meso-Diphenylbacteriochlorins: Macrocyclic Dyes ... - ACS Publications

Mar 8, 2017 - We herein report the synthesis, UV–vis absorption, fluorescence emission, and redox properties of three novel meso-diphenylbacteriochl...
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meso-Diphenylbacteriochlorins: Macrocyclic Dyes with Rare Colors for Dye-Sensitized Solar Cells Subrata Chakraborty, Huei-Chi You, Chung-Kai Huang, Bo-Zhi Lin, Chin-Li Wang, Ming-Chi Tsai, Chia-Lin Liu, and Ching-Yao Lin* Department of Applied Chemistry, National Chi Nan University, Puli, Nantou Hsien, Taiwan 54561 S Supporting Information *

ABSTRACT: We herein report the synthesis, UV−vis absorption, fluorescence emission, and redox properties of three novel meso-diphenylbacteriochlorins. Significantly, we show that substituents at the meso-positions of an air-stable bacteriochlorin can be manipulated for the first time. With proper design, this allows tailor-made bacteriochlorins to exhibit suitable properties for a chosen application. As an example, photovoltaic properties of two such bacteriochlorins in dye-sensitized solar cells are investigated. The results show that the bacteriochlorin dyes outperform a reference porphyrin dye. Intriguingly, the anodes sensitized with the new dyes display rare colorsblue and pink.

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INTRODUCTION Bacteriochlorins are planar, highly conjugated macrocyclic molecules. They give rise to three groups of intense absorptions in the near-UV, visible, and near-IR region. Together with chlorins and isobacteriochlorins, these chromophores are known as hydroporphyrins.1 There are naturally occurring bacteriochlorins in biological systems. For example, crystal structures of bacterial photosynthetic reaction centers reveal bacteriochlorophylls (Mg2+ complexes) and bacteriopheophytins (free-base bacteriochlorins) in the systems.2−4 The crucial roles of these molecules in purple bacteria include light harvesting, energy transduction, and charge separation.5,6 Protected by amino acids, bacteriochlorophylls and bacteriopheophytins are stable within the proteins. Without the amino acids, however, these molecules become unstable because the pyrroline rings in the chemical structures are susceptible to oxidation reactions. This increases the difficulty of studying them and limits the potential of using bacteriochlorins for various applications. Thus, there have been reports of synthetic bacteriochlorins. Possibly the most convenient way to make a bacteriochlorin is to carry out a diimide reaction on a porphyrin.7 The problem remains. Bacteriochlorins generated by this method are not stable, either. To improve stability, there have been studies of saturating βpositions of the pyrroline rings with alkyl groups. These bacteriochlorins are stable in air.8−10 For naturally occurring bacteriochlorins and most of the synthetic air-stable bacteriochlorins, the substituents are located at β-positions of the pyrrole/pyrroline rings. To effectively implement bacteriochlorins in a wide range of studies and applications, meso-substitution may be more beneficial for the following reasons. Steric hindrance of the meso-substituents © 2017 American Chemical Society

could provide protection against undesired reactions at the sites. By changing the meso-substituents, it is relatively easy to achieve push−pull (or donor−acceptor), push−push, or pull− pull effects for various purposes. The importance and benefits of using meso-substituents have been well-demonstrated for porphyrins in solar cells,11−14 catalysis,15,16 metal−organic framework,17,18 electron transfer,19,20 sensors,21,22 molecular motors,23 molecular electronics,24−26 nonlinear optical materials,26−28 two-photon absorption studies,28,29 photodynamic therapy,30,31 and polymer memory devices.32 Similar benefits are expected for bacteriochlorins. In other words, the ability to synthesize symmetrically or asymmetrically meso-diphenylbacteriochlorins may have profound influences in the aforementioned studies and applications. In this work, we report the synthesis and properties of three meso-diphenylbacteriochlorins. Most importantly, the synthesis shows that meso-substitution of an air-stable bacteriochlorin can be controlled for the first time. This achievement allows easy preparation of suitable derivatives for the applications mentioned above. Figure 1 depicts the structures of the bacteriochlorins and a reference porphyrin in this report, denoted as LS-00, LS-01, LS-11, and H2PE1, respectively. As shown for the LS bacteriochlorins, the upper-left and lowerright pyrroline rings are each saturated with two methyl groups at one of the β-positions, and the top and down meso-positions each bear a phenyl ring. For LS-00, the left and right mesopositions each bear a proton. For LS-01, a proton and an ethynyl benzoic acid occupy the left and right meso-position, Received: January 4, 2017 Revised: March 3, 2017 Published: March 8, 2017 7081

