Triggering Gel Formation and Luminescence through Donor-Acceptor Interactions in a C3-Symmetric Tris(pyrene) System

Article abstract of DOI:10.1002/chem.201600072

Straightforward modulation of the gelation, absorption and luminescent properties of a tris(pyrene) organogelator containing a C₃-symmetric benzene-1,3,5-tricarboxamide central unit functionalized by three 3,3′-diamino-2,2′-bipyridine fragments

Full text of DOI:10.1002/chem.201600072

DOI: 10.1002/chem.201600072
Communication
&
Donor–Acceptor Systems |Hot Paper|
Triggering Gel Formation and Luminescence through Donor–
Acceptor Interactions in a C3-Symmetric Tris(pyrene) System
Thanh-Loan Lai, Flavia Pop, Caroline Melan, David Canevet,* Marc SallØ, and
Narcis Avarvari*[a]
ing the preparation of redox-active gels and xerogels,[17] or hel-
Abstract: Straightforward modulation of the gelation, ab-
sorption and luminescent properties of a tris(pyrene) orga-
ical fibres.[18] However, to the best of our knowledge, this plat-
form has never been appended with luminescent moieties. We
nogelator containing a C3-symmetric benzene-1,3,5-tricar-
describe in this communication the C3-symmetric tris(pyrene)
boxamide central unit functionalized by three 3,3’-diami-
no-2,2’-bipyridine fragments is achieved through donor–
acceptor interactions in the presence of tetracyanoquino-
organogelator 1 (Scheme 1) together with the remarkable
modulation of its gelation and emission properties by addition
of the tetracyanoquinodimethane (TCNQ) acceptor.
dimethane.
The synthesis of the tris(pyrene) compound 1 started with
the conversion of the commercially available pyrenebutyric
acid into its acid chloride 3 upon treatment with oxalyl chlo-
Organic supramolecular gels and derived xerogels have been
increasingly studied especially in the last decade,[1] thanks to
their propensity to be easily structured[2] and appropriately
functionalized for a large area of applications such as charge
transport,[3] organic electronics,[4] sensing,[5] pharmaceutics,[6]
etc. In this respect, luminescent organogels occupy a promi-
nent place in view of their potential photonic applications.[7]
The pyrene (Pyr) unit is known to possess interesting lumines-
cent properties,[8] and was therefore introduced in the struc-
ture of organogelators,[9] including a bis(urea) system,[10] which
we very recently reported. Another interest of the pyrene plat-
form lies on its electroactivity, as it can be readily oxidized into
radical cation species or be involved in charge-transfer com-
plexes.[11] Redox active organogels constitute indeed an impor-
tant family of soft materials as many of them respond revers-
ibly to redox stimuli.[12] Accordingly, donor–acceptor gels con-
taining pyrene-based organogelators and acceptors such as tri-
nitrofluorenone,[13] porphyrins[14] or graphene[15] have been de-
scribed. In these systems, where intermolecular hydrogen
bonding or p–p stacking are accompanied by charge transfer
bands, modulation of the emission and gelation properties
occurs. A particularly interesting platform prone to favour gela-
tion is the C3 symmetry system containing a benzene-1,3,5-tri-
carboxamide central unit functionalized by three 3,3’-diamino-
2,2’-bipyridine wedges, initially described by Meijer and col-
leagues.[16] Some of us reported the functionalization of this
disk shape core with tetrathiafulvalene units (TTF), thus allow-
ride, followed by the monoacylation reaction with 3,3’-diami-
no-2,2’-bipyridine[19] to obtain amine 4. The latter was then
treated with a substoichiometric amount of trimesic acid chlo-
ride to afford analytically pure 1 as a white solid upon precipi-
tation from the reaction mixture, filtration and then washing
1
with methylene chloride and pentane. H NMR characterization
performed in deuterated 1,1,2,2-tetrachloroethane (C2D2Cl4)
clearly indicates the presence of two strongly deshielded sets
of three amidic protons at 15.26 and 13.60 ppm (see the Sup-
porting Information).
As the C3-symmetric platform showed good propensity to
provide organogels in chlorinated solvents,[17] we investigated
the gelation behaviour of compound 1 in 1,1,2,2-tetrachloro-
ethane (C2H2Cl4). A critical gelation concentration (CGC) of
65 mgmLÀ1 (4.26”10À2 m) was found, at which 1 forms instan-
taneously an opaque white gel upon cooling a hot solution, as
attested by the inverted-vial method (Figure 1). The latter
slowly evolves within one month towards a yellow material
(Figure 1), which, however, does not show any degradation of
the compound, as attested by 1H NMR spectroscopy.
