Optical waveguides from 4-aryl-4H-1,2,4-triazole-based supramolecular structures

Article abstract of DOI:10.1039/c2cc37381e

Ribbon-like supramolecular structures prepared by the organized aggregation of 4-aryl-4H-1,2,4-triazoles act as optical waveguides that propagate photoluminescence.

Full text of DOI:10.1039/c2cc37381e

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Optical Waveguide from 4-Aryl-4H-1,2,4-triazole-based Supramolecular
Structures
David Cáceres a, Cristina Cebrián a, Antonio M. Rodríguez a, José R. Carrillo a, Ángel Díaz-Ortiz a, Pilar
Prieto a*, Fátima Aparicio,b Fátima García,b Luis Sánchezb*
5
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
50 tris(dodecyloxy)-5-(2-iodoethynyl)benzene13a and 4-aryl-4H-
The ribbon-like supramolecular structures prepared by the
organized aggregation of 4-aryl-4H-1,2,4-triazoles act as
optical waveguides that propagate photoluminiscence.
1,2,4-triazole13b by a direct arylation reaction catalyzed by
copper14 in 62% and 68% yield, respectively (Scheme S1 in
Supporting Information).
10 The self-assembly of π-conjugated systems provides valuable,
organized supramolecular structures that can exhibit enhanced
optical and/or electronic properties in comparison to the isolated
molecules.1 The chemical structure of the molecular building
blocks and the processing conditions strongly affect the
15 morphology of the aggregates. Thus, the aggregation of
conjugated molecules in an ample variety of morphologies like
spherical or cilindrical micelles,2 helical tubes,3 or flat lamellae4
has been reported. Among these morphologies, 1D micro- and
nanostructures have been applied as the active layer of thin
20 devices like logic gates,5 photodetectors,6 and chemical sensors.7
A challenging application of organized 1D supramolecular
structures consists in optical waveguiding, that is, in efficiently
propagating and handling the incident light on these structures, a
process that plays a pivotal role in the implementation of
25 photonic devices.8
Unlike many optical waveguides developed from inorganic
materials,9 those based on organic compounds are still scarce
despite their high luminescence efficiency and facile
processability. Some significant examples of organic-based
30 optical waveguides have been reported for the supramolecular
structures formed upon the self-assembly of 9,10-
bis(phenylethynyl)anthracene,10a 2,4,5-triphenylimidazole,10b or
1,4-bis(1,2’:6’,1’’-bis(3-butyl-1H-3,4,5-triazolyl)pyridin-4’-yl)-
benzene.10c A common feature of these two latter examples of
35 organic waveguides is the presence of heteroaromatic rings with
nitrogen atoms which exert a strong influence on the formation of
supramolecular structures.
55 Fig. 1 (a) Chemical structure of the 4-aryl-4H-1,2,4-triazoles 1 and 2. (b)
Calculated geometries for the monomeric units 1 and 2. (c) Columnar
arrangement calculated for a tetramer of triazole 1 (side view; the H-
bonds and the π-π aromatic interactions are depicted as red and black
dotted lines, respectively). (d) Intercolumnar interaction in compound 1
60 (the π-π aromatic interactions between the aryl groups attached to the N at
position 4 are depicted as dotted lines).
Herein, we report on the synthesis of two 4-aryl-4H-1,2,4-
triazoles (compounds 1 and 2 in Figure 1) and their self-assembly
40 into ribbon-like aggregates. These as-prepared aggregates exhibit
waveguiding properties propagating the incident light along the
length of the supramolecular structures. The unnatural 4-aryl-4H-
1,2,4-triazole moiety has been selected as molecular building
block to prepare the optical waveguides because of the easy and
45 environmentally friendly synthetic protocol utilized11 and, more
importantly, because of its optical, electronic and geometrical
chracteristics.12
The self-assembly of triazoles 1 and 2 has been firstly studied
1
by concentration dependent H NMR experiments in deuterated
1
chloroform as solvent. The H NMR spectra for both triazoles 1
65 and 2 show negligible changes in all the aromatic and aliphatic
resonances. These findings suggest the lack of remarkable
supramolecular interactions in these experimental conditions
(Figures S1 and S2). Unfortunately, both compounds 1 and 2 are
insoluble in more polar solvents like acetonitrile —which would
70 favor the aggregation of these heterocycles by a solvophobic
effect15— that impedes performing these 1H NMR studies.
