Abstract
The excited-state heterolysis of acridinol-based derivatives leads to the release of the OH- ion and the formation of the corresponding acridinium cations. To evaluate the parameters that control the reaction barriers, the kinetics of excited-state OH- release from a series of acridinol photobases were studied using transient absorption spectroscopy. The rate constants were obtained in three solvents (methanol, butanol, and isobutanol), and the data were modeled using Marcus theory. The intrinsic reorganization energies obtained from these fits were found to correlate well with the solvent reorganization energies calculated using dielectric continuum model, suggesting that the excited-state OH- release occurs along the solvent reaction coordinate. Furthermore, the ability of acridinol photobases to photoinitiate chemical reactions was demonstrated using the Michael reaction between dimethylmalonate and nitrostyrene.
Highlights
Photoacids are a group of molecules, usually containing the aromatic alcohol motif, whose acidity significantly increases upon electronic excitation.[1,2,3] The excitation of photoacids in the presence of appropriate base acceptors enables mechanistic studies of fast proton transfer (PT) reactions in solution using time-resolved laser techniques.[4,5,6,7,8,9] These experimental results provided a valuable platform to test and improve current theoretical models for PT
The intrinsic reorganization energies obtained from these fits were found to correlate well with the solvent reorganization energies calculated using dielectric continuum model, suggesting that the excited-state OH− release occurs along the solvent reaction coordinate
The results indicate that the OH− release from acridinol models in their ground state is thermodynamically unfavorable in all solvents (Table 1)
Summary
Photoacids are a group of molecules, usually containing the aromatic alcohol motif, whose acidity significantly increases upon electronic excitation (up to 32 pKa units).[1,2,3] The excitation of photoacids in the presence of appropriate base acceptors enables mechanistic studies of fast proton transfer (PT) reactions in solution using time-resolved laser techniques.[4,5,6,7,8,9] These experimental results provided a valuable platform to test and improve current theoretical models for PT. To evaluate the parameters that control the reaction barriers, the kinetics of excited-state OH− release from a series of acridinol photobases were studied using transient absorption spectroscopy. The intrinsic reorganization energies obtained from these fits were found to correlate well with the solvent reorganization energies calculated using dielectric continuum model, suggesting that the excited-state OH− release occurs along the solvent reaction coordinate.
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