Experimental section

Synthesis. The zinc porphyrins used in this study Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A were synthesized in three steps by literature reported methods such mixed condensation,¹⁹ Zn²⁺ metalation,²⁰ and hydrolysis.²¹ The mixed condensation of 4-methylformyl benzoate, 2-thiophenecarboxyaldehyde, and pyrrole in the presence of boron trifluoride-diethyl etherate as catalyst, followed by subsequent oxidation by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) gave a mixture of six porphyrins, which were purified by column chromatography. Out of these six porphyrins, the four studied porphyrins were dissolved separately in CH2Cl2 and refluxed with zinc acetate in MeOH to obtain corresponding zinc complexes. The purification by column chromatography and subsequent hydrolysis by KOH isolated analytical pure products.

All of the porphyrins were fully characterized by optical spectroscopy, ATR-FTIR, NMR, and high-resolution mass spectrometry.

Optical Spectroscopy. Transmittance and reflection UV−visible absorption spectra of the porphyrins in THF and porphyrins adsorbed on TiO₂ electrodes were recorded using a JASCO V-670 UV−visible/NIR spectrophotometer. For the thin film TiO₂ absorption spectra, 1 × 1 cm² area and ~1 µm thickness films were prepared to obtain accurate shape and position of peaks.³ The films were immersed in THF solution of 2 × 10^-4 M porphyrin for 12 h, the films were rinsed with THF, dried and measure the absorbance. Steady-state fluorescence spectra were acquired by using a Varian Cary Eclipse fluorescence spectrophotometer.

Cyclic Voltammetry. The cyclic voltammetry measurements of all porphyrins were carried out on CHI 600D electrochemical analyzer (CH Instruments, Austin, TX, USA) in degassed THF

17 containing 0.2 M tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) as the supporting electrolyte. The cell assembly consists of a platinum working electrode, a Ag/AgCl reference electrode, and a platinum wire as the auxiliary electrode. The scan rate for all measurements was fixed at 100 mV/_s. A ferrocene^+1/0 couple (0.56 V vs SCE) is used as the internal reference for correcting the applied potential.

Fluorescence Lifetime Measurements. The picosecond fluorescence transients were measured with time-correlated single photon counting (PicoQuant, FluoTime 200). The samples were excited at 445 nm with a picosecond laser diode, and fluorescence decays were monitored at the maximum of fluorescence Q(0,0) band for all zinc porphyrins (1 × 10⁻⁵ M) dissolved in atmospheric THF. The fluorescence lifetimes in THF solution were fit by single exponential function.

DFT Calculations. Geometric optimization and electronic structure of the porphyrins were performed with DFT in B3LYP level and the 6-31G* basis set in the Gaussian 03 program package.

ATR-FTIR Measurements. ATR-FTIR spectra for the zinc porphyrins were recorded on a VERTEX 70 spectrometer by using Golden Gate diamond ATR accessory on solid powder of porphyrin samples. For the preparation of samples with zinc porphyrins adsorbed on TiO2, THF solution containing 5 ×10^-4 M porphyrin was mixed with 5 mg TiO2 powder and kept for 12 h.

Excess solvent were dripped out by pipet. TiO2 powder was washed twice by THF and dried in vacuo, and the obtained powder sample was used for measurement. ATR-FTIR spectra for zinc porphyrin adsorbed on TiO2 were recorded at a resolution of 4 cm⁻¹ and 320 scans.

