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Selenium Containing Fused Bicyclic Heterocycle - Diselenolodiselenole: Field Effect Transistor Study and Structure-Property Relationship Sashi Debnath, Sundaresan Chithiravel, Sagar Sharma, Anjan Bedi, Kothandam Krishnamoorthy, and Sanjio S. Zade ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02154 • Publication Date (Web): 29 Jun 2016 Downloaded from http://pubs.acs.org on June 29, 2016

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ACS Applied Materials & Interfaces

Selenium Containing Fused Bicyclic Heterocycle Diselenolodiselenole: Field Effect Transistor Study and StructureProperty Relationship

Sashi Debnatha,† Sundaresan Chithiravelb,† Sagar Sharmac, Anjan Bedia, Kothandam Krishnamoorthyb, Sanjio S. Zadea*

a

Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India

b

Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, CSIRNetwork of Institutes for Solar Energy, Dr Homi Bhabha Road, Pune 411008, India

c

Physical Sciences Division, Institute of Advanced Study in Science and Technology (IASST), Paschim Boragaon, Guwahati 781035, India [*] email: [email protected]

ABSTRACT: First application of diselenolodiselenole (C4Se4) heterocycle as active OFET materials is demonstrated here. C4Se4-derivatives (2a-2d) were obtained by using a newlydeveloped straightforward diselenocyclization protocol, which include the reaction of diynes with selenium powder at elevated temperature. C4Se4-derivatives exhibit strong donor characteristics and planar structure (except 2d). Atomic force microscopic analysis and thinfilm X-ray diffraction pattern of compounds 2a-2d indicated the formation of distinct crystalline films which contains large domains. Scanning electron microscopy study of compound 2b showed development of symmetrical grains with average diameter of 150 nm. Interestingly 2b exhibited superior hole mobility approaching 0.027 cm2 V−1 s−1 with

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transconductance of 9.2 µS. This study correlate the effect of -stacking, Se…Se intermolecular interaction and planarity with the charge transport properties and performance in the FET devices. We have shown that the planarity in C4Se4-derivatives was achieved by varying end-groups attached to C4Se4-core. In turn, optoelectronic properties can also be tuned for all these derivatives by end-group variation.

KEYWORDS:

Diselenolodiselenole,

organic

field

effect

transistor,

selenium,

diselenocyclization, spectroelectrochemistry, charge transport.

INTRODUCTION Organic field-effect transistors (OFETs) have been the main focus of enormous research activity because of their advantages of lightweight, cost-effective, and flexible processability and as a future alternative to silicon-based transistors.1,2,3 Presently, rapid progress in OFETs has been made, thanks to their prospective implementations in flexible display drivers, like radio frequency identification tags, electronic papers, memory cards, sensors and displays.4,5,6 The primary impediment of OFETs to realistic implementations is the inferior hole or electron mobility. Therefore continuous efforts have been given both in the synthesis of promising organic semiconductors and standardization of cheap device fabrication methods which can enhanced the field-effect mobility (µFET) appreciably.7,8,9 Achieving large-scale low-cost OFETs requires high-performance and solution-processable materials, like conducting, semiconducting, and dielectric components for low cost fabrication in electronic devices. Solubility in common organic solvents makes small molecules more efficacious in organic electronic devices as simple and conventional deposition methods such as spin coating, spray coating and printing can be employed.10,11,12 The contact between small


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molecules in the solid state and degree of conjugation determined the charge-carrier mobility.13,14 Among the several sets of conjugated materials, sulfur-based conjugated systems15,16,17,18 are highly explored as p-type semiconducting materials.19,20,21 Recently, selenium-based conjugated systems are being explored.22,23,24,25,26,27 Selenium containing analogues such as bis(ethylenedithio)tetraselenafulvalene

