ORIGINAL PAPER

Synthesis, Crystal Structure and Electrical Studies of Naphthoyl-Thiourea as Potential Organic Light Emitting Diode

Siti Maryam Jasman1 • Wan M. Khairul1 • Tei Tagg1 • K. KuBulat1 •

Rafizah Rahamathullah1,4 • Suhana Arshad2 • Ibrahim Abdul Razak2 •

Mohamed Ibrahim Mohamed Tahir3

Received: 25 March 2015 / Accepted: 11 June 2015 / Published online: 18 June 2015

� Springer Science+Business Media New York 2015

Abstract A new type of naphthoyl-thiourea derivative

namely N-(5-methylpyridine)-N0-(1-naphthoyl) thiourea

(NT) was successfully synthesized prior to form conduc-

tive layer in organic light emitting diode (OLED). The

structure of compound was determined via single crystal

X-ray crystallography analysis and spectroscopically

characterized by infrared spectroscopy, 1H and 13C nuclear

magnetic resonance, UV–Vis, UV-fluorescence, cyclic

voltammetry analysis as well evaluated theoretically via

Gaussian 09 software employing DFT approach with set of

basis function B3LYP/6-31G (d,p). In turn, the compound

was deposited onto ITO substrate through electrochemical

deposition method prior the electrical conductivity and

performance as OLED was investigated via Four Point

Probe and Two Point Probe. From the crystal structure, NT

crystallizes as triclinic crystal system in P-1 space group,

unit cell parameters a = 7.4916(5) A, b = 9.4050(7) A,

c = 12.0584(9) A, a = 69.685 (7)�, b = 82.130 (6)� and

c = 71.917 (7)�. The conductivity analysis of NT per-

formed better and exhibited semiconductor material;

0.231 Scm-1 under dark condition which indicates this

single molecular system can act as potential OLEDs.

Graphical Abstract This contribution reports on the

design, preparation, and characterization of new type of

naphthoyl-thiourea derivative namely N-(5-methylpyr-

idine)-N0-(1-naphthoyl) thiourea (NT) prior acting as

potential Organic Light Emitting Diode (OLED).

Keywords Thiourea � Crystal structure � Organic light

emitting diode (OLED)

Introduction

Nowadays, organic light emitting diode (OLED) is one of

rapidly developing technology and becomes as an exciting

approach for efficient, high contrast, power saving and low-

cost devices to grab the challenge of the rising demand for

green and clear light supply [1, 2]. Within these concerns,

significant progress has been made to develop an active

material featuring conjugated polymer and small molecule

organic semiconductors [3–5]. This is due to the fact that

these compounds offer unique properties which consist

conjugated molecular unit and excellent electronic

behaviours.

& Wan M. Khairul

wmkhairul@umt.edu.my

1 School of Fundamental Science, Universiti Malaysia

Terengganu, 21030 Kuala Terengganu, Malaysia

2 School of Physics, Universiti Sains Malaysia,

11800 USM Penang, Malaysia

3 Department of Chemistry, Faculty of Science, Universiti

Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

4 Faculty of Engineering Technology, Universiti Malaysia

Perlis, 02600 Arau, Perlis, Malaysia

123

J Chem Crystallogr (2015) 45:338–349

DOI 10.1007/s10870-015-0599-6

Recently, significant contributions of organic conjugated

molecular wires in wide range of the molecular electronic

applications have been extensively studied and proven to

show great performances [6, 7]. The organic molecular wire

has the unique characteristics of electronic delocalisation in

its extended p-orbital system, through which electrons can

move along the molecules [8, 9]. By making use of these

characteristics in the molecules, the design and the prepa-

ration of molecules should have been extensively studied

and modified with the goal of making them potentially

useful in numerous approaches such as in OLEDs.

In this context, thiourea derivatives are widely known to

be versatile ligand due to their ability of electronic trans-

portation that arises from rigid p-conjugated systems on

resonance structures which have been studied in numerous

applications [10–12], including in molecular electronics

[13, 14]. The uniqueness of thiourea derivatives are due to

their lone pair of N, S and O atoms which involve in the

resonance that leads to electronic delocalization throughout

the molecular backbone of conjugated molecular system

which lead to the effective electrical conductivity proper-

ties [15–17].

Moving towards this interest, lots of previous studies

reported on the exploration of 1-acyl-thiourea family

especially 1-(2-naphthoyl) substituted thiourea to deter-

mine their various properties such as synthesis and char-

acterization, crystal structure, thermal behavior and its

usage in various applications [18–20]. Due to this matter

with highly interest in developing single molecule OLEDs,

we are presenting naphthoyl-thiourea derivative as organic

material candidate to be applied as conductive layer in

OLED as shown in Fig. 1. This study also involved a

contribution of molecular modeling by Gaussian 09 in

order to predict conductivities as well crystal structure and

examination of its potential to act as single layer OLED.

