Rate-limiting action of a proton crane in long-range intramolecular proton transfer

Abstract

The photo-induced enol-keto tautomerization in the compounds 7-hydroxy,8-(N-morpholinomethyl)quinoline, HMMQ, and 7-hydroxy,8-(N,N-dimethylaminomethyl)quinoline, HDMQ, involves the long-distance intramolecular transfer of the proton from t h e hydroxyl group to the endocyclic N atom. This is accomplished by t h e action of t h e amine in the mobile substituent attached to the aromatic ring system which is referred to as the proton crane.The excited-state intramolecular proton transfer, ESIPT, proceeds via two sequential adiabatic proton-transfer steps, as revealed by time-resolved fluorescence spectroscopy. The isoviscous and isothermal behaviour of the time dependence of the fluorescence of solutions in a series of n-alkanols and alkanenitriles has been studied on a picosecond time-scale. No deuterium kinetic isotope effect on the ESlPT process has been found. The overall rate constant for t h e ESIPT process is found to be controlled by the rotational diffusion of the proton crane over an energy barrier, E, , with a height of 5.6 kJ mol-', independent of solvent and viscosity. There is a deuterium isotope effect on t h e decay of the electronically excited keto tautomer, K*. This suggests that motion along the N-H bond between the proton and endocyclic N atom acts a s an efficient dissipative mode in the radiationless decay of K*. The ground-state reverse tautomerization has been studied on a nanosecond timescale by transient absorption spectroscopy. This process turns out to be controlled by the rotational diffusion of the crane over a barrier with a height of <I6 kJ mol-'.

Excited‐state Intramolecular Proton–transfer‐induced Charge Transfer of Polyquinoline

“…Whether hydrogen‐atom transfer from the hydroxyl group to the pyridinic nitrogen atom occurs via one‐step hydrogen‐atom transfer or rate‐determining charge transfer following fast proton transfer can be clarified by examining the kinetic isotope effects of the protic hydrogen atom. One‐step hydrogen‐atom transfer would have a significant deuterium kinetic isotope effect, whereas rate‐determining charge transfer following fast proton transfer would exhibit no deuterium kinetic isotope effect (39,40). Figure 3b shows that the rate constants of both the ESIPT‐induced charge transfer and the reverse charge transfer remain invariant regardless of the isotope exchange of the protic hydrogen atom.…”

“…The rate constants of both the ESIPT‐induced charge transfers and the reverse charge transfer remain invariant regardless of the isotope exchange of the protic hydrogen atom. The lack of a kinetic isotope effect in both forward and reverse charge‐transfer reactions implies that the charge transfers following the ESIPT of the intramolecularly H‐bonded cis ‐E form are nuclear–mass‐independent processes indeed (39,40). Thus, the plausible tautomerization reaction of an intramolecularly H‐bonded cis ‐E conformer occurs sequentially involving two mechanistically distinct proton‐transfer and charge‐transfer steps (21–25).…”

8-(Pyridin-2-yl)quinolin-7-ol as a Platform for Conjugated Proton Cranes: A DFT Structural Design

“…Several classes of compound could be mentioned as typical examples. Historically, the term “proton crane” was used for the first time by Varma et al [ 15 , 16 , 17 , 18 ], who designed a single bond axle system, 8-(morpholinomethyl)quinolin-7-ol ( 1 in Scheme 2 ), where the proton is exchanged between O and N in the 7-hydroxyquinoline upon irradiation [ 19 ]. Proton crane 1 and its mimics [ 12 , 20 , 21 ] are based on an excited state proton transfer pathway upon irradiation as the only possibility to reach the energetically unfavorable end state.…”

Unraveling the Mechanism of a Reversible Photoactivated Molecular Proton Crane

“…Varma and co-workers were the first to demonstrate the proton-craning behavior of this molecule . The proton crane has been studied previously by steady-state fluorescence, time-resolved fluorescence, and transient absorption spectroscopies as well as simulations. − Interestingly, the intermolecular hydrogen-bond breaking that should precede the craning has never been observed. The ground-state retautomerization has also never been observed, as it was thought to occur too fast. , The experimental techniques employed previously are intrinsically not capable of providing a complete picture of the structural changes that occur during the different steps of the photocycle of the proton crane.…”

“…The proton crane has been studied previously by steady-state fluorescence, time-resolved fluorescence, and transient absorption spectroscopies as well as simulations. − Interestingly, the intermolecular hydrogen-bond breaking that should precede the craning has never been observed. The ground-state retautomerization has also never been observed, as it was thought to occur too fast. , The experimental techniques employed previously are intrinsically not capable of providing a complete picture of the structural changes that occur during the different steps of the photocycle of the proton crane. Techniques based on emission require that the fluorescence bands of the various intermediate stages of the craning process are different, but in practice it has been found that they are difficult to separate.…”