Electronic properties of phenanthrimidazoles as hole transport materials in organic light emitting devices and in photoelectron transfer to ZnO nanoparticles

Article abstract of DOI:10.1002/poc.3100

Phenanthrimidazoles as hole transport materials have been synthesized, characterized, and applied as nondoping emitters in organic light emitting devices. The synthesized molecules possess high fluorescent quantum yield and thermal properties and display film forming abilities. The highest occupied molecular orbital (HOMO) energies of these materials are shallower than the reported tris(8-hydroxyquinoline)aluminum (Alq₃), which enables the hole transport ability of these phenanthrimidazoles. Taking advantage of the thermal stability and hole transporting ability, these compounds can be used as a functional layer between NPB [4,4-bis(N-(1-naphthyl)-N-phenylamino)biphenyl] and Alq₃ layers and show that these phenanthrimidazoles can be alternatively used as novel hole transport materials and to improve the device performances. Geometrical, optical, electrical, and electroluminescent properties of these molecules have been probed. Further, natural bond orbital, nonlinear optical materials (NLO), molecular electrostatic potential, and HOMO-lowest unoccupied molecular orbital (LMO) energy analysis have been made by density functional theory (DFT) method to support the experimental results. Hyperpolarizability analysis reveals that the synthesized phenanthrimidazoles possess NLO behavior. The chemical potential, hardness, and electrophilicity index of phenanthrimidazoles have also been computed by DFT method. Photoinduced electron transfer explains the enhancement of fluorescence by nanoparticulate ZnO, and the apparent binding constant has been obtained. Adsorption of the fluorophore on ZnO nanoparticle lowers the HOMO and LUMO energy levels of the fluorophore. The strong adsorption of the phenanthrimidazoles on the surface of ZnO nanocrystals is likely due to the chemical affinity of the nitrogen atom of the organic molecule to Zn(II) on the surface of nanocrystal. Copyright

Full text of DOI:10.1002/poc.3100

Research Article
Received: 7 December 2012,
Revised: 18 January 2013,
Accepted: 27 January 2013,
Published online in Wiley Online Library: 2 April 2013
(wileyonlinelibrary.com) DOI: 10.1002/poc.3100
Electronic properties of phenanthrimidazoles
as hole transport materials in organic light
emitting devices and in photoelectron transfer
to ZnO nanoparticles
Chockalingam Karunakarana, Jayaraman Jayabharathia*,
Marimuthu Venkatesh Perumala, Venugopal Thanikachalamb
and Prasoon Kumar Thakurc
Phenanthrimidazoles as hole transport materials have been synthesized, characterized, and applied as nondoping
emitters in organic light emitting devices. The synthesized molecules possess high fluorescent quantum yield and
thermal properties and display film forming abilities. The highest occupied molecular orbital (HOMO) energies of these
materials are shallower than the reported tris(8-hydroxyquinoline)aluminum (Alq3), which enables the hole transport
ability of these phenanthrimidazoles. Taking advantage of the thermal stability and hole transporting ability, these
compounds can be used as a functional layer between NPB [4,4-bis(N-(1-naphthyl)-N-phenylamino)biphenyl] and
Alq3 layers and show that these phenanthrimidazoles can be alternatively used as novel hole transport materials and
to improve the device performances. Geometrical, optical, electrical, and electroluminescent properties of these
molecules have been probed. Further, natural bond orbital, nonlinear optical materials (NLO), molecular electrostatic
potential, and HOMO–lowest unoccupied molecular orbital (LMO) energy analysis have been made by density
functional theory (DFT) method to support the experimental results. Hyperpolarizability analysis reveals that the
synthesized phenanthrimidazoles possess NLO behavior. The chemical potential, hardness, and electrophilicity index
of phenanthrimidazoles have also been computed by DFT method. Photoinduced electron transfer explains the
enhancement of fluorescence by nanoparticulate ZnO, and the apparent binding constant has been obtained. Adsorption
of the fluorophore on ZnO nanoparticle lowers the HOMO and LUMO energy levels of the fluorophore. The strong
adsorption of the phenanthrimidazoles on the surface of ZnO nanocrystals is likely due to the chemical affinity of
the nitrogen atom of the organic molecule to Zn(II) on the surface of nanocrystal. Copyright © 2013 John Wiley & Sons, Ltd.
Keywords: phenanthrimidazole; hole transport materials; optical properties; nanosemiconductor; lifetime
of the device leading to high power efficiency. In addition, the
relative mobilities of electron and hole in the same material can
INTRODUCTION
A great deal of effort has been made on the studies of organic
electroluminescent (EL) materials and devices.[1–4] Because of
the optical and conductive properties, conjugated materials
containing thiophene, imidazole, and phenanthroline heterocy-
cles have found many applications.[5–12] These aryl-imidazo-
phenanthrolines play an important role in materials science
and medicinal chemistry due to their optoelectronic properties
and possess high thermal stabilities. Intensive research has been
carried out to find materials with high light emitting efficiencies,
high thermal stability, and good amorphous film formation
property.[13–16] The optoelectronic properties for organic light
emitting devices (OLEDs) depend on appropriate highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy levels and suitable electron and hole mobil-
ities. Although the guidelines for designing small molecules with
the desirable photo and thermal properties[17–19] are well-known,
analogous guidelines on the mobilities of charge carriers in
organic materials are limited because of the scarce of experimen-
tal data. Nevertheless, the mobilities are important in optimizing
the performance of OLED; high mobilities reduce the resistance
also affect the power efficiency.
Oxadiazoles, triphenylamine, and their derivatives are known
electron and hole transporting materials and are used for the
construction of hetero-junction multi-layer OLED.[20,21] A new
class of materials for EL devices having both the processability
* Correspondence to: J. Jayabharathi, Professor of Chemistry, Department of
Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India.
E-mail: jtchalam2005@yahoo.co.in
a C. Karunakaran, J. Jayabharathi, M. Venkatesh Perumal
Department of Chemistry, Annamalai University, Annamalainagar, Tamilnadu,
India
b V. Thanikachalam
Department of Chemistry Wing (DDE), Annamalai University, Annamalainagar,
Tamilnadu, India
c P. Kumar Thakur
Division of Nematology, Indian Agricultural Research Institute, New Delhi-12,
India
J. Phys. Org. Chem. 2013, 26 386–406
Copyright © 2013 John Wiley & Sons, Ltd.

MATERIALS SCIENCE
of polymers and the defined optical and electrical properties of
low molecular weight materials are of interest; spiro and branched
derivatives containing oxadiazoles as well as dendrimers are being
used as electron-transporting and hole-blocking materials.[22–24]
Shirota[25,26] has recently suggested a new method for the
molecular design of low molecular weight amorphous materials,
which involve increasing the number of conformers by lowering
the symmetry of nonplanar molecules. Bazan and co-workers[27]
demonstrated that tetrahedral oligophenylenevinylene tetramer
acted as an efficient OLED. Quite recently, amorphous blue light-
emitting materials based on tetraphenylsilane compounds or bis
(spirobifluorenyl) anthracenes and morphologically stable carba-
zole with peripheral aryl amines possessing dual functions, light
emitting, and hole transporting have been reported.[28–31] The
devices, however, have been prepared by sequential vacuum
deposition method. As of today, the development of organic
molecules with hole transport property is less than those with
electron transport properties. Thus, development of better emis-
sive materials with hole transport properties is an important
approach for performance enhancement. Our experimental find-
ings prompted us to develop new hole-transporting materials
with high glass transition temperature (Tg) using a new core
molecule for OLEDs.
Synthesis of phenanthrimidazoles
1,10-Phenanthroline-5,6-dione was synthesized and purified according
to the reported literature procedure.[65] The various substituted
phenanthrimidazoles[66–68] were prepared by four components conden-
sation of 1,10-phenanthroline-5,6-dione (40 mmol), ammonium acetate
(30 mmol), arylaldehyde (30 mmol), and arylamine (30 mmol). These four
components were refluxed in ethanol (20 mL) at 80 ꢀC. The completion of
the reaction was monitored by thin layer chromatography. The reaction
mixture was extracted with dichloromethane and purified by column
chromatography using benzene : ethyl acetate (9:1) as the eluent.
1-(3,5-Dimethoxyphenyl)-2-phenyl-1H-imidazo[4,5-f][1,10]phenan-
throline (1)
Yield: 55%. Anal. Calcd for C27H20N4O2: C, 74.98; H, 4.66; N, 12.95. Found:
C, 74.68; H, 4.53; N, 12.57. 1H NMR (500 MHz, CDCl3): d 4.01 (s, 6H), 6.50
(d, 2H), 7.59–7.81 (m, 3H), 8.36 (s, 1H), 8.69 (t, 2H), 8.96 (d, H-aryl), 9.26
(d, 2H), 9.38 (d, 2H). 13C (100 MHz, CDCl3): d 55.58, 92.33, 96.46, 102.88,
116.19, 118.01, 119.17, 122.97, 123.44, 124.07, 124.46, 124.99, 126.98,
127.32, 128.58, 129.08, 130.78, 131.53, 132.41, 134.88, 137.82, 138.31,
139.33, 140.70, 143.32, 144.84, 147.40, 148.31, 149.64, 156.72, 158.91,
163.72, 164.55. MS: m/z. 431.17 [M ꢁ 1].
Study of interfacial electron transfer dynamics between
molecular adsorbates and nanocrystalline semiconductors is of
current interest[32–37] because of its essential role in dye-sensitized
solar cells,[38,39] photocatalysis,[40] and molecular electronics.[41]
The kinetics of charge injection from dye excited state to semicon-
ductor nanoparticles and recombination are found to be, in
general, nonsingle exponential, suggesting a heterogeneous
distribution of interfacial electron transfer rates. The nonexponential
kinetics could result from static heterogeneities in energetics of
the adsorbate and semiconductor and their electronic coupling as
well as dynamic fluctuation of these quantities. Nanocrystalline zinc
oxide (nano-ZnO) is a wide band gap semiconductor possessing a
large excitonic binding energy and a unique photoluminescence
(PL) spectrum. These properties make nano-ZnO a promising
material for use in the areas of photonics, electronics, and sensors.
For example, the ability of nano-ZnO to act as a gas sensor
has been demonstrated by utilizing changes in its electrical
resistivity. To our knowledge, most researchers have studied
the ligand effects on the optical properties of nanocrystals,[42–52]
and comparison of the photoelectric properties of different ligands
with ZnO nanocrystals is rare. In this paper, we examine electron
transfer from photoexcited phenanthrimidazole to ZnO nanocrys-
tal. We have analyzed these systems with PL and time-resolved
PL decay spectra. Our results show that, unlike the widely reported
quenching of fluorescence of different ligands by various
nanoparticulate semiconductors[53–63] including ZnO,[64] the
fluorescence of synthesized phenanthrimidazoles is enhanced
by nano-ZnO.
2-(4-Fluorophenyl)-1-(3, 5-dimethoxyphenyl)-1H-imidazo [4, 5-f]
[1,10]phenanthroline (2)
Yield: 55%. Anal. Calcd for C27H19N4FO2: C, 71.99; H, 4.25; N, 12.44.
Found: C, 71.78; H, 4.21; N, 12.18. 1H NMR (500 MHz, CDCl3): d 3.81 (s,
6H), 7.80–7.87 (m, 4H), 8.46 dd, 2H), 8.72 (s, 1H), 9.06 (dd, 2H, J = 8.4 Hz),
9.33 (dd, 2H, J = 4.4 Hz), 9.38 (dd, 2H, J = 4.4 Hz). 13C (100 MHz, CDCl3):
d 52.16, 121.07, 124.39, 124.48, 125.48, 125.57, 132.55, 137.91, 144.33,
146.27, 147.72, 151.56, 153.65, 158.87, 162.56, 163.95. MS: m/z. 451.17
[M + 1], 453.23 [M + 2].
2-(2, 4-Difluorophenyl)-1-(3,5-dimethoxyphenyl)-1H-imidazo[4,5-f]
[1,10]phenanthroline (3)
Yield: 60%. Anal. Calcd for C24H18F2N4O2: C, 69.23; H, 3.87; N, 11.96.
Found: C, 69.97; H, 3.63; N, 11.69. 1H NMR (400 MHz, CDCl3): d 3.95 (s,
3H), 7.81–7.87 (m, 3H), 8.46 (d, 2H), 8.73 (s, 1H), 9.06 (dd, 2H, J = 8.8 Hz),
9.33 (dd, 2H, J = 4.4 Hz), 9.38 (dd, 2H, J = 4.4 Hz). 13C (400 MHz, CDCl3):
d 55.60, 121.08, 124.38, 124.47, 125.49, 125.56, 132.54, 137.90, 144.34,
146.29, 147.73, 151.56, 153.64, 159.63, 160.32. MS: m/z. 469.42 [M + 1].
2-(4-(Trifluoromethyl)phenyl)-1-(3,5-dimethoxyphenyl)-1H-imidazo
[4,5-f][1,10]phenanthroline (4)
Yield: 60%. Anal. Calcd for C28H19F3N4O2: C, 67.20; H, 3.83; N, 11.19.
Found: C, 67.03; H, 3.75; N, 11.04. 1H NMR (400 MHz, CDCl3): d 3.65 (s,
3H), 7.78–7.84 (m, 4H), 8.44 (d, 2H), 8.68 (s, 1H), 9.02 (dd, 2H, J = 8.4 Hz),
9.30 (dd, 2H, J = 3.2 Hz), 9.35 (dd, 2H, J = 3.2 Hz). 13C (400 MHz, CDCl3):
d 55.62, 90.90, 95.07, 97.72, 121.02, 123.01, 124.64, 125.39, 126.28,
128.32, 128.60, 129.54, 130.33, 131.07, 132.50, 135.83, 137.23, 137.89,
144.26, 145.62, 146.21, 147.66, 150.15, 151.48, 152.57, 153.60, 159.49,
161.72. MS: m/z. 501.58 [M + 1].
EXPERIMENTAL
Chemicals
Synthesis of nanocrystalline ZnO by sol–gel method
4-Fluorobenzaldehyde, 2,4-difluorobenzaldehyde, and 4-trifluoromethyl-
benzaldehyde were supplied by Sigma Aldrich (St. Louis, USA). Benzaldehyde
and 3,5-dimethoxyaniline used were of analytical grade and received
from S.D.Fine (Mumbai, India). The solvents used for cyclic voltammetric,
UV-visible spectroscopic, PL studies and lifetime measurements were of
spectroscopic grade supplied by Himedia (Chennai, India).
To zinc acetate (0.1 g) solution under continuous stirring, aqueous
ammonia (1:1) was added dropwise to reach a pH of 7, and the
stirring was continued for another 30 min. The formed glassy-like white
gel was allowed to age overnight. It was filtered, washed with water
and ethanol, dried at 100 ꢀC for 12 h, calcinated at 500 ꢀC for 2 h[69] at a
heating rate 10 ꢀC minꢁ1
.
J. Phys. Org. Chem. 2013, 26 386–406
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

