From Solution to Thin Film: Molecular Assembly of π-Conjugated Systems and Impact on (Opto)electronic Properties.

The assembly of conjugated organic molecules from solution to solid-state plays a critical role in determining the thin film morphology and optoelectronic properties of solution-processed organic electronics and photovoltaics. During evaporative solution processing, π-conjugated systems can assemble via various forms of intermolecular interactions, forming distinct aggregate structures that can drastically tune the charge transport landscape in the solid-state. In blend systems composed of donor polymer and acceptor molecules, assembly of neat materials couples with phase separation and crystallization processes, leading to complex phase transition pathways which govern the blend film morphology. In this review, we provide an in-depth review of molecular assembly processes in neat conjugated polymers and nonfullerene small molecule acceptors and discuss their impact on the thin film morphology and optoelectronic properties. We then shift our focus to blend systems relevant to organic solar cells and discuss the fundamentals of phase transition and highlight how the assembly of neat materials and processing conditions can affect blend morphology and device performance.

Nanoscale Dodecahedral and Fullerene-Type Organoboroxine and Borazine Cages from Planar Building Units.

Boroxine- and borazine-cage analogs to C20, C60, and C70 were calculated and compared in terms of structure, strain indicators, and physical properties relevant to nanoscale applications. The results show C60 and C70 type cages are less strained than the smaller congener, primarily due to minimized bending in the B-arylene-B segments. The smallest cage calculated has a diameter of 2.4 nm, which increases up to 4.9 nm by either variation of the polyhedron (C20 < C60 < C70-type cage) or organic spacer elongation between boron centers. All calculated cages are porous (apertures ranging from 0.6 to 1.9 nm). Molecular electrostatic potential and Hirshfeld population analysis revealed both nucleophilic and electrophilic sites in the interior and exterior cage surfaces. HOMO-LUMO gaps range from 3.98 to 4.89 eV and 5.10-5.18 eV for the boroxine- and borazine-cages, respectively. Our findings provide insights into the design and properties of highly porous boroxine and borazine cages for nanoscience.

Mechanical properties and peculiarities of molecular crystals.

In the last century, molecular crystals functioned predominantly as a means for determining the molecular structures via X-ray diffraction, albeit as the century came to a close the response of molecular crystals to electric, magnetic, and light fields revealed that the physical properties of molecular crystals were as rich as the diversity of molecules themselves. In this century, the mechanical properties of molecular crystals have continued to enhance our understanding of the colligative responses of weakly bound molecules to internal frustration and applied forces. Here, the authors review the main themes of research that have developed in recent decades, prefaced by an overview of the particular considerations that distinguish molecular crystals from traditional materials such as metals and ceramics. Many molecular crystals will deform themselves as they grow under some conditions. Whether they respond to intrinsic stress or external forces or interactions among the fields of growing crystals remains an open question. Photoreactivity in single crystals has been a leading theme in organic solid-state chemistry; however, the focus of research has been traditionally on reaction stereo- and regio-specificity. However, as light-induced chemistry builds stress in crystals anisotropically, all types of motions can be actuated. The correlation between photochemistry and the responses of single crystals-jumping, twisting, fracturing, delaminating, rocking, and rolling-has become a well-defined field of research in its own right: photomechanics. The advancement of our understanding requires theoretical and high-performance computations. Computational crystallography not only supports interpretations of mechanical responses, but predicts the responses itself. This requires the engagement of classical force-field based molecular dynamics simulations, density functional theory-based approaches, and the use of machine learning to divine patterns to which algorithms can be better suited than people. The integration of mechanics with the transport of electrons and photons is considered for practical applications in flexible organic electronics and photonics. Dynamic crystals that respond rapidly and reversibly to heat and light can function as switches and actuators. Progress in identifying efficient shape-shifting crystals is also discussed. Finally, the importance of mechanical properties to milling and tableting of pharmaceuticals in an industry still dominated by active ingredients composed of small molecule crystals is reviewed. A dearth of data on the strength, hardness, Young's modulus, and fracture toughness of molecular crystals underscores the need for refinement of measurement techniques and conceptual tools. The need for benchmark data is emphasized throughout.

Mechanochemical Mediated Coexistence of B←N Coordination and Hydrogen Bonding.

Mechanochemistry afforded a photoactive cocrystal via coexisting (B)O-H⋅⋅⋅N hydrogen bonds and B←N coordination. Specifically, solvent-free mechanochemical ball mill grinding and liquid-assisted grinding of a boronic acid and an alkene resulted in mixtures of hydrogen-bonded and coordinated complexes akin to mixtures of noncovalent complexes that can be obtained in solution in equilibria processes. The alkenes of the hydrogen-bonded assembly undergo an intermolecular [2+2] photodimerization in quantitative conversion, effectively reporting the outcome of the self-assembly processes. Our results suggest that interplay involving noncovalent bonds subjected to mechanochemical conditions can lead to functional solids where, in the current case, the structure composed of the weaker hydrogen bonding interactions predominates.

