Seite - 66 - in Advanced Chemical Kinetics

mandatoryexigence is tohaveoneof themolecules involved in the complex formation in the electronic excited state. The success of collisionswill give thenumber of intermediates in the excited states that present the ideal characteristics for energy transfer. These excited state complexesareclassifieddependingonthe identityof their components [2–4]: 1. Excimersare theexcitedstatecomplexes thatare formedbytwosimilar compounds.They present the same absorption electronic spectra as the isolated molecules, but emission spectrabroaderandred-shifted than theemissionexpected for the isolatedmolecule.The emissionspectrumis the result of theemissionofanewcompound, the complex, formed duringtheexcitedstateof themolecule thatabsorbedtheelectromagnetic radiationandis formed by collision. Excimers present several distinct orientations, from the totally overlappedorientation, called sandwichexcimer, to somepartiallyoverlappedand the t- shapedexcimer.Figure5presents theseconfigurations. 2. Exciplexesare thecomplexes formedbydistinctcompounds,withoneof thembeingat the electronic excited state. They are also governed bydiffusion rates, but in a very specific manner, since itdependsonefficient simultaneouscollisions.Theirabsorptionspectraare similar to that observed for the isolated absorber, but the emissions are very difficult to predict, sinceseveral competingpathwaysofdeactivation,withkinetics influencedbythe environment and the interaction forces acting to keep the exciplex together, during the excited state of the complex. This is the case of exciplexes involved in supramolecular photochemical reactions, asexemplifies inFigure6. 5.Fromphotophysical tophotochemicalprocesses All thesephotophysical processesmodulate the energyand the characteristics of the interme- diates prior to theoccurrence ofphotochemicalmodifications. Theyoccur in typical amounts of time; thus, light absorption is the determining step and it takes femtoseconds (10 15 sec- onds) tooccur.The radiativedeactivationof the lowest excited state to reach thegroundstate is the fluorescence, which occurs in nanoseconds (10 9 seconds) timescale; its occurrence informsabout theelectronic excitedstate lifetimeand, therefore, about its stability. If it is long enough, severalprocesses canoccurand the radiativedeactivation isnotobservedor itsyield is diminished. From there, reactive intermediates can be formed in the excited state and, if funnelsor interconversionsituationsareavoidedby, for instance,guaranteeingthat theenergy barrier is too high to be superposed, then the final product, result of all photophysical and photochemical processes that occur during the lifetime of the electronic excited state, is the excited product. The ground state product is obtainedwhen the excess energy is released as radiativeemission [3]. Nevertheless, if the energybarrier is superposedand funnels are formed, the reactive excited state intermediate cannot be formed and the chemical reaction occurs in the ground state. Theseevents canbesummarized inFigure7. The rate constants and the probabilities of these processes determinewhichpath can lead to the product formation. Todescribe the excited states and the changes that occur to yield the Advanced Chemical Kinetics66