Incorporation of Anthracene into Zeolites: Confinement Effect on the Recombination Rate of Photoinduced Radical Cation-Electron Pair

Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation

Silica-based materials (SBMs) are widely used in catalysis, photonics, and drug delivery. Their pores and cavities act as hosts of diverse guests ranging from classical dyes to drugs and quantum dots, allowing changes in the photochemical behavior of the confined guests. The heterogeneity of the guest populations as well as the confinement provided by these hosts affect the behavior of the formed hybrid materials. As a consequence, the observed reaction dynamics becomes significantly different and complex. Studying their photobehavior requires advanced laser-based spectroscopy and microscopy techniques as well as computational methods. Thanks to the development of ultrafast (spectroscopy and imaging) tools, we are witnessing an increasing interest of the scientific community to explore the intimate photobehavior of these composites. Here, we review the recent theoretical and ultrafast experimental studies of their photodynamics and discuss the results in comparison to those in homogeneous media. The discussion of the confined dynamics includes solvation and intra- and intermolecular proton-, electron-, and energy transfer events of the guest within the SBMs. Several examples of applications in photocatalysis, (photo)sensors, photonics, photovoltaics, and drug delivery demonstrate the vast potential of the SBMs in modern science and technology.

Activation energies of photoinduced unimolecular, bimolecular and termolecular processes on silica gel surfaces

Activation energies for energy and electron transfer have been measured in various systems on silica gel. In the case of ion-electron recombination, a facile technique involving fluorescence recovery is described which complements diffuse reflectance spectroscopy in the study of these systems. In bimolecular anthracene/azulene systems, activation energies have been shown to be independent of pre-treatment temperature in the range 25-210 °C, demonstrating that physisorbed water plays little role in determining diffusion rates on silica gel. In a ternary anthracene/azulene/perylene system, we have for the first time presented comparative activation energies for the diffusion of azulene and its radical cation, and have shown a greater activation energy for diffusion of the latter species.

Incorporation and electron transfer of anthracene in pores of ZSM-5 zeolites. Effect of Brønsted acid site density

The sorption course of anthracene (ACENE-3) into Brønsted-acidic medium pore MFI zeolites was monitored by in situ EPR and diffuse reflectance UV-visible absorption over one year. Weighed amounts of solid ACENE-3 were merely exposed to H(n)ZSM-5 (H(n)(AlO(2))(n)(SiO(2))(96-n)), with the following Brønsted acid site (BAS) densities, n = 0.0, 0.17, 0.57, 0.95, 2.0, 3.4, 6.6, dehydrated at 623 K under argon. The weighed amounts correspond to 1 ACENE-3 per zeolite unit cell. ACENE-3 is found to be incorporated as intact molecules in purely siliceous MFI (silicalite-1). Monte Carlo simulations indicate that ACENE-3 lies in the intersection of straight and zigzag channels. In contrast, the presence of BASs on the inner surface of channels induces spontaneous ionization of ACENE-3 (ionization potential = 7.44 eV). The charge separation as ACENE-3*(+)@H(n)ZSM-5*(-) is caused by the strong Coulombic field gradient of Si-O(-)(H(+))-Al BAS in the absence of any Lewis acid site. The rate and yield of ionization are found to increase dramatically with BAS density increase. The stabilization of ACENE-3*(+)@H(n)ZSM-5*(-) is explained by the tight fit between the rod-shape ACENE-3 and the channel dimensions and especially by the compartmentalization of ejected electrons as AlO(4)H*(-) centers away from the initial site of ionization. The final charge recombination occurs after more than one year and leads to ACENE-3 occluded in the straight channel in close proximity to BAS without any protonation of ACENE-3 (pK(a) = -13.5).

Synthesis, photophysical, electrochemical, and electrochemiluminescent properties of 5,15-bis(9-anthracenyl)porphyrin derivatives

Novel 5,15-bis(9-anthracenyl)porphyrin derivatives (, ) were synthesized by stepwise Suzuki-type coupling reactions using anthracenyl-boronates bearing various electronically active moieties. Absorption spectra of these porphyrin conjugates reveal some degree of delocalisation with the directly linked chromophores, particularly in the case of anthracenyl-porphyrin bearing dimethylanilino moieties at the two extremities. Fluorescence and 77 K phosphorescence properties indicate that the excitation energy is invariably funnelled to the lowest singlet and triplet states of the porphyrin chromophore. The latter levels have been probed also by transient absorption spectroscopy, showing the typical triplet features detected in meso-substituted porphyrins. Extensive electrochemical studies have been performed to unravel the electronic properties of the newly synthesized porphyrins. Low-temperature cyclic voltammetry investigations showed that the anthracenyl-porphyrins are capable of undergoing as many as four electron transfer processes. In particular, by means of UV-Vis-NIR spectroelectrochemical measurements, a NIR-centred intramolecular photoinduced intervalence charge transfer (IV-CT) from a neutral N,N-dimethylanilino moiety to the N,N-dimethylanilino radical cation has been observed for the doubly-oxidised porphyrin (2+). The molecules also showed unexpected electrogenerated chemiluminescence properties, which revealed to be largely controlled by the electronic characteristics of the peripheral anthracenyl substituents. The structural and the electronic properties of these complexes have been also characterised by DFT calculations, as well as by X-ray crystallographic analyses.