All chemicals were used as received from commercial sources without purification. Solvents for chemical synthesis, including toluene, dichloromethane (DCM), MeOH and tetrahydrofuran (THF), were purified by distillation. High-resolution mass spectra were recorded on either a Q-TOF or a GC-TOF mass spectrometer. ¹H and ¹³C NMR spectra were recorded on either a Varian Inova-400 spectrometer (400 MHz for ¹H; 100 MHz for ¹³C) or a Bruker Avance II-400 spectrometer (400 MHz for ¹H; 100 MHz for ¹³C).

A screen-printing technique was used to fabricate the TiO₂ films. First, the paste was deposited on a fluorine-doped tin oxide conductive glass (FTO, Asahi Glass Co., Ltd.; sheet resistance: 10 ohm/sq). The film was then sintered at 450˚C for 30 min in atmospheric air, immersed in 40 mM TiCl₄ solution for 30 min at 70˚C, rinsed with water and ethanol, and sintered at 500˚C for 30 min. The film was dipped into D1, D2, and D3 dye solutions (0.6 mM in THF) for 18 h after cooling to 80˚C. Finally, dye-sensitized TiO₂ photoelectrodes (thickness: 12 μm) were obtained. The organic electrolyte was composed of 0.06 M LiI, 0.03 M I₂, 0.1 M guanidinium thiocyanate, 0.6 M 1-propyl-3-methylimidazolium iodide (PMII), and 0.5 M t-butyl-pyridine in acetonitrile. The active area of the DSSCs was 0.36 cm². DSSCs devices were assembled with counter electrodes (thermally platinized FTO) using a thermoplastic frame (Surlyn, thickness: 60 μm).

Absorption and emission spectra were respectively recorded by HP8453 (USA) and PTI700 (USA) instruments. Electrochemical measurements were carried out

on a BAS100W (USA) electrochemistry workstation. The irradiation source for photocurrent?voltage (J-V) measurements was an AM 1.5 solar simulator (16S-002, Solar Light Co.Ltd., USA). The incident light intensity was 100 mW/cm⁻² and calibrated against a standard silicon solar cell. The J-V curve was obtained by the linear sweep voltammetry (LSV) method using an electrochemical workstation (LK9805, Lanlike Co. Ltd., China). Incident photon-to-current conversion efficiency (IPCE) measurements were performed on a Hypermonolight instrument (SM-25, Jasco Co. Ltd., Japan).

Dyes D1, D2, and D3 were synthesized following the steps depicted in Scheme 1.

3-1, (yellow solid, 53% yield), mp 167˚C - 168˚C. ¹H NMR (400 MHz, CDCl₃) δ 9.02 (s, 1H), 8.31 - 8.27 (m, 2H), 8.09 (d, J = 8.4 Hz, 1H), 7.97 (dd, J = 8.8, 1.7 Hz, 1H), 7.43 (dd, J = 8.8, 2.4 Hz, 1H), 7.35 - 7.31 (m, 4H), 7.21 (d, J = 8.0 Hz, 4H), 7.13 (d, J = 7.2 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃) δ 153.1, 152.2, 148.3, 147.1, 138.3, 134.7, 130.9, 129.6, 127.4, 126.7, 125.2, 124.6, 124.5, 124.4, 124.1, 117.7, 92.3. HRMS (EI, m/z): calcd. for C₂₆H₁₆N₃SI, 529.0110 [M]; found, 529.0107.

3-2, (yellow solid, 62% yield), mp 154˚C - 155˚C. ¹H NMR (400 MHz, CDCl₃) δ 9.00 (d, J = 2.0 Hz, 1H), 8.25 - 8.22 (m, 2H), 8.07 (d, J = 8.8 Hz, 1H), 7.95 (dd,

Scheme 1. The synthetic routes of D1, D2, and D3.

J = 8.8, 2.0 Hz, 1H), 7.38 (dd, J = 8.8, 2.4 Hz, 1H), 7.15 - 7.10 (m, 8H), 2.61 (t, J = 8.0 Hz, 4H), 1.67 - 1.59 (m, 4H), 1.45 - 1.35 (m, 4H), 0.96 (t, J = 7.3 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃) δ 153.1, 152.2, 148.7, 144.7, 138.9, 138.1, 134.6, 131.0, 129.7, 127.3, 126.5, 125.4, 124.5, 124.3, 124.0, 123.6, 116.4, 92.1, 35.3, 33.8, 22.7, 14.2. HRMS (EI, m/z): calcd. for C₃₄H₃₂N₃SI, 641.1362 [M]; found, 641.1356.

