Polymorph crystal packing effects on charge transfer emission in the solid state

Substituent-Controlled Photophysical Responses in Dihydropyridine Derivatives and Their Application in the Detection of Volatile Organic Contaminants

In the ever-expanding realm of organic fluorophores, structurally simple and synthetically straightforward molecules with unique photophysical properties have received special attention. Among these, 1,4-dihydropyridine (DHP) is an important scaffold that permits fine-tuning of their photophysical properties through substituents on the periphery. Herein, we describe a series of solid-emissive N-substituted 2,6-dimethyl-4-methylene-1,4-dihydropyridine derivatives appended with electron-withdrawing substituents (dicyanomethylene or 2-dicyanomethylene-3-cyano-2,5-dihydrofuran) at the C-4 position and alkyl or alkylaryl groups on the DHP nitrogen. Electronic and steric tuning exerted by these substituents resulted in interesting photophysical properties such as negative solvatochromism, solidstate, and aggregation-induced emission (AIE). Theoretical calculations were carried out to explain the solvatochromic properties. Insight into the AIE properties was obtained through variable-temperature nuclear magnetic resonance and viscosity- and temperature-dependent emission studies. The variations in molecular packing in the crystal lattice with changes in the N-substituents contributed to the tuning of solid state emission properties. Detection of aromatic volatile organic compounds (VOCs) was achieved using the aggregates of the DHP derivatives. Among the VOCs, p-xylene elicited a significant enhancement in emission, allowing its detection at submicromolar levels.

Polymorphs with Remarkably Distinct Physical and/or Chemical Properties

Polymorphism in crystals is known since 1822 and the credit goes to Mitscherlich who realized the existence of different crystal structures of the same compound while working with some arsenate and phosphate salts. Later on, this phenomenon was observed also in organic crystals. With the advent of different technologies, especially the easy availability of single crystal XRD instruments, polymorphism in crystals has become a common phenomenon. Almost 37 % of compounds (single component) are polymorphic to date. As the energies of the different polymorphic forms are very close to each other, small changes in crystallization conditions might lead to different polymorphic structures. As a result, sometimes it is difficult to control polymorphism. For this reason, it is considered to be a nuisance to crystal engineering. It has been realized that the property of a material depends not only on the molecular structure but also on its crystal structure. Therefore, it is not only of interest to academia but also has widespread applications in the materials science as well as pharmaceutical industries. In this review, we have discussed polymorphism which causes significant changes in materials properties in different fields of solid-state science, such as electrical, magnetic, SHG, thermal expansion, mechanical, luminescence, color, and pharmaceutical. Therefore, this review will interest researchers from supramolecular chemistry, materials science as well as medicinal chemistry.

Solid-State Fluorescence of A Quasi-Isostructural Polymorphic Biphenyl Based Michael Addition Product

Polymorphic materials have gained significant attention owing to their fascinating physicochemical properties. Herein, a biphenyl based Michael addition product (Compound A) with an active methylene group (dimedone) was synthesized. Compound...

Multi-color mechanochromic luminescence of three polymorphic crystals of a donor–acceptor-type benzothiadiazole derivative

The three polymorphic crystals of a donor–acceptor dye exhibited different luminescence colors, which changed in response to mechanical grinding.

Improved quantitative crystal-structure comparison using powder diffractograms via anisotropic volume correction

A new anisotropic volume correction improves quantitative crystal structure comparison. Benchmarking against the 6th crystal structure prediction blind test data results in identification of two previously uncredited matching structures.

Polymorphism Dependent 9-Phosphoanthracene Derivative Exhibiting Thermally Activated Delayed Fluorescence: A Computational Investigation

Polymorphs of anthracene derivatives exhibit diverse photophysical properties that can help to develop efficient organic-based photovoltaic devices. 10-Anthryl-9-phosphoanthracene (10-APA) shows different photophysical behaviors for the solid state due to its variety in crystalline arrangement. Herein, we investigate the ground and excited-state properties of the monomer and two different polymorphs of 10-APA from first-principles. Calculations reveal that strong spin-orbit coupling (SOC) between first excited singlet state (S₁) and triplet manifolds at their S₁-optimized geometries enabling the reverse intersystem crossing (RISC). The electron-vibration coupling (Huang-Rhys factor) in the excited state is the most relevant factor here. For both ISC and RISC, a similarity in Huang-Rhys factors for the molecular vibration along the π···π stacking at low-frequency region makes the rates effective. On the other side, the nonvanishing vibronic relaxation modes provide a relatively slower RISC rate in the red crystal. However, for the red crystal, small reorganization energy (λ) and large Huang-Rhys factor toward S₁ → S₀ conversion reduce nonradiative decay, leading to a prompt fluorescence. As the feasibility of S₁ ↔ T₁ conversion increases in the yellow dimer, it allows a delay in fluorescence emission, leading to thermally activated delayed fluorescence (TADF).

