3-Ethynyltriimidazo[1,2-a:1',2'-c:1″,2″-e][1,3,5]triazine Dual Short- and Long-Lived Emissions with Crystallization-Enhanced Feature: Role of Hydrogen Bonds and π-π Interactions.

Organic room temperature phosphorescent (ORTP) materials with stimuli-responsive, multicomponent emissive behaviour are extremely desirable for various applications. The derivative of cyclic triimidazole (TT) functionalized with an ethynyl group, TT-CCH, is isolated and investigated. The compound possesses crystallization-enhanced emission (CEE) comprising dual fluorescence and dual phosphorescence of both molecular and supramolecular origin with aggregation-induced components highly sensitive to grinding. The mechanisms involved in the emissions have been disclosed thanks to combined structural, spectroscopic and computational investigations. In particular, strong CH⋯N hydrogen bonds are deemed responsible, for the first time in the TT family, together with frequently observed π⋯π stacking interactions, for the aggregated fluorescence and phosphorescence.

Semi-Transparent Luminescent Solar Concentrators Based on Intramolecular Energy Transfer in Polyurethane Matrices.

Luminescent solar concentrators (LSCs) are spectral conversion devices offering interesting opportunities for the integration of photovoltaics into the built environment and portable systems. The Förster-resonance energy transfer (FRET) process can boost the optical response of LSCs by reducing energy losses typically associated to non-radiative processes occurring within the device under operation. In this work, a new class of FRET-based thin-film LSC devices is presented, in which the synthetic versatility of linear polyurethanes (PU) is exploited to control the photophysical properties and the device performance of the resulting LSCs. A series of luminescent linear PUs are synthesized in the presence of two novel bis-hydroxyl-functionalized luminophores of suitable optical properties, used as chain extenders during the step-growth polyaddition reaction for the formation of the linear macromolecular network. By synthetically tuning their composition, the obtained luminescent PUs can achieve a high energy transfer efficiency (≈90%) between the covalently linked luminophores. The corresponding LSC devices exhibit excellent photonic response, with external and internal photon efficiencies as high as ≈4% and ≈37%, respectively. Furthermore, their optimized power conversion efficiency combined with their enhanced average visible-light transmittance highlight their suitability for potential use as transparent solar energy devices.

Enhancement of Photoluminescence Properties via Polymer Infiltration in a Colloidal Photonic Glass.

Photonic glasses (PGs) based on the self-assembly of monosized nanoparticles can be an effective tool for realizing disordered structures capable of tailoring light diffusion due to the establishment of Mie resonances. In particular, the wavelength position of these resonances depends mainly on the morphology (dimension) and optical properties (refractive index) of the building blocks. In this study, we report the fabrication and optical characterization of photonic glasses obtained via a self-assembling technique. Furthermore, we have demonstrated that the infiltration of these systems with a green-emitting polymer enhances the properties of the polymer, resulting in a large increase in its photoluminescence quantum yield and a 3 ps growing time of the photoluminescence time decay Finally, the development of the aforementioned system can serve as a suitable low-cost platform for the realization of lasers and fluorescence-based bio-sensors.

A Facile Access to Green Fluorescent Albumin Derivatives.

A Morita-Baylis-Hillman Adduct (MBHA) derivative bearing a triphenylamine moiety was found to react with human serum albumin (HSA) shifting its emission from the blue to the green-yellow thus leading to green fluorescent albumin (GFA) derivatives and enlarging the platform of probes for aggregation-induced fluorescent-based detection techniques. A possible interaction of MBHA derivative 7 with a lipophilic pocket within the HSA structure was suggested by docking studies. DLS experiments showed that the reaction with HSA induce a conformational change of the protein contributing to the aggregation process of GFA derivatives. The results of investigations on the biological properties suggested that GFA retained the ability of binding drug molecules such as warfarin and diazepam. Finally, cytotoxicity evaluation studies suggested that, although the MBHA derivative 7 at 0.1 µg/mL affected the percentage of cell viability in comparison to the negative control, it cannot be considered cytotoxic, whereas at all the other concentrations≥0.5 µg/mL resulted cytotoxic at different extent.

Activation of Solid-State Emission and Photostability through Molecular Confinement: The Case of Triptycene-Fused Quinacridone Dyes.

