Corroles as triplet photosensitizers

Coordination Chemistry Reviews xxx (2017) xxx–xxx

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Review

Corroles as triplet photosensitizers Atif Mahammed ⇑, Zeev Gross ⇑ Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 3200008, Israel

a r t i c l e

i n f o

Article history: Received 27 July 2017 Received in revised form 29 August 2017 Accepted 30 August 2017 Available online xxxx Keywords: Metallocorroles Fluorescence Phosphorescence Triplet photosensitizer Photoredox catalyst Photodynamic therapy Photodynamic inactivation of microorganisms Singlet oxygen

a b s t r a c t Despite the similarity with porphyrins, the focus on the photophysical properties of corroles and their chelates with transition and post-transition metal ions (metallocorroles) started quite late. Free base corroles are much more fluorescent than related macrocycles; and this feature can be easily changed and tuned by facile synthetic manipulations. Metallocorroles may be designed as to display desired properties such as delayed fluorescence, RT phosphorescence, high yield of intersystem crossing, efficient singlet oxygen generation, and more. The practical utility of metallocorroles has been exemplified by their use as photosensitizers in photodynamic therapy for fighting cancer, photodynamic inactivation of microorganisms, as catalysts for photo-assisted organic reactions and energy-relevant inorganic transformations. Key to success in the above applications is the ability to delicately control the photophysical properties, redox potentials, and the selective positioning of substituents on the corrole macrocycle. Ó 2017 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5.

6. 7. 8.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Singlet oxygen generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoinduced oxygenation of thioanisoles and hydrocarbons by molecular oxygen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photodynamic therapy reagents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photodynamic inactivation (PDI). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. PDI of mold fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. PDI of green algae growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photooxidation of inorganic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoredox organic reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks and future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Abbreviations: PDT, photodynamic therapy; PDI, photodynamic inactivation; TPS, triplet photosensitizers; ISC, intersystem crossing; SOC, spin–orbit coupling; UFl, fluorescence quantum yield; UT, triplet quantum yield; UD, singlet oxygen quantum yield; ROS, reactive oxygen species. ⇑ Corresponding authors. E-mail addresses: [email protected] (A. Mahammed), [email protected] (Z. Gross).

00 00 00 00 00 00 00 00 00 00 00 00

1. Introduction One main use of photosensitizers is as triplet energy donors to initiate photochemical and photophysical processes, which are utilized for many applications such as photodynamic therapy (PDT) or imaging of cancer [1–4], and for catalyzing organic and inorganic

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reactions [5–10]. Good triplet photosensitizers (TPS) should have large molar absorption coefficients of visible light at the excitation wavelength, efficient intersystem crossing (ISC) to produce triplet excited state with high quantum yield, high photostability, and long-lived triplet excited states for allowing easy detection of phosphorescence or for increasing the probability of bimolecular reactions [11–13]. One straightforward approach to TPS that fulfill those conditions is to prepare chromophores that contain elements with a large atomic number. These heavy atoms induce strong spin–orbit coupling (SOC), and consequentially enhance ISC [14]. Typical heavy atoms include transition metals such as Ru, Rh, Re, Os, Ir, Pt, Au, and the halides [15–21]. Many of these complexes have however small molar absorption coefficients in the desired wavelengths and short-lived triplet excited state, which limits their suitability as TPS [14,22–24]. Thus, development of TPSs that strongly absorb visible light and display long-lived triplets is highly desired. Quite surprisingly, excitation of free base porphyrins leads to both fluorescence and population of the triplet state, even in the absence of the heavy atom effect [1]. This is clear evidence for quite a high yield of ISC, but these porphyrins are nevertheless not phosphorescent at RT [25]. The ISC mechanism of porphyrin is in fact somewhat controversial: recent studies suggest that both vibration modes and excited state intramolecular proton transfer (tautomerization) may facilitate the ISC [26,27]. Free base corroles, tetrapyrrolic macrocycles with one meso carbon less than porphyrins [28– 33], also yield some triplet excited states upon photoexcitation [34–37]. More unique is that they and their post-transition metal chelates have fluorescence quantum yields that are much higher than those of porphyrins and other macrocycles [38]. For instance, metallation of corrole by gallium and more so by phosphorus or aluminum leads to very large increase in fluorescence quantum yields (UFl) [38–40]. In fact, the UFl of 0.76 for aluminum corrole is a record value for oligopyrrolic macrocycles [38]. The photophysical properties of corroles may be easily tuned by metallation of the corroles with heavy post-transition metals, such as Sb and Sn or by replacing the hydrogen atoms of the macrocycle via halogenation with bromine or iodine [41–45]. These modifications

lower the fluorescence quantum yield and result in an increased phosphorescence quantum yield. In some cases the metallocorroles are phosphorescent even at room temperature [28,44,46,47]. The heavy-atom-effect applied on aluminum corroles is a good example for that. While 1-Al exhibits high UF and shows no triplet dynamics in time-resolved EPR experiments, CAH by CABr replacement increases the UP/UF ratio and the brominated analog 1-Al-Br8 exhibits a rich time-resolved EPR spectrum which was attributed to a triplet state [48]. Also, selective iodination of the corrole skeleton of 1-Al leads to compounds with long-lived triplet excited states and phosphorescence emission even at ambient temperatures [42]. The ability of antimony corroles to efficiently produce singlet oxygen was confirmed by photoinduced oxygenation of thioanisoles and hydrocarbons by molecular oxygen [49]. The results obtained by this catalyst outperform all previous reports in terms of absolute catalytic turnover numbers, selectivity, and catalyst stability. The potential of other metallocorroles as TPS was also demonstrated in the various applications that are outlined in this review, but prior to that, we summarize in Table 1 all the reported singlet oxygen quantum yields of corroles. 2. Singlet oxygen generation Formation of singlet oxygen via irradiation of photosensitizercontaining aerobic solutions is the most straightforward indication for efficient transformation of the photosensitizer into a long lived triplet excited state. The existing photophysical data for corroles and their corresponding chelates with transition- and posttransition elements (Schemes 1–4) are summarized in Table 1. All the compounds depicted in Schemes 1–4 are simple corroles, with the exception of those with one and two cyclodextrins. Table 1 provides the maximal wavelengths of fluorescence, the corresponding quantum yields, triplet quantum yields, and singlet oxygen quantum yields. The corroles listed in the table are confined to those reported to generate moderate to high yield of singlet oxygen upon visible light irradiation. We divided these corroles into four sets on the basis of the number of pentafluorophenyl groups on

Table 1 The maximal wavelengths of fluorescence (kmax Fl), the corresponding quantum yields (UFl), the triplet quantum yield (UT), and the singlet oxygen quantum yields (UD) of corroles and metallocorroles. Corrole set

Compound

kmax Fl (nm)

UFl%

UT%

UD%

Ref.

