Solvent Directed Morphogenesis and Electrical Properties of a Peptide

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Solvent Directed Morphogenesis and Electrical Properties of a Peptide-Perylenediimide Conjugate Sahnawaz Ahmed, Kandan Natarajan Amba Sankar, Bapan Pramanik, Kallol Mohanta, and Debapratim Das Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01750 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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Solvent Directed Morphogenesis and Electrical Properties of a Peptide-Perylenediimide Conjugate Sahnawaz Ahmed,†a Kandan Natarajan Amba Sankar,†b Bapan Pramanik,a Kallol Mohanta*b and Debapratim Das*a a

Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India

b

Department of Physics, PSG College of Technology and Nanotech Research Innovation and

Incubation Centre (NRIIC), PSG Institute of Advanced Studies, Avinashi Road, Coimbatore, 641004, TN, India

ACS Paragon Plus Environment

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ABSTRACT Molecular organization of electron-deficient aromatic systems like perylenediimides (PDI) is extremely appealing, as they are potential candidates for organic electronics. Performance of these molecules in such applications primarily depends on the self-organization of the molecules. However, any correlation between the morphology of these self-assembled semiconducting molecules and their electrical performances have not yet been formulated. Herein, for the first time, we have made an effort to find such a correlation by studying the self-assembly, morphology and their conducting properties for a peptide-PDI conjugate. The PDI-conjugate formed fiber like morphology in relatively non-polar solvents (THF and CHCl3) while in more polar solvents (HFIP, MeOH, ACN, Acetone), spherical morphology could be found. Interestingly, the self-assembly and the morphologies showed clear dependence on the solvent polarity. In polar solvents, the conjugate aggregates more efficiently than in the non-polar solvents and with decrease in solvent polarity, the dimension of the nano-structures increased. However, in all the tested solvents, irrespective of their polarity, the PDI-peptide conjugate adopts a right handed helicity. To find a correlation between the morphologies with the conducting property, detailed electrical characterization of these nano-structures were carried out. While no significant change could be observed for the dc conductivities of these nanostructures, the ac conductivities show prominent difference at low frequency region. A dispersion of conductivity was observed for the nano-spheres due to polarization effect. A critical correlation between the nano-structures and the activation energy was observed as with decrease in radii of curvature of the aggregates the activation energy increases with an exception in case of MeOH. The observed results suggest that the long range transport of charge carriers is less favorable when the aggregates are small and closely packed.


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INTRODUCTION In recent years, use of organic semiconductors in electronics has gained unprecedented importance. Organic semiconductors have been used in areas like photovoltaics, light-emitting diodes, electrochromic devices, memories and thin film transistors (TFT) driving the liquid crystal pixels in active-matrix liquid crystal displays.1-2 These organic semiconductors are able to conduct charges by means of partial delocalization or charge hopping through the molecules that are coupled by relatively weak non-covalent supramolecular forces.3 Amongst the popular organic semiconductors, perylenediimide (PDI) derivatives have been of interest owing to their electron-accepting and transporting properties (n-type characteristics).4-10 Tang, for the first time in 1986, reported PDI as an electron acceptor for two-layer organic solar cell devices.11 A decade later, Horowitz et al. had proved that PDI show n-type conduction when incorporated into a thin film transistor, in which a positive gate bias was essential to induce an increase in the source–drain current under positive drain voltage.12 Since then, a plethora of studies have been reported to elucidate the underlying fundamentals.13 Control of molecular organization into well-defined nano-structures from electronically and optically active small molecules is considered as an important approach to fabricate next generation nanoscale electronic devices. It is observed that the aggregation of these semiconductors is the key toward modulating their electrical properties and performance.14-15 Substantial effort have been made to control the aggregation pattern and electron transport properties by proper functionalization of the PDI moiety.16-22 Core substitution significantly affect the electronic states of the PDI core and subsequently the aggregation of the derivatives.19,

