Synthesis, optical and electrochemical properties, and photovoltaic performance of a panchromatic and near-infrared (D)₂–π–A type BODIPY dye with pyridyl group or cyanoacrylic acid

Abstract

(D)₂–π–A type boron dipyrromethene (BODIPY) dyes OMK-PY and OMK-CA bearing a pyridyl group or cyanoacrylic acid group, respectively, at the end of 8-positions on the BODIPY core, as an electron-withdrawing anchoring group to adsorb onto the TiO₂ electrode and two diphenylamine–thienylcarbazole moieties as an electron-donating unit at the 3- and 5-positions on the BODIPY core, were designed and developed as a photosensitizer for dye-sensitized solar cells (DSSCs). It was found that the two BODIPY dyes adsorbed on a TiO₂ film show a strong and broad absorption band in the range of 600 to 850 nm, and the onset of the absorption band reached 900 nm, that is, OMK-PY and OMK-CA possess the near-infrared (NIR) adsorption ability as well as the panchromatic adsorption ability, and good adsorption ability onto the TiO₂ electrode. Moreover, cyclic voltammetry demonstrated that the two BODIPY dyes show two reversible oxidation waves, thus indicating that the redox processes of OMK-PY and OMK-CA are very stable. On the basis of the experimental results and density functional theory calculation, we propose that the (D)₂–π–A BODIPY structure with two diphenylamine–thienylcarbazole moieties as strong electron-donating units at the 3- and 5-positions on the BODIPY core is an effective strategy to lead a high light-harvesting efficiency (LHE) in the range of visible light to NIR light, although the DSSCs based on (D)₂–π–A type BODIPY dyes that have been developed in this current stage showed low photovoltaic performances.

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Introduction

Dye-sensitized solar cells (DSSCs) employing dye-adsorbed TiO₂ electrodes have made a hopeful impact on chemists, physicists, and engineers as one of the most promising new renewable photovoltaic cells in the last quarter of a century,^1–8 since Grätzel and co-workers reported high-performance DSSCs employing a Ru-complex dye-adsorbed TiO₂ electrode in 1991.⁹ In particular, much effort has been made on the development of various types of D–π–A organic dye sensitizers which are constructed of a diphenyl- or dialkylamino group as the electron donor (D) and a carboxyl group,^10–16 2-(1,1-dicyanomethylene)rhodanine,¹⁷ pyridine,^18–21 or 8-hydroxylquinoline²² as the electron acceptor (A) and anchor linked by π-conjugated bridges, and there has been a gradual accumulation of information about the relationship between the chemical structures and the photovoltaic performances of DSSCs. As a result, DSSCs have achieved a solar energy-to-electricity conversion yield (η) of up to 13%.^11c,d However, one of the challenges to further improve the photovoltaic performances of DSSCs lies in the enhancement of light-harvesting efficiency (LHE) in the near-infrared (NIR) region (780–1000 nm) as well as visible region of the solar spectrum. Some boron dipyrromethene (BODIPY) dyes which are electrochemically modified through the introduction of electron-donating and electron-accepting groups onto the BODIPY core, possess panchromatic absorption ability because of their high photoabsorption coefficient in the visible region of the solar spectrum.^23–25 Actually, Akkaya et al. have designed and developed a series of D–A-substituted BODIPY dye sensitizers, which have two triphenylamines as electron donors at the 3- and 5-positions and benzoic acid as electron acceptor at the 8-position. The D–A-substituted BODIPY dyes showed a strong and broad absorption band in the range of 650 to 850 nm with the absorption maxima at around 720–760 nm (molar extinction coefficient (ε) = 60 000–70 000 M⁻¹ cm⁻¹).^23d However, almost all BODIPY photosensitizers reported so far show the photoabsorption band in wavelength region shorter than 700 nm.

Thus, in this work, to explore effective BODIPY dye sensitizers possessing a high LHE in the range of visible light to NIR light, we have designed and developed (D)₂–π–A type BODIPY dyes OMK-PY and OMK-CA bearing pyridyl group and cyanoacrylic acid group, respectively, at the end of 8-positions on the BODIPY core, as an electron-withdrawing anchoring group to adsorb onto the TiO₂ electrode and two diphenylamine–thienylcarbazole moieties as an electron-donating unit at the 3- and 5-positions on the BODIPY core (Fig. 1). Here we reveal the photophysical and electrochemical properties of the (D)₂–π–A BODIPY dye sensitizers and their photovoltaic performances in DSSCs, based on photoabsorption (dye solution and dye-adsorbed TiO₂ film) and electrochemical measurements (cyclic voltammetry), density functional theory (DFT) calculation, FT-IR spectroscopy of the dye-adsorbed TiO₂ film, photocurrent–voltage (I–V) curves, and incident photon-to-current conversion efficiency (IPCE) spectra. We demonstrate that the (D)₂–π–A type BODIPY dyes possess the NIR adsorption ability as well as the panchromatic adsorption ability.

