SH-Substituted Porphyrins As Sensitizer Candidates for Dye

Jan 12, 2010 - School of Chemistry and Chemical Engineering, Liaocheng University, ... the acceptor G as novel sensitizer candidates have been designe...
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J. Phys. Chem. A 2010, 114, 1973–1979

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Theoretical Screening of -NH2-, -OH-, -CH3-, -F-, and -SH-Substituted Porphyrins As Sensitizer Candidates for Dye-Sensitized Solar Cells Ruimin Ma,† Ping Guo,† Linlin Yang,† Lianshun Guo,† Xianxi Zhang,*,†,‡ Mohammad K. Nazeeruddin,*,‡ and Michael Gra¨tzel*,‡ School of Chemistry and Chemical Engineering, Liaocheng UniVersity, Liaocheng 252059, China and Laboratory for Photonics and Interfaces, Swiss Federal Institute of Technology, CH 1015, Lausanne, Switzerland ReceiVed: October 12, 2009; ReVised Manuscript ReceiVed: December 21, 2009

Following former studies, the donor-acceptor combinations of -NH2-substituted porphyrin donor and the acceptors C, D, E, F, H and G, those of -OH-, -CH3- and -Ph-substituted porphyrins as well as porphine donors and the acceptors E, G, and H, and those of -F- and -SH-substituted porphyrin donors and the acceptor G as novel sensitizer candidates have been designed and calculated at the density functional B3LYP level. The result shows that -NH2-, -OH- and -CH3-substituted porphyrins as donors combined with the acceptor G are very promising to provide good performances as sensitizers because of their smaller HOMO-LUMO gaps, much red-shifted absorption bands, and good frontier molecular orbital spatial distributions. They are all promising to challenge the current photon-to-current conversion efficiency record 7.1% of porphyrin-sensitized solar cells in which the -NH2-substituted porphyrins as donors combined with the acceptor G are the best systems. 1. Introduction Dye-sensitized solar cells (DSSCs) have attracted much interest as the most promising alternative to the conventional silicon-based solar cells.1-6 The most successful sensitizers employed in these cells are ruthenium sensitizers, which yield photon-to-current conversion efficiencies of the corresponding nanocrystalline TiO2 solar cells more than 11% at AM 1.5 sunlight.7 However, ruthenium is not readily available. The complexes derived from metals that are common in nature or the free-metal compounds become more and more important, in which porphyrin dyes play an important role.8-17 Among the alternative porphyrin dyes, the donor-acceptor porphyrin sensitizers have attracted much more attention because of the relative ease with which functional groups can be attached to its basic structure and their relatively better performance than the common porphyrin compounds.11-13 The typical porphyrins used as sensitizers in DSSCs mostly have porphyrin moieties as donors and carboxyl-containing groups as acceptors which adsorb the dyes onto the TiO2 surface.14 W. M. Campbell et al. found the β-substituted monoporphyrin carboxylic acid derivative 4-trans-2′-(2′′-(5′′,10′′,15′′,20′′-tetraphenylporphyrinatozinc(II)yl)ethen-1′-yl)-1-benzoic acid gives an overall photon-tocurrent conversion efficiency of 4.2% under AM1.5 conditions in an unoptimized Gra¨tzel cell.15 Gra¨tzel’s group reported the synthesis and characterization of a series of zinc metalloporphyrins, and several of them were calculated at the density functional theory (DFT) level using the 3-21g* basis set.16 The results show that the highest overall conversion efficiency of cyano-3-(2′-(5′,10′,15′,20′-tetraphenylporphyrinatozinc(II))yl)acrylic acid (Zn-3) is 5.6%.16 Recently, this record was further increased to 7.1%.17 All these sensitizers with good photon-tocurrent conversion efficiencies are formed with the typical * To whom correspondence should be addressed. Phone: +86 635 8230680. Fax: +86 635 8239121. E-mail: [email protected]. † Liaocheng University. ‡ Swiss Federal Institute of Technology.

