Excellent chirality transcription in two-component photochromic organogels assembled through J-aggregation

Article abstract of DOI:10.1039/c2cc38221k

Organogels made of pyridine-end oligo-p-phenylenevinylenes with tartaric acid exhibit remarkable J-aggregation induced red-shifts (Δλ = 55 nm) and notable chirality transcription. Induction of liquid-crystalline behavior is also tuned in the supramolecula

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Excellent Chirality Transcription in TwoꢀComponent Photochromic
Organogels Assembled through JꢀAggregation
Suman K. Samanta,a Santanu Bhattacharya*a,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
excess ethanol, indicating a crucial hydrophilic/hydrophobic
50 balance is operational for the selfꢀassembly that leads to gelation.
Organogels made of pyridineꢀend oligoꢀpꢀphenylenevinylenes
with tartaric acid exhibit remarkable Jꢀaggregation induced
redꢀshifts (∆λ = 55 nm) and notable chirality transcription.
Induction of liquidꢀcrystalline behavior is also tuned in the
10 supramolecular assembly.
55
Supramolecular organization to a wellꢀdefined architecture may
occur by selective association of complementary components
through molecular recognition.1 The information stored in the
individual components are expressed in the assembly.
15 Development of chiral supramolecular assemblies from achiral
monomers through transcription of chirality have attracted much
attention in mimicking the biological helical structures, liquid
crystals, physical gels and recently the synthesis of selfꢀ
assembled oneꢀdimensional stacks.2 Supramolecular gelation via
20 molecular recognition in a specific solvent or solvent mixtures
could be a vehicle for generating such chiral nanomaterials3 as a
consequence of various nonꢀcovalent interactions.4 Specifically,
πꢀstacking interactions play a major role in the gelation of
conjugated chromophoric molecules, where the chromogenic part
25 offers an extra handle to probe their aggregationꢀinduced changes
in the optoelectronic properties.5 In particular, Park et al. reported
gelationꢀinduced enhanced emission and chirality transcription in
the Hꢀbonded tartaric acid (TA) with a pyridine based nonꢀ
fluorescent molecule.6 However, controlled organization of such
30 molecules dictated by another component to generate specific and
predictable chiral nanoꢀstructures still remains a challenge.
Herein, we report organogelation of new chromophoric pyridineꢀ
end oligoꢀpꢀphenylenevinylenes (OPV, 1 and 2, Scheme 1) in
assistance with isomeric TAs viz. Lꢀtartaric acid (LꢀTA) and Dꢀ
35 tartaric acid (DꢀTA) as well as chirality transcription in the
supramolecular assembly via Jꢀaggregate formation. Also,
thermal mesophases of the complexes could be tuned depending
on the TA isomers used.
60
Scheme 1 The preꢀgelators 1 and 2 and three different TA used here for
the gelation studies. Snapshot of a typical gel and sol of 1+LꢀTA in 10:1
toluene/ethanol (v/v) mixture.
Although, gelation occurred with 1+LꢀTA and 1+DꢀTA
65 complexes with a minimum gelator concentration (MGC) of 0.93
and 1.1 wt% respectively in 10:1 toluene/ethanol mixture, 1+Mꢀ
TA (mesoꢀtartaric acid) did not form gel at all. This suggests that
the spatial orientation of the ꢀCOOH moiety in the chiral TA
holds the key to favor the mode of aggregation that is conducive
70 for gel formation.6 Thus, a 1:1 mixture of 1+LꢀTA and 1+DꢀTA
(above individual MGCs) did not form gel probably due to the
interꢀgelator interactions. However, the gelation was observed in
each of the complexes (1:1) of 2 with LꢀTA, DꢀTA and MꢀTA
having MGC 0.87, 0.92 and 1.14 wt% respectively presumably
75 due to the enhanced πꢀstacking and van der Waals interactions
through the five aromatic rings and six nꢀhexadecyl chains of 2
respectively. In contrast, 1 and 2 alone failed to form a gel.
Presence of hydroxyl groups on the TA moiety is also crucial, as
the gelation did not occur when tested with mixtures of 1 with
80 either oxalic or succinic acid. Protonation of the pyridineꢀends of
1 and 2 by TA was evident from the IR stretching frequency of
the free –COOH (1730 cmꢀ1) which decreased to 1612 cmꢀ1 upon
complexation with 1 or 2 indicating formation of –COO– species
(Fig. S1†). Each gel was optically transparent (Fig. S2†) and a
Preꢀgelators 1 and 2 were synthesized using Wittig reaction
40 and were characterized unambiguously (Scheme S1, ESI†). TA
complexes of 1 and 2 in 1:1 molar ratio were prepared and tested
for gelation. It was observed that none of the complexes were
soluble in nonꢀpolar solvents like toluene or nꢀheptane. Although
soluble in polar solvents like ethanol or THF they did not form
45 gel either. However, gelation was observed in a mixture of
toluene and ethanol at a particular ratio (10:1 v/v respectively).