C. KARUNAKARAN ET AL.
method, using B3LYP/6-31G (d,p) basis set. The density functional theory
was used to calculate the dipole moment (m), mean polarizability (a),
and the total first static hyperpolarizability (b)[74,72,75–82] of these
phenanthrimidazole derivatives in terms of x, y, and z components using
the following equations:
Thermal analysis
Thermal analysis of the phenanthrimidazoles 1–4 was made with
NETZSCH-Geratebau Gmbh thermal analysis STA 409 PCO. The differen-
tial scanning calorimetric (DSC) and thermogravimetric (TG) analyses
were made under nitrogen atmosphere (100 mL minꢁ1). The sensitivity
of the instrument was set at 0.01 mg, and the sample ꢁ(110 mg) was heated
1
ꢀ
ꢁ
ꢀ
ꢁ
m ¼ m2 þ m2 þ m2 2; a ¼ 1=3 axxþayyþazz ; b
¼
x
y
z
tot
from 30 to 700 ꢀC at the rate of 10 or 15 or 20 K min
.
ꢂ
ꢃ
1=2
b2x þ b2y þ b2z
(1)
(2)
Spectral measurements
hꢂ
ꢃ
ꢂ
ꢃ
2
btot
¼
bxxx þ bxyy þ bxzz 2 þ byyy þ byzz þ byxx
The infrared spectra were recorded with an Avatar 330-Thermo Nicolet
Fourier transform infrared (FT-IR) spectrometer (Thermo, America). The
proton spectra at 400 MHz were obtained at room temperature using a
Bruker 400 MHz NMR spectrometer (Bruker biospin, California, USA).
Proton decoupled 13C NMR spectra were also recorded at room temper-
ature employing a Bruker 400 MHz NMR spectrometer operating at
100 MHz. The mass spectra of the samples were obtained using a Thermo
Fischer LC-Mass spectrometer in fast atom bombardment (FAB) mode
(Thermo, France). The cyclic voltammetry (CV) analyses were performed
with a CH Instrument electrochemical analyzer 604C (CH Instrument elec-
trochemical analyzer, USA) at scan rate of 100 mV sꢁ1 using 0.1 M
tetra-(n-butyl)-ammonium hexafluorophosphate as supporting electrolyte
with Ag/Ag+ (0.01 M AgNO3) as the reference electrode and Pt electrode
as the working electrode under nitrogen atmosphere at room tempera-
ture. The UV–Vis absorption and fluorescence spectra were recorded with
a PerkinElmer Lambda 35 spectrophotometer (Lambda 35, PerkinElmer,
Singapore) and a PerkinElmer LS55 spectrofluorimeter (LS55, PerkinElmer,
Singapore), respectively. Solid state emission spectra were recorded using a
Fluoromax 2 ISA SPEX fluorimeter with Xenon-arc lamp as light source. Fluo-
rescence lifetime measurements were carried out with a nanosecond time
correlated single photon counting spectrometer Horiba Fluorocube-01-NL life-
time system (Horiba, UK) with NanoLED (pulsed diode excitation source) as
the excitation source and TBX-PS as detector. The slit width was 8 nm, and
the excitation wavelength was 282 nm. The fluorescence decay was analyzed
using DAS6 software. The PL quantum yield for all phenanthrimidazoles
were measured in dichloromethane using coumarin 47 in ethanol as the
standard.[70,71] The radiative and nonradiative rate constants, kr and knr,
were deduced from the phosphorescence yield (Φp) and phosphorescence
lifetime (t) using the equation,Φp = Φisc{kr/(kr + knr)} where Φisc is the
ꢂ
ꢃ i
1=2
2
þ bzzz þ bzxx þ bzyy
The reported b components of Gaussian output are in atomic units, and
the calculated values are in e.s.u. units (1a.u. = 8.3693ꢂ 10ꢁ33 e.s.u.). The
second order Fock matrix was carried out to evaluate the donor–acceptor
interactions in the natural bond orbital (NBO) analysis.[83–92] For each
donor (i) and acceptor (j), the stabilization energy E(2) associated with the
Fði;jÞ
delocalization i ! j is estimated as, E(2) = qi
2 , where qi is the donor
ejꢁei
orbital occupancy, ei and ej are diagonal elements, and F(i, j) is the off
diagonal NBO Fock matrix element. The larger the E(2) value, the more
intensive is the interaction between electron donors and electron accep-
tors. To find out the heat of formation of phenanthrimidazole with
ZnO nanoparticle, structures of the molecules were generated using
ChemSketch software and converted into 3D models.[93] Molecular network
(MN) format converter was used to convert .mol2 file to .pdb file format
available at http://www.molecular-networks.com. The quantum mechanical
(QM) calculations were performed by semi-empirical molecular orbital cal-
culations MOPAC 7.01 with AM1(Hamiltonian) implementation of Vega zz
3.0.0.[94] All calculations were carried out on an Intel pentium4, 2.4 GHz-
based machine running MS Windows XP SP2 as operating system. Visualiza-
tion of the optimized structure was performed on PyMol molecular
graphics program, a comprehensive software package for rendering and
animating three-dimensional structures that produces high-quality three-
dimensional images of small molecules.[95]
intersystem crossing yield,[72] kr = Φp/t, knr = 1/t ꢁ Φp/t; t = (kr + knr)ꢁ1
.
RESULTS AND DISCUSSION
Photophysical studies of the phenanthrimidazoles
Device fabrication
UV–Vis absorption and fluorescence spectral studies of
phenanthrimidazoles are presented in Figs 1 and 2, respectively.
All the four phenanthrimidazoles show the main absorption
band around 275 nm; compounds 1 and 2 at 275 nm, 3 at
270 nm, and 4 at 265 nm. In addition, some weaker bands are
also observed around 325 nm. All the observed absorption
bands do not show remarkable difference between them and
suggest very similar optical energy gap. However, the fluores-
cence emission differs significantly; compounds 1 and 2 emit
around 400 nm, 3 at 410 nm, and 4 at 450 nm. It is interesting
to note that the fluorescence emission of compounds 1 and 2
are at the same wavelength, whereas the emissions of 3 and 4
are red shifted with respect to the parent molecule 1. The fact
that compounds 1 and 2 absorb at the same wavelength
(275 nm) and also emit at identical frequencies indicates that
their optical energy gaps are similar. The emission of compound
4 is red shifted in comparison with compounds 1–3, and this red
shifted emission indicates that the polarity of the excited state is
higher than the ground state. These results suggest an important
geometrical rearrangement in the S1 excited state.
The EL devices based on the phenanthrimidazoles were fabricated by
vacuum deposition of the materials at 5 ꢂ 10ꢁ6 torr onto a clean glass
precoated with a layer of indium tin oxide (ITO) as the substrate. The glass
was cleaned by sonication successively in a detergent solution, acetone,
methanol, and deionized water before use. Organic layers were deposited
onto the substrate at a rate of 0.1nmsꢁ1. LiF and Alq3 were thermally evap-
orated onto the surface of organic layer. The thickness of the organic mate-
rials and the cathode layers were controlled using a quartz crystal thickness
monitor. A series of devices (I, II, III, and IV) of configuration [ITO/NPB
(10 nm)/buffer layer (30 nm)/Alq3 (60nm)/LiF (1nm)/Al] with compounds
1–4 as buffer layer were fabricated, where NPB is the hole-transporting
layer and Alq3 was the electron-transporting layer as well as the emitting
layer (EML). Compounds 1–4 as film was the buffer layer in devices I, II, III,
and IV, respectively. For comparison, the reference device (device V) with
the configuration of [ITO/NPB (40nm)/Alq3 (60nm)/LiF/Al] was fabricated.
Measurements of current, voltage, and light intensity of a series of devices
were made simultaneously using a Keithley 2400 sourcemeter (Keithley,
Cleveland, Ohio). The EL spectra and luminance of the devices were carried
out in ambient atmosphere without further encapsulations.
Computational details
All the calculations were performed with Gaussian 03[73] package. The
geometry of all involved structures was fully optimized with the DFT
All the four phenanthrimidazoles absorb at 270 ꢃ 5 nm but
emit at longer wavelength (400–450 nm). This suggests the
existence of a-twist in the molecule as given in Fig. 3. The
wileyonlinelibrary.com/journal/poc
Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 386–406

