Dendrimers and dendrons represent an important class of
molecules which act as gel precursors since their self-replicat-
ing structure provides a suitable structural scaffold for multi-
ple intermolecular interactions, leading to effective gelation
and efficient sensing.7 However, most of the reported cases of
dendrimer based gel systems involves the presence of con-
ventional gelation motifs such as long alkyl chains, steriodal
groups, and a large number of amide groups for facilitating
the intermolecular interactions.8 Low molecular weight,
nonconventional dendritic gelators with light emitting prop-
erties are rarely reported in literature.9
Scheme 1. Reaction between Anthraldehyde and Hydrazide of
First Generation AB3 Type Poly(aryl ether) Dendron
Here, we have shown that poly(aryl ether) dendrons with
an anthracene moiety attached to the core through a acylhy-
drazone linkage can form an ‘instant gel’ in the absence of the
above-mentioned conventional gelation motifs. Most impor-
tantly, the resulting gel undergoes a gel-to-sol transition,
accompanied by a color change from deep yellow to bright
red, in the presence of fluoride ions. The onset of the color
change was noticed at a fluoride ion concentration as low as
0.1 equiv with respect to the gelator concentration. This
provides an opportunity for the ‘naked eye’ detection of
fluoride ions, which is one of the targeted anions due to its
important role in biological systems. While a few anion-
sensing organogels are reported,10 herein we demonstrate the
first example of a dendrimer based fluorescent LMWG which
undergoes a reversible solꢀgel transition in the presence of
fluoride ions at very low concentrations of the analyte.
We have synthesized the first and second generations
(G1 and G2, respectively) of AB2 and AB3 type poly(aryl
ether) dendrons according to a reported procedure,9d and
anthracene was attached to the dendron through a acylhy-
drazone spacer unit (Scheme 1) (synthetic procedure is
given in the Supporting Information (SI)). The structures
of the poly(aryl ether) dendron derivatives utilized in the
present study are shown in Figure 1.
(6) (a) Babu, S. S.; Kartha, K. K.; Ajayaghosh, A. J. Phys. Chem.
Lett. 2010, 1, 3413. (b) Praveen, V. K; George, S. J; Varghese, R.;
Vijayakumar, C; Ajayaghosh, A. J. Am. Chem. Soc. 2006, 128, 7542. (c)
Srinivasan, S.; Babu, P. A.; Mahesh, S.; Ajayaghosh., A. J. Am. Chem.
Soc. 2009, 131, 15122. (d) Diring, S; Camerel, F.; Donnio, B.; Dintzer,
T.; Toffanin, S.; Capelli, R.; Muccini, M.; Ziessel, R. J. Am. Chem. Soc.
2009, 131, 18177. (e) Sugiyasu, K.; Fujita, N.; Shinkai, S. J. Mater.
Chem. 2005, 15, 2747. (f) Wang, X.-J.; Xing, L.-B.; Cao, W.-N.; Li,
X.-B.; Chen, B.; Tung, C.-H.; Wu, L.-Z. Langmuir 2011, 27, 774.
(7) Rosen, B. M.; Wilson, C. J.; Wilson, D. A.; Peterca, M.; Imam,
M. R.; Percec, V. Chem. Rev. 2009, 109, 6275.
(8) (a) Ji, Y.; Luo, Y. F.; Jia, X. R.; Chen, E. Q.; Huang, Y.; Ye, C.;
Wang, B. B.; Zhou, Q. F.; Wei., Y. Angew. Chem., Int. Ed. 2005, 44,
6025. (b) Kuang, G.; Ji, Y.; Jia, X.; Chen, E.; Gao, M.; Yeh, J.; Wei, Y.
Chem. Mater. 2009, 21, 456. (c) Kim, C.; Lee, S. J.; Lee, I. H.; Kim, K. T.;
Song, H. H.; Jeon, H. J. Chem. Mater. 2003, 15, 3638.
Figure 1. Structure of the anthracene cored AB3 and AB2 type
poly(aryl ether) dendrons examined in the present study.
A mixture of the poly(aryl ether) dendron derivative and
anthraldehyde (1:1 equiv) was stirred in a chloroformꢀ
methanol mixture (1:1% v/v) for 1 h at room temperature
to yield a robust gel in the reaction flask. The product was
then characterized by NMR, mass, and FT-IR spectro-
scopic techniques (SI Figures S1 to S8), and the analysis
suggests that the reactants were completely transformed to
the acylhydrazone derivative.
The gelation propensity of the dendrons in other di-
electric media was examined in a wide range of solvents
and solvent mixtures. The ‘instant gel’ from the chloro-
formꢀmethanol mixture was dried under vacuum and
then dissolved in a selected solvent or solvent mixtures
by heating. The homogeneous solution is then sonicated
and cooled to room temperature to obtain the gel. The
critical gel concentration (CGC) of the compounds was
determined in various solvents as well as solvent mixtures,
and the values are given in Table S1. AB3 type poly(aryl
ether) dendrons form gel in the presence of several organic
solvents at a very low CGC (0.4 wt %). For example, 4 mg
(9) (a) Chin, Q.; Feng, Y.; Zhang, D.; Zhang, G.; Fan, Q.; Sun, S.;
Zhu, D. Adv. Funct. Mater. 2010, 20, 36. (b) Chin, Q.; Zhang, D.; Zhang,
G.; Yang, X.; Feng, Y.; Fan, Q; Zhu, D. Adv. Funct. Mater. 2010, 20,
3244. (c) Yang, X.; Lu, R.; Gai, F.; Xue, P.; Zhan, Y. Chem. Commun.
2010, 1088. (d) Rajamalli, P.; Prasad, E. New J. Chem. 2011,
DOI:10.1039/c1nj20136k.
(10) (a) Zhang, Y.-M.; Lin, Q.; Wei, T.-B.; Qin, X.-P.; Li, Y. Chem.
Commun. 2009, 6074. (b) Liu, J.-W.; Yang, Y.; Chen, C.-F.; Ma, J.-T.
Langmuir 2010, 26, 9040. (c) Piepenbrock, M.-O. M.; Lloyd, G. O.;
ꢀ
ꢁ
Clarke, N.; Steed, J. W. Chem. Commun. 2008, 2644. (d) Dzolic, Z.;
ꢀ
ꢁ
Cametti, M.; Cort, A. D.; Mandolini, L.; Zinic, M. Chem. Commun.
2007, 3535. (e) Lloyd, G. O.; Steed, J. W. Nat. Chem. 2009, 1, 437. (f)
Foster, J. A.; Piepenbrock, M.-O. M.; Lloyd, G. O.; Clarke, N.;
Howard, J. A. K.; Steed, J. W. Nat. Chem 2010, 2, 1037. (g) Wang, C.;
Zhang, D.; Zhu, D. Langmuir 2007, 23, 1478. (h) Varghese, R.; George,
S. J.; Ajayaghosh, A. Chem. Commun. 2005, 593.
Org. Lett., Vol. 13, No. 14, 2011
3715