3-3, (yellow solid, 49% yield), mp 207˚C - 208˚C. ¹H NMR (400 MHz, CDCl₃) δ 8.98 (d, J = 1.6 Hz, 1H), 8.19 (d, J = 9.2 Hz, 1H), 8.11 (d, J = 2.8 Hz, 1H), 8.04 (d, J = 8.8 Hz, 1H), 7.93 (dd, J = 8.4, 1.6 Hz, 1H), 7.29 (dd, J = 9.2, 2.8 Hz, 1H), 7.19 - 7.15 (m, 4H), 6.92 - 6.88 (m, 4H), 3.85 (s, 6H). ¹³C NMR (100 MHz, CDCl₃) δ 156.7, 150.3, 124.5, 120.2, 116.3, 108.6, 106.9, 104.4, 95.2, 94.5, 94.0, 90.1, 87.9, 82.9, 79.6, 23.0. HRMS (EI, m/z): calcd. for C₂₈H₂₀N₃O₂SI, 589.0321 [M]; found, 589.0314.

4-1, (yellow solid, 80% yield), mp 173˚C - 174˚C. ¹H NMR (400 MHz, d6-DMSO) δ 10.10 (s, 1H), 8.85 (s, 1H), 8.70 (d, J = 8.8 Hz, 1H), 8.65 (d, J = 9.2 Hz, 1H), 8.18 (dd, J = 8.6, 1.9 Hz, 1H), 8.12 (d, J = 8.4 Hz, 2H), 8.06 (d, J = 8.4 Hz, 2H), 8.02 (d, J = 5.6 Hz, 2H), 7.45 - 7.41 (m, 4H), 7.23 (d, J = 8.3 Hz, 6H). ¹³C NMR (100 MHz, d6-DMSO) δ 193.3, 153.0, 148.4, 146.8, 145.0, 144.9, 136.3, 135.9, 130.8, 130.7, 130.5, 129.2, 128.4, 127.9, 126.4, 126.2, 126.0, 125.1, 124.6, 123.9, 123.6, 115.7. HRMS (EI, m/z): calcd. for C₃₃H₂₁N₃OS, 507.1405 [M]⁺; found, 507.1414.

4-2, (yellow solid, 92% yield), mp 134˚C - 135˚C. ¹H NMR (400 MHz, CDCl₃) δ 10.10 (s, 1H), 8.96 (s, 1H), 8.46 (d, J = 8.4 Hz, 1H), 8.33 - 8.29 (m, 2H), 8.04 - 7.97 (m, 5H), 7.41 (dd, J = 8.8, 2.0 Hz, 1H), 7.17-7.11 (m, 8H), 2.62 (t, J = 8.0 Hz, 4H), 1.67 - 1.60 (m, 4H), 1.45-1.36 (m, 4H), 0.97 (t, J = 7.3 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃) δ 192.1, 153.6, 153.5, 148.7, 146.0, 144.7, 139.0, 137.7, 135.5, 131.9, 130.6, 129.7, 128.4, 127.7, 127.5, 125.4, 124.7, 124.5, 124.3, 123.7, 116.4, 35.3, 33.8, 22.7, 14.2. HRMS (EI, m/z): calcd. for C₄₁H₃₇N₃OS, 619.2657 [M]⁺; found, 619.2653.

4-3, (yellow solid, 80% yield), mp 107˚C - 108˚C. ¹H NMR (400 MHz, CDCl3) δ 10.01 (s, 1H), 8.95 (s, 1H), 8.44 (d, J = 8.8 Hz, 1H), 8.29 (d, J = 9.2 Hz, 1H), 8.16 (d, J = 2.8 Hz, 1H), 8.03-7.95 (m, 5H), 7.34 (dd, J = 8.8, 2.8 Hz, 1H), 7.21 - 7.17 (m, 4H), 6.93 - 6.89 (m, 4H), 3.84 (s, 6H). ¹³C NMR (100 MHz, CDCl₃) δ 192.0, 156.7, 153.6, 153.5, 149.1, 146.0, 140.1, 137.4, 135.4, 131.9, 130.5, 128.3, 127.6, 127.4, 125.3, 124.7, 124.5, 123.6, 123.4, 121.8, 115.1, 114.2, 29.9. HRMS (EI, m/z): calcd. for C₃₅H₂₅N₃O₃S, 567.1617 [M]⁺; found, 567.1619.

