Symmetrically backfolded molecules emulating the self-similar features of a Sierpinski triangle

Article abstract of DOI:10.1039/c9ob00958b

We synthesized self-similar molecules (G3 and G2; based on phenylalkynyl backbones) with symmetrically backfolded shapes inspired by the famous fractal of a Sierpinski triangle. Unlike the more traditional, starburst dendrimers, the centripetal-shaped Sierpinski molecules feature side branches symmetrically bent away from the growth direction of the main branch, thus contrasting the natural-tree shape. Molecule G3 exhibits three distinct levels of the structural hierarchy comprising the primary, secondary and tertiary branches, while the smaller G2 contains only features of the 1^st and 2^nd orders. In spite of the much larger conjugated backbone of G3, its solution UV-vis absorption and fluorescence exhibit no red shift relative to G2. In a test of nitrobenzene sensing, a thin film of G3 deposited from THF was more sensitively quenched in fluorescence than the smaller G2.

Full text of DOI:10.1039/c9ob00958b

View Article Online
View Journal
Organic &
Biomolecular
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: J. Hu, Y. Sun, R.
XIAO, S. Cheng, J. He, M. Zeller, W. Wong and Z. Xu, Org. Biomol. Chem., 2019, DOI:
10.1039/C9OB00958B.
This is an Accepted Manuscript, which has been through the
Volume 14 Number
1 7 January 2016 Pages 1–372
Royal Society of Chemistry peer review process and has been
accepted for publication.
Organic &
Biomolecular
Chemistry
www.rsc.org/obc
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1477-0520
COMMUNICATION
Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
rsc.li/obc

Please do not adjust margins
Organic & Biomolecular Chemistry
Page 1 of 5
View Article Online
DOI: 10.1039/C9OB00958B
ARTICLE
Symmetrically Backfolded Molecules Emulating the Self-Similar
Features of Sierpinski Triangle†
Jieying Hu,a Yan-Qiong Sun,b Ran Xiao,c Shengxian Cheng,c Jun He,*a Matthias Zeller,d Wai-Yeung
Received 00th January 20xx,
Accepted 00th January 20xx
Wong,*e and Zhengtao Xu*c
DOI: 10.1039/x0xx00000x
We synthesize self-similar molecules (G3 and G2; based on phenylalkynyl backbones) with symmetrically backfolded shapes
inspired by the famous fractal of the Sierpinski triangle. Unlike the more traditional, starburst dendrimers, the centripetal-
shaped Sierpinski molecules feature side branches symmetrically bent away from the growth direction of the main branch,
thus contrasting the natural-tree shape. Molecule G3 exhibits three distinct levels of structural hierarchy comprising the
primary, secondary and tertiary branches, while the smaller G2 contains only features of the 1st and 2nd orders. In spite of
the much larger conjugated backbone of G3, its solution UV-vis absorption and fluorescence exhibits no red shift relative to
G2. In a test of nitrobenzene sensing, the thin film of G3 deposited from THF was more sensitively quenched in fluorescence
than the smaller G2.
radiating, starburst shape,13-20 but with the growing numbers of
Introduction
The relevance of the mathematically famous fractal of the
Sierpinski triangle to molecular design is illustrated in Fig. 1. The
hierarchy (or iteration) of the triangles is superimposable with
a ternary tree that features a symmetrically backfolded
geometry,1-5 in which the subbranches are bent away from the
pointing direction of the main branches. Such a backfolded
geometry contrasts with the starburst shape of conventional
dendrimers, with all branches in the latter radiating away from
the center (e.g., H6L3; Fig. 1), mimicking the growth direction of
a biological tree. Even though symmetrically backfolded
molecules have been studied by chemists (e.g., L1 and L2 in Fig.
1 and other examples6-12), most examples remain rather simple
systems consisting of only primary and secondary branches
(e.g., L1 and L2). The synthesis of Sierpinski molecules of higher
iterations (e.g., G3 in Fig. 2, with three generations of branches)
has not been reported.
