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Gold Tris(carboxyphenyl)corroles as Multifunctional Materials: Room Temperature Near-IR Phosphorescence and Applications to Photodynamic Therapy and Dye-Sensitized Solar Cells Abraham B. Alemayehu,† Nicholas U. Day,‡ Tomoyasu Mani,§ Alexander B. Rudine,‡ Kolle E. Thomas,† Odrun A. Gederaas,*,∥ Sergei A. Vinogradov,*,§ Carl C. Wamser,*,‡ and Abhik Ghosh*,† †

Department of Chemistry and Center for Theoretical and Computational Chemistry, UiT − The Arctic University of Norway, N-9037 Tromsø, Norway ‡ Department of Chemistry, Portland State University, Portland, Oregon 97207-0751, United States § Departments of Biochemistry and Biophysics and Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ∥ Department of Cancer Research and Molecular Medicine, Norwegian University of Science and Technology, N-7491 Trondheim, Norway S Supporting Information *

ABSTRACT: Two amphiphilic corroles5,10,15-tris(3-carboxyphenyl)corrole (H3[mTCPC]) and 5,10,15-tris(4-carboxyphenyl)corrole (H3[pTCPC])and their gold complexes have been synthesized, and their photophysical properties and photovoltaic behavior have been investigated. Like other nonpolar gold corroles, Au[mTCPC] and Au[pTCPC] were both found to exhibit room temperature phosphorescence in deoxygenated solutions with quantum yields of ∼0.3% and triplet lifetimes of ∼75 μs. Both compounds exhibited significant activity as dyes in photodynamic therapy experiments and in dye-sensitized solar cells. Upon irradiation at 435 nm, both Au corroles exhibited significant phototoxicity against AY27 rat bladder cancer cells while the free-base corroles proved inactive. Dye-sensitized solar cells constructed using the free bases H3[mTCPC] and H3[pTCPC] exhibited low efficiencies (≪1%), well under that obtained with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin, H2[pTCPP] (1.9%, cf. N719 9.5%). Likewise, Au[pTCPC] proved inefficient, with an efficiency of ∼0.2%. By contrast, Au[mTCPC] proved remarkably effective, exhibiting an open-circuit voltage (Voc) of 0.56 V, a short-circuit current of 8.7 mA cm−2, a fill factor of 0.72, and an efficiency of 3.5%. KEYWORDS: dye-sensitized solar cell, photodynamic therapy, photovoltaic, near-IR phosphorescence, gold corrole, metallocorrole

1. INTRODUCTION Recent years have seen remarkable advances in the synthesis of 5d metallocorroles,1,2 a novel class of sterically mismatched complexes that combine a large 5d metal ion and a sterically constrained corrole ligand. Initially rather capricious, the syntheses have been greatly optimized in recent years.3−9 As potential functional materials, these complexes offer attractive features such as unusually high chemical and photochemical stability. Some of these metallocorroles are known to exhibit room temperature phosphorescence.10−12 These characteristics invite applications in the biomedical field, particularly as sensitizers in photodynamic therapy (PDT).13−15 Gold corroles have also been successfully applied in organic solar cells.16 In this study, we evaluated two new amphiphilic gold tris(3/ 4-carboxyphenyl)corrole derivatives Au[mTCPC] and Au[pTCPC] (Figure 1), as well as the corresponding free bases H3[mTCPC] and H3[pTCPC], with respect to their photophysical and photovoltaic properties and performance in photodynamic therapy (PDT) experiments. For a broader © XXXX American Chemical Society

perspective, photophysical measurements were also carried out for several previously reported, nonpolar tris(p-X-phenyl)corrole derivatives, M[TpXPC], where M = H3,17 Cu,18 Ag,9 and Au9 and X = CF3, F, H, Me, and OMe. The Au corroles were all found to exhibit room temperature phosphorescence at ∼800 nm. The two amphiphilic Au corrolesAu[mTCPC] and Au[pTCPC]also exhibited substantial phototoxicity against AY27 rat bladder cancer cells. Remarkably, these two structurally similar Au complexes were found to differ dramatically in their photovoltaic activity. Dye-sensitized solar cells (DSSCs) based on Au[mTCPC] proved to be far more efficient than those based on Au[pTCPC]. Overall, our results underscore the considerable potential of gold corroles as photoactive materials in a variety of arenas. Received: April 11, 2016 Accepted: June 28, 2016

A

DOI: 10.1021/acsami.6b04269 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. UV−vis absorption spectra of amphiphilic corroles in 0.15 M aqueous K2CO3.

Figure 1. Amphiphilic gold corroles studied in this work.

