Solid State Optical Properties of 4-Alkoxy-pyridine Butadiene Derivatives: Reversible Thermal Switching of Luminescence

The synthesis and optical properties of a series of alkoxyphenyl-pyridyl butadiene derivatives in solution and in the solid state are described. All the derivatives were practically nonfluorescent in solution but showed good fluorescence in the solid-state. The role of molecular packing in controlling the solid-state fluorescence was investigated by studying the X-ray crystal structure of these molecules. One of the derivatives, 4-((1E,3E)-4-(4-butoxyphenyl)buta-1,3-dienyl)pyridine exhibited polymorphism, with the different polymorphs exhibiting visually distinguishable fluorescence. In the natural state it existed as a polymorph exhibiting blue fluorescence, while it’s cooled melt emitted yellow light. The difference could be attributed to a transformation in the molecular packing of the material from a herringbone to a brickstone arrangement, resulting in a change from monomer to J-type aggregate fluorescence. The polymorph exhibiting yellow fluorescence was fairly stable (>6 months) but could be converted back to the original form by keeping the film at 110 °C for a short period of time (∼8–10 min) before slowly cooling to room temperature. The thermally induced changes in fluorescence behavior were clearly reproducible over several cycles, indicating the utility of this material for thermal imaging applications

Correlation between Solid-State Photophysical Properties and Molecular Packing in a Series of Indane-1,3-dione Containing Butadiene Derivatives

The solid-state photophysical and photochromic properties and the molecular packing in single crystals of a series of donor−acceptor-substituted butadiene derivatives with alkoxy groups as donor and indane-1,3-dione as acceptor are reported. These materials showed significant enhancement and red-shift in fluorescence in the solid state compared to that in solution. The single crystal analysis of these derivatives indicated that these effects could be attributed to both improved intramolecular charge transfer due to planarization of the molecules and to intermolecular exciton coupling between adjacent molecules leading to aggregate fluorescence. The character of the aggregate formed (H- or J-type) and extent of aggregation were strongly dependent on the length of the alkyl substituent, and these differences could be correlated to variations in the molecular packing observed in their single crystals. Some of the derivatives could be super cooled to a metastable glassy state with significantly different optical properties. Transformation from crystalline to a highly stable glassy form could also be induced by light, making these materials useful for recording optical images

Correlation between Solid-State Photophysical Properties and Molecular Packing in a Series of Indane-1,3-dione Containing Butadiene Derivatives

The solid-state photophysical and photochromic properties and the molecular packing in single crystals of a series of donor−acceptor-substituted butadiene derivatives with alkoxy groups as donor and indane-1,3-dione as acceptor are reported. These materials showed significant enhancement and red-shift in fluorescence in the solid state compared to that in solution. The single crystal analysis of these derivatives indicated that these effects could be attributed to both improved intramolecular charge transfer due to planarization of the molecules and to intermolecular exciton coupling between adjacent molecules leading to aggregate fluorescence. The character of the aggregate formed (H- or J-type) and extent of aggregation were strongly dependent on the length of the alkyl substituent, and these differences could be correlated to variations in the molecular packing observed in their single crystals. Some of the derivatives could be super cooled to a metastable glassy state with significantly different optical properties. Transformation from crystalline to a highly stable glassy form could also be induced by light, making these materials useful for recording optical images

Supergelation via Purely Aromatic π–π Driven Self-Assembly of Pseudodiscotic Oxadiazole Mesogens

A series of highly luminescent oxadiazole-based stilbene molecules (OXD4, OXD8, OXD10, and OXD12) exhibiting interesting enantiotropic liquid crystalline and gelation properties have been synthesized and characterized. The molecules possessing longer alkyl substituents, OXD10 and OXD12, possess a pseudodisc shape and are capable of behaving as supergelators in nonpolar solvents, forming self-standing gels with very high thermal and mechanical stability. Notably the self-assembly of these molecules, which do not possess any hydrogen-bonding motifs normally observed in most reported supergelators, is driven purely by π-stacking interactions of the constituent molecules. The <i>d</i>-spacing ratios estimated from XRD analysis of OXD derivatives possessing longer alkyl chains show that the molecules are arranged in a columnar fashion in the mesogens and the self-assembled nanofibers formed in the gelation process

Tailor-Made Highly Luminescent and Ambipolar Transporting Organic Mixed Stacked Charge-Transfer Crystals: An Isometric Donor–Acceptor Approach

We have rationally designed a densely packed 1:1 donor–acceptor (<b>D</b>–<b>A</b>) cocrystal system comprising two isometric distyrylbenzene- and dicyanodistyrylbenzene-based molecules, forming regular one-dimensional mixed stacks. The crystal exhibits strongly red-shifted, bright photoluminescence originating from an intermolecular charge-transfer state. The peculiar electronic situation gives rise to high and ambipolar p-/n-type field-effect mobility up to 6.7 × 10^–3 and 6.7 × 10^–2 cm² V^–1 s^–1, respectively, as observed in single-crystalline OFETs prepared via solvent vapor annealing process. The unique combination of favorable electric and optical properties arising from an appropriate design concept of isometric <b>D</b>–<b>A</b> cocrystal has been demonstrated as a promising candidate for next generation (opto-)­electronic materials