The lack of synthetic efforts on higher-generation Sierpinski
molecules is curious, and stands in sharp contrast not only with
the numerous reports on conventional dendrimers of the
Fig. 1 Two backfolded molecules (L1 and L2), the starburst dendrimer
H6L3, and a Sierpinski triangle (with the equivalent ternary tree
shown in red). L2 resembles the Sierpinski triangle, whereas H6L3 has a
radiant, geometrically different form. The backfolded molecules (L1 and
L2) can be deconstructed into tritopic subunits attached to the inner
fragments (shown in green).
a. School of Chemical Engineering and Light Industry, Guangdong University of
Technology, Guangzhou 510006, Guangdong, China
b. College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, China
c. Department of Chemistry, City University of Hong Kong, 83 Tat Chee Avenue,
Kowloon, Hong Kong, China. E-mail: zhengtao@cityu.edu.hk.
d. Department of Chemistry, 560 Oval Drives, Purdue University, West Lafayette,
Indiana, 47907 United States.
supramolecular assemblies related to the Sierpinski fractal.21-28
On a more technical plane, the backfolded shape of Sierpinski
molecules offers unique opportunities in molecular design (as
outlined in earlier works).1-5, 29 A comparison of backfolded L2
and starburst H6L3 is illustrative. While the end groups of H6L3
consist of six chemically equivalent, separate carboxyl groups,
L2 offers two sets of terminal sites: the –CN groups on the three
e. Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Hong
Kong, China; HKBU Institute of Research and Continuing Education, Shenzhen
Virtual University Park, Shenzhen, 518057, China. E-mail:
rwywong@hkbu.edu.hk
† Electronic Supplementary Information (ESI) available: experimental details
including synthetic procedures, NMR, IR, MS and elemental analysis data, cyclic
voltammetry curve and tabulated crystallographic data. CCDC 1570192 contains
the supplementary crystallographic data for G2. See DOI: 10.1039/x0xx00000x
Please do not adjust margins

Please do not adjust margins
Organic & Biomolecular Chemistry
Page 2 of 5
ARTICLE
Journal Name
impacts photophysical properties (e.g., fluorescVeienwcAer/tieclxecOintloinne
lifetime, charge/energy transfer). In DOadI:d10it.1io03n9,/Ct9hOeB00e9i5g8hBt
thioether groups at the G3 branches can serve as metal-binding
sites; while the long, floppy alkyl chains might also impart liquid
crystal properties in combination with the rigid Sierpinski
backbone.
Experimental
This is provided in the Supporting Information.
Results and discussion
The synthesis of G3 builds on a convergent strategy extensively
utilizing the well-established Sonogashira reaction,30-36 in
conjunction with the masking/demasking of the iodo and
terminal alkyne groups by means of the triazene37-40 and the
trimethylsilyl (TMS) groups, respectively.41 Among the many
precursors, molecule 9 is a cornerstone for the overall assembly
scheme. As shown in Fig. 3, the synthesis of 9 begins with a
carboxylic acid (2) formation from 3,5-dibromo-4-
iodobenzonitrile (1), followed by esterification with 1-decanol
to give 4 in 80% yield (via the acid chloride 3). Sonogashira
cross-coupling of 4 with 1 equivalent of trimethylsilylacetylene
(TMSA) selectively displaces the iodo group to provide 5 in 76%
yield, indicating that the higher reactivity of the iodo group
overrides the steric hindrance from the flanking Br atoms. The
two Br- groups in 5 were then reacted with 2 equivalents of 4-
triazenephenylacetylene (S4, see Fig. S1 for its preparation) at
90 °C to afford compound 6 in 60% yield. After unmasking the
TMS group of 6 with tetrabutylammonium fluoride (TBAF), the
resultant terminal alkyne compound 7 was coupled with the
iodo compound S5 to afford 8 in 56% yield. Heating 8 in methyl
iodide at 120 °C in a sealed vessel then replaces the triazene
units by the iodo groups, generating 9 in 75% yield.
Fig. 2 a) A schematic model of molecule G3: a self-similar Sierpinski molecule
featuring two carboxylate ester groups at the primary branch, together with
six ester groups and 8 thioether groups at the secondary and tertiary
branches, respectively. b) An energy-minimized conformation of molecule
G3. Selected cofacial C···C contacts (3.438-3.643 Å) are shown by dashed
lines. The geometry was optimized by classical molecular mechanics using the
FORCITE package of the Materials Studio software. The Universal force field
was selected, with the electrostatic and van der Waals terms being set as
follows: Summation method: atom-based; Truncation method: cubic spline;
Cutoff distance: 12.5 Å; Spline width: 1 Å; Buffer width: 0.5 Å.
