Ionization-Induced Reversible Aggregation of Self-Assembled

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Ionization–Induced Reversible Aggregation of Self-Assembled Polycarbonyl Hydrazide Nanoparticles: A Potential Candidate for Turn-On Base Sensor & pH-Switchable Materials Rewati Raman Ujjwal, Debashis Panda, Anand Pratap Singh, Anuj Kumar, Umaprasana Ojha, and Kumar Kishore ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00039 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Applied Nano Materials

Ionization–Induced Reversible Aggregation of Self-Assembled Polycarbonyl Hydrazide Nanoparticles: A Potential Candidate for Turn-On Base Sensor & pH-Switchable Materials Rewati Raman Ujjwal1, Debashis Panda1, Anand Pratap Singh1, Anuj Kumar1, Umaprasana Ojha1,* & Kumar Kishore2 1

Department of Chemistry, Rajiv Gandhi Institute of Petroleum Technology Jais, Amethi, UP229304, India, 2Asylum Research and Oxford instruments, Oxford Instruments India Private Limited, Mumbai, 400072

KEYWORDS: Hierarchical Self-assembly, pH responsiveness, aza-Michael adduct, anion-π interaction, base sensor, pH-switchable material Abstract Hierarchical assembly of nanostructures remains as one of the desirable targets in nanoscience. Herewith, we report a hydrogen bond promoted polymeric nanoparticle system that reversibly aggregates into different microstructures upon varying the concentration of the base in the medium. PBTH, a polyaza-Michael adduct formed uniform spherical nanoparticles in solution owing to the presence of inherent CO---HNCO hydrogen bond based physical crosslinks in the system. In presence of the base, the CONH groups ionized to form the corresponding nitranions and the resulting anion-π interaction between the ionic polymer nanoparticles promoted the secondary aggregation to different shapes and sizes in micro-domain. The shape of the aggregated microparticles gradually changed from spherical to fiber through flakes on gradually increasing the base concentration in the medium. The modulus of these superstructures

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notably decreased compared to that of the original unionized nanoparticles suggesting the involvement of anion-π interaction and loss of hydrogen bonding in the system. Importantly, these dynamic shape changes in sub-micron range was reversible and addition of protic solvent or acid recovered the original shape and size. PBTH in sufficiently low concentration (40 µg/mL) is capable of detecting various organic and inorganic bases in ppm level and pH in between 8.4 to 11.4 with 1.0 precision. The polymer is also a promising candidate for pHswitchable applications. Introduction Immense expectations are pinned on the stimuli responsive dynamic self-assembly of macromolecular architectures induced by hydrogen bond,1,2 ionic3,4 and charge transfer interaction5,6 to develop materials emulating the nuances and efficiency of naturally occurring systems.7,8 Hierarchical self-assembly of nano-objects to generate organized superstructures,9 though a relatively new area has garnered interest in the last decade due to their potential applications in nano devices,10 biomaterials, nano machines,11 spintronics, sensing, photonic materials,12 membranes and configurable catalysts.13,14 Moreover, self-assembly of nanostructures is important to address several fundamental issues ranging from crystal growth, nano-patterning, lithographic technique to scaling of material properties down to molecular dimensions.15 Noncovalent inter-particle forces such as electrostatic, dipole-dipole, π-π, van der waals, hydrophobic and hydrogen bond interaction provide an attractive avenue to self-assemble these nanoparticles (NP) to 1D and 2D microstructures with controllable geometry. For example, anisotropic NPtemplated deoxyribonucleic acid (DNA) were utilized to generate various superlattice architectures.16 Similarly, a lipid-bilayer assisted self-assembly procedure was used to assemble DNA nanostructures into 2D lattices.17 Polarity of the medium was also utilized to successfully


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achieve reversible hierarchical self-assembly of polypeptide based polymer micelles in solution.18,19 Since the self-assembly process is constrained by relaxation to thermodynamic equilibrium, fast aggregation rate owing to a strong thermodynamic driving force may result in substantial amount of defects and large polydispersity index (PDI) of the self-assembled particles.11 Slow aggregation rate may hinder quantitative completion of the assembly process and pose a concern towards implementation of the method for commercial applications. Therefore, precise control over the extent of above physical interactions is necessary to control the aggregation process and achieve ordered assembly with low PDI. Electrostatic forces is considered as one of the viable option to achieve NP self-assembly, since this particular interaction can be both attractive or repulsive in nature.20,14 For example, anion-π interaction between anionic surfactants and calixarene derivatives guided the formation of dynamic vesicles with narrow size distribution.21 Similarly, the trade-off between electrostatic attraction and repulsion was employed to achieve the aggregation of gold NPs.22 Electrostatic forces in conjunction with other interactive forces were also efficiently utilized to guide the hierarchical self-assembly of NP.23,24 Therefore, introduction of anion-π interactions in a precise and reversible manner is anticipated to result in ordered self-assembly and disassembly of NPs with narrow PDI. In this report, we design to achieve the above by inducting charge into a polymeric NP system with the help of an external stimuli. We further plan to understand the effect of anion-π interaction density on the self-assembly pattern and size distribution in a polymeric system. Fascinating use of versatile amine-ene25,26 and thiol-ene27,28 click additions to develop a range of useful polymeric architectures is well documented in literature.29,30 Carbonyl hydrazide functionality is known to possess swift reactivity towards a range of complementary



