Combined Experimental and Theoretical Study of the Transient IR Spectroscopy of 7-Hydroxyquinoline in the First Electronically Excited Singlet State

Abstract

The photophysics of 7-hydroxyquinoline (7HQ) in protic media results from an interplay of acid-base chemistry, prompted by the effects of photoacidity of the hydroxyl group and photobasicity of the nitrogen atom in the quinoline aromatic system. With ultrafast IR spectroscopic measurements, we follow the proton transfer dynamics of 7HQ in its four possible charged forms in methanol solution. Using deuterated methanol as solvent, we determine deuteron transfer rates from the neutral to the zwitterionic form to be 330 ps, those from the cationic form to the zwitterionic form to be 170 ps, and those from the anionic form to the zwitterionic form to be 600 ps. We compare the observed IR-active fingerprint marker patterns in the electronic ground state and the first electronically excited L-state with those calculated using density functional theory and time-dependent density functional theory, respectively, and find good correspondence between experimental and calculated transitions. The calculations provide insight into the nature of electronic excitation of these four different charged forms of 7HQ, suggesting the key role of electronic charge distribution changes upon electronic excitation of 7HQ and hydrogen bond changes at the donor hydroxyl and acceptor nitrogen moieties.

Ultrafast Proton Transport between a Hydroxy Acid and a Nitrogen Base along Solvent Bridges Governed by the Hydroxide/Methoxide Transfer Mechanism

“… a Reference ( 33 ) from femtosecond UV/IR measurements. b Reference ( 29 ) from time-correlated single photon counting (TCSPC) measurements.…”

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

“…It should be noted that concentration effect also plays a substantial role [ 73 , 80 ] in the PT through intermolecular interactions of species. There is a large number of papers where the solvent assisted PT mechanism has been proven by a variety of experimental measurements [ 81 , 82 , 83 , 84 ] and theoretical simulations [ 61 , 62 , 85 , 86 ].…”

Ultrafast proton transport in water-methanol mixtures

“…Photoacids have been utilized to much effect in time-resolved studies of ultrafast proton transfer dynamics [1][2][3]. A special class consists of so-called bifunctional photoacids [4][5][6], where both proton donating and proton accepting groups are part of the same molecule. Bifunctional photoacids allow for a well-defined distance along which proton transport can occur and -thus -defining a highly specific number of solvent molecules that can function as solvent bridge (for protic solvents such as water or methanol often denoted as 'solvent wire') through which the proton transport is understood to occur either in concerted or sequential von Grotthuss-type fashion.…”

“…Hydroxyquinolines have been a favourite choice as bifunctional photoacids [5][6], where the proton donating hydroxyl-group behaves as is known of naphthol photoacid compounds. In addition to that the quinoline nitrogen acts through its lone pair as a photobase, i.e.…”

“…Here 7HQ is excited in its neutral form N*, that then converts to the zwitterionic tautomer Z* on picosecond time scales. We have reported in a UV/IR pump-probe study the time constants of particular proton transfer steps of differently charged forms of 7HQ in deuterated methanol [6]. Here we expand on our previous results by measuring femtosecond UV/IR pump-spectra of 7HQ in D2O/CD3OD mixtures ( Figure 1b To investigate further the proton transfer dynamics of 7HQ, we correlate obtained time constants with excited-state pKa*-values using reported free-energy-reactivity correlation relationships [2,3] (closely related to Marcus theory for electron transfer) and investigating trends in derived pKa*-values as a function of solvent composition.…”