Photophysical and electrochemical investigation of highly conjugated pyridine based diphenylamine materials

Article abstract of DOI:10.1016/j.molstruc.2017.10.058

Three simple and small donor-acceptor type conjugated moieties, namely (2Z, 2′Z)-3,3'-((hexylazanediyl)bis (4,1-phenylene))bis (2-(pyridin-2-yl)acrylonitrile) (DPA-PA-1), (2Z, 2′Z)-3,3'-((dodecylazanediyl)bis (4,1-phenylene))bis (2-(pyridin-2-yl)acrylonit

Full text of DOI:10.1016/j.molstruc.2017.10.058

Accepted Manuscript
Photophysical and Electrochemical investigation of Highly Conjugated Pyridine
based diphenylamine materials
K. Mahesh, V. Priyanka, S.Vijai Anand, S. Karpagam
PII:
S0022-2860(17)31404-7
10.1016/j.molstruc.2017.10.058
MOLSTR 24429
DOI:
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
12 September 2017
16 October 2017
16 October 2017
Please cite this article as: K. Mahesh, V. Priyanka, S.Vijai Anand, S. Karpagam, Photophysical and
Electrochemical investigation of Highly Conjugated Pyridine based diphenylamine materials,
Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.10.058
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ACCEPTED MANUSCRIPT
Highlights:
 Three diphenylamine and pyridine based conjugated moieties are synthesized from
easiest Knoevenagel condensation route

Excellence greenish to yellow fluorescence shift was noted from solution state to thin
film state
 Low electrochemical band gap (1.9 and 1.8 eV ) was observed which is suitable for
photovoltaic applications.

ACCEPTED MANUSCRIPT
Photophysical and Electrochemical investigation of Highly Conjugated
Pyridine based diphenylamine materials
K. Mahesh, V. Priyanka, S. Vijai Anand, and S. Karpagam*
Department of Chemistry,
School of Advanced Sciences,
VIT University,
Vellore-14, TamilNadu, India.
*skarpagam80@yahoo.com
Abstract
Three simple and small donor-acceptor type conjugated moieties, namely (2Z,2'Z)-3,3'-
((hexylazanediyl)bis(4,1-phenylene))bis(2-(pyridin-2-yl)acrylonitrile) (DPA-PA-1), (2Z,2'Z)-
3,3'-((dodecylazanediyl)bis(4,1-phenylene))bis(2-(pyridin-2-yl)acrylonitrile)
(DPA-PA-2),
(2Z,2'Z)-3,3'-((allylazanediyl)bis(4,1-phenylene))bis(2-(pyridin-2-yl)acrylonitrile) DPA-PA-3
have been synthesized according to the Knoevenagel condensation. Here alkyl (hexyl, dodecyl,
allyl) diphenylamine (DPA) moieties acts as an electron donor and pyridine-acetonitrile (PA)
moiety acts as an electron acceptor. These moieties are recently showing great interest in
1
optoelectronic applications. The structures of the DPA-PA-1-3 were confirmed by FT-IR, H
NMR and 13C NMR. The final products showed great solubility in common organic solvents
such as toluene, tetrahydrofuran, ethyl acetate, dichloromethane, chloroform etc due to the alkyl
chains. The absorption maximum of DPA-PA-1-3 appeared at 440, 433, 447 nm in chloroform
solution. The optical band gaps are 2.33, 2.29, and 2.25 eV calculated from thin film absorption
edges. The photoluminescence spectra of three molecules were exhibited a maximum peak at
511, 513, 529 nm with greenish fluorescence in chloroform solution and 553, 541, 554 nm as in
thin film state. DPA-PA-1-3 showed a delay fluorescence decay time (τ1) of 35, 16 and 14 µs
respectively. The lower electrochemical band gaps 1.90 and 1.80 eV was observed by cyclic
voltammetry. The morphological images were indicated that spherical shaped particles were
observed with lower surface roughness. These types of low bandgap materials have much
attention for their various potential applications in optoelectronic devices.
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Keywords: Knoevenagel condensation, Diphenylamine, Pyridine-acetonitrile, Greenish
fluorescence, Optoelectronic devices.
1. Introduction
The design and synthesis of low band gap donor-acceptor based conjugated small molecules
have been much focus of intense research over a last 10 years. Compared to conjugated small
molecules, oligomers and long-chain polymers are having some difficulties such as purity and
polydispersity. So it is required to give more interest for smaller systems of molecules, which
possess clear chemical characteristics and spectroscopic data. The general attention of the
conjugated small molecules is due to their electrochemical, optical, morphological and electrical
properties [1,2]. These conjugated small molecule materials may be used in various
optoelectronic devices such as organic light-emitting diodes (OLEDs) [3,4] organic thin film
transistors (OTFTs) [5] and organic solar cells or organic photovoltaic devices (OPVs) [6].