A

1 1-I4 1-Ga 1-Sb-F2 1-Sb-O PCor SbCor 1-Au 1-Au-I4

645

64

603, 657 600 600 595 613

10.1 <0.11 30.5

58 46 52 86 87 58 66 46 26

[34] [52] [34] [53] [54] [55] [55] [52] [52]

B

2 2-Ga 5 6 7 8 10

654 613, 659 645 647 647 659, 720 626

8.6 24.5 11 6.5 2.1 14 21

73 77 63 87 91

70 57 50 78 86 77 29

[34] [34] [34] [34] [34] [35] [56]

C

9 11

653 626

12.6 27

68

61 32

[34] [56]

D

16 17 18 19 PCor+ SbCor+

664 670, 730 668, 752 652, 710 591 609

9.1 22 21 19 16.5 2.4

36

34 61 64 52 81 67

[34] [35] [35] [35] [55] [55]

69 95–99

19.6 3.4 <0.11 <0.11

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A. Mahammed, Z. Gross / Coordination Chemistry Reviews xxx (2017) xxx–xxx

M

L1

L2

X1

X2

X3

X4

Y

1

3H

---

---

H

H

H

H

H

1-Al

Al

Pyridine

Pyridine

H

H

H

H

H

1-Al-Br8

Al

Pyridine

Pyridine

Br

Br

Br

Br

Br

1-Al-I3

Al

Pyridine

Pyridine

I

I

H

I

H

1-I4

3H

---

---

I

I

I

I

H

1-Al-I4

Al

Pyridine

Pyridine

I

I

I

I

H

1-Ga

Ga

Pyridine

---

H

H

H

H

H

1- Ga-Br8

Ga

Pyridine

Pyridine

Br

Br

Br

Br

Br

1- Ga-I3

Ga

Pyridine

Pyridine

I

I

H

I

H

1- Ga-I4

Ga

Pyridine

Pyridine

I

I

I

I

H

1-Sb-py

Sb

Pyridine

---

H

H

H

H

H

1-Sb-F2

Sb

F

F

H

H

H

H

H

1-Sb-O

Sb

O

---

H

H

H

H

H

1- Sb-Br8

Sb

OH

OH

Br

Br

Br

Br

Br

1-Au

Au

---

---

H

H

H

H

H

1-Au-I3

Au

---

---

I

I

H

I

H

1-Au-I4

Au

---

---

I

I

I

I

H

1-Au-Br4

Au

---

---

Br

Br

Br

Br

H

1-Au-(CF3)3

Au

---

---

CF3

CF3

H

CF3

H

1-Au-(CF3)4

Au

---

---

CF3

CF3

CF3

CF3

H

1-P-Br8

P

OH

OH

Br

Br

Br

Br

Br

1-Co-Br8

Co

Pyridine

Pyridine

Br

Br

Br

Br

Br

GaCor¯

Ga

H 2O

---

SO3¯ Na+

H

SO3¯ Na+

H

H

+

¯

+

PCor

¯

SbCor¯

¯

P

OH

OH

SO3 Na

H

SO3 Na

H

H

Sb

O

---

SO3¯ Na+

H

SO3¯ Na+

H

H

Scheme 1. Chemical structures of substituted 5,10,15-trispentafluorophenylcorrole (1) and its metal complexes (Set A).

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R

M

L1

L2

2

phenyl

3H

---

---

2-Ga

phenyl

Ga

pyridine

---

3

2-hydroxyphenyl

3H

---

---

4-P

4-hydroxyphenyl

P

OH

OH

5

5-fluoro-2-hydroxyphenyl

3H

---

---

6

5-Chloro-2-hydroxyphenyl

3H

---

---

7

2-hydoxy-5-Iodophenyl

3H

---

---

8

tolyl

3H

---

---

10

2,3,5,6-tetrafluoro-4-β-cyclodextrin

3H

---

---

12

4-formylphenyl

3H

---

---

12-Ga

4-formylphenyl

Ga

pyridine

---

13

4-pyridyl

3H

---

---

Scheme 2. Chemical structures of corroles and metallocorroles substituted by pentafluorophenyl groups on two of the meso–C atoms, C5 and C15 (Set B).

Scheme 3. Chemical structures of corroles 9, 11, and 14 bearing a pentafluorophenyl group on one of the meso–C atoms, C10 (Set C).

the meso-C positions of the corrole skeleton (C5, C10, C15): set A with three, set B with two (C5, C10), set C with one (C10), and set D with zero pentafluorophenyl groups. The corroles in set A (Scheme 1) are derivatives of trispentafluorophenylcorrole (1), modified on either the b-pyrrole position or/and metallated by transition or post transition elements. Metallation of the free base corrole 1 by gallium(III) leads to 1-Ga, in which the UFl is enhanced and the UD is decreased. The decrease in the UD value may be due to inefficient quenching of the triplet state by oxygen [50]. Supportive of this hypothesis is that the UT value (69%) of 1-Ga is higher than its UD (52%) and also than the UT of 1 (64%). Covalent attachment of four iodine atoms to the macrocycle of 1 via C-H/C-I replacements, i.e., 1-I4, is one way

for introducing the heavy atom effect. The UFl of 1-I4 is quenched indeed, 0.11% rather than 10.1% in 1, but the UD (46%) did not increase (58% for 1). Metallation of 1 with the heavy post transition metal antimony (1-Sb-F2, 1-Sb-O, SbCor ) does lead to increased UD, into the range of 58–87%. On the other hand, insertion of the light non-metal phosphorus to the corrole macrocycle (PCor ) increased the UFl relative to 1, but did not affect the UD. Complete fluorescence quenching is obtained in the gold(III) corroles 1-Au, 1-Au-I4, consistent with expectations due to the heavy atom effect. However the UD is rather low, 46% and 26% respectively, which might be attributed to inefficient quenching of the triplet state by oxygen, or low efficiency of ISC, and/or fast non-radiative decay of the triplet excited states. The quite significant phosphorescence