Structures and Reactivities of Cocrystals Involving Diboronic Acids and Bipyridines: In Situ Linker Reaction and 1D-to-2D Dimensionality Change via Crystal-to-Crystal Photodimerization.

Cocrystallizations of diboronic acids [1,3-benzenediboronic acid (1,3-bdba), 1,4-benzenediboronic acid (1,4-bdba) and 4,4'-biphenyldiboronic acid (4,4'-bphdba)] and bipyridines [1,2-bis(4-pyridyl)ethylene (bpe) and 1,2-bis(4-pyridyl)ethane (bpeta)] generated the hydrogen-bonded 1 : 2 cocrystals [(1,4-bdba)(bpe)2 ] (1), [(1,4-bdba)(bpeta)2 ] (2), [(1,3-bdba)(bpe)2 (H2 O)2 ] (3) and [(1,3-bdba)(bpeta)2 (H2 O)] (4), wherein 1,3-bdba involved hydrated assemblies. The linear extended 4,4'-bphdba exhibited the formation of 1 : 1 cocrystals [(4,4'-bphdba)(bpe)] (5) and [(4,4'-bphdba-me)(bpeta)] (6). For 6, a hemiester was generated by an in-situ linker transformation. Single-crystal X-ray diffraction revealed all structures to be sustained by B(O)-H⋅⋅⋅N, B(O)-H⋅⋅⋅O, Ow -H⋅⋅⋅O, Ow -H⋅⋅⋅N, C-H⋅⋅⋅O, C-H⋅⋅⋅N, π⋅⋅⋅π, and C-H⋅⋅⋅π interactions. The cocrystals comprise 1D, 2D, and 3D hydrogen-bonded frameworks with components that display reactivities upon cocrystal formation and within the solids. In 1 and 3, the C=C bonds of the bpe molecules undergo a [2+2] photodimerization. UV radiation of each compound resulted in quantitative conversion of bpe into cyclobutane tpcb. The reactivity involving 1 occurred via 1D-to-2D single-crystal-to-single-crystal (SCSC) transformation. Our work supports the feasibility of the diboronic acids as formidable structural and reactivity building blocks for cocrystal construction.

Modulation of π-stacking modes and photophysical properties of an organic semiconductor through isosteric cocrystallization.

We report on the control of π-stacking modes (herringbone vs slipped-stack) and photophysical properties of 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene (BP4VA), an anthracene-based organic semiconductor (OSC), by isosteric cocrystallization (i.e., the replacement of one functional group in a coformer with another of "similar" electronic structure) with 2,4,6-trihalophenols (3X-ph-OH, where X = Cl, Br, and I). Specifically, BP4VA organizes as slipped-stacks when cocrystallized with 3Cl-ph-OH and 3Br-ph-OH, while cocrystallization with 3I-ph-OH results in a herringbone mode. The photoluminescence and molecular frontier orbital energy levels of BP4VA were effectively modulated by the presence of 3X-ph-OH through cocrystallization. We envisage that the cocrystallization of OSCs with minimal changes in cocrystal formers can provide access to convenient structural and property diversification for advanced single-crystal electronics.

Phototriggered Guest Release from a Nonporous Organic Crystal: Remarkable Single-Crystal-to-Single-Crystal Transformation of a Binary Cocrystal Solvate to a Ternary Cocrystal.

The development of organic solids for applications in materials science requires a fundamental understanding of how close packing of molecules can affect structure and function. We report here nonporous organic crystals that release entrapped guest molecules upon application of UV light. We show components of binary cocrystal solvates to undergo an intermolecular photoreaction to generate ternary cocrystals that results in release of entrapped solvent molecules. The phototriggered guest release occurs in a single-crystal-to-single-crystal transformation that is in the absence pores and channels in the solid. The cocrystals are composed of a tetratopic hydrogen-bond-acceptor molecule synthesized in the solid state. The UV-light results in [2 + 2] photodimerization of an isocoumarin to generate a ternary cocrystal with cyclobutane molecules that support guest release.

Exploiting Aurophilic Interactions in a [2 + 2] Photocycloaddition: Single-Crystal Reactivity with Changes to Surface Morphology.

A single-crystal-to-single-crystal (SCSC) photodimerization is achieved using Au(I) coordination and aurophilic interactions. The rigid Au2(dppbz)(CF3COO)2 precursor (dppbz = 1,2-bis(diphenylphosphino)benzene) self-assembles with 1,2-trans-bis(4-pyridyl)ethylene (bpe) to afford a discrete [Au4(dppbz)2(bpe)2]4+ macrocycle in the solid state. The alkene undergoes a [2 + 2] photocycloaddition reaction. The photoreaction proceeds via a rare SCSC transformation in quantitative yield that generates [Au4(dppbz)2(4,4'-tpcb)]4+ (4,4'-tpcb = rctt-tetrakis(4-pyridyl)cyclobutane) stereoselectively. Mechanical strain induced by the photoreaction is evidenced by the formation of ramp features on single-crystal surfaces using scanning electron microscopy.