porphyrin moieties as donors and the carboxyl-containing groups as acceptors. In an attempt to tune the molecular orbital energy levels and further improve the light absorption properties to get higher conversion efficiencies of the porphyrin dyes, systematical design and screening of promising sensitizer candidates were performed assisted with the concepts of attribute axis and attribute coordinate system.18 The term attribute here may indicate any property of molecules. These corresponding attribute axes combine together with the electron-withdrawing or -donating ability axis to form the corresponding attribute coordinate systems in which each compound has its own coordinates. Since the scopes of each axis are unlimited, designs can be provided as many as possible, from which promising candidates with anticipated properties may be screened. Assisted with these attribute coordinate systems, with a wider range of the electron-withdrawing or -donating substituents along the axis being checked, the way may be found to tune the molecular orbital energy levels of porphyrins. In this study porphine and 11 kinds of bridge carbon-substituted porphyrins as donors and 9 common acceptors A-I were designed and calculated at the density functional B3LYP level.18 The results show that the candidates selected are very promising to provide better performance as sensitizers in which ZnTPPG is promising to challenge the photon-to-current conversion efficiency record 7.1% of ZnTMPPI.17-19 Besides the combinations of the tetraphenylporphyrin donors and acceptors E, G, H, and I reported in previous work,18 the combinations of 4-(trans-2′-(2′′-(5′′,10′′,15′′,20′′tetraaminoporphyrin)ethen-1′-yl))-1,2-benzenedicarboxylic acid (Por-NH2C), 3-(trans-(5′-(2′′-(5′′,10′′,15′′,20′′-tetraaminoporphyrin)thiophen-2′-yl))-acrylic acid (Por-NH2D), 2-cyano3-(5′-(2′′-(5′′,10′′,15′′,20′′-tetraaminoporphyrin)thiophen-2′yl))-acrylic acid (Por-NH2E), 3-[trans-5′-(trans-2′′-(2′′′(5′′′,10′′′,15′′′,20′′′-tetraaminoporphyrin)ethen-1′′-yl)-thiophen2′-yl]-acrylic acid (Por-NH2F), 2-cyano-3-[5′-(trans-2′′-(2′′′(5′′′,10′′′,15′′′,20′′′-tetraaminoporphyrin)ethen-1′′-yl)-thiophen-

10.1021/jp909787t  2010 American Chemical Society Published on Web 01/12/2010

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Figure 1. Structures of donors (R ) hydrogen; X ) hydrogen, -NH2, -OH, -CH3, -Ph, -F, -SH), acceptors (C-H), and porphyrin sensitizers (R ) C, D, E, F, G, H; X ) hydrogen, -NH2, -OH, -CH3, -Ph, -F, -SH).

2′-yl]-acrylic acid (Por-NH2G), and 2-cyano-3-(2′-(5′,10′,15′,20′tetraaminoporphyrin)-acrylic acid (Por-NH2H), those of 2-cyano-3-(5′-(2′′-(5′′,10′′,15′′,20′′-tetrahydroxyporphyrin)thiophen-2′-yl))-acrylic acid (Por-OHE), 2-cyano-3-[5′-(trans2′′-(2′′′-(5′′′,10′′′,15′′′,20′′′-tetrahydroxyporphyrin)ethen-1′′yl)-thiophen-2′-yl]-acrylic acid (Por-OHG), and 2-cyano-3(2′-(5′,10′,15′,20′-tetrahydroxyporphyrin)-acrylic acid (PorOHH), those of 2-cyano-3-(5′-(2′′-(5′′,10′′,15′′,20′′-tetramethylporphyrin)thiophen-2′-yl))-acrylic acid (Por-CH3E), 2-cyano-3-[5′-(trans-2′′-(2′′′-(5′′′,10′′′,15′′′,20′′′-tetramethylporphyrin)ethen-1′′-yl)-thiophen-2′-yl]-acrylic acid (Por-CH3G), and 2-cyano-3-(2′-(5′,10′,15′,20′-tetramethylporphyrin)-acrylic acid (Por-CH3H), and those of 2-cyano-3-(5′-(2′′-(5′′,10′′,15′′,20′′tetraphenylporphyrin)thiophen-2′-yl))-acrylic acid (Por-PhE), 2-cyano-3-[5′-(trans-2′′-(2′′′-(5′′′,10′′′,15′′′,20′′′-tetraphenylporphyrin)ethen-1′′-yl)-thiophen-2′-yl]-acrylic acid (PorPhG), and 2-cyano-3-(2′-(5′,10′,15′,20′-tetraphenylporphyrin)acrylic acid (Por-PhH), those of 2-cyano-3-(5′-(2′′- dihydrogenporphyrin)thiophen-2′-yl))-acrylic acid (Por-E), 2-cyano-3-[5′(trans-2′′-(2′′′- dihydrogenporphyrin)ethen-1′′-yl)-thiophen2′-yl]-acrylic acid (Por-G), and 2-cyano-3-(2′- dihydrogenporphyrin)-acrylic acid (Por-H), and those of 2-cyano-3-[5′(trans-2′′-(2′′′-(5′′′,10′′′,15′′′,20′′′-tetrafluoroporphyrin)ethen1′′-yl)-thiophen-2′-yl]-acrylic acid (Por-FG) and 2-cyano-3[5′-(trans-2′′-(2′′′-(5′′′,10′′′,15′′′,20′′′tetrasulfhydrylporphyrin)ethen-1′′-yl)-thiophen-2′-yl]acrylic acid (Por-SHG) porphyrins were also suggested to be promising as sensitizer candidates for DSSCs. These combinations were designed and calculated at the density functional B3LYP level to provide further complementary information on this series of compounds in the current work. 2. Computational Methods The structures of different porphyrin donors and acceptor moieties used in this study are shown in Figure 1. Considering the influences of the central ions, the two central H+ ions were also replaced by Zn2+ ion for -NH2-, -OH-, and -CH3substituted porphyrin donors and acceptor G as preferred sensitizer candidates. Full geometry optimizations and frequency/ intensity calculations were performed at the density functional B3LYP level using the 6-31G (d) basis set. The electronic absorption spectra of the novel porphyrin sensitizers were calculated with the time-dependent density functional theory (TDDFT) method in THF solvent. All calculations were performed using the Gaussian03 program20 on the IBM P690 system in the Shandong Province High Performance Computer Center.

