Influence of Solvent Relaxation on Ultrafast Excited-State Proton Transfer to Solvent

Abstract

A thorough understanding of the microscopic mechanism of excited-state proton transfer (ESPT) and the influence of the solvent environment on its dynamics are of great fundamental interest. We present here a detailed investigation of an ESPT to solvent (DMSO) using time-resolved broadband fluorescence and transient absorption spectroscopies. All excited-state species are resolved spectrally and kinetically using a global target analysis based on the two-step Eigen-Weller model. Reversibility of the initial short-range proton transfer producing excited contact ion pairs (CIP*) is observed unambiguously in fluorescence and must be explicitly considered to obtain the individual rate constants. Close inspection of the early dynamics suggests that the relative populations of the protonated form (ROH*) and CIP* are governed by solvent relaxation that influences the relative energies of the excited states. This constitutes a breakdown of the Eigen-Weller model, although the overall agreement between the data and the analysis using classical rate equations is excellent.

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

“…1A) to DMSO. 35 In the global analysis, each of the uorescent species is modeled as a single timedependent log-normal function. 22 The main advantage of the global analysis is that the solvent relaxation can be modeled as frequency downshis of the log-normal functions whereas the population dynamics can be independently accessed through the band integrals.…”

“…Secondly, the CIP* uorescence was unambiguously identied in the time-resolved spectra. 35 The photoacid, SHONI, exhibits an excited-state pK a of À1.9 and is able to deprotonate in several organic solvents such as DMSO and alcohols. 36 The choice of the solvent can impact ESPT in several ways.…”

“…The absorption spectra show two main bands attributed to the S 1 ) S 0 and S 2 ) S 0 transitions. 35,36 In the RO À form, both bands are shied to a lower frequency and the energy gap between the bands increases. The shapes and frequencies of all spectra are relatively similar with the exception of the RO À absorption in DMSO.…”

“…When the reaction rate approaches the time scale of the solvent dynamics, the reaction becomes solvent-controlled and is limited by the relaxation time of the solvent. [29][30][31][32][33][34][35] This complicates the analysis because the singlewavelength uorescence decays become contaminated by the spectral shis due to solvent relaxation. Therefore, the individual contributions from the spectral shis and population dynamics must be resolved separately for a detailed quantitative analysis.…”

Propyl acetate/butyronitrile mixture is ideally suited for investigating the effect of dielectric stabilization on (photo)chemical reactions

“…1,2 In addition to "static" or equilibrium solvent effects, solvation dynamics play an important role in many ultrafast photochemical reactions in solutions. [3][4][5][6][7][8][9] Understanding the effect of the macroscopic solvent parameters on chemical reactions is of great fundamental importance but it is challenging to investigate one single parameter at a time. The main problem arises from the fact that most solvents do not have a single property that is different from the properties of other solvents.…”