Dye-sensitized solar cell based on an inclusion complex of a cyclic porphyrin dimer bearing four 4-pyridyl groups and fullerene C₆₀

Abstract

Cyclic free-base porphyrin dimers (H₄-C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO)) linked by butadiyne or phenothiazine bearing four 4-pyridyl groups and their inclusion complexes (C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO)) with fullerene C₆₀ have been applied to dye-sensitized solar cells (DSSCs) as a new class of porphyrin dye sensitizers with pyridyl anchoring groups for attachment on a TiO₂ electrode. The FTIR spectra of the porphyrin dimers adsorbed on TiO₂ nanoparticles demonstrated that these porphyrin dimers are adsorbed on the TiO₂ surface through the formation of hydrogen bonding of pyridyl groups and/or pyridinium ions at Brønsted acid sites on the TiO₂ surface. The adsorption amount of the porphyrin dimers adsorbed on the TiO₂ electrode is 2.0 × 10¹⁷ molecules per cm², that is, the adsorption amount of the porphyrin unit is 4.0 × 10¹⁷ cm⁻², which is higher than that of dye sensitizers with pyridyl groups reported so far. The photovoltaic performance of DSSCs based on phenothiazine-bridged cyclic porphyrin dimer H₄-Ptz-CPD_Py(TEO) is higher than that of DSSCs based on butadiyne-linked cyclic porphyrin dimer H₄-C₄-CPD_Py(TEO). Moreover, the photovoltaic performances of DSSCs based on cyclic free-base porphyrin dimers are higher than those of DSSCs based on their C₆₀ inclusion complexes C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO). On the basis of the electrochemical measurements (voltammetry and electrochemical impedance spectroscopy) and the transient absorption spectroscopy, the differences in the photovoltaic performances among these cyclic free-base porphyrin dimers are discussed from kinetic and thermodynamic considerations concerning the electron transfer processes in DSSCs.

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Introduction

Dye-sensitized solar cells (DSSCs) employing dye-adsorbed TiO₂ electrodes are one of the most promising new renewable photovoltaic cells utilizing the sun as a free and inexhaustible energy source because of their interesting construction and operational principles, and low cost of production, since Grätzel and co-workers produced high-performance DSSCs based on a Ru-complex dye, which showed a solar energy-to-electricity conversion yield (η) of 11%.¹ To further improve the photovoltaic performance of DSSCs, many kinds of ruthenium (Ru) dyes, porphyrin dyes, phthalocyanine dyes and organic dyes bearing carboxyl groups as anchoring groups, which are adsorbed on the TiO₂ electrode through the bidentate bridging linkage between the carboxyl group of the dye and Brønsted acid sites (surface-bound hydroxyl groups, Ti-OH) on the TiO₂ surface, have been developed as dye sensitizers during the last two decades.^2–9 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. Much effort in molecular design and development of porphyrin dye sensitizers having carboxyl group have been made to further improve the photovoltaic performances of DSSCs so far.^10–15 Consequently, DSSCs based on donor–π–acceptor (D–π–A) porphyrin dyes bearing the diarylamino group as an electron donor and the benzothiadiazole-benzoic acid moiety as an electron acceptor, which exhibited good absorption features (bathochromic shift and broadening of the Soret and Q bands), have achieved η value of up to ca. 13%.^11d,e

On the other hand, we have reported that a new type of D–π–A dye sensitizers bearing pyridyl group as electron-withdrawing anchoring group were predominantly adsorbed on the TiO₂ electrode through coordinate bonding between the pyridyl group of the dye and the Lewis acid site (exposed Tiⁿ⁺ cations) on the TiO₂ surface.^16,17 It was demonstrated that the new-type of D–π–A dye sensitizers can inject electrons efficiently from the pyridyl group to the conduction band (CB) of the TiO₂ electrode through the coordinate bonding, rather than the bidentate bridging linkages of conventional D–π–A dye sensitizers bearing carboxyl group. Recently, Wang et al. reported that DSSCs based on D–π–A porphyrin dye bearing a pyridyl group reached solar energy-to-electricity conversion yield (η) of 3.96%.^18a On the other hand, Goutsolelos et al. reported the η value of 3.9% for DSSC based on porphyrin dye bearing four pyridyl groups.^18c However, the adsorption amounts (<5.0 × 10¹⁶ molecules per cm²) of these porphyrin dye sensitizers bearing pyridyl group adsorbed on TiO₂ electrode are much lower than those of porphyrin dye sensitizers bearing carboxyl group, and thus the low dye loading leads to low light-harvesting efficiency (LHE) and poor surface coverage of the TiO₂ electrode, resulting in lowering of the photovoltaic performances of DSSCs.^18,19 More recently, Goutsolelos et al. have designed and synthesized “spider-shaped” porphyrin dye sensitizer bearing oligophenylenevinylene moieties, long dodecyloxy chains, and four pyridyl groups, which showed high dye loading value (1.9 × 10¹⁷ molecules per cm²) due to an increase in the basicity of the pyridyl groups. As the results, the DSSC based on the “spider-shaped” porphyrin dye sensitizer reached the η value of 5.12%.^18e