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Figure 2. Molecular orbital energy level graphs of amino porphyrin derivatives.

TABLE 1: Molecular Orbital Compositions of the Amino Porphyrin Derivatives Por-NH2C

Por-NH2D

Por-NH2E

HOMO LUMO HOMO LUMO HOMO LOMO substituent acceptor -CN

23.5% 1.0%

37.2%

Por-NH2F

24.1% 1.0%

48.6%

Por-NH2G

23.9% 1.1% 0.04%

69.9% 4.5%

Por-NH2H

HOMO LUMO HOMO LUMO HOMO LUMO substituent acceptor -CN

23.3% 1.0%

53.3%

23.3% 1.3% 0.04%

66.6% 3.5%

23.4% 1.8% 0.29%

28.4% 3.5%

3. Results and Discussion 3.1. Molecular Orbitals of the Selected Novel Porphyrins. The ground-state electronic structures were calculated to determine the energy levels and compositions of the molecular orbitals. There is no imaginary vibration predicted in the frequency calculations, indicating that the energy minimum structures of all the selected novel porphyrins are true energy minima. 3.1.1. Molecular Orbitals of Amino Porphyrin DeriWatiWes. The molecular orbital energy level graphs from LUMO+3 to HOMO-5 for amino porphyrin derivatives are shown in Figure 2, and the data of the HOMO, LUMO, and HOMO-LUMO gaps are listed in Table S1 (Supporting Information). The minimal energy gap between HOMO and LUMO is 1.328 eV for Por-NH2G, which is the smallest energy gap among all the porphyrin candidates in this study. Por-NH2C has the largest energy gap, 1.749 eV, among these amino porphyrin derivatives. According to the previous study,18 the acceptor G has a cyan group and a longer conjugated chain with a thiophene ring, which lowers the LUMO, and the donor Por-NH2 has the highest HOMO among the substituted porphyrins. The energy gap between HOMO and LUMO for Por-NH2G is thus the smallest. In order to obtain more information about the LUMO and HOMO for these amino porphyrins, the compositions of them for all of these compounds were calculated and listed in Table 1. For amino porphyrin derivatives, the contributions of the amino groups to the HOMO of the molecules are around 23%. However, the contributions of different acceptors to the LUMO of the molecules are obviously different. The largest contribution of acceptor E is 69.9%, and the minimal contribution is 28.4% from acceptor H. In addition, acceptors E, G, and H all have

Theoretical Screening of Substituted Porphyrins

J. Phys. Chem. A, Vol. 114, No. 4, 2010 1975 TABLE 2: Orbital Compositions of the Hydroxyl and Methyl Porphyrin Derivatives Por-OHE HOMO LUMO substituent 17.4% acceptor 1.6% -CN 0.08%

52.3% 2.8%

Por-CH3E HOMO substituent 6.6% acceptor 0.19% -CN 0.0004%

Figure 3. Molecular orbital spatial distribution of amino porphyrin derivatives.