DOI: 10.1021/acs.jpcc.7b00097 J. Phys. Chem. C 2017, 121, 7081−7087

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The Journal of Physical Chemistry C

chlorins with desired structures or properties. The synthetic details are put in the Supporting Information. UV−Vis Absorption and Fluorescence Emission Spectra. Figure 2 overlays UV−vis and fluorescence spectra of the LS bacteriochlorins and H2PE1 porphyrin. The related parameters are put in Table 1.

Figure 1. Chemical structures of the bacteriochlorins and a porphyrin.

respectively. For LS-11, an electron-donating group is added to the left meso-position to enhance push−pull tendency of the molecule. A known porphyrin, H2PE1,33 is used as a reference compound to compare with LS-01 because of the similarity in the chemical structures. As one of the possible applications, we applied LS-01 and LS-11 in a dye-sensitized solar cell (DSSC). For LS-01, LS-11, and H2PE1, the caboxylic acid group serves as an anchoring group for DSSC applications. DSSC has been of interest in recent decades because of the advantages of low cost, convenient fabrication processes, and versatile design of the dyes.34,35 Ruthenium complexes,36,37 porphyrins,11,12,38 and organic dyes39,40 all have been shown to achieve high efficiencies. Among the dyes, porphyrins have been intensively studied owing to their strong absorption bands in the visible region. However, it proves to be quite challenging for porphyrin-based DSSCs to harvest solar energy in the nearIR region.41 Bacteriochlorins and the derivatives should be more advantageous in this aspect because of their intrinsic nearIR absorption bands.10,42−44

Figure 2. UV−vis absorption (solid lines) and fluorescence emission (dashed curves) spectra of the LS bacteriochlorins and porphyrin H2PE1 in THF.

The absorption spectrum of H2PE1 is consistent with that of a porphyrin,1 i.e., strong B bands at 431 nm and much weaker Q bands around 500−600 nm. For LS-00, LS-01, and LS-11, these macrocycles give rise to typical bacteriochlorin absorption spectra,1 and they are very different from that of H2PE1. The B bands appear in the near-UV region from 300 to 400 nm, the Qx bands locate in the visible region from 500 to 650 nm, and the Qy bands are in the near-IR region from 700 to 800 nm. Comparison of LS-01 and H2PE1 shows that the B bands of LS-01 are noticeably blue-shifted, the Qx bands are located in the similar region, and the Qy bands are largely red-shifted from those of H2PE1. In addition, the Q bands of LS-01 are more intense than those of H2PE1. These characteristics are consistent with literature reports.1,8−10 Among the LS bacteriochlorins, a trend of LS-11 > LS-01 > LS-00 is observed for the absorption wavelengths. The longer wavelengths of LS11 and LS-01 are consistent with their having more conjugated double bonds in the chemical structures. Additionally, the Qx bands are noticeably broadened from LS-00 to LS-11. This phenomenon is consistent with other bacteriochlorin systems45 Note that the number of the meso-substituents is increased from LS-00 to LS-11. The broadening of the Qx bands may be related to the increasing interactions between the bacteriochlorin core and the meso-substituents along the x-axis.46,47 Fluorescence emissions of the LS bacteriochlorins are mirror images of their corresponding Qy absorptions. The emission wavelengths also show a trend of LS-11 > LS-01 > LS-00. Fluorescence quantum yields of the LS bacteriochlorins are comparable with other bacteriochlorins, and they are greater than that of H2PE1.8−10,48 UV−vis spectra of H2PE1, LS-01, and LS-11 in THF were also compared with those on TiO2 films (Figure S2 in the Supporting Information). For H2PE1, the absorption bands of the film spectra are considerably broadened and shifted relative to their solution counterparts. This may be attributed to

