Porphyrins with a carbosilane dendrimer periphery as synthetic components for supramolecular self-assembly †

The preparation of the shape-persistent carbosilane-functionalized porphyrins H2TPP(4-SiRR’Me)4, Zn(II)TPP(4-SiRR’Me)4 (R = R’ = Me, CH2CHvCH2, CH2CH2CH2OH; R = Me, R’ = CH2CHvCH2, CH2CH2CH2OH; TPP = tetraphenyl porphyrin), H2TPP(4-Si(C6H4-1,4-SiRR’Me)3)4, and Zn(II)-TPP(4-Si(C6H4-1,4-SiRR’Me)3)4 (R = R’ = Me, CH2CHvCH2; R = Me, R’ = CH2CHvCH2) using the Lindsey condensation methodology is described. For a series of five samples their structures in the solid state were determined by single crystal X-ray structure analysis. The appropriate 0 and 1 generation porphyrin-based 1,4-phenylene carbosilanes form 2D and 3D supramolecular network structures, primarily controlled by either π–π interactions (between pyrrole units and neighboring phenylene rings) or directional molecular hydrogen recognition and zinc–oxygen bond formation in the appropriate hydroxyl-functionalized molecules. UV-Vis spectroscopic studies were carried out in order to analyze the effect of the dendritic branches on the optical properties of the porphyrin ring.


Introduction
Highly branched macromolecular architectures as well as selfassembly processes have become very popular and represent fascinating research areas both in natural sciences and engineering. 1 In this respect, dendrimers and metallo-dendrimers, repetitive branched molecules of structural perfection, have attracted considerable attention as nanoscale molecular materials due to their novel properties. 2 Dendrimers possess a three-dimensional and well-designed highly symmetric spherical arrangement with flexible structures employing isotropic assembling processes. 3 In the past, also snowflake-shaped dendrimers containing rigid backbones within the dendrimer side chains were prepared. 4 Such systems were used, for example, as mediators in electron-transfer and energy-transfer processes, 5 as dendritic boxes 6 or as drug carrier systems. 7 Porphyrins can successfully be used as core molecules, as branching units, or, for example, in the stepwise synthesis of cross-shaped covalent assemblies. 8 The micro-environments set-up by such molecules can be used among others to tune and control both optical and electrochemical properties of the appropriate porphyrin building block. 8,9 Out of this, porphyrins are very useful molecules to probe and hence to characterize dendritic local environments. Recently, metallo-porphyrins tailored at a dendrimer core have been developed as synthetic models to mimic naturally occurring systems including lightharvesting and electron transfer processes (i.e., chlorophyll), 8b,10 molecular oxygen storage and transport phenomena (i.e., hemoglobin) as well as oxidation enzymes (i.e., cytochrome c). 11,12 In such systems, the porphyrin building blocks have site isolation effects imposed by the dendritic shell. This makes it possible to utilize such species in diverse applications including homogeneous catalysis, 13 drug delivery 14 and singlet oxygen generation. 11a,14b,15 In addition, these compounds can be applied as non-linear optical, 16 and light-emitting 17 materials, as molecular sensors 18 or photo-active systems which can be considered as artificial antennae devoted to solar energy conversion. 19 † Electronic supplementary information (ESI) available: X-ray crystal structure data. Fig. S1/S2 and Fig. S3 display the crystal structure of 3c and 6a, respectively, with respect to the orientation of the unit cell. Table S1 gives crystal and intensity collection data of 3b·1/4CH 2 Cl 2 , 3c, 4a, 6a·2thf and 9b·3.5EtOH. Fig. S4-S6 illustrate T-shaped π-π interactions in the crystal structure of 9b·3.5EtOH. Table S2 gives selected geometric features of intermolecular hydrogen bonds of 6a. Table S3 gives structural parameters of the porphyrin cores of 3b, 3c, 4a, 6a and 9b. Fig. S7 illustrates geometrical features of saddling distorted porphyrins. Fig. S8 shows the atom labelling for the NMR data. CCDC 976300-976304. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/ c3dt53535e Porphyrins 3b and 3c, respectively, with their terminal SiCH 2 CHvCH 2 units could successfully be converted to the corresponding Si-propanolic-functionalized porphyrins 5a and 5b by a consecutive hydroboration-oxidation procedure (Scheme 1, see the Experimental section). Hydroboration of the appropriate end-grafted allyl groups with [BH 3 ·dms] (dms = dimethyl sulfoxide) in thf gave the corresponding BH 2 -functionalized systems, which on addition of hydrogen peroxide were oxidized to the respective alcohols H 2 TPP-(4-SiRR″Me) 4 (5a, R = Me, R″ = CH 2 CH 2 CH 2 OH; 5b, R = R″ = CH 2 CH 2 CH 2 OH) (Scheme 1). These porphyrins produce, when reacted with the transition metal salt [Zn(OAc) 2 ·2H 2 O], the expected zinc(II) species Zn(II)-TPP(4-SiRR″Me) 4 (6a, R = Me, R″ = CH 2 CH 2 CH 2 OH; 6b, R = R″ = CH 2 CH 2 CH 2 OH) in virtually quantitative yield (Scheme 1, see the Experimental section). Zinc porphyrins 6a and 6b dissolve in most common organic solvents.
After appropriate work-up, aldehydes 8a-c were obtained in excellent yield, and porphyrins 3, 5 and 9 in yields between 25 and 40%, while the formation of the respective zinc(II)-porphyrins 4, 6 and 10 was quantitative (see the Experimental section). All carbosilane-functionalized (metallo)porphyrins are, as the aldehyde starting materials, dark red colored solids soluble in most common polar organic solvents. They are airand moisture-stable with decomposition or melting points between 200 and 350°C (see the Experimental section). Aldehydes 8 and porphyrins 3-6 and 9-10 were characterized by elemental analysis, IR, UV-Vis and NMR spectroscopy ( 1 H, 13 C{ 1 H}, 29 Si{ 1 H}) (see the Experimental section). ESI-TOF mass spectrometric measurements were additionally carried out with selected metal-free and zinc(II) metallated samples (2c, 3b,c, 4a,b, 5a,b, 6a,b and 8a,c). The identity of 3b,c, 4a, 6a and 9b in the solid state was confirmed by single X-ray diffraction studies (vide infra).
The IR spectra of the newly synthesized allyl carbosilanebased porphyrins (Schemes 1 and 2) show a characteristic ν CvC vibration at ca. 1630 cm −1 together with one or two typical absorptions in the range of 800-840 cm −1 for the Si-C stretching vibrations (see the Experimental section). The CH 3 bending vibration of the SiMe n entities (n = 1, 2, 3) is observed at ca. 1250 cm −1 . These findings are in agreement with allylfunctionalized carbosilanes, i.e. Si(CH 2 CHvCH 2 ) 4 (ref. 25). Further characteristic broad absorptions are observed at ca. 3315 and 3400 cm −1 , which can be assigned to the NH as well as OH groups. The aldehyde functionalities present in 2 and 8 gave characteristic bands at 2732 and 2820 cm −1 for the CH and at 1705 cm −1 for the CO moieties. In addition, IR spectroscopy can be applied to monitor the progress of the hydroboration of 3 with [BH 3 ·SMe 2 ], since the ν CvC vibration of the allylic units in the respective starting compounds (ca. 1630 cm −1 ) disappears in the course of the reaction. After H 2 O 2 treatment new bands for the terminal hydroxyl functionalities in 5a and 5b are found at ca. 3400 cm −1 , which is typical for primary alcohols. 26 Solely broad absorptions are observed in the IR spectra; hydrogen-bridge formation and hence formation of molecular networks are the most obvious. 27 The 1 H and 13 C{ 1 H} NMR spectra of all compounds are characterized by well-resolved resonance signals for the organic groups present (see the Experimental section). Most typical for aldehydes 2b,c and 8a-c is the resonance signal at 10.07 ppm. This functionality allows the monitoring of the progress of the appropriate porphyrin formation because this signal disappears during the course of the reaction. Further evidence for the successful formation of the porphyrins is the appearance of a singlet at 8.92 ppm, which can be assigned to the pyrrole hydrogen atoms. 