Figure 4. Molecular orbital energy level graphs of hydroxyl and methyl porphyrin derivatives.

cyan groups whose contributions to the HOMO of the molecules are no more than 0.29% while to the LUMO are no less than 3.5%. Moreover, in order to validate the possibility of the electronic transition in amino porphyrin derivatives, the molecular orbital spatial distribution of these porphyrin derivatives were also calculated. According to previous discussions,18 the HOMO of the whole molecule should be mainly dominated by the HOMO of the donor and the LUMO be dominated by the LUMO of the acceptor. When the donor absorbs light energy it injects electrons into the LUMO of the acceptor, which then injects it into the conduction band of TiO2. In the current calculations, all paired amino porphyrin derivatives have their HOMO mainly localized in the donor region and LUMO much more localized in the acceptor region as shown in Figure 3, which indicates good electron-separated states. This corresponds well with anticipation from the analysis of the separated donor and acceptor moieties. 3.1.2. Molecular Orbitals of the Hydroxyl and Methyl Porphyrin DeriWatiWes. Similarly, the energy level graphs from LUMO+3 to HOMO-5 of hydroxyl and methyl porphyrin derivatives are shown in Figure 4, and related data of the HOMO, LUMO, and HOMO-LUMO gaps are listed in Table S2 (Supporting Information). The minimal energy gap between the HOMO and the LUMO is 1.509 eV for Por-OHG, which is smaller than those of the amino porphyrin derivatives except Por-NH2G. It may be related to the reason that the energy gap

Por-OHG

Por-OHH

HOMO

LUMO HOMO LOMO

17.5% 0.26% 0.002%

62.9% 3.2%

Por-CH3G

23.3% 1.4% 0.21%

28.5% 3.5%

Por-CH3H

LUMO HOMO LUMO HOMO LUMO 40.5% 2.7%

6.3% 2.4% 0.09%

59.4% 3.1%

6.5% 1.1% 0.18%

24.9% 3.1%

of Por-OH (2.168 eV) and Por-NH2 (1.997 eV) is similar, and the energy gap of G is much smaller than C, D, E, F, and H.18 If the donor and acceptor both have small HOMO-LUMO gaps, the donor and acceptor pairs would be more likely to have small HOMO-LUMO gaps.19 Herein, all of the HOMO-LUMO gaps of the hydroxyl porphyrin derivatives are smaller than those of the methyl pophyrins. When the same donor was paired with different acceptors, the gaps of G derivatives are much smaller than the other acceptor derivatives. The compositions of the LUMO and HOMO of the hydroxyl and methyl porphyrin derivatives are shown in Table 2. For the hydroxyl porphyrin derivatives, the contributions of the hydroxyl group to the HOMO of the molecules, are around 17%, and the contributions of the acceptors to the LUMO range from 28.5% of H to 62.9% of G. For the methyl porphyrin derivatives, the contributions of the methyl groups to the HOMO of the molecules are around 6.5%. The contributions of the acceptor G to LUMO are the largest, while the contribution of the acceptor H is also the smallest. The contributions of the cyano groups to the HOMO of the molecules are all below 0.21%; however, the LUMO is composed of around 3% cyan group for all of the hydroxyl and methyl porphyrin derivatives. The molecular orbital spatial distributions of the hydroxyl and methyl porphyrin derivatives are shown in Figure 5, which indicate that the hydroxyl and methyl porphyrin derivatives also have good electron-separated states. This also corresponds well with anticipation from the analysis of the separated donor and acceptor moieties. The HOMO mainly dominated by the hydroxyl- and methyl-substituted porphyrin donors and the LUMO are much more dominated by acceptors E, G, and H.

Figure 5. Molecular orbital spatial distribution of the hydroxyl and methyl porphyrin derivatives.

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Figure 6. LUMO and HOMO energy level graphs of phenyl-, sulfhydryl-, fluoro-, and hydrogen-substituted porphyrin derivatives.

TABLE 3: Orbital Compositions of Porphine as well as the Phenyl-, Sulfhydryl-, and Fluoro-Substituted Porphyrin Derivatives Por-PhE