functionalities, such as aldehydes, carboxylates, acids and active double bonds.31,32 Exemplary utilization of carbonyl hydrazide, especially through the formation of acyl hydrazone33,34 linkage clearly outlines the potential of the above functionality as a key component in material synthesis.35,36 In the past, we have utilized carbonyl hydrazide functionality to develop epoxy networks,37 injectable gels38 and drug encapsulated nanogels39. Carbonyl hydrazide functionality is already reported to be a versatile Michael-donor in literature.40 Therefore, aza-Michael addition of carbonyl hydrazide with activated doubles bonds offers a viable pathway to develop various polymeric architectures. Herewith, suitable Michael-acceptor possessing highly activated double bonds is utilized to carry out the polymerization under catalyst free conditions.41 Hydrogen bond promoted self-assembly of the resulting polymer in solution is studied. Ionization of the polymer in presence of base and subsequent aggregation of the polymer selfassembly is analyzed. The reversibility of the secondary aggregation with respect to the charge neutralization is accessed. Experimental Materials: Thiodiglycolic acid (Acros Organics, 98.0%), diethyl malonate (DEM, s-d fine chem., 98%) N, N-dimethyl formamide (DMF, Merck, ≥ 99.8%), acetonitrile (ACN, Spectrochem, 99.5%), N-methyl-2-pyrrolidone (NMP, SRL-chemicals, 99.5%), sodium chloride (Qualigens, >99.9%), hydrazine hydrate (s-d fine chem., 99%), sodium nitrite (Merck, ≥98.0%), sodium hydroxide (NaOH, Qualigens, 98.0%), hydrochloric acid (HCl, s-d fine chem., 35-38%), ammonium hydroxide (Qualigens, 25-30%), D2O (Sigma Aldrich, 99.0%), chloroform (CHCl3, s-d fine chem., 99.5%), CHCl3-D (CDCl3, Sigma Aldrich, 99.8 atom %D), dimethyl sulfoxide-D6 (DMSO-D6, Sigma Aldrich, 99.9 atom %D), methanol (Qualigens, 99.0%), sulfuric acid (Merck, 98%), ethyl acetate (Merck, ≥99.5%), diethyl oxalate (s-d fine chem., 99%), 1-bromobutane (s-d


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fine chem., 98%), potassium carbonate (Qualigens, 98.0%), terephthalaldehyde (Alfa Aesar), piperidine (Qualigens, 99.0%), acetic acid (s-d fine chem., 99.5%), 3,4-hexadione (Acros Organics, 96%), ethanol (Merck, 99.9%), Na metal (s-d fine chem., >98%), tetrabutyl ammonium hydroxide (TBAH, Qualigens, 25-27% in methanol), tertiary-butanol (t-BuOH, Qualigens, 98.0%), potassium-t-butoxide (t-BuOK, Spectrochem, 99.0%), sodium methoxide (NaOMe, s-d fine chem., 98.0%) were used as received. Tetrahydrofuran (THF, Qualigens, 99.0%) was refluxed over sodium metal and benzophenone overnight and distilled under a nitrogen atmosphere prior to use. Characterization:

13

C and 1H NMR spectra were recorded using a Bruker AMX-500

spectrometer at a probe temperature of 26 °C. The 1H and 13C NMR spectra were recorded at 500 and 125 MHz respectively. 1H NMR spectra of solutions in DMSO-D6 or CDCl3 were calibrated to tetramethylsilane (TMS) as internal standard (δH 0.00). Quantitative

13

C NMR spectra were

acquired using low (30°) flip angle and a long relaxation delay (10 sec). Polynomial fourth-order baseline correction was performed before manual integration of all NMR spectra. 1H-1H homonuclear gradient COSY 2D NMR spectra were obtained using 256 increments of 1K data points, 16 scans, and 4 dummy scans with a recycle delay of 1 sec. The Perkin Elmer Spectrum Two FT-IR spectrometer was used to record the FT-IR spectra of the samples as either solid or thin films. All the samples were recorded using “Attenuated Total Reflectance” (ATR) mode. The PIKE MIRacleTM ATR accessory equipped with ZnSe ATR crystal was used for recording spectra. Lab India UV-VIS 3200 was used to record the UV-Vis Spectra of the sample. UV-Vis spectra were recorded at 1 nm/min scan rate. Cary Eclipse Fluorescence Spectrophotometer (serial no. MY14270004) was used to record the fluorescence spectra of the sample. The spectrophotometer used a xenon flash lamp for superior sensitivity, high signal-to-noise ratio,



and fast kinetics. Fluorescence spectra were recorded at a scan rate of nm/min. The fluorescence spectra of polymer in presence of different NaOMe concentrations were recorded by exciting the solutions at their respective absorption maximum (λmax = 302 to 430 nm). Circular Dichroism (CD) spectroscopic data were recorded within a µM concentration range in 1 mm quartz cell on J-815, JASCO. CD spectra were recorded as θ in units of millidegrees (mdeg). The quantity θ was converted to ∆ε using the equation ∆ε = θ / (32982*c*l), where ∆ε is the difference in molar absorptivity for oppositely polarized light expressed in M-1 cm-1, c is the concentration of the sample (µmol/L), and l is the path length through the cell (cm). Field emission scanning electron microscopic (FESEM) images were recorded using a Carl Zeiss-Sigma field-emission microscope operating at an acceleration voltage of 3 kV. Samples were prepared by drop-casting the polymer solution on silver foil. The samples were sputtered with gold before recording the images. Cryo Transmission Electron Microscope (TEM) analysis was conducted using FEI TecnaiTM G2 12 Twin operated at 120 kV. The specimens were prepared by drop-casting the PBTH solution with or without base in ACN on a copper grid coated with transparent graphite. The solvent was evaporated at 25 °C and the sample was stored at 25 °C for 72 h to allow sufficient time to the self-assembled structures to attend stable conformation prior to recording the images. The size distribution curves of the particles present in the solutions were measured by dynamic light scattering (DLS) approach using a Zetasizer Nano-ZS (Malvern) equipped with green laser (523 nm). Intensity of scattered light was detected at the angle of 173°. For each sample, 5 measurements were performed. The data processing was carried out using the Zetasizer software 7.10 (Malvern Instruments). The size distributions were reported as volume and number distributions. The ground state geometries of chromophoric segments of PBTH were optimized using the density functional theory (DFT) method with the