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R1, R2

M

L1

L2

15

R1 = R2 = 4-pyridyl

3H

---

---

16

R1 = R2 = phenyl

3H

---

---

17

R1 = R2 = 4-CF3-phenyl

3H

---

---

18

R1 = R2 = 2,5-bis-trifluoromethyl-phenyl

3H

---

---

19

R1 = 2,4,6-trimethylphenyl, R2 = p-cyanophenyl

3H

---

---

20

R1 = R2 = 3-carboxyphenyl

3H

---

---

20-Au

R1 = R2 = 3-carboxyphenyl

Au

---

---

21

R1 = R2 = 4-carboxyphenyl

3H

---

---

21-Au

R1 = R2 = 4-carboxyphenyl

Au

---

---

GaCor+

R1 = R2 = 4-methyl-pyridyl

Ga

H 2O

---

PCor+

R1 = R2 = 2-methyl-pyridyl

P

OH

OH

R1 = R2 = 2-methyl-pyridyl

Sb

O

---

SbCor

+

Scheme 4. Chemical structures of the corroles and metallocorroles without any pentafluorophenyl group attached to the meso–C atoms (Set D).

yield at RT reported for these gold corroles serve to indicate that the two last aspects are not the dominant factors. All the corroles in set B (Scheme 2) contain two pentafluorophenyl groups attached to the meso carbon atoms at positions 5 and 15 (C5 and C15) of the corrole, but they have different aryls group on C10. The photophysical data of corrole 2, with a phenyl group on C10, disclose a little decrease in the UFl value and small increases of UD and UT values relative to corrole 1. Corroles 5, 6, and 7 have a 2-hydroxy-5-halogenophenyl on C10. The photophysical data of 5 with F in the variable position are similar to those of corrole 1, while the fluorescence emission yield decreased and the UT and UD increased in the heavier-halogen-containing 6 and 7. The iodide present in 7 decreased the UFl and increased the UT and UD more than the chloride present in 6. It may hence be concluded that substitution of the C10 phenyl group by even quite remote heavy halide atoms enhances the fluorescence decay and increases the UT and UD of the corrole. Insertion of gallium into corrole 2 leads to 2-Ga, with an enhanced UFl and reduced UD. The decrease in the UD value is most likely due to inefficient quenching of the triplet state by oxygen [50], as the UT (77%) value of 2-Ga is higher than its UD (57%) and higher than the UT of 2 (73%). These results are similar to those obtained when corrole 1 was metallated by gallium(III) in set A. The corroles in set C (Scheme 3) contain one pentafluorophenyl group at C10 and either two phenyl groups (9) or two tetrafluorophenyl groups that are further substituted by b-cyclodextrin on the para-aryl positions (11). Comparison of the photophysical properties of these corroles with those of tris-pentafluorophenylcorrole 1 reveals that replacement of two pentafluorophenyl groups by two phenyl groups on meso-C5 and C15 positions does not affect the UFl,

UT, and UD, dramatically. On the other hand, substitution of the

para-fluoro substituent in corrole 1 by b-cyclodextrin increased the UFl and decreased the UD. Set D (Scheme 4) consists of 5,10,15-triarylcorroles, in which none of the aryls is a pentafluorophenyl group. The free base corroles 17, 18, 19, 20 and 21 in this set have aryls that are substituted by electron withdrawing groups; and their UD values are similar to those of corrole 1, 2 and 9 that have at least one pentafluorophenyl group. On the other hand, corrole 16 with its three meso-phenyl groups has a much lower UD than corrole 1. The emerging picture is that aryl groups substituted by electron-withdrawing groups apparently enhance the ISC process. To account for this, we recall that the most direct effect of electron-poor aryl groups attached to free-base corroles is on the acidity of the inner NH protons. This could affect the vibration modes and excited state intramolecular proton transfer phenomena (tautomerization) and in turn also the ISC. The last, but not least interesting, comparison is between PCor+ and SbCor+, which have the same corrole macrocycle with 2-methylpyridyl groups attached to the three meso-C atoms. PCor+ has a higher UFl than SbCor+ due to the much larger heavy atom effect in the latter. But contrary to expectation, the UD of SbCor+ is actually lower than that of PCor+. This apparently reflects inefficient quenching of the triplet state of SbCor+ by oxygen, probably because the relevant corrode orbitals are lowered in energy due to very strong electron-withdrawing effect of the chelated antimony. This is most easily appreciated by comparing the first oxidation potentials of 1-M-Br8 complexes (Scheme 1): for M = Al, Ga, P, and Sb, the trend is 1-Al-Br8 (+1.00 V) <1-Ga-Br8 (+1.14 V) <1-P-Br8 (+1.40 V) <1-Sb-Br8 (+1.64 V) [51].

