Unveiling the Reversibility of Crystalline Ì¶ Amorphous Nanostructures

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C: Physical Processes in Nanomaterials and Nanostructures

Unveiling the Reversibility of Crystalline # Amorphous Nanostructures via Sonication Induced Protonation Madoori Mrinalini, Balahoju Shiva Prasad Achary, Samrat Ghosh, D. Koteshwar, Seelam Prasanthkumar, and Lingamallu Giribabu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01807 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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The Journal of Physical Chemistry

Unveiling the Reversibility of Crystalline ̶ Amorphous Nanostructures via Sonication Induced Protonation Madoori Mrinalini,†,§ B. Shiva Prasad Achary,†,§ Samrat Ghosh, ‡,§ D. Koteshwar,†,§ Seelam Prasanthkumar,*,†,§ Lingamallu Giribabu*,†,§ †

Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology (IICT), Tarnaka, Hyderabad-500007, Telangana, India

‡

Photosciences and Photonics Section, Chemical Sciences and Technology Division, CSIRNational Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Thiruvananthapuram-695019, (India)

§

Academy of Scientific and Innovation Research (AcSIR), New Delhi

ACS Paragon Plus Environment

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ABSTRACT

Self-assembled π-conjugated molecules exhibit switching between crystalline-amorphous nanostructures attract significant interest in the field of organic electronics, particularly memory devices. Herein, we reported ferrocene appended tetratolylporphyrin, H2TTP-Fc undergoes protonation in 1, 2-dichloroethane (DCE) via sonication and reverse to original state by deprotonation about time, confirmed by optical and electrochemical properties. Absorption spectra revealed selectivity of reversible and irreversible protonation of H2TTP-Fc in halogenated solvents and mineral acids. Microscopic analysis suggested that H2TTP-Fc aggregates exhibits crystalline flower like morphology from the joining of 2D micro sheets, while H4TTP-Fc form nanospheres with average diameter of 150 – 200 nm upon methanol vapour diffusion approach (MVD). Electrochemical properties of H4TTP-Fc films revealed ease of oxidation when compared to solution state as a result high current generation at less work function. Thereby, these novel features aid to design efficient organic redox active materials for hazardous pollutant detection and organic-electronic applications.

INTRODUCTION Diversity of crystalline and amorphous nanostructures adopt self-assembly by non-covalent interactions possess potential applications such as photoactive switches,1-3

light-harvesting

systems,4-8 sensors9-12 and field effect transistors.13-18 Despite the fact that numerous organic materials exhibit well defined nanostructures, crystalline-to-amorphous transitions rarely explored.19-22 Consequently, redox active system regulate the crystalline behaviour aid to diverse electronic properties.23-27 In this context, ferrocene is considered one of the important redox active material due to ease of oxidation, when covalently linked with aromatic systems depict


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remarkable influence on optical, electrochemical and self-assembled properties.28-30 Among various aromatic π-conjugated systems, porphyrin consisting of nitrogen centred four pyrrole rings and highly delocalized π-electron cloud serve as good redox system and their planar structure assist aggregation behaviour, thus, exceptional derivative in organic electronics.31-36 Recently, porphyrin-ferrocene based donor-acceptor systems extensively investigated optical and electrochemical properties however self-assembled studies still unexplored.28 Therefore, we reported the tetratolylporphyrin attached ferrocene system (H2TTP-Fc) undergo protonation in the presence of sonication and deprotonated by means of time (Figure 1).37 Electron microscopy revealed H2TTP-Fc forms microcrystalline flower like morphology and H4TTP-Fc displayed amorphous nanospheres. To the best of our knowledge, reversibility of crystalline – amorphous nanostructures via protonation/deprotonation mechanism an exceptional phenomenon.

EXPERIMENTAL SECTION Synthesis: Tetratolylporphyrin attached ferrocene system (H2TTP-Fc) was synthesized and characterized as per literature reported method. 37 Characterization

Techniques:

1

H Nuclear

magnetic resonance

(NMR) spectra of

protonated/deprotonated samples recorded on a 400 MHz INOVA spectrometer using CDCl3 as internal reference. Matrix-assisted laser desorption ionization time-of-flight (MALDI–TOF) mass spectrometry performed on Shimadzu Biotech Axima Performance 2.9.3.20110624: Mode Reflectron-HiRes, Power: 85. UV-visible and Fluorescence Measurements: Electronic absorption spectra were recorded on a Shimadzu (Model UV-3600) spectrophotometer. Steady-state fluorescence spectra of solutions


