Synthesis and Band Gap Analysis of Meso-Arylporphyrins Containing Exclusively Electron Donating or Withdrawing Groups

Abstract

Tetra-aryl substituted A4-type porphyrins (TP, TD, TA) and trans-A2B2 porphyrins (DDP1, AAP1) with electron-donating or withdrawing groups were synthesized. The band gap energy of those porphyrins was calculated from their UV-Vis spectra and CV data. With an electron-withdrawing group, the band gap energy of porphyrin TA increased via the LUMO energy up. Meanwhile, the introduction of an electron-donating group decreased the band gap of porphyrin by HOMO level up as as in the case of porphyrin TD. The band gap (2.19-2.28 eV) of metalloporphyrin PP-Ni was greater than those (1.81-2.06 eV) of non-metalloporphyrins PP due to the LUMO level up.

INTRODUCTION

The band gap energy (E_g) of organic materials is the energy difference between the HOMO and the LUMO energy of those. The band gap is known to be one of the important properties for application in the organic light-emitting device (OLED) and organic photovoltaic (OPV) devices.^1,2 Depending on the band gap of organic electronic materials, the emission or absorption wavelength, charge injection or transfer, and the device’s efficiency could be adjusted. To obtain a long-wavelengthabsorption in photovoltaics and electroluminescence, the structural modification of copolymers as well as counter monomers are processed leading to low band gap energy.^2-4

Porphyrin has been applied as a sensitizer in P3HT [poly(3-hexylthiophene)]-PCBM(phenyl-C61-butyric acid methyl ester) based organic heterojunction solar cells in a role of a light-harvesting with a high extinction coefficient at an adequate wavelength.^5-7 To avoid overlap with the absorption by the donor, a proper band gap (HOMO and LUMO energy level) of porphyrins is required for efficient electron and hole transfer between the donor and acceptor. The band gap of porphyrins has been known to be dependent on the variation of substituent, incorporating metal, length of the conjugation, and working environment.^3,8

Previously, we have reported the substituent effect on the band gap of porphyrin (PP) derivatives.⁸ In a continuation effort, we designed meso-tetraarylporphyrins (TP, TD, and TA) in which only electron-donating (EDG) or accepting (EWG) groups are attached to the aryl groups as in Fig. 1. Here, we report the synthesis of tetra-aryl A4-porphyrins (TP1, TP2, TD, TA) and A2B2 porphyrin (DDP1, AAP1) derivatives with metalloporphyrin (PP-Ni) and their band gap with the HOMO-LUMO energy analyzed from the electro and spectroscopic data.

Figure 1. Chemical structures of tetra-arylporphyrin (PP) and nickel-contained porphyrin (PP-Ni) derivatives.

EXPERIMENTAL

General Information

All reactions were carried out under atmospheric dry N₂ pressure. The solvent CHCl₃ was dried by distillation with P₂O₅. For solution-state NMR data, JNM-LAMDA (300 MHz) was used with CDCl₃ as solvent and Mass data were collected from JMS-700 (Jeol) for FAB ionization at the Korea Research Institute of Chemical Technology. UV-Visible and emission spectroscopy were collected on Evolution 60S (Thermo Scientific) and Cary eclipse fluorescence spectrophotometer (Varian) respectively. Cyclic voltammetry was measured on a Versa STAT3 (AMETEK) at Korea University (Sejong). A three-electrode system was used and consisted of a reference electrode (Ag/AgCl), a working electrode (sample-coated TiO₂ films and Pt plate), and a counter electrode (Pt wire). The redox potentials of porphyrins were measured in tetrahydrofuran with 0.1M TBAPF₆ with a scan rate of 50 mV s^-1.

For the organic synthesis of all porphyrin derivatives, similar procedures in literature⁸ were applied with minor modifications unless specified. All the structure identification was confirmed using NMR (300 MHz, CDCl₃) and FAB Mass spectroscopic data.

2,2′-((Phenyl)methylene)bis(1H-pyrrole) (2P1). Yield; 95%, yellow solid: ¹H NMR δ (ppm) 7.87 (s, 2H, NH), 7.13-7.27 (m, 5H, ArH), 6.62-6.64 (dd, J = 3 Hz, 3 Hz, 2H, α-pyrrole), 6.07-6.10 (dd, J = 3 Hz, 3 Hz, 2H, β-pyrrole), 5.85 (s, 2H, β-pyrrole), 5.41 (s, 2H, CH).

2,2′-((3,5-Di-tert-phenyl)methylene)bis(1H-pyrrole) (2P2). Yield; 73%, brown solid: ¹H NMR δ (ppm) 7.91 (s, 2H, NH), 7.31 (s, 1H, p-ArH), 7.05-7.06 (d, J = 3 Hz, 2H, o-ArH), 6.67-6.68 (d, J = 3 Hz, 2H, α-pyrrole), 6.13-6.16 (dd, J = 3 Hz, 3 Hz, 2H, β-pyrrole), 5.93 (s, 2H, β-pyrrole), 5.44 (s, 1H, CH), 1.25-1.28 (d, J = 9 Hz, 18H, CH₃).