It must be noted that at
a lower concentration of
1 (8 mgmLÀ1), gelation is also observed but after several
weeks, which suggests a gelation mechanism with slow kinet-
ics. The xerogel prepared by the slow evaporation of the sol-
vent from the gel ([1]=CGC) has a fibrous bundle-like aspect,
and the dry material obtained by the drop-casting method of
a solution of 1 with a much lower concentration (CGC/100)
shows a similar texture (Figure S1 in the Supporting Informa-
tion).
[a] Dr. T.-L. Lai, Dr. F. Pop, Dr. C. Melan, Dr. D. Canevet, Prof. M. SallØ,
Dr. N. Avarvari
The C3-symmetric compound 1 is an excellent candidate for
tuning emission and gelation properties, as it possesses three
luminescent pyrene units around a benzene tris(diamidobipyri-
dine) core which has the propensity to self-assemble into
fibres. The use of a planar electron acceptor is thus likely to
favour the establishment of pyrene–acceptor interactions po-
tentially contributing to the gelation mechanism. When mixing
Laboratoire MOLTECH-Anjou, UniversitØ d’Angers, CNRS UMR 6200
2 bd Lavoisier, 49045 Angers Cedex (France)
E-mail: david.canevet@univ-angers.fr
narcis.avarvari@univ-angers.fr
Supporting information and the ORCID identification number for the
author of this article can be found under http://dx.doi.org/10.1002/
chem.201600072.
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Scheme 1. Synthesis of the C3-symmetric tris(pyrene) 1 and chemical structure of TCNQ.
a solution of 1 in C2H2Cl4 at a concentration of 8 mgmLÀ1
(5.24”10À3 m), which is about one order of magnitude less
than the CGC (4.26”10À2 m), with three equivalents of the
well-known electron acceptor tetracyanoquinodimethane
(TCNQ), a robust dark brown gel (1·(TCNQ)3) spontaneously
formed when the hot solution was cooled to room tempera-
ture (Figure 1). The threefold quantity of acceptor related to
the C3 compound 1 corresponds to the critical amount of
TCNQ necessary for gelling the solvent in these experimental
conditions. The donor–acceptor gel is particularly stable, as no
solvent flow was observed within weeks. The gel-to-sol transi-
tion temperature of composite gels formed between 1 and
TCNQ was then studied as a function of their respective ratio.
This experiment also confirmed the key role of TCNQ mole-
cules along the gelation process since the Tgel–sol transition
temperature could be increased from 408C ([TCNQ]=3”[1]) to
608C upon further addition of acceptor ([TCNQ]>4”[1]). Alto-
gether, these results suggest the involvement of the three
Figure 1. a) Freshly prepared gel of 1 in C2H2Cl4 (65 mgmLÀ1), and b) the
same sample after one month at RT. c) Solution of 1 in C2H2Cl4 (8 mgmLÀ1
pyrene units in donor–acceptor (D–A) interactions, and thus
the formation of alternate D–A stacks. The materials obtained
after evaporation of the solvent contain networks of micromet-
ric size whirling fibres which polarize light, as shown by scan-
ning electron microscopy (SEM) and polarized optical micros-
copy (POM) images (Figure 2), respectively, underlining a certain
long range order possibly related to the p–p D–A interactions.
In addition, these microstructures appear much better defined
than the ones observed for compound 1 alone prepared at
65 mgmLÀ1 (CGC; Figure S1).
)
,
at RT, and d) effect of the temperature on a 1·(TCNQ)3 gel ([1]=8 mgmLÀ1
TCNQ: 3 equiv).
the C3 central platform. The synthetic strategy for 5 follows the
same steps as those for the pyrene derivative 1 starting from
hexanoic acid chloride (Scheme S1). We then proceeded to ab-
sorption/emission/excitation investigations of 1 and 5 in
C2H2Cl4 at the reference concentrations of 5.24”10À3 m, corre-
sponding to the CGC for the D–A 1·(TCNQ)3 composite and of
a pyrene solution at a three-times higher concentration. The
absorption spectra of 1, showing a broad band between 450
and 800 nm, clearly indicate aggregation in these conditions,
while 5 and Pyr are completely dissolved (Figure S2 in the Sup-
porting Information). However, the three compounds strongly
absorb below 400 nm due to p–p* transitions. Excitation of
1 in C2H2Cl4 (5.24”10À3 m) at lexc =344 nm, a wavelength for
As previously discussed, one of the main interests of the
pyrene unit lies on its emission properties which can be of mo-
nomer or excimer type, when Pyr···Pyr interactions take place
in the excited state.[10] Since the bipyridine units can also show
emissive properties, we also prepared the reference tris(hexa-
noyl) derivative 5 (Scheme S1 in the Supporting Information),
where pyrenebutanoyl substituents were replaced with hexa-
noyl units, in order to disclose the luminescence behaviour of
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Figure 2. SEM (top) and POM (bottom) images of the xerogel 1·(TCNQ)3 pre-
pared at a concentration of 8 mgmLÀ1 for 1 in C2H2Cl4 and 3 equiv of TCNQ.
