Structural Features of FrÃ©chet-Type Dendrons and Dendrimers in

DOI: 10.1021/cg100445y

Structural Features of Frechet-Type Dendrons and Dendrimers in Single Crystals

2010, Vol. 10 5050–5065

Adrian-Mihail Stadler*,†,‡ † Institut de Science et d’Ing enierie Supramol eculaires, Universit e de Strasbourg, 8 All ee Gaspard Monge, Strasbourg 67083, France, and ‡Karlsruhe Institute of Technology (KIT), Forschungszentrum Karlsruhe (FZK), Institute for Nanotechnology (INT), Postfach 3640, 76021 Karlsruhe, Germany

Received April 2, 2010; Revised Manuscript Received August 27, 2010

ABSTRACT: In the present systematic study, the solid-state conformational features of widely utilized Frechet-type dendrimers and dendrons derived from 3,5-di(benzyloxy)benzene are discussed on the basis of X-ray structures reported up now. Their conformations are described in terms of the six-torsion-angle sequences u1-u2-u3-u30 -u20 -u10 of the 3,5-di(benzyloxy)benzene moiety, and of the conformational descriptors Ap (antiperiplanar), Ac (anticlinal), Sc (synclinal), and Sp (synperiplanar). Of the 10 possible conformational types of u3-u30 sequences, only six types were observed in single crystals, and out of 136 possible types of conformations of u2-u3-u30 -u20 sequences, only 14. Ap-Sp and Ap-Ap are the preferred conformations of sequences u3-u30 , and Ap-Ap-Sp-Ap and Ap-Ap-Ap-Ap those of the sequences u2-u3-u30 -u20 . The presence of a substituent at position 2 of the focal phenyl ring generally induces a preference for the Ap-Sp conformation of u3-u30 sequences and for the Ap-Ap-Sp-Ap conformation of u2-u3-u30 -u20 sequences. Examples of solid-state interactions are discussed in order to analyze their complex and diverse roles in the stabilization of different conformational types. Particular attention is paid to the comparison of structural data, with theoretical results reported in the literature.

1. Introduction Dendritic molecules1 consist of a central core with attached dendrons;a fact that induces their treelike appearance νδFο = tree);and are monodisperse. Interest in their (Gr. δε structural features is connected to the increasing development of the field of research that concerns their properties, encompassing numerous and diverse aspects such as biological2 and medical3 applications, mechanically interlocked molecules,4 controlled elaboration of organic-inorganic materials,5 metallosupramolecular architectures (metallo-dendrimers6), organic light-emitting diodes,7 liquid crystalline dendrimers,8 catalysis,9 etc. The interest in structural aspects of dendrimers is expressed through numerous studies, given that the conformation of dendritic molecules influences their properties.10 Conformational aspects are crucial in the determination of planarity, global shape, and spatial demand of dendrons and dendrimers.11 Structural and retrostructural analysis of dendrons was performed in the group of Percec12 in view of the design of hybrid amphiphilic self-assembling dendrons. It allows understanding of the conformation and self-assembly process of these dendrons, a process that leads to bi- and tridimensional supramolecular architectures. Particular attention should be paid to recent works that have opened perspectives in the emerging field of dendrimerbased crystal engineering. Networks based on weak hydrogen bonding generated by aliphatic polyester dendrimers were reported by the group of Rissanen.13 Dendrimeric tectons were used by the group of Metrangolo and Resnati to generate halogen bonding-based crystals.14 The group of Lukin reported an intermolecular interaction mode consisting of an anchor-type packing of complementary second-generation *To whom correspondence should be addressed. E-mail: mstadler@ isis.u-strasbg.fr. pubs.acs.org/crystal

Published on Web 10/26/2010

branches of neighboring molecules.15 Supertritopic dendrimers were used in the group of Xu to generate networks of hexagonal hierarchy.16 In the field of crystal engineering, small conformational changes may induce significant changes in the network. A particular class of dendritic molecules is that of poly(arylalkyl ether)-based dendrimers, often termed “Frechet-type” dendrimers or dendrons, as the convergent approach17 to their synthesis (first reported in 1990), as well as numerous further developments, were achieved in the group of Frechet. An important and widely studied type of high generation dendrimer is that based on 1-substituted-3,5-di(benzyloxy)benzene (Figure 1). Although;due to the difficulty of obtaining single crystals of dendrimeric molecules;only structures13b,14,15,18 of G1 and G2 dendrimers (vide infra) have been reported to date, the examination of their structural and especially conformational features is rich in information. Other methods, such as powder19 X-ray diffraction and REDOR (rotational-echo double-resonance) solid state NMR,20 have also been utilized to explore dendrimer shapes in the solid state. The solid-state in-crystal structural features of benzyl ether dendrons and dendrimers are of interest for several reasons: (i) together with surface studies,21 they enable elucidation of the organization of molecules containing dendritic fragments on surfaces. The role of the dendron wedge effect in the cyclic self-assembly of chlorophyll dyes on HOPG has recently been investigated;22 (ii) as basic structural data required when the influence of a given parameter (like the solvent, the nature of substituents) on the conformation of a series of dendritic molecules, is investigated; (iii) as data complementary to conformational studies in solution,23 that may concern, for example, the molecular volume (size) in solution or the conformation required to make possible a specific interaction, etc. Although it r 2010 American Chemical Society

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Crystal Growth & Design, Vol. 10, No. 12, 2010

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Figure 1. Schematic representation of a G1 (a) and G2 (b) Frechet-type dendrimer based on 1-substituted-3,5-di(benzyloxy)benzene.

Figure 2. Torsion angles and the corresponding distances in poly(arylalkyl ether) dendrimers based on 1-substituted-3,5-di(benzyloxy)benzene. Up are shown Newman projections, bottom are shown the corresponding planar representations. The arrows indicate the front atom in the Newman projections.

appears possible that dendrimers might be very flexible in solution and adopt a wide range of conformations, there are overall preferred conformational orientations incrystals (vide infra); (iv) as data complementary to molecular dynamic simulations24 performed in order to estimate the size and conformation of poly(arylalkyl ether)-based dendrtitc molecules; (v) for the study of the solid-state structural features that are of particular importance in catalysis, especially in heterogeneous25 and solid-supported catalysis involving Frechet-type dendrimers, and particularly in the case of high generation dendrimers, where the structural aspects are crucial for the access of the substrate to the catalytic site;26 (vi) as data complementary to solid state nuclear magnetic resonance studies;20 (vii) for analytical purposes, in order to confirm the expected structure and to understand the structure of

these molecules in the solid state, to answer questions such as “are they planar?”, “how do they pack?”, and “which are the conformations most often present?”, thus opening perspectives for further projects related to the solid state, such as crystal engineering (vide supra), chemistry of materials, and sensors.27 All these considerations clearly show that a broad class of chemists should be interested in solid-state conformational studies on dendrimers and therefore motivated us to perform and report herein the first study of structural typology concerning the conformations of Frechet-type dendrimers derived from 1-substituted-3,5-di(benzyloxy)benzene (Figure 1). The aims of this article are as follows: • to establish a typology of solid-state conformations of Frechet-type dendrimers on the basis of X-ray structures reported to date; • to identify the solid-state conformational preferences of these dendrons and dendrimers;

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Figure 3. X-ray molecular structures (and CSD references codes) of representative types of G 1 Frechet-type 1-substituted-3,5-di(benzyloxy)benzene-based dendrimers and dendrons. A circle marks a part of molecule omitted for clarity. HEVXEM, JESZIR and WIRXOK were seen as G1 dendrons with respect to phenyl rings. Protons, solvent molecules and/or anions are omitted for clarity.