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Becke3LYP functional42,43 as implemented in GAUSSIAN 09w software package.44 The default options for the self-consistent field (SCF) convergence and threshold limits in the optimization were used. Time dependent density functional theory (TDDFT) calculations were performed on the gas phase optimized geometry of the ground state (S0).45 Vertical excitations were carefully analyzed by inspecting the corresponding Kohn-Sham orbital contours. The gel permeation chromatography (GPC) (Viscotek pump; two ViscoGel I-Series G4000 columns; Viscotek refractive index (RI) detector operating at λ = 660 nm, Viscotek model 270 series platform consisting of a laser light scattering detector at 3 mW with λ = 670 nm, detection angles of 7°and 90°, and a four-capillary viscometer) was used to determine molecular weights and molecular weight distributions, using a flow rate of 1 mL/min. The system was calibrated with polystyrene (PS) standards of narrow molecular weight distribution. The Atomic force microscopy (AFM) topography images were recorded under ambient conditions on an Asylum MFP-3D classic AFM. The samples were prepared by evaporating the solutions at room temperature. The ACN solutions of PBTH in presence of different [NaOMe] were left undisturbed for 5 days before drop-casting to allow the samples sufficient time to attain a stable conformation. The drop-cast samples were dried under ambient conditions for 3 more days before recording the images. The spring constant of the cantilever was calibrated using a similar procedure reported in literature.46 The images were obtained using standard silicon cantilevers (with resonance frequency of 300 KHz and spring constant of 40 N/m) with AC mode. The silicon AFM probe mounted on the amplitude modulation (AM)–frequency modulation (FM) cantilever holder was used for nanomechanical characterization under viscoelastic mapping mode. The AM-FM mode involves the detection of two cantilever resonances simultaneously. With increase in load, the calculated indentation decreased and eventually ended up with a



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negative value. The indentation depth with respect to tip geometry was then fitted using Hertz model based on the equation below to obtain the modulus value. 2 = tan 1 − ν

whereby ν is the Poisson’s ratio, δ is the indentation depth and α is the half cone angle of the probe (36°). Herewith, the un-corrected data is presented in a relative scale and the difference between highest and lowest value is considered as the modulus of the material. Diethyl-2,2´-thiodiacetate,

diethyl-3,4-dihydroxythiophene-2,5-dicarboxylate,

and

diethyl-3,4-dibutoxythiophene-2,5-dicarboxylate, were synthesized using procedures reported earlier.47 Tetraethyl-1,4-phenylbis(methanylylidene) dimalonate (TPMD): Terepthalaldehyde (3.0 g, 22 mmol) was dissolved in anhydrous ethanol (50 ml) in a round bottom flask. To it DEM (8.9 g, 56 mmol) was added followed by piperidine (0.2 g, 2 mmol) and acetic acid (0.1 g, 2 mmol). The resulting mixture was refluxed overnight under inert atmosphere. The temperature of the solution was then decreased to room temperature and the solvent was allowed to evaporate slowly to crystallize the product. The final product was obtained as white crystals (4.5 g) in 48% yield. 1H NMR (500 MHz, CDCl3) δ (ppm): 7.7 (s, 2H, CH=C(COO)2), 7.5 (s, 4H, Ar H), 4.3 (q, 8H, -O-CH2-), 1.3 (m, 12H, -O-CH2-CH3).

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C NMR (125 MHz, CDCl3): δ (ppm) 164.5 (-

COOCH2), 140.7 (-CH=C-(COO)2), 134.6 (Ar C), 130.1 (-CH=C-(COO)2), 127.4 (Ar C), 60.2 (O-CH2-), 14.0 (-O-CH2-CH3). FT-IR (thin film, cm-1): 770 (m, p-substitution), 1230 (s, C-O), 1427 (m, C=C), 1460 (w, C-H), 1624 (m, C=C, Ar), 1718 (s, C=O), 2875 (w, C-H), 2971 (m, CH). 3,4-Dibutoxythiophene-2,5-dicarbonylhydrazide (BTH): A solution of dimethyl-3,4dibutoxythiophene-2,5-dicarboxylate (6.5 g, 19 mmol) and hydrazine hydrate (10.4 g, 208 mmol)


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in ethanol (50 ml) was reﬂuxed for 4 h. The resulting solution was concentrated by rotary evaporation and the residue was poured into water (100 ml). The precipitate settled at the bottom was filtered using Whatman filter paper (pore size = 11 µm) and recrystallized from ethanol/water (50:50, v:v) solution to obtain the final product (4.1 g, 12 mmol) as a pale white solid in 63% yield. 1H NMR (500 MHz, CDCl3) δ (ppm): 8.3 (s, 2H, -CONH), 4.2 (t, 4H, -OCH2-), 4.1 (s, 4H, -NH2), 1.8 (m, 4H, -CH2-), 1.5 (m, 4H, -CH2-), 1.0 (m, 6H, -CH3). 13C NMR (125 MHz, CDCl3) δ (ppm): 162.0 (-CO-NH-), 147.4 (-S-C=C-O-), 123.9 (-S-C=C-O-), 74.3 (O-CH2-), 31.9 (-CH2-), 19.1 (-CH2-CH3), 13.8 (-CH3). FT-IR (thin film, cm-1): 1301 (s, C-S), 1511 (s, C=C), 1645 (s, C=O), 2871 (m, C-H), 2966 (m, C-H), 3203 (m, C-H), 3048 (m, C-H), 3307 (m, N-H). Polycarbonyl hydrazide (PBTH): BTH (1.5 g, 4 mmol) and TPMD (1.8 g, 4 mmol) were dissolved in dry THF (10 ml) and stirred for 24 h under N2 atmosphere at 25 ºC. The product obtained was precipitated and repeatedly washed with methanol to remove the low molecular weight impurities. The sticky solid obtained was dried under vacuum before further characterization. 1H NMR (500 MHz, CDCl3) δ (ppm): 8.4 (br, 2H, -NH), 7.4 (m, 4H, Ar H), 4.8 (br, 2H, -CH-CH-(CO-)2), 4.3 (m, 12H, -CH2-O-), 4.2 [br, 2H, -CH-CH(CO-)2], 3.8 (s, 2H, -NHCH-), 1.8 (br, 8H, -CH2-), 1.3 (m, 12H, -CH3), 0.9 (m, 6H, -CH3), 13C NMR (125 MHz, CDCl3): δ (ppm) 166.7 (-COO-), 163.9 (-CONH-), 140.7 (-S-C=C-O-), 133.0 (-S-C=C-O-), 129.8 (Ar C), 128.1 (Ar C), 77.4 (-O-CH2-), 62.0 (-COO-CH2-), 61.8 (-CH-CH-(COO-)2), 61.6 (-CH-CH(COO-)2), 19.3 (-CH2-), 14.2 (-CH3), 13.9 (-CH2-CH3), 13.8 (-CH2-CH3). FT-IR (thin film, cm1

): 1063 (s, C-S), 1500 (s, C=C), 1652 (m, C=O), 1732 (s, C=O), 2965 (m, C-H), 3288 (s, N-H),

GPC (THF): Mn = 6200 g/mol, polydispersity index (PDI) = Mw/Mn = 1.9. Results & Discussion



The Knoevenagel condensation between terephthalaldehyde and DEM was utilized to synthesize TPMD (Scheme 1).48 BTH was synthesized using a procedure similar to that of the reported in literature.47 PBTH was synthesized by employing facile aza-Michael type polyaddition between the activated double bonds of TPMD and carbonyl hydrazide functionality of BTH under catalyst free and ambient conditions (Scheme 1).

