The synthesis of O‐doped polyaromatic hydro‐ carbons in which two polycyclic aromatic hydrocarbon sub units are bridged through one or two O atoms has been achieved. This includes high‐yield ...ring‐closure key steps that, depending on the reaction conditions, result in the formation of furanyl or pyranopyranyl linkages through intramolecular C−O bond formation. Comprehensive photophysical measurements in solution showed that these compounds have exceptionally high emission yields and tunable absorption properties throughout the UV/Vis spectral region. Electrochemical investigations showed that in all cases O annulation increases the electron‐donor capabilities by raising the HOMO energy level, whereas the LUMO energy level is less affected. Moreover, third‐order nonlinear optical (NLO) measurements on solutions or thin films containing the dyes showed very good values of the second hyperpolarizability. Importantly, poly(methyl methacrylate) films containing the pyranopyranyl derivatives exhibited weak linear absorption and NLO absorption compared to the nonlinearity and NLO refraction, respectively, and thus revealed them to be exceptional organic materials for photonic devices.
Staring at the rainbow: π‐extended O‐doped polycyclic aromatic hydrocarbons were synthesized by high‐yielding ring‐closure reactions leading to the formation of furanyl or pyranopyranyl frameworks. Photophysical and electro chemical characterization showed that these compounds exhibit finely tunable optoelectronic properties (see figure), while third‐order nonlinear optical measurements revealed very good second hyperpolarizability values, which make them exceptional candidates for photonic applications.
A dendrimer bearing two cyclam units linked by an azobenzene moiety, and luminescent naphthalene units at the periphery performs three different functions (light-harvesting, photoisomerization and ...coordination of metal ions) which can cooperate or interfere depending on the nature of the metal ion. It is thus an example of light controlled molecular tweezers in which Zn(II) coordination allows 100% efficient photosensitization of azobenzene switching, while Cu(II) shuts down azobenzene isomerization.
A shape‐persistent molecule, featuring four bipyridinium units, has been synthesized that upon reduction undergoes intermolecular pimerization because of the rigid architecture of the molecule. The ...pimerization process has been investigated by a variety of techniques, such as absorption measurements, EPR spectroscopy, as well as gamma and pulse radiolysis, and compared with the behavior of a model compound. Computational studies have also been performed to support the experimental data. The most interesting feature of the tetramer is that pimerization occurs only above a threshold concentration of monoreduced species, on the contrary to the model compound. Furthermore, there is an increase of the apparent pimerization constant by increasing the concentration of reduced bipyridinium units. These results have been interpreted by the fact that pimerization is favored in the tetrahedrally shaped molecule because of a cooperative mechanism. Each multiply reduced molecule can indeed undergo multiple intermolecular interactions that enhance the stabilization of the system, also leading to hierarchical supramolecular growth. The resulting supramolecular system formed by such intermolecular pimerization should exhibit a diamond‐like structure, as suggested by a simplified modeling approach. The intermolecular nature of the pimerization process occurring in the tetramer has been demonstrated by measuring the corresponding bimolecular rate constant by pulsed radiolysis experiments.
Stay in shape: A shape‐persistent molecule made of four tetrahedrally arranged bipyridinium units undergoes intermolecular pimerization upon reduction, leading to hierarchical supramolecular growth (see figure).
The electrochemistry, photophysics, and electrochemically generated chemiluminescence (ECL) of a family of polysulfurated dendrimers with a pyrene core have been thoroughly investigated and ...complemented by theoretical calculations. The redox and luminescence properties of dendrimers are dependent on the generation number. From low to higher generation it is both easier to reduce and oxidize them and the emission efficiency increases along the family, with respect to the polysulfurated pyrene core. The analysis of such data evidences that the formation of the singlet excited state by cation–anion annihilation is an energy‐deficient process and, thus, the ECL has been justified through the triplet–triplet annihilation pathway. The study of the dynamics of the ECL emission was achieved both experimentally and theoretically by molecular mechanics and quantum chemical calculations. It has allowed rationalization of a possible mechanism and the experimental dependence of the transient ECL on the dendrimer generation. The theoretically calculated Marcus electron‐transfer rate constant compares very well with that obtained by the finite element simulation of the whole ECL mechanism. This highlights the role played by the thioether dendrons in modulating the redox and photophysical properties, responsible for the occurrence and dynamics of the electron transfer involved in the ECL. Thus, the combination of experimental and computational results allows understanding of the dendrimer size dependence of the ECL transient signal as a result of factors affecting the annihilation electron transfer.
Generation gap: Pyrene‐based dendrimers show stable and intense electrochemiluminescence (ECL) emission thanks to their excellent photophysical and electrochemical properties (see figure; kann=rate of electron transfer in the annihilation process). They are therefore an ideal benchmark to establish how and to what extent structural and electronic parameters affect and control the dynamics of the ECL properties.
Recent inve-stigations have shown that coupling luminescence with dendrimer chemistry can lead to systems capable of exhibiting quite unusual and interesting properties. In this trend article we ...focus on: (i) interactions of luminescent units within a dendrimer, (ii) quenching of dendrimer luminescence by external species, (iii) sensitization of luminescent metal ions, and (iv) sensitization and quenching of dye luminescence. Several examples of dendrimers are discussed, including compounds capable of: (a) performing excimer and/or exciplex emission, (b) exploiting energy transfer for harvesting light, (c) undergoing electron transfer processes, (d) protecting with their branches the luminescence of the core, (e) down and up converting light frequency, (f) playing the role of ligands for luminescent and non-luminescent metal ions, and (g) performing as hosts for luminescent dyes.