• to compare these solid-state conformational preferences with theoretical and experimental data obtained on model molecules whose motifs are incorporated in Frechet-type dendrimers; • to identify the various classes of factors that may be responsible for the conformation of Frechet-type dendrimers; • to discuss examples of the influence of the conformation on the molecular dimensions.

2. Conformational Characterization of Dendrimers The conformation of a Frechet-type dendrimer (Figure 1) is governed by three types of torsion angles um (m = 1, 2, 3; Figure 2) to which correspond three distances xm (m = 1, 2, 3; Figure 2) between the two end atoms of each unit of torsion angle um. Describing the conformation of a given dendrimer requires specification of these angles. This can be done in a

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Figure 4. X-ray molecular structures (and CSD references codes) of G2 Frechet-type dendrimers of 1-substituted 3,5-di(benzyloxy)benzenebased dendrimers. A circle denotes a part of the molecule omitted for clarity. Protons, solvent molecules, and/or anions are omitted for clarity.

As well, only the absolute value (modulus) |um| is considered, unless otherwise stated. 2.1.2. Data Analysis. The results of the measurement of um and xm in structures reported to date of dendrons or dendrimers based on 1-substituted-3,5-di(benzyloxy)benzene are summarized in Table S1 (Supporting Information), columns 1 to 15. The structures of the corresponding molecules are shown in Figures 3 and 4. The plots of the distances xm versus the corresponding torsion angle um (m = 1, 2, 3) are shown in Figure 5. From these data, the following can be inferred: • The value range of the angle u1 (or u10 ) is between 0 and 90 (Figure 5a). It does not show any remarkable discontinuity within this domain. The corresponding distance x1 (or x10 ) lies between 2.6 A˚ and 3.2 A˚. • Most values of the angle u2 (or u20 ) lie in two domains, namely between 70 and 100, and between 160 and 180 (Figure 5b), with the corresponding distances x2 (or x20 ) being from 3 A˚ to 3.4 A˚, and respectively from 3.6 A˚ to 3.8 A˚. • A similar two-domain distribution is observed for u3 (or u30 ), i.e. between 0 and 20, and between 160 and 180, with x3 (or x30 ) being from 2.8 A˚ to 2.9 A˚ and from 3.6 A˚ to 3.7 A˚, respectively (Figure 5c). Figure 5. Plots of measured distances xm versus the corresponding torsion angle um (m = 1, 2, 3; see Figure 2) in 1-substituted-3,5di(benzyloxy)benzene-based dendrons and dendrimers: (a) x1 versus u1; (b) x2 versus u2; (c) x3 versus u3.

“quantitative” manner by the means of exact values of the distances xm and angles um, but also in a “semiquantitative” manner by the means of the nomenclature used to describe torsion angles. 2.1. Quantitative Conformational Descriptors. 2.1.1. Utilization of Sets of Three Kinds of Torsion Angles. The three kinds of torsion angles um to be considered in the case of 1-substituted-3,5-di(benzyloxy)benzene can be defined by sequences of four atoms. The torsion angles are as follows: u1 (C2 or 6Ph(n)-C1Ph(n)-Csp3-O, where the central bond is C1Ph(n)-Csp3), u2 (C1Ph(n)-Csp3-O-C3 or 5Ph(n-1), where the central bond is Csp3-O), and u3 (Csp3-O-C3 or 5Ph(n-1)C4Ph(n-1), where the central bond is O-C3Ph(n-1)) (Figure 2). The subscripts Ph(n) and Ph(n-1) correspond to a phenyl of level n, respectively n-1, of the dendrimer. For a phenyl ring that has two O atoms at positions 3 and 5, there are two angles u3: u3 (that contains C3Ph(n-1)) and u30 (that contains C5Ph(n-1)), and two corresponding distances x3 and x30 . The same holds for u2 and u1. For u1, two angles should be been taken into account, namely C2Ph(n)-C1Ph(n)-Csp3-O and C6Ph(n)-C1Ph(n)-Csp3-O, but as they are supplementary angles, only the smallest one (always between 0 and 90) will be cited in the following analysis.

The angle um is directly correlated with the distance xm, by the mean of a relation of type xm = [k1m - k2m cos(um)]1/2. These data show the higher frequencies of certain values of the torsion angles u2 (or u20 ) and u3 (or u30 ) in the solid state. u1 (or u10 ) seems to be regularly distributed between 0 and 90. This quantitative approach is completed by the semiquantitative one. 2.2. Semiquantitative Conformation Descriptors. 2.2.1. Observed Structural Typology. This approach makes use of the nomenclature: synperiplanar (Sp, where 0 < |u| e 30), synclinal (Sc, where 30 < |u| e 90), anticlinal (Ac, where 90 < |u| < 150), and antiperiplanar (Ap, where 150 e |u|e180). To each 1-substituted-3,5-di(benzyloxy)benzene motif can be attributed a sequence u1-u2-u3-u30 -u20 -u10 of angle values (for example: 2.39-177.58-2.05-178.87-169.3947.32) which can be transformed into a sequence of descriptors (in the above example: Sp-Ap-Sp-Ap-Ap-Sc). There are three sequences that may describe the conformational solid-state typology of 1-substituted-3,5-di(benzyloxy)benzene-based dendrimers. The first sequence is u3-u30 . Six u3-u30 types were observed in the X-ray structures that were analyzed. They are denoted from 1 to 6 (Figure 6). For example, type 1 corresponds to the Ap-Ap sequence and type 6 corresponds to Sp-Sp. The second sequence is obtained from u3-u30 by adding u2 and u20 , i.e. u2-u3-u30 -u20 . To the number indicating the type

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Figure 6. Observed structural typology of u3-u30 sequences and of u2-u3-u30 -u20 sequences derived from the types 1-6. Protons not shown. The types can be read from the left to the right or from the right from the left (e.g., Sc-Ap-Sp-Ap = Ap-Sp-Ap-Sc).

