Transfer, amplification, and inversion of helical chirality mediated by concerted interactions of C3-supramolecular dendrimers

ARTICLE

pubs.acs.org/JACS

Transfer, Amplification, and Inversion of Helical Chirality Mediated

by Concerted Interactions of C₃-Supramolecular Dendrimers

Mihai Peterca,^†,‡Mohammad R. Imam,^†Cheol-Hee Ahn,^†Venkatachalapathy S. K. Balagurusamy,^†

Daniela A. Wilson,^†Brad M. Rosen,^†and Virgil Percec*^,†

^†Roy & Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia,

Pennsylvania 19104-6323, United States

^‡Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6396, United States

S Supporting Information

b

ABSTRACT: The synthesis, structural, and retrostructural anal

ysis of two libraries containing 16 ﬁrst and second generation

C₃-symmetric self-assembling dendrimers based on dendrons

connected at their apex via trisesters and trisamides of 1,3,5-

benzenetricarboxylic acid is reported. A combination of X-ray

diﬀraction and CD/UV analysis methods demonstrated that

their C₃-symmetry modulates diﬀerent degrees of packing on

the periphery of supramolecular structures that are responsible

for the formation of chiral helical supramolecular columns and

spheres self-organizable in a diversity of three-dimensional (3D) columnar, tetragonal, and cubic lattices. Two of these periodic

arrays, a 3D columnar hexagonal superlattice and a 3D columnar simple orthorhombic chiral lattice with P222₁symmetry, are

unprecedented for supramolecular dendrimers. A thermal-reversible inversion of chirality was discovered in helical supramolecular

columns. This inversion is induced, on heating, by the change in symmetry from a 3D columnar simple orthorhombic chiral lattice to

a 3D columnar hexagonal array and, on cooling, by the change in symmetry from a 2D hexagonal to a 2D centered rectangular lattice,

both exhibiting intracolumnar order. A ﬁrst-order transition from coupled columns with long helical pitch, to weakly or uncorrelated

columns with short helical pitch that generates a molecular rotator, was also discovered. The torsion angles of the molecular rotator

are proportional to the change in temperature, and this eﬀect is ampliﬁed in the case of the C₃-symmetric trisamide supramolecular

dendrimers forming H-bonds along their column. The structural changes reported here can be used to design complex functions

based on helical supramolecular dendrimers with diﬀerent degree of packing on their periphery.

’ INTRODUCTION

structural information from the molecular to supramolecular

level, and from the supramolecular to the three-dimensional

periodic or quasi-periodic level. Using a combination of powder

and oriented ﬁber X-ray diﬀraction (XRD), electron density Fourier

reconstructions,¹⁶numerical and molecular model based simula-

tions of the XRD,^2ediﬀerential scanning calorimetry (DSC),

circular dichroism, and UV spectroscopy (CD/UV-vis) techni-

ques applied in solid state, we identiﬁed an unexpected diversity

of three-dimensional (3D) columnar, tetragonal, and cubic arrays

self-organized from C₃-symmetric dendrimers. This combination

of techniques facilitated the elucidation of the mechanism of

transfer, ampliﬁcation, and inversion of helical chirality that

occurs upon heating at the transition from a new orthorhombic

chiral phase with P222₁symmetry to a 3D hexagonal columnar

phase Φ_h^3D. The inversion of chirality¹⁷is thermal-reversible,

occurring upon cooling at the transition from columnar hexago-

nal to columnar centered rectangular phase. This inversion is

determined by the symmetry of the 3D periodic organization,

Supramolecular helical structures generated from self-assem-

bling dendrons,¹dendrimers,²and other macromolecules³are of

interest for a diversity of areas, including optics,⁴molecular rec-

ognition,⁵nanotubes,⁶folding,^3a,7nanomachines,⁸porous pro-

tein mimic,¹and organic electronics.⁹Interest in these supramo-

lecular assemblies is fueled by the high level of control that can be

achieved in the design of the building blocks,³which can facilitate

a precise incorporation of functional groups in various parts of

the helix.^1,10Nevertheless, there are many examples of helical

structures that have the potential to act as scaﬀolds for nanoscale

applications,¹¹but which upon subsequent functionalization

might lose their original spatial organization.¹²Therefore, full

exploration of the potential of these systems requires a complete

understanding of how helicity emerges through transfer of

structural information¹³from the molecular to the supramolec-

ular,¹⁴and from the supramolecular to the periodic or quasi-

periodic array levels.

Herein, we exploit the intrinsic propensity of self-assembling

dendrimers with 3-fold symmetry¹⁵to form helical supramole-

cular structures^2eand investigate the mechanism of transfer of

Received: November 30, 2010

Published: January 31, 2011

r

2011 American Chemical Society

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|

Journal of the American Chemical Society

ARTICLE

Scheme 1. Structure and Synthesis of 1,3,5-Bz-Trisester and 1,3,5-Bz-Trisamide Dendrimers^a

^aReagents and conditions: (i) DMAP, CH₂Cl₂, room temperature, 8-12 h, 50-70%; (ii) DTBMP, CH₂Cl₂, room temperature, 6 h,

65-75%.

illustrating the importance of chirality transfer at the lattice level.

In addition, such 3D correlations between helical columns limit

the helical growth and template unusual slow advancing helices

characterized by a long pitch. Furthermore, a molecular rotator¹⁸

was demonstrated to occur via temperature change at the ﬁrst-

order transition from coupled helical columns with long helical

pitch to uncoupled helical columns with short helical pitch. The

motion of the molecular rotator is reversible and can be tuned via

the incorporation of amide groups at the dendrimer apex to

amplify the conformational changes via the formation of direc-

tional intracolumnar H-bonds. The transfer, ampliﬁcation, and

inversion of chirality reported here provide a fundamentally new

approach to the ampliﬁcation of chirality.¹⁹Other helical supra-

molecular systems have been found to exhibit inversion of helical

chirality in response to changes in temperature,^17a,17bsolvent,^17b

or the concentration of the chiral dopant molecules used in the

coassembly.^19eNevertheless, to our knowledge, this is the ﬁrst

example of chirality inversion templated by the symmetry of the

self-organized 3D packing. While previous examples of chiral

inversion could be rationalized on the basis of thermodynamics

and kinetics, the detailed structural information provided in this

solid-state inversion of chirality provides the ﬁrst case wherein

a molecular mechanism of the inversion processes is demon-

strated.

The trisester and trisamide dendrimers form chiral spherical

supramolecular structures by pathways similar to those of other

C₃-symmetric dendrimers, such as dendronized cyclotriveratry-

lene and triphenylene.^15c,15dThe unexpected formation of a

diversity of columnar and helical structures, with or without inter-

columnar correlations, is attributed to the less-densely packed

periphery of the supramolecular dendrimers and to the increased

conformational freedom of the dendritic branches of the trisester

and trisamide dendrimers by contrast with dendrimers with

3-fold symmetry characterized by a more densely packed per-

iphery generated via dendritic hexasubstitution of cyclotrivera-

trylene^2e,15cand triphenylene.^15d

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ARTICLE

Scheme 2. Synthesis of Chiral Amine (3,4,5)dm8*G1-NH₂(4)^a

^aReagents and conditions: (i) K₂CO₃, DMF, 60 °C, 4 h; (ii) SiO₂HNO₃, CH₂CI₂, room temperature, 15 min; (iii) NH₂NH₂H₂O, graphite,

3

EtOH, reﬂux, 24 h.

Scheme 3. Synthesis of Second Generation Dendritic Amines^a

^aReagents and conditions: (i) K₂CO₃, DMF, 70 °C, 12 h; (ii) NH₂NH₂H₂O, graphite, EtOH-THF, reﬂux, 24 h; (iii) NH₄OH, 50 °C, 12 h; (iv)

3

KOAc, AcOH, reﬂux, 1.5 h; (v) NH₂NH₂H₂O, graphite, EtOH-THF, reﬂux, 2 h.