2. RESULTS AND DISCUSSION 2.1. Photophysical and Electrochemical Properties. Table 1 presents the photophysical properties of the various corroles studied. As expected, the free-base corroles were found to be fluorescent (with quantum yields of 6−13% and fluorescence lifetimes of ∼5 ns),19 whereas the Cu and Ag corroles proved to be (essentially) nonluminescent.9 In contrast, the gold corroles were found to exhibit near-IR phosphorescence at ambient temperatures with phosphorescence lifetimes of ∼75 μs and quantum yields of ∼0.3%. An interesting feature of gold corroles is that their phosphorescence maxima were found to shift bathochromically with increasing electron-donating character of the meso-aryl para substituents X, varying from 788 nm for Au[TpCF3PC] to 804 nm for Au[TpOMePC]. The two amphiphilic Au corroles did not exhibit any unusual photophysical characteristics (Figures 2 and 3) relative to the nonpolar Au corroles. To gain insight into the varying performance of the various amphiphilic corroles as photosensitizers in DSSCs, we also carried out cyclic voltammetry measurements on selected compounds. Unfortunately, free-base corrole derivatives, including those with free carboxyl groups, yielded complex cyclic voltammograms with irreversible features that could not be interpreted. Accordingly, we carried out electrochemical measurements on the methyl esters Au[mTCPC-Me] and Au[pTCPC-Me], which we chose as models for the actual dyes used in the DSSCs. Given that the exact protonation states of

Figure 3. Phosphorescence emission spectra of the two Au corroles in deoxygenated DMA (λex = 560 nm, 22 °C).

the semiconductor-bound dyes are unknown, the use of the methyl esters, in our opinion, is entirely legitimate. Estimates for corrole-centered redox potentials for the free-base corrole esters H3[mTCPC-Me] and H3[pTCPC-Me] were obtained by extrapolation from a Hammett analysis reported by Kadish and co-workers.20,21 Also, to gain insight into the generally poor performance of copper corroles in DSSCs,22,23 we carried out cyclic voltammetry measurements on the Cu derivatives Cu[mTCPC-Me] and Cu[pTCPC-Me].18 The results of these analyses are presented in Figure 4 and Table 2. The key point to note in this connection is that although Cu and Au

Table 1. Photophysical Properties of Selected Free-Base and Gold Corroles λmax,abs (log ε)a

compound H3[TpCF3PC] H3[TpFPC] H3[TpMePC] H3[TpOMePC] Au[TpCF3PC] Au[TpFPC] Au[TPC] Au[TpMePC] Au[TpOMePC] Au[mTCPC] Au[pTCPC]

425 420 422 424 424 422 423 422 425 418 418

(5.21), (5.34), (5.77), (5.16), (5.05), (5.66), (5.20), (5.71), (5.24), (4.87), (4.78),

577 572 564 564 564 561 562 562 562 573 573

(4.41), (4.41), (4.87), (4.34), (4.31), (4.94), (4.44), (4.95), (4.45), (4.11) (3.91)

620 618 621 621 576 577 577 580 584

(4.23), (4.27), (4.77), (4.28), (4.38) (4.98) (4.55) (5.08) (4.62)

651 652 654 659

(4.13) (4.27) (4.80) (4.29)

emission type

λmax,em

ϕb (%)

τc

fluo fluo fluo fluo phos phos phos phos phos phos phos

670 668 674 678 788 788 794 800 804 804 800

10.1 10.5 12.8 6.1 0.19 0.18 0.18 0.24 0.16 0.35 0.29

5.34 ns 5.05 ns 4.88 ns 4.69 ns 98 μs 91 μs 86 μs 83 μs 76 μs 75 μs 75 μs

a Molar extinction coefficients were measured in toluene for all the nonpolar corrole derivatives and in 0.15 M aqueous K2CO3 for the amphiphilic Au corroles. bQuantum yields are reported relative to that for fluorescence of Rhodamine 6G in EtOH (0.94).24 cThe phosphorescence lifetimes and quantum yields of all the nonpolar corrole derivatives were measured in deoxygenated toluene, whereas those of the amphiphilic Au corroles were measured in deoxygenated dimethylacetamide.

B


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Figure 5. Viability of rat bladder cancer cells (AY27) as a function of concentration of free-base corroles (H3[mTCPC], H3[pTCPC]) and gold corroles (Au[mTCPC], Au[pTCPC]) in the absence of light. The cell viability was measured using the MTT assay (see Experimental Section) and normalized to the viability of the cells incubated in a corrole-free medium (control). The results are shown as mean values ± SD from three separate experiments, with duplicates or triplicates in each experiment.

Figure 4. Cyclic voltammograms of Au and Cu m/pTCPC-Me derivatives in CH2Cl2.

Table 2. Redox Potentials (V vs SCE) for TCPC-Me Derivatives corrole Au[mTCPCMe] Au[pTCPCMe] H3[mTCPCMe]a H3[pTCPCMe]a Cu[mTCPCMe]b Cu[pTCPCMe]b

Eox

description

Ered

Cor3−/Cor•2−

−1.34

Cor3−/Cor•4−

+1.00

Cor3−/Cor•2−

−1.39

Cor3−/Cor•4−

+0.51

Cor3−/Cor•2−

−1.20

Cor3−/Cor•4−

+0.54

Cor3−/Cor•2−

−1.16

Cor3−/Cor•4−

+0.86

CuIICor•2−/ CuIICor− CuIICor•2−/ CuIICor−

−0.13

CuIICor•2−/ CuIICor3− CuIICor•2−/ CuIICor3−

+0.91

−0.10

10 μM solutions (24 h, 37 °C). According to the dose− response curves (Figure 6), which plot relative cell viability

description

+0.88

a

These values were obtained with linear free energy relationships reported for free-base triarylcorroles20 and applying the appropriate Hammett substituent constants21 for m-CO2Me (+0.35) and pCO2Me (+0.44). bCopper corroles are noninnocent and are best described as CuII−corrole•2−. See section 2.3 for additional comments.