of the sequence u3-u30 , a capital letter is added (Figure 6). For example, Ap-Sp belongs to the u3-u30 type 4 and Ap-Ap-SpAp belongs to the u2-u3-u30 -u20 type 4A. Similarly, for the third sequence u1-u2-u3-u30 -u20 -u10 , a small letter will be added to the precedent two, in order to indicate the type corresponding to u1 and u10 (see Figures S3, S4 and S5 from the Supporting Information). For example, Sc-Ap-Ap-Sp-Ap-Sc belongs to the type 4Aa. 2.2.2. Theoretically Possible Conformational Types. Comparison between the Observed and Theoretically Possible Number of Types. It is now interesting to determine how many types are theoretically possible for each of three sequences u3-u30 , u2-u3-u30 -u20 , and u1-u2-u3-u30 -u20 -u10 , in order to compare the theoretical possibilities with the observed data. A given sequence E of an even number of conformational descriptors may or may not have a plane of symmetry. For example, the sequence Ap-Ap has such a plane, but the sequence Ap-Sp has not. This symmetry can be distinct from the molecular symmetry. Let n be the number of conformational types that can be adopted by an angle um or um0 . Let us consider a general sequence um-E-um0 . If E is unsymmetrical, and um and um0 have each n possible conformational types, then there are n n = n2 possible conformational types of the um-E-um0 sequence, with all of them being unsymmetrical. For example, if E = u3-u30 = Ap-Sp, then there are 16 possible conformational types for u2-Ap-Sp-u20 (e.g. Ap-Ap-Sp-Ap, Ap-Ap-Sp-Ac, Ap-ApSp-Sc, etc.), given that u2 and u20 may each adopt four conformational types (Ac, Ap, Sc, Sp). If E is symmetrical, then there are n(n þ 1)/2 possible umE-um0 sequences, of which n are symmetrical (the conformation descriptor for um is the same as that for um0 ) and n(n 1)/2 are unsymmetrical (the conformation descriptor for um is distinct from that for um0 ). For example, if E = Ap-Ap, then for the sequence u2-E-u20 there are four symmetrical conformational types (Ap-Ap-Ap-Ap, Ac-Ap-Ap-Ac, Sc-Ap-Ap-Sc, and Sp-Ap-Ap-Sp), and six unsymmetrical

conformational types (Ap-Ap-Ap-Ac, Ap-Ap-Ap-Sc, Ap-ApAp-Sp, Ac-Ap-Ap-Sc, Ac-Ap-Ap-Sp, and Sc-Ap-Ap-Sp). (a) The first sequence to be considered is u3-u30 , i.e. um-E-um0 , where m = 3 and E = 0 (no torsion angle between u3 and u30 ). n = 4, as each torsion angle u3 or u30 may have four possible conformational types (Ap, Ac, Sp, Sc). As E = 0, E is symmetrical. So there are n(n þ 1)/2 = 10 possible conformational types, of which n = 4 are symmetrical (Ap-Ap, Ac-Ac, Sp-Sp, Sc-Sc) and n(n - 1)/2 = 6 are unsymmetrical (Ap-Ac, Ap-Sp, Ap-Sc, Ac-Sp, Ac-Sc, Sp-Sc). Only six types (Ap-Ap, Ac-Ap, Ac-Sp, Ap-Sp, Sc-Sp, Sp-Sp) were found in the 84 sequences u3-u30 (belonging to the 54 structures herein considered) that could be fully characterized. (b) The second sequence to be considered is u2-u3-u30 -u20 , also written u2-E-u20 , where E = u3-u30 . Each torsion angle u2 and u20 may adopt four (so, n = 4) conformational types (Ac, Ap, Sc, Sp). Where E = u3-u30 is unsymmetrical, there are n2 = 16 possible conformational types for each u2-E-u20 sequence with a given E. But there are six possible unsymmetrical types for E = u3-u30 (vide supra). This gives 16 6 = 96 possible conformational types for u2-u3-u30 -u20 sequences that have an unsymmetrical u3u30 sequence. Where E = u3-u30 is symmetrical, there are n(n þ 1)/2 = 10 possible conformational types for u2-E-u20 , of which (i) n=4 possible conformational types for each symmetrical u2-E-u20 (symmetrical u2-u3-u30 -u20 with symmetrical u3-u30 ), so 4 4 = 16 for all, and (ii) n(n - 1)/2 = 6 possibilities for each unsymmetrical u2-E-u20 (unsymmetrical u2-u3-u30 -u20 with symmetrical E = u3-u30 and unsymmetrical u2-[...]-u20 ), so 4 6 = 24 for all. So, there are 96 þ 24 = 120 unsymmetrical possible conformational types and 16 symmetrical ones for the sequence type u2-u3-u30 -u20 , so a total of 136 possible conformational types. Out of them, only 14 types (10.3%) were observed (Figure 6) in the 54 structures herein considered that contain 82 sequences u2-u3u30 -u20 that could be fully analyzed (see Table 2).

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(c) The third sequence to be considered is u1-u2-u3-u30 u2 -u10 or u1-E-u10 , where E = u2-u3-u30 -u20 . Here, n = 2, because u1, u10 ∈ {Sp, Sc}. If E is unsymmetrical, there are n2 = 4 possible conformational types for each u1-E-u10 . Given that there are 120 unsymmetrical possible conformational types for the sequence u2-u3-u30 -u20 , there will be 120 4 = 480 types.

If E is symmetrical, there are n = 2 symmetrical possible conformational types and n(n - 1)/2 = 1 unsymmetrical possibility for each u1-E-u10 , so a total of 3 16 = 48 possible conformational types. Overall, there are 480 þ 48 = 528 possible conformational types for the sequence u1-u2-u3-u30 -u20 -u10 . In the 54 structures (77 sequences u1-u2-u3-u30 -u20 -u10 that could be fully analyzed; see Table 2) were observed 26 conformational types (4.9% of the 528 possible ones). 2.3. Conformational Distribution and Preferences. 2.3.1. 1-Substituted-3,5-di(benzyloxy)benzene-Based Dendrimers or Dendrons. Statistically (Table 1), u1 “moderately” prefers the synclinal conformation (59%) over the synperiplanar one (41%). A strong tendency to favor the antiperiplanar orientation is observed for angles u2 (79.5%) and u3 (66.7%). The second preference is synclinal (18.1%) for u2 and synperiplanar (30.2%) for u3. Anticlinal (2.4%) is residual for u2, and anticlinal (2.3%) and synclinal (0.8%) are residual for u3. In terms of sequence conformations, there are two major tendencies for u3-u30 (Table 2). Out of 63 motifs for which the measurement of all these two angles was possible, 30 (47.6%) are of type 4 (Ap-Sp), and 26 (41.3%) are of type 1 (Ap-Ap).