MATERIALS SCIENCE
(1)
(2)
1.00
0.75
0.1
1.0
0.7
0.1
0.25
0.25
0
0
265
270
275
280
265
270
275
280
wavelength (nm)
wavelength (nm)
(3)
(4)
1.00
0.75
0.1
1.00
0.7
0.1
0.25
0.25
0
0
21
260
270
280
255
260
265
270
wavelength (nm)
wavelength (nm)
n-Hexane
1,4-dioxane
Benzene
Dichloromethane
1-Butanol
Ethyl acetate
DMSO
Ethanol
Acetonitrile
1-Propanol
Chloroform
Methanol
Figure 1. Absorption spectra of phenanthrimidazoles 1–4 (1 ꢂ 10ꢁ8 M) in different solvents
(1)
(2)
1.00
0.75
0.1
1.00
0.75
0.1
0.25
0.25
0
0
380
380
390
400
410
420
390
400
410
420
wavelength (nm)
wavelength (nm)
(3)
(4)
1.00
0.75
0.1
1.00
0.75
0.1
0.25
0.25
0
0
360
380
400
420
440
400
420
440
460
480
wavelength (nm)
wavelength (nm)
n-Hexane
1,4-dioxane
Benzene
Dichloromethane
1-Butanol
Ethyl acetate
DMSO
Ethanol
Acetonitrile
1-Propanol
Chloroform
Methanol
Figure 2. Emission spectra of phenanthrimidazoles 1–4 (1 ꢂ 10ꢁ8 M) in different solvents
J. Phys. Org. Chem. 2013, 26 386–406
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

C. KARUNAKARAN ET AL.
an ideal compromise between the fluorescent quantum yield
in solution and the aggregation induced quenching in solid.
As shown in Fig. 4, the lifetime of these derivatives lies
between 2.6 and 5.8 ns. The fluorescence decays for the
phenanthrimidazoles 1–4 fit satisfactorily to a biexponential
kinetics, S(t) = a1eꢁt/t1 + a2eꢁt/t2; where ai and ti are amplitudes
and time constants of the ith (= 1, 2) exponential component.
From these fitting parameters, the amplitude weighted average
time constant, tave, can be calculated, tave = (a1t1 + a2 t2)/(a1 + a2).
These fitting parameters, the average time constants, radiative
(kr), and nonradiative rate (knr) constants are listed in Table 1. From
the table, it could be easily seen that the radiative rate constant
increases with the number of fluoro substituent.
The distortion of the geometry in the excited state implies a
decrease in the resonance energy so that the fluorescence band
is bathochromically shifted to a larger extent than the absorption
band. Moreover, the loss of planarity in the excited state of the
phenanthrimidazole derivatives could explain the lower fluores-
cence quantum yield in apolar solvents owing to an increase in
the nonradiative processes. The nonradiative rate constants of
the four phenanthrimidazoles in different solvents have been
deduced. Figure 5 shows a fair correlation between the knr values
and the fluorescence wavenumbers of the phenanthrimidazoles
1–4 in different solvents. This linear relationship indicates the
influence of solvent parameters on the photophysical character-
istics of phenanthrimidazoles, as have been observed for
other aromatic systems.[96–99] The distorted geometry of the
phenanthrimidazoles is also confirmed by potential energy
surface (PES) diagram; during the calculation, all the geometrical
parameters were simultaneously relaxed, whereas the C21-
C22-O22-C25 torsional angles were varied in steps of 10ꢀ from
0ꢀ to 360ꢀ. The PES diagrams of 1–4 as displayed in Fig. 6 show
Figure 3. Molecular modeling of phenanthrimidazoles 1–4 using
Gaussian-03
correlation of the emission wavelength with the a-twist indicates
the importance of noncoplanarity between the imidazole and
the aryl ring. This may be due to the variation of dihedral angles.
The measured fluorescence quantum yields in dioxane solution
are between 0.25 and 0.36; 0.25 for 1, 0.31 for 2, 0.36 for 3,
and 0.33 for 4. That is, a smaller dihedral angle will give a more
extended conjugation system and a higher fluorescent quantum
yield in solution. Stronger intermolecular interaction in the
solid state increases the quenching. Therefore, it is important
to design a molecule with a suitable dihedral angle to achieve
Figure 4. Time resolved fluorescence decay spectra of phenanthrimidazoles 1–4
wileyonlinelibrary.com/journal/poc
Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 386–406

MATERIALS SCIENCE
Table 1. Bi-exponential fitting parameter for fluorescence decay of 1–4
Compound
Фf
t1 (ns)
a1 (%)
t2 (ns)
a2 (%)
w2
kr
knr
ket ꢂ 108 sꢁ1
1
0.25
0.22
0.31
0.36
0.33
2.6
0.12
3.3
2.3
2.8
1.46
67.84
2.74
17.26
68.40
5.2
7.6
5.3
5.1
5.8
98.54
32.16
97.26
82.74
41.60
1.03
1.23
1.05
1.23
1.96
0.05
0.03
0.06
0.08
0.10
0.14
0.12
0.13
0.15
0.21
—
6.84
—
—
—
1 + ZnO
2
3
4
1
2
3
4
Figure 5. Correlation of nonradiative deactivation rate with the fluorescence wavenumbers of phenanthrimidazoles 1–4 in different solvents
that the minimum energy during the rotation about C21-C22-
O22-C25 torsional angles are 117.51 kcal molꢁ1 (1) or
139.23 kcal molꢁ1 (2) or 43.93 kcal molꢁ1 (3) or ꢁ22.30 kcal
molꢁ1 (4). In these minimum energy conformation, the tilting
angle of 2,5-dimethoxyaniline ring attached to the nitrogen
atom (N1) about C5-N1-C18-C19 bond is 87.66ꢀ (1 and 2) or
103.04ꢀ (3) or 98.35ꢀ (4), whereas tilting angle of aldehydic phe-
nyl ring attached to the carbon atom (C2) about N3-C2-C26-C31
is found to be 0.23ꢀ (1 and 2) or ꢁ49.16ꢀ (3) or ꢁ25.67ꢀ (4).
moderate and does not play any important role in absorption
as well as fluorescence. The electron releasing ability or basicity
of the solvent (Cb or CSB) has a negative value. With the increase
of electron-donating ability of the solvent, the absorption as well
as fluorescence bands are shifted to lower energies. For 3 and 4,
the basicity of solvent (Cb or CSB) is the lowest, and hence, the
solvent basicity does not affect the absorption and fluores-
cence. The coefficient of the correlation with the H-donor
capacity or acidity of the solvent (Ca or CSA) is of negative
value. This suggests that with increasing acidity character of
the solvent, the absorption and fluorescence bands are shifted
to lower energies. This effect can also be interpreted in terms
of the resonance stabilization of the chromophore that is
shown in Fig. 8. For 1 and 2, the resonance structure (b) has
positive charge located at the nitrogen atom, and it will be
stabilized in basic solvents. This resonance structure is pre-
dominant in the S1 state and the stabilization of the S1 state
with the solvent basicity is more important than that of the
S0 state. Consequently, the energy gap between S1 and S0
states decreases. With increasing solvent basicity, the absorp-
tion and fluorescence wavelengths are shifted to longer wave-
lengths. Thus, the polar solvents stabilize the S0 state more
extensively than the S1 state and thereby increase the energy
gap between both the states and explain the solvatochromic
Taft and Catalan solvatochromism in the
phenanthrimidazoles
The multi-parameter correlation analysis employed is linear
correlation of a physicochemical property with several solvent
parameters. Figure 7a and b shows the obtained correlation
between the absorption or fluorescence wavenumbers calcu-
lated using the multi-component linear regression by employing
Taft[100]and Catalan[101] solvent parameters and the experimen-
tal results. Table 2 presents the coefficients of multi-parameter
correlation. The absorption and fluorescence bands of the
phenanthrimidazoles 1 and 2 show that the solvatochromic
shifts are associated with the solvent polarity. The H-donor
capacity or acidity of the solvent coefficient (Ca or CSA) is only
J. Phys. Org. Chem. 2013, 26 386–406
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

C. KARUNAKARAN ET AL.
1
2
4
3
Figure 6. Potential energy surface of phenanthrimidazoles 1–4
shifts.[69] In the case of 3 and 4, because of the electrostatic
interaction between the lone pair of electrons and acidic
solvents, the resonance structure (a) will be stabilized in acidic
solvents. This resonance structure is predominant in the S1
state, and the stabilization of the S1 state with the solvent
acidity is more important than that of the S0 state.
wileyonlinelibrary.com/journal/poc
Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 386–406

MATERIALS SCIENCE
1
1
a
2
b
2
a
3
b
3
a
4
b
4
a
b
Figure 7. a Taft correlation of the observed absorption (a) and fluorescence wavenumber (b) with the predicted values of phenanthrimidazoles 1 and
2. b. Taft correlation of the observed absorption (a) and fluorescence wavenumber (b) with the predicted values of phenanthrimidazoles 3 and 4
Optical properties of the phenanthrimidazoles
of the molecules as shown in Table 3 and optimized geometries
of 1–4 are given in Fig. 3. Their solid state PL spectra are red
shifted by about 30 to 40 nm with respect to those in solution.
The larger red shifts can be explained by their stronger p–p
stacking interactions of the aryl ring.
The absorption and PL spectra of the phenanthrimidazole mate-
rials 1–4 have been measured in solid state and in solutions.
Intensive emission peak ranging from 401 to 428 nm (for solids)
and 390 to 453 nm (for solutions) are observed. It is apparent
that the phenanthrimidazoles have similar absorption and
emission in solution, whereas they differ in their bathochromic
shifts in the solid state. In CH2Cl2, all the compounds show simi-
lar absorption peaks around 275 nm, which may originate from
the aryl rings,[102] and the absorption band that appeared
between 325 and 330 nm is assigned to the p–p* electronic tran-
sition in the phenanthrimidazoles. The observation that the
absorption and the emission spectra of the phenanthrimidazole
materials 1–4 shows relatively small differences in solutions, and
solid state suggests that the bulky and noncoplanar parts of
these compounds have successfully restrained intermolecular
aggregation. It can also be correlated to the dihedral parameters
The electrochemical properties of the phenanthrimidazoles
(1–4) have been examined by cyclic voltammetry, and the redox
potentials have been measured from the plot of potential versus
current that is shown in Fig. 9. The energies of the HOMO and
LUMO have been calculated using the relation, EHOMO = 4.4 +
Eoxi; ELUMO = EHOMO ꢁ 1239/lonset, and the calculated values are
½
given in Table 4. The LUMO energies have been deduced from
the HOMO energies and the lowest-energy absorption edges of
the UV–Vis absorption spectra.[103,104] The calculated energy
gap (Eg = EHOMO ꢁ ELUMO) of the phenanthrimidazoles 1–4 are
3.33, 3.36, 3.38, and 3.40 eV. Therefore, the HOMO stability and
the emission energy gap are controlled by the nature and
substituent present in the phenanthrimidazole moiety. The
J. Phys. Org. Chem. 2013, 26 386–406
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

