Meso-tetraphenylporphyrin with a Pi-system Extended by Fusion with Anthraquinone †

Fusion with a 9,10-anthraquinone moiety was achieved to extend porphyrin's π-system. A bridged di-hydroisoindole derivative was used to prepare the corresponding meso-tetraphenyltetraanthraquinono-porphyrin (Ph 4 TAQP) 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.


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 applicationsfrom 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). 2A 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. 9irst 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. 10Theoretical 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 lightharvesting efficiency of this kind of molecular skeleton to reach 90% in the range of 300-800 nm. 11This 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) catalystscompounds having the ability to accommodate and transfer multiple electrons to reaction substrates at one time. 12espite 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′-tetrasulfolenoporphyrin 13 and an excess of benzoquinone. 10erein we report a synthetic approach to meso-tetraphenyltetraanthraquinonoporphyrin (Ph 4 TAQP) based on a bridged dihydroisoindole precursor.In addition we describe the basic optical properties of the newly synthesized Ph 4 TAQP 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 aromatization 15 and thermal retro-Diels-Alder reaction. 16s is shown in Scheme 1, the use of the oxidative aromatization approach for the synthesis of tetraanthraquinono-porphyrin 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).
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. 17Treatment 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. 18ione 1 is known to form a deprotonated dihydronaphthoquinone irreversibly upon treatment with bases. 7Treatment 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. 19iacetate 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 tertbutoxides, HMDS), solvents and temperature regimes failed to deliver the target product.It is known that aromatization of cyclohexadienes can be incurred by strong bases. 21However, 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 1,3cyclohexadiene 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 isocyanoacetate 24 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. 22As shown in Scheme 3, pyrrole 11 reacted with benzaldehyde in CH 2 Cl 2 in the presence of BF 3 •OEt 2 , followed by oxidation with 2,3-dichloro-5,6-dicyanobenzoquinone Scheme 1 Retrosynthetical analysis of a TAQP system.
Scheme 2 Synthesis of a TAQP pyrrole precursor.
(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).
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 1 H NMR spectrum.It is noteworthy that well-resolved 1 H and 13 C 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 Ph 4 TAQP (ESI †).
The absorption and emission spectra of porphyrins 12, Ph 4 TAQP 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 CH 2 Cl 2 (for comparison, tetraphenyltetracyclohexenoporphyrin free base: Soret band 439 nm, Q-bands 537, 580, 606, 674 nm). 20The 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).Ph 4 TAQP exhibits strongly red-shifted Soret and Q-bands (Fig. 1B).The vibronic structure in the Q-band region is wellresolved.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 placethe  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 Ph 4 TAQP shows multiple maxima that may be associated either with excimer formation or formation of charge-transfer excited states.Solutions of Ph 4 TAQP and its metal complexes do not decompose noticeably when exposed to daylight for several days, indicating good photostability compared to other π-extended porphyrins. 23omparison of the absorption spectra of Ph 4 TAQPPd with those of palladium(II) tetraphenyltetrabenzo-and tetraphenyltetranaphthoporphyrins (Ph 4 TBPPd and Ph 4 TNPPd 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 S 1 and S 2 states of the molecule is manifested by the pronounced red shift of the Soret and Q-bands.While in the case of Ph 4 TBPPd and Ph 4 TNPPd 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 Ph 4 TAQPPd as a sensitizer for the TTA-UC process that will be reported in a separate study.

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 Ph 4 TAQP 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.
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.

Ph 4 TAQP 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 CH 2 Cl 2 to separate soluble porphyrin from the inorganic solid, the resulting solution was dried with Na 2 SO 4 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 Na 2 SO 3 , dried over Na 2 SO 4 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 CH 2 Cl 2 and purified on a silica gel column (eluent CH 2 Cl 2 , then CH 2 Cl 2 -THF, purple band collected).Additional purification by repetitive precipitation from CH 2 Cl 2 -Et 2 O delivered the title product (24 mg, 65%) as a purple powder.Ph 4 TAQP-Pd was obtained in 75% yield after heating a mixture of the free-base porphyrin, excess PdCl 2 (PhCN) 2 (2 eq.) and Et 3 N (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 CH 2 Cl 2 ) and evaporation of the filtrate.UV/vis (CH 2 Cl 2 ) λ max (log ε): 501 (5.05), 629 (4.11)