Kinetic Evidence for the Necessity of Two Proton Donor Molecules for Successful Excited State Proton Transfer by a Photobase

Abstract

Photobases are molecules that convert light to proton transfer drive and therefore have potential applications in many areas of chemistry. Previously, we studied the photobasicity of quinolines and explored their applications. While it is possible to tether a photobase near a target proton donor, for the sake of versatility it is desirable to explore their capability to deprotonate molecules dispersed in a solution. Previous evidence suggested that in this scenario at least two proton donors were necessary for successful excited state proton transfer: one to donate a proton and the second to stabilize the photogenerated donor anion. Here we report kinetic evidence from transient absorption (TA) and time-correlated single photon counting (TCSPC) in support of this hypothesis. We used 5-methoxyquinoline as the photobase and 2,2,2-trifluoroethanol (TFE), a low pK a alcohol, as the proton donor. A constant concentration of the photobase was used for a range of proton-donor dilutions spanning several orders of magnitude in an aprotic background solvent. Absorption spectra confirm that over most of the studied range the majority of the photobase population is hydrogen bonded to at least one donor. Short-pulse TA was used to measure the faster (2–500 ps) dynamics, while TSCPC was used to measure the slower (>500 ps) dynamics. The measured proton transfer time constants varied as a function of donor concentration over a wide range. A log–log plot of the proton transfer rate constant as a function of proton-donor concentration shows two regimes: nondiffusive at high donor concentrations where multiple proton donors are near the photobase and diffusive at low donor concentrations where proton donors are more dilute. The nondiffusive regime has a slope of approximately one, suggesting that the proton transfer process is dependent on one donor molecule in addition to the donor molecule already hydrogen bonded with the photobase. The diffusive regime reasonably follows diffusion kinetics. We propose a model for how the second proton-donor molecule may interact with the photogenerated alkoxide to stabilize it. This work highlights the importance of inducing irreversible changes, in this case solvation of the alkoxide, after proton transfer. Understanding of such details is likely to be important in applications of photobases.

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

“…[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.…”

Quinoline Photobasicity: Investigation within Water‐Soluble Light‐Responsive Copolymers

“…We assign this process to “anion escape” from the local solvent cage, that is, the separation of the protonated photobase and the acid anion. Such anion escape has not been observed in ultrafast proton transfer studies involving quinoline photobases before, likely due to the limited delay‐time window of 600 ps employed in studies so far [11, 12, 23] . Nevertheless, a sequential proton transfer was previously identified during ESPT of a quinone cyanine 9 photoacid [24] and a Schiff base super‐photobase [6] .…”

“…Such anion escape has not been observed in ultrafastp roton transfer studies involvingq uinoline photobases before, likely due to the limited delay-time window of 600 ps employed in studies so far. [11,12,23] Nevertheless,asequential proton transfer was previously identified duringE SPT of aq uinonec yanine 9 photoacid [24] and aS chiff base super-photobase. [6] The anion escape is prolonged in the polymeric photobase, for example, characterized by at ime constant of 90 ps in NQMAm compared to approx.…”

Steric effects in light-induced solvent proton abstraction

“…Studies carried out on 5methoxyquinoline have concluded that a cluster of at least two solvent molecules is required to enable ESPT. 7,8 However, these investigations were not able to assess steric restrictions posed by secondary and tertiary alcohols given the fact that 5-methoxyquinoline deprotonates only low-pK a primary alcohols (e.g., halogenated ethanol). 3 To that end, we focus here on the steric properties of the solvent-solute complex, required for ESPT to occur, using the much stronger photobase FR0-SB as an example.…”