Mixed Molecular Crystals Research Articles

Luminescent anthracene-based elastic mixed molecular crystals for flexible and FRET-assisted optical waveguides

Abstract Optical waveguides based on elastic molecular crystals are of interest as flexible and compact optical communication materials. Low emission efficiency is often a problem in terms of communication signal strength, and an increase in the loss factor α due to fluorescence reabsorption in the crystal reduces the photon transport efficiency. Here, we report the development and improvement of Förster resonance energy transfer (FRET)-assisted optical waveguides using anthracene-based elastic mixed molecular crystals. 9,10-Dicyanoanthracene was selected as the dopant for solution-grown crystallization of 9,10-dibromoanthracene. 1H NMR measurements of the obtained crystals showed that the acceptor doping to be 1% to 5%. Elastic behavior was observed even when doped with a few percent of the acceptor. The quantum efficiency was 0.016, a dramatic improvement over the previous luminescent elastic mixed molecular crystals (0.004). The α value (92 dB/cm) of this crystal containing 1%-doped 9,10-dicyanoanthracene is much lower than that of the crystal consisting of only 9,10-dibromoanthracene (1258 dB/cm) due to the reduced reabsorption in the FRET system. We have demonstrated a practical approach toward developing improved fluorescent, highly efficient, and flexible optical waveguides by constructing the mixed crystal structure by selecting appropriate acceptor molecules and their low doping ratios.

Flexible Förster resonance energy transfer‐assisted optical waveguide based on elastic mixed molecular crystals

AbstractFlexible molecular crystal waveguides based on elastic molecular crystals (EMCs) are essential in flexible and compact optical materials. An increased loss coefficient α due to self‐absorption is often a problem in optical waveguides (OWGs) of fluorescent chemical materials waveguiding photons in active mode. Herein, the development of anthracene‐based elastic mixed molecular crystals (EMMCs) is reported for Förster Resonance Energy Transfer (FRET)‐assisted OWG. To yield a FRET crystal system based on elastic molecular crystals, 1%–5% acceptor doping for fluorescent molecular crystals of 9,10‐dibromoanthracene 1 was successful by selecting the same regioisomer having electron‐withdrawing group, 9,10‐diformylanthracene 2, as a dopant. In addition to conversion to the mixed system, there is a difference in the elastic modulus and hardness in EMC C1 and EMMC C2@1. However, the elastic behaviour was also shown in a few percent doping of the acceptor. The α value of this EMMC, composed of 1 including 1% of 2 (0.0077 dB/μm), is much lower than that of EMC composed of 1 (0.1258 dB/μm) because of reducing self‐absorption in the FRET system. An efficient and flexible OWG was successfully developed by selecting an appropriate acceptor molecule and its low doping rate for mixed crystal construction. This method is a practical approach in various functional and flexible crystal systems.

Biogenic Guanine Crystals Are Solid Solutions of Guanine and Other Purine Metabolites.

Highly reflective crystals of the nucleotide base guanine are widely distributed in animal coloration and visual systems. Organisms precisely control the morphology and organization of the crystals to optimize different optical effects, but little is known about how this is achieved. Here we examine a fundamental question that has remained unanswered after over 100 years of research on guanine: what are the crystals made of? Using solution-state and solid-state chemical techniques coupled with structural analysis by powder XRD and solid-state NMR, we compare the purine compositions and the structures of seven biogenic guanine crystals with different crystal morphologies, testing the hypothesis that intracrystalline dopants influence the crystal shape. We find that biogenic “guanine” crystals are not pure crystals but molecular alloys (aka solid solutions and mixed crystals) of guanine, hypoxanthine, and sometimes xanthine. Guanine host crystals occlude homogeneous mixtures of other purines, sometimes in remarkably large amounts (up to 20% of hypoxanthine), without significantly altering the crystal structure of the guanine host. We find no correlation between the biogenic crystal morphology and dopant content and conclude that dopants do not dictate the crystal morphology of the guanine host. The ability of guanine crystals to host other molecules enables animals to build physiologically “cheaper” crystals from mixtures of metabolically available purines, without impeding optical functionality. The exceptional levels of doping in biogenic guanine offer inspiration for the design of mixed molecular crystals that incorporate multiple functionalities in a single material.