With less amount of ethanol in the mixture, the complexes could
not be solubilized properly while it remained as a solution with
o
85 typical gel e.g. 1+LꢀTA displayed a red color at 25 C while it
transformed into a yellow sol when heated to 60 oC (Scheme 1).
Morphological analysis of the complexes using transmission
electron microscopy revealed the presence of fibrillar networks
with diameter ~200ꢀ300 nm. With increasing concentration, a
90 nucleation induced aggregated structures were observed under the
fluorescence and scanning electron microscopy (Fig. S3ꢀS5†).
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Circular dichroism (CD) spectral studies verified the chiral
transcription abilities of the selfꢀassembly of the isomeric TAs
with each of the preꢀgelators 1 and 2. A uniform cast film of the
decay of 1. Both 1 and 2 showed solvatochromic effect and the
solventꢀinduced shifts were greater in the emission maximum
than in the absorption maximum indicating that the excited states
layered gels of 1+TA revealed a bisignated Cotton effect 60 are more sensitive towards the solvent polarity (Fig. S9†). Thus,
5
producing a strong CD signal close to 500 nm with zero crossings
at 366 and 293 nm (Fig. 1). This leads to virtually mirror image
spectra for the complexes with two enantiomers (Lꢀ and DꢀTA)
indicating the transfer of chiral information of TA to the selfꢀ
by changing the polarity of the solvent from nꢀheptane to
chloroform, the emission maximum shifted from 442 nm to 465
nm (∆λ = 23 nm) for 1 and 506 nm to 527 nm (∆λ = 21 nm) for 2.
Interestingly, in the gel phase, the emission spectra showed
assembled chromophores in a helical sense.1 Similar mirrorꢀ 65 remarkable redꢀshifts. The emission maximum of a gel of 1+Lꢀ
10 image spectra were observed in the CD signals of the 2+TA
complexes showing a strong band at 520 nm with zero crossings
at 610, 466 and 265 nm. The DꢀTA complexes showed a peak
first followed by a trough which indicates a rightꢀhanded helical
TA appeared at 575 nm (λex 380 nm) and for 2+LꢀTA it was 660
o
nm (λex 540 nm) at 25 C (Fig. S10†). Corresponding emission
o
maximum of the ‘sol’ at 60 C appeared at 520 nm and 625 nm
respectively. Variable temperature fluorescence spectral studies
assembly (P) and conversely a leftꢀhanded helical assembly (M) 70 (Fig. 2) showed gradual hypochromic redꢀshifts in the emission
15 for LꢀTA complexes was indicated.7 However, the complexes
containing MꢀTA for both 1 and 2 showed no helical bias.
maximum from 520 nm to 575 nm (∆λ = 55 nm) for 1+LꢀTA and
625 nm to 660 nm (∆λ = 35 nm) for 2+LꢀTA respectively on
transformation from solꢀtoꢀgel indicating a Jꢀtype of aggregate
formation (Fig. S11†).9 A plot of the intensity at the emission
75 maximum vs. temperature produces a sigmoidal curve showing a
1200
1+D-TA
1+M-TA
1+L-TA
(b)
(a)
200
150
100
50
2+D-TA
2+M-TA
2+L-TA
900
600
o
300
solꢀtoꢀgel transition temperature at ~40 C at this concentration
0
0
(1.15 wt% for both, Fig. S12†). The gels under a 365 nm UVꢀ
lamp showed a lightꢀred emission for 1+LꢀTA and a red emission
for 2+LꢀTA (Fig. 2 and S2†). However, the sol at 60 oC showed a
80 bright yellow emission for 1+LꢀTA while a bright orange
emission for 2+LTA.
20
-300
-600
-900
-1200
-50
-100
-150
-200
200 250 300 350 400 450 500 550 600
Wavelength (nm)
200 300 400 500 600 700 800
Wavelength (nm)
(c)
(i)
(ii)
(iii)
P
25
160
200
(a)
(b)
250
200
150
100
50
200
150
100
50
60oC
20oC
60oC
20oC
160
120
80
120
80
40
0
P
85
90
95
M
M
P
40
30 Fig. 1 Circular dichroism spectra of (a) 1+TA and (b) 2+TA complexes.
(c) AFM topographic images showing (i) Mꢀhelix for 1+LꢀTA, (ii) Pꢀ
helix for 1+DꢀTA and (iii) mixture of M and Pꢀhelices for 1+MꢀTA.
