Metastable-State Photoacid: A Comprehensive Theoretical Study of the Photoreaction, Reverse Reaction, Thermal Isomerization, and TD-DFT Absorption in Solvent
Alharbi, Arwa Wasel Hamed
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The main objective of the research is to investigate the fundamentals of novel metastable-state photoacids. These photoacids can reversibly produce a large concentration of protons while undergoing photo-irradiation. This change in proton concentration can induce and catalyze chemical reactions, biological functions, and material properties that do not occur in the dark. (IMAGE) Scheme 1. Energy diagram of the photoreaction. (The red route is the forward photoreaction; the blue route is the reverse reaction; and the process that involves the proton dissociation of the photoacid in the dark is the green route; "t-" means trans- and "c-" means cis-). Scheme 1 shows the predicted energy diagram of the conversion between merocyanine (t-MEH) and spiropyran (SP) using DFT and TD-DFT calculations with B3LYP/6-31G* basis set. The photoreaction involves the t-MEH reactant (the low acidity state), the SP + H+ products (the high acidity metastable-state), and two intermediates tME, and c-ME. The excited state t-MEH* releases its proton through the ground state transition state because the excited state proton dissociation transition state is too high and then proceeds downhill along with the ground state mechanism. The energy absorbed by the t-MEH is sufficient to make the photoreaction proceed to the closed ring without observing any of the intermediate species. The rate constant of the proton dissociation process is 3.32X1010 s-1 . In the reverse reaction after irradiation, SP proceeds to the t-MEH and the mechanism involves one intermediate species (c-MEH) where the cis-trans isomerization is the rate determining step. The theoretical activation energy ΔG* = 12.41 kcal/mol that was obtained from the predicted reverse mechanism agrees with the kinetic data obtained for mPAH1 in water (experimental ΔG* = 12.45 kcal/mol). In the dark, t-MEH undergoes a proton dissociation process through an energy barrier of 11.2 kcal/mol. The theoretical pKa = 7.39 obtained from ΔG =Gt-ME–Gt-MEH and agrees with the estimated pKa of mPAH1 in water is 7.4. The TD-DFT calculations with B3LYP/6-31G* basis set successfully predicted the maximum absorption of the open metastable-state photoacid (mPAH1) isomer and its related derivatives within ~ 10 nm error. The results show that the peak observed experimentally at 527 nm for mPAH1 is associated with the c-ME which has a lower EHOMO and a lower ΔEHOMO-LUMO than the t-ME isomer by 24 kcal/mol.