Intramolecular Exciton-Coupled Squaraine Dyes for Dye-Sensitized

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C: Energy Conversion and Storage; Energy and Charge Transport

Intramolecular Exciton-Coupled Squaraine Dyes for Dye-Sensitized Solar Cells Takeshi Maeda, Tay Van Nguyen, Yuki Kuwano, Xixi Chen, Kyohei Miyanaga, Hiroyuki Nakazumi, Shigeyuki Yagi, Suraj Soman, and Ayyappanpillai Ajayaghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06131 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 8, 2018

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The Journal of Physical Chemistry

Intramolecular Exciton-Coupled Squaraine Dyes for Dye-Sensitized Solar Cells Takeshi Maeda,1* Tay V. Nguyen, 1 Yuki Kuwano, 1 Xixi Chen, 1 Kyohei Miyanaga, 1 Hiroyuki Nakazumi, 1 Shigeyuki Yagi,1 Suraj Soman,2 Ayyappanpillai Ajayaghosh2* 1

Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture

University, Naka-ku, Sakai 599-8531, Japan 2

Photosciences and Photonics Section, Chemical Sciences and Technology Division,

CSIR-

National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram 695019, Kerala, India.

E-MAIL [email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT

V-shaped squaraine dyes with carbazole cores in the center and indolenine groups at the end were designed to broaden the light-harvesting range of dye-sensitized solar cells (DSSCs) by taking advantage of intramolecular exciton coupling. V-shaped squaraines containing carboxy group on the indolenine moiety and 4-phenyl-2-cyanoacrylic acid groups on the carbazole cores as anchoring groups for chemical adsorption onto TiO2 were synthesized by the utilization of Stille cross-coupling as a key reaction. Although conventional squaraine dyes show typically narrow absorption bands, V-shaped squaraine dyes exhibited split absorption bands due to the intramolecular exciton coupling and thereby enabled the absorption of a broad range of light. The exciton coupling between squaraine chromophores obliquely installed in the molecule was theoretically supported by the Kasha exciton coupling model, in which the coupling energy was well-fitted with the experimental absorption spectra. As the result of intramolecular exciton coupling, the DSSCs based on exciton-coupled squaraine dyes exhibited a spectral response in a much wider range compared to that of a conventional linear counterpart, giving the higher energy conversion efficiency in comparison to that of the single chromophoric counterpart.


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INTRODUCTION Towards the increasing demand for renewable and alternative energy resources, dye-sensitized solar cells (DSSCs) have shown promise ever since they were first reported by O'Regan and Grätzel in 1991.1 Researchers have made great efforts working on different aspects of DSSCs such as sensitizers, nanostructured metal oxide electrodes, electrolytes and more.2 Among these aspects, the development of sensitizers appears to provide a way to achieve a significant boost in the final efficiency of DSSCs. Metal complexes (typified by ruthenium polypyridyl complexes) have been demonstrated to be successful sensitizers for TiO2-based DSSCs.3–5 Metal-free organic sensitizers have shown cost-effectiveness, excellent molar extinction coefficients, and flexibility in molecular design, allowing modifications of their electronic and steric structures through molecular engineering.6–8 A variety of metal-free sensitizers such as coumarin dyes,9 indoline dyes,10 cyanine dyes,11 and π-conjugated compounds in which donor and acceptor components were linked π-linkers (D-π-A dyes) have been reported. D-π-A dyes in particular have contributed to the significant improvement of DSSC performance to date because of their great flexibility in molecular design.12–17 Squaraine dyes, which are synthesized by the condensation of squaric acid with electron-rich components, exhibited outstanding electronic absorption in the near-infrared (IR) region as proven by their molar absorptivity coefficient, which is as high as 105-order.18,19 In addition, the potential of squaraine dyes also depends on their tunable optical properties, which can be obtained by introducing different heterocyclic and aromatic components into two sides of the squaric acid residue.20,21 In general, research on squaraine dyes for DSSCs has targeted the further increase in their light-harvesting properties in the longer wavelength regions, as well as the suppression of their biggest drawback, i.e., dye aggregation.22–27 The enhancement of light-