Experimental Section

Materials

All reagents, including materials and solvents were com-

mercially purchased from various standard suppliers and

used as received without further purification. All reactions

were carried out under an ambient atmosphere and no pre-

caution was taken to exclude air or moisture during work up.

In this present work, chemicals used namely 1- naphthoyl

chloride, ammonium thiocyanate and 2-amino-5-methyl-

pyridine were purchased from Sigma-Aldrich, Merck and R

& M Chemical. Whilst, solvents used in this study namely

acetone, methanol, dichloromethane, chloroform, diethyl

ether, acetonitrile and dimethylsulphoxide were supplied by

Merck, Fisher scientific and R & M chemicals.

Characterization and Instrumentation

The infrared (IR) spectrum was recorded on Perkin Elmer

100 Fourier Transform Infrared Spectroscopy by using

potassium bromide (KBr) pellets in the spectral range of

4000–400 cm-1. Meanwhile, 1H and 13C NMR spectra were

recorded using Bruker Avance III 400 Spectrometer in

CDCl3 as solvent and internal standard at room temperature

in the range between dH 0–15 and dC 0–200 ppm, respec-

tively. For UV–Vis analysis, the compound was recorded by

Shimadzu UV–Vis in 1 cm3 cuvette. The emissions of the

compound was characterised by using Shimadzu UV-Fluo-

rescence in 1 cm3 cuvette. The crystallographic structure for

X-ray analysis was performed on Oxford Diffraction Gemini

diffractometers using Cu Ka radiation. Thermogravimetric

analysis was performed using Perkin–Elmer TGA analyzer

from 30 to 700 �C at a heating rate of 10 �C/min under

nitrogen atmosphere. Afterwards, cyclic voltammetry (CV)

analysis was carried out via Electrochemical Impedance

Spectroscopy (EIS) PGSTAT302. Finally, the targeted

molecule was optimized using Gaussian 09 quantum

mechanical software package at the theoretical level of DFT

(B3LYP)/6-31G (d,p) to calculate energy band gap and

predict the conductivity value.

Synthesis of Naphthoyl-Thiourea (NT)

The experimental details regarding to the synthesis of NT

was adapted from the methods carried out by Douglas-Dains

[21] involving reactants of 1-naphthoyl chloride (1.5 mL,

1 mol) with equimolar amount of ammonium thiocyanate

(0.76 g, 1 mol) in ca. 50 mL acetone in 100 mL two-necked

round-bottom flask. The reaction mixture was put at reflux

with continuous stirring for ca. 5 h. Then, a solution of

2-amino-5-methylpyridine (1.07 g, 1 mol) in ca. 50 mL

acetone was added to the reaction mixture and was put at

reflux with continuous stirring for ca. 7 h. The progress of

the reaction was monitored by TLC (hexane:CH2Cl2; 3:2).

Once the reaction has completed, the reaction mixture was

cooled to room temperature and filtered into a beaker con-

taining some ice cubes. The resulting light yellow crystalline

solid obtained was recrystallized from acetone to afford the

title compound (98 % yield). The synthetic route of NT is

NNH

HN

S

O

Fig. 1 The molecular structure

of N-(5-methylpyridine)-N0-(1-naphthoyl) thiourea

(NT)

J Chem Crystallogr (2015) 45:338–349 339

123

presented in Scheme 1. C18H15N3OS requires: C, 67.27; H,

4.70; N, 13.07; S, 9.98. Found: C, 68.83; H, 4.80; N, 13.24;

S, 9.33 %. 1H NMR (CDCl3, 400.11 MHz): d 2.38 (s, 3H,

CH3); 7.48–7.57 (m, 3JHH = 9 Hz, 5H, C10H7 ? C5H3);

7.82 (d, 3JHH = 7 Hz, 1H 9 C5H3); 7.92 (d, 1H x C5H3);

8.05 (d, 3JHH = 8 Hz, 1H x C10H7); 8.69(d, 3JHH = 8 Hz,

1H 9 CH); 8.29, 9.19 (2 9 s, 1H, NH).13C NMR (CDCl3,

100.61 MHz): d 18.03 (CH3); 124.49, 124.59, 126.57,

126.85, 127.20, 128.42, 128.94, 129.87, 130.03, 133.69

(C10H7), 133.90, 135.03, 143.42, 143.88, 146.59 (C5H3);

148.27 (C = S); 148.87 (C = O).