C. KARUNAKARAN ET AL.
wileyonlinelibrary.com/journal/poc
Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 386–406

MATERIALS SCIENCE
decreased HOMO–LUMO energy gap is attributed to the more
planar configuration between the aryl ring and the imidazole
ring in 1–4. On the other hand, the lone electron pair in the
nitrogen atom may undergo an n–p* transition with lower
energy absorption with the more coplanar structure.[105,106] This
indicates that conjugation of the aryl ring to the imidazole ring
plays a key role in controlling the HOMO energy of the molecule.
With the shallow HOMO, it can effectively lower the hole-
injection barrier from the 4,4-bis(N-(1-naphthyl)-N-phenylamino)
biphenyl (NPB) layer (i.e., HOMO= ꢁ5.4 eV). Furthermore, the
shallow HOMOs of the phenanthrimidazoles also imply that it is
more difficult for the holes to leak into the EML. All these
phenanthrimidazoles have HOMO levels similar or shallower than
the classical hole transport/injection materials[107–109] facilitating
the hole injection/transport from the hole transport layer.
Furthermore, the much shallower HOMO level than those of the
classical electron transport materials[110,111] effectively prevents
the leakage of holes into the EML. With these advantages, the
devices based on these materials are expected to show lower
onset and operation voltages compared with devices with recently
reported emitters.
the phenanthroline ring and on the carbon atoms (C2, C4, C25,
C26, C28, and C30) of the imidazole and aldehydic phenyl rings.
In compound 2, whereas the HOMO is on the imidazole ring
and partly on the phenanthroline and aldehydic phenyl rings,
the LUMO is located partly on the phenanthroline ring and on
the carbon atoms (C2 and C4) of the imidazole ring. In com-
pound 3, the HOMO is on the imidazole ring, partly on the
phenanthroline and aldehydic phenyl rings, and the LUMO is
partly on the phenanthroline ring and on the carbon atoms
(C2, C4 and C5) of the imidazole ring. In compound 4, whereas
the HOMO is on the imidazole ring, partly on the phenanthroline
and aldehydic phenyl rings, the LUMO is on the phenanthroline
ring and on the nitrogen, carbon atoms (N1, C2, N3, C4, C25, C26,
C28, and C30) of the imidazole and aldehydic phenyl ring. The
HOMO ! LUMO transition implies that intramolecular charge
transfer takes place within the molecule.[112] The energy gap
(Eg) of these phenanthrimidazoles has been calculated from
the HOMO and LUMO energy levels. The energy gap explains
the probable charge transfer (CD) inside the chromophores.
The potential barrier (PB) and potential drop (PD) computed
using the theoretically deduced HOMO–LUMO energies have
been discussed in detail in the later section.
The electron density distribution has been studied using the
frontier molecular orbitals (FMOs) by density function theory
(DFT) analysis. The 3D plots of the frontier orbitals HOMO and
LUMO for the phenanthrimidazoles 1–4 are shown in Fig. 10.
The HOMO orbital acts as an electron donor, and the LUMO
orbital acts as an acceptor. In compound 1, the HOMO is located
on the imidazole ring and partly on the phenanthroline and
aldehydic phenyl rings, whereas the LUMO is located partly on
4
3
N
N
N
N
N
H3CO
H3CO
N
N
N
R2
R2
2
1
H3CO
H3CO
R1
R1
a
b
1, R1 = R2 =H
2, R1 = F; R2=H
3, R1 = F; R2=F
4, R1 =CF3; R2=H
-0.4
-0.6
-0.8
-1.0
-1.2
Potential / V
Figure 8. Resonance structures of the phenanthrimidazole chromo-
phores 1–4
Figure 9. Cyclic voltammogram of phenanthrimidazoles 1–4
Table 3. Dihedral angles, electric dipole moment (m in Debye), polarizability (a in 10ꢁ24 e.s.u) and hyperpolarisability (btot in
10ꢁ32 e.s.u) of 1–4
Compound
Anglea (ꢀ)
Angleb (ꢀ)
Anglec (ꢀ)
mtotal
atot
btot
1
2
3
4
99.38
99.45
103.04
99.05
151.93
152.67
131.98
153.85
ꢁ2.64
ꢁ2.89
ꢁ3.17
ꢁ3.15
7.04
6.03
6.98
5.48
26.28
27.58
28.16
30.38
237.42
212.55
230.98
281.16
aDihedral angle between the imidazole ring and 3,4-dimethoxyphenyl ring; dihedral angle between the imidazole ring and aryl
ring; dihedral angle between the phenyl ring and aryl ring.
b
c
J. Phys. Org. Chem. 2013, 26 386–406
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

C. KARUNAKARAN ET AL.
Table 4. Photophysical, thermal, and electrochemical data of 1–4
t (ns)
lemi (nm) Tg/Tm/Td (ꢀC) HOMOd (eV) LUMOe (eV)
c
Compound
labsa (nm) lemia (nm) Фfb t1
t2
labs (nm)
1
2
3
4
275, 322
275, 322
267, 327
266, 331
392
390
418
453
0.28 2.6 5.2 272, 328, 384
0.31 3.3 5.3 278, 330, 385
0.36 2.3 5.1 272, 334, 391
0.33 2.8 5.8 271, 337, 396
428
421
417
445
98/248/ 310
103/242/335
106/237/320
110/228/315
ꢁ5.39
ꢁ5.41
ꢁ5.45
ꢁ5.48
ꢁ2.06
ꢁ2.05
ꢁ2.07
ꢁ2.08
aMeasured in CH2Cl2 solvent at room temperature. Measured in acetonitrile using coumarin 47 in ethanol as a reference. Solid
b
c
state thin-film samples on quartz substrate. dBy measuring ionization potential (IP) with cyclic voltammetry studies.
eLUMO = HOMO + Egap
.
DTA / (mW/mg)
TG%
Compound
HOMO
LUMO
Exo
100
95
90
85
80
75
1
2
3
4
1
50
100 150 200 250 300 350 400 450 500
Temperature/ °C
DSC
mW%
2
3
4
3
2
1
50
100
150
200
250
300
Temperature/ °C
Figure 11. Thermogravimetric-differential thermal analysis (TG-DTA)
(upper) and differential scanning calorimetric (ꢁb1ottom) curves of
phenanthrimidazoles 1–4 at a heating rate of 20 ꢀCmin
4
(Table 4). There appears to be a glass transition temperature (Tg)
in the range of 98 to 107 ꢀC. The melting points (Tm) of com-
pounds 1, 2, 3, and 4 measured by DSC examinations are 248,
242, 237, and 228 ꢀC, respectively. The high Tm and Td values
indicate that the compounds 1–4 are thermally stable and are
able to undergo the vacuum thermal sublimation process. There-
fore, these derivatives could be used in EL devices because the
high Tm and Tg values improve the lifetime of the devices.[113]
Figure 10. Frontier molecular orbitals (highest occupied molecular orbital
and lowest unoccupied molecular orbital) of phenanthrimidazoles 1–4
Thermal analysis
The thermal properties of the phenanthrimidazoles 1–4 have
been investigated by DSC and thermogravimetric analyses under
nitrogen atmosphere. All the phenanthrimidazoles exhibit good
thermal stability as displayed by Fig. 11. Decomposition temper-
atures (Td), defined as the temperature at which the material
showed a 5% weight loss, have been measured to be 310, 335,
320, and 315 ꢀC for compounds 1, 2, 3, and 4, respectively
Electroluminescence and device studies
The electroluminescence properties have been investigated by
using these materials as an organic buffer layer. A series of
devices (I, II, III, and IV) with NPB as the hole-transporting layer,
Alq3 as the electron-transporting layer, and the EML have been
fabricated. Compounds 1–4 as films were the buffer layers in
wileyonlinelibrary.com/journal/poc
Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 386–406

MATERIALS SCIENCE
devices I, II, III, and IV, respectively. For comparison, the refer-
ence device (device V) is also fabricated. The current density–
brightness–voltage and the luminous efficiency are listed in
Table 5. The devices exhibit emission between 400–430 nm,
and Fig. 12 displays the same. The emission is independent
of the applied voltages in the range of 3 to 16 V. From the
device study, it is observed that all the compounds show
higher brightness and current efficiency at low voltage. From the
electrochemical data, the HOMO energy level of compounds 1–4
is obtained as ca. ꢁ5.6eV, and this is in between that of NPB
(ꢁ5.4 eV) and Alq3 (ꢁ6.0eV) that could regulate the hole injection.
Thus, the synthesized series of phenanthrimidazole derivatives can
prevent excessive holes to enter the EML increasing the efficiency.
Comparison of HOMO values of already reported emitters[114–117]
with phenanthrimidazole derivatives 1–4 shows that these
compounds have HOMO levels similar or shallower than the classi-
cal hole transport/injection materials. Therefore, the EL studies
show an alternative idea to design novel hole transport materials
and also provided a convenient way to improve the performance
of devices by inserting a suitable buffer layer.
DFT analysis
The molecular geometry of 1–4 has been further examined by
DFT calculation [DFT/B3LYP/6-31G(d,p)]. Figure 3 shows the
optimized structures of the phenanthrimidazoles 1–4. NBO
analysis has been performed for the phenanthrimidazoles at the
DFT/B3LYP/6-31G (d,p) level in order to elucidate the intramolecu-
lar charge transfer, within the molecule. Several donor–acceptor
interactions are observed in these phenanthrimidazoles (1–4),
and among the strongly occupied NBOs, the most important delo-
calization sites are in the p system and in the lone pairs (n) of the
oxygen, fluorine, and nitrogen atoms as displayed in Table 6. The
s system shows some contribution to the delocalization, and the
important contributions to the delocalization that correspond to
the donor–acceptor interactions in the phenanthrimidazoles 1–4
are C6–C11 ! C12–C17, N13–C14 ! C12–C17, N13–C14 ! C15–
C16, N3–C2! C4–C5, N3–C2! C26–C27, LpN1! N3–C2, C28–
C29 ! C30–C31.The charge distribution has been calculated from
the Mulliken atomic charges by NBO that reveal that among the ni-
trogen atoms (N1 and N3), oxygen and fluorine atom, N1 is consid-
ered as more basic site. Further, compared with all the nitrogen
atoms (N1, N3, N10, and N13), oxygen atoms are less electronega-
tive.[118] This shows that more negative charge is concentrated on
N1 atom in all these phenanthrimidazoles. The percentage of s and
p-character in each NBO natural atomic hybrid orbital for the
phenanthrimidazoles 1–4 are displayed in Table 7. Analysis of
HOMO–LUMO and NBO explains the charge transfer within the
molecule. Therefore, it is of interest to test their hyperpolarizability
and to correlate with absorption. The microscopic nonlinear
responses in 1–4 have been obtained by DFT calculation. From
Table 8, it is suggested that the phenanthrimidazoles (1–4) are polar
having nonzero dipole moment and nonzero hyperpolarizabilities
and hence have good microscopic NLO behavior.[119] The larger
mbo values are attributed to the positive contribution of their
conjugation. The hyperpolarizability is a strong function of the
absorption maximum. Because even a small absorption at the
operating wavelength of optic devices can be detrimental, it is
important to make NLO chromophores as transparent as possi-
ble without compromising the molecule’s nonlinearity. The
computed b values may be correlated with UV-visible spectro-
scopic data in order to understand the molecular structure
and NLO relationship in view of a future optimization of the
microscopic NLO properties. The phenanthrimidazoles 1–4
possess more appropriate ratio of off-diagonal versus diagonal
b tensorial component (r = bxyy/bxxx) that reflects the inplane
nonlinearity anisotropy (r = bxyy/bxxx) [r = 0.357 (1), 0.047 (2),
0.125 (3), and 0.159 (4)] and the largest mbo values. The b-tensor
Table 5. Devices I, II, III, and IV based on phenanthrimidazoles
1, 2, 3, and 4 as buffer layer with comparison device V (NPB
as HTL)
Device EL (nm) L (cd Aꢁ1 a V (V)b ꢀc (cd Aꢁ1) CIE (x,y)@ 9 V
)
I
II
III
IV
V
435
431
423
452
520
4.7
4.1
3.7
3.2
2.7
34870
35630
32450
30780
37360
5.87
4.35
4.07
4.45
3.22
(0.35, 0.54)
(0.33, 0.52)
(0.32, 0.53)
(0.33, 0.52)
(0.32, 0.52)
HTL, hole-transporting layer. aThe brightness (L), current
b
efficiency (ꢀc), voltage (V) required for 1 cdm-2; I, II, III, and
IV-[ITO/NPB (10 nm)/buffer layer (30 nm)/Alq3 (60 nm)/LiF
(1 nm)/Al]; V-[ITO/NPB (40 nm)/Alq3 (60 nm)/LiF/Al].
1
2
ꢀ
ꢁ
2D
can be decomposed in to a sum of dipolar J¼1b and octupolar
ꢀ
ꢁ
1.00
0.75
0.50
0.25
0
3
4
NPB
2D
J¼3b tensorial components, and the ratio of these two compo-
nents strongly depends on their ‘r’ ratios. The zone for r > r2 and
r < r1 corresponds to a molecule of octupolar and dipolar,
respectively. The critical values for r1 and r2 are (1 ꢁ √3)/
√3√3 + 1) = ꢁ0.16 and (√3 + 1)/√3(√3 ꢁ 1) = 2.15, respectively.
ꢄ
ꢅ
2D
b
b
kJ¼3
k
The parameter r2, D [r2D] =
is convenient to compare
2D
kJ¼1
k
the relative magnitudes of the octupolar and dipolar compo-
nents of b. The observed positive small r2D values [11.63 (1),
3.54 (2), 3.98 (3), 6.72 (4)] reveal that the biii component cannot
be zero, and these are dipolar component. Because most of the
practical applications for second-order NLO chromophores are
based on their dipolar components, this strategy is more appro-
priate for designing highly efficient NLO chromophores.
300
350
400
450
500
550
wavelength (nm)
Figure 12. Electroluminescence spectra of devices I–IV (buffer layer 1–4)
with comparison device V (NPB as hole-transporting layer)
J. Phys. Org. Chem. 2013, 26 386–406
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