(BETS),

and

tetramethyltetraselenafulvalene

(TMTSF) have exhibited better organic superconductors than tetrathiafulvalene (TTF) derivatives.28 Similarly selenium containing fused homologue 2,6-Diphenylbenzo[1,2-b:4,5b΄]diselenophenes showed twofold increased OFET performance than that of its sulfur analogue.25 Because of larger size and higher polarizability of Se, incorporation of selenium in place of sulphur in the conjugated systems is expected to result in (a) stronger intermolecular interactions, (b) lower redox potentials and (c) lower band gap of the resulting conjugated systems. Regardless of their potential advantages, selenium based electroactive small molecules have acquired very less attention. Only a limited number of synthetic methods of Se-based conjugated systems have been developed24,25,26,27 and their instabilities in the charged states have hampered their analysis and applications. Therefore, new types of conducting materials are strongly desired. In our search for new and better organic semiconductors, we are interested to introduce multiple selenium containing fused conjugated systems. In this regard, recently we have reported a series of organic semiconductors, namely diselenolodiselenole (C4Se4) derivatives.29 Thiophene substituted C4Se4 showed superior optical properties and nearly planar rigid backbone compared to phenyl substituted C4Se4. Here we incorporated the synthesis of two new heterocycle substituted diselenolodiselenole 2a and 2c to study their optoelectronic properties, solid state morphology and charge carrier mobility in OFET devices along with compounds 2b and 2d. Interestingly thiophene substituted C4Se4 (2b) in 3 ACS Paragon Plus Environment


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the best transistor devices showed highest hole mobility 0.027 cm2/(Vs) which is one of the preeminent result among selenium containing fused heterocyclic small molecules.25,26

RESULTS AND DISCUSSION Synthesis. Compound 2a and 2c were synthesized from corresponding diynes and selenium powder as shown in Scheme 1 with 17% and 23% yields, respectively. Compound 2b and 2d were synthesized by previously reported procedure from our group.29 The precursor diyne compounds 1a and 1c were prepared as described in Scheme S1 (see SI).

Scheme 1. Synthesis of 2a, 2b, 2c and 2d from substituted diynes (1a-1d) in neat condition.

X-ray Crystal Structural Analysis. 4 ACS Paragon Plus Environment

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Single crystals of 2a and 2c were obtained by slow evaporation of dichloromethane (DCM) solutions at ambient condition. Solid state molecular structure and packing patterns of 2a and 2c are represented in Figures 1 and 2 and the crystallographic parameters are given in Table S2 (SI). In the crystal structure of 2a, the dihedral angle between the middle C4Se4 unit and the terminal selenophene rings is found to be ~7° (C3–C4–C5–C6 = 172.9(7)) (Fig. 1b). Similarly in 2c, the dihedral angle between the terminal 3-hexyl substituted thiophene rings and the middle C4Se4 unit is only ~6° (C4–C3–C2–C1 = 173.6(7)) (Fig. 2b). These dihedral angles are smaller than that was observed in 2d (~53°)29 which indicates that, substitution of C4Se4 core with chalcogenophenes decreases the dihedral angle significantly compared to substitution with phenyl groups.30,31 Se…Se covalent and spatial distances were found to be 2.32 Å (Se2–Se3) and 3.07 Å (Se1–Se3) in 2a, whereas in 2c Se…Se covalent distance were found to be 2.34 Å (Se1–Se2) and S…Se spatial distance were found to be 3.02 Å (S1–Se1). Molecular packing of 2a (Figure 1c) indicates the formation of two perpendicular dimers by four different Se…Se interactions (Se1…Se1 = 3.67 Å, Se3…Se3 = 3.70 Å, Se2…Se3 = 3.54 Å, and Se2…Se2 = 3.72 Å) with its neighbouring molecules. Thus four molecules connected by Se…Se interaction in two directions perpendicular to each other to construct a 2D packing. π−π stacking were also observed in 2a with slipped fashion and the distances of the neighbouring face-to-face molecules were around 4.35 Å which results in typical herringbone structure (Figure 1c). In 2c, the interdigitation of hexyl chains creates through multiple CH…CH dispersion interactions along with Se1…C1 (3.58 Å) and Se2…H (3.04 Å) interactions (Figure S3). π−π interections (3.52Å) between the molecular planes (Figure 2b) organized them in parallel face to face orientation in bulk (Figure 2c). Higher degree of nonbonding interactions were achieved by the involvement of multiple heteroatoms which could be constructive for charge transport properties.32,33



