Unveiling MOF-808 photocycle and its interaction with luminescent guests.

The world of metal-organic frameworks (MOFs) has become a hot topic in recent years due to the extreme variety and tunability of their structures. There is evidence of MOFs that exhibit intrinsic luminescence properties that arise directly from their organic components or from the interaction between them and metallic counterparts. A new perspective is to exploit the porous nature of MOFs by encapsulating luminescent guests, such as organic dyes, in order to explore possible changes in the luminescence activity of the combined systems. This work is focused on the optical study of zirconium-based MOF-808 and its interaction with encapsulated rhodamine B molecules. Using a plethora of different techniques, we were able to unravel its photocycle. MOF-808 displays intrinsic luminescence activity that derives from an energy transfer process from the linker to the metal sites occurring in 300 ps. The emission is a singlet-singlet transition in aqueous solution, and it is a triplet transition in powdered form. After exploring the bare MOF, we combined it with rhodamine B molecules, following an easy post-synthetic process. Rhodamine B molecules were found to be encapsulated in MOF pores and interact with the MOF's matrix through nanosecond energy transfer. We created a totally new dual-emitting system and suggested a way, based on the time-resolved studies, to clearly unravel the photocycle of MOFs from the very first photoexcitation.

Photoinduced charge separation in functional carbon-silver nanohybrids.

In recent times, nanoscience is devoting growing interest to the easy assembly of well-established nanomaterials into hybrid nanostructures displaying new emerging features. Here, we study the photophysicochemical response of binary nanohybrids obtained by the spontaneous coupling of luminescent carbon dots to silver nanoparticles with controlled surface charge. Evidence of the successful coupling is obtained by steady-state and time resolved optical measurements and further confirmed by direct imaging. We demonstrate strong interactions within nanohybrids, which can be modelled in terms of a sub-picosecond electron transfer from photoexcited carbon dots to silver nanoparticles. Accordingly, newly designed nanohybrids display significant photocatalytic performance demonstrated by the photodegradation of methylene blue under ultraviolet-visible light. Our results provide an exhaustive picture of the optical response of these self-assembled carbon-silver nanohybrids and show their promise as a new class of eco-friendly materials for light-driven catalytic applications.

Isolation of the CH3˙ rotor in a thermally stable inert matrix: first characterization of the gradual transition from classical to quantum behaviour at low temperatures.

Matrix isolation is a method which plays a key role in isolating and characterizing highly reactive molecular radicals. However, the isolation matrices, usually composed of noble gases or small diamagnetic molecules, are stable only at very low temperatures, as they begin to desegregate even above a few tens of Kelvin. Here we report on the successful isolation of CH3˙ radicals in the cages of a nearly inert clathrate-SiO2 matrix. This host is found to exhibit a comparable inertness with respect to that of most conventional noble gas matrices but it is characterized by a peculiar thermal stability. The latter property is related to the covalent nature of the host material and gives the opportunity to study the confined radicals from a few degrees of Kelvin up to at least room temperature. Thanks to this advantage we were able to explore with continuity for the first time the CH3˙ rotor properties by electron paramagnetic resonance spectroscopy, starting from the quantum rotations which are observable only at the lowest temperatures (T ≈ 4 K), going through the gradual transition to the classical motion (4 K < T < 30 K), and ending with the properties of the fully classical rotor (T > 30 K). The method of isolation presented here is found to be very effective and promising, as it is expected to be applicable to a large variety of different molecular radicals.

Stimuli-responsive photoluminescent and structural properties of MIL-53(Al) MOF for sensing applications.

Metal-organic frameworks (MOFs) are an intriguing group of porous materials due to their potential influence on the development of indispensable technologies like luminescent sensors and solid-state light devices, luminescent multifunctional nanomaterials. In this research work we explored MIL-53(Al), an exceptional class of MOF that, along with guest adsorption, undergoes structural transitions exhibiting breathing behavior between narrow pore and large pore under temperature and mechanical stress. Therefore, we opted for the time resolved luminescence and FT-Raman spectroscopy to investigate the mechanochromic and thermochromic response of this material under external stimuli. Intriguingly, when subjected to temperature changes, MIL-53(Al) exhibited a ratiometric fluorescence behavior related to the reversible relationship of photoluminescence emission intensity with respect to temperature. Moreover, under higher mechanical stress MIL-53(Al) displayed turn-on behavior in emission intensity, hence offering a thrilling avenue for the application in mechanically deformed-based luminescent sensors and ratiometric fluorescence temperature sensors.

The role of impurities in the irradiation induced densification of amorphous SiO(2).

In a recent work (Buscarino et al 2009 Phys. Rev. B 80 094202), by studying the properties of the (29)Si hyperfine structure of the E'(γ) point defect, we have proposed a model able to describe quantitatively the densification process taking place upon electron irradiation in amorphous SiO(2) (a-SiO(2)). In particular, we have shown that it proceeds heterogeneously, through the nucleation of confined densified regions statistically dispersed into the whole volume of the material. In the present experimental investigation, by using a similar approach on a wider set of materials, we explore how this process is influenced by impurities, such as OH and Cl, typically involved in relevant concentrations in commercial materials. Our results indicate that the degree of local densification within the structurally modified regions is influenced by the OH content of the material: the higher the OH content, the lower the local degree of densification of the irradiated materials. In contrast, no relevant contribution to the densification process induced by electron irradiation in a-SiO(2) can be ascribed to Cl impurities up to [Formula: see text] ppm by weight.

(29)Si hyperfine structure of the E(')(alpha) center in amorphous silicon dioxide.

We report a study by electron paramagnetic resonance on the E'(alpha) point defect in amorphous silicon dioxide (a-SiO(2)). Our experiments were performed on gamma-ray irradiated oxygen-deficient materials and pointed out that the (29)Si hyperfine structure of the E'(alpha) consists of a pair of lines split by approximately 49 mT. On the basis of the experimental results, a microscopic model is proposed for the E'(alpha) center, consisting of a hole trapped in an oxygen vacancy with the unpaired electron sp(3) orbital pointing away from the vacancy in a back-projected configuration and interacting with an extra oxygen atom of the a-SiO(2) matrix.

Delocalized nature of the E'delta center in amorphous silicon dioxide.

We report an experimental study by electron paramagnetic resonance (EPR) of E(')(delta) point defect induced by gamma-ray irradiation in amorphous SiO2. We obtained an estimation of the intensity of the 10 mT doublet characterizing the EPR spectrum of such a defect arising from hyperfine interaction of the unpaired electron with a 29Si (I=1/2) nucleus. Moreover, determining the intensity ratio between this hyperfine doublet and the main resonance line of E(')(delta) center, we pointed out that the unpaired electron wave function of this center is actually delocalized over four nearly equivalent silicon atoms.