Effect of Protonation on the Molecular Structure and Reactivity of a Typical Merocyanine Dye: Experimental and Theoretical Investigation

Article abstract of DOI:10.1021/j100115a017

The molecular reactivity of 1-methyl-2-(4-hydroxystyryl)pyridiniumbetaine (M) is affected upon protonation in both ground and excited states.In aqueous solution, the protonated trans form is photochemically active and isomerizes to give the cis form.The quantum yields Φ_tc and Φ_ct are determined.The rate constant and the thermodynamic parameters of the reverse cis -> trans thermal reaction, from the unprotonated cis form, are also calculated.Due to the irreversibility of the thermal reaction, a complete molecular reaction cycle is performed in one direction.The excited molecule exhibits more acidic character than in the ground state.To correlate with the experimental work, ASED-MO calculations are applied for both unprotonated and protonated forms.The photochemical isomerization and the thermal reactions are discussed in terms of changes of the geometrical structure from the quinonoid to the benzenoid form upon protonation in both ground and excited states.Upon excitation, it seems that the molecular polarity character decreases in the case of M but increases for MH(+).Protonation is found as an exothermic, downhill reaction in the ground state and as an endothermic, uphill reaction in the excited state.It is shown that the lowest electronic transition is ? -> ?* which is higher for MH(+) than for M.