Effect of the linkage location in double branched organic dyes on the photovoltaic performance of DSSCs

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/268528688 Effect of the linkage location in double branched organic dyes on the photovoltaic performance of DSSCs Article · November 2014 DOI: 10.1039/C4TA05652C CITATIONS READS 11 108 9 authors, including: Zafar Iqbal Lingyun Wang 21 PUBLICATIONS 184 CITATIONS 117 PUBLICATIONS 1,489 CITATIONS Pakistan Council of Scientific and Industrial R… SEE PROFILE South China University of Technology SEE PROFILE Herbert Meier Derong Cao 306 PUBLICATIONS 4,098 CITATIONS 163 PUBLICATIONS 2,067 CITATIONS Johannes Gutenberg-Universität Mainz SEE PROFILE South China University of Technology SEE PROFILE Some of the authors of this publication are also working on these related projects: synthetic and natural dyes for dye sensitized solar cells View project All content following this page was uploaded by Derong Cao on 11 June 2015. The user has requested enhancement of the downloaded file. Journal of Materials Chemistry A PAPER Cite this: J. Mater. Chem. A, 2015, 3, 1333 Effect of the linkage location in double branched organic dyes on the photovoltaic performance of DSSCs Zu-Sheng Huang,a Cheng Cai,b Xu-Feng Zang,a Zafar Iqbal,c Heping Zeng,a Dai-Bin Kuang,*b Lingyun Wang,a Herbert Meierd and Derong Cao*a Two novel double branched D–p–A organic dyes (DB dyes) are synthesized to investigate the influence of the linkage location in DB dyes on the performance of dye-sensitized solar cells (DSSCs), where phenothiazine is introduced as a donor, thiophene–benzotriazole unit as the p-bridge and cyanoacrylic acid as the electron-acceptor. The photophysical, electrochemical and photovoltaic properties of the dyes are systematically investigated. The results show that the location of the linkage unit has a small effect on the physical and electrochemical properties of the dyes. However, when the dyes are applied in DSSCs, an obvious decline of short-circuit current (Jsc) and open-circuit voltage (Voc) is found by moving the linkage unit from the donor part to the p-bridge part. The DSSC based on the dye DB-D with the linkage unit in the donor obtains an overall power conversion efficiency of 6.13%, which is Received 22nd October 2014 Accepted 19th November 2014 about 68% higher than that (3.65%) of the DSSC based on the dye DB-B with the linkage unit in the p- DOI: 10.1039/c4ta05652c bridge. The DB-B based device exhibits a lower efficiency due to its serious aggregation and short electron lifetime. The results indicate that the linkage location of the dyes has a big effect on the www.rsc.org/MaterialsA performance of the DSSCs. Introduction Dye-sensitized solar cells (DSSCs) currently attract worldwide scientic research attention owing to their easy fabrication and low cost as compared to the traditional silicon-based photovoltaic devices.1 The sensitizer, one of the most important components in DSSC devices, absorbs sunlight and injects electrons into the TiO2 semiconductor. Recently the TiO2 nanowire array based photoelectrode has shown signicant efficiency for DSSCs.2–5 To date, some cells with ruthenium complexes as sensitizers have achieved high power conversion efficiency (PCE) of over 10–11%.2,6–8 Moreover, the zincporphyrin sensitized DSSCs have reached the efficiency of over 12%.9–11 However, concerning the cost and limited ruthenium resources and the purication difficulty for the zinc-porphyrin dyes, metal-free organic dyes have gained more and more attention due to their advantages of high molar extinction coefficients, relatively low cost, ease of structure tuning and a School of Chemistry and Chemical Engineering, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510641, China. E-mail: drcao@scut.edu.cn; Fax: +86 20 87110245; Tel: +86 20 87110245 b MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China. E-mail: kuangdb@mail.sysu. edu.cn; Tel: +86 20 84113015 c Applied Chemistry Research Centre, PCSIR Labs Complex, Lahore 54000, Pakistan d Institute of Organic Chemistry, University of Mainz, Mainz 55099, Germany This journal is © The Royal Society of Chemistry 2015 environmental friendliness.12–15 The metal-free organic sensitizers always contain a donor–p–bridge–acceptor (D–p–A) conguration for facilitating photo-induced charge separation. To date, sensitizers with D–p–A conguration have been extensively explored for DSSCs with promising performances.16–21 It is generally believed that increasing the light harvesting ability of the sensitizer is of great importance to obtain better overall power conversion efficiency. This can be achieved through increasing the molar extinction coefficient, enlarging the absorption region and increasing the loading amount of the dyes on TiO2 lms.22 Recently, some groups have introduced the electron-withdrawing unit benzotriazole as the p-bridge to induce a red-shi of the charge transfer absorption band, improve the photo-to-electricity conversion efficiency and enhance the photo-stability of the solar cell devices.23–25 Also, our group has developed a series of double D–p–A branched organic dyes (DB dyes) with a non-conjugated alkyl linkage to connect the two separate D–p–A segments.26–28 It was found that the double D–p–A branched dye has a larger adsorption amount of the D–p–A segment on the TiO2 lm compared to the single D–p–A dye. Thus the double D–p–A branched dye based cells exhibited better short-circuit current (Jsc), open-circuit voltage (Voc) and PCE. Wang and co-workers synthesized a novel thiophene-bridged double D–p–A dye for efficient DSSCs.29 It is easy to understand that the structure modication of the organic dyes can greatly inuence the performance of the sensitizer J. Mater. Chem. A, 2015, 3, 1333–1344 | 1333 Journal of Materials Chemistry A Paper based DSSCs.30–32 In order to better understand the structure– performance relationship of DB dyes, it is necessary to study the effect of the linkage location on the performance of the DSSCs. It is expected that the linkage location of DB dyes might inuence the geometrical conguration of the dyes on the TiO2 lms, affecting the performance of the device. According to the above analysis, we aimed to construct two novel DB dyes (DB-D and DB-B) with benzotriazole as the pbridge and hexylene chain as the linkage which was linked in the donor part and in the p-bridge part, respectively. In addition, two octyl chains were incorporated at the N-position of the dyes, which not only improve the solubility of the dyes, but also reduce the intermolecular aggregation and restrain the charge recombination on the TiO2 surface. The corresponding single D–p–A branched dye SB-B was also synthesized for comparison. The chemical structures of the three dyes are shown in Fig. 1. Their photophysical, electrochemical and solar cell performances were systematically investigated. Results and discussion Synthesis of the materials The synthesis procedure of SB-B, DB-D and DB-B is outlined in Scheme 1. 