Photovoltaic Measurements. To characterize the photovoltaic performance of the DSSC devices, a fluorine-doped tin oxide (FTO; 30 Ω/sq, Sinonar, Taiwan) glass (typical size 1.0 × 2.0 cm²), used as a cathode, was coated with Pt particles by using the thermal platinum nanocluster catalyst method. The Pt catalyst was deposited from a precursor solution composed of 5 × 10^‒4 M solution of hexachloroplatinic acid in anhydrous isopropanol. The precursor solution was spin-coated on FTO glass (10 L/cm²) and dried in air for 3 min. The coated Pt electrode was placed in an oven, and the temperature was gradually increased to 360 °C in 15 min. The porphyrin/TiO2 layer was served as a working electrode (anode). We immersed the TiO2 coated FTO (TiO2 thickness 10 μm, active size 0.4 × 0.4 cm²) in a THF solution containing Zn3S1A,

18 trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A (1 × 10^-4 M) with 3 × 10^-4 M of chenodeoxycholic acid (CDCA) at 40 °C for 3 h. To fabricate the DSSC device, the two electrodes were assembled into a sandwich-type cell, spaced, and sealed with a hot-melt film (SX1170, Solaronix, thickness 30 μm). The thin layer of electrolyte was introduced into the space between the two electrodes. A typical redox electrolyte contained lithium iodide (LiI, 0.1 M), diiodine (I2, 0.01 M), 4-tert-butylpyridine (TBP, 0.5 M), 1-butyl-3-methylimidazolium iodide (BMII, 0.6 M), and guanidinium thiocyanate (GuNCS, 0.1 M) in a mixture of acetonitrile and valeronitrile (15/1, v/v). The photocurrent and voltage curves were recorded with a digital source meter (Keithley 2400) under AM1.5 one-sun irradiation from a solar simulator (Sanei Electric XES-502S) calibrated with a Si-based reference cell (Hamamatsu S1133). IPCE measurements were carried out with a homebuilt system, which includes a Xe lamp (PTi A-1010, 150 W), a monochromator (Dongwoo DM150i, 1200 gr/mm blazed at 500 nm), and a source meter (Keithley 2400). A standard Si photodiode (ThorLabs FDS1010) was used as a reference to calibrate the power density of the light source at each wavelength so that the IPCE of the DSSC device could be obtained.

19 2.3 Synthesis of sensitizers

All chemicals were purchased from Acros Organics and Sigma Aldrich and used without further purification. ¹H NMR spectra were recorded on a Bruker 400 MHz spectrometer and performed in CDCl₃ (δ = 7.26 ppm) or DMSO-D₆ (δ = 2.50 ppm) solution. Chemical shifts are reported in ppm. Coupling constants J are reported in Hz. The signals are described as s: singlet; d: doublet;

dd: doublet of doublet. HRMS-FAB or HRMS-ESI was conducted on a JMS-700 double focusing mass spectrometer (JEOL, Tokyo, Japan). Flash chromatography was carried out by using silica gel (40‒63 μm, Merck). Analytical TLC was performed on Merck silica gel plates.

Melting points were recorded using an Electrothermal capillary melting point apparatus.

The zinc porphyrins used in this study Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, and Zn1S3A were synthesized by the simplest possible route in three steps, mixed condensation,¹⁹ zinc metalation,²⁰ and hydrolysis,²¹ by literature reported methods: Condensation of pyrrole, methyl 4-formylbenzoate, and thiophene-2-carboxyaldehyde under Lindsey’s conditions catalyzed by boron trifluoride-diethyl etherate followed by subsequent oxidation by DDQ afforded the mixture of six porphyrins. All six porphyrins were purified and characterized by various spectroscopic techniques. Each porphyrin can be distinguished by its unique splitting pattern in NMR spectroscopy. Notably, tran-2S2E and cis-2S2E porphyrins in Table 1, display characteristic splitting patterns for the resonances of β-pyrrolic protons. In the case of the tran-2S2E, eight protons on pyrrole rings split as two resonances, 9.87 (d, J = 4.8 Hz, 4H) and 8.87 (d, J = 4.8 Hz, 4H), whereas in the case of the cis-2S2E, four resonances, 9.08 (s, 2H), 9.07 (d, J = 4.8 Hz, 2H), 8.78 (d, J = 2.8 Hz, 2H), and 8.80 (s, 2H), were observed. The IR spectra of porphyrins shows stretching frequency at around 1720 cm⁻¹ supporting the presence of ester carbonyl group. Zinc metalation has been readily achieved in high yields by reacting free base porphyrins with zinc acetate. The success of zinc metalation was confirmed through the complete disappearance of the NMR resonance of inner NH with slight upfield shifts for all remaining protons. Hydrolysis of metal complexes has been achieved by reacting metal complexes in a mixture of THF and methanol with excess aqueous KOH. The success of hydrolysis was confirmed through complete disappearance of methyl protons of ester group and appearance of acidic protons of p-carboxyphenyl in highly downfield rejoin at around 13 ppm. ATR-FTIR spectra of final acid products show shifting of carbonyl peaks in the range of 1675−1700 cm⁻¹ because of intermolecular hydrogen bonding.