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(a) 7.03°

7.03°

b c

a

(c)

(b)

a

c b

(d) Figure 1. (a) ORTEP diagram, (b) torsional angle, (c) packing structure and (d) typical herringbone structure of 2a. Ellipsoids are presented here at 50% probability level.

S1

(a) 6.33

6.33

(b) 6 ACS Paragon Plus Environment

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c

b

a

(c)

Figure 2. (a) ORTEP diagram, (b) torsional angle and (c) packing structure (along ‘c’ axis ) of 2c. Ellipsoids are drawn here at 50% probability level and Hydrogen atoms were removed for clarification in (c).

Optical and Electrochemical Properties: Absorption spectra of these compounds were measured in dichloromethane solutions (10−5 M) and also in solid state by spin coating on an ITO coated clean glass slide. The absorption spectra of 2a‒2d (Figure 3a) exhibited two bands; higher energy bands (240-280 nm) corresponding to the conjugation (π‒π* transition) and lower energy band (440−500 nm) attributable to the charge-transfer transition (HOMO  LUMO).31,34 The absorption spectra of 2a-2d show the effect of different substituents on C4Se4 core on their optoelectronic properties. However, hexyl chain substitution did not show any significant change in the absorption peak for 2c compared to that of unsubstituted 2b. The low energy absorption band of selenophene- substituted 2a showed red shift as compared to thiophene- substituted 2b due to electron rich selenium atoms. Higher polarizability and lower electronegativity of selenium atoms decreases the HOMO and LUMO energy gap, thus the absorption maxima moved to a longer wavelength region. The absorption maxima for the thin films of all C4Se4 derivatives were found to be red shifted (Figure 3b, Table 1) compared to their solution spectra, which indicated the presence of solid state intermolecular interactions.35,36



The electrochemical properties of 2a‒2d were studied in dichloromethane, where Ag/AgCl was placed as a reference electrode and tetrabutylammonium hexafluorophosphate (TBAPF6) was added as supporting electrolyte (Figure 4). All C4Se4 derivatives were found to have double reversible oxidations with E1/2 values in the range of 0.30 - 0.50 V for the first oxidation and 0.85 V to 1.05 V for the second oxidation (Figure 4). The CV and UV-vis data were used to calculate HOMO/LUMO and Eg values, which are summarized in Table 1. Lower oxidation potential of 2d compared to 2a, 2b and 2c may be due to the electron rich thiophene and selenophene in planar backbone could reinforce the electron delocalization over the molecular framework which reduces the electron cloud on C4Se4 core. Compound 2a showed lower oxidation potentials (

0.43 and 0.89 V) than 2b (

0.47 and 0.98

V) which could be attributed to the more polarizable selenium atom present in the selenophene ring which facilitated the oxidation process. (b)

1.0

2a

0.8

2b 2c

29

2d

29

0.6 0.4 0.2

Normalized absorbance

(a) Normalized absorbance

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0.0 300

400

500

600

1.0 0.8 0.6 0.4 0.2 0.0

700

2a 2b 2c 2d

400

500

600

700

Wavelength (nm)

Wavelength (nm)

Figure 3. (a) UV−vis absorption spectra of dilute solution of compounds 2a‒2d in dichloromethane; (b) UV−vis absorption spectra in solid state by spin coating on an ITO coated glass slide. Table 1 Optical and electrochemical properties of 2a–2d


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Compounds

(nm) Solution

(nm) Film

vs.

