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- Synthesis and photophysical properties of tetraphenylethylene derivatives as luminescent downshifting materials for organic photovoltaic applicationsPublication . Barros, Helio; Esteves, M. Alexandra; Brites, Maria JoãoABSTRACT: Luminescent Down-Shifting (LDS) is an optical approach applied in several photovoltaic (PV) technologies in which high energy solar radiation is converted to a wavelength region where the response of the photovoltaic devices is better. The use of LDS layers on organic photovoltaics (OPV) could serve two purposes: to prevent cell degradation by filtering the incident ultraviolet (UV) radiation and to improve the spectral response of PV cells at short-wavelength. This work reports, the design and synthesis of a series of tetraphenylethylene (TPE) derivatives based on TPE-A or A-pi-TPE-pi-A molecular structure featuring various electron-acceptor (A) groups. The photo-physical properties of the new LDS compounds were systematically studied in 1,4-dioxane solution and film (Zeonex) by UV-visible absorption and fluorescence spectroscopy, and electrochemical properties studied by cyclic voltammetry. Thermal stability of the new LDS compounds was evaluated by Thermogravimetric analysis (TGA). Theoretical computational studies provided evidence of existence of intramolecular charge-transfer (ICT) between frontier orbitals of donor and acceptor moieties. The good photophysical and thermal properties of the synthesized TPE derivatives, associated with high molar absorption coefficients in UV spectrum and emission maximum in the range of 476-531 nm, make them promising candidates for LDS layers in OPV application.
- Synthesis and photophysical behavior of fluorescent benzazole dyes and fluorescent microparticles: Their use as fingerprint developerPublication . Barros, Helio; Stefani, ValterABSTRACT: Novel benzazole derivatives were synthesized and their photophysical properties were determined experimentally in different types of solvents also by theoretical calculations using Density Functional Theory (DFT). The results of the theoretical computational studies were well consistent with those observed experimentally, evidencing the type of electronic transitions that occurs between the frontiers orbitals. All synthesized derivatives are highly fluorescent in the violet-green region (363-536 nm) under ultraviolet light of 365 nm with large Stokes shift due to excited state intramolecular proton transfer (ESIPT) mechanism. Furthermore, the thermogravimetric and differential thermogravimetric analysis showed their good thermal stability, with decomposition temperature up to 250 degrees C. The fluorescent benzazole derivative was successfully entrapped into starch-based microparticles and applied for visualization of latent fingerprints on different types of surfaces common to forensic cases (metal, plastic, paper, wood, and ceramic).