Fig. 1 Chemical structures of (D)₂–π–A type BODIPY dyes OMK-PY and OMK-CA.

Results and discussion

Synthesis

(D)₂–π–A BODIPY dye sensitizers OMK-PY and OMK-CA studied in this work were synthesized according to a stepwise synthetic protocol (Scheme 1). Indeed, the (D)₂–π–A BODIPY dyes were successfully prepared by using Stille coupling to introduce the electron-donating unit (two diphenylamine–thienylcarbazole moieties), and then by using Knoevenagel condensation to introduce the electron-withdrawing anchoring group (pyridyl group or cyanoacrylic acid group). Dipyrromethane 2 was prepared by condensation of compound 1 (ref. 26) with pyrrole with TFA added as a catalyst. The dichlorinated compound 3 was prepared by chlorination of 2 with N-chlorosuccinimide (NCS) followed by oxidation with DDQ. Compound 3 was reacted with NEt₃ and treated with BF₃–OEt₂ to afford the BODIPY 4. Compound 6 was prepared by Stille coupling of 4 with stannanyl compound 5 (ref. 18g). Compound 6 is hydrolyzed by treatment with acid to generate the aldehyde 7. BODIPY dye OMK-PY was prepared by reacting 4-pyridylacetonitrile hydrochloride with the compound 7 in the presence of NEt₃.²⁷ BODIPY dye OMK-CA was prepared by typical Knoevenagel condensation of the compound 7 with cyanoacetic acid in the presence of piperidine.

Scheme 1 Synthesis of OMK-PY and OMK-CA.

Photophysical properties

The photoabsorption spectra of OMK-PY and OMK-CA in THF and adsorbed on the TiO₂ film are shown in Fig. 2a and their optical data are summarized in Table 1. The two dyes in THF show a strong and broad absorption band in the range of 600 to 800 nm, which is assigned to the intramolecular charge-transfer (ICT) excitation from the electron donor moiety (two diphenylamine–thienylcarbazole moieties) through the BODIPY core to the electron acceptor moiety (2-(pyridin-4-yl)acrylonitrile moiety for OMK-PY and cyanoacetic acid group for OMK-CA, respectively). For OMK-PY the molar extinction coefficient (ε) at the absorption peak wavelength (λ_abs = 753 nm) is 95 400 M⁻¹ cm⁻¹, which is higher than that of OMK-CA (ε = 83 100 M⁻¹ cm⁻¹ at λ_abs = 754 nm). Moreover, for the two dyes a strong absorption peak at around 400 nm with the ε value of ca. 70 000–80 000 M⁻¹ cm⁻¹, which can be assigned to the ICT band due to the diphenylamine–thienylcarbazole moiety, was also observed. Interestingly, for the absorption spectra of OMK-PY and OMK-CA adsorbed on TiO₂ film the longest wavelength absorption band in the range of 600 to 850 nm is significantly broadened, and the onset of the absorption band reached 900 nm, compared with that in THF. As shown in Fig. 2b, the colors of the two dyes in THF are blackish green and the color of dye-adsorbed TiO₂ film is almost black. Thus, this result indicates that the (D)₂–π–A type BODIPY dyes possess a good LHE in the range of visible light to NIR light and good adsorption ability onto TiO₂ electrode, that is, OMK-PY and OMK-CA possess the NIR adsorption ability as well as the panchromatic adsorption ability.

Fig. 2 (a) Photoabsorption spectra of OMK-PY and OMK-CA in THF and adsorbed on TiO₂ film. (b) Photographs of the dyes in THF and adsorbed on TiO₂ film.

