Daniele Marinotto

Ultrafast resonant optical switching in isolated polyfluorenes chains

Using ultrafast spectroscopy, we report evidence of monodimensional photophysics coming from isolated chains of poly(9,9-dioctylfluorene) dispersed in a polymethylmethacrylate matrix. We identify intrachain charge carriers dynamics, the decay taking place in about 1 ps. Using a three-pulse experiment we bring evidence for charges generation from higher lying states and we report an ultrafast optical gain switching in the stimulated emission region over a broad wavelength range (around 100 nm). Our experiments suggest that the expected maximum rate of on–off switching in such a molecular device can be as high as 300 GHz.

Polymorphism-dependent aggregation induced emission of a push-pull dye and its multi-stimuli responsive behavior†

A comprehensive optical investigation of 1,1-dicyano-2,2-bis(4-dimethylaminophenyl)ethylene (1) is presented. The compound crystallizes in four different forms all displaying AIE behavior. The crystalline forms A and B are yellow-orange-emissive, while C and D are green-emissive. On the basis of X-ray structural analysis, the weak intermolecular interactions account for restricted internal rotations, leading to fluorescence enhancement in the crystals; however, the difference in emission color is ascribed to the various conformations of the molecules in the four crystalline forms. In addition, the emission color of crystals of A can be tuned by heating and grinding, that of B by grinding only, while crystals of C show chronochromic behavior. An explanation for such a rich variety of luminescence behavior is formulated here through the use of steady state and time resolved photoluminescence, X-ray diffraction analysis and DFT and TDDFT calculations. The involved chromic mechanism appears to be mainly associated with surface defects induced by the external stimuli rather than an amorphization process, as frequently observed for other stimuli responsive compounds.

H-Aggregates Granting Crystallization-Induced Emissive Behavior and Ultralong Phosphorescence from a Pure Organic Molecule

Solid-state luminescent materials with long lifetimes are the subject of ever-growing interest from both a scientific and a technological point of view. However, when dealing with organic compounds, the achievement of highly efficient materials is limited by aggregation-caused quenching (ACQ) phenomena on one side and by ultrafast deactivation of the excited states on the other. Here, we report on a simple organic molecule, namely, cyclic triimidazole (C9H6N6), 1, showing crystallization-induced emissive (CIE) behavior and, in particular, ultralong phosphorescence due to strong coupling in H-aggregated molecules. Our experimental data reveal that luminescence lifetimes up to 1 s, which are several orders of magnitude longer than those of conventional organic fluorophores, can be realized under ambient conditions, thus expanding the class of organic materials for phosphorescence applications.

4D–π–1A type β-substituted ZnII-porphyrins: ideal green sensitizers for building-integrated photovoltaics

Two novel green β-substituted ZnII-porphyrins, G1 and G2, based on a 4D-π-1A type substitution pattern have been synthesized. Their enhanced push-pull character, by reduction of H-L energy gaps, promotes broadening and red-shifting of absorption bands. The effective synthetic pathway and the remarkable spectroscopic properties make G2 ideal for BIPV application.

Cyclic Triimidazole Derivatives: Intriguing Examples of Multiple Emissions and Ultralong Phosphorescence at Room Temperature

The performance of solid luminogens depends on both their inherent electronic properties and their packing status. Intermolecular interactions have been exploited to achieve persistent room-temperature phosphorescence (RTP) from organic molecules. However, the design of organic materials with bright RTP and the rationalization of the role of interchromophoric electronic coupling remain challenging tasks. Cyclic triimidazole has been shown to be a promising scaffold for such purposes owing to its crystallization-induced room-temperature ultralong phosphorescence (RTUP), which has been associated with H-aggregation. Herein, we report three triimidazole derivatives as significant examples of multifaceted emission. In particular, dual fluorescence, RTUP, and phosphorescence from the molecular and supramolecular units were observed. H-aggregation is responsible for the red RTUP, and Br substituents favor yellow molecular phosphorescence while halogen-bonded Br⋅⋅⋅Br tetrameric units are involved in the blue-green phosphorescence.