Scheme 1. Synthetic scheme for semi aromatic polyaza-Michael adduct (PBTH) The FT-IR band at 1718 cm-1 (-C=O) assigned to TPMD shifted to 1732 cm-1 and the intensity of band at 1624 cm-1 (C=C) substantially decreased after polymerization suggesting formation of the polyaza-Michael adduct (SI, Figure S1). The band at 1645 cm-1 accountable to CONHNH2 of BTH shifted to 1652 cm-1 in PBTH supporting polymerization. In 1H NMR spectrum of PBTH, new resonances at 4.2, 4.8, and 8.4 ppm accountable to –CH-CH(COO)2, – NH-CH-CH(CO-)2 and –CONH- respectively appeared and the integration of peak at 7.7 ppm


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accountable to Ph-CH=C- of TPMD substantially decreased suggesting effective polyaddition (Figure 1A). The 2D COSY NMR spectrum of PBTH showed the correlation of resonance at 3.8 ppm (CONHNH) with both the peaks at 4.8 and 8.4 ppm suggesting the peak at 4.8 ppm is accountable to –NH-CH-CH(CO-)2 (SI, Figure S2). The Mn of PBTH determined from the GPC analysis was found to be 6200 g/mol with a PDI of 1.9. The polymer was readily soluble in a range of common organic solvents such as DMF, CHCl3, THF, ACN and NMP. b

d

a

C

c

TPMD * e

B BTH

b

a

c d

f

i

h

f O

HN HN

A

O

O

e

O

S O

O

O Ha N N Hg

c

a 8.0

7.0

6.0

k d O

c

b

PBTH

e

f

*

9.0

a

* j

10.0

b

d

c

5.0

h, i, j, k b

de f

g

4.0

3.0

2.0

1.0

0.0

Chemical Shift (ppm) Figure 1. 1H NMR spectra of (A) PBTH and precursors (B) BTH and (C) TPMD recorded in CDCl3 solvent, *the peak marked with “asterisk” is assigned to the solvent. The peaks labeled as “a ̶ k" in “A” are assigned to the protons present in PBTH. Modulation of Optical Properties via ionization



The optoelectronic properties of PBTH was studied in various non-polar (CHCl3 & THF) and polar (ACN & NMP) solvents to understand the origin of and factors affecting the photophysical properties. The solution of PBTH in ACN was colorless and exhibited an absorption maximum (λmax) at 295 nm (Figures 2A & D). Interestingly, with the addition of base (NaOMe), the λmax of PBTH got red-shifted from 295 to 375 nm and emergence of a peak at 443 nm was noticed. The color of the solution turned yellow and the intensity of color gradually increased with the concentration of NaOMe (Figure 2A). The gradual bathochromic shift in λmax was associated with two notable jumps for a change in [NaOMe] from 45 to 60 µmol/L (red shift ≈ 21 nm) and 300 to 500 µmol/L (red shift ≈ 77 nm) respectively (Figure 2B). The first red shift (21 nm) at low concentration of base can be attributed to the reorganization of polymeric structure leading to the exposure of chromophoric unit to a more polar environment and a loss of acidic hydrogen (-1CONHNH2) resulting in formation of mono-anionic PBTH. The λmax of precursor BTH shifted from 295 to 350 nm in presence of NaOMe supporting the formation of nitranion (SI, Figure S3). Interestingly, in presence of high concentration of NaOMe, the intensity of λmax at 443 nm notably increased (Figure 2D). Possibly, the ionization of both the acidic protons (-2CONHNH2) present on either side of the 2,5-biscarbonyl thiophene moiety at high [NaOMe] resulted in the formation of di-anionic PBTH and the λmax further red shifted (Figure 2F). A similar trend in absorption spectra was observed for PBTH in apolar solvents such as THF and CHCl3 (SI, Figures S4 & S5). Notably, a distinct isosbestic point for the acidbase reaction was observed in NMP solvent (SI, Figure S6). The causes of nonappearance of isosbestic zones in other solvents may be attributed to the fluctuation of total concentration of the components in solution through specific interaction with solvents at molecular level.


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Figure 2. (A) The color of the PBTH solutions in presence of different [NaOMe], (B) & (C) the plots of [NaOMe] versus λmax and fluorescence intensity (FI) respectively in solution, inset C precipitation of PBTH at high [NaOMe] (D) UV-Vis and (E1 & E2) fluorescence spectra of PBTH recorded in presence of different concentrations of NaOMe (µmol/L, I0→ I18: 0, 5, 10, 15, 25, 35, 45, 60, 90, 100, 110, 120, 150, 200, 300, 400, 500, 600 & 750) in ACN and (F) Equilibrium between PBTH and the corresponding polynitranions along with their delocalized structures. To further support the formation of nitranions, 1H NMR and FT-IR spectra of precursor BTH were recorded in presence of NaOMe. The peak at 4.6 ppm accountable to CONHNH2



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notably shifted to 3.5 ppm in presence of base suggesting the formation of –CON− nitranion and shielding of the –NH2 protons by the corresponding negative charge (Figure 3). The decrease in intensity and marginal shielding of the resonance at 8.8 ppm assigned to –CONH- supported the above. The 13C NMR resonance for CONHNH2 shifted from 159 to 164 ppm after treatment with base suggesting ionization (SI, Figure S7). Shifting of the IR band at 1651 (C=O) to 1591 cm-1 in presence of base also supported the formation of –CON− ion and resonance delocalization of loan pair on “N” to the carbonyl moiety (SI, Figure S8). *

f

e

d a

A

a

H2N

f

c O

O

H N

H N S

O

b

c

b

d e

NH2

*

O J = 6.6 Hz

e' a'

