Development of a functionally separated D – π -A ﬂ uorescent dye with a pyrazyl group as an electron-accepting group for dye-sensitized solar cells †

A functionally separated D – π -A dye OUK - 3 with a pyrazyl group as an electron-withdrawing anchoring group and a carboxyl group as an additional anchoring group has been newly developed as a photosensitizer for dye-sensitized solar cells. The optical and electrochemical properties, adsorption states on TiO 2 nanoparticles, and photovoltaic performances in dye-sensitized solar cells (DSSCs) were investigated. It was found that the maximum adsorption amount of dye adsorbed on the TiO 2 electrode for OUK - 3 is 3 times as much as that of the D – π -A dye sensitizer OUK - 1 with a pyrazyl group as an electron-withdrawing anchoring group. Moreover, this work revealed that the DSSC based on the dye OUK-3 exhibits a higher photovoltage ( V oc ) value than the DSSC based on the dye OUK - 1 . On the basis of the FTIR spectra of the dyes adsorbed on TiO 2 nanoparticles and the electrochemical impedance spectroscopy (EIS) analysis of DSSCs based on the two dyes OUK - 1 and OUK - 3 , the di ﬀ erences of photovoltaic performances between the two dyes are discussed by taking into account the adsorption states of the dyes adsorbed on the TiO 2 surface. This work demonstrates that functionally separated D – π -A dye sensitizers can achieve e ﬀ ective surface coverage of the TiO 2 electrode due to their high adsorption ability onto the TiO 2 electrode, leading to not only the improvement of light-harvesting e ﬃ ciency (LHE), but also an increase in the number of injected electrons in the CB of TiO 2 , which is responsible for the higher V oc value of functionally separated D – π -A dye sensitizers.


Introduction
][3][4][5][6][7][8][9][10] Much effort in molecular design and development of efficient dye sensitizers have been made to further improve the photovoltaic performances of DSSCs so far.][6][7][8][9][10] Consequently, a new type of D-π-A dye sensitizers with 2-(1,1-dicyanomethylene)rhodanine, 11 pyridine, 12,13 and 8-hydroxylquinoline 14 as an electron-withdrawing anchoring group, as well as conventional D-π-A dye sensitizers with a carboxyl group have been designed and developed so far.][6][7][8][9][10] On the other hand, we have demonstrated that dye sensitizers with a pyridyl group are predominantly adsorbed on the TiO 2 through coordinate bonding between the pyridyl group of the dye and the Lewis acid site (exposed Ti n+ cations) on the TiO 2 surface. 12It was found that D-π-A dye sensitizers with a pyridyl group can inject electrons efficiently to the conduction band (CB) of the TiO 2 electrode through coordinate bonding, rather than conventional D-π-A dye sensitizers with carboxyl groups.Recently, DSSCs based on porphyrin dyes and D-π-A dyes with pyridyl groups have been reported by some research groups. 13oreover, to seek an epoch-making molecular design of D-π-A dye sensitizers for high-performance DSSCs, some research groups designed and synthesized the functionally separated D-π-A dye sensitizers with a pyridyl group, 12a,c a cyano group, 15,16 a nitro group 17 or an aldehyde 18 as an electron-accepting group and a carboxyl group as an additional anchoring group.They investigated the effects of the interaction between the electron-accepting group of the dye and the TiO 2 surface on the photovoltaic performances of DSSCs.It was revealed that the biggest advantage of the functionally separated D-π-A dye sensitizers is that one can introduce both strong electron-withdrawing groups into the electron acceptor moiety and an additional anchoring group, leading to not only high adsorption ability onto the TiO 2 film, but also the bathochromic shift and broadening of the photoabsorption property and efficient electron injection from the dye to the CB of the TiO 2 electrode.For example, Sun and co-workers designed and synthesized efficient near infrared D-π-A dye sensitizers, with strong electron-withdrawing groups as the electron acceptors, in which the anchoring group (carboxyl group) in these dyes was separated from the electron acceptor unit (dicyanomethylene). 15They also developed the functionally separated D-π-A dye sensitizer with a nitro group as the electron-accepting group and a carboxyl group as the anchoring group.They demonstrated the effects of the interaction between the nitro group of the dye and the TiO 2 surface on the photovoltaic performances and UV-Vis absorption properties. 17Thus, the functionally separated D-π-A dye sensitizers would also be expected to be one of the most promising classes of organic dye sensitizers for DSSCs.
In this work, a functionally separated D-π-A dye sensitizer OUK-3 with a pyrazyl group as an electron-withdrawing anchoring group and a carboxyl group as an additional anchoring group has been newly developed.The optical and electrochemical properties, adsorption states on TiO 2 nanoparticles, and photovoltaic performances in dye-sensitized solar cells (DSSCs) were investigated (Scheme 1).It was found that the maximum adsorption amount of dyes adsorbed on the TiO 2 electrode for OUK-3 is 3 times as much as that of the D-π-A dye sensitizer OUK-1 19 with a pyrazyl group as an elec-tron-withdrawing anchoring group.Moreover, this work revealed that the DSSC based on the dye OUK-3 exhibits a higher photovoltage (V oc ) value than the DSSC based on the dye OUK-1.On the basis of the FTIR spectra of the dyes adsorbed on TiO 2 nanoparticles and the electrochemical impedance spectroscopy (EIS) analysis of DSSCs based on the two dyes OUK-1 and OUK-3, the differences of photovoltaic performances between the two dyes are discussed by taking into account the adsorption states of the dyes adsorbed on the TiO 2 surface.