In the donor-acceptor conjugated system, we selected diphenylamine (DPA) as electron
donor because of the good hole transporting ability. The previous literature proved that
triphenylamine and diphenylamine derivatives are good candidates for photovoltaics because of
their multifunctional properties, such as most soluble in organic solvents, two-photon absorption,
hole-conducting properties, high electron mobility, and good optoelectronic properties. The DPA
exhibited high thermal, electrochemical, and photochemical stability. The rigid and non-planar
structure of DPA increases the solubility [7-9].
Electron transfer is the main role in the chemistry and also biology [10] Many conjugated
systems that carry aromatic heterocyclic rings. This helps to increase the hyperpolarizability
because of the delocalization energy of heteroaromatics. The fluorescence and nonlinear optical
(NLO) properties are increased due to electron-donor units along with electron withdrawing units
in the main conjugated chain [11]. Therefore, the diphenylamine moiety should be connected
with strong electron acceptor for increasing of fluorescence along with lower electrochemical
band gap.
A pyridine ring is an electron deficient molecule and it is highly electron-withdrawing
moiety. If it is introduced in the conjugated system, it exhibits good electron-transporting
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abilities and also optical properties. The low-lying lowest unoccupied molecular orbital (LUMO)
reduces the environmental oxidation of the molecule, leading to a stable conjugated system.
Along with pyridine ring, cyano group helps to reduce the both highest occupied molecular
orbital (HOMO) and LUMO energy levels because of its withdrawing nature [12-14].
The current article deals with the synthesis and optoelectronic properties of low band gap
diphenylamine-pyridyl acetonitrile (DPA-PA) based conjugated molecules. In this conjugated
system, electron-donating diphenylamine moiety was connected with electron withdrawing
cyano-pyridyl ring through Knoevenagel condensation. In this, donor and acceptor were linked
directly with cyano-vinylene linkages. These linkages increase the flexibility, energy levels,
increases the planarity of the π-conjugated system and also tuning optoelectronic properties [15].
Cyano-substituted conjugated molecules exhibit excellent electrical and optical properties by
reason of their favorable electron affinities [16]. Moreover, we investigated the optical,
electrochemical and morphological studies of the conjugated molecules.
2 Experimental section
2.1. Reagents and solvents
Diphenylamine, hexyl bromide, dodecyl bromide, allyl bromide, phosphoryl chloride, 2-
acetonitrile pyridine, piperidine, sodium hydride, was purchased from Sigma-Aldrich, Bangalore,
India. Solvents such as DMF ethyl acetate, DCM, toluene, respectively were purchased from Sd-
fine chemicals, India. All solvents were dried and distilled before the synthesis as per standard
methods.
2.2. Characterization method
1
FT-IR analysis was recorded on Thermo Nicolet 330 FT-IR spectrometer. The H NMR
and 13C NMR spectra were obtained on a Bruker Advanced III 400 MHz spectrometer. The UV-
visible absorption spectra were recorded on Hitachi U2910 spectrophotometer. The fluorescence
emissions were performed with Perkin-Elmer LS45 fluorescence spectrometer. The lifetime
measurements were carried out with Jobin Yvon FLUOROLOG-FL3-111 spectrofluorometer.
The thin films are spin-coated on a silica glass substrate by using apex spin NXG-P1 instrument
and observed the luminescence and surface morphology. The morphological images were
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observed on Nanosurf Easyscan 2 AFM instrument. The cyclic voltammogram peaks were
recorded on CH-I 660C instrument. HRMS values of final products were obtained on Joel GC
Mate II GC-Mass Spectrometer.
2.3. Synthetic procedure
2.3.1. General procedure for the synthesis of alkyl diphenylamine (2)
Sodium hydride (0.0059 mol) was added to the solution of diphenylamine (0.0059 mol)
in DMF (10 mL). This mixture was stirred for approximately 15 min. After alkyl bromide (hexyl,
dodecyl, allyl) (0.0059 mol) was slowly added to the reaction mixture and the mixture was
o
stirred for 5 h at 70 C temperature. After completion of the reaction, the crude product was
diluted with water and extracted with ethyl acetate and dried over anhydrous Na2SO4. The crude
product was purified by column chromatography using ethyl acetate and hexane as a mobile
phase.