Recently, we have designed and prepared cyclic free-base porphyrin dimers (H₄–C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO)) linked by butadiyne or phenothiazine bearing four 4-pyridyl groups and their inclusion complexes (C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO)) with fullerene C₆₀.²⁰ It was found that these porphyrin dimers have favorable photochemical and electrochemical properties for DSSC through the electrochemical measurements and the transient absorption spectroscopy. Moreover, as for phenothiazine derivative H₄-Ptz-CPD_Py(TEO), it would be expected that the phenothiazine unit possessing electron donating ability can provide a unidirectional flow of electrons toward the pyridyl anchoring group upon photoexcitation of the porphyrin, leading to the efficient electron injection from the photoexcited dye to the CB of TiO₂ electrode. Thus, in this work, to achieve high dye loading and high surface coverage of the TiO₂ electrode for DSSCs based on porphyrin dye sensitizers bearing pyridyl group, the cyclic free-base porphyrin dimers H₄-C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO) and their C₆₀ inclusion complexes C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO) have been applied to DSSCs as a new class of porphyrin dye sensitizers bearing pyridyl anchoring groups for attachment on TiO₂ electrode (Scheme 1). It was demonstrated that these porphyrin dimers are adsorbed on the TiO₂ surface through the formations of hydrogen bonding of pyridyl groups and/or pyridinium ion at Brønsted acid sites on the TiO₂ surface. Here we reveal the photovoltaic performances of DSSCs based on these porphyrin dimers from kinetic and thermodynamic consideration concerning the electron transfer processes in DSSCs, based on the electrochemical measurements (voltammetry and electrochemical impedance spectroscopy) and the transient absorption spectroscopy.

Scheme 1 Cyclic free-base porphyrin dimers (H₄-C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO)) bearing 4-pyridyl groups.

Results and discussion

Photoabsorption properties of cyclic free-base porphyrin dimers and their inclusion complexes with fullerene C₆₀

The synthesis of H₄-Ptz-CPD_Py(TEO) has been reported elsewhere.^20e The synthetic pathway for H₄-C₄-CPD_Py(TEO) is shown in Scheme 2. The inclusion complexes C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO) with C₆₀ were prepared by slowly evaporating the mixed solution of the corresponding cyclic free-base porphyrin dimer in chloroform (0.1 mM, 5 mL) and C₆₀ in toluene (0.1 mM, 5 mL). The UV/vis absorption spectra of cyclic free-base porphyrin dimers H₄-C₄-CPD_Py(TEO) and H₄-C₄-CPD_Py(TEO) in benzonitrile are shown in Fig. 1 and their spectral data are summarized in Table 1. The porphyrin dimers H₄-C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO) exhibit strong Soret band at around 420 nm and relatively weak Q band in the range 500–650 nm. For the C₆₀ inclusion complexes C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO), it is difficult to obtain their exact absorption spectra because the 1 : 1 complex of H₄-C₄-CPD_Py(TEO) or H₄-Ptz-CPD_Py(TEO) with C₆₀ is in dissociation equilibrium in solution of 10⁻⁵ to 10⁻⁶ M concentration which is suitable for the measurement of absorption spectra of porphyrins. In our previous work, however, we have demonstrated that upon addition of C₆₀ to the benzonitrile solution of the cyclic porphyrin dimers, their Soret bands were redshifted with a decrease in intensity, whereas their Q bands were slightly redshifted but increased in intensity.²⁰

Scheme 2 Synthetic pathway for H₄-C₄-CPD_Py(TEO). (i) TFA, DDQ, CH₂Cl₂; (ii) Zn(OAc)·2H₂O, MeOH, CH₂Cl₂; (iii) KF, DMF; (iv) CuCl, pyridine, air; (v) HCl aq.

Fig. 1 UV/vis absorption spectra of H₄-C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO) in benzonitrile.

Table 1 Optical, electrochemical data and HOMO and LUMO energy levels of H₄-C₄-CPD_Py(TEO), C₆₀⊂H₄-C₄-CPD_Py(TEO), H₄-Ptz-CPD_Py(TEO), and C₆₀⊂H₄-Ptz-CPD_Py(TEO)

Porphyrin dimers	λ ^absmax/nm (ε/M^−1 cm^−1)^a	E ^oxpa ^b/V vs. Fc/Fc^+	HOMO^c/eV	LUMO^c/eV
a In benzonitrile. b Anodic (Epa) peak potentials for oxidation were recorded in benzonitrile/Bu4NPF6 (0.1 M) solution for the porphyrin unit in H4-C4-CPDPy(TEO), C60⊂H4-C4-CPDPy(TEO), H4-Ptz-CPDPy(TEO) and C60⊂H4-Ptz-CPDPy(TEO). c HOMO and LUMO energy levels of the porphyrin unit.
H4-C4-CPDPy(TEO)	422 (646 000), 517 (30 800), 551 (9800), 590 (9600), 647 (4300)	0.71	−5.51	−3.61
C60⊂H4-C4-CPDPy(TEO)	—	0.74	−5.54	−3.64
H4-Ptz-CPDPy(TEO)	421 (55 800), 516 (32 100), 550 (10 500), 590 (9500), 646 (5600)	0.72	−5.52	−3.62
C60⊂H4-Ptz-CPDPy(TEO)	—	0.75	−5.55	−3.65