Close-Packed Arrangements of Flat-On Free-Base Porphyrins Driven by van der Waals Epitaxy

The functionality of low dimensional phases of porphyrins in optical, chemical, electrical, and multimodal combinational devices is strictly related to the control of molecular orientation within the produced solid layers. A promising strategy to drive the growth of adlayers with predictable structural properties relies on the template effect exerted by the substrate. Tetraphenyl porphyrins, being disc-shaped objects, can be adsorbed on a crystal surface by taking on different geometries. An edge-on configuration is adopted when the interactions among molecules overtake those between molecules and substrate, whereas a flat-on configuration is adopted when molecule-substrate interaction is dominant, with the weaker intermolecular interaction driving a close-packed geometry in the adlayer. For this latter reason, square and/or hexagonal lattice symmetries of physisorbed porphyrin layers are disclosed on highly interacting metal substrates such as Au(111). Unfortunately, metal substrates modify the intrinsic properties of porphyrins by suppressing many of their functionalities. To overcome this drawback, here we report the selective growth of porphyrins in a flat-on arrangement on the chiral (110) cleavage surface of the mixed molecular organic crystal formed by 2,5-diketopiperazine and fumaric acid in a 1:1 mole ratio. The energetic advantage ensured by the interaction with the insulating substrate drives the prevalent formation of domains with a square symmetry, which is retained from monolayer to multilayers. However, rare domains with a hexagonal symmetry are revealed and analyzed by high-resolution scanning probe microscopic techniques. The experimental structural analysis performed at the nanoscale, combined with ab initio calculations, allowed us to demonstrate that the molecular architectures we found arise from the simultaneous fulfillment of site adsorption energy maximization driven by peculiar molecular motifs of the selected substrate, close-packing criteria, and epitaxial locking to the substrate surface by weak van der Waals interactions.

Electrical Conductivity-Relay between Organic Charge-Transfer and Radical Salts toward Conductive Additive-Free Rechargeable Battery.

In recent years, organic electrode materials have been strongly considered for use in sustainable batteries. However, most organic electrode materials have low electrical conductivity and require a lot of conductive additives, which decrease the effective capacity based on the entire electrode weight/volume. In this study, we propose a novel electrical conductivity-relay system that imparts electrical conductivity to organic small molecular electrodes without any conductive additive throughout the charge/discharge cycles. It consists of the combination of the charge-transfer phenomenon in a pristine state and the formation of organic radical salts in redox states. Herein, we demonstrate this electrical conductivity-relay system using a simply mixed molecular crystal couple of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) as a cathode without any conductive additive and aqueous sodium bromide as an electrolyte. During charge/discharge, the electrical conductivity of the cathode is supported by charge-transfer at the TTF/TCNQ interface and (TTF)Brn (0.7 ≤ n ≤ 0.8) and NaTCNQ radical salts, and the cathode exhibits a specific capacity of 112 mAh g-1 and a retention rate of 80.7% at the 30th cycle. Furthermore, the molecular crystal couple electrode of TTF and TCNQ shows better charge/discharge performance than the pure charge-transfer complex electrode, indicating that this system expands candidates for organic electrode materials to various pairs and mixing ratios of small molecules that do not form charge-transfer complexes.

Electrochemical synthesis of phthalocyanine–molecular mixed crystals in dilute solution

We report the synthesis of a new phthalocyanine–molecular mixed crystal, TPP[CrxCo1−x(Pc)(CN)2]2, by an electrochemical method using a dilute solution. For the whole range of x, the chemical composition of the obtained single crystals is homogeneous and nearly equal to the nominal one in the dilution solution, as revealed by energy-dispersive X-ray spectroscopy. We have found that the concentrations of the starting materials play a vital role in the homogeneous growth of mixed crystals.