With the key precursor 9 in hand, the assembly of G3
becomes relatively straightforward, which involves installing
the tertiary branches and a Glaser coupling that oxidatively
homocouples the terminal alkyne (Fig. 4). Specifically, molecule
11 was prepared by coupling 4-ethynylthioanisole (S8) with the
above-mentioned molecule 5 (followed by the removal of the
primary branches and the –MeS units on the six secondary
branches, with the backfolded configuration arranging the –
MeS groups in three chelation pairs. The backfolded Sierpinski
molecules have also proven interesting building blocks for
extended networks, e.g., each molecule behaves as a collection
of starburst units (e.g., the regular tritopic unit; see also L1 and
L2 in Fig. 1); and the resultant networks, in spite of their unusual
complexity, can be deconstructed into subnets corresponding
to the starburst units of the molecule.29
To fill in the gap in the study of higher-generation Sierpinski
molecules, and to draw more attention to this unconventional
class of self-similar molecules, we here report the synthesis of
molecule G3 (Fig. 2). G3 was designed with consideration to
synthetic feasibility and functionality. For example, a total of six
long alkyl groups (i.e., n-decyl, n-C10H11-) were installed at the
(two) primary and (four) secondary termini to improve
solubility; and extra phenylacetylene spacer units are built into
the primary and secondary branches to alleviate steric
hindrance. Also, the large, conjugated phenylene-acetylene
backbone might help future studies on how the Sierpinski shape
O
OH
O
OC10H21
Pd(PPh3)4, CuI
O
OC10H21
CN
KOH
H2O2
C10H21OH
SOCl2
100%
3
Br
Br
Br
Br
Br
Br
Br
Br
pyridine
80%
TMSA, Et3N,
PPh3,90°C,
76%
100%
I
4
I
1
I
2
5
TMS
OC10H21
NEt2
N
O
O
OC10H21
O
Pd(PPh3)2Cl2
CuI, PPh3, Et3N
N
TBAF
80%
+
+
5
7
90 °C
60%
Et2N3
N3Et2
Et2N3
N3Et2
H
TMS
H
S4
6
7
OC10H21
O
OC10H21
I
Pd(PPh3)2Cl2
CuI, PPh3, Et3N
CH3I,
120 °C
90 °C
75%
Et2N3
N3Et2
I
56%
I
TMS
S5
8
9
TMS
TMS
Fig. 3 The synthetic steps for molecule 9.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 5
Organic & Biomolecular Chemistry
Please do not adjust margins
Journal Name
ARTICLE
View Article Online
DOI: 10.1039/C9OB00958B
O
OR
O
OR
CuI, PPh3
S
Pd(PPh3)2Cl2
TBAF
85%
+
5
Et3N, 90°C
80%
S
S
S
S
H
TMS
H
S8
10
11
9
+
37%
Pd(PPh3)2Cl2
CuI, PPh3,Et3N,
90°C
Cu(OAc)2
80°C
85%
O
OR
S
S
O
OR
O
O
S
S
OR
S
S
OR
S
S
TMS
12
CuCl, DMF
RO
O
G3
G2
Fig. 4 Synthetic steps for G3 and G2 (R is an n-decyl group: n-C10H21-).
TMS group to recover the terminal alkyne function, with overall
yields over 60%). Subsequently, 11 was subjected to a
Sonogashira coupling with 9 to generate the precursor 12.
Notice however that the reaction between 11 and 9 required
the rather stringent conditions of refluxing for 40 h, and the
yield was modest (37%). Finally, compound 12 (without
removing the TMS group) directly underwent a homocoupling
reaction in the presence of CuCl in N,N-dimethylformamide
(DMF) to afford the target G3 as an orange solid in 52% yield.
The product G3 was extensively characterized, e.g., by
Fig. 5 A stack (panel a) and a layer (panel b) of molecules in the crystal
structure of G2. The π···π interactions are shown as pink sticks and other
intermolecular contacts are shown as black dotted lines. Color code: S,
orange; O, red; C, grey.