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were recorded on a Fluorolog-3 spectrofluorometer (Spex model, JobinYvon) at wavelength of excitation (λexc) = 420 nm, 445 nm, 550 nm and 650 nm. Fluoroscence lifetime measurements measured on a picoseconds time-correlated single photon (TCSPC) setup (Fluoro Log3-Triple Illuminator, IBH Horiba JobinYvon) employing a picoseconds light emitting diode laser (NanoLED, λexc = 440 nm). The samples for the analyses were prepared in DCE and evaluated using 1 cm cuvette at 25 °C. Electrochemical studies: Cyclic voltammetry experiments were performed on a PC-controlled CH instruments model CHI 620C electrochemical analyzer. The pink and green coloured solutions of H2TTP-Fe recorded in DCE at scan rate of 200 mV/s using 0.1 M tetrabutylammonium hexafluorophosphate (NBu4PF6)). The working electrode is glassy carbon, standard calomel electrode (SCE) is reference electrode and platinum wire is an auxiliary electrode. Thereafter, the aggregates were coated onto the glassy carbon electrode using 0.1 weight % of Nafion in ethanol: water.

CV performed in acetonitrile using 0.1 M

tetrabutylammonium hexafluorophosphate (NBu4PF6)) at scan rate of 200 mV/s in comparison to solution state. Powder X-ray Diffraction (PXRD) studies: Powder X-ray diffraction studies of H2TTP-Fc and H4TTP-Fc aggregates were prepared by methanol vapour diffusion approach for two days at 25 °C in DCE, transferred onto a glass slide, which was allowed to dry slowly. The X-ray diffractograms of the dried films were recorded on a Simens D5000 X-ray diffractometer using Cu Kα radiation. Microscopic Analysis: Transmission Electron Microscope (TEM) measurements were carried out using FEI (TECNAI G2 30 S-TWIN) with an accelerating voltage of 100 kV. Samples were


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prepared by drop casting the aggregates of H2TTP-Fc and H4TTP-Fc in DCE on carbon coated copper grid and images were obtained without staining. Atomic Force Microscopy (AFM) images recorded under ambient conditions using a NTEGRA (NT-MDT) operating with a tapping mode regime. Micro-fabricated TiN cantilever tips (NSG10) with a resonance frequency of 299 kHz and a spring constant of 20-80 Nm-1 used for measurement. AFM section analysis was done offline. Scanning electron microscopy (SEM) measurements were performed using HITACHI-S 3000 N and recorded the images by drop casting the aggregates on copper substrate directly at 25 °C. RESULTS AND DISCUSSIONS Sonication Induced Protonation In this report, H2TTP-Fc comprised of tetratolylporphyrin linked with ferrocene utilized as a redox active system37 and examined their optical, electrochemical and self-assembled properties by sonication and time. Absorption and emission studies of H2TTP-Fc were investigated in 1, 2dichloroethane (DCE) at a concentration of 1 × 10-4 M. UV-vis absorption spectrum of pink coloured solution of H2TTP-Fc displayed Soret band at 420 nm and Q bands at 517 - 648 nm. Interestingly, pink coloured solution turned to green while sonication and exhibits two new sharp and broad absorption bands at 445 and 700 nm together with characteristic peaks of H2TTP-Fc. Subsequently, green colour solution reverse to pink colour by disappearance of new bands whilst maintenance after 8 h, suggests the sonication induced protonation of H2TTP-Fc and retained to initial state about time (Figure 2a).38 Consequently, kinetically controlled experiments performed to confirm the saturation time of reversibility of H2TTP-Fc in 1,2-DCE by recording absorption every consecutive 10 min and 1 h (Figure S1). Figure 2b represents the time against absorbance at 445 nm exposed deprotonation occurred at 8 h and remains same after prolonged time.


