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.Heads can consist of organic, of silicon organic, but also of coordina-tion compounds.In some cases, it is desirable to use heads that bear reactive  arms  so that in an additional step they can interact with each other to forma   monolayer polymer. Acceptor-heads should in general be strongly luminescent molecules with alarge spectral overlap with the donor molecules located inside of the channels.Since luminescence is quenched by dimerization, the structure must be suchthat the chromophores do not interact electronically with each other.Nearly allstrongly luminescent organic chromophores can be considered since it is alwayspossible to attach inert (e.g., aliphatic) groups so that the head cannot enter thechannels.In reality, many of these chromophores will turn out to be less interest-ing because of stability, shape, toxicity, and so on.A very interesting aspect of theprinciple discovered by us is that the head can be chosen or functionalized inorder to realize the desired properties.The difference of the donor-heads withrespect to the acceptors is that they must be able to transfer their excitation energy(by a radiationless process, in general dipole dipole coupling) to acceptorslocated inside of the channels.III.TRANSFER OF ELECTRONIC EXCITATION ENERGYImportant transfer and transformation processes of electronic excitation energythat can take place in dye loaded zeolite materials areAbsorption emission of a photon.Transformation into chemical energy and the reverse.Transformation into heat.Radiationless and radiative transfer to an acceptor.Stimulated emission.Upconversion.Energy-transfer processes in which free photons exist as intermediates aresometimes referred to as   trivial  transfer mechanism.This term is misleadingin the sense that such processes (e.g., in combination with internal reflection) cancause very complex and interesting phenomena [61, 65 67].Radiationlessenergy-transfer processes have been studied extensively since the pioneeringwork of Förster [68, 69] and Dexter [70] (see, e.g., [40, 67, 71 73]).Here, weconcentrate on the description of one-photon events, specifically with respectto radiationless energy-transfer processes.TRANSFER OF ELECTRONIC EXCITATION ENERGY 27A.Radiationless Energy TransferExcitation transfer requires some interaction between unexcited and excitedmolecules M and M*, respectively.We denote the two molecules underconsideration as M and M0.The interaction can then be expressed as occurringbetween the two configurations MM0* and M*M0:1ci¼pffiffiffiðcð1Þcð2Þ cð2Þcð1ÞÞð23ÞM M0 M M021pffiffifficf¼ ðcð1Þcð2Þ cð2Þcð1ÞÞð24ÞMM0 M M02where i and f denote the initial and final state, respectively, and (1) and (2) refer toelectrons.By using H0for the interaction Hamiltonian, the interaction energy b isb¼u ex ð25Þwhere u denotes the Coulombic interaction and ex the exchange interaction:u¼hcð1Þcð2ÞjH0jcð1Þcð2Þið26ÞM M0 MM0ex¼hcð1Þcð2ÞjH0jcð2Þcð1Þið27ÞM M0 MM0If more than two electrons should be involved, these expressions can be extendedaccordingly.The transfer rate kel for electronic excitation energyM M0!MM ð28Þcan be expressed by means of the Golden Rule as follows:2pkel¼ b2r ð29Þhwhere r is the density of states.We consider situations for which the orbital overlap between M and M0 isnegligible, this means situations as depicted in the upper and middle part ofFigure 1.8, which makes sense because we focus on strongly luminescentmaterials.In general, orbital overlap causes fast radiationless decay for organicmolecules as, for example, observed in dimers [56, 57].The exchange part ex28 SUPRAMOLECULARLY ORGANIZED LUMINESCENT DYE MOLECULES OF ZEOLITE Lvanishes in absence of orbital overlap.In this case, the initial and the final statescan be simply expressed asci¼cM cM0 ð30Þcf¼cMcM ð31ÞBy following the arguments of Förster [74], we distinguish between strong,medium, and weak coupling.Weak interaction means that the interaction energyu is less than the vibrational bandwidth.In this case, energy transfer must betreated as a nonradiative transition between two configurations with continuousenergy and the so-called Förster equation as used in the Section III.B can bederived [68 70, 72].B.Förster Energy Transfer in Dye Loaded Zeolite LBy using the geometrical concepts explained in Section II [ Pobierz caÅ‚ość w formacie PDF ]