Hybrid organic-inorganic perovskites (PVKs) are among the most promising materials for optoelectronic applications thanks to their outstanding photophysical properties and easy synthesis. Herein, a new PVK-based thermochromic composite is demonstrated. It can reversibly switch from a transparent state (transmittance > 80%) at room temperature to a colored state (transmittance < 10%) at high temperature, with very fast kinetics, taking only a few seconds to go from the bleached to the colored state (and vice versa). X-ray diffraction, Fourier-transform infrared spectroscopy, differential scanning calometry, rheological, and optical measurements carried out during heating/cooling cycles reveal that thermochromism in the material is based on a reversible process of PVK disassembly/assembly mediated by intercalating polymeric chains, through the formation and breaking of hydrogen bonds between polymer and perovskite. Therefore, differently from other thermochromic perovskites, that generally work with the adsorption/desorption of volatile molecules, the system is able to perform several heating/cooling cycles regardless of environmental conditions. The color and transition temperature (from 70 to 120 °C) can be tuned depending on the type of perovskite. Moreover, this thermochromic material is printable and can be deposited by cheap techniques, paving the way for a new class of smart coatings with an unprecedented range of colors.
Bio-based polymers are attracting great interest due to their potential for several applications in place of conventional polymers. In the field of electrochemical devices, the electrolyte is a fundamental element that determines their performance, and polymers represent good candidates for developing solid-state and gel-based electrolytes toward the development of full-solid-state devices. In this context, the fabrication and characterization of uncrosslinked and physically cross-linked collagen membranes are reported to test their potential as a polymeric matrix for the development of a gel electrolyte. The evaluation of the membrane's stability in water and aqueous electrolyte and the mechanical characterization demonstrated that cross-linked samples showed a good compromise in terms of water absorption capability and resistance. The optical characteristics and the ionic conductivity of the cross-linked membrane, after overnight dipping in sulfuric acid solution, demonstrated the potential of the reported membrane as an electrolyte for electrochromic devices. As proof of concept, an electrochromic device was fabricated by sandwiching the membrane (after sulfuric acid dipping) between a glass/ITO/PEDOT:PSS substrate and a glass/ITO/SnO2 substrate. The results in terms of optical modulation and kinetic performance of such a device demonstrated that the reported cross-linked collagen membrane could represent a valid candidate as a water-based gel and bio-based electrolyte for full-solid-state electrochromic devices.
Control of photophysical properties is crucial for the continued development of electroluminescent devices and luminescent materials. Preparation and study of original molecules uncovers design rules towards efficient materials and devices. Here we have prepared 7 new compounds based on the popular donor-acceptor design used in thermally activated delayed fluorescence emitters. We introduce for the first time benzofuro[3,2-e]-1,2,4-triazine and benzothieno[3,2-e]-1,2,4-triazine acceptors which were connected to several common donors: phenoxazine, phenothiazine, carbazole and 3,6-di-tert-butylcarbazole. DFT calculations, and steady-state and time-resolved photophysical studies were conducted in solution and in solid states. While derivatives with azine moieties are non-emissive in any form, the compounds comprising 3,6-di-tert-butylcarbazole display TADF in all cases. More interestingly, the two derivatives substituted with a carbazole donor are TADF active when dispersed in a polymer matrix and phosphorescent at room temperature in neat films (microcrystalline form).
Carbazoles , Luminescence , Crystallization , Triazines
The photophysical and electrochemical properties of a new class of fluorinated benzonitrile compounds substituted with mixed phenoxazine and carbazole units have been investigated. When absorbing in a large range of the UV-vis spectrum due to both localized and charge-transfer absorptions, these compounds show dual broad emission in solution and intense emission in PMMA films, with photoluminescence quantum yields changing from a few percent in solution to 18% in a more rigid environment. The compounds also exhibit thermally activated delayed fluorescence demonstrated by the role of oxygen in the quenching of delayed fluorescence and by time-resolved luminescence studies, with an efficiency directly related to the number of phenoxazine substituents. Electrochemistry reveals dramatic changes in the reduction mechanisms according to the number of remaining fluorine atoms on the benzonitrile core. All these results demonstrate how it is possible to tune the photophysical and electrochemical properties of easily synthesizable derivatives by controlling the nature and relative number of the substituents on a simple aromatic platform.
In this work, we investigate the optical and structural properties of the well-known triplet emitter bis(4',6'-difluorophenylpyridinato)-iridium(III) picolinate (FIrpic), showing that its ability to pack in two different ordered crystal structures promotes attractive photophysical properties that are useful for solid-state lighting applications. This approach allows the detrimental effects of the nonradiative pathways on the luminescence performance in highly concentrated organic active materials to be weakened. The remarkable electro-optical behavior of sky-blue phosphorescent organic light-emitting diodes incorporating crystal domains of FIrpic, dispersed into an appropriate matrix as an active layer, has also been reported as well as the X-ray diffraction, nuclear magnetic resonance, electro-ionization mass spectrometry, and scanning electron microscopy analyses of the crystalline samples. We consider this result as a crucial starting point for further research aimed at the use of a crystal triplet emitter in optoelectronic devices to overcome the long-standing issue of luminescence self-quenching.
The irreversible reaction of methyl triflate with neutral Re(I) tetrazolato complexes of the type fac-[Re(diim)(CO)3(L)], where diim is either 1,10-phenanthroline or 2,2'-bipyridine and L is a para substituted 5-aryltetrazolate, yielded the corresponding cationic methylated complexes. While methylation occurred regioselectively at the N4 position of the tetrazole ring, the cationic complexes were found to exist in solution as equilibrating mixtures of linkage isomers, where the Re(i) centre was bound to either the N1 or N2 atom of the tetrazole ring. The existence of these isomers was highlighted both by NMR and X-ray crystallography studies. On the other hand, the two isomers appeared indistinguishable by IR, UV-Vis and luminescence spectroscopy. The prepared cationic complexes are all brightly phosphorescent in fluid and rigid solutions, with emission originating from triplet metal-to-ligand charge transfer excited states. Compared to their neutral precursors, which emit from admixtures of triplet metal-to-ligand and ligand-to-ligand charge transfer states, the methylated complexes exhibit blue-shifted emission characterised by elongated excited state lifetimes and increased quantum yields. The nature of the excited states for both the neutral and the methylated complexes was probed by resonance Raman spectroscopy and with the aid of time-dependent density functional theory calculations. Lastly, both the neutral and the methylated species were used as emitting phosphors in the fabrication of Organic Light Emitting Diodes and Light Emitting Electrochemical Cells.