Tailor-Made Highly Luminescent and Ambipolar Transporting Organic Mixed Stacked Charge-Transfer Crystals: An Isometric Donor–Acceptor Approach

We have rationally designed a densely packed 1:1 donor–acceptor (<b>D</b>–<b>A</b>) cocrystal system comprising two isometric distyrylbenzene- and dicyanodistyrylbenzene-based molecules, forming regular one-dimensional mixed stacks. The crystal exhibits strongly red-shifted, bright photoluminescence originating from an intermolecular charge-transfer state. The peculiar electronic situation gives rise to high and ambipolar p-/n-type field-effect mobility up to 6.7 × 10^–3 and 6.7 × 10^–2 cm² V^–1 s^–1, respectively, as observed in single-crystalline OFETs prepared via solvent vapor annealing process. The unique combination of favorable electric and optical properties arising from an appropriate design concept of isometric <b>D</b>–<b>A</b> cocrystal has been demonstrated as a promising candidate for next generation (opto-)­electronic materials

Green Phosphorescence and Electroluminescence of Sulfur Pentafluoride-Functionalized Cationic Iridium(III) Complexes

We report on four cationic iridium­(III) complexes [Ir­(C^N)₂(d<i>t</i>Bubpy)]­(PF₆) that have sulfur pentafluoride-modified 1-phenylpyrazole and 2-phenylpyridine cyclometalating (C^N) ligands (d<i>t</i>Bubpy = 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridyl). Three of the complexes were characterized by single-crystal X-ray structure analysis. In cyclic voltammetry, the complexes undergo reversible oxidation of iridium­(III) and irreversible reduction of the SF₅ group. They emit bright green phosphorescence in acetonitrile solution and in thin films at room temperature, with emission maxima in the range of 482–519 nm and photoluminescence quantum yields of up to 79%. The electron-withdrawing sulfur pentafluoride group on the cyclometalating ligands increases the oxidation potential and the redox gap and blue-shifts the phosphorescence of the iridium complexes more so than the commonly employed fluoro and trifluoromethyl groups. The irreversible reduction of the SF₅ group may be a problem in organic electronics; for example, the complexes do not exhibit electroluminescence in light-emitting electrochemical cells (LEECs). Nevertheless, the complexes exhibit green to yellow-green electroluminescence in doped multilayer organic light-emitting diodes (OLEDs) with emission maxima ranging from 501 nm to 520 nm and with an external quantum efficiency (EQE) of up to 1.7% in solution-processed devices

Stimulated Emission Properties of Sterically Modified Distyrylbenzene-Based H‑Aggregate Single Crystals

J-aggregation has been shown to be beneficial for light amplification in single crystals of π-conjugated organic molecules. In the case of H-aggregation, the criteria for such processes are still under debate. It has also been shown that H-aggregate arrangements with considerable π–π overlap are detrimental for light amplification. We show here that a proper alignment of the molecules in the crystal lattice, which minimizes π–π overlap between adjacent molecules, gives rise to (random) stimulated emission from cofacial arrangements similar to that of the herringbone aggregates

Self-Assembling and Luminescent Properties of Chiral Bisoxadiazole Derivatives in Solution and Liquid-Crystalline Phases

Herein, we report the synthesis, self-assembly, and electroluminescence characteristics of a new green-emitting, pseudodiscoid chiral molecule, OXDC, containing an electron-donating stilbene core and an electron-accepting oxadiazole substituent. The helical organization and specific interaction of the chiral pseudodiscoid molecule resulted in the formation of self-assembled nanofibers with a columnar superstructure. Macroscopic chirality was observed in both the liquid-crystalline phases and the self-assembled nanofibers of OXDC, a feature which was absent in the analogous achiral oxadiazole derivative reported earlier [Sivadas, A. P.; Supergelation via Purely Aromatic π-π Driven Self-Assembly of Pseudodiscotic Oxadiazole Mesogens. J. Am. Chem. Soc. 2014, 136, 5416−5423]. A high-performance organic light-emitting device was demonstrated using OXDC as the emitting material, with a luminous intensity of 10 115 cd m^–2 at 5 V and chromaticity coordinates of (0.32, 0.51)

Excited State Features and Dynamics in a Distyrylbenzene-Based Mixed Stack Donor–Acceptor Cocrystal with Luminescent Charge Transfer Characteristics

Combined structural, photophysical, and quantum-chemical studies at the quantum mechanics/molecular mechanics (QM/MM) level precisely reveal the structure–property relationships in a mixed-stack donor–acceptor cocrystal, which displays vibronically structured fluorescence, strongly red-shifted against the spectra of the parent donor and acceptor, with high quantum yield despite the pronounced CT character of the emitting state. The study elucidates the reasons for this unusual combination, quantifies the ordering and nature of the collective excited singlet and triplet state manifold, and details the deactivation pathways of the initially created Franck–Condon state