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Therefore, pink – green and vice versa were observed while sonication and time gap 8h. Afterwards, fluorescence spectra recorded for three solutions and emission maxima observed at 653 and 716 nm upon excitation wavelength at 420 nm (Figure 2c). However, fluorescence intensity quenched for sonicated sample when compared to others, indicates protonation of H2TTP-Fc aid to enhance electron transfer between ferrocene to porphyrin. Likewise, emission spectra recorded at different excitation wavelengths of 445, 550 and 650 nm and found identical to λexc; 420 nm (Figure S2). Furthermore, time correlated single photon counting (TCSPC) revealed sonicated H2TTP-Fc gave biexponential decay profile with apparent values of 1.99 and 5.26 ns, whereas initial and reverse solutions displayed 5.63 and 10.87 ns upon excitation at 440 nm nanoLED source and monitored at emission maximum of 698 nm. Thus, protonation of H2TTP-Fc induces the difference in energy levels between ferrocene and porphyrin trigger efficient electron transfer from ferrocene to porphyrin when compared to deprotonated state (Figure 2d). Therefore, optical properties confirmed the changes in visual and electronic features via sonication facilitates hydrochloric acid formation in DCE leads protonation of the core nitrogen atoms of the H2TTP-Fc and reverse back to original state by deprotonation suggest switching of electronic behaviour of redox materials under external stimuli aid to yield efficient optoelectronic devices in near future. In order to understand the protonation/deprotonation of H2TTP-Fc affect on electrochemical properties, cyclic voltammetry (CV) were performed on pink and green coloured solutions of H2TTP-Fc by using supporting electrolyte (0.1 M, tetrabutylammonium hexafluorophosphate (NBu4PF6)) with glassy carbon, calomel electrode and platinum (Pt) wire as the respective working, reference and auxiliary electrodes at a scan rate of 200 mV/s. The pink coloured solution of H2TTP-Fc displayed oxidation potentials of 0.78 V and 1.22 V, respectively.


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However, green coloured solution exhibits the values of 0.77 V and 1.19 V indicates protonation of H2TTP-Fc (Figure 2e). Subsequently, reverse process displayed similar potentials like initial state of H2TTP-Fc. Optical and electrochemical properties support the changes in electronic nature of porphyrin and ferrocene via protonation/deprotonation during sonication and time. Furthermore protonation/deprotonation of H2TTP-Fc confirmed by proton nuclear magnetic resonance (1H NMR) and matrix assisted laser desorption ionization – time of flight (MALDITOF) studies. 1H NMR spectrum of H2TTP-Fc displayed the peaks at 8.88 - 8.90 ppm and 7.55 8.26 ppm corresponding to the β-pyrrolic protons which are near and far to the periphery position of phenyl moiety linked to the ferrocene group. On the other hand, sonicated sample exhibit broad proton peaks at the 8.4-8.7 ppm and downfield shift proton peaks at 7.7-7.9 ppm alongside typical proton peaks of H2TTP-Fc indicates protonation on core nitrogen leads H4TTP-Fc.38 Meanwhile, reversible process also recorded and their proton peaks matches with the initial spectrum of H2TTP-Fc suggest deprotonation after time 8 h (Figure S3). Subsequently, MALDI spectra showed mass of H2TTP-Fc at 883.06, whereas protonated sample found at 885.37 results H4TTP-Fc. However, mass of reverse sample showed 883.26 alike initial H2TTP-Fc (Figure S4). Thus, sonication and time lag induces protonation/deprotonation phenomenon in H2TTP-Fc results colour change from pink – green and vice versa, confirmed by optical, electrochemical and spectroscopic analyses. Interestingly, protonation/deprotonation of H2TTP-Fc reflects the selectivity of hazardous halogenated solvents, which is confirmed by optical properties of H2TTP-Fc in 1,2 dibromoethane (C2H4Br2), 1,2 diiodoethane (C2H4I2), 2,2,2 trifluoroethanol (C2H3F3O), carbon tetrachloride (CCl4), chloroform (CHCl3), dichloromethane (CH2Cl2), acetic acid (CH3COOH) and mineral acids (Figure S5-S18) under sonication and time. UV-vis absorption and emission


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spectra displayed reversibility of protonation/deprotonation in C2H4Br2, C2H4I2, and C2H3F3O, whereas other solvents only protonation takes place and remains same regarding time, suggests the selectively detection of toxic solvents by reversible/irreversible protonation of H2TTP-Fc under sonication and time. Nonetheless, optical studies in CCl4 (Figure S15 and S16) determine no change in the spectral properties revealed hydrogen containing halogenated solvents played a crucial role to attain the protonation of H2TTP-Fc. Figure 2f represents the absorbance at 445 nm against several cycles revealed reproducibility of protonation/deprotonation of H2TTP-Fc in DCE upon sonication and time suggest the applications for memory devices.