X-ray Data Collection and Structure Refinement

Single crystal of NT was found to be suitable for X-ray

analysis and the analysis was performed on Oxford

Diffraction Gemini diffractometer using Cu Ka radiation.

The crystal structure was determined by single crystal X-ray

diffraction from data collected at low temperature using the

oxford cryosystem open-flow nitrogen cryostat [22]. Data

collection, cell refinement and data reduction was performed

under the CrysAlisPro software [23]. Absorption correction

was applied to the final crystal data by using the CrysAlisPro

software [23]. The crystal structure was solved by direct

method using the program SHELXTL [24] and was refined

by full-matrix least squares technique on F2 using aniso-

tropic displacement parameters using SHELXTL [24]. All

geometrical calculations were carried out using the program

PLATON [25]. The molecular graphics were drawn using

SHELXTL [24] and Mercury program [26]. The non-hy-

drogen atoms were refined anisotropically. The hydrogen

atoms which bounded to the nitrogen atom were found from

the difference fourier maps and refined with a bond restraint

N–H = 0.85 (2) A [refined distance: N1–H1N1 = 0.847(9)

A and N2–H1N2 = 0.852(9) A]. All the other hydrogen

atoms were positioned geometrically (C–H = 0.93 or

0.96 A) and refined using riding model Uiso(H) = 1.2 or 1.5

Ueq(C). A rotating group model was applied to the methyl

group. A summary of crystal data and relevant refinement

parameters of the title compound is given in Table 1.

Preparation of NT on ITO Substrate

The preparation of NT on ITO substrate has been deposited

by applying electrochemical deposition (ECD) method in

the following conditions: 0.05 V, 0.05 V/S, potential

range: 0.85 V until 1.5 V in 50 mL acetonitrile, 0.5 M

sulphuric acid (as supporting electrolyte) with sample

concentration of 1 9 10-3 M.

Electrical Conductivity of Thin Films

Four Point Probe was used to determine the conductivity of

the thin film. The sheet resistivity in produced film was

measured by using Four Point Probing System consists of

the Jandel Universal Probe combined with a Jandel RM3

Test Unit and calculated using Eq. 1.

Sheet resistivity (Rs) calculation for wafer and film

RS ¼ 4:532 � V=I ð1Þ

Four probes were aligned and lowered onto the sample. The

two outer probes supplied a voltage difference that drove a

current through the film while the two inner probes picked up a

voltage difference. Where, RS is the sheet resistance (resis-

tivity), 4.532 is the correction factor,V is the voltage measured

and I is the current applied from the test unit. Then, electrical

conductivity can be determined which it is the reciprocal

(inverse) of the electrical resistivity, r as shown in Eq. 2.

Cl

O

+ NH4SCN NCS

OAcetone

Reflux

1-Naphthoyl chloride

Ammonium thiocyanate

Naphthoyl thiocyanate

Acetone

Reflux2-amino-5-methylpyridine

N-(5-methylpyridine)-N'-(1-naphthoyl) thiourea (NT)

N

+

NNH

HN

S

O H2N

Scheme 1 Synthetic approach

for the synthesis of NT

340 J Chem Crystallogr (2015) 45:338–349

123

Electrical conductivity calculation

r ¼ 1=Rs ð2Þ

where r is electrical conductivity and RS is sheet resistance

(resistivity).

Current–Voltage Diode Characterization

The performance of organic diode was measured by using

Keithley 4200 SCS Semiconductor Characterization

System and Probe Station. In this study, ITO substrate act

as hole collecting layer (anode) while NT act as hole and

electron carriers. The coated ITO substrates were masked

with low work function metal which is aluminium which

acts as cathode. The IV characteristic curve shows the

relationship between the currents and voltages gradient

associated with the different current terminal (anode and

cathode) of the diode. The obtained curve displays the

forward current, reverse current, knee voltage and break-

down voltage of the diode. The fabricated diodes with NT

are illustrated in Fig. 2.

Results and Discussion

Spectroscopic Studies

The infrared spectrum of NT revealed all the expected

bands of interest namely m(N–H), m(C = O), m(C–N), m(C–

O) and m(C = S). Two N–H stretching modes can be

identified at 3369 and 3114 cm-1 due to the existence of

C = O���H–N intramolecular hydrogen bonding [27–30].

The C–H stretching vibration for methyl v(C–H) can be

seen at 2922 cm-1. The strong absorption band at

1673 cm-1 was ascribed to the stretching of carbonyl

group. The decrease in the wavenumber compared to the

typical carbonyl absorption (1710 cm-1) was due to the

conjugated resonance with the phenyl ring and formation

of molecular hydrogen bonding with N–H [31, 32]. The

v(C–N) band can be observed at around 1289 cm-1 due to

resonance affect of the double bond character between the

ring and the attached nitrogen atoms [33]. Additionally, the

existence of C = S stretching band can be assigned at low

frequency of 790 cm-1 due to less double bond character

and lower nucleophilic character of sulfur atom that were

in close agreement with previously reported series of

thiourea derivatives [34–36].1H NMR spectrum of NT showed methyl resonance at

dH 2.38 ppm. Whilst, the overlapping distinctive unre-

solved signal of the aromatic protons can be observed at

around dH 7.48–8.69 ppm. These resonances characteristic

are strongly influenced by the o and p-substituents posi-

tions of methyl groups at the pyridine rings [37]. The

resonance for NH(1)C = O and NH(2)C = S can be

observed at around dH 8.29 ppm and dH 9.19 ppm

Fig. 2 Diode arrangements for I–V characteristic measurement

Table 1 Crystal data and structure refinement for naphthoyl-thiourea

(NT)

Compound NT

CCDC deposition numbers 1048526

Molecular formula C18H15N3OS

Molecular weight 321.39

Crystal system Triclinic

Space group P-1

a (A) 7.4916 (5)

b (A) 9.4050 (7)

c (A) 12.0584 (9)

a (�) 69.685 (7)

b (�) 82.130 (6)

c (�) 71.917 (7)

V (A3) 757.01 (9)

Z 2

l (mm-1) 1.96

Crystal size (mm) 0.28 9 0.19 9 0.12

Dcalc (Mg m-3) 1.410

Crystal dimensions (mm) 0.38 9 0.36 9 0.12

l/mm-1 2.84

Radiation, k (A) 1.54180

F(000) 336

Tmin/Tmax 0.610/0.799

Reflections measured 2910

Ranges/indices (h, k, l) h = -8 ? 9

k = -10 ? 11

l = -0 ? 14

h limit (�) 3.9–71.5

Unique reflections 2910

Observed reflections (I[ 2r(I)) 2677

Parameters/restraints 217/2

R1a, wR2 [I C 2r(I)]b 0.038, 0.104

Goodness of fit (GOF) on F2c 1.05

Largest diff. peak and hole, e/A-3 0.32 and -0.27

For NT, w = 1/[r2(Fo2) ? (0.0604P)2 ? 0.3749P], where P = (Fo

2 ?

2Fc2)/3

a R1 = R||Fo| - |Fc||/R|Fo|b Rw = {wR(|Fo| - |Fc|)

2/Rw|Fo|2}}1/2

c GOF = {Rw(|Fo| - |Fc|)2/(n–p)}1/2, where n is the number of

reflections and p the total number of parameters refined

J Chem Crystallogr (2015) 45:338–349 341

123

respectively. The resonance for NH(1)C = O slightly

deshielded to higher chemical shifts compared to

NH(2)C = S was due to oxygen atom presence which is

known to be more electronegative than sulphur. These

signals are different in term of chemical shift which is due

to the intramolecular hydrogen bonded N–H bonds in the

trans and cis conformations, respectively [31, 38]. The 13C

NMR of NT showed methyl resonance can be clearly

observed at dC 18.03 ppm. Whilst, resonances for aromatic

rings can be found in the range of dC 116.13–144.03 ppm

and the carbon for C = O and C = S can be identified

at dC 148.27 ppm and 148.87 ppm respectively. The for-

mation of intra-molecular hydrogen bonding of the com-

pound and the increasing electronegativity of oxygen and

sulphur caused the deshielding effect to higher chemical

shifts [39, 40].

Optical Properties

The electronic absorption spectrum of NT revealed two

principal bands which were expected to arise from C = O,

C = S and phenyl moieties at around kmax 243 nm and

275 nm. The absorption band of C = O chromophore can

be identified in the range of kmax 240 nm to 243 nm due to

the effect of p-conjugation of carbonyl with the phenyl

group, thus the transition of p ? p* shifted to the longer

wavelength. Strong absorption band centered at around

kmax 275 nm can be attributed to p ? p* and n ? p*

transitions of C = S and naphthoyl in thiourea moiety. The

broad absorption band observed in the region was due to p-

conjugation of this compound with the phenyl rings (p - p*

transition) and orbitals overlapping between C = O and

C = S.

