Homochiral coordination polymeric gel: Zn2+-induced conformational changes leading to J-aggregated helical fibres formation

Article abstract of DOI:10.1039/c3cc47359g

A novel chiral coordination polymeric gel based on an amino acid derived symmetrical ligand, specific base, and Zn²⁺ has been described. Zn²⁺ induced conformational reorganization in the ligand leads to gelation via J-aggregated helical fibres. The vital role played by π-π stacking in gelation has been monitored by fluorescence and NMR spectroscopy.

Full text of DOI:10.1039/c3cc47359g

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Homochiral coordination polymeric gel:
Zn2+-induced conformational changes leading
to J-aggregated helical fibres formation†
Cite this: Chem. Commun., 2014,
50, 1675
Received 26th September 2013,
Accepted 28th October 2013
Mrigendra Dubey, Ashish Kumar and Daya Shankar Pandey*
DOI: 10.1039/c3cc47359g
www.rsc.org/chemcomm
A novel chiral coordination polymeric gel based on an amino acid derivatives,8,9a such an observation has not been made with
derived symmetrical ligand, specific base, and Zn2+ has been described. coordination polymeric gels.
Zn2+ induced conformational reorganization in the ligand leads to
Considering these points, through this contribution we describe
gelation via J-aggregated helical fibres. The vital role played by p–p homochiral coordination polymeric gel based on L-tyrosine derived
stacking in gelation has been monitored by fluorescence and NMR ligand 1 (H4TL-tyr), KOH and Zn2+. Herein, metal coordination
spectroscopy.
not only influences conformational changes in the ligand
but, also induces gelation accompanied by J-aggregated helical
Gels have attracted immense current interest because of their fibre formation and remarkable fluorescence enhancement.
potential application in diverse areas.1 The majority of gelators The Zn2+ induced conformational reorganization in the ligand 1
are long chain amphiphiles based on amino acids,2 sugars,3 leading to gelation has been monitored by fluorescence, CD,
and pyridine4a possessing amide head groups. Usually the and NMR spectroscopy.9b Moreover, the twisted helical fibrous
gelators form a hydrogel or organogel alone or together with morphology has been revealed by AFM.
neutral organic molecules as additives.4b Further, organic hydro-
The ligands 1–3 were synthesized in good yield and char-
organogels have been more extensively investigated relative to acterization data are consistent with proposed formulations
metal–organic gels; metallogelators5a,b or coordination polymers.5c (Scheme S1, ESI†). Addition of a methanolic solution of
Essentially, there is no obvious difference between an organo- Zn(NO3)2ꢀ6H2O to a deprotonated solution (using KOH or NaOH)
gelator and a metal–organic gel. However, the incorporation of of 1 forms CP gel 4 (0.9%, w/v) instantly at room temperature,
a metal centre, particularly a transition metal, influences the which was confirmed by the inverted test tube method (Fig. 1).
self-aggregation modes and allows additional scope for tuning The CP complex 4 was well characterized by elemental analysis,
the gel properties.1b,5
IR, ESI-mass spectrometry and TGA. In addition, 1 can also be
Coordination-based gels may be associated with additional gelated using various zinc counter ions such as Clꢁ, ClO4 and
physicochemical properties such as magnetism, colour, rheology, OAcꢁ indicating that the CP gel formation with 1 does not strongly
adsorption, emission, catalytic activity and redox behaviour.5 depend on the nature of the anion. Interestingly, gelation was
Recently, Vittal et al., reported a chiral coordination polymeric
ꢁ
(CP) gel based on a glycine derived ligand and Zn2+ which
displayed enormous fluorescence enhancement.6 In addition,
chiral gelators have attracted immense interest because gelation
often leads to helical twisting of the fibres,7 inducing chiro-optical
effects.5d Though there are few reports on the conformational
change pertaining to helix formation involving organic
Department of Chemistry, Faculty of science, Banaras Hindu University,
Varanasi-221005, U.P., India. E-mail: dspbhu@bhu.ac.in; Fax: +91 542 2368174;
Tel: +91 542 6702480
† Electronic supplementary information (ESI) available: Experimental proce-
Fig. 1 (A) Photograph of the CP gel 4 in an inverted vial, (B) AFM image
showing single helical twisted fibres, (C) phase mode image, (D) magnified
image of two twisted helical fibres intertwined with each other and
(E) CD spectra for 1 (blue line) and diluted CP gel 4 (red line).
