Thermodynamics and Photodynamics of a Monoprotonated Porphyrin Directly Stabilized by Hydrogen Bonding with Polar Protic Solvents

Abstract

Addition of 1 equiv of TFA to an acetone solution containing dodecaphenylporphyrin (H DPP) in the presence of 10 % MeOH (v/v) resulted in selective formation of a monoprotonated form (H DPP ), in sharp contrast to protonation of H DPP directly affording a diprotonated form (H DPP ) in acetone in the absence of MeOH. The crucial role of MeOH for selective H DPP formation was interpreted as hydrogen-bonding stabilization of H DPP , since a MeOH molecule was found to form hydrogen bonds with an NH proton of H DPP in the crystal. The selectivity of H DPP formation was evaluated by the formation yield of H DPP , which increased when elevating the portion of MeOH (0-10 %) in acetone with saturation behavior, suggesting that H DPP is stabilized by hydrogen bonding with MeOH even in solution, together with the thermodynamic parameters determined from a van't Hoff plot based on the spectroscopic titration. Femto- and nanosecond laser flash photolysis allowed us to elucidate the photodynamics of H DPP in intermolecular photoinduced electron transfer (ET) from ferrocene derivatives as one-electron donors to the triplet excited state of H DPP as an electron acceptor. The second-order rate constants of the ET reactions were evaluated in light of the Marcus theory of ET. The reorganization energy of ET was determined to be 1.87 eV, which is slightly larger than that of H DPP in acetonitrile (1.69 eV), due to larger structural change upon ET than that of H DPP .

Water Molecules Gating a Photoinduced One‐Electron Two‐Protons Transfer in a Tyrosine/Histidine (Tyr/His) Model of Photosystem II

“…The bathochromic shift of the Soret band can be assigned to the protonation of the porphyrin site acting as ad istant base (see Figure S19 for comparison). [15] Hence,i ta ppears clearly that water is responsible to convey the released proton to the porphyrin N 4 coordinating cavity.…”

Molecular Engineering of Free‐Base Porphyrins as Ligands—The N−H⋅⋅⋅X Binding Motif in Tetrapyrroles

“…H 2 DPP (64)also formed astable monoacid complex 69 in the presence of methanol and trifluoroacetic acid in dichloromethane. [107] Thec rucial role of methanol is highlighted as stabilizing the monoacid through hydrogen bonding with an out-of-plane NÀHg roup and an imine moiety at the same time,preventing the second acid-base reaction of the macrocycle (Figure 17 B). Furthermore,Almarsson and co-workers were the first to observe am onoprotonated meso-tetraphenylporphyrin, which was capped on one face so that the access of atrifluoroacetate anion to the second protonation site was restricted.…”

“…where the monoacid H 3 DPP + is stabilized by hydrogen bonding with methanol. [58,107] C: Supramolecular hetero-triad 70 and its structure in the crystal (RARQEJ). [58,108] This occurred as stronger acids provided weaker corresponding bases that were weaker H-bond acceptors (Figure 17 C).…”

A Mechanistic Dichotomy in Two‐Electron Reduction of Dioxygen Catalyzed by N,N’‐Dimethylated Porphyrin Isomers

“…Next, to characterize two‐electron‐reduced species of syn ‐Me 2 P or anti ‐Me 2 P in the presence of excess TFA (p K a =12.6 in CH 3 CN) under Ar, chemical reduction by Me 8 Fc was scrutinized by UV/Vis and NMR measurements at 298 K (Schemes and 4). Upon addition of Me 8 Fc (1.0 m m ) to a deaerated CH 3 CN solution containing syn ‐Me 2 P (0.010 m m ) with TFA (3.0 m m ), absorption bands at 503 and 730 nm derived from syn ‐H 2 Me 2 P 2+ , which was formed by diprotonation of syn ‐Me 2 P with TFA (Figure S2), disappeared with simultaneous appearance of an absorption band at 450 nm as shown in Figure .…”

“…On the basis of Equation (1), the second‐order rate constant ( k ) was determined to be (4.0±0.2) M −1 s −1 from the slope in Figure b. When dichloroacetic acid (DCA, p K a =15.9 in CH 3 CN) was used as acids instead of TFA, the second‐order rate constant ( k ) decreased to (1.6±0.4) M −1 s −1 due to the lower acidity of DCA than that of TFA (Figure S5). This result indicates that the isomerization rates of anti ‐Me 2 Iph depends on not only the concentration of acids but also the p K a values of acids used as shown in Figure b and Figure S5.…”