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The first non biologically relevant application of metallocorroles as TPS was reported for the antimony corroles 1-Sb-py, 1-Sb-F2 and 1-Sb-O [49]. These complexes displayed high catalytic activity for the photo-induced oxygenation of thioanisole by molecular oxygen. The best results were obtained in ethanolic solutions, while no reaction occurred in benzene. The corresponding sulfoxide was always the only product with no further oxidation to sulfone. 1-Sb-F2 displayed the best catalytic efficiency, followed by 1Sb-O and 1-Sb-py. Full conversion of thioanisole to its sulfoxide was obtained with as little as 0.02 mol% of 1-Sb-F2 (5000 catalytic turnovers) without any indications for catalyst bleaching [32]. No reaction took place when thioanisole was replaced by diphenylsulfide as substrate, known to be inert toward oxidation by singlet oxygen [57]. This result strongly pointed toward singlet oxygen as the active oxidant, generated by the photoexcited antimony corroles. 1-Sb-F2 was further examined as a potential photocatalyst for the aerobic oxidation of hydrocarbons, one of the ‘‘holy grails” in catalysis. The results disclosed absolute selectivity to CAH vs. C@C bond oxidation: the allylic CAH bonds of cyclohexene and cyclooctene were exclusively oxidized while the double bonds therein and that of styrene remained unreacted. This very efficient ‘‘ene” reactions are perfectly consistent with the reactivity profile of singlet oxygen [58]. Of the non-olefin-containing substrates–et hylbenzene, adamantane and cumene – that were also examined, only cumene was oxidized. This selectivity reflects the mechanistic aspects of reactions that proceed with CAH abstraction as the ratelimiting step, with singlet oxygen as the oxidizing reagent [59]. No oxidation of any of the substrates took place when any of the constituents required for efficient formation of singlet oxygen is omitted. In all the above CAH oxidation cases, the corresponding hydroperoxides were obtained as sole products (determined by in situ 1H NMR spectroscopy). The best performing photocatalyst 1-Sb-F2 was not bleached or modified during continuous irradiation, and up to 78% conversion was obtained when the ratio of catalyst:substrate was decreased to 1:2000. These results exceed all previous reports in terms of absolute catalytic turnover numbers and selectivity [60]. The efficiency of singlet oxygen production by 1-Sb-F2 and 1-Sb-O was later measured, in dichloromethane solutions, disclosing promising UD values: 86% for 1-Sb-F2 and 87% for 1-Sb-O [54]. In fact, the triplet formation yields for antimony corrole complexes are even higher than their measured singlet oxygen yields. The ultrafast dynamics measurements for photoexcited 1-Sb-F2 show that the intersystem crossing yield from the Q-band excitation to the triplet is between 95 and 99%. This efficient triplet formation was attributed to the spin–orbit coupling induced by the heavy antimony ion [53]. The high UD explains the excellent results obtained using the antimony corrole complexes as photosensitizers for oxygenation of thioanisoles and hydrocarbons by molecular oxygen. The apparent potential of main group corroles as photosensitizers for biological applications that rely on singlet oxygen, and possibly also for conversion of light to electricity, is hence likely to be further advanced in the near future.

4. Photodynamic therapy reagents The IUPAC definition of the photodynamic effect is ‘‘a term used in photobiology to refer to photoinduced damage requiring the simultaneous presence of light, photosensitizer and molecular oxygen” [61]. The outcome is usually singlet oxygen, produced via the exothermic and spin-allowed reaction of the PS’s photoexcited

Ó Elsevier 2012

3. Photoinduced oxygenation of thioanisoles and hydrocarbons by molecular oxygen

Fig. 1. Cytotoxicity of HerGa in MDA-MB-435 cells with and without irradiation at 424 nm: (a) cell survival after HerGa treatment, with and without 424 nm irradiation, at the indicated concentrations and (b) morphological changes of cancer cells after HerGa treatment with and without light. PBS: Phosphate Buffer Solution. Reprinted with permission from Ref. [67].

triplet state with molecular oxygen in its most stable triplet state form. Application of the photodynamic effect for killing cancer cells is called photodynamic therapy (PDT), whose utility has been realized since the beginning of the twentieth century and is currently used in various fields of medicine like oncology, ophthalmology and dermatology [62,63]. Numerous porphyrinoids have been investigated as PDT photosensitizers after the approval of Photofrin for clinical usage, a world wide effort that led to the approval of 8 more derivatives [64]. Corroles have also been considered as good PDT photosensitizer because of their relatively intense absorption of long visible wavelengths and efficient singlet oxygen generation thereby [28]. Of the derivatives studied so far, gallium(III) corroles have been demonstrated as most promising anti-cancer agents [65,66]. Specifically, the negatively charged water soluble gallium(III) 2,1 7-bis(sulfonato)-5,10,15-tris(pentafluorophenyl)corrole (GaCor ) binds tightly to a cancer cell targeting protein (HerPBK10) and the protein-corrole bioconjugate can penetrate tumor cells and exhibit high toxicity against breast cancer cells (MDA-MB-435) [66]. The above dark toxicity of the gallium corrole was enhanced by 2 orders of magnitudes via irradiation (see Fig. 1) [30,67]. Positively charged water soluble tris-(N-methyl-pyridyl)corrole gallium(III) complex (GaCor+) and some neutral gallium corroles were also shown to exhibit good PDT activity against tumor cells [68]. Cheng et al. used 10-(4-formylphenyl)-5,15-bis(pentafluoro phenyl)corrole 12 and its gallium complex 12-Ga to investigate their photocytotoxicity in liver cancer (BEL-7402), lung cancer (A549) and cervical cancer (Siha, Hela) cell lines [69]. They found that among all the tested tumor cells, Siha tumor cells were the most sensitive to photodynamic therapy (PDT) treatment by 12 and 12-Ga. The PDT induced IC50 values of 12 and 12-Ga toward Siha tumor cell were 1.3 and 0.8 lmol/L respectively, which is 10-fold lower than that of cis-platin. They also suggested that 12 and 12-Ga might pass across the tumor cell membrane smoothly

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without the help of any carrier protein, and that they are mainly localized in the cytoplasm of tumor cell. After PDT treatment, they found the intracellular reactive oxygen species (ROS) level to be increased and the mitochondrial membrane potential to be significantly decreased. This apparently leads to apoptosis of tumor cells. Wang et al. studied the PDT activity of phosphorus(V) monohydroxyl corrole 4-P and this complex also exhibited promising features [70]. Much progress has also been made in the utilization of free base corroles as TPSs for PDT. As early as 2006, Chang et al. found that the 2-hydroxyphenyl-substituted corrole 3 exhibits good PDT activity against nasopharyngeal carcinoma cancer cell lines [71]. Later, meso-pyridyl-substituted corroles 13, 14, and 15 were also found to display good PDT activity [72], while corroles 10 and 11 with their b-cyclodextrin moieties showed low PDT activity against tumor cells (HeLa) [56]. More recently, Alemayehu et al. measured the chemotoxicity and phototoxicity of free base amphiphilic corroles 20, and 21 and their gold complexes (20-Au, and 21-Au) against AY27 rat bladder cancer cells [47]. In the absence of light, the corroles did exhibit relatively low toxicity: LD50 values of about 300 lM for the free-base corroles and about 100 lM for the gold corroles. Encouraged by these dark toxicity measurements, they examined the light-induced toxicity of the corroles using 10 lM solutions (24 h incubation time, at 37 °C). The cells treated with either of the two free-base corroles exhibit little difference relative to untreated cells. In contrast, 21-Au exhibited substantial phototoxicity, leading to 50% cell death (LD50) after about 17 min and complete cell death after 40 min of irradiation. 20-Au was found to be even more effective, resulting in as much as 50% cell death (LD50) after 2.5 min of irradiation. These results confirm the hypothesis regarding the importance of the heavy atom effect (gold in this case) for effective production of singlet oxygen and subsequent cancer cell killing. There remain however many application-relevant concerns that were merely addressed so far, except for the earlier mentioned negatively charged gallium corroles. This includes interaction with native proteins present in human serum, which could be either beneficial or deleterious, specificity to cancer vs. healthy cells, cell penetration mechanisms and retention, excitation of the PS by tissue-penetration wavelengths, and more. Nevertheless, the results obtained so far are promising enough for justification of more focused research on corroles as TPS for PDT.