Table 1 Optical and electrochemical data, HOMO and LUMO energy levels, and DSSC performance parameters of OMK-PY and OMK-CA

Dye	λ^absmax/nm (ε^a/M^−1 cm^−1)	E^ox1/2^b/V	HOMO^c/V	LUMO^c/V	Molecules^d/cm^−2	Jsc^e/mA cm^−2	Voc^e/mV	ff^e	η^e (%)
a In THF.b Half-wave potentials versus Fc/Fc^+ of oxidation (E^ox1/2) for OMK-PY and OMK-CA were recorded in DMF/Bu4NClO4 (0.1 M) solution.c Versus normal hydrogen electrode (NHE).d Adsorption amount of dye molecules per unit area of the TiO2 electrode, when the 9 μm thick TiO2 electrode was immersed into 0.1 mM dye solution in THF.e Under a simulated solar light (AM 1.5, 100 mW cm^−2).
OMK-PY	385 (80 800), 574 (35 000), 753 (95 400)	0.28	1.00	−0.44	6.9 × 10^16	1.08	335	0.53	0.19
OMK-CA	385 (70 600), 574 (28 500), 754 (83 100)	0.29	1.01	−0.43	1.0 × 10^17	0.68	319	0.51	0.11

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Electrochemical properties

The electrochemical properties of OMK-PY and OMK-CA were determined by cyclic voltammetry (CV) in DMF containing 0.1 M tetrabutylammonium perchlorate (Bu₄NClO₄) (Fig. 3). The potentials were referenced to ferrocene/ferrocenium (Fc/Fc⁺) as the internal reference. Two oxidation waves were observed at 0.34 V and 0.48 V for OMK-PY and 0.33 V and 0.48 V for OMK-CA, corresponding to the oxidations of the BODIPY core and the diphenylamino group, respectively. The corresponding reduction wave to the first oxidation wave appeared at 0.22 V for OMK-PY and 0.25 V for OMK-CA, respectively, whereas for the two dyes the corresponding reduction wave (ca. 0.42 V) to the second oxidation wave was not clearly observed. These results show that for the two dyes the first redox process is electrochemically reversible, but the second redox process is electrochemically irreversible. The HOMO energy levels of the two dyes were evaluated from the half-wave potential for oxidation (E^ox_1/2 = 0.28 V for OMK-PY and 0.29 V for OMK-CA, respectively, Table 1). The HOMO energy level was 1.00 V for OMK-PY and 1.01 V for OMK-CA, respectively, versus the normal hydrogen electrode (NHE). Thus, the two dyes have comparable HOMO energy levels. This result shows that the HOMO energy levels of the two dyes are more positive than the I₃⁻/I⁻ redox potential (0.4 V), and thus this ensures an efficient regeneration of the oxidized dyes by electron transfer from the I₃⁻/I⁻ redox couple in the electrolyte. The LUMO energy level was estimated from the E^ox_1/2 and the onset of photoabsorption spectra (860 nm; 1.44 eV for both OMK-PY and OMK-CA) in THF. The LUMO energy level was −0.44 V for OMK-PY and −0.43 V for OMK-CA, respectively, and thus the two dyes have comparable LUMO energy levels. However, the LUMO energy levels of the two dyes are lower than the energy level (E_cb) of the conduction band (CB) of TiO₂ electrode (−0.5 V). This suggests that the electron injection from the photoexcited dye to the CB of TiO₂ electrode is thermodynamically difficult.

Fig. 3 Cyclic voltammograms of OMK-PY and OMK-CA in DMF containing 0.1 M Bu₄NClO₄. The arrow denotes the direction of the potential scan.

Theoretical calculations

To examine the electronic structures of OMK-PY and OMK-CA, the molecular orbitals of the two dyes were calculated using density functional theory (DFT) at the B3LYP/6-31G(d,p) level.²⁸ As shown in Fig. 4, for the two dyes the HOMO and HOMO−1 are mostly localized on the BODIPY core containing the two diphenylamine–thienylcarbazole moieties and the two diphenylamine–carbazole moieties, respectively. The LUMO is mostly localized on the BODIPY core containing thiophene-2-(pyridin-4-yl)acrylonitrile moiety for OMK-PY and thiophene–cyanoacetic acid group for OMK-CA, respectively, and the LUMO+1 is mostly localized on thiophene-2-(pyridin-4-yl)acrylonitrile moiety for OMK-PY and thiophene–cyanoacetic acid group for OMK-CA, respectively. Accordingly, the DFT calculations reveal that for both OMK-PY and OMK-CA the dye excitations upon light irradiation induce a strong ICT from the electron donor moiety (two diphenylamine–carbazole moieties) through the BODIPY core to the electron acceptor moiety (2-(pyridin-4-yl)acrylonitrile moiety for OMK-PY and cyanoacetic acid group for OMK-CA, respectively).

Fig. 4 HOMO−1, HOMO, LUMO and LUMO+1 of (a) OMK-PY and (b) OMK-CA derived from the DFT calculations at B3LYP/6-31G(d,p) level.