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RESULTS AND DISCUSSION Structural Design and Synthesis. In our design, the top and down meso-positions of the LS bacteriochlorins are each blocked with a phenyl group, whereas the left and right mesopositions remain available for further modification. To block the top and down meso-positions, we first synthesized a chiral nitroethylpyrrole precursor, 2-(2-nitro-1-phenylethyl)pyrrole. An α-keto acetal was then attached to the chiral precursor via Michael addition, followed by cyclization to give a dihydrodiyrrin-acetal. Acid-catalyzed self-condensation of the dihydrodiyrrin-acetal afforded LS-00 at 37% yield. The last three steps of the synthesis were based on the literature reports.9,10 Significantly, monobromination of LS-00 was successfully carried out at 85% yield. Such a reaction was reported disastrous for other air-stable bacteriochlorins.9 Monobrominated LS-00 was used to prepare LS-01 via Sonogashira crosscoupling reaction at 75% yield. For LS-11, monobrominated LS-00 was first cross-coupled with the electron-donating group. After a secondary bromination of the remaining meso-position, the monobromo−monodonor precursor was cross-coupled with the anchoring group to afford LS-11 at 46% yield. As such, we show that substitution reactions at the meso-positions of a bacteriochlorin can be controlled to generate bacterio7082

DOI: 10.1021/acs.jpcc.7b00097 J. Phys. Chem. C 2017, 121, 7081−7087

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The Journal of Physical Chemistry C

Table 1. Absorption Wavelengths, Fluorescence Maxima, and the First Macrocyclic Ring Redox Potentials in THF/TBAP E1/2 /V vs SCE absorption/nm (log ε, M−1 cm−1)

entry LS-00 LS-01 LS-11 H2PE1

348 372 388 431

(5.14), (5.05), (5.08), (5.56),

370 551 576 526

(5.27), (4.64), (4.51), (4.19),

503 745 615 566

emission/nma (ϕf, %)b

(4.71), 718 (5.09) (4.98) (4.54), 769 (5.05) (4.47), 604 (3.88), 661 (4.02)d

724 748 774 663

(9.83) (7.92) (8.32) (2.10)

Ox(1) 0.65 0.73 0.75 1.16

(130) (110) (90) ()e

Red(1) c

−1.23 −1.05 −0.95 −1.01

(150) (110) (110) (120)

a

Excitation wavelength/nm: LS-00 (370), LS-01 (372), LS-11 (388), H2PE1 (431). bThe quantum yields of the bacteriochlorins were estimated by comparing with 1,3,3,1′,3′,3′,-hexamethyl-2,2′-indotricarbocyanine iodide (HITCI) at 688 nm. The quantum yield of H2PE1 was estimated by comparing with rhodamine 6G (R6G) at 505 nm. cPeak-to-peak separation in millivolts. dTaken from ref 33. ePotential determined by differential pulse voltammetry due to poorer reversibility.

aggregation of the dye molecules on TiO2 surfaces.49,50 In contrast, the absorption bands of the film spectra of LS-01 and LS-11 are only slightly shifted and broadened compared to those in THF. This implies less aggregation of LS-01 and LS-11 on TiO2 surfaces, which is a welcome property of the dyes for DSSC applications (see below). Electrochemistry and Molecular Orbital Patterns. Figure 3 collects the cyclic voltammograms (CV) of LS-00,