For comparison, the lower generation Sierpinski molecule
G2 was synthesized (see Fig. 4 and ESI for the procedure). The
G2 crystal structure (space group P-1; Table 1) features the
molecule in the achiral, Ci-symmetric butterfly conformation
(cf. the C2-symmetric, dihedral conformation found in a related
system1, see Fig. S3 for the butterfly and dihedral
atropisomers). The side-arm phenyl units from the two butterfly
halves of G2 form face-to-face overlaps with C···C (e.g., 3.421
and 3.500 Å) and C···S (e.g., 3.669 Å) contacts. The molecules of
G2 stack along the crystallographic a axis: the neighboring
molecules are significantly offset (i.e., slipped by 67.5°), with
face-to-face contacts between the backbone phenylacetylene
units (e.g., C···C distances: 3.315 and 3.433 Å; Fig. 5a). The G2
stacks as such were aligned along the c axis to form layers of the
aromatics, with distinct C···S (3.811 Å) interactions across the
individual stacks. The layers are piled along the b axis, with
dimeric aggregates of the long aliphatic chains found in-
between (Fig. 5b). Across the layers, no short contacts were
observed between the aromatics and the S···S distance (4.259
Å) is also well above the VDW diameter of sulfur (3.6 Å). The
lamellar character (along the b or 010 plane) is also reflected in
the intensity profile of the PXRD patterns (Fig. 6), as in the
weakened 001 peak relative to the 010 peak.
A comparison of the solution UV-vis absorption and
emission spectra of G2 and G3 (Fig. 7) unveils some surprises.
The UV-vis wavelengths of maximum absorption (λmax) of
conjugated systems generally arise from π→π* transitions from
the HOMO to the LUMO, and often red-shift significantly with
added conjugation. In spite of the larger π-electron backbone of
G3, its λmax is, surprisingly close to that of G2 (with λmax,G3 = 328
nm and λmax,G2 = 320 nm; Fig. 7a; molar extinction coefficients:
εG3 = 2.1x105; εG2 = 8.03x104 M-1cm-1). A bathochromic side band
(most likely from n→π* transitions) was also observed for both
1
solution H NMR, 13C NMR, 2D COSY, DEPT spectra and cyclic
voltammetry (Fig. S2 and elsewhere in ESI). In addition, high-
resolution mass spectrometry HRMS found, e.g., 3478.4020
(100%) for M + H+ (3478.435 calculated for C234H218O12S8 + H+),
and the CHN elemental analysis results are also consistent with
the proposed structure, i.e., calculated for C234H218O12S8: C,
80.79; H, 6.32; found: C, 80.42; H, 6.57.
Table 1 Crystal Data and Structure Refinement Parameters for G2.
Compound
Chemical Formula
Formula weight
Temperature (K)
Crystal size (mm)
Space group
a (Å)
G2
C74H74O4S4
1155.57
100 (2)
0.50 × 0.17 × 0.10
P -1
8.960 (2)
11.752 (3)
15.156 (4)
90.717 (4),
103.972 (3)
92.265 (4)
1547.0 (6)
1
1.240
1.033
0.0438
0.1037
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
ρcalcd (g cm-1)
GOF
R1a [I >2σ(I)]
wR2b [I >2σ(I)]
aR1=(F0- Fc)/F0; bwR2=[w(F02- Fc2)2/w(F02)2]1/2
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Organic & Biomolecular Chemistry
Page 4 of 5
ARTICLE
Journal Name
emission spectra (e.g., Fig. 7b; with two close peaks with λmax
=
View Article Online
510 and 525 nm) are similar to that of thDeOsIo: 1lu0t.1i0o3n9/sCa9mOpBl0e0.9T5h8Be
persistent crystalline phase of G2 (e.g., even in fast precipitation
from the rotary evaporator; as revealed by the PXRD pattern of
Fig. 6c) can be understood in light of the stacking and packing
forces unveiled from the above-mentioned single crystal
structure (Fig. 5). In particular, the distinct π-π interactions
between the side-arm phenyl units help to forge the well-
defined, rigid conformation of the G2 molecules; the
intermolecular π-π as well as CH-π interactions, on the other
hand, direct the packing motif of their super-structure in the
solid state.