C. KARUNAKARAN ET AL.
wileyonlinelibrary.com/journal/poc
Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 386–406

MATERIALS SCIENCE
From the absorption analysis, it can be seen that the
phenanthrimidazoles absorb around 280 nm (ΔE ~ 1.99 eV), and
at the same time, oscillator strength for absorption is very strong.
From the analysis of m and Δm, it can be seen that both the values
are high, and this indicates that the phenanthrimidazoles are
polar and also confirmed charge transfer during excitation. The
nature of charge transfer of the phenanthrimidazoles is analyzed
from the FMOs participating in the process. Analysis shows that
the excitation of HOMO to LUMO is associated with the intramo-
lecular charge transfer within the molecule. Dipole moment
analysis of the b0 indicates its dipolar nature, and the dipole is
almost unidirectional with little contribution from my and mz
because of the asymmetric nature of the molecule. Similar types
of situation can also be visualized from the vectorial components
of b, which show that the maximum contribution to the total
b0 of the molecule is from the x-direction. In view of this, direc-
tional hyperpolarizability is calculated from the individual
tensorial components of b using Eqns 1 and 2. Reasonably good
value of b0 is then explained in the light of two-state model,
b0a fΔm/ΔE3, where, f is the oscillator strength of ground to
excited state transition, Δm = me ꢁ mg is the state dipole difference
between the excited and ground state, and ΔE is the excitation
energy for ground to first excited state. Fitting the spectroscopic
parameters for phenanthrimidazoles f, Δm, and ΔE in the afore-
mentioned equation, large value of b0 can be well explained.
Analyzing all the values, it can be concluded that the large Δm
and strong oscillator strength for the phenanthrimidazoles are
mainly responsible for its large hyperpolarizability (Table 8).
The electron affinity for an N-electron system is equal to the
negative of the LUMO energy, and it is a measure of susceptibility
of molecule towards attack by nucleophiles.[120] The global
hardness is defined as ꢀ = ½ (d2E/dN2) V(r) or ½ (EHOMO ꢁ ELUMO).
Electronegativity of the phenanthrimidazoles is calculated using
the equation: m = ꢁw = ꢁ(dE/dN) V(r), where E is the energy, N is
the number of electrons, and V(r) is the constant external potential.
By combining the aforementioned equation with the work of
Ickowski and Margrave,[121] assuming a quadrate relationship
between E and N and in a finite difference approximation,
the equation can be rewritten as, w = ꢁm = (I + A)/2; wKoopaman’s
=
(EHOMO + ELUMO)/2. The electrophilicity index of a molecule can
be calculated by the equation, o = m2/2ꢀ, which can measure the
capacity of electrophile to accept the maximal number of electrons
in a neighboring reservoir of Electron Sea. By using the aforemen-
tioned equations, the calculated chemical potential (m) are
ꢁ0.2780 (1), ꢁ0.2590 (2), ꢁ0.2760 (3), and ꢁ0.2775 (4). The hard-
ness (ꢀ) are ꢁ0.0510 (1), ꢁ0.0510 (2), ꢁ0.0500 (3), and ꢁ0.0510
(4), and the electrophilicity index (o) are ꢁ0.7576 (1), ꢁ0.6576
(2), ꢁ0.7618 (3), and ꢁ0.7550 (4).
Phenanthrimidazoles versus electron transport properties
Delocalized nature of the FMOs can be attributed to the planar
structure of the molecule that probably facilitates the easier
delocalization of the orbitals. According to hypothesis, in an
electronic circuit Fermi level of the electrode contact lies approx-
imately in the middle of the HLG (HOMO–LUMO Gap).[122,123]
Now in a sufficient applied voltage (the applied voltage must
be sufficient enough to raise the energy of electron in the Fermi
level of contact to that of the LUMO) one electron will be
injected from the cathode contact to the LUMO of the molecule.
Because of the delocalized nature of the vacant LUMO, the
injected electron in the LUMO can travel through the molecule
J. Phys. Org. Chem. 2013, 26 386–406
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

C. KARUNAKARAN ET AL.
Table 7. Percentage of s and p-character on each natural atomic hybrid of the natural bond orbital of 1–4
1
2
Bond (A–B)
ED/energy
(a.u.)
EDA%
EDB%
s%
p%
Bond (A–B)
ED/energy
(a.u.)
EDA%
EDB%
s%
p%
N1–C2
0.7986
0.6019
.07645
0.6447
0.6473
0.7622
0.7139
0.7003
0.6105
0.7920
0.7911
.6117
0.7063
0.7079
0.6459
0.7635
0.6467
0.7627
0.5725
0.8199
0.8256
0.5643
0.7186
0.6954
63.77
—
58.44
—
41.90
—
50.96
—
37.28
—
62.58
—
49.89
—
41.71
—
41.83
—
32.78
—
68.16
—
51.64
—
36.23
—
41.56
—
58.10
—
49.04
—
62.72
—
37.42
—
50.11
—
58.29
—
58.17
—
67.22
—
31.84
—
48.36
—
33.01
29.31
35.33
32.81
29.59
32.84
34.18
34.10
27.33
32.74
34.16
26.26
37.84
31.07
30.67
35.46
30.72
35.21
25.06
33.35
28.42
20.88
33.86
34.11
66.96
70.58
64.45
67.15
70.36
66.96
65.78
65.86
72.57
67.23
65.81
73.63
62.12
68.89
69.28
64.37
69.23
64.62
74.73
66.58
71.52
78.85
66.11
65.81
N1–C2
C2–N3
0.7982
0.6023
0.7643
0.6449
0.6471
0.7624
0.7139
0.7003
0.6103
0.7921
0.7911
0.6117
0.7053
0.7090
0.6459
0.7634
0.5230
0.8523
0.5726
0.8198
0.8257
0.5641
0.7175
0.6966
63.72
—
58.41
—
41.87
—
50.96
—
37.25
—
62.58
-
49.74
—
41.72
—
27.35
—
32.79
—
68.18
—
51.48
—
36.28
—
41.59
—
58.13
—
49.04
—
62.75
—
37.42
—
50.26
—
58.28
—
72.65
—
67.21
—
31.82
—
48.52
—
32.97
29.37
35.30
32.84
29.57
32.87
34.20
34.10
27.30
32.77
34.16
26.25
37.74
31.38
30.68
35.45
22.15
30.68
25.07
33.36
28.41
20.86
33.67
34.44
29.37
70.51
64.48
67.11
70.38
66.93
65.76
65.86
72.60
67.20
65.81
73.64
62.22
68.58
69.27
64.37
77.54
69.24
74.72
66.57
71.52
78.87
66.30
65.52
C2–N3
N3–C4
N3–C4
C4–C5
C4–C5
N1–C5
N1–C5
N1–C18
C2–C26
N10–C11
N13–C12
C22–O22
O22–C25
C26–C31
N1–C18
C2–C26
N10–C11
C29–F32
C22–O22
O22–C25
C26–C31
3
4
Bond (A–B)
ED/energy
(a.u.)
EDA%
EDB%
s%
p%
Bond (A–B)
N1–C2
ED/energy
(a.u.)
EDA%
EDB%
s%
p%
N1–C2
0.7985
0.6020
0.7634
0.6460
0.6478
0.7618
0.7142
0.6999
0.6100
0.7924
0.7915
0.6111
0.7011
0.7130
0.6459
0.7634
0.6468
0.7627
0.5725
0.8199
0.8257
0.5641
0.7182
0.6958
0.5240
0.8517
63.76
—
58.27
—
41.97
—
51.01
—
37.21
—
62.65
—
49.16
—
41.72
—
41.84
—
32.77
—
68.18
—
51.58
—
36.24
—
41.73
—
58.03
—
48.99
—
62.79
—
37.35
—
50.84
—
58.28
—
58.16
—
67.23
—
31.82
—
48.42
—
32.67
29.35
35.49
33.57
29.54
32.37
34.23
34.09
27.29
33.02
34.25
26.23
37.02
32.04
30.66
35.44
30.73
35.20
25.05
33.34
28.42
20.87
32.95
38.78
22.30
30.67
67.30
70.53
64.29
66.38
70.41
67.43
65.73
65.87
72.61
66.95
65.72
73.66
62.94
67.92
69.28
64.38
69.22
64.62
74.74
66.59
71.52
78.86
67.01
61.18
77.39
69.24
0.7980
0.6026
0.7643
0.6449
0.6467
0.7628
0.7139
0.7002
0.6102
0.7922
0.7916
0.6111
0.7049
0.7093
0.6460
0.7633
0.6469
0.7626
0.5727
0.8197
0.8259
0.5638
0.7197
0.6943
0.5140
0.8578
63.68
—
58.41
—
41.82
—
50.97
—
37.24
—
62.66
—
49.69
—
41.73
—
41.84
—
32.80
—
68.21
—
51.80
—
36.32
—
41.59
—
58.18
—
49.03
—
62.76
—
37.34
—
50.31
—
58.27
—
58.16
—
67.20
—
31.79
—
48.20
—
32.90
29.47
35.23
32.82
29.56
32.97
34.16
34.07
27.35
32.82
34.17
26.16
37.65
31.28
30.68
35.44
30.74
35.20
25.10
33.38
28.42
20.83
28.95
36.08
21.34
29.10
67.07
70.41
64.55
67.13
70.39
66.83
65.80
65.88
72.55
67.15
65.80
73.74
62.31
68.68
69.27
64.38
69.21
64.62
74.69
66.55
71.51
78.90
70.99
63.85
78.22
70.81
C2–N3
C2–N3
N3–C4
N3–C4
C4–C5
C4–C5
N1–C5
N1–C5
N1–C18
C2–C26
N10–C11
N13–C12
C22–O22
O22–C25
C26–C31
C29–F32
N1–C18
C2–C26
N10–C11
N13–C12
C22–O22
O22–C25
C29–C32
C32–F33
27.45
—
72.55
—
26.42
—
73.58
—
wileyonlinelibrary.com/journal/poc
Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 386–406

