcium Sulfate for the Conservation of Gypsum Plasterwork

In this work, the potential of bio-inspired strategies for the synthesis of calcium sulfate (CaSO4·nH2O) materials for heritage conservation is explored. For this, a nonclassical multi-step crystallization mechanism to understand the effect of calcein- a fluorescent chelating agent with a high affinity for divalent cations- on the nucleation and growth of calcium sulfate phases is proposed. Moving from the nano- to the macro-scale, this strategy sets the basis for the design and production of fluorescent nano-bassanite (NB-C; CaSO4·0.5H2O), with application as a fully compatible consolidant for the conservation of historic plasterwork. Once applied to gypsum (CaSO4·2H2O) plaster specimens, cementation upon hydration of nano-bassanite results in a significant increase in mechanical strength, while intracrystalline occlusion of calcein in newly-formed gypsum cement improves its weathering resistance. Furthermore, under UV irradiation, the luminescence produced by calcein molecules occluded in gypsum crystals formed upon nano-bassanite hydration allows the easy identification of the newly deposited consolidant within the treated gypsum plaster without altering the substrate's appearance.

Ferroelastic Control of the Multicolor Emission from a Triply Doped Organic Crystal

Emission from crystalline organic solids is often quenched by nonemissive energy-transfer deexcitation processes. While dispersion of fluorophores in polymers or other hosts has been used to enhance the emission intensity, this strategy results in randomization of guest orientation and optical losses at grain boundaries. Here, we report the doping of inherently nonemissive single crystals of anilinium bromide with three fluorescent organic molecules. The doping process equips the crystal with emission characteristics that tune from blue to deep orange. The emission intensity can be reversibly modulated by ferroelastic twinning, which causes the material to function as a multiemissive force sensor. This approach opens up new pathways in the manipulation of emissive properties in organic crystals and may have substantial implications for optoelectronic devices and sensors.

Enhancement of Hydrate Stability through Substitutional Defects

Cytosine monohydrate (CM) and anhydrate crystal forms reversibly interconvert under high temperatures or high humidity conditions. Here, we demonstrate through defect engineering the ability to expand the thermal stability range of CM through the targeted creation of quantifiable defects in low-level concentrations. Twelve different molecular dyes with a variety of core structures and charges were screened as potential dopants in CM. CM-dye phases prepared with Congo red (CR), Evans blue (EB), and Azocarmine G (AG) exhibited the highest inclusion levels (up to 1.1 wt %). In these doped isomorphous materials, each dye is presumed to substitute for 4-7 cytosine molecules within the low-rugosity (102) planes of the CM matrixes, thereby creating a quantifiable substitutional defect and an impediment to the cooperative molecular motions which enable the transformation to the anhydrate. Dehydration of materials with these engineered defects requires significantly higher temperatures and proceeds with slower kinetics compared to pure CM. The CM-dye phases also exhibit a reduction in the thermal expansion along key crystallographic axes and yield dehydration products with altered particle morphologies.

Multi-layered Barium and Strontium Carbonate Structures Induced by the Small Organic Dye Acid Orange 7

The crystal growth behavior induced by small molecular additives is commonly assumed to be far less complex and rich in comparison to that obtained when using macromolecules. Herein, we demonstrate that the small organic molecule Acid Orange 7 can induce a large diversity of multi-layered barium carbonate structures. These multi-layered structures stem from the small molecule imperfectly blocking the fastest growing crystal face. By tuning the balance of growth and inhibition, we control the layer shape and thickness of the structures. Extending these strategies to strontium carbonate enables the precipitation of large quasi two-dimensional multi-layer sheets. Collectively, these findings highlight the unforeseen potential for using small organic molecules to induce the formation of complex inorganic structures.

Chiroptical anisotropy of crystals and molecules

Optical activity, a foundational part of chemistry, is not restricted to chiral molecules although generations have been instructed otherwise. A more inclusive view of optical activity is valuable because it clarifies structure-property relationships however, this view only comes into focus in measurements of oriented molecules, commonly found in crystals. Unfortunately, measurements of optical rotatory dispersion or circular dichroism in anisotropic single crystals have challenged scientists for more than two centuries. New polarimetric methods for unpacking the optical activity of crystals in general directions are still needed. Such methods are reviewed as well as some of the 'nourishment' they provide, thereby inviting to new researchers. Methods for fitting intensity measurements in terms of the constitutive tensor that manifests as the differential refraction and absorption of circularly polarized light, are described, and examples are illustrated. Single oriented molecules, as opposed to single oriented crystals, can be treated computationally. Structure-property correlations for such achiral molecules with comparatively simple electronic structures are considered as a heuristic foundation for the response of crystals that may be subject to measurement.

