Prospects for using photoinduced intramolecular proton transfer to study the dynamics of conformational changes in flexible molecular chains

Photoinduced excited-state intramolecular proton transfer in 2-hydroxy-1-(N-morpholinomethyl)naphthalene (HMMN) and in 7-hydroxy-8-(N-morpholinomethyl)quinoline (HMMQ) has been studied by means of stationary and time-resolved fluorescence. Both molecular systems have an intramolecular hydrogen bond. This causes a very fast ESIPT process to occur directly upon excitation with a rate constant >2 × 1011 s–1. HMMN is known to show dual fluorescence; however, HMMQ in neat polar solvents is found to exhibit three fluorescence bands. The bands at the short- and long-wavelength side of the spectrum are assigned, respectively, to emission from the excited initial tautomeric form (enol form) and the adiabatically produced excited keto form. This agrees with the fact that these two bands are also observed in the fluorescence spectrum of solutions of 7-hydroxyquinoline in neat alkanols, in which the excited keto form is known to be formed adiabatically as a result of active solvent participation in the ESIPT process. The third band originates from an excited zwitterionic species in which the proton is located on the N atom in the morpholino group. The side group acts as a molecular crane, picking up the proton from the initial binding-site and depositing it at the final proton binding-site. The rate constant for the transport of the proton from the initial to the final location is determined by the rate constant for the required motion of the free end of the side group. This rate constant has been studied as a function of viscosity η of the solvent and is found to be proportional to η–c, with c= 0.6 and 0.4 in the case of n-alkanols and alkanenitriles as solvents, respectively. A process consisting of two sequential ESIPT processes, as in HMMQ, offers precise control of initial and final conformation in the study of the dynamics of flexible molecular chains. The prospects for using such a double ESIPT process as a tool to study chain dynamics are discussed.