Prospects for using photoinduced intramolecular proton transfer to study the dynamics of conformational changes in flexible molecular chains

Rotational Brownian motion of the proton crane operating in a photoinduced long-distance intramolecular proton transfer

“…1). 2 In the electronic ground state, HMMQ exists only in the enol form (E), which has an intramolecular hydrogen bond between the hydroxyl group and the nitrogen atom N(2) in the morpholino ring. In the first excited singlet electronic state (E*) the acidity of the hydroxy proton is enhanced relative to the ground state.…”

“…The protonation of N( 1) proceeds adiabatically and yields the electronically excited keto tautomer (K*) of HMMQ. Of course, the protonation of N(l) is feasible only after completion of the rotation of the side group over 180", as required to establish a hydrogen bond between protonated N (2) and N(1). This new hydrogen bond defines the final state of the rotational motion, whereas the initial state is defined by the hydrogen bond in E. Both the first (i.e.…”

A Bistable Molecular Switch Driven by Photoinduced Hydrogen‐Atom Transfer

“…[18] The qualitative difference between the photophysics of HMMQ and HMQC is that the tautomeric form of the former system was adiabatically produced in the excited singlet state, while participation of nonadiabatic transition at the S 1 -S 0 conical intersection is anticipated for the latter. Thus the enol-to-keto tautomerization of HMMQ was photophysically irreversible, while we expect such reversibility in the case of HMQC.…”

“…The calculated absorption spectrum of enol form I is in a good agreement with experimental observation for the similar HMMQ system. [18] On the other hand, the absorption spectra visualized in Figures S1 and S2 of the Supporting Information clearly show that HMQC is not yet the ideal system for optically driven tautomerization. While one can expect complete "burning out" of product III by irradiation at longer wavelengths, both reactant I and product III absorb at shorter wavelengths.…”

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.…”