However, heating up a dispersion of triazoles 1 or 2 in
acetonitrile gives rise to a white precipitate diagnostic of the
The synthesis of the 4-aryl-4H-1,2,4-triazoles 1 and 2 has been
straightforwardly performed from previously reported 1,2,3-
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formation of aggregates. The absorption and photoluminescence
(PL) spectra of compounds 1 and 2 are collected in Figures 2 and
S3, respectively. Both triazoles show an absorption band at 328 40 computational calculations at the B3LYP/6-31G* and
The geometry of triazoles 1 and 2 together with a model of the
aggregation of compound 1 have been elaborated by using
nm, ascribable to a π→ π* transition, and a narrow featureless
peak in the PL spectra at 503 and 519 nm for 1 and 2,
respectively. The white precipitate formed by 1 or 2 in
acetonitrile displays a higher-energy symmetric emission peak at
MPWB1K/6-31G** level. Both structures display a planar, π-
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5
conjugated core that comprises the triazole ring as well as the
phenylethynyl fragments with the peripheral paraffinic chains as
the flexible part of the molecule. The aryl substituent at the
414 nm in compound 1 and 432 nm in compound 2. The 45 position 4 of the triazole remains out of the molecular plane,
hypsochromic shift in the PL maximum for these compounds is
10 indicative of an efficient self-assembly phenomenon.16
being the phenyl ring rotated 55.21º in compound 1 and the
biphenyl unit 54.67º in 2 (Figure 1b and S6). The planar
geometry of the 3,5-bis(2-(3,4,5-tris(alkyloxy)phenyl)ethynyl)-
1,2,4-triazole units favors their intermolecular interaction by the
50 operation of π-stacking. Thus, compounds 1 and 2 self-assemble
into columnar stacks in which the adjacent molecules are
separated by 3.8
Å (Figures 1c and 1d). The molecular
electrostatic potentials calculated for 1 and 2 reveal a strong
electronic density located at the nitrogen atoms of the triazole
55 ring at positions 1 and 2 (Figure S7). To avoid the electrostatic
repulsion between these electron-rich fragments of the molecule
upon self-assembly, the adjacent molecules constitutive of the
columnar aggregates are rotated 150 º (Figure 1c). Importantly,
the rotated aromatic ring on the nitrogen of position 4 of the
60 triazole moieties plays a crucial role in the stabilization of the
columnar aggregates. In very good agreement with previously
reported X-ray diffraction data for referable 4-phenyl-4H-1,2,4-
triazoles,17 the aromatic C-H group at the ortho position of one
molecule is able to form weak H-bonding arrays with the sp2
65 nitrogen atoms of the heterocycle of the adjacent molecules
within the same stack (Figure 1d). The computed H-bond value is
2.7 Å. Furthermore, the offset face-to-face π-stacking on this
aromatic unit between adjacent columnar stack reinforces the
three-dimensional growth of the aggregates (Figure 1d). The π-
70 stacking between the aromatic units on the nitrogen of position 4
of the triazole ring is more efficient in the case of the biphenyl
units exhibited by compound 2 which results in thicker and more
robust aggregates.
Fig. 2 Normalized UV-Vis and PL spectra of the triazoles 1 in solution
(298 K, CHCl3, 1 x 10-4 M) (black and red lines, respectively) and PL the
aggregates formed by 1 in acetonitrile (blue line).