MATERIALS SCIENCE
Table 8. Excitation energy (ΔE) in eV, absorption maximum (lmax) in nm, oscillator strength (f), and ground to excited state dipole
difference (Δm) in Debye, Ci coefficient for the transition
Compound
btot
lmax
ΔE
f
Δm
Major transition
Ci
m
1
2
3
4
237.4
212.2
231.0
281.2
280
282
278
280
1.92
1.91
1.99
1.92
1.07
1.11
1.09
1.07
5.03
4.92
4.99
4.25
HOMO ! LUMO
HOMO ! LUMO
HOMO ! LUMO
HOMO ! LUMO
0.52
0.49
0.55
0.53
7.05
6.04
6.98
5.48
Table 9. Frontier molecular orbital energies and PB values
Compound
HOMO LUMO LUMO + 1 Eg
PB
PD
1
2
3
4
ꢁ8.32 ꢁ5.63
ꢁ8.40 ꢁ5.58
ꢁ8.46 ꢁ5.50
ꢁ8.50 ꢁ5.45
ꢁ4.38
ꢁ4.03
ꢁ3.76
ꢁ3.62
2.69 1.34 1.25
2.82 1.41 1.55
2.96 1.48 1.74
3.05 1.52 1.83
from one end to the other, thus showing the conductance ability
of the molecule. This applied voltage, which is required to bring
the molecule to behave as a conductor, is called the PB for conduc-
tion. So to gage the electron transport property in a molecule
through FMO analysis, PB for the molecules is calculated using
equation, PB= ½ HLG + q, when LUMO is the conduction channel
q = 0 and q = ΔLUMO for the conduction channel other than LUMO.
Lowest unoccupied molecular orbital is the delocalized low
lying vacant orbital for all the molecules and can serve the
channel for conduction process. So using the extreme condition
q = 0, for all the molecules, we have calculated the PB required
for the conduction. Along with the HOMO and LUMO orbital
energies, PB values for all the molecules are shown in Table 9.
In Table 9, it is clear that increasing the number of electron
withdrawing substituent increase the PB value. Because of such
trend of PB required to achieve the conductance at molecular
level, poly-phenylene and poly-thiophene-like systems are studied
Figure 14. Highest occupied molecular orbital–lowest unoccupied
molecular orbital energy levels of isolated phenanthrimidazoles,
conduction, and valence bands of nano-ZnO
as potential molecular wires.[124,125] In phenanthrimidazole deriva-
tives, although the decrease in PB is not large, when such type
molecules tailored into polymeric type of system, an overall large
a
b
Figure 13. (a) Enhancement of absorption spectra of phenanthrimidazoles by nano-ZnO. (b) Enhancement of emission spectra of
phenanthrimidazoles by nano-ZnO
J. Phys. Org. Chem. 2013, 26 386–406
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

C. KARUNAKARAN ET AL.
decrease in PB value can be expected and in this regard the afore-
mentioned structure–property analysis will be helpful in designing
new polymeric molecules for molecular electronics.
The PD is one of the parameters to gage the ability of the
molecule to show rectification, can be calculated from the orbital
energy difference between the LUMO and the LUMO + 1. As the
LUMO and LUMO + 1 are localized on different parts of the
molecule, the tunneling of electron is possible and the rectifying
ability of the molecule can be expected. Rectification on electron
transport property can be achieved where there is differential
population localization of the FMOs.[126] It has been suggested
that when such a molecule is placed in between the two
electrodes and voltage is applied, there will be an inelastic
tunneling of electron from the acceptor side to the donor
side.[126] In considering the FMOs, the resonant transport of
electron from the LUMO to the next unoccupied level localized
on the donor part could be explained. The rectifying property
of the molecular systems, PD across the molecular rectifier can
be calculated as, PD (ΔLUMO) = ELUMO ꢁ ELUMO + 1. This increase in
PD and PB is important because the rectifier molecule can be
used for a higher range of applied voltage.
Electron transfer from phenanthrimidazoles to
nanocrystalline ZnO
ZnO nanoparticles enhance the absorbance of the
phenanthrimidazoles remarkably without shifting its absorption
maximum at 275 nm and it is displayed in Fig. 13a. This indicates
that the nanocrystals do not modify the excitation process of the
phenanthrimidazoles. The enhanced absorption at 275 nm
observed with the dispersed semiconductor nanoparticles is due
to adsorption of the phenanthrimidazoles on semiconductor
surface. This is because of effective transfer of electron from the
excited state of the phenanthrimidazole to the conduction band
of the semiconductor nanoparticle as visualized in photoinduced
electron transfer mechanism.
The emission spectra of the phenanthrimidazole in the pres-
ence of ZnO nanoparticles dispersed at different loading and
also in their absence are displayed in Fig. 13b. The nanoparticle
Figure 15. Mechanism of fluorescence enhancement of phenanthrimidazoles
by nano-ZnO
Figure 16. Molecular electrostatic potential surface of phenanthrimidazoles 1–4
wileyonlinelibrary.com/journal/poc
Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 386–406

MATERIALS SCIENCE
enhances the emission of the phenanthrimidazole remarkably
without shifting its emission maximum at 403 nm. This indicates
that the nanocrystals do not modify the excitation process of
the phenanthrimidazole. The enhanced emission at 403 nm
observed with the dispersed semiconductor nanoparticle is due
to the adsorption of the phenanthrimidazole on semiconductor
surface. This is because of effective transfer of electron from
the excited state of the phenanthrimidazole molecule to the
conduction band of the semiconductor nanoparticle. The fluo-
rescence enhancement arises because of the formation of the
fluorophore–nanoparticulate ZnO complex and the binding
constant (K) has been evaluated as 5.03 ꢂ 103. Figure 14 presents
the HOMO and LUMO energy levels of an isolated imidazole
molecule along with the conduction band and valence band edges
of ZnO nanoparticle. The energy levels presented in Fig. 15
suggests enhancement of fluorescence of the phenanthrimidazole
molecule by ZnO nanocrystal. On illumination at 275 nm, both the
ligand and nanosemiconductor are excited. Duel emission is
expected because of LUMO! HOMO and CB! VB electron
transfer. Also possible is the electron jump from the excited
organic molecule to the nanocrystal; the electron in the LUMO of
the excited ligand is of higher energy compared with that in the
CB of ZnO nanocrystals. This should lead to quenching of
fluorescence in the phenanthrimidazole molecule. However,
contrary to the expectations, enhancement of fluorescence is
observed in the presence of ZnO nanocrystal. This may be because
of the lowering of the HOMO and LUMO energy levels of the
phenanthrimidazole molecule due to adsorption on ZnO nanopar-
ticle. The polar ZnO surface enhances the delocalization of the
Figure 17. Visualized mode of attachment of phenanthrimidazole
with ZnO
Figure 18. Time resolved fluorescence decay spectra of phenanthrimidazole with nano-ZnO
Table 10. Selected parameters obtained from MOPAC calculations
Parameter
Ligand
ZnO (Wurzite)
Conjugate
Heat of formation (kcal)
Electronic energy (eV)
Core–core repulsion (eV)
Dipole(debye)
276.204720
ꢁ45436.98896
40247.149971
4.90349
536.623765
ꢁ3051.723501
1673.737891
0.30906
780.006160
ꢁ57946.333560
51377.085678
19.81556
96
No. of filled levels
80
16
Ionization potential (eV)
Molecular weight
8.659489
432.481
7.699572
325.518
6.905978
757.998
SCF calculations
1
1
1
Ref: MOPAC 93: J.J.P. Stewart, Fujitsu, 1993. MOPAC 7: I. Cserny, Linux Public Domain: ftp://esca.atomki.hu, or ftp://
infomeister.osc.edu
J. Phys. Org. Chem. 2013, 26 386–406
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

C. KARUNAKARAN ET AL.
Figure 19. AM1 optimized molecular structure of (a) phenanthrimidazole, (b) nano-ZnO, and (c) adsorption of phenanthrimidazole on ZnO nanopar-
ticle. Color code: C, green; N, blue; O, red; Zn, gray. Hydrogen atoms were omitted for clarity and black dotted line showing surface interaction
p electrons and lowers the HOMO and LUMO energy levels due to ad-
sorption.[127] The azomethine nitrogen atom of phenanthrimidazole is
more basic, and the molecular electrostatic potential shown in
Fig. 16 supports the same; the nitrogen, fluorine, and oxygen
atoms represent the most negative potential region. The chemical
affinity between the zinc ion on the surface of the nano-oxide and
the azomethine nitrogen atom of phenanthrimidazole may be a
reason for strong adsorption of phenanthrimidazole molecule on
ZnO nanoparticle that causes the enhancement. The visualized
mode of attachment is displayed in Fig. 17.
nonradiative energy transfer theory, the energy transfer effi-
ciency (E) can be defined by the following equations:
E=1 ꢁ = R0 /(R60 + r0 ); R06 = 8.8 ꢂ 1023[қ2n-4ΦD J(l)] in Å6; (l)=
6
6
F
Fo
1
FD (l) eA (l) l4 dl, where E is the efficiency of transfer
0
between the donor and the acceptor and R0 is the critical dis-
tance when the efficiency of transfer is 50%. FD (l) is the
corrected fluorescence intensity of the donor at wavelength l
to (l + Δl), with the total intensity normalized to unity and
eA(l) is the molar extinction coefficient of the acceptor at wave-
length l. The Forster distance (R0) has been calculated assuming
random orientation of the donor and acceptor molecules. In the
present case, қ2 = 2/3, n = 1.334, ΦD = 0.25, and from the available
data, it results that J(l) = 7.75 ꢂ 10ꢁ12 cm3 L molꢁ1, E = 0.63,
R0 = 8.13 ꢂ 10ꢁ7 cm and r = 0.741 nm. The donor-to-acceptor
distance is less than 8 nm that indicates that the energy
could transfer from phenanthrimidazole to ZnO nanoparticle[135]
with high probability and the distance obtained by FRET with
higher accuracy.
The thermodynamic feasibility of excited state electron
transfer reaction has been confirmed by the calculation of free
energy change by employing the well known Rehm-Weller
expression,[128] ΔGet = E1/2 (ox) ꢁ E1(/r2ed) ꢁ Es + C, where, E1/2
is
(ox)
the oxidation potential of phenanthrimidazole (0.19 V), E1/2 is
(red)
the reduction potential of ZnO nanoparticle, i.e., the conduction
band potential of nano-ZnO, Es is the excited state energy of
phenanthrimidazole molecule and C is the coulombic term.
Because the phenanthrimidazole is neutral and the solvent used
is polar in nature, the coulombic term in the aforementioned
expression can be neglected.[129] The values of ΔGet is calculated
as ꢁ2.80 eV. The high negative value indicates the thermody-
namic feasibility of the electron transfer process.[130,131]
Computational support
In order to further confirm the adsorption of phenanthrimidazole
on semiconductor ZnO nanoparticle surface, the semiempirical
AM1 MO calculations were carried out with the MOPAC 93[136]
program package using a single self-consistent-field method
(1SCF).The structure was subjected to geometry optimization in
two stages, whereby first just the hydrogen atoms were relaxed
and second all the atoms were relaxed. The parametrization of
this technique has been developed to best reproduce the
geometry, heat of formation, and ionization potential of their
ground state. The results are as shown in Table 10. It reveals
the increase of heat of formation which indicates that the
phenanthrimidazole molecule adsorbs strongly on ZnO nanopar-
ticle which was shown in Fig. 19, in a shorter lifetime (tads).The
electronic energy also supports the proposition. The low ioniza-
tion potential of the adsorbed species is in agreement with the
adsorption of phenanthrimidazole on ZnO.
The phenanthrimidazoles adsorbed on the semiconductor
particle surface had significantly shorter fluorescence lifetime
than the unadsorbed molecules; this decrease in lifetime can
be correlated with the electron transfer process.[132,133] The
fluorescence decay of phenanthrimidazole–ZnO nanoparticle is
shown in Fig. 18. This emission fits into a bi-exponential decay
with short-lived and a long-lived components of life time 0.12
and 7.6 ns, respectively. The long-lived state may correspond to
the free phenanthrimidazoles molecule and the short-lived one
may be adsorbed on ZnO nanoparticles. The populations of the
two states are 67.8% and 32.2%, respectively. In the absence of
ZnO nanocrystals also, phenanthrimidazoles decay bi-
exponentially with lifetime of 2.6 and 5.2 ns. The respective
populations are 1.46% and 98.54%. This clearly shows that the mol-
ecule in the excited state exists in two forms. The major one is likely
to be involved in the adsorption process. The shorter lifetime of the
fluorophore bound to the nanoparticle is likely due to the electron
transfer process from HOMO of phenanthrimidazole molecule to
conduction band of ZnO by way of emission of light. The rate
constant for electron transfer[134] (ket) from the excited state
sensitizer into semiconductor can be calculated by using the rela-
tion ket = 1/(tads ꢁ 1/t). The calculated value of ket is found to be
6.8 ꢂ 108 sꢁ1. The distance between the phenanthrimidazole
and the ZnO nanoparticle can be estimated by Forster’s
nonradiative energy transfer theory. According to Forster’s
CONCLUSION
In summary, we have synthesized a series of phenanthrimidazoles
(hole transport materials) with increasing number of fluoro-
substituents. The photophysical, electrical, optical, thermal, and
device properties of the phenanthrimidazole molecules have been
studied and explained. From the physicochemical studies on the
phenanthrimidazoles, it is concluded that molecules of higher
wileyonlinelibrary.com/journal/poc
Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 386–406

