meso-Tetraphenylporphyrin with a pi-system extended by fusion with anthraquinone

Abstract

Fusion with a 9,10-anthraquinone moiety was achieved to extend porphyrin's π-system. A bridged dihydroisoindole derivative was used to prepare the corresponding meso-tetraphenyltetraanthraquinonoporphyrin (Ph₄TAQP) via a thermal retro-Diels–Alder reaction. The basic optical properties of the prepared new anthraquinonoporphyrin and its complexes with Zn and Pd were studied.

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Introduction

Porphyrins with aromatic rings fused to the tetrapyrrolic core, so-called π-extended porphyrins, have attracted much attention in recent years as materials for numerous applications – from biomedical sensing and imaging to organic optoelectronics.¹ Metallated π-extended porphyrins are particularly important for the process of triplet–triplet annihilation photon energy upconversion (TTA-UC).² A variety of π-extended porphyrins have been synthesized by fusing benzene,³ naphthalene,⁴ pyrene,⁵ azulene,⁶ anthracene,⁷ corannulene,⁸ and other aromatic moieties to the meso- and β-positions of the macrocycle. Fusion of aromatic rings to all four pyrrole residues results in particularly strong effects on the π-system, leading to enhanced light absorption and efficient emission in the near-infrared (IR-A) region of the spectrum.⁹

First reported by Krautler and co-workers, a conjugation of naphthoquinone to a porphyrin has a remarkable effect on its properties. Particularly, resulting materials exhibit optical properties which resemble those of nanoscopic carbon materials with extended π-systems, such as graphene, graphite, and nanotubes.¹⁰ Theoretical studies of tetranaphthoquinonoporphyrin (TNQP) revealed that introduction of the carbonyl groups into the π-system results in strong alternations of bonds and a transformation of the conjugation from “benzene-type” to “butadiene-type”. Unidirectional photon-induced current associated with p–π conjugation enables light-harvesting efficiency of this kind of molecular skeleton to reach 90% in the range of 300–800 nm.¹¹ This makes TNQPs attractive materials for panchromatic dye-sensitized solar cells. Moreover, porphyrins fused with quinone moieties are expected to exhibit interesting electrochemical properties, since they are able to accept a load of at least 8 electrons per molecule. Such materials clearly promise to expand the range of multi-electron transfer (MET) catalysts – compounds having the ability to accommodate and transfer multiple electrons to reaction substrates at one time.¹²

Despite promising properties, tetraquinonoporphyrins (TQP) are almost unknown because the available synthetic methods in the field of π-extended porphyrins chemistry have been very limited until recently. To the best of our knowledge, the only representative of a porphyrin directly fused with four quinone fragments was obtained by Krautler and co-workers, using the [4 + 2] cycloaddition reaction between β,β′-tetrasulfolenoporphyrin¹³ and an excess of benzoquinone.¹⁰

Herein we report a synthetic approach to meso-tetraphenyltetraanthraquinonoporphyrin (Ph₄TAQP) based on a bridged dihydroisoindole precursor. In addition we describe the basic optical properties of the newly synthesized Ph₄TAQP free-base and its metal complexes.

Results and discussion

Due to the instability of isoindole and its π-expanded analogues,¹⁴ the formation of a fully conjugated π-system has to be performed after the formation of the porphyrin macrocycle. So far, two general synthetic methods have been employed to construct the extended porphyrin architecture: oxidative aromatization¹⁵ and thermal retro-Diels–Alder reaction.¹⁶

As is shown in Scheme 1, the use of the oxidative aromatization approach for the synthesis of tetraanthraquinonoporphyrin requires the corresponding dihydroisoindole derivative (Scheme 1, route A). According to the thermal retro-Diels–Alder approach, the target molecule can be prepared from bicyclo[2.2.2]octadiene-annelated porphyrin which can undergo thermal extrusion of ethylene (route B).

Scheme 1 Retrosynthetical analysis of a TAQP system.