Whilst, the energy band gap (Eg) values was calculated

from the absorption edge (koff-set) of the compound in the

range of kmax 200–600 nm region by using (Eg = hc/k)

equation. From the UV–Vis, the predicted HOMO–LUMO

energy band gap for NT was 4.07 eV. In comparison, the

theoretical value was calculated using Gaussian 09 at the

theoretical level of DFT/B3LYP 6-31G (d,p) and the result

was in agreement with experimental which the energy band

gap, Eg was around 4.05 eV.

Meanwhile, the absorption and fluorescence spectrum

of NT was carried out in methanol (1 9 10-6 M). The

fluorescence emission spectrum exhibited almost mirror-

image of the corresponding absorption spectra and the

remarkable mirror image relationship observed between

the spectra should be noted with a small Stokes shift

(90 nm) [41]. The absorption spectrum of NT has intense,

short wavelength band at 294 nm and the emission

spectra showed one maxima wavelength at 370 nm

respectively.

Crystal Structure Determination of NT

The single crystal of NT was obtained from slow evapo-

ration process in acetone at room temperature. NT crys-

tallizes as triclinic crystal system in P-1 space group, unit

cell parameters a = 7.4916(5) A, b = 9.4050(7) A,

c = 12.0584(9) A, a = 69.685 (7)�, b = 82.130 (6)� and

c = 71.917 (7)�. Table 2 shows the selected bond lengths

and angles of NT in comparison with the corresponding

values observed for related thiourea derivatives [42–44].

All geometrical parameters of NT are within the normal

ranges [45] and comparable with the corresponding values

tabulated in Table 2.

The molecular structure of NT is shown in Fig. 3a with

displacement ellipsoids plotted at 50 % probability level.

The compound adopts trans–cis configuration with respect

to the position of naphthalene (C1–C10) and picoline (N3/

C13–C18) moieties relative to the S1 atom across their

C12–N1 and C12–N2 bonds. The molecule is stabilized by

intramolecular N2–H1N2���O1 [46] and C14–H14A���S1

hydrogen bonds (Table 3), forming two six-membered

rings with the graph-set notation S(6) [47]. The non-

bonding distances of N2���O1 and C14���S1 are 2.6487 (18)

and 3.2571 (18) A, respectively in agreement with

intramolecular hydrogen bond observed in other related

structures [42, 48], where the reported values are in the

range of 2.590–3.210 A. Formation of these S(6) ring

motifs is important for molecular conformations because it

prevents free rotation within the central carbonyl thiourea

moiety and locks its atoms in a nearly planar arrangement

[48]. Additionally, the planar structure of the central –

C(O)NHC(S)NH– moiety with opposite orientation

between the C = O and C = S bonds (S-form) is reported

to be the most stable conformation in comparison to the

non-planar synclinal conformer (U-form) [49].

The O1–C11 bond distance of 1.226(2) A indicates

double bond character in agreement with literature data

(Table 2). The observed C–N [N1–C11 = 1.379(2), N1–

C12 = 1.407(2) and N2–C12 = 1.340(2) A] and C–S [S1–

C12 = 1.6610(16) A] bond lengths of NT are all shorter

than the average single C–N (1.48 A) and C–S (1.82 A)

bond lengths, indicating partial double bond character.

Thus, it can be deduced that this thiourea makes up a multi-

electron conjugated p-bond [50] and also the existence of

resonance interactions extended over the whole planar –

C(O)NHC(S)NH– moiety [51, 52].

The essentially planar –C(O)NHC(S)NH– moiety with

maximum deviation of 0.0904(5) A at S1 atom forms

dihedral angles of 54.74(6) and 16.20(8)� with the terminal

naphthalene [C1–C10; maximum deviation of 0.036(2) A

at atom C9] and pyridine [N3/C13–C17; maximum devi-

ation of 0.007(2) A at atom C15 and C16] rings, respec-

tively. The naphthalene ring and the methyl substituted

342 J Chem Crystallogr (2015) 45:338–349

123

Table 2 Selected geometrical parameters (bond lengths and angles) of NT

Parameters NT Aydin et al. [42] Aydin et al. [43] Arslan et al. [44]

Bond lengths (A)

S1–C12 1.6610 (16) 1.6521 (14) 1.672 (4)

1.658 (4)

1.6696 (17)

O1–C11 1.226 (2) 1.2241 (18) 1.224 (5)

1.228 (5)

1.224 (2)

N1–C11 1.379 (2) 1.3748 (18) 1.374 (5)

1.363 (5)

1.377 (2)

N1–C12 1.407 (2) 1.3931 (17) 1.384 (5)

1.394 (5)

1.3974 (19)

N2–C12 1.340 (2) 1.3450 (17) 1.317 (5)

1.324 (5)

1.327 (2)

