Singlet oxygen generation properties of an inclusion complex of cyclic free-base porphyrin dimer and fullerene C₆₀

Abstract

To gain insight into the singlet oxygen (¹O₂) generation properties of supramolecular complexes of cyclic free-base porphyrin dimer with fullerene C₆₀, we evaluated the ¹O₂ quantum yield (Φ_Δ) and rate constant (K_obs) of ¹O₂ generation for a cyclic free-base porphyrin dimer (CPD) linked by butadiyne bearing four 4-pyridyl groups and its inclusion complex (C₆₀⊂CPD) with C₆₀. We demonstrate that CPD and C₆₀⊂CPD possess the ability to generate ¹O₂ under visible light irradiation. Moreover, it was found that the Φ_Δ value of C₆₀⊂CPD is lower than that of CPD. Based on the kinetic and thermodynamic consideration concerning the electron transfer processes between the porphyrin dimer and C₆₀, this work revealed that the lower Φ_Δ value of the C₆₀ inclusion complex would be attributed to the formation of the charge-separated state C₆₀^·−-CPD^·+, leading to a low intersystem crossing (ISC) efficiency for the formation of the triplet excited state ³(CPD)*.

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Introduction

Photosensitizers possessing the ability to generate singlet oxygen (¹O₂) have created considerable interest in recent years from the viewpoint of fundamental studies in photochemistry and their potential applications in photodynamic therapy (PDT).^1–6 These photosensitizers generally produce ¹O₂ through the following processes: initially the photosensitizer absorbs light (hν) to generate the singlet excited state of the photosensitizer (¹S*), then the photoexcited sensitizer (¹S*) undergoes intersystem crossing (ISC) to generate the triplet excited state (³S*). Subsequent energy transfer from the photoexcited sensitizer (³S*) to triplet oxygen (³O₂) produces ¹O₂. Thus, to generate ³S* efficiency is one of the most effective strategies to give high ¹O₂ quantum yield (Φ_Δ). In particular, porphyrin dyes have been regarded as promising candidates for photosensitizers as a result of their strong Soret (400–500 nm) and moderate Q band (500–700 nm) absorption properties, as well as their electrochemical, photochemical and thermal stabilities. The typical photosensitizer, free-base tetraphenylporphyrin (H₂TPP) exhibits Φ_Δ value of 0.6–0.7 (in benzene).⁷ Much effort in molecular design and development of porphyrin photosensitizers have been made to further improve the Φ_Δ value so far.^7–11 Some researchers reported that porphyrin–fullerene C₆₀ dyads as well as boron dipyrromethene (BODIPY)-C₆₀ dyads exhibits a higher Φ_Δ value than each porphyrin or C₆₀, which is attributed to the formation of long-lived triplet excited state (³S*) by the effective ISC due to intramolecular energy transfer or electron transfer (charge separation) between photosensitizer and C₆₀.^11,12

Recently, we have designed and developed cyclic free-base porphyrin dimer CPD linked by butadiyne bearing four 4-pyridyl groups and its inclusion complex C₆₀⊂CPD with C₆₀ (Fig. 1).¹³ It was expected that C₆₀⊂CPD has favorable photochemical and electrochemical properties for PDT through the electrochemical measurements and the transient absorption spectroscopy, based on the fact that the singlet excited state C₆₀-¹(CPD)* undergoes intrasupramolecular electron transfer to give a completely charge-separated state C₆₀^·−-CPD^·+. Thus, in this work, to gain insight into the ¹O₂ generation properties of supramolecular complex of cyclic free-base porphyrin dimer with C₆₀, we evaluated the Φ_Δ and rate constant (K_obs) of ¹O₂ generation for CPD and C₆₀⊂CPD. Here we reveal that cyclic free-base porphyrin dimer and its inclusion complex with fullerene C₆₀ possess the ability to generate ¹O₂ under visible light irradiation, based on the kinetic and thermodynamic consideration concerning the electron transfer processes between the porphyrin dimer and C₆₀.

Fig. 1 Chemical structures of cyclic free-base porphyrin dimer (CPD) linked by butadiyne bearing four 4-pyridyl groups, its inclusion complex (C₆₀⊂CPD) with C₆₀ and ABAB porphyrin monomer H2PyP as a reference.