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The UV/vis absorption spectra of cyclic free-base porphyrin dimers and their C₆₀ inclusion complexes adsorbed on TiO₂ film are shown in Fig. 2. It is worth mentioning here that the adsorption amount of the porphyrin dimers adsorbed on TiO₂ film is 2.0 × 10¹⁷ molecules per cm², that is, the adsorption amount of porphyrin unit is 4.0 × 10¹⁷ cm⁻², which is higher than those (<2.0 × 10¹⁷ molecules per cm²) of porphyrin dye sensitizers and D–π–A dye sensitizer bearing pyridyl groups reported so far.^16,18 The Soret bands of C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO) were redshifted compared to those of H₄-C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO), although there is little difference in the Q band between the cyclic free-base porphyrin dimers and their C₆₀ inclusion complexes. Thus, the UV/vis absorption spectra of these porphyrin dimers-adsorbed TiO₂ films are in good agreement with those in benzonitrile.

Fig. 2 UV/vis absorption spectra of (a) H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-C₄-CPD_Py(TEO) and (b) H₄-Ptz-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO) adsorbed on TiO₂ film (3 μm).

Electrochemical and photochemical properties of cyclic free-base porphyrin dimers and their inclusion complexes with fullerene C₆₀

The electrochemical properties of cyclic free-base porphyrin dimers and their C₆₀ inclusion complexes were determined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) (see Fig. S7–S10 in ESI†).²⁰ The oxidation potential of the porphyrin unit in C₆₀⊂H₄-C₄-CPD_Py(TEO) (0.74 V vs. Fc/Fc⁺) showed anodic shift by 0.03 V, compared with that of H₄-C₄-CPD_Py(TEO) (0.71 V) (Table 1). The oxidation potential of the porphyrin unit in C₆₀⊂H₄-Ptz-CPD_Py(TEO) (0.75 V) also showed anodic shift by 0.03 V, compared with that of H₄-Ptz-CPD_Py(TEO) (0.72 V). The reduction potentials corresponding to the reduction of the fullerene entity of C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO) were observed at −0.94 V and −0.96 V, respectively, which is cathodically shifted by 0.02 V and 0.04 V, respectively, compared to pristine C₆₀ (−0.92 V).^20e The small anodic shift of the oxidation potential of the porphyrin and the small cathodic shift of the reduction potential of C₆₀ compared with their reference compounds is indicative of the charge transfer interaction between the porphyrins and C₆₀. The HOMO and LUMO energy levels were evaluated from the oxidation wave and the singlet excited energy of these porphyrin dimers (1.90 eV) based on the Q absorption band and fluorescence band in benzonitrile (ca. 650 nm), respectively. The HOMO and LUMO energy levels of these porphyrin dimers was ca. −5.5 eV and ca. −3.6 eV, respectively (Table 1). Thus, this result shows that the HOMO energy levels are more positive than the I₃⁻/I⁻ redox potential (−4.9 eV), and thus this indicates that an efficient regeneration of the oxidized porphyrin dimers by electron transfer from the I₃⁻/I⁻ redox couple in the electrolyte is thermodynamically feasible. Evidently, the LUMO energy levels of these porphyrin dimers are higher than the energy level (E_cb) of the CB of TiO₂ (−4.0 eV), suggesting that an electron injection to the CB of TiO₂ is thermodynamically feasible (some researchers have proposed that an energy gap of over 0.2–0.3 eV is necessary for efficient electron injection).^2–7

Time-resolved absorption spectroscopy for cyclic free-base porphyrin dimers and their C₆₀ inclusion complexes was performed by femtosecond laser flash photolysis after photoexcitation.²⁰ The photodynamics of these porphyrin dimers is summarized in Fig. 3. The decay of photoexcited state of C₆₀⊂H₄-C₄-CPD_Py(TEO) has two steps: the first step has a lifetime of 18 ps, which corresponds to the disappearance of the singlet excited state of the ¹H₄-C₄-CPD_Py(TEO)* (ca. −3.6 eV), that is, the ¹H₄-C₄-CPD_Py(TEO)* undergoes intrasupramolecular electron transfer to give a completely charge-separated state C₆₀˙⁻-H₄-C₄-CPD_Py(TEO)˙⁺ (ca. −3.7 eV). The C₆₀˙⁻-H₄-C₄-CPD_Py(TEO)˙⁺ decays with a lifetime of 470 ps to the ground state. The decay of photoexcited state of C₆₀⊂H₄-Ptz-CPD_Py(TEO) also includes the charge-separated states C₆₀˙⁻-H₄-Ptz-CPD_Py(TEO)˙⁺ (ca. −3.8 eV), but has multiple steps to the ground state via the triplet charge separated state ³(C₆₀˙⁻-H₄-Ptz˙⁺-CPD_Py(TEO)) (ca. −4.3 eV) with a lifetime of 0.71 ms: the first step has a lifetime of 20 ps, which is similar to that (18 ps) of C₆₀⊂H₄-C₄-CPD_Py(TEO). On the other hand, the energy diagrams for the photochemical events in these porphyrin dimes revealed that the energy levels of charge-separated states is very close to or lower than the E_cb of the CB of TiO₂ electrode. This suggests that the electron injection from the C₆₀˙⁻ in the charge-separated states to the CB of TiO₂ electrode is thermodynamically difficult.

