Catalytic-Type Excited-State N-H Proton-Transfer Reaction in 7-Aminoquinoline and Its Derivatives.

ABSTRACT

7-Aminoquinoline (7AQ) and various amino derivatives thereof (-NHR) have been strategically designed and synthesized to study their excited-state proton-transfer (ESPT) properties. Due to the large separation between the proton donor -NHR and the acceptor -N- site, ESPT in 7AQ derivatives, if available, should proceed under protic solvent catalysis. ESPT is found to be influenced by the acidity of -NHR and the basicity of the proton-acceptor -N- in the quinoline moiety. The latter is varied by the resonance effect at the quinoline -N- site induced by the -NHR substituent. For those 7AQ derivatives undergoing ESPT, increased quinoline basicity results in a faster rate of ESPT, implying that proton donation from methanol to the quinoline moiety may serve as a key step in the process. Our studies also indicate the existence of an equilibrium between cis and trans arrangements of -NHR in terms of its hydrogen-bond (H-bond) configuration with methanol, whereby only the cis-H-bonded form undergoes methanol-assisted ESPT. With one exception, the interconversion between cis and trans configurations is much faster than the rate of ESPT, yielding amino-type (normal form) and imine-type (proton-transfer tautomer) emissions with distinct relaxation dynamics.