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state to another vibrationalmodewithin the same electronic state. This process is very fast, taking around 10�14–10�11 seconds. It usually takes place immediately following absorbance and, since it occurs between vibrational levels, generally it does not result in electronic level changes [1, 2]. 2.5. Internal conversion Anon-radiative process that promotes the conversion of a singlet electronic excited state of higherenergy intoanother singlet electronic stateof lowerenergy is the internal conversion. It can involve any twosinglet states and,whenoccurringbetween the singlet electronic excited state of lower energy and the singlet ground state, it competeswith fluorescence, being one reason for a decrease in fluorescence quantumyield. It occurs rapidlywith release of kinetic energy [1,2]. 2.6. Intersystemcrossing Thenon-radiativeprocess of conversionof anelectronic excited singlet state intoa triplet one through an isoenergetic process is the intersystem crossing. This is a very slow process, because it is forbidden by spin multiplicity selection rules and it only takes places if an effective spin-orbit couplingoccurs [1, 2]. These radiative and non-radiative processes are unimolecular, involving only the electronic statesofasinglemolecule.Nevertheless, thereareseveralotherbimolecularprocesses, charac- terizingenergy transferprocessesorevenchemical reactions. 3.Energy transferprocesses Energy transfer canoccurbetweensimilarmoleculesordistinct compoundsandtheway they interactwill define themoreappropriate transfermechanismfor eachcase.Dependingon the mechanism and the energetic characteristics of the energy transferred, the transfer can be classifiedas [1]: 1. Hole transfer: When a positively chargedmolecule interacts with another molecule to achieve its energetic equilibrium and resulting in the second molecule to present the positivecharge. ð2Þ 2. Electron transfer: Similarly, if a negatively chargedmolecule interacts in somewaywith anotherneutralmolecule to result in thesecondmoleculenowasnegativelycharged. ð3Þ 3. Energy transfer:When the interaction betweenmolecules, one of them in the electronic excited state and the other occupying the electronic ground state results in the second New Materials to Solve Energy Issues through Photochemical and Photophysical Processes: The Kinetics Involved http://dx.doi.org/10.5772/intechopen.70467 61