Proton‐Transfer Dynamics of Photoacidic Merocyanines in Aqueous Solution

Abstract

Photoacids attract increasing scientific attention, as they are valuable tools to spatiotemporally control protonrelease 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 À 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 orthonitro group is suggested that is involved in a rapid relaxation into the ground state.

Large, Tunable, and Reversible pH Changes by Merocyanine Photoacids

“…The p K a h ν value determined under irradiation can be viewed as the p K a value of the cis -MCH form because the equilibrium between cis- MC and SP is shifted entirely to SP when irradiated. The cis -MC form is short-lived and can only be detected by transient absorption spectroscopy, , whereas cis -MCH is stable at low pH values (determined by p K a h ν ). , In earlier studies, the cis -MCH was assigned as the protonated spiropyran (SPH). ,,, More recent work has shown that the assignment as cis -MCH is most appropriate . From a practical perspective, cis -MCH behaves mechanistically as SPH due to an apparent barrierless transition from SP to cis -MCH at low pH …”

Inactivation of Competitive Decay Channels Leads to Enhanced Coumarin Photochemistry

“…The absence of a hydrogen-atom can also prevent possible non-radiative decay upon hydrogen-bonding with protic solvents [34][35][36][37]39] and eliminates a potentially photoacidic site. [40,41] The synthesis of compounds 7 a-d started from commercially available m-aminophenol 2 (see Figure 3a). In the first step, the hydroxy group of 2 was protected with tertbutyldimethylsilyl (TBDMS) in a quantitative yield.…”

“…This exhibits an electron donating character compared to the proton in 7 a and 7 c and should thus enhance the charge transfer character formation originating from the nitrogen that is necessary for uncaging. The absence of a hydrogen‐atom can also prevent possible non‐radiative decay upon hydrogen‐bonding with protic solvents [34–37,39] and eliminates a potentially photoacidic site [40,41] …”

Reversible photo control of proton chemistry

“…Transient absorption spectroscopy was also used by Wachtveitl's group to study the photoreaction of a pyridinyl merocyanine photoacid. 14 Physicochemical properties, e.g. acidity, reverse reaction rate etc.…”