Figure 6. Cytotoxic effects of blue light illumination (435 nm, 13 mW/cm2, 0−40 min) on rat bladder cancer cells (AY27) incubated in the presence of free-base corroles (H3[mTCPC] or H3[pTCPC]) and Au corroles (Au[mTCPC] or Au[pTCPC]) (10 μM, 24 h, 37 °C). The cell viability was measured using the MTT assay 24 h postillumination (see Experimental Section) and normalized to control cells that were incubated in a corrole-free medium in the absence of light. The results are shown as mean values ± SD from three separate experiments, with duplicates or triplicates in each experiment.

corroles exhibit very similar oxidation potentials, the Cu corroles exhibit far more positive reduction potentials (i.e., are easier to reduce) than the Au corroles. We believe that this difference has major implications for the varying performance of the compounds as dyes in DSSCs; this point is discussed in section 2.3. 2.2. Photodynamic Therapy Experiments. All four amphiphilic corrole derivatives, H3[mTCPC], H3[pTCPC], Au[mTCPC], and Au[pTCPC], were evaluated for their phototoxicity against AY27 rat bladder cancer cells. Cell viability studies were initially carried out via incubation with corrole solutions of different concentrations (0.05−1000 μM) in the dark, thus revealing their chemotoxicity. Each data point in the plots (Figure 5) represents metabolic activity measured relative to the cells incubated in the corrole-free medium (see Experimental Section for details). In the absence of light, the corroles exhibit relatively low toxicity. Up to 90% of the cells were found to be metabolically active after incubation in 50 μM corrole solutions (24 h, 37 °C). However, this fraction was reduced to about 40−60% in 200 μM solution. Incubation in 1000 μM solutions resulted in quantitative cell death. Therefore, the LD50 dose in the dark appears to be ∼300 μM for the free-base corroles and ∼100 μM for the gold corroles. On the basis of the above dark toxicity measurements, we chose to examine the light-induced toxicity of the corroles using

against illumination times, the cells treated with either of the two free-base corroles exhibit little difference relative to untreated cells. In contrast, Au[pTCPC] exhibited substantial phototoxicity, leading to 50% cell death (LD50) after about 17 min and complete cell death after 40 min of irradiation. Au[mTCPC] was found to be more effective, resulting in as much as 50% cell death (LD50) after 2.5 min of irradiation.25 2.3. Dye-Sensitized Solar Cells. Porphyrins have been extensively used in solar cells and reviewed regularly.26−30 Indeed, the most efficient dye-sensitized solar cells (DSSCs), which reached efficiencies of 13%, used porphyrins as photosensitizers.31 In contrast, corroles have been rarely employed as components in solar cells. In a recent report, gold corroles were employed in organic solar cells, with the best example reaching an efficiency of 6.0%.9 As a photosensitizer for DSSCs, the most efficient corrole has been a gallium corrole with β-sulfonate anchoring groups, which gave an efficiency of 1.6%, referenced to a standard DSSC dye N3, which gave an efficiency of 3.1%.32 C


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ACS Applied Materials & Interfaces In this study, all four amphiphilic corroles, H3[mTCPC], H3[pTCPC], Au[mTCPC], and Au[pTCPC], were evaluated as photosensitizers in DSSCs (Figure 7 and Table 3). Three of

Figure 7. Current−voltage curves for selected DSSCs.

Table 3. Photovoltaic Parameters for Selected DSSCs dye

Voc (V)

Isc (mA/cm )

fill factor

efficiency (%)

N719 H2[pTCPP] H3[mTCPC] H3[pTCPC] Au[mTCPC] Au[pTCPC]

−0.83 −0.53 −0.43 −0.45 −0.56 −0.46

22.3 6.4 0.27 0.42 8.7 0.48

0.57 0.72 0.72 0.73 0.72 0.75

10.5 2.5 0.08 0.14 3.5 0.17

2

Figure 8. Energy-level diagram for corrole-sensitized TiO2 solar cells.