Synthesis
The synthesis of OUK-1 has been reported elsewhere. 19The synthetic pathway of OUK-3 is shown in Scheme 2. The starting material 1 19 was converted to stannyl compound 2 by treatment with nBuLi and then Me 3 SnCl.Compound 3 was prepared by the Stille coupling of 2 with 2-iodopyrazine.The reaction of 3 with ethyl 7-bromoheptanoate by using sodium hydride yielded the compound 4, and then the hydrolysis of 4 with a base gave the functionally separated D-π-A dye sensitizer OUK-3.

Optical properties
The absorption and fluorescence spectra of OUK-3 in 1,4dioxane are shown in Fig. 1a (see Fig. S1a in ESI † for OUK-1).The two dyes show the absorption maximum (λ abs max ) at 402 nm, which is assigned to the intramolecular charge-transfer (ICT) excitation from the electron donor moiety (diphenylamino group) to the electron acceptor moiety ( pyrazyl group).The molar extinction coefficient (ε) for the ICT band is 45 400 M −1 cm −1 for OUK-1 and 48 300 M −1 cm −1 for OUK-3, respectively.The corresponding fluorescence bands for the two dyes occur at around 480 nm, and the fluorescence quantum yield (Φ fl ) is 0.46 for OUK-1 and 0.48 for OUK-3, respectively (Table 1).
The absorption spectrum of the dye OUK-3 adsorbed on the TiO 2 film is shown in Fig. 1b (see Fig. S1b in ESI † for OUK-1).The absorption bands of OUK-1 and OUK-3 adsorbed on the TiO 2 film are broadened compared with those in 1,4-dioxane.However, the absorption peak wavelengths of OUK-1 and OUK-3 adsorbed on TiO 2 are similar to those in 1,4-dioxane.Thus, this result indicates that the two dyes form weak π-stacked aggregates on the TiO 2 surface.