0
0
0
300 350 400 450 500 550 600 650 700
450 500 550 600 650 700 750
Wavelength(nm)
Wavelength (nm)
(d)
(c)
Under an atomic force microscope, ‘rope’ꢀlike Mꢀhelical fibers
from 1+LꢀTA and Pꢀhelices from 1+DꢀTA were clearly visible
35 with high aspect ratios (Fig. 1 and S6†). The fiber diameters
ranged within 200ꢀ300 nm. Interestingly, 1+MꢀTA showed
presence of both P and M helices together, a resultant of which
averaged out in the CD spectra. The helix formation in the achiral
1.0
0.5
0.0
Solution
'Sol'
'Gel'
(i)
(iii)
(ii)
450
500
550
600
650
700
Wavelength (nm)
components of 1+MꢀTA could be
a
consequence of
Fig. 2 Variable temperature excitation and emission spectra of (a) 1+Lꢀ
40 supramolecular selfꢀassembly with no preferable handedness. In
addition to the fibrillar morphologies, it also furnished ringꢀlike
assemblies of average diameter of ~500 nm. The complexes of
2+LꢀTA and 2+DꢀTA showed M and P type helical fibers
respectively with the average fiber diameters of 100ꢀ150 nm.
45 Thus, the information stored in the individual TA components
was expressed specifically in the assembly through chirality
transcription, a phenomenon of particular interest.6,8
Photophysical studies of 1 (10 ꢁM) in 10:1 toluene/ethanol
mixture showed absorption maxima at 400 and 330 nm and an
50 emission maximum at 468 nm (λex 380 nm, Fig. S7†).
Corresponding absorption and emission maximum of 2 appeared
at higher wavelengths at 456 and 525 nm respectively (λex 450
nm) due to a greater conjugation length in 2. The excitation
spectra of both 1 and 2 are consistent with their respective
55 absorption spectra (Fig. S8†). A higher quantum yield of 1 (0.78)
over 2 (0.50) indicates a favorable nonꢀradiative fluorescence
TA (λex 380 nm, λem 575) and (b) 2+LꢀTA (λex 470 nm, λem 660). (c)
o
100 Snapshots showing (i) 1 as solution, (ii) 1+LꢀTA as ‘sol’ at 60 C, (iii)
1+LꢀTA as gel at 25 oC and (d) corresponding emission spectra.
Concentration 1.15 wt% in 10:1 toluene/ethanol mixture in each case.
However, this type of spectral shift on temperature variation
was not observed for the solution of 1 alone at an identical
105 concentration (1.15 wt%, Fig. S10c†). It showed an emission
maximum at 482 nm which corresponds to a bright skyꢀblue
emission (Fig. 2c). The emission color changed to bright yellow
o
in the ‘sol’ of 1+LꢀTA at 60 C leading to a redꢀshifted emission
band at 520 nm (Fig. 2d). Finally the emission color changed to
o
110 lightꢀred in the gel at 25 C showing an emission maximum at
575 nm. Therefore, these emission changes and the remarkable
redꢀshifts occur as a consequence of two consecutive phenomenaꢀ
(i) protonation of the pyridine N by TA leading to a shift from
482 to 520 nm (∆λ = 38 nm) and (ii) Jꢀaggregation induces a
2
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of Pꢀ and Mꢀhelical fibers for the complexes containing DꢀTA
55 nm). Theoretical calculations were undertaken to explain the 60 and LꢀTA respectively while both helices exist in the complexes
further shift in the emission maximum from 520 to 575 nm (∆λ =
effect of protonation on the redꢀshifts in the emission spectra (Fig
S13, S14†). It showed that upon protonation, the HOMO and
LUMO energy gap decreases significantly which accounts for a
higher emission λmax (Table S1†). It may be further noted that the
derived from MꢀTA. Notable redꢀshifts in the emission maxima
are observed as a consequence of protonation and Jꢀaggregation
induced gel formation. Thermal mesophases could also be tuned
depending on the spatial orientation of different TA isomers.
5
systems attain planarity on protonation and these planar aromatic 65 Such tunable systems deserve further research for the generation
surfaces may offer effective πꢀstacking interactions leading to
excellent Jꢀaggregation.
of specific and predictable chiral supramolecular stacks through
chiral induction.