spectroscopic band gaps indicated by the UV−vis absorption spectra. The phosphorescence spectra also allowed the determination of excited triplet state energy levels. In the cases of the gold corroles, the triplet energy levels are approximately 1.55 eV (corresponding to their ∼800 nm phosphorescence) above that of the ground state (measured by the ground state oxidation potential). The triplet energy levels are substantially lower than the singlet levels, consistent with large S1−T1 gaps that have been noted before for metallocorroles.35 All the corrole oxidation potentials (Table 2) clearly indicate that the classic DSSC redox electrolyte (I−/I3−) is sufficiently reducing to regenerate any of the corroles from its oxidized state. More significant is the variation in reduction potentials. The excited singlet states of the free-base and the gold corroles apparently have more than sufficient energy to inject electrons into the TiO2 conduction band. On the other hand, the excited states of the copper corroles have insufficient energy to act as effective DSSC dyes, consistent with reported calculations and DSSC studies on related copper corroles.22,23 The efficiency of a solar cell is determined by the ratio of electrical power output to solar power input, where power output is the product of short-circuit photocurrent (Isc), opencircuit voltage (Voc), and fill factor.29 The relatively high efficiency of Au[mTCPC] is manifested primarily in higher photocurrents; photovoltages are only slightly higher, and fill factors are all essentially constant in the corrole series. One potential cause of low photocurrents could be an insufficient amount of dye to absorb the majority of the available photons. In fact, all the corroles adsorbed well onto TiO2 and gave comparably dark electrodes that were capable of absorbing the majority of the visible photons. Given comparable light absorption, the most plausible cause of higher photocurrent is a higher efficiency of electron transfer from the excited state of Au[mTCPC] to the conduction band of TiO2. From the point of view of DSSC efficiencies, the key difference between the free-base and gold corroles is the availability of a long-lived triplet state for the latter. Although

the compounds showed efficiencies well below 1%. An exceptional result, however, was obtained for Au[mTCPC], which yielded a DSSC with an efficiency of 3.5%. For reference, we also obtained DSSC data under identical conditions for N719,33 a standard ruthenium photosensitizer, and 5,10,15,20tetrakis(4-carboxyphenyl)porphyrin (H2[pTCPP]), a widely studied porphyrin dye.29,34 The generally poor performance of corroles in DSSCs has been analyzed recently, through both experimental and computational studies of a series of triarylcorroles with a single β-carboxy anchoring group.22,23 The conclusion was that the energy levels of corroles are poorly aligned for successful electron injection into the TiO2. Copper corroles were also studied, and as mentioned above, they too performed poorly, with the same reasoning applied. Cyclic voltammetry data (Table 2) allowed the construction of an energy level diagram for the corrole dyes in a DSSC (Figure 8), i.e., the corrole energy levels relative to the TiO2 conduction band into which an electron must be injected (about −0.74 V vs SCE) and the redox level of the iodide/ triiodide couple, which must provide an electron back to the oxidized dye (about +0.16 V vs SCE).29 The fact that the electrochemical studies were performed on the carbomethoxy esters, whereas the DSSC measurements used the free carboxylic acids, is not a concern, since the corrole energy levels are expected to be very similar for the two carboxylic acid derivatives. Excited singlet state energy levels were derived directly from the ground state reduction potentials, an assumption supported by the excellent agreement between the electrochemical HOMO−LUMO gaps (Eox − Ered) and the D


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corrole on the surface is also a crucial factor. The superior performance of Au[mTCPC] relative to Au[pTCPC] may indeed derive from the particular anchoring of the meta-carboxy groups.

the excited singlet states of free-base corroles have sufficient energy for electron injection into the conduction band of TiO2, they do not appear to do so within their ∼5 ns lifetimes. Effective DSSC photosensitizers can accomplish nearly quantitative electron injection in much faster time scales,36 but corroles simply do not. The availability of a long-lived triplet state opens up a much wider kinetic regime in which electron injection can compete successfully, even when it is energetically slightly uphill, as it is in the case of the Au corroles (Figure 8). Although electron transfer rates depend on a wide variety of factors, energetics are clearly crucial.37 Nevertheless, there are ample literature examples of successful uphill electron transfer; for example, a Pt porphyrin derivative with a 25 μs triplet lifetime undergoes reversible electron transfer to a covalently attached acceptor within 1 μs even though the electron transfer is unfavorable by about 200 mV.38 The observation that the m-carboxycorrole Au[mTCPC] exhibits a much higher DSSC efficiency than its para isomer Au[pTCPC] implies that the availability of a long-lived triplet state alone is not sufficient to generate DSSC efficiency; the meta substitution clearly plays a significant role. The underlying causes for the differences between Au[mTCPC] and Au[pTCPC] may be electronic or steric or both. Previous studies of carboxyphenylporphyrins have uncovered clear differences in efficiency that were ascribed to different positional anchoring of the porphyrin to the TiO2 surface. Whereas p-carboxyphenyl anchoring groups are generally considered to hold the dye upright, the corresponding meta or ortho anchoring groups leave the dye plane tilted with respect to the surface, with distinct improvement of the electron injection efficiency.30,39−41 Taking this into consideration, we visualize that Au[mTCPC] is anchored to the TiO2 surface by two, or possibly three, m-carboxyphenyl groups, which hold the corrole macrocycle plane above and relatively close to the surface of the TiO2. In contrast, Au[pTCPC] is expected to be held in an upright position by two p-carboxyphenyl groups. The CV data suggest that there may also be an energetic factor favoring Au[mTCPC] over Au[pTCPC]. A carboxy (or carbomethoxy) substituent in a para position exerts a greater electron-withdrawing effect relative to meta. Thus, the ground state and the triplet state of Au[pTCPC] lie somewhat lower than those of Au[mTCPC], as illustrated in Figure 8. Though both have comparably long triplet state lifetimes, the meta isomer has a somewhat more favorable energy alignment. Indeed, appending additional electron-donating substituents on the mTCPC skeleton may lead to even more effective 5d metallocorrole-based photosensitizers.