5,10,15,20-Tetrakis(phenyl)porphyrin (TP1). To the solution of bispyrrole, 2P1 (0.33 g, 1.5 mmol) and benzaldehyde (0.16 g, 1.5 mmol) in CHCl₃ (10 ml) was added BF₃·OEt₂ (0.20 ml of 2.5 M solution in CHCl₃, 0.49 mmol) at room temperature. After being stirred for 1 h, 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) (0.51 g, 2.25 mmol) was added and stirred for 1h more. The solution was evaporated, then added CH₂Cl₂ and 0.1N aqueous NaOH solution. The organic layer was dried with MgSO₄ and then evaporated. Isolation using column chromatography (SiO₂, 10% CH₂Cl₂ in n-hexane) gave purple solid TP1(65%): ¹H NMR d (ppm) 8.77 (s, 8H, β-pyrrole), 8.13 - 8.15 (d, J = 6 Hz, 8H, o-ArH-PP), 7.65-7.69 (dd, J = 6 Hz, 6 Hz, 12H, m,p-ArH-PP), -2.85 (s, 2H, inner NH). MS (FAB) m/z; calcd exact mass for C₄₄H₃₀N₄ 614.75, obs 615.

5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)porphyrin (TP2). Yield; 62%, purple solid: ¹H NMR δ (ppm) 8.82 (s, 8H, β-pyrrole), 8.01 (s, 8H, o-ArH-PP), 7.71 (s, 4H, p-ArH-PP), 1.44 (s, 72H, CH₃) -2.75 (s, 2H, inner NH). MS (FAB) m/z; calcd exact mass for C₇₆H₉₄N₄ 1063.62, obs 1063.

5,10,15,20-Tetrakis(4-(diphenylamino)phenyl)porphyrin (TD). Yield; 44%, purple solid: ¹H NMR δ (ppm) 8.98 (s, 8H, β-pyrrole), 8.31-8.34 (d, J = 9 Hz, 8H, PP-ArH-N), 8.06-8.08 (d, J = 9 Hz, 8H, PP-ArH-N), 7.40-7.46 (m, 40H, ArH-PP), -2.69 (s, 2H, inner NH). MS (FAB) m/z; calcd exact mass for C₉₂H₆₆N₈ 1283.60, obs 1284.

5,10,15,20-Tetrakis(pentafluorophenyl)porphyrin (TA). Yield; 47%, purple solid: ¹H NMR δ (ppm) 8.84 (s, 8H, β-pyrrole), -2.99 (s, 2H, inner NH). MS (FAB) m/z; calcd exact mass for C₄₄H₁₀F₂₀N₄ 974.56, obs 975.

5,15-Bis(4-(diphenylamino)phenyl)-10,20-bis(phenyl)-porphyrin (DDP1). Yield; 31%, purple solid: ¹H NMR δ (ppm) 8.99-9.00 (m, 4H, β-pyrrole), 8.84 (m, 4H, β-pyrrole), 8.21 - 8.23 (m, 8H, o-C₆H₅-N), 8.06-8.09 (m, 4H, o-PhH-PP), 7.77 (m, 6H, m,p-PhH-PP), 7.52-7.55 (m, 4H, PP-ArH-N), 7.40-7.46 (m, 12H, m, p-C₆H₅-N), 7.14-7.15 (m, 4H, PP-ArH-N), 2.75 (s, 2H, inner NH). MS (FAB) m/z; calcd exact mass for C₆₈H₄₈N₆ 949.17, obs 950.

5,15-Bis(pentafluorophenyl)-10,20-bis(phenyl)porphyrin (AAP1). Yield; 67%, purple solid: ¹H NMR δ (ppm) 8.87- 8.89 (d, J = 6 Hz, 4H, β-pyrrole), 8.72-8.74 (d, J = 6 Hz, 4H, β-pyrrole), 8.13-8.16 (dd, J = 3 Hz, 3 Hz, 4H, o-ArH-PP), 7.69-7.76 (dd, J = 6 Hz, 6 Hz, 6H, m, p-ArH-PP), -2.91 (s, 2H, inner NH). MS (FAB) m/z; calcd exact mass for C₄₄H₂₀F₁₀N₄ 794.66, obs 795.

