Excited‐State Proton Transfer Dynamics of a Super‐Photoacid in Acetone‐Water Mixtures

Abstract

Super‐photoacids, that is, photoacids with a negative pKnormala value in the electronically excited state, can trigger an excited‐state proton transfer (ESPT) to the solvent. For the neutral pyranine‐derived super‐photoacid studied here, even indications for ESPT in acetoneous solution are reported. The characteristics of ESPT in this environment, that is, which intermediates exist and what the impact of cosolvents is, remain unsettled though. In this work, we study ESPT in acetone‐water mixtures by steady‐state and time‐resolved fluorescence spectroscopy. Various effects are observed: First, the addition of water supports the formation of a hydrogen‐bonded ground‐state complex comprising one water molecule and the photoacid, whose excitation triggers the formation of a hydrogen‐bonded ion pair on a sub‐ns time scale. Second, water has an overall accelerating effect on the fluorescence dynamics of the involved emitting species, whose contributions are disentangled in a global analysis scheme, enabling the identification of emission from the free photoacid, a photoacid‐water complex, a hydrogen‐bonded ion pair, and the deprotonated photoacid. At least two water molecules are necessary for ESPT in the environment. Third, additional acidification thwarts an efficient ground‐state complex formation of the photoacid and water. However, upon excitation, complexation may occur on a timescale faster than the photoacid's excited‐state lifetime, so that emission from a nascent complex emerges.

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

“…Furthermore, the experimental results have shown that specific molecular interactions between the photoacid B and the solvent or other proton acceptors become relevant in acetone solution. 60 This motivated us to include explicit solvent molecules in the first solvation shell and to study their effect on the electronic excitation energies of this photoacid.…”

“…55 As an aprotic and only moderately basic solvent, acetone is a quite unusual medium for PT reactions. 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.…”

“…For further reference, the results in bulk water (e = 78.30) are also included. Experimental values are taken from ref 60.…”

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.…”

Prediction of acid pKa values in the solvent acetone based on COSMO‐RS

“…Thereby, we followed closely the computational procedure described in References [24–26]. This is in particular of interest in light of recent studies of super photoacids in this solvent, 35,36 where previous implementations of COSMO excited‐state energy gradients for efficient wave function‐based methods 37,38 offer the possibility to employ these LFER parameters for COSMO in future applications on .…”