3

’ RESULTS AND DISCUSSION

and 3D phases they will form.²⁰For the synthesis of all den-

drimers, a convergent iterative process based on accelerated

synthetic methods elaborated in our laboratory was employed.²¹

The ﬁrst step of the synthesis of self-assembling trisester den-

drimers requires dendrons possessing a benzyl alcohol at their

apex. These benzyl alcohols were esteriﬁed with 1,3,5-benze-

netricarbonyl trichloride in CH₂Cl₂using catalytic 4-dimethy-

lamino pyridine (DMAP) in moderate yields. Similarly, the

ﬁrst step of the synthesis of self-assembling trisamide dendri-

mers required dendrons possessing an aryl amine at their apex.

These aryl amines were amidated with 1,3,5-benzenetricarbonxyl

Synthesis. Two libraries of self-assembling dendrimers con-

taining nine trisester (A2-9, A6*) and seven trisamide groups at

their apex (B1-4, 6, 7, and B2*) were synthesized through the

symmetric esterification or amidation of nine different achiral

and chiral dendrons to a central 1,3,5-benzenetricarboxylic acid

core (Scheme 1).

The nine self-assembling dendrons forming the periphery of

dendrimers were designed with primary structures that will probe

the eﬀect of branching pattern on the type of 2D liquid crystalline

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ARTICLE

Table 1. Thermal Analysis of the Supramolecular Dendrimers Self-Assembled from Trisesters and Trisamides

a

phase transition (°C) and corresponding enthalpy changes (kcal mol^-1

)

compound

heating

cooling

1,3,5-Bz-[tris(3,4,5)12G1]-

trisester (A2)

Φ_x49.0 (33.64) i Φ_x1.1 (2.59) Φ_x39.8 (-9.10) Φ_x49.0

(29.76) i

i 31.0 (-12.04) Φ_x-2.6 (-2.46) Φ_x

io

1,3,5-Bz-[tris(4-3,4,5)12G1]-

trisester (A3)

Φ_h^3D-16.3 (9.77) Φ_h^3D46.6 (0.89) Φ_h^io192.5 (20.18) i

Φ_h^3D-6.7 (8.45) Φ_h^3D71.6 (1.09) Φ_h^io191.9 (18.42) i

Φ_h^io-13.3 (16.53) Φ_h^io147.6 (1.93) Φ_h161.6 (9.44) i

Φ_h^io-11.2 (16.30) Φ_h^io146.4 (1.93) Φ_h161.3 (9.22) i

Φ_h^io46.2 (0.96) Φ_h^io116.4 (1.48) Tet 149.9 (0.39) i

Φ_h^io117.2 (2.62) Tet 140.1 (0.65) i

i 184.6 (-18.65) Φ_h^io64.2 (-1.01) Φ_h

3D

-11.0 (-6.75) Φ_h

io

1,3,5-Bz-[tris(4-3,4-3,5)12G2]-

trisester (A7)

i 159.2 (-9.14) Φ_h145.2 (-1.45) Φ_h

io

-25.5 (-38.55) Φ_h

io

1,3,5-Bz-[tris(4-3,4,5-3,5)12G2]-

trisester (A8)

i 134.2 (-0.53) Tet 104.0 (-2.43) Φ_h

io

1,3,5-Bz-[tris(4-3,4-3,4,5)12G2]-

trisester (A9)

Φ_h^io-16.4 (14.89) Φ_h^io99.1 (1.79) Cub 209.5 (0.52)

Tet 253^bdec

dec 195.9 (-0.01) Cub 144.6 (-1.75) Φ_h

Φ_h^io-14.8 (11.28) Φ_h^io110.4 (2.10) Cub 165.2 (0.87)

Cub 207.6 (0.15) Tet 250^bdec

Φ_h^3D-137.4 (2.23) Φ_h^3D-267.5 (26.75) Φ_h^3D-393.0 (13.11)^ci

Φ_h^3D-12.2 (13.44) Φ_h^3D-220.8 (1.32) Φ_h^3D-393.2 (15.25)^ci

Φ_h42.8 (5.59) Φ_h^3D-SL70.1 (29.17) i

Φ_h38.6 (-11.18) Φ_h^3D-SL70.6 (35.35) i

x -14.1 (5.85) g^d42.1 (2.67) Cub 121.4 (1.32) i

x -16.6 (15.60) g^d19.0 (4.48) Cub 121.3 (1.38) i

T_g7.6 Cub 74.7 (1.25) i

3D-2

1,3,5-Bz-[tris(3,4-3,5)12G2]-

trisester (A4)

i 86.1 (-13.93) Φ_h^3D-318.5 (-1.25) Φ_h

3D-1

-3.7 (-11.36) Φ_h

1,3,5-Bz-[tris(3,4,5-3,5)12G2]-

trisester (A5)

1,3,5-Bz-[tris(3,4,5)²12G2]-

i 56.3 (-0.99) Φ_r-c42.3 (-2.98) Φ_h27.0

(-2.17) Φ_h

i 47.9 (-0.48) Cub 3.0 g^d

trisester (A6)

1,3,5-Bz-[tris(3,4,5)²dm8*G2]-

trisester (A6*)

i 109.2 (-0.94) Cub -20.1 (-16.47) T_g

T_g11.4 Cub 74.7 (1.15) i

Φ_x80.2 (24.91) Φ_h^io250.7 (2.55) i

1,3,5-Bz-[tris(3,4)12G1]-

trisamide (B1)

i 247.7 (-2.34) Φ_h^io51.3 (-0.64) Φ_x

-6.2 (-2.44) Φ_x

Φ_x-1.2 (4.65) Φ_x78.6 (0.93) Φ_h^io250.5 (2.42) i

1,3,5-Bz-[tris(3,4,5)12G1]-

trisamide (B2)

Φ_r-s^o-1.5 (9.29) Φ_r-s^o57.4 (0.65) Φ_h^3D94^bΦ_h^io178.8 (6.29) i i 175.0 (-6.13) Φ_h^io82^dΦ_h^3D-9.6

Φ_r-s^o-3.9 (10.63) Φ_r-s^o61.0 (0.53) Φ_h^3D103.6 (0.53)

Φ_h^io179.0 (6.50) i

(-5.00) Φ_r-s

o

1,3,5-Bz-[tris(3,4,5)dm8*G1]-

trisamide (B2*)

Φ_s-o^o40^bΦ_h^3D88.3 (0.63) Φ_h^io108.3 (2.39) i

Φ_s-o^o40^bΦ_h^3D86.0 (0.46) Φ_h^io108.3 (1.95) i

Φ_r-s^o174.6 (1.17) Φ_h212.7 (3.83) i

Φ_r-s^o152.0 (0.79) Φ_h204.1 (1.25) i

Φ_h^o44.7 (12.83) T_g81.9 Φ_h116.4 (2.10) i

Φ_h^o6.0 (10.76) T_g78.8 Φ_h116.4 (2.05) i

i 102.6 (-1.92) Φ_h^io78.7 (-0.53)

