Bent-core liquid crystalline cyanostilbenes : fluorescence switching and thermochromism

The authors from ICMA greatly appreciate financial support from the Spanish Government (MINECO-FEDER project MAT2012-38538-C03-01), the Aragon's Government and FSE (project E04) and the Jae PreDoc-CSIC (M. M.-A.) fellowship program.Fluorescent bent-core molecules, bearing one or two cyanostilbene units in the lateral structure and different positions of the cyano group (α- or β-isomers), are described with the aim of modulating the molecular packing and fluorescence properties. These compounds give rise to a variety of crystal polymorphs and bent-core liquid crystalline phases (SmCP, Colr and B6), offering the unique chance to study the fluorescence properties of the cyanostilbene structure in different phases. Experimental and computational studies elucidate geometrical and electronic properties of these bent-core structures but especially the fluorescence properties (spectral positions, quantum yields and decay curves), in a detailed comparison between diluted solutions, in dichloromethane (DCM) or poly(methylmethacrylate) (PMMA), and condensed phases. Quantum yields as high as 70% have been obtained in some diluted solutions (PMMA) and condensed phases. Remarkably, the quantum yield values depend on the position of the cyano group, being higher for β- than for the α-isomers due to the higher radiative rates and lower non-radiative rates of the former. The photophysical characterization in the condensed phase focuses on RT studies with solid samples and different processing, and show that, upon aggregation, interactions between the cyanostilbene groups result in changes of the emission spectra and dynamics compared to the diluted systems in DCM and PMMA, giving rise to H-aggregations of varying strength. Furthermore, the compounds exhibit thermochromism, showing a green-yellow fluorescence in the pristine crystalline phase that changes to blue on heating to the liquid crystal phase. This journal isPostprintPeer reviewe

Near-infrared fluorescence of silicon phthalocyanine carboxylate esters

EZ-C acknowledges the University of St Andrews for financial support. We are grateful to the EPSRC for financial support from grant EP/M02105X/1 and the European Research Council for financial support from grant 321305. I.D.W.S. acknowledges a Royal Society Wolfson Research Merit Award.Seven silicon(IV) phthalocyanine carboxylate esters (SiPcs, 1–7 ) with non-, partially- and per-fluorinated aliphatic (linear or branched at the alpha-carbon) and aromatic ester groups have been synthesized, their solid-state structures determined and their optoelectronic properties characterized. The SiPcs exhibit quasi-reversible oxidation waves (vs. Fc+/Fc) at 0.58–0.75 V and reduction waves at −0.97 to −1.16 V centered on the phthalocyanine ring with a narrow redox gap range of 1.70–1.75 V. Strong absorbance in the near-infrared (NIR) region is observed for 1–7 with the lowest-energy absorption maximum (Q band) varying little as a function of ester between 682 and 691 nm. SiPcs 1–7 fluorescence in the near-infrared with emission maxima at 691–700 nm. The photoluminescence quantum yields range from 40 to 52%. As a function of esterification, the SiPcs 1–7 exhibit moderate-to-good solubility in chlorinated solvents, such as 1,2-dichlorobenzene and chloroform.Publisher PDFPeer reviewe

Solution-processable silicon phthalocyanines in electroluminescent and photovoltaic devices

E.Z.-C. acknowledges the University of St. Andrews for financial support. The authors thank the EPSRC UK National Mass Spectrometry Facility at Swansea University for analytical services. I.D.W.S. acknowledges support from the EPSRC (grant EP/J01771X), the European Research Council (grant 321305), and a Royal Society Wolfson Research Merit Award.Phthalocyanines and their main group and metal complexes are important classes of organic semiconductor materials, but are usually highly insoluble so frequently need to be processed by vacuum deposition in devices. We report two highly soluble silicon phthalocyanine (SiPc) diester compounds and demonstrate their potential as organic semiconductor materials. Near-infrared (λEL = 698-709 nm) solution-processed organic light- emitting diodes (OLEDs) were fabricated and exhibited external quantum efficiencies (EQEs) of up to 1.4%. Binary bulk heterojunction solar cells employing P3HT or PTB7 as the donor and the SiPc as the acceptor provided power conversion efficiencies (PCE) of up to 2.7% under simulated solar illumination. Our results show that soluble SiPcs are promising materials for organic electronics.Publisher PDFPeer reviewe

