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Proton‐Transfer Dynamics of Photoacidic Merocyanines in Aqueous Solution
Author(s) -
Kaiser Christoph,
Halbritter Thomas,
Heckel Alexander,
Wachtveitl Josef
Publication year - 2021
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.202100168
Subject(s) - merocyanine , chemistry , photochemistry , photoisomerization , protonation , proton , excited state , intramolecular force , ultrafast laser spectroscopy , aqueous solution , methanol , relaxation (psychology) , ground state , isomerization , organic chemistry , spectroscopy , atomic physics , ion , social psychology , psychology , physics , quantum mechanics , photochromism , catalysis
Photoacids attract increasing scientific attention, as they are valuable tools to spatiotemporally control proton‐release reactions and pH values of solutions. We present the first time‐resolved spectroscopic study of the excited state and proton‐release dynamics of prominent merocyanine representatives. Femtosecond transient absorption measurements of a pyridine merocyanine with two distinct protonation sites revealed dissimilar proton‐release mechanisms: one site acts as a photoacid generator as its pK a value is modulated in the ground state after photoisomerization, while the other functions as an excited state photoacid which releases its proton within 1.1 ps. With a pK a drop of 8.7 units to −5.5 upon excitation, the latter phenolic site is regarded a super‐photoacid. The 6‐nitro derivative exhibits only a phenolic site with similar, yet slightly less photoacidic characteristics and both compounds transfer their proton to methanol and ethanol. In contrast, for the related 6,8‐dinitro compound an intramolecular proton transfer to the ortho ‐nitro group is suggested that is involved in a rapid relaxation into the ground state.