1
1
Compound-2a: H NMR (400 MHz, CDCl , ppm): 7.04 (m, 4H, aromatic H), 6.58 (m, 2H,
훿
3
1
1
aromatic H), 6.43 (m, 4H, aromatic H) 3.06 (t, 2H, -N-CH2-), 1.52-0.96 (m, 11H, -CH2 and –
CH3). 13C NMR (100 MHz, CDCl , ppm): 143.2, 129.4, 118, 117.9 (12C, aromatic C), 58.4,
훿
3
32.2, 29.2, 27.7, 23.1, 14.0 (hexyl chain C).
1
1
Compound-2b: H NMR (400 MHz, CDCl , ppm): 7.02 (m, 4H, aromatic H), 6.51 (m, 2H,
훿
3
1
1
aromatic H), 6.39 (m, 4H, aromatic H) 3.02 (t, 2H, -N-CH2-), 1.50-0.96 (m, 23H, -CH2 and –
CH3). 13C NMR (100 MHz, CDCl , ppm): 143.1, 129.3, 118.1, 117.7 (12C, aromatic C), 58.4,
훿
3
32.5, 29.2, 30.3, 23.1, 14.0 (dodecyl chain C).
1
1
Compound-2c: H NMR (400 MHz, CDCl3, ppm): 7.06 (m, 4H, aromatic H), 6.58 (m, 2H,
훿
aromatic 1H), 6.45 (m, 4H, aromatic 1H), 3.98, 3.83, 3.81 (m, 5H, allyl H). 13C NMR (100 MHz,
CDCl , ppm): 143.2, 129.4, 118, 117.9 (12C, aromatic C), 118.3, 106.8, 54.3 (allyl C)
훿
3
2.3.2. General Procedure for Synthesis of dicarboxaldehyde hexyl diphenylamine (Compound-
3a)
The phosphorus oxychloride (0.3555 mol) was added dropwise to DMF (0.3555 mol) at 0
oC over a period of 15 min. Then hexyl diphenylamine (2a) (0.0237 mol) was added to the above
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solution and stirred at 90 oC for 20 h. After completion of the reaction, the solution was cooled to
room temperature, poured into ice-water, neutralized to pH 6-7 by the dropwise addition of
sodium carbonate solution and extracted with ethyl acetate. The organic layer was dried over
anhydrous sodium sulfate. The crude product was purified by column chromatography using
ethyl acetate and hexane as eluent. 1H NMR (400 MHz, CDCl , ppm): 9.87 (s, 2H, -CHO), 7.85
훿
3
(d, 4H, aromatic H), 7.18 (d, 4H, aromatic H), 3.85 (t, 2H, N-CH2), 1.71-1.28 (m, 8H, -CH2),
0.90 (t, 3H, CH ). 13C NMR (100 MHz, CDCl , ppm): 190.79 (-CHO-), 152.01, 131.65, 130.65,
훿
3
3
120.81 (12C, aromatic C), 52.32, 31.60, 27.51, 26.53, 22.58, 14.01 (6C, hexyl C)
2.3.3. Synthesis of dicarboxaldehyde dodecyl diphenylamine (Compound-3b)
Compound-3b was synthesized from dodecyl diphenylamine (2b), POCl3 and DMF in a
similar way as described above. 1H NMR (400 MHz, CDCl , ppm): 9.87(s, 2H, -CHO), 7.79 (d,
훿
3
4H, aromatic H), 7.16 (d, 4H, aromatic H), 3.83 (t, 2H, N-CH2), 1.73 (m, 2H, N-CH2CH2-), 1.44-
13
1.28 (m, 18H, -CH ), 0.88 (t, 3H, CH ). C NMR (100 MHz, CDCl , ppm): 190.74 (-CHO-),
훿
2
3
3
151.93, 131.58, 130.61, 120.73 (12C, aromatic C), 52.56, 31.90, 29.60, 29.53, 29.32, 27.50,
26.94, 22.68, 14.11 (12C, dodecyl C).
2.3.4. Synthesis of dicarboxaldehyde allyl diphenylamine (Compound-3c)
Compound-3c was synthesized from allyl diphenylamine (2c), POCl3, and DMF in a
similar way as described above. 1H NMR (400 MHz, CDCl , ppm): 9.86(s, 2H, -CHO), 7.81(d,
훿
3
13
4H, aromatic H), 7.19 (d, 4H, aromatic H), 3.98, 3.89, 3.85 (m, 5H, allyl H). C NMR (100
MHz, CDCl , ppm): 190.78 (-CHO-), 149.5, 135.4, 120, 119.2 (12C, aromatic C), 118.9, 108.3,
훿
3
54.9 (allyl C)
2.3.5. General Synthetic procedure for Diphenylamine - Pyridyl acetonitrile based molecule
(DPA-PA-1-3)
Compound-3a or 3b or 3c (0.0006 mol) was dissolved in 20 mL of toluene. To this 2-
acetonitrile pyridine (0.0013 mol) and piperidine (0.0013 mol) was added and the reaction
o
mixture was stirred approximately at 120 C for 2 h. After completion of the reaction, the
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mixture was diluted with water extracted with DCM. The crude product was purified by column
chromatography using ethyl acetate and hexane as a mobile phase.