A pyrrole derivative containing a naphthoquinone moiety represents a direct precursor for the synthesis of TAQP through route A. We first examined the possibility to apply directly 1,4,4a,9a-tetrahydro-anthraquinone 1 (Scheme 2) for the synthesis of the corresponding pyrrole from vinyl or allyl sulfones via a Barton–Zard reaction.¹⁷ Treatment of 1 with PhSCl, followed by oxidation with Oxone led to the chlorosulfone derivative 2. Further reaction with DBU yielded 2-phenylsulfonylanthraquinone 3, rather than the expected vinyl sulfone. An attempt to introduce 3 into Barton–Zard synthesis was unsuccessful and delivered mixture of products arising from the reduction of the quinone moiety. Thus, a protection of the reactive quinonic moiety was necessary to avoid side reactions during the pyrrole synthesis. Conversion of the quinone into corresponding hydroquinone diacetates was preferable over reductive methylation since it requires mild conditions for further deprotection.¹⁸

Scheme 2 Synthesis of a TAQP pyrrole precursor.

Dione 1 is known to form a deprotonated dihydronaphthoquinone irreversibly upon treatment with bases.⁷ Treatment of 1 with DBU and acetic anhydride provided diacetate 4. It should be noted that this procedure was found to give higher yields than previously reported aromatization of the dione ring by boiling with acetic anhydride and acetic acid in the presence of p-toluenesulfonic acid as a catalyst.¹⁹

Diacetate was then used for the preparation of allylsulfone 5, employing a previously established procedure. As expected, compound 5 was formed in good yield. However, under the conditions of Barton–Zard reaction (t-BuOK, THF, isocyanoacetate),²⁰ no formation of the corresponding pyrrole compound was observed. Diacetoxyanthracene 6 was the only isolated product. Attempts to optimize the reaction conditions: changing the base (DBU, potassium and sodium tert-butoxides, HMDS), solvents and temperature regimes failed to deliver the target product. It is known that aromatization of cyclohexadienes can be incurred by strong bases.²¹ However, taking into account that a similar sulfone derivative containing butoxy-groups instead or acetoxy-groups was previously successfully used in the pyrrole synthesis,⁷ it is interesting that sulfone 6 behaves so differently under basic conditions, when elimination is the predominant pathway.

Thus we focused further efforts on the thermal retro-Diels–Alder approach. 1,4-Naphthoquinone was reacted with 1,3-cyclohexadiene to obtain dione precursor 7. Its acetylation gave 8, which was used for the preparation of the corresponding sulfone 9. As expected, the Barton–Zard reaction with isocyanoacetate synthesis delivered pyrrole 10.

In this case tert-butyl isocyanoacetate²⁴ was used, since for pyrrole tert-butyl esters a decarboxylation reaction can be performed via solvolysis in neat trifluoroacetic acid. These conditions were expected to secure the hydroquinone moiety from deprotection. Indeed, treatment with TFA for 30 min delivered pyrrole 11 in good yield (68%).

With pyrrole 11 in hand, we succeeded to prepare intermediate porphyrin 12 according to the conventional Lindsey condensation.²² As shown in Scheme 3, pyrrole 11 reacted with benzaldehyde in CH₂Cl₂ in the presence of BF₃·OEt₂, followed by oxidation with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) at room temperature for additional 3 hours to afford porphyrin 12 in 18% yield after purification. After further treatment of the obtained porphyrin 12 with KOH and oxidation by DDQ the resulting crude intermediate was heated at 200 °C in vacuum for 4 h. Target tetraanthraquinonoporphyrin was isolated in 65% yield after chromatographic purification and recrystallization. To our surprise, instead of the expected problems with poor solubility due to π-stacking, we observed a rather good solubility (as compared to tetranaphtho- or tetraanthraporphyrins) of the obtained product in common organic solvents (chlorohydrocarbons, aromatics, THF).

Scheme 3 Synthesis of Ph₄TAQP.

The aromatization was clearly observed by the disappearance of methylene groups and the appearance of a new singlet peak in the aromatic region corresponding to eight protons on the anthraquinone rings in the ¹H NMR spectrum. It is noteworthy that well-resolved ¹H and ¹³C NMR spectra were obtained after addition of trifluoroacetic acid (TFA) which converted the porphyrin into a dication form. MALDI-TOF mass spectra gave the additional evidence for the formation of Ph₄TAQP (ESI†).

