Proton-transfer dynamics of photoacidic merocyanines in aqueous solution

  • 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 pKa 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 pKa 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.
Author:Christoph Kaiser, Thomas Halbritter, Alexander HeckelORCiD, Josef WachtveitlORCiDGND
Parent Title (English):Chemistry
Place of publication:Weinheim
Document Type:Article
Date of Publication (online):2021/04/30
Date of first Publication:2021/04/30
Publishing Institution:Universitätsbibliothek Johann Christian Senckenberg
Release Date:2022/04/13
Tag:cyanines; photoacid; proton transfer; spiro compounds; ultrafast spectroscopy
Page Number:14
First Page:9160
Last Page:9173
We thank the Deutsche Forschungsgemeinschaft (DFG) for funding through, SFB 902 „Molecular Principles of RNA-based Regulation“, GRK 1986 “CLiC – Complex Light Control” and grant number WA 1850/4-2. Open access funding enabled and organized by Projekt DEAL.
Institutes:Biochemie, Chemie und Pharmazie
Dewey Decimal Classification:5 Naturwissenschaften und Mathematik / 54 Chemie / 540 Chemie und zugeordnete Wissenschaften
Licence (English):License LogoCreative Commons - Namensnennung-Nicht kommerziell 4.0