Figure 3. Temperature dependence of the emission spectra of 1 (C2H2Cl4,
8 mgmLÀ1, 5.24”10À3 m) upon increasing the temperature from 30 to 908C,
for lexc =344 nm (top) and the corresponding excitation spectra for
lem =515 nm (bottom).
which the intensity of the pyrene excimer emission band is
usually enhanced,[20] results in an emission band centred at l=
515 nm, for which the intensity decreases upon heating,
though with no appearance of pyrene monomer emission
(Figure 3).
at 450 nm, as mentioned above, while the emission band of
the TCNQ acceptor appears at 480 nm, both intensities de-
creasing by increasing the temperature (Figure S3 in the Sup-
porting Information). Note that the TCNQ fluorescence has
been only scarcely discussed in the literature, although it is an
electron acceptor widely used in the molecular materials
field.[22] On the other hand, the pyrene/TCNQ 1:1 solution
shows, quite surprisingly, two emission bands centred at 490
and 570 nm (Figure S4 in the Supporting Information), corre-
sponding to TCNQ and, possibly, to a {pyrene–TCNQ}* exciplex,
although the latter has not been previously mentioned in the
literature, to the best of our knowledge. Note however that for
single crystals of pyrene–TCNQ, fluorescence has been ob-
served at 840 nm when irradiated in the charge transfer
band.[23] In the absorption spectra of the gel 1·(TCNQ)3, two
new broad bands centred at 480 and 790 nm appear, indica-
tive of the charge transfer between the pyrene and TCNQ
units (Figure 5).[23] The intensity of the bands decreases upon
heating, as expected, since the donor–acceptor interactions
are less favourable. At a fixed 208C temperature, a steady in-
crease of the absorption is also observed with up to ten equiv-
alents of acceptor, when the saturation is reached (Figure S5 in
the Supporting Information).
However, the excitation spectra measured for the emission
at 515 nm (Figure 3) clearly show that the latter is mainly due
to an absorption at 405 nm, where the pyrene and compound
5 alone practically do not absorb, but compound 1 does (Fig-
ure S2). Irradiation of the same solution of 1 at lexc =405 nm
results in two emission bands centred at 450 and 515 nm, re-
spectively, which decrease upon heating (Figure 4). The emis-
sion spectra of pyrene and compound 5 excited at the same
wavelengths (Figure 4) suggest that the band at 450 nm corre-
sponds to {Pyr–Pyr}* excimer formation, while the one at
515 nm is due to the diamidobipyridine units, as previously ob-
served in similar C3 derivatives.[21] The fact that only the latter
is observed for an excitation at 344 nm indicates that an
energy transfer from pyrene to bipyridine is likely to occur.
How do the luminescence properties of
1 (C2H2Cl4,
8 mgmLÀ1, 5.24”10À3 m) change by adding a TCNQ solution (3
equivalents) leading to donor–acceptor gel formation? Since
compound 1 contains two fluorophore units, that is, pyrene
and diamidobipyridine, we first analysed the temperature de-
pendence emission of separate solutions of pristine pyrene
and TCNQ, together with their mixture. By excitation at
405 nm the pyrene solution shows an emission band centred
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Figure 4. Temperature dependence of the emission spectra of 1 (C2H2Cl4,
8 mgmLÀ1, 5.24”10À3 m) upon increasing the temperature from 30 to 908C,
for lexc =405 nm (top) and RT emission spectra of pyrene (C2H2Cl4,
3”(5.24”10À3 m)) and 5 (C2H2Cl4, 5.24”10À3 m) for lexc =405 nm (bottom).
Figure 5. UV/Vis absorption spectra (C2H2Cl4) of 1 (black)
([1]=8 mgmLÀ1 =5.24”10À3 m), TCNQ (red) ([TCNQ]=15.72”10À3 m) and
the corresponding gel 1·(TCNQ)3 at different temperatures upon heating
(top); temperature-dependent emission spectra (C2H2Cl4) of 1·(TCNQ)3 at
lexc =344 nm (bottom) upon heating.