21-22

However, imide substitution also

affect the aggregation pattern of the PDI molecules and with attachment of proper functionality, desired nano-structures can be obtained.17-18,

20

Another controlling factor is the choice of


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solvent. The solvent polarity plays a prominent role not only to control the opto-electric properties, but also the aggregation pattern of PDI based molecules.10, 18, 23-24 An easy and effective approach to achieve the desired aggregation pattern as well as nanomorphology could be utilizing the inherent self-assembling properties of peptides. The ability of peptides to adopt specific self-assembled structures as well as availability of wide range of functional groups offers unique opportunity to design and create nanoscale materials with desired shape and size.25-26 Short peptide sequences of known self-assembly pattern can be conjugated to these semiconductor units to create targeted nano-architectures and consequently achieve the anticipated electrical property. Though relatively new, this approach have already been proved successful.27-28 Bio-organic ﬁeld-effect transistor is successfully fabricated by Hodgkiss et al. using a conjugate of PDI and a 8-mer peptide derived from an interface of the peroxiredoxin family of self-assembling proteins.29 In a recent work, Joon and coworkers showed morphogenesis and optoelectronic properties of self-assemblies of a conjugate of PDI and Phenylalanine (Phe) in a binary solvent system.30 However, there are reports on fine tuning of the emission properties of amino acid/peptide conjugates of PDI in different solvents.31-34 Importantly, any detailed study correlating the solvent induced changes in self-assembly, morphology, and consequently their electrical properties is still missing for these organic semiconducting group. In a recent work, we have shown how the solvent polarity affect the morphology of a short peptide-PDI conjugate (P-1, Figure 1).35 P-1 adopts a helical nano-fiber like structure in THF while in presence of 90% water, the morphology changes to nano-ring. These two morphologies showed prominently different conducting properties. Herein, we report the effect of solvent polarity on the self-assembly and consequently on the electrical properties of P-1. The


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aggregation of the conjugate was found to be dependent on the solvent polarity. In polar solvents, it forms spherical aggregates while in relatively non-polar solvents, fibrous morphology could be found. A detailed electrical characterization of these morphologies has been carried out to understand the effect of the morphology on the electrical behavior. The results from these studies show clear dependence of the conducting property on the size and shape of the nanostructures. EXPERIMENTAL SECTION General Information and Materials: All chemicals and reagents were purchased from Sigma-Aldrich (USA) and used without further purification. All solvents were procured from Merck (India). To prepare samples, Milli-Q water with a conductivity of less than 2 µS cm-1 was used. UV-Visible spectra were recorded on a PerkinElmer Lambda 750 spectrometer, while fluorescence measurements were performed on a Cary Eclipse (Agilent) spectrophotometer. Standard 10 mm-path quartz cuvettes were used for all spectroscopic measurements. 1H NMR spectra were recorded with a Bruker Ascend 600 MHz (Bruker, Coventry, UK) spectrometer or an Oxford AS400 (Varian) spectrometer and referenced to deuterated solvents. ESI-MS was performed with a Q-tof-Micro Quadrupole mass spectrophotometer (Micromass). Synthesis of P-1: P-1 was synthesized according to our previously reported procedure35 and the synthetic protocol and spectroscopic characterization data are provided in the Supporting Information. UV−Visible and Fluorescence Spectroscopic Studies: All the samples were prepared in a 10 mL of volumetric flask by weighing appropriate amount of the compound and dissolving in the corresponding solvent. These stock solutions were diluted to the concentrations required for the experiments. Unless otherwise mentioned, all the working solutions were kept undisturbed for at


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72 h prior to perform any experiment. Emission spectra of P-1 were measured in different solvents by exciting the solutions at 485 nm. Dynamic Light Scattering (DLS) Studies: The particle sizes of the samples were measured at 298 K using a 632.8 nm He − Ne laser using Zetasizer Nano-ZS90 (Malvern). The samples were prepared in the corresponding solvent and filtered through appropriate filters to remove dust particles if present, and then allowed to settle for 72 h before the measurement. Circular Dichroism (CD) Spectroscopy: The CD spectra of all the samples were recorded on a J-1500 (JASCO, U.S.A.) instrument at room temperature. The data were collected at 1 nm intervals with 2 nm band width. All measurements were done in 0.2 cm path length cuvette with 400 µL sample volume. Each CD profile is an average of 3 scans of the same sample collected at a scan rate 100 nm min-1, with a proper baseline correction from the respective solvents. Scans were performed over 190 to 600 nm. Time-Resolved Fluorescence Lifetime Measurements: Lifetime measurements were done using Edinburgh FSP920 spectrophotometer equipped with a pulse diode laser (PDL). Field Emission Scanning Electron Microscopy (FESEM): For FESEM imaging of the materials, 1 micron thick films were made by drop casting from 72 h matured 10 µM solutions on Al electrode coated glass substrate. FESEM images were taken on a Gemini SEM 300 (Sigma Zeiss) instrument. Transmission Electron Microscopy (TEM): A 5 µL of 10 µM solution of the sample was casted on the carbon coated copper grid (300 mesh Cu grid with thick carbon film from Pacific Grid Tech, USA) and allowed to air dry for 2 minutes and then the excess sample was bloated with a tissue paper. The grid was then air dried for 1 day. The samples with desired solvent were


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prepared and incubated for at least 3 days before casting on the grid. TEM images were taken in JEOL JEM-2100 microscopes. Quantum Yield measurement: The fluorescence quantum yields of P-1 in different solvents were determined by Parker-Rees method using Rhodamine-6G as a standard fluorophore. The Parker-Rees equation36 is given below,

∅ = ∅

(1)

where, ɸs = Quantum Yield of standard fluorophore = 0.95 in EtOH, ɸu = Quantum Yield of unknown fluorophore, As = the absorbance of standard fluorophore at the excitation wavelength, Au = the absorbance of unknown fluorophore at the excitation wavelength, Fs = the area of integrated fluorescence intensity of the reference sample when excited at the same excitation wavelength, Fu = the total area of integrated fluorescence intensity for the unknown sample when excited at the same excitation wavelength, The refractive indices of the solvents for the unknown and the standard samples are denoted by nu and ns respectively. To minimize the reabsorption of the fluorescence light passing through the samples their absorption maximum was kept 0.1. Powder XRD (PXRD): For XRD measurements, a few drops of the solutions were placed on silicon surfaces, and the solvents were evaporated at room temperature. These sample was used for XRD measurements on a Bruker D2 Phaser X-ray diffractometer (30 kV, 10 mA). The Bragg peak λ was extracted from the XRD data and the layer thickness d could be obtained according to the Bragg equation d = λ/2sinθ, λ = 0.15405 nm. Atomic Force Microscopy (AFM): For AFM imaging of the materials, films were made by spin casting from 72 h matured 10 µM solutions on commercial Indium Tin Oxide coated glass substrate (ITO). The typical spin casting condition was 1500 rpm for 60 s. Prior to film


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deposition the ITO substrates were cleaned atomically which ensures a cleaned, flat and conducting surface of roughness around 4-8 nm. Images were taken by NTMDT NTEGRA Aura AFM. A SiN tip was used to probe the thin films of the samples in Non-Contact mode. The setpoint used as “Auto Set”. Electrical characterization: For electrical characterization of the materials, 1 micron thick films were made by drop casting from 2 mg/ml solutions on Al electrode coated glass substrate. The process has to be repeated for several times to get desired thickness. Photos of the solutions and the deposited films are given as Fig. S1. On the top of the film another Al electrode deposited via thermal evaporator. The cross-section area of the devices is 10 mm2. I-V characteristics and dc measurements were taken by a Keithley 2450 source-measure unit. The impedance and ac characteristics were recorded by Keysight E4990A impedance analyzer.

RESULTS AND DISCUSSION Solvent Polarity Directed Aggregation of P-1 The PDI-peptide conjugate P-1 showed moderate to good solubility in organic solvents like tetrahydrofuran (THF), acetonitrile (ACN), acetone, chloroform (CHCl3), methanol (MeOH) and 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP). The self-aggregation of P-1 was studied in these organic solvents. To understand the self-assembly behavior of P-1 in these solvents, the absorption and emission behavior of P-1 are recorded (0.5-50 µM). Concentration dependent absorption and emission profiles of P-1 in these solvents are provided in the Supporting Information (Figures S1-6). In the absorption spectra, P-1 exhibited three characteristic vibronic bands at ~517, ~482 and a shoulder at ~460 nm which are attributed to the 0-0, 0-1 and 0-2 vibrational transitions respectively. The emission behavior of the compound is the mirror image


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of the absorption spectra in all the solvents. Three emission bands at ~ 540, ~570, and ~620 nm were observed in all cases. At higher concentration, the absorption value crossed the limit of 1 in some of the solvents and deviates from linearity when plotted as a function of concentration (Figures S1-6). Thus, in continuation to our previous report35 on the self-assembly of P-1 in THF-water binary solvent mixtures, the experiments for the present work are performed at 10 µM concentration. Figure 1 shows the absorption and normalized emission spectra of P-1 in these solvents. The positions of the bands varied with solvents as can be seen in Figure 1B. Interestingly, as we move from HFIP to THF, there is a ~28 nm blue shift of the absorption bands (Table 1) noted. Importantly, the blue shift followed the polarity index of these solvents. The polarity of the solvents follow the order of THF 1 µmb

1.41

0.82

4.34

0.49

5.17

THF

37.4

510

472

521

Right handed Helical fiber

> 1 µmb

1.44

0.89

4.30

0.28

8.83

Solvent

ET(30)

λ0-0

λ0-1

λem

(kcalmol-1)37

(nm)

(nm)

(nm)

HFIP

65.3

538

496

MeOH

55.4

531

ACN

45.6

Acetone

a

from DLS measurements. bfrom Microscopic measurements.


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Schemes and Figures

Figure 1. A) Chemical structure of the PDI-peptide conjugate, P-1; B) UV-Visible and C) Normalized Emission spectra of 72 h matured samples of P-1 in different solvents measured at room temperature. [P-1] = 10 µM.


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Figure 2. A) Wave number (at absorbance maxima) and wave number (at emission maxima) versus ET(30) of solvents. B) Quantum yield and fluorescence life time versus ET(30) of solvents plots.

Figure 3. FESEM images from 72 h matured samples of P-1 (10 µM) in different solvents.


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Figure 4. TEM images from 72 h matured samples of P-1 (10 µM) in different solvents.

Figure 5. AFM images from 72 h matured samples of P-1 (10 µM) in different solvents.


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Figure 6. A) Intensity weighted distribution of particles obtained from DLS measurements of 72 h matured samples of P-1 (10 µM) in different solvents. B) Powder X-ray diffraction analysis of dried samples prepared from 72 h matured 10 µM solutions of P-1 in different solvents.


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Figure 7. Variation of ac conductivity of nanostructures formed by 10 µM P-1 in different solvents with frequency. The characteristics were taken at room temperature.


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Figure 8. ac conductivity Vs temperature, in semi-log scale for nanostructures by 10 µM P-1 in different solvents. From the slope of the profiles activation energy is calculated.


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Figure 9. A) Current-voltage characteristics for nanostructures by 10 µM P-1 in different solvents. The arrows indicate the path of currents. B) On/off ratio Vs voltage for different nanostructures by 10 µM P-1 in different solvents. The values in the parentheses at legend are the on/off ratio of nanostructure formed in that solvent.


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Table of Content Graphic


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Solvent Directed Morphogenesis and Electrical Properties of a Peptide

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