B

a'

c'

*

f' d'

f' b'

c'

b'

*

d' e'

J = 6.6 Hz

10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

Chemical Shift (ppm) Figure 3. 1H NMR spectra of (A) BTH and (B) BTH + NaOMe mixture recorded in DMSO-D6. The peaks marked with “*” is assigned to the solvent. “J” stands for the coupling constant of the triplet. The peaks labeled as “a ̶ f” in “A” and “a' ̶ f'” in “B” are assigned to the protons present in BTH and ionized BTH respectively. The PBTH alone showed a very feeble and relatively broad emission spectrum with a peak centered at 410 nm irrespective of polarity of solvents. However, with successive addition of NaOMe to the PBTH solution, the emission peak red shifted up to 470 nm. This was in


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accordance with the ionization-induced conformational change of the polymer resulting in accessibility of chromophoric units to a more polar environment. Moreover, the intensity of the emission band was dependent on the NaOMe concentration (Figure 2C). The intensity of the emission band of PBTH solution gradually decreased on increasing the [NaOMe] beyond 300 µmol/L supporting the aggregation of polymer chains (Inset, Figure 2C). This decrease in fluorescence intensity at relatively high concentration of NaOMe may be primarily due to the aggregation-induced static quenching of fluorescence. To bolster this hypothesis, Resonance Light Scattering (RLS) data of PBTH solution in presence of different NaOMe amount was recorded (SI, Figure S9). RLS study has become an important tool to identify the formation of aggregates. When aggregates are formed, the RLS intensity which is proportional to the square of volume of the scattering particle, is expected to increase. In presence of [NaOMe] above 300 µmol/L, the scattering intensity was very high suggesting the formation of aggregates. PBTH precipitated out in solution in presence of 750 µmol/L of NaOMe confirming large scale aggregation. In order to understand the nature of the temporal properties of ionized PBTH, TCSPC measurements were carried out. An analysis of the fluorescence decay traces revealed that the lifetime value of ionized PBTH gradually decreased from 0.23 to 0.18 ns on increasing the [NaOMe] from 10 to 100 µmol/L (SI, Figure S10, Table S1). This short life time may be attributed to the fast non-radiative process, such as swift conformational change of the flexible chromophoric unit associated with the polymeric structure. Evidence on ionization-induced aggregation hypothesis Ionization-induced aggregation was anticipated to primarily depend on the concentration of NaOMe in solution and ionization extent of PBTH. To the solution of PBTH (6.4 µmol/L) and NaOMe (7.5 mmol/L) in CHCl3, a controlled amount of methanol was gradually added and the



effect was monitored. The λmax hypsochromically shifted from 435 to 350 nm on increasing the methanol amount up to 3.8 mol/L in solution suggesting the shifting of equilibrium from dianionic towards mono-anionic repeating unit of PBTH (Figure 4A). The characteristic isosbestic point for the above reversible transition was visible at 390 nm. The average particle size (Davg) of ionized PBTH self-assembly decreased from 1.0 to 0.3 µm on increasing the amount of methanol up to 1.0 mol/L suggesting the aggregation is reversible and extent of aggregation is dependent on the degree of ionization of the polymer chains in solution (Figure 4B).

Figure 4. (A) Effect of methanol (mol/L: 0.01, 0.02, 0.05, 0.1, 0.2, 0.4, 0.6, 0.9, 1.2, 1.5, 2.4 & 3.8) on the UV-Vis traces of the PBTH (6.4 µmol/L) in CHCl3 containing 7.5 mmol/L of NaOMe, (B) the DLS traces of PBTH (6.4 µmol/L) and NaOMe (7.5 mmol/L) in presence of different amounts of methanol in CHCl3. The standard error is based on three experiments, Inset; the photograph of the PBTH and NaOMe solution in presence of different methanol amount. Quantum Chemical Computations The geometries of the mono-anionic, and di-anionic forms of the chromophoric repeating unit of PBTH were optimized independently using DFT (SI, Figures S12-14). The predicted λmax values obtained from TDDFT analysis were in reasonable agreement with that of the


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experimental data for both mono and di-anionic repeating units. The vertical excitations in lower energy region was computed at the TDDFT/B3LYP/6-311G+(d,p) // B3LYP /6-311G+(d,p) level for the ionic forms of chromophoric units of PBTH in gas phase. The calculations predicted three vertical transitions with acceptable oscillator strength (Figure 5). The major peak red shifted by 18 nm compared to that of the experimental λmax of the mono-anionic form (Table 1). Overall, the spectral features of absorption spectra highlighted from the computational results were in excellent agreement with that of the experimental observations (Figure 5).

0.8 100

0.4

300

500 400 Wavelength (nm)

0 600

0.0050

0.0025

Oscillator Strength

200

Epsilon

1.2

0.0

0.0075

Theoretical Experimental 300 Oscillator strength

1.6

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0000

Figure 5. Experimental (red line) and theoretical (violet line) absorption spectra of monoanionic form of chromophore, PBTH, Computed vertical excitations ( ψ0 →ψv) in lower energy region computed at the TDDFT/B3LYP/6-311+G (d,p)// B3LYP /6-311+G (d,p) level (green vertical line). Deprotonation of PBTH to the di-anionic form was anticipated to cause further red shift of the λmax. The calculated oscillator strength for di-anionic segment supported the significant spectral shift and characteristics (SI, Figure S11). However, a notable underestimation of computed excitation energies for di-anionic segment compared to that of the experimental value was witnessed, which may be attributed to the increased overall charge density, omission of