5. Photodynamic inactivation (PDI) The principles of photodynamic inactivation (PDI) are exactly the same as for PDT, but the target is different: instead of human cells, the focus is on pathogenic microorganisms such as Grampositive and Gram-negative bacteria, yeasts and fungi [73]. During the last two decades there is an increased interest to investigate the utilization of photosensitizers for PDI of bacteria, as well as of yeast and fungi [74,75]. Nevertheless, only few efforts have been made to adopt PDI for inhibition of phototrophic organisms such as green algae and cyanobacteria. The photostability of photosensitizers investigated up to date with regard to PDI of phototrophic organisms for prevention of biofilm formation is an issue that must be addressed for long-time use [76]. The photophysical properties and photostability of corroles suggested that these macrocycles could be promising photosensitizers against microorganisms and for long-term photodynamic inhibition of phototrophic organisms such as green algae. In line with that, Preuß et al. investigated the photophysical properties of positively and negatively charged antimony and phosphorus corroles (SbCor , SbCor+, PCor , and PCor+), in order to use them as photosensitizers for PDI of bacteria [55,77]. All of

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these complexes display typical corrole absorption and fluorescence spectra with Soret bands between 412 and 420 nm, Q bands in the 500–600 nm region, and emission maxima at 591–613 nm. In addition, they were disclosed to generate singlet oxygen with high quantum yields when irradiated with a daylight-mimicking bulb. The positively charged phosphorus corrole, PCor+, has been determined to have the highest UD (81%), followed by SbCor+ (67%), SbCor (66%), and PCor (58%). These encouraging results suggested that such photosensitizers could be suitable for using them for the PDI of microorganism.

5.1. PDI of mold fungi Both dark toxicity and photoinduced toxicity of the corroles SbCor , SbCor+, PCor , and PCor+ to the mold fungi A. niger (AN), Cladosporium cladosporoides (CC) and Penicillium purpurgenum (PP) were investigated by Preuß et al. [77]. The fungi were incubated with the above corroles either in the darkness or under illumination. Due to the asexual life cycle of mold fungi, the illuminated samples were incubated for additional 3 days in the darkness as to gain insight into the survival of the conidia by observation of the fungus recovery. Both dark toxicity and phototoxicity of the new photosensitizers were investigated by observation of the mold fungi growth, caused by the germination and following development of new fungus mycelium from the conidia. None of the corroles did inhibit the fungi growth under dark conditions, i.e., without any illumination after application of the TPS. But all of them did induce fungi growth inhibition under illumination. The conclusion was that all four compounds are photodynamically active but not toxic to the fungi in the dark. The two anionic complexes – SbCor and PCor – induced growth reduction of CC and PP and complete growth inhibition of AN. The fungi did however recover (i.e., resumed growth) after an additional incubation period of 3 days in the dark, which proves that the fungi spores were not damaged by the PDI treatment. The conclusion was that even though the anionic TPSs are able to reduce fungi growth under illumination, the fungi conidia were apparently not affected by photodynamic treatment. On the other hand, the cationic corroles SbCor+ and PCor+ were much more efficient photo-fungicides: they not only completely inhibited the growth of AN, CC, and PP, but also induced complete inactivation of the conidia. This came into effect by realizing that the fungi were not able to recover even after an additional incubation period of 3 days in the darkness. Fig. 2 shows some representative images of the results of these experiments. The photographs on the left-hand side demonstrate the growth of the fungi after three days under illumination without any TPS (Ref). All fungi display their characteristic mycelium and the development of new conidia. The photographs in column [A] in Fig. 2 show examples of the characteristic growth inhibition of AN, CC, and PP incubated with the anionic TPS (PCor ). Column [B1] shows the recovery of these exact mold fungi after the additional incubation in darkness. The last column [B2] illustrates the photodynamic effect of a cationic TPS (PCor+) on the fungi. After 3 days of illumination and the additional period in darkness none of the mold fungi does show any visible growth. The photodynamic treatment inhibited the growth under illumination and also inactivated the conidia, which is the reason that even in darkness no recovery of the fungi was detected. This could be related to the better interaction of cationic TPSs with the fungicidal cell wall [78]. Despite the fact that the central metal in the two cationic corroles is different (antimony vs. phosphorus) their phototoxicity to the mold fungi is similar. Considering that antimony is an environmental toxin [79], the cationic phosphorous corroles appear much more appealing for further application as photodynamic agents against wall-growing mold fungi and other microorganism.

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Ó Elsevier 2014

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Ó Wiley 2015

Fig. 2. Phototoxicity of PCor+ and PCor to the mold fungi Aspergillus niger (AN), Cladosporium cladosporoides (CC), or Penicillium purpurgenum (PP). The reference (Ref) contains no photosensitizer. [A]-Samples are incubated for 3 days under illumination with a fluence rate of 9 mW cm 2. [B]-Samples are the same samples incubated for 3 days under illumination and additionally 3 days in darkness. Reprinted with permission from Ref. [77].

Fig. 3. Macroscopic (a) and microscopic (b) photographic documentation of phototoxicity tests on green algae after 18 days of incubation. Reprinted with permission from Ref. [55].