The absorption and emission spectra of porphyrins 12, Ph₄TAQP and its metal complexes are compared in Fig. 1. Electronic absorption spectra of 12 are similar to other tetratetraphenyl-β-octaalkylporphyrins, such as the derivatives of octaethylporphyrin (OEP) showing a Soret band at 434 nm and Q-bands at 523, 607, 675 nm in CH₂Cl₂ (for comparison, tetraphenyltetracyclohexenoporphyrin free base: Soret band 439 nm, Q-bands 537, 580, 606, 674 nm).²⁰ The fluorescence spectrum of 12 is also consistent with this type of porphyrin skeleton, showing a maximum at 718 nm and a low quantum yield of emission (φ_fl < 0.01 in toluene, λ_exc = 638 nm).

Fig. 1 Absorption (black) and emission (red lines) of (A) porphyrin 12, (B) Ph₄TAQP free base, (C) Ph₄TAQP-Zn, and (D) Ph₄TAQP-Pd. Solvent: toluene.

Ph₄TAQP exhibits strongly red-shifted Soret and Q-bands (Fig. 1B). The vibronic structure in the Q-band region is well-resolved. The lowest energy Q-band (752 nm) is red-shifted by 77 nm relative to the corresponding transition of the porphyrin 12 due to the effect of extended π-conjugation. At the same time, intensification of Q-bands is taking place – the maximum absorption ratio of the Q-band to the Soret band is enhanced from 0.09 (in 12) to 0.35. The free-base shows much stronger emission (φ_fl = 0.08) than the parent compound 12, with a small Stokes shift (9 nm). Metal insertion has a profound effect on optical properties. The absorption spectra of Zn and Pd-complexes are shown in Fig. 1C and D. Very strong blue-shift by 66 nm upon palladium insertion and 25 nm upon zinc insertion are observed for the lowest energy Q-band. Both complexes show relatively strong emission (φ_em = 0.11 and 0.06 for Zn and Pd-complexes respectively). The emission of Ph₄TAQP shows multiple maxima that may be associated either with excimer formation or formation of charge-transfer excited states. Solutions of Ph₄TAQP and its metal complexes do not decompose noticeably when exposed to daylight for several days, indicating good photostability compared to other π-extended porphyrins.²³

Comparison of the absorption spectra of Ph₄TAQPPd with those of palladium(II) tetraphenyltetrabenzo- and tetraphenyltetranaphthoporphyrins (Ph₄TBPPd and Ph₄TNPPd respectively, Fig. 2) demonstrates the effect of anthraquinone fusion on the porphyrin core with respect to annelation of extra benzo-rings. The strong effect on the energies of S₁ and S₂ states of the molecule is manifested by the pronounced red shift of the Soret and Q-bands. While in the case of Ph₄TBPPd and Ph₄TNPPd the Soret band is shifted only by 20–30 nm with respect to parent palladium(II) tetraphenylporphyrin, fusion of anthracenes causes 100 nm red shift. Nevertheless, a “spectral window” between the Soret and Q-bands allows for the application of Ph₄TAQPPd as a sensitizer for the TTA-UC process that will be reported in a separate study.

Fig. 2 UV-Vis absorption spectra of Ph₄TBPPd (black), Ph₄TAQPPd (red) and Ph₄TNPPd (blue).

Conclusions

Two approaches towards the synthesis of TAQP were explored: the one based on the hydroisoindole precursor and bridged dihydroisoindole. The latter was found to be suitable for the synthesis of a target compound using the Barton–Zard reaction. The strategy based on oxidative aromatization of the dihydroisoindole precursor failed to deliver the target compound due to side reactions in the course of pyrrole synthesis. The optical properties of Ph₄TAQP indicate electronic features that call for theoretical studies, as well as for better characterization using photophysical and electrochemical experiments. Indeed, new quinonoporphyrins are expected to exhibit interesting electrochemical properties as a result of the directly conjugated porphyrin and quinone moieties. Such materials appear to be of interest in photon energy conversion systems and in other applications. We relay a detailed discussion of the photophysical properties of variously substituted TAQP for a separate study.

