Acylhydrazone Subunits as a Proton Cargo Delivery System in 7‐Hydroxyquinoline

Abstract

The reimagined concept of long‐range tautomeric proton transfer using crane subunits is shown by designing and synthesising two new acylhydrazones containing a 7‐hydroxyquinoline (7‐OHQ) platform. The acylhydrazone subunits attached to the 7‐OHQ at the 8th position act as crane arms for delivering proton cargo to the quinoline nitrogen. Light‐induced tautomerization to their keto forms leads to Z/E isomerization of the C=C axle bond, followed by proton delivery to the quinoline nitrogen by the formation of covalent or hydrogen bonds. The axle‘s being either an imine or ketimine bond is the structural difference between the studied compounds. The −CH3 group in the latter provides steric strain, resulting in different proton transport pathways. Both compounds show long thermal stability in the switched state, which creates a tuneable action of bidirectional proton cargo transport by using different wavelengths of irradiation. Upon the addition of acid, the quinoline nitrogen is protonated; this results in E/Z configuration switching of the acylhydrazone subunits. This was proven by single‐crystal X‐ray structure analysis and NMR spectroscopy.

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(ii) complexes with 1-hydroxy-1H-imidazole-based ligands

“…[109][110][111][112] Interestingly, the proton-transfer properties of these systems make them analogous to the C-OH -CvO tautomeric systems. [113][114][115][116][117][118] The ability of 1-hydroxy-1H-imidazoles to form N-hydroxy and N-oxide tautomers had not been used in the engineering of ESIPT-fluorophores until our recent works. 105,106,119 It is well known that the proton transfer abilities and emission properties of ESIPT-capable molecules can be tuned by substitution on the proton-donating and protonaccepting cores, as well as by the degree of π-conjugation.…”

N-Hydroxy–N-oxide photoinduced tautomerization and excitation wavelength dependent luminescence of ESIPT-capable zinc(ii) complexes with a rationally designed 1-hydroxy-2,4-di(pyridin-2-yl)-1H-imidazole ESIPT-ligand

“…The interplay of acid-base catalysis and the shape of the crane arm in the 7-hydroxyquinoline system appeared to be responsible for the crane motion. 126 A similar role can be played by the coordination of metal ions to ESIPTligands. For example, decreasing the basicity of the donor atoms in the metal binding site of an ESIPT-ligand can trigger metal coordination to the ESIPT-site with the subsequent proton transfer to the former metal binding site.…”

“…89 This energy difference is close to the value reported by Nakashima, Georgiev and Antonov et al for 7-hydroxyquinoline C–OH – CO systems. 126…”

“…The sequence of elementary steps in this proton cargo delivery process can include (i) the ESIPT photoreaction transferring the proton from the N 1 -OH group to the crane arm, (ii) the turn of the crane and (iii) the delivery of the proton from the crane arm to the N 3 atom. Considering that the design of long-range proton cargo delivery systems is limited to C-OH -CvO tautomeric systems, [126][127][128][129][130][131] such 1-hydroxy-1H-imidazole-based systems exploiting the N-OH -N-O crane-assisted tautomerism could form a new family of long-range proton cargo delivery systems. The 1-hydroxy-1Himidazole-based fluorophore HL can potentially act as a representative example of N-OH -N-O crane-assisted long-range proton delivery systems.…”

“…89 This energy difference is close to the value reported by Nakashima, Georgiev and Antonov et al for 7-hydroxyquinoline C-OH -CvO systems. 126 Intramolecular motions in molecular switches can be triggered by various factors, including different light wavelengths and chemical fuels. The interplay of acid-base catalysis and the shape of the crane arm in the 7-hydroxyquinoline system appeared to be responsible for the crane motion.…”