0

Table 1. Angle Distribution in 1-Substituted 3,5-Di(benzyloxy)benzeneBased Dendrimers and Dendrons in the Solid State angle u1 (or u10 ) u2 (or u20 ) u3 (or u30 )

Ap (%)

Ac (%)

Sc (%)

Sp (%)

Substituted 3,5-Di(benzyloxy)benzene 59 41 79.5 2.4 18.1 0 66.7 2.3 0.8 30.2

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average (deg) 41.4 154.4 121.7

1,2-Disubstituted or 2-Substituted 3,5-Di(benzyloxy)benzene 59.4 40.6 40.1 u1 (or u10 ) 90.6 0 9.4 0 165.9 u2 (or u20 ) 40.6 0 0 59.4 73.7 u3 (or u30 ) 1,2,6-Trisubstituted or 2,6-Disubstituted 3,5-Di(benzyloxy)benzene 60 40 48 u1 (or u10 ) 90 0 10 0 161.1 u2 (or u20 ) 0 0 0 100 7.8 u3 (or u30 )

Table 2. Statistics of Conformational Types in Frechet-Type Dendrimers and Dendronsa u3-u30 types

1 (Ap-Ap)

2 (Ac-Ap) 3 (Ac-Sp) 4 (Ap-Sp)

5 (Sc-Sp) 6 (Sp-Sp) subtotal 4

6 subtotal

NOM

%

u2-u3-u30 -u20 types

NOM

%

u1-u2-u3-u30 -u20 -u10 types

1-Substituted 3,5-Di(benzyloxy)benzene-Based Dendrimers and Dendrons (38 Structures; Table S1) 41.3 1A (Ap-Ap-Ap-Ap) 13 21.3 1Aa (Sc-Ap-Ap-Ap-Ap-Sc) 1Ab (Sc-Ap-Ap-Ap-Ap-Sp) 1Ac (Sp-Ap-Ap-Ap-Ap-Sp) 1B (Ac-Ap-Ap-Ap) 2 3.3 1Ba (Sc-Ac-Ap-Ap-Ap-Sc) 1C (Ap-Ap-Ap-Sc) 8 13.1 1Ca (Sc-Ap-Ap-Ap-Sc-Sc) 1Cb (Sc-Ap-Ap-Ap-Sc-Sp) 1Cc (Sp-Ap-Ap-Ap-Sc-Sc) 1Cd (Sp-Ap-Ap-Ap-Sc-Sp) 1D (Sc-Ap-Ap-Sc) 2 3.3 1 Da (Sc-Sc-Ap-Ap-Sc-Sc) 2 3.2 2A (Ap-Ac-Ap-Ap) 1 1.6 2Aa (Sp-Ap-Ac-Ap-Ap-Sp) 1 1.6 3A (Sc-Ac-Sp-Ap) 1 1.6 3Aa (Sp-Sc-Ac-Sp-Ap-Sp) 30 47.6 4A (Ap-Ap-Sp-Ap) 21 34.4 4Aa (Sc-Ap-Ap-Sp-Ap-Sc) 4Ab (Sp-Ap-Ap-Sp-Ap-Sc) 4Ac (Sc-Ap-Ap-Sp-Ap-Sp) 4Ad (Sp-Ap-Ap-Sp-Ap-Sp) 4B (Ac-Ap-Sp-Ap) 1 1.6 4Ba (Sp-Ac-Ap-Sp-Ap-Sp) 4C (Ap-Ap-Sp-Sc) 5 8.2 4Ca (Sc-Ap-Ap-Sp-Sc-Sp) 4Cb (Sp-Ap-Ap-Sp-Sc-Sp) 4D (Ap-Sp-Ap-Sc) 1 1.6 4 Da (Sc-Ap-Sp-Ap-Sc-Sc) 4E (Sc-Sp-Ap-Sc) 2 3.3 4Ea (Sc-Sc-Sp-Ap-Sc-Sc) 1 1.6 5Aa (Sc-Ap-Sc-Sp-Ap-Sc) 1 1.6 5A (Ap-Sc-Sp-Ap) 3 4.8 6A (Ap-Sp-Sp-Ap) 3 4.9 6Aa (Sc-Ap-Sp-Sp-Ap-Sc) 6Ab (Sc-Ap-Sp-Sp-Ap-Sp) 6Ac (Sp-Ap-Sp-Sp-Ap-Sp) 63 61 26

NOM

%

4 3 5 2 4 2 1 1 2 1 1 8 4 5 3 1 1 1 1 2 1 1 1 1 56

7.1 5.4 8.9 3.6 7.1 3.6 1.8 1.8 3.6 1.8 1.8 14.3 7.1 8.9 5.4 1.8 1.8 1.8 1.8 3.6 1.8 1.8 1.8 1.8

1,2-Disubstituted or 2-Substituted 3,5-Di(benzyloxy)benzene-Based Dendrimers and Dendrons (12 Structures; Table S2) 12 75 4A 9 56.3 4Aa 2 4Ab 3 4Ac 4 4C 1 6.3 4Cb 1 4D 2 12.5 4 Da 1 4Db (Sc-Ap-Sp-Ap-Sc-Sp) 1 4 25 6A 4 25 6Aa 2 6Ab 1 6Ac 1 16 16 16

1,2,6-Trisubstituted or 2,6-Disubstituted 3,5-Di(benzyloxy)benzene-Based Dendrimers and Dendrons (Four Structures; Table S3) 5 100 6A 4 80 6Ab 4 6B (Ap-Sp-Sp-Sc) 1 20 6Ba (Sc-Ap-Sp-Sp-Sc-Sc) 1 subtotal 5 5 5 total 84 82 77

6

a

12.5 18.8 25 6.3 6.3 6.3 12.5 6.3 6.3

80 20

The percentages are rounded up and are relative to each subtotal. NOM means the number of motifs. Tables S1, S2, and S3 are from the Supporting Information.

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Figure 8. Small model molecules. The X-ray structure of the model molecule with CSD reference code NASSIK is shown.

Figure 7. Plots of measured distances xm versus the corresponding torsion angle um (m = 1, 2, 3; see Figure 2) in 1,2- or 2-substituted3,5-di(benzyloxy)benzene-based dendrons or dendrimers: (a) x1 versus u1; (b) x2 versus u2; (c) x3 versus u3.

Concerning the sequence u2-u3-u30 -u20 , out of 61 motifs, the most preferred type is 4A (Ap-Ap-Sp-Ap; 34.4%); then come 1A (Ap-Ap-Ap-Ap; 21.3%), 1C (Ap-Ap-Ap-Sc; 13.1%), and 4C (Ap-Ap-Sp-Sc; 8.2%) (see Table 2). Concerning the sequence u1-u2-u3-u30 -u20 -u10 , the most preferred conformations out of 53 motifs are 4Aa (Sc-ApAp-Sp-Ap-Sc; 8 motifs; 14.3%), 1Ac (Sp-Ap-Ap-Ap-Ap-Sp; 5 motifs; 8.9%), and 4Ac (Sc-Ap-Ap-Sp-Ap-Sp; 5 motifs; 8.9%) (see Table 2). 2.3.2. Supplementary Substituents. The presence of substituents at position 2 (see Table 1 and Figures 7 and 10) produces the following results in terms of preferred conformations of torsion angles um: (i) u1 slightly prefers the conformation Sc (59.4%) over Sp (40.6%), (ii) u2 clearly favors Ap (90.6%), and (iii) u3 prefers Sp (59.4%) rather than Ap (40.6%). Out of 16 motifs that were analyzed, the following types are predominant: (i) among u3-u30 types, 4 predominates (Ap-Sp; 75%), (ii) among u2-u3-u30 -u20 types, 4A predominates (Ap-Ap-Sp-Ap; 56.3%), and (iii) among u1-u2-u3-u30 -u20 -u10 types, 4Ac (Sc-Ap-Ap-Sp-Ap-Sc; 25%) and 4Ab (Sp-Ap-Ap-Sp-Ap-Sc; 18.8%) predominate (Table 2). Out of five motifs where substituents are present at both positions 2 and 6, only type 6 (Sp-Sp) was observed for the sequence u3-u30 (Table 2). A supplementary substituent at position 4, i.e. between the O atoms meta-oriented on the phenyl ring (Figure 12), induces the Ap-Ap conformation of the u3-u30 sequence, i.e. the u3-u30 type 1. 3. Factors That Influence the Conformation in the Solid State On one hand, intramolecular factors should be considered. These can be (i) intrinsic to 3,5-di(benzyloxy)benzyl, the basic