Experimental

1,4,4a,9a-Tetrahydroanthraquinone⁷ and tert-butyl isocyanoacetate²⁴ were prepared according to published synthetic protocols. DBU, thiophenol, bis(benzonitrile)palladium(II) chloride, DDQ, N-chlorosuccinimide, Oxone, 1,4-naphthoquinone, trifluoroacetic acid, benzaldehyde, boron trifluoride etherate and extra dry THF were purchased from Sigma-Aldrich. The handling of all air/water sensitive materials was carried out using standard high vacuum techniques. All solvents and reagents were obtained from commercial sources and used as received. Where mixtures of solvents were used, ratios are reported by volume. Column chromatography was carried out on silica gel 60 at normal pressure. NMR spectra were recorded on Bruker DPX 250, Bruker AC300 NMR and Bruker Avance 500 spectrometers, with the solvent proton or carbon signal as an internal standard. Elemental analysis was carried out using a Foss Heraeus Vario EL. Electronic absorption spectra were recorded on a Perkin Elmer Lambda 25 instrument. MALDI-TOF spectra were recorded on a Bruker Reflex spectrometer III instrument using dithranol as a matrix. Melting points were determined on a Büchi hot stage apparatus and are uncorrected. Emission spectra were recorded using a Fluoromax-2 instrument. Emission quantum yields of the compounds were measured relative to the fluorescence of free-base tetraphenylporphyrin (φ_fl = 0.11)²⁵ in deoxygenated toluene.

2-Benzenesulfonyl-3-chloro-1,2,3,4-tetrahydro-anthra-quinone 2

¹H NMR δ_H (300 MHz, CD₂Cl₂) 8.06 (2H, m), 7.97 (2H, m), 7.79–7.59 (5H, m), 5.03 (1H, q, J = 3.3 Hz), 4.05 (1H, m), 3.42–2.92 (4H, m). ¹³C NMR δ_C (75 MHz, CD₂Cl₂) 184.04, 140.57, 140.45, 139.49, 135.24, 134.59, 134.56, 133.21, 130.52, 129.97, 127.01, 126.94, 62.80, 51.53, 30.72, 20.30. Anal. calcd for C₂₀H₁₇ClO₄S: C, 61.77; H, 4.41; found: C, 61.23; H, 4.65.

2-Benzenesulfonyl-anthraquinone 3

¹H NMR δ_H (300 MHz, CD₂Cl₂) 8.76 (1H, t, J = 1.2 Hz), 8.4 (2H, t, J = 1.1 Hz), 8.28 (2H, m), 8.09–8.03 (2H, m), 7.90–7.82 (2H, m), 7.64–7.53 (2H, m). ¹³C NMR δ_C (75 MHz, CD₂Cl₂) 182.06, 148.44, 142.38, 137.36, 135.57, 135.41, 135.37, 134.94, 134.73, 134.61, 134.57, 133.18, 130.62, 129.26, 129.16, 128.02, 127.05. Anal. calcd for C₂₀H₁₂O₄S: C, 68.95; H, 3.47; found: C, 68.32; H, 3.72.

9,10-Diacetoxy-1,4-dihydro-anthracene 4

The title compound was prepared following a modified literature procedure.²⁶ 1,8-Diazabicycloundec-7-ene (10.5 mL, 70 mmol) was added to a stirred solution of 1,4,4a,9a-tetrahydroanthraquinone (6.36 g, 30 mmol) and THF (100 mL) at room temperature. The mixture was cooled in an ice bath and acetic anhydride (8.5 mL, 90 mmol) was added dropwise over a period of 10 min and the resulting solution was stirred for 2 hours. Then diethyl ether (100 mL) was added to precipitate the product. The solid so formed was filtered and washed with ether (50 mL) to give 8.44 g (95%) of the product as a white powder (m.p. 255–257 °C, lit. 256–258 °C).²⁶¹H NMR δ_H (300 MHz, CD₂Cl₂) 7.75 (2H, m), 7.51 (2H, m), 5.95 (2H, m), 3.37 (4H, br. s), 2.49 (6H, s). ¹³C NMR δ_C (75 MHz, CD₂Cl₂) 169.57, 142.2, 126.97, 126.52, 125.59, 123.54, 121.62, 25.0, 20.96.