addition of an electron-donating group has a greater influence to the reduction potential than to the oxidation potential. In addition, a second oxidation wave is observed at +0.84 V versus SCE for LS-11, overlapping with the first oxidation wave. As a result, a much larger current is observed. These oxidative waves can be better resolved by differential pulse voltammetry, and the second oxidation potential of LS-11 is consistent with that of the electron-donating substituent being oxidized.41,48,50 For DSSC studies (vide infra), the first redox potentials of the dyes are used to estimated the HOMO and LUMO levels of each dye to compare with the conduction bands (CB) of TiO2, and the redox energy of the electrolyte (Figure S4a in the Supporting Information). For comparison, the first oxidation potentials were also used to estimate the ground-to-oxidized states (S0/S+). The zero−zero excitation energies (E0−0) obtained from the intersection of the corresponding normalized absorption and emission spectra were used to estimate the energy gaps between the S*/S+ and the first excited-to-oxidized (S0/S+) states.34 A diagram comparing the S0/S+, S*/S+ states of each dye, the conduction bands (CB) of TiO2, and the redox energy of the electrolyte is provided in the Supporting Information (Figure S4b). Most importantly, the LUMO or S*/S+ levels of the dyes are considerably higher than the conduction bands of TiO2 and the HOMO or S0/S+ levels are noticeably lower than the redox energy of the electrolyte. Therefore, all dyes should be capable of injecting electrons to the CB of TiO2 upon excitation and the resulting cations should be efficiently regenerated by the electrolyte. Additionally, time-dependent density functional theory (TD-DFT) was also used to calculated the HOMO and LUMO levels of the dyes (Figure S4c in the Supporting Information). The results show that the trend of the TD-DFT results is consistent with the experimental results. To help understand the electrochemistry, we carried out calculations by density functional theory (DFT) at the B3LYP/ 6-31G(d,p) level.54 Figure 4 depicts the frontier orbital patterns of the LS bacteriochlorins and H2PE1 porphyrin. Four orbitals are illustrated for each compound, from one orbital below the HOMOs to one level above the LUMOs (or from HOMO − 1 to LUMO + 1). These patterns represent the pi-electron densities/probabilities of each orbital. For H2PE1, the patterns of the frontier orbitals are consistent with those of Gouterman’s four-orbital model.1 For example, the HOMO − 1 and HOMO patterns resemble those of the a1u and a2u orbitals, respectively. The LUMO and LUMO + 1 patterns are similar to those of the eg orbitals. Delocalization of the patterns is observed at the HOMO and LUMO levels. This may be attributed to the presence of a conjugated anchoring group and a lowered molecular symmetry.

Figure 3. Cyclic voltammograms of LS-00, LS-01, LS-11, and H2PE1 in THF/TBAP. For H2PE1 and LS-11, differential pulse voltammograms are shown as thin solid lines.

LS-01, LS-11, and H2PE1 in THF. Selected redox potentials are listed in Table 1. For H2PE1, the first porphyrin-ring reduction and oxidation reactions occur at −1.01 and +1.16 V versus SCE, respectively.33 These reactions are consistent with the formation of a porphyrin anion and cation radical.51 An illshaped reduction wave around −0.8 V has been suggested to be the reduction of the carboxylic acid group.52 Similar reactions are also observed for LS-01 and LS-11, but not for LS-00. For all LS bacteriochlorins, the first macrocyclic reduction and oxidation potentials are consistent with the formation of a bacteriochlorin anion and cation radical, respectively.53 The first macrocyclic reduction potential of LS-01 is similar to that of H2PE1, suggesting that the differences in the chemical structures do not largely affect the energy levels of the lowest unoccupied molecular orbitals (or LUMOs). In contrast, the first oxidation potentials are very different, suggesting that the highest occupied molecular orbitals (or HOMOs) are largely affected. For LS-11, the first macrocyclic reduction and oxidation are found at −0.95 and +0.75 V versus SCE, respectively. Comparison of LS-01 and LS-11 shows that the 7083

DOI: 10.1021/acs.jpcc.7b00097 J. Phys. Chem. C 2017, 121, 7081−7087

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Figure 4. Frontier orbital patterns of the LS bacteriochlorins and H2PE1 porphyrin, calculated by density functional theory (DFT) at the B3LYP/6-31G(d,p) level.

