Expanding dendrons. The photoisomerism of folded azobenzene dendrons

Article abstract of DOI:10.1021/ja066126z

Irradiation of a folded azobenzene dendron system triggers a large, reversible structural expansion. The dendrons adopt a compact helical conformation in the stable E form owing to the E geometry of azo linkage and the syn-syn preference of the dendritic

Full text of DOI:10.1021/ja066126z

Published on Web 10/04/2006
Expanding Dendrons. The Photoisomerism of Folded Azobenzene Dendrons
Christopher J. Gabriel and Jon R. Parquette*
Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue, Columbus, Ohio 43210
Received August 23, 2006; E-mail: parquett@chemistry.ohio-state.edu
Many of the potential applications envisaged for dendrimers1
require a capacity to extrinsically manipulate global structural
properties via the long-range action of a local trigger.2 Azobenzene
chromophores have been extensively studied as conformational
triggers in dendrimers3 because the photochemical E f Z isomer-
ization induces a large change in structure and dipole moment.4
However, whereas significant conformational transitions have been
induced in peptides,5 and synthetic helical polymers6 and oligomers,7
relatively small photoinduced structural changes have been observed
Figure 1. Photoswitchable dendrons: photoisomerization induces a revers-
in azobenzene functionalized dendrimers because the flexible nature
of their structures decouples local and global motions.8 In contrast,
larger structural changes were observed in rigid polyphenylene9
and phenylacetylene10 dendrimers. These systems uniformly experi-
ence a collapse in hydrodynamic volume (up to 38%9) upon
irradiation that decreases with generation. We reasoned that the
structural consequence of a local perturbation would be maximal
within a dynamically folded dendron exhibiting a compact, folded
conformational state.11 Herein, we report a folded azobenzene
dendron system that exhibits a compact helical conformation in
the stable E form that expands significantly upon exposure to light.
The extent of the structural response to light depends on the type
of folding and increases with generation.
ible disruption of the helical folded state.
The helical conformational preference of the dendrons is
established by the E geometry of the azo linkage in conjunction
with the syn-syn conformational preferences of the 2-methoxy-
isophthalamide and pyridine-2,6-dicarboxamide dendritic repeat
units. The 2-methoxyisophthalamides (2-OMe-IPA) exhibit a syn-
syn conformational preference similar to that of the pyridine-2,6-
dicarboxamides, owing to intramolecular hydrogen-bonding inter-
actions between the amide-NHs and the 2-methoxy oxygen.12
Photoisomerization from the E to Z form disrupts this folded state
in a manner that decreases packing efficiency and leads to an
increase in hydrodynamic volume (Figure 1).
Monte Carlo conformational searching (AMBER) of 3 and 4
(Supporting Information) indicated that both the E-2- and E-3-
acylaminophenylazo linkages position the 2-methoxyisopthalamide
branch points above and below the plane defined by the focal
pyridine-2,6-dicarboxamide. This helical conformation places the
B ring protons at the first shell in close proximity to those of ring
C present on the adjacent dendritic branch on the second shell.
Two-dimensional NOESY 1H NMR spectroscopy in CDCl3 revealed
the presence of close contacts between protons Hb and Hc in 3
and between H4b and Hc in 4 (Figures 2 and 3). Similar close
contacts were evident between protons H1b and Hc on adjacent
branches of the first and second shells of third generation dendron
5. Additionally, NOESY cross-peaks occurred between H2b and
H3b of the first shell with Hd on ring D of the third shell. The
corresponding cross-peaks between protons on rings A and B of
the focal phenylazo linkage in 2- and 3-CBzN-C6H4N2-[G1] (1a/
b) and 2-CBzN-C6H4N2-[G2] (2) were not observed, consistent with
this conformational preference. NOESY cross-peaks between the
amide NH protons and both the methoxyl and aromatic protons of
the 2-OMe-IPA linkages were also observed in all dendrons. These
Figure 2. NOESY 1H NMR cross-peaks for photoswitchable dendrons 1-5.
close contacts reflect the occurrence of both the syn-syn and syn-
anti forms in the conformational ensemble that occur as a
consequence of the greater flexibility of 2-OMe-IPA linkage as
compared with the 2,6-pyridine-2,6-dicarboxamide branch point,
which does not exhibit these cross-peaks in 3 and 5.13
Whereas the m-acylamino substituted azobenzene linkages in 1b
and 4 exhibit π f π* and very weak n f π* absorptions at ca.
320 and 440 nm, respectively,14 the o-acylamino substitution in
structures 1a, 2, 3, and 5 structures induces a large red-shift of the
π f π* band to ca. 400 nm (Supporting Information). This causes
the long wavelength component of this transition to overlap with
the n f π* band centered near 440 nm.15 Irradiation of the dendrons
with 350 nm light induced an E f Z isomerization to take place,
as evidenced by a decrease in the 320/400 nm absorption. The
amount of Z-isomer at the photostationary state (PSS) was
1
determined by H NMR for 1-4, and by UV-vis for 5,16 using
the formula (A0 - APSS(320/400))/A0) where A0 and APSS(320/400)
9
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J. AM. CHEM. SOC. 2006, 128, 13708-13709
10.1021/ja066126z CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
disrupts this packing arrangement in a manner that induces a larger
increase in volume compared with that of the o-linked dendron 3.
In conclusion, we have described a series of azobenzene dendrons
that adopt compact helical conformations in the E-azo form.
Photoisomerization disrupts this folded conformation in a manner
that decreases the packing efficiency and results in large structural
expansions. Folded photoresponsive materials provide an op-
portunity to investigate how local conformational fluctuations can
be correlated with global structural motions to maximize the
structural response to a triggering event.
Acknowledgment. This work was supported by the National
Science Foundation (Grant CHE-0239871).
1
Figure 3. 2D H-1H NOESY spectra of 4 and 5.
Table 1. Hydrodynamic Volumesa of E State (Dark) and at the
PSS
Supporting Information Available: Synthetic procedures and
characterization, NOESY and UV-vis spectra, and thermal Z f E
activation parameters/rates for 1-5; stereodepictions of lowest energy
conformers of 3 and 4. This material is available free of charge via the
Internet at http://pubs.acs.org.
darkc
irradiated at 350 nm
%Zb
UV
%Zb
NMR
Vv
Vh
Vh(PSS)
(Å3)
Vh/Vvw
(PSS)
compd
(Å3)
(Å3)
Vh/Vvw
Vh/Vh(PSS)
1a
1b
2
3
4
58
23
39
38
33
20
59
63
610
757 1.24
827
564
2056
1877
2632
5364
1.36
0.83
1.6
1.7
2.12
2.19
1.13 ( 0.035
0.52 ( 0.120
0.97 ( 0.045
1.32 ( 0.048
1.77 ( 0.140
1.47 ( 0.016
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678 1076 1.59
41 1285 2228 1.73
38 1103 1371 1.24
53 1238 1496 1.21
2453 3666 1.49
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1
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