Por-PhG

Por-PhH

HOMO LUMO HOMO LUMO HOMO LOMO substituent 13.6% acceptor 0.72% -CN 0.04%

41.1% 2.6%

11.8% 3.6% 0.16%

Por-E

56.9% 3.1%

13.8% 0.8% 0.2%

Por-G

20.5% 2.4%

Por-H

HOMO LUMO HOMO LUMO HOMO LUMO substituent acceptor -CN

2.8% 0.16%

50.3% 3.2%

8.2% 0.37%

Por-FG substituent acceptor -CN

57.1% 3.0%

0.9% 0.2%

24.6 3.1%

Por-SHG

HOMO

LOMO

HOMO

LUMO

10.2% 3.8% 0.2%

47.6% 2.4%

22.1% 1.4% 0.05%

39.0% 2.0%

3.1.3. Molecular Orbitals of Porphine as well as the Phenyl-, Sulfhydryl-, and Fluoro-Substituted Porphyrin DeriWatiWes. The orbital energy level graphs from LUMO+3 to HOMO-5 of porphine as well as phenyl-, sulfhydryl-, and fluoro-substituted porphyrin derivatives are shown in Figure 6 and related data of the HOMO, LUMO, and HOMO-LUMO gaps are listed in Table S3 (Supporting Information). The energy gap between the HOMO and the LUMO is 2.499 eV for Por-E, which is the largest among all these porphyrin derivatives. Similar to the amino, hydroxyl, and methyl porphyrin derivatives, when porphine and phenyl-substituted porphyrin donors are paired with different acceptors, those compounds with acceptor G have the smallest HOMO-LUMO gaps. For all compounds shown in Figure 6, although Por-FG and Por-SHG have smaller HOMO-LUMO gaps than porphine and phenylsubstituted porphyrin, their HOMO-LUMO gaps are all above 2.0 eV, which are larger than those of the amino- and hydroxylsubstituted porphyrin derivatives. The compositions of the LUMO and HOMO of porphine as well as phenyl-, sulfhydryl-, and fluoro-substituted porphyrin derivatives are shown in Table 3. For the phenyl-substituted derivatives, the contributions of the phenyl groups to the HOMO of the molecules are around 13%, which are much smaller than those of the amino and hydroxyl substituents. The contribution

Figure 7. Molecular orbital spatial distribution of phenyl-, sulfhydryl-, fluoro-, and hydrogen-substituted porphyrin derivatives.

of G to the LUMO is also the largest. For the porphine donor derivatives, the contribution of the acceptor H to the LUMO of the molecule is smallest and that of the acceptor G is the largest. For Por-FG, the HOMO is composed of 10.2% fluorine and the LUMO consists of 47.6% G character, which includes 2.4% -CN. For Por-SHG, the HOMO consists of 22.1% sulfhydryl and the LUMO consists of 39.0% G character, which includes 2.0% -CN. The molecular orbital spatial distributions of porphine as well as the phenyl-, sulfhydryl-, and fluoro-substituted porphyrin derivatives are shown in Figure 7. All these porphyrin derivatives have good electron-separated states and correspond well with what is anticipated from analysis of the separated donor and acceptor moieties. The HOMO mainly dominated by the porphyrin donors and the LUMO are much more dominated by the acceptors. According to the discussion above and the calculation data, the energy gaps of the same donor and different acceptors follow the trend G < E < H < F < D < C. That is, the donors paired with the acceptor G have the smallest energy gap and those with C have the largest. For different donors with the same acceptor, the order of the energy gap is Por-NH2 < Por-OH < Por-CH3 < Por-F < Por-Ph < Por-SH < Por. 3.1.4. Molecular Orbitals of the Amino-, Hydroxyl-, and Methyl-Substituted Zinc Metalloporphyrin DeriWatiWes. The orbital energy level graphs from LUMO+3 to HOMO-5 of the amino-, hydroxyl-, and methyl-substituted zinc metalloporphyrin derivatives are shown in Figure 8, and related data of the HOMO, LUMO, and HOMO-LUMO gaps are listed in Table S4 (Supporting Information). The energy gap between the HOMO and the LUMO is 1.402 eV for 2-cyano-3-[5′-(trans2′′-(2′′′-(5′′′,10′′′,15′′′,20′′′-tetraaminoporphyrinatozinc(II)yl)ethen1′′-yl)-thiophen-2′-yl]-acrylic acid (ZnPor-NH2G), which is larger than corresponding Por-NH2G, 1.328 eV. Similarly, the HOMO-LUMO gap of 2-cyano-3-[5′-(trans-2′′-(2′′′(5′′′,10′′′,15′′′,20′′′-tetrahydroxyporphyrinatozinc(II)yl)ethen-1′′yl)-thiophen-2′-yl]-acrylic acid (ZnPor-OHG) is 1.627 eV, which is larger than that of Por-OHG, 1.509 eV. It is 2.234 eV for 2-cyano-3-[5′-(trans-2′′-(2′′′-(5′′′,10′′′,15′′′,20′′′-tetramethylporphyrinatozinc(II)yl)ethen-1′′-yl)-thiophen-2′-yl]-acrylic acid (ZnPor-CH3G), which is also larger than Por-CH3G, 2.083 eV. The HOMO-LUMO energy gaps of these zinc metalloporphyrins are a little higher than the corresponding dihydroporphyrins. The HOMO energy levels of zinc metalloporphyrin compounds

Theoretical Screening of Substituted Porphyrins

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Figure 8. Molecular orbital energy level graphs of the amino-, hydroxyl-, and methyl-substituted zinc porphyrin derivatives.

