Multiple‐Porphyrin Functionalized Hexabenzocoronenes

Abstract Porphyrin–hexabenzocoronene architectures serve as good model compounds to study light‐harvesting systems. Herein, the synthesis of porphyrin functionalized hexa‐peri‐hexabenzocoronenes (HBCs), in which one or more porphyrins are covalently linked to a central HBC core, is presented. A series of hexaphenylbenzenes (HPBs) was prepared and reacted under oxidative coupling conditions. The transformation to the respective HBC derivatives worked well with mono‐ and tri‐porphyrin‐substituted HPBs. However, if more porphyrins are attached to the HPB core, Scholl oxidations are hampered or completely suppressed. Hence, a change of the synthetic strategy was necessary to first preform the HBC core, followed by the introduction of the porphyrins. All products were fully characterized, including, if possible, single‐crystal XRD. UV/Vis absorption spectra of porphyrin‐HBCs showed, depending on the number of porphyrins as well as with respect to the substitution pattern, variations in their spectral features with strong distortions of the porphyrins’ B‐band.


Results and Discussion
First, mono-porphyrin-HBC 6, [33,38] previously made by us through ad ifferent route, was prepared as ar eference com- Figure 1. HBC as am odel system for nanographenes with porphyrinsdirectly attached to the periphery.Influenceoft he geometrical alignment (previous work, left side) and the amount (this work, right side) of porphyrins on the HBC core.
Next, we aimed to prepare ap orphyrin-saturated HBC compound,w hich is substituted by six porphyrins. For that, we plannedt ot ransform ah exa-porphyrin-substitutedH PB 10, which is well accessible and already known in the literature, [22,29,31] to the respective HBC derivative 11 (Scheme 2). First, di-porphyrin-tolane 9·Ni 2 [31] was prepared in aS onogashira cross-coupling reaction and subjected to ac obalt-catalyzed cyclotrimerization reaction. Hexa-porphyrin-substituted HPB 10 was obtained, after demetallation, in 62 %y ield. The final transformation to the target molecule hexa-porphyrin-HBC 11,h owever,d id not proceed as expected. The conditions we typicallya pply for Scholl oxidation( FeCl 3 /CH 3 NO 2 in CH 2 Cl 2 ), failed to deliver the central hexabenzocoronene. Instead, precursor 10 was recovered quantitatively.V ariationso f the reaction conditions (reaction time, amount of FeCl 3 , change of oxidant/Lewisa cid) did not lead to any successful formation of the desired HBC product 11.
Although many different oxidant/Lewis acid combinations for Scholl oxidations exist [46,47] and the mechanism is still not fully understood, [48][49][50][51][52] we suspect that the reason behind the failure of HBCf ormation of 10 stems from its structure. On the one hand, the HPB core is sterically well shielded due to the arrangemento ft he bulky porphyrins aroundi t. Additionally, given that the porphyrins are in their free-base form, protonation occurs under the acidic reactionc onditions, leading to positivelyc harged porphyrin-HPB conjugates. Coulombic repulsion between the charged porphyrins and the oxidantF e 3+ + furtherh ampers the reactivity.T oa void electrostatic effects, we introduced metals to the porphyrin core. First, zinc was tested and hexa-zinc-porphyrin-HPB 10·Zn 6 wasp repared and reacted in aS choll oxidation. However,z inc did not endure the acidic reactionc onditions and therefore, demetallationo ccurred as the only reaction, [35,40] yieldingf ree-base porphyrin-HPB 10 as the product. Given that more robust metal complexesw ere required, nickel was inserted into the porphyrins. Hexa-nickelporphyrin-HPB 10·Ni 6 was subjected to the conditions usedf or Schollr eactions andi nc ontrast to all previous attempts, reactivity waso bserved and new product spots were detected by TLC. However, rather than formingt he desired HBC product, it seemed that the peripheral porphyrins have reacted with their meso-3,5-di-tert-butylphenyl substituents (Scheme 3), as it has already been shown for other 3,5-di-tert-butylphenyl-substituted nickel porphyrins. [53][54][55] Owing to the possibility of forming up to three differentisomers i1, i2, i3 per porphyrin, an inseparable mixture of products was obtained, which showedn either in NMR nor in UV/Vis spectroscopy indications of an HBC formation ( Figures S40 and S41, Supporting Information). All reaction conditions, which were tested to initiate the Scholl oxidation, are summarized in Ta ble 1. To get ab etter understanding about the failure of HBC formation of 10,c alculations were performed (see Figure S6). From at heoretical perspective, HBC formation seems to be unlikely with hexa-porphyrin-HPB 10, because most of the highest-occupied molecular orbital (HOMO) electron density is located on the porphyrins and only little on the HPB unit. Thus,t he preferred point of oxidation is rather at the porphyrins than at the HPB.
Given that transformationso fh exa-porphyrin-HPB 10 to the corresponding HBC could not be achieved, an ew strategy was developed in which the HBC was preformed before the attachment of the porphyrins (Scheme 4). Therefore, hexa-iodo-HPB 12 [56,57] was prepared and oxidized to the HBC derivative 13. [58,59] Then, HBC 13 was reactedw ith the boronic ester porphyrin [40,60] 14·Zn in as ix-fold Suzuki reaction to the desired product 11.I ns pite of the spatially restricted situation of the product and the very low solubility of hexa-iodo-HBC 13,t he six-fold Suzuki reactionw orked sufficiently and produced hexa-porphyrin-HBC 11,after demetallation, in 36 %yield.
The successful formation of hexa-porphyrin-HBC 11 wasverified by NMR spectroscopy,w hich shows clear differences between the HPB and HBC species( see 1 HNMR, Figure 2). Characteristic for directly linked porphyrin-HBC compounds [33-35, 38, 40] are the most downfield-shifted signals around 10 ppm, which originate from the HBC protons close to the porphyrin. Due to the high symmetry of 11 (D 6h )t he NMR spectrum is rather simple foram olecule with am olar mass M = 5762 Da. The proton signal for the HBC core for example, is only noticeable as one singlet at 10.33 ppm. Ac omparison between the spectra of 10 and 11 shows that the resonances of the protons close to the HPB/HBC core experience the strongest chemical shifts. For example, signals of the b-pyrrolicp rotons of HPB 10 show up at 7.97 ppm whereas the most downfield-shifted ones of the HBC derivative are found at 9.06 ppm. Furthermore, for HPB system 10,t he resonanceso ft he porphyrins' outwards pointing aryl rings (Figure 2, drawn in yellow) appear sharp whereas the signals of the other aryl rings are broadened. [22] This is also true for the tert-butyl groups,w hich almostv anish. The broadening of some signals is due to the sterically stressed situation in 10 resulting in restricted rota-Scheme3.Suggestedreaction products of Scholl oxidationw ith hexa-nickel-porphyrin-HPB 10·Ni 6 .The reaction is simplifiedb ys howing only one porphyrin of 10·Ni 6 .For asingle-bond formation,upt othree different isomersp er porphyrinare feasible.  [22] For the HBC derivative 11,o nt he other hand, the rigid HBC core resultsi ns harp appearance of all signals. X-ray diffractiona nalysis( XRD) with single crystals of 11 and 11·Zn 6 turned out to be challenging. Due to the size of the molecules with the chemicalc ompositiono fC 414 H 450 N 24 (11)a nd C 414 H 438 N 24 Zn 6 (11·Zn 6 ) and the therefore expected large unit cells, only weak diffraction patternsw ere obtained. To get sufficient crystallographic data, either high X-ray intensities or large crystalsw ere required. Given that the crystallization attempts of zinc-porphyrin-HBC 11·Zn 6 generated only small crystals, synchrotron X-ray intensities [61] were required to obtain suitable crystallographic data (Figure 3). Molecules 11·Zn 6 are arranged in am onoclinic crystal system with au nit-cell volumeo fa pproximately 53 000 3 .S olvent molecules incorporated into the crystal were highly disordered and therefore removed during refinement. However,M eOH,w hich wasu sed as the anti-solvent, was properly modelled for each porphyrin because it is coordinated to the central zinc ion. Crystallization attempts of the free-base form of 11,i nc ontrast, yielded large crystals (0.7 0.5 0.5 mm 3 ), which were suitable for measurements on as tandard X-ray diffractometer ( Figure 4). [62] In this case, the molecules are oriented in ah exagonal crystal system with a unit-cellv olume of approximately 130 000 3 .Ac omparisono ft he crystal packing shows ad ifferencei n solid-state arrangemento fm olecules 11 and 11·Zn 6 , respectively,w ith an interlocked packing motif for the zinc derivative (Figure 3b,c )a nd ac olumnar arrangementf or the free-base form (Figure4b, c). Althought he datasets of both structures are not suit-   6 with MeOH coordinated to eachcentralzinc atom; [32] b) top view of eight molecules of 11·Zn 6 ;c)side view of four molecules of 11·Zn 6 ;b), c) 3,5-di-tert-butylphenyl groups and hydrogen atoms as well as disordered groupsa re omitted for clarity. able for detailed structural discussions, they unambiguously verify the successful formation of the desired hexa-porphyrinsubstituted HBCs 11 and 11·Zn 6 .
UV/Vis absorption spectra were recorded for all porphyrin-HPB and HBC compounds and showed variationsi nt heir spectral features. Depending on the number of porphyrins, their substitution pattern, and core unit (HPB or HBC), different photophysicalc haracteristics wereo bserved.F or the hexaphenylbenzene linked conjugates 3, 4, 5,a nd 10 the B-band (Soret band) absorptions appear as sharp signals with full width at half maximum (FWHM) values of 11.6-13.8nma nd molar extinctionc oefficients per porphyrin e B-band /n(porphyrin) of 4.00-5.00·10 5 m À1 cm À1 . [31] The transformation to the corresponding porphyrin-HBCs, significantly changes the photophysical properties. [33,38] As an example, the spectra of mono-porphyrin-HPB 3 and HBC 6 are depicted in Figure 5( blue and purple lines). The B-band of mono-porphyrin-HBC 6 is redshifted, decreased in intensity,a nd significantly broadened with respectt ot he HPB precursor 3 (see Ta ble 2). Furthermore,a na dditional absorptionb and, originating from the newly formed HBCs' psystem,a rises at 357 nm. Ac omparison of the UV/Vis absorption features of hexabenzocoronenec entered compounds 6, 7, 8, 11 ( Figure 5) shows that the number of porphyrins and the substitution pattern conspicuously influence the spectralp roperties.C ompared with the HPB analogues,t he porphyrins' Bband is split and considerably broadened, if more than one porphyrin is attached to the HBC core. Similar characteristics were observed for recently reported bis-porphyrin-substituted HBCs, [40] which showed,d epending on the substitution geometry,s plit B-band absorptions as well ( Figure S11, Supporting Information). Interestingly,t he right (lower-energy)m aximum is alwaysl ocated at the same position, 432 nm, for 7, 8, 11,a s well as for para-bis-porphyrin-HBC [40] and only the left (higherenergy) maximum is shifted. The positiono ft he Q-bands, and therefore the optical band gap, remains unaffected by the number of porphyrins.T he intensity of the HBCs' b-absorption, however,i ss trongly dependent on the number of porphyrins. The most pronounced b-band was found for mono-porphyrin-HBC 6,w hereas for tri-porphyrin-HBCs 7 and 8 ad ecreased, and for hexa-porphyrin-HBC 11,n od istinct maximum, due the superposition with the porphyrins' absorption, were observed.