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5.2. PDI of green algae growth Encouraged by the high photodynamic efficiency of antimony and phosphorus corroles against mold fungi, Pohl et al. investigated them as photosensitizers for inactivation of green algae [55]. The anionic corroles SbCor and PCor had a weak growth inhibition effect on green algae when illuminated with visible light. In contrast, both photoactivated cationic corroles, SbCor+ and PCor+, prevented algae growth and even caused effective reduction of biomass in all treated samples. As the singlet oxygen quantum yields of both SbCor and PCor are in the same range as that of SbCor+, it is unlikely that insufficient generation of singlet oxygen is responsible for these results. Instead, Pohl et al. suggested that the interaction of anionic corroles with green algae is unfavorable. They supported this assumption by the unsatisfactory results obtained by the negatively charged sulfonated phthalocyanines used for PDI of similar strains of green algae and cyanobacteria. The anionic corroles led to 45–50% growth reduction of green algae Chlorella fusca var. vacuolata and Stichococcus bacillaris relative to control during the first 7 days of the experiment (Fig. 3). Under the same conditions, treatment with the cationic corroles resulted in truly negligible growth rates. The inhibitory effect of the cationic corroles exceeds previously reported results by far, further confirming the hypothesis of preference for positively charged TPSs for photodynamic inactivation of green algae. One concern is that dark phases without illumination in accordance with a natural day/night rhythm could allow for algae growth due to interruption of photoinduced generation of singlet oxygen and stimulating effect on biomass production regulated by the circadian rhythm of the algae [80]. But, Pohl et al. were able to show for the first time that even interrupted illumination with white light, as may be expected under weathering conditions of building materials, provoked no recovery of the algae cultures as obtained for photoactive surfaces. Further studies looking into the photodynamic efficiency of this positively charged metallocorroles on biofilms of cyanobacteria, bacteria and lichens are very much in demand. Their successful application to green algae in addition to the ability to destroy mold fungi conidia already identifies these positively charged corroles as promising candidates for inhibition or removal of biofilms on surfaces that are subject to weathering or biodeterioration. 6. Photooxidation of inorganic compounds The future of modern society crucially depends on our ability to use reasonable and clean energy for driving endergonic reactions. The best-known example is water splitting to hydrogen and oxygen, which would resolve all global energy problems if it could be initiated by sunlight as the exclusive energy source [81,82]. Simpler light-driven chemical transformations should however also be considered because the photocatalytic water splitting is a very complicated process. One such example is the aerobic oxidation of bromide to bromine [83], whose importance might be illustrated by the advantages of H2/Br2 fuel cells relative to the hydrogen/air fuel cells and to the power generators that are fired by fossil fuels [84,85]. The chemical energy stored in the H2/Br2 reactants may be converted to electricity with up to 90% efficacy, compared to only about 50% for state-of-the-art hydrogen/air fuel cells, and about 40% for most power generators that are fired by fossil fuels [85]. The main industrial process for the production of bromine still relies on the oxidation of bromide by chlorine, which is quite preposterous from the chemical (as opposed to business) point of view [86]. In 2015, Mahammed and Gross studied the ability of metallocorroles to act as photosensitizers for catalyzing the aerobic oxidation of bromide to bromine using sunlight only [51]. Since the

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maximal visible light absorbance of the corroles ranges between 400 and 450 nm, they used a blue-light-emitting diode lamp (LED, kmax(emission) = 450 nm, power = 150 mW cm 2) for the photoexcitation of the corroles. The LED lamp has the advantage of emitting only the relevant wavelengths and not warm radiation that heats the solution, and also of consuming much less energy. Metallocorroles that were used are 1-Sb-F2, 1-P-Br8, 1-Al-Br8, and 1-Ga-Br8 whose kmax (Soret band) are 408, 428, 437, and 438 nm, respectively (Fig. 1) and have ground state oxidation potentials of +1.47 V, +1.40 V, +1.0, and +1.14 V vs. Ag/AgCl, respectively. These data suggested that all complexes will be photoexcited by the LED lamp and that the photoexcited state should be high enough in energy as to oxidize bromide. The experiments were performed in acetonitrile solutions containing the metallocorrole, trifluoroacetic acid (TFA), and tetrabutylammonium bromide (TBABr). All complexes remained stable during the irradiation with no bleaching or structural changes. The catalytic efficiency for photooxidation of bromide by the post-transition metallocorroles was 1-Ga-Br8 > 1-Al-Br8  1-P-Br8 > 1-Sb-F2, with turnover frequencies (TOFs) of 341 h 1, 290 h 1, 93 h 1, 72 h 1, respectively. This led to the following conclusions: (1) the metallocorroles whose absorption is better matched with the 450 nm wavelength emitted by the LED light perform better; and (2) the complexes that have more positive redox potentials are more efficient catalysts. The latter variable comes into effect in the comparison between the brominated gallium(III) and aluminum(III) corroles. While their electronic spectra are practically identical, the performance of 1-Ga-Br8 is better because its oxidation potential is higher than that of 1-Al-Br8. Control experiments showed that the absence of one of the components- light, oxygen, acid, and photosensitizing catalyst inhibits the catalytic reaction. For example, no significant amounts of bromine were formed when the photosensitizer was replaced by the transition-metal analog 1-Co-Br8, which is not photosensitizer, or when the reactions were performed under nitrogen. Irradiation of the acetonitrile solution of 1-Sb-F2/TBABr/TFA under nitrogen led to an about 50% catalyst- bleaching and partial reduction of the antimony(V) corrole into the antimony(III) corrole. This result uncovered the importance of the oxygen, required for closing the catalytic cycle by acting as electron acceptor from the photoreduced photosensitizer. The authors suggested a plausible mechanism for the photocatalysis of bromide oxidation based on the obtained results: irradiation of the photosensitizer by visible light creates a hole in its HOMO which is reduced by the bromide, while the single electron in the half-filled LUMO is donated to oxygen (Scheme 5). The redox potential of these photocatalysts provides valuable information about their oxidizing and reducing powers, considering that the electrochemical HOMO–LUMO gap of corroles is quite constant (about 2.1 eV). A more positive redox potential of the photosensitizer leads to a more oxidizing HOMO and a less reducing LUMO. These features are tunable by changing the identity of the corrole metal center and by bromination of the macrocycle. They also gave the following explanation about the importance of the acid in this system: it assists the O2 reduction, which becomes much more feasible when the one-electron product superoxide anion radical is protonated to produce HO2. The formed HO2 undergoes very efficient disproportionation to oxygen and hydrogen peroxide, the latter of which may also react with bromide to produce bromine and water. The alternative mechanism that depends on the photocatalyzed formation of singlet oxygen was disfavored, because it is known that bromide ions do not react with the short-lived singlet oxygen [87]. They also mentioned that the prior coordination of bromide to the catalyst, which was confirmed for the Ga complex in the dark, might safely be predicted to contribute to the fast redox process that leads to bromine.