Electrochemical properties
The electrochemical properties of OUK-1 and OUK-3 were determined by cyclic voltammetry (Fig. 2 and Table 1, see Fig. S2 in ESI † for OUK-1).The reversible oxidation waves for the two dyes were observed at 0.42 V vs. ferrocene/ferrocenium (Fc/Fc + ).The corresponding reduction waves for the two dyes appeared at 0.35 V, thus showing that the oxidized states of the two dyes are stable.The HOMO energy level was evaluated from the half-wave potential for oxidation (E ox 1/2 = 0.39 V for the two dyes).The HOMO energy level was 1.11 V vs. the normal Scheme 1 Chemical structures of the D-π-A dye sensitizer OUK-1 and the functionally separated D-π-A dye sensitizer OUK-3.
hydrogen electrode (NHE), thus indicating that the HOMO energy level is more positive than the I 3 -/I -redox potential (0.4 V).This assures efficient regeneration of the oxidized dyes by electron transfer from the I 3 -/I -in the electrolyte.The LUMO energy level was estimated from the E ox 1/2 and an intersection of the absorption and fluorescence spectra (445 nm; 2.79 eV for OUK-1 and 443 nm; 2.80 eV for OUK-3).The LUMO energy levels of OUK-1 and OUK-3 were −1.68 and −1.69 V, respectively.Evidently, the LUMO energy levels of OUK-1 and OUK-3 are higher than the energy level (E cb ) of the CB of TiO 2 (-0.5 V), suggesting that an electron injection to the CB of TiO 2 is thermodynamically feasible.Accordingly, the optical and electrochemical properties of OUK-1 and OUK-3 resemble each other very closely, showing that the effect of N-alkylation of the carbazole ring on the photophysical and electrochemical properties is negligible.

Theoretical calculations
To examine the electronic structures of OUK-1 and OUK-3, the molecular orbitals of the two dyes were calculated using density functional theory (DFT) at the B3LYP/6-31G(d, p) level.The DFT calculations indicate that for the two dyes the HOMOs were mostly localized on the diphenylamine-carbazole moiety containing a thiophene ring (Fig. 3, see Fig. S3 in ESI † for OUK-1).On the other hand, the LUMOs were mostly localized on the thienylpyrazine moiety.Accordingly, the DFT calculations reveal that dye excitations upon light irradiation induce a strong ICT from the diphenylamine-carbazole moiety to the pyrazine moiety.Table 1 Optical and electrochemical data, HOMO and LUMO energy levels, and DSSC performance parameters of OUK-1 and OUK-3

FTIR spectra
To elucidate the adsorption states of OUK-1 and OUK-3 on TiO 2 nanoparticles, we measured the FTIR spectra of the dye powders and the dyes adsorbed on TiO 2 nanoparticles (Fig. 4, see Fig. S4 in ESI † for OUK-1).For the dye powders of OUK-3, the CvO stretching band of the carboxyl group was observed at 1724 cm −1 .When the dye OUK-3 was adsorbed on the TiO 2 surface, the CvO 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.In addition, for the powders of the two dyes the CvN stretching band of the pyrazine ring was clearly observed at around 1490 cm −1 .Inter-estingly, when the two dyes were adsorbed on the TiO 2 surface, where the adsorption amount of the dye adsorbed on the TiO 2 electrode is low (0.8 × 10 16 and 1.5 × 10 16 molecules per cm 2 for OUK-1 and OUK-3 respectively), the band at 1490 cm −1 disappeared completely and a new band appeared at around 1650 cm −1 , which indicates the formation of a pyrazinium ion with Brønsted acid sites on the TiO 2 surface. 19,20More interestingly, when the adsorption amount of the dye adsorbed on the TiO 2 electrode is high (3.0 × 10 16 and 9.6 × 10 16 molecules per cm 2 for OUK-1 and OUK-3 respectively), the CvN stretching band at 1490 cm −1 is shifted by 6 and 1 cm −1 for OUK-1 and OUK-3, respectively, to a higher wavenumber, that is, the band can be assigned to the hydrogen-bonded pyrazyl group to Brønsted acid sites on the TiO 2 surface.These observations indicate that in the initial stage of dye adsorption both the dyes OUK-1 and OUK-3 are predominantly adsorbed on the TiO 2 surface through the formation of a pyrazinium ion with Brønsted acid sites.However, both the dyes are adsorbed on the TiO 2 surface through the hydrogen bonding at Brønsted acid sites as the dye adsorption progresses, although there still remain dye molecules adsorbed on the TiO 2 surface through the formation of a pyrazinium ion with Brønsted acid sites.Consequently, the dye OUK-1 is adsorbed on the TiO 2 surface through formations of both hydrogen bonding of the pyrazyl group and the pyrazinium ion at Brønsted acid sites on the TiO 2 surface.On the other hand, the dye OUK-3 is adsorbed on the TiO 2 surface through not only the bidentate bridging linkage of the carboxyl group but also the formations of hydrogen bonding of the pyrazyl group and the pyrazinium ion at Brønsted acid sites on the TiO 2 surface.Thus, these results indicate that effective surface coverage of the TiO 2 electrode is achieved successfully by employing the functionally separated D-π-A dyes with two functional groups (cyclic azine and carboxyl group) possessing bonding ability to two points on Brønsted acid sites on the TiO 2 surface.Moreover, it is worth mentioning here that the functionally separated D-π-A dye sensitizers with a pyridyl group and a carboxyl group as an additional anchoring group was adsorbed on the TiO 2 surface through not only the formation of a bidentate bridging linkage between the carboxyl group of the dye and the Brønsted acid site on the TiO 2 surface, but also the coordinate bonding between the pyridyl group of the dye and the Lewis acid site on the TiO 2 surface.12c Our work suggest that the binding modes of D-π-A dye sensitizers with cyclic azine on the TiO 2 surface can be changed by controlling the basicity and the electron density of cyclic azine.