D1, (red solid, 68% yield), mp 214˚C - 215˚C. ¹H NMR (400 MHz, d6-DMSO) δ 8.73 (s, 1H), 8.57 (d, J = 9.2 Hz, 1H), 8.53 (d, J = 9.2 Hz, 1H), 8.32 (s, 1H), 8.14 (d, J = 8.4 Hz, 2H), 8.02 (d, J = 8.4 Hz, 1H), 8.02 (d, J = 8.4 Hz, 2H), 7.94 (d, J = 2.4 Hz, 1H), 7.44 - 7.40 (m, 4H), 7.29 (dd, J = 8.8, 2.4 Hz, 1H), 7.23 - 7.21 (m, 6H). ¹³C NMR (100 MHz, d6-DMSO) δ 163.7, 153.2, 152.9, 152.5, 148.1, 146.8, 143.0, 137.4, 131.7, 131.5, 131.4, 130.6, 130.4, 128.8, 127.7, 126.9, 126.1, 126.0, 125.1, 125.0, 124.7, 124.5, 123.5, 123.3, 117.4, 115.5. HRMS (ESI, m/z): calcd. for C₃₆H₂₂N₄O₂S, 574.1463 [M]⁺; found, 574.1459.

D2, (red solid, 76% yield), mp 131˚C - 132˚C. ¹H NMR (400 MHz, d6-DMSO ) δ 8.77 (s, 1H), 8.57 (d, J = 8.8 Hz, 1H), 8.50 (d, J = 9.2 Hz, 1H), 8.37 (s, 1H), 8.18 (d, J = 8.4 Hz, 2H), 8.11 (d, J = 8.4 Hz, 1H), 8.06 (d, J = 8.3 Hz, 2H), 7.89 (d, J = 2.5 Hz, 1H), 7.23 - 7.10 (m, 9H). ¹³C NMR (100 MHz, d6-DMSO) δ 163.4, 152.8, 152.3, 152.0, 147.3, 143.7, 142.8, 138.4, 135.9, 131.2, 130.6, 129.3, 126.8, 126.0, 125.4, 124.0, 122.8, 116.3, 113.4, 103.5, 34.4, 33.0, 21.9, 13.7. HRMS (ESI, m/z): calcd. for C₄₄H₃₈N₄O₂S, 686.2715 [M]⁺; found, 686.2710.

D3, (red solid, 57% yield), mp 190˚C - 191˚C. ¹H NMR (400 MHz, d6-DMSO ) δ 8.62 (s, 1H), 8.47 (s, 1H), 8.39 (s, 1H), 8.05 - 7.83 (m, 6H), 7.72 (s, 1H), 7.20 (d, J = 8.4 Hz, 4H), 7.09 (d, J = 8.8 Hz, 1H), 7.00 (d, J = 8.8 Hz, 4H), 3.80 (s, 6H). ¹³C NMR (100 MHz, d6-DMSO) δ 119.1, 115.6, 114.7, 111.6, 102.4, 100.6, 97.1, 93.6, 89.7, 88.8, 86.8, 86.7, 85.6, 84.2, 77.5, 76.6, 18.0. HRMS (ESI, m/z): calcd. for C₃₈H₂₆N₄O₄S, 634.1675 [M]⁺; found, 634.1667.

UV-vis absorption spectra of three dyes in CH₂Cl₂/CH₃OH (10/1) solution are depicted in Figure 2. All dyes in solutions gave two distinct absorption bands: one relatively weak band in the near-ultraviolet region (300 nm to 340 nm) that corresponds to the π-π* electron transition and another with a strong absorption in the visible region (370 nm to 450 nm). The bands can be assigned to an intramolecular charge transfer between the triarylamine donating unit and the cyanoacrylic acid anchoring moiety, thereby producing an efficient charge-separated state. The absorption data of D1 (384 nm, 26,960 M⁻¹∙cm⁻¹), D2 (395 nm, 27007 M⁻¹∙cm⁻¹) and D3 (397 nm, 25625 M⁻¹∙cm⁻¹) are shown in Table 1.