Results and discussion

The cyclic free-base porphyrin dimer CPD in CH₂Cl₂/MeOH exhibits strong Soret band at around 420 nm and relatively weak Q band in the range 500–650 nm (Fig. 2, λ^abs_max/nm (ε/M⁻¹ cm⁻¹) = 416 (708 000), 514 (31 200), 548 (7400), 587 (9000), 642 (2900)). The molar extinction coefficients (ε) of Soret and Q bands for CPD are higher than those of H2PyP (λ^abs_max/nm (ε/M⁻¹ cm⁻¹) = 418 (419 000), 513 (19 200), 548 (6200), 588 (6000), 643 (3000))^13e as an ABAB porphyrin monomer with two pyridyl groups and two phenyl groups. The fact is attributed to the porphyrin dimer structure of CPD with two porphyrin units. For the C₆₀ inclusion complex C₆₀⊂CPD, it is difficult to obtain its exact absorption spectra because the 1 : 1 complex of CPD with C₆₀ is in dissociation equilibrium in solution of 10⁻⁵ to 10⁻⁶ M concentration which is suitable for the measurement of photoabsorption spectra of porphyrins. In our previous work, however, we have demonstrated that upon addition of C₆₀ to the solution of the cyclic porphyrin dimer, its Soret band was redshifted with a decrease in intensity, whereas its Q band was slightly redshifted but increased in intensity.^{13 1}O₂ generation by CPD, C₆₀⊂CPD or H2PyP in CH₂Cl₂/MeOH (=1/1, v/v) was evaluated by monitoring the photoabsorption spectral change of the known ¹O₂ scavenger 1,3-diphenylisobenzofuran (DPBF) accompanied by the reaction of DPBF with the generated ¹O₂, that is, DPBF can trap ¹O₂ through its photooxidation.¹⁴ CH₂Cl₂/MeOH was bubbled with air for 15 min. The air-saturated solution containing CPD, C₆₀⊂CPD or H2PyP and DPBF was irradiated with 509 nm (300 μW cm⁻², ε = 27 300 M⁻¹ cm⁻¹@λ^abs = 509 nm for CPD and ε = 17 000 M⁻¹ cm⁻¹@λ^abs = 509 nm for H2PyP, respectively) obtained by passage of xenon light through monochromator. For both CPD and C₆₀⊂CPD as well as H2PyP the absorption band of DPBF at around 410 nm decreased with the increase in the photoirradiation time (Fig. 3), which indicate the reaction of DPBF with ¹O₂ generated upon the excitation of the porphyrin dimer. To gain insight into the effect of the cyclic porphyrin dimers on the efficiency of DPBF photooxidation, the changes in optical density (ΔOD) of DPBF are plotted against the photoirradiation time (Fig. 4a), and the slope (m_sl) is used to estimate the Φ_Δ value for CPD, C₆₀⊂CPD and H2PyP. The m_sl value (−1.5 × 10⁻²) of H2PyP is larger than those of CPD (−1.2 × 10⁻²) and C₆₀⊂CPD (−9.8 × 10⁻³). Moreover, it was revealed that the m_sl value of CPD is larger than that of C₆₀⊂CPD. Thus, the Φ_Δ values of CPD, C₆₀⊂CPD and H2PyP were estimated by the relative method using Rose Bengal (RB) (Φ_Δ = 0.80, m_sl = −1.5 × 10⁻², see Fig. S1†) in methanol¹⁵ as the standard (Table 1). The Φ_Δ value of CPD, C₆₀⊂CPD and H2PyP is 0.62, 0.52 and 0.91 respectively, which is in good agreement with the m_sl value. This result suggests that as for the ABAB porphyrin monomer H2PyP the ISC efficiency from ¹S* to the ³S* may be higher than in the cyclic free-base porphyrin dimer CPD. Moreover, it is worth noting that the Φ_Δ value of C₆₀⊂CPD is lower than that of CPD. Our previous work demonstrates that the decay of photoexcited state of C₆₀⊂CPD has two steps (Fig. 5): the first step has a lifetime of 18 ps, which corresponds to the disappearance of the singlet excited state of C₆₀-¹(CPD)* (ca. −3.6 eV), that is, C₆₀-¹(CPD)* undergoes intrasupramolecular electron transfer to give a completely charge-separated state C₆₀^·−-CPD^·+ (ca. −3.7 eV).¹³ C₆₀^·−-CPD^·+ decays with a lifetime of 470 ps to the ground state. The singlet excited state C₆₀-¹(CPD)* has a slower ISC to its triplet excited state C₆₀-³(CPD)* (ca. −4.0 eV), in addition, the C₆₀-³(CPD)* would undergo energy transfer to C₆₀, leading to the formation of ³C₆₀*-(CPD). Thus, on the basis of the photodynamics of the cyclic free-base porphyrin dimer and its inclusion complex with C₆₀, the lower Φ_Δ value of the C₆₀ inclusion complex would be attributed to the formation of charge-separated state, leading to low ISC efficiency because the ISC is in kinetically competition with the intrasupramolecular electron transfer, that is, the formation of triplet excited state ³(CPD)* is in kinetically unfavorable compared to that of the charge-separated state C₆₀^·−-CPD^·+.