The four-orbital patterns of LS-00 are also consistent with the Gouterman’s model. The HOMO − 1 and HOMO resemble the a2u and a1u orbitals, respectively, and the LUMO and LUMO + 1 resemble the eg orbitals. Note that no patterns appear at the β-carbons of the upper-left and lower-right pyrroline rings. This is consistent with the lack of carbon− carbon double bonds at the said locations. For LS-01, the patterns are similar to those of LS-00. Delocalization of the patterns can be seen at the HOMO − 1, LUMO, and LUMO + 1 levels. The four-orbital patterns of LS-11 are similar to those of LS-01. For delocalization, the electron densities populate at the left side of the molecule at the HOMO − 1 and HOMO levels, whereas the electron probabilities concentrate at the right side of the molecule at the LUMO and LUMO + 1 levels. This translates to a better push−pull tendency of the dye upon excitation, pushing the electron density from the donor group toward the anchoring group. This is a welcome merit of a dye. The DFT-calculated dipole moments of H2PE1, LS-00, LS-01, and LS-11 are 3.37, 0.05, 4.08, and 8.69 D, respectively. These values are consistent with the suggestion that LS-11 has a stronger push−pull tendency. Photovoltaic Properties. The purpose of the photovoltaic measurements is to demonstrate the benefits of using these dyes, not to set a new record. Also, we chose a known dye, H2PE1, instead of synthesizing a new porphyrin in hope to keep the focus on the bacteriochlorins. LS-00 is not included in these tests because it lacks of the necessary anchoring group, i.e., the carboxylic group. Figure 5 shows (a) the current density−voltage (J−V) curves under the irradiance of 100 mW/cm2 simulated AM1.5 sunlight (solid lines) and in the dark (dotted lines), (b) the plots of incident photon-to-electron conversion efficiency (or IPCE) as a function of wavelengths, and (c) the pictures of anodes sensitized with the LS-01 and LS-11 dyes. Related parameters are collected in Table 2. For the overall efficiencies, a trend of LS-11(5.36%) > LS01(4.67%) > H2PE1(2.06%) is observed. These values are comparable with other bacteriochlorin systems reported in the literature.33,42−44 Significantly, we observed more than 2-fold improvement from the H2PE1 to LS-11 cells. This clearly exhibits the benefits of using bacteriochlorin dyes. Superior photovoltaic performance of the LS cells can be attributed to their greater current densities. The greater current densities are consistent with the stronger IPCE responses across the entire

Figure 5. (a) Current density−voltage (J−V) curves under the irradiance of 100 mW/cm2 simulated AM1.5 sunlight (solid lines) and in dark (dotted lines) and (b) the IPCE spectra of the solar cells sensitized with the LS-01, LS-11, and H2PE1 dyes. The overall efficiencies of the specific cells are 2.18% for H2PE1, 4.92% for LS-01, 5.69% for LS-11. (c) Photos of the LS-01 and LS-11 anodes.

