O comportamento eletroquímico do ácido cafeico (H 3 CAF) em meio aquoso foi estudado na faixa de pH 2,0-8,5 aplicando-se as técnicas voltametria cíclica, eletrólise com potencial controlado e espectroscopia UV-vis. A eletro-oxidação envolve a transferência reversível de dois elétrons e de dois prótons em soluções de pH até 5,5, de acordo com o mecanismo uma etapa-dois elétrons. Em soluções de pH superiores a 5,5 a eletro-oxidação do H 3 CAF segue um mecanismo EC i . O principal produto desta oxidação é a o-quinona correspondente (o-HCAF), a qual decompõe-se rapidamente em soluções de pH superior a 7,4, obedecendo a uma cinética de primeira ordem. Na faixa de pH 2,0-8,5, o potencial formal (E 0' ) varia linearmente com o pH, gerando uma reta com coeficiente angular de -60,83 mV/pH. Em paralelo, a corrente de pico anódica (i pa ) diminui de modo não-linear. Os processos são controlados por difusão em toda a faixa de pH estudada.The electrochemical behavior of caffeic acid (H 3 CAF) in aqueous solutions with pH 2.0 to 8.5 was studied by cyclic voltammetry, controlled potential electrolysis and UV-vis spectroscopy. The electrooxidation of H 3 CAF involves a reversible transfer of two electrons and two protons in solutions of pH up to 5.5, in agreement with the one step-two electron mechanism. In solutions of pH higher than 5.5, the process of electro-oxidation of H 3 CAF follows an EC i mechanism. The main oxidation product is the corresponding o-quinone (o-HCAF), which is decomposed quickly at pH higher than 7.4 obeying a first order kinetics. In the pH range investigated, the formal potential (E 0' ) varies linearly with pH, generating a straight line with an angular coefficient of -60.83 mV/pH. In parallel, the anodic peak current (i pa ) decreases in a nonlinear mode. The processes are controlled by diffusion over the whole pH range studied.

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Two series of flexible dicyanomethylene compounds, specifically, class 1 and class 2 compounds, have been designed and synthesised. In class 1 compounds, the dicyanomethylene groups are separated by glycol chain spacers of different lengths, whereas, in class 2 compounds, the spacers are alkyl linkers of different lengths. The notion underlying the design of these compounds is that in class 1 molecules, the spacers contain donor oxygen atoms that could not only form hydrogen bonds during the course of crystal packing but also promote withdrawing effects that modify the photophysical and electrochemical properties of these molecules in solution; in contrast, these effects would be absent for class 2 molecules. However, this study revealed that, with respect to crystal packing, the size of the spacers and their even and odd numbers of atoms are more important than their chemical nature. All of the synthesised compounds exhibited blue emission in the solid state and in CH2Cl2 solutions. The photophysical and electrochemical properties of these compounds in solution were not significantly affected by the type and length of the spacer that was used in each molecule. In the solid state, however, the compound with the shortest spacer showed the highest Stokes shift. The electronic transitions for the synthesized compounds in solution were explained by density functional theory and time-dependent density functional theory calculations, which indicated that the methylene moieties control the properties of both classes of compounds and that the spacers do not conjugate with the end groups. These two series of flexible dicyanomethylene compounds could be utilised as molecular building blocks for the development of new solids with novel properties.

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