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Evidence is provided that transformation of the electronically excited enol form E* of 7-hydroxy,8-( N -morpholinomethyl)quinoline (HMMQ) into the excited state K* of its keto form requires formation of a hydrogen bonded complex of E* with a solvent molecule. The excited zwitterionic form Z* generated after a first intramolecular proton transfer has the mobile proton carrier (morpholino group) initially locked in position by an intramolecular hydrogen bond with the deprotonated O atom. It is concluded that participation of the proton in an additional hydrogen bond, involving a solvent molecule, is the main reason why the proton carrier can get into motion to deliver the proton at the remote site. The conclusion has been achieved by studying the spectral variations in the fluorescence spectra of HMMQ in various binary solvent mixtures, containing at least one component able to form hydrogen bonds with a proton donor. In the case of solutions in n -hexane/THF there is clear evidence of 1 : 1 complex formation between Z* and THF, as has been also obtained previously in the case of solutions of HMMQ in n -hexane/1,4-dioxane. Evidence is obtained that dichloromethane is causing conversion of Z* into K* through H-bonding with Z*, but that it quenches the fluorescence from K* completely. |
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