Defocused wide-field fluorescence microscopy was used to follow the 3D molecular rotational diffusion of a fluorescent probe molecule in a polymer thin film. The technique allows for visualizing the ...molecular reorientation both in-plane and out-of-plane. The local environmental change driven by heterogeneous dynamics of the polymer can be probed on a scale of 1μm as demonstrated by parallel imaging of several molecules. A multi-component rotational diffusion decay is observed which might reflect both different relaxation regimes of the polymer as well as rapid changes of the local environment.
In recent years, there has been significant research on integrated microfluidic devices. Microfluidics offer an advantageous platform for the parallel laminar flow of adjacent solvents of potential ...use in modern chemistry and biology. To reach that aim, we worked towards the realization of a buried microfluidic Lab-on-a-Chip which enables the separation of the two components by exploiting the non-mixing properties of laminar flow. To fabricate the aforementioned chip, we employed a femtosecond laser irradiation technique followed by chemical etching. To optimize the configuration of the chip, several geometrical and structural parameters were taken into account. The diffusive mass transfer between the two fluids was estimated and the optimal chip configuration for low diffusion rate of the components was defined.
The development of nanoscale systems capable to perform specific functions under external control is a challenging task and a fascinating objective in Chemistry. Photochromic compounds undergo ...radical changes in their physico‐chemical properties upon light excitation, for this reason they are valuable building blocks for the construction of photo‐controllable molecular devices, machines and materials. The E–Z photoisomerization of azobenzene has been known for almost 80 years and – owing to its high efficiency and excellent reversibility – has been widely employed to introduce an element of photo‐control in a large variety of compounds, biomolecules, nanosystems and materials. Here we present some of our research results highlighting how this outstanding photochrome can be utilized to develop systems with light‐induced functionalities.
In this Account we report some examples in which azobenzene moieties are contained into supramolecular structures and how the photoisomerization achieves precise and clean control on some of their properties. These results highlight how this outstanding photochrome can be used to develop molecular devices and machines with light‐induced functionalities.
We have synthesized a novel class of dendrimers, consisting of a polysulfurated pyrene core with appended poly(thiophenylene) dendrons (PyG0, PyG1, and PyG2, see Scheme 1), which exhibit remarkable ...photophysical and redox properties. In dichloromethane or cyclohexane solution they show a strong, dendron‐localized absorption band with a maximum at around 260 nm and a band in the visible region with a maximum at 435 nm, which can be assigned to the pyrene core strongly perturbed by the four sulfur substituents. The dendrimers exhibit a strong (Φ=0.6), short‐lived (τ=2.5 ns) core‐localized fluorescence band with maximum at approximately 460 nm in cyclohexane solution at 293 K. A strong fluorescence is also observed in dichloromethane solution at 293 K, in dichloromethane/chloroform rigid matrix at 77 K, and in the solid state (powder) at room temperature. The dendrimers undergo reversible chemical and electrochemical one‐electron oxidation with formation of a strongly colored deep blue radical cation. A second, reversible one‐electron oxidation is observed at more positive potential values. The photophysical and redox properties of the three dendrimers are finely tuned by the length of their branches. The strong blue fluorescence and the yellow to deep blue color change upon reversible one‐electron oxidation can be exploited for optoelectronic devices.
Optoelectronic devices: Dendrimers containing a pyrene core and polysulfurated branches show intense blue luminescence and a reversible color change from light yellow to deep blue upon mono‐oxidation by chemical (AuCl4−) or electrochemical methods (see picture). These properties could be exploited for optoelectronic devices.
Silicon nanocrystals with an average diameter of 5 nm and functionalized with 4,7-di(2-thienyl)-2,1,3-benzothiadiazole chromophores (TBT) and dodecyl chains exhibit near-infrared emission upon ...one-photon (1P) excitation at 515 nm and two-photon (2P) excitation at 960 nm. By using TBT chromophores as an antenna, we were able to enhance both 1P and 2P absorption cross-sections of the silicon nanocrystals to more efficiently excite their long-lived luminescence. These results chart a path to 2P-excitable imaging probes with long-lived oxygen-independent luminescence, a rare combination of properties that should allow for a substantial increase in imaging contrast.
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•Silicon nanocrystals (SiNCs) are weak light absorbers but are highly luminescent•Excitation of benzothiadiazole chromophores attached to SiNCs sensitizes core emission•An antenna by two-photon excitation is coupled to long-lived SiNC luminescence
Silicon nanocrystals (SiNCs) occupy a niche in the realm of quantum dots and offer several advantages: silicon is abundant, is essentially non-toxic, and can form robust chemical bonds with ligands at the nanocrystal surface. From the optical point of view, SiNCs are strong light emitters but weak absorbers. The latter drawback was circumvented by their surface functionalization with chromophores, which are strong light absorbers and are able to efficiently transfer the excitation energy to the silicon core. Here, we present a step further: functionalization of SiNCs with multiple two-photon (2P) absorbing chromophores forming a 2P light-harvesting antenna. The 2P antenna enables excitation of SiNCs in the near-infrared region of the spectrum, charting a path to applications in bioimaging. Long-lived oxygen-insensitive luminescence and 2P excitability are a rare combination of properties, which could allow unprecedentedly high contrast in images from scattering biological tissues.
Silicon nanocrystals with an average diameter of 5 nm have been functionalized with dye molecules at their surfaces. The resulting nanostructures work as molecular light-harvesting antennae: upon one-photon excitation at 515 nm or two-photon excitation at 960 nm of the peripheral dyes, the excitation energy is funneled to the silicon core and results in sensitized emission in the near-infrared spectral region. The resulting emission is long lived and oxygen independent, two important properties for a substantial increase in imaging contrast.
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