Fig. 2 Photoabsorption spectra of CPD and H2PyP in CH₂Cl₂/MeOH.

Fig. 3 Photoabsorption spectral changes for the photooxidation of DPBF (Abs. = ca. 1.0) using (a) CPD (1.3 × 10⁻⁶ M), (b) C₆₀⊂CPD (1.0 × 10⁻⁶ M) and (c) H2PyP (1.4 × 10⁻⁶ M) as photosensitizer under photoirradiation with 509 nm (300 μW cm⁻²) in CH₂Cl₂/MeOH (=1/1, v/v).

Fig. 4 (a) Plots of ΔOD for DPBF against the photoirradiation time for the photooxidation of DPBF using CPD, C₆₀⊂CPD, H2PyP or Rose Bengal as photosensitizers under photoirradiation with 509 nm (300 μW cm⁻²) in CH₂Cl₂/MeOH (=1/1, v/v) and MeOH, respectively. (b) Plots of ln(C_t/C₀) for DHN against the photoirradiation time for the photooxidation of DHN using CPD, C₆₀⊂CPD, H2PyP or Rose Bengal as photosensitizers under photoirradiation with visible light (>385 nm, 30 mW cm⁻²) in CH₂Cl₂/MeOH (=1/1, v/v).

Table 1 ¹O₂ quantum yield (Φ_Δ) and first-order rate constant (K_obs) for the photooxidation of DPBF and DHN in CH₂Cl₂/MeOH (=1/1, v/v), respectively, using CPD, C₆₀⊂CPD or H2PyP as photosensitizer

Photosensitizer	ΦΔ^a	Kobs^b/min^−1
a ^1O2 quantum yield (relative decomposition rate of DPBF), with Rose Bengal (RB) as standard (ΦΔ = 0.80 in methanol,^15 see Fig. S1) and 3-diphenylisobenzofuran (DPBF) as ^1O2 scavenger.b First-order rate constant for the reaction of DHN with ^1O2 generated upon photoexcitation of the photosensitizer. The Kobs for RB is 5.3 × 10^−3 min^−1 (see Fig. S2).
CPD	0.62	6.6 × 10^−3
C60⊂CPD	0.52	5.8 × 10^−3
H2PyP	0.91	9.5 × 10^−3

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Fig. 5 The photodynamics of C₆₀⊂CPD and ¹O₂ generation.

In order to evaluate the photosensitizing ability of the cyclic free-base porphyrin dimer and its inclusion complex with C₆₀, the ln(C_t/C₀) is plotted against the photoirradiation time, where C_t is a concentration of 1,5-dihydroxynaphthalene (DHN) at the reaction time (t) and C₀ is the initial concentration of DHN before photoirradiation. CH₂Cl₂/MeOH (=1/1, v/v) were bubbled with air for 15 min. The air-saturated solution containing CPD or C₆₀⊂CPD and DHN was irradiated with visible light (>385 nm, 30 mW cm⁻²) obtained by passage of xenon light through a 385 nm long path filter. The photoabsorption spectral changes for the photooxidation of DHN using CPD, C₆₀⊂CPD or H2PyP under photoirradiation with the visible light in CH₂Cl₂/MeOH (=1/1, v/v) are shown in Fig. 6. Evidently, the absorption band of DHN at around 300 nm decreased with the increase in the photoirradiation time. The plots of ln(C_t/C₀) against the photoirradiation time indicate that for CPD, C₆₀⊂CPD and H2PyP the ln(C_t/C₀) decreased almost linearly with the increase in the photoirradiation time (Fig. 4b). Thus, this result indicates the ln(C_t/C₀) bears a linear relationship with the photoirradiation time to provide the first-order rate constants (K_obs) for the photooxidation of DHN using the cyclic free-base porphyrin dimer or its inclusion complex with C₆₀ as the photosensitizer (Table 1). Obviously, the higher K_obs values of the cyclic free-base porphyrin dimer and its inclusion complex with C₆₀ relative to RB (see Fig. S2†) are due to the contribution of the strong Soret band of the cyclic free-base porphyrin skeleton, although the K_obs values of CPD and C₆₀⊂CPD are lower than that of H2PyP (9.5 × 10⁻³ min⁻¹). It is worth noting here that the K_obs value (6.6 × 10⁻³ min⁻¹) of CPD is greater than that (5.8 × 10⁻³ min⁻¹) of C₆₀⊂CPD. Therefore, this result demonstrates that CPD exhibits more efficient photosensitizing ability due to the effective ISC compared to C₆₀⊂CPD.