FT-IR spectra

To elucidate the adsorption states of OMK-PY and OMK-CA on TiO₂ nanoparticles, we measured the FTIR spectra of the dye powders and the dyes adsorbed on TiO₂ film (Fig. 5). For the dye powder of OMK-PY the C=N stretching band of the pyridyl group was observed at around 1590 cm⁻¹. When OMK-PY was adsorbed on the TiO₂ film, the C=N stretching band was slightly shifted by 1 cm⁻¹, to higher wavenumber compared with the dye powders, that is, the band can be assigned to the hydrogen-bonded pyridyl group to Brønsted acid sites on the TiO₂ surface.¹⁸ For the dye powder of OMK-CA, the C=O stretching band of the carboxyl group was observed at around 1690 cm⁻¹. On the other hand, in the FTIR spectrum of OMK-CA adsorbed onto TiO₂ film showed that the C=O stretching bands of the carboxyl group disappeared completely; this indicates the formation of a bidentate bridging linkage between the carboxyl group of the dye and the Brønsted acid site on the TiO₂ surface. Consequently, this result demonstrates that OMK-PY and OMK-CA was adsorbed on the TiO₂ film through the hydrogen bond and bidentate bridging linkage, respectively, at the Brønsted-acidic sites on the TiO₂ surface.

Fig. 5 FTIR spectra of the dye powder and the dye-adsorbed TiO₂ film. (a) OMK-PY and (b) OMK-CA.

Photovoltaic performances

The DSSCs based on OMK-PY and OMK-CA were fabricated by 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 the electrolyte. The I–V characteristics were measured under simulated solar light (AM 1.5, 100 mW cm⁻²). The IPCE spectra and the I–V curves are shown in Fig. 6. The photovoltaic performance parameters (a short circuit photocurrent density (J_sc), an open-circuit photovoltage (V_oc), a fill factor (ff) and a solar energy-to-electricity conversion yield (η)) are collected in Table 1. The adsorption amount (1.0 × 10¹⁷ molecules per cm²) of OMK-CA adsorbed on TiO₂ electrode is larger than that of OMK-PY (6.9 × 10¹⁶ molecules per cm²). The high adsorption ability of OMK-CA relative to OMK-PY is attributed to the formation of the bidentate bridging linkage between the carboxyl group of OMK-CA and the Brønsted acid sites on the TiO₂ surface. The IPCE spectra showed that the maximum IPCE values of OMK-PY and OMK-CA are 13% and 9%, respectively. It is worth mentioning here that for the two dyes the IPCE value in the range of 600 to 800 nm is notably much lower than that at around 400 nm, that is, the ICT band from the electron donor moiety through the BODIPY core to the electron acceptor moiety make little contribution to the IPCE spectra. The I–V curves show that the J_sc and η values are 1.08 mA cm⁻² and 0.19% for OMK-PY and 0.68 mA cm⁻² and 0.11% for OMK-CA, respectively. Accordingly, the relatively low photovoltaic performances of OMK-PY and OMK-CA are due to the low LUMO level which makes it thermodynamically difficult for the photoexcited dye to inject the electron into the CB of TiO₂ electrode. In addition, the V_oc value of BODIPY dye sensitizers is generally lower than that of the conventional organic dye sensitizers.²³ It is assumed that the low V_oc value (ca. 330 mV) for OMK-PY and OMK-CA is attributed to faster charge recombination between the injected electrons in the CB of TiO₂ electrode and I₃⁻ ions in the electrolyte, arising from the approach of I₃⁻ ions to the TiO₂ surface due to the electrostatic interactions between the BODIPY core and I₃⁻ ions. Consequently, the low photovoltaic performances of DSSCs based on OMK-PY and OMK-CA are attributed to the low electron-injection efficiency due to the low LUMO level and the faster charge recombination due to the BODIPY core, leading to the low J_sc and V_oc values, respectively.

Fig. 6 (a) IPCE spectra and (b) I–V curves of DSSCs based on OMK-PY and OMK-CA.

Conclusions

To explore effective BODIPY dye sensitizers possessing a high LHE in the range of visible light to NIR light, we have designed and developed (D)₂–π–A type BODIPY dyes OMK-PY and OMK-CA bearing pyridyl group and cyanoacrylic acid group, respectively, at the end of 8-positions on the BODIPY core, as an electron-withdrawing anchoring group to adsorb onto the TiO₂ electrode and two diphenylamine–thienylcarbazole moieties as an electron-donating unit at the 3- and 5-positions on the BODIPY core. It was found that for the absorption spectra of OMK-PY and OMK-CA adsorbed on TiO₂ film the two dyes show a strong and broad absorption band in the range of 600 to 850 nm, and the onset of the absorption band reached 900 nm, that is, OMK-PY and OMK-CA possess the NIR adsorption ability as well as the panchromatic adsorption ability. Consequently, we demonstrate that (D)₂–π–A BODIPY structure having two diphenylamine–thienylcarbazole moieties as strong electron-donating units at the 3- and 5-positions on the BODIPY core is an effective strategy to lead a high LHE in the range of visible light to NIR light, although the DSSCs based on (D)₂–π–A type BODIPY dyes that have been developed in this current stage showed low photovoltaic performances due to the low LUMO level.

