Engineering the Excited-State Dynamics of 3-Aminoquinoline by Chemical Modification and Temperature Variation

The role of the amino group in the excited-state dynamics of 3-aminoquinoline (3AQ) has been investigated by comparison with its synthetic derivative 3-(piperidin-1-yl)quinoline (3PQ). The absence of amino hydrogen atoms in 3PQ eliminates, to a large extent, the complexity of the excited-state processes observed in 3AQ. The polarity of the medium is found to be the most important determinant in the nonradiative rate constants of 3PQ, unlike in 3AQ where hydrogen bonding plays the most significant role. The nonradiative rate constants decrease with increase in micropolarity. This trend is opposite to what is usually observed with dipolar states. Temperature dependence of the fluorescence spectra and lifetime has been studied to understand this unexpected observation. An unusual redshift in the emission of 3AQ and 3PQ is observed in nonpolar media at low temperatures. This is surprising, as a process involving a barrier is expected to be hindered at low temperatures and be manifested in a blueshift of the spectra, due to the predominance of the locally excited (LE) state. Moreover, the variation of emission maxima of 3AQ with temperature is sigmoidal in nature, indicating the involvement of two distinct states. The counterintuitive observation of the predominance of the state with comparatively lower emission energy, at low temperatures, establishes the following: the photophysics in 3AQ is dominated by a LE state at room temperature in nonpolar media. This state is associated with rapid flip-flop of the amino group, which provides an efficient nonradiative channel of deactivation. At low temperatures, this flip-flop is hindered and the molecule can undergo intramolecular charge transfer (ICT), whereby the lower energy state is populated. In the case of 3PQ, the ICT state is the only one present, owing to the tertiary amino group.