Fig. 6 Photoabsorption spectral changes for the photooxidation of DHN (1.0 × 10⁻⁴ M) using (a) CPD (2.5 × 10⁻⁶ M), (b) C₆₀⊂CPD (2.5 × 10⁻⁶ M) and (c) H2PyP (5.0 × 10⁻⁶ M) as photosensitizer under photoirradiation with visible light (>385 nm, 30 mW cm⁻²) in CH₂Cl₂/MeOH (=1/1, v/v).

In addition, we performed an electron paramagnetic resonance (EPR) method with 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TEMP) as the spin-trapping agent, which can react with ¹O₂ to produce 4-oxo-TEMPO as a stable nitroxide radical.¹⁶ When the air-saturated solution containing CPD, C₆₀⊂CPD or H2PyP and 4-oxo-TEMP was irradiated with visible light (>385 nm, 30 mW cm⁻²) obtained by passage of xenon light through a 385 nm long path filter, for both the free-base porphyrin dimer and its inclusion complex with C₆₀ as well as H2PyP the ESR spectra of 4-oxo-TEMPO were clearly observed as a characteristic 1 : 1 : 1 triplet (Fig. 7). Consequently, this work demonstrated that the cyclic free-base porphyrin dimer and its inclusion complex with C₆₀ possess the ability to generate ¹O₂ under visible light irradiation.

Fig. 7 The ESR spectra of 4-oxo-TEMPO which is formed by the reaction of 4-oxo-TEMP with ¹O₂ which was generated by (a) CPD, (b) C₆₀⊂CPD and (c) H2PyP under irradiation with visible light (temperature 298 K, microwave power 1 mW, microwave frequency 9.439 GHz, and field modulation 0.2 mT at 100 kHz). The air-saturated CH₂Cl₂ solution containing CPD (2.5 × 10⁻⁶ M), C₆₀⊂CPD (2.5 × 10⁻⁶ M) or H2PyP (5.0 × 10⁻⁶ M) as the photosensitizer and 4-oxo-TEMP (50 mM) as the spin-trapping agent was irradiated with visible light (>385 nm, 30 mW cm⁻² for 1 h) obtained by passage of xenon light through a 385 nm long path filter, where CH₂Cl₂ as low polar solvent was used because it was difficult to obtain a clear ESR signal in polar solvent such as CH₂Cl₂/MeOH (=1/1, v/v).

Conclusions

To investigate singlet oxygen (¹O₂) generation properties of cyclic free-base porphyrin dimer and its inclusion complex with fullerene C₆₀, we evaluated the ¹O₂ quantum yield (Φ_Δ) and rate constant (K_obs) of ¹O₂ generation for cyclic free-base porphyrin dimer CPD and its inclusion complex C₆₀⊂CPD with C₆₀. It was found that the Φ_Δ value of C₆₀⊂CPD is lower than that of CPD. The lower Φ_Δ value of the C₆₀ inclusion complex would be attributed to the formation of charge-separated state C₆₀^·−-CPD^·+, leading to low intersystem crossing (ISC) efficiency for the formation of triplet excited state ³(CPD)*, although it was expected that the formation of C₆₀^·−-CPD^·+ is favorable for ¹O₂ generation. Consequently, this work demonstrates that the cyclic free-base porphyrin dimer and its supramolecular complex with C₆₀ possess the ability to generate ¹O₂ under visible light irradiation.