DPA-PA-1: Yield: 75%; 1H NMR (400 MHz, CDCl , ppm): 8.54 (d, 2H, pyridyl CH ), 8.31 (s,
훿
3
2
2H, cyano-vinylene H), 7.89 (d, 4H, diphenylamine H), 7.64 (m, 4H, pyridine H), 7.15 (m, 2H,
pyridine H), 7.06 (d, 4H, diphenylamine H), 3.75 (t, 2H, N-CH2), 1.97 (m, 2H, N-CH2CH2-),
1.64-1.20 (m, 6H, -CH2), 0.83 (t, 3H, CH3). 13C NMR (100 MHz, CDCl , ppm): 151.53,
훿
3
149.52, 148.75, 148.46, 144.12, 137.22, 131.77, 126.83, 122.92, 120.75, 120.67 (aromatic C),
118.46, 106.77 (vinylene C), 52.28 (-N-CH2-), 31.50, 27.54, 26.62, 22.60, 14.03 (hexyl C).
HRMS-ESI (m/z) calculated for C34H31N5 [M-H]+ = 509.2579, found = 509.2575.
1
DPA-PA-2: Yield: 70%; H NMR (400 MHz, CDCl , ppm): 8.48 (d, 2H, pyridyl CH ), 8.26 (s,
훿
3
2
2H, cyano-vinylene H), 7.86 (d, 4H, diphenylamine H), 7.59 (m, 4H, pyridine H), 7.09 (m, 2H,
pyridine H), 7.02 (d, 4H, diphenylamine H), 3.69 (t, 2H, N-CH2), 1.59 (m, 2H, N-CH2CH2-),
1.55-1.20 (m, 18H, -CH2), 1.16 (t, 3H, CH3). 13C NMR (100 MHz, CDCl , ppm): 151.57,
훿
3
149.52, 149.13, 144.20, 137.28, 131.81, 126.87, 122.95, 120.86, 120.71, 120.14 (aromatic C),
118.50, 106.76 (vinylene C), 52.34 (-N-CH2-), 31.92, 29.72, 29.65, 29.64, 29.62, 29.58, 29.36,
27.62, 27.01, 22.71, 14.17 (dodecyl C). HRMS-ESI (m/z) calculated for C40H43N5 [M-H]+ =
593.3518, found = 593.3515.
DPA-PA-3: Yield: 73%; 1H NMR (400 MHz, CDCl , ppm): 8.61 (d, 2H, pyridyl CH ), 8.39 (s,
훿
3
2
2H, cyano-vinylene H), 7.96 (d, 4H, diphenylamine H), 7.74 (m, 4H, pyridine H), 7.25 (m, 2H,
13
pyridine H), 7.15 (d, 4H, diphenylamine H), 3.93, 3.83, 3.81 (m, 5H, allyl H). C NMR (100
MHz, CDCl , ppm): 151.65, 150.28, 149.99, 149.55, 149.21, 144.29, 137.30, 131.80, 126.92,
훿
3
122.96, 122.22, 120.95 (aromatic C), 120.20, 118.54, 106.86 (vinylene C), 52.40 (-N-CH2-).
HRMS-ESI (m/z) calculated for C31H23N5 [M-H]+ = 465.1953, found = 465.1950.
3. Results and Discussion
3.1. Characterization of donor and DPA-PA-1-3
The general synthetic scheme for dicarboxaldehyde diphenylamine (3) and final molecule
(DPA-PA-1-3) was shown in Scheme 1. The structure of the intermediate and final products was
6

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1
confirmed by FT-IR, H & 13C NMR. Firstly, the intermediate (2) was synthesized from
diphenylamine (1) using alkyl bromides (hexyl, dodecyl, allyl) and sodium hydride as a base.
Newly formed alkyl substitutions appeared signals at 3.06 ppm and 1.60- 1.20 ppm as multiplets
1
in H NMR. The Compound 3a-3c was synthesized from alkyl substituted compound-2 in
presence of POCl3 and DMF through Vilsmeier-Haack formylation [8]. The newly formed
1
13
aldehyde protons appeared at 1678 cm-1 in FT-IR, 9.87, 190.7 ppm was appeared in H & C
NMR, which confirms the newly formed aldehyde protons.
