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.

References

1. Medforth, C. J.; Senge, M. O.; Smith, K. M.; Sparks, L. D.; Shelnutt, J. A. J. Am. Chem. Soc. 1992, 114, 9859-9869.
2. Honda, T.; Kojima, T.; Fukuzumi, S. Chem. Commun. 2009, 4994-4996.
3. Nakanishi, T.; Ohkubo, K.; Kojima, T.; Fukuzumi, S. J. Am. Chem. Soc. 2009, 131, 577–584.