Fig. 3 Energy diagrams for the photochemical events in C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO).

FTIR spectra of cyclic free-base porphyrin dimers and their inclusion complexes with fullerene C₆₀

To elucidate the adsorption states of cyclic free-base porphyrin dimers and their C₆₀ inclusion complexes on TiO₂ nanoparticles, we measured the FTIR spectra of the porphyrin dimer powders and the porphyrin dimers adsorbed on TiO₂ nanoparticles (Fig. 4). For the powders of all the four porphyrin dimers H₄-C₄-CPD_Py(TEO), H₄-Ptz-CPD_Py(TEO), C₆₀⊂H₄-C₄-CPD_Py(TEO), and C₆₀⊂H₄-Ptz-CPD_Py(TEO), the C=N stretching band of pyridyl group was clearly observed at around 1590 cm⁻¹. Interestingly, when these porphyrin dimers were adsorbed on the TiO₂ surface, a new band appeared at around 1650 cm⁻¹, which indicates the formation of a pyridinium ion with Brønsted acid sites on TiO₂ surface.²¹ In addition, the C=N stretching band at around 1590 cm⁻¹ is shifted by 3–5 cm⁻¹ to higher wavenumber, that is, the resulting band can be assigned to the hydrogen-bonded pyridyl group to Brønsted acid sites on the TiO₂ surface. Consequently, these observations demonstrate that these porphyrin dimers are adsorbed on the TiO₂ surface through the formations of hydrogen bonding of pyridyl groups and/or pyridinium ion at Brønsted acid sites on the TiO₂ surface. Thus, the UV/vis absorption and the FTIR spectra of the porphyrin dimers adsorbed on TiO₂ film indicate that high dye loading and high surface coverage of the TiO₂ electrode for DSSCs based on porphyrin dye sensitizers bearing pyridyl group are achieved by employing the cyclic porphyrin dimers bearing four pyridyl anchoring groups possessing the bonding ability to the two points on Brønsted acid sites on TiO₂ surface.

Fig. 4 FTIR spectra of the porphyrin dimer powders and the porphyrin dimers adsorbed on TiO₂ nanoparticles for (a) H₄-C₄-CPD_Py(TEO), (b) C₆₀⊂H₄-C₄-CPD_Py(TEO), (c) H₄-Ptz-CPD_Py(TEO), and (d) C₆₀⊂H₄-Ptz-CPD_Py(TEO).

Photovoltaic performances of DSSCs based on cyclic free-base porphyrin dimers and their inclusion complexes with fullerene C₆₀

The DSSC was prepared using the dye-adsorbed TiO₂ electrode (9 μm), Pt-coated glass as a counter electrode, and an acetonitrile solution with iodine (0.05 M), lithium iodide (0.1 M), and 1,2-dimethyl-3-propylimidazolium iodide (0.6 M) as an electrolyte. The photocurrent–voltage (I–V) characteristics were measured under simulated solar light (AM 1.5, 100 mW cm⁻²). The incident photon-to-current conversion efficiency (IPCE) spectra and the I–V curves are shown in Fig. 5. The photovoltaic performance parameters are collected in Table 2. The IPCE values corresponding to the Soret band (29% at 424 nm) and the Q band (6% at 520 nm) for the phenothiazine-bridged cyclic porphyrin dimer H₄-Ptz-CPD_Py(TEO) are higher than those (15% at 420 nm for the Soret band and 2% at 522 nm for the Q band) of the butadiyne-linked cyclic porphyrin dimer H₄-C₄-CPD_Py(TEO) (Fig. 5a). The I–V curves show that the short-circuit photocurrent density (J_sc) and η values of H₄-Ptz-CPD_Py(TEO) (1.40 mA cm⁻² and 0.35%) are higher than those of H₄–C₄-CPD_Py(TEO) (0.79 mA cm⁻² and 0.19%) (Fig. 5b). The higher photovoltaic performance for DSSC based on H₄-Ptz-CPD_Py(TEO) may be attributed to the efficient electron injection from the photoexcited dye (¹H₄-Ptz-CPD_Py(TEO)*) to the CB of TiO₂ electrode because the phenothiazine unit possessing electron donating ability can provide a unidirectional flow of electrons toward the pyridyl anchoring group upon photoexcitation of the porphyrin. Interestingly, the photovoltaic performances of DSSC based on the C₆₀ inclusion complexes C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO) are lower than those of H₄-C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO). The electron injection (≈100 ps) from the photoexcited porphyrin to the CB of TiO₂ electrode is in kinetically competition with the intrasupramolecular electron transfer (18–20 ps), that is, the formation of charge-separated state C₆₀˙⁻-H₄-C₄-CPD_Py(TEO)˙⁺ and C₆₀˙⁻-H₄-Ptz-CPD_Py(TEO)˙⁺ (Fig. 3). Therefore, the lower photovoltaic performances for DSSC based on the C₆₀ inclusion complexes would be attributed to the formation of charge-separated state, leading to low electron-injection efficiency from the photoexcited porphyrin to CB of TiO₂ electrode. On the other hand, the photovoltaic performance of DSSC based on C₆₀⊂H₄-Ptz-CPD_Py(TEO) is slightly lower than that of C₆₀⊂H₄-C₄-CPD_Py(TEO), which may be attributed to the thermodynamically difficult electron-injection due to the lower triplet charge separated state ³(C₆₀˙⁻-H₄-Ptz˙⁺- CPD_Py(TEO)).