Φ_r-c^io40^bΦ_s-o

o

1,3,5-Bz-[tris(4-3,4,5)12G1]-

amide (B3)

i 208.6 (-1.20) Φ_h146.5 (-0.48) Φ_r-s

o

1,3,5-Bz-[tris(3,4-3,5)12G2]-

trisamide (B4)

i 111.2 (-2.66) Φ_h1.0 (-8.48) Φ_h

o

1,3,5-Bz-[tris(4-3,4-3,5)12G2]-trisamide (B7) Φ_h^o-9.9 (7.76) Φ_h^o58.2 (0.55) Φ_h^io189.7 (3.00) i

i 151.8 (-2.38) Φ_h^io-13.0 (-8.78) Φ_h

Φ_h^o-6.1 (7.55) Φ_h^io159.1 (2.81) i

1,3,5-Bz-[tris(3,4,5)²12G2]-

g^d-8.8 (23.47) g^d95.4 (2.59) Cub 178.1 (1.10) i

g^d-11.0 (23.24) Cub 177^bi

i 169^bCub -15.0 (-27.32) g^d

trisamide (B6)

^aData from the ﬁrst heating and cooling scans are on the ﬁrst line, and data from the second heating are on the second line; Φ_h, hexagonal

columnar phase; Φ_h^io, columnar hexagonal phase with intracolumnar order; Φ_h^3D, columnar hexagonal phase with three-dimensional order;

Φ_h^3D-SL, three-dimensional columnar hexagonal superlattice; Φ_x, unidentiﬁed columnar lattice; Φ_r-c, centered rectangular columnar phase;

Φ_r-c^io, centered rectangular columnar phase with intracolumnar order; Φ_r-s^o, simple rectangular ordered columnar phase; Φ_s-o^o, simple

orthorhombic ordered columnar phase; Tet, P4₂/mnm tetragonal LC phase; Cub, Pm3n cubic phase; T_g, glass transition; i, isotropic; dec,

decomposition. ^bThis transition is observed only by TOPM and XRD. ^cSum of enthalpies from overlapped peaks. ^dGlassy Pm3n cubic phase as

observed by XRD.

trichloride in CH₂Cl₂using 2,6-di-tert-butyl-4-methylpyridine

(DTBMP) as base.

The three new dendritic amines required for the synthesis

of trisamides B4, B6, and B7 were prepared through similar

methods (Scheme 3). 3,4,5-Tris(3,4,5-tris-dodecyloxy-benzyloxy)-

phenyl amine, (3,4,5)²12G2-NH₂(8), was obtained in two

steps from the dendritic chloride, 5, and the known^22a,22c

compound 1,2,3-trihydroxy-5-nitrobenzene (6). The aryl nitro

The synthesis of the three dendritic amines employed in the con-

struction of compounds B1, B2, and B3 was reported previously.²²

The chiral amine for the triamide B2* was synthesized following the

identicalliteratureprocedure(Scheme2).^22aSpeciﬁcally, pyrogallol

was trisalkylated with (R)-2,7-dimethyloctyl (dm8*) bromide in

excellent yield (94%). Nitration of the alkylated pyrogallol pro-

ceeded rapidly using SiO₂pretreated with nitric acid (85% yield).

Finally, (3,4,5)dm8*G1-NH₂was achieved in very good yield

(93%) through reduction of nitro group with hydrazine hydrate.

group from intermediate 7 was reduced by NH₂NH₂H₂O using

3

graphite catalyst to give the dendritic amine 8 in 82% yield.

The N-protected dihydroxy aniline 11 was obtained from

phloroglucinol 9 via successive monoamination with ammo-

nium hydroxide and imidization with phthalic anhydride

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Table 2. Phases, Lattice Parameters, and Measured d-Spacing of the Trisester Dendrimers

d₁₀₀, d₁₁₀, d₂₀₀, d₂₁₀, d₃₀₀, d₂₂₀, d₃₁₀, d₄₀₀, d₃₂₀(Å)^c

d₄₁₀, d₄₀₁, d₃₂₁, d₁₁₃, d₃₀₃, d₂₂₃, d₄₀₃, d₀₀₁₀(Å)^d

d₀₀₂, d₁₁₂, d₂₀₂, d₀₀₃, d₀₀₄, d₂₀₅, d₀₀₇, d₀₀₈, d₀₀₉, d₀₀₁₇(Å)^e

d₁₀, d₁₁, d₂₀, d₂₁, d₃₀, d₂₂(Å) ^f

d₀₀₂, d₄₁₀, d₃₃₀, d₄₁₁, d₄₂₁(Å)^g

d₁₁₀, d₂₀₀, d₂₁₀, d₂₁₁, d₂₂₀, d₃₁₀, d₃₂₁, d₄₀₀(Å)^h

d₂₀₀, d₂₂₀, d₄₀₀, d₃₀₁, d₁₀₂(Å)ⁱ

dendrimer

T (°C)

phase^a

a, b, c (Å)^b

d₂₀, d₀₄, d₂₄, d₃₃, d₄₀

k

36.7, -, 18.4, 13.9, 12.2, 10.6, 10.2, 9.2, 8.4^c

8.0, 8.9, 8.2, 9.9, 8.2, 7.7, 7.1, 3.4^d

38.9, -, 19.4, 14.7, 12.9, 11.2^f

29.1^j

32.4^j

50.0, 28.8, 24.9 ^f

48.3, 27.8, 24.0 ^f

46.1, 26.5, 23.0 ^f

44.3, 25.5, 22.1^f

48.2, 40.6, 39.4, 37.4^g

47.6, 27.3, 23.7 ^f

3D

1,3,5-Bz-[tris(4-3,4,5)12G1]-trisester

25

Φ_h

42.4, -, 33.5

io

170

45

Φ_h

Φ_x

Φ_h

44.9

1,3,5-Bz-[tris(3,4,5)12G1]-trisester

20

io

1,3,5-Bz-[tris(4-3,4-3,5)12G2]-trisester

25

57.6

120

155

80

55.6

53.1

io

1,3,5-Bz-[tris(4-3,4,5-3,5)12G2]-trisester

1,3,5-Bz-[tris(4-3,4-3,4,5)12G2]-trisester

51.2

140

25

P4₂/mnm

io

167.2, -, 96.4

54.8

Φ_h

210

240

-25

Pm3n

102.3

-, 51.2, 45.8, 41.7, 36.2, -, 27.2, 25.5^h

48.3, 43.6, -, 39.8, 37.1^g

38.5, 22.3, 19.3^c

11.5, -, 9.9, 7.7, -, -, 3.3^e

38.6, 22.3, 19.3^c

-, -, -, -, 13.9, 9.6, -, 7.0, 6.2, 3.3^e

38.8, 22.4, 19.4^c

-, 10.5, -, -, -, -, 3.4^e

38.1, 21.9, 19.0^f

P4₂/mnm

3D-1

179.9, -, 96.5

44.5, -, 23.0

1,3,5-Bz-[tris(3,4-3,5)12G2]-trisester

Φ_h

3D-2

12

80

Φ_h

44.6, -, 55.6

44.8, -, 23.8

3D-3

Φ_h

1,3,5-Bz-[tris(3,4,5-3,5)12G2]-trisester

10

57

53^l

Φ_h

43.9

3D-SL

Φ_h

87.7, -, 71.9

74.3, 103.9

37.9, 21.9, 18.9, 23.9, 32.5ⁱ

37.2, 26.0, 21.3, 20.1, 18.6^k

36.2^j

Φ_r-c

X

1,3,5-Bz-[tris(3,4,5)²12G2]- trisester

1,3,5-Bz-[tris(3,4,5)²dm8*G2]-trisester

25

105

20

Pm3n

72.4

63.3

62.8

51.2, 36.2, 32.4, 29.6, 25.6, 22.9, 19.3, 18.1^h

-, 31.6, 28.3, 25.8^h

65

-, 31.4, 28.1, 25.6^h

^aPhase notation: Φ_h, columnar hexagonal phase; Φ_h^io, columnar hexagonal phase with intracolumnar order; Φ_h, columnar hexagonal ordered

phase; Φ_h^3D, Φ_h^3D-1, Φ_h^3D-2, and Φ_h^3D-3, three-dimensional columnar hexagonal phases; Φ_r-c, centered rectangular columnar phase; Φ_x,

unidentiﬁed columnar phase; x, unidentiﬁed phase; P4₂/mnm, tetragonal phase; Pm3n, cubic phase; Φ_h^3D-SL, three-dimensional columnar

hexagonal superlattice. ^bLattice parameters calculated using d_hkl= 1/(4(h²þ k²þ hk)/(3a²) þ l²/c²)^1/2for hexagonal phases, d_hk= 1/(h²/a²þ k²/

b²)^1/2for rectangular phases, d_hkl= a/(h²þ k²þ l²)^1/2for cubic phases, and d_hkl= 1/((h²þ k²)/a²þ l²/c²)^1/2for tetragonal phases. ^c,d,e,f,id-spacing for

columnar hexagonal phases. ^gd-spacing for tetragonal phases. ^hd-spacing for cubic phases. ^jd-spacing for unidentiﬁed phase. ^kd-spacing for

centered rectangular phases. ^lΦ_r-cphase observed only upon cooling from the isotropic phase.