20 (I) Mixed condensation: In a 1000 ml round bottom flask equipped with a magnetic stirrer, nitrogen inlet and outlet, dichloromethane (750 ml) was added and purged for 10 min. Pyrrole (0.500 g, 7.5 mmol), methyl 4-formylbenzoate (0.615 g, 3.75 mmol), and thiophene-2-carboxyaldehyde (2.62 g, 3.75 mmol) were added and degassed. Then, 48% boron trifluoride-diethyl ether (10 mol%, 0.142 mL) was added. The reaction mixture was protected from light.

After stirring at room temperature for 1h, DDQ (1.70 g, 7.5 mmol) was added and the solution was continuously stirred at room temperature for another 1 h in open atmosphere. Excess dichloromethane was removed completely on rotary evaporator. The dark powder was eluted through short silica gel column with 2% methanol/dichloromethane as the eluent. The porphyrin fraction was concentrated and then separated on a second silica gel column eluted with a solvent gradient from 7:3 dichloromethane/hexane to pure dichloromethane. If necessary, mixed fraction of tran-2S2E and cis-2S2E was further separated on a third silica gel column eluted with 1:1 dichloromethane/hexane. The yields of the obtained porphyrins 3S1E, tran-2S2E, cis-2S2E and 1S3E are listed in Table1.

Reaction conditions: (i) methyl 4-formylbenzoate (3.75 mmol), thiophene-2-carboxyaldehyde (3.75 mmol), pyrrole, BF3.OEt2 (10 mol%), CH2Cl2 (750 ml), 1 h (ii) DDQ (7.5 mmol), 1 h. ^b Yield of analytically pure product.

21 (II) Zn metalation: The respective free-base porphyrin 3S1E, tran-2S2E, cis-2S2E, or 1S3E (60 mg) was dissolved in 30 ml dichloromethane. To this solution, Zn(OAc)2·2H2O (1.5 equivalent) dissolved in 10 mL methanol was added. Reaction mixture was refluxed for 1 h. Reaction progress was monitored by TLC and UV‒visible spectroscopy. The solvent was removed under reduced pressure and purified directly by silica gel column chromatography eluted with dichloromethane to give zinc(II) porphyrins Zn3S1E, tran-Zn2S2E, cis-Zn2S2E, or Zn1S3E. The yields of the reactions are reported in Table 2.

a Reaction conditions: free base porphyrin (60 mg), Zn(OAc)2·2H2O (1.5 equivalent), CH2Cl2/MeOH (3/1, v/v), reflux 1 h, ^b Yields of analytically pure product.

22 (III) Hydrolysis: The respective zinc(II) porphyrin Zn3S1E, tran-Zn2S2E, cis-Zn2S2E or Zn1S3E (50 mg) was dissolved in 40 ml solvent (THF/MeOH, V/ V, 3/1), to this solution 20 equiv. of 1 M aqueous KOH was added and refluxed for 10 h. After cooling, the reaction mixture was treated slowly with 0.1 M HCl. The precipitation formed was filtered off and washed with distilled water. The residue remained was dried on vacuum to yield analytically pure Zn3S1A, trans-Zn2S2A, cis-Zn2S2A, or Zn1S3A in good yields as reported in Table 3.

a Reaction conditions: zinc(II) porphyrin (50 mg), 1 M KOH (20 equivalent), THF/MeOH (3/1, v/v),

reflux, 10 h. ^b Yield of analytically pure product. The R¹, R², R³, R⁴ are same as in Table 2.