9,10-Diacetoxy-2-benzenesulfonyl-1,2-dihydro-anthracene 5

The title compound was prepared following a modified literature procedure.²⁷ Thiophenol (2 mL, 2.2 g, 20 mmol) was added dropwise to a suspension of N-chlorosuccinimide (2.67 g, 20 mmol) in CH₂Cl₂ (20 mL) under cooling in an ice bath. The mixture was stirred for 1 h at r.t. and the resulting orange solution was added dropwise to a stirred solution of 9,10-diacetoxy-1,4-dihydro-anthracene (5.92 g, 20 mmol) in CH₂Cl₂ (150 mL) at 0 °C. The mixture was stirred at room temperature for 2 h and then evaporated in a vacuum. The residue was dissolved in methanol (60 mL) and a suspension of Oxone (12.3 g, 20 mmol) in water (30 mL) was added under vigorous stirring. The mixture was stirred at room temperature for 2 days, diluted with water (100 mL) and extracted with CH₂Cl₂. The combined organic layers were dried with Na₂SO₄ and evaporated to dryness. The resulting solid was dissolved in CH₂Cl₂ (50 mL), and DBU (3 mL, 20 mmol) was added dropwise over a period of 10 min at 0 °C. The mixture was stirred for 1 h at room temperature, washed with water, dried with Na₂SO₄ and evaporated in a vacuum. The solid residue was recrystallized from MeOH to give 6.1 g (70%) of the title compound as a white powder (m.p. 155–157 °C). ¹H NMR δ_H (300 MHz, CD₂Cl₂) 7.69 (4H, m), 7.51 (2H, m), 7.3 (3H, d, J = 6.9 Hz), 6.84 (1H, dd, J = 9.9 Hz), 6.18 (1H, dd, J = 9.9 Hz), 4.09 (1H, m), 3.48 (1H, m), 3.11 (1H, m), 2.49 (3H, s), 2.46 (3H, s). ¹³C NMR δ_C (75 MHz, CD₂Cl₂) 169.17, 136.76, 134.33, 129.88, 129.11, 128.06, 128.0, 127.54, 127.43, 127.06, 122.41, 122.09, 121.85, 121.72, 121.51, 23.1, 20.93. Anal. calcd for C₂₄H₂₀O₆S: C, 66.04; H, 4.62; found: C, 66.32; H, 4.85.

1,2,3,4,4a,9a-Hexahydro-1,4-etheno-anthraquinone 7

A mixture of 1,4-naphthoquinone (10 g, 63 mmol), 1,3-cyclohexadiene (9.5 mL, 100 mmol) and 2,6-di-tert-butylphenol (0.05 g, 0.24 mmol) was dissolved in CHCl₃ and refluxed for 24 h under argon. The resulting mixture was evaporated in a vacuum and the residue was recrystallized from EtOH to give 12.7 g (85%) of the title compound as a white powder (m.p. 83–85 °C). ¹H NMR δ_H (300 MHz, CD₂Cl₂) 7.98 (2H, m), 7.69 (2H, m), 6.13 (2H, m), 3.3 (2H, m), 3.21 (2H, t, J = 1.3 Hz), 1.78 (2H, m), 1.38 (2H, m). ¹³C NMR δ_C (75 MHz, CD₂Cl₂) 198.12, 181.74, 151.07, 136.28, 134.47, 134.33, 134.24, 133.83, 133.0, 127.14, 126.59, 50.99, 36.26, 34.72, 25.44, 25.11. Anal. calcd C, 80.65; H, 5.92; O, 13.43; found: C, 80.12; H, 6.04.

9,10-Acetoxy-1,2,3,4-tetrahydro-1,4-etheno-anthracene 8

The title compound was obtained according to the procedure described for 4. Yield: 90%. White powder with m.p. 232–233 °C. ¹H NMR δ_H (300 MHz, CD₂Cl₂) 7.8 (2H, m), 7.5 (2H, m), 6.55 (2H, m), 4.08 (2H, m), 2.52 (2H, m), 1.58 (4H, s). ¹³C NMR δ_C (75 MHz, CD₂Cl₂) 170.01, 137.63, 135.28, 134.36, 126.61, 126.37, 121.80, 34.95, 24.94, 21.03. Anal. calcd for C₂₀H₁₈O₄: C, 74.52; H, 5.63; found: C, 74.87; H, 5.85.

