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

* (D) 2 – p – 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 TiO 2 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 TiO 2 ﬁ lm 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 TiO 2 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 – p – 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 e ﬀ ective strategy to lead a high light-harvesting e ﬃ ciency (LHE) in the range of visible light to NIR light, although the DSSCs based on (D) 2 – p – A type BODIPY dyes that have been developed in this current stage showed low photovoltaic performances.


Introduction
Dye-sensitized solar cells (DSSCs) employing dye-adsorbed TiO 2 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][2][3][4][5][6][7][8] since Grätzel and co-workers reported high-performance DSSCs employing a Ru-complex dye-adsorbed TiO 2 electrode in 1991. 9n particular, much effort has been made on the development of various types of D-p-A organic dye sensitizers which are constructed of a diphenyl-or dialkylamino group as the electron donor (D) and a carboxyl group, [10][11][12][13][14][15][16] 2-(1,1-dicyanomethylene) rhodanine, 17 pyridine, 18-21 or 8-hydroxylquinoline 22 as the electron acceptor (A) and anchor linked by p-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 (h) 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.4][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 3and 5-positions and benzoic acid as electron acceptor at the 8position.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 (3) ¼ 60 000-70 000 M À1 cm À1 ).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 -p-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 TiO 2 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 -p-A BODIPY dye sensitizers and their photovoltaic performances in DSSCs, based on photoabsorption (dye solution and dye-adsorbed TiO 2 lm) and electrochemical measurements (cyclic voltammetry), density functional theory (DFT) calculation, FT-IR spectroscopy of the dye-adsorbed TiO 2 lm, photocurrent-voltage (I-V) curves, and incident photon-tocurrent conversion efficiency (IPCE) spectra.We demonstrate that the (D) 2 -p-A type BODIPY dyes possess the NIR adsorption ability as well as the panchromatic adsorption ability.

Synthesis
(D) 2 -p-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 -p-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 NEt 3 and treated with BF 3 -OEt 2 to afford the BODIPY 4.

Photophysical properties
The photoabsorption spectra of OMK-PY and OMK-CA in THF and adsorbed on the TiO 2 lm 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 (3) at the absorption peak wavelength (l abs ¼ 753 nm) is 95 400 M À1 cm À1 , which is higher than that of OMK-CA (3 ¼ 83 100 M À1 cm À1 at l abs ¼ 754 nm).Moreover, for the two dyes a strong absorption peak at around 400 nm with the 3 value of ca.70 000-80 000 M À1 cm À1 , 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 TiO 2 lm the longest wavelength absorption band in the range of 600 to 850 nm is signicantly 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 TiO 2 lm is almost black.Thus, this result indicates that the (D) 2 -p-A type BODIPY dyes possess a good LHE in the range of visible light to NIR light and good adsorption ability onto TiO 2 electrode, that is, OMK-PY and OMK-CA possess the NIR adsorption ability as well as the panchromatic adsorption ability.

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 (Bu 4 NClO 4 ) (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 rst 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 rst 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 ¼ 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 I 3 À / I À redox potential (0.4 V), and thus this ensures an efficient regeneration of the oxidized dyes by electron transfer from the I 3 À / I À 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 À0.44 V for OMK-PY and À0.43 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 TiO 2 electrode (À0.5 V).This suggests that the electron injection from the photoexcited dye to the CB of TiO 2 electrode is thermodynamically difficult.

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. 28As shown in Fig. 4, for the two dyes the HOMO and HOMOÀ1 are mostly localized on the BODIPY core containing the two diphenylaminethienylcarbazole moieties and the two diphenylamine-carbazole moieties, respectively.The LUMO is mostly localized on the Table 1 Optical and electrochemical data, HOMO and LUMO energy levels, and DSSC performance parameters of OMK-PY and OMK-CA 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 diphenylaminecarbazole 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).

FT-IR spectra
To elucidate the adsorption states of OMK-PY and OMK-CA on TiO 2 nanoparticles, we measured the FTIR spectra of the dye powders and the dyes adsorbed on TiO 2 lm (Fig. 5).For the dye powder of OMK-PY the C]N stretching band of the pyridyl group was observed at around 1590 cm À1 .When OMK-PY was adsorbed on the TiO 2 lm, the C]N stretching band was slightly shied by 1 cm À1 , 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 TiO 2 surface. 18For the dye powder of OMK-CA, the C]O stretching band of the carboxyl group was observed at around 1690 cm À1 .On the other hand, in the FTIR spectrum of OMK-CA adsorbed onto TiO 2 lm 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 TiO 2 surface.Consequently, this result demonstrates that OMK-PY and OMK-CA was adsorbed on the TiO 2 lm through the hydrogen bond and bidentate bridging linkage, respectively, at the Brønstedacidic sites on the TiO 2 surface.

Preparation of DSSCs
The TiO 2 paste (JGC Catalysts and Chemicals Ltd., PST-18NR) was deposited on a uorine-doped-tin-oxide (FTO) substrate by doctor-blading, and sintered for 50 min at 450 C. The 9 mm thick TiO 2 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 TiO 2 electrode (0.5 Â 0.5 cm 2 in photoactive area) thus prepared, Ptcoated 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-3propylimidazolium 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 À2 ).IPCE spectra were measured under monochromatic irradiation with a tungsten-halogen lamp and a monochromator.The amount of dye molecules adsorbed on TiO 2 lm was determined form the calibration curve by absorption spectral measurement of the concentration change of the dye solution before and aer adsorption.The quantication of dye was made based on the molar extinction coefficient for l abs max of dye in the above solution.Absorption spectra of the dyes adsorbed on TiO 2 lm (3 mm) were recorded on the dyesadsorbed TiO 2 lm in the transmission mode with a calibrated integrating sphere system.
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 NEt 3 . 27BODIPY dye OMK-CA was prepared by typical Knoevenagel condensation of the compound 7 with cyanoacetic acid in the presence of piperidine.

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

Fig. 3
Fig. 3 Cyclic voltammograms of OMK-PY and OMK-CA in DMF containing 0.1 M Bu 4 NClO 4 .The arrow denotes the direction of the potential scan.
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 TiO 2 electrode and I 3 À ions in the electrolyte, arising from the approach of I 3 À ions to the TiO 2 surface due to the electrostatic interactions between the BODIPY core and I 3 À 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.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 -p-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 electronwithdrawing anchoring group to adsorb onto the TiO 2 electrode and two diphenylamine-thienylcarbazole moieties as an electrondonating 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 TiO 2 lm 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 -p-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 -p-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 SHI-MADZU IRAffinity-1 spectrometer by ATR method.Highresolution mass spectral data were acquired on a Thermo Fisher Scientic LTQ Orbitrap XL. 1 H NMR and 13 C 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.
Adsorption amount of dye molecules per unit area of the TiO 2 electrode, when the 9 mm thick TiO 2 electrode was immersed into 0.1 mM dye solution in THF.
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 MgSO 4 , ltrated, 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