Experimental

General

Melting points were measured with a Yanaco micro melting point apparatus MP model. IR spectra were recorded on a SHIMADZU IRAffinity-1 spectrometer by ATR method. High-resolution mass spectral data were acquired on a Thermo Fisher Scientific LTQ Orbitrap XL. ¹H NMR and ¹³C NMR spectra were recorded on a Varian-400 (400 MHz) or a Varian-500 (500 MHz) FT NMR spectrometer. Absorption spectra were observed with a Shimadzu UV-3150 spectrophotometer. Cyclic voltammetry (CV) curves were recorded in DMF/Bu₄NClO₄ (0.1 M) solution with a three-electrode system consisting of Ag/Ag⁺ as reference electrode, Pt plate as working electrode, and Pt wire as counter electrode by using a AMETEK Versa STAT 4 potentiostat. The potentials were referenced to ferrocene/ferrocenium (Fc/Fc⁺) as the internal reference. The HOMO and LUMO energy levels of OMK-PY and OMK-CA were evaluated from the spectral analyses and the CV data. Thus, the HOMO energy level was evaluated from the E^ox_1/2. The LUMO energy level was estimated from the E^ox_1/2 and the onset of photoabsorption spectra (860 nm; 1.44 eV for both OMK-PY and OMK-CA) in THF, that is, the LUMO energy level was obtained through eq. −[E_0–0 − HOMO], where E_0–0 transition energy is the onset of photoabsorption spectra corresponding to the energy gap between HOMO and LUMO.

Synthesis

2,2′-((5-(4,4,5,5-Tetramethyl-1,3-dioxolan-2-yl)thiophen-2-yl)methylene)bis(1H-pyrrole) (2). To a dichloromethane solution (40 ml) of 1 (ref. 26) (0.1 g, 0.42 mmol) and pyrrole (0.06 ml, 0.88 mmol) under an argon atmosphere was added trifluoroacetic acid (0.0005 ml) and stirred for 2 h at room temperature. The reaction mixture was diluted with water, and then the solution was extracted with dichloromethane. The dichloromethane extract was dried over MgSO₄, filtrated, and concentrated. The residue was chromatographed on silica gel (dichloromethane–ethyl acetate = 9 : 1 as eluent) to give 2 (0.07 g, yield 50%) as a yellow viscous solid; IR (ATR): [small nu, Greek, tilde] = 3358, 2976, 1670, 1553 cm⁻¹; ¹H NMR (500 MHz, CDCl₃) δ = 1.29 (d, 12H), 5.71 (s, 1H), 6.04 (br, 2H), 6.12 (s, 1H), 6.14–6.16 (m, 2H), 6.69–6.70 (m, 2H), 6.74 (d, J = 3.6 Hz, 1H), 7.00 (d, J = 3.6 Hz, 1H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ = 22.22, 24.54, 39.54, 83.17, 96.86, 107.22, 108.59, 117.54, 125.30, 126.43, 131.74, 142.59, 146.80 ppm; HRMS (ESI): m/z (%): [M + H⁺] calcd for C₂₀H₂₅N₂O₂S, 357.16313; found 357.16336.

(E)-2-Chloro-5-((5-chloro-2H-pyrrol-2-ylidene)(5-(4,4,5,5-tetramethyl-1,3-dioxolan-2-yl)thiophen-2-yl)methyl)-1H-pyrrole (3). To compound 2 (2.68 g, 7.52 mmol) under an argon atmosphere was added THF (40 ml) and then stirred at −75 °C. Next, a solution of N-chlorosuccinimide (2.01 g, 15.1 mmol) in THF (30 ml) was added dropwise and the solution was stirred at −75 °C for 1 h and then at room temperature for 2 h. After concentrating under reduced pressure, the resulting residue was dissolved in dichloromethane and washed with water. To the dichloromethane extract (150 ml) was added DDQ (1.71 g, 7.52 mmol) and the solution was stirred at room temperature for 2 h. After concentrating under reduced pressure, the residue was chromatographed on silica gel (dichloromethane as eluent) to give 3 (1.54 g, yield 49%) as a reddish brown solid; mp 89–91 °C; IR (ATR): [small nu, Greek, tilde] = 2967, 1560, 1420 cm⁻¹; ¹H NMR (500 MHz, CDCl₃) δ = 1.33 (d, 12H), 6.21 (s, 1H), 6.29 (d, J = 4.3 Hz, 2H), 6.90 (d, J = 4.3 Hz, 2H), 7.19 (br, 2H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ = 22.20, 24.40, 83.57, 96.58, 117.18, 126.28, 130.33, 131.53, 132.40, 136.98, 138.44, 142.12, 147.64 ppm; HRMS (ESI): m/z (%): [M + H⁺] calcd for C₂₀H₂₁N₂O₂Cl₂S, 423.06953; found 423.06921.