^aAbsorption and emission spectra were measured in CH₂Cl₂/CH₃OH (10/1), with a concentration of 1.0 × 10⁻⁵ M at room temperature. ^bThe oxidation potential of the dyes was measured under the following conditions: working electrode, Pt; electrolyte, 0.1 M tetrabutylammonium hexafluorophosphate, n-Bu₄NPF₆ in THF; scan rate, 0.1 V/s. Potentials measured vs Fe⁺/Fe were converted to NHE by addition of +0.63 V. ^cThe E_0-0 energies were estimated from the intercept of the normalized absorption and emission spectra.

The redox behavior of these dyes was studied by cyclic voltammetry (Figure 3). The cyclic voltammograms of D1, D2, and D3 were measured in a solution of 0.1˚M n-Bu₄NPF₆ in CH₂Cl₂/CH₃OH (10/1). A three-electrode cell containing a Pt-coil working electrode, a Pt wire counter electrode, and a Ag/AgCl reference electrode was employed. The ferrocene/ferricenium redox couple was used as an internal reference. The examined highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) levels were collected, as shown in Table 1. HOMO values (0.94 V to 1.22 V vs. NHE) were more positive than the I⁻/I³⁻ redox couple (0.4 V vs. NHE), thus suggesting that the oxidized dyes can thermodynamically accept electrons from I⁻ ion in iodide/triiodide electrolyte for regeneration. Electron injection from the excited sensitizers to the conduction band of TiO₂ should be energetically favorable because of the more negative LUMO values (−1.29 V to −1.50 V vs. NHE) compared with the conduction band edge energy level of the TiO₂ electrode (at approximately −0.5 V vs. NHE). Table 1 shows that the introduction of the methoyl and methyl groups on arylamine can change the HOMO?LUMO energy gaps of the dyes narrowly. These results clearly demonstrate that dyes D1, D2, and D3 are potentially efficient dyes for DSSCs.

The action spectrum, or the IPCE as a function of wavelength, was measured to evaluate the photoresponse of the photoelectrode in the whole spectral region. D1, D2, and D3 sensitizers were used to manufacture solar cell devices. Figure 4 shows the IPCE obtained with 0.06 M LiI, 0.03 M I₂, 0.1 M guanidinium thiocyanate, 0.6 M PMII, and 0.5 M tert-butyl-pyridine in acetonitrile as redox electrolyte. The three dyes can efficiently convert visible light into photocurrent in the region of 300 nm to 600 nm. A solar cell based on D3 showed the highest IPCE value of 75% at 491 nm. In addition, the cell exhibited a broad IPCE spectrum with IPCE values (>70%) ranging from 390 nm to 520 nm. The IPCE spectra of D1 and D2 were slightly low, with a maximum IPCE of 68% at 450 nm and 69% at 445 nm, respectively.

Figure 5 shows the J-V curves of the DSSCs based on the D1, D2, and D3 dyes under standard global AM 1.5 G solar irradiation, and the results are provided in Table 2. The DSSCs based on the D3 dye showed a short-circuit photocurrent density of 9.70 mA/cm⁻², an open-circuit voltage of 0.77 V, and a fill

^aThe DSSCs had an active area of ~0.36 cm² and used an electrolyte composed of 0.06 M LiI, 0.03 M I₂, 0.1 M guanidinium thiocyanate, 0.6 M 1-propyl-3-methylimidazolium iodide (PMII), and 0.5 M t-butyl-pyridine in acetonitrile.

factor of 0.70, thereby yielding an overall conversion efficiency (η) of 5.23%. Under the same measurement conditions, cells that were sensitized with D1 and D2 exhibit Jsc of 9.41 and 8.52 mA/cm⁻², Voc of 0.73, and 0.73 V, and ff of 0.67 and 0.69, corresponding to η of 4.60% and 4.29%, respectively. By comparison, the N719-sensitized TiO₂ solar cell showed an efficiency of 7.47%, with a J SC of 15.04 mA/cm⁻², a Voc of 0.72 V, and a ff of 0.69. The conversion efficiency of D3 reached 70% of the N719 cell efficiency.