Correlating structure and photophysical properties in thiazolo[5,4-d]thiazole crystal derivatives for use in solid-state photonic and fluorescence-based optical devices

There is a growing demand for new fluorescent small molecule dyes for solid state applications in the photonics and optoelectronics industry. Thiazolo[5,4-d]thiazole (TTz) is an organic heterocycle moiety which has previously shown remarkable properties as a conjugated polymer and in solution-based studies. For TTz-based small molecules to be incorporated in solid-state fluorescence-based optical devices, a thorough elucidation of their structure-photophysical properties needs to be established. Herein, we have studied four TTz-based materials functionalized with alkyl appendages of varying carbon chain lengths. We report the single crystal structures of the TTz derivatives, three of which were previously unknown. The packing modes of the crystals reveal that molecular arrangements are largely governed by a chorus of synergistic intermolecular non-covalent interactions. Three crystals packed in herringbone mode and one crystal packed in slipped stacks proving that alkyl appendages modulate structural organization in TTz-based materials. Steady state and time-resolved photophysical properties of these crystals were studied via diffuse-reflectance, micro-Raman, and photoluminescence spectroscopy. The crystals fluoresce from orange-red to blue spanning through the whole gamut of the visible spectrum. We have established that photophysical properties are a function of crystal packing in symmetrically substituted TTz-based materials. This correlation was then utilized to fabricate crystalline blends. We demonstrate, for the first time, that symmetrically substituted donor-acceptor-donor TTz-based materials can be used for phosphor-converted color-tuning and white-light emission. Given the cost effectiveness, ease of synthesis and now a structure-photophysics correlation, we present a compelling case for the adoption of TTz-based materials in solid-state photonic and fluorescence-based optical devices.

Dynamic Fluorescent Anti-Counterfeiting Labels Based on Conjugated Polymers Confined in Submicron Fibrous Membranes

Developing a new anti-counterfeiting strategy is of great significance to combating the global counterfeiting problem. Here we report the construction of a dynamic fluorescence response system for anti-counterfeiting by combining the photochromism induced by the ring-opening of spiropyran (SP) to merocyanine (MC) with the fluorescence resonance energy transfer (FRET) between the conjugated polymer and MC. After elucidating the design principle, a new conjugated polymer, PPETE-SP, consisting of a poly[p-(phenylene ethynylene)-alt-(thienylene-ethynylene)] (PPETE) backbone with pendant SP, was synthesized and characterized. With poly(methyl methacrylate) (PMMA) as the matrix, the PPETE-SP/PMMA fibrous membrane was prepared via electrospinning. Under the irradiation of UV light, the fluorescent color of the membrane dynamically changed from green to light green, then light pink, and finally pink, and this process was reversible under visible light. The fluorescence emission switch was examined for 10 cycles and proved to have good repeatability, indicating that the membrane can be directly used as an anti-counterfeiting label for multiple verifications. The FRET efficiency was found to be about 61% based on the FRET study with confocal laser scanning microscopy. The covalent bonding between PPETE backbone and SP, the confinement of PPETE-SP chains in the fibrous membrane, as well as employing PMMA as the matrix were demonstrated to be crucial in realizing the photochromism and the FRET. Different anti-counterfeiting modes were proposed, providing rich selections for operation of verification. Such facile-to-operate and hard-to-imitate dynamic fluorescent responsive materials are very promising for use in practical anti-counterfeiting applications.

Self-assembled organic and hybrid materials derived from oligo-(p-phenyleneethynylenes)

Oligo-(p-Phenyleneethynylenes) (OPEs) have garnered widespread interest over the past three decades due to their excellent opto-electronic properties. However, the chief focus has been on the use of mainly small molecules or polymeric systems for the study of their structural diversity in opto-electronic applications. Recently, researchers have started delving deeper into their utility in material applications. Purely organic materials such as supramolecular polymers, self-assembled nanostructures, nanostructured organogels and single-crystalline materials derived from OPEs have already been developed and researched. Chirality has also been introduced into these systems. Additionally, these have shown physical properties such as polymorphism, liquid crystallinity, melt formation, mechanochromism, etc. All these materials have also shown excellent luminescence properties with high quantum yield and some have even shown energy harvesting properties. There have also been sporadic reports on OPE linker based hybrid systems such as metallogels and metal-organic framework (MOF) structures where structural analysis reveals the origin of tunable emission in these materials. Furthermore, by innovative structural design, unexplored properties of OPEs such as water repellency, bioimaging, drug delivery, photocatalysis, energy transfer, nanomorphology control, photoconductivity, and colour tunability could be achieved. This feature article will, therefore, encompass a detailed discussion on the development of this field as well as the analysis of the properties realized in OPE derived self-assembled supramolecular materials. The main focus will be on the following classes of materials: soft supramolecular materials, crystalline supramolecular π-systems, nanoscale metal-organic frameworks (NMOFs) and bulk metal-organic frameworks (MOFs) and how their application horizon has been expanded by integrating OPEs into their structures.