N2–C13 1.414 (2) 1.4114 (17) 1.454 (5)

1.449 (5)

1.4408 (19)

C10–C11 1.494 (2) 1.4826 (19) 1.483 (6)

1.482 (6)

1.488 (2)

Bond angles (�)C11–N1–C12 129.13 (14) 129.20 (12) 129.6 (4)

130.3 (4)

128.11 (13)

C12–N2–C13 131.14 (14) 129.66 (12) 124.2 (4)

123.5 (4)

123.59 (13)

C9–C10–C11 119.36 (14) 122.71 (12) 125.6 (4)

117.7 (4)

121.68 (14)

C1–C10–C11 119.79 (14) 117.77 (13) 117.7 (4)

124.1 (4)

118.24 (14)

O1–C11–N1 122.57 (15) 121.61 (13) 121.9 (4)

121.1 (4)

122.77 (14)

O1–C11–C10 122.30 (14) 121.95 (13) 122.5 (4)

121.6 (4)

121.33 (14)

N1–C11–C10 115.12 (13) 116.44 (12) 115.6 (4)

117.3 (4)

115.88 (13)

N2–C12–N1 114.11 (14) 114.01 (12) 117.8 (4)

116.9 (4)

116.72 (14)

N2–C12–S1 128.65 (13) 127.60 (10) 123.9 (4)

125.2 (4)

124.47 (12)

N1–C12–S1 117.20 (12) 118.38 (10) 118.3 (3)

117.9 (3)

118.81 (11)

C14–C13–N2 125.48 (14) 123.97 (13) 114.9 (4)

112.4 (4)

118.36 (14)

Torsion angles (�)C12–N1–C11–O1 2.8 (3) 3.8 (3) 0.2 (7)

-6.3 (7)

-4.8 (3)

C12–N1–C11–C10 -177.62 (15) -176.61 (14) 179.9 (4)

-6.3 (7)

173.38 (14)

C9–C10–C11–O1 129.48 (17) -156.35 (17) 164.6 (5)

17.4 (6)

140.14 (17)

C1–C10–C11–O1 -46.5 (2) 20.8 (2) -13.6 (7)

-165.0 (5)

-32.9 (2)

J Chem Crystallogr (2015) 45:338–349 343

123

pyridine ring makes dihedral angle of 39.14(7)� to each

other. Additionally, the naphthalene ring is twisted at C10–

C11 bond with C1–C10–C11–O1 torsion angle value of

46.5 (2)� as shown in Fig. 3b.

In the crystal packing, the molecules are connected into

infinite one-dimensional chain along the a-axis via inter-

molecular C8–H8A���O1 and C9–H9A���N3 hydrogen

bonds [symmetry code: x - 1, y, z; Table 3]. As shown in

Fig. 4a, both C–H���O and C–H���N hydrogen bonds lead to

formation of R22(11) ring motifs [47]. This chain is inter-

connected with the adjacent chain by C18–H18A���Cg1

(symmetry code: -x?2, -y?2, -z?1; Table 3) interac-

tions, where Cg1 is the centroid of the C1–C6 ring. This

type of interaction extend along [100] to form a

supramolecular column (Fig. 4b). Furthermore, the crystal

structure is stabilized by p���p interactions (symmetry code:

1-x,1-y,1-z; Fig. 4c), involving the centroid of Cg2 ring

(C1/C6–C10) [53, 54]. The interplanar and centroid to

centroid distances of Cg2���Cg2 interaction are 3.7096(7)

and 3.8332(11) A, respectively, with a slip angle (the angle

between the centroid vector and the normal to the slip

plane) of 14.59�. The p���p interactions further link the

supramolecular columns into a two dimensional sheet

parallel to ab-plane (Fig. 4c). Figure 4d shows the full

molecular structure arrangement of NT where the C–H���O,

C–H���N, C–H���p and p���p interactions connect the

molecules into a supramolecular two-dimensional sheet

architecture.

Fig. 3 a The molecular structure of N-(5-methylpyridine)-N0-(1-

naphthoyl) thiourea (NT) showing the atomic numbering scheme.

Displacement ellipsoids are drawn at the 50 % probability level.