4. EXPERIMENTAL SECTION 4.1. Materials. The nonpolar triarylcorrole derivatives M[TpXPC], where M = H3, Cu,18 Ag,9 and Au,9 were prepared as previously described, as was free-base H3[pTCPC]. For the syntheses of H3[mTCPC] and the amphiphilic gold corroles Au[mTCPC] and Au[pTCPC], the raw materials were sourced as follows. Gold(III) acetate was obtained from Alfa Aesar. Sodium carbonate (granulated), methyl 3-formylbenzoate, methyl 4-formylbenzoate, sodium hydroxide, tetrahydrofuran (THF), and pyridine were purchased from SigmaAldrich and used as received. Silica gel 60 (0.04−0.063 mm particle size, 230−400 mesh, Merck) was employed for flash chromatography. For the PDT experiments, RPMI-1640 medium, L-glutamine, fetal bovine serum (FBS), sodium pyruvate, nonessential amino acids, trypsin, and Dulbecco’s phosphate-buffered saline (DPBS) were obtained from Gibco BRL, Life Technologies (Inchinnan, Scotland). Gentamicin sulfate was purchased from Schering Corp. (Kenilworth, NJ), absolute ethanol was from Arcus A/S (Oslo, Norway), and MTT solution, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, was obtained from Sigma-Aldrich (St. Louis, MO). The syngeneic rat bladder cancer cell line AY27 was cultured in Sarstedt 60 mm × 15 mm dishes (Thermo Fisher Nunc, Denmark) and grown in RPMI-1640 medium, containing 10% v/v fetal bovine serum (FBS), L-glutamine (80 mg/L), penicillin (100 U/mL), streptomycin (100 U/mL), and fungizone (0.25 mg/mL). The cell cultures were maintained at 37 °C in an incubator in an atmosphere of 5% CO2/95% air, subcultured approximately twice a week. The cell line was kindly provided by Professor S. Selman of the University of Ohio, USA. For the DSSC experiments, acetonitrile, valeronitrile, ethylene glycol, potassium iodide, 4-tert-butylpyridine, guanidine thiocyanate, deoxycholic acid, and chenodeoxycholic acid were purchased from Aldrich and used as received. Iodine was purchased from Acros and was sublimed under vacuum at 40 °C. Butylmethylimidazolium iodide and H2[TCPP] were purchased from TCI America, hexachloroplatinic acid was from Alfa Aesar, and absolute ethanol was purchased from Pharmco-AAPER. Fluorine-doped SnO2 (FTO) glass (Tec 7:7 ohm/ □, 2.5 mm thickness, and Tec 15:15 ohm/□, 3.5 mm thickness) was obtained from Pilkington Glass, and the TiO2 powder was Degussa P25, with approximately 20 nm particle size. 4.2. Instrumental Methods. 1H NMR spectra were recorded on 400 MHz Bruker Avance III HD equipped with a 5 mm SmartProbe BB/1H (BB = 19F, 31P, 15N) spectrometer at 298 K in CDCl3 and acetone-d6 and referenced to 7.26 (residual CHCl3) and 2.05 ppm, respectively. High resolution electrospray ionization (HR-ESI) mass spectra were recorded on an LTQ Orbitrap XL spectrometer. UV−vis absorption spectra were recorded on a PerkinElmer Lambda 35 UV−vis spectrophotometer. Fluorescence and phosphorescence measurements were performed on a FS900 spectrofluorometer (Edinburgh Instruments, UK), equipped with R2658P redsensitive PMT (Hamamatsu). Both the excitation and the emission optical paths in the fluorometer were calibrated using a lamp with NIST-traceable spectral radiant flux (RS-15-50, Gamma Scientific, SN HL1956). The corresponding correction curves were used in all emission measurements. For quantum yield measurements, the absorbances of the samples at the excitation wavelengths were kept below 0.03 OD. The excitation and emission slits on the fluorometer were set for 1 nm bandpass. Data analysis was performed with Origin 7.0 (OriginLab). Time-resolved phosphorescence measurements were carried out with a custom-made fiber-optic phosphorometer constructed around a multichannel data acquisition board (USB NI-6361, National Instruments) operating at 2 MHz digitizer frequency. The excitation sources in the instrument are light-emitting diodes (LEDs) (Ledengin, LZ1 series), and the detector is an avalanche photodiode module (C12703-

3. CONCLUSIONS Our photophysical measurements show that free-base corroles form excited singlet states with lifetimes of ∼5 ns, detected by fluorescence with a quantum yield of ∼10%. In contrast, gold corroles exhibit phosphorescence and not fluorescence. The observed photoactivity of the gold corroles is thought to originate from triplet states with long lifetimes of about 75 μs. PDT mediates its activity via the generation of singlet oxygen.6 The long triplet lifetimes of the gold corroles are sufficient for diffusional interaction with O2. Photovoltaic activity leading to functional DSSCs similarly requires a bimolecular interaction involving the corrole excited state, i.e., electron injection into TiO2. In a DSSC, however, the corrole is adsorbed on a surface and does not require diffusion. The long triplet lifetime is again likely to be highly beneficial, but the specific orientation of the E