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harvesting properties in the near-IR region can be achieved by extending the π-conjugation28–32 and adding different heterocyclic components.33–35 Although conventional squaraine dyes show intense absorption that contributes to the efficient light-harvesting of DSSCs, the extremely sharp and narrow absorption band of squaraine chromophores causes the missing of photons that are outside their absorption bands. Grätzel and Nazeeruddin et al. and El-Sayed and Marder et al. reported high performance squaraine-based sensitizers in which a cyanoacryl group was incorporated on the terminal of squaraine chromophores through thiophene and fused thiophene linkages.36,37 The absorption bands of these dyes was broadened and extended in the near-IR region, thanks to the terminal auxochrome group, resulting in the improvement of their lightharvesting properties. In addition to the auxochromic approach, the aggregation of squaraine dyes might lead to the broadening of absorption bands of squaraine chromophores due to the exciton coupling between the dyes participating in the aggregation. The aggregation of dyes may cause the promotion of a nonradiative decay of the excited state to the ground state, resulting in the loss of electron injection into TiO2. Therefore, dye aggregation on TiO2 has generally been suppressed by a nonchromophoric coadsorbent, as typified by chenodeoxycholic acid. However, the exciton coupling should have intrinsic value for extending the light-harvesting capability of squaraine sensitizers on DSSCs. The exciton coupling model, introduced by Kasha et al., states that upon the interaction of two transition dipole moments, the original excited state will split into two new excited states in which one state locates at higher energy and the other one locates at lower energy compared to that of the original excited state.38,39 This can create a spectral change in the absorption band of sensitizers in which the strength of the splitting greatly depends on the strength of the transition dipole moments for the single chromophore and the orientation between two or more


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chromophores. The exciton coupling between chromophores was found not only in supramolecules40 but also at the molecular level in the case in which two or more chromophores are inherently installed in single molecules.41,42 The intramolecular exciton coupling can cause the absorption band to be split and consequently provide the possibility to broaden the absorption band as indicated in several reports. Lambert et al. introduced a polysquaraine exhibiting lowband-gap behavior to the application of bulk heterojuntion solar cells by considering the intramolecular exciton coupling phenomena.43 The resulting squaraine polymers showed a photo-response range as wide as from 300–850 nm for their solar cells. In the same manner, Würthner et al. reported spectral changes inside perylene bisimide molecules, which was explained by the angle-dependent oblique exciton coupling model.44 These two reports demonstrated a promising strategy of introducing intramolecular exciton coupling for targeting broader absorption bands and better light-harvesting properties. In the present study, we took the advantage of intramolecular exciton coupling in designing novel squaraine dyes for application -in dye-sensitized solar cells (DSSCs). The V-shaped structure was designed based on the use of a carbazole moiety which is able not only to create intramolecular exciton coupling between two branches with an appropriate angle to possibly increase the coupling interaction strength, but also to constitute a donor component for squaraine chromophores. By connecting the carbazole moiety with two semisquaraine components, a novel V-shaped squaraine dye BSQ and BSQC were formed and exhibited a significant enhancement in light-harvesting properties compared to that of the conventional non-branched squaraine MSQ (Figure 1). As a result, a significant improvement in performance of DSSCs could be observed in BSQC in comparison with the single-chromophoric squaraine sensitizer SQ. We also relocated


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the anchoring position on the indolenine component terminal to the carbazole moiety (SQY-a) to examine the influence of the position of anchoring functionality.

Figure 1. Molecular structures of novel V-shaped squaraine sensitizers and the conventional non-branched squaraines.

EXPERIMENTAL SECTION General

All starting materials, catalysts, and dehydrated solvents were purchased from

Tokyo Chemical Industry (Tokyo), Wako Pure Chemicals (Osaka, Japan) and Aldrich (St. Louis, MO, USA). Silica gel (SiO2, spherically-shaped, neutral) for the flash chromatography was purchased from Kanto Chemical (Tokyo). Bio-Beads SX-3 for the size exclusion column


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chromatography (SEC) was purchased from Bio-Rad Laboratories (Hercules, CA). 3-(1Methylethoxy)-4-(tri-n-butylstannyl)-3-cyclobutene-1,2-dione was prepared as described.45,46 Detailed syntheses of compounds 2, 5, 6 and 7 are reported in the Supporting Information. The NMR spectra were obtained using ECX-400 and ECS-400 spectrometers (JEOL, Tokyo) operating at 400 MHz for 1H-NMR and 100 MHz for 13C-NMR. Chemical shifts were reported in parts per million (δ) downfield from tetramethylsilane (TMS) as an internal standard in CDCl3, DMSO-d6. The electronspray ionization mass spectra (ESI-MS) were recorded on a JEOL JMS-T100CS spectrometer. The matrix-assisted laser desorption mass spectra (MALDI TOF-MS) were recorded on a Kompact Axima-CFR Plus spectrometer (Shimadzu, Kyoto, Japan). The elemental analyses were performed on a Yanaco CHN Corder JM-10 analyzer (Yanagimoto, Tokyo). The FT-IR spectra were recorded using a Shimadzu FT-IR 8400S spectrophotometer. The absorption spectra and fluorescence emission spectra were measured in a 1.0-cm quartz cell on a Shimadzu UV-3100 spectrophotometer and an FP-6600 spectrofluorometer (Jasco, Tokyo). Fluorescence lifetimes were measured using a Horiba Jobin Yvon FluoroCube spectroanalyzer with a 625 nm LED light source for excitation. Colloidal silica suspension in water was used as scatterer to determine the instrumental response. Fluorescence quantum yields were measured in CHCl3 at 25 °C using a Hamamatsu Photonics C9920 PL quantum yield measurement system. The oxidation potential of the dye was measured on an HZ-5000 electrochemical measurement system (Hokuto Denko, Tokyo) at a scanning rate of 100 mV s−1, equipped with a normal one-compartment cell with a glassy carbon disk working electrode, a Pt counter electrode, a nanoaqueous Ag/AgNO3 reference electrode and an acetone solution including 0.1 M tetrabutylammonium hexafluorophosphate as a supporting electrolyte. The fabrication of DSSCs is described in the Supporting Information.