(eV)

Ag/AgCl

HOMO

LUMO

(eV)

(eV)

(V) 2a

282, 498

335, 568

0.43, 0.89

2.13

−4.87

−2.74

2b29

275, 484

342, 525

0.47, 0.98

2.21

−4.91

−2.70

2c

277, 485

334, 523

0.44, 0.94

2.23

−4.88

−2.65

2d29

240, 440

0.30, 0.86

2.39

−4.74

−2.35

= 1240/

352 = − (4.44 +

,

,

=

+

.

3.3 A

2a

0

7.6A

2b

Current (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

2c

1.9A

2d 0

0

6.1 A

2a 29

2b 2c

29

2d

0.0

0.5 1.0 Voltage [V]

1.5

Figure 4. Cyclic voltammograms of compounds 2a–2d in 0.1 M TBAPF6 in dry DCM as solvent.

Conjugated materials that experience considerable redox induced transformations in their absorption

properties

have

verified

utility

in

electrochromic

devices.37,38

Spectroelectrochemistry of C4Se4 derivatives were studied to demonstrate their suitability as electrochromic materials by assessing their spectral variations in neutral and oxidized states.39,40 Solution state spectroelectrochemical studies were investigated in-situ for compound 2d in 0.1 M TBAPF6 solutions in dichloromethane by using a Pt mesh as working 9 ACS Paragon Plus Environment


electrode, a AgCl coated Ag wire as the reference electrode and a platinum wire as the counter electrode (Figure 5). With the increasing applied potential from 0.0 - 1.1 V, two new bands were observed at 480 and 760 nm; while the band at 440 nm for the neutral compound slowly disappeared. The colour of the solution turned to faint brown on oxidation which was yellow in the neutral state. The absorption bands appeared in the longer wavelength region upon oxidation indicated the formation of radical cation.41 1.0

Normalized Absorbance

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0.8

Neutral 0.6

Radical Cation

0.4 0.2 0.0

Baseline 400

500

600

700

800

Wavelength(nm)

Figure 5. Absorption spectra during the various redox processes of a solution of 2d in 0.1 M TBAPF6 in dry DCM solution (reference electrode Ag/AgCl).

Field Effect Transistor: The charge transport properties were measured from prefabricated Silicon/Silicon oxide based filed effect transistors. The substrate has gold electrodes as contact. Thin films of the molecules were spun on the OFET substrates from chlorobenzene solution. The output characteristics were measured by sweeping the drain voltage (VD) between 0 and −80 V, while biasing the gate voltage (VG) at various constant negative potentials (Fig 6a). The output and transfer characteristic curves were recorded in argon filled glove box with Keithley 4200SCS semiconductor analysis. The output characteristic curves showed well defined linear and saturation regimes irrespective of gate voltage. The drain current (ID) 10 ACS Paragon Plus Environment

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started increasing while the VD was increased, and then saturated at higher VD. ID was found to increase as a function of gate voltage (VG) which indicates typical FET characteristics (Figure 6). Compounds (2b-2d) exhibited p-type charge transport. Field-effect mobility (µ) was calculated in linear and saturation regimes. The µ value was found to be 0.018 cm2/(Vs) in the linear regime and 0.027 cm2/(Vs) in the saturation regime for molecule 2b. Transconductance (gm) is calculated in the saturation regime with a constant VDS, which was found to be 9.2 µS for 2b. The hole carrier mobility and transconductance values are highest for 2b. The increased µ value in case of 2b is likely due to better packing between the molecules. The molecule 2c is structurally similar to 2b, but 2c has alkyl chains. The crystal structure analysis shows interdigitation of alkyl chains (Figure 2c), which is known to increase charge carrier mobility. However, the  (6.7 x 10-3 cm2/Vs) was found to be lower than that measured for 2b (Table 2). This is likely due to higher interplanar spacing (3.52 Å) between the molecules of 2c. The compound 2d also showed decreased  (1.7 x 10-3 cm2/Vs) compared 2b (0.01 cm2/Vs). The decreased  is due to the presence of phenyl terminal moieties. The torsional angle between heterocycles and phenyl ring is higher than that observed between a thiophene and heterocycle. This has affected the charge transport. Interestingly, the compound 2a didn't show output and transfer characteristic curves that indicates that there was no charge transport. This is presumably due to very high interplanar spacing of 4.35 Å between the molecules of 2a.