dures, photophysical, NMR titration, TGA, rheological data, crystal structure,
and schematic representations of the CP gel 4. CCDC 962462. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc47359g
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not observed in other common organic solvents or in presence when 1 was deprotonated using LiOHꢀH2O, complex formation
of the metal ions including Co2+, Ni2+ and Cu2+. Furthermore, occurs by the same phenomenon however, the AIEE was not
deprotonation of 1 with LiOHꢀH2O could not lead to gel formation. observed and has been supported by comparative fluorescence
The loss of gelation was also observed when the hydrophobic polar studies using K+ and Li+ (Fig. S5 and S6, ESI†). The IR spectra of
arm of 1 (L-tyrosine) was replaced by other hydrophobic non polar 5 shows a shift in the nas(COO) (B12 cmꢁ1) band relative to 4 which
amino acids like L-leucine or L-phenylalanine. In fact, ligand 2 and clearly indicates the 4CQOꢀꢀꢀLi+ interaction. The Li+ interaction
3 afforded precipitate and gelatinous precipitate respectively, indi- probably interrupts gelation and forms the sol. The effect of alkali
cating that a small change in gelator could affect the gelation metal ions on gel13a or self-assembly processes using amino acid
properties. All the isolated complexes were insoluble in water and derivatives has already been investigated.13b Fluorescence emission
common organic solvents. The CP gel 4 shows thermo-reversibility studies were performed to follow the effect of other metal ions on
which is an uncommon property for coordination polymer gel aggregation behaviour. Neither gel formation nor aggregation occurs
systems.6
in the presence of Co2+, Ni2+ and Cu2+ ions. Further, gradual and
Lucid twisted helical fibrous morphology was revealed by controlled addition of the aforesaid metal ions leads to quenching
AFM (Fig. 1 and Fig. S2–S4, ESI†). The formation of helical fibres of the emission band at 306 without any appreciable shift
is most likely due to the helical arrangement of molecules within (Fig. S8, ESI†). The non-attendant red shift in the presence of the
the assembly which is supported by the crystal structure of said metal ions may be attributed to the involvement of the phenol
three dimensional CP Cu(II) complex 6 (Fig. S14–S16, ESI†). as a fifth axial ligand, in turn, segregation of the p–p stacking.
The structural similarities of 4 and 6 have been thoroughly Therefore, it may be concluded that selective Zn2+ coordination to 1
established by elemental analyses, ESI-MS and TGA. The CD enforces conformational changes and thereby aggregation.
spectra of 1 in methanol (1 ꢂ 10ꢁ5 M) shows a bisignated Cotton
The thermo-reversible nature of CP gel 4 was established by
effect, that is negative at lower (lmax = 280 nm) and positive at variable temperature fluorescence studies. Upon heating up to
higher energy (lmax = 252 nm) along with a zero crossing at 64 1C, it illustrated gradual quenching and returns after cooling to
265 nm (Fig. 1). On the other hand, diluted CP gel 4 (2 ꢂ 10ꢁ5 M) 20 1C (Fig. S9, ESI†). The band due to p–p* exhibited substantial
shows intense CD signals (lmax = 303, 280 and 252 nm) with a blue shift (B32 nm) with a huge drop off in intensity (B70%)
negative Cotton effect. Enhancement of the signal intensity at upon addition of dilute HCl to the CP gel 4 (Fig. S5, ESI†). It
252 nm with a negative Cotton effect indicates that chirality suggested disruption of the gel into sol and may be attributed to
transfer becomes more effective with the growth of p–p stacking protonation of the ligand at low pH, thereby blocking the coordi-
of the phenolic units. The solid state CD signal of the xerogel nation sites and, in turn altering the aggregation pattern of the gel.
