(Invited) Photoinduced Electron-Transfer Dynamics of a Monoprotonated Saddle-Distorted Porphyrin
Authors: Kojima Takahiko , Suzuki Wataru , Kotani Hiroaki , Ishizuka Tomoya , Ohkubo Kei , Fukuzumi Shunichi 
Published: February 2020

Protonation of a free-base porphyrin (H2P) has been widely investigated because of the crucial influences on characteristic redox and photophysical properties of H2P. Protonation of H2P under strongly acidic conditions resulted in formation of diprotonated porphyrin (H4P2+) without any detectable monoprotonated porphyrin (H3P+) as an intermediate. Thus, there are only a few reports on the formation of H3P+ because of the higher basicity of saddle distorted H3P+ rather than that of planer H2P. On the other hand, dodecaphenylporphyrin (H2DPP),1 shows a saddle-distorted structure due to the steric repulsion among the phenyl rings. Previously, we reported a crystal structure of a monoprotonated dodecaphenylporphyrin (H3DPP+) by using 2-anthracene sulfonic acid (2-AN-SO3H) as a proton source.2 Herein, we report selective formation of H3DPP+ in an acetone solution with a polar protic solvent such as methanol. Hydrogen bonding between H3DPP+ and methanol has been revealed to be important for stabilization of the monoprotonated porphyrin. In this presentation, we will focus on photoinduced electron-transfer (ET) reactions from ferrocene derivatives to the triplet excited state of H3DPP+ (3[H3DPP+]*), which have been scrutinized by femto- and nano-second laser flash photolysis for the first time.
Protonated products of saddle-distorted H2DPP with trifluoroacetic acid (TFA) in acetone were revealed to be controlled by addition of methanol as a protic polar solvent. Addition of 1 eq of TFA to an acetone solution containing H2DPP in the presence of 3% methanol (v/v) resulted in selective formation of a monoprotonated form (H3DPP+) as confirmed by spectroscopic measurements and X-ray crystallography. The crucial role of methanol for the selective H3DPP+ formation was interpreted as stabilization of H3DPP+ by hydrogen bonding with methanol; because a methanol molecule was found to form two-point hydrogen bonding with an NH proton and the non-protonated nitrogen atom of H3DPP+in the crystal.
The reduction potential (E
red, V vs Fc/Fc+) of H3DPP+(CF3COO–) was determined to be –1.05 V in acetone/methanol (3%) containing 0.1 M [(n-butyl)4N]BPh4 as an electrolyte at 298 K by electrochemical measurements. The E
red value of H3DPP+(CF3COO–) was lower than that of H4DPP2+(CF3COO–)2 (–0.83 V) and higher than that of H2DPP (–1.41 V). Then, absorption and emission spectra of H3DPP+(CF3COO–) were measured to determine the energy levels of the singlet and triplet excited states of H3DPP+(CF3COO–) in acetone/methanol (3%). As a result, the energy levels of 1[H3DPP+]* and 3[H3DPP+]* were determined to be 1.66 eV and 1.44 eV, respectively.
Femto- and nano-second laser flash photolysis was applied to elucidate the photodynamics of intermolecular photoinduced ET reactions from ferrocene derivatives as one-electron donors to H3DPP+ as an electron acceptor. Femto-second laser flash photolysis of H3DPP+ in acetone/methanol (3%) with laser excitation at 500 nm revealed a transient absorption spectrum with a peak at 575 nm assigned to the singlet excited state of H3DPP+ (1[H3DPP+]*). The decay time profile at 575 nm exhibited a mono-exponential decay with the lifetime of 200 ps, which is ascribed to intersystem crossing from 1[H3DPP+]* to the triplet excited state of H3DPP+ (3[H3DPP+]*). The decay time profiles of 3[H3DPP+]* were measured by nano-second laser flash photolysis in the absence and presence of ferrocene derivatives as one-electron donors. As a result, we obtained the lifetime of 3[H3DPP+]* (τ = 42 μs) and second-order rate constants (k
et) of the ET reactions, respectively. The k
et values were analyzed in light of the Marcus theory of electron transfer (Figure 1). The reorganization energy (λ) of electron transfer was determined to be 1.85 eV, which is slightly larger than that of H4DPP2+ in acetonitrile (1.69 eV),3 due to the larger structural change after electron transfer than that of H4DPP2+.
In conclusion, we have succeeded in selective formation of H3DPP+ in the presence of methanol. We have also revealed the photodynamics of H3DPP+ in photoinduced ET by analyzing intermolecular ET from ferrocene derivatives as electron donor to 3[H3DPP+]* as an electron acceptor to determine the λ value of intermolecular ET for H3DPP+to be 1.85 eV. This is the first report on a reorganization energy of a monoprotonated porphyrin in ET.

Figure 1. Driving-force dependence of log k
et for ET from electron donors to 3[H3DPP+]* and a fitting curve based on the Marcus theory of ET (black). Gray: H4DPP2+(Cl–)2 in acetonitrile.3
1: Fc, 2: Me2Fc, 3: Me5Fc, 4: Me8Fc, 5: Me10Fc.

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Figure 1