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solvent polarity and specific interactions such as H-bonding between the chains. Inspection of Kohn-Sham orbital contours for the frontier MOs revealed that the highest intensity transition is π-π* type and occurs predominantly between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbitals (LUMOs) (SI, Figure S12). The electron density of HOMO was localized on the pivotal CON ̶ ion while the LUMO orbitals were visible on the 2,5carboxythiophene moiety of the mono-anionic chromophoric unit of PBTH. For di-anionic form, the electron density of HOMO resided on the thiophenyl ring while LUMO+1 was visualized on benzyl moiety ring. Such complexity in unsymmetrical distribution of electron density may be resolved further by using mixed quantum-classical dynamics, i.e. Surface hopping, Density matrix evolution etc. Table 1: The experimental and theoretical band gap values of PBTH S. No. Species

1

2

Energy Exp#

Mono-anionic 3.43 (361)

Di-anionic

2.75(450)

Transition

Oscillator Strength

Theo* 2.70 (461) HOMO → LUMO

0.0041

3.15 (393) HOMO → LUMO+2

0.0008

3.27 (379) HOMO → LUMO+3

0.0066

2.09 (591)

HOMO → LUMO+1

0.004

2.05 (602)

HOMO → LUMO+2

0.002

Exp# and Theo* are experimental and theoretical values respectively. Solvatochromism The UV-Vis and fluorescence spectra of PBTH (6.4 µmol/L) in presence of NaOMe (0.12 mmol/L) were recorded in different solvents and compared to each other. The λmax got red shifted with increase in polarity of the medium. Notably, the variation of λmax with polarity followed an almost linear trend (Figure 6). Considerable Stokes’ shifts between absorption and emission


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maxima were observed in all the solvents. Stokes shift up to 155 nm was recorded in non-polar solvent (CHCl3), whereas, in polar solvents (ACN, and DMF) the value was limited to 115 nm. PBTH in DMF displayed an orange color and the color changed to yellow in ACN solution (Figure 6, Inset). However, no apparent color was visible for the solution of PBTH in CHCl3. This suggested that the emissive state of the ionized species is significantly stabilized with respect to the ground state in polar solvents.

Figure 6. The UV-Vis and fluorescence spectroscopic traces and (Inset) photographs of PBTH (6.4 µmol/L) and NaOMe (0.12 mmol/L) in different solvents Morphological Transformation Aided by the inherent CONH⋅⋅⋅OC hydrogen bonds, PBTH self-assembled into uniform spherical particles in an inverse micellar manner with the hydrophobic alkyl chains forming the periphery and the hydrogen bond forming carbonyl hydrazide moieties remaining towards the core (Figures 7 & 8). Formation of micelles in the self-assembled spherical NPs was visible in the TEM image of PBTH drop-cast from ACN solution (SI, Figure S15A). The DLS analysis revealed reasonably narrow size distribution (PDI = 0.37) of the particles with Davg around ~190



nm (Figure 7A). The Davg increased marginally (~1.5 times) on increasing the concentration of PBTH solution in ACN from 0.8 to 6.4 µmol/L suggesting secondary aggregation is negligible and concentration has minor effect on the self-assembly of the polymer (Figure 7A). Similarly, the Davg of PBTH self-assembly remained least affected by the change in polarity of the medium (SI, Figure S16). The AFM images of PBTH drop cast from ACN solution (6.4 µmol/L) displayed uniform spherical particles with diameters in the range of 30 ─ 120 nm (Figure 7D1). The self-assembly pattern of PBTH changed markedly in presence of different concentrations of NaOMe (Figures 7B & D2-D6). The spherical PBTH NPs started aggregating and the Davg gradually increased on increasing the amount of NaOMe in the medium (Figure 7B). In the presence of moderate concentration of NaOMe (100 µmol/L), the Davg increased to ~300 nm. The AFM images of the sample drop-cast from PBTH (6.4 µmol/L) and NaOMe (100 µmol/L) solution displayed uniform microspheres with increased diameter (250 − 350 nm) compared to that of the unionized PBTH (~30-120 nm) NPs (Figures 7D1 & D3). Possibly, the formation of nitranions in presence of base altered the conformation of the chains and aided the secondary aggregation of the smaller spherical PBTH particles into bigger ones through anion-π interaction. Similar interactions is already known to regulate the self-assembly of substrates in solution.49 On further increasing the [NaOMe] in solution up to 200 µmol/L, the shape of the aggregates changed from spherical to flower petals (Figure 7D4). The shape change process continued with further increase in amount of NaOMe in solution. In presence of ~300 µmol/L of NaOMe, the average height of the resulting flakes decreased to ~1.5 nm (Figure 7D5). The Davg value obtained from DLS analysis also increased to 450 nm (Figure 7B). Presumably, in presence of high [NaOMe] (≥ 300 µmol/L), both the CONH moieties present in each repeating unit ionized to form the di-anionic species and the existing hydrogen bond based physical crosslinks


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substantially decreased in the system. The resulting anion-π interaction between the ionic PBTH chains controlled and altered the self-assembly pattern of the polymer.

Figure 7. DLS traces of (A) different concentrations of PBTH in ACN and (B) PBTH solution (6.4 µmol/L) containing different amount of NaOMe. The standard error is based on three experiments, (C) CD traces of PBTH in presence of different [NaOMe]. AFM images of the self-assembly of PBTH (6.4 µmol/L) in ACN solution in presence of (D1) 0, (D2) 20, (D3) 100, (D4) 200, (D5) 300 & (D6) 800 µmol/L of NaOMe. Under very high concentration of NaOMe (800 µmol/L), the Davg substantially increased to ~1 µm (Figure 7B). Interestingly, the AFM images of the above solution revealed fibers of



~150-250 nm width. Probably, high concentration of charge in the polymer chains enhanced the hydrophilicity and promoted bulk aggregation of PBTH (Figures 2 & 8). The precipitation of the polymer in ACN in presence of high [NaOMe] supported the above. Importantly, the aggregation was reversible and neutralization of charge by adding a protic solvent or weak acid again decreased the Davg value, which became comparable to that of the unionized polymer (Figure 4B). However, the TEM data failed to further characterize the internal structures of the second order self-assembled structures of different shapes formed in presence of base (SI, Figures S15B & C). Possibly, the involvement of anion-π interaction along with the existing hydrogen bond interaction complicated the aggregation process.