TABLE 4: Orbital Compositions of the Amino-, Hydroxyl-, and Methyl-Substituted Zinc Metalloporphyrin Derivatives ZnPor-NH2G

ZnPor-OHG

HOMO LUMO HOMO

LUMO

Zn 1.93% 0.07% 1.85% 0.005% Substituent 28.65% 22.54% Acceptor 0.95% 78.44% 1.42% 76.78%

ZnPor-CH3G

Figure 10. Electronic absorption spectra of the amino porphyrin derivatives.

HOMO LOMO 1.54% 8.84% 3.66%

0.10% 68.68%

are much lower than the corresponding free base porphyrin derivatives. However, the LUMO energies of zinc metalloporphyrin compounds are not much higher than the corresponding free base porphyrin derivatives. The compositions of LUMO and HOMO of the zinc metalloporphyrin derivatives are shown in Table 4. For all the zinc metalloporphyrin compounds, the contributions of the zinc ions to the HOMO are above 1.50% but to the LUMO are below 0.1%. This coincides with the HOMO energy levels of zinc metalloporphyrin compounds which are much lower than the corresponding free base porphyrin derivatives. However, the LUMO energy levels of the zinc metalloporphyrin compounds are not much higher than the corresponding free base porphyrin derivatives. That is, the central zinc ion has much more influence on the HOMO than on the LUMO. In addition, the contributions of the substituents to the zinc metalloporphyrin compounds are larger than to the corresponding free base porphyrin derivatives. For example, the contribution of the amino groups to the HOMO of ZnPor-NH2G is 28%, but to the corresponding free base porphyrin Por-NH2G it is only 23%. The contributions of the acceptor G to the LUMO of the zinc metalloporphyrin compounds are also larger than the corresponding free base porphyrin derivatives.

Figure 9. Molecular orbital spatial distribution of the amino-, hydroxyl-, and methyl-substituted zinc metalloporphyrin derivatives.

The molecular orbital spatial distributions of the amino-, hydroxyl-, and methyl-substituted zinc metalloporphyrin derivatives are shown in Figure 9. These compounds also have good electron-separated states, which corresponds well with the anticipation from the analysis of the separated donor and acceptor moieties. The HOMO mainly dominated by the amino-, hydroxyl- and methyl-substituted zinc metalloporphyrin donors and the LUMO are much more dominated by the acceptor G. The order of the energy gaps is ZnTPPG (2.194 eV) < ZnTMPPI (2.288 eV) < ZnTPPH (2.321 eV) < ZnTPPE (2.338 eV) < ZnTPPA (2.567 eV) according to our previous results,18 while the experimental photon-to-current conversion efficiencies reported in previous research are ZnTMPPI (7.1%)17 > ZnTPPH (5.2%)16 > ZnTPPI (5.1%)17 > ZnTPPA (4.11%).14 It seems to some extent that the smaller the HOMO-LUMO gap of the sensitizer is, the higher the efficiency of the corresponding solar cell. ZnTMPPI has the highest photon-to-current conversion efficiencies, 7.1%, and the HOMO-LUMO gap is 2.288 eV calculated at the B3LYP/6-31G(d) level. Most of the aminoand hydroxyl-substituted porphyrin derivatives have HOMOLUMO gaps smaller than 2.288 eV for ZnTMPPI and even smaller than 2.194 eV for ZnTPPG according to the current calculations, which indicates that these candidates are also very promising to challenge the current conversion efficiency record of 7.1% of ZnTMPPI. 3.2. Electronic Absorption Spectra. The calculated wavelengths, oscillator strengths, transition energies, and molecular orbital excitations for the most relevant electron transitions of the electronic absorption bands of the free base porphyrin derivatives and the selected zinc metalloporphyrins were obtained through TDDFT calculations in tetrahydrofuran (THF). The electronic spectra are simulated by fitting to the Lorentzian line shape with a half-width at half-maximum of 12 nm. The UV-vis spectra simulated are shown in Figures 10-13, and the most representative calculated wavelengths and electron transitions of the novel porphyrins are collected in Tables S5-S8 (Supporting Information). 3.2.1. Electronic Absorption Spectra of the Amino Porphyrin DeriWatiWes. The electronic absorption spectra of the amino porphyrin derivatives simulated in THF solvent are shown in Figure 10, and the most representative calculated wavelengths and optical transitions are collected in Table S5 (Supporting

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Figure 11. Electronic absorption spectra of the hydroxyl and methyl porphyrin derivatives.

Figure 12. Electronic absorption spectra of porphine as well as the phenyl-, sulfhydryl-, and fluoro-substituted porphyrin derivatives.