o

(35% two-step yield). The etheriﬁcation of 11 with dendritic

chloride 12 in the presence of anhydrous K₂CO₃in DMF

provided compound 13, which was then reduced to the desired

amine, 3,5-bis-[3,4-bis-(dodecyloxy)-benzyloxy] aniline, (3,4-

3,5)12G2-NH₂(14), in 73% yield. Similarly, 3,5-bis-[3,4-bis-

(4-dodecyloxy-benzyloxy)-benzyloxy] aniline, (4-3,4-3,5)12G2-

NH₂(17), was synthesized from 11 and chloride 15 in good

yield.

Structural and Retrostructural Analysis. Tables 1, 2, and 3

summarize the combined analysis of the self-assembling C₃-

symmetric dendrimers in the solid state via a combination of

differential scanning calorimetry (DSC), thermal polarized op-

tical microscopy, small (SAXS), and wide (WAXS)-angle powder

and oriented fiber X-ray diffraction experiments. Assignment

of various new and unusual columnar lattices reported in

Tables 1, 2, and 3 was facilitated by X-ray diffraction data methods,

simulation techniques established previously for the columnar

hexagonal and centered rectangular phases with intracolumnar

order (Φ_hand Φ_r-c^io, respectively),^2d,7,16and through the

io

development of new Fourier series reconstructions of the

electron density distributions for the columnar hexagonal phase

with 3D order (Φ_h^3D), columnar hexagonal superlattice with

3D order (Φ_h^3D-SL), and the simple columnar orthorhombic

phase with 3D order (Φ_s-o^3D). In combination with thin film

CD/UV spectroscopy and molecular modeling, the mechanism

of self-assembly and self-organization process into a variety of

helical supramolecular columnar and spherical assemblies was

elucidated.

On the basis of the phases presented in Tables 1, 2, and 3, the

self-assembly process of C₃dendrimers can be directly correlated

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Table 3. Phases, Lattice Parameters, and Measured d-Spacing of the Trisamide Dendrimers

d₁₀, d₁₁, d₂₀, d₂₁(Å)^c

d₁₀₀, d₁₀₁, d₁₁₁, d₁₀₂(Å)^d

d₀₁, d₁₀, d₁₁, d₂₁(Å)^e

d₂₀₀, d₂₁₀, d₂₁₁, d₂₂₀, d₃₁₀, d₃₂₁, d₄₀₀(Å) ^f

d₀₁₀, d₁₁₀, d₂₁₀, d₄₀₁, d₂₀₂, d₃₁₁, d₄₀₂(Å)^g

d₀₀₆, d₀₀₈, d₀₀₁₂(Å)^h

dendrimer

T (°C)

phase^a

a, b, c (Å)^b

32.4

d₂₀, d₁₁(Å)ⁱ

io

1,3,5-Bz-[tris(3,4)12G1]-trisamide

95

35

Φ_h

28.2, 16.2^c

Φ_x

32.5ⁱ

o

1,3,5-Bz-[tris(3,4,5)12G1]-trisamide

20

Φ_r-s

66.1, 30.1

36.4, -, 54.2

34.4

30.1, -, 27.4, 22.3^e

31.5, 27.2, 17.3, 20.6^d

29.8, 17.2^c

39.3, 29.9^e

33.9^c

44.2, 25.4, 22.2^c

43.7, 25.2, 21.8^c

38.0^c

3D

60

Φ_h

io

110

140

170

20

Φ_h

o

1,3,5-Bz-[tris(4-3,4,5)12G1]-trisamide

1,3,5-Bz-[tris(4-3,4-3,5)12G2]-trisamide

1,3,5-Bz-[tris(3,4-3,5)12G2]-trisamide

1,3,5-Bz-[tris(3,4,5)²12G2]-trisamide

1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide

Φ_r-s

29.9, 39.3

29.9

Φ_h

o

Φ_h

51.0

io

140

25

Φ_h

50.5

io

Φ_h

43.8

90

Φ_h

42.9

37.1^c

25

Pm3n

69.8

34.3, 31.0, 28.4^f

36.9, 33.0, 30.1, 25.9, 19.6, 18.5^f

26.1, 25.5, 23.8, 25.0, 23.3, 19.9, 19.1^g

8.5, 6.4, 4.2^h

27.0, 25.5, -, 22.1^d

25.1, 14.5^c

120

25

73.6

3D

Φ_s-o

115.0, 26.1, 50.9

3D

75

95

65^j

Φ_h

31.2, -, 77.3

29.0

io

Φ_h

52.2, 26.9ⁱ

io

Φ_r-c

52.2, 26.9

^aPhase notation: Φ_h, columnar hexagonal phase; Φ_h^io, columnar hexagonal phase with intracolumnar order; Φ_h^3D, three-dimensional columnar

hexagonal phases; Pm3n, cubic phase; Φ_x, unidentiﬁed phase; Φ_r-c^io, centered rectangular columnar phase with intracolumnar order; Φ_r-s

,

o

simple rectangular ordered columnar phase; and Φ_s-o^3D, simple orthorhombic 3D ordered columnar phase with P222₁symmetry. ^bLattice

parameters calculated using d_hkl= 1/(4(h²þ k²þ hk)/(3a²) þ l²/c²)^1/2for hexagonal phases, d_hk= 1/(h²/a²þ k²/b²)^1/2for rectangular phases,

d

_hkl= a/(h²þ k²þ l²)^1/2for cubic phases, and d_hkl= 1/(h²/a²þ k²/b²þ l²/c²)^1/2for the simple orthorhombic columnar phase with P222₁symmetry.

^c,dd-spacing for hexagonal phases. ^ed-spacing for simple rectangular phases. ^fd-spacing for cubic phases. ^g,hd-spacing for the simple

orthorhombic phase. ⁱd-spacing for centered rectangular phase. ^jPhase observed only upon cooling from the isotropic phase.

with the primary structure of their dendritic branches. Columnar

hexagonal, rectangular, and orthorhombic phases are predomi-

nantly formed by self-organization of dendrimers based on the

ﬁrst generation dendrons (Tables 1, 2, and 3). C₃dendrimers

based on second generation dendrons also self-assemble into

columnar phases at low temperatures, with the exception of the

two trisester and one trisamide dendrimers based on the (3,4,5)²-

G2 dendrons, which exhibit the Pm3n cubic phase.^21c,23,24

Remarkably, two dendrimers based on second generation den-

drons, 1,3,5-Bz-[tris(4-3,4,5-3,5)12G2]-trisester and 1,3,5-

Bz-[tris(4-3,4-3,4,5)12G2]-trisester, exhibit at high tempera-

tures the tetragonal phase with P4₂/mnm symmetry, which is

rarely observed in self-assembling dendrons,^25adendrimers,^15c,d

and other macromolecules.^25b,25c

L = 0, 2, 5, and 7 (Figure 1c). These two indexing schemes

correspond to two possible helices, triple-30₁and triple-21₁,^2e,7

respectively (Figure 1c).