Figure 8: Schematics showing the morphological transformation of PBTH self-assembly in presence of different NaOMe amounts in solution. The FESEM data of PBTH in presence of different amounts of NaOMe displayed a morphological transformation pattern similar to that of the AFM data and supported the formation of hierarchical super-structures (Figure 9). PBTH drop-cast from ACN displayed


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uniform spherical NPs with sizes in the range of 40-100 nm (Figure 9A). In presence of 100 µmol/L of NaOMe, the diameter of the self-assembled particles increased to 200-300 nm suggesting second order self-assembly (Figure 9B). The self-assembly pattern changed to petals in presence of 200 µmol/L of NaOMe, which assumed the shape of flakes (width ≈ 100 nm) in presence of 300 µmol/L of NaOMe (Figures 9C, D1 &D2). In presence of high amount of NaOMe (800 µmol/L), fibrillar pattern of the self-assembly was observed (Figure 9E). Since the repeating unit of PBTH contains chiral center, CD data of the polymer was recorded in presence of different concentrations of NaOMe to support the above hypothesized conformational change of the polymer chains on ionization. The CD trace of PBTH displayed a characteristic peak at 210 nm and revealed irregular conformation of the chains in solution (Figure 7C). Exciton splitting of the π →π* transition may have resulted in appearance of the peak at 210 nm for PBTH.50 In presence of 10 µmol/L of NaOMe, a positive peak at ~205 nm appeared along with the negative peak at 210 nm. On further increasing the [NaOMe] to 100 µmol/L, the sign of CD trace got inverted and a positive peak was observed at 208 nm suggesting a possible inversion of the configuration at chiral center in the PBTH repeating unit upon ionization.51 However, the CD signal became weak in presence of high amount of NaOMe and almost disappeared in presence of 500 µmol/L NaOMe. This could be attributed to the notable decrease in the amount of hydrogen bonds present in the system through ionization and subsequent loss of secondary structure of the polymer chain. The microscale aggregation of the stiffened polymer chains due to formation of the extended chromophore through delocalization of the charge on “N” atom to the thiophene ring may also have contributed towards the loss in optical activity (Figure 8). Since the presence of NaOMe in solution promoted the aggregation of mono and dianionic PBTH, the critical aggregation concentrations (CACs) of these ionic forms were



determined in presence of different [NaOMe]. The onset of change of slope in absorbance versus [PBTH] plots corresponding to three different [NaOMe] was used to determine the CACs (SI, Figure S17).52 The CACs of ionized PBTH were calculated to be 14.5, 9.3 and 2.3 µmol/L in presence of 45, 120 and 400 µmol/L of NaOMe respectively. Since the morphological transformation study was carried out with 6.4 µmol/L of PBTH, weak aggregation was noticed for mono-anionic (CAC ≈ 9.3 µmol/L) polymer and strong aggregation was observed for the dianionic (CAC ≈ 2.3 µmol/L) PBTH in solution. The DLS, scattering data and other microscopic data supported the above observation (Figures 7B and S9).

Figure 9: FESEM images of drop-cast samples of (A) PBTH from ACN solution and PBTH in presence of (B) 100, (C) 200, (D1 & D2) 300 & (E) 800 µmol/L of NaOMe in ACN. To further understand the second order aggregation of self-assembled PBTH NPs, nanomechanical analysis was conducted. The spherical PBTH particles before treatment with base exhibited a Young’s modulus value of ~4 GPa (Figure 10A). The high value of modulus


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supported the presence of CO⋅⋅⋅HNCO hydrogen bond based physical cross-links in the system (Figure 8). The modulus of the PBTH NPs notably decreased after ionization. In presence of 100 µmol/L of NaOMe, the modulus decreased to 1.4 GPa suggesting a possible loss in the extent of hydrogen bonds in the system (Figures 10B, S18). The average height (2.3 nm) of above particles also decreased by ~60% compared to that of the PBTH spheres (5.3 nm) supporting the rearrangement of the self-assembled network due to the loss of hydrogen bond based physical crosslinks (Figure 7D1& D3). The modulus of PBTH self-assembly reached a minimum value (~1.0 GPa) in presence of 300 µmol/L of NaOMe suggesting substantial removal of the hydrogen bonds through ionization (Figures 10C & D). A similar modulus value was observed for the aggregated fibers ((~1.3 GPa) formed in presence of a very high amount of NaOMe (800 µmol/L) (Figure 10E). This residual strength of the fibers could be attributed to the anion-π interactions present in the ionized system. Based on the AFM, FESEM, TEM, DLS, CD and nanomechanical analysis data, a possible mechanism for the shape change may be proposed as schematically shown in Figure 8. PBTH self-assembled into spherical NPs in solution owing to the presence of hydrophobic pendant groups on thiophene moiety and hydrophilic CONHNH2 groups in each repeating unit. The –C=O---HNCO- hydrogen bond stabilized the NPs in solution. In presence of low [NaOMe] (100 µmol/L), the PBTH NPs started to ionize and the resulting anion-π interaction between the NPs promoted the second order aggregation and the NPs merged into microspheres (Davg = 300 nm) in solution (Figures 7D3 & 9B). The increase in hydrophilicity of the polymer through ionization may have aided the aggregation process in organic solvent. With increase in [NaOMe] to 200 µmol/L, further ionization of the microspheres promoted the formation of multi-spherical particles. Simultaneously, since the PBTH self-assembly lost notable amount of hydrogen bonds



due to ionization and the mechanical strength of the spheres decreased, the multi-spherical particles collapsed and assumed the shape of flower petals (Figures 7D4 & 9C). In presence of a high amount of NaOMe (~300 µmol/L), the polymer lost a critical amount of hydrogen bond and the particles became flat and the average height of the resulting flakes decreased from 2.3 to 1.5 nm (Figures 7D5 & 9D1). In presence of very high amount of NaOMe (~800 µmol/L), the system lost most of the hydrogen bonds and the existing self-assembly dissolved. The chain conformation also changed due to simultaneous formation of extended chromophores through delocalization of the charge from nitranion towards the biscarboxy-thiophene moiety (Figure 8). Subsequently, a fiber type self-assembly pattern formed aided by the anion-π interaction between the nitranions and thiophene moieties. Substantial increase in the hydrophilicity of strongly ionized PBTH facilitated the aggregation process further and the particle size notably increased to ~900 nm.