Information). These porphyrin derivatives show a series of bands between 230 and 550 nm due to the conjugated macrocycle, and they all have absorptions between 620 and 710 nm. In particular, the strongest absorption at 547 nm belonging to the HOMO-1 to LUMO (87%) of Por-NH2G is red shifted with respect to the other amino porphyrin derivatives with the same transition. Herein, the absorption at 442 nm of Por-NH2C being assigned to the electronic transition from HOMO-1 to LUMO is also hypochromatic shifted compared to any other amino porphyrin derivatives. All of these changes in the electronic absorption spectra can be understood according to the calculated orbital energy levels for the amino porphyrin derivatives in Table S1 (Supporting Information). 3.2.2. Electronic Absorption Spectra of the Hydroxyl and Methyl Porphyrin DeriWatiWes. Electronic absorption spectra of the hydroxyl and methyl porphyrin derivatives simulated in THF solvent are shown in Figure 11, and the most representative calculated wavelengths and optical transitions are collected in Table S6 (Supporting Information). For Por-OHG, the absorptions are red shifted from the other hydroxyl and methyl porphyrin derivatives due to its smaller energy gap. The energy gap of Por-OH is smaller than that of Por-CH3, and the acceptor

Ma et al.

Figure 13. Electronic absorption spectra of the amino-, hydroxyl-, and methyl-substituted zinc metalloporphyrin derivatives.

G has a smaller energy gap than E, which is then smaller than H. The absorptions of the compounds had a bathochromic shift from Por-CH3H to Por-OHG gradually. Their primary absorptions centralize at 250-540 nm, while Por-CH3G has a stronger absorption at 689 nm, being assigned to electronic transition from the HOMO to the LUMO (87%). This corresponds well with the calculated orbital energy levels for the hydroxyl and methyl porphyrin derivatives listed in Table S2 (Supporting Information). 3.2.3. Electronic Absorption Spectra of Porphine as well as the Phenyl-, Sulfhydryl-, and Fluoro-Substituted Porphyrin DeriWatiWes. Similarly, these compounds display intense absorptions in the simulated electronic absorption spectra in the region of 300-540 nm and even longer wavelength. For example, Por-G has absorptions at 620 (f ) 0.50) and 603 nm (f ) 0.31). For these porphyrin derivatives, their strongest absorptions are all near 400 nm. All of these can be seen from the electronic absorption spectra of porphine as well as phenyl-, sulfhydryl-, and fluoro-substituted porphyrin derivatives simulated in THF solvent in Figure 12. The most representative calculated wavelengths and optical transitions are collected in Table S7 (Supporting Information). For the same acceptor with different donors, the amino porphyrin derivatives have more absorption peaks and bathochromic shift with respect to the other compounds. For the same donor with different acceptors, such as amino-substituted compounds, they all have strong absorptions between 230 and 550 nm and even 620 and 710 nm. Those compounds with G acceptor have the largest bathochromic shift. The order of their strongest absorption bands is Por-NH2G (547 nm) > Por-OHG (538 nm) > Por-SHG (528 nm) > Por-FG (522 nm) > Por-CH3G (461 nm) > Por-PhG (392 nm) > Por-G (366 nm). Hydroxylsubstituted porphyrin derivatives have the same phenomenon that a compound with G acceptor has a longer absorption wavelength. It is in line with their calculated molecular orbital energy levels. 3.2.4. Electronic Absorption Spectra of the Amino-, Hydroxyl-, and Methyl-Substituted Zinc Metalloporphyrin DeriWatiWes. Electronic absorption spectra of the amino-, hydroxyl-, and methyl-substituted zinc metalloporphyrin derivatives simulated in THF solvent are shown in Figure 13, and the most representative calculated wavelengths and optical transitions are collected in Table S8 (Supporting Information). The strongest

