Synthesis of 2D-π-2A porphyrin sensitizers

All chemicals were purchased from Acros Organics or 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) or Methanol-D4 (δ = 3.31 ppm) solutions.

Studied 2D-π2A porphyrins Zn2T2A, cis-Zn2U2A, cis-Zn2TH2A, cis-Zn2TC2A, cis-Zn2BC2A, and cis-Zn2TPA2A were synthesized in three steps, mixed condensation,¹¹ zinc(II) metalation,¹²

69 and Hydrolysis.¹³ Condensation of pyrrole, methyl 4-formylbenzoate, and a corresponding aldehyde under Lindsey’s conditions catalyzed by boron trifluoride-diethyl etherate followed by subsequent oxidation by DDQ afforded the mixture of six porphyrins. All of the porphyrins were fully characterized by optical spectroscopy, ATR-FTIR, NMR, and high-resolution mass spectrometry. The cis ester derivative of porphyrins cis-2T2E, cis-2U2E, cis-2S2E, cis-2TH2E, cis-2TC2E, cis-2BC2E, and cis-2TPA2E obtained from each separate reaction were used for next steps. In ATR-FTIR all the porphyrins show a stretching frequency at around 1720 cm⁻¹ supporting the presence of ester carbonyl group. In UV‒visible spectra as expected the free base porphyrins shows single strong Soret band and four moderate Q bands. The subsequent step of zinc metalation has been readily achieved in high yields by reacting free base porphyrin with zinc acetate. The success of zinc metalation of all the porphyrin Zn2T2E, Zn2U2E, cis-Zn2S2E, cis-Zn2TH2E, cis-Zn2TC2E, cis-Zn2BC2E, and cis-Zn2TPA2E was confirmed by UV‒

visible spectra, high resolution mass and NMR spectra. UV‒visible spectra showed one Soret bands and two Q bands only whereas in NMR spectroscopy the inner NH signal disappeared completely with slight upfield shifts for all remaining protons. Hydrolysis of metal complexes has been achieved easily by reacting metal complexes in a mixture solution of THF and methanol with excess aqueous KOH. The final products have been confirmed by the disappearance of methylene protons of ester in NMR and high resolution mass spectra. ATR-FTIR spectra of final acid products Zn2T2A, Zn2U2A, Zn2TH2A, Zn2TC2A, Zn2BC2A, and cis-Zn2TPA2A showed shifting of carbonyl peaks in the range of 1675−1700 cm⁻¹ because of intermolecular hydrogen bondings. Synthetic details and characterization data of 2S2E, cis-Zn2S2E, cis-Zn2S2A, is as discussed in 2^nd chapter.

(I) Mixed condensation: In a 1000 ml round bottom flask equipped with a magnetic stirrer, nitrogen inlet and outlet, CH2Cl2 (750 ml) was added and purged with nitrogen gas for 10 min.

Pyrrole (0.500 g, 7.5 mmol), methyl 4-formylbenzoate (0.615 g, 3.75 mmol), and the required carboxyaldehyde (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 stirred at room temperature for 1 h, 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 a rotary evaporator. The dark powder dissolved in minimum amount of CH2Cl2 was eluted through short silica gel column with 2% methanol in

70 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 (v/v) dichloromethane/hexane to pure dichloromethane. The yields of the obtained porphyrins cis-2T2E, cis-2U2E, cis-2TH2E, cis-2TC2E, cis-2BC2E, or cis-2TPA2E are reported in Table 1.

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

(II) Zn metalation: The respective free base porphyrin 2T2E, 2U2E, 2TH2E, cis-2TC2E, cis-2BC2E, or cis-2TPA2E (60 mg) was dissolved in 30 ml dichloromethane. To this solution Zn(OAc)2·2H2O (1.5 equiv) 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 solution was concentrated under reduced pressure and purified directly by silica gel column

71 chromatography eluted with dichloromethane to give Zn(II) porphyrins Zn2T2E, cis-Zn2U2E, cis-Zn2TH2E, cis-Zn2TC2E, cis-Zn2BC2E, or cis-Zn2TPA2E The yields of the reactions are reported in Table 2.