Green phosphorescence and electroluminescence of sulfur pentafluoride-functionalized cationic iridium(III) complexes

EZ-C acknowledges the University of St Andrews for financial support.We report four cationic iridium(III) complexes [Ir(C^N)2(dtBubpy)](PF6) that have sulfurpentafluoride-modified 1-phenylpyrazole and 2-phenylpyridine cyclometalating (C^N) ligands (dtBubpy = 4,4'-di-tert-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 between 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 than do 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 501–520 nm and with an external quantum efficiency (EQE) of up to 1.7% in solution-processed devices.PostprintPeer reviewe

The influence of crystal packing on the solid state fluorescence behavior of alkyloxy substituted phenyleneethynylenes

The study reports the solid state photophysical properties of a series of alkyloxy-substituted oligo(phenyleneethynylene)s, methoxy to hexyloxy, supported by a detailed analysis of molecular packing obtained from single crystal X-ray diffraction data. While the emission peaks are highly red shifted (by as much as 114 nm) in the solid state, all molecules exhibit similar absorption and emission in dilute solutions. The red shift is maximum in ethoxy and minimum in methoxy, while other crystalline films exhibit intermediate values. In the crystal structures, the spacing between the molecular pairs forming the J-aggregates is varied between 3.48 to 4.65 Å, with no systematic dependence on the chain length. The red shifted emission maximum is found to vary linearly with the spacing between the interacting molecules in the J-aggregate. Thus, the emission in the solid state is determined by the extent of dipolar coupling between the molecules, the alkyl chain length influencing the properties only indirectly

Hierarchical self-assembly of donor-acceptor-substituted butadiene amphiphiles into photoresponsive vesicles and gels

It all gels well: Amphiphilic donor-acceptor-substituted butadienes undergo spontaneous concentration-dependent self-assembly into highly fluorescent vesicles and gels. The vesicles and gels exhibit light-induced destruction and self-repair as a result of the photochromic nature of the butadiene chromophore

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

Luminescent assemblies of pyrene-containing bent-core mesogens: liquid crystals, π-gels and nanotubes

Bent-shaped molecules incorporating terminal pyrene moieties were designed and synthesized aiming at promoting a variety of luminescent self-assembled materials. The novel compounds are fluorescent both in solution and in condensed phases, achieving brighter fluorescence in the last case with fluorescence quantum yields up to 60%. Depending on the number of pyrene units (1 or 2) and/or the presence or absence of a long and flexible linker between the bent-core and the pyrene structures, bent-core liquid crystal phases, physical gels and nanoassemblies were obtained. Their properties depend on the molecular design, unveiling a synergistic and versatile 'tandem' of the bi-functional system in supramolecular functional materials chemistry. Long hydrocarbon spacers connecting bent-core and pyrene structures were necessary to display mesomorphic properties, achieving monotropic rectangular columnar mesophases, being for one compound stable at room temperature and displaying a glassy transition below room temperature, as revealed by POM, DSC and XRD. Liquid crystal phases exhibited excimer-like emission. In addition, organogels were obtained without the help of hydrogen bonding functionalities, exhibiting in one case supergelator behavior. Different nanoassemblies were obtained from these non-amphiphilic bent-core structures, in particular unprecedented organic nanotubes with beveled ends formed by a single bilayer.The authors from INMA greatly appreciate financial support from projects of the Spanish Government PGC2018-093761-B-C31 [MCIU/AEI/FEDER, UE] and MAT2015-66208-C3-1 [MINECO/FEDER, UE], the Gobierno de Aragón/FEDER (research group E47_20R) and the JAE.PREDOC-CSIC (M. M.-A.) fellowship program. Thanks are given to the nuclear magnetic resonance, mass spectrometry, and thermal analysis services of the INMA (Univ. Zaragoza-CSIC), the LMA (Univ. Zaragoza) for TEM images and Servicio General de Apoyo a la Investigación-SAI (Univ. Zaragoza) for SEM images. The work at IMDEA was supported by the Spanish Government (MINECO-FEDER project CTQ2017-87054, MICINN project CEX2020-001039-S) and by the Campus of International Excellence (CEI) UAM+CSIC.Peer reviewe