spectrum. This suggestion is supported by the same trend of the integrated current densities derived from the IPCE spectra. The stronger IPCE responses and current densities of the LS cells may also benefit from suppressed dye aggregation of LS-01 and LS-11 on the surfaces of TiO2 anodes. As discussed above in the UV−Vis Absorption and Fluorescence Emission Spectra section and Figure S2 in the Supporting Information, the film samples of LS-01 and LS-11 give rise to less broadened absorption bands, i.e., less molecular aggregation. As a result, this may contribute to higher photovoltaic performance of the LS cells.49,50 Upon closer inspection, the IPCE spectra of the H2PE1 and LS cells are consistent with their corresponding absorption spectra. Importantly, IPCE intensities of the LS cells are clearly greater than those of the H2PE1 cell, especially in the longer wavelength region. Between LS-01 and LS-11, the IPCE spectrum of the LS-11 cell exhibits a further intensified and red-shifted curve, likely due to the electron-donating substituent. For fill factors, the value of the LS-11 cell is poorer than that of the LS-01 and H2PE1 cells. This obviously lowers photovoltaic performance of the LS-11 cell. For Voc, the values are very similar for all three dyes. Nonetheless, the values show a trend of H2PE1 > LS-01 > LS11. This is somewhat puzzling because (a) the Jsc value of the H2PE1 cell is smaller than those of the LS-cells and (b) the S*/ S+ energy level of H2PE1 is not higher than those of LS-01 and LS-11 (Table 1 and Figure S4). As for the stability, the LS cells show a larger decay comparing with the H2PE1 cells. Over a period of 1000 h, we observed 31%, 54%, and 46% decreases in the overall efficiencies for the H2PE1, LS-01, and LS-11 cells, respectively (Supporting Information). The decrease in the overall efficiencies is mainly related to the decrease in the Jsc values. This might imply desorption of the dye molecules from the TiO2 surface over a long period of time. 7084

DOI: 10.1021/acs.jpcc.7b00097 J. Phys. Chem. C 2017, 121, 7081−7087

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The Journal of Physical Chemistry C Table 2. Parameters of the LS- and H2PE1-Sensitized Solar Cellsa dye

JscIPCE (mA/cm2)b

Jsc (mA/cm2)

Voc (V)

FF

η (%)

H2PE1 LS-01 LS-11

4.47 ± 0.33 10.29 ± 0.57 14.99 ± 1.16

5.26 ± 0.25 12.58 ± 0.51 16.13 ± 0.76

0.54 ± 0.00 0.53 ± 0.00 0.52 ± 0.01

0.73 ± 0.01 0.70 ± 0.01 0.64 ± 0.01

2.06 ± 0.12 4.67 ± 0.19 5.36 ± 0.21

a

The photovoltaic parameters were obtained under simulated AM-1.5G illumination (power density 100 mW/cm2). The active area was 0.25 cm2 with a black mask of area 0.16 cm2 for each cell. These parameters are averaged values of 5, 8, and 11 cells for H2PE1, LS-01, and LS-11, respectively. b To compare with the JSC obtained from the J−V measurements, JscIPCE is derived via wavelength integration of the IPCE spectra.

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Photovoltaic performance of the LS dyes can be improved via modification of the chemical structures in the future. We have seen similar development of the porphyrin dyes in the past decade.11−14 Therefore, it is safe to suggest the same may be expected for bacteriochlorin dyes. On the other hand, mesodiphenylbacteriochlorins already show their potentials even at this early stage of development. First, energy conversion of the DSSCs easily reaches 870 nm with a simple push−pull bacteriochlorin. This took a very complicated porphyrin system to achieve.41 Second, the colors of bacteriochlorins are bright and versatile. Figure 5c displays the pictures of the anodes sensitized by LS-01 and LS-11. Amazingly, the color changes from pink of LS-01 to blue of LS-11 by merely adding an electron-donating substituent. To the best of our knowledge, LS-01 could be the first pink dye for dye-sensitized solar cells with decent photovoltaic performance.

Corresponding Author

*Fax: +886-49-2917956. Phone: +886-49-2910960, ext. 4152. E-mail: [email protected]. ORCID

Ching-Yao Lin: 0000-0002-3963-5244 Author Contributions

S.C. developed the synthetic route and helped prepare the Supporting Information. S.C., H.-C.Y., C.-K.H., C.-L.W., and M.-C.T. prepared large amounts of the bacteriochlorins, collected the characterization data, and performed the spectral analyses. C.-L.L. gathered the electrochemical data. B.-Z.L. measured the photovoltaic properties. C.-Y.L. proposed the research, oversaw the project, and wrote the paper.

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Notes

The authors declare no competing financial interest.