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A. Mahammed, Z. Gross / Coordination Chemistry Reviews xxx (2017) xxx–xxx

Scheme 5. Proposed mechanism for photocatalytic oxidation of bromide to bromine, with metallocorroles as photosensitizers.

Scheme 6. Chemical structures of the chromophores based on precious Ru and Ir and the non-precious chromophores based on porphyrin.

Table 2 Electrochemical and photophysical properties of precious metal complexes (Ru and Ir) and non-precious metal complexes of porphyrins and corroles. Photocatalyst

Ru(bpy)2+ 3 Ir(Fppy)2(dtbbpy)+ TPP ZnTPP 1-Al 1-Al-I3 1-Al-I4 1-Ga 1-Ga-I3 1-Ga-I4 1-Sb-F2 1-Sb-O 1-Au 1-Au-I3 1-Au-I4 1-Au-Br4 1-Au-(CF3)3 1-Au-(CF3)4 a b c d e

Excited state redox potentials, V vs. SCE

Ground state redox potentials, V vs. SCE

(M+/M*) V

(M*/M )

(M+/M)

(M/M )

0.81a 0.94a 0.91b 1.18b 1.48b 1.38b 1.33b 1.43b 1.21b 1.13b 0.65b 1.00b 0.52a 0.34a 0.3a 0.23a 0.03a 0.18a

0.77 0.77 0.91b (0.42)a 0.72b (0.27)a 0.36b

1.29 1.33 1.03 0.86 0.51c 0.66c 0.69c 0.64c 0.82c 0.87c 1.42c 1.06c 1.13d 1.25d 1.28d 1.33d 1.49d 1.66d

1.33 1.50 1.03 1.32 1.63 Irreversible Irreversible 1.60c Irreversible Irreversible 0.82c 0.69c 1.09d 0.85d 0.77d 0.78d 0.54d 0.35d

( 0.42)a ( 0.73)a ( 0.87)a ( 0.82)a

0.46b ( ( ( (

0.71)a 0.63)a 0.15)a 0.53)a

1.26b (0.76)a 1.38b (0.9)a 0.55a 0.74a 0.81a 0.83a 0.98a 1.14a

kmax (nm) [e  10

CV CV CV CV

452 380 418 424 412 407 410 398 410 412 412 406 421 430 427 422 417 416

4

(M

1

cm

1

)]

[1.46] [6.16] [26.7], 514 [1.8], 548 [1.16] [45.2], 550 [1.21], 588 [0.28] [5.1], 432 [29.4], 620 [3.7] [4.32], 430 [17.07], 603 [3.53] [3.31], 433 [11.64], 596 [3.01] [5.3], 420 [28.4], 594 [2.36] [3.66], 433 [14.73], 608 [2.78] [4.59], 435 [14.84], 611 [3.18] [32.05], 588 [3.73] [9.37], 590 [1.43] [14.33], 530 [0.8] [12.07], 538 [1.04] [12.82], 542 [1.19] [15.84], 534 [1.18] [11.18], 540 [1.19] [5.68], 534 [1.19]

s triplet, (ls)

Ref.

0.81 2.3 >10 >10

[104–106] [106,107] [103], [108] [103], [108] [38], [109] [41] [41] [109], [40] [41] [41] [49] [49] [52,110] [110] [52,110] [110] [110] [110]

192 192 192 192 300e >100 >100

In the triplet state. In the singlet state. Recalculated potentials vs. SCE from reported data vs. Ag/AgCl by subtracting 0.045 V. Recalculated potentials vs. SCE from reported data vs. Ag in a 0.01 M AgNO3 and 0.1 M TBAP by adding 0.298 V. Unpublished result.

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7. Photoredox organic reactions The development of visible-light-promoted-methodologies for chemical transformations, such as efficient CAC bond formation, in a green, nontoxic, and inexpensive way has always been a challenge [88,89]. In particular, scientists have studied the excitedstate redox properties of noble metal (Ru, Ir, Pt, Au) complexes in photocatalysis [16,90–93]. The applications of these rare-earthbased photocatalysts include visible light induced water splitting, photovoltaic cells, and energy storage [2,94–98]. Despite some very good successes, the high price, low abundance, and some toxicity concerns of these complexes limit their large-scale utilization [99,100]. Therefore, the development of non-precious-metal photocatalysts with low toxicity is the highly sought after [11]. Surprisingly, despite the very well established utility of porphyrins (and to a lesser extent corroles as well) for PDT [1,15,101,102], there is almost no prior art about using them as photocatalysts for organic reactions. In fact, the 2016 publication by D. Gryko cites that ‘‘To the best of our knowledge, this is the first example of the use of porphyrins as photoredox catalysis in CAC bond forming reactions” [103]. We now present data that we consider as decisive for the design of new photoredox catalyst. Knowing the redox potentials of photocatalyst in the excited state is crucial for understanding existing and designing new light-induced reactions. This information may be deduced from the differences between the measured first redox potentials of the ground state of the photocatalyst and the energy of the maximum fluorescence or phosphorescence emission intensity, presented in electronvolts (eV). Table 2 summarizes the calculated redox potentials in the excited states, the measured redox potentials in the ground states, the maximum wavelength absorbance, and the triplet life times (s) of corroles and other leading photocatalysts. From the data in Table 2 it appears that metallocorroles are actually perfectly suited for serving as both photooxidants and photoreductants and may hence be anticipated useful for replacing the extensively studied Ru and Ir bipyridine complexes. Comparison between the photophysical data listed in Table 1 of the well + known and extensively studied Ru(bpy)2+ 3 and Ir(Fppy)2(dtbbpy) (Scheme 6) and those of corroles reveals that: 1. The molar absorption coefficients of the corroles are much higher than those of the Ru and Ir complexes, in the desired wavelengths. 2. The corrole triplet excited state life time is in the order of hundred microseconds while the Ru and Ir complexes have shortlived triplet excited states, which make them less suitable as TPSs. 3. The corroles metallated by abundant and not toxic nontransition metals such as phosphorous are non-precious and more environment-friendly than Ru and Ir complexes. 4. The corroles may function as both photooxidant and photoreductant, as their redox properties can be easily tuned by the electronic effects of their macrocycle substituents and by changing the coordinated axial ligand ligated to the metal center [28,109]. The last statement is exemplified by the following. The excitedstate oxidation potential of 1-Al is 1.48 V, which is more negative + than that of Ru(bpy)2+ 3 ( 0.81 V) and Ir(Fppy)2(dtbbpy) ( 0.94 V). As these Ru and Ir complexes are good photocatalysts for CAC bond formation [111], the aluminum corrole may be expected to be even better according to this criterion (but only if its excited lifetime could be increased). The very simple reaction of 1-Al with iodine produces the complexes 1-Al-I3 and 1-Al-I4, whose long tripletexited-state life-time makes them suitable as TPSs. Bromination of 1-Al to produce 1-Al-Br8 shifts its oxidation potential by