[5,10,15,20-Tetrakis(phenyl)porphyrin]nickel(II) complex (TP1-Ni). Yield; 56%, red solid: ¹H NMR δ (ppm) 8.74 (s, 8H, β-pyrrole), 8.22 (m, 8H, o-ArH-PP), 7.69 (s, 12H, m,p-ArH-PP). MS (FAB) m/z; calcd exact mass for C₄₄H₂₈N₄Ni 671.43, obs 670.

[5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)porphyrin]nickel(II) complex (TP2-Ni). Yield; 52%, red solid: ¹H NMR δ (ppm) 8.73 (s, 8H, β-pyrrole), 7.80 (s, 8H, o-ArH-PP), 7.63 (s, 4H, p-ArH-PP), 1.39 (s, 72H, CH₃). MS (FAB) m/z; calcd exact mass for C₇₆H₉₂N₄Ni 1120.29, obs 1120.

[5,10,15,20-Tetrakis(pentafluorophenyl)porphyrin]nickel(II) complex (TA-Ni). Yield; 37%, purple solid: ¹H-NMR δ (ppm) 8.81 (s, 8H, β-pyrrole). MS (FAB) m/z; calcd exact mass for C₄₄H₈F₂₀N₄Ni 1031.24, obs 941(M-90).

[5,15-Bis(pentafluorophenyl)-10,20-bis(phenyl) porphyrin]nickel(II) complex (AAP1-Ni). Yield; 53%, red solid: ¹H NMR δ (ppm) 8.89 (s, 4H, β-pyrrole), 8.74 (s, 4H, β-pyrrole), 8.14 (m, 4H, o-ArH-PP), 7.70 (m, 6 Hz, 6H, m, p-ArH-PP). MS (FAB) m/z; calcd exact mass for C₄₄H₁₈F₁₀N₄Ni 851.33, obs 852.

RESULTS AND DISCUSSION

Synthesis

As shown in Scheme 1, tetra-substituted A₄-type porphyrins (TP1, TP2, TD, TA) were synthesized by McDonald coupling of bispyrrole 2 with the corresponding benzaldehyde 1 as a key step with minor modification of the known procedures.^8,10,11 Bispyrrole 2 was prepared by condensation of the corresponding aryl aldehyde 1 with an excess amount of pyrrole using an InCl₃ catalyst over 70% yield. Coupling of bispyrrole 2 with benzaldehyde 1 proceeded under BF₃ in CHCl₃ and was followed by in situ DDQ oxidation to give reddish fluorescent A₄-porphyrin TP1, TP2, TD and TA as well as trans-A₂B₂ porphyrin derivativesin 30-60% yield. All porphyrin derivatives were confirmed using the NMR and mass spectroscopic data. Internal ring-proton in porphyrins was assigned by a peak that appeared at a negative chemical shift (near -2.8 ppm). Metalloporphyrins were prepared by reaction with Ni(OAc)₂ in the chloroform-DMF solvent in about 37-56% yield. The metalation was characterized by the disappearance of the negative NMR peak. Metalloporphyrin TD-Ni, however, could not be afforded for unidentified reasons.

Scheme 1. Synthetic route for meso-tetraaryl porphyrins.

Symmetric trans-A2B2 porphyrins (DDP1, AAP1) and their Ni-incorporates were also prepared similarly to the methods used in A4-porphyrin. The condensation followed by oxidation of bispyrrol 2D and 2A with benzaldehyde gave DDP1 and AAP1 in 31, 67% respectively. The incorporation of Ni cation proceeded by the reaction with Ni(OAc)₂ in chloroform-DMF to give metalloporphyrin AAP1-Ni in 53% yield. However, metalloporphyrin DDP1-Ni could not be prepared as in the case of TD-Ni.

Spectroscopic Analysis of Band Gap

The UV-Vis and emission spectra of the porphyrin derivatives (PP, PP-Ni) were measured at 3.3×10^-5 M in CHCl₃ as listed in Table 1. The band gap energy (E_g) was calculated using Eq. (1); the results are also shown in Table 1.

Table 1. Absorption, emission, and CV analysis and the band gap energy of porphyrin derivatives PP and PP-Ni

^aUV absorption was measured at 3.3×10^-5 M in CHCl₃.

^bThe absorbance values are shown in parentheses.

^cEmission was measured with excitation at 420 nm.

^dThe intersection of the tangent line of an absorption peak at the longest wavelength with the baseline of UV spectrum.

^eFrom the equation 1 was calculated.

^fThe voltage at the oxidation begins in CV diagram.

^gThe HOMO-LUMO energy values are calculated from Eqs. (2) and (3).

^hAll data are from the reference [17], UV absorption was measured at 5×10^-6 M in CHCl₃.

ⁱNo significant emission intensity was observed.