The atomistic simulations^2e,7of the triple-30₁and triple-21₁

supramolecular columns (Figure 1c) demonstrate that the posi-

tion of the maxima of the layer lines observed in the experimental

XRD ﬁber pattern (Figure 1b) is well ﬁt by both simulations.

Therefore, the deﬁnitive assessment of the helical packing of the

supramolecular columns self-assembled from 1,3,5-Bz-[tris-

(4-3,4,5)12G1]-trisester required the simulation of the oriented

ﬁber patterns calculated from the two alternative molecular models

(Supporting Information, Figure SF1). From the simulation, the

triple-30₁(Figure 2) helical packing provided the best ﬁt of the

io

experimental ﬁber WAXS data of the Φ_hphase (Figure 1b).

Furthermore, the average correlation length of the intracolumnar

order, ξ, calculated from the width of the π-stacking features

indicated in Figure 1a and b is ξ ≈ 60 column strata for both

Molecular Design Principles for Supramolecular Columns

with Long Helical Pitch. The analysis of the wide-angle X-ray

diffraction (WAXS) patterns collected from the oriented fibers of

1,3,5-Bz-[tris(4-3,4,5)12G1]-trisester, presented in Figure 1,

demonstrated the formation of a Φ_h^3Dphase at low temperatures

3D

io

the Φ_hand the Φ_hphases (Figure 1d). These results

demonstrate that upon the ﬁrst-order transition from the

io

3D

(25 °C), and a Φ_hphase at high temperatures (170 °C).

Φ_hphase, formed by strongly coupled helical columns, to

Interestingly, the diffraction features of the fiber WAXS patterns

the Φ_h^iophase, formed by weakly or uncoupled helical columns,

the internal helical packing of the supramolecular columns is

unchanged.

can be indexed as L = 0, 3, 7, and 10, even upon heating through

3D

io

the first-order transition from Φ_h(Figure 1a) to Φ_h

(Figure 1b). Alternatively, the layer lines observed in the WAXS

fiber pattern of the Φ_hphase (Figure 1b) can be indexed as

The unusual slow advancing helical arrangement (Figure 2a)

can be attributed to the speciﬁc topology of the dendritic unit,

io

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Figure 1. Wide-angle X-ray scattering patterns collected from the oriented ﬁber of 1,3,5-Bz-[tris(4-3,4,5)12G1]-trisester in the Φ_h^3D(a) and Φ_h^io(b)

phases. Simulations of WAXS ﬁber pattern from (b) based on the two simpliﬁed atomistic models (c). Meridional plots of the corresponding WAXS ﬁber

patterns (d) and plot integrating the region of the ﬁrst layer line with nonzero intensity of the WAXS ﬁber pattern shown in b (e). In parts a,b, the layer

lines absent or with very weak intensity proﬁles are colored in gray.

Figure 2. Detail of the supramolecular helical columns of 1,3,5-Bz-[tris(4-3,4,5)12G1]-trisester in the Φ_h^iophase (a) established from atomistic and

molecular model-based simulation (b) of the WAXS oriented ﬁber patterns.

which facilitates a smaller angle of rotation of the adjacent layers,

j = 12°. As a comparison, the peripheries of these trisubstituted

C₃-symmetric dendrimers are less packed than the periphery of

the hexasubstituted dendronized cyclotriveratrylene and triphe-

nylene reported previously.^15c,15dTherefore, the helical param-

eter j of the C₃-symmetric dendrimers with “ﬁlled” periphery^15c,15d

varied between 30 and 60°, whereas in the case of 1,3,5-Bz-[tris-

(4-3,4,5)12G1]-trisester this parameter is at least 3 times

smaller (j = 12°, Figures 1 and 2). Similarly, slow advancing

helical packing in combination with strong column-to-column

correlations²⁶can explain the remarkable 3D columnar hexago-

nal superlattice, Φ_h^3D-SL, and the columnar hexagonal 3D phases

observed, respectively, in the second generation 1,3,5-Bz-[tris-

(3,4,5-3,5)12G2]-trisester and 1,3,5-Bz-[tris(3,4-3,5)12G2]-

trisester dendrimers (Figures 3, 4, and Supporting Information

Figure SF6).

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3D-SL

Figure 3. SAXS pattern collected from the oriented sample of 1,3,5-Bz-[tris(3,4,5-3,5)12G2]-trisester in the Φ_h

phase (a), corresponding

intensity versus scattering vector plot (b), reconstructed relative electron density distribution (c), and two representative planar cross sections of the

volumetric distribution from (c) are detailed in (d) and (e), as indicated.

The formation of the 3D columnar hexagonal superlattice can

be attributed to the close packing of undulated columns,²⁶as

suggested by the reconstructed relative electron density distribu-

tion and maps presented in Figure 3c-e. The oriented ﬁber XRD

pattern collected at small angles from 1,3,5-Bz-[tris(3,4,5-3,5)-

12G2]-trisester (Figure 3a) exhibits very weak and broad (hkl)

diﬀraction peaks in the case of l ¼ 0, demonstrating that the long-

range column-to-column correlations, which span in average about

10-14 ꢀ c unit cells, are relatively weak. On the other hand, the

oriented ﬁber WAXS pattern exhibits strong oﬀ-meridional fea-

tures on the L = 6 layer, demonstrating stronger short-range

column-to-column correlations (Supporting Information Figure

SF7). Therefore, the hexagonal columnar superlattice is gener-

ated via coupling of undulated columns that are highly ordered

within the length scale of the unit cell. Interestingly, after “remov-

ing”²⁶one alkyl chain from the dendritic building block, the 1,3,5-

Bz-[tris(3,4-3,5)12G2]-trisester exhibits a 3D columnar hexa-

gonal phase with a shorterc axis of55.6Å (Figure4), in comparison

with c = 71.9 Å observed for the 1,3,5-Bz-[tris(3,4,5-3,5)12G2]-

trisester. At the same time, the average thickness of the column

stratum was reduced from 4.4 to 3.4 Å. These results demonstrate

that the rotational angle of the adjacent column stratum was slightly

reduced from 120°/16 = 7.5° to 120°/17 = 7.1°. Furthermore,

the torsion angle θ₁characterizing the propeller like conformation

of the dendron, which will be discussed in the next section, was

reduced in direct correlation to the degree of the intramolecular

steric constraints generated by a less-packed alkyl periphery.

The 1,3,5-Bz-[tris(3,4-3,5)12G2]-trisester exhibits an un-

usual sequence of 3D columnar hexagonal phases. Figure 4 illus-

trates the Φ_h^3D-2-Φ_h^3D-3transition, which is accompanied by a

reduction of the c-axis of the 3D columnar hexagonal phases from

55.6 to 23.8 Å. This reduction can be associated with an increase

of the rotational angle of the adjacent column strata associated

with increasing temperature. Such an increase is typically corre-

lated with a larger dendron solid angle^16a,27and possibly with the

increase of its θ₁torsion angle upon the increase of temperature.

Additional examples of C₃-symmetric dendrimers that exhibit an

increase of the rotational angle of the adjacent column strata and

of the θ₁torsion angle upon the increase of temperature will be

discussed in the next section.

Thermal-Reversible Inversion of Chirality in Supramole-

cular Columns Mediated by Lattice Symmetry Changes.

The achiral 1,3,5-Bz-[tris(3,4,5)12G1]-trisamide dendrimer self-

organizes at low temperatures into a 3D columnar hexagonal

phase (Figure 5) and at high temperatures into a columnar

hexagonal phase with intracolumnar order (Φ_h^io). Both WAXS

fiber patterns exhibit similar diffuse and broad features in the

wide-angle region (Figure 5a and Supporting Information Figure

SF3a), which demonstrate that in both phases the supramole-

cular columns are jacketed by an aliphatic region with a liquid-

like structure. Remarkably, when the same dendritic branching

pattern is decorated with chiral chains on the periphery, 1,3,5-

Bz-[tris(3,4,5)dm8*G1]-trisamide, the self-organization pro-

3D

cess templates a new simple orthorhombic 3D phase, Φ_s-o

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Figure 5. Small and wide-angle X-ray scattering patterns collected in

the Φ_h^3Dphase of 1,3,5-Bz-[tris(3,4,5)12G1]-trisamide at the indicated

temperature (a) and the corresponding plot (b).

Figure 4. Wide-angle X-ray scattering patterns collected in the second

heating of the oriented ﬁber of 1,3,5-Bz-[tris(3,4-3,5)12G2]-trisester at

the indicated temperatures (a) and corresponding q_yplots (b).

exhibits a Cotton eﬀect with the maximum at 222 nm and a

negative one with the maximum at 203 nm. At the same time, the

UV-vis spectrum displays a maximum absorption positioned at

212 nm, at the midpoint between these two Cotton eﬀects with

opposite sign (Figure 8b,c). These results indicate a positive

exciton coupling²⁸and demonstrate that the supramolecular

columns self-organized in the Φ_s-o^3Dphase are helical.

(Figure 6), at low temperatures, while preserving the Φ_h^3Dand

Φ_h^iophases observed at higher temperature (Figure 7).

Figure 6 presents the wide- and small-angle XRD data, as well

as the approximate relative electron density distribution within

the z = 0 plane of the new Φ_s-o^3Dphase. The indexing of the small

angle XRD ﬁber pattern (Figure 6a,c) is consistent with P222₁

symmetry. The proposed relative electron density distribution

(Figure 6d) can only be approximated because the Φ_s-o^3Dphase

is noncentrosymmetric¹⁶and only the amplitudes of the (hk0)

diﬀraction peaks were included in the Fourier series reconstruc-

tion. In contrast with the high temperature Φ_h^3Dand Φ_h^iophases

(Figure 7), the wide-angle region of the ﬁber pattern collected in

the low temperature simple orthorhombic ordered phase exhibits

sharp diﬀraction features (Figure 6b and Supporting Information

Figures SF4, SF5). These features demonstrate the presence of

long-range intra- and intercolumnar correlations and also that

the aliphatic region jacketing the columns has signiﬁcantly lower

Remarkably, upon subsequent heating, above ∼35 °C in the

Φ_h^3Dphase, the exciton coupling changes its sign from positive

to negative (Figure 9a). The negative exciton coupling is pre-

served upon subsequent heating, above the temperature of the

Φ_h^3D-Φ_h^ioﬁrst-order phase transition, but the intensities of the

two Cotton eﬀects observed at 198 and 224 nm, respectively, are

reduced by a factor of ∼6. In combination with the WAXS ﬁber

data (Figure 7), these results suggest that the supramolecular

3D

io

columns of the Φ_hand Φ_hphase are helical and that the

Φ_h^3Dphase was templated by undulations of the soft liquid-like

aliphatic periphery. Precedent for the helical arrangement of

the aliphatic region on the periphery of supramolecular columns

is available.²⁹These undulations are generated by the helical

packing of the aromatic core region of the supramolecular

columns, which translate into periodic variations of the aliphatic

periphery. Furthermore, in the low temperature simple orthor-

hombic phase, the degree of column-to-column correlation

increased to the extent of favoring a structure with P222₁chiral

symmetry, exhibiting the opposite positive exciton coupling

(Figures 9 and 10).

3D

conformational freedom in the Φ_s-ophase. Therefore, the

addition of chiral centers to the alkyl chains induced strong cou-

pling of the helical supramolecular columns and templates the

formation of the new simple orthorhombic lattice with P222₁

chiral symmetry.

Figure 8a details the DSC traces collected with 10°/min from

the 1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide. Interestingly, the

Φ_s-o^3D-Φ_h^3Dtransition upon heating and the Φ_h^io-Φ_r-c^iotran-

sition upon cooling indicated in Figure 8a were observed in the

XRD and CD experiments (Figures 6, 7, and 9) but not in the

DSC traces collected with either slow (1°/min) or fast (10°/min)

rates. CD and XRD experiments were used to study upon heating

and cooling the chiral 1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide

dendrimer in the solid state (Figures 8b,c and 9). No linear

dichroism was observed in these perfectly reproducible experi-

ments. In the low temperature Φ_s-o^3Dphase, the CD spectrum

io

Upon cooling through the Φ_h^io-Φ_r-ctransition tempera-

ture, the CD spectra exhibit another inversion of chirality, from a

positive to a negative exciton coupling. The intensity of the two

Cotton eﬀects (Figure 9b) is weak in both phases, in direct

correlation with the diﬀuse and broad features observed in the

wide-angle region of their oriented ﬁber XRD patterns (Figure 7).

Upon further cooling, the exciton coupling stays positive in the

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Figure 6. SAXS (a) and WAXS (b) patterns collected from the oriented sample of 1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide in the Φ_s-o^3Dphase with

P222₁symmetry observed at T < 35 °C. Intensity versus scattering vector plot of the ﬁber pattern from (a), diﬀraction peaks indexing, and lattice

parameters (c). Two alternative electron density maps of the plane z = 0 of the Φ_s-o^3Dphase (d).

Φ_s-o^3Dphase, as expected, but the two Cotton eﬀects exhibit an

increase of their intensity by a factor of ∼5. Through the com-

bination of the electron density maps, phase assignment, and the

sign of the exciton coupling (Figure 10), a direct correlation

between the chirality of the supramolecular columns and the

symmetry of their three-dimensional packing is found. Consis-

methylcyclohexane, dodecane, buthanol, nor from the good

solvents tetrahydrofuran and chloroform. While previously

documented by Meijer,^15athe absence of any CD response in

solution is somewhat surprising considering the signiﬁcant

intensity of the Cotton eﬀects observed in solid state and the

typically observed direct correspondence between helical self-

assembly of chiral supramolecular dendrimers in solution and

in thin ﬁlm.^1,15c,15d

3D

tently, in the “triangular” phases, Φ_hand Φ_h^io, which are

formed by weakly or uncoupled helical columns, their chirality

3D

is negative, whereas in the rectangular phases, Φ_s-oand

The analysis of the WAXS ﬁber data shown (Figures 7 and

SF5), demonstrated that upon the Φ_h^3D-Φ_h^ioﬁrst-order phase

transition the π-stacking distance is shifted from ∼3.6 to ∼3.9 Å.

This result, in combination with CD spectroscopy on ﬁlm

(Figures 8 and 9) and molecular modeling, provided the limit of

the torsion angles proposed in Figures 11 and 12. Upon the

transition from the Φ_h^3Dphase, formed by coupled columns, to

Φ_r-c^io, their chirality is positive. These results imply that the

stronger column-to-column correlations established in the

Φ_s-o^3Dphase play an important role in favoring one dominant

handedness.

Nearly all the self-organizable C₃-symmetric dendrimers re-

ported here are achiral and would therefore provide no dichroic

signal in their helical supramolecular structure. However, 1,3,5-

Bz-[tris(3,4,5)dm8*G1]-trisamide (B2*), for example, is chiral,

and, therefore, its self-assembly in solution was probed by

CD/UV-vis experiments performed in solvophobic solvents.

