Proton Abstraction Mediates Interactions between the Super Photobase FR0-SB and Surrounding Alcohol Solvent

Abstract

We report on the motional and proton transfer dynamics of the super photobase FR0-SB in the series of normal alcohols C1 (methanol) through C8 (n-octanol) and ethylene glycol. Steady-state and time-resolved fluorescence data reveal that the proton abstraction dynamics of excited FR0-SB depend on the identity of the solvent and that the transfer of the proton from solvent to FR0-SB*, forming FR0-HSB+*, fundamentally alters the nature of interactions between the excited molecule and its surroundings. In its unprotonated state, solvent interactions with FR0-SB* are consistent with slip limit behavior, and in its protonated form, intermolecular interactions are consistent with a much stronger interaction of FR0-HSB+* with the deprotonated solvent RO–. We understand the excited-state population dynamics in the context of a kinetic model involving a transition state wherein FR0-HSB+* is still bound to the negatively charged alkoxide, prior to solvation of the two charged species. Data acquired in ethylene glycol confirm the hypothesis that the rotational diffusion dynamics of FR0-SB* are largely mediated by solvent viscosity while proton transfer dynamics are mediated by the lifetime of the transition state. Taken collectively, our results demonstrate that FR0-SB* extracts solvent protons efficiently and in a predictable manner, consistent with a ca. 3-fold increase in dipole moment upon photoexcitation as determined by ab initio calculations based on the equation-of-motion coupled-cluster theory.

Steric effects in light-induced solvent proton abstraction

“…For the primary alcohol solvents there is a monotonic decrease in FR0-HSB + * fluorescence intensity with increasing solvent aliphatic chain length, which is directly proportional to solvent [-OH]. 9 Secondary alcohols exhibit a markedly reduced propensity for proton donation relative to that seen for primary alcohols, despite the fact that the pK a values of primary and secondary alcohols, differing by structural isomerism, are similar (e.g., pK a = 16.1 for n-propanol 25 and 16.5 for i-propanol 26 ). We note that cyclopentanol has a higher protonation probability than i-propanol despite the lower -OH concentration.…”

“…The trends observed in the primary alcohols can be understood in terms of the relative concentration of -OH in each solvent. 9 The time-resolved emission increase and subsequent decay for FR0-HSB + * in the primary alcohols, methanol through n-octanol, is shown in Fig. 4a.…”

“…Our analysis of the time-resolved data is based on a kinetic scheme used in our previous work involving linear alcohols, 9 modified slightly and schematized in Fig. 5.…”

“…1b). 9 As shown in Fig. 1b, where we plot the S 1 − S 0 total electron density difference, calculated using the CC/ EOMCC one-electron reduced density matrices resulting from our earlier computations, 9 the cause of this significant increase in dipole moment upon photoexcitation is an overall intramolecular migration of a relatively small amount of electron density over a long distance, from the diethylamino nitrogen to the imine nitrogen.…”

“…In a recent report, we presented experimental evidence for the formation of a persistent interaction between solvent primary alcohols and FR0-SB* based on rotational diffusion dynamics measurements. 9 The high-level ab initio calculations presented in that report, based on the coupled-cluster (CC) theory 10 and its equation-of-motion (EOM) excited-state extension, 11 indicated that this persistent interaction, which leads to ESPT, is a consequence of the ca. 3-fold increase in the static dipole moment of FR0-SB upon excitation from its ground electronic state (S 0 ) to its first-excited singlet state (S 1 ) (cf.…”