By contrast, the solid samples of the more complex G3,
however, exhibits poor crystallinity—even in solid samples
obtained from carefully controlled diffusion experiments (Fig. 6,
pattern d). The resistance of G3 molecules against
crystallization can be rationalized by the complex branched
structure, which allows many energetically similar
conformations to exist both in solution and in the solid state,
thereby frustrating periodic ordering in the aggregates of the
G3 molecules. For illustrating the diverse and flexible
conformations of G3, the computationally optimized geometry
of G3 (see Fig. 2, panel b) is informative. In particular, the
conformation in Fig. 2b feature a pair of cofacially-stacked
bezenoid units of the tertiary branches; but, unlike the
decidedly rigidifying cofacial pairs in the smaller G2 molecule
(Fig. 5b), the cofacial pair here does not prevent the rotation of
the side arms in the larger structure of G3 (e.g., the secondary
branches can readily turn, and pull the pair apart, as is in the
case of the left moiety of the G3 molecules in Fig. 2b). Also, the
more numerous and irregularly arranged alkyl chains of G3
molecules further complicate the packing motifs of the
molecules to disfavor the ordered array required by
crystallization.
Fig. 6 X-ray powder patterns (Cu Kα, λ=1.5418 Å): (a) calculated from the
single crystal structure of G2; (b) a powder sample obtained from rotary-
evaporating a dichloromethane solution of G2; (c) a ground crystal sample--
the crystals were obtained by slowly evaporating a dichloromethane (e.g., 3.0
mL) solution of G2 (e.g., 15 mg); (d) a powder sample of G3 obtained from
slowly diffusing acetonitrile (1.0 mL) into a toluene (2.0 mL) solution of G3 (10
mg) in glass tube (diameter: 8.0 mm) over three days. The pattern was
measured on a Rigaku SmartLab X-ray diffractometer operating at 40 kV, 30
mA, and a scan rate of 10 °/min.
cases, with the absorption edge of G2 slightly red-shifted
relative to the larger G3. Unexpected observations continue in
the fluorescent spectra. In THF, the small G2 exhibits a broad
emission peaked at 516 nm, while G3 exhibits a major emission
at 498 nm and a broad shoulder at about 525 nm: as shown in
Fig. 7b, the emission profile of G2 is red-shifted relative to G3.
G2 solid samples from various solvents/conditions form the
same crystalline phase (e.g., Fig. 7, patterns b and c), and their
Fig. 7 (a) UV-vis spectra of G2 (red graph) and G3 (black graph) in THF (10-5 M; slit width: 20 nm); (b) Emission spectra of G2 in THF (λex= 360 nm), G2 in the
solid state (λex= 380 nm), G3 in THF (λex= 380 nm); (c) Emission spectra of G3 solid samples deposited from THF, ethyl acetate (EA), CHCl3 and ethanol (λex= 360
nm); (4) Emission spectra of G2 and G3 solid samples (both deposited from THF) before and after nitrobenzene (NB) exposure (λex= 360 nm), with corresponding
photographs of the samples under UV radiation (λ= 365 nm) shown in panels (e) and (f), respectively. Slit width for excitation beam: 2 nm.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 5
Organic & Biomolecular Chemistry
Please do not adjust margins
Journal Name
ARTICLE
3. Y.-Q. Sun, C. Yang, Z. Xu, M. Zeller and A. D. Hunter, Cryst. Growth Des., 2009, 9,
As the luminescent characteristics of the molecules closely
depend on their conformations as well as the intermolecular
arrangements, the highly changeable emission features as
observed of G3 molecules in the solid state (Fig. 7c) are not
unexpected. For example, their emission features depend on
the solvents used for deposition. As shown in Fig. 7c, the solid
sample from THF fluoresces at distinctly longer wavelengths
(relative to other solvents). The variable solid state emission
profiles of G3 can be (tentatively) ascribed to the diverse
View Article Online
1663-1665.
DOI: 10.1039/C9OB00958B
4. J. He, M. Zeller, A. D. Hunter and Z. Xu, CrystEngComm, 2015, 17, 9254-9263.
5. J. Cui, Y.-L. Wong, M. Zeller, A. D. Hunter and Z. Xu, Angew. Chem., Int. Ed., 2014,
53, 14438-14442.
6. C. R. Woods, M. Benaglia, S. Toyota, K. Hardcastle and J. S. Siegel, Angew. Chem. Int.
Ed., 2001, 40, 749-751.
7. R. Nandy, M. Subramoni, B. Varghese and S. Sankararaman, J. Org. Chem., 2007, 72,
938-944.