In agreement with previous studies performed in heptane at

10^-4-10^-5M,^15aB2* did not exhibit any Cotton eﬀects

when the CD/UV-vis spectra were collected in the same range

of concentration in the temperature range 8-25 °C from

a variety of other solvophobic solvents, including hexane,

io

the Φ_hphase, formed by weakly or uncoupled columns, the

decoupling of columns release the steric constraints, which

limited the helical parameter to values of j<15°, and driven by

the tendency to form directional hydrogen bonds, this parameter

io

increases in the Φ_hphase to j = 60°. At the same time, the

torsion angle θ₁is increased from values smaller than 30° to

io

about 45° (Figures 11 and 12). Therefore, upon the Φ_h^3D-Φ_h

ﬁrst-order phase transition, the supramolecular dendrimers under-

go rotation mediated by the formation of directional H-bonds

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Figure 7. WAXS and SAXS patterns collected from the oriented sample

of 1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide in the Φ_h^3D(a) and Φ_h^io(b)

phases. Reconstructed relative electron density distributions of the Φ_h

Figure 8. Diﬀerential scanning calorimetry traces collected with 10°/

min from 1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide (a). CD (b) and UV

(c) spectra of 1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide in spin-coated

ﬁlm cast from hexane (concentration = 2.0%, w/v) collected on heating.

In (a) phases, phase transition temperatures and associated enthalpy

changes (in brackets in kcal/mol) are indicated. In (b), for clarity only

four representative CD spectra are shown to illustrate the change of the

sign of the two Cotton eﬀects associated with the maximum of UV

absorption from 212 nm.

3D

phase (c).

and by the decoupling of bundles of helical columns, providing a

reversible “molecular rotator” motion controlled by temperature.

Because this molecular rotator motion is generated by changes

within the packing of bundles of helical columns,^8,30it is most

probable that these changes are concerted.

electronic materials.³¹Therefore, the proposed mechanism of

self-assembly (Figure 13) has the potential, via appropriate

dendritic functionalization of other aromatic groups, which exhibit

increased charge transport, to reveal which combination of

π-stacking distance and of j and θ₁angles facilitates the maximum

value of the charge carrier mobility.³¹

One possible mechanism that can account for the inversion of

chirality at the transition from orthorhombic to hexagonal

lattices is illustrated in Figure 13. The strongest possible coupling

of the columns is achieved via interdigitation of the undulated

periphery of helical columns with opposite handedness. In a

triangular lattice, such as Φ_h^3D, symmetry constraints limit the

column-to-column correlations in the case when all the columns

have the same handedness (Figure 13). Furthermore, the sym-

metry of the hexagonal lattice disfavors close packing of helical

columns with opposite handedness. For example, if one left and

one right handed column are closely packed as shown in

Figure 13, the third one, either left or right handed, cannot form

a triangular close packing with the ﬁrst two, because such packing

will be frustrated by either the left or, respectively, the right

handed column forming the initial close-packing. In the case of

a lattice with “rectangular” symmetry (i.e., 2-fold, 4-fold sym-

metry), such packing frustrations of helical columns do not

occur. Therefore, the inversion of chirality is templated by close-

Chiral Supramolecular Spheres. The second generation

dendrimer 1,3,5-Bz-[tris(3,4,5)²dm8*G2]-trisester functionalized

with chiral chains self-organizes into the Pm3n cubic lattice.^21c,23,24

The relative electron density distribution (Figure 14), recon-

structed using the amplitudes calculated from the powder XRD

data (Figure 14a), illustrates the spherical shape of the supra-

molecular structures self-assembled from 1,3,5-Bz-[tris(3,4,5)²-

dm8*G2]-trisester. The CD and UV-vis spectra collected from

spin-coated films exhibit the signature of a negative exciton cou-

pling²⁸(Figure 14c,d). The two Cotton effects observed, positive

at 202 nm and negative at 217 nm, have roughly one-half of the

intensity of the two Cotton effects observed for the first genera-

tion 1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide dendrimer in the

3D

packing of helical columns in the low temperature Φ_s-oand

io

Φ_r-c^iophases, as suggested by the smaller lattice parameter b =

high temperature Φ_hphase (Figure 9). The maximum of the

26.1 and 26.9 Å of the Φ_s-o^3Dand Φ_r-c^iophases, respectively, in

UV absorption, which accompanies the two Cotton effects

associated with the negative exciton coupling, is observed at

212 nm in both structures (Figures 9 and 14d). These results

demonstrate that the negative chirality established for the helical

supramolecular columns self-assembled from the first generation

1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide dendrimers is preserved

3D

io

comparison with a = 31.2 and 29.1 Å in the Φ_hand Φ_h

phases, respectively (Figure 10).

The types of conformational changes shown in Figures 11, 12,

and 13 have been shown theoretically to inﬂuence the properties

of the structures, in particular of the charge transport of organic

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Figure 9. Temperature dependence of the CD spectra of 1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide in spin-coated ﬁlm cast from hexane (concentration =

2.0%, w/v) on heating (a) and cooling (b). Maximum of the UV absorption, representative SAXS ﬁber patterns, and phase assignments are indicated.

in the case of the chiral spheres self-assembled from the second

generation 1,3,5-Bz-[tris(3,4,5)²dm8*G2]-trisester dendrimers

(Figure 14e). The relatively weak Cotton effects observed in

the later structure suggest that most probably the observed CD

spectrum was generated by uncorrelated chiral spheres. In other

words, there is almost no chirality transfer between the supra-

molecular spheres. This is also supported by the observation that

the maximum correlation length, ξ, of the helical packing of the

1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide dendrimer in the high

temperature Φ_h^iois about 8 column strata. Considering that the

intensity of the Cotton effects observed for the second generation

1,3,5-Bz-[tris(3,4,5)²dm8*G2]-trisester dendrimer is roughly one-

half of those observed for the first generation 1,3,5-Bz-[tris-

(3,4,5)dm8*G1]-trisamide dendrimer (Figures 9 and 14), then

the ξ ≈ 8 dendrimers of the later structure translate into a

correlation length of the helical packing of the chiral spheres of

about 4 dendrimers.

intramolecular overcrowdedness^15c,15dgenerated by their aro-

matic substitution pattern. These intramolecular steric con-

strains, indicated in Figure 15 by the color surfaces, distort the

2D close to planar conformation of the dendrimers and template a

3D propeller- (Figure 13) or crown-like^2e(Figure 16) conforma-

tion, which in turn favor self-organization into phases with 3D

periodicity (Tables 1-3).

Helical Packing of C₃-Symmetric Dendrimers. Figure 17

summarizes the packing of columnar hexagonal phases self-

organized from helical supramolecular columns self-assembled

from dendrimers with C₃symmetry. In the case of columnar

hexagonal phases without intercolumnar order, such as Φ_hand

Φ_h^io, there are no limitations on the length of the helix. Further-

more, the two parameters c and j, which uniquely define a

specific type of helix,^2eare templated only by intracolumnar inter-

actions. The two representative examples shown in Figure 17a

illustrate that in the presence of H-bonding interactions the

helical parameter c increases from 3.4 to 3.8 Å and the helical

parameter j significantly increases from 12° to 60°, respectively.

In the case of columnar hexagonal phases with intercolumnar

order, such as the various Φ_h^3Dphases (Tables 1-3), the presence

Figure 15 illustrates the direct correlation between the self-

organization into columnar and cubic phases, and the primary

structure of the dendrimer. Cubic phases are observed only

in the second generation dendrimers with a high degree of

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3D

Figure 11. Molecular models of the aromatic core region in the Φ_h

phases self-assembled from 1,3,5-Bz-[tris(3,4,5)-G1]-trisamide dendri-

mers: three representative sets of the torsion angles θ₁and θ₂(a);

intercolumnar correlations limit the dendrimer rotation j from layer i to

i þ 1 (b); top (c) and side (d) views of two consecutive layers. In parts c

and d, the H-bonding (dotted green lines) and the steric monitoring

distances are indicated.