Synthesis of donor moiety:
POCl3, 90 oC
DMF, 20 h
O
O
NaH, DMF, 5 h
DMF, 20 h
N
H
N
N
R
3
R
1
2
R = C6H13
R = C12H25
Compound-2a
R = C6H13
Compound-3a
Compound-3b
Compound-3c
Compound-2b
Compound-2c
R = C12H25
R =
R =
Synthesis of donor-acceptor conjugated molecule:
piperidine, 120 oC
Toluene, 2 h
O
O
N
N
NC
N
R
N
CN
CN
N
R
Compound-3a 3b 3c
or or
DPA-PA
R = C6H13
DPA-PA-1
DPA-PA-2
DPA-PA-3
R = C12H25
R =
Scheme. 1 Synthesis route to the donor and conjugated DPA-PA molecule.
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The final product was synthesized from Compound 3a-3c and 2-Acetonitrile pyridine in
presence of piperidine. The FT-IR spectrum of DPA-PA-1-3 as shown in Fig. 1. The FT-IR
spectrum of the final product (DPA-PA-1-3) displayed a peak at 2215 cm-1, which represents the
1
presence of cyano groups. In H NMR newly formed cyano vinylene protons of DPA-PA-1-3
was observed as a singlet at 8.3 ppm which was shown in Fig. 2-4. Based on these results, we
successfully synthesized and characterized the target molecules. The HRMS results showed
excellent agreement between calculated mass and obtained mass which was shown in Fig. 5 The
final molecules (DPA-PA-1-3) are having excellent solubility in common organic solvents such
as toluene, diethyl ether, dichloromethane, tetrahydrofuran, chloroform etc due to the
introduction of long hexyl, dodecyl, and allyl chains in the compounds.
3.2. Absorption studies
The optical and photophysical properties of DPA-PA-1-3 were investigated by UV-
visible absorption spectroscopy in dilute THF solution (10-5 M) and thin film, which was shown
in Fig. 6. The DPA-PA-1-3 has exhibited two types of absorption peaks. This is the common
feature in donor-acceptor based conjugated molecules due to donor-acceptor orbital
hybridization [17, 18]. The first absorption peak was observed at lower wavelength side, which
was appeared at 270 nm for DPA-PA-1, 260 nm for DPA-PA-2, 267 nm for DPA-PA-3, and
second absorption peak was observed at higher wavelength side, which was appeared at 440 nm
for DPA-PA-1, 433 nm for DPA-PA-2, 444 nm for DPA-PA-3 (Fig. 6a). The higher energy
peak of 274-285 nm was due to π-π* electronic transition of the conjugation. The lower energy
peak of 440-447 nm was attributed to intramolecular charge transfer (n-π*) between the electron
donating diphenylamine and electron withdrawing cyanopyridine groups. It was higher
wavelength than previously reported diphenyl amine-carbazole based molecules [19]. Donor
(compound-3a, 3b, 3c) absorption peaks appeared at 349, 351, 354 nm respectively. When
compared to donors, final molecules (DPA-PA-1-3) exhibited 91, 82, 90 nm red shift due to
strong accepting nature of cyanopyridine acceptor.
Upon going to thin film absorption spectra appeared as broad peak and red-shifted when
contrasted with solution absorption spectra displayed in Fig. 6b. This establishes the thin film
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state having strong intermolecular interactions between the molecules aggregated on the silica
substrate glass surface [20]. The optical bandgap value of the product was calculated from the
absorption onset value of thin film according to the equation 표푝푡 = 1240/absorption onset value
퐸
푔
[21]. The optical band gap values were calculated as 2.33 eV, 2.29 eV and 2.25 eV for DPA-PA-
1-3. Moreover, we measured absorption values of the DPA-PA-1-3 in different solvents showed
in Table. 1.
Table. 1 UV-visible absorption maxima changes of DPA-PA-1-3 in different solvents.