Characterization data

5-(4-methoxycarbonylphenyl)-10,15,20-tris(2-thienyl)porphyrin (3S1E). mp ˃ 300 ^oC; ¹H NMR (400 MHz, CDCl3) δ = 9.05 (m, 6H), 8.77 (d, J = 4.5 Hz, 2H), 8.45 (d, J = 8.8 Hz, 2H), 8.29 (d, J = 8.8 Hz, 2H ), 7.92 (d, J = 2.4 Hz, 3H), 7.86 (d, J = 5.2 Hz, 3H), 7.50 (dd, J = 4.4, 6.0

23 Hz, 3H), 4.12 (s, 3H), -2.66 (s, 2H); IR (Neat, cm^-1): 3318, 1710, 1604, 1403, 1276, 1177, 1100, 973, 857, 721; λabs/nm (CH2Cl2): 424, 520, 559, 596, 655; HRMS-ESI calcd for C40H26N4O2S3([M+H]⁺): 691.1296, found 691.1294.

5,15-bis(4-methoxycarbonylphenyl)-10,20-bis(2-thienyl)porphyrin (trans-2S2E). mp ˃ 300

oC; ¹H NMR (400 MHz, CDCl3) δ = 9.87 (d, J = 4.8 Hz, 4H), 8.87 (d, J = 4.8 Hz, 4H), 8.45 (d, J

= 7.6 Hz, 4H), 8.29 (d, J = 7.6 Hz, 4H ), 7.92 (d, J = 3.6 Hz, 2H), 7.85 (d, J = 4.8 Hz, 2H), 7.50 (dd, J = 3.2, 5.4 Hz, 2H), 4.12 (s, 6H), -2.70 (s, 2H); IR (Neat, cm^-1): 3319, 1713, 1604, 1473, 1279, 1111, 1020, 949, 796, 706; λabs/nm (CH2Cl2): 422, 519, 556, 595, 652; HRMS-ESI calcd for C44H30N4O4S2([M+H]⁺):743.1787, found 743.1793.

5,10-bis(4-methoxycarbonylphenyl)-15,20-bis(2-thienyl)porphyrin (cis-2S2E). mp ˃ 300 ^oC;

1H NMR (400 MHz, CDCl3) δ = 9.08 (s, 2H), 9.07 (d, J = 4.8 Hz, 2H), 8.78 (d, J = 2.8 Hz, 2H), 8.80 (s, 2H), 8.44 (d, J = 7.6 Hz, 4H), 8.29 (d, J = 8.4 Hz, 4H ), 7.92 (d, J = 3.2 Hz, 2H), 7.86 (d, J = 5.6 Hz, 2H), 7.50 (dd, J = 2.8, 5.2 Hz, 2H), 4.12 (s, 6H), -2.63 (s, 2H); IR (Neat, cm^-1): 3313, 1716, 1606, 1473, 1271, 1176, 1099, 978, 818, 679; λabs/nm (CH2Cl2): 421, 518, 556, 594, 650;

HRMS-ESI calcd for C44H30N4O4S2([M+H]⁺): 743.1787, found 743.1780.

