Photoinduced Long-Distance Hydrogen-Atom Transfer in Molecules with a 7-Hydroxyquinoline Frame and a Carbaldehyde or Aldoxime Group as the Intramolecular Hydrogen Transporting Crane

Abstract

The photochemical properties of monomeric 7-hydroxyquinoline substituted at position 8 with carbaldehyde or aldoxime groups were studied for the molecules isolated in solid Ar low-temperature matrices (at 10 K). It was experimentally demonstrated that upon UV excitation, both carbaldehyde and aldoxime groups act as intramolecular cranes transmitting hydrogen atoms from the hydroxyl group to the remote nitrogen atom of the quinoline ring. Furthermore, in the case of 7-hydroxyquinoline-8-aldoxime (and its derivatives), the second photochemical channel was activated upon UV (λ > 360 nm) excitation. This process involves syn–anti isomerization around the double C=N bond in the aldoxime group. The structures of the reactant hydroxy tautomeric form and the photoproduced isomers of the studied molecules were unequivocally determined by means of IR spectroscopy combined with theoretical predictions of the IR spectra of the candidate structures.

Reversible Switching Based on Truly Intramolecular Long-Range Proton Transfer─Turning the Theoretical Concept into Experimental Reality

“…On the other side, the large barrier for the thermal ground-state relaxation back to the off -state, requiring twisting around a double bond, makes the back-reaction slow and the overall process possible to be investigated by conventional spectroscopy and even monitored by the naked eyes. Most of the investigations related to conjugated proton cranes are only theoretical, where the effect of the rotor (modified pyridines or CHO group) on the excited-state energy landscape has been studied. ,− Experimental data are available for several 7-hydroxyquinoline-8-carbaldehydes ( HQC , Scheme B), showing that both on- and off- states coexist in solution at room temperature, which does not allow a clean switching. , The mechanism of long-range PT in these compounds upon UV irradiation was revealed, very recently, by using matrix-isolated IR spectroscopy . In the past few years, we have synthesized and studied a series of Schiff bases ( 7HQ-SB , Scheme B), where twist-assisted ESIPT occurs under suitable structural conditions, − but its efficiency is low due to the flexibility of the rotor, allowing competitive E/Z isomerization of the azomethine bond in the excited state .…”

Intramolecular Catalytic Hydrogen Atom Transfer (CHAT)

“…the systematization in ref ) is considered to occur via an initial photostimulated H-transfer of the hydroxyl group to the oxygen of the carbaldehyde moiety to form a protonated (“loaded”) crane group (first step in Scheme ). This is followed by an ethylene-type rotation of the formed vinyl-hydroxy group about the double bond with the aromatic (quinolone) ring, and by the second H-transfer from the crane group to the N-center (3rd step) to conclude the long-range tautomerization …”

“…Another, even more relevant example of experimentally explored CHAT -related processes is the so-called photoinduced bistable molecular switch, where a substituent group, such as carbaldehyde or acylhydrazone, serves as an intramolecular H-transporting crane in a long-range tautomerization process. − This process is illustrated in Scheme based on the data of ref .…”

Luminescence of ESIPT-capable zinc(ii) complexes with a 1-hydroxy-1H-imidazole-based ligand: exploring the impact of substitution in the proton-donating moiety

“…Proton transfer processes are among the most widespread chemical and photochemical reactions. 1–7 In compounds having short and strong intramolecular hydrogen bonds, such reactions can take place within a single molecule. 8–20 In this case, two forms of the molecule, i.e.…”

“…Proton transfer processes are among the most widespread chemical and photochemical reactions. [1][2][3][4][5][6][7] In compounds having short and strong intramolecular hydrogen bonds, such reactions can take place within a single molecule. [8][9][10][11][12][13][14][15][16][17][18][19][20] In this case, two forms of the molecule, i.e., the normal (N-form) and tautomeric (T-form) ones, can be interconverted to each other through a set of processes shown in Scheme 1.…”