Dashed lines represent the intramolecular hydrogen bond. b Projection

of the twisted naphthalene ring

Table 3 Hydrogen bonding geometry of NT

Bond Bond length (A) Angle (�)

D–H���A D–H H���A D���A D–H���A

N2–H1N2���O1 0.85 (1) 1.93 (1) 2.6487 (18) 142 (2)

C14–H14A���S1 0.93 2.66 3.2571 (18) 123

C8–H8A���O1i 0.93 2.50 3.407 (2) 166

C9–H9A���N3i 0.93 2.61 3.523 (2) 168

C18–H18A���Cg1ii 0.96 2.79 3.740 (2) 170

Symmetry codes: ix - 1, y, z; ii-x?2, -y?2, -z?1

Cg1 is the centroid of the C1–C6 ring

Table 2 continued

Parameters NT Aydin et al. [42] Aydin et al. [43] Arslan et al. [44]

C9–C10–C11–N1 -50.1 (2) 24.0 (2) -15.1 (7)

-163.6 (4)

-38.0 (2)

C1–C10–C11–N1 133.87 (15) -158.86 (14) 166.7 (5)

14.1 (6)

148.88 (15)

C13–N2–C12–N1 178.69 (15) -177.42 (14) 175.5 (4)

-177.5 (4)

176.77 (14)

C13–N2–C12–S1 1.2 (3) 1.7 (2) -3.5 (6)

2.2 (6)

-3.9 (2)

C11–N1–C12–N2 -8.0 (2) 6.0 (2) 3.0 (7)

-177.5 (4)

-4.2 (2)

C11–N1–C12–S1 169.83 (13) -173.22 (13) -178.0 (4)

2.2 (6)

176.37 (13)

C12–N2–C13–C14 21.4 (3) -28.7 (3) 85.4 (5)

106.5 (5)

-103.42 (19)

344 J Chem Crystallogr (2015) 45:338–349

123

Fig. 4 Hydrogen bonding interactions in NT a intermolecular C–

H���O and C–H���N hydrogen bonds observed in the crystal packing,

b formation of column via C–H���p interactions, c p ���p interactions

stabilized the crystal structure d the crystal packing of NT connected

into a two-dimensional network. Dashed lines show the intermolec-

ular hydrogen bonds. H atoms not involved in hydrogen bonding are

omitted for clarity

J Chem Crystallogr (2015) 45:338–349 345

123

Thermogravimetric Analysis (TGA)

Thermal stability of the material is important to be inves-

tigated for fabrication of any thin films application. Ther-

mal properties of NT was evaluated via TGA at heating

rate of 10 �C/min under nitrogen atmosphere with tem-

perature range of 30–700 �C as shown in Fig. 5. Based on

the thermogram data, there was no mass loss occurred

below 100 �C and it showed that there was no presence of

water or any solvent in the sample. From the thermogram,

the degradation process showed that NT has the highest

onset which started to degrade at around 200 �C (Tonset)

and ended at 320 �C (Toffset) in a single step. During the

decomposition of this stage, a minor shoulder between 240

to 260 �C was also present in the thermogram and the total

mass loss was about 92 %. Therefore, it can be summa-

rized that NT was thermally stable according on its onset

and it gave good indication that they can be potentially

good coating material for the fabrication of thin film at

high temperature.

Cyclic Voltammetry

The electrochemical behaviour of NT as a focalpoint

compound in this study was investigated further by using

CV analysis in order to determine the redox reaction and

potential range of different electrochemical processes. The

oxidation peak for NT occurred at DEpa = 1.30 V while

the reduction peak occurred at DEpc = 1.15 V as depicted

in Fig. 6. The concentration decreased rapidly at the

electrode surface as the anodic current decreased which

resulted the current to peak. There was a report claimed

that the first oxidation of thiourea is defined at *0.7 V

which represents to the formation of formamidine disul-

phide [55]. In this study, NT strongly oxides to NT2? at

*1.15 V and the current started to decay as NT became

more depleted and NT2? surrounded the electrode. As the

anodic current continues, the electrode is sufficiently strong

to reductant to reduce NT2? which has accumulated

adjacent to the electrode surface. Thus, the cathodic current

rapidly increased and the surface concentration of NT2?

was diminished causing the current to peak (*1.3 V) and

the current started to decay as solution surrounding the

electrode was depleted of NT2?. Thus, it can be concluded

that the redox potential of NT appeared in the positive

region with its electro-oxidation stage with a separation

about 1 V.

Theoretical Evaluation: Prediction of Electrical

Conductivity

Theoretically, to predict the relation between the sub-

stituent of NT with the conductivity, Mulliken charge

values was measured where Ln v2 for the first and the

second benzene rings of NT was evaluated at the theoret-

ical level of B3LYP/6-31G (d,p). In this study the natural

logarithm of v2 were employed as a measure of the degree

of delocalization among the benzene rings of the targeted

molecule. As a standard, the value of the total Ln v2 for the

benzene ring evaluated at the same theoretical level is -31.

