10-Hydroxybenzo[h]quinoline: switching between single- and double-well proton transfer through structural modifications

Abstract

Proton transfer in 10-hydroxybenzo[h]quinoline (HBQ) and structurally modified compounds was investigated experimentally (steady state UV-Vis absorption and emission spectroscopy, NMR and advanced chemometric techniques) and theoretically (DFT and TD-DFT M06-2X/TZVP calculations) in the ground and excited singlet state. We observed that the incorporation of electron acceptor substituents on position 7 of the HBQ backbone led to appearance of a keto tautomer in ground state and changes in the excited state potential energy surface. Both processes were strongly solvent dependent. In the ground state the equilibrium could be driven from the enol to the keto form by change of solvent. The theoretical calculations explain the substitution-determined transition from a single-to a double-well proton transfer mechanism.

Dynamics of excited state proton transfer in nitro substituted 10-hydroxybenzo[h]quinolines

“…The title compounds are synthesized and purified according to the already described procedures. 13,14 They are dissolved with a concentration in the order of 10 -4 M in acetonitrile of Uvasol quality (Merck). Fused silica cuvettes with a thickness of 1 mm are employed as sample cells.…”

“…It shows very good results in predicting the position of tautomeric equilibria for compounds with intramolecular hydrogen bonds. 14,[24][25][26][27] For comparison some additional functionals as implemented in Gaussian 09 D.01 (B3LYP 28 , MN12SX 29 , BHandH 30 ) were used as well. Second and higher order Møller-Plesset perturbation theory (MP2-4) 31 and coupled-cluster theory with single, double, and perturbative triple excitations (CCSD(T)) 32 were used in addition for calculations of the ground state.…”

“…The spectral properties of substituted HBQs are usually reported for nonpolar solvents, 43 which makes the comparison difficult, because no keto absorption is observed for 2 and 3 in some of them. 14 However, analogues of 2 and 3 in which the nitro group(s) are replaced by CN group(s) do not show an absorption band around 440 nm in ethyl acetate 44 , while the keto form is clearly seen in the same solvent in the case of 2 and 3 14 . Compounds 2 and 3 have previously been investigated by NMR in DMF-d 7 and it was found that 3 existed to a large extent in the proton transfer form.…”

Sugar‐Based Self‐Assembly of Hydrogel Nanotubes Manifesting ESIPT: Theoretical Insight and Application in Live Cell Imaging

“…The ‘keto‐enol’ tautomers (Scheme ) behave quite different under UV‐irradiation. In the ground state, the BTHBC present as enol form (E) and this is stabilized by the intramolecular hydrogen bonding interaction between hydrogen atom of phenolic –OH group and nitrogen atom (N T ) of HBT or imine nitrogen (N C ) of chitosan –. Upon excitation, the hydrogel in the excited‐state enol form (E*) transform to the excited‐state keto form (K*) through intramolecular proton transfer.…”

“…The maximum fluorescence intensity can be achieved when the thickness of BTHBC is 0.2 cm. The thickness dependence of fluorescence intensity may be due to a certain thickness of the hydrogel with the maximum irradiation depth at a given excitation wavelength …”