Theoretical Screening of Substituted Porphyrins absorption of zinc metalloporphyrin derivatives has a bathochromic shift compared to the corresponding free base porphyrin derivatives. The strongest absorption of ZnPor-NH2G at 562 nm, assigned to an electronic transition from HOMO-1 to LUMO (87%), bathochromic shifts 15 nm compared to 547 nm for Por-NH2G with the same transition. Similarly, the strongest absorption of ZnPor-OHG at 544 nm bathochromic shifts compared to Por-OHG at 538 nm and 473 nm for ZnPor-CH3G, which is also more bathochromic shifted than Por-CH3G at 461 nm. In addition, these zinc metalloporphyrin derivatives show a series of strong absorption bands between 230 and 700 nm, which indicates the possibility of wide range absorption in the solar spectrum and high photon-to-current conversion efficiency. The -NH2-substituted porphyrin donors combined with G acceptor are found to be the best systems from the discussions above. Further synthesis of these promising sensitizer candidates is in progress in our group. They will be manufactured into DSSC devices and tested comparatively with the best porphyrin sensitizer ZnTMPPI and the N3 dye at the same conditions to check if the theoretical prediction is correct or not. 4. Conclusions We present here density functional theory calculations of the novel sensitizer candidates formed by -NH2-, -OH-, -CH3-, -F-, and -SH-substituted porphyrin donors and corresponding acceptors. The results show that the porphyrin donors with amino, hydroxyl, and methyl groups can significantly boost the HOMO energy level, which have better performance than the other donors. The acceptor G can significantly reduce the LUMO energy level due to its longer conjugated chain with a thiophene ring and the cyan group, which has better performance than the other acceptors. The HOMO-LUMO gaps of most of the -NH2-, -OH- and -CH3-substituted porphyrin sensitizer candidates are much smaller than that of ZnTMPPI (2.288 eV), in which the smallest energy gap of Por-NH2G is only 1.327 eV, indicating that a higher efficiency than 7.1% may be obtained with these novel porphyrin derivatives. This further suggests that the concepts of attribute axis and attribute coordinate system are very helpful for tuning the molecular properties and rational design of functional molecules with anticipated good properties.

J. Phys. Chem. A, Vol. 114, No. 4, 2010 1979 Acknowledgment. The authors thank the National Natural Science Foundation of China (Grant No. 20501011) and TaiShan Scholar Research Fund for financial support. Supporting Information Available: Calculated data of the HOMO, LUMO, and HOMO-LUMO gaps, wavelength, oscillator strength, transition energy, and molecular orbital excitations for the novel porphyrin sensitizer candidates. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Hagfledt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (3) Gra¨tzel, M. Nature 2001, 414, 338. (4) Robertson, N. Angew. Chem., Int. Ed. 2006, 45, 2–10. (5) Hamann, T. W.; Jensen, R. A.; Martinson, A. B. F.; Ryswyk, H. V.; Hupp, J. T. Energy EnViron. Sci. 2008, 1, 66–78. (6) Nazeeruddin, Md. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Grat¨zel, M. J. Am. Chem. Soc. 2005, 127, 16835. (7) Gra¨tzel, M. J. Photochem. Photobiol. C 2003, 4, 145–153. (8) Mizuseki, H.; Niimura, K.; Majumder, C.; et al. Mol. Cryst. Liq. Cryst. 2003, 406, 11/[205]. (9) Wamser, C. C.; Kim, H.-S.; Lee, J.-K. Opt. Mater. 2002, 21, 221. (10) Kroeze, J. E.; Savenije, T. J.; Warman, J. M. J. Photochem. Photobiol. A 2002, 148, 49. (11) Rai, S.; Ravikan, M. Tetrahedron 2007, 63, 2455. (12) Flamigni, L.; Ventura, B.; Tasior, M.; Gryko, D. T. Inorg. Chim. Acta 2007, 360, 803. (13) Wienke, J.; Schaafsma, T. J. J. Phys. Chem. B 1999, 103, 2702. (14) Nazeeruddin, Md. K.; Humphry-Baker, R.; Officer, D. L.; Campbell, W. M.; Burrel, A. K.; Grat¨zel, M. Langmuir 2004, 20, 6514–6517. (15) Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W. Coord. Chem. ReV. 2004, 248, 1363–1379. (16) Wang, Q.; Campbell, W. M.; Bonfantani, E. E.; Jolley, K. W.; Officer, D. L.; Walsh, P. J.; Gordon, K.; Humphry-Baker, R.; Nazeeruddin, Md. K.; Grat¨zel, M. J. Phys. Chem. B 2005, 109, 15397. (17) Campbell, W. M.; Jolley, K. W.; Wagner, P.; Wagner, K.; Walsh, P. J.; Gordon, K. C.; Schmidt-Mende, L.; Nazeeruddin, Md. K.; Wang, Q.; Grat¨zel, M.; Officer, D. L. J. Phys. Chem. C 2007, 111, 11760. (18) Ma, R.; Guo, P.; Cui, H.; Zhang, X.; Nazeeruddin, Md. K.; Gra¨tzel, M. J. Phys. Chem. A 2009, 113, 10119–10124. (19) Balanay, M. P.; Dipaling, C. V. P.; Lee, S. H.; Kim, D. H.; Lee, K. H. Sol. Energy Mater. Sol. Cells 2007, 91, 1775–1781. (20) Frisch M. J. Gaussian′03, revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003.

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