Ultrafast transient absorption and solvation of a super-photoacid in acetoneous environments

Abstract

The phenomenon of photoacidity, i.e., an increase in acidity by several orders of magnitude upon electronic excitation, is frequently encountered in aromatic alcohols capable of transferring a proton to a suitable acceptor. A promising new class of neutral super-photoacids based on pyranine derivatives has been shown to exhibit pronounced solvatochromic effects. To disclose the underlying mechanisms contributing to excited-state proton transfer (ESPT) and the temporal characteristics of solvation and ESPT, we scrutinize the associated ultrafast dynamics of the strongest photoacid of this class, namely tris(1,1,1,3,3,3-hexafluoropropan-2-yl)8-hydroxypyrene-1,3,6-trisulfonate, in acetoneous environment, thereby finding experimental evidence for ESPT even under these adverse conditions for proton transfer. Juxtaposing results from time-correlated single-photon counting and femtosecond transient absorption measurements combined with a complete decomposition of all signal components, i.e., absorption of ground and excited states as well as stimulated emission, we disclose dynamics of solvation, rotational diffusion, and radiative relaxation processes in acetone and identify the relevant steps of ESPT along with the associated time scales. Graphical abstract

Photoacidity of Indolinospirobenzopyrans in Water

“…[61][62] In other words, any MCH can be used to perturb acid-base equilibria featuring an equilibrium constant that is higher than that of its ground state, but lower than that of its metastable state. [63] A recent work by Wachtveitl and co-workers, [25] however, suggests that the phenolic site of MCHs can be regarded to as a "superphotoacid" [64] and deprotonation precedes any other thermal decay (see below for a more detailed discussion). Hence, the unrivalled capability of MCHs to display persistent proton dissociation in water results from the possibility to photochemically populate a conjugate base that is metastable.…”

Computational investigation of explicit solvent effects and specific interactions of hydroxypyrene photoacids in acetone, DMSO, and water

“…Similarly, the study in neat acetone did not fully answer the question regarding the actual proton acceptor, keeping the possibility of ESPT to acetone still unsettled. 61 In a previous computational study, we have applied the Fo ¨rster cycle (Fig. 2) to determine the pK Ã a values of the photoacids A-F in the related solvent DMSO through the ground-state pK a and electronic transition energies.…”

“…Most recently, the ESPT of photoacid B has been studied experimentally in acetone–water mixtures 60 and neat acetone. 61 The study in acetone–water mixtures showed that a reaction complex between the photoacid and water—formed either already in the ground state or directly after excitation—becomes crucial for the ESPT dynamics of photoacid B in these solvent mixtures. 60 In particular, association constants could be determined from the changes in the UV/Vis absorption spectra, but more detailed spectroscopic information on such intermediates was not directly accessible.…”

“…Similarly, the study in neat acetone did not fully answer the question regarding the actual proton acceptor, keeping the possibility of ESPT to acetone still unsettled. 61…”

Theoretical Study on the Photoacidity of Hydroxypyrene Derivatives in DMSO Using ADC(2) and CC2

“…The solvatochromic properties as well as ESPT kinetics , have been studied experimentally in a variety of solvents including water, methanol, ethanol, and DMSO. Recently, the dynamics and ESPT intermediates of a strongly hydrogen-bonded complex of compound A with a trialkylphosphine oxide in nitrile solution were elucidated. , Furthermore, for the strongest superphotoacid B of that class, namely tris­(1,1,1,3,3,3-hexafluoropropan-2-yl)-8-hydroxypyrene-1,3,6-trisulfonate (p K a * ≈ −4 in water), the possibility of ESPT in acetone–water mixtures and in neat acetone were demonstrated . Despite the wealth of experimental knowledge regarding this photoacid, only limited information on the molecular origin of the exceptionally high photoacidity is yet available in the literature.…”