MATERIALS SCIENCE
[26] Y. Shirota, M. Kinoshita, T. Noda, K. Okumoto, T. Ohara, J. Am.
Chem. Soc. 2000, 122, 11021–11022.
[27] M. R. Robinson, S. Wang, G. C. Bazan, Y. Cao, Adv. Mater. 2000,
hyperpolarizability have larger dipole moments and could be used
as potential NLO molecules. On the basis of the PES scan study, the
minimum energy conformation of the molecule is drawn. From the
electrochemical data of the phenanthrimidazoles, the HOMO
energy level of 1–4 is deduced as ꢁ5.6eV, which is in between that
of NPB (ꢁ5.4 eV) and Alq3 (ꢁ6.0 eV) and could regulate the hole
injection. These phenanthrimidazoles can prevent excessive holes
to enter the EML and increase the efficiency. The high Tm and Td
values indicate that these compounds are thermally stable and
can be used to fabricate efficient EL devices. EL data strongly
support that these phenanthrimidazoles are better hole transport
materials. Photoinduced electron transfer explains the enhance-
ment of fluorescence by nanoparticulate ZnO and the apparent
binding constant has been obtained. Adsorption of the fluorophore
on ZnO nanoparticle lowers the HOMO and LUMO energy levels of
the fluorophore. The strong adsorption of the phenanthrimidazoles
on the surface of ZnO nanocrystals is likely due to the chem-
ical affinity of the nitrogen atom of the organic molecule to
Zn(II) on the surface of nanocrystal. Theoretical calculations
also support the experimental results.
12, 1701–1704.
[28] L. H. Chan, H. C. Yeh, C. T. Chen, Adv. Mater. 2001, 13, 1637–1641.
[29] Y. H. Kim, D. C. Shin, S. H. Kim, C. H. Ko, H. S. Yu, Y. S. Chae,
S. K. Kwon, Adv. Mater. 2001, 13, 1690–1693.
[30] K. R. J. Thomas, J. T. Lin, Y. T. Tao, C. W. Ko, J. Am. Chem. Soc. 2001,
123, 9404–9411.
[31] S. Grigalevicius, G. Blazys, J. Ostrauskaite, J. V. Grazulevicius, V.
Gaidelis, V. Jankauskas, E. Montrimas, Synth. Met. 2002, 128, 127–131.
[32] F. Willig, Surface electron transfer processes, VCH Publishers, Inc.,
New York, 1995.
[33] A. Hagfeldt, M. Gratzel, Chem. Rev. 1995, 95, 49–68.
[34] P. V. Kamat, Chem. Rev. 1993, 93, 267–300.
[35] P. V. Kamat, Prog. React. Kinet. 1994, 19, 277–316.
[36] P. V. Kamat, D. Meisel, Semiconductor Nanoclusters - Physical,
Chemical, and Catalytic Aspects, Elsevier, Amsterdam, 1997, 103.
[37] N. A. Anderson, T. Lian, Annu. Rev. Phys. Chem. 2005, 56, 491–519.
[38] B. O’Regan, M. Gratzel, Nature 1991, 353, 737–740.
[39] W. U. Huynh, J. J. Dittmer, A. P. Alivisatos, Science 2002, 295,
2425–2427.
[40] N. Serpone, E. Pelizzetti, Photocatalysis, Fundamentals and Applica-
tions, John Wiley & Sons, New York 1989.
[41] A. Nitzan, M. A. Ratner, Science 2003, 300, 1384–1389.
[42] O. Schmelz, A. Mews, T. Basche, A. Herrmann, K. Mullen, Langmuir
2001, 17, 2861–2865.
[43] M. Tomasulo, I. Yildiz, S. L. Kaanumalle, F. M. Raymo, Langmuir
2006, 2, 10284–10290.
[44] C. Bullen, P. Mulvaney, Langmuir 2006, 22, 30073013.
[45] J. Locklin, D. Patton, S. Deng, A. Baba, M. Millan, R. C. Advincula,
Chem. Mater. 2004, 16, 5187–5193.
[46] Q. Shang, H. Wang, H. Yu, G. Shan, R. Yan, Colloids Surf., A 2007,
294, 86–91.
[47] M. Y. Odoi, N. I. Hammer, K. Sill, T. Emrick, M. D. Barnes, J. Am.
REFERENCES
[1] D. E. Loy, B. E. Koene, M. E. Thompson, Adv. Funct. Mater. 2002, 12,
245–249.
[2] C. W. Tang, S. A. Vanslyke, Appl. Phys. Lett. 1987, 51, 913–915.
[3] C. Adachi, K. Nagai, N. Tamoto Appl. Phys. Lett. 1995, 66, 2679–2681.
[4] D. F. O’Brien, P. E. Burrows, S. R. Forrest, B. E. Koene, D. E. Loy, M. E.
Thompson, Adv. Mater. 1998, 10, 1108–1112.
[5] W. S. Hung, J. T. Lin, C. H. Chien, Y. T. Tao, S. S. Sun, Y. S. Wen, Chem.
Mater. 2004, 16, 2480–2488.
[6] X. R. Bu, H. Li, D. V. Derveer, E. A. Mintz, Tetrahedron Lett. 1996, 37,
7331–7334.
Chem. Soc. 2006, 128, 3506–3507.
[48] M. Anni, L. Manna, R. Cingolani, D. Valerini, A. Creti, M. Lomascolo,
Appl. Phys. Lett. 2004, 85, 4169–4171.
[49] N. I. Hammer, K. T. Early, K. Sill, M. Y. Odoi, T. Emrick, M. D. Barnes, J.
Phys. Chem. B 2006, 110, 14167–14171.
[7] S. Wang, L. Zhao, Z. Xu, C. Wu, S. Cheng, Mater. Lett. 2002, 56,
1035–1038.
[50] C. Landes, C. Burda, M. Braun, M. A. El-Sayed, J. Phys. Chem. B 2001,
105, 2981–2986.
[51] B. R. Fisher, H. J. Eisler, N. E. Stott, M. G. Bawendi, J. Phys. Chem. B
2004, 108, 143–148.
[52] S. F. Wuister, C. M. Donega, A. Meijerink, J. Phys. Chem. B 2004,
108, 17393–17397.
[8] C. R. Moylan, R. D. Miller, R. J. Twieg, K. M. Betterton, V. Y. Lee, T. J.
Matray, C. Nguyen, Chem. Mater. 1993, 5, 1499–1508.
[9] J. Santos, E. A. Mintz, O. Zehnder, C. Bosshard, X. R. Bu, P. Günter,
Tetrahedron Lett. 2001, 42, 805–808.
[10] S. Wang, L. Zhao, Z. Xu, C. Wu, S. Cheng, Mater. Lett. 2002, 56,
1035–1038.
[53] A. Kathiravan, M. Chandramohan, R. Renganathan, S. Sekar,
[11] F. Effenberger, F. Wuerthner, F. Steybe, J. Org. Chem. 1995, 60,
Spectrochim. Acta A 2009, 72, 496–501.
[54] A. Kathiravan, R. Renganathan, Spectrochim. Acta A 2008, 71,
1106–1109.
2082–2091.
[12] F. Mariani, S. Pina, A. Santarelli, A. Tamayo, B. Valtancolia, Dalton
Trans. 2006, 33, 4000–4010.
[55] A. Kathiravan, V. Anbazhagan, M. Asha Jhonsi, R. Renganathan,
[13] Y. Shirota, K. Okumoto, H. Inada, Synth. Met. 2000, 111, 387–391.
[14] I. Y. Wu, J. T. Lin, Y. T. Tao, E. Balasubramaniam, Y. Z. Su, C.-W. Ko,
Chem. Mater. 2001, 13, 2626–2631.
[15] J. Salbeck, N. Yu, J. Bauer, F. Weissotel, H. Bestgen, Synth. Met.
1997, 91, 209.
Spectrochim. Acta A 2008, 70, 615–618.
[56] A. Kathiravan, R. Renganathan, Polyhedron 2009, 28¸ 1374–1378.
[57] A. Kathiravan, R. Renganathan, J. Colloid Interface Sci. 2009,
331, 401–407.
[58] M. Asha Jhonsi, A. Kathiravan, R. Renganathan, J. Lumin. 2009,
[16] B. C. Maiti, S. Z. Wang, C. P. Cheng, D. J. Huang, C.-I. Chao, J. Chin.
Chem. Soc. 2001, 48, 1059–1062.
[17] E. L. Wehry, Effects of Molecular Structure on Fluorescence and
Phosphorescence, in: Practical Fluorescence, 2nd ed. (Ed.: G. C.
Guilbault), Marcel Dekker, New York, 1990.
129, 854–860.
[59] A. Kathiravan, R. Renganathan, J. Colloid Interface Sci. 2009,
335, 196–202.
[60] M. Asha Jhonsi, A. Kathiravan, R. Renganathan, Colloids Surf., B
2009, 72, 167–172.
[18] B. Valeur, Molecular Fluorescence Principles and Applications, Wiley-
VCH, Weinheim, Germany, 2002.
[19] T. Yasuda, I. Yamaguchi, T. Yamamoto, Adv. Mater. 2003, 15, 293–296.
[20] L. Tao, J. Li, Y. Yu, Y. Jiang, C. S. Zhou, S. T. Lee, X. Zhang, O. Kwon,
Chem. Mater. 2009, 21, 1284–1287.
[61] A. Kathiravan, R. Renganathan, Colloid Surf., A 2008, 324, 176–180.
[62] M. Asha Jhonsi, A. Kathiravan, R. Renganathan, J. Mol. Struct. 2009,
921, 279–284.
[63] J. M. Chen, J. H. Hou, Y. F. Li, X. W. Zhou, J. B. Zhang, X. P. Li,
X. R. Xiao, Y. Lin, Chinese Sci. Bull. 2009, 54, 1669–1676.
[64] A. Kathiravan, G. Paramaguru, R. Renganathan, J. Mol. Struct. 2009,
921, 279–284.
[21] P. I. Shih, C. H. Chien, F. I. Wu, C. F. Shu, Adv. Funct. Mater. 17, 2007,
3514–3520.
[22] K. H. Weinfurtner, F. Weissortel, G. Harmgarth, J. Salbeck, Proc. SPIE
Int. Soc. Opt. Eng. 1998, 3476, 40–48.
[23] S. Berleb, W. Brutting, M. Schwoerer, R. Wehrmann, A. Elschner, J.
Appl. Phys. 1998, 83, 4403–4409.
[65] M. Yamada, Y. Tanaka, Y. Yoshimoto, S. Kuroda, I. Shimao, Bull.
Chem. Soc. Jpn. 1992, 65, 1006–1011.
[66] I. Sjoholm, B. Ekman, A. Kober, I. Ljungstedt-Pahlman, B. Seiving,
T. Sjodin, Mol. Pharmacol. 1979, 16, 767–767.
[24] J. Bettenhausen, M. Greczmiel, M. Jandke, P. Strohriegl, Synth. Met.
1997, 91, 223–228.
[25] Y. Shirota, J. Mater. Chem. 2000, 10, 1–25.
[67] Y. Marcus, Chem. Soc. Rev. 1993, 22, 409–416.
[68] P. Wang, P. Zhu, W. Wu, H. Kang, C. Ye, Phys. Chem. Chem. Phys.
1999, 1, 3519–3525.
J. Phys. Org. Chem. 2013, 26 386–406
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