DPA-PA
Toluene Diethyl
DCM
(nm)
THF
(nm) acetate
(nm)
Ethyl
Acetone CH3CN DMF
(nm)
ether
(nm)
(nm)
(nm)
(nm)
DPA-PA-1
π-π*
285
442
276
434
274
442
270
440
272
440
266
440
315
440
276
447
n-π*
DPA-PA-2
π-π*
290
433
276
427
275
433
260
433
275
433
272
433
275
433
262
441
n-π*
DPA-PA-3
π-π*
284
447
296
447
274
448
267
444
273
442
278
443
273
441
277
447
n-π*
3.3. Fluorescence studies
The fluorescence study of DPA-PA-1-3 was observed in different solvents (nonpolar to
polar) such as toluene, diethyl ether, dichloromethane, tetrahydrofuran, ethyl acetate, acetone,
acetonitrile, and DMF. The products displayed high emission fluorescence behavior in various
solvents when irradiated with UV light (λmax = 365 nm) showed in Fig. 7. The product displayed
a strong solvatochromic effect [22]. It was exhibited green fluorescence color in nonpolar
solvents. While increasing the polarity of solvents green fluorescence was shifted to yellow and
orange fluorescence, it was clearly depicted in Fig. 7. The fluorescence spectra of DPA-PA-1-3
in different solvents were shown in Fig. 8. For DPA-PA-1-3, the emission maximum was
observed at 498, 495, 499 nm in a nonpolar solvent (toluene). While increasing solvent polarity
the emission maxima also red-shifted to 530, 529, 529 nm in DMF, it was supported by
fluorescence images (Fig. 7). This type of solvatochromism is called as a positive
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solvatochromism. This is influenced by three things, one is the interaction between compound
dipole and a solvent molecule, second is the energy of excited state and ground state were also
leading to wavelength shift, third is the stabilization of the charge transfer transition in polar
solvents [23]. Due to one kind of the chromophore, products exhibited pure fluorescence
emission spectra without any shoulder peak. All three compounds exhibited higher emission
maxima than previously reported thieno [3, 2-c] pyridine derivatives [24].
The thin films were prepared by spin coating on silica glass from THF solution and
exhibited excellent yellowish fluorescence emission which was depicted in Fig. 9. The thin film
emission spectrum of DPA-PA-1-3 (Fig. 8) was and broad emission peaks due to strong
intramolecular interactions. The emission maxima of the thin films were observed at 553, 541,
554 nm for DPA-PA-1-3. Among these three small molecules, DPA-PA-3 showed little higher
emission maxima with bright fluorescence due to N-substituted allyl chain. These types of
solvatochromic fluorescent small molecules were very useful to interact with nucleotide analogs
[25] and human serum proteins [26-29].
Stokes shift is the difference between the point of the emission maxima and absorption
maxima. If the Stokes shift is very less, the overlying of absorption and emission spectra will be
more, it leads to decreases the luminescent efficiency [30]. In our study, Stoke shift of DPA-PA-
1-3 was found to have 113, 108, 110 nm in THF solution. These are higher Stokes shift
compounds when compared to our previously reported thiophene oligomers and carbazole
polymers [17,31]. So this type of longer Stokes shift compounds considered as the best
compound for organic light-emitting diodes.
3.4. Fluorescence lifetime measurement
Fig. 10 shows typical fluorescence lifetime spectrum for the DPA-PA-1-3 with the
emission wavelengths of 512 nm, 515 nm, 530 nm in chloroform solution. For the three
derivatives, microsecond time (delayed lifetime) profiles were measured utilizing a time-
correlated single photon counting method [32]. Investigation of the decay curve demonstrated
that it could be fitted effectively with that single exponential decay function of form 1.
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푡
-
F(t) = A exp
(1)
( )
휏
Where ‘A’ is the amplitude of fluorescence intensity at time zero and ‘τ’ is fluorescence decay
time. The calculated parameters from the single exponential curves for DPA-PA-1-3 were
displayed in Table 2. The assertive first lifetime (τ) of DPA-PA-1 is 35 µs has the lowest χ2 fit
(0.948). The reduced lifetime (τ) for DPA-PA-2 is 16 µs has the 0.973 χ2 fit and for DPA-PA-3
is 14 µs has the 0.977 χ2 fit. The nature of the fit was assessed by the examination of the
residuals, and the value of the decreased χ2 ratio [32].
Table. 2 Fluorescence lifetime analysis data.
DPA-PA
Analysis
Lifetime (µs)
Amplitude
A
χ2
τ
DPA-PA-1
DPA-PA-2
DPA-PA-3
Single exponential
Single exponential
Single exponential
35
16
14
36240.13
15914.01
11329.27
0.948
0.973
0.977
3.5. Measurement of Quantum Yield
The significance and essential properties of photoluminescent materials were performed
by quantum yield. The variation between the number of emitted photons and number of
absorbed photons of the fluorophore is defined as quantum yield. Different approaches have
been explored to evaluate this quantity, contingent on whether the fluorophore is in solid, thin
film or in solution phase. The effectiveness of organic devices was mainly dependent on the
emission quantum yield, lifetime and fluorescence emission range. This is pertinent for organic
optoelectronic devices such as organic photovoltaic OLEDs [33].
In this study, the quantum yield can be deliberate by using quinine sulfate with 0.1 M
H2SO4 solution as standard reference material. The standard reference having absorption maxima
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and quantum yield was 350 nm and 0.54, which was used to estimate the relative quantum yield
of our derivatives. The relative quantum yields of DPA-PA-1-3 were calculated on -the basis of
Equation. 2 [34].