also shows a negative Cotton effect (Fig. S10, ESI†). The CD
In the 1H NMR spectrum, phenolic protons of the free ligand
1 displayed broadening and significant downfield shift
signal transformation at 252 nm supported conformational
reorganization of 1 in a helical supramolecular structure upon (Dd = 0.2 and 0.1 ppm) upon addition of Zn2+ (1 equiv.). It
metal coordination.7b,9
supported the occurrence of p–p stacking between phenolic
The fluorescence spectrum of 1 (1 ꢂ 10ꢁ4 M, deprotonated rings (Fig. S11, ESI†). Further, the aromatic ring (B7.18 ppm)
with KOH) displayed a broad band with an emission maximum as well as aliphatic protons did not show any significant shift
at 306 nm (lex = 280 nm, Stokes shift = 26 nm) corresponding to except broadening of the respective signals which strongly
a p–p* transition. Aliquot addition of Zn2+ to a methanolic supported the aggregation involving phenolic ring (p–p) stack-
solution of 1 led to a gradual decrease in the intensity of the ing.14 In the case of Cu2+, no significant shifts for any protons
band at 306 nm and appearance of a new band at 338 nm were observed which eliminates the possibility of p–p stacking
(Stokes shift = 58 nm) along with an isoemissive point at and axial coordination of the phenolic groups. It is consistent
315 nm (Fig. S5, ESI†). Notably, the p–p* emission band with the observations made from fluorescence titration studies
exhibited a red shift of B32 nm which may be due to p–p (Fig. S8 and S11, ESI†). This also supports that p–p stacking
stacking between phenolic rings like excimer formation.10 This between phenolic rings plays a role in the gelation process. In
unique red shift upon addition of the Zn2+ indicates J-type addition, a conformational change in the ligand 1 upon coordina-
aggregation which was further attested by UV-vis spectral tion with the metal has been affirmed by geometry optimizations
studies (Fig. S10, ESI†).11 The remarkable fluorescence and its crystal structure (Fig. S17, ESI†). Based on morphology,
enhancement arising from p–p stacking and various hydrogen results from spectral and structural studies we summarize results
bonds created by CH3OH, N–H donor groups and O atoms as for CP gel formation in Fig. S18 (ESI†).
hydrogen bond acceptors can be ascribed to the aggregation
A gel must show specific rheology viz. mechanical or elastic
induced enhanced emission (AIEE) effect.12 The critical role of strength. The mechanical strength of a gel is measured by its
phenolic –OH in aggregation towards gelation cannot be ruled storage modulus (G0) and loss modulus (G00) which depend on
out. Such packing can easily be extended to form a CP gel by the gelator concentration. The rheological experiments have been
accommodating methanol molecules tightly in the interstitial performed using freshly prepared CP gel 4 at fixed concentration
solvophilic pockets, together with hydrogen bond donors and (0.9%, w/v). The storage modulus G0 and loss modulus G00 were
acceptors. The crystal lattice of 3D CP 6 further supports the above measured as a function of shear stress at 20 1C, at a frequency of
observation (Fig. S15, ESI†). Job’s plot signifies complex formation 1 rad sꢁ1. From the available data it is clear that initially G0 is
between 1 and Zn2+ in 1 : 1 ratio (Fig. S10, ESI†). On the other hand, higher relative to G00 by 1 order of magnitude and maintained the
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dramatic fluorescence enhancement. The J-aggregation and fluores-
cence intensity enhancement is mainly due to the p–p stacking
interaction between phenolic rings. Moreover, AFM and CD analysis
provided evidence of the formation of twisted helical fibres.
Rheological studies revealed phase transition via gel–semi sol–
solid at Tgel and gel–sol at yield stress. The present gel may find
applications in chiro-optical, optoelectronic devices and also 1
can be used for sensing metal cations.
D. S. P. thanks DST New Delhi for providing a Fluorescence
Spectrometer through the scheme SR/S1/IC-25/2011 and M. Dubey
acknowledges the University Grants Commission, New Delhi,
India for a Dr. D. S. Kothari Postdoctoral Fellowship.
Fig. 2 (A) Primary axis: dynamic frequency sweep measurements of G0
and G00 for CP gel 4, at strain of 0.5%. Secondary axis: complex viscosity
measurements. (B) Dynamic temperature ramp G0 and G00 for CP gel 4 at a
Notes and references
heating rate of 1 1C minꢁ1, strain of 0.5% and frequency of 1 rad sꢁ1
.
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Chem. Commun., 2014, 50, 1675--1677 | 1677