9,10-Diacetoxy-12-benzenesulfonyl-1,2,3,4-tetrahydro-1,4-etheno-anthracene 9

The title compound was obtained according to the procedure described for 5. Yield: 65%. White powder with m.p. 213–214 °C. ¹H NMR δ_H (300 MHz, CD₂Cl₂) 7.81 (3H, m), 7.69 (1H, m), 7.62 (1H, m), 7.51 (5H, m), 4.34 (2H, m), 2.52 (3H, s), 2.42 (3H, s), 1.65 (4H, m). ¹³C NMR δ_C (75 MHz, CD₂Cl₂) 169.80, 169.50, 147.74, 144.23, 139.86, 138.46, 138.14, 134.08, 131.21, 131.18, 129.84, 128.32, 127.23, 127.15, 126.75, 126.56, 122.06, 121.93, 36.5, 35.59, 25.60, 24.81, 21.0, 20.86. Anal. calcd for C₂₆H₂₂O₆S: C, 67.52; H, 4.79; found: C, 67.89; H, 5.04.

5,10-Diacetoxy-4,11-etheno-2H-naphtho[2,3-f]isoindole-1-carboxylic acid tert-butyl ester 10

The title compound was obtained according to a previously published general procedure.²⁴ Yield: 78%. White powder with m.p. 186–187 °C. ¹H NMR δ_H (300 MHz, CD₂Cl₂) 8.62 (1H, br. s), 7.78 (2H, m), 7.5 (2H, m), 6.7 (1H, d, J = 2.7 Hz), 4.93 (1H, m), 4.42 (1H, m), 2.54 (3H, s), 2.53 (3H, s), 1.77 (4H, m), 1.61 (9H, s). ¹³C NMR δ_C (75 MHz, CD₂Cl₂) 169.93, 169.87, 161.40, 138.45, 137.96, 134.75, 134.21, 132.88, 129.02, 126.83, 126.56, 126.55, 121.86, 117.12, 114.22, 81.06, 32.54, 32.29, 28.84, 28.51, 27.20, 26.59, 21.14, 21.06. Anal. calcd for C₂₇H₂₇NO₆: C, 70.27; H, 5.90; N, 3.03; found: C, 69.89; H, 6.14; N, 2.87.

5,10-Diacetoxy-4,11-etheno-2H-naphtho[2,3-f]isoindole 11

Compound 10 (1 g, 2.2 mmol) was dissolved in TFA (30 mL), and the solution was stirred for 30 min under Ar at room temperature. After the addition of CH₂Cl₂ (50 mL), the mixture was washed with water, then with 10% solution of Na₂CO₃, dried with Na₂SO₄ and evaporated in a vacuum. The residue was passed through a layer of silica using CH₂Cl₂ as the eluent. The solvent was evaporated to give 0.53 g (68%) of the title compound as a gray solid (m.p. 130–132 °C). ¹H NMR δ_H (300 MHz, CD₂Cl₂) 7.76 (2H, m), 7.47 (2H, m), 6.58 (2H, d, J = 2.4 Hz), 4.41 (2H, t, J = 1.3 Hz), 2.53 (6H, s), 1.75 (4H, m). ¹³C NMR δ_C (75 MHz, CD₂Cl₂) 169.96, 137.94, 135.36, 126.91, 126.64, 126.53, 121.82, 109.94, 32.28, 31.05, 27.65, 21.07. Anal. calcd for C₂₂H₁₉NO₄: C, 73.12; H, 5.30; N, 3.88; found: C, 72.65; H, 5.14; N, 3.47.

Porphyrin 12

5,10-Diacetoxy-4,11-etheno-2H-naphtho[2,3-f]isoindole (0.3 g, 0.83 mmol) was dissolved in CH₂Cl₂ (83 mL) freshly distilled from CaH₂, and benzaldehyde (0.088 g, 0.83 mmol) was added. The mixture was stirred under nitrogen for 10 min in the dark at room temperature. BF₃·Et₂O (10 μL, 0.083 mmol) was added in one portion, and the mixture was stirred for an additional 2 h. DDQ (0.141 g, 0.62 mmol) was added followed by additional stirring for 2 h. The resulting mixture was washed with aqueous Na₂SO₃, dried over Na₂SO₄ and concentrated in a vacuum. The residue was purified on a silica gel column (eluent CH₂Cl₂, then CH₂Cl₂–HOAc, green band collected). Additional purification by recrystallization from CH₂Cl₂–Et₂O delivered the title product (67 mg, 18%) as a dark-green powder. ¹H NMR δ_H (300 MHz, CD₂Cl₂–TFA) 8.96–6.85 (36H, m), 4.74–4.17 (8H, m), 3.22–2.74 (24H, m), 2.15–1.84 (16H, m). ¹³C NMR δ_C (75 MHz, CD₂Cl₂–TFA) 170.26, 137.89, 135.43, 134.96, 132.05, 127.12, 126.98, 126.66, 126.27, 121.71, 120.45, 110.42, 32.59, 31.16, 27.34, 21.45. UV/vis (CH₂Cl₂) λ_max (log ε): 434 (5.2), 523 (4.21), 607 (3.93), 675 (3.84). MALDI-TOF: m/z found 1791.61, calcd for [M+] C₁₁₆H₈₆N₄O₁₆ 1791.60. Anal. calcd for C₉₂H₄₆N₄O₈: C, 77.75; H, 4.84, N, 3.13; found: C, 78.58; H, 5.36; N, 3.41.