Reversible Polymorphism After confirming the protonation/deprotonation of H2TTP-Fc, we prepared aggregates by diffusing methanol vapours into the pink and green coloured solutions of H2TTP-Fc and H4TTP-Fc. Figure 3a represents the real colour images of aggregates of H2TTP-Fc and H4TTPFc illustrated differentiation in molecular packing leads distinct coloured solids. UV-vis optical absorption spectra of H2TTP-Fc film displayed 4 nm blue shift in soret band maxima and new scattering band at 450 nm when compared to solution state indicates H-aggregates. In contrast, H4TTP-Fc showed broad spectra with red shift in soret band maxima and new band at higher wavelength region suggest J- aggregates (Figure 3b). Furthermore, surface morphology of H2TTP-Fc and H4TTP-Fc aggregates were carried out by microscopic analyses. Transmission electron microscopic (TEM) images of H2TTP-Fc showed crystalline flower like morphology with average dimensions of micrometer in width and length on carbon coated copper substrate. From the careful examination, 2D micro sheets diverging from a specific point directs flower like morphology (Figure 4a, 4b and S19).


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Subsequently, atomic force microscopic (AFM) images recorded on freshly cleaved mica surface (Figure 4c, 4d and S20) and observed the similar morphology alike TEM. On the other hand, H4TTP-Fc have shown nanospheres with average diameter of 150 - 200 nm, confirmed by TEM and AFM. These studies revealed that microcrystalline structures formed from the systematic packing of H2TTP-Fc facilitated by H-aggregates, while nanospheres observed from Jaggregates of H4TTP-Fc. Consequently, powder X-ray diffraction (PXRD) analysis performed to analyze the molecular packing in self-assembled films of H2TTP-Fc and H4TTP-Fc (Figure 5a). XRD revealed that H2TTP-Fc displayed sharp diffraction peaks at small and wide angle region, hence crystalline nature. The peak at the d-spacing value of 3.69 Å corresponds to intermolecular interactions of H2TTP-Fc. Whereas an additional peak found at the d-spacing value of 6.72 Å indicates tolyl group at the end position of porphyrin stack with another tolyl moiety of H2TTP-Fc and their length corresponding to the 32.04 Å, thereby two molecules coalesce forms bilayers and stack upon each other through π-π stacking leads 2D micro sheets which further produce crystalline flower like microstructure (Figure 5b). In contrast, H4TTP-Fc displayed broad peak and found d-spacing value of 6.70 Å correspond to the bilayer formation of H4TTP-Fc. However, protonation in H4TTP-Fc direct slight tilting of molecules joined together gave nanospheres (Figure 5c). Schematic illustration of protonation/deprotonation directly affect the molecular stacking leads H- and J-aggregates as a result micro flowers and nanospheres and its reversible phenomenon (Figure 6). Furthermore, electrochemical potentials of micro flowers (H2TTP-Fc) and nanospheres (H4TTP-Fc) were estimated by CV upon coated onto the glassy carbon electrode using 0.1 wt % of Nafion solution (Figure 7). CV performed in acetonitrile using 0.1 M tetrabutylammonium


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hexafluorophosphate (NBu4PF6)) at scan rate of 200 mV/s in order to compare the solution state. The resultant data suggest that H2TTP-Fc displayed two oxidation potentials 0.40 V and 1.16 V corresponding to the ferrocene and porphyrin moieties. However, H4TTP-Fc showed considerable shift in the oxidation potentials of 0.20 V and 1.14 V suggest that ease of oxidation while protonated nanospheres facilitates to generate efficient optoelectronic devices. Moreover, self-assembled films exhibit superior performance than solution state.

CONCLUSIONS In conclusion, reversibility of protonation/deprotonation of H2TTP-Fc found in DCE under sonication and time lag undergo self-assembly leads amorphous nanospheres facilitated by Jaggregates of H4TTP-Fc and reverse back to crystalline flower like morphology via Haggregates from H2TTP-Fc. Optical properties demonstrated the rational approach of selective detection of hazardous halogenated solvents considered highly important for environmental safety.

Electrochemical data proved that self-assembled films exhibit enhanced current

generation than solution state which paves the way for the design of redox active materials assist hierarchical growth of controlled nanostructures for future generated organic electronics.

ASSOCIATED CONTENT UV-vis-NIR, Emission, 1H NMR, MALDI-TOF, and morphological studies of H2TTP-Fc in various solvents. “This material is available free of charge via the Internet at http://pubs.acs.org.”