Dye-sensitized solar cells
The DSSC was prepared using the dye-adsorbed TiO 2 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 I-V curve and the incident photon-to-current conversion efficiency (IPCE) spectrum of DSSC based on the dye OUK-3   are shown in Fig. 5 (see Fig. S5 in ESI † for OUK-1).The photovoltaic performance parameters are collected in Table 1.The maximum adsorption amount of dyes adsorbed on TiO 2 for OUK-3 is 3 times as much as that of OUK-1 (3.0 × 10 17 and 9.6 × 10 16 molecules per cm 2 for OUK-1 and OUK-3 respectively).The high adsorption ability of OUK-3 relative to OUK-1 is attributed to the formation of a bidentate bridging linkage between the carboxyl group of OUK-3 and Brønsted acid sites on the TiO 2 surface.The I-V curves show that the short-circuit photocurrent density ( J sc ) and solar energy-to-electricity conversion yield (η) of OUK-3 (4.48 mA cm −2 and 1.58%) are higher than that of OUK-1 (2.99 mA cm −2 and 0.89%) (Fig. 5a).
The maximum IPCE value of OUK-3 is 65% at 420 nm, which is higher than that (38% at 420 nm) of OUK-1 (Fig. 5b).Moreover, it is worth mentioning here that the V oc value of OUK-3 (550 mV) is higher than that of OUK-1 (448 mV).Thus, electrochemical impedance spectroscopy (EIS) analysis was performed to study the electron recombination process in DSSCs based on OUK-1 and OUK-3 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, see Fig. S6a in ESI † for OUK-1), which corresponds to the midfrequency peaks in the Bode phase plots, represents the charge recombination between the injected electrons in TiO 2 and I 3 − ions in the electrolyte, that is, the charge-transfer resistances at the TiO 2 /dye/electrolyte interface.The Nyquist plots show that the resistance value for the large semicircle for OUK-3 (50 Ω) is larger than that of OUK-1 (26 Ω), indicating that the electron recombination resistance of OUK-3 is higher than that of OUK-1.The electron recombination lifetimes (τ e ) expressing the electron recombination between the injected electrons in TiO 2 and I 3 − ions in the electrolyte, extracted from the angular frequency (ω rec ) at the midfrequency peak in the Bode phase plot (Fig. 6b, see Fig. S6b in ESI † for OUK-1) using τ e = 1/ω rec , are 11 ms for DSSC based on OUK-3, which is slightly larger than 10 ms for DSSC based on OUK-1.This result revealed that the charge recombination between the injected electrons in TiO 2 and I 3 − ions in the electrolyte is not the major reason for the difference in the V oc value between the two dyes, but the increase in the number of injected electrons in the CB of TiO 2 and/or a negative shift of the E cb of TiO 2 by the formation of hydrogen bonding of the pyrazyl group at Brønsted acid sites on the TiO 2 surface with the increasing dye loading of OUK-3 on the TiO 2 electrode may result in a higher V oc value for OUK-3. 4,6Consequently, the higher photovoltaic performance of OUK-3 is attributed to the improvement of light-harvesting efficiency (LHE) and the increase in the number of injected electrons in the CB of TiO 2 due to a high adsorption ability of the functionally separated D-π-A dye onto the TiO 2 electrode.