15
Organized supramolecular structures of triazoles 1 and 2 have
been obtained by utilizing a slow diffusion technique. Thus, a
diluted solution of (~1 x 10-4 M) of 1 or 2 in a good solvent like
CHCl3 was gently placed on a vessel containing acetonitrile that
acts as bad solvent. After several days of slow diffusion, a white
20 precipitate appears in the vial. SEM images of the precipitates
obtained from compounds 1 and 2 show ribbon-like structures
aggregated around a nucleation centre (Figure 3). The presence of
the larger biphenyl unit in triazole 2 exerts a significant role in
the thickness of the ribbons obtained from this compound. The
25 aggregates formed by the self-assembly of 1 appear as thin and
flexible ribbons with a uniform width of several micrometers and
tens of micrometers length (Figure 3a and S4). In contrast, the
self-assembly of 2 in these conditions gives rise to thicker ribbons
with well-defined edges with typical thickness of ~ 1 µm and ~
30 100 µm length (Figures 3b, S4 and S5). The thickness of the
observed ribbons clearly suggests the aggregation of compounds
1 and 2 into stratified sheets stacked together by the operation of
non-covalent forces, mainly π-π stacking between the aromatic
units and the interdigitation of the peripheric alkyl chains.
The optical waveguiding behaviour of the ribbons formed upon
1 and 2 has been
75 self-assembly of triazole derivatives
investigated by utilizing confocal optical microscopy coupled to a
camera. The irradiation of the ribbon-like aggregates with a laser
beam with UV, blue or green light shows the intense luminescent
character of the supramolecular structures formed by triazoles 1
80 and 2 (Figures 4a and 4b). A closer inspection of the PL images
shown in Figures 4a and 4b demonstrates that the ends and the
edges of the ribbons emit light which could be indicative of the
ability of these aggregates to absorb the light and propagate PL
toward the ends, thus behaving as optical waveguiding.
85 Considering that compounds 1 and 2 do not exhibit any
absorbance at wavelengths of 488 nm (blue) and 543 nm (green),
the appearance of molecular excitation due to the laser energy is
avoided. Pointing the ribbon-like aggregates of 1 with a laser
beam at 488 nm or 543 nm generates a strong emission in the
90 irradiated area. In addition, a brilliant emission point is clearly
observed in the extreme and edges of the aggregates due to the
propagation of laser light (Figure 4c). The ability of triazole 2 to
propagate the laser light is more efficient than that observed for 1
(Figure 4d). Thus, the laser irradiation of the thick filament
95 generated upon self-assembly of triazole
2 provokes the
longitudinal propagation of the light and highly brilliant spots are
visible at the opposite extremes of the ribbons.
35
Fig. 3 SEM images (298 K, 1 x 10-4 M, glass substrate) of the ribbons
formed by the self-assembly of 1 (a) and 2 (b).
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supramolecular structures, which can be ascribed to the large
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85
Financial support by UCM (GR42/10-962027), the MINECO
25 of Spain (CTQ2011-22581 and CTQ2011-22410), and Consejería
de Educación y Ciencia, JCCM through project PII2I09-0100 is
acknowledged. F. A. and F. G. are indebted to MEC for a FPU
studentship. The support from High Performance Computing
Service of University of Castilla-La Mancha and the CAI of
30 cytometry and microscopy of fluorescence at UCM are gratefully
acknowledged.
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Notes and references
a Departamento de Química Inorgánica, Orgánica y Bioquímica,
Facultad de Ciencias y Tecnologías Químicas, Universidad de Castilla-
35 La Mancha, 13071 Ciudad Real (Spain).Fax: (+) 34926295318.
bDepartamento de Química Orgánica, Facultad de Ciencias Químicas,
Universidad Complutense de Madrid, 28040 Madrid (Spain).Fax: (+)
34913944103; E-mail: lusamar@quim.ucm.es
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† Electronic Supplementary Information (ESI) available: suplementary
40 figures, S1-S12 and experimental section. See DOI: 10.1039/b000000x/
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Optical Waveguide from 4-Phenyl-4H-1,2,4-
triazole-based Supramolecular Structures
View Article Online
5
David Cáceres,a Cristina Cebrián,a Antonio M.
Rodríguez,a José. R. Carrillo, a Ángel Díaz-Ortiz,a
Pilar Prieto,a* Fátima Aparicio,b Fátima García,b
Luis Sánchezb*
10 The ribbon-like supramolecular structures prepared by the organized aggregation of 4-aryl-4H-1,2,4-triazoles act as optical waveguides
that propagate photoluminiscence.
4 | Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]