Fig. 5 (a) IPCE spectra and (b) I–V curves of DSSCs based on H₄-C₄-CPD_Py(TEO), C₆₀⊂H₄–C₄-CPD_Py(TEO), H₄-Ptz-CPD_Py(TEO), and C₆₀⊂H₄-Ptz-CPD_Py(TEO).

Table 2 DSSC performance parameters of H₄-C₄-CPD_Py(TEO), C₆₀⊂H₄-C₄-CPD_Py(TEO), H₄-Ptz-CPD_Py(TEO), and C₆₀⊂H₄-Ptz-CPD_Py(TEO)

Porphyrin dimers^a	J sc/mA cm^−2^b	V oc ^b/mV	ff^b	η ^b (%)
a The adsorption amount of the porphyrin dimers adsorbed on TiO2 electrode is 2.0 × 10^17 molecules per cm^2 under the adsorption condition of 0.1 mM porphyrin dimer solution in chloroform. The amount of adsorbed dye on TiO2 electrode was determined form the calibration curve by absorption spectral measurement of the concentration change of the dye solution before and after adsorption. b Under a simulated solar light (AM 1.5, 100 mW cm^−2).
H4-C4-CPDPy(TEO)	0.79	376	0.64	0.19
C60⊂H4-C4-CPDPy(TEO)	0.27	315	0.69	0.06
H4-Ptz-CPDPy(TEO)	1.40	396	0.63	0.35
C60⊂H4-Ptz-CPDPy(TEO)	0.24	304	0.54	0.04

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Moreover, it is worth mentioning here that the open-circuit photovoltage (V_oc) values of the C₆₀ inclusion complexes C₆₀⊂H₄-C₄-CPD_Py(TEO) (315 mV) and C₆₀⊂H₄-Ptz-CPD_Py(TEO) (304 mV) are lower than those of H₄-C₄-CPD_Py(TEO) (376 mV) and H₄-Ptz-CPD_Py(TEO) (396 mV). Thus, electrochemical impedance spectroscopy (EIS) analysis was performed to study the electron recombination process in DSSCs based on these porphyrin dimers in the dark under a forward bias of −0.60 V with a frequency range of 10 mHz to 100 kHz. The large semicircle in the Nyquist plot (Fig. 6a), which corresponds to the midfrequency peaks in the Bode phase plots, represents the charge recombination between the injected electrons in TiO₂ and I₃⁻ ions in the electrolyte, that is, the charge-transfer resistances at the TiO₂/dye/electrolyte interface. The Nyquist plots show that the resistance values of the large semicircle for H₄-C₄-CPD_Py(TEO) (13 Ω) and H₄-Ptz-CPD_Py(TEO) (14 Ω) are higher than those of C₆₀⊂H₄-C₄-CPD_Py(TEO) (5 Ω) and C₆₀⊂H₄-Ptz-CPD_Py(TEO) (5 Ω), indicating that the electron recombination resistances for H₄-C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO) are larger those of C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO). The electron recombination lifetimes (τ_e) expressing the electron recombination between the injected electrons in TiO₂ and I₃⁻ ions in the electrolyte, extracted from the angular frequency (ω_rec) at the midfrequency peak in the Bode phase plot (Fig. 6b) using τ_e = 1/ω_rec, increase in the order of C₆₀⊂H₄-C₄-CPD_Py(TEO) (0.1 ms) and C₆₀⊂H₄-Ptz-CPD_Py(TEO) (0.1 ms) < H₄-C₄-CPD_Py(TEO) (2.2 ms) < H₄-Ptz-CPD_Py(TEO) (2.5 ms), respectively, which are consistent with the sequence of V_oc values in the DSSCs. Consequently, this is attributed to the fact that the formation of charge-separated state C₆₀˙⁻-H₄-C₄-CPD_Py(TEO)˙⁺ and C₆₀˙⁻-H₄-Ptz-CPD_Py(TEO)˙⁺ facilitates the approach of I₃⁻ ions to the TiO₂ surface because of complexation between the charge-separated state and I₃⁻ ions, leading to faster charge recombination and thus resulting in a lower V_oc value.^3,6

Fig. 6 (a) Nyquist plots and (b) Bode phase plots of DSSCs based on H₄-C₄-CPD_Py(TEO), C₆₀⊂H₄-C₄-CPD_Py(TEO), H₄-Ptz-CPD_Py(TEO), and C₆₀⊂H₄-Ptz-CPD_Py(TEO).