3,7-Dichloro-5,5-difluoro-10-(5-(4,4,5,5-tetramethyl-1,3-dioxolan-2-yl)thiophen-2-yl)-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinine (4). To a compound 3 (1.5 g, 3.55 mmol) under an argon atmosphere was added dichloromethane (100 ml) and triethylamine (4.72 ml, 35.5 mmol) and stirred for 30 min at room temperature. Next, BF₃–OEt₂ (6.25 ml, 53.3 mmol) was added dropwise and the solution was stirred at room temperature for 12 h. The reaction mixture was washed with water and extracted with dichloromethane. The dichloromethane extract was dried over MgSO₄, filtrated, and concentrated. The residue was chromatographed on silica gel (dichloromethane–hexane = 1 : 2 as eluent) to give 4 (1.11 g, yield 67%) as a dark red solid; mp 185–187 °C; IR (ATR): [small nu, Greek, tilde] = 2982, 1545, 1381 cm⁻¹; ¹H NMR (500 MHz, acetone-d₆) δ = 1.31 (s, 12H), 6.26 (s, 1H), 6.71 (d, J = 4.4 Hz, 2H), 7.42 (d, J = 3.8 Hz, 1H), 7.43 (d, J = 4.4 Hz, 2H), 7.65 (d, J = 3.8 Hz, 1H) ppm; ¹³C NMR (125 MHz, acetone-d₆) δ = 22.34, 24.51, 84.17, 96.90, 119.96, 128.16, 133.17, 133.34, 133.95, 134.73, 144.76, 152.63, 153.81 ppm; HRMS (ESI): m/z (%): [M + Na⁺] calcd for C₂₀H₁₉N₂O₂BF₂Cl₂SNa, 493.04976; found 493.04974.

7,7′-((5,5-Difluoro-10-(5-(4,4,5,5-tetramethyl-1,3-dioxolan-2-yl)thiophen-2-yl)-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinine-3,7-diyl)bis(thiophene-5,2-diyl))bis(9-butyl-N,N-diphenyl-9H-carbazol-2-amine) (6). A solution of 4 (1.0 g, 2.13 mmol), 5 (ref. 18g) (2.71 g, 4.25 mmol), and Pd(PPh₃)₄ (2.94 g, 2.54 mmol) in toluene (270 ml) was stirred for 12 h at 110 °C under an argon atmosphere. After concentrating under reduced pressure, the resulting residue was dissolved in dichloromethane and washed with water. The dichloromethane extract was dried over MgSO₄, filtrated, and concentrated. The residue was chromatographed on silica gel (chloroform–hexane = 1 : 2 as eluent) to give 6 (1.99 g, yield 70%) as a blackish green solid; mp 176–177 °C; IR (ATR): [small nu, Greek, tilde] = 2955, 1591, 1431 cm⁻¹; ¹H NMR (400 MHz, acetone-d₆) δ = 0.88 (t, J = 7.4 Hz, 6H), 1.32–1.34 (m, 16H), 1.77–1.84 (m, 4H), 4.37 (t, J = 7.0 Hz, 4H), 6.29 (s, 1H), 6.94 (dd, J = 1.8 and 8.4 Hz, 2H), 7.04–7.08 (m, 4H), 7.13–7.14 (m, 10H), 7.23 (d, J = 1.8 Hz, 2H), 7.30–7.34 (m, 10H), 7.40 (d, J = 3.8 Hz, 1H), 7.55 (d, J = 3.7 Hz, 1H), 7.64 (dd, J = 1.5 and 8.1 Hz, 2H), 7.76 (d, J = 4.1 Hz, 2H), 7.95 (d, J = 1.2 Hz, 2H), 8.06 (d, J = 8.4 Hz, 2H), 8.15 (d, J = 8.0 Hz, 2H), 8.31 (d, J = 4.1 Hz, 2H) ppm; ¹³C NMR (125 MHz, acetone-d₆) δ = 14.18, 21.02, 22.38, 24.60, 31.84, 43.07, 84.02, 97.12, 105.79, 106.95, 117.90, 118.44, 119.25, 121.22, 121.60, 122.04, 123.65, 124.26, 124.88, 126.15, 126.47, 127.67, 130.18, 130.79, 131.21, 132.77, 133.72, 133.91, 135.69, 137.40, 142.26, 143.43, 147.74, 149.12, 150.10, 150.32, 150.98 ppm; HRMS (ESI): m/z (%): [M + Na⁺] calcd for C₈₄H₇₃N₆O₂BF₂S₃Na, 1365.49105; found 1365.49304.