Effect of 2,4-dinitrophenol dye doping on tristhioureazinc(II) sulfate single crystals: a potential nonlinear optical material

Good quality single crystals of 2,4-dinitrophenol (DNP)-doped tristhioureazinc(II) sulfate (ZTS) were successfully grown by employing the simple and cost effective slow-evaporation solution technique. To study the effect of doping on various device properties, the grown single crystals were subjected to powder X-ray diffraction (PXRD), high-resolution XRD, thermogravimetric analysis (TGA), Vickers hardness testing, and UV–visible, photoluminescence (PL) and Fourier transform IR (FTIR) spectroscopy techniques. The crystal structure of DNP-doped ZTS bulk single crystals remained the same as the crystal structure of ZTS. However, the changes in intensities of the diffraction peaks in the PXRD spectra indicated the incorporation of dopants into the crystalline matrix. FTIR studies confirm the incorporation of dopants into the crystalline matrix, shown by the shifting of certain prominent absorption bands towards higher energy. This also indicated the induced useful strain due to doping, leading to charge transfer and the enhancement of nonlinear optical properties. The cut-off wavelength and optical band gap energy of pure ZTS and DNP-doped ZTS crystals were studied by UV–visible absorption spectroscopy, revealing a slight reduction in the optical band gap energy due to doping, which in turn revealed the enhancement of the optical range. PL studies revealed an enhanced optical range of photoluminescence in ZTS crystals. Second harmonic generation (SGH) studies carried out by the Kurtz powder technique revealed the enhancement of SHG value due to DNP doping. To ensure the thermal stability and mechanical strength of the grown crystals with doping (required from the point of view of device applications), TGA and Vicker's hardness studies were performed.

Monitoring the evolution of intersite and interexciton coherence in electronic excitation transfer via wave-packet interferometry

We detail an experimental strategy for tracking the generation and time-development of electronic coherence within the singly excited manifold of an energy-transfer dimer. The technique requires that the two monomers have nonparallel electronic transition-dipole moments and that these possess fixed orientations in space. It makes use of two-dimensional wave-packet interferometry (WPI or whoopee) measurements in which the A, B, C, and D pulses have respective polarizations e, e, e, and e'. In the case of energy-transfer coupling that is weak or strong compared to electronic-nuclear interactions, it is convenient to follow the evolution of intersite or interexciton coherence, respectively. Under weak coupling, e could be perpendicular to the acceptor chromophore's transition dipole moment and the unit vector e' would be perpendicular to the donor's transition dipole. Under strong coupling, e could be perpendicular to the ground-to-excited transition dipole to the lower exciton level and e' would be perpendicular to the ground-to-excited transition dipole to the upper exciton level. If the required spatial orientation can be realized for an entire ensemble, experiments of the kind proposed could be performed by either conventional four-wave-mixing or fluorescence-detected WPI methods. Alternatively, fluorescence-detected whoopee experiments of this kind could be carried out on a single energy-transfer dimer of fixed orientation. We exhibit detailed theoretical expressions for the desired WPI signal, explain the physical origin of electronic coherence detection, and show calculated observed-coherence signals for model dimers with one, two, or three internal vibrational modes per monomer and both weak and strong energy-transfer coupling.

Visualization of the effect of additives on the nanostructures of individual bio-inspired calcite crystals

Soluble additives provide a versatile strategy for controlling crystallization processes, enabling selection of properties including crystal sizes, morphologies, and structures. The additive species can also be incorporated within the crystal lattice, leading for example to enhanced mechanical properties. However, while many techniques are available for analyzing particle shape and structure, it remains challenging to characterize the structural inhomogeneities and defects introduced into individual crystals by these additives, where these govern many important material properties. Here, we exploit Bragg coherent diffraction imaging to visualize the effects of soluble additives on the internal structures of individual crystals on the nanoscale. Investigation of bio-inspired calcite crystals grown in the presence of lysine or magnesium ions reveals that while a single dislocation is observed in calcite crystals grown in the presence of lysine, magnesium ions generate complex strain patterns. Indeed, in addition to the expected homogeneous solid solution of Mg ions in the calcite lattice, we observe two zones comprising alternating lattice contractions and relaxation, where comparable alternating layers of high magnesium calcite have been observed in many magnesium calcite biominerals. Such insight into the structures of nanocomposite crystals will ultimately enable us to understand and control their properties.