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450 mV (to 0.96 V vs. SCE) which suggest that 1-Al-Br8 may act as strong photooxidant catalyst. Changing the aluminum in 1-Al by antimony (1-Sb-F2) shifts the oxidation potential to a much more positive direction (1.42 V) [49] which is shifted to even more positive potentials by bromination of the corrole macrocycle of 1Sb-F2 to produce 1-Sb-Br8 with an oxidation potential of 1.60 V [51]. 8. Concluding remarks and future directions The data compiled in this review clearly show that corroles are good triplet photosensitizers that may be used as electron- or energy-donors for various photochemical and photophysical processes. They can be applied for different applications such as photodynamic imaging, photodynamic therapy, photodynamic inactivation of bacteria, and as photocatalysts for organic and inorganic reactions. The synthesis of corroles is facile and their photochemical, photophysical, and electrochemical properties can be easily tuned by metallation of the corrole core by different metals and/or substitution of electron-withdrawing/donating groups on the corrole skeleton. Despite these promising properties of corroles they are still much less frequently used as photocatalysts than metalloporphyrins or rare transition metal complexes such as ruthenium bipyridine. The examples provided in this review serve to predict a great future for photocatalysis by metallocorroles. Acknowledgments This work has been supported by the Pazy Foundation. References [1] A.E. O’Connor, W.M. Gallagher, A.T. Byrne, Photochem. Photobiol. 85 (2009) 1053–1074. [2] Y. Cakmak, S. Kolemen, S. Duman, Y. Dede, Y. Dolen, B. Kilic, Z. Kostereli, L.T. Yildirim, A.L. Dogan, D. Guc, E.U. Akkaya, Angew. Chem. Int. Ed. 50 (2011) 11937–11941. [3] A. Kamkaew, S.H. Lim, H.B. Lee, L.V. Kiew, L.Y. Chung, K. Burgess, Chem. Soc. Rev. 42 (2013) 77–88. [4] Q. Zhao, F. Li, C. Huang, Chem. Soc. Rev. 39 (2010) 3007–3030. [5] D. Ravelli, M. Fagnoni, A. Albini, Chem. Soc. Rev. 42 (2013) 97–113. [6] M. Neumann, S. Füldner, B. König, K. Zeitler, Angew. Chem. Int. Ed. 50 (2011) 951–954. [7] Y.-Q. Zou, L.-Q. Lu, L. Fu, N.-J. Chang, J. Rong, J.-R. Chen, W.-J. Xiao, Angew. Chem. Int. Ed. 50 (2011) 7171–7175. [8] M. Ertl, E. Wo, G. Knor, Photochem. Photobiol. Sci. 14 (2015) 1826–1830. [9] G. Knör, Coord. Chem. Rev. 304 (2015) 102–108. [10] J. Prock, C. Strabler, W. Viertl, H. Kopacka, D. Obendorf, T. Muller, E. Tordin, S. Salzl, G. Knor, M. Mauro, L. De Cola, P. Bruggeller, Dalton Trans. 44 (2015) 20936–20948. [11] Y. You, W. Nam, Chem. Soc. Rev. 41 (2012) 7061–7084. [12] J. Zhao, S. Ji, W. Wu, W. Wu, H. Guo, J. Sun, H. Sun, Y. Liu, Q. Li, L. Huang, RSC Adv. 2 (2012) 1712–1728. [13] J.I. Goldsmith, W.R. Hudson, M.S. Lowry, T.H. Anderson, S. Bernhard, J. Am. Chem. Soc. 127 (2005) 7502–7510. [14] X. Cui, J. Zhao, Z. Mohmood, C. Zhang, Chem. Rec. 16 (2016) 173–188. [15] N.J. Turro, V. Ramamurthy, J.C. Scaiano, Principles of Molecular Photochemistry: An Introduction, University Science Books, Sausalito, CA, 2009. [16] H. Xiang, J. Cheng, X. Ma, X. Zhou, J.J. Chruma, Chem. Soc. Rev. 42 (2013) 6128–6185. [17] T. Yogo, Y. Urano, Y. Ishitsuka, F. Maniwa, T. Nagano, J. Am. Chem. Soc. 127 (2005) 12162–12163. [18] S.G. Awuah, Y. You, RSC Adv. 2 (2012) 11169–11183. [19] S.G. Awuah, J. Polreis, V. Biradar, Y. You, Org. Lett. 13 (2011) 3884–3887. [20] F. Julia, D. Bautista, P. Gonzalez-Herrero, Chem. Commun. 52 (2016) 1657– 1660. [21] N. Armaroli, ChemPhysChem 9 (2008) 371–373. [22] J.A.G. Williams, Photochemistry and photophysics of coordination compounds: platinum, in: V. Balzani, S. Campagna (Eds.), Photochemistry and Photophysics of Coordination Compounds II, Springer Berlin Heidelberg, Berlin, Heidelberg, 2007, pp. 205–268. [23] J.C. Deaton, R.H. Young, J.R. Lenhard, M. Rajeswaran, S. Huo, Inorg. Chem. 49 (2010) 9151–9161. [24] J. Zhao, W. Wu, J. Sun, S. Guo, Chem. Soc. Rev. 42 (2013) 5323–5351.

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Please cite this article in press as: A. Mahammed, Z. Gross, Corroles as triplet photosensitizers, Coord. Chem. Rev. (2017), http://dx.doi.org/10.1016/j. ccr.2017.08.028