5-(3,7-Bis(5-(9-butyl-7-(diphenylamino)-9H-carbazol-2-yl)thiophen-2-yl)-5,5-difluoro-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-10yl)thiophene-2-carbaldehyde (7). To a solution of 6 (1.85 g, 1.38 mmol) in THF (20 ml) was added 2 N HCl (13.8 ml) and refluxed for 12 h. The reaction mixture was washed with water and extracted with dichloromethane. The dichloromethane extract was dried over MgSO₄, filtrated, and concentrated. The residue was chromatographed on silica gel (dichloromethane–hexane = 1 : 1 as eluent) to give 7 (1.71 g, yield 99%) as a dark green solid; mp 171–172 °C; IR (ATR): [small nu, Greek, tilde] = 2953, 1665, 1585 cm⁻¹; ¹H NMR (500 MHz, acetone-d₆) δ = 0.88 (t, J = 7.4 Hz, 6H), 1.29–1.36 (m, 4H), 1.78–1.83 (m, 4H), 4.37 (t, J = 6.9 Hz, 4H), 6.94 (dd, J = 1.9 and 8.4 Hz, 2H), 7.05–7.07 (m, 4H), 7.13–7.15 (m, 8H), 7.17 (d, J = 4.8 Hz, 2H), 7.23 (d, J = 1.8 Hz, 2H), 7.28–7.33 (m, 10H), 7.65 (dd, J = 1.5 and 8.1 Hz, 2H), 7.78 (d, J = 4.0 Hz, 2H), 7.81 (d, J = 3.5 Hz, 1H), 7.96 (d, J = 1.0 Hz, 2H), 8.07 (d, J = 8.4 Hz, 2H), 8.15 (d, J = 8.1 Hz, 2H), 8.18 (d, J = 3.5 Hz, 1H), 8.35 (d, J = 3.8 Hz, 2H), 10.12 (s, –CHO) ppm; ¹³C NMR (125 MHz, acetone-d₆) δ = 14.18, 21.02, 31.85, 43.08, 105.77, 107.01, 117.91, 118.46, 119.21, 121.24, 122.07, 123.67, 124.37, 124.89, 126.29, 130.18, 130.56, 131.11, 133.53, 133.82, 137.44, 137.61, 142.25, 143.46, 143.57, 147.81, 149.10, 150.99, 151.65, 184.42 ppm (four aromatic carbon signals were not observed owing to overlapping resonances); HRMS (ESI): m/z (%): [M + H⁺] calcd for C₇₆H₆₂N₆OBF₂S₃, 1243.42029; found 1243.42139.

