Abstract
With the aid of stationary and time-resolved fluorescence spectroscopy, we investigated substituent effects on the intramolecular fluorescence quenching process in naphthylalanine derivatives (model compounds)1 that have naphthyl (electron acceptor) and dialkylamino (electron donor) groups at both ends of a propionamide chain. The effects of alkyl substituents (that were introduced into the donor moiety)on the rate and free-energy change (ΔGex) for the formation of a singlet-exciplex intermediate in methanol (MeOH) were explained in terms of steric and electronic effects of the alkyl substituents on the hydrogen-bonding solvation of the donor nitrogen in the locally excited singlet state and the stability of the exciplex intermediate. In addition, the alkyl substituent effects on the viscosity dependence of the A Ge. supported the view that the solvation of both the locally excited-state 1 and the exciplex intermediate plays a significant role in controlling the rate of the exciplex formation. On the other hand, the results of MM2 calculations and H-D exchange reactions for the model 1 and its reference compound substantiated the possibility that the spacer moiety interconnecting the acceptor and donor groups adopts a relatively rigid conformation and thereby requires at best only a small conformational change on production of a given exciplex intermediate. Activation parameters for the singlet exciplex-forming process in MeOH provided strong evidence for the occurrence of hydrogen-bonding solvation in the excited singlet-state model compounds. The enhanced steric hindrance of the alkyl groups attached to the tertiary amino nitrogen resulted in a decrease in the ability of this nitrogen atom to form a hydrogen bond with a MeOH molecule, being reflected in the decreased activation enthalpy and entropy. However, replacement of the 1-naphthyl acceptor in 1 by the 2-naphthyl exerted a negligible effect on the activation parameters.