motif of the dendrons, considered as unsubstituted (i.e. stereoelectronic and energetic properties), and (ii) factors extrinsic to that motif, such as substituents placed on the focal ring or on the ends. On the other hand, there are intermolecular factors, extrinsic to 3,5-di(benzyloxy)benzyl, namely the in-crystal interactions. 3.1. Factors Intrinsic to the 3,5-Di(benzyloxy)benzyl Motif. 3.1.1. Intrinsic Torsion Angle Preferences. Analysis of um Conformations through Theoretical and Experimental Data on Model Molecules. Data on such molecules are available in the chemical literature and are reviewed below. Three kinds of torsion angles have to be analyzed, i.e. u1 (u10 ), u2 (u20 ), and u3 (u30 ). These angles are as well present in smaller model molecules (Figure 8) that contain structural motifs close to those of Frechet-type dendrimers. 3.1.1.1. Conformation of u1 (or u10 ). While the conformation of u1 does not seem of great importance for G1 dendrons, it is crucial for higher generation dendrimers, where this phenyl ring becomes the focal ring of other dendrons. The torsion angle u1 is a Csp2-Csp2-Csp3-O angle, with a central Csp2-Csp3 bond. The simplest models are ethylbenzene (EB), benzyl alcohol (BA), and benzyl-methylether (BME). In the case of benzyl alcohol (BA), there is some ambiguity concerning this angle. NMR studies in CCl4 and (CD3)2SO solutions provided data consistent with a Csp2-Csp2-Csp3-O (u1) angle of 60 or a freely rotating model.28 Electron diffraction data on gaseous benzyl alcohol29 provided u1 = 54. IR spectroscopy studies in CCl4 and CS2 revealed that two Csp2-Csp2-Csp3-O conformers were present, with u1 equal to 0 or 60, a third one where u1 =90 still remaining possible.30 The minimum energy structure of BA obtained by STO-3G calculations has an u1 of ≈43.31 Supersonic jet mass resolved excitation spectroscopy studies32 showed that the Sc (90) conformation of the angle Csp2-Csp2-Csp3-O is preferred, with the rotational barrier being mainly due to the internal hydrogen bond between the H atom of the OH group and the π-system of the aromatic ring, a similar stabilizing interaction but of CH/π type being found in benzyl-methyl-ether (BME).33 The (1 þ 1) REMPI (resonance enhanced multiphoton ionization) spectrum of benzyl alcohol (BA)34 displayed one weak OH stretching frequency at 3585 cm-1, that was attributed to the conformer with an OH-π hydrogen bond and a syn (gauche) u1, while the OH band at 3650 cm-1 was attributed to a planar conformer (u1 = 0). IR-UV ion dip spectroscopy35 results together with MP2 6-31þG** optimizations were found to be consistent with a dihedral angle Csp2-Csp2-Csp3-O between 35 and 60,

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while the microwave spectrum36 of benzyl alcohol was found to be consistent with an angle of about 60. (1 þ 1) one-color REMPI, ICL-PAS (intracavity laser photoacoustic spectroscopy), and jet-cooled NID (nonresonant ionization detected) spectral data37 were found to be consistent with a gauche conformation of u1 = 55.4. In ethylbenzene (EB), the consequence of the steric hindrance between the CH3 group and the H atoms at ortho positions is that the synclinal perpendicular38 (90; Sc) conformation is preferred over the synperiplanar one (0; Sp), the rotational barrier being of 1.3 kcal 3 mol-1, according to experimental data,39 and by 2.2 kcal 3 mol-1 (ref 40) or 1.07 kcal 3 mol-1 (ref 41), according to theoretical calculations. The contribution of σ-π hyperconjugation to the rotational barrier was estimated at 0.29 kcal 3 mol-1 (ref 42). The preference of u1 for synclinal (59%) over synperiplanar (41%) orientations as observed in the 1-substituted-3,5-di(benzyloxy)benzene-based structures analyzed herein (Table 1) follows the same trend as with the preference of the above presented small models for conformations where the Csp2Csp2-Csp3-O torsion angle lies between 35 and 90. 3.1.1.2. Conformation of u2 (or u20 ). The central bond of the unit defining the torsion angle u2 (u20 ) in Ph-CH2-O-Ph is the Csp3-O single bond, which preferentially adopts an anti (or s-trans) conformation in Frechet-type dendrimers and dendrons. There are two elements to be considered in order to understand this conformational preference: the central single bond and the role of Ph groups. Central Single Bond. The same type of Csp3-O central single bond is found in ethyl-methyl-ether (EME)43 (Figure 8). Gas phase electron diffraction studies44 have shown that, at 20 C, EME is a mixture of anti (or trans; 180) and syn (or gauche; 84) conformers that contains about 80% anti; the free energy difference ΔG between syn and anti is about 1.23 kcal 3 mol-1. The ΔH value determined from the temperature dependence of IR spectral intensities45 was found to be about 1.5 kcal 3 mol-1. Torsional data from vibrational spectra of gaseous EME gave an enthalpy difference between the gauche and s-trans conformers of 1.11 kcal 3 mol-1.46 From far-infrared spectra of gaseous EME, the trans to gauche and gauche to trans barriers and energy difference were calculated to be 2.75 kcal 3 mol-1, 1.80 kcal 3 mol-1, and 1.57 kcal 3 mol-1, respectively.47 The energy difference between the gauche and trans isomers in the liquid state as determined from the Raman spectroscopy was found to be 1.1 kcal 3 mol-1 (ref 48) or, by IR and Raman spectroscopy of a dilute solution in CS2, to be 1.35 kcal 3 mol-1.49 STO-3G calculations50 carried out for liquid EME resulted in an anti (180) to syn (90) energy difference of 1.96 kcal 3 mol-1 and in an anti to syn rotation barrier of 2.33 kcal 3 mol-1. The same calculations showed that EME contains 93.9% anti (180) for the ideal gas and 92.9% anti (180) for the liquid. Other STO-3G calculations gave a relative energy of gauche with respect to trans of 2.04 kcal 3 mol-1, while, from the 4-31G calculation, this value is 1.85 kcal 3 mol-1.51 Other 4-31G calculations resulted in the following energies for the eclipsed, gauche, skew, and trans conformations: 8.35, 2.74, 3.31, and 0.00 kcal 3 mol-1.52 The relative ΔH between the two conformers gauche and trans was also found to be 1.27 kcal 3 mol-1 (by CCSD(T)/ccpVTZ) and 1.37 kcal 3 mol-1 (by MP2/cc-pVTZ).53 Ab initio molecular orbital calculations with electron correlation energy correction by the Moller-Plesset methods (MP2, MP3, and MP4(SDTQ)), coupled cluster calculations (CCD,


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Figure 9. Conformations of the Csp2-Csp3-O-Csp2 angle in ethylmethyl-ether (EME).

