Proton Capture Dynamics in Quinoline Photobases: Substituent Effect and Involvement of Triplet States

Abstract

Converting light into chemical energy often occurs through redox reactions that require transfer of several electrons and protons. Using light to control proton transfer has the potential for driving otherwise unfavorable protonation reactions or producing transient pH changes. Photoacids and photobases are fundamental functional elements that could serve this purpose. Previously, we have reported the thermodynamic drive for proton removal in a series of quinoline photobases using Forster cycle analysis of the singlet states. Because the existence of thermodynamic drive does not imply that the molecules can indeed capture protons in the excited state, in this work we report the kinetics of proton removal from water by 5-R-quinolines, R = {NH, OCH, H, Cl, Br, CN}, using ultrafast transient absorption spectroscopy. We found that the time constants and mechanisms of proton capture from water are highly sensitive to the substituent. In some cases, proton transfer occurs within the singlet manifold, whereas in some others intersystem crossing competes with this process. We have evidence that the triplet states are also capable of proton capture in two of the compounds. This renders the excited state proton transfer process more complicated than can be captured by the linear free energy relationships inferred from the energetics of the singlet states. We have measured proton capture times in this family to be in the range of several tens of picoseconds with no discernible trend with respect to the Hammett parameter of the substituents. This wide range of mechanisms is attributed to the high density of excited electronic states in the singlet and triplet manifolds. The ordering between these states is expected to change by substituent, solvent, and hydrogen bonding, thus making the rate of intersystem crossing and proton transfer very sensitive to these parameters. These results are necessary fundamental steps to assess the capabilities of photobases in prospective applications such as photomediated proton removal in redox reactions, steady state optical regulation of local pH, and pOH jump kinetics experiments.

Structure–Photochemical Function Relationships in Nitrogen-Containing Heterocyclic Aromatic Photobases Derived from Quinoline

“…Using transient absorption spectroscopy, Dawlaty and coworkers identified evidence of SCT undergoing intersystem crossing into the triplet manifold for a series of 5-R quinolines. 21 In particular, for 5-aminoquinoline, the strongest photobase that was experimentally investigated, This data, which was calculated using the Tamm-Dancoff approximation to minimize triplet instability problems, is summarized in Tables S13-S16 in the Supporting Information. 43 For the acridine and 1-azaanthracene compounds, the average N O PQR − 5 67 N is 0.52 eV and 0.71 eV for the basic and acidic forms respectively, where Tmin is chosen as the triplet state closest in energy to SCT.…”

“…[12][13][14][15][16][17][18][19] Recent experimental studies performed in the Dawlaty lab have investigated the photochemical properties of a family of 5-R quinoline derivatives. [20][21][22][23][24][25] These compounds were all shown to be photobases, with the magnitude of the photobasicity depending strongly on the identity of the substituent. Specifically, photoexcitation results in Kb increasing by over 10 orders of magnitude when the substituent is the electron-donating NH2 group but only approximately 2 orders of magnitude when the substituent is the electron-withdrawing CN group.…”

Photobase effect for just-in-time delivery in photocatalytic hydrogen generation

“…Hence, such molecules offer an opportunity to control the proton transfer by light 7,8 . For instance, quinoline photobases have been shown to form protonated species within tens of picoseconds of the light excitation 9 . Effectively, the photobase can ensure that the proton promptly and selectively arrives at the reduction site.…”

Structure-Photochemical Function Relationships in Nitrogen-Containing Heterocyclic Aromatic Photobases Derived from Quinoline

“…Using transient absorption spectroscopy, Dawlaty and coworkers identified evidence of SCT undergoing intersystem crossing into the triplet manifold for a series of 5-R quinolines. 21 In particular, for 5-aminoquinoline, the strongest photobase that was experimentally investigated, the spectroscopic evidence points to excited state protonation occurring on a timescale of 41 ps followed by relaxation into a triplet state approximately 36 ps later. Such a process does not necessarily invalidate the applicability of the Förster mechanism to the photobasicity of 5aminoquinoline as the intersystem crossing only occurs after the excited state protonation.…”

“…[12][13][14][15][16][17][18][19] Recent experimental studies performed in the Dawlaty lab have investigated the photochemical properties of a family of 5-R quinoline derivatives. [20][21][22][23][24][25] These compounds were all shown to be photobases, with the magnitude of the photobasicity depending strongly on the identity of the substituent. Specifically, photoexcitation results in Kb increasing by over 10 orders of magnitude when the substituent is the electron-donating NH2 group but only approximately 2 orders of magnitude when the substituent is the electron-withdrawing CN group.…”