I ODR n2
  R
IR OD nR
(2)
Where,
Ф = quantum yield,
I = Integrated intensity of the emission spectra,
OD = absorbance at the excitation wavelength,
n = refractive index,
R = reference fluorophore with a known quantum yield [35].
The quantum yield values of DPA-PA-1-3 were found to be 0.34, 0.35 and 0.37,
respectively in THF solution. The quantum efficiency of DPA-PA-3 is slightly higher than that
of DPA-PA-1-2 due to the lower energy of the emitting states, which may, in turn, support
nonradiative pathways [36]. According to electron transfer mechanism, the moderate quantum
yield was noticed, which is owed to the photoinduced intramolecular charge transfer mechanism
to take place between diphenylamine and cyanopyridine. This is a familiar aspect observed in
donor-acceptor based conjugated molecules [37].
3.6. Surface morphological studies
Morphology of the conjugated molecules is very important to design photovoltaic
devices. The surface morphology of DPA-PA-1-3 was explored by Atomic force microscopy
(AFM). The high-resolution images of the surface of compounds were analyzed under tapping
mode and images are shown in Fig. 11. The thin films exhibited mainly three types of
interactions, those are molecule-molecule, molecule-solvent, and molecule-surface.
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The AFM images of three compounds (DPA-PA-1-3) were exhibited nanostructural
images. The DPA-PA-1-3 displayed sub-angular shaped particles. The particle size of DPA-PA-
1 is 0.42 µm, for DPA-PA-2 is 0.58 µm, for DPA-PA-3 is 0.9 µm. The AFM topography images
are essential for calculating the surface roughness (Sq), average surface roughness (Sa), root
mean square (RMS), maximum peak height (Sp), maximum peak-valley depth and peak-peak
height (sy). These all surface parameters of three compounds were displayed in the Table. 3. The
average surface roughness is determined as the average value of the complete distance of the
surface points from the mean plane [38]. While the alkyl chain length increased, Sa and Sq have
shown gradual decreases. The lower Sa values of DPA-PA-1-3 were determined as 2.01 nm,
3.82 nm, and 1.18 nm. The lower surface roughness of the compounds leads to increases the
short-circuit current (Jsc) in solar cells [39].
Table. 3 Surface morphology measurements of DPA-PA-1-3.
Average
surface
roughness
(Sa)
RMS
roughness
(Sq)
Peak-
peak
height
(Sy)
Maximum
peak height
(Sp)
Maximum
peak-valley
depth (Sv)
(nm)
Maximum
peak-valley
(Sp-Sv) (nm)
DPA-PA
(nm)
(nm)
(nm)
(nm)
DPA-PA-1
DPA-PA-2
DPA-PA-3
2.01
5.02
7.37
2.65
108.06
12.83
83.84
51.77
-95.22
-16.50
-10.04
108.05
100.34
61.81
3.82
1.18
100.34
61.82
3.7. Electrochemical studies
The electrochemical property of DPA-PA-1-3 was explored by cyclic voltammetry
measurements as shown in Fig. 12. The onset oxidation and onset reduction potentials have been
measured with three electrode system. Ag/AgCl electrode used as a reference electrode, the
platinum wire used as a counter electrode, DPA-PA coated ITO glass used as working electrode
and tetrabutylammonium hexafluorophosphate (Bu4NPF6) (0.1 M in CH3CN) as the electrolyte.
The HOMO and LUMO energy levels were calculated from following empirical equation [40].
EHOMO = -(Eox+ 4.4).
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ELUMO = -(Ered+ 4.4).
Eg= EHOMO -ELUMO.
(3)
Table. 4. shows the electrochemical energy level values of DPA-PA-1-3. The onset
oxidation and reduction potentials of DPA-PA-1-2 were 1.0 V and -0.9 V respectively. The
HOMO and LUMO energy levels are calculated as -5.4 and -3.5 eV respectively. As well as the
onset oxidation and reduction potentials of DPA-PA-3 were 1.0 V and -0.80 V respectively. The
HOMO and LUMO energy levels were calculated as -5.4 and -3.6 eV respectively. The DPA-
PA-1-3 compounds exhibited lower oxidation and reduction values when compared with
thiadiazole [3,4-c] pyridine-based copolymer [41]. The electrochemical band gaps of DPA-PA-
1-3 were calculated as 1.90, 1.90, 1.80 eV from HOMO and LUMO values. Successfully we
achieved very lower band gap values for smaller molecules when compared to diphenylamine-
malononitrile compounds [42]. Even these three small compounds were exhibited extremely
lower band gap values, when compared with MDMO-PPV, MEH-PPV and P3HT polymers,
which was represented in the Fig. 13. The difference between absorption band gap and
electrochemical band gaps for DPA-PA-1 is 0.43 eV, for DPA-PA-2 is 0.39 eV and for DPA-
PA-3 is 0.45 eV. Still, these different values were within the range of error (0.2-0.5 eV) [43].