28 Also very distinctive is the resonance signal of the NH units at −2.8 ppm, while zincation leads to the disappearance of this signal and hence this unit can also be used to monitor the formation of the appropriate zinc porphyrins. Further indicative groups are the SiMe, Si(CH 2 ) 3 Me, and SiCH 2 CHvCH 2 entities (see the Experimental section). Particularly the latter group is best suited to study the progress of the consecutive hydroboration-oxidation processes since new resonances for the Si(CH 2 ) 3 OH building blocks are found (see the Experimental section). Similar to the IR spectra, the representative resonance signals for the SiCH 2 CHvCH 2 groups in 3b and 3c at ca. 2.0 (SiCH 2 ), 5.0 (H 2 Cv) and 6.0 ppm (CHv) (in CDCl 3 ) disappear on hydroboration and after oxidation with H 2 O 2 new signals can be found at 0.9 (SiCH 2 CH 2 CH 2 OH), 1.7 (SiCH 2 CH 2 CH 2 OH), 3.5 (SiCH 2 CH 2 CH 2 OH) and 4.5 ppm (SiCH 2 CH 2 CH 2 OH) for alcohols 5a and 5b (in dmso-d 6 ), respectively. Similar observations were made in the 13 C{ 1 H} NMR spectroscopic studies (see the Experimental section).
Additionally, the 29 Si{ 1 H} spectra of the carbosilane-based porphyrins 3b,c, 4a,b, 9b,c and 10a,b and the aldehydes 2b,c and 8b,c were measured. For example, the 29 Si{ 1 H} NMR spectra of 9 and 10 (in CDCl 3 ) show, as expected, two resonance signals at ca. −14.5 ppm and between −3.9 and −5.7 ppm, which can be assigned to the core and terminal silicon atoms (see the Experimental section). 24 The values for the inner silicon atoms are in good agreement with tetraphenyl silane (−14.98 ppm). 29 ESI-TOF mass spectrometric studies were carried out for all aldehyde derivatives and the 0 th generation porphyrins. Compounds 2c, 3b,c and 5a show the protonated molecular ion peak [M + H] + , while for 4a,b [M] + is characteristic. Compounds 5b, 6a,b and 8a,c could successfully be ionized by doping with KSCN and hence the ion [M + K] + could be detected (see the Experimental section).
UV-Vis absorption spectra were additionally recorded for porphyrins 3b,c, 4a,b, 5a,b, 6a,b, 9a-c and 10a-c, in order to analyze the effect of the dendritic branches on the optical properties of the porphyrin ring. The spectra were measured in CH 2 Cl 2 and thf as solvents ( Table 1). The porphyrin core of 3b shows one Soret band at 420 nm in CH 2 Cl 2 and four Q bands at 517, 552, 592 and 648 nm (Fig. 1a). 30 Metallation of 3b with zinc(II) did not influence the position of the soret band (4a, 421 nm, in CH 2 Cl 2 ) ( Fig. 1b) but has a significant impact on the shape of the Q band pattern. Two characteristic bands at 549 and 588 nm are observed in CH 2 Cl 2 , which is typical for metallo-porphyrins. 31 Fig. 1 also shows the difference between the UV-Vis spectra of the corresponding 0 th and 1 st generation dendritic porphyrins. Conspicuous is that the transitions typical for the 1 st generation dendritic porphyrins, for example, 9b, are nearly meeting the shape of the bands of the appropriate 0 th generation systems, as, for example 3b, with just little enhancement of the band intensities and a very small red shift in the Soret band (Fig. 1a). Similar observations were made for zinc porphyrins 4a and 10b (Fig. 1b). The increasing shielding effect of growing dendrons around the porphyrin core as, for example, described by Aida and coworkers for an aryl ether scaffold 30b,c does not appear in the case of our systems. The reason therefore is apparent when looking at the molecular structure of the 1 st generation 9b ( Fig. 10/11). Compared to aryl ether dendrons, the here reported aryl silyl dendrons (e.g. 9b) are more rigid. A backfolding, as observed for the aryl ether dendrons, is impossible and that implies that the porphyrin core plane is even in the 1 st generation type compounds easily accessible from above and below by solvents. A dendritic effect is therefore not observed by comparing the UV-Vis data of the 0 th and 1 st generation molecules. The data correlate well with reported literature spectra for silyl-functionalized porphyrins. 32

X-ray investigations
Single crystals of 3b (as 3b·1/4CH 2 Cl 2 ), 3c, 4a and 9b (as 9b·3.5EtOH) were grown by slow diffusion of CH 2 Cl 2 into EtOH solutions containing the respective compounds, while single crystals of 6a (as 6a·2thf ) were obtained by layering a thf solution containing 6a with n pentane at ambient temperature. The molecular structures of 3b,c, 4a and 6a are displayed in Fig. 2, 6, 3 and 8. In the case of 9b the asymmetric unit comprises two crystallographically independent centrosymmetric halves of 9b, denoted as 9bA and 9bB. Their molecular structures are depicted in Fig. 10 and 11. Crystal and intensity collection data of 3b·1/4CH 2 Cl 2 , 3c, 4a, 6a·2thf and 9b·3.5EtOH are summarized in Table S1, † while selected bond lengths and angles are given in Tables 2 and 3, respectively.
It should be emphasised that meso-tetraphenylporphyrins carrying at the para position any kind of Si-containing groups have been sparingly characterised by single crystal X-ray diffraction studies so far. The solid state structures of 5,10,15,20tetrakis(4-pentamethyldisilanyl)phenyl)porphyrin 32a and of 5,10,15,20-tetrakis [4-(diethoxymethylsilyl)phenyl]porphyrin 32b are already described in the literature.
However, experimentally observed bond lengths and angles for the end-grafted carbosilane branches of all functionalized meso-tetraphenylporphyrins described here are in agreement with parameters typically found for phenylene-based carbosilanes. 24,33 The molecular structures of H 2 TPP(4-SiMe 2 (CH 2 CHvCH 2 )) 4 (3b) and its related zinc(II) species Zn(II)-TPP(4-SiMe 2 (CH 2 CHvCH 2 )) 4 (4a) in the solid state are depicted in    (Table S1 †). For 3b a partially occupied packing solvent molecule of CH 2 Cl 2 is observed in the crystal structure, which should be found for 4a as well. However, no electron density peaks of 4a could be used for the refinement of an analogous packing solvent molecule. Despite this, 3b and 4a can be regarded as structurally isomorphic to each other. The asymmetric unit of both 3b and 4a possesses one crystallographically independent porphyrin molecule. Related bond lengths and angles of the C 20 N 4 cores of 3b and 4a can be considered as identical to each other within standard deviations (Tables 2 and 3). Not surprisingly, the N⋯N distances of opposite nitrogen atoms of 3b are significantly longer, when compared with 4a (3B, N1⋯N3/N2⋯N4 = 4.160(5)/4.113(5) Å Symmetry code: . Furthermore, the Zn1 atom is located practically in plane with respect to its N 4 environment as it deviates by just 0.007(4) Å of the calculated mean plane of the atoms N1 to N4 (root-mean-square deviation from planarity (rmsd) = 0.030 Å, highest deviation from planarity (hdp) observed for N4 with 0.031(2) Å).
Porphyrins 3b and 4a are structurally isomorphic to each other (vide supra) and consequently their crystal structures are identical. For both porphyrins a 3D network structure is observed in the solid state of which a selected part has been illustrated in Fig. 4 (3b) and Fig. 5 (4a). Thereby it is observed that all four crystallographically different C 6 H 4 rings are involved in T-shaped π-π contacts 34 with the respective C 20 N 4 cores ( Fig. 4 and 5). Geometrical features of these π-π contacts of 3b and 4a ( Fig. 4 and 5) are in good agreement with each other, when comparing both molecules.
The molecular structure of H 2 TPP(4-SiMe(CH 2 CHvCH 2 ) 2 ) (3c) in the solid state is depicted in Fig. 6. The replacement of one methyl group of the carbosilane SiMe 2 (CH 2 CHvCH 2 ) moiety in 3b by an allyl unit, as characteristic for 3c, induces considerable changes. In contrast to 3b, porphyrin 3c crystallizes in the triclinic space group P1 with crystallographically imposed inversion symmetry. Thus, the asymmetric unit of 3c comprises just half of the molecule, while the 2 nd half is generated by an inversion center which is located at the crossing point of the atoms N1/N1A and N2/N2A (Fig. 3). Furthermore, it is astonishing to notice that related bond lengths of the C 20 N 4 core of 3c, when compared with those of 3b and 4a and those of 6a and 9b as well, are significantly elongated ( Table 2). As a wrongly determined space group may cause such deviations the structural refinement of 3c was checked with the utmost precision; however, there are no indications that the unit cell dimensions and space group of 3c are not accurate (Table S1 †). For example, the N⋯N distances of opposite N atoms of 3c (N1⋯N1A/N2⋯N2A = 4.325(5)/4.228(5) Å) are by far the longest ones of the here described porphyrins (Table 2). A possible explanation of this unexpected observation for 3c might be drawn from the crystal structure of 3c, which is illustrated in Fig. 7. In contrast to the observation of 3D network structures for 3b and 4a ( Fig. 4 and 6) which are induced by T-shaped π-π interactions, for 3c the formation of 2D layers has been noted. The 2D layers are formed along the crystallographic a-and b-axes, but not along the crystallographic c-axes as depicted in Fig. S1 and S2. † Moreover, only the C 6 H 4 aromatic group comprising the atoms C11 to C16 and symmetry generated analogues is involved in T-shaped π-π interactions with the C 20 N 4 core of adjacent molecules.  Thereby, comparatively large centroid-to-centroid distances are observed (Fig. 7). To deduce that the different crystal structure of 3c is responsible for the observation of significantly different bond lengths of 3c, when compared to those of 3b and 4a, is certainly a more qualitative description. Hence, further work, e.g. quantum chemical calculations, is required to figure out the origin of this remarkable phenomenon. The molecular structure of Zn(II)-TPP(4-SiMe 2 ((CH 2 ) 3 OH)) 4 (6a) is depicted in Fig. 8. Porphyrin 6a crystallizes in the triclinic system P1 with one molecule of 6a in the asymmetric unit cell. As indicated before, bond lengths and angles of the central C 20 N 4 core of 6a compare well with those of 3b, 4a and 9b (Tables 2 and 3 The exchange of the terminal allyl groups of 4a by 3-propyloxy functionalities, as present in 6a, resulted in a completely different packing mode. There are no π-π interactions of any kind observed for 6a in the solid state. Instead, the crystal structure is exclusively governed by reciprocal formation of intermolecular O(H)-Zn contacts along the crystallographic a-axes together with formation of intermolecular O(H)⋯O hydrogen bonds along the crystallographic b-axes. Fig. S3 and Table S2 † show bond lengths and angles for the characteristic intermolecular hydrogen bonds. A part of the thus formed 2D Fig. 4 Above: Graphical illustration of a selected part of the 3D network formed by 3b due to intermolecular π-π interactions. Labels A-C refer to a 1 st -3 rd symmetry generated molecule of 3b. All hydrogen atoms and terminal substituents at the silicon atoms are omitted for clarity. Below: graphical illustration of the four different types of intermolecular π-π interactions between the aromatic C 6 H 4 groups with respective parts of the porphyrin core. The sign ∢ refers to calculated interplanar angles between differently colored functionalities. Dotted lines indicate the shortest observed distances between the geometrical centroids of the C 6 H 4 groups and the respective centroids/atoms of the porphyrin core with D1 = centroid of C21-C26, D3 = centroid of C27-C32, D4 = centroid of C33-C38, D6 = centroid of C39-C44, D2 = centroid of N1 and C2, D5 = centroid of C12-C14 and symmetry generated related atoms/fragments. layers is furthermore graphically illustrated in Fig. 9. It is surprising to note that the formation of 1D chains, as a part of the 2D network structure, due to mutual intermolecular O(H)-Zn contacts as observed for 6a, has not been observed so far for any kind of O-functionalised metalloporphyrins possessing 3d transition metal ions. Related metalloporphyrins functionalised by any kind of O-donor atoms, with the oxygen atoms belonging to alcohol, ether, carbonic acid and/or carbonyl functionalities, form either dimers, 35 trimers 36 or polymeric 2D 37 and 3D 38 networks, respectively.
Porphyrin H 2 TPP(4-Si(C 6 H 4 -4-Si(CH 2 CHvCH 2 )Me 2 ) 3 ) 4 (9b) crystallises in the triclinic space group P1. The asymmetric unit contains half of two crystallographically independent molecules of 9b, denoted as 9bA and 9bB. Both 9bA and 9bB possess in the solid state crystallographically imposed inversion symmetry, whereby the molecular structures of both individual molecules are depicted in Fig. 10 and 11. The bond distances and angles of the C 20 N 4 cores of 9bA and 9bB do not only compare well with each other, they can be even considered as closely related to analogous data reported here for 3b, 4a and 6a but not 3c (see above and Tables 2 and 3).
Due to the presence of sixteen crystallographically independent C 6 H 4 aromatic rings of both 9bA and 9bB, determination of the crystal structure of 9b with respect to possible π-π interactions is rather complicated. 39 However, it was found that a 3D network is formed by 9b in the solid state due to T-shaped Above: graphical illustration of a selected part of the 3D network formed by 4a due to intermolecular π-π interactions. Labels A-C refer to a 1 st to the 3 rd symmetry generated molecule of 4a. All hydrogen atoms and terminal substituents at the silicon atoms are omitted for clarity. Below: graphical illustration of the four different types of intermolecular π-π interactions between the aromatic C 6 H 4 groups with the respective parts of the porphyrin core. The sign ∢ refers to the calculated interplanar angles between differently colored functionalities. Dotted lines indicate the shortest observed distances between the geometrical centroids of the C 6 H 4 groups and the respective centroids/atoms of the porphyrin core with D1 = centroid of C21-C26, D3 = centroid of C32-C37, D4 = centroid of C43-C48, D6 = centroid of C54-C59, D2 = centroid of N2 and C6, D5 = centroid of C16-C18, D7 = centroid of C12-C14 and symmetry generated related atoms/fragments. π-π interactions. This 3D network can be understood as being formed of 2D layers of molecules of 9bA and 9bB of which a part is illustrated in Fig. 12. Further descriptions of the individual interactions are given in Fig. 14 and S4-S6. † The 2D layers interact then with each other by means of T-shaped π-π interactions exclusively between molecules of either 9bA or 9bB (Fig. 13). The respective π-π interactions being responsible for the formation of the 2D layers and of the 3D network are then separately illustrated in Fig. 14. The inter-layer distance between 2D layers corresponds to 25.1408 Å (=b), (Fig. 13).
Different types of non-planar distortions commonly observed for porphyrins have been already explicitly discussed. 41 In the case of here structurally described porphyrins it can be determined that the central C 20 N 4 porphyrin cores can be regarded as planar which can be concluded from, for example, the sum of angles around the 5,10,15,20-anellated atoms of the C 20 N 4 cores ( Table 3) and further data are given in Table S3 and Fig. S7, † including accompanying remarks.
In summary, the observation of the formation of 3D (3b, 4a and 9b) or 2D networks (3c) resulting from intermolecular Fig. 6 ORTEP diagram (25% ellipsoid probability) of the molecular structure of 3c. All C-bonded hydrogen atoms are omitted for clarity. Of disordered atoms only one atomic position is displayed. The sign ∢ refers to the calculated interplanar angles between terminal C 6 H 4 groups and the central C 20 N 4 H 2 core. Symmetry code for A: −x, −y + 2, −z + 1. Fig. 7 Above: graphical illustration of a selected part of one 2D layer formed by 3c due to intermolecular π-π interactions. Labels ', '', * and # refer to symmetry generated atoms of crystallographically independent molecules of the asymmetric unit of 3c, label A to symmetry generated atoms of the asymmetric unit of 3c and labels B-E to symmetry generated atoms of A labelled atoms. All C-bonded hydrogen atoms and terminal substituents at the silicon atoms are omitted for clarity. Below: graphical illustration of the intermolecular π-π interactions between the aromatic C 6 H 4 groups with the respective parts of the porphyrin core. The sign ∢ refers to the calculated interplanar angles between differently coloured functionalities. Dotted lines indicate the shortest observed distances between the geometrical centroids of the C 6 H 4 groups and the respective centroids/atoms of the porphyrin core with D1 = centroid of C11-C16 and D2 = centroid of C6-C8, respectively, and symmetry generated related atoms/fragments. T-shaped π-π interactions is not a specific feature of the here reported porphyrins. There are already two crystallographically described meso-tetraphenylporphyrins known bearing terminal Si-functionalities at the para-position of the phenyl rings, namely 5,10,15,20-tetrakis(4-pentamethyldisilanyl)phenyl)porphyrin 32a and 5,10,15,20-tetrakis[4-(diethoxymethylsilyl)phenyl]porphyrin, 32b which can be regarded as closely related to the here reported porphyrins. Bond lengths and angles of the C 20 N 4 porphyrin cores of these two reported samples are in very good agreement with the corresponding data observed for 3b, 4a, 6a and 9b, respectively.
Especially for 5,10,15,20-tetrakis[4-(diethoxymethylsilyl)phenyl]porphyrin it is indicated that "no significant short contacts such as π-stacking" could be observed, which is attributed to the steric hindrance of the terminal silyl functionalities. 32b It seems, however, likely, that only sandwich type π-π interactions have been ruled out.
In the discussion of the crystal structures of Zn(TPP) and Ag(TPP), 42 which are isomorphous to H 2 TPP, T-shaped π-π interactions have been explicitly mentioned. Interplanar angles between interacting aromatic units are given, although no centroid-to-centroid distances and the final type of assemblies formed have been mentioned. 42 In the case of a report on the crystallographic characterisation of Fe(TPP) as a toluene solvate, the formation of 1D chains, due to sandwich type π-π interactions, is observed. 43 One can thus assume that for nonfunctionalised M(TPP) (M = 3d metal ion) type porphyrins, especially for those in which the central metal ion is not coordinated by one and/or two donor molecules, π-π interactions play a significant role in the crystal structures. Indeed, for  Zn(TPP), being co-crystallized with different guest molecules, a review by Byrn et al. 44 for over 200 different cases mentions explicitly the observation of intermolecular T-shaped π-π interactions, although no geometrical features or types of assemblies were given.