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Synthesis of BSQC

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A mixture of compound 2 (100 mmol, 0.28 mmol) and HCl aq. (6 M, 2

mL) in THF (20 mL) was heated at 60°C for 2 days. The solvent was removed on a rotary evaporator to obtain a light yellow solid (compound 3). The crude product of 3 (70 mg, 0.17 mmol) and 1-butyl-5-carboxy-2,3,3-trimethyl-3H-indol-1-ium iodide (145 mg, 0.42 mmol) was then dissolved in 4 mL of butanol/toluene (1/1 v/v). After one drop of quinolone was added, the reaction mixture was then heated and refluxed at 110°C using a Dean-Stark trap for 24 hr. After the reaction, the solvent was evaporated under a vacuum and the residue was injected into a SEC column with THF as the eluent to obtain a purple liquid. After removal of the solvent under reduced pressure, the crude product was purified by recrystallization from CHCl3-hexane to give BSQC as a purple solid (56 mg, 37 %). 1H NMR (DMSO-d6, 400 MHz): δ 8.96 (s, 2H), 8.36 (d, 2H, J = 7.2 Hz), 8.25 (s, 2H), 8.10 (d, 2H, J = 8.4 Hz), 7.80–7.86 (m, 4H), 6.36 (s, 2H), 4.42– 4.51 (m, 6H), 1.78–1.86 (m, 18H), 1.34–1.48 (m, 6H), 0.97 (t, 6H, J = 7.4 Hz), 0.92 (t, 3H, J = 7.4 Hz). 13C NMR (CDCl3/CD3OD = 24/1, v/v, 100 MHz): δ 185.49, 180.68, 178.67, 177.14, 167.70, 144.70, 143.24, 137.50, 132.78, 131.96, 130.99, 128.92, 127.12, 124.21, 121.92, 113.69, 111.54, 110.10, 91.42, 51.18, 45.26, 43.41, 31.09, 29.78, 26.04, 20.44, 20.15, 13.81, 13.72. FTIR (KBr, cm−1): 2222, 1556, 1396, 1321, 1283. MS (ESI-TOF) m/z: [M]− Calcd for C56H54N3O8 896.39; Found 896.55. Anal. Calcd for C56H55N3O8: C, 74.90; H, 6.17; N, 4.68. Found: C, 74.92; H, 5.98; N, 4.93. Synthesis of BSQ

A mixture of compound 2 (100 mmol, 0.28 mmol) and HCl aq. (6 M, 2

mL) in THF (20 mL) was heated at 60°C for 2 days. The solvent was removed on a rotary evaporator to obtain crude 3 as a light yellow solid. The crude 3 (100 mg, 1.1 mmol) and 1butyl-2,3,3-trimethyl-3H-indolium iodide (247 mg, 0.73 mmol) were then dissolved in butanol/toluene (4/1, v/v, 5 mL), and one drop of quinoline was added into the mixture. The