3-Bromo-10-octyl-10H-phenothiazine,33 3-bromo10H-phenothiazine,34 4,7-dibromo-2-octyl-2H-benzo[d][1,2,3]triazole,35 and 1,6-bis(2H-benzo[d][1,2,3]triazol-2-yl)hexane36 were synthesized according to the references. 2 was synthesized according to the traditional method: N-alkylation, while the two phenothiazine borates 1 and 3 were synthesized through a Suzuki–Miyaura coupling reaction. The Suzuki coupling reaction of 4,7-dibromo-2-octyl-2H-benzo[d][1,2,3]triazole with thiophen-2-ylboronic acid gave 4. Aldehyde 5 was prepared from 4 by the Vilsmeier–Haack reaction and then converted to 6 through bromination with N-bromosuccinimide (NBS). In the next step, Suzuki coupling of 6 with compounds 1 and 3 gave aldehydes 7 and 8, respectively. Knoevenagel condensation reactions of 7 and 8 with tert-butyl 2-cyanoacetate afforded cyanoacetates, and then the cyanoacetates were hydrolyzed by triuoroacetic acid to give SB-B and DB-D, respectively. The synthesis of dye DB-B with the hexyl chain linked in the pspacer was started from 1,6-bis(2H-benzo[d][1,2,3]triazol-2-yl) hexane. The bromination of 1,6-bis(2H-benzo[d][1,2,3]triazol-2yl)hexane with bromine gave 9, in which the Suzuki coupling reaction of 9 with thiophen-2-ylboronic acid afforded 10 in a good yield. Next, 10 was converted to its cyanoacetate derivative Fig. 1 Chemical structures of SB-B, DB-D and DB-B. 1334 | J. Mater. Chem. A, 2015, 3, 1333–1344 Scheme 1 Synthesis route to SB-B, DB-D and DB-B. 11 through a Vilsmeier–Haack reaction and a Knoevenagel condensation reaction. The bromination of 11 with NBS gave 12, and then the Suzuki coupling reaction of 12 with borate 1 afforded 13. Finally, the nal acid DB-B was achieved via hydrolysis in the presence of triuoroacetic acid. All the new compounds were characterized by 1H NMR, 13C NMR and HRMS. Photophysical properties The UV-Vis absorption and emission spectra of SB-B, DB-D and DB-B both in CH2Cl2/THF (1 : 1) solutions and on TiO2 lms are shown in Fig. 2, and the characteristic data are recorded in Table 1. In the solution UV-Vis spectra, two distinct bands can be found for the three dyes. The rst band at a shorter wavelength is attributed to the p–p* electron transition of the conjugated skeleton, and the other at a longer wavelength is ascribed to the intramolecular charge transfer (ICT) from the donor to acceptor.37 The lmax (absorption maximum wavelength) of SB-B, DB-D and DB-B are 488 (molar extinction coefficient (3) ¼ 41 185 M 1 cm 1), 492 (3 ¼ 88 790 M 1 cm 1) and 492 nm (3 ¼ 79 225 M 1 cm 1), respectively. The lmax of the three dyes are almost the same to each other due to the alike D–p–A conjugated skeleton. The molar extinction coefficients of the three dyes (4.1  104 to 8.9  104 M 1 cm 1) are higher than the well-known sensitizer N719 (1.4  104 M 1 cm 1), indicating a better ability of light harvesting of these three metal-free This journal is © The Royal Society of Chemistry 2015 Paper Journal of Materials Chemistry A In order to understand the aggregation of the three dyes on TiO2 lms, a set of deprotonation experiments were conducted on three dyes in solution (CH2Cl2/THF, v/v ¼ 1 : 1) by addition of an excess amount of triethylamine (TEA). As shown in Fig. 3, the maximum wavelengths of charge transfer (CT) absorption bands of the three dyes are blue-shied to similar 467 nm in comparison with those in solution without TEA, which are induced by the deprotonation effect of TEA. Obviously, the lmax of DB-D with TEA in the solution is similar to that on the TiO2 lm. That is to say, the blue-shi in the CT absorption band of DB-D during the adsorption process on TiO2 should be predominated by the deprotonation effect of the –COOH group rather than the intermolecular aggregation.41 However, the lmax of DB-B on the TiO2 lm shows still blue-shi about 14 nm with respect to that with TEA in the solution. This result means that the blue-shi in the CT absorption band of DB-B during the adsorption process on TiO2 should be ascribed to both the deprotonation effect of the –COOH group and the aggregation effect of dye molecules.41 In fact, DB-B is a relative rigid molecule in comparison with DB-D, resulting in easier aggregation (see below). Electrochemical properties Absorption spectra of SB-B, DB-D and DB-B in DCM/THF solutions (a), on TiO2 films (b), and emission spectra of the dyes in DCM/THF solutions (c). Fig. 2 organic dyes than that of N719.38 Owing to the double D–p–A structure, the 3 of the double branched dyes DB-D and DB-B is nearly twice as high as that of SB-B. Fig. 2b shows the normalized absorption spectra of SB-B, DB-D and DB-B on 16 mm thick TiO2 lms. Upon adsorption on the TiO2 surface, the lmax of SB-B, DB-D and DB-B are blue shied 34, 25 and 39 nm, respectively, as compared to those in solution (Table 1). This is ascribed to the deprotonation of the dyes or formation of H-aggregation on the TiO2 lms.39,40 Table 1 To investigate the possibility of electron injection from the excited state of the organic dyes to the conduction band (CB) of TiO2 and the regeneration of the dyes, the redox behavior was studied by cyclic voltammetry (CV) (Fig. 4, Table 1). The rst oxidation versus normal hydrogen electrode (vs. NHE) was calibrated by Fc/Fc+ (0.63 V vs. NHE) corresponding to the Absorption spectra of SB-B, DB-D and DB-B with triethylamine in DCM/THF solutions. Fig. 3 Photophysical and electrochemical parameters of SB-B, DB-D and DB-B Dye lmax (nm)a/(3 M SB-B DB-D DB-B 488 (41 185) 492 (88 790) 492 (79 225) 1 cm 1) lmax on TiO2 (nm) lb (nm) HOMOc (vs. NHE) (V) 454 467 453 557 564 561 0.89 0.90 0.90 LUMOd (vs. NHE) (V) 1.34 1.30 1.31 E0–0e (eV) 2.23 2.20 2.21 a Absorption maximum of the dyes measured in DCM/THF (1 : 1) with a concentration of 2  10 5 M, 3: molar extinction coefficient at lmax. b l intersection obtained from the cross point of normalized absorption and emission spectra in CH2Cl2/THF (1 : 1) solution. c HOMO of the dyes by cyclic voltammetry in 0.1 M tetrabutylammonium perchlorate in MeCN solutions as the supporting electrolyte, Ag/AgCl as the reference electrode and Pt as the counter electrode, scanning rate: 50 mV s 1. d LUMO was calculated by HOMO E0–0. e E0–0 ¼ 1240/l intersection. This journal is © The Royal Society of Chemistry 2015 J. Mater. Chem. A, 2015, 3, 1333–1344 | 1335 Journal of Materials Chemistry A Paper Adsorption amount Fig. 4 Cyclic voltammograms of SB-B, DB-D and DB-B. highest occupied molecular orbital (HOMO) level of the dye. As shown in Table 1, the HOMO levels of SB-B, DB-D and DB-B are 0.89, 0.90 and 0.90 V, respectively. They are more positive than the redox potential of I /I3 (0.4 V vs. NHE),13 which means that the oxidized dyes can be regenerated effectively. The energy gaps (E0–0) between the HOMO and LUMO level calculated from the intersection points of normalized absorption and emission spectra for SB-B, DB-D and DB-B are 2.23, 2.20 and 2.21 V, respectively. The estimated excited state potentials corresponding to the lowest unoccupied molecular orbital (LUMO) level of the dyes are 1.34 V for SB-B, 1.30 V for DB-D and 1.31 V for DB-B calculated from HOMO E0–0. The LUMO levels of the three dyes are more negative than the conduction band of TiO2 ( 0.5 V vs. NHE),42 demonstrating that the three dyes have sufficient driving force for electron transfer from the excited dye molecule to the TiO2 surface. By combining the information from the photophysical and electrochemical measurements, we thereby summarize that the position of the linkage unit in the double branched dyes has a less pronounced effect on the physical and electrochemical properties. Molecular orbital calculations To gain further insight into the electron distribution of the dyes, the three dyes were optimized by density functional theory (DFT) at the B3LYP/6-31G* level with the Gaussian 03W program package. Optimized structures and the electronic distribution in HOMO and LUMO levels are listed in Table 2. The electron distributions in the HOMOs of the three dyes are largely distributed along the phenothiazine–thiophene–benzotriazole system and the LUMOs are mainly concentrated at the benzotriazole–thiophene–cyanoacrylic acid system. The well overlapped HOMO and LUMO orbits on the benzotriazole unit suggest that the benzotriazole serves as an electron-trap in facilitating electron transfer from the donor to the acceptor/ anchor.23 The HOMO and LUMO electronic states allow the effective electron injection from the dyes to the TiO2 surface excited aer the photoexcitation. 1336 | J. Mater. Chem. A, 2015, 3, 1333–1344 The dye loading amounts of the three dyes on TiO2 lms (16 mm) were obtained by dipping them into 0.1 M aqueous solution of NaOH and THF (1 : 1) and measuring the absorbance of the desorbed dye solutions.28 As listed in Table 3, the dye loading amounts of SB-B, DB-D and DB-B are 3.83  10 7, 2.58  10 7 and 3.02  10 7 mol cm 2, respectively. As we all know, the dye adsorption amounts are directly related to the molecular size of the dyes. The dye SB-B containing the D–p–A structure exhibits the highest loading on the TiO2 lm due to its smallest size. However, the number of light-harvesting units of the double D– p–A branched dyes is higher than the single D–p–A branched molecules (for example, 2  2.58  10 7 for DB-D). In comparison with dyes DB-D and DB-B, it can be found that the dye with the alkyl chain linkage in the p-spacer shows a higher adsorption amount than the dye with the alkyl chain connected in the donor part. This phenomenon suggests that the nonconjugation connector linking at different places of double branched dyes may show different conguration on the TiO2 lm, which will inuence the adsorption amounts of dyes. Fig. 5 presents the plausible schematic illustration of the steric demands of DB-D and DB-B adsorbed on the TiO2 lms. The connector hexylene linked on the p-bridge makes the whole DBB molecule some relatively rigid, like “H” type, requiring smaller space of the surface of the TiO2 lm. DB-D with hexylene linked on the donor position shows less rigid conguration, like “X” type, requiring more space of the surface of the TiO2 lm. In addition, according to the pioneering work,43 a large conguration moiety near the acceptor decreases the adsorbed amount of dyes on the TiO2 lm when compared to the other dyes with a less steric demanding moiety near the acceptor. Thus, in comparison with DB-B, the two octyl chains on DB-D near the acceptor demand more TiO2 surface. The two factors above may cause the higher dye loading amount of DB-B than DB-D. Photovoltaic performance of DSSCs The light harvesting efficiency of the DSSCs sensitized by the three dyes was evaluated by the incident photo-to-current conversion efficiency (IPCE) spectra, as shown in Fig. 6. It can be found that all the three dyes can efficiently convert visible light into photocurrent in the region from 400 to 700 nm. It is easy to nd that DB-D shows a higher IPCE value than the other two dyes. Specically, the IPCE value for the DSSC sensitized by DBD is more than 60% in the region of 430–590 nm and with a maximum IPCE value of 67% at 490 nm, while the IPCE values of SB-B and DB-B reach a maximum 61% at 470 nm and 58% at 470 nm. This result is in good accordance with the Jsc variation tendency obtained in J–V (current–voltage) measurements. IPCE is a product of the electron injection efficiency, light-harvesting efficiency and charge collection efficiency. For a dye-loaded lm with a thickness over 10 mm, light-harvesting efficiency can be considered to be 100%.19 Since DB-D and DB-B have similar HOMO and LUMO energy levels, the different IPCE values for the two dyes may result from their different charge injection efficiencies. Dye DB-B with the connector linked in the p-bridge This journal is © The Royal Society of Chemistry 2015 Paper Table 2 Journal of Materials Chemistry A Optimized structures and electron distributions in HOMO and LUMO levels of SB-B, DB-D and DB-B Dye Optimized structure HOMO LUMO SB-B DB-D DB-B Photovoltaic performance parameters of the DSSCs based on SB-B, DB-D and DB-B Table 3 Dye Jsc (mA cm 2) Voc (mV) h (%) FF Dye loading amount (mol cm 2) SB-B DB-D DB-B 10.60 14.39 9.40 683 718 657 4.32 6.13 3.65 0.60 0.59 0.59 3.83  10 2.58  10 3.02  10 7 7 7 Fig. 6 Fig. 5 Schematic representation of the steric demands of double D– p–A dyes adsorbed on TiO2 films. shows serious intermolecular aggregation, leading to lower charge injection efficiency,44 resulting in a lower IPCE value than DB-D. On the basis of aforementioned photo-physical property analysis, DB-B adsorbed on TiO2 shows larger blueshi appearance than DB-D by comparison of lmax of two dyes in the solution. This journal is © The Royal Society of Chemistry 2015 IPCE spectra of the DSSCs based on SB-B, DB-D and DB-B. The detailed photovoltaic parameters of short-circuit photocurrent (Jsc), open-circuit photovoltage (Voc), ll factor (FF) and overall conversion efficiency (h) are listed in Fig. 7 and Table 3. The DSSC sensitized by DB-D exhibits the highest h of 6.13% with a Jsc of 14.39 mA cm 2, a Voc of 718 mV and a FF of 0.59. The other two devices based on sensitizers SB-B and DB-B show efficiencies of 4.32% and 3.65%, respectively. Apparently, the dye DB-D yields the highest conversion efficiency due to its highest Jsc and Voc. In comparison with single D–p–A branched dye SB-B, the efficiency of double D–p–A branched dye DB-D increases 42% due to its larger amount of light-harvesting units J. Mater. Chem. A, 2015, 3, 1333–1344 | 1337 Journal of Materials Chemistry A Paper Fig. 7 J–V curves of the DSSCs based on SB-B, DB-D and DB-B. and nearly no aggregation on the TiO2 lm. However, it is worth noting that the efficiency of the double D–p–A branched dye DBB with the exible alkyl chain connected in the p-spacer is even lower than SB-B. In particular, the low Jsc for DB-B can be mainly correlated to the serious aggregation. This aggregation has a deleterious effect on electron transfer from the donor to the TiO2 lm, leading to lower electron injection efficiency.45 On the other hand, the lowest value of Voc of DB-B can be attributed to the recombination of the injected electron with I /I3 in the electrolyte on the TiO2 surface and the larger extent of aggregation. These results may suggest that the connector alkyl chain should be linked in the donor part when used in the construction of non-conjugated double D–p–A branched dyes. It has been veried that the transparent organic compound CDCA (chenodeoxycholic acid) can hinder the formation of dye aggregates, then enhance the performance of the DSSCs.46,47 Thus, the performances of the DSSCs in the presence of CDCA with various concentrations were studied. The results are summarized in Table 4, and the corresponding J–V curves are shown in Fig. 8. A concentration of 1 mM CDCA did not efficiently improve the performance of the device based on DB-D. However, when the concentration increased to 10 mM, the power conversion efficiency decreased. A possible explanation is that the amount of dye adsorbed on the TiO2 lm was reduced Table 4 Photovoltaic performance parameters of the DSSCs based on SB-B, DB-D and DB-B with CDCA Dye CDCA Jsc (mA cm 2) Voc (mV) h (%) FF SB-B 0 mM 1 mM 10 mM 0 mM 1 mM 10 mM 0 mM 1 mM 10 mM 10.60 11.26 12.03 14.39 13.37 13.37 9.40 13.33 13.30 683 706 715 718 718 721 657 691 695 4.32 5.30 5.53 6.13 6.21 5.98 3.65 5.88 5.79 0.60 0.67 0.64 0.59 0.65 0.62 0.59 0.64 0.63 DB-D DB-B 1338 | J. Mater. Chem. A, 2015, 3, 1333–1344 Fig. 8 J–V curves of the DSSCs based on SB-B, DB-D and DB-B with 1 mM CDCA (a) and with 10 mM CDCA (b). by the co-adsorption of CDCA, resulting in a loss of active lightharvesting.26 However, upon co-adsorption with CDCA, the Jsc and Voc values of the devices based on SB-B and DB-B were improved remarkably, which can be attributed to the reduction of dye aggregation. These results indicate that DB-D with the alkyl chain linked in the donor part nearly did not aggregate on the TiO2 surface. However, the connector alkyl chain linking at the p-spacer position of DB-B causes serious aggregation. A reason is that the conguration of DB-B on the lm of TiO2 shows some relatively rigid like “H” type, leading to easier aggregation. Another reason is that the long alkyl chain introduced in the donor part cannot prevent aggregation effectively.48 Thus, in comparison with DB-D, the serious aggregation of DB-B might be attributed to both effects of octyl chain linking in the donor part and the hexylene connector linking in the pbridge part. To further understand the electron recombination in DSSCs, electrochemical impedance spectroscopy (EIS) was carried out in the dark. The Nyquist plots for the DSSCs based on the dyes without CDCA are displayed in Fig. 9, and the corresponding parameters are listed in Table 5. The rst semicircle (Rce) is assigned to charge transfer at the counter electrode/electrolyte interface, while the second semicircle (Rrec) is accorded to charge transfer at the TiO2/dye/electrolyte interface.49 The Rce values of the three dyes are similar to each other due to the use of the same counter electrode and electrolyte. The Rrec values are 138.8, 115.5 and 143.1 U cm 2, respectively. The This journal is © The Royal Society of Chemistry 2015 Paper Journal of Materials Chemistry A and thus efficiently improve the short-circuit photocurrent and the open-circuit photovoltage, leading to a higher photoelectric conversion efficiency of the DSSC. The work will pave a way for further improvement of the double D–p–A branched based organic sensitizers in the future. Experimental Materials and instruments Electrochemical impedance spectra (Nyquist plot) of the DSSCs measured in the dark. Fig. 9 Table 5 Parameters obtained by fitting the impedance spectra of the DSSCs Dye Rrec (U cm 2) CPE2 (mF) sr (ms) SB-B DB-D DB-B 138.8 115.5 143.1 879 1269 814 122 147 116 corresponding electron lifetimes for SB-B, DB-D and DB-B are 122, 147 and 116 ms, respectively, which are calculated by tting the equation sr ¼ Rrec  CPE2 (CPE2, chemical capacitance).50 The electron lifetime trend in order of DB-B < SB-B < DB-D is in good accordance with the Voc results above. This result suggests that the recombination of the injected electron with I /I3 in the electrolyte can be more effectively suppressed by introducing the connector on the donor position in the double D–p–A branched dyes. Conclusions In summary, two novel benzotriazole based double D–p–A branched organic dyes (DB-D and DB-B) with the hexylene connector linked in the donor and the p-spacer position, respectively, were synthesized and applied in DSSCs. The results indicate that the linkage location of DB-D and DB-B has a small effect on the physical and electrochemical properties, but a signicant impact on the dye adsorption amount, dye aggregation degree and light harvesting ability, thus leading to distinctive photovoltaic performance. In comparison with the PCE (4.32%) of the DSSC based on the single D–p–A branched analogue dye SB-B, the PCE (6.13%) of the DSSC with DB-D is enhanced to about 42%. However, the PCE (3.65%) of the DSSC with DB-B is dropped to about 16%. The hexylene chain connector linking at the p-spacer shows serious intermolecular aggregation, leading to lower electron injection efficiency and short electron lifetime. However, the hexylene chain connector linking at the donor-position can signicantly restrain intermolecular aggregation and suppress the charge recombination, This journal is © The Royal Society of Chemistry 2015 All reagents were purchased from Adamas, Aladdin and J&K in analytical grade. THF, dioxane and toluene were distilled over sodium. 1,2-Dichloroethane and DMF were dried over a 4Å molecular sieve before use. Other solvents and reagents were of analytical grade and used without further purication. All reactions were performed under an argon atmosphere and monitored by TLC. Chromatographic separations were carried out on silica gel (200–300 or 300–400 mesh). 1 H and 13C NMR spectra were recorded on a Bruker 400 MHz instrument in CDCl3, DMSO-d6 and THF-d8. HRMS spectra were recorded on an Agilent Technologies 1290 Innity mass spectrophotometer. The melting point was measured on a SGW X-4B microscopic melting point apparatus. Ultraviolet-visible (UV-Vis) spectra of the dyes in solution were recorded on a Shimadzu UV2450 spectrophotometer. The PL spectra were recorded on a Fluorolog III photoluminescence spectrometer. The absorption spectra of the dyes adsorbed on TiO2 lms were recorded on a UV-3010 spectrophotometer. Electrochemical redox potentials were obtained by cyclic voltammetry (CV) using a three electrode cell on an electrochemistry workstation (e-corder (ED 401) potentiostat) at a scan rate of 50 mV s 1. The dye adsorbed TiO2 lms were used as working electrodes. Ag/AgCl (3 M in KCl) was used as a reference electrode and a platinum wire was utilized as the counter electrode. Tetrabutylammoniumhexauorophosphate (TBAPF6, 0.1 M in dry acetonitrile) was used as the supporting electrolyte. The ferrocene/ ferrocenium (Fc/Fc+) redox couple acted as an internal potential reference. The electrochemical redox potentials of dyes versus NHE were calibrated by the addition of 0.63 V to the potentials versus (Fc/ Fc+). The incident monochromatic photon-to-current conversion efficiency (IPCE) spectra were recorded on a Spectral Products DK240 monochromator from the 400 to 800 nm region. The current–voltage characteristics were recorded on a Keithley 2400 source meter under simulated AM 1.5G (100 mV cm 2) illumination with a standard solar light simulator (Oriel, Model: 91192). The electrochemical impedance spectra (EIS) were recorded on a Zahner Zennium electrochemical workstation under dark conditions. Synthesis of dyes 10-Octyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-10Hphenothiazine (1). To a mixture of 3-bromo-10-octyl-10Hphenothiazine (1.17 g, 3 mmol), 4,4,40 ,40 ,5,5,50 ,50 -octamethyl2,20 -bi(1,3,2-dioxaborolane) (1.52 g, 6 mmol), and KOAc (588 mg, 6 mmol) in dioxane, Pd(dppf)Cl2 (150 mg, 0.2 mmol) was added. The reaction mixture was stirred under an argon atmosphere at 100  C for 24 h. Aer cooling to room temperature, the solvent was removed under reduced pressure. The residue was puried by column chromatography on silica gel (petroleum J. Mater. Chem. A, 2015, 3, 1333–1344 | 1339 Journal of Materials Chemistry A ether/ethyl acetate, v/v ¼ 20 : 1) to give 1 as a light yellow liquid in 77% yield (1.01 g). 1H NMR (400 MHz, DMSO-d6) d 7.49–7.47 (m, 1H), 7.33 (s, 1H), 7.21–7.17 (m, 1H), 7.14–7.12 (m, 1H), 7.01– 6.92 (m, 3H), 3.88–3.85 (t, J ¼ 6.1 Hz, 2H), 1.67–1.64 (m, 2H), 1.35 (m, 2H), 1.26 (s, 12H), 1.18 (m, 8H), 0.81–0.79 (t, J ¼ 6.8 Hz, 3H). 13C NMR (100 MHz, DMSO-d6) d 147.4, 144.1, 134.2, 132.9, 127.6, 127.1, 123.4, 122.9, 122.7, 116.0, 115.3, 83.5, 46.4, 31.0, 28.5, 28.4, 26.0, 25.9, 24.6, 22.0, 13.9. HRMS (ESI, m/z): [M + H]+ calcd for C26H37BNO2S: 438.2637, found: 438.2640. 1,6-Bis(3-bromo-10H-phenothiazin-10-yl)hexane (2). 3-Bromo-10H-phenothiazine (2.78 g, 10 mmol) and potassium hydroxide (1.68 g, 30 mmol) were added in DMSO (25 mL) under an argon atmosphere. The reaction mixture was stirred for 30 min, and then 1,6-dibromohexane (0.77 mL, 5 mmol) was added. The mixture was stirred for another 48 h at room temperature. Aer completion of the reaction, the reaction mixture was poured into ice-water, ltered and washed with water three times. The collected solid was then dissolved in CH2Cl2, dried over MgSO4. Aer evaporation of the solvent under reduced pressure, the residue was puried by column chromatography on a silica gel column with acetone/petroleum ether (v/v ¼ 1/30) as the eluent. 2 was obtained as a white solid in 86% yield (2.74 g), mp 125–127  C. 1H NMR (400 MHz, CDCl3) d 7.22–7.20 (m, 4H), 7.16–7.09 (m, 4H), 6.93–6.90 (m, 2H), 6.81– 6.79 (m, 2H), 6.65–6.63 (m, 2H), 3.77 (t, J ¼ 6.8 Hz, 4H), 1.74 (m, 4H), 1.43–1.40 (m, 4H). 13C NMR (100 MHz, CDCl3) d 145.0, 144.5, 129.9, 129.7, 127.6, 127.5, 127.4, 124.4, 122.7, 116.6, 115.6, 114.5, 47.1, 26.6, 26.3. HRMS (ESI, m/z): [M + Na]+ calcd for C30H2679Br79Br N2NaS2: 658.9796, found: 658.9796; [M + Na]+ calcd for C30H2679Br81Br N2NaS2: 660.9777, found: 660.9775. 1,6-Bis(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-10Hphenothiazin-10-yl)hexane (3). The synthesis procedure for 3 was similar to that of 1. The faint yellow solid of 3 was obtained with the yield of 65%, mp 102–104  C. 1H NMR (400 MHz, CDCl3) d 7.57–7.54 (m, 4H), 7.14–7.06 (m, 4H), 6.90–6.87 (m, 2H), 6.80–6.78 (m, 4H), 3.81 (t, J ¼ 6.8 Hz, 4H), 1.76–1.74 (m, 4H), 1.42 (m, 4H), 1.31 (s, 24H). 13C NMR (100 MHz, CDCl3) d 147.9, 144.7, 134.1, 133.9, 127.5, 127.1, 125.0, 124.2, 122.6, 115.5, 114.7, 83.7, 47.1, 26.7, 26.4, 24.9. HRMS (ESI, m/z): [M + H]+ calcd for C42H51B2N2O4S2: 733.3485, found: 733.3494. 2-Octyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole (4). To a mixture of 4,7-dibromo-2-octyl-2H-benzo[d][1,2,3]triazole (1.43 g, 3.67 mmol), K2CO3 aqueous solution (2 M, 11 mL) and thiophen-2-ylboronic acid (1.127 g, 8.81 mmol) in THF (50 mL), Pd(PPh3)4 (211 mg, 0.18 mmol) was added. The reaction mixture was stirred for 24 h under an argon atmosphere at 70  C. Aer cooling to room temperature, the reaction was quenched by water, extracted with CH2Cl2 and dried over MgSO4. Aer evaporation of the solvent under reduced pressure, the residue was puried by column chromatography on a silica gel column with ethyl acetate/petroleum ether (v/v ¼ 1 : 20) as the eluent. 4 was obtained as a light yellow solid in 84% yield (1.25 g), mp 74– 76  C. 1H NMR (400 MHz, CDCl3) d 8.09–8.08 (m, 2H), 7.61 (s, 2H), 7.37–7.36 (m, 2H), 7.19–7.17 (m, 2H), 4.80 (t, J ¼ 7.2 Hz, 2H), 2.22–2.15 (m, 2H), 1.45–1.34 (m, 4H), 1.33–1.27 (m, 6H), 0.88–0.85 (t, J ¼ 6.0 Hz, 3H). 13C NMR (100 MHz, CDCl3) d 142.1, 1340 | J. Mater. Chem. A, 2015, 3, 1333–1344 Paper 140.0, 128.1, 127.0, 125.5, 123.6, 122.8, 56.9, 31.8, 30.1, 29.1, 29.0, 26.6, 22.6, 14.1. HRMS (ESI, m/z): [M + Na]+ calcd for C22H25N3NaS2: 418.1382, found: 418.1376. 5-(2-Octyl-7-(thiophen-2-yl)-2H-benzo[d][1,2,3]triazol-4-yl) thiophene-2-carbaldehyde (5). To a solution of 4 (1.1 g, 2.7 mmol) and dry DMF (1.0 mL, 13.5 mmol) in 1,2-dichloroethane (40 mL), POCl3 (1.3 mL, 13.5 mmol) was added slowly over 30 min at 0  C under an argon atmosphere. Then the bath was heated to 70  C and maintained for 7 h. Aer cooling to room temperature, dilute aqueous solution of sodium hydroxide was added, and the mixture was extracted with CH2Cl2 three times. The combined organic fractions were washed with brine and dried over MgSO4. The solvent was removed under reduced pressure, and the residue was puried by using silica gel column chromatography with ethyl acetate/petroleum ether (v/v ¼ 1 : 15) as the eluent. 5 was obtained as a yellow solid in 94% yield (1.08 g), mp 88–90  C. 1H NMR (400 MHz, CDCl3) d 9.95 (s, 1H), 8.15–8.14 (m, 2H), 7.82–7.81 (m, 1H), 7.76–7.74 (m, 1H), 7.66–7.65 (m, 1H), 7.43–7.42 (m, 1H), 7.21–7.19 (m, 1H), 4.82 (t, J ¼ 7.2 Hz, 2H), 2.26–2.12 (m, 2H), 1.45–1.38 (m, 4H), 1.34–1.27 (m, 6H), 0.88–0.85 (t, J ¼ 6.8 Hz, 3H). 13C NMR (100 MHz, CDCl3) d 183.0, 149.6, 142.5, 142.1, 142.0, 139.5, 137.2, 128.2, 127.9, 127.4, 126.5, 125.9, 124.4, 122.4, 121.9, 57.0, 31.8, 30.1, 29.1, 29.0, 26.6, 22.6, 14.1. HRMS (ESI, m/z): [M + Na]+ calcd for C23H25N3NaOS2: 446.1331, found: 446.1325. 5-(7-(5-Bromothiophen-2-yl)-2-octyl-2H-benzo[d][1,2,3]triazol-4-yl)thiophene-2-carbaldehyde (6). To a solution of 5 (212 mg, 0.5 mmol) in THF (10 mL), NBS (267 mg, 1.5 mmol) was added in one portion. Aer that the reaction mixture was stirred for 12 h at room temperature, and then water was added to quench the reaction. The reaction mixture was extracted with CH2Cl2 three times. The combined organic solution was washed with brine and dried over MgSO4. Aer removal of the solvent under reduced pressure, the residue was puried by silica gel column chromatography with ethyl acetate/petroleum ether (v/v ¼ 1 : 10) as the eluent. 6 was obtained as a yellow solid in 80% yield (201 mg), mp 103–105  C. 1H NMR (400 MHz, CDCl3) d 9.95 (s, 1H), 8.15–8.14 (m, 1H), 7.83–7.80 (m, 2H), 7.72 (d, J ¼ 7.6 Hz, 1H), 7.54 (d, J ¼ 7.6 Hz, 1H), 7.14–7.13 (m, 1H), 4.81 (t, J ¼ 7.2 Hz, 2H), 2.23–2.16 (m, 2H), 1.42–1.40 (m, 4H), 1.30–1.28 (m, 6H), 0.88–0.85 (m, 3H). 13C NMR (100 MHz, CDCl3) d 182.9, 149.3, 142.7, 142.0, 141.7, 140.9, 137.2, 131.0, 127.6, 127.5, 124.9, 124.2, 122.3, 121.9, 114.1, 57.1, 31.7, 30.1, 29.1, 29.0, 26.6, 22.6, 14.1. HRMS (ESI, m/z): [M + Na]+ calcd for C23H2479BrN3NaOS2: 524.0436, found: 524.0428; [M + Na]+ calcd for C23H2481BrN3NaOS2: 526.0417, found: 526.0410. 5-(2-Octyl-7-(5-(10-octyl-10H-phenothiazin-3-yl)thiophen-2-yl)2H-benzo[d][1,2,3]triazol-4-yl)thiophene-2-carbaldehyde (7). To a mixture of 1 (157 mg, 0.36 mmol), 6 (120 mg, 0.24 mmol), and K2CO3 aqueous solution (2 M, 0.36 mL) in THF (15 mL), Pd(PPh3)4 (28 mg, 0.024 mmol) was added. The reaction mixture was stirred under an argon atmosphere at 70  C for 18 h. Aer cooling to room temperature, the reaction was quenched by water, extracted with CH2Cl2 and dried over MgSO4. Aer removal of the solvent under reduced pressure, the residue was puried by column chromatography on a silica gel column with ethyl acetate/petroleum ether (v/v ¼ 1 : 10) as the eluent. 7 was This journal is © The Royal Society of Chemistry 2015 Paper obtained as a red solid in 74% yield (130 mg), mp 77–79  C. 1H NMR (400 MHz, CDCl3) d 9.94 (s, 1H), 8.15 (d, J ¼ 4.0 Hz, 1H), 8.10–8.09 (m, 1H), 7.81 (d, J ¼ 4.0 Hz, 1H), 7.76–7.74 (m, 1H), 7.64–7.62 (m, 1H), 7.46–7.44 (m, 2H), 7.28–7.27 (m, 1H), 7.18– 7.14 (m, 2H), 6.95–6.91 (m, 1H), 6.88–6.85 (m, 2H), 4.84 (t, J ¼ 7.2 Hz, 2H), 3.86 (t, J ¼ 7.2 Hz, 2H), 2.25–2.18 (m, 2H), 1.86–1.79 (m, 2H), 1.46–1.43 (m, 6H), 1.33–1.26 (m, 14H), 0.88–0.85 (m, 6H). 13C NMR (100 MHz, CDCl3) d 182.9, 149.7, 144.8, 144.7, 144.4, 142.3, 142.0, 141.7, 137.8, 137.3, 129.1, 128.5, 127.5, 127.4, 127.2, 125.8, 125.2, 124.8, 124.3, 124.3, 124.0, 123.3, 122.6, 121.8, 121.5, 115.4, 115.4, 57.0, 47.6, 31.8, 31.8, 30.1, 29.3, 29.3, 29.2, 29.1, 27.0, 26.9, 26.7, 22.7, 22.7, 14.2, 14.1. HRMS (APCI, m/z): [M + H]+ calcd for C43H49N4OS3: 733.3063, found: 733.3046. 5,5 0 -(7,70 -(5,50 -(10,100 -(Hexane-1,6-diyl)bis(10H-phenothiazine-10,3-diyl))bis(thiophene-5,2-diyl))bis(2-octyl-2H-benzo[d] [1,2,3]triazole-7,4-diyl))bis(thiophene-2-carbaldehyde) (8). To a mixture of 3 (79 mg, 0.11 mmol), 6 (164 mg, 0.33 mmol) and 2 M aqueous K2CO3 solution (0.33 mL) in THF (15 mL), Pd(PPh3)4 (25 mg, 0.022 mmol) was added. The reaction mixture was stirred under an argon atmosphere at 70  C for 18 h. The puried procedure was similar to 7, and CH2Cl2 was used as the eluent. 8 was obtained as a deep red color solid in 44% yield (64 mg), mp 115–117  C. 1H NMR (400 MHz, CDCl3) d 9.92 (s, 2H), 8.08–8.07 (m, 2H), 8.03–8.02 (m, 2H), 7.76–7.75 (m, 2H), 7.64– 7.61 (m, 2H), 7.51–7.48 (m, 2H), 7.41–7.38 (m, 4H), 7.22–7.21 (m, 2H), 7.17–7.13 (m, 4H), 6.95–6.91 (m, 2H), 6.86–6.84 (m, 2H), 6.81–6.78 (m, 2H), 4.81–4.78 (m, 4H), 3.87–3.84 (m, 4H), 2.22–2.15 (m, 4H), 1.82–1.81 (m, 4H), 1.51 (m, 4H), 1.42–1.27 (m, 20H), 0.88–0.85 (m, 6H). 13C NMR (100 MHz, CDCl3) d 182.9, 149.7, 144.8, 144.4, 142.3, 142.0, 141.7, 137.9, 137.2, 129.1, 128.6, 127.5, 127.4, 127.2, 125.8, 125.4, 124.8, 124.5, 124.3, 123.4, 122.7, 121.8, 121.5, 115.5, 57.0, 47.2, 31.8, 30.1, 29.1, 29.0, 26.6, 26.5, 26.2, 22.6, 14.1. HRMS (ESI, m/z): [M + Na]+ calcd for C76H74N8NaO2S6: 1345.4151, found: 1345.4147. 1,6-Bis(4,7-dibromo-2H-benzo[d][1,2,3]triazol-2-yl)hexane (9). 1,6-Bis(2H-benzo[d][1,2,3]triazol-2-yl)hexane (480 mg, 1.5 mmol) and an aqueous HBr solution (15 mL) were added to a two-necked, round-bottom ask. The mixture was stirred for 1 h at 100  C. Then, bromine (0.61 mL, 12 mmol) was added dropwise, and the reaction was carried out under vigorous stirring for another 12 h at 135  C. Aer cooling to room temperature, the reaction mixture was poured into an aqueous solution of sodium hydroxide. The product was extracted with chloroform, and the solvent was removed under reduced pressure. The crude product was puried by silica gel column chromatography with CH2Cl2/petroleum ether (v/v ¼ 2 : 1) as the eluent. 9 was obtained as a white solid in 52% yield, mp > 300  C. 1H NMR (400 MHz, CDCl3) d 7.45 (s, 4H), 4.78 (t, J ¼ 7.2 Hz, 4H), 2.20–2.13 (m, 4H), 1.48–1.44 (m, 4H). 13C NMR (100 MHz, CDCl3) d 143.8, 129.6, 110.0, 57.1, 29.8, 25.9. HRMS (ESI, m/z): [M + Na]+ calcd for C18H1679Br79Br79Br79BrN6Na: 654.8062, found: 654.8060; [M + Na]+ calcd for C18H1679Br81Br79Br79BrN6Na: 656.8042, found: 656.8039; [M + Na]+ calcd for C18H1679Br81Br81Br79BrN6Na: 658.8022, found: 658.8017; [M + Na]+ calcd for C18H1679Br81Br81Br81Br N6Na: 660.8003, found: 660.8006. This journal is © The Royal Society of Chemistry 2015 Journal of Materials Chemistry A 1,6-Bis(4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazol-2-yl) hexane (10). To a mixture of 9 (480 mg, 0.75 mmol), 2 M aqueous K2CO3 solution (4.5 mL) and thiophen-2-ylboronic acid (580 mg, 4.5 mmol) in THF (20 mL), Pd(PPh3)4 (166 mg) was added. The reaction was carried out in a similar manner as 4. The crude product was chromatographed on a silica gel column with CH2Cl2/ petroleum ether (v/v ¼ 1 : 1) as the eluent. 10 was obtained as a yellow solid in 60% yield (294 mg), mp 171–173  C. 1H NMR (400 MHz, CDCl3) d 8.10–8.09 (m, 4H), 7.65 (s, 4H), 7.39–7.38 (m, 4H), 7.20–7.18 (m, 4H), 4.85 (t, J ¼ 7.1 Hz, 4H), 2.30–2.23 (m, 4H), 1.61– 1.58 (m, 4H). 13C NMR (100 MHz, CDCl3) d 142.1, 139.9, 128.1, 126.9, 125.6, 123.6, 122.8, 56.6, 29.8, 26.1. HRMS (ESI, m/z): [M + Na]+ calcd for C34H28N6NaS4: 671.1150, found: 671.1141. (2E,20 E)-Di-tert-butyl 3,30 -(5,50 -(2,20 -(hexane-1,6-diyl)bis(7(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole-4,2-diyl))bis(thiophene-5,2-diyl))bis(2-cyanoacrylate) (11). To a solution of 10 (649 mg, 1 mmol) and dry DMF (0.77 mL, 10 mmol) in 1,2dichloroethane (50 mL), POCl3 (0.93 mL, 10 mmol) was added slowly over 30 min at 0  C under an argon atmosphere. Then the bath was heated to 75  C and maintained for 24 h. Aer cooling to room temperature, the mixture was poured into an aqueous solution of sodium hydroxide. The crude product was extracted with CHCl3. Aer the solvent was removed under reduced pressure, the residue was used for the next Knoevenagel reaction without purication. The unpuried mixture was reacted with tert-butyl 2-cyanoacetate (847 mg, 6 mmol), ammonium acetate (462 mg, 6 mmol), and acetic acid (2 mL) in toluene (50 mL) under an argon atmosphere at 130  C for 3 h. Aer cooling to room temperature, water was added and the mixture was extracted with CH2Cl2. The combined organic layers were dried over MgSO4 and evaporated under reduced pressure. The crude product was puried by silica gel column chromatography with CH2Cl2 as the eluent. 11 was obtained as a red solid in 54% yield for two steps (513 mg), mp 192–194  C. 1H NMR (400 MHz, CDCl3) d 8.19 (s, 2H), 8.12–8.09 (m, 4H), 7.76–7.72 (m, 4H), 7.62– 7.60 (m, 2H), 7.41–7.40 (m, 2H), 7.17–7.15 (m, 2H), 4.82 (t, J ¼ 6.9 Hz, 4H), 2.28–2.22 (m, 4H), 1.62 (m, 4H), 1.59 (s, 18H). 13C NMR (100 MHz, CDCl3) d 161.9, 149.2, 145.5, 142.0, 141.9, 139.4, 138.2, 135.3, 128.3, 127.9, 127.8, 126.6, 125.9, 124.6, 122.5, 121.6, 116.3, 99.8, 83.4, 56.8, 29.5, 28.1, 26.0. HRMS (ESI, m/z): [M + Na]+ calcd for C50H46N8NaO4S4: 973.2417, found: 973.2411. (2E,20 E)-Di-tert-butyl 3,30 -(5,50 -(2,20 -(hexane-1,6-diyl)bis(7-(5bromothiophen-2-yl)-2H-benzo[d][1,2,3]triazole-4,2-diyl))bis(thiophene-5,2-diyl))bis(2-cyanoacrylate) (12). To a solution of 11 (100 mg, 0.105 mmol) in THF (25 mL), NBS (112 mg, 0.635 mmol) was added. Aer that the reaction mixture was stirred for 36 h at room temperature and water was added to quench the reaction. The solution was extracted with CH2Cl2 for three times, the combined organic solution was washed with brine and dried over MgSO4. Aer removal of the solvent under reduced pressure, the residue was puried by silica gel column chromatography with CH2Cl2 as the eluent. 12 was obtained as a red solid in 90% yield (105 mg), mp 181–183  C. 1H NMR (400 MHz, CDCl3) d 8.18 (s, 2H), 8.06–8.05 (m, 2H), 7.74–7.72 (m, 4H), 7.65–7.62 (m, 2H), 7.47–7.44 (m, 2H), 7.07–7.06 (m, 2H), 4.79 (t, J ¼ 6.8 Hz, 4H), 2.23–2.21 (m, 4H), 1.59 (s, 18H), 1.52 (m, 4H). 13C NMR (100 MHz, CDCl3) d 161.8, 148.9, J. Mater. Chem. A, 2015, 3, 1333–1344 | 1341 Journal of Materials Chemistry A 145.4, 141.8, 141.5, 140.8, 138.2, 135.5, 131.0, 127.9, 127.6, 124.7, 124.4, 122.0, 122.0, 116.3, 114.2, 100.0, 83.4, 56.8, 29.4, 28.1, 26.0. HRMS (ESI, m/z): [M + Na]+ calcd for C50H4479Br79Br N8NaO4S4: 1129.0627, found: 1129.0641; [M + Na]+ calcd for C50H4479Br81Br N8NaO4S4: 1131.0631, found: 1131.0624. (2E,20 E)-Di-tert-butyl 3,30 -(5,50 -(2,20 -(hexane-1,6-diyl)bis(7-(5(10-octyl-10H-phenothiazin-3-yl)thiophen-2-yl)-2H-benzo[d] [1,2,3]triazole-4,2-diyl))bis(thiophene-5,2-diyl))bis(2-cyanoacrylate) (13). To a mixture of 1 (124 mg, 0.285 mmol), 12 (105 mg, 0.095 mmol) and 2 M aqueous K2CO3 solution (0.29 mL) in THF, Pd(PPh3)4 (23 mg, 0.02 mmol) was added. The reaction was carried out in a similar manner as the preparation of 7. The crude product was chromatographed on a silica gel column with CH2Cl2 as the eluent. 13 was obtained as a deep red solid in 73% yield (108 mg), mp 95–97  C. 1H NMR (400 MHz, CDCl3) d 8.13 (s, 2H), 8.02–8.01 (m, 2H), 7.92–7.91 (m, 2H), 7.65–7.64 (m, 2H), 7.57 (d, J ¼ 7.7 Hz, 2H), 7.43 (d, J ¼ 7.7 Hz, 2H), 7.34–7.32 (m, 4H), 7.18–7.11 (m, 6H), 6.94–6.90 (m, 2H), 6.85–6.83 (m, 2H), 6.76–6.74 (m, 2H), 4.79 (t, J ¼ 6.8 Hz, 4H), 3.79 (t, J ¼ 7.1 Hz, 4H), 2.25–2.23 (m, 4H), 1.82–1.74 (m, 4H), 1.58 (s, 18H), 1.41–1.38 (m, 4H), 1.30–1.26 (m, 20H), 0.87 (t, J ¼ 6.7 Hz, 6H). 13C NMR (100 MHz, CDCl3) d 161.9, 149.4, 145.5, 144.7, 144.7, 144.5, 141.8, 141.6, 138.4, 137.7, 135.1, 129.0, 128.4, 127.7, 127.4, 127.4, 125.7, 125.1, 124.7, 124.5, 124.2, 124.0, 123.2, 122.5, 121.8, 121.1, 116.4, 115.4, 115.3, 99.5, 83.3, 56.7, 47.6, 31.8, 29.5, 29.3, 29.3, 28.1, 27.0, 26.8, 26.0, 22.7, 14.1. HRMS (ESI, m/z): [M + H]+ calcd for C90H93N10O4S6: 1569.5700, found: 1569.5718. (E)-2-Cyano-3-(5-(2-octyl-7-(5-(10-octyl-10H-phenothiazin-3-yl) thiophen-2-yl)-2H-benzo[d][1,2,3]triazol-4-yl)thiophen-2-yl) acrylic acid (SB-B). A mixture of 7 (117 mg, 0.16 mmol), tertbutyl 2-cyanoacetate (67.5 mg, 0.48 mmol), ammonium acetate (37 mg, 0.48 mmol) and acetic acid (2 mL) in toluene was stirred under an argon atmosphere at 130  C for 3 h. Aer cooling to room temperature, the reaction was quenched by water and the mixture was extracted with CH2Cl2. The organic layers were dried over MgSO4 and evaporated by reduced pressure. The residue was puried by column chromatography on silica gel (ethyl acetate/petroleum ether, v/v ¼ 1 : 10) to give a black solid. The resulting black solid was dissolved in triuoroacetic acid (15 mL), and stirred at room temperature for 4 h. Aer that deionized water (100 mL) was added, and the resulting black solid SB-B was collected by ltration. The black solid was then washed with pure water (100 mL) three times and the nal product was obtained. The yield for the two steps is about 57% (72.9 mg), mp 179–181  C. 1H NMR (400 MHz, DMSO-d6) d 8.44 (s, 1H), 8.20–8.19 (m, 1H), 8.10–8.09 (m, 1H), 8.02–8.01 (m, 1H), 7.92–7.90 (m, 1H), 7.78–7.76 (m, 1H), 7.54–7.49 (m, 3H), 7.23–7.20 (m, 1H), 7.17– 7.15 (m, 1H), 7.03–6.95 (m, 3H), 4.87 (d, J ¼ 6.4 Hz, 2H), 3.87 (d, J ¼ 6.0 Hz, 2H), 2.15–2.09 (m, 2H), 1.72–1.65 (m, 2H), 1.37– 1.21 (m, 20H), 0.83–0.77 (m, 6H). 13C NMR (100 MHz, DMSOd6) d 163.5, 147.2, 145.0, 144.3, 144.1, 143.8, 141.1, 140.8, 139.3, 136.9, 135.7, 129.0, 127.7, 127.7, 127.4, 127.1, 124.7, 124.5, 124.3, 124.3, 123.9, 123.4, 122.8, 122.6, 121.9, 120.8, 117.0, 115.9, 115.8, 100.7, 56.4, 46.5, 31.1, 31.1, 29.1, 28.6, 28.5, 28.5, 28.3, 26.1, 26.0, 25.9, 22.0, 22.0, 13.9, 13.8. HRMS (ESI, m/z): [M]+ calcd for C46H49N5O2S3: 799.3043, found: 799.3025. 1342 | J. Mater. Chem. A, 2015, 3, 1333–1344 Paper (2E,20 E)-3,30 -(5,50 -(7,70 -(5,50 -(10,100 -(Hexane-1,6-diyl)bis(10Hphenothiazine-10,3-diyl))bis(thiophene-5,2-diyl))bis(2-octyl-2Hbenzo[d][1,2,3]triazole-7,4-diyl))bis(thiophene-5,2-diyl))bis(2cyanoacrylic acid) (DB-D). DB-D was synthesized according to the same procedure as that of SB-B as a black solid, mp 190–192  C. 1H NMR (400 MHz, THF-d8) d 8.35 (s, 2H), 8.20–8.19 (m, 2H), 8.08 (d, J ¼ 3.8 Hz, 2H), 7.89–7.88 (m, 2H), 7.74–7.72 (m, 2H), 7.58–7.56 (m, 2H), 7.44–7.42 (m, 4H), 7.31 (d, J ¼ 3.8 Hz, 2H), 7.15–7.09 (m, 4H), 6.92–6.88 (m, 6H), 4.84 (t, J ¼ 7.1 Hz, 4H), 3.89 (t, J ¼ 6.7 Hz, 4H), 2.20–2.15 (m, 4H), 1.80–1.79 (m, 4H), 1.52 (m, 4H), 1.42–1.28 (m, 20H), 0.87–0.84 (m, 6H). 13C NMR (100 MHz, THF-d8) d 164.1, 149.9, 146.4, 145.9, 145.9, 145.4, 142.8, 142.6, 139.2, 138.8, 136.6, 130.1, 129.6, 128.6, 128.1, 128.0, 126.6, 126.5, 125.6, 125.2, 125.2, 124.9, 124.3, 123.3, 122.6, 122.3, 116.7, 116.5, 116.5, 100.0, 57.6, 47.9, 32.7, 30.8, 30.1, 30.0, 27.5, 27.4, 27.1, 23.5, 14.4. HRMS (ESI, m/z): [M 2H]2 calcd for C82H74N10O4S6: 727.2115, found: 727.2087. (2E,2 0 E)-3,30 -(5,50 -(2,2 0 -(Hexane-1,6-diyl)bis(7-(5-(10-octyl10H-phenothiazin-3-yl)thiophen-2-yl)-2H-benzo[d][1,2,3]triazole-4,2-diyl))bis(thiophene-5,2-diyl))bis(2-cyanoacrylic acid) (DB-B). A mixture of 13 (100 mg, 0.064 mmol) in CF3COOH (20 mL) was stirred for 4 h at room temperature. Then the mixture was poured into water. The black solid was collected, washed with water for three times, and dried to give the nal product DB-B in 82% yield (77 mg), mp 220–222  C. 1H NMR (400 MHz, THF-d8) d 8.30 (s, 2H), 8.16–8.15 (m, 2H), 8.07–8.06 (m, 2H), 7.82–7.81 (m, 2H), 7.77–7.76 (m, 2H), 7.61–7.59 (m, 2H), 7.44–7.41 (m, 4H), 7.28– 7.27 (m, 2H), 7.16–7.09 (m, 4H), 6.94–6.87 (m, 6H), 4.87 (t, J ¼ 6.8 Hz, 4H), 3.88 (t, J ¼ 7.0 Hz, 4H), 2.28–2.25 (m, 4H), 1.82–1.77 (m, 4H), 1.58 (m, 4H), 1.46–1.43 (m, 4H), 1.29–1.26 (m, 16H), 0.88– 0.85 (m, 6H). 13C NMR (100 MHz, THF-d8) d 164.1, 149.8, 146.4, 145.9, 145.9, 145.4, 142.8, 142.6, 139.3, 138.7, 136.6, 130.1, 129.5, 128.4, 128.1, 128.0, 126.6, 126.4, 125.5, 125.2, 125.2, 124.8, 124.2, 123.3, 122.6, 122.3, 116.7, 116.5, 116.4, 99.9, 57.5, 48.1, 32.7, 30.6, 30.3, 30.2, 27.8, 27.7, 26.8, 23.5, 14.4. HRMS (ESI, m/z): [M 2H]2 calcd for C82H74N10O4S6: 727.2115, found: 727.2082. Fabrication of dye-sensitized solar cells The TiO2 lms (16 mm in thickness) were fabricated according to a previous procedure.51 The TiO2 electrodes were immersed in a solution of the dyes for 24 h in the dark (0.5 mM dye in DCM/THF (1 : 1)). The dye-adsorbed TiO2 lms were washed with THF and dried. The dye-sensitized TiO2/FTO glass lms were assembled into a sandwiched type together with the Pt/FTO counter electrode. The electrolyte (0.6 M 1-metyl-3-propylimidazoliumiodide (PMII), 0.05 M LiI, 0.10 M guanidiniumthiocyanate, 0.03 M I2 and 0.5 M tert-butylpridine) in acetonitrile/valeronitrile (85 : 15) was injected from a hole made on the counter electrode into the space between the sandwiched cells. The active area of the dye coated TiO2 was 0.16 cm2. 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