Conclusions

In this work, to seek a direction in molecular design toward creating a new class of porphyrin dye sensitizers bearing pyridyl anchoring groups for achieving high dye loading and high surface coverage of the TiO₂ electrode for dye-sensitized solar cells (DSSCs), cyclic free-base porphyrin dimers (H₄-C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO)) linked by butadiyne or phenothiazine bearing 4-pyridyl groups and their C₆₀ inclusion complexes (C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO)) have been applied to DSSCs as the dye sensitizers. The Soret bands of C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO) were redshifted with a decrease in intensity, whereas their Q bands were slightly redshifted but increased in intensity, compared to H₄-C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO). These porphyrin dimers are adsorbed on the TiO₂ surface through the formations of hydrogen bonding of pyridyl groups and/or pyridinium ion at Brønsted acid sites on the TiO₂ surface. It was found that the adsorption amount of the porphyrin dimers adsorbed on TiO₂ film is 2.0 × 10¹⁷ molecules per cm², that is, the adsorption amount of porphyrin unit is 4.0 × 10¹⁷ cm⁻², which is higher than those (<2.0 × 10¹⁷ molecules per cm²) of porphyrin dye sensitizers and D–π–A dye sensitizer bearing pyridyl groups reported so far. The higher photovoltaic performance for DSSC based on H₄-Ptz-CPD_Py(TEO) relative to H₄-C₄-CPD_Py(TEO) may be attributed to the efficient electron injection from the photoexcited dye (¹H₄-Ptz-CPD_Py(TEO)*) to the CB of TiO₂ because the phenothiazine unit possessing electron donating ability can provide a unidirectional flow of electrons toward the pyridyl anchoring group upon photoexcitation of the porphyrin. On the other hand, the photovoltaic performances of DSSC based on C₆₀⊂H₄-C₄-CPD_Py(TEO) and C₆₀⊂H₄-Ptz-CPD_Py(TEO) are lower than those of H₄-C₄-CPD_Py(TEO) and H₄-Ptz-CPD_Py(TEO). The transient absorption spectroscopy and electrochemical measurements revealed that the lower photovoltaic performances for DSSC based on the C₆₀ inclusion complexes would be attributed to the formation of charge-separated state between the photoexcited porphyrin and C₆₀, leading to low electron-injection efficiency from the photoexcited porphyrin to TiO₂ electrode. Consequently, this work provides that the cyclic porphyrin dimers with four pyridyl anchoring groups possessing bonding ability to the two points on Brønsted acid sites on TiO₂ surface, which can achieve the high dye loading and the high surface coverage of the TiO₂ electrode, would be expected to be one of the most promising classes of porphyrin dye sensitizers with pyridyl group.

Experimental

General

Infrared (IR) absorption spectra were recorded on a Perkin Elmer Spectrum One FT-IR spectrometer by ATR method and a Bio-Rad FTS6000 spectrometer by KBr disk method. Ultraviolet/visible (UV/vis) absorption spectra were observed with Shimadzu UV-3150 and UV-2500PC spectrophotometers. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AVANCE III 600 spectrometer. The resonance frequencies are 600 MHz for ¹H and 151 MHz for ¹³C. Chemical shifts were reported as δ values in ppm relative to tetramethylsilane. High-resolution fast atom bombardment mass spectra (HR-FAB-MS) were measured with 3-nitrobenzyl alcohol (NBA) as a matrix and recorded on a JEOL LMS-HX-110 spectrometer. Melting point (Mp) measurements were carried out on a Yanaco MP-S3 melting point meter. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried out on an ALS630C (BAS, Japan) electrochemical analyzer in deaerated benzonitrile containing 0.1 M tetra-n-butylammonium hexafluorophosphate (Bu₄NPF₆) as a supporting electrolyte at room temperature under an Ar atmosphere. A conventional three-electrode cell was used with a platinum working electrode (surface area of 0.3 mm²) and a platinum wire as a counter electrode. The measured potentials were recorded with respect to an Ag/AgNO₃ (0.01 M) reference electrode. All potentials (vs. Ag⁺/Ag) were converted to values with respect to that of the Fc⁺/Fc redox couple, which was measured under the same conditions. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of porphyrin dimers were evaluated from the spectral analyses and the DPV data. The HOMO energy level was evaluated from the oxidation wave for porphyrin dimers. The LUMO energy level was estimated from the singlet excited energy of these porphyrin dimers (1.90 eV) based on the Q absorption band nm and fluorescence band in benzonitrile.