8. V. Narayanan, S. Sankararaman and H. Hopf, Eur. J. Org. Chem., 2005, 2740-2746.
9. J. A. Marsden, M. J. O'Connor and M. M. Haley, Org. Lett., 2004, 6, 2385-2388.
10. J. D. Bradshaw, L. Guo, C. A. Tessier and W. J. Youngs, Organometallics, 1996, 15,
2582-2584.
packing motifs engendered by its complex branched shape. On 11. M. Laskoski, W. Steffen, J. G. M. Morton, M. D. Smith and U. H. F. Bunz, J.
Organomet. Chem., 2003, 673, 25-39.
12. W. Yang, G. Longhi, S. Abbate, A. Lucotti, M. Tommasini, C. Villani, V. J. Catalano, A.
the other hand, the absence of red shift in the solution spectra
of G3 (relative to the much smaller G2) runs counter to the
general trend of higher-generation dendrimers to absorb and
emit at longer wavelengths42-44 (albeit to a lesser degree for
meta-conjugated systems45, 46). We suspect that the diacetylene
bridge in G2 offers an efficient push-pull pathway between the
sulfur and the carboxyl groups to facilitate π-electron
delocalization, while such a push-pull effect becomes weaker in
the longer, zigzag path offered by G3 (see Fig. S4). Further
studies on the excitation dynamics of Sierpinski molecules are
needed for elucidating the unexpected absence of red shift
observed in the solution spectra of G3, and for uncovering other
potentially interesting photophysical properties.
O. Lykhin, S. A. Varganov and W. A. Chalifoux, J. Am. Chem. Soc., 2017, 139, 13102-
13109.
13. E. Buhleier, W. Wehner and F. Vögtle, Synthesis, 1978, 155-158.
14. D. A. Tomalia, A. M. Naylor and W. A. Goddard, III, Angew. Chem., 1990, 102, 119-
157.
15. J. S. Moore, Acc. Chem. Res., 1997, 30, 402-413.
16. A. W. Bosman, H. M. Janssen and E. W. Meijer, Chem. Rev., 1999, 99, 1665-1688.
17. C. C. Lee, J. A. MacKay, J. M. J. Fréchet and F. C. Szoka, Nat. Biotechnol., 2005, 23,
1517-1526.
18. M. Gingras, J.-M. Raimundo and Y. M. Chabre, Angew. Chem. Int. Ed., 2007, 46,
1010-1017.
19. S.-H. Hwang, C. D. Shreiner, C. N. Moorefield and G. R. Newkome, New J. Chem.,
2007, 31, 1192-1217.
20. M. Kozaki, S. Morita, S. Suzuki and K. Okada, J. Org. Chem., 2012, 77, 9447-9457.
21. G. R. Newkome, P. Wang, C. N. Moorefield, T. J. Cho, P. P. Mohapatra, S. Li, S.-H.
Hwang, O. Lukoyanova, L. Echegoyen, J. A. Palagallo, V. Iancu and S.-W. Hla, Science,
2006, 312, 1782-1785.
The complex branched shape of G3 may also account for its
enhanced sensitivity to nitrobenzene (e.g., by allowing for
easier guest penetration into the solid). As shown in Fig. 7d-f,
the fluorescence of the G3 thin film (drop-cast onto filter paper
from a THF solution: 0.3 mg/mL; 1.0 µL) was largely quenched
after being suspended over nitrobenzene in a capped vial (e.g.,
at 80 °C for 5 minutes); by comparison, a G2 thin film (similarly
cast from THF: 0.3 mg/mL; 1.0 µL) shows lesser fluorescence
quenching, as is both visually and spectroscopically observed.
22. P. W. K. Rothemund, N. Papadakis and E. Winfree, PLoS Biol., 2004, 2, 2041-2053.
23. J. Shang, Y. Wang, M. Chen, J. Dai, X. Zhou, J. Kuttner, G. Hilt, X. Shao, J. M. Gottfried
and K. Wu, Nat. Chem., 2015, 7, 389-393.
24. D. Nieckarz and P. Szabelski, Chem. Commun., 2016, 52, 11642-11645.
25. X. Zhang, N. Li, G.-C. Gu, H. Wang, D. Nieckarz, P. Szabelski, Y. He, Y. Wang, C. Xie,
Z.-Y. Shen, J.-T. Lu, H. Tang, L.-M. Peng, S.-M. Hou, K. Wu and Y.-F. Wang, ACS Nano,
2015, 9, 11909-11915.