Conclusions
We have designed and synthesized a functionally separated D-π-A dye sensitizer OUK-3 with a pyrazyl group as an electron-withdrawing anchoring group and a carboxyl group as an additional anchoring group for dye-sensitized solar cells.The dye OUK-3 is adsorbed on the TiO 2 surface through not only the bidentate bridging linkage of the carboxyl group but also the formation of hydrogen bonding of the pyrazyl group and the pyrazinium ion at Brønsted acid sites on the TiO 2 surface.This work revealed that functionally separated D-π-A dye sensitizers can achieve effective surface coverage of the TiO 2 electrode due to its high adsorption ability onto the TiO 2 electrode, leading to not only the improvement of lightharvesting efficiency, but also the increase in the number of injected electrons in the CB of TiO 2 , which is responsible for the higher V oc value of functionally separated D-π-A dye sensitizers.This work also shows that the functionally separated D-π-A dyes with two functional groups (cyclic azine and carboxyl group) possessing bonding ability to two points on the acid sites (Brønsted acid site and Lewis acid site) on the TiO 2 surface would be expected to be one of the most promising classes of organic dye sensitizers for dye-sensitized solar cells.However, to further improve the photovoltaic performances of dye-sensitized solar cells based on the functionally separated D-π-A dye sensitizers, the extension of the absorption spectrum of functionally separated D-π-A dye sensitizers is necessary; for example, the destabilization of the HOMO level by the introduction of a stronger electron-donating group such as starburst triarylamine and the expansion of the π-conjugated system by the introduction of a long π-bridge such as terthio- phene is one of the most effective strategies to improve the light-harvesting efficiency.

General
Melting points were measured with a Yanaco micro melting point apparatus MP model.TG-DTA was carried out on a SII EXTRA6000 thermal analyser.IR spectra were recorded on a Perkin Elmer Spectrum One FT-IR spectrometer by the ATR method.High-resolution mass spectral data were acquired on a Thermo Fisher Scientific LTQ Orbitrap XL. 1 H NMR spectra were recorded on a Varian-400 (400 MHz) FT NMR spectrometer with tetramethylsilane as an internal standard.Absorption spectra were observed with a Shimadzu UV-3150 spectrophotometer and fluorescence spectra were measured with a HORIBA FluoroMax-4 spectrofluorometer.The fluorescence quantum yields in solution were determined by a HORIBA FluoroMax-4 spectrofluorometer by using a calibrated integrating sphere system (λ ex = 402 nm for OUK-1 and OUK-3).

Fig. 2
Fig. 2 Cyclic voltammogram of OUK-1 (0.1 mM) in DMF containing 0.1 M Bu 4 NClO 4 at a scan rate of 10 mV s −1 .The arrow denotes the direction of the potential scan.
Preparation of DSSCs based on dyes OUK-1 and OUK-3The TiO 2 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 TiO 2 electrode was immersed into a 1 mM dye solution in THF for 15 hours sufficient to adsorb the photosensitizer.The DSSCs were fabricated by using the TiO 2 electrode (0.5 × 0.5 cm 2 in the photoactive area) thus prepared, with 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 the 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 dye-coated film was immersed in a mixed solvent of THF-DMSO-NaOH aq. 1 M (5 : 4 : 1), which was used to determine the amount of dye molecules adsorbed onto the film by measuring the absorbance.The quantification of the dye was made based on the λ max (405 nm for OUK-1 and 402 nm for OUK-3) and the molar extinction coefficient of the dye in the above solution.Absorption spectra of the dyes adsorbed on TiO 2 nanoparticles were recorded on the dye-adsorbed TiO 2 film in the transmission mode with a calibrated integrating sphere system.