10
Presence of thermotropic mesophases in the preꢀgelators and in
complexes were investigated using differential scanning
calorimetry and polarized optical microscopy (POM). Compound
S.B. thanks J.C. Bose Fellowship (DST) for funding this work.
o
1 alone showed a sharp phaseꢀtransition at 111.3 C for melting
70 Notes and references
and at 102.3 oC for freezing (Fig. S15†). The phaseꢀtransition
a Department of Organic Chemistry, Indian Institute of Science,
Bangalore-560012, India. Fax: +91-80-23600529; Tel: +91-80-
22932664; E-mail: sb@orgchem.iisc.ernet.in
15 temperature of 1+TA showed a decrease of ~7 oC for the heating
cycle and ~11 oC for the cooling cycle (Table S2†). Similar
extent of decrease in the phase transition temperature was also
observed for 2+TA complexes than 2 alone indicating a change in
the supramolecular organization10 in the TA complexes.
20
b Chemical Biology Unit, Jawaharlal Nehru Centre for Advanced
75 Scientific Research, Bangalore-560064, India.
† Electronic Supplementary Information (ESI) available: Experimental
section,
synthesis
and
supporting
figures
(S1ꢀS19).
See
DOI: 10.1039/b000000x/
(a)
(b)
(c)
(d)
1
2
J.ꢀM. Lehn, Angen. Chem. Int. Ed. Engl., 1990, 29, 1304; T. Gulicꢀ
Krzywicki, C. Fouquey, J.ꢀM. Lehn, Proc. Nati. Acad. Sci. USA,
1993, 90, 163; R. Oda, I. Huc, M. Schmutz, S. J. Candau, F. C.
MacKintosh, Nature, 1999, 399, 566; C. C. Lee, C. Grenier, E. W.
Meijer, A. P. H. J. Schenning, Chem. Soc. Rev., 2009, 38, 671; S. J.
George, Z. Tomovic, A. P. H. J. Schenning, E. W. Meijer, Chem.
Commun., 2011, 47, 3451.
S. J. George, R. de Bruijn, Z. Tomovic, B. V. Averbeke, D. Beljonne,
R. Lazzaroni, A. P. H. J. Schenning, E. W. Meijer, J. Am. Chem.
Soc., 2012, 134, 17789; H. Nakade, B. J. Jordan, S. Srivastava, H.
Xu, X. Yu, M. A. Pollier, G. Cooke, V. M. Rotello, Supramol.
Chem., 2010, 22, 691; R. Oda, F. Artzner, M. Laguerre, I. Huc, J.
Am. Chem. Soc., 2008, 130, 14705; J. J. D. de Jong, L. N. Lucas, R.
M. Kellogg, J. H. van Esch, B. L. Feringa, Science, 2004, 304, 278;
R. Custelcean, M. D. Ward, Angew. Chem., Int. Ed., 2002, 41, 1724.
B. Adhikari, J. Nanda, A. Banerjee, Soft Matter, 2011, 7, 8913; X.
Zhu, P. Duan, L. Zhang, M. Liu, Chem. Eur. J. 2011, 17, 3429–3437;
S. K. Batabyal, W. L. Leong, J. J. Vittal, Langmuir, 2010, 26, 7464;
C. Bao, R. Lu, M. Jin, P. Xue, C. Tan, G. Liu, Y. Zhao, Org. Biomol.
Chem., 2005, 3, 2508.
80
85
(f)
(e)
(g)
(h)
25
Fig. 3 POM images of (a) 1, (b) 1+LꢀTA, (c) 1+DꢀTA, (d) 1+MꢀTA and
30 (e) 2, (f) 2+LꢀTA, (g) 2+DꢀTA, (h) 2+MꢀTA.
90
POM images showed birefringent textures from 1, 2 and their
TA complexes which were monitored by taking snapshots upon
progressively decreasing temperature of the isotropic melts (Fig.
3, S16ꢀS19†). In case of 1, only a crystalline phase appeared all at
3
95
o
35 once at ~107 C while for 1+LꢀTA, initially a rodꢀlike texture
o
appeared at 120 C followed by another type of smaller domain
o
of distinct morphology at ~110 C and finally a crystalline phase
4
5
S. Bhattacharya, S. K. Samanta, Langmuir, 2009, 25, 8378; M.
George, R. G. Weiss, Acc. Chem. Res., 2006, 39, 489; S.