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7.9 Hz, 10-o-Ph), 8.31 (d, 4H, 3JHH = 8.2 Hz, 5,15-m-Ph), 8.21 (d, 2H, JHH = 8.2 Hz, 10-m-Ph), 4.13 (s, 6H, 5,15-p-CO2CH3), 4.12 (s, 3H, 10-p-CO2CH3). HRESI-MS: M+ = 894.1755 (expt), 894.1752 (calcd for C43H29N4O6Au). Elemental analysis (%): calcd C 57.73, H 3.27, N 6.26; found C 58.24, H 3.72, N 5.80. Gold Tris(m-carbomethoxyphenyl)corrole (Au[mTCPC-Me]). Yield 40.8 mg (32.12%). UV−vis (CH2Cl2) λmax (nm), [ε × 10−4 (M−1 cm−1)]: 420 (17.38), 531 (1.02), 561 (3.08), 573 (3.62). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 9.07 (d, 2H, 3JHH = 4.5 Hz, β-H), 8.97− 8.93 (d, 2H, 3JHH = 4.3 Hz, β-H and s, 2H, 5, 15-o1-Ph, overlapping), 8.88 (s, 1H, 10-o1-Ph), 8.73 (d, 4H, 3JHH = 4.6 Hz, β-H); 8.46 (d, 3H, 3 JHH = 8.2 Hz, 5,10,15-o2-Ph), 8.31 (d, 3H, 3JHH = 7.9 Hz, 5,10,15-pPh), 8.22 (d, 3H, 3JHH = 8.3 Hz, 5,10,15-m-Ph),4.02 (s, 6H, 5,15-mCO2CH3), 4.00 (s, 3H, 10-m-CO2CH3). HRESI-MS: M+ = 894.1755 (expt), 894.1752 (calcd for C43H29N4O6Au). Elemental analysis (%): calcd C 57.73, H 3.27, N 6.26 ; found C 58.05, H 3.76, N 5.79. Hydrolysis of Free Base Tris(m/p-carbomethoxyphenyl)corrole and Gold Tris(m/p-carbomethoxyphenyl)corrole. To tris(m/pcarbomethoxyphenyl)corrole or gold tris(m/p-carbomethoxyphenyl)corrole (0.067 mmol) dissolved in THF (60 mL) was added 0.067 M sodium hydroxide solution (15 mL), and the mixture was stirred under reflux for 24 h. At that point, HRESI-MS analysis indicated no evidence of unhydrolyzed or partially hydrolyzed starting materials. Upon cooling to room temperature, the reaction mixture was concentrated by rotary evaporation. The remaining aqueous phase was neutralized by slow addition of 0.67 M HCl (10 mL), which resulted in precipitation of the carboxycorroles. The solution was left to stand for 1 h at room temperature to ensure complete precipitation of the corroles. The precipitate was filtered, repeatedly washed with distilled water, and air-dried, affording the pure carboxycorroles. Tris(p-carboxyphenyl)corrole (H3[pTCPC]). Yield 38 mg (86.0%). UV−vis (0.15 M K2CO3 in H2O) λmax (nm), [ε × 10−4 (M−1 cm−1)]: 420 (6.11), 585 (0.75), 630 (2.04). 1H NMR (acetone-d6): δ = 9.08 (d, 2H, 3JHH = 4.4 Hz, β-H), 8.79 (d, 2H, 3JHH = 4.6 Hz, β-H), 8.75 (d, 2H, 3JHH = 4.3 Hz, β-H), 8.64 (d, 2H, 3JHH = 4.6 Hz, β-H), 8.62 (d, 4H, 3JHH = 8.2 Hz, 5, 15-o-Ph), 8.57 (d, 2H, 3JHH = 8.0 Hz, 10-o-Ph), 8.50 (d, 4H, 3JHH = 8.2 Hz, 5, 15-m-Ph), 8.48 (d, 2H, 3JHH = 8.2 Hz, 10-m-Ph). 2.85 (br s, NH). HRESI-MS: M+ = 659.1920 (expt), 659.1925 (calcd for C40H27O6N4). Tris(m-carboxyphenyl)corrole (H3[mTCPC]). Yield 39 mg (88.3%). UV−vis (0.15 M K2CO3 in H2O) λmax (nm), [ε × 10−4 (M−1 cm−1)]: 416 (7.77), 434 (5.42), 581 (0.83), 625 (2.21). 1H NMR (acetone-d6): δ = 8.96 (d, 2H, 3JHH = 4.3 Hz, β-H), 8.90 (s, 2H, 5,15-o1-Ph), 8.72 (d, 2H, 3JHH = 4.6 Hz, β-H), 8.72 (s, 1H, 10-o1-Ph), 8.50 (d, 2H, 3JHH = 8.2 Hz, 5, 15-o2-Ph), 8.41 (d, 2H, 3JHH = 4.6 Hz, β-H), 8.37 (d, 2H, 3 JHH = 4.3 Hz, β-H), 8.34 (d, 1H, 3JHH = 8.1 Hz, 10-o2-Ph), 8.21 (d, 2H, 3JHH = 8.2 Hz, 5,15-p-Ph), 7.88 (t, 2H, 5,15-m-Ph), 7.81 (t, 1H, 10-m-Ph), 7.54 (d,1H, 3JHH = 8.2 Hz, 10-p-Ph), 2.85 (br s, NH). HRESI-MS: M+ = 659.1920 (expt), 659.1925 (calcd for C40H27O6N4). Gold Tris(p-carboxyphenyl)corrole. Yield 52 mg (91.2%). UV−vis (0.15 M K2CO3 in H2O) λmax (nm), [ε × 10−4 (M−1 cm−1)]: 405 (6.11), 577 (0.81). M+ = 852.1283 (expt), 852.1283 (calcd for C43H29N4O6Au). Satisfactory 1H NMR spectra could not be obtained due to poor solubility in common NMR solvents. Gold Tris(m-carboxyphenyl)corrole. Yield 51.5 mg (90.3%). UV− vis (0.15 M K2CO3 in H2O) λmax (nm), [ε × 10−4 (M−1 cm−1)]: 403 (7.42), 571 (1.33). M+ = 852.1283 (expt), 852.1283 (calcd for C43H29N4O6Au). Satisfactory 1H NMR spectra could not be obtained due to poor solubility in common NMR solvents. 4.4. Photodynamic Therapy Experiments. Blue Light Source for in Vitro Cell Experiments. Culture Petri dishes (6 cm wide, Nunc, Denmark) were illuminated from below at room temperature by means of a LumiSource blue light box (PCI Biotech, Norway) consisting of four Osram tubes (18 W, peak wavelength 435 nm). The light intensity at the level of the cells was 13 mW/cm2, as measured with an Optometer UDT model 161 radiometer−photometer (United Detector Technology, Inc., Culver City, CA), giving a total light dose of 7.8 J/cm2 at the cell level during a 10 min illumination period. The light was detected near the bottom of and just outside the dishes. The