We report the facile, metal-free convergent synthesis and the characterization of novel quinacridone dyes in which two triptycene units end-cap and sterically confine the quinacridone chromophore. A precise comparison of the confined dyes with their known homologues reveals that the reduction of π-π interactions in triptycene-fused quinacridone dyes compared to classical quinacridone results not only in an increase of solubility and processability but also in an enhancement of fluorescence quantum yield and photostability in the solid state.

Stimuli Responsive Features of Organic RTP Materials: An Intriguing Carbazole-Cyclic Triimidazole Derivative.

Stimuli responsive luminescent materials possessing room temperature phosphorescence (RTP) are extremely desirable for various applications. The here investigated derivative of cyclic triimidazole (TT) functionalized with carbazole (Cz), namely TT-Ph-Cz, belongs to this class. TT-Ph-Cz possesses high conformational freedom resulting in rigidochromic and multi-stimuli responsive emissive behavior. It has been isolated as MeOH-solvated and de-solvated forms characterized by distinctive emissive features. In particular, the solvated form, in which hydrogen bonds with MeOH inhibit competitive non-radiative deactivation channels, possesses a higher quantum yield associated with a strong phosphorescence contribution which is preserved in DMSO/water solutions.

Impact of chiral ligands on photophysical and electro-optical properties of ß-diketonate europium complexes in circularly polarized OLEDs.

Luminescent lanthanide complexes exhibiting chiroptical properties are attracting attention for their application in chiral optoelectronics and photonics, thanks to their unique optical properties, allied to intraconfigurational f-f transitions, which are generally electric-dipole-forbidden and can be magnetic dipole-allowed, which in an appropriate environment can lead to high dissymmetry factors and strong luminescence, in the presence of an antenna ligand. However, because luminescence and chiroptical activity are governed by different selection rules, their successful application in commonly used technologies is still an expectation. Recently, we showed that europium complexes bearing ß-diketonates acting as luminescence sensitizers, and chiral bis(oxazolinyl) pyridine derivatives as the chirality inducer, reasonably perform in circularly polarized (CP) organic light-emitting devices (OLEDs). Indeed, europium ß-diketonate complexes are an interesting molecular starting point, given their strong luminescence and their established use in conventional (i.e., nonpolarized) OLEDs. In this context, it is interesting to investigate in detail the impact of the ancillary chiral ligand on complex emission properties and the performances of corresponding CP-OLEDs. Here we show that, by incorporating the chiral compound as emitter in the architecture of solution processed electroluminescent devices, CP emission is retained, and the efficiency of the device is comparable to reference unpolarized OLED. The observed remarkable dissymmetry values strengthen the position of chiral lanthanide-OLEDs as CP-emitting devices.

Anti-Oxidation Agents to Prevent Dye Degradation in Organic-Based Host-Guest Systems Suitable for Luminescent Solar Concentrators.

Luminescent solar concentrators (LSCs) have been extensively studied as they offer a practical solution to increase the efficiency of silicon-based photovoltaics (PVs). In this context, the use of natural and organic luminescent materials is desirable in order to obtain sustainable and environmentally friendly devices. Moreover, solution-processable organic host-guest systems based on Foerster Resonant Energy Transfer (FRET) processes offer the possibility to exploit a low-cost technique to obtain an efficient energy downshift from the UV-visible to red or deep red emissions in order to concentrate the radiation in the area of maximum efficiency of the PV device. Nevertheless, organic materials are subjected to photodegradation that reduces their optical properties when exposed to UV light and oxygen. In this work, we incorporated two different antioxidant molecules (i.e., octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Octa) and L-ascorbic acid (L-Asc)) in a three-dye host-guest system and studied the corresponding optical properties after prolonged irradiation times in air. It was found that the presence of the antioxidants, especially L-Asc, slowed the system's photodegradation down whilst at the same time retaining high emission efficiencies and without interfering with the cascade Resonant Energy Transfer processes among the dyes inserted in the nanochannels of the host.

Ultrasensitive Detection of Cu(II) and Pb(II) Using a Water-Soluble Perylene Probe.