C. KARUNAKARAN ET AL.
[69] C. Karunakaran, V. Rajeswari, P. Gomathisankar, Mater. Sci.
Semicond. Process. 2011, 14, 133–138.
[70] S. Okada, K. Okinaka, H. Iwawaki, M. Furugori, M. Hashimoto,
T. Mukaide, J. Kamatani, S. Igawa, A. Tsuboyama, T. Takiguchi,
K. Ueno, Dalton Trans. 2005, 9, 1583–1590.
[96] R. Dunsbach, R. Schmidt, J. Photochem. Photobiol., A 1994, 83,
7–13.
[97] L. Biczok, T. Berces, F. Marta, J. Phys. Chem. 1993, 97, 8895–8899.
[98] P. O. Andersson, S. M. Bachilo, R. L. Chen, T. Gillbro, J. Phys. Chem.
1995, 99, 16199–16209.
[71] H. Y. Chen, Y. Chi, C. S. Liu, J. K. Yu, Y. M. Cheng, K. S. Chen,
P. T. Chou, S. M. Peng, G. H. Lee, A. J. Carty, S. J. Yeh, C. T. Chen,
Adv. Funct. Mater. 2005, 15, 567–574.
[72] C. Anbuselvan, J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy,
Spectrochim. Acta Part A 2012, 96, 848–853.
[99] Y. Porter, K. M. Ok, N. S. P. Bhuvanesh, P. S. Halasyamani, Chem.
Mater. 2001, 13, 1910–1915.
[100] M. J. Kamlet, R. W. Taft, J. Am. Chem. Soc. 1976, 98, 377–383.
[101] J. Catalan, V. Lopez, P. Perez, J. Fluoresc. 1996, 6, 15–22.
[102] Y. Zhang, S. L. Lai, Q. X. Tong, M. Y. Chan, T. W. Ng, Z. C. Wen,
G. Q. Zhang, S. T. Lee, H. L. Kwong, C. S. Lee, J. Mater. Chem.
2011, 21, 8206–8214.
[103] S. Ranjan, S. Y. Lin, K. C. Hwang, Y. Chi, W. L. Ching, C. S. Liu,
Y. T. Tao, C. H. Chien, S. M. Peng, G. H. Lee, Inorg. Chem.
2003, 42, 1248–1255.
[104] S. L. Lin, L. H. Chan, R. H. Lee, M. Y. Yen, W. J. Kuo, C. T. Chen,
R. J. Jeng, Adv. Mater. 2008, 20, 3947–3952.
[105] A. P. Kulkarni, A. P. Gifford, C. J. Tonzola, S. A. Jenekhe, Appl. Phys.
Lett. 2005, 86, 061106.
[106] W. L. Yu, J. Pei, Y. Cao, W. Huang, J. Appl. Phys. 2001, 89, 2343–2350.
[107] J. Staudigel, M. Stossel, F. Steuber, J. Simmerer, J. Appl. Phys. 1999,
86, 3895–3910.
[108] Y. Kawabe, J. Abe, Appl. Phys. Lett. 2002, 81, 493–495.
[109] M. T. Lee, H. H. Chen, C. H. Liao, C. H. Tsai, C. H. Chen, Appl. Phys.
Lett. 2004, 85, 3301–3303.
[110] Q. X. Tong, S. L. Lai, M. Y. Chan, Y. C. Zhou, H. L. Kwong, C. S. Lee,
S. T. Lee, J. Phys. Chem. C 2009, 113, 6227–6230.
[111] S. Tokito, H. Tanaka, K. Noda, A. Okada, Y. Taga, Appl. Phys. Lett.
1997, 70, 1929–1931.
[73] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery, T. Vreven, Jr., K. N. Kudin,
J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,
B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson,
H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
M. Klene, X. Li, J. E. Knox, P. Hratchian, J. B. Cross, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma,
G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,
A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul,
S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian 03, Revision C.02,
Gaussian, Inc., Wallingford, CT, 2004.
[74] M. Wagener, J. Sadowsky, J. Gasteiger, J. Am. Chem. Soc. 1995,
117, 7769–7775.
[75] N. Vijayan, N. Rani, G. Bhagavannarayana, D. Haranath,
J. Jayabharathi, M. A. Wahab, S. Das, Spectrochim. Acta A 2012,
93, 75–80.
[76] J. Jayabharathi, V. Thanikachalam, M. Vennila, K. Jayamoorthy,
Spectrochim. Acta A 2012, 95, 446–451.
[112] V. Crasta, V. Ravindrachary, R. F. Bharantri, R. Gonsalves, J. Cryst.
Growth 2004, 267, 129–133.
[113] Y. Zhang, S. L. Lai, Q. X. Tong, M. Y. Chan, T. W. Ng, Z. C. Wen, G. Q.
Zhang, S. T. Lee, H. L. Kwonge, C. S. Lee, J. Mater. Chem. 2011, 21,
8206–8214.
[77] J. Jayabharathi, V. Thanikachalam, M. Vennila, K. Jayamoorthy,
Spectrochim. Acta A 2012, 95, 589–595.
[78] J. Jayabharathi, V. Thanikachalam, N. Srinivasan, K. Jayamoorthy,
Spectrochim. Acta A 2011, 83, 329–336.
[79] J. Jayabharathi, V. Thanikachalam, N. Vijayan, N. Srinivasan,
M. Venkatesh Perumal, Struct. Chem. Commun. 2010, 1, 46–52.
[80] J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy, M. Venkatesh
Perumal, Spectrochim. Acta A 2011, 79, 6–16.
[81] J. Jayabharathi, A. Manimekalai, M. Padmavathy, Med. Chem. Res.
2011, 20, 981–995.
[114] Y. Yang, W. J. Zhang, X. M. Gao, Int. J. Quant. Chem. 2006,
106, 1199–1207.
[115] M. Szafran, A. Komasa, E. B. Adamska, J. Mol. Struct. 2007, 827, 101–107.
[116] G. Wang, F. Lian, Z. Xie, G. Su, L. Wang, X. Jing, F. Wang, Synth. Met.
2002, 131, 1–5.
[117] K. Fukui, T. Yonezawa, H. Shingu, J. Chem. Phys. 1952, 20, 722–725.
[118] F. Bergstrom, I. Mikhalyov, P. Hagglof, R. Wortmann, T. Ny,
L. B. Johansson, J. Am. Chem. Soc. 2002, 124, 196–204.
[119] P. Wang, P. Zhu, W. Wu, H. Kang, C. Ye, Phys. Chem. Chem. Phys.
1999, 1, 3519–3525.
[82] N. Subramanian, N. Sundaraganesan, J. Jayabharathi, Spectrochim.
Acta A 2010, 76, 259–269.
[83] Y. Yang, W. J. Zhang, X. M. Gao, Int. J. Quantum Chem. 2006,
[120] R. G. Parr, R. A. Donnelly, M. Levy, W. E. Palke, J. Chem. Phys.
1978, 68, 3801–3807.
[121] R. P. Iczkowski, J. L. Margrave, J. Am. Chem. Soc. 1961, 83, 3547–3551.
[122] K. Sohlberg, N. Vedova Brook, J. Comput. Theo. Nanosci. 2004, 1,
256–260.
[123] H. Chen, J. Q. Lu, J. Wu, R. Note, H. Mizuseki, Y. Kawazoe, Phys. Rev.
B 2003, 67, 113408.
[124] J. M. Tour, Polym. News 2000, 25, 329–336.
[125] X. Lu, M. Grobis, K. H. Khoo, S. G. Louie, M. F. Crommie, Phys. Rev.
Lett. 2003, 90, 096802–1.
106, 1199–1207.
[84] J. Jayabharathi, V. Thanikachalam, R. Sathishkumar, Spectrochim.
Acta Part A 2012, 97, 582–588.
[85] J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy, Spectrochim.
Acta Part A 2012, 97, 131–136.
[86] J. Jayabharathi, V. Thanikachalam, M. Venkatesh Perumal,
Spectrochim. Acta A 2011, 92, 113–121.
[87] J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy, Spectrochim.
Acta Part A 2012, 89, 301–307.
[88] J. Jayabharathi, V. Thanikachalam, K. Brindha Devi, M. Venkatesh
Perumal, Spectrochim. Acta A 2012, 86, 69–75.
[89] J. Jayabharathi, V. Thanikachalam, N. Srinivasan, M. Venkatesh
Perumal, K. Jayamoorthy, Spectrochim. Acta A 2011, 79, 137–147.
[90] J. Jayabharathi, M. Venkatesh Perumal, V. Thanikachalam,
Spectrochim. Acta Part A 2012, 95, 497–504.
[126] C. Majumder, H. Mizuseki, Y. Kawazoe, J. Phys. Chem. A 2001,
105, 9454–9459.
[127] H. M. Cheng, W. F. Hsich, Nanotech. 2010, 21, 485202–485209.
[128] G. J. Kavarnos, N. J. Turro, Chem. Rev. 1986, 86, 401–449.
[129] S. Parret, F. M. Savary, J. P. Fouassier, P. Ramamurthy, J. Photochem.
Photobiol. A 1994, 83, 205–209.
[130] K. Kikuchi, T. Niwa, Y. Takahashi, H. Ikeda, T. Miyashi, J. Phys. Chem.
1993, 97, 5070–5073.
[91] J. Jayabharathi, V. Thanikachalam, M. Venkatesh Perumal,
Spectrochim. Acta Part A 2012, 95, 614–621.
[92] J. Jayabharathi, V. Thanikachalam, M. Venkatesh Perumal,
Spectrochim. Acta A 2012, 85, 31–37.
[131] S. Nath, H. Pal, D. K. Palit, A. V. Sapre, J. P. Mittal, J. Phys. Chem.
A 1998, 102, 5822–5830.
[132] P. V. Kamat, J. Phys. Chem. 1989, 93, 859.
[133] Y. N. Yan, W. L. Pan, H. C. Song, Dyes Pigm. 2010, 86, 249–258.
[134] H. A. Benesi, J. H. Hildebrand, J. Am. Chem. Soc. 1949, 71,
2703–2707.
[93] Z. Li, H. Wan, Y. Shi, P. Ouyang, J. Chem. Inf. Comput. Sci. 2004,
44, 1886–1890.
[94] A. Pedretti, L. Villa, G. Vistoli, J. Comput. Aided Mol. Des. 2004, 18,
167–173.
[95] M. A. Lill, M. L. Danielson, J. Comput. Aided Mol. Des. 2011, 25, 13–19.
[135] M. M. Yang, X. L. Xi, P. Yang, Chin. J. Chem. 2006, 24, 642–648.
[136] MOPAC 93: J.J.P. Stewart, Fujitsu, 1993. MOPAC 7: I. Cserny, Linux
Public Domain: ftp://esca.atomki.hu, or ftp://infomeister.osc.edu.
wileyonlinelibrary.com/journal/poc
Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 386–406

Featured Compounds
1
4
2
Compound Details
Structure Search
Compound Details
Structure Search
Compound Details
Structure Search
3
Compound Details
Structure Search
FC-1