Tuning the π-π overlap and charge transport in single crystals of an organic semiconductor via solvation and polymorphism

Polymorphism is a central phenomenon in materials science that often results in important differences of the electronic properties of organic crystals due to slight variations in intermolecular distances and positions. Although a large number of π-conjugated organic compounds can grow as polymorphs, it is necessary to have at disposal a series of several polymorphs of the same molecule to establish clear and predictive structure-property relationships. We report here on the occurrence of two solvates and three polymorphs in single crystalline form of the organic p-type semiconductor 2,2',6,6'-tetraphenyldipyranylidene (DIPO). When grown from chlorobenzene or toluene, the DIPO crystals spontaneously capture solvent molecules to form two pseudopolymorphic 1 : 1 binary solvates. Independently, three solvent-free DIPO polymorphs are obtained either from the vapor phase or from acetonitrile and benzene. Surprisingly, single crystal field-effect transistors (SC-FETs) reveal that the DIPO 1 : 1 binary solvate grown from chlorobenzene possesses a higher hole mobility (1.1 cm² V^-1 s^-1) than the three solvent-free polymorphs (0.02-0.64 cm² V^-1 s^-1). A refined crystallographic analysis combined with a theoretical transport model clearly shows that the higher mobility of the solvate results from an improved π-π overlap. Our observations demonstrate that solvation allows to tune the π-π overlap and transport properties of organic semiconductors by selecting appropriate solvents.

Can the solvent enhance the rate of chemical reactions through C-H/π interactions? insights from theory

The current computational study with density functional theory (DFT) shows that the rate of chemical reactions can be influenced through non-covalent C-H/π interactions between substrates and the solvent. It is shown that intramolecular carbon-carbon interaction and CO2 activation by a low valent silicon complex are both favourably affected by the explicit presence of the solvent toluene, due to C-H/π interactions between toluene and the silicon complex. Furthermore, ab initio molecular dynamics (AIMD) simulations demonstrate that even if the C-H/π interacting solvent molecule is displaced from the complex, another would quickly take its place, thus maintaining the interaction. Hence, the current work shows how non-covalent interactions between solvent and substrate can enhance the rate of the reaction and expands our understanding of the role and influence of the solvent in effecting important chemical transformations.

Generation and Reactivity Studies of Diarylmethyl Radical Pairs in Crystalline Tetraarylacetones via Laser Flash Photolysis Using Nanocrystalline Suspensions

The nanosecond electronic spectra and kinetics of the radical pairs from various crystalline tetraarylacetones were obtained using transmission laser flash photolysis methods by taking advantage of aqueous nanocrystalline suspensions in the presence of submicellar CTAB, which acts as a surface passivator. After showing that all tetraarylacetones react efficiently by a photodecarbonylation reaction in the crystalline state, we were able to detect the intermediate radical pairs within the ca. 8 ns laser pulse of our laser setup. We showed that the solid-state spectra of the radical pairs are very similar to those detected in solution, with λ_max in the 330-360 nm range. Kinetics in the solid state was observed to be biexponential and impervious to the presence of oxygen or variations in laser power. A relatively short-lived component (0.3-1.7 μs) accounts for only 3-8% of the total decay with a longer-lived component having a time constant in the range of 40-90 μs depending on the nature of the substituents.

Understanding the polymorphism-dependent emission properties of molecular crystals using a refined QM/MM approach

A refined QM/MM approach demonstrated that a monomer model is suitable for describing the emission spectra of crystals without the ππ stacking interaction. Whereas for the crystals with notable intermolecular ππ stacking interaction, the most stable trimer model (or at least a dimer model) should be used for accurately describing the corresponding emission spectra. This approach is applied to understand the emission properties of two kinds of organic polymorphs.

One-Pot Synthesis of a Mono-O,B,N-strapped BODIPY Derivative Displaying Bright Fluorescence in the Solid State

Tin(IV) catalysis allows isolation of a boron dipyrromethene derivative bearing a solitary strap around the boron center. The conditions favor internal cyclization without contamination by side products and provide high yields of product in good purity. A phenolate-based strap imposes chirality and causes geometrical distortion of the dipyrrin. Relatively strong fluorescence is observed for single crystals, evaporated films, and adsorbed layers. Single-crystal absorption and emission spectra resemble those observed from solution with contributions from a dimer.

Anion-controlled dimer distance induced unique solid-state fluorescence of cyano substituted styrene pyridinium

Molecular packing arrangements play a key role in dominating the photophysical properties of luminophores in aggregated state but fine control of the molecular packing is a great challenge. This article describes a unique cyano substituted styrene pyridinium with interesting solid-state fluorescence that can be finely tuned by simple change of counteranions. The dilute solutions of the organic salts (PyCl, PyNO₃, PyOTs and PyPh₄B) exhibit very weak fluorescence. The crystals of the organic salts (PyCl, PyNO₃, and PyOTs) show much enhanced fluorescence compared with their dilute solutions. It is interesting that the emissions changed from bluish-green to deep-blue and fluorescence quantum yields increase from 2.5% to 13.1% with the increasing of steric hindrance of the anions from chloridion, nitrate, to p-toluenesulfonate. Crystal and DFT studies reveal that the enhanced fluorescence is ascribed to the formation of dimers and bigger anions induce larger molecular separation in dimers. Tetraphenylboron anion with very large steric hindrance impedes the formation of dimers and thus results in non-fluorescent salt (PyPh₄B). Meanwhile, this unique dimeric packing endows the crystal of PyNO₃ with anisotropic fluorescence.