The selective detection of metal ions in water, using sustainable detection systems, is of crescent importance for monitoring water environments and drinking water safety. One of the key elements of future chemical sciences is the use of sustainable approaches in the design of new materials. In this study, we design and synthesize a low-cost, water-soluble potassium salt of 3,4,9,10-perylene tetracarboxylic acid (PTAS), which shows a selective optical response on the addition of Cu2+ and Pb2+ ions in aqueous solutions. By using a water-soluble chromophore, the interactions with the metal ions are definitely more intimate and efficient, with respect to standard methods employing cosolvents. The detection limits of PTAS for both Cu2+ and Pb2+ are found to be 2 µM by using a simple absorbance mode, and even lower (1 µM) with NMR experiments, indicating that this analyte-probe system is sensitive enough for the detection of copper ions in drinking water and lead ions in waste water. The complexation of PTAS with both ions is supported with NMR studies, which reveal the formation of new species between PTAS and analytes. By combining a low-cost water-soluble chromophore with efficient analyte-probe interactions due to the use of aqueous solutions, the results here obtained provide a basis for designing sustainable sensing systems.

Asymmetric AZA-BODIPY with Optical Gain in the Near-Infrared Region.

In recent years, there has been a lot of interest in the development of organic compounds emitting in the near-infrared (NIR) region due to their stimulating applications, such as biosensing and light detection and ranging (LiDAR). Moreover, a lot of effort has been devoted to finding organic emitters with optical gain in the NIR region for lasing applications. In this paper, we present the ultrafast spectroscopy of an asymmetric AZA-BODIPY molecule that shows relevant photophysical changes moving from a diluted solution to a concentrated solution and to a spin-coated film. The diluted solution and the spin-coated film show a bleaching band and a stimulated emission band in the visible region, while the very concentrated solution displays a broad (150 nm) and long-living (more than 400 ps) optical gain band in the NIR region, centered at 900 nm. Our results pave the way for a new organic laser system in a near-infrared spectral region.

Solvent-Free Preparation and Moderate Congruent Melting Temperature of Layered Lead Iodide Perovskites for Thin-Film Formation.

Lead halide 2D hybrid perovskites (HP) have emerged as promising materials for photovoltaic and lighting applications. Solvent-free preparations offer greener route, but require congruent melting of halide perovskite for thin films, which has been demonstrated only for monolayered HP (n=1 of the (A)2 (MA)n-1 Pbn I3n+1 series) at the quite high temperature Tm (Tmelting ) of 171 °C. Here, we report on the solvent-free preparation (n=2, 4), thermal behaviour and melt-processed thin films of a series of HP (GABA)2 (MA)n-1 Pbn I3n+1 (n=1, 2, 4; GABA+ : 4-ammoniumbutyric acid cation). The n=1 and n=2 HP have an exceptional low congruent Tm of 126 °C and 136 °C, respectively, and, for n=1, a very good stability in the molten state. The liquid-solid reaction of molten (GABA)2 PbI4 or (GABA)2 (MA)Pb2 I7 with MAPbI3 in 1/1 (T=130 °C) or 1/2 (T=135 °C) ratio leads to pure phases of the n=2 HP and (GABA)2 (MA)3 Pb4 I11 (n=4, Tm =185 °C), respectively. Melt-processed thin films of n=1, n=2 and n=4 HP have also been prepared.

Prompt and Long-Lived Anti-Kasha Emission from Organic Dyes.

Anti-Kasha behavior has been the subject of intense debate in the last few years, as demonstrated by the high number of papers appearing in the literature on this topic, dealing with both mechanistic and applicative aspects of this phenomenon. Examples of anomalous emitters reported in the last 10 years are collected in the present review, which is focused on strictly anti-Kasha organic molecules displaying radiative deactivation from Sn and/or Tn, with n greater than 1.

Tunable Linear and Nonlinear Optical Properties from Room Temperature Phosphorescent Cyclic Triimidazole-Pyrene Bio-Probe.

Organic materials with multiple emissions tunable by external stimuli represent a great challenge. TTPyr, crystallizing in different polymorphs, shows a very rich photophyisics comprising excitation-dependent fluorescence and phosphorescence at ambient conditions, and mechanochromic and thermochromic behavior. Transformation among the different species has been followed by thermal and X-ray diffraction analyses and the emissive features interpreted through structural results and DFT/TDDFT calculations. Particularly intriguing is the polymorph TTPyr(HT), serendipitously obtained at high temperature but stable also at room temperature, whose non-centrosymmetric structure guarantees an SHG efficiency 10 times higher than that of standard urea. Its crystal packing, where only the TT units are strongly rigidified by π-π stacking interactions while the Pyr moieties possess partial conformational freedom, is responsible for the observed dual fluorescence. The potentialities of TTPyr for bioimaging have been successfully established.