Table 2: OFET device metrics of 2a-2d



Compounds

Mobility (µ)

Mobility (µ)

[cm2/(Vs)] Linear

[cm2/(Vs)]

conductance

regime

Saturation regime

(gm) µS

2a

‒

No field Effect

‒

‒

‒

2b

0.0180

0.0270

−5

2.33  102

9.20

(0.0083)

(0.0182)

0.0067

0.0024

−2

1.13 102

4.01

(0.0050)

(0.0015)

0.0017

0.0018

−10

5.64 102

0.34

(8.87  10-4)

(7.71  10−4)

2c

2d

(a) -6.0x10-5 (5a)

10

-6

ID (A)

ID (A)

-5

-5

I (A) D 1/2 1/2 I (A) D

0.0 0

-20

-40

-60

-2

-8.0x10

-3

-4.0x10

-3

0

-80

VD (V)

-20

-40

-60

-80

1/2

-2.0x10

10

-1.2x10 V = 40 V D

(A)

-5

Trans

Ion/off

1/2

-4.0x10

(b) 10-4 (5b)

0V -10 V -20 V -30 V -40 V -50 V

VT (V)

(ID)

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0.0

VG (V)

Figure 6. (a) OTS-modified output and (b) transfer characteristics of compound 2b.

Computational investigation of charge transfer properties: DFT calculated charge transport properties gave more insight about OFET performance (see supporting information for computational details). The crystal geometries were taken as initial geometry for the gas phase geometry optimization. The charge transport properties are 12 ACS Paragon Plus Environment

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evaluated from the hopping model, using the dimers of different hopping pathways in the crystal structures. The evaluation of charge carrier mobility requires estimation of reorganization energies and the hole reorganization energies for 2a-2d were found to be similar with the values ranging from 0.19 – 0.20 eV (Table 3). The lower values of λh for the compounds are in conformity with their relatively rigid and planar crystal structures. The details of transfer integrals for compounds 2a-2dalong different pathways with the centroid to centroid distance between the monomers are shown in Table 3 and the major charge hopping pathways are shown at Fig 7 and Fig S4-S5(in SI). For compounds 2a and 2b, the values of transfer integrals are highest for the hopping paths where dimers are oriented in face to edge motifs (pathways b and c). For compound 2c and 2d, the face to face packing has the major contribution towards transfer integral (pathways a, b, d for 2c and pathway c for 2d). The calculated hole mobilities for all compounds are of similar order i.e., 0.092 cm2V-1s-1 for 2a and 0.083 cm2V-1s-1 for 2b, while slightly higher for 2c (0.239 cm2V1 -1

s ) and 2d (0.137 cm2V-1s-1).

Figure 7.Charge transport pathways for compound 2b(a) andcompound 2c (b).

Table 3: Calculated reorganization energies (λh), distance (d) between the monomers for different pathways and the corresponding transfer integrals (t), and average hole mobilities (µ)



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Compounds λh (eV) Pathways d (Å) t (meV) µ (cm2V-1s-1) 2a

2b

2c

2d

0.192

0.197

0.199

0.203

a

5.64

-9

b

6.22

-26

c

6.22

-26

a

5.57

-6

b

6.21

-25

c

6.21

-25

a

9.46

-30

b

10.7

9

c

14.2

-2

d

8.09

9

a

9.60

-2

b

7.34

-7

c

5.86

-36

d

7.34

-7

0.092

0.083

0.239

0.137

Thin Film Morphologies To comprehend the OFET performance of 2a‒2d, their microstructural arrangements in film state were investigated by scanning electron microscopy (SEM), film state X-ray diffraction (XRD), and tapping-mode atomic force microscopy (TMAFM). For thin film XRD analysis, 14 ACS Paragon Plus Environment