Self-Assembly of an Amphiphilic Bile Acid Dimer: A Combined Experimental and Theoretical Study of Its Medium-Responsive Fluorescence

This work describes the synthesis and aggregation behavior of a dimeric bile acid derivative in which two steroid cores are bridged by a p-di(phenylethynyl)phenylene fluorophore. The studied compound contains three key characteristics: (a) restricted conformational equilibrium in solution, (b) efficient fluorescence conferred by the bridge, and (c) medium responsiveness encoded in the steroid fragments. The incorporation of the three components afforded a compound that generates nano- and micrometric spherical particles with aggregation-responsive fluorescence emission. The observed self-assembly process of the featured molecule was induced by the gradual addition of water to the tetrahydrofuran (THF) solution. This aggregation led to significant changes in fluorescence that went from two bands at λ_em values of 370 and 390 nm in pure THF to a new spectrum with two maxima at λ_em values of 395 and 418 nm at high water contents, without a decrease in emission. The observed changes can be ascribed to weakly coupled aggregation, a hypothesis supported by multiscale molecular modeling, which sheds light on the mechanism of the self-assembly of this unconventional amphiphile.

Dicyanodistyrylbenzene based positional isomers: a comparative study of AIEE and stimuli responsive multicolour fluorescence switching

Positional isomers of α-dicyanodistyrylbenzene core systems were synthesized and studied for the AIEE effect and stimuli responsive fluorescence colour switching.

Studies of cyanomethylcarbamoyl-bridged anthracene and pyrene fluorophores

Cyanomethylcarbamoyl-bridged anthracene and pyrene derivatives were prepared as functional fluorophores with diverse structural, electronic, and fluorescence properties.

Exciton Coupling and Conformational Changes Impacting the Excited State Properties of Metal Organic Frameworks

In recent years, the photophysical properties of crystalline metal-organic frameworks (MOFs) have become increasingly relevant for their potential application in light-emitting devices, photovoltaics, nonlinear optics and sensing. The availability of high-quality experimental data for such systems makes them ideally suited for a validation of quantum mechanical simulations, aiming at an in-depth atomistic understanding of photophysical phenomena. Here we present a computational DFT study of the absorption and emission characteristics of a Zn-based surface-anchored metal-organic framework (Zn-SURMOF-2) containing anthracenedibenzoic acid (ADB) as linker. Combining band-structure and cluster-based simulations on ADB chromophores in various conformations and aggregation states, we are able to provide a detailed explanation of the experimentally observed photophysical properties of Zn-ADB SURMOF-2: The unexpected (weak) red-shift of the absorption maxima upon incorporating ADB chromophores into SURMOF-2 can be explained by a combination of excitonic coupling effects with conformational changes of the chromophores already in their ground state. As far as the unusually large red-shift of the emission of Zn-ADB SURMOF-2 is concerned, based on our simulations, we attribute it to a modification of the exciton coupling compared to conventional H-aggregates, which results from a relative slip of the centers of neighboring chromophores upon incorporation in Zn-ADB SURMOF-2.

Highly Efficient Deep-Blue Electroluminescence Based on a Solution-Processable A-π-D-π-A Oligo(p-phenyleneethynylene) Small Molecule