Figure 10. Relative electron density maps of 1,3,5-Bz-[tris(3,4,5)-

dm8*G1]-trisamide in the columnar phases observed upon heating

(a) and cooling (b). Temperature, phase, unit cell lattice dimensions

(dashed lines), and correlation of the lattice symmetry with the sign of

chirality established in CD/UV are indicated.

of stronger or weaker column-to-column correlations limits the

helical parameter j. This limitation is generated by a complex

mechanism that cooperatively minimizes the free energy both

at the intra- and at the intercolumnar levels (Figure 17b). In

Figure 17b, helical “growth” of the coupled columns, which is

limited by the column-to-column steric constrains, can be followed

from left to right. When this limit is achieved, the helical growth

continues via two possible routes of passing this discontinuity of

the helix. In the case of achiral dendrimers, the helical packing

continues either by inverting the handedness or via a step-like

discontinuity δ (Figure 17b, bottom). In the case of chiral den-

drimers, the presence of the chiral centers typically favors one

handedness. Therefore, their helical packing continues preferen-

tially via a δ step-like discontinuity.

Figure 12. Molecular models illustrating the conformation of the

aromatic core region of the supramolecular helical columns forming

io

the Φ_hphase self-assembled from 1,3,5-Bz-[tris(3,4,5)12G1]-trisa-

mide and 1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide dendrimers.

discontinuity of the helical packing at the midpoint of the helical

supramolecular column along the c-axis (Figures 7c and SF2b).

The “normal” discontinuity of the helical packing, which corre-

sponds to the two “ends” of the column that template the c-axis

repeat unit, can be attributed to one of the two possible mecha-

nisms (Figure 17b). Yet this additional “mid-point” discontinu-

ity, which exhibits a somewhat stronger modulation of the

Interestingly, the relative electron density distributions of the

Φ_h^3Dphases observed in the chiral and achiral dendrimers, 1,3,5-

Bz-[tris(3,4,5)dm8*G1]-trisamide and 1,3,5-Bz-[tris(3,4,5)12G1]-

trisamide, respectively, demonstrate that there is an additional

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Figure 13. Changes at the molecular and supramolecular level observed upon the Φ_s-o^3D-Φ_h^3Dand Φ_h^3D-Φ_h^iotransitions of the supramolecular

helical columns self-assembled from C₃dendrimers.

Figure 14. Powder XRD data (a), relative electron density distribution illustrating the spherical shape of the high electron density region of the

supramolecular assemblies (b), CD (c), and UV-vis (d) spectra of 1,3,5-Bz-[tris(3,4,5)²dm8*G2]-trisester in spin-coated ﬁlm cast from hexane

(concentraton = 1.2%, w/v) in the Pm3n cubic phase and molecular model of the chiral supramolecular spherical assemblies (e).

electron density in the case of the chiral dendrimer, remains un-

resolved. If in the case of an achiral structure the two disconti-

nuity points of the helical packing can be attributed to helix

inversion and to a helix δ step-like discontinuity, respectively,

such assignment is prohibited in the case of the 1,3,5-Bz-[tris-

(3,4,5)dm8*G1]-trisamide dendrimer by the CD spectra (Figure 9).

In addition, the increase of the c-axis with more than 43%, from

54.2 Å, in the case of the 1,3,5-Bz-[tris(3,4,5)12G1]-trisamide, to

77.3 Å, in the case of 1,3,5-Bz-[tris(3,4,5)dm8*G1]-trisamide,

also suggests that the two helical fragments forming the unit

cell of the former structure have the same handedness. There-

fore, the midpoint discontinuity is most probable a second

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Figure 15. Correlation of the dendron substitution pattern with the self-assembly and self-organization into columnar and cubic phases for the libraries

of second generation dendrimers.

Figure 16. Molecular models of selected examples of C₃-dendrimers supporting their crown-like rather than conical conformation (a), helical

supramolecular columns (b), and the column to sphere shape change at the transition from columnar hexagonal to tetragonal phase (c).

io

δ-step that can arise from periodic ﬂuctuations of the helical

packing generated by deviations of c, j, and θ₁parameters.

For example, the WAXS ﬁber patterns collected in the Φ_h

and Φ_hphases of the 1,3,5-Bz-[tris(3,4,5)-G1]-trisamide

dendrimers exhibit diﬀuse broad features in the region of π-

stacking distance (Figures 5 and 7). This demonstrates that

3D

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Figure 17. Helical packing of dendrimers with 3-fold symmetry self-assembled into columnar hexagonal phases without (a) and with (b) column-to-

column correlations.

the stacking of the dendrimers has only short-range correla-

tions.

primary structure of the building block. This strategy can be

used to design other molecular rotators, mechanical-like func-

tions, and novel molecular recognition process, as well as to

correlate charge transport properties³¹with changes of the mole-

cular conformation and 3D packing. The new supramolecular

structures reported here demonstrate the capability of the

topology of the self-assembling dendrimer as a new and simple

design strategy that is complementary to generational^7,20and

deconstruction²⁶design strategies. These design strategies are

required for the discovery of primary structures that provide access

to new supramolecular dendrimer architectures and periodic

arrays.

’ CONCLUSIONS

The self-assembly of an unexpected diversity of three-dimen-

sional columnar, cubic, and tetragonal periodic arrays from simple

trisester and trisamide dendrimers with C₃-symmetry was de-

monstrated to be determined by diﬀerent degrees of packing on

the periphery of their supramolecular structures. By comparison

with other classes of C₃-symmetric dendrimers,^15c,15dthe struc-

tures reported here have a signiﬁcantly less-packed peripheral

alkyl region and an increased conformational freedom. The com-

bination of these two main structural characteristics was shown

to generate molecular rotators, chiral spheres, helical columns

with unusual long helical pitch, and facilitate the transfer,

ampliﬁcation, and inversion of helical chirality via transitions

from strongly coupled helical columns with long helical pitch to

weakly or uncoupled helical columns with short helical pitch. The

mechanism of the thermal-reversible inversion of helical chirality

was demonstrated to be due to the change of the lattice sym-

metry: negative chirality for 2D and 3D phases with triangular

symmetry (columnar hexagonal) and positive chirality for 2D

and 3D phases with rectangular symmetry (columnar rectangular

and orthorhombic). To our knowledge, this is the ﬁrst example of

a reversible inversion of helical chirality in supramolecular den-

drimers for which the mechanism was elucidated. The mecha-

nism of self-assembly into the coupled or uncoupled helical columns

demonstrated here provides additional pathways to tune and

control the packing of helical structures via changes of the

’ ASSOCIATED CONTENT

S

Supporting Information. Experimental procedures with

b

complete, spectral, structural, and retrostructural analysis, and

complete ref 2i. This material is available free of charge via the

Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION

Corresponding Author

percec@sas.upenn.edu

’ ACKNOWLEDGMENT

Financial support by the National Science Foundation

(DMR-0548559 and DMR-0520020) and the P. Roy Vagelos

Chair at Penn is gratefully acknowledged. B.M.R. gratefully

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acknowledges funding from an NSF Graduate Research Fel-

lowship and ACS Division of Organic Chemistry Graduate

Fellowship (Roche).

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dx.doi.org/10.1021/ja110753s |J. Am. Chem. Soc. 2011, 133, 2311–2328

Article Doi

Transfer, amplification, and inversion of helical chirality mediated by concerted interactions of C3-supramolecular dendrimers

DOI: 10.1021/ja110753s

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Full text of DOI:10.1021/ja110753s

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