The 1·(TCNQ)3 system shows luminescence only when irradi-
ated at 344 nm (Figure 5). Indeed, two emission bands centred
at 515 and 590 nm are observed, the former corresponding to
the bipyridine units. The latter is very similar to the one mea-
sured for the pyrene/TCNQ 1:1 solution (vide supra) and does
not correspond to any of the individual fluorophores in the
system (Figure S6 in the Supporting Information). Moreover,
interactions are mainly intramolecular.[16] The luminescence
properties of the system are rather rich due to the concomi-
tant presence of the pyrene and diamidobipyridine chromo-
phores, as either the pyrene excimer and bipyridine emissions,
or both of them, can be observed depending on the excitation
energy. In the donor–acceptor gel 1·(TCNQ)3, the pyrene exci-
mer luminescence vanishes to the profit of bipyridine and,
possibly, pyrene–TCNQ exciplex emissions. However, more in-
vestigations are needed to unveil the origin of this last emis-
sion band. These results open up very interesting perspectives
towards the preparation of luminescent donor–acceptor orga-
nogels with a whole range of acceptors and also of chiral lumi-
nescent gels and fibres in which the supramolecular chirality of
the helical aggregates would be controlled by the introduction
of stereogenic centres on the pyrene units.
the choice of the excitation wavelength is critical, as for lexc
=
405 (Figure S7 in the Supporting Information), 480 and
788 nm, the two latter corresponding to the charge-transfer
absorption bands, no significant emission was observed. It has
been recently shown that the fluorescence of charge transfer
complexes of pyrene based materials with TCNQ is highly de-
pendent on the excitation wavelength and the solvent.[24]
In summary we have synthesized a first C3-symmetric system
based on the tris(3,3’-diamido-2,2’-bipyridine)-benzene-1,3,5-tri-
carboxamide core and pyrene units as a new luminescent elec-
troactive organogelator. Remarkably, its gelation capacity in
1,1,2,2-tetrachloroethane increases by an order of magnitude
upon addition of TCNQ through the establishment of intermo-
lecular donor–acceptor interactions. The latter play very likely
a determining role in the formation mechanism of the gel.
Indeed, as discussed above, the p–p interactions drive the self-
assembly of the tris(3,3’-diamido-2,2’-bipyridine)-benzene-1,3,5-
tricarboxamide based C3 compounds, while hydrogen bonding
Experimental Section
Compound 1
A cold solution of trimesic acid chloride (284 mg, 1.07 mmol,
0.2 equiv) in dry dichloromethane was added dropwise to a three-
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tesanz, L. Sanchez, Chem. Commun. 2012, 48, 5757–5759; c) D. K.
Kumar, J. W. Steed, Chem. Soc. Rev. 2014, 43, 2080–2088.
necked flask containing 4 (2.44 g, 5.4 mmol, 1 equiv), triethylamine
(722 mL, 5.35 mmol, 1 equiv) and dry DCM (70 mL) at 08C. The reac-
tion mixture was allowed to stir at 08C for 2 h, and then at room
temperature, overnight. The pale yellow precipitate was filtered,
washed with DCM, pentane, then dried in vacuo to obtain a white
solid (m=1.39 g, 85%). 1H NMR (300 MHz, [D2]1,1,2,2-tetrachloro-
ethane, TMS): d=15.26 (s, 3H), 13.60 (s, 3H); d=9.34–7.14 (m,
48H, Ar-H); d=3.44, 2.55, 2.34 ppm (br, 18H); 13C NMR (300 MHz,
[D2]1,1,2,2-tetrachloroethane, TMS): d=171.8, 163.3, 141.2, 141.0,
140.4, 139.9, 136.8, 136.7, 135.5, 135.4, 131.0, 130.5, 129.6, 129.2,
128.9, 128.4, 127.3, 127.2, 126.6, 125.8, 124.9, 124.7, 124.5, 124.1,
123.5, 123.1, 37.9, 32.2, 26.9 ppm; IR-ATR: n˜ =2927(br), 1671(C=O),
1564 cmÀ1 (C=C); m.p. 188–1908C; C99H72N12O6 (528.35): calcd: C
77.93, H 4.76, N 11.02; found: C 76.79, H 4.64, N 10.73. Full experi-
mental details and characterization are provided in the Supporting
Information.
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Acknowledgements
The authors gratefully acknowledge the Pays de la Loire
Region for funding the PHOTOGEL Project, the Ministry of Edu-
cation and Research (grant to C.M.), the National Agency for
Research (ANR, Project 09-BLAN-0045-01) and the CNRS. I.
Freuze and C. MØzi›re are thanked for the MS analysis and V.
Bonnin for the elemental analyses.
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Keywords: C3 symmetry
luminescence · organogels · pyrene
·
donor–acceptor systems
·
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Received: January 7, 2016
Published online on February 29, 2016
Chem. Eur. J. 2016, 22, 5839 – 5843
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5843
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