ST4CCD, CCSD, and CCSD(T)), and configuration interaction energy calculations (CISD, QCISD, and QCISD(T)) using the 6-31G* basis set54a,b gave the following energies relative to the trans conformation: for the gauche one, from 1.28 to 1.46 kcal/mol, for the eclipsed one, from 6.77 to 6.84 kcal 3 mol-1, and for the skew one, from 2.55 to 2.67 kcal 3 mol-1. The following equilibria and the corresponding ΔΔG and constant K values describe the system involving the four interconvertible conformers trans, gauche, skew, and eclipsed (Figure 9): trans / gauche

ΔΔG1

K 1 ¼ e - ΔΔG1 =RT

trans / eclipsed

ΔΔG2

K 2 ¼ e - ΔΔG2 =RT

trans / skew

ΔΔG3

K 3 ¼ e - ΔΔG3 =RT

The percentages of each of these conformers are as follows: % trans = 100/(K1 þ K2 þ K3 þ 1), % gauche = 100K1/(K1 þ K2 þ K3 þ 1), % eclipsed = 100K2/(K1 þ K2 þ K3 þ 1), % skew = 100K3/(K1 þ K2 þ K3 þ 1). Introduction in these equations of the average energy values54c from the above theoretical calculations on EME, namely ΔΔG1=1.4 kcal 3 mol-1, ΔΔG2=6.8 kcal 3 mol-1, and ΔΔG3 = 2.6 kcal 3 mol-1, gave the following percentages of conformation at 25 C % trans = 90.4, % gauche= 8.5, % eclipsed anisole > 1,4dimethoxybenzene > 1,2-dimethoxybenzene. There is a good correlation between π charge densities on the carbon atom to which the methoxy is bonded and rotational barriers.70b 1,3-Dimethoxybenzene (1,3-DMB) may in principle adopt three planar conformations: two symmetrical ones (Ap-Ap and Sp-Sp) and an unsymmetrical one (Ap-Sp). Its low melting point (-52 C) makes crystallization and consequently single crystal X-ray diffraction almost impossible and requires other methods of investigation. Its 13C CP MAS NMR (charge polarized magic angle spinning nuclear magnetic resonance) spectrum showed that it adopts the Ap-Sp conformation in the solid state.72 A study of the conformational equilibrium of 1,3-dimethoxybenzene in solution by the nonlinear dielectric effect method73 shows the predominance of the Ap-Sp conformer (85%). Theoretical calculations74 by the DFT/B3LYP method with the 6-311þþG** basis set showed that the Sp-Sp and Ap-Ap planar conformers are, respectively, 0.75 kcal 3 mol-1 and 0.47 kcal 3 mol-1 less stable than the Ap-Sp conformer. The steric hindrance between the two methoxy groups and the H atom located between them is a destabilizing factor of the Sp-Sp conformation. The possibility of a kind of hydrogen bonds between the O atoms and the neighboring H atoms (for example, where OCH3 at C1 of the Ph ring is Ap, a hydrogen bond is established between the O atom at C1 and the H atom at C2, and where OCH3 at C3 is Sp, the hydrogen bond is between the O at C3 and the H at C4) has also been considered as a stabilizing factor.74 These stereoenergetic considerations can be extended to the OCH2 groups of the dendrimers. One may notice that there is concordance between them and the preference of the u3-u30 sequence for the conformational types 4 (Ap-Sp; 47.6%) and 1 (Ap-Ap; 41.3%) in 1-substituted-3,5-di(benzyloxy)benzene-based motifs. The preference of the u3-u30 sequence for the Ap-Sp conformation is, as well, confirmed by the X-ray structure of a model molecule (namely (3,5-dimethoxyphenyl)methanol, structure NASSIK19a) that was reported by the group of Constable. It is structurally close to 1,3-DMB. It adopts an Ap-Sp conformation in the solid state (Figure 8), where the values of u3 and u30 are 12.73 and 176.34. There are cases, e.g. the structures M, HAVDIS, MAFSAO, and RISZIC01, where both conformations Ap-Ap and Ap-Sp of the u3-u30 sequence are present in the same crystal. This situation is in agreement with the small energy difference between these conformations. In view of these considerations, as well as of the statistical data presented above, it can be concluded that the u3-u30 sequence conformational types 4 and 1 can be considered as “current” or “normal” ones, while the other types are less common and may be induced by specific intra- or intermolecular factors. Other factors;the extrinsic ones;such as the substituents at the focal point or at the dendrimer surface, in-crystal interactions, have an influence on such conformations governed by low torsional energy barriers. 3.2. Factors Extrinsic to the 3,5-Di(benzyloxy)benzyl Motif. 3.2.1. Nature of the Substituents Located at Position 1 of the Focal Benzene Ring. The role of substituents on the central ring as conformational influences is not readily analyzed. The same structural type can be observed for different

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Figure 10. X-ray molecular structures (and CSD references codes) of representative types of Frechet-type 1,2/1,2,6/2-substituted-3,5di(benzyloxy)benzene-based dendrons. Protons, solvent molecules, and/or anions are omitted for clarity.