Expanding the Scope of Molecular Mixed Crystals Enabled by Three Component Solid Solutions

Crystalline solid solutions (mixed crystals) formed from molecular compounds are a long-known class of multicomponent solids whose composition can be varied, at least in principle, in continuum. Should such control occur in mixed crystals over a wide range of compositions, then it follows that some structural and physical properties would also be tuned in continuum. However, the potential utility of mixed crystals as molecular materials has been hindered by their limited accessibility across a broad range of compositions and difficulties associated with their preparation. We address these matters herein through the use of mechanochemistry and solution crystallization to study three-component mixed crystals. In addition to expanding the composition range of previously known mixed crystals, we demonstrate that anthracene (ant) and phenazine (pnz), which tend to crystallize as separate phases, form mixed crystals with a third molecule which is miscible in both, namely, acridine (acr). The three-component mix...

Kitaigorodsky revisited: polymorphism and mixed crystals of acridine/phenazine.

Non-stoichiometric molecular mixed crystals have potential as functional materials, the properties of which can be tailored by rationally changing their composition. The guidelines for their preparation were summarized over thirty years ago by Alexander Kitaigorodsky. Here those principles are revised in light of new studies on the acridine/phenazine system, and solvent-assisted grinding is presented as a convenient synthetic procedure to afford a more homogeneous product than traditional solvent-evaporation methods. Finally, the proposed prerequisite of crystal isostructurality/isomorphicity for the pure compounds, which seems to be violated in the present case, is discussed.

Orientational glass: Full replica symmetry breaking in generalized spin glass-like models without reflection symmetry

We investigate near the point of glass transition the expansion of the free energy corresponding to the generalized Sherrington--Kirkpatrick model with arbitrary diagonal operators U standing instead of Ising spins. We focus on the case when U is an operator with broken reflection symmetry. Such a consideration is important for understanding the behavior of spin-glass-like phases in a number of real physical systems, mainly in orientational glasses in mixed molecular crystals which present just the case. We build explicitly a full replica symmetry breaking (FRSB) solution of the equations for the orientational glass order parameters when the non-symmetric part of U is small. This particular result presents a counterexample in the context of usually adopted conjecture of the absence of FRSB solution in systems with no reflection symmetry.

Room-temperature solid-state maser

The invention of the laser has resulted in many innovations, and the device has become ubiquitous. However, the maser, which amplifies microwave radiation rather than visible light, has not had as large an impact, despite being instrumental in the laser's birth. The maser's relative obscurity has mainly been due to the inconvenience of the operating conditions needed for its various realizations: atomic and free-electron masers require vacuum chambers and pumping; and solid-state masers, although they excel as low-noise amplifiers and are occasionally incorporated in ultrastable oscillators, typically require cryogenic refrigeration. Most realizations of masers also require strong magnets, magnetic shielding or both. Overcoming these various obstacles would pave the way for improvements such as more-sensitive chemical assays, more-precise determinations of biomolecular structure and function, and more-accurate medical diagnostics (including tomography) based on enhanced magnetic resonance spectrometers incorporating maser amplifiers and oscillators. Here we report the experimental demonstration of a solid-state maser operating at room temperature in pulsed mode. It works on a laboratory bench, in air, in the terrestrial magnetic field and amplifies at around 1.45 gigahertz. In contrast to the cryogenic ruby maser, in our maser the gain medium is an organic mixed molecular crystal, p-terphenyl doped with pentacene, the latter being photo-excited by yellow light. The maser's pumping mechanism exploits spin-selective molecular intersystem crossing into pentacene's triplet ground state. When configured as an oscillator, the solid-state maser's measured output power of around -10 decibel milliwatts is approximately 100 million times greater than that of an atomic hydrogen maser, which oscillates at a similar frequency (about 1.42 gigahertz). By exploiting the high levels of spin polarization readily generated by intersystem crossing in photo-excited pentacene and other aromatic molecules, this new type of maser seems to be capable of amplifying with a residual noise temperature far below room temperature.