The development of solution-processable fluorescent small molecules with highly efficient deep-blue electroluminescence is of growing interest for organic light-emitting diode (OLED) applications. However, high-performance deep-blue fluorescent emitters with external quantum efficiencies (EQEs) over 5% are still scarce in OLEDs. Herein, a novel highly soluble oligo(p-phenyleneethynylene)-based small molecule, 1,4-bis((2-cyanophenyl)ethynyl)-2,5-bis(2-ethylhexyloxy)benzene (2EHO-CNPE), is designed, synthesized, and fully characterized as a wide band gap (2.98 eV) and highly fluorescent (Φ_PL = 0.90 (solution) and 0.51 (solid-state)) deep-blue emitter. The new molecule is functionalized with cyano (-CN)/2-ethylhexyloxy (-OCH₂CH(C₂H₅)C₄H₉) electron-withdrawing/-donating substituents, and ethynylene is used as a π-spacer to form an acceptor (A)-π-donor (D)-π-acceptor (A) molecular architecture with hybridized local and charge transfer (HLCT) excited states. Physicochemical and optoelectronic characterizations of the new emitter were performed in detail, and the single-crystal structure was determined. The new molecule adopts a nearly coplanar π-conjugated framework packed via intermolecular "C-H···π" and "C-H···N" hydrogen bonding interactions without any π-π stacking. The OLED device based on 2EHO-CNPE shows an EQE_max of 7.06% (EQE = 6.30% at 200 cd/m²) and a maximum current efficiency (CE_max) of 5.91 cd/A (CE = 5.34 cd/A at 200 cd/m²) with a deep-blue emission at CIE of (0.15, 0.09). The electroluminescence performances achieved here are among the highest reported to date for a solution-processed deep-blue fluorescent small molecule, and, to the best of our knowledge, it is the first time that a deep-blue OLED is reported based on the oligo(p-phenyleneethynylene) π-framework. TDDFT calculations point to facile reverse intersystem crossing (RISC) processes in 2EHO-CNPE from high-lying triplet states to the first singlet excited state (T₂/T₃ → S₁) (hot-exciton channels) that enable a high radiative exciton yield (η_r ∼ 69%) breaking the theoretical limit of 25% in conventional fluorescent OLEDs. These results demonstrate that properly designed fluorescent oligo(p-phenyleneethynylenes) can be a key player in high-performance deep-blue OLEDs.

Molecular Packing and Solid-State Photophysical Properties of 1,3,6,8-Tetraalkylpyrenes

The relationship between the photophysical properties and molecular orientation of 1,3,6,8-tetraalkylpyrenes in the solid state is described herein. The introduction of alkyl groups with different chain structures (in terms of length and branching) did not affect the photophysical properties in solution, but significantly shifted the emission wavelengths and fluorescence quantum yields in the solid state for some samples. Pyrenes bearing ethyl, isobutyl, or neopentyl groups at the 1-, 3-, 6-, and 8-positions showed similar emission profiles in both the solution and solid states. In contrast, pyrenes bearing other alkyl groups exhibited an excimer emission in the solid state, similar to that of the parent pyrene. On studying the photophysical properties in the solid state with respect to the obtained crystal structures, the observed solid-state photophysical properties were found to depend on the relative position of the pyrene chromophores. The solid-state photophysical properties can be controlled by the alkyl groups, which provide changing crystal packing. Among the pyrenes tested, 1,3,6,8-tetraethylpyrene showed the highest fluorescence quantum yield of 0.88 in the solid state.

Fluorine-containing bistolanes as light-emitting liquid crystalline molecules

We synthesised a series of dissymmetric bistolane derivatives and evaluated their liquid-crystalline (LC) and photoluminescence properties in detail. In measuring LC behaviours, rational structural design based on the dissymmetric molecular structure and electron-density distribution facilitated the production of the LC phase with a wide temperature range (up to 97 °C). In addition, dissymmetric bistolane derivatives were shown to strongly emit blue-photoluminescence in dilute solution and in crystalline states. It was found that dissymmetric bistolanes possess emissive features in even the LC phase and photoluminescence behaviours such as emission intensity and colour were sensitively switched depending on the molecular aggregate structure caused by applying a thermal stimulus.

Harnessing solid-state packing for selective detection of chloride in a macrocyclic anionophore

We report the synthesis of an inherently fluorescent macrocyclic receptor for chloride. The use of a disulphide tether provides for an excellent yield in the macrocyclization step. This compound binds chloride in the solution and solid state, and while unstable over time in aqueous solution, shows a selective response toward chloride over other anions in the solid state due to intermolecular interactions between fluorophore backbones. Surprisingly, the optoelectronic response to anions differs in solution and the films, with a distinct colorimetric response observed only in the film.

Pure white light emission and charge transfer in organogels of symmetrical and unsymmetrical π-chromophoric oligo-p-(phenyleneethynylene) bola-amphiphiles

The gelation behavior of two novel π-chromophoric bola-amphiphiles has been studied and solution processable pure white light emitting and CT gels have been realized.

Bio-degradable highly fluorescent conjugated polymer nanoparticles for bio-medical imaging applications

Conjugated polymer nanoparticles exhibit strong fluorescence and have been applied for biological fluorescence imaging in cell culture and in small animals. However, conjugated polymer particles are hydrophobic and often chemically inert materials with diameters ranging from below 50 nm to several microns. As such, conjugated polymer nanoparticles cannot be excreted through the renal system. This drawback has prevented their application for clinical bio-medical imaging. Here, we present fully conjugated polymer nanoparticles based on imidazole units. These nanoparticles can be bio-degraded by activated macrophages. Reactive oxygen species induce scission of the conjugated polymer backbone at the imidazole unit, leading to complete decomposition of the particles into soluble low molecular weight fragments. Furthermore, the nanoparticles can be surface functionalized for directed targeting. The approach opens a wide range of opportunities for conjugated polymer particles in the fields of medical imaging, drug-delivery, and theranostics.