Dislocations in molecular crystals

Dislocations in molecular crystals remain terra incognita. Owing to the complexity of molecular structure, dislocations in molecular crystals can be difficult to understand using only the foundational concepts devised over decades for hard materials. Herein, we review the generation, structure, and physicochemical consequences of dislocations in molecular crystals. Unlike metals, ceramics, and semiconductors, molecular crystals are often characterized by flexible building units of low symmetry, thereby limiting analysis, complicating modeling, and prompting new approaches to elucidate their role in crystallography from growth to mechanics. Such considerations affect applications ranging from plastic electronics and mechanical actuators to the tableting of pharmaceuticals.

Amino Acid Assisted Incorporation of Dye Molecules within Calcite Crystals

Biomineralisation processes invariably occur in the presence of multiple organic additives, which act in combination to give exceptional control over structures and properties. However, few synthetic studies have investigated the cooperative effects of soluble additives. This work addresses this challenge and focuses on the combined effects of amino acids and coloured dye molecules. The experiments demonstrate that strongly coloured calcite crystals only form in the presence of Brilliant Blue R (BBR) and four of the seventeen soluble amino acids, as compared with almost colourless crystals using the dye alone. The active amino acids are identified as those which themselves effectively occlude in calcite, suggesting a mechanism where they can act as chaperones for individual molecules or even aggregates of dyes molecules. These results provide new insight into crystal–additive interactions and suggest a novel strategy for generating materials with target properties.

Revisiting polarimetry near the isotropic point of an optically active, non-enantiomorphous, molecular crystal

Accurate polarimetric measurements of the optical activity of crystals along low symmetry directions are facilitated by isotropic points, frequencies where dispersion curves of eigenrays cross and the linear birefringence disappears. We report here the optical properties and structure of achiral, uniaxial (point group D_2d ) potassium trihydrogen di-(cis-4-cyclohexene-1,2-dicarboxylate) dihydrate, whose isotropic point was previously detected (S. A. Kim, C. Grieswatch, H. Küppers, Zeit. Krist. 1993; 208:219-222) and exploited for a singular measurement of optical activity normal to the optic axis. The crystal structure associated with the aforementioned study was never published. We report it here, confirming the space group assignment I 4¯c2, along with the frequency dependence of the fundamental optical properties and the constitutive tensors by fitting optical dispersion relations to measured Mueller matrix spectra. k-Space maps of circular birefringence and of the Mueller matrix near the isotropic wavelength are measured and simulated. The signs of optical rotation are correlated with the absolute crystallographic directions.

A H-shaped heterometallic Sn4Au4 system with guest-tuneable multicolour and selective luminescence sensing properties

A H-shaped heterometallic Sn₄Au₄ system displays guest-dependent optical properties and its stimuli-responsive methylred-rich crystals are sensitive towards pH variations.

Effect of phenol red dye on monocrystal growth, crystalline perfection, and optical and dielectric properties of zinc (tris) thiourea sulfate

In this work, the growth of large size (∼25 × 29 × 5 mm and ∼25 × 24 × 6 mm) colorful single crystals of zinc (tris) thiourea sulfate (ZTS) in the presence of 0.05–2 wt% phenol red (PR) dye was achieved using a simple and low-cost technique. Powder X-ray diffraction patterns confirm the presence of PR dye, which is indicated by an enhancement of the Raman peak intensities, a shift in their position and the appearance of a few extra peaks. The quality of the grown crystals was assessed by high-resolution X-ray diffraction, which shows that the crystalline perfection of 1 wt% PR-dyed ZTS crystals is better than that of 2 wt% PR-dyed crystals. The measured UV–vis absorbance spectra show two additional, strong absorption bands at ∼430 and 558 nm in the dyed crystals, due to the presence of PR dye, along with a band at ∼276 nm which is present for all crystals but is slightly shifted for the dyed crystals. Photoluminescence spectra were recorded at two excitation wavelengths (λexc= 310 and 385 nm). The luminescence intensity is found to be enriched in dyed crystals, with some extra emission bands. An enhancement in the value of the dielectric constant and a.c. electrical conductivity was also observed in the dyed ZTS crystals.