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CONCLUSIONS In this work, we report three novel meso-diphenylbacteriochlorins. For the synthesis, we demonstrate that symmetrical or asymmetrical substitution at the meso-positions of an air-stable bacteriochlorin can be controlled for the first time. Very recently, meso-ditolylbacteriochlorins with additional ester groups are also reported.55 Further structural modification of those bacteriochlorins is not yet available. In comparison with a reference porphyrin, H2PE1, the LS bacteriochlorins give rise to blue-shifted B band absorptions from 300 to 450 nm, Qx bands from 500 to 650 nm, and largely red-shifted/intensified Qy bands from 700 to 800 nm. The fluorescence emissions are found from 700 to 800 nm and are mirror images of their corresponding Qy absorptions. For the electrochemistry, comparison of LS-01 and H2PE1 shows that the first reduction potentials are similar, whereas the oxidation potentials are largely affected. These phenomena are consistent with other bacteriochlorin systems and the DFT calculations. For the photovoltaic properties, the overall efficiencies show that LS-11 (5.36%) and LS-01 (4.67%) significantly outperform H2PE1 (2.06%) in DSSCs. Finally and most intriguingly, the LS dyes exhibit beautiful and rare colors on the anodes, pink and blue. Although the consensus has been that black dyes could give rise to higher photovoltaic performance owing to their panchromatic absorption of visible light, the rare colors of LS-01 and LS-11 provide diversity in the colors of dyes. This may widen color variety of the DSSCs for indoor and portable applications.

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AUTHOR INFORMATION

ACKNOWLEDGMENTS This project is supported by Ministry of Science and Technology, Taiwan (MOST 104-2119-M-260-001 and MOST 105-2119-M-260-003-MY3). S.C. is supported by Ministry of Science and Technology, Taiwan (MOST 1042811-M-260-003 and MOST 105-2811-M-260-002). We thank Professor Nagao Kobayashi for useful discussion and Professor Ming-Yu Kuo and Miss Rong Yang for assistance of the TDDFT calculations.

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ABBREVIATIONS DSSC, dye-sensitized solar cell; CV, cyclic voltammogram; DPV, differential pulse voltammogram; IPCE, incident photonto-current efficiency

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REFERENCES

(1) Gouterman, M. Spectra of Porphyrins. J. Mol. Spectrosc. 1961, 6, 138−163. (2) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. Structure of the Protein Subunits in the Photosynthetic Reaction Centre of Rhodopseudomonas Viridis at 3Å Resolution. Nature 1985, 318, 618−624. (3) Allen, J. P.; Feher, G.; Yeates, T. O.; Komiya, H.; Rees, D. C. Structure of the Reaction Center from Rhodobacter sphaeroides R-26: the Protein Subunits. Proc. Natl. Acad. Sci. U. S. A. 1987, 84, 6162− 6166. (4) McDermott, G.; Prince, S. M.; Freer, A. A.; HawthornthwaiteLawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Crystal Structure of an Integral Membrane Light-Harvesting Complex from Photosynthetic Bacteria. Nature 1995, 374, 517−521. (5) Scheer, H. Chlorophylls; CRC Press: Boca Raton, FL, 1991. (6) Barber, J.; Andersson, B. Revealing the Blueprint of Photosynthesis. Nature 1994, 370, 31−34. (7) Whitlock, H. W., Jr.; Hanauer, R.; Oester, M. Y.; Bower, B. K. Diimide Reduction of Porphyrins. J. Am. Chem. Soc. 1969, 91, 7485− 7489.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00097. Details of the synthesis and characterization of the LS bacteriochlorins, DSSC fabrication, and photovoltaic characterization (J−V, IPCE, EIS, stability tests) (PDF) 7085

DOI: 10.1021/acs.jpcc.7b00097 J. Phys. Chem. C 2017, 121, 7081−7087

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The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.7b00097 J. Phys. Chem. C 2017, 121, 7081−7087

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DOI: 10.1021/acs.jpcc.7b00097 J. Phys. Chem. C 2017, 121, 7081−7087