Exploiting Boron Coordination: B←N Bond Supports a [2+2] Photodimerization in the Solid State and Generation of a Diboron Bis-Tweezer for Benzene/Thiophene Separation.

B←N coordination supports a [2+2] photodimerization in the solid state. The bond is defined by an orthogonal interaction between stilbazole and a phenylboronic ester to enable a stereocontrolled and rapid photoreaction. The cyclobutane photoproduct affords a novel diboron bis-tweezer adduct that is used to separate a mixture of benzene and thiophene upon crystallization.

Chromic and dynamic: soft crystals of platinum(II) complexes pave the way for multi-responsive materials.

The development of smart, stimuli-responsive materials has received increased attention in the past decade for their applications as sensing technologies. This commentary discusses a timely topical review by Kato [(2024). IUCrJ, 11, 442-452] on the fabrication of multi-stimuli responsive crystals comprised of luminescent platinum(II) complexes, which exhibit intriguing chromic phenomena in response to stimuli.

Engineering Lipophilic Aggregation of Adapalene and Adamantane-Based Cocrystals via van der Waals Forces and Hydrogen Bonding.

Lipophilic aggregation using adamantanes is a widely exploited molecular property in medicinal and materials chemistry. Adamantanes are traditionally installed to molecular units via covalent bonds. However, the noncovalent installation of adamantanes has been relatively underexplored and presents the potential to bring properties associated with adamantanes to molecules without affecting their intrinsic properties (e.g., pharmacophores). Here, we systematically study a series of adamantanecarboxylic acids with varying substitution levels of methyl groups and their cocrystals with bipyridines. Specifically, single-crystal X-ray diffraction shows that while the directionality of single-component adamantanes is notably sensitive to changes in methyl substitution, hydrogen-bonded cocrystals with bipyridines show consistent and robust packing due to π-stacking predominance. Our observations are supported by Hirshfeld surface and energy framework analyses. The applicability of cocrystal formation of adamantanes bearing carboxylic acids was used to generate the first cocrystals of adapalene, an adamantane-bearing retinoid used for treating acne vulgaris. We envisage our study to inspire noncovalent (i.e., cocrystal) installation of adamantanes to generate lipophilic aggregation in multicomponent systems.

Self-Assembly of Diboronic Esters with U-Shaped Bipyridines: "Plug-in-Socket" Assemblies.

Self-assembled complexes utilizing the ditopic dative bond acceptor 1,3-diboronic acid with catechol and complementary U-shaped donors in the form of 1,8-dipyridylnaphthalenes (1,8-bis(4-pyridyl)naphthalene (DPN), 1,8-bis(4-ethylenylpyridyl)naphthalene (DEPN), and 1,8-bis(4-ethynylpyridyl)naphthalene (DAPN)) yielded discrete two-component structures. The assemblies exhibit "plug-in-socket" geometries. DFT calculations are consistent with the donor pyridyl and acceptor catecholate being electron poor and rich, respectively. The assemblies pack via π-π interactions and support the inclusion of a solvent (i.e., DPN, DAPN). The materials may form a basis for the design of complex B-based structures (e.g., supramolecular dyads).

Channel Confinement of Aromatic Petrochemicals via Aryl-Perfluoroaryl Interactions With a B←N Host.

We report channel confinement properties of an electron-deficient boron host derived from the orthogonal B←N interaction between a boronic ester and trans-pentafluorostilbazole. The boron host forms one-dimensional channels in the crystalline solid state when crystallized with common electron-rich aromatic petrochemicals (i.e., benzene, toluene, o-xylene) to form solvates and a cocrystal with stilbene. Molecular confinement of the electron-rich molecules in the solids is achieved through a combination of aryl-perfluoroaryl interactions (π-πF) and hydrogen bonds.

Exploiting the Hydrogen-Bonding Capacity of Organoboronic Acids to Direct Covalent Bond Formation in the Solid State: Templation and Catalysis of the [2 + 2] Photodimerization.

The ability of organoboronic acids to direct intermolecular [2 + 2] photodimerizations in the solid state is identified. The reactivity of 1,2-bis(4-pyridyl)ethylene (bpe) within cocrystals of discrete hydrogen-bonded molecular assemblies generates rctt-tetrakis(4-pyridyl)cyclobutane (tpcb) stereoselectively and in up to quantitative yield. Dry grinding of the boronic acids and excess bpe reveal the acids to function as supramolecular catalysts.