Page - 70 - in Advanced Chemical Kinetics

may give some important information on the photochemical characteristics of several other β-diketones inwater. The energy transfer in organic systems can also beused tomonitor distinct environments by enabling severalmechanismsof tracking the changes in the electronic excited states involved in thephotophysical orphotochemicalprocesses. Sensingand imagingare, therefore,ways to collect informationondistinct environments. In our researchgroup [25],wehave focusedon theproposal of newmaterials that are able to efficientlyformenergytransfercomplexesandgiverisetonewphotophysicalcharacteristicsthat are very sensitive to specific environmental changes. An example is a newmaterial based on supramolecular structuresof adipeptide,diphenylalanine, composinganexciplexwitha chro- mophore, coumarin. In distinct proportions, this systemwas able tomodulate the coumarin sensibilitytoO2(g)dissolvedinwater,presentingdistinct fluorescencespectrafromthatexpected forcoumarin,whichwasaresultoftheenergytransfercomplexformationandthenewelectronic excitedstatesthatresultedfromtheinteractionsbetweenthecomponents.Wangetal. [8],onthe other hand, developed amethod for monitoring photochemical reaction kinetics, presenting spatialresolution, thelaser-excitedmuonpump-probespinspectroscopy(photo-μSR).Withthis, they expected to monitor the dynamic of excitations and to explore the mechanism of photophysical andphotochemical processes. Using pentacene as subject, they temporally and spatiallymappedtheseprocessesat thesingle-carbonlevelandobservedthat thephotochemical reactivityofaspecificcarbonatomismodifiedinthepresenceofaspecificexcitedstate. Energy conversion can also be based on hole transfers or proton transfers and can involve photophysical processes, photochemical reactions or both processes in a collaborative way. Elbin andBazan [7] proposed a newelectron-deficient compoundbased on three-coordinate boryl substituents adjacent to highly conjugated distyrylbenzene derivative (DSB) or poly (aryleneethynylene)s (PAE). In thesematerials, boron atomprovides a vacant pz-orbital that confers thema strong electron acceptor character, enabling a significant delocalization. They showed that due to the distinct photophysical characteristics of the constituents, the excited state migration by FRET is modulated and, depending on the substituent, light of distinct colors are emitted from these systems. Based on that, they believed that thesematerials can findapplication indisplays. Also based on hole transfer to promote energy conversion is the electrochemical energy conversion in a system called fuel cell. It consists of an additional way for chemical energy conversion, without photocatalytic effect. It is an electrochemical system which converts chemical energy into electricity through the oxidation of a fuel [26, 27],which takes place in theanodeof thecell, andthereductionof theoxygenfromatmosphere inthecathode.Someof these fuel cells are classifiedby temperatureoperation [28], especially,ProtonExchangeMem- brane Fuel Cells (PEMFCs) work at low temperatures (from room to 100 C) [29] with a Nafion®membrane electrolyte. Low temperatures requires a very active catalyst in the elec- trodes, usually beingplatinum (Pt) [30].Adirect ethanol fuel cell (DEFC) is a veryattractive electrochemical energy converter [31], and its unitary fuel cell scheme is shown in Figure 8. Thefuel is supplied intotheanodesideandtheair (orpureO2(g)) is supplied intothecathode. The electrolyte carries protons from the anode to the cathode and the electrons are availed at anexternal electrical circuit toproducework. Advanced Chemical Kinetics70