Optical Gain in Semiconducting Polymer Nano and Mesoparticles.

The presence of excited-states and charge-separated species was identified through UV and visible laser pump and visible/near-infrared probe femtosecond transient absorption spectroscopy in spin coated films of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) nanoparticles and mesoparticles. Optical gain in the mesoparticle films is observed after excitation at both 400 and 610 nm. In the mesoparticle film, charge generation after UV excitation appears after around 50 ps, but little is observed after visible pump excitation. In the nanoparticle film, as for a uniform film of the pure polymer, charge formation was efficiently induced by UV excitation pump, while excitation of the low energetic absorption states (at 610 nm) induces in the nanoparticle film a large optical gain region reducing the charge formation efficiency. It is proposed that the different intermolecular interactions and molecular order within the nanoparticles and mesoparticles are responsible for their markedly different photophysical behavior. These results therefore demonstrate the possibility of a hitherto unexplored route to stimulated emission in a conjugated polymer that has relatively undemanding film preparation requirements.

Poly(vinylidenefluoride) polymers and copolymers as versatile hosts for luminescent solar concentrators: compositional tuning for enhanced performance.

Novel host matrices based on fluoropolymers blended with poly(methyl methacrylate) (PMMA) are presented in this work for application in efficient and photochemically stable thin-film luminescent solar concentrators (LSCs). These systems consist of blends of PMMA with three different partially fluorinated polymers in different proportions: polyvinylidenefluoride homopolymer, a copolymer of vinylidenefluoride and chloro-trifluoro-ethylene, and a terpolymer of vinylidenefluoride, hexafluoropropylene and hydroxyl-ethyl acetate. A detailed chemical, physical and structural characterization of the obtained materials allowed us to shed light on the structure-property relationships underlying the response of such blends as a LSC component, revealing the effect of the degree of crystallinity of the polymers on their functional characteristics. An optimization study of the optical and photovoltaic (PV) performance of these fluoropolymer-based LSC systems was carried out by investigating the effect of blend chemical composition, luminophore concentration and film thickness on LSC device output. LSCs featuring copolymer/PMMA blends as the host matrix were found to outperform their homopolymer- and terpolymer-based blend counterparts, attaining efficiencies comparable to those of reference PMMA-based LSC/PV assemblies. All optimized LSC systems were subjected to weathering tests for over 1000 h of continuous light exposure to evaluate the effect of the host matrix system on LSC performance decline and to correlate chemical composition with photochemical durability. It was found that all fluoropolymer/PMMA-based LSCs outperformed reference PMMA-based LSCs in terms of long-term operational lifetime. This work provides the first demonstration of thermoplastic fluoropolymer/PMMA blends for application as host matrices in efficient and stable LSCs and widens the scope of high-performance thermoplastic materials for the PV field.

The Key Role of the Interface in the Highly Sensitive Mechanochromic Luminescence Properties of Hybrid Perovskites.

Hybrid perovskite (HP) materials are of interest in photovoltaics and lighting applications. Here we report that hybrid perovskite composites, as crystallized powders, can behave as intelligent materials showing highly sensitive and reversible mechanochromic luminescence (MCL). Composites consisting of monolayered 2D HP and 3D HP components exhibit reversible tunable color emission upon mechanical strain. The bluish-whitish emission of the 2D HP turns into orange in the composite owing to an energy transfer process. The bright green emission, observed as soon as the composite is slightly crushed, originates from the 3D HP after efficient energy funneling from the multi-layered 2D HP produced at the 2D/3D interface by the mechanical treatment. Besides highlighting the key role of the interfaces in light emission of HP, our findings pave the way for hybrid perovskites as highly sensitive MCL smart materials for mechanosensors, security papers, or optical storage applications.

Unravelling the intricate photophysical behavior of 3-(pyridin-2-yl)triimidazotriazine AIE and RTP polymorphs.