This can be attributed to the formation of free ions in an electrochemical experiment which
distinguishes with optical absorption spectra. The lower reduction of DPA-PA-1-3 due to an
electron-withdrawing molecule (pyridyl-acetonitrile) was attached to the conjugated system. The
electron density in the π system of the conjugated backbone will be decreased due to pyridyl
acetonitrile. Subsequently, the conjugated molecule (DPA-PA) will be stabilized and its
oxidation value will be elevated. These types of low bandgap conjugated molecules will be
worked as a good capability in the area of photovoltaic applications [44].
Table. 4 Electrochemical properties of DPA-PA-1-3.
풐풙
풓풆풅
푬푬품풄
(eV)a
DPA-PA
HOMO (eV)
LUMO (eV)
(V)
푬풐풏풔풆풕
푬풐풏풔풆풕
(V)
DPA-PA-1
1.0
-0.90
-5.40
-3.50
1.90
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DPA-PA-2
DPA-PA-3
1.0
1.0
-0.90
-0.80
-5.40
-5.40
-3.50
-3.60
1.90
1.80
a Electrical band gaps were calculated from using the equation, Eg= EHOMO -ELUMO.
b Optical band gaps were calculated from absorption edge of the thin film spectra according to
퐸푔
= 1240/absorption onset
4. Conclusion
We have successfully synthesized and characterized the diphenylamine-pyridyl
acetonitrile-based donor-acceptor based conjugated small molecule through Knoevenagel
condensation. We have achieved excellent solubility in all solvents due to successful
incorporation of the hexyl, dodecyl and allyl chains. The UV-visible absorption maxima of
DPA-PA were observed at 440-447 nm. Three small molecules can exhibit excellent green color
fluorescence at 498-499 nm in nonpolar solvents. The fluorescence emission was increased to
orange color while increasing the solvent polarity. The delay fluorescence decay time (τ1) of
DPA-PA-1-3 is 35, 16 and 14 µs respectively with high quantum yield. The HOMO and LUMO
values suggest that diphenylamine and pyridine-acetonitrile can be used as an electron donor and
acceptor materials. The lower electrochemical band gaps were 1.9 and 1.8 eV achieved in small
molecules by the incorporation of the pyridine-acetonitrile acceptor. The lower surface
roughness and the lower band gap of synthesized conjugated small molecules will be promised
for future photovoltaic applications.
Acknowledgement
This work was financially supported by DST-SERB, New Delhi (Grant no.SB/FT/CS-
140/2013). The authors are grateful to DST/VIT-FIST, VIT University for providing
instrumental facilities
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List of Figure caption:
Fig. 1. FT-IR spectra of the (a) DPA-PA-1, (b) DPA-PA-2 (c) DPA-PA-3
Fig. 2. 1H and 13C NMR spectra of the DPA-PA-1
Fig. 3. 1H and 13C NMR spectra of the DPA-PA-2
Fig. 4. 1H and 13C NMR spectra of the DPA-PA-3
Fig. 5. HRMS spectrum of the DPA-PA-1-3
Fig. 6. UV-visible absorption spectra of DPA-PA-1-3 in (a) solvents and (b) thin films
Fig. 7. Visual changes of fluorescence color photos of DPA-PA in different solvents
under UV light irradiation (365 nm) (a) DPA-PA-1, (b) DPA-PA-2, (c) DPA-PA-3
Fig. 8. Changes of fluorescence emission spectra of DPA-PA in different solvents and thin films
(a) DPA-PA-1, (b) DPA-PA-2, (c) DPA-PA-2
Fig. 9. Fluorescence emission photos of DPA-PA in thin film state under UV light irradiation
(365 nm) (a) DPA-PA-1, (b) DPA-PA-2, (c) DPA-PA-3
Fig. 10. Fluorescence Lifetime spectrum of (a) DPA-PA-1, (b) DPA-PA-2, (c) DPA-PA-3
Fig. 11. AFM morphology images of (a) DPA-PA-1, (b) DPA-PA-2, (c) DPA-PA-3
Fig. 12. Cyclic voltammogram of thin film on ITO glass in 0.1 M Bu4NPF6, CH3CN solution
with a scan rate 50 mV/s (a) DPA-PA-1, (b) DPA-PA-2, (c) DPA-PA-3
Fig. 13. Schematic diagram of HOMO/LUMO and band gap energy levels of DPA-PA-1-3 and
comparison with MDMO-PPV, MEH-PPV, and P3HT polymers
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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Fig. 6
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Fig. 7
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Fig. 8
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Fig. 9
28