substituents, while for a common substituent there are cases where two or three different conformations are observed (conformational diversity or multiplicity). Thus, in each of the followings structures, there are two or more u2-u3-u30 -u20 conformational types for the same substituent at the focal point: BEQQUK (1A, 1C, 4A), HAVDIS (4A, 6A), HIDKIP (3A, 4B), MAFSES (4A, 4C), REMQEG (1A, 1C, 6A), VEFKIB (1A, 2A), ZUDDIL (4A, 4C, 4D), M (1A, 4A). A given substituent may be considered to have an intramolecular influence on conformation where it undergoes interactions with the OCH2Ph arms of the dendrimer. This seems to be the case for structures where the substituent occupies a region of space needed if another conformation were to be adopted (e.g., as in FAVGIT, Figure 3). 3.2.2. Supplementary Substituents. They may be located either at the focal phenyl ring or at dendrimer surface rings. 3.2.2.1. Substituents Located at the Focal Ring. Two kinds of positions may bear supplementary substituents: positions 2 and 6, and position 4. 3.2.2.1.1. 1,2-Di- and 1,2,6-Trisubstituted-3,5-di(benzyloxy)benzenes. In each of the reported structures, where a supplementary substituent is located at position 2 (respectively 6) of the benzene ring (Figures 10 and 11), the sequence u2-u3 related to position 3 (respectively 5) adopts the Ap-Sp conformation (except in one case, LUDXAJ, where it is Sc-Sp). In all cases, the conformation of the related u3 torsion angle is synperiplanar. When substituents are located at both positions 2 and 6, then the sequence u3-u30 is Sp-Sp (type 6), as in JATZUA, LUDXAJ, YARGEE, and QEMBUG (Figure 10). It can be concluded that the presence of a vicinal substituent at position 2 (respectively 6) induces the Sp conformation of the u3 angle related to position 3 (respectively 5). Structures such as MIPMEE (Figure 10), and BOPJIA, COTJOL and COTJUR (not shown), where the focal phenyl ring has a substituent at position 2 (but no substituent at position 1) can be included in this structural family. This behavior can be explained by the sterically destabilizing intramolecular interaction between the protons of the CH2 group of the u3 that would adopt an Ap conformation and its vicinal substituent (Figure 11). 3.2.2.1.2. 1,4-Substituted-3,5-di(benzyloxy)benzene. The only type of u3-u30 conformation observed in the X-ray structures of G1 dendrons having a methoxy group at position 4 of the focal benzene ring (structures RIPVOC and RIPVUI75) is Ap-Ap (Figure 12). For, most likely, steric reasons (i.e. hindrance between the H atom of a CH2 group and the C atom of the methoxy group at position 4) conformation Sp-Sp or Ap-Sp of the sequence u3-u30 is blocked. 3.2.2.2. Substituents Located at Dendrimer Termini (i.e. at the Dendrimer Surface Rings). There are only a few X-ray structures of 1-substituted-3,5-di(benzyloxy)benzene-based dendrons that have substituents at the dendrimer surface

Figure 11. (a) Ap-Sp conformation of a u2-u3 sequence in a dendrimer or dendron possessing a 2- or 6-substituted phenyl ring. (b) Sterical hindrance in the Ap-Ap conformation of a u2-u3 sequence in a dendrimer or dendron possessing a 2- or 6-substituted phenyl ring. R2(6) is of type -X-A where X and A are atoms or groups of atoms.

Figure 12. X-ray molecular structures (and CSD references codes) of G1 Frechet-type 1,4-disubstituted-3,5-di(benzyloxy)benzenebased dendrons. Protons are omitted for clarity.

rings (e.g. BEQQUK, MAWNUU, and M). BEQQUK (COOCH3 at position 4 of the surface phenyl) and M (tBu at position 4 of the surface phenyl) show in one lattice both Ap-Ap (type 1) and Ap-Sp (type 4) conformations of the sequence u3-u30 . However, in MAWNUU (OCH3 in positions 3, 4, and 5 of the surface phenyl), the sequence u3-u30 adopts only the Sp-Sp conformation. The small number of such structures does not permit formulation of a general conclusion about the predictable influence of these substituents on the conformation. 3.2.3. Other Factors that determine dendrimer conformation can be the in-crystal interactions, such as π-π stacking, CH/π interactions, hydrogen bonds, and the nature of cocrystallizing molecules. Several examples of such interactions are discussed below. These factors may reinforce or may be in competition with the intrinsic factors, and the balance of these two factors determines the final in-crystal conformation. (a) π-π stacking. Frechet-type dendritic molecules possess at least three phenyl rings per molecule, and consequently, π-π stacking76 interactions are expected to be observed in the solid state. Indeed, in the structure BAGXAJ (Figure 13a) the Sp conformation of u3 from

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Stadler

Figure 13. Types of interactions that govern the conformation of Frechet-type dendrimers in the solid state: (a) π-π stacking and OH 3 3 3 O hydrogen bonds; (b and c) CH/π interactions; (d and e) CH 3 3 3 O hydrogen bonds; (f) alkyl-alkyl interaction. In parts b and d, protons are omitted for clarity.

the sequence u3-u30 of type 4 (Ap-Sp), as well as the Sc conformation of the corresponding u1, seems to be stabilized by partial intra- (≈ 3.7 A˚) and intermolecular (≈ 3.6 A˚) π-π stacking between the phenyl of the substituent located at position 2 and the phenyl of the benzyloxy group located at position 3. The Sc conformation of one u2 seems to be due to the intermolecular hydrogen bond between the H atom of an OH group and the O of the benzyloxy group located at position 5 (the H 3 3 3 O distance is ≈2.5 A˚). (b) CH/π interactions77 may also be responsible for selecting or stabilizing conformational preferences. In the structure WIBXEL, such CH/π interactions are observed (Figure 13c): between a Csp3H of molecule A and a phenyl of molecule B (dCsp3H-Ph center = 2.51 A˚, and dCsp3-Ph center = 3.45 A˚) and between a CPhH of molecule B and a phenyl of molecule C (dCH-Ph center = 3.04 A˚, and dC-Ph center = 3.68 A˚). These interactions can be seen as locking or rigidifying somehow the conformation of u1 related to the phenyl B. Another example of similar interactions is found in VEFKIB, where dCsp3H-Ph center=2.83 A˚ and dCsp3-Ph center = 3.77 A˚ (Figure 13b). (c) Hydrogen bonds78 may also contribute to the stabilization of a given conformation. This is the case of the less usual weak C-H 3 3 3 O bonds79 observed in the structure UDUTIX (Figure 13d), where dH 3 3 3 O = 2.63 A˚ (respectively 3.11 A˚), dC-H 3 3 3 O = 3.54 A˚ (respectively 4.02 A˚), and — CHO = 162.2 (respectively 150.4), which stabilize the conformation Sc-Sc-Ap of a sequence u1-u2-u3. A similar interaction is apparently found (Figure 13e) in TUWCOD, where it contributes to the Sc conformation of a u1 (dH 3 3 3 O = 2.45 A˚, dC-H 3 3 3 O = 3.22 A˚, — CHO = 133.6). (d) The influence of the cocrystallizing molecules (solvent molecules) is manifested in the structure WIBXEL,

where the presence of a molecule of CH2Cl2 (Figure 13c) induces the Sp conformation of the angle u3 located close to it. Comparative crystallization experiments utilizing different solvents have not been reported to date. (e) Self-assembly in the solid state, through hydrophobic alkyl-alkyl interaction, of structures that can be seen as G1 dendrons with respect to phenyl rings, but as G2 dendrons with respect to octyloxy groups (HEVXEM, JESZIR, WIRXOK), generates planar layers that involve the following: (i) a planar conformation of the u1-u2-u3-u30 -u20 -u10 sequences and (ii) a space that could be filled by the interpenetrating alkyl chains. These requirements are compatible with an Sp-Ap-Ap-ApAp-Sp conformational sequence, i.e. a sequence of type 1Ac (Figure 13f; see also Table S1 from Supporting Information). Thus, the self-assembly seems to stabilize or even to direct this conformation. 4. From in-Crystal Data to Molecular Geometric Parameters. Dependence of Molecular Geometric Parameters on Torsion Angles Several intramolecular distances are important parameters used to estimate the size of molecular or supramolecular objects where these dendrons may be involved. In order to estimate the space demand of a G2 dendron (RISZIC or RISZIC01), Schl€ uter et al.80 evaluated the distances between carbon C1 of the phenyl ring of the G0 level and carbon C3 or C4 of the phenyl rings of the G2 level (Figure 14a). These distances gave the radius and the diameter of the cylinder partially filled by the dendron. Thus, this radius depends on the conformation of several of the torsion angles. This dependence could be expressed through a mathematical function. A simpler example is the case that deals with the radius r of a G1 dendron, defined between carbon C1 of a phenyl ring