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spin coating procedure was followed to form the films on glass slides from dichloromethane solution while for SEM and AFM study, thin films were prepared under identical conditions that of the device fabrication. In thin film XRD measurements of 2a and 2b, sharp peaks were isolated at 2θ of 7.69° and 7.78°, which equate with the d-spacing of 11.48 and 11.35 Å (Figure 8, Table S1, SI). The molecular lengths of 2a and 2b were measured as 11.16 and 10.99 Å from the crystallographic analysis, which designates the arrangement of the molecules along the molecular length in solid state. X-ray crystal structure analyses were also in agreement with this observation. Compound 2c showed a d-spacing of 13.18 and 8.80 Å with core molecular length 11.07 Å in the thin film XRD pattern resulting a planar molecular packing. Compound 2d showed one major peak at 2θ = 26.48° in wider angle region with dspacing of 3.36 Å which can be assigned to the intermolecular π‒π stacking distance (3.39 Å) between the substituted phenyl rings (Figure S4 in SI).

2a

Intensity (a.u.)

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


2b

2c

2d

10

20

30

40

2 Theta (degree) Figure 8. XRD patterns of the compounds 2a‒2d as-cast film at room temperature.

Tapping mode height images (Figure S1 in SI) of 2a‒2d films were obtained through atomic force microscopy (AFM) at room temperature. Compound 2b showed a uniform grain like aggregation with the root mean square roughness (RMS) of 12.23 nm. Strong 15 ACS Paragon Plus Environment


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intermolecular interactions might be responsible for this type of arrangement which leads to the proficient device performance. Compound 2c showed spot-like features that consist of small domains with RMS roughness of 14.44 nm. Scanning electron microscopy of 2b and 2c (Figure S9) showed that the phase separated circular grains consist of smaller nanoparticles with average diameter of 150 and 120 nm for 2b and 2c, respectively. However, the thin films of 2d were found to be quite different and exhibited heavy aggregation, and no such symmetrical nanoparticles could be found. But in case of 2a inferior aggregation was observed which could be the reason for featureless device performance of compound 2a.

EXPERIMENTAL SECTION Required reagents and solvents were purchased from production resources (Aldrich or Merck) of reagent grade and used as received. Selenophene and 3-hexylthiophene were received from TCI. Syntheses of all the C4Se4 derivatives have been done with one common diselenocyclizationin process in neat state. Synthesis of 2b and 2d were published by our group in recent times.29 Synthesis of compounds 2a and 2c were described in SI. JEOL JNMECS400 MHz spectrometer was used to collect all the NMR spectra and chemical shifts were mentioned with respect to tetramethylsilane (TMS) internal standard in parts per million (δ scale). Product purification was done by silica gel column chromatography (100-200 mesh, Merck, India). Absorption spectra were collected on HITACHI U-4100 UV-vis spectrophotometer. ITO-coated glass plates were spin coated by the compounds from dichloromethane solution for film-state absorption study. Electrochemical analyses were performed by using a three electrode system on a Princeton Applied Research 263A electrochemical workstation. A platinum (Pt) wire and a Pt disk (dia. 1.6 mm) were used as counter electrode and as working electrode respectively, whereas Ag/AgCl was used as the reference electrode. Ferrocene was applied as an internal standard. TBAPF6 (0.1 M in dry


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DCM) was utilised as the supporting electrolyte and the calculated half-wave potential for forferrocene/ferrocenium couple was found to be 0.32 V. Every electrochemical analysis was executed in inert atmosphere. An NT-MDT instrument (model AP-0100) was used for AFM analysis to record tapping mode atomic force microscopic images. Scanning electron microscopic (SEM) analysis was performed by using a Carl Ziess Sigma SEM instrument. Slow evaporation of 2a and 2c from DCM solution gave the single crystals for crystallographic analysis. Fine crystals were processed on a Super Nova, Dual, and Cu/Mo at zero, Eos diffractometer. Structure was solved by Using Olex2,42 in Superflip43 structure solution programme by Charge Flipping and refined with the Least Squares minimization ShelXL44 refinement package.

