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The dynamics of the elementary electron transfer step between pheophytin and primary ubiquinone in bacterial photosynthetic reaction centers is investigated by using a discrete state approach, including only the intramolecular normal modes of vibration of the two redox partners. The whole set of normal coordinates of the acceptor and donor groups have been employed in the computations of the Hamiltonian matrix, to reliably account both for shifts and mixing of the normal coordinates, and for changes in vibrational frequencies upon ET. It is shown that intramolecular modes provide not only a discrete set of states more strongly coupled to the initial state but also a quasicontinuum of weakly coupled states, which account for the spreading of the wave packet after ET. The computed transition probabilities are sufficiently high for asserting that electron transfer from bacteriopheophytin to the primary quinone can occur via tunneling solely promoted by intramolecular modes; the transition times, computed for different values of the electronic energy difference and coupling term, are of the same order of magnitude (10(2) ps) of the observed one.

Four conjugated donor-acceptor compounds featuring a central dithieno[3,2-b:2′,3′-d]thiophene-4,4′-dioxide (do-DTT) core, with either a symmetric (DTT-H2, O2, S2) or nonsymmetric (DTT-H1) structure, have been designed based on DFT computational investigations and prepared using direct arylation reactions as the key C-C bond-forming steps. Spectroscopic analysis of the compounds in solution, carried out with both stationary and time-resolved techniques, confirmed that they have properties compatible with application as fluorescent emitters in Luminescent Solar Concentrators (LSCs). Accordingly, their performances were initially screened in thin-film LSCs employing poly(methyl methacrylate) (PMMA) as host matrix. The devices fabricated with the emitter DTT-S2, featuring thiomethyl-substituted donor groups, appeared very promising, with a good external photon efficiency (ηext) of up to 5.6%, accompanied by a notable internal photon efficiency (ηint) of up to 43%. Due to these favorable characteristics, this compound was selected as the emitter for PMMA-based slab LSC devices (5 × 5 × 0.3 cm3) fabricated using regenerated MMA. Remarkably, a ηext of 6.7% was reached together with a fluorescence quantum yield (Φfl) of 90%, which resulted in a device efficiency of 0.74% once the LSC was coupled with a Si-PV cell. In addition, a preliminary stability assessment of the doped slabs, conducted by an accelerated protocol, provided encouraging results, with the nonsymmetric emitter DTT-H1 being able to retain >90% of its initial emission intensity after 960 h of simulated time.

Fast and efficient intramolecular energy transfer takes place in the umbelliferone–alizarin bichromophore; the process is well described by the Förster mechanism.

Luminescent solar concentrators (LSCs) is a technology developed since the 1970s[1] with the aim of obtaining large-area, semi-transparent and cheap photovoltaic devices capable of concentrating solar radiation on small solar cells at their margins. Specifically, they consist of a panel of a standard plastic material (e. g., poly(methyl methacrylate), PMMA) in which a fluorescent compound, able of absorbing direct and indirect sunlight radiation and emitting it at a different, usually longer wavelength, is dispersed. Commonly used fluorescent compounds can be quantum dots, perovskites, rare-earth complexes and organic molecules[2]. Thanks to the different refractive indexes of air and the plastic material, the emitted radiation is mainly concentrated via total internal reflection at the edge of the panel, where the solar cells are usually placed, making the device less dependent on light orientation. This, together with the aesthetic characteristics (colour and shape tunability), allows their use in building-integrated photovoltaics (BIPVs)[3]. In order to obtain high-performance LSC devices, a careful study of the materials used for their assembly must be performed, both concerning the selection of the fluorophore and the plastic material in which it is dispersed[2]. We recently synthetized and investigated the properties of a series of organic fluorophores with donor-acceptor-donor (D-A-D) structure, characterized by a benzo[1,2-d:4,5-d']bisthiazole[4] and quinoxaline[5] as acceptor core. The optical properties of the molecules were investigated in solution as well as after dispersion in PMMA and its copolymer with more apolar cyclohexyl methacrylate repeating units. The variation of the absorption and emission maxima, the fluorescence quantum yields and the optical efficiency of the corresponding LSC devices were eventually determined. Due to the very good fluorophores compatibility with the polymeric matrices, LSCs with optical features superior to the state-of-the-art were obtained[4,5].

In recent years visible pump/mid-infrared (IR) probe spectroscopy has established itself as a key technology to unravel structure-function relationships underlying the photo-dynamics of complex molecular systems. In this contribution we review the most important applications of mid-infrared absorption difference spectroscopy with sub-picosecond time-resolution to photosynthetic complexes. Considering several examples, such as energy transfer in photosynthetic antennas and electron transfer in reaction centers and even more intact structures, we show that the acquisition of ultrafast time resolved mid-IR spectra has led to new insights into the photo-dynamics of the considered systems and allows establishing a direct link between dynamics and structure, further strengthened by the possibility of investigating the protein response signal to the energy or electron transfer processes. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Luminescent solar concentrators (LSCs) are a class of optical devices able to harvest, downshift, and concentrate sunlight, thanks to the presence of emitting materials embedded in a polymer matrix. Use of LSCs in combination with silicon-based photovoltaic (PV) devices has been proposed as a viable strategy to enhance their ability to harvest diffuse light and facilitate their integration in the built environment. LSC performances can be improved by employing organic fluorophores with strong light absorption in the center of the solar spectrum and intense, red-shifted emission. In this work, we present the design, synthesis, characterization, and application in LSCs of a series of orange/red organic emitters featuring a benzo[1,2-b:4,5-b']dithiophene 1,1,5,5-tetraoxide central core as an acceptor (A) unit. The latter was connected to different donor (D) and acceptor (A') moieties by means of Pd-catalyzed direct arylation reactions, yielding compounds with either symmetric (D-A-D) or non-symmetric (D-A-A') structures. We found that upon light absorption, the compounds attained excited states with a strong intramolecular charge-transfer character, whose evolution was greatly influenced by the nature of the substituents. In general, symmetric structures showed better photophysical properties for the application in LSCs than their non-symmetric counterparts, and using a donor group of moderate strength such as triphenylamine was found preferable. The best LSC built with these compounds presented photonic (external quantum efficiency of 8.4 ± 0.1%) and PV (device efficiency of 0.94 ± 0.06%) performances close to the state-of-the-art, coupled with a sufficient stability in accelerated aging tests.