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The conformational preferences of the u3-u30 sequence in dendrons or dendrimers are the same as those due to the stereoelectronic factors that already appear in smaller model molecules such as 1,3-dimethoxybenzene or (3,5-dimethoxyphenyl)methanol. A substituent at position 2 (or respectively 6) on the focal phenyl ring generally induces the Ap-Sp conformation of the u2-u3 (or u20 -u30 ) sequence related to the neighboring position 3 (or respectively 5). As a consequence, the most encountered conformation of the u3-u30 sequence of these substituted motifs is Ap-Sp. The simultaneous presence of substituents at both the 2 and 6 positions generates the Sp-Sp conformation of the u3-u30 sequence. However, the full conformational roles of various substituents and of solid state in-crystal interactions appear to be complex and have not been totally elucidated. Consequently, these questions should be discussed in further studies. In some cases, the same dendron adopts in the same crystal conformations that belong to different types. This conformational diversity or multiplicity is possible thanks to the conformational mobility66 (flexibility) of Frechet-type dendrimers or dendrons due to the low rotational barrier about single bonds. 6. Experimental Section

Figure 14. (a) Distance z between C1 of a G0 phenyl and C4 of a G2 phenyl measured on a G2 dendron. (b) Distance r between C1 of a G0 phenyl and C4 of a G1 phenyl measured on a G1 dendron. (c) Radius r in the space demand cylinder of a G1 dendron.

of the G0 level and carbon C4 of one of the two phenyl rings of the G1 level (Figure 14b and c). Among the three torsion angles u1, u2, and u3, the distance r does not depend on u1 (because one end is located on the rotational axis of this angle), but only on u2 and u3. A function r(u2,u3) expresses this dependence. The conformations where the radius r adopts extrema are the following: r(0,0) = 6.20 A˚, r((180,0) = 8.73 A˚, r(0,(180) = 3.93 A˚, r((180,(180) = 8.45 A˚, r(-159.81,54.2) = r(159.81,54.2) = 8.76 A˚.81 Of course, in-crystal molecular geometrical parameters can be different from the ones in solution.

The X-ray structures of the dendrimers and dendrons discussed herein are published and can be obtained from the Cambridge Crystallographic Data Centre. The CSD version used was 5.31 (November 2009). The reference codes of the structures of 1-substituted-3,5-di(benzyloxy)benzene are as follows: BEQQUK,82 CAZVAB,83 EDEWUG,84 FAVGIT,85 GOZJOU,86 HAVDIS,87 HEVXEM,21j HIDKIP,88 IXEKIF,89 IXEKOL,89 JESZIR,21j LIHPAU,90 LIHPEY,90 MAFROB,91 MAFRUH,91 MAFSAO,91 MAFSES,91 MAWNUU,19b NIFQEZ,92 NIFQID,92 NOLXOB,93 OJUQOZ,94 QENPUU,95 QOTQUM,96 REMQEG,97 RIBZEH,98 RIBZIL,98 RISZIC01,99 TUWCOD,100 UDUTIX,101 VEFKIB,102 VOQHIT,103 WIBXEL,104 WIRXOK,105 YALNIJ,21h YIQBOP,106 ZUDDIL107 (see Table S1 of the Supporting Information). Structure M was published previously.108 The reference codes of the structures of 2-, 1,2-, 1,4-, or 1,2,6-substituted-3,5-di(benzyloxy)benzene are as follows: BAGDIW,109 BAGXAJ,110 BAGXIR,111 BOPJIA,112 COTJOL,113 COTJUR,114 JATZUA,115 LUDXAJ,116 MIPMEE,117 NAWBOC, 118 QEMBUG, 119 QIBMEU, 120 REBHEM, 121 VOJDUU, 122 YARGEE 123 (see Tables S2 and S3 of the Supporting Information). Supporting Information Available: Data for quantitative and semiquantitative solid-state description of 1-substituted, 2-substituted, 1,2-disubstituted, 2,6-disubstituted, or 1,2,6-trisubstituted 3,5-di(benzyloxy)benzene-based dendrimers and dendrons; structural formulas of the dendrons and dendrimers discussed in the paper; and observed topologies of sequences. This material is available free of charge via the Internet at http://pubs.acs.org.

References 5. Conclusion The present study on the structural features of Frechet-type dendrons and dendrimers in the solid-state shows that, within 1-substituted-3,5-di(benzyloxy)benzene-based motifs, the antiperiplanar (Ap) conformation of the torsion angles u2 (CPh(n)-C sp3-O-CPh(n-1)) and u3 (Csp3-O-CPh-C4Ph) is preferred. Overall, the angle u1 (CPh(n)-C1Ph(n)-Csp3-O) slightly prefers the conformation Sc (about 60%) over Sp (about 40%). The preferred conformations of the sequence u3-u30 are ApSp and then Ap-Ap. The Ap-Ap-Ap-Ap and Ap-Ap-Sp-Ap conformations of the sequence u2-u3-u30 -u20 are also preferred.

(1) For reviews and books, see: (a) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III. Angew. Chem., Int. Ed. Engl. 1990, 29, 138– 175. (b) Tomalia, D. A.; Durst, H. D. Top. Curr. Chem. 1993, 165, 193–313. (c) Newkome, G. R., Ed.; Advances in Dendritic Macromolecules; JAI Press: London, 1994-1996; Vols. 1-3. (d) Frechet, J. M. J. Science 1994, 263, 1710–1715. (e) Issberner, J.; Moors, R.; V€ogtle, F. Angew. Chem., Int. Ed. Engl. 1994, 33, 2413–2420. (f) Newkome, G. R.; Moorefield, C.; V€ogtle, F. Dendritic Molecules: Concepts, Syntheses, Perspectives; VCH: Weinheim, 1996. (g) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681–1712. (h) Peerling, H. W. I.; Meijer, E. W. Chem.;Eur. J. 1997, 3, 1563–1570. (i) Archut, A.; V€ogtle, F. Chem. Soc. Rev. 1998, 27, 233–240. (j) Narayanan, V. V.; Newkome, G. R. Top. Curr. Chem. 1998, 197, 19–77. (k) Majoral, J.-P.; Caminade, A.-M. Top. Curr. Chem. 1998, 197, 79–124. (l) Seebach, D.; Rheiner, P. B.; Greiveldinger, G.; Butz, T.;

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(2) (3) (4)

(5) (6)

(7) (8) (9)

(10)

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