Conjugated polymer nanoparticles have been applied for biological fluorescence imaging in cell culture and in small animals, but cannot readily be excreted through the renal system. Here the authors show fully conjugated polymer nanoparticles based on imidazole units that can be bio-degraded by activated macrophages.

Rotational Diffusion of a New Large Non Polar Dye Molecule in Alkanes

Rotational reorientation times of a newly synthesized 2,5-bis(phenylethynyl)1,4-bis(dodecyloxy) benzene (DDPE) are experimentally determined in series of n-alkanes by employing steady state and time resolved fluorescence depolarization technique with a view to understand rotational dynamics of large non-polar solute molecule in non-polar solvents and few general solvents of different sizes and varying viscosity. It is observed that rotational reorientation times vary linearly as function of viscosity. The hydrodynamic stick condition describes the experimental results at low viscosities while the results tend to deviate significantly from it at higher viscosities. This is attributed to the possibility of long chains in solvents hosting a variety of chain defects (end-gauche, double-gauche, all-trans, kink, etc.) thereby reducing the effective length of the molecule, leading to a slightly reduced friction. The experimental results are compared with the predictions of Stokes-Einstein-Debye (SED) hydrodynamic theory as well as the quasi-hydrodynamic theories of Gierer-Wirtz (GW) and Dote-Kivelson-Shwartz (DKS). The predictions from these theories underestimate τr in the solvents employed in the study.

Significant effect of alkyl chain length on fluorescent thermochromism of 9,10-bis(p-alkoxystyryl)anthracenes

9,10-bis(p-alkoxystyryl)anthracenes show excellent fluorescent thermochromism and alkyl chain length was found to have significant effect on their fluorescent thermochromic behaviours.

Mechanochromism and aggregation induced emission in benzothiazole substituted tetraphenylethylenes: a structure function correlation

The mechanochromic and aggregation induced emission (AIE) properties of benzothiazole (BT) substituted tetraphenylethylene (TPE) 3a–3c were found to be dependent on the linkage between BT and TPE.

Theoretical study on molecular packing and electronic structure of bi-1,3,4-oxadiazole derivatives

One of the energy-minimum structures predicted by the intermolecular interaction potential energy surface computed using the M062x/6-31G** method closely resembled the crystal packing.

Double N-arylation reaction of polyhalogenated 4,4'-bipyridines. Expedious synthesis of functionalized 2,7-diazacarbazoles

Unusual 2,7-diazacarbazoles were prepared in one step from readily available tetra-halogenated 4,4'-bipyridines by using a double N-arylation reaction in the presence of the Pd-XPhos catalyst system. Moderate to good yields were obtained in this site-selective Buchwald-Hartwig double amination. The functionalization of these tricyclic derivatives was performed by using Pd-catalyzed cross-coupling reactions such as the Stille and Suzuki couplings. Two compounds were analyzed by X-ray diffraction and show π-π stacking involving the diazacarbazole moieties and the phenyl rings of functionalized groups.

Role of Molecular Packing in Determining Solid-State Optical Properties of π-Conjugated Materials

The optical properties of π-conjugated organic molecules in their solid state are critically important in determining performance efficiencies of optoelectronic devices such as organic light-emitting diodes and organic thin-film transistors. This Perspective discusses some recent systematic explorations aimed toward arriving at an understanding of the role that molecular packing plays in determining these properties.

Multistimuli two-color luminescence switching via different slip-stacking of highly fluorescent molecular sheets

Color tuning and switching of the solid-state luminescence of organic materials are attractive subjects for both the fundamental research and practical applications such as optical recording. We report herein cyanostilbene-based highly luminescent molecular sheets which exhibit two-color fluorescence switching in response to pressure, temperature, and solvent vapor. The origin for the multistimuli luminescence switching is the two-directional shear-sliding capability of molecular sheets, which are formed via intermolecular multiple C-H···N and C-H···O hydrogen bonds. The resulting two distinctive crystal phases are promoted by different modes of local dipole coupling, which cause a substantial alternation of π-π overlap. These changes can be directly correlated with the subsequent intermolecular excitonic and excimeric coupling in both phases, as demonstrated by an in-depth theory-assisted spectroscopic and structural study. Finally, we have prepared a first device demonstrator for rewritable fluorescent optical recording media which showed multistimuli luminescence tuning with fast response. Our multistimuli responsive system is unique in terms of the slip-stacking of molecular sheets and thus provides a novel concept of rewritable fluorescent optical recording media.