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

(III) Hydrolysis: The respective Zn(II)porphyrin Zn2T2E, Zn2U2E, cis-Zn2TH2E, cis-Zn2TC2E, cis-Zn2BC2E, or cis-Zn2TPA2E (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

cis-72 Zn2T2A, cis-Zn2U2A, cis-Zn2TH2A, cis-Zn2TC2A, cis-Zn2BC2A, and cis-Zn2TPA2A in good yields as reported in Table 3.

a Reaction conditions: zinc porphyrin (50 mg), KOH (20 equiv., 1 M), THF/MeOH (3/1, v/v), reflux, 10 h. ^b Yields of analytically pure products.

Characterization data

5,10-bis(4-methoxycarbonylphenyl)15,20-bis(4-methylphenyl)porphyrin (cis-2T2E). mp ˃ 300 ^oC; ¹H NMR (300 MHz, CDCl3) δ = 8.90-8.88 (m, 4H), 8.80-8.77 (m, 4H), 8.44 (d, J = 803 Hz, 4H), 8.30 (d, J = 8.3 Hz, 4H ), 8.09 (d, J = 7.7 Hz, 4H), 7.56 (d, J = 7.7 Hz, 4H), 4.11 (s, 6H), 2.71 (s, 6H), -2.70 (s, 2H); IR (Neat, cm^-1): 3314, 1725, 1606, 1433, 1270, 1181, 1109, 1099, 1070, 1021, 995, 982, 796, 759; λabs/nm (CH2Cl2): 420, 516, 554, 588, 646; HRMS-ESI calcd for C50H38N4O4 ([M+H]⁺): 759.2971, found 759.2979.

73 5,10-bis(4-methoxycarbonylphenyl)15,20-bis(undecyl)porphyrin (cis-2U2E). mp ˃ 300 ^oC;

1H NMR (300 MHz, CDCl3) δ = 9.57 (s, 2H), 9.46 (d, J = 8.7 Hz, 2H), 8.80 (d, J = 4.6 Hz, 2H ), cm^-1): 3317, 1722, 1606, 1588, 1550, 1483, 1361, 1274, 1263, 1178, 1110, 975, 800, 763, 750;

λabs/nm (CH2Cl2): 424, 521, 559, 593, 652; HRMS-ESI calcd for C84H76N6O4S2 ([M+H]⁺):

74

75 9.46 (s, 2H), 9.38 (d, J = 4.6 Hz, 2H), 8.96 (d, J = 4.6 Hz, 2H), 8.88 (s, 2H), 8.41 (d, J = 8.1 Hz, 4H), 8.32 (d, J= 8.1 Hz, 4H), 8.22 (d, J= 1.7 Hz, 4H), 7.98 (d, J = 3.6 Hz, 2H ), 7.88 (d, J = 8.6 Hz, 4H ), 7.68-7.65 (m, 4H), 7.62 (d, J = 3.6 Hz, 2H), 4.08 (s, 6H), 1.53 (s, 36H) ; IR (Neat, cm

-1): 1722, 1608, 1552, 1488, 1465, 1363, 1274, 1176, 1112, 1072, 1000, 979, 875, 792, 763, 750, 715; λabs/nm (CH2Cl2): 426, 553, 596; HRMS-ESI calcd for C84H74N6O4S2Zn ([M+H]⁺):

1359.4583, found 1359.4630.

5,10-bis(4-methoxycarbonylphenyl)15,20-bis(3,6-di-tert-butyl-9-phenyl-9H-carbazole)porphyrinato zinc(II) (cis-Zn2BC2E). mp ˃ 300 ^oC; ¹H NMR (400 MHz, CDCl3) δ

= 9.20 (s, 2H), 9.17 (d, J = 4.7 Hz, 2H), 9.00 (d, J = 4.7 Hz, 2H), 8.95 (s, 2H), 8.46 (d, J = 8.1

