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.

Cyano Derivatives of 7‐Aminoquinoline That Are Highly Emissive in Water: Potential for Sensing Applications

“…An ideal peptide sensor for sensing proteinase K should exhibit a fluorescence restoration upon being digested. Previous work has proved that amidation of 7AQ , forming amide derivatives, gives very low PLQY due to the dominant non‐radiative decay rate ( k nr ) caused by fast internal conversion [1] . Based on the hydrophobic interaction with biomembranes, a hydrophobic penetration peptide PFVYLI [47] was then introduced to modify 6CN−7AQ .…”

“…In summary, we report highly emissive 6CN−7AQ and 3CN−7AQ in water. This, together with the commonly very low PLQY upon amidation of 7AQ , [1] makes powerful the exploitation of their azide and peptide conjugated derivatives toward the sensing application. Two prototypical experiments are demonstrated, in which the azide derivatives 6CN−7N 3 Q and 3CN−7N 3 Q selectively recognized Na 2 S in aqueous solution with high sensitivity of 48.3 nM and 259.2 nM, respectively, and fast response.…”

“…FTIR spectra were recorded on a Bomen MB‐100 FTIR spectrometer. Compound 2 and 8 was prepared according to our previously report [1] …”

“…Recently, we have designed and synthesized a series of 7‐aminoquinoline ( 7AQ ) derivatives ( 7AQ s) by replacing one of the amino protons in the amino site with either an electron donating or withdrawing group to study their excited‐state proton transfer (ESPT) reaction in methanol, where ESPT is assisted/catalyzed by the relay of methanol molecules [1] . Our results indicate that the increase of the electronic withdrawing group increases the acidity of the amino proton and hence facilitates ESPT.…”

4-Azidocinnoline—Cinnoline-4-amine Pair as a New Fluorogenic and Fluorochromic Environment-Sensitive Probe

“…[17,36] We suppose that the main fluorescence mechanism for amine 6 in isopropanol changes from ICT to excited state intermolecular proton transfer (ESPT) (Scheme 2). Thus, ESPT with the formation of quinoid-type emissive excited states was studied for different heterocyclic systems [37][38][39][40] and a competition between ICT and ESPT for heterocyclic D-π-A molecular architecture has been reported recently [41].…”