26. G. Gu, N. Li, L. Liu, X. Zhang, Q. Wu, D. Nieckarz, P. Szabelski, L. Peng, B. K. Teo, S.
Hou and Y. Wang, RSC Adv., 2016, 6, 66548-66552.
27. X. Zhang, N. Li, L. Liu, G. Gu, C. Li, H. Tang, L. Peng, S. Hou and Y. Wang, Chem.
Commun., 2016, 52, 10578-10581.
28. N. Li, G. Gu, X. Zhang, D. Song, Y. Zhang, B. K. Teo, L.-m. Peng, S. Hou and Y. Wang,
Chem. Commun., 2017, 53, 3469-3472.
29. C. Yang, Y.-L. Wong, R. Xiao, M. Zeller, A. D. Hunter, S.-M. Yiu and Z. Xu,
ChemistrySelect, 2016, 1, 4075-4081.
Conflicts of interest
There are no conflicts to declare.
30. Z. Luo, N. Zhu and D. Zhao, Chem. - Eur. J., 2016, 22, 11028-11034.
31. D. Zhao and J. S. Moore, Chem. Commun., 2003, 807-818.
32. Y. Jin, Q. Wang, P. Taynton and W. Zhang, Acc. Chem. Res., 2014, 47, 1575-1586.
33. R. Chinchilla and C. Najera, Chem. Rev., 2007, 107, 874-922.
34. R. Chinchilla and C. Najera, Chem. Soc. Rev., 2011, 40, 5084-5121.
35. N. M. Jenny, M. Mayor and T. R. Eaton, Eur. J. Org. Chem., 2011, 2011, 4965-4983.
36. A. M. Thomas, A. Sujatha and G. Anilkumar, RSC Adv., 2014, 4, 21688-21698.
37. D. B. Kimball and M. M. Haley, Angew. Chem., Int. Ed., 2002, 41, 3338-3351.
38. Y. Zhang, D. Cao, W. Liu, H. Hu, X. Zhang and C. Liu, Curr. Org. Chem., 2015, 19, 151-
178.
39. Z. Wu and J. S. Moore, Tetrahedron Lett., 1994, 35, 5539-5542.
40. B. T. Holmes, W. T. Pennington and T. W. Hanks, Synth. Commun., 2003, 33, 2447-
2461.
41. Z. Xu, M. Kahr, K. L. Walker, C. L. Wilkins and J. S. Moore, J. Am. Chem. Soc., 1994,
116, 4537-4550.
Acknowledgements
This work was supported by an SRG grant of City University of
Hong Kong (Project 7004820), the National Natural Science
Foundation of China (21871061), Local Innovative and Research
Teams Project of Guangdong Pearl River Talents Program and
Science
and
Technology
Program
of
Guangzhou (201807010026). W.-Y.W. thanks the financial
support from Hong Kong Research Grants Council (HKBU
12302114). M.Z. thanks the United States National Science
Foundation (Grant CHE 0087210), Ohio Board of Regents Grant
CAP-491, and Youngstown State University for funding for the
single crystal diffractometer.
42. J. S. Melinger, Y. Pan, V. D. Kleiman, Z. Peng, B. L. Davis, D. McMorrow and M. Lu, J.
Am. Chem. Soc., 2002, 124, 12002-12012.
43. Z. Peng, Y. Pan, B. Xu and J. Zhang, J. Am. Chem. Soc., 2000, 122, 6619-6623.
44. M. Lu, Y. Pan and Z. Peng, Tetrahedron Lett., 2002, 43, 7903-7906.
45. K. M. Gaab, A. L. Thompson, J. Xu, T. J. Martinez and C. J. Bardeen, J. Am. Chem.
Soc., 2003, 125, 9288-9289.
46. R. Kopelman, M. Shortreed, Z.-Y. Shi, W. Tan, Z. Xu, J. S. Moore, A. Bar-Haim and J.
Klafter, Phys. Rev. Lett., 1997, 78, 1239-1242.
Notes and references
1. Y.-Q. Sun, J. He, Z. Xu, G. Huang, X.-P. Zhou, M. Zeller and A. D. Hunter, Chem.
Commun., 2007, 4779-4781.
2. Z. Xu, Y.-Q. Sun, J. He, M. Zeller and A. D. Hunter, in The concept and use of a class
of branchy molecules with centripetal self-similarity, Nova Science Publishers, Inc.,
Hauppauge, NY 2008.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins