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Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents.

ABSTRACT: Excited-state proton transfer (ESPT) to solvent is often explained according to the two-step Eigen-Weller model including a contact ion pair (CIP*) as an intermediate, but general applicability of the model has not been thoroughly examined. Furthermore, examples of the spectral identification of CIP* are scarce. Here, we report on a detailed investigation of ESPT to protic (H₂O, D₂O, MeOH and EtOH) and aprotic (DMSO) solvents utilizing a broadband fluorescence technique with sub-200 fs time resolution. The time-resolved spectra are decomposed into contributions from the protonated and deprotonated species and a clear signature of CIP* is identified in DMSO and MeOH. Interestingly, the CIP* intermediate is not observable in aqueous environment although the dynamics in all solvents are multi-exponential. Global analysis based on the Eigen-Weller model is satisfactory in all solvents, but the marked mechanistic differences between aqueous and organic solvents cast doubt on the physical validity of the rate constants obtained.

SUBMITTER: Verma P

PROVIDER: S-EPMC8163259 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Choline-based electrolytes have been proposed as environmentally friendly and low-cost alternatives for secondary zinc air batteries. Choline acetate [Ch]+ [OAc]- in protic (D2 O) and aprotic (DMSO-d6 ) solvents has been studied by means of concentration-dependent 1 H NMR, viscosity, and density measurements. The viscosities have been calculated on the basis of the Jones-Dole equation and showed that the dominant contribution originates from short-range ion-solvent interactions. Site-specific association affinities were assigned from NMR chemical shift titrations. In DMSO-d6 , the hydroxyl group of choline was found to have the smallest dissociation constant followed by the methyl group of acetate. The corresponding Gibbs energies at low concentration were found to be in agreement with a solvent-separated ion pair (2SIP) configuration, whereas at concentrations above 300 mM, a solvent-shared ion pair (SIP) configuration was assigned. For [Ch]+ [OAc]- in D2 O, association effects were found to be weaker, attributed to the high dielectric constant of the solvent. On time scales on the order of 100 ms, NMR linewidth perturbations indicated a change in the local rotational dynamics of the ions, attributed to short-range cation-solvent interactions and not to solvent viscosity. At 184 mM, ∼ 40 % of the cations in DMSO-d6 and ∼ 10 % in D2 O were found to exhibit short-range interactions, as indicated by the linewidth perturbations. It was found that at about 300 mM, the ions in DMSO-d6 exhibit a transition from free to collective translational dynamics on time scales on the order of 400 ms. In DMSO-d6 , both ions were found to be almost equally solvated, whereas in D2 O solvation of acetate was stronger, as indicated by the obtained effective hydrodynamic radii. For [Ch]+ [OAc]- in DMSO-d6 , the results suggest a solvent-shared ion association with weak H-bonding interactions for concentrations between 0.3-1 M. Overall, the extent of ion association in solvents such as DMSO is not expected to significantly limit charge transport and hinder the performance of choline-based electrolytes.

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Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents.

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