5,10,15-tris(4-methoxycarbonylphenyl)-20-(2-thienyl)porphyrin (1S3E). mp ˃ 300 ^oC; ¹H NMR (400 MHz, CDCl3) δ = 9.09 (m, 2H), 8.82 (s, 2H), 8.80 (d, J = 5.6 Hz, 4H), 8.45 (d, J =

24

5-bis(4-carboxyphenyl)-10,15,20-bis(2-thienyl)porphyrinato zinc(II) (Zn3S1A). mp ˃ 300

oC; ¹H NMR (400 MHz, DMSO-D6) δ = 13.26 (s, 1H) 9.02-8.99 (m, 6H), 8.77 (d, J = 4.4 Hz,

5,15-bis(4-carboxyphenyl)-10,20-bis(2-thienyl)porphyrinato zinc(II) (trans-Zn2S2A). mp ˃ 300 ^oC; ¹H NMR (400 MHz, DMSO-D6) δ = 13.25 (s, 2H) 9.01 (d, J = 4.4 Hz, 4H), 8.78 (d, J = 4.8 Hz, 4H), 8.37 (d, J = 8.0 Hz, 4H), 8.30 (d, J = 8.0 Hz, 4H ), 8.11 (d, J = 5.2 Hz, 2H), 7.95 (d,

25 J = 3.2 Hz, 2H), 7.57 (dd, J = 3.1, 4.9 Hz, 2H); IR (Neat, cm^-1): 3512, 3378, 2920, 2851, 1681, 1605, 1421, 1278, 1230, 1045, 994, 809; λabs/nm (THF), (ε/10³ M^-1 cm^-1): 426 (296), 560 (13.2), 600 (4.8); λem/nm (THF): 613, 661; HRMS-FAB calcd for C42H24N4O4S2Zn ([M⁺]): 776.0530, found 776.0537.

5,10-bis(4-carboxylphenyl)-15,20-bis(2-thienyl)porphyrinato zinc(II) (cis-Zn2S2A). mp ˃ 300 ^oC; ¹H NMR (400 MHz, DMSO-D6) δ = 13.28 (s, 2H), 9.00 (s, 2H), 9.00 (d, J = 4.6 Hz, 2H), 9.78 (d, J = 4.4 Hz, 2H), 8.77 (s, 2H ), 8.37 (d, J = 8.0 Hz, 4H), 8.30 (d, J = 8.0 Hz, 4H), 8.11 (d, J = 5.2 Hz, 2H), 7.95 (d, J = 8.4 Hz, 2H), 7.57 (dd, J = 3.6, 5.0 Hz, 2H); IR (Neat, cm^-1):

3550,3367, 2924, 2853, 1687, 1403, 1259, 1174, 1100, 999, 980,822, 791; λabs/nm (THF), (ε/10³ M^-1 cm^-1): 424 (243), 558 (10.7), 600 (3.4); λem/nm (THF): 612, 659 HRMS-FAB calcd for C42H24N4O4S2Zn ([M⁺]): 776.0530, found 776.0527.

5,10,15-tris(4-carboxylphenyl)-20-(2-thienyl)porphyrinato zinc(II) (Zn1S3A). mp ˃ 300 ^oC;

1H NMR (400 MHz, DMSO-D6) δ = 13.26 (s, 3H), 9.023 (d J = 4.4 Hz, 2H), 8.80-8.77 (m, 6H),

The UV−visible peak positions of the Soret and Q bands and the molar absorption coefficient (ε) of Zn3S1A, trans-Zn2S2A, cis-Zn2S2A and Zn1S3A in THF are listed in Table 4. The UV−visible spectra of the studied zinc porphyrins displayed in Figure 2a show typical metalloporphyrin features. Because of inductive effect from meso-thienyl unit, a slight red shift in Soret and Q bands with the increase number of meso-thienyl groups has observed. Specifically, the Zn3S1A (428, 560, 600 nm) shows a red-shift over Zn1S3A (425, 558, 600 nm), whereas trans-Zn2S2A (426, 560, 600 nm) and cis-Zn2S2A (426, 560, 600 nm) exhibit comparable absorption wavelengths. Noticeably, the extinction coefficients in the Soret band region for the Zn1S3A and trans-Zn2S2A are significantly higher than Zn3S1A and cis-Zn2S2A. The steady-state fluorescence spectra of all zinc porphyrins (Figure 2b) measured in THF by excitation at the