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2.Photophysicalprocesses 2.1.Absorption The initial photophysical process that gives rise to excited states fromwhere every photo- physicalandsubsequentphotochemicalprocessesoccur intheradiativeabsorptionofphotons topromote an electron to ahigher electronic energy state. The accessed excited state is deter- mined by selection rules that involve symmetry and spin conservation, existence of a dipole moment andmust occur to an ideal vibrationalmodewavefunction in the excited state over- lapped in some extentwith the low energy vibrationalmode of the ground electronic state, enabling some probability of transition, as predicted by the Franck-Condon principle. The magnitude of this overlap influences the moment transition in absorption and every other photophysicalprocesses [2].Theexpression thatdescribes the transition is: ð1Þ Where the second integral is theoverlap integral. Fromthis expression, it is evident that there mustbeaprobabilityofawavefunction fromalowerelectronic state toabsorbenoughenergy to be converted in another wavefunction that describes a higher electronic state and that if there is no overlap between the vibrational states expected to be involved in the transition, then the electronic transition is forbidden. It evidences the vibronic nature of the electronic state, inwhichelectronic statesarecoupled tovibrational states.Figure2presents theFranck- Condonabsorption fromthegroundelectronic state toavibronic stateofhigherenergy. The absorption process populates electronic excited states fromwhere all deactivation pro- cesseswill occur.Themost significantphotophysicaldeactivationprocessesare: 2.2.Fluorescence Thephotophysicalprocess inwhichtheelectronicexcitedstate isradiativelydeactivated, involv- ingsingletexcitedandgroundstates, isthefluorescence.Itspontaneouslyoccursfromthesinglet excited state of lower energy, as predicted byLewis andKasha [3], through the emission of a photon and the energy involved in this process is similar to the absorbed energy, if no other competingprocessofdeactivationoccurs. It occursvery rapidly ina timescale thatdependson thesystemidentitybutbetween10�6and10�10secondsforseveralorganiccompounds.If longer timescales are observed, it may evidence the occurrence of another process that results in a similarspectrum,butoccursaftersomeotherphotophysicaldeactivationprocessesthatpopulate the singlet electronic state of lower energy. This is the delayed fluorescence and it only can be distinguishedfromthefluorescencebytime-resolvedmeasurements. 2.3.Phosphorescence Phosphorescence isaradiativedeactivationprocesscharacterizedbyared-shiftof theemission spectrum. It is aprocess that occur fromanelectronic excited statewith less energy than that fromwhere fluorescenceoccurs. In fact, itoccurs fromatripletelectronicstatewith lessenergy New Materials to Solve Energy Issues through Photochemical and Photophysical Processes: The Kinetics Involved http://dx.doi.org/10.5772/intechopen.70467 59