01, Hamamatsu). The digitized luminescence decays were analyzed by the nonlinear least-squares method. The time constant of the instrument is ∼3 μs (fwhm). Time-resolved fluorescence measurements were performed by the time-correlated single photon counting method (TCSPC). The TCSPC system consisted of a picosecond diode laser (PicoQuant LDH-C-400, λmax = 408 nm, fwhm ∼ 100 ps, 40 MHz repetition rate), multichannel-plate PMT (Hamamatsu R2809U), and a TCSPC board (Becker & Hickl, SPC-730). Quartz fluorometric cells (1 cm optical path length) were used in all optical experiments. The samples were deoxygenated by Ar (Airgas, grade 5) bubbling. Quantum yields were measured against the fluorescence of Rhodamine 6G in EtOH (ϕfl = 0.94) using rigorously deoxygenated solutions.24 Cyclic voltammetry was carried out in anhydrous CH2Cl2 (Aldrich) at 298 K with an EG&G Model 263A potentiostat equipped with a three-electrode system, including a glassy carbon working electrode, a platinum wire counter electrode, and a saturated calomel reference electrode (SCE). Tetra(n-butyl)ammonium perchlorate, recrystallized twice from absolute ethanol and dried in a desiccator for at least 2 weeks, was used as the supporting electrolyte. The reference electrode was separated from the bulk solution by a fritted-glass bridge filled with the solvent/supporting electrolyte mixture. The electrolyte solution was purged with argon for at least 2 min, and all measurements were conducted under an argon blanket. All measurements were carried out at a scan rate of 100 mV/s. All potentials were referenced to the SCE. Elemental analyses for Au[mTCPC-Me] and Au[pTCPC-Me] were obtained from Atlantic Microlab Inc., USA. Accurate elemental analyses could not be obtained for the amphiphilic corroles, presumably on account of their hygroscopic character. 4.3. Synthesis and Characterization of New Compounds. Free-Base Tris(m-carbomethoxyphenyl)corrole. Methyl 3-formylbenzoate (820.8 mg, 5 mmol) and pyrrole (697 μL, 10 mmol) were dissolved in 200 mL of methanol, to which was added 200 mL of water and 4.25 mL of concentrated HCl. The resulting solution was stirred at room temperature for 3 h. The reaction mixture was extracted with chloroform (300 mL), and the organic phase was washed twice with distilled water, dried over Na2SO4, and filtered. To the filtrate was added p-chloranil (1.23 g, 5 mmol), and the resulting mixture was refluxed for 1 h. The reaction mixture was then evaporated to dryness. The crude residue was dissolved in CH2Cl2, loaded onto a silica gel column, and eluted with pure dichloromethane until the eluate was free of reduced p-chloranil, as judged by the disappearance of its absorbance at ∼308 nm. The eluent was then replaced by 98:2 toluene/ethyl acetate. All green fractions were collected and rotary evaporated to dryness. Crystallization of the residue from CH2Cl2/n-hexane afforded the pure free-base corrole (220 mg, 18.41%). UV−vis (CH2Cl2) λmax (nm), [ε × 10−4 (M−1 cm−1)]: 417 (12.11), 574 (1.81), 615 (1.33), 658 (0.80). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 8.86 (br s, 8H, β-H), 8.44 (br s, 10H, Ph), 8.87 (br s, 2H, Ph), 4.01 (s, 6H, 5,15-p-CO2CH3), 4.00 (s, 3H, 10-p-CO2CH3). HRESI-MS: M+ = 701.2396 (expt), 701.2395 (calcd for C43H32N4O6). General Procedure for the Synthesis of Gold Tris(m/pcarbomethoxyphenyl)corrole. To free base tris(m/p-carbomethoxyphenyl)corrole (100 mg, 0.142 mmol) dissolved in 10 mL of pyridine was added 3 equiv of gold(III) acetate (160 mg, 0.427 mmol). The reaction mixture was stirred overnight, >16 h. The resulting reddishbrown mixture was rotary evaporated to dryness, and the brown residue obtained was chromatographed on a silica gel column with 80:20:1 dichloromethane/toluene/ethyl acetate as eluent. The gold corrole eluted as the first light-red band and was obtained as a dark-red solid after rotary evaporation. Gold Tris(p-carbomethoxyphenyl)corrole (Au[pTCPC-Me]). Yield 38.2 mg (30.1%). UV−vis (CH2Cl2) λmax (nm), [ε × 10−4 (M−1 cm−1)]: 423 (13.38), 533 (0.85), 576 (2.96). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 9.05 (d, 2H, 3JHH = 4.4 Hz, β-H), 8.98 (d, 2H, 3JHH = 4.6 Hz, β-H), 8.76 (d, 2H, 3JHH = 4.3 Hz, β-H), 8.74 (d, 2H, 3JHH = 4.6 Hz, β-H), 8.49 (d, 4H, 3JHH = 8.2 Hz, 5,15-o-Ph), 8.45 (d, 2H, 3JHH =

