FR0-SB, a Schiff base chromophore, is capable of reversible proton abstraction from linear alcohols upon photoexcitation. A pK_a change of 14 units in first electronic excited state (S₁), derived from steady-state spectra using the Förster equation, yields a pK_a of ~21 in S₁. The photoinduced proton abstraction is driven by the change in the electronic structure of the system in the S₁ in comparison to the ground state, depicted by a 4.5 times increase of the dipole moment value in S₁. The excited state proton transfer (ESPT) propagates through an intermediate complex which comprises of a vibrationally relaxed FR0-SB in S₁ sharing a proton with an alcohol molecule (FR0-SB . . . H—OR). Computational studies of the ESPT reaction coordinate, where the intermediate complex is the reactant while the protonated photobase (FR0-HSB⁺) along with the alkoxide (⁻OR) are the products, reveal the need for at least 3 alcohol molecules to complete the reaction. The two additional alcohol molecules are essential to the stabilization of the intermediate complex as well as the product ion-pairs. The ESPT in secondary alcohols is significantly diminished, which emphasizes the importance of the proximity of the alcohol hydroxy- proton to the imine nitrogen of the photobase. The presentation will highlight our efforts towards understanding the different aspects of the proton transfer dynamics following one- and two-photon excitation of the novel fluorene-based “super” photobase molecule FR0-SB.

Steady-state absorption and fluorescence of <b>FR0</b>-SB in alcohols, normalized to their respective maximum value. The supressed <b>FR0</b>-HSB<sup>+</sup>* emission (~15870 cm<sup>−1</sup>) in secondary alcohols indicates moderated excited state proton transfer (ESPT) compared to that in analogous primary alcohols. The optimized geometries of the intermediate complex in <i>n</i>-propanol and <i>i</i>-propanol, obtained from quantum mechanical calculations, underscore the significance of proximity and proper alignment of the proton donor (hydroxy-group of alcohol) and proton acceptor (imine group of <b>FR0</b>-SB) for ESPT.

Steady-state absorption and fluorescence of FR0-SB in alcohols, normalized to their respective maximum value. The supressed FR0-HSB⁺* emission (~15870 cm⁻¹) in secondary alcohols indicates moderated excited state proton transfer (ESPT) compared to that in analogous primary alcohols. The optimized geometries of the intermediate complex in n-propanol and i-propanol, obtained from quantum mechanical calculations, underscore the significance of proximity and proper alignment of the proton donor (hydroxy-group of alcohol) and proton acceptor (imine group of FR0-SB) for ESPT.

Mehdi Moemeni

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