E_g [eV] = h×c/ (λ_onset×10⁻⁹)×(1.6×10⁻¹⁹) = 1240/λ_onset       (1)

h = 6.626 ×10^-34 J/s, 1eV = 1.6 ×10^-19 J

c = 3.0×10⁸ m/s

λ_onset = cut-off wavelength

To determine the HOMO-LUMO energy level of porphyrins PP and PP-M, the cyclic voltammetry (CV) of those was measured. THF solutions containing porphy rins were coated as a film on TiO₂ films and Pt plate, with Ag/AgCl as reference electrode and ferrocene was used for potential standard.¹¹ Appling Eqs. (2) and (3), E_HOMO and E_LUMO values were calculated¹² and results are shown in Table 1.

E_HOMO [eV] = -4.8 – [E_onset – E_1/2(ferrocene)]      (2)

E_1/2(ferrocene) = 1/2(E_pc +E_pa) = 1/2(776 + 506) = 641(mV)

Epc, Epa: cathodic and anodic peak potential

E_g [eV] = E_HOMO – E_LUMO       (3)

In the absorption spectra of all porphyrin derivatives, a strong Soret-band was observed at 413-450 nm. Without EDG, EWG, the absorption of PP appeared near 430 nm as in the case of TP1, TP2 (entry 1,2). With a substitution of EWG, the Soret-absorption of TA showed a blue shift (entry 4, 424 nm) from 450 nm of TD (entry 3). This result suggested that the band gap of PP increases with EWG and decreases with EDG. The incorporation of nickel resulted in a distinct blue-shifted Soret band (PP-Ni, 415-426 nm) from those of the corresponding PP with one main Q-band peak near 530 nm. The emission of PP derivatives appeared with two peaks at 650-732 nm. Meanwhile, no significant emission peak of PP-Ni was observed.

As suggested from the UV-Vis spectroscopic data, the band gap (E_g) of PP derivatives depends on the substituent of its capacity of electron donation and withdrawal. For comparison, the energy diagram of representative PP and PP-Ni is represented in Fig. 2. With EWG, the band gap of tetra-aryl PP increased in the order of TA (2.06) > TP1 (1.85) > TD (1.80). A similar result was also observed in A₂B₂ porphyrins (entry 5-8) such as AAP1 (1.87) > DDP1 (1.85). These results were considered to attribute that EWG in porphyrin TA induced an increase in LUMO energy (0.17 eV) with a decrease of HOMO energy (-0.03 eV) from those of TP1 as presented in Fig. 2. Meanwhile, EDG in TD increased both HOMO (0.31 eV) and LUMO (0.25 eV) levels. Thus, the net result is a decreased band gap (1.81 eV) of TD from that of TP1 (1.85 eV).

Figure 2. The band gap (E_g) and HOMO-LUMO energy diagram of porphyrin PP (a) and PP-Ni (b) derivatives. E_g values are shown near the arrow and the numbers in parenthesis are MO’s energy value in eV unit. The other numbers are the energy difference between the HOMOs or LUMOs of porphyrins in comparison.

The substituent’s electron push-pull tendency seems to affect HOMO and LUMO levels nonlinearly. This means a substituent effect on the up and down of HOMO and LUMO levels via inconsistent patterns. Thus, the band gap in porphyrin PP is the result of a net value of MO’s energy. The more electron donation gave a smaller E_oneset (oxidation) value resulting in a higher HOMO energy level; TD (685 mV, -4.84 eV) < TP1 (988 mV, -5.15 eV) < TA (1020 mV, -5.18 eV).

Metalation of PP elevated LUMO energy (-3.23 − -2.86) compared with the nonmetallic PP (-3.29 − -3.45). Thus, the band gap energy (-2.15 − -2.28 eV) of metalloporphyrin PP-Ni was greater than those of nonmetallic PP (1.80-2.06 eV). Both donor (entry 13) and acceptor (entry 11, 12, 14) groups in PP-Ni resulted in a decrease in the band gap from the TP-Ni (entry 9, 10). The substitution of EWG to PP-Ni lowered the LUMO level (-0.15) more than the HOMO energy (-0.07) to give decreased E_g (2.19 eV) of TA-Ni as in Fig. 2. Consequently, the acceptor group A(-C₆F₅) mediated LUMO energy decrease was considered the most significant effect on the band gap in metalloporphyrin PP-Ni.

CONCLUSION

As a summary, we have synthesized A₄-porphyrin derivatives TP1, TP2, TA and TD and metalloporphyrins PP-Ni. The band gap with HOMO-LUMO levels of those PP and PP-Ni were analyzed from the UV-Vis and CV data. With EWG, the band gap of PP increased via the LUMO level up as the most remarkable effect. With an EDG, the band gap energy of PP decreased because of more HOMO energy up than the LUMO up. In the case of metalloporphyrin TP-Ni, the substitution of both EWG and EDG decreased the LUMO energy level and resulting in a reduced band gap from the tetraaryl porphyrins (TP1, TP2).