Ph₄TAQP free base

Porphyrin 12 (50 mg) was dissolved in THF (10 mL) and a solution of KOH (0.25 g) in EtOH (5 mL) was added. The mixture was stirred at room temperature for 12 h, then concentrated HCl (1 ml) was added and the solution was evaporated in a vacuum. The residue was washed several times with CH₂Cl₂ to separate soluble porphyrin from the inorganic solid, the resulting solution was dried with Na₂SO₄ and filtered. DDQ (0.188 g, 0.83 mmol) was then added and the mixture was stirred for 6 h. The resulting mixture was washed with aqueous Na₂SO₃, dried over Na₂SO₄ and concentrated in a vacuum. The residual solid was heated in a vacuum oven at 200 °C for 4 h. Then it was dissolved in CH₂Cl₂ and purified on a silica gel column (eluent CH₂Cl₂, then CH₂Cl₂–THF, purple band collected). Additional purification by repetitive precipitation from CH₂Cl₂–Et₂O delivered the title product (24 mg, 65%) as a purple powder. ¹H NMR δ_H (500 MHz, C₂D₂Cl₄–TFA) 8.69 (8H, m), 8.44 (8H, s), 8.38 (4H, m), 8.29–8.17 (16H, m), 7.83 (8H, m), 4.05 (4H, br. s). ¹³C NMR δ_C (125 MHz, C₂D₂Cl₄–TFA) 181.73, 142.53, 138.04, 135.86, 135.52, 134.56, 133.53, 133.12, 133.11, 130.39, 127.33, 124.70, 117.37. UV/vis (CH₂Cl₂) λ_max (log ε): 534 (5.18), 642 (4.22), 699 (4.72), 752 (4.22). MALDI-TOF: m/z found 1355.33, calcd for [M+] C₉₂H₄₆N₄O₈ 1355.33. Anal. calcd for C₉₂H₄₆N₄O₈: C, 82.75; H, 3.47, N, 4.20; found: C, 83.57; H, 3.98; N, 4.74.

Ph ₄ TAQP-Pd was obtained in 75% yield after heating a mixture of the free-base porphyrin, excess PdCl₂(PhCN)₂ (2 eq.) and Et₃N (10 eq.) in benzonitrile at 160 °C for 0.5–3 h (control by UV-Vis spectroscopy), with subsequent filtration through a layer of silica (eluent CH₂Cl₂) and evaporation of the filtrate. UV/vis (CH₂Cl₂) λ_max (log ε): 501 (5.05), 629 (4.11), 686 (4.97). MALDI-TOF: m/z found 1439.2361, calcd for [M+] C₉₂H₄₄N₄O₈Pd 1439.22.

Ph ₄ TAQP-Zn was obtained in 90% yield after the treatment of a free-base in THF with an excess of Zn(OAc)₂·2H₂O, followed by subsequent precipitation with MeOH, filtration and drying in a vacuum. UV/vis (CH₂Cl₂) λ_max (log ε): 552 (5.12), 677 (4.35), 727 (4.84). MALDI-TOF: m/z found 1397.24, calcd for [M+] C₉₂H₄₄N₄O₈Zn 1397.24.

Acknowledgements

M.A.F. acknowledges POLINNOVA project (FP7-REGPOT-2012-2013-1) for the financial support. Financial support from the Bulgarian Science Fund (DFNI E 02/11 – SunStore-project) and the European Commission FCFP FRIAS COFUND Fellowship Programme (FP7-MCA-609305) for S.B. are highly appreciated.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: NMR and mass-spectroscopy data. See DOI: 10.1039/c5ob00884k

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