76

5,10-bis(4-carboxylphenyl)15,20-bis(3,6-di-tert-butyl-9-(thiophen-2-yl)-9H-carbazole)porphyrinato zinc(II) (cis-Zn2TC2A). mp ˃ 300 ^oC; ¹H NMR (400 MHz, DMSO-D6) δ = 13.26 (s, 2H), 9.38 (s, 2H), 9.31 (d, J = 4.6 Hz, 2H), 8.88 (d, J = 4.6 Hz, 2H), 8.79 (s,

77

Synthesis of Zn3TPA1A. The procedure to synthesize Zn3TPA1A is as shown in Scheme 1. The synthetic details for condensation reaction, Zn metallation and hydrolysis are the same as reported in table 1, 2, and 3 respectively. The free-base porphyrin 3TPA1E synthesized from a mixed condensation is obtained in 0.93% yield. Zinc metalation of which gives 99% Zn3TPA1E.

Hydrolysis of Zn3TPA1E by KOH gives final product Zn3TPA1A in 98% yield.

78 porphyrins with electron donating substituents Zn2S2A, Zn2TH2A, Zn2TC2A, cis-Zn2BC2A, and cis-Zn2TPA2A are slightly red shifted (426‒430 nm) compared to cis-Zn2T2A and cis-Zn2U2A (424‒425 nm). The presence of two triphenylamine groups in cis-Zn2TPA2A resulted in a broad Soret band. The molar absorption coefficient of cis-Zn2BC2A is the highest among all porphyrins which has significant influence on its overall conversion efficiency (vide infra). The steady state fluorescence spectra of all porphyrins in THF, shown in Figure 1b, upon excitation at Soret band displayed similar pattern like UV‒visible spectra with red-shifting on electron donating group substituted complexes. Noticeably, the dramatic red shifting in fluorescence spectra of cis-Zn2TC2A in comparison with the cis-Zn2BC2A suggests that the

Table 4. Optical and electrochemical properties of (2D-π-2A) porphyrins.

Dye λabsa/nm

79

aAbsorption maximum of porphyrin in THF. ^bEmission maximum measured in THF by exciting at Soret band. ^cFirst oxidation potentials determined by using cyclic voltammetry in THF. ^d E0-0 values were estimated from the intersection of the absorption and emission spectra. ^eExcited state oxidation potentials approximated from Eox and E0-0.

bridging thiophene can better communicates the electron density through likely better coplanar of the substituents with the porphyrin conjugation system. The adsorption spectra of these porphyrins as thin films on TiO₂ were studied in order to understand the adsorption behavior of porphyrins on TiO₂.The UV‒visible spectra of studied porphyrins on TiO2 are shown in Figure 2. The position and shape of porphyrin peaks differ largely on TiO₂ thin films. Red shift of 2‒13 nm in Soret band and 2‒4 nm in Q band is observed with broadening of peaks. In specific, the spectra of cis-Zn2TPA/TiO₂ and cis-Zn2TH2A/TiO₂ are significantly broadened and red shifted 8 nm and 13 nm, respectively, compared to its monomer in THF whereas those of cis-Zn2T2A/TiO2, cis-Zn2S2A/TiO2, cis-Zn2TC2A/TiO2, and cis-Zn2BC2A/TiO2 are also broadened significantly with 2‒6 nm red shift. These results suggest a J-aggregation type side-by-side porphyrin arrangement when studied compounds are absorbed on TiO2.^14,15 Interestingly cis-Zn2U2A/TiO2, shows splitting of Soret band with blue and red shift compare to its monomer in THF indicating both H-aggregation and J-aggregation has taken place.^15,16

80 Figure 1 (a). UV‒visible spectra of Zn2T2A, Zn2U2A, Zn2S2A, Zn2TH2A, cis-Zn2TC2A, cis-Zn2BC2A, and cis-Zn2TPA2A. Inset displaying enlarged spectra of longer wavelength. (b) Fluorescence spectra of 2D-π-2A porphyrins.

Figure 2. UV‒visible spectra of porphyrin sensitizers cis-Zn2T2A, cis-Zn2U2A, cis-Zn2S2A, cis-Zn2TH2A, cis-Zn2TC2A, cis-Zn2BC2A, and cis-Zn2TPA2A on TiO2.