3-(5-(3,7-Bis(5-(9-butyl-7-(diphenylamino)-9H-carbazol-2-yl)thiophen-2-yl)-5,5-difluoro-5H-4dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-10-yl)thiophen-2-yl)-2-(pyridin-4-yl)acrylonitrile (OMK-PY). To a solution of 4-pyridylacetonitrile hydrochloride (0.05 g, 0.322 mmol) in dichloromethane (3 ml) was added triethylamine (0.21 ml, 1.61 mmol) and stirred for 20 min at 0 °C. Next, 7 (0.4 g, 0.322 mmol) was added and the solution was stirred at room temperature for 5 days. The reaction mixture was washed with water and extracted with dichloromethane. The dichloromethane extract was dried over MgSO₄, filtrated, and concentrated. The residue was chromatographed on silica gel (dichloromethane and then ethyl acetate as eluent) to give OMK-PY (0.171 g, yield 40%) as a dark green solid; mp 169–170 °C; IR (ATR): [small nu, Greek, tilde] = 2924, 1585, 1429 cm⁻¹; ¹H NMR (500 MHz, acetone-d₆) δ = 0.88 (t, J = 7.4 Hz, 6H), 1.30–1.35 (m, 4H), 1.79–1.82 (m, 4H), 4.37 (t, J = 6.6 Hz, 4H), 6.94 (d, J = 8.3 Hz, 2H), 7.04–7.07 (m, 4H), 7.13–7.14 (m, 8H), 7.18 (br, 2H), 7.23 (s, 2H), 7.30–7.33 (m, 10H), 7.64 (d, J = 8.9 Hz, 2H), 7.75–7.78 (m, 5H), 7.96 (s, 2H), 8.05–8.07 (m, 3H), 8.15 (d, J = 8.9 Hz, 2H), 8.34 (br, 2H), 8.58 (s, 1H), 8.73 (d, J = 5.9 Hz, 2H) ppm; ¹³C NMR (125 MHz, acetone-d₆) δ = 14.18, 21.02, 31.85, 43.09, 105.78, 107.01, 117.49, 117.74, 117.92, 118.46, 119.22, 120.53, 121.25, 122.07, 123.67, 124.36, 124.89, 125.99, 126.28, 129.64, 130.18, 130.51, 131.15, 133.63, 133.77, 134.27, 137.93, 142.26, 143.46, 147.80, 149.11, 151.65 ppm (seven aromatic carbon signals were not observed owing to overlapping resonances); HRMS (ESI): m/z (%): [M + H⁺] calcd for C₈₅H₆₆N₈BF₂S₃, 1343.46282; found 1343.46300.

3-(5-(3,7-Bis(5-(9-butyl-7-(diphenylamino)-9H-carbazol-2-yl)thiophen-2-yl)-5,5-difluoro-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-10-yl)thiophen-2-yl)-2-cyanoacrylic acid (OMK-CA). A solution of 7 (0.4 g, 0.322 mmol), cyanoacetic acid (0.11 g, 0.644 mmol) and piperidine (0.032 ml, 0.322 mmol) in chloroform/acetonitrile (1 : 1, 65 ml) was refluxed for 22 h. After concentrating under reduced pressure, the resulting residue was chromatographed on silica gel (ethyl acetate and then ethyl acetate containing 5% acetic acid as eluent) to give the mixture OMK-CA and unreacted cyanoacetic acid. The mixture was dissolved in dichloromethane and washed with water. The dichloromethane extract was dried over MgSO₄, filtrated, and concentrated to give OMK-CA (0.256 g, yield 61%) as a dark green solid; mp 176–179 °C; IR (ATR): [small nu, Greek, tilde] = 2922, 1688, 1578, 1424 cm⁻¹; ¹H NMR (500 MHz, DMSO-d₆) δ = 0.79 (t, J = 7.5 Hz, 6H), 1.17–1.23 (m, 4H), 1.62–1.68 (m, 4H), 4.29 (t, J = 6.8 Hz, 4H), 6.87 (dd, J = 1.6 and 8.5 Hz, 2H), 7.03–7.08 (m, 12H), 7.15 (d, J = 1.5 Hz, 2H), 7.26 (d, J = 4.5 Hz, 2H), 7.29–7.33 (m, 10H), 7.60 (d, J = 9.1 Hz, 2H), 7.84 (d, J = 3.9 Hz, 1H), 7.90 (d, J = 4.1 Hz, 2H), 7.95 (s, 2H), 8.07 (d, J = 8.5 Hz, 2H), 8.13–8.15 (m, 3H), 8.24 (d, J = 4.2 Hz, 2H), 8.58 (br, –COOH) ppm; unfortunately, it was difficult to obtain the ¹³C NMR spectrum of OMK-CA because of its poor solubility in deuterated solvent; HRMS (ESI): m/z (%): [M⁺] calcd for C₈₁H₆₂N₇O₂BF₂S₃, 1309.41827; found 1309.42188.

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 dye solution in THF for 15 hours enough to adsorb the dye sensitizers (OMK-PY and OMK-CA). 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 dye molecules adsorbed on TiO₂ film was determined form the calibration curve by absorption spectral measurement of the concentration change of the dye solution before and after adsorption. The quantification of dye was made based on the molar extinction coefficient for λ^abs_max of dye in the above solution. Absorption spectra of the dyes adsorbed on TiO₂ film (3 μm) were recorded on the dyes-adsorbed TiO₂ film in the transmission mode with a calibrated integrating sphere system.

Acknowledgements

This work was supported by Grants-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 15H03859, by Matching Planner Program (MP27115659061) from Japan Science and Technology Agency (JST) and by Yazaki Memorial Foundation for Science and Technology.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra00799j

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