Synthesis

5,15-Bis{3-[2-(2-ethoxyethoxy)ethoxy]-5-[(trimethylsilyl)ethynyl]phenyl}-10,20-di{4-pyridyl}porphine (1). A solution of 3-[2-(2-ethoxyethoxy)ethoxy]-5-[(trimethylsilyl)ethynyl]benz-aldehyde²² (4.63 g, 13.8 mmol) and meso-(4-pyridyl)-dipyrromethane²³ (3.08 g, 13.9 mmol) in CH₂Cl₂ (1.4 L) was degassed with a stream of N₂ for 30 min. To this solution, trifluoroacetic acid (TFA) (12.3 mL, 166 mmol) was added and stirred under a N₂ atmosphere. After 30 min, the resultant mixture was neutralized with NEt₃ (23 mL, 0.17 mol) and then 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (4.7 g, 21 mmol) dissolved in benzene (250 mL) was added to the mixture. After an overnight stirring, the resultant mixture was purified by column chromatography (silica-gel, CHCl₃/EtOH = 50/1) then recrystallized from CHCl₃/i-PrOH to give 1 as a purple solid (1.27 g, 18%). ¹H NMR (CDCl₃, 600 MHz): δ −2.91 (br-s, 2H, –NH), 0.27 (s, 18H, –Si(CH₃)₃), 1.18 (t, J = 7.2 Hz, 6H, –O(CH₂CH₂O)₂CH₂CH̲₃), 3.52 (q, J = 7.2 Hz, 4H, –O(CH₂CH₂O)₂CH̲₂CH₃), 3.63 (dd, J = 4.8 and 6.0 Hz, 4H, –O(CH₂)₂OCH₂CH̲₂OC₂H₅), 3.76 (t, J = 4.8 Hz, 4H, –O(CH₂)₂OCH̲₂CH₂OC₂H₅), 3.96 (t, J = 4.8 Hz, 4H, –OCH₂CH̲₂O(CH₂)₂OC₂H₅), 4.36 (t, J = 4.8 Hz, 4H, –OCH̲₂CH₂O(CH₂)OC₂H₅), 7.47 (br-s, 2H, Ar–H), 7.75 (br-s, 2H, Ar–H), 7.93 (br-s, 2H. Ar–H), 8.17 (d, J = 5.4 Hz, 4H, Ar–H), 8.81 (d, J = 4.2 Hz, 4H, pyrrole β–H), 8.92 (d, J = 4.8 Hz, 4H, pyrrole β–H), 9.06 (d, J = 6.0 Hz, 4H, Ar–H); ¹³C NMR (CDCl₃, 151 MHz): δ 0.1, 15.3, 66.9, 68.1, 69.9, 70.0, 71.2, 94.7, 104.9, 117.2, 117.4, 119.7, 122.6, 122.6, 129.6, 131.1, 143.1, 148.5, 150.3, 157.2; HR-FAB-MS (NBA): m/z calcd for C₆₄H₆₈N₆O₆Si₂: 1072.4739; found: 1072.4718; UV/vis (CHCl₃): λ_max (ε, cm⁻¹ M⁻¹) 421 (392 000), 515 (18 200), 549 (5400), 588 (5400), 645 (2100); IR (KBr): ν = 3439, 3317, 3092, 3022, 2956, 2928, 2867, 2714, 2614, 2533, 2156, 1591, 1582, 1476, 1420, 1404, 352, 1323, 1297, 1247, 1196, 1173, 1161, 1114, 1069, 976, 927, 847, 800, 788, 760, 730, 697, 660, 563, 411 cm⁻¹; Mp: >300 °C.

{5,15-Di{3-[2-(2-ethoxyethoxy)ethoxy]-5-ethynyl]phenyl}-10,20-di{4-pyridyl}porphinato}zinc(II) (2). Zn(OAc)₂·2H₂O (2.4 g, 11 mmol) dissolved in MeOH (60 mL) was added to a solution of 1 (1.16 g, 1.08 mmol) in CH₂Cl₂ (1.1 L). The resultant mixture was refluxed overnight under N₂ atmosphere and then cooled to room temperature. The reaction mixture was neutralized with saturated aqueous solution of NaHCO₃ (200 mL), washed with water (200 mL, 2 times), dried over Na₂SO₄ and evaporated. The residue and KF·2H₂O (0.63 g, 6.7 mmol) were dissolved in DMF (40 mL) and the mixture was stirred overnight under a N₂ atmosphere. After the stirring, the resultant mixture was diluted with CHCl₃ (100 mL) and washed with water (500 mL, 2 times). The organic layer was dried over Na₂SO₄ and evaporated to give the product as a purple solid (1.00 g, 99%). ¹H NMR (pyridine-d₅, 600 MHz): δ 1.00 (t, J = 7.2 Hz, 6H, –O(CH₂CH₂O)₂CH₂CH̲₃), 3.33 (q, J = 6.6 Hz, 4H, –O(CH₂CH₂O)₂CH̲₂CH₃), 3.52 (t, J = 4.8 Hz, 4H, –O(CH₂)₂OCH₂CH̲₂OC₂H₅), 3.67 (t, J = 4.8 Hz, 4H, –O(CH₂)₂OCH̲₂CH₂OC₂H₅), 3.87 (t, J = 4.8 Hz, 4H, –OCH₂CH̲₂O(CH₂)₂OC₂H₅), 4.15 (s, 2H, –C≡CH) 4.31 (t, J = 4.8 Hz, 4H, –OCH̲₂CH₂O(CH₂)OC₂H₅), 7.68 (d, J = 0.6 Hz, 2H, Ar–H), 8.05 (br-s, 2H, Ar–H), 8.23 (d, J = 4.2 Hz, 2H, Ar–H), 8.26 (d, J = 4.2 Hz, 4H, Ar–H), 9.02 (d, J = 4.8 Hz, 4H, pyrrole β–H), 9.11 (d, J = 5.4 Hz, 4H, Ar–H), 9.15 (d, J = 4.8 Hz, 4H, pyrrole β–H); ¹³C NMR (pyridine-d₅, 151 MHz): δ 15.5, 66.5, 68.5, 70.0, 70.3, 71.2, 79.6, 84.5, 117.6, 118.7, 120.6, 122.0, 130.1, 131.7, 132.2, 132.9, 145.2, 148.7, 149.8, 150.3, 150.7, 151.4, 157.7; HR-FAB-MS (NBA): m/z calcd for C₅₈H₅₀N₆O₆Zn: 990.3083; found: 990.3082; UV/vis (CHCl₃): λ_max (ε, cm⁻¹ M⁻¹) 425 (418 000), 555 (16 200), 595 (2400); IR (KBr): ν = 3433, 3283, 2972, 2928, 2872, 1592, 1581, 1540, 1523, 1488, 1456, 1419, 1344, 1321, 1298, 1283, 1246, 1204, 1181, 1113, 1071, 1048, 998, 938, 874, 819, 796, 763, 719, 675, 665, 569, 545, 436, 419 cm⁻¹; Mp: >300 °C.