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AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (ORCID: 0000-0001-6287-1977) [email protected] (ORCID: 0000-0001-5936-7729) Author Contributions M. M. S. P. and L. G. designed the experiment and wrote the manuscript. M. M. performed the spectroscopic measurements, characterization of H2TTP-Fc/H4TTP-Fc. B. S. P. A and D. K. performed H2TTP-Fc synthesis. S. G. performed microscopic analyses. All authors participated in the discussion of results and revision of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the SERI, Department of Science and Technology, Government of India. S. P. thanks to the DST-Inspire Faculty fellowship. M. M, B. S. P and S. G. are thankful to the CSIR for research fellowships.


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(37) Gokulnath, S.; Achary, B. S.; Kumar, C. K.; Trivedi, R.; Sridhar, B.; Giribabu, L. Synthesis, Structure and Photophysical Properties of Ferrocenyl or Mixed Sandwich Cobaltocenyl Ester Linked meso‐Tetratolylporphyrin Dyads. J. Photochem. Photobiol. 2015, 91, 33 – 41. (38) Mhuircheartaigh, M. É. Ní.; Blaua, W. J.; Pratob, M.; Giordani, S. Spectroscopic Changes Induced by Sonication of Porphyrin-Carbon Nanotube Composites in Chlorinated Solvents. Carbon. 2007, 45, 2665 – 2671.


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Page 18 of 25

FIGURES

Figure 1. Molecular structures of H2TTP-Fc and H4TTP-Fc depicting the pink – green transition and vice versa during sonication and time lapse of 8 h.


18

Page 19 of 25

(a)

(b) 2.0

0.4 On Sonication Reverse

1.5 Absorbance

Absorbance/445 nm

H2TTP-Fc

1.0 0.5

0.3 0.2 0.1 0.0

0.0 300

(c)

400

500 600 700 Wavelength (nm)

800

0

2

4 6 Time (h)

8

10

(d) 4

100

10

Prompt Decay of H2TTP-Fc

H2TTP-Fc On Sonication Reverse

3

Decay on Sonication Decay on Reverse

10

60

Counts

Fl. Intensity

80

40

2

10

1

10

20 0 600

0

700 Wavelength (nm)

800

(e)

10

0.0

0.5

1.0 1.5 Time (ns)

2.0

(f)

6

H2TTP-Fc

4

On Sonication Reverse

2

Absorbance/445 nm

Current (µ A)

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 -2 -4

ON

ON

OFF

OFF

0

1

ON

OFF

OFF

ON

OFF

-6 -2

-1

0 Voltage (V)

1

2

2 3 No. of Cycles

4

Figure 2. Optical and electrochemical properties of H2TPP-Fc in DCE at a concentration of 1 x 10-4 M and performed experiments at initial, sonication and time conditions

a) UV-vis

absorption spectra. b) Plot of time Vs absorbance at 445 nm. c) Emission spectra. d) photoluminescence decay profile. e) Cyclic voltammetry. f) Plot of absorbance at 445 nm Vs number of cycles while sonication and time gap.


19


(a)

H2TTP-Fc

H4TTP-Fc

(b) Normalized 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

Page 20 of 25

1.2 H2TTP-Fc H4TTP-Fc

0.8

0.4

0.0 300

400

500 600 700 Wavelength (nm)

800

Figure 3. a) Real images of aggregates of H2TTP-Fc and H4TTP-Fc. b) Normalized UV-vis absorption spectra of H2TTP-Fc and H4TTP-Fc films on glass slide.


20

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a)

b)

50 nm

1 µm

1 µm

c)

d)

1 µm

1 µm

Figure 4. a, b) TEM images of an air-dried suspension of H2TTP-Fc and H4TTP-Fc dropcasted on carbon coated grid at 25 °C. c, d) AFM images of H2TTP-Fc and H4TTP-Fc aggregates on freshly cleaved mica substrate.


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Page 22 of 25

Figure 5. a) Powder XRD of H2TTP-Fc and H4TTP-Fc films recorded at 25 °C. b, c) Molecular packing of H2TTP-Fc and H4TTP-Fc leads H-and J-aggregates from the represented d-spacing values.


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Figure 6. Schematic illustration of switching between H2TTP-Fc and H4TTP-Fc via sonication and time, undergo polymorphism by methanol vapour diffusion approach.


23


20 H2TTP-Fc Current (µA)

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

Page 24 of 25

H4TTP-Fc 10

0

-10 -2

-1

0 1 Voltage (V)

2

Figure 7. Cyclic voltammetry of H2TTP-Fc and H4TTP-Fc aggregates coated onto the glassy carbon electrode and recorded in acetonitrile solution.


24

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TOC GRAPHIC


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Unveiling the Reversibility of Crystalline Ì¶ Amorphous Nanostructures

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