When there is a substituent existed in the ring, these values

should increase to a certain number which represent how

far the electron delocalization deviates from the aro-

maticity of the benzene ring.

In this study, the calculated total value of -R Ln (v2)

was -70.087, in which the first ring (naphtyl) contributed

the highest values of -47.405 and the second ring (pyr-

idine) contributed as -22.682. Based on theoretical find-

ings, it can be summarized that the conductivity depended

on the delocalization of the aromatic portions of NT

derivative. Thus, we believed the performance of the

electrical conductivity showed positive results under dark

condition was caused by the conjugation and delocalization

of the first benzene ring and the movement of electron from

the para-position along the molecular framework.

Performance of Diode and Emitting Light

with Different Voltage from ITO Substrate

The IV characteristic curve of NT was investigated to

determine their performances as diode which the optimum

values of voltage applied into the circuit for diode in this

study was in the range of -9.0E-6 to -3.0E-6 V. The

shape of the curve determined the transport charge carrier

through the depletion layer or depletion region that exists at

the P–N junction which referred to the semiconductor

properties. P–N junction formed when the electrons diffuse

from N-doped region (positively charge donor) into

P-doped region (negatively charge acceptor) which that theFig. 5 The thermogram of NT

346 J Chem Crystallogr (2015) 45:338–349

123

electrons recombined. Additionally, the knee voltage of

NT was around 0.7 V where the values lower than the knee

voltages of Schottky diode (0.2–0.3 V) and pn junction

diode (0.55–0.60 V) [56]. Thus, it can be summarized that

NT exhibited a semiconductor properties and it can be used

in rectification application such as OLED as the electric

current can pass through the P–N junction. Afterwards,

ITO substrates were tested with direct current and different

voltage in the range of 15–30 V in order to investigate

diode performance in various conditions, as depicted in

Fig. 7. The output of light was emitted from ITO substrates

and increased by the increasing amount of current voltage.

Therefore, this proposed material gave good indication for

conductive layer in fabrication of thin film to be applied as

OLED.

Conclusion

The performance of single molecule compound of naph-

thoyl-thiourea derivative namely N-(5-methylpyridine)-N0-(1-naphthoyl) thiourea (NT) has been successfully syn-

thesized with good yields prior to form conductive layer in

OLED. Conformational and structural properties were

characterized via several spectroscopic technique and sin-

gle crystal X-ray diffraction analysis. The X-ray structure

3.80E-05

4.30E-05

4.80E-05

5.30E-05

1.1 1.121.141.161.18 1.2

Curr

ent,

I (A)

Potential, E (V)

scan 1

scan 2

scan 3

scan 4

scan 5

Epa(A)

9.00E-05

1.40E-04

1.90E-04

1.2 1.25 1.3 1.35 1.4

Curr

ent,

I (A)

Potential, E (V)

scan 1scan 2scan 3

Epc(B)Fig. 6 Cyclic voltammogram

of NT in (1 9 10-3 M

acetonitrile ? 0.5 M sulphuric

acid), 298 K at 0.05 V, 0.05

Vs-1

15V 20V 25V 30V

Fig. 7 The emission of light of NT with different voltages

J Chem Crystallogr (2015) 45:338–349 347

123

shows the usual value for bond lengths and bond angles

and adopts trans–cis configuration. The molecular confor-

mation of NT stabilized by strong intramolecular N–

H���O = C hydrogen bond occurs between the carbonyl

and thioamide groups which forming a planar six-mem-

bered ring with respect to the hydrogen bonding. In the

crystal packing, supramolecular arrangement of NT formed

two-dimensional sheet parallel to the ab-plane via C–

H���O, C–H���N, C–H���p and p���p interactions. From the

evaluation of conductivity study, NT/ITO thin film can

conduct electricity better under dark condition and the

emission of light from ITOs can be observed. In conclu-

sion, this type of system featuring naphthoyl-thiourea

exhibits wide potential to be applied as conductive layer in

OLED.

Supplementary Materials

This data CCDC: 1048526 can be obtained free of charge

at www.ccdc.cam.ac.uk.conts/retrieving.html/ or from the

Cambridge Crystallographic Data Centre (CCDC), 12

Union Road, Cambridge CB2 IEZ, UK; fax: ?44(0)

1223-336033; e-mail: deposit@ccdc.cam.ac.uk.

Acknowledgments The authors would like to acknowledge MOE

for Fundamental Research Grant Schemes (FRGS), Grant No. 59253

and MyBrain15 Fund for postgraduate student’s fellowship and the

School of Fundamental Science, Universiti Malaysia Terengganu for

instrumentations and characterisations facilities.

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