Magnetic interactions in nonalternant mixed molecular radical crystals and mixed ion radical crystals

AbstractThe character and the magnitude of magnetic interaction in 1‐D stacks of mixed molecular radical crystals (MMRCs) and mixed molecular ion radical crystals (MMIRCs) of alternant aromatic hydrocarbons have already been studied theoretically. In this work nonalternant 1‐D stacks built of π‐electronic subunits are considered. Some of the elementary units are singly or doubly charged (MMIRCs), others are neutral (MMRCs); alternant and nonalternant subunits are used as building blocks. Qualitative investigations based on the band theory of exchange effects in solid organic open‐shell systems are carried out. The contributions to the effective exchange integral Jeff: direct (Hund's), kinetic and indirect, are calculated. Structures of 1:1 MMRCs and MMIRCs with positive value of the integral Jeff, accounting for the spin exchange of the electrons in the half‐filled band, i.e., with ferromagnetic ground state, are suggested. The influence of different parameters (molecular size, presence of heteroatoms, localization versus delocalization, alternant versus nonalternant, charge, etc.) on the magnitude of the net exchange is discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Structure and Magnetic Interaction in Organic Radical Crystals. 6. Spin-Transfer Crystals: A Theoretical Study

The nature of the spin exchange interaction in half-filled bands of one-dimensional (1-D) stacks of mixed molecular radical crystals (MMRC) with the general formula ...R * ....H....R * ....H.... (R * is a π radical, and H is a diamagnetic polycyclic molecule) has been investigated theoretically. The intermolecular interaction is accompanied by spin transfer from R * to H and determines the spin-spin interaction within the 1-D stacks of the MMRCs. The nature and the magnitude of the magnetic interaction in these structures are investigated using the band theory. At a given topology and geometry of the MMRC stacks the antiferromagnetic interaction in the parent molecular radical crystals...R * ....R * .... becomes ferromagnetic in the MMRCs and vice versa. It was found that the effective exchange integral J e f f of MMRCs, belonging to a given group with one and the same radical but different H, is proportional to the diamagnetic susceptibility Χ(H). A new topological effect of the exchange interaction in MMRCs is formulated.

Migration of triplet excitations of complex molecules in disordered media and in systems with a confined geometry

The paper presents the results of analysis of migration of a triplet excitation in inhomogeneous media such as mixed molecular crystals, solid solutions of organic compounds in low-molecular solvents and polymers, as well as porous matrices activated by complex molecules. The experimental data obtained under conditions of steady-state excitation and time resolution are analyzed within different approaches used for describing the energy transport in disordered systems.

Nature of the Magnetic Interaction in Organic Radical Crystals. 4. Magnetic Interaction in Mixed Molecular Radical Crystals

The nature of the spin exchange interaction within the half-filled band of one-dimensional stacks of 1-D mixed molecular radical crystals (MMRC) with the general formula: ······PAH····R•·····PAH····R•·····PAH····R•·····PAH····R•·····(R•, radical; PAH, diamagnetic polycyclic aromatic hydrocarbon) was investigated theoretically. The various contributions to the Heisenberg effective exchange integral, the direct (Coulomb), kinetic, and indirect spin exchange for the MMRCs, are evaluated quantitatively, using band theory, and are compared with those of the corresponding parent molecular radical crystals (MRC, without hydrocarbons). The nature and the magnitude of the exchange interaction in MMRCs are significantly different in comparison with the parent MRCs: the ferromagnetic interaction in MRC changes into antiferromagnetic in MMRCs and vice versa.

Solid-state study of cyclic thiohydroxamic acids: 1-hydroxy-2(1H)-pyridinethione and 3-hydroxy-4-methyl-2(3H)- thiazolethione

Cyclic thiohydroxamic acids such as 1-hydroxy-2(1H)-pyridinethione (1) and 3-hydroxy-4-methyl-2(3H)-thiazolethione (2) find diverse applications as fungicides and alkoxy-radical precursors. Selected physico-chemical properties such as thermal stability can be rationalized by examination of their solid-state structures. We report here a new hydrated form of 2 (denoted 3) that displays enhanced thermal stability and solubility compared with the anhydrous form. These enhanced properties may be attributed to the ionic nature of 3 in the solid state. The oxidized form of 1, 2,2′-dithiobis(pyridine-1-oxide) (DTPO, 4) also finds application as a fungicide. Attempts to produce an analogous disulfide from 2 produced a new thioether product, 3-hydroxy-4-{[3-oxo-4-methyl-2(3H)-thiazol-2-yl]thio}-2(3H)-thiazolethione (5). Co-crystallization of 1 with 2 produced a mixed molecular crystal (6) containing both DTPO and a new thioether derivative of 2, 5,5′-thiobis[3-hydroxy-4-methyl-2(3H)-thiazolethione]. The supramolecular complementarity of the two components gives rise to discrete hydrogen-bonded dimers within the co-crystal. Copyright © 2000 John Wiley & Sons, Ltd. Additional material for this paper is available from the epoc website at http://www.wiley.com/epoc