The spectral variations of porphyrin-HBCs compared with the HPB-linked derivatives are much more pronounced, due to the HBCs' conjugated p-system, which is in contrastt ot he flexible HPB bridge, more effective in elongating the pconjugation pathway.O nt he one hand, the large aromatic p-systemo ft he HBC itself influences the porphyrins' electronic characteristics (compare mono-porphyrin HPB 3 vs. HBC 6)a nd on the other hand, the HBC unit facilitates electronic interaction between the porphyrins. In our study with bis-porphyrin substituted HBCs [40] we already investigated the communication ability across HBC bridges and suggested that the degree of the B-bands' distortion can be used as aq ualitative tool to measure the electronic interaction between the porphyrins. [40] The herein presented porphyrin-HBCsf ollow this trend, for example,t ri-porphyrin substituted HBC 8 has ab roader (FWHM 30.7 nm) and more split (12 nm) B-band absorption as the isomer 7.This is because two of the porphyrins in 8 are arranged in a para geometry,allowing the best in- Figure 5. UV/Vis absorptionspectra [63] of molecules 3, 6, 7, 8,and 11 in THF.Insertss how magnificationso fthe b-( left side)a nd Q-band( right side) absorptions. teraction, [40] whereas in 7 all of the porphyrinsa re aligned in a meta fashion.T he number of porphyrins per HBC has also a significant influence on photophysical properties. Ac omparison between porphyrin-HBCs 6, 7, 8, 11 as well as bis-porphyrin-HBCs [40] (Figure 5, FigureS11,S upporting Information), shows that an increasing number of porphyrinsp er HBC leads to ah igher degree of B-band distortion, hence enhanced communication between the porphyrins. Thus, the strongeste lectronic interaction was found for hexa-porphyrin-HBC 11 with a split in the Soret-band of 16 nm and aF WHM value of 37.4 nm. Steady-state fluorescences pectraw erem easured ( Figure S12, Supporting Information) and showedu pon excitation of the HBCs' b-banda ne fficient energy transfer to the porphyrins yielding only their fluorescencea t6 53 and 718 nm. [33][34][35]38] Generally,t he influence of substitutionp attern and number of porphyrins on the steady-state fluorescence characteristics were less pronouncedc ompared with the changes of the UV/Vis absorption properties. All spectroscopic data are summarized in Ta ble2.

Conclusions
Porphyrin-HBC conjugates bearing one and three porphyrins per HBC were prepared through Scholl oxidation reactions of the respective HPB precursors. With respect to previously reported bis-porphyrin-HBCs, we come to the conclusiont hat Scholl transformations are an excellent, high yieldingc hoice for the preparation of porphyrin-HBCs with up to three porphyrins per molecule. Clearly,e ffects that might hinder Scholl reactions, such as sterics hielding or electrostatic repulsion, are less pronounced andt herefore do not influence the reaction outcomei fn ot more than three porphyrinsa re attached to a central HPB core. However, the synthesis of highers ubstituted porphyrin-HBCs through this synthetic route is not advisable, because the reactivity of the respective precursors under typical Scholl reaction conditions is hampered. The lack of reactivity was clearly demonstrated by the attempts to transform hexa-porphyrin-HPB 10 into the HBC compound 11.S everal reaction conditions, as well as different metalation states of the porphyrins, were tested and all of them failed to form the HBC core of 11.P reliminary resultsf urther suggest,t hat already tetra-porphyrin substituted HPBs have ad ecreased reactivity, making the transformation to the respective HBC derivatives inefficient. [64] Given that hexa-porphyrin-HBC 11 could not be obtainedt hrough aS choll reaction-based route, the synthetic strategyw as changed to the preparationo ft he HBC core prior to the porphyrins' introduction. The spectroscopicd ata of porphyrin-HBCs within this project furtherc omplements the series of multiple-porphyrin-substituted HBCs. [40] UV/Vis absorption spectra showedt hat an increased number of porphyrinsp er HBC leads to ah igherd egree of the B-bands'd istortion, which was attributed to an enhanced electronicc ommunication between the porphyrins. With respecttothe design and the characteristics of light-harvesting arrays, we concludet hat unlike in nonconjugated HPB architectures, the electronic communication across al arge p-system( HBC) significantly changes with the substitution pattern and the number of chromophores, which is reflectedi nt he FWHM values of the Soret band of the porphyrins. Although the broadened and split B-band of multiple-porphyrin-HBCs can be ascribed to intramolecular electronic interactions, detailed understanding of the photophysical properties is still lacking and therefore at opic of current investigations. Additionally,c ompounds like hexa-porphyrin-HBC 11 are tested for the buildupo fs elf-assembled supramolecular architectures with guest molecules such as C 60 -fullerene.

Experimental Section
Experimental procedures, characterization data, X-ray crystallographic data and copies of HRMS and NMR spectra can be found in the Supporting Information.