Conclusions
The preparation of a series of 0 th and 1 st generation carbosilane dendrimer-based porphyrins of types H 2 TPP(4-SiRR′Me) 4 , Zn(II)-TPP(4-SiRR′Me) 4 (R = R′ = Me, CH 2 CHvCH 2 , Fig. 11 ORTEP diagram (20% ellipsoid probability) of the molecular structure of 9bB. All carbon-bonded hydrogen atoms are omitted for clarity. Of disordered atoms only one atomic position is displayed. The sign ∢ refers to the calculated interplanar angles between the directly porphyrin bonded C 6 H 4 groups and the central C 20 N 4 H 2 core. Symmetry code for A: −x + 1, −y and for #:, −z + 1. 40a CH 2 CH 2 CH 2 OH; R = Me, R′ = CH 2 CHvCH 2 , CH 2 CH 2 CH 2 OH; TPP = tetraphenyl porphyrin), H 2 TPP(4-Si(C 6 H 4 -1,4-SiRR′ Me) 3 ) 4 , and Zn(II)-TPP(4-Si(C 6 H 4 -1,4-SiRR′Me) 3 ) 4 (R = R′ = Me, CH 2 CHvCH 2 ; R = Me, R′ = CH 2 CHvCH 2 ) using the Lindsey condensation methodology is described. Functionalization of TPP with the carbosilane dendrons leads to a slight bathochromic shift of the Soret and Q bands in the UV-Vis spectra, which is in agreement with the literature. 30 The structures of five samples (3b,c, 4a, 6a, 9b) in the solid state have been determined by single X-ray structure determination. The supramolecular structures of the allyl 0 th generation species 3b,c, 4a and the 1 st generation carbosilane-containing porphyrin 9b are primarily controlled by π-π interactions, while in the hydroxylfunctionalized porphyrin 6a the network formation is exclusively set by zinc-oxygen coordination and hydrogen bonding intermolecular interactions. Conspicuously, porphyrin 3b and its analogous zinc-metallated system 4a possess an identical 3D supramolecular structure as both compounds Fig. 12 Selected part of one 2D layer formed by 9b in the solid state due to T-shaped π-π interactions between molecules of 9bA and 9bB. 40b Fig. 13 Graphical illustration of the mutual T-shaped π-π interactions between molecules of 9bA and 9bB, being responsible for the connection of the 2D layers of 9b to give a 3D network structure. 40b can be regarded as isomorphic in the solid state. On the other hand, the eight allyl groups in 3c, instead of the four allyl units present in 3b, modify the 3D network into a 2D network which might be responsible for the observation of significantly larger bond lengths of its C 20 N 4 core in comparison with the related bond lengths of the other here described porphyrins.
For meso-tetraphenylporphyrins bearing any kind of substituent at the phenyl rings it seems less likely that intermolecular sandwich type π-π interactions can be observed, although T-shaped π-π interactions might be found. For such cases we do not find, to the best of our knowledge, precise comments for crystallographically described representatives. We assume, however, that especially T-shaped π-π interactions should be observed, as reported here for 3b, 3c, 4a and 9b, at least in such cases where the central metal ions are not coordinated by additional donor solvents or the two N(H)-protons of the central porphyrin rings are not involved in hydrogen bridges with protic donor solvents.

General data
All reactions were carried out under an atmosphere of nitrogen using standard Schlenk techniques. Tetrahydrofuran, toluene and n pentane were purified by distillation from sodium-benzophenone ketyl; CH 2 Cl 2 and CHCl 3 were purified by distillation from calcium hydride. Diethylamine and diisopropylamine were distilled from KOH; absolute MeOH was obtained by distillation from magnesium.

4-Diallylmethylsilylbenzaldehyde (2c)
t BuLi (1.7 M, 34.7 mmol, 20.4 mL, n pentane) was added dropwise to a Et 2 O (75 mL) solution containing 1c (17.37 mmol, 4.88 g) at −78°C. After 1 h of stirring at this temperature the resulting solution was drop-wise transferred via a cannula to dimethyl formamide (52.12 mmol, 3.81 g, 4.04 mL) in thf (50 mL) at 0°C and the obtained reaction mixture was kept at this temperature for 15 min. Afterwards, it was warmed to ambient temperature, stirring was continued for 2 h, and then it was quenched with aqueous HCl (3 N, 80 mL). The obtained residue was extracted with Et 2 O (100 mL) and the organic layer was washed with water (2 × 60 mL), saturated NaHCO 3 (60 mL) and brine (60 mL) and was then dried over MgSO 4 . Afterwards, all volatiles were removed in oil-pump vacuum. The crude product was purified by column chromatography (a) n hexane, (b) 20 vol% CH 2 Cl 2 -n hexane to afford 2c as colorless oil (14.86 mmol, 3.42 g, 86% based on 1c).