Thermoadjustable molecular alloys for energy storage and thermal protection: fundamental aspects and applications

The formation, stability, and phase change behaviour of molecular mixed crystals have been studied. More than 100 phase diagrams have been determined within various families of substances. This permits the correlation of thermodynamic properties of A 1-x B x with the degree of isomorphism of the pure substances A and B. This work has led to the development of important rules which allow the prediction of syncrystallisation. Moreover, if A and B correctly chosen it is possible to obtain molecular alloys which have a high latent heat of phase transition. They are thermoadjustable by means of the composition. They can provide suitable molecular alloy phse change materials in the field of energy storage and/or thermal protection

Optical Spectroscopy of Single Pentacene Molecules in a Naphthalene Crystal

Abstract Dilute mixed molecular crystals of pentacene in naphthalene were prepared by cosublimation of the two compounds. Single pentacene molecules could be detected in the thin platelets by fluorescence excitation spectroscopy. By analyzing the exponential decay of the fluorescence intensity autocorrelation function for a single molecule in the microsecond time regime we could determine the population (k23) and depopulation (k31) rate of the metastable triplet state. The increase of the population rate k23 by a factor of ∼ 4 when compared to pentacene in p-terphenyl is responsible for the weaker fluorescence signals of single pentacene molecules in naphthalene crystals.

Microscopic approach to the quadrupolar-glass problem.

We report results of the microscopic approach to the quadrupolar-glass problem in mixed molecular crystals formed by linear and spherical molecules. The temperature-concentration analysis of the orientational order parameter derived from NMR spectroscopy data is given on the two-component vector model of site-diluted ortho-para-hydrogen-type mixtures. Two features of the short-range orientational ordering, characteristically different from the spin-glass case, namely, incomplete freezing and residual ordering, respectively, at low and high temperatures are discussed. It is established that the first effect is due to spatial correlated local molecular and reaction fields and the second caused by quadrupolar intrinsic field. A zero-field freezing temperature is estimated taking into account the threshold concentration for quadrupolar glass formation. \textcopyright{} 1996 The American Physical Society.

Theory and calculation of singlet excitation energy transfer in mixed molecular crystals

A theoretical study of excitation energy transport among dipole–dipole interacting guests in mixed molecular crystals is carried out. To describe the temporal evolution of the excited state population, we derive a microscopic theory which treats an ensemble of dipole–dipole interacting guest molecules homogeneously distributed among two inequivalent sites of a host lattice. The theory is based on a generalized effective Hamiltonian accounting for intramolecular excited state depopulation and excitation energy transfer. The results are applied to the analysis of experimental data obtained from transient grating experiments in p-terphenyl:pentacene mixed molecular crystals.

Molecular mixed crystals of neopentane derivatives. A comparative analysis of three binary systems showing crossed isodimorphism

A thermodynamic analysis is presented for three binary systems out of the four neopentane derivatives: neopentylglycol (NPG), pentaglycerin (PG), pentaerythritol (PE) and tris(hydroxymethyl) aminomethane (TRIS). These substances give rise to a plastic crystalline state, which is bcc for TRIS and fcc for NPG, PG and PE. In each of the three systems considered TRIS is one of the components: they are examples of crossed isodimorphism and the estimation of metastable melting points is an essential part of the analysis. Unlike the plastic-crystalline state, miscibility in the ordered solid state is poor, with the exception of the PE side in the system PE/TRIS. In the analysis, the liquid mixtures were taken as ideal and for the plastic crystalline solutions the excess Gibbs energy was expressed in terms of the (A,B,⊖) model.