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Figure 10. Nano-mechanical data of the PBTH self-assembled particles solvent-cast from ACN solutions (6.4 µmol/L) possessing different amounts of NaOMe; (A) 0, (B) 100, (C) 200, (D) 300, and (E) 800 µmol/L, (F) the Young's modulus versus [NaOMe] plot of the PBTH selfassembled particles, error bar for each value is included in the graph. The study revealed that, the secondary aggregation of these polymeric NP systems may be reversibly controlled by altering the concentration of base in solution. This ability of PBTH may further be utilized to use the polymer as an efficient carrier and release on demand applications and for hierarchical self-assembly of metal nanoparticles in future. The material is also potentially useful for base sensing application. Potential Applications of PBTH: Turn-on base sensor and pH-switchable material The pH responsiveness of self-assembled nanoparticles are already utilized in the past to sense basic pH in turn-on fashion through a fluorescence resonance energy transfer mechanism.53 The instant coloring of PBTH in presence of various bases through ionization offered the possibility of using this material as turn-on pH and base sensors. In fact, a change in color of the DMF solution of PBTH was visible to the necked eye with respect to a change in pH between 8.4 to 11.4 with a precision of 1.0 (Figure 11A). The sensing efficiency of PBTH towards various organic and inorganic bases was ascertained using UV-Vis spectroscopic procedure. The minimum detection limit was determined by using the following equation;54 Limit of detection (LOD) = 3 × (Sy/S) Where, “Sy” is the standard deviation of response and “S” is the slope of the [analyte] versus absorbance curve. The [PBTH] was fixed at 6.4 µmol/L for all the measurements. A typical calibration curve for calculating the LOD of t-BuOK in ACN is shown in Figure S19 (SI).



The detection limit depended on the strength and solubility of the base in the medium. Most of the bases apart from NaOH were detectable up to < 10 ppm in ACN, THF and CHCl3 (Figure 11B). NaOH was detectable only up to ~20 ppm in CHCl3, which could be attributed to the low solubility of the base in above medium. The detection limit was generally lower in ACN compared to that of the THF and CHCl3. Importantly, a reasonably low amount (6.4 µmol/L) of PBTH was sufficient to sense the basic compounds in ppm level in organic solvents.

Figure 11. (A) Color of PBTH under different pH conditions in DMF (B) detectable concentrations of different basic compounds in PBTH solution as determined by UV-Vis spectroscopic analysis, (C) photographs showing the change in color of the script after exposing to different pH solutions, (D) Six consecutive cycles showing the pH-switchability of PBTH solution in DMF. *pH of the solution is 11.4, #pH of the solution is 7.0. Especially, pH-switchable self-assembled materials are important from the prospective of sensing, functional coatings, imaging, delivery and other applications as outlined in literature.55,56


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Recently a dithienylethene based system was developed for photo-switchable application, in which irradiation of light developed the hidden structure under room temperature conditions.57 Similarly, PBTH and analogous structures could be used to store information, which may be developed later by simply altering the pH of the medium as exemplified in Figure 11C. The script written from the ACN solution of PBTH became visible on dipping in 0.5 N NaOH solution for 30 sec and again disappeared instantly on exposing to 0.5 N HCl solution. The efficiency of the pH-switchability was accessed by observing the λmax of a typical PBTH solution (6.4 µmol/L) in DMF for six consecutive cycles through alternatively switching the pH of the medium between 11.4 and 7.0 (Figure 11D). The λmax shift from 450 (pH = 11.4) to 370 nm (pH = 7.0) occurred instantly with the change in pH and absorbance remained least-affected during the process suggesting that the polymer is a promising candidate for pH-switchable materials. Conclusion Aza-Michael type polyaddition between carbonyl hydrazide and highly activated double bond can be utilized to synthesize polycarbonyl hydrazides under catalyst free and ambient conditions. Driven by the inherent hydrogen bonds, these polymers form uniform spherical NPs in solution. The self-assembled spherical NPs of the polymer reversibly aggregate to various superstructures such as micro-flakes and fibers on modulating the base strength of the medium. Anion-π interaction is presumed to aid the controlled secondary aggregation of the NPs and the base strength can be modulated to tailor the shape of above hierarchical self-assembly. Further studies are in progress to elucidate the mechanism of this shape modulation process. The mechanical properties of the self-assembled particles are also tunable since the extent of hydrogen bond and anion-π interaction in the system changes with the amount of base in the medium. The reversibility of secondary aggregation may be utilized in future to use the material



for release, pH and base sensing, pH-switchable device and shape control of plasmonic NP applications. Substituents on the thiophene moiety could be altered to tailor the optical properties further in the resulting polymers. Supporting Information: FT-IR spectra of PBTH, UV-Vis traces in different solvents, 2D NMR spectrum of PBTH, dilution studies, absorbance versus concentration plots, fluorescence lifetime data, TEM figures, AFM figures and viscoelastic mapping data, CEC graphs, Calibration curve for determination of detection limit and Table containing the DFT data. “The material is available free of charge via the Internet at http://pubs.acs.org.” Author information Corresponding Author *E-mail: [email protected], Tel: 09451959597, Fax: 0535-221-1888, Department of Chemistry, Rajiv Gandhi Institute of Petroleum Technology, Jais, Amethi, 229304 ORCIDID (Umaprasana Ojha):0000-0001-8933-6579 Acknowledgement AK acknowledges CSIR-India for the Junior Research Fellowship. We are thankful to Priyadarsi De, IISER Kolkata for the GPC analysis. We acknowledge research grants from DST (EMR/2016/006464) and DAE [36(4)/14/81/2014-BRNS] for providing partial financial assistance for the work. Notes: The Authors declare no competing financial interest.

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Reversible Aggregation of Polymer Nanoparticles

Hydrogen bond

Hydrogen bond & anion-π interaction

[NaOMe] = 300 µM

Sphere

Anion-π interaction [NaOMe] = 800 µM

Flake


Fiber

Ionization-Induced Reversible Aggregation of Self-Assembled

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