In situ growth in a PMMA film of oriented nanocrystals of the hybrid inorganic–organic acentric material [(E)-N,N-dimethylamino-N′-methylstilbazolium][Cu5I6]

A poly(methyl methacrylate) (PMMA) composite film containing 4 wt% of the strongly second order NLO active ionic material [(E)-N,N-dimethylamino-N′-methylstilbazonium][Cu5I6] (1), obtained by spin coating of a CH3CN solution on a glass substrate, shows by a two step process (electric poling followed by controlled thermal annealing up to 140 °C) in situgrowth of nanocrystals, some of which are aligned, with a size of less than 400 nm as supported by TEM and GIXRD evidence. A long lasting second harmonic generation (SHG) is produced which is stable even after exposure to water vapour for one night.

Highly efficient acido-triggered reversible luminescent and nonlinear optical switch based on 5-π-delocalized-donor-1,3-di(2-pyridyl)benzenes

The acidochromic behaviour of trans-5-(p-(N,N-diphenylamino)styryl)-1,3-di(2-pyridyl)benzene (1) and 5-(p-(N,N-diphenylamino)phenylethynyl)-1,3-di(2-pyridyl) benzene (2) was investigated, both in solution and doped into films, studying their linear and nonlinear optical properties. Remarkably, the efficient luminescence of 1 drops drastically upon protonation and is restored upon deprotonation – both in solution and in the solid state – leading to an excellent on/off emissive acido-triggered reversible switch. In parallel, protonation leads to a large enhancement of the second-order NLO response, as determined by the EFISH technique, which is reversible upon deprotonation. The related methylated salts of 1 and 2 are similarly characterized by a high NLO response.

Spontaneous in situ thermal growth of oriented acentric nanocrystals of [(E)-N,N-dimethylamino-N′-methylstilbazolium][p-toluenesulfonate] embedded in a PMMA film on ITO generating stable SHG

A poly(methyl methacrylate) (PMMA) composite film containing 4 wt% of the strongly second order NLO active ionic material [(E)-N,N-dimethylamino-N′-methylstilbazolium][p-toluenesulfonate] (DAST) deposited on an ITO substrate shows by a two step process (electric poling followed by controlled thermal annealing) a nonlinear optical coefficient d33 higher than the one obtained on a glass substrate. This higher value is the result of a polar order induced on the dye-doped polymeric film at the ITO–film interface, indicating that the polarity of the ITO surface is relevant in controlling the orientation process. By exploiting such induced polar order, oriented dipolar acentric nanocrystals of DAST can be grown in situ by solely and careful annealing, thus avoiding the detrimental effects often induced by the presence of a strong electric field. This leads not only to a spontaneous one step process of self-orientation but also to a better second order NLO response. In addition, different thermal pretreatments of ITO itself can induce different SHG responses from the DAST/PMMA films, the highest response being observed by preheating the ITO substrate at 370 °C for 24 hours.

Effect of Salicylic Acid and 5-Sulfosalicylic Acid on UV-Vis Spectroscopic Characteristics, Morphology, and Contact Angles of Spin Coated Polyaniline and Poly(4-aminodiphenylaniline) Thin Films

Polyaniline and poly(4-aminodiphenyl)aniline have been prepared following two different synthetic protocols (a traditional method and a “green” method). Both the polymers have been spin coated with salicylic acid and 5-sulfosalicylic acid as the dopants, in order to obtain them in form of thin films. These materials have been characterized, thereof achieving important information on their water contact angles and surface morphology.

A nonlinear optical active polymer film based on Pd(II) dithione/dithiolate second-order NLO chromophores

A film of [Pd(R2pipdt)(dmit)] (1), where R2pipdt = 1,4-didodecyl-piperazine-2,3-dithione (acceptor) and dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate (donor) incorporated into a polymethylmethacrylate (PMMA) matrix, showing a good second-harmonic generation, has been prepared for the first time in the class of dithione-dithiolate 2nd order NLO-chromophores. Moreover full characterization of 1, including molecular second-order NLO properties in solution, is reported.