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mixture was reflexed at 100°C until most of the reactant was converted to the product, which was monitored by the electronic absorption spectroscopy. After removal of the solvent under reduced pressure, the crude product was purified by silica gel column chromatography (eluent: CHCl3/CH3OH = 10/1). The combined fractions containing the target compound were evaporated, and the residue was further purified by the SEC, followed by recrystallization in CHCl3/hexane to give BSQ as a deep blue solid (35 mg, 0.043 mmol, 18%). 1H NMR (CDCl3, 400 MHz): δ 9.20 (s, 2H), 8.56 (d, 2H, J = 8.8 Hz), 7.51(d, 2H, J = 7.2 Hz), 7.45 (d, 2H, J = 8.8 Hz), 7.43–7.32 (m, 4H), 7.23 (d, 2H, J = 8.0 Hz), 6.40 (s, 2H), 4.34–4.22 (m, 6H), 1.95-1.86 (m, 18H), 1.58–1.39 (m, 6H), 1.03 (t, 6H, J = 7.2 Hz), 0.98 (t, 3H, J = 7.6 Hz). 13C NMR (CDCl3, 100 MHz): δ 192.83, 184.61, 180.70, 177.62, 173.13, 143.39, 142.58, 141.26, 128.30, 126.92, 126.65, 124.91, 123.99, 122.74, 121.66, 111.63, 109.65, 90.52, 51.45, 44.88, 43.30, 31.13, 29.77, 26.21, 20.52, 20.26, 13.85, 13.82. IR (KBr, cm−1): 1730, 1610, 1551, 1497, 1468, 1396, 1286, 1199. MS (ESI-TOF) m/z: [M + Na]+ Calcd for C54H55N3O4Na 832.41; Found 832.47. Anal. Calcd for C54H55N3O4: C, 80.07; H, 6.84; N, 5.19. Found: C, 79.94; H, 7.09; N, 5.03. Synthesis of SQY-a To a 100-mL flask we added 7 (100 mg, 0.19 mmol), 1-butyl-2,3,3trimethyl-3H-indolium iodide (195 mg, 0.57 mmol). Then, butanol/benzene (4/1, v/v, 40 mL) was added, and the mixture was refluxed at 90°C for 90 min after 4 drops of quinoline were added. The reaction mixture was cooled to room temperature, the solvent was evaporated under reduced pressure, The residue was purified by the SEC column with THF as an eluent. The combined fractions was further purified by recrystallizing from ethanol-THF-hexane to give SQY-a as a green solid (37 mg, 0.040 mmol, 21%). 1H NMR (DMSO-d6, 400 MHz): δ 8.94 (s, 2H), 8.44 (s, 1H), 8.35 (d, 2H, J = 9 Hz), 8.27 (d, 2H, J = 9 Hz), 7.94 (d, 2H, J = 9 Hz), 7.73 (d, 4H, J = 10 Hz), 7.60 (d, 2H, J = 9 Hz), 7.54–7.41 (m, 4H), 6.33 (s, 2H), 4.45 (t, 4H, J = 10 Hz),


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1.86–1.75 (m, 16H), 1.42 (m, 4H), 0.94 (t, 6H, J = 9 Hz). 13C NMR (CDCl3/CD3OD = 24/1, v/v, 100 MHz): δ 185.38, 180.83, 178.72, 169.44, 163.85, 153.29, 143.66, 142.08, 141.14, 140.55, 132.92, 131.14, 128.58, 127.43, 127.39, 127.35, 127.15, 125.36, 124.77, 122.90, 121.46, 121.43, 115.86, 112.22, 110.75, 104.46, 91.67, 51.74, 45.38, 30.02, 25.93, 20.24, 13.81. FT-IR (KBr, cm−1): 2222, 1556, 1396, 1321, 1283. HRMS (ESI-TOF) m/z: [M−H]− Calcd for C60H51N4O6 923.3809; Found 923.3812. Anal. Calcd for C60H52N4O6: C, 77.90; H, 5.67; N, 6.06. Found: C, 77.76; H, 5.88; N, 5.92.

RESULTS AND DISCUSSION Synthesis of novel V-shaped squaraine dyes BSQC bearing carboxy groups and its reference dye BSQ were prepared according to Scheme 1. More detailed synthetic pathways are reported in the Supporting Information. The synthesis basically started with a Stille cross-coupling reaction to introduce squaraine units to Nbutylcarbazole skeletons. The semisquarate propyl ester 2 was hydrolyzed under acidic conditions to obtain the semi-squaraines 3. V-shaped dyes BSQC and BSQ were obtained by the condensation of 3 with corresponding indolenium salts.

Scheme 1. Synthesis of V-shaped squaraine dyes BSQ and BSQC.


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SQY-a in which the anchoring groups were designed to be located on the carbazole moiety were synthesized according to Scheme 2. N-(4-formylpheynyl)carbazole moiety 4 reacted with stannylcyclobutenedione derivative under the Stille condition to afford semisquarate 5. The aldehyde group was converted to a cyanoacrylic acid group by Knoevenagel condensation with cyanoacetic acid to give the semisquarate 6, which was hydrolyzed to be the semi-squaraine 7 bearing cyanoacyrlic acid group. It was condensed with indolenium derivative to give SQY-a. The synthetic procedure of reference dye MSQ was the same as that used for BSQC, in which diiodocarbazole derivative 1 was replaced by a mono-bromocarbazole derivative (Supporting Information). A reference dye SQ was also synthesized as described for a comparison of photovoltaic performance.34

Scheme 2. Synthesis of V-shaped squaraine dyes SQY-a.


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Absorption and fluorescence properties The UV/vis/near-infrared (NIR) absorption spectra of V-shaped squaraine dyes BSQ, BSQC, SQY-a, and the reference dye MSQ in CHCl3 are shown in Figure 2A, and the results are summarized in Table 1. In general, the absorption bands of squaraine dyes are extremely sharp and unimodal in the range from the visible to far-red region, and their absorption intensities are quite high. Monochromophoric MSQ with an unsymmetrical structure consisting of carbazole and indolenine components showed an intense and sharp absorption with a maximum at 595 nm, originating from the intramolecular charge transfer in squaraine chromophores as reported.47 In contrast, V-shaped squaraine BSQ, in which two indolenine-based semi-squaraine branches are linked by a carbazole core, had an absorption property entirely different from a monochromophoric analogue of MSQ. The dye exhibited intense absorption with two peaks at 538 nm and 655 nm. The difference of lower and higher energy absorption bands was 3320 cm−1. BSQC and SQY-a bearing carboxy groups as the anchoring functionality for the adsorption on TiO2 also exhibited split absorption with maxima at 551 and 676 nm for BSQC and at 539 and 632 nm for SQY-a. Electronic absorptions of V-shaped squaraines were intensive as indicated in the 105 order of molar absorptivity coefficient. Thus, V-shaped squaraines, especially BSQC showed light harvesting capability in the wide range. Upon excitation in the higher energy absorption band of the exciton manifold, BSQ and SQY-a exhibited fluorescence emission in the lower energy side of the lower energy absorption band in CHCl3 as shown in Figure 2B. This indicated the fluorescence emission occurred only from the lowest exciton state according to Kasha’s rule. In contrast, BSQC showed a weak fluorescence emission that overlap with the absorption band together with a strong fluorescence emission in the lower energy side of


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the lowest energy absorption band. The peculiar fluorescence behavior was also observed in the exciton-coupled squaraine heterotrimers reported by Lambert et al.41 It is noted that fluorescence lifetime of aggregates is decreased in inverse proportion to the delocalization length of excitons.48 The lifetime of fluorescence emission at the lower energy emission band of BSQC (0.28 ns), in which the exciton was practically delocalized in two chromophores, was almost two times lower than that at the higher energy emission band (0.53 ns). In addition, the higher energy emission was observed in the almost same region as that for the monomeric chromophore MSQ. From these experiments, we speculate that the monomeric states/species are involved in the weak and higher energy emission found in BSQC.


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Figure 2. Absorption spectra (A) and emission spectra (B) of V-shaped dyes BSQ, BSQC, SQY-a, and a monochoromophoric counterpart MSQ in CHCl3 (5 µM). Excitation of V-shaped dyes was at absorption maxima of higher energy absorption bands of split absorption. Excitation of MSQ was at 560 nm.


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Table 1. Optical and electrochemical parameters of the studied compounds. λmaxa /nm

logε at the λmax

538

4.89

655

5.08

551

4.74

BSQ

λemc

E0-0

Eoxf

Eox–E0-0

/ cm−1

/nm

/ ns

/V

/V vs NHE

/V vs NHE

3320

674

0.32

0.18

1.87

0.96

−0.91

628

0.53

693

0.28

0.12

1.81

0.93

−0.88

2730

652

0.45

0.18

1.93

0.99

−0.94

-

619

0.14

0.095

2.05

0.98

−1.07

ΦF

3355

BSQC 676

4.88

539

4.98

632

5.06

595

5.08

SQY-a MSQ

τd

△Eb

e

a

Measured in CHCl3 (5 µM). bThe exciton splitting energy. cMeasured in CHCl3. dLifetimes were determined by fitting the decay curves with a single exponential decay function. The quantities that express the mismatch between data and fitted function (χ2) are 1.01 for BSQ, 1.01 for MSQ, 1.00, 1.02 for BSQC, and 1.01 for SQY-a. eQuantum yield. fThe oxidation potential was measured on 0.1 M tetrabutylammonium hexafluorophosphate in CH2Cl2 (working electrode: glassy carbon disk; counter electrode: Pt; reference electrode: nonaqueous Ag/AgNO3 calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal reference and converted to NHE by the addition of 0.63 V).

The absorption spectra of V-shaped squaraine dyes on a TiO2 surface are displayed in Figure 3. The dyeing processes were carried out by immersing a 2.1-µm-thick TiO2 thin-film into the dye solution of BSQC 120 µM with 6 mM chenodeoxycholic acid (CDCA) and SQY-a 60 µM with 1.2 mM CDCA in acetonitrile/t-butanol solvent mixture. The absorption spectra of dyes on TiO2 were significantly broader in comparison to that of the absorption in solution. This indicates a strong interaction between dye molecules and TiO2 and among sensitizer molecules upon immobilization on the TiO2 surface.30 Although the absorbance value of TiO2 films with BSQC was slightly lower than that with SQY-a in the dying condition, the spectrum of BSQC on TiO2 became much broader compared to SQY-a. The light harvesting efficiency (LHE = 1−10A) is a fundamental factor to decide the incident photon to current conversion efficiency (IPCE) of


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DSSCs.49 The LHE of dye-loaded TiO2 films estimated from their absorption spectra was close to unity for in the wide range from 700 nm to 480 nm for BSQC and from 660 nm to 480 nm for SQY-a due to the split absorption band.

Figure 3. Absorption spectra and the plot of the light harvesting efficiency (LHE) of TiO2 thin films dyed with BSQC, SQY-a. The LHE was determined by the absorbance of given dyes immobilized on TiO2 films; LHE = 1−10A.

Theoretical study of absorption properties In the case in which a chromophore was proximally positioned to another chromophore, the excited state of the chromophore split into two excited states in which one is located at higher energy and the other is located at lower energy compared to the original excited state. The split of excited states occurs as a consequence of electronic coupling between chromophores. If two transition dipole moments of chromophores are parallel, only one transition is allowed, leading to either a blue or red shift in the absorption spectra depending on the orientation of the chromophores (Figure 4a,b). In an oblique coupling case, transitions from the ground state to two excited states arising from electronic coupling are allowed since the transition dipole moments


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do not entirely cancel out each other and the intensity of the absorption to each new level is angle-dependent (Figure 4c).

Figure 4. Schematic energy level diagram with allowed transitions due to exciton coupling in (a) a face-to-face system, (b) a head-to-tail system, and (c) an oblique system.

The split absorption band of the present V-shaped squaraine dyes should be expected to be caused by the oblique system of the intramolecular coupling interaction. We carried out the calculation of exciton splitting energy and transition moments from the ground state to lower and higher excited states to confirm the origin of the band splitting. The data of absorption spectra of reference V-shaped BSQ and its monochromophoric counterpart MSQ were used for the calculation based on the exciton coupling theory (Supporting Information). The transition dipole moment (µ) for the electronic absorption of the MSQ in the range from 14450 to 22684 cm−1 (440–692 nm) was calculated to be 11.0 D according to Eq. S1. The transition moments of BSQ for the first absorption band between 17580 and 12748 cm−1 (569–785 nm) (µ'exp) and the second band between 17580 and 23912 cm−1 (418–569 nm) (µ''exp) were calculated as 11.4 D and 10.3 D, respectively. The energy gap (∆εexp) between the first and second absorption bands of BSQ in THF was 3320 cm−1. The exciton splitting energy, corresponding to the separation ∆εcalc is given by Eq. 1:38,39


17


∆ε =

2µ

r

3

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2

(cos α + 3cos 2 θ )

(1)

where µ is the transition moment for the singlet-singlet transition in the monochromophoric squaraine, r is the center-to-center distance between squaraine chromophores, α is the angle between polarization axes for the components' absorption units and θ is the angle made by the polarization axes of the unit chromophores with the line of the chromophores' center. The splitting energy (∆εcalc) of V-shaped squaraine yielded 3435 cm−1 (r = 8.137 Å, µ = 11.0 D, α= 92.9° and θ = 43.9°) according to Eq. 1 (Figure 5). The calculation value of splitting energy was well-fitted to the experimental value (∆εexp = 3320 cm−1). Moreover, transition moments to the new exciton states µ' and µ'' are given by the following equations:

µ ' = 2 µ cos θ

(2)

µ '' = 2µ sin θ

(3)

The calculated transition moments of first absorption band (µ'calc) and the second band (µ''calc) of BSQ were calculated as 11.2 D and 10.8 D, respectively. Since experimental values regarding exciton coupling are well fitted to that of the calculation (Table S1), it confirmed that the splitting absorption band of the V-shaped dye originates from the exciton coupling between two oblique squaraine chromophores.


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Figure 5. (A) An energy level diagram for V-shaped squaraine dyes along with parameters of transition dipole moment (µ; monochromophoric, µ’, µ’’; V-shaped dye) and the splitting energy (∆ε = E’’− E’). (B) Geometrical parameters of the V-shaped dye BSQ used for the calculations.

DFT calculation We used the density functional theory (DFT) calculation based on the B3LYP/6-31G(d) level of theory to analyze the electron distribution of the frontier molecular orbitals of the studied compound. The isodensity plots of frontier orbitals for these compounds and the energy level diagram are shown in Figure 6 and Figure S2, respectively. HOMO of BSQ and BSQC was delocalized through the carbazole and cyclobutenedione cores. Both LUMO and LUMO+1 orbitals for BSQ and BSQC were delocalized through the squaraine chromophore, and have lower and higher energy, respectively, in comparison to the LUMO energy of the single chromophoric counterpart MSQ. This result supports the excitonic coupling found in their absorption spectra. The LUMO and LUMO+1 of BSQC display the substantial electron density on carboxy groups at indolenine components which link dyes on the TiO2 surface, suggesting that efficient electron transfer could occur upon photoexcitation from BSQC to TiO2 on DSSCs due to the strong electronic coupling between LUMO/LUMO+1 and the conduction band of TiO2.24,25 The calculation for SQY-a indicated distortion between the N-phenyl groups bearing cyanoacrylic acid and carbazol-based squaraine chromophores. Considering the distortion, Nphenyl groups are scarcely conjugated to the squaraine chromophore. Although the electron


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density distribution of HOMO for SQY-a is almost identical to that of BSQC, LUMO was delocalized mainly on the N-phenyl group and cyanoacrylic acid. LUMO+1 and LUMO+2 in

SQY-a were attributed to the splitting of excited states through excitonic coupling. For the efficient electron injection from dyes to TiO2, LUMO+1/LUMO+2 of SQY-a were required to be placed near TiO2. However, the unconjugated 2-cyano-3-phenylacrylic acid group kept LUMO+1/LLUMO+2 away from TiO2 surface. In this regard, the anchoring position of SQY-a is supposed to be undesirable to the efficient electron injection.

Figure 6. Frontier molecular orbitals of BSQ, BSQC, SQY-a and MSQ calculated at the B3LYP/6-31G(d) level of theory.

Electrochemical properties The electrochemical properties of these dyes were studied by cyclic voltammetry using a glassy carbon disk working electrode, a Pt counter electrode, a non-aqueous Ag/AgNO3 reference electrode and 0.1 M tetrabutylammonium hexafluorophosphate in CH2Cl2 as a supporting electrolyte. The measurement was carried out at scan rate of 100 mVs−1 (Figure 7A).


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As can be seen from the cyclic voltammograms, BSQ and MSQ exhibited reversible redox processes in the potential range of −0.3–0.75 V (vs. Fc/Fc+). MSQ showed one oxidation waves in the potential range, whereas, BSQ showed two oxidation waves due to the interaction of two squaraine chromphores.43 Unlike BSQ, the cyclic voltammograms of V-shaped dyes bearing carboxy groups BSQC, SQY-a, were irreversible and oxidation waves were broad in comparison to BSQ. Thus, carboxy groups on the V-shaped dyes influenced on their electrochemical properties. The onset of the first oxidation potential for BSQC is more negative than those of

BSQ and MSQ, suggesting that the carboxy groups influenced on the electrochemical oxidation. The first oxidation potentials of SQY-a was observed in the more positive potential in comparison to BSQC. The energy diagram of dyes are displayed in Figure 7B, together with the energy level of the TiO2 conduction band and the I−/I3− redox potential. The HOMO levels of dyes were estimated from the oxidation potential derived from the onset of the oxidation wave in cyclic voltammograms. LUMO levels were then subsequently calculated from the HOMO levels and optical band gap values which were obtained from the onset of absorption spectra. The energy level values are introduced in Table 1. It can be observed that the oxidation potentials of all dyes are more positive than that of the I−/I3− redox potential (0.4 V vs. NHE), suggesting that all studied dyes are thermodynamically sufficient to accept a regenerated electron from the I− ion. Moreover, the excited-state potentials of these dyes are calculated as more negative than that of the TiO2 conduction band (−0.5 vs. NHE), indicating that the electron injection processes are thermodynamically favorable. As a result, all dyes are thermodynamically applicable for TiO2based DSSCs with the iodine-based electrolyte.


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Figure 7. (A) Cyclic voltammograms of dyes recorded in CH2Cl2; 0.1 M tetrabutylammonium hexafluorophosphate was used as a supporting electrolyte. The arrow indicates the beginning and sweep direction of the first segment. (B) Energy level diagram of the studied compounds.

Performance of V-shaped squaraine dyes in DSSCs We fabricated DSSCs based on BSQC, SQY-a and the reference SQ and measured their performances under simulated sunlight (AM 1.5 G, 100 mW cm−2). Chenodeoxycholic acid (CDCA) was used as a co-absorbent to prevent dye aggregation and surface coverage to prevent undesired charge recombination. The concentrations of sensitizers, CDCA and each component of iodine-based electrolytes were optimized accordingly (Supporting Information). Figure 8 displays the obtained J-V curves and IPCE spectra of the DSSCs based on the studied dyes. All of the performance parameters are summarized in Table 2. From the IPCE spectra, the DSSCs


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based on V-shaped BSQC exhibited spectral responses in the entire range of the visible region, whereas the linear SQ sensitized cells showed a spectral response only from 500 nm to 700 nm. The DSSC based on the V-shaped dye BSQC gave the highest Jsc of 13.01 mA·cm−2 among the three dyes with a Voc of 0.48 V and ff of 0.57, corresponding to an overall conversion efficiency of 3.52%, exhibiting much better photovoltaic performance than the efficiency of the linear dye

SQ at 2.66% (Jsc = 11.04 mA·cm−2, Voc = 0.47 V, ff = 0.51). Thus the enlarged range of absorption of the V-shaped dyes originating from the intramolecular exciton coupling contributed to the enhancement of their light-harvesting ability in DSSCs.

Figure 8. J-V curves (A) and IPCE spectra (B) of DSSCs based on BSQC, SQY-a, and SQ.


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The SQY-a sensitized cell showed a quite low IPCE, resulting in a low Jsc, which limited the conversion efficiency. Although BSQC and SQY-a possess similar exciton-coupled V-shaped structures, the SQY-a showed a much lower photovoltaic performance. As mentioned in DFT calculations, 2-cyano-3-phenylacrylic acid group was unconjugated to the chromophore of SQY-

a. The unconjugated anchoring group inhibited the electronic coupling between LUMO+1/LLUMO+2 of SQY-a and the conduction band of TiO2, leading to the decrease of the current density. As shown in the absorption spectra on TiO2 (Figure 3), the BSQC-sensitized TiO2 film showed relatively broader absorption compared to that of SQY-a in the absorption spectra, which indicated that double carboxylic anchors in the terminal indolenine components of

BSQC enhance the incorporation of the dye onto TiO2 and promote the electron injection. Moreover, the relatively higher LUMO level of BSQC (as shown in the energy diagram in Figure 7B) would be anticipated to exhibit a higher probability of electron injection from the excited dye to TiO2, leading to a better short-circuit current (Jsc). Thus, strong incorporation between TiO2 and dyes and a thermodynamically favorable injection of electrons of BSQC contributed to the achievement of much better photovoltaic performance than exciton-coupled

SQY-a, bearing only one anchoring group on the central carbazole component in DSSCs.

Table 2. Photovoltaic performance parameters of DSSCs based on the studied dyesa Jsc / mA cm−2

Voc / V

ff

η%

BSQC

13.01

0.48

0.57

3.52

SQY-ac

4.46

0.46

0.60

1.25

SQd

11.0

0.47

0.51

2.67

Dye b

a

Illumination: 100 mW cm−2 simulated AM 1.5 G solar light; electrolyte: 0.6 M DMPII, 0.03 M I2, 0.05 M LiI, 0.05 M GuNCS in CH3CN / BuCN (85/15, v/v); TiO2 electrodes: 20-µm-thickness. TiO2 electrodes were immersed into dye solutions for 4 h, at RT. bDye solution: 60 µM, 15 mM CDCA. c Dye solution: 60 µM, 12 mM CDCA. dDye solution: 60 µM, 1.2 mM CDCA.


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CONCLUSION Aiming at the expansion of the light-harvesting range of squaraine dyes which conventionally show narrow and sharp absorption, we designed and synthesized novel V-shaped squaraine dyes based on central carbazole core. Compared with a counterpart dye with only a single-squaraine chromophore (MSQ), V-shaped squaraines, BSQC and SQY-a exhibited split and intense absorption covering the entire visible region. The exciton coupling energy obtained by the theoretical calculation based on the absorption spectrum of the single-chromophoric counterpart

MSQ is in great agreement with the experimental absorption data of the V-shaped dye BSQ. This indicates that the origin of the observed intense and splitting absorption band originated from the intramolecular exciton coupling between squaraine chromophores incorporated on carbazole cores. The intramolecular exciton coupling on the V-shaped dyes was supported by DFT calculations, which revealed that the excited states of these dyes were split. The DSSC based on the V-shaped dye BSQC exhibited a wider spectral response due to the split of its absorption band, showing excellent light-harvesting properties, resulting in the higher performance of DSSCs compared to the standard squaraine dye SQ with a narrow absorption band. The V-shaped dye SQY-a having cyanoacrylic acid group on the carbazole component, despite showing an excellent broad and panchromatic optical response, yielded only moderate performance. Thus, split absorption bands caused by the intramolecular exciton interaction contributed to the enlarged range of spectral response of DSSCs.

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Number JP16H06048 and DST-JSPS Joint Research Project (DST/INT/JSPS/P-194/2015).


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SUPPORTING INFORMATION Supporting information for publication contains synthetic details of BSQ, BSQC, SQY-a, and

MSQ, calculations of exciton coupling, DFT calculations, fabrication and optimization of DSSCs, and NMR and MS spectra.

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Intramolecular Exciton-Coupled Squaraine Dyes for Dye-Sensitized

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