H₄-C₄-CPD_Py(TEO) . The free-base dimer was prepared from 2 (397 mg, 0.4 mmol) with CuCl (8.0 g, 80 mmol) in pyridine (1.2 L) according to the reported procedure.^20a,d The crude product was purified by flash column chromatography (silica-gel, CHCl₃/EtOH = 100/1 to 10/1) and recrystallized from CHCl₃/i-PrOH to give the product as a purple powder (48 mg, 13%). ¹H NMR (CDCl₃, 600 MHz): δ −3.04 (br-s, 4H, –NH), 1.21 (t, J = 7.2 Hz, 12H, –O(CH₂CH₂O)₂CH₂CH̲₃), 3.55 (q, J = 7.2 Hz, 8H, –O(CH₂CH₂O)₂CH̲₂CH₃), 3.67 (dd, J = 4.8 and 6.0 Hz, 8H, –O(CH₂)₂OCH₂CH̲₂OC₂H₅), 3.81 (dd, J = 4.8 and 6.0 Hz, 8H, –O(CH₂)₂OCH̲₂CH₂OC₂H₅), 4.03 (t, J = 4.8 Hz, 8H, –OCH₂CH̲₂O(CH₂)₂OC₂H₅), 4.42 (t, J = 4.8 Hz, 8H, –OCH̲₂CH₂O(CH₂)OC₂H₅), 6.88 (s, 4H, Ar–H), 7.25 (dd, J = 0.6 and 1.8 Hz, 4H, Ar–H), 7.94 (br-s, 8H, Ar–H), 8.22 (t, J = 1.8 Hz, 4H, Ar–H), 8.62 (d, J = 4.2 Hz, 8H, pyrrole β–H), 8.70 (d, J = 4.2 Hz, 8H, pyrrole β–H), 8.96 (br-s, 8H, Ar–H); ¹³C NMR (CDCl₃, 151 MHz): δ 15.3, 66.9, 68.3, 70.0, 70.1, 71.3, 74.5, 83.1, 114.9, 117.1, 118.6, 120.4, 120.8, 129.3, 135.3, 143.0, 148.4, 149.9, 157.8; HR-FAB-MS (NBA): m/z calcd for C₁₁₆H₁₀₀N₁₂O₁₂: 1852.7584; found: 1852.7545; UV/vis (PhCN): λ_max (ε, cm⁻¹ M⁻¹); 422 (646 000), 517 (30 800), 551 (9800), 590 (9600), 647 (4300); IR (ATR): ν = 3312, 2971, 2920, 2865, 1578, 1474, 1455, 1415, 1372, 1348, 1285, 1254, 1221, 1195, 1172, 1108, 1062, 993, 974, 930, 871, 849, 797, 728, 692, 659, 563, 540, 517, 467, 411 cm⁻¹; Mp: >300 °C.

Preparation of DSSCs

The TiO₂ paste (JGC Catalysts and Chemicals Ltd., PST-18NR) was deposited on a fluorine-doped-tin-oxide (FTO) substrate by doctor-blading, and sintered for 50 min at 450 °C. The 9 μm thick TiO₂ electrode was immersed into 0.1 mM porphyrin dimer solution in chloroform for 15 hours enough to adsorb the dye sensitizers. The DSSCs were fabricated by using the TiO₂ electrode (0.5 × 0.5 cm² in photoactive area) thus prepared, Pt-coated glass as a counter electrode, and a solution of 0.05 M iodine, 0.1 M lithium iodide, and 0.6 M 1,2-dimethyl-3-propylimidazolium iodide in acetonitrile as electrolyte. The photocurrent–voltage characteristics were measured using a potentiostat under a simulated solar light (AM 1.5, 100 mW cm⁻²). IPCE spectra were measured under monochromatic irradiation with a tungsten-halogen lamp and a monochromator. The amount of adsorbed dye on TiO₂ nanoparticles was determined form the calibration curve by absorption spectral measurement of the concentration change of the porphyrin dye solution before and after adsorption. Absorption spectra of the dyes adsorbed on TiO₂ nanoparticles were recorded on the porphyrin dyes-adsorbed TiO₂ film (thickness of 3 μm) in the transmission mode with a calibrated integrating sphere system. Electrochemical impedance spectroscopy (EIS) for DSSCs in the dark under a forward bias of −0.60 V with a frequency range of 10 mHz to 100 kHz was measured with a AMETEK Versa STAT 3.

Acknowledgements

This work was supported by Grants-in-Aid (No. 20108009 and 15K05432 to F. T. and No. 15H03859 to Y. O.) from Ministry of Education, Culture, Sports, Science and Technology of Japan and Research Grants to F. T. from Tokuyama Science and Technology Foundation and Iketani Science and Technology Foundation.

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR and ¹³C NMR for compounds 1, 2 and H₄-C₄-CPD_Py(TEO). See DOI: 10.1039/c6ra01131d

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