Device Fabrication. The OFET substrates were ultrasonicated for 2-5 min in acetone and iso-propanol for cleaning purpose. The SiO2 surface was modified with octadecyltrichlorosilane (OTS) in anhydrous chloroform at room temperature for 1 h. A 100 μl solution of 10 mg/ml compounds (2a-2d) in chlorobenzene was spun on the devices and the films were allowed to dry. All experiments were done under argon filled glove box with a Keithley 4200-SCS semiconductor parameter characterization system. The OFET devices had a channel length (L) varying from 2.5 to 20 μm and a channel width (W) of 10 mm. The charge carrier mobility μ, was calculated according to the equation ISD = (W /L) Ciμ( VG – VT )VD for linear and ISD = (W /2L) Ciμ( VG – VT )2 for saturation from the data in the linear and saturated regime, where ISD is the drain current, W is the channel width and L is the channel length. VG is the gate voltage and VT is the threshold voltage, Ci (Ci = 14.9 nF) is the capacitance per unit area of the gate dielectric layer. VG– VT of the device was determined at the saturation regime from the relationship with the square root of ISD.



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CONCLUSIONS Multiple selenium containing fused bi-cyclic semiconductors (diselenolodiselenole derivatives) have been synthesized via straightforward procedure and studied for their physicochemical properties. All C4Se4-derivatives showed two reversible oxidations. Optoelectronic properties showed that they have small HOMO−LUMO energy gap (∼2.2 eV). Due to presence of Se∙∙∙Se interactions, diselenolodiselenole derivatives were arranged into the interesting 2D-crystalline motifs through utilisation of multiple intermolecular and π−π interections. Chalcogenophene- substituted C4Se4 possess nearly planar rigid backbone compared to phenyl-substitution which facilitate the charge transport. In charge transport studies, thin films of thiophene substituted C4Se4 showed maximum hole mobility of 0.027 cm2V−1 s−1 compared to that of other C4Se4 derivatives. The inferior film forming nature was observed for 2a in SEM and AFM study which could be the reason for featureless device performance of 2a. Domination of intermolecular Se∙∙∙Se/Se∙∙∙H interactions might be responsible for mediocre response of these molecules in FET devices compared to the BDT derivatives.45,46 Therefore, delicate interplay is needed between various factors such as nonbonding interactions, - stacking and herringbone interactions for better performance in FET devices. ASSOCIATED CONTENT Supporting Information Complete synthetic procedures of 1a, 1c, 2a and 2c; 1H NMR and

13

C NMR spectra of all

compounds, XRD data table, Crystallographic data and refinement parameters, AFM topographic images and output and transfer representative curves of FET devices.

AUTHOR INFORMATION 18 ACS Paragon Plus Environment

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Corresponding Authors *E-mail: [email protected].

ACKNOWLEDGMENTS S.S.Z is thankfully acknowledges CSIR-India for funding (Project No. 02(0122)/13/EMR-II). S.D. is thankful to UGC-India for fellowship.

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TABLE OF CONTENTS

(b) 10-4 (5b) -5

10

-6

-8.0x10

-3

-4.0x10

-3

1/2

-20

-40

-60

-80

1/2

0

(A)

I (A) D 1/2 1/2 I (A) D

µ h=0.027 cm2 V−1 s−1 -80

-2

V = 40 V D

ID (A)

10

-1.2x10

(ID)

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 40 41 -60 42 D (V) 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

VG (V)


Selenium-Containing Fused Bicyclic Heterocycle ... - ACS Publications

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