Bhattacharya, S. K. Samanta, Chem. Eur. J., 2012, 18, 16632.
S.ꢀJ. Yoon, J. H. Kim, J. W. Chung, S. Y. Park, J. Mater. Chem.,
2011, 21, 18971; S. K. Samanta, A. Pal, S. Bhattacharya, Langmuir,
2009, 25, 8567; J. F. Hulvat, M. Sofos, K. Tajima, S. I. Stupp, J. Am.
Chem. Soc., 2005, 127, 366; S. K. Samanta, S. Bhattacharya, J.
Mater. Chem., 2012, 22, 25277.
o
100
105
110
115
120
appeared rapidly at ~83 C. However, 1+DꢀTA showed only the
smaller domain morphology followed by a crystalline phase.
40 Interestingly, 1+MꢀTA revealed focalꢀconic morphology possibly
due to the formation of smectic phase along with smaller
domains.11 The birefringent textures were liquidꢀcrystalline in
nature before appearing the rapidly growing phase starting at ~83
o
o
oC for 1+LꢀTA, ~90 C for 1+DꢀTA and ~85 C for 1+MꢀTA
45 complexes. These observations suggest that the appearance of
smaller domains are predominantly due to the helical fibers (for
1+LꢀTA and 1+DꢀTA) while the focalꢀconic morphology is
possibly due to the ringꢀlike structures of 1+MꢀTA. Interestingly,
an opposite phenomena appeared in case of 2 which showed
50 liquidꢀcrystalline behavior upto ~130 oC while none of the
complexes of TAs with 2 showed any liquidꢀcrystalline behavior,
rather they all showed crystalline phases. Thus the birefringent
textures are dictated by the spatial orientations of TA isomers.
In conclusion, it is shown for the first time, that pyridineꢀend
55 OPVs form supramolecular organogels via molecular recognition
of the chiral forms of TA. The chiral information stored in TA
induces exactly opposite sense of chirality in the complexes for
two optically active TA (Lꢀ and Dꢀ). This leads to the formation
6
7
8
J. Seo, J. W. Chung, E.ꢀH. Jo, S. Y. Park, Chem. Commun., 2008,
2794.
S. J. George, A. Ajayaghosh, P. Jonkheijm, A. P. H. J. Schenning, E.
W. Meijer, Angew. Chem. Int. Ed. 2004, 43, 3422.
S. J. George, Z. Tomovic, M. M. J. Smulders, T. F. A. de Greef, P. E.
L. G. Leclere, E. W. Meijer, A. P. H. J. Schenning, Angew. Chem.
Int. Ed. 2007, 46, 8206; R. Amemiya, M. Mizutani, M. Yamaguchi,
Angew. Chem. Int. Ed., 2010, 49, 1995; M. R. Molla, A. Das, S.
Ghosh, Chem. Commun., 2011, 47, 8934.
9
F. Wurthner, T. E. Kaiser, C. R. SahaꢀMoller, Angew. Chem. Int. Ed.,
2011, 50, 3376; H. Wu, L. Xue, Y. Shi, Y. Chen, X. Li, Langmuir,
2011, 27, 3074.
10 S. K. Samanta, A. Pal, S. Bhattacharya, C. N. R. Rao, J. Mater.
Chem., 2010, 20, 6881; S. K. Samanta, K. S. Subrahmanyam, S.
Bhattacharya, C. N. R. Rao, Chem. Eur., J. 2012, 18, 2890.
11 V. N. Vijayakumar, K. Murugadass, M. L. N. Madhu Mohan, Mol.
Cryst. Liq. Cryst., 2010, 517, 43.
This journal is © The Royal Society of Chemistry [year]
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Graphical Abstract
Excellent Chirality Transcription in Two-Component Photochromic
Organogels Assembled through J-Aggregation
Suman K. Samanta,a Santanu Bhattacharya*a,b
aDepartment of Organic Chemistry, Indian Institute of Science, Bangalore-560012, India.
Fax: +91-80-23600529; Tel: +91-80-22932664; E-mail: sb@orgchem.iisc.ernet.in
bChemical Biology Unit, Jawaharlal Nehru Centre for Advanced Scientific Research,
Bangalore-560064, India.
1200
900
D-TA
600
300
M-TA
0
-300
-600
L-TA
-900
-1200
200 300 400 500 600 700 800
Wavelength(nm)
Gelation of optically-active tartaric acid and pyridine-end oligo-phenylenevinylenes brings
about selective chirality transcription, fluorescence modulation and formation of altered
mesophases.