3

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Research Article

ACS Applied Materials & Interfaces experimental setups, including the blue light source, were shielded from additional ambient lighting by covering with aluminum foil. Viability Assays and PDT Experiments on the AY27 Cancer Cell Line. AY27 cells were seeded in 6 cm Petri dishes at a density of (0.3− 0.4) × 106 cells per dish and grown with a regular culture medium 1 day prior to the experiment. Stock solutions (1 mM) of the four amphiphilic corroles were prepared in distilled water (for the free-base corroles) or in 0.15 M potassium carbonate solution (for the Au− corroles). These were diluted with the growth medium to produce 10 μM solutions. After the cells were washed with phosphate-buffered saline (PBS), the corrole solutions were added in the dark, followed by incubation for 24 h at 37 °C. Cell survival measurements were then carried out by the MTT assay.42 For the photochemical experiments, the corrole incubations (10 μM, 24 h, 37 °C) were performed in the same manner, and the cells were washed (twice with PBS) before blue light exposure (0−40 min, 435 nm, 13 mW/cm2). After illumination, cells were grown overnight in the standard culture medium prior to MTT assay. Corrole-free samples, which were not illuminated, were used as controls. Dishes containing corroles that were not illuminated were used as “dark toxicity” controls. Statistical Analyses. Each cell survival experiment (MTT assay) was run as duplicates or triplicates. Data are presented as mean value (±SD) from three independent experiments, each with 1−3 culture dishes. 4.5. DSSC Experiments. DSSC fabrication procedures and photoelectrochemical measurement procedures are described in detail in the Supporting Information as well as in a concurrent publication.43 All DSSC fabrication procedures were adaptations of Grätzel et al.,44 with the exception of a modified TiO2 preparation; a paste consisting of TiO2 powder, Ti(OiPr)4, citric acid, and ethylene glycol was prepared following a modifed Peccini sol−gel method.45 For all experiments involving porphyrins and corroles, the electrolyte consisted of 0.05 M I2, 0.6 M butylmethylimidizolium iodide, 0.1 M lithium iodide, and 0.5 M 4-tert-butylpyridine in 85:15 acetonitrile/ valeronitrile. For experiments involving the N719 reference dye, the electrolyte was identical except that LiI was omitted.21 All dyes gave reproducible and uniform dye loading, leading to similarly dark electrodes. Current−voltage data were reproducible for duplicate cells; the reported data are for the best-performing cells for each dye tested.

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(ONAMI), grant CHE-0911186 of the US National Science Foundation (C.C.W.) and the grants EB018464 and 1R24NS092986-01 from the US National Institiutes of Health (S.A.V.). The DOE SCGF Program was made possible in part by the American Recovery and Reinvestment Act of 2009 and is administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by Oak Ridge Associated Universities (ORAU) under DOE Contract DE-AC05-06OR23100. All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of DOE, ORAU, or ORISE.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b04269. Details of DSSC fabrication procedures and photoelectrochemical measurements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail *E-mail *E-mail *E-mail

REFERENCES

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[email protected] (O.A.A.). [email protected] (S.A.V.). [email protected] (C.C.W.). [email protected] (A.G.).

Present Address

T.M.: Department of Chemistry, University of Connecticut, Storrs, CT 06269. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by FRINATEK grant 231086 of the Research Council of Norway (A.G., A.B.A.), the Cancer Research Foundation of St. Olav’s Hospital (Trondheim, Norway; O.A.G.), a U.S. Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF; A.B.R.), the Oregon Nanoscience and Microtechnologies Institute G


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