Experimental

Evaluation of ¹O₂ quantum yield

Quantum yield (Φ_Δ) for singlet oxygen (¹O₂) generation by cyclic free-base porphyrin dimer CPD, its inclusion complex C₆₀⊂CPD with fullerene C₆₀ and H2PyP in CH₂Cl₂/MeOH (=1/1, v/v) was evaluated by monitoring the photoabsorption spectral change of the known ¹O₂ scavenger 1,3-diphenylisobenzofuran (DPBF) accompanied by the reaction of DPBF with the generated ¹O₂, that is, DPBF can trap ¹O₂ through its photooxidation. CH₂Cl₂/MeOH was bubbled with air for 15 min. The absorbance of DPBF was adjusted to around 1.0 in air-saturated solvent. Concentration of CPD, C₆₀⊂CPD or H2PyP was adjusted with an absorbance of ca. 0.03 at the irradiation wavelength (509 nm). The air-saturated solution containing the photosensitizer (CPD, C₆₀⊂CPD or H2PyP) and DPBF was irradiated with 509 nm (300 μW cm⁻²) obtained by passage of xenon light through monochromator. The photoabsorption spectral change of DPBF with the photoirradiation was monitored with an interval of 1 min up to 10 min. The absorption band of DPBF at around 410 nm decreased with the increase in the photoirradiation time. The changes in optical density (ΔOD) of DPBF are plotted against the photoirradiation time, and the slope is used to estimate the Φ_Δ values of CPD, C₆₀⊂CPD and H2PyP. The Φ_Δ values of CPD, C₆₀⊂CPD and H2PyP were estimated by the relative method using Rose Bengal (RB) (Φ_Δ = 0.80) in methanol as the standard. Therefore, the ¹Φ_Δ values were calculated according to the following eqn (1):

Φ_Δsam = Φ_Δref × [(m_sam/m_ref) × (L_ref/L_sam)] (1)

where Φ_Δsam and Φ_Δref are the ¹O₂ quantum yield of photosensitizer (CPD, C₆₀⊂CPD or H2PyP) and RB, respectively, m_sam and m_ref are the slope of the difference (ΔOD) in the change in the absorption maximum wavelength of DPBF (around 410 nm) which are plotted against the photoirradiation time, L_sam and L_ref are the light harvesting efficiency, which is given by L = 1 − 10^−A (“A” is the absorbance at the photoirradiation wavelength).

Photosensitizing ability

Photosensitizing ability of CPD, C₆₀⊂CPD and H2PyP in CH₂Cl₂/MeOH was evaluated by plotting the ln(C_t/C₀) against the photoirradiation time, where C_t is a concentration of 1,5-dihydroxynaphthalene (DHN) at the reaction time (t) and C₀ is the initial concentration of DHN before photoirradiation. CH₂Cl₂/MeOH was bubbled with air for 15 min. The air-saturated solution containing the photosensitizer (2.5 × 10⁻⁶ M for CPD and C₆₀⊂CPD, 5.0 × 10⁻⁶ M for H2PyP and 2.5 × 10⁻⁶ M for RB) and DHN (1.0 × 10⁻⁴ M) was irradiated with visible light (>385 nm, 30 mW cm⁻²) obtained by passage of xenon light through a 385 nm long path filter. The photooxidation of DHN with the photoirradiation was monitored by following the decrease in the photoabsorption at around 300 nm with an interval of 1 min up to 10 min and then an interval of 5 min up to 30 min. The concentration (C_t) of DHN at the reaction time (t) was calculated based on Lambert–Beer law (A_DPBF = εcl). The ln(C_t/C₀) decreased almost linearly with the increase in the photoirradiation time due to the photooxidation of DHN, that is, the slope was used to estimate the rate constants (K_obs).

¹O₂ detection by EPR spin-trapping method with 4-oxo-TEMP

The EPR spectra were recorded on a JEOL JES-RE1X spectrometer under the following experimental conditions: temperature 298 K, microwave power 1 mW, microwave frequency 9.439 GHz, and field modulation 0.2 mT at 100 kHz. The air-saturated CH₂Cl₂ solution containing CPD (2.5 × 10⁻⁶ M), C₆₀⊂CPD (2.5 × 10⁻⁶ M) or H2PyP (5.0 × 10⁻⁶ M) as the photosensitizer and 4-oxo-TEMP (50 mM) as the spin-trapping agent was irradiated with visible light (>385 nm, 30 mW cm⁻² for 1 h) obtained by passage of xenon light through a 385 nm long path filter. The ESR spectrum of 4-oxo-TEMPO which is formed by the reaction of 4-oxo-TEMP with ¹O₂, was clearly observed as a characteristic 1 : 1 : 1 triplet (Fig. 7).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra02699d

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