The development of purely organic materials showing multicolor fluorescent and phosphorescent behaviour represents a formidable challenge in view of practical applications. Herein the rich photophysical behaviour of 3-(pyridin-2-yl)triimidazotriazine (TT-Py) organic molecule, comprising excitation-dependent fluorescence and phosphorescence under ambient conditions in both blended films and the crystalline phase, is investigated by means of steady state, time resolved and ultrafast spectroscopies and interpreted on the basis of X-ray diffraction studies and DFT/TDDFT calculations. In particular, by proper excitation wavelength, dual fluorescence and dual phosphorescence of molecular origin can be observed together with low energy phosphorescences resulting from aggregate species. It is demonstrated that the multiple emission properties originate from the copresence, in the investigated system, of an extended polycyclic nitrogen-rich moiety (TT), strongly rigidified by π-π stacking interactions and short C-H···N hydrogen bonds, and a fragment (Py) having partial conformational freedom.

One-Pot Regiodirected Annulations for the Rapid Synthesis of π-Extended Oligomers.

We demonstrate the broad applicability of the annulation protocol combining, in one pot, a direct arylation and cross aldol condensation for the straightforward synthesis at gram-scale of π-extended thiophene-based scaffolds. The regiospecific direct arylation drives the subsequent cross-aldol condensation proceed under the same basic conditions, and the overall protocol has broad applicability in the synthesis of extended aromatics wherein the thiophene ring is annulated with furans, pyridines, indoles, benzothiophenes, and benzofurans. These scaffolds can be further elaborated into π-extended, highly fluorescent oligomers with a central deficient benzothiadiazole unit with up to nine aromatic rings through coupling reactions.

Physicochemical Properties of A New PEGylated Polybenzofulvene Brush for Drug Encapsulation.

A new polymer brush was synthesized by spontaneous polymerization of benzofulvene macromonomer 6-MOEG-9-T-BF3k bearing a nona(ethylene glycol) side chain linked to the 3-phenylindene scaffold by means of a triazole heterocycle. The polymer structure was studied by SEC-MALS, NMR spectroscopy, and MALDI-TOF MS techniques, and the results supported the role of oligomeric initiatory species in the spontaneous polymerization of polybenzofulvene derivatives. The aggregation features of high molecular weight poly-6-MOEG-9-T-BF3k-FE were investigated by pyrene fluorescence analysis, dynamic light scattering studies, and transmission electron microscopy, which suggested a tendency towards the formation of spherical objects showing dimensions in the range of 20-200 nm. Moreover, poly-6-MOEG-9-T-BF3k-FE showed an interesting cytocompatibility in the whole concentration range tested that, besides its aggregation features, makes this polybenzofulvene brush a good polymer candidate for nanoencapsulation and delivery of drug molecules. Finally, the photo-physical features of poly-6-MOEG-9-T-BF3k-FE could allow the biodistribution of the resulting drug delivery systems to be monitored by fluorescence microscopy techniques.

Solid State Room Temperature Dual Phosphorescence from 3-(2-Fluoropyridin-4-yl)triimidazo[1,2-a:1',2'-c:1″,2″-e][1,3,5]triazine.

Organic room temperature persistent luminescence is a fascinating but still largely unexplored phenomenon. Cyclic-triimidazole and its halogenated (Br, I) derivatives have recently revealed as intriguing phosphors characterized by multifaceted emissive behavior including room temperature ultralong phosphorescence (RTUP) associated with the presence of H-aggregates in their crystal structure. Here, we move towards a multicomponent system by incorporating a fluoropyridinic fragment on the cyclic-triimidazole scaffold. Such chromophore enhances the molecular properties resulting in a high photoluminescence quantum yield (PL QY) in solution but preserves the solid-state RTUP. By means of X-ray diffraction (XRD) analysis, theoretical calculations, steady-state and time-resolved spectroscopy on solutions, polymethylmethacrylate (PMMA) blends and crystals, the nature of the different radiative deactivation channels of the compound has been disclosed. In particular, the molecular fluorescence and phosphorescence, this latter observed in frozen solution and in PMMA blends, are associated to deactivation from S1 and T1 respectively, while the low energy RTUP, observed only for crystals, is interpreted as originated from H aggregates.