New blue emissive conjugated small molecules with low lying HOMO energy levels for optoelectronic applications

Article abstract of DOI:10.1016/j.saa.2016.12.035

Versatile conjugated small molecules bearing cyanopyridone core (CP1–5), composed of various donor/acceptor moieties at position ??4 and ??6 have been designed, developed and characterized. Their solvatochromic studies were conducted and analyzed using Li

Full text of DOI:10.1016/j.saa.2016.12.035

Accepted Manuscript
New blue emissive conjugated small molecules with low lying
HOMO energy levels for optoelectronic applications
C. Trupthi Devaiah, B. Hemavathi, T.N. Ahipa
PII:
DOI:
Reference:
S1386-1425(16)30759-4
doi: 10.1016/j.saa.2016.12.035
SAA 14848
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
2 September 2016
20 December 2016
20 December 2016
Please cite this article as: C. Trupthi Devaiah, B. Hemavathi, T.N. Ahipa , New blue
emissive conjugated small molecules with low lying HOMO energy levels for
optoelectronic applications. The address for the corresponding author was captured as
affiliation for all authors. Please check if appropriate. Saa(2016), doi: 10.1016/
j.saa.2016.12.035
This is a PDF file of an unedited manuscript that has been accepted for publication. As
a service to our customers we are providing this early version of the manuscript. The
manuscript will undergo copyediting, typesetting, and review of the resulting proof before
it is published in its final form. Please note that during the production process errors may
be discovered which could affect the content, and all legal disclaimers that apply to the
journal pertain.

ACCEPTED MANUSCRIPT
New Blue Emissive Conjugated Small Molecules with Low Lying
HOMO Energy Levels for Optoelectronic Applications
Trupthi Devaiah C., Hemavathi B., and Ahipa T.N.*
Nanostructured Hybrid Functional Materials and Devices, Centre for Nano and Material
Sciences, Jain University, Jain Global Campus, Kanakapura, Ramanagaram,
Bangalore – 562 112, India.
* Corresponding Author e-mail address: tn.ahipa@jainuniversity.ac.in
Abstract
Versatile conjugated small molecules bearing cyanopyridone core (CP1-5), composed of
various donor/ acceptor moieties at position -4 and -6 have been designed, developed and
characterized. Their solvatochromic studies were conducted and analyzed using Lippert-Mataga,
Kamlet-Taft and Catalan solvent scales and interesting results were obtained. The polarizability/
dipolarity of the solvent greatly influenced the spectra. The electrochemical studies were carried
out using cyclic voltammetry to calculate the HOMO-LUMO energy levels. The study revealed
that the synthesized conjugated small molecules possess low lying HOMO energy levels which
can be exploited for application in various fields of optoelectronics.
Keywords: Cyanopyridone; Photophysical Properties; Solvatochromisim; Cyclic Voltammetry.
1

ACCEPTED MANUSCRIPT
Introduction
Conjugated small molecules have been a subject of intensive research in the recent years
owing to its vast application in biological imaging probes, sensors, lasers, light emitting devices
[1-3] and light-energy harvesting [4, 5]. Thus, the development of novel conjugated small
molecules (CSM) has attracted a great attention. Of the various types of CSM’s, the molecules
containing electron donors (D) and acceptors (A) connected by a σ bond or by a -electron
bridge (D-A-D or D--A) are of substantial interest for optoelectronic devices [6, 7] such as
light emitting diodes [8-10], solar cells [11-13] or electro generated chemiluminescence [14, 15].
In fact, synthetic [16] and theoretical [17, 18] investigations have clearly indicated that the
introduction of aromatic and hetero aromatic systems bearing different electronic properties
while designing a donor-acceptor (D-A) molecular frameworks may lead to the development of
new CSM materials possessing unusual electrochromic, photochromic or luminescent behaviors.
In this context, D-A systems show excellent photophysical properties because of their push-pull
nature. Here, the molecular property of chromophores depends mainly on the strength of “push-
pull” effect (i.e. the ability of the donor to provide electrons and the acceptor to withdraw
electrons).
Pyridines are privileged structures because of their ubiquity in a wide span of natural
products [19, 20]. Additionally, their chemical reactivity as a result of the ring aromaticity,
presence of ring nitrogen atom, -deficient nature of the ring, large permanent dipole moment,
presence of electron deficient carbon atom which centers at the  andposition invites attention
for it to be probed as an acceptor [20]. Interestingly, cyanopyridines and cyanopyridones belong
to the class of substituted pyridines which are known to display excellent thermal and
photochemical stability [21, 22], high luminescence efficiency [21] and novel optoelectronic
2

ACCEPTED MANUSCRIPT
properties [23]. In fact, the cyano group is known for its good electron withdrawing ability, small
size and excellent stability under UV irradiation which will further increase the ability of the
acceptor [1]. In view of their considerable photophysical potential, 3-cyanopyridines have been
investigated for their viability as photoactive materials for use in a variety of applications [24-
27]. As well, pyridine-2-ones are well known for their uniqueness of their tautomeric structure
[10] [28] and fluorescence efficiency with high quantum yield [10, 21, 29].
The characterization of the compounds using simple photophysical studies may reveal
fascinating facts about their luminescent properties. In this perspective, solvatochromic study of
the compounds is necessary in order to understand the phenomenon of solute-solvent interactions
and photophysical characteristics of a D-A molecule. These studies may be well examined by the
utilization of solvent scales like Lippert-Mattaga [30-33], Kamlet-Taft and Catalán scales [34-
37].
In the present work, we report five new cyanopyridone-based CSM’s with tunable
photophysical properties such as molar absorptivity, Stokes shift and quantum yield. These CSM
scaffold possess easily-interchangeable electron-donating and electron-accepting groups in order
to provide the obligatory architectures with tunable photophysical behavior. The photophysical
properties of cyanopyridone derivatives were examined in a wide variety of solvents, including
apolar, polar-protic and polar-aprotic solvents. Further, the effect of the solvent on the absorption
and emission spectra are analyzed by a multi-component linear regression, wherein, several
solvent parameters are simultaneously analyzed. Finally, the fluorescence quantum yield, the
Stokes shift, optical band gap, electrochemical band gap as well as their HOMO-LUMO energy
levels are analyzed to look for the suitability of these derivatives in optoelectronic devices.
3

ACCEPTED MANUSCRIPT
Experimental Section
Materials and Methods
All the chemicals used in the present work were procured from commercial source. The
reagents and solvent were used without further purification. The target compounds were purified
and recrystallized from analytical grade solvents. UV-visible absorption and emission properties
were measured using UV-1800 SHIMADZU UV-spectrophotometer and RF-5301 PC,
SHIMADZU spectrophotometer equipped with a Xe-lamp as an excitation source, respectively.
IR spectra were collected using Bruker ALPHA eco-ATR-IR on ZnSe Crystal. All the
1
13
compounds were characterized by H NMR (400 MHz) and C NMR (100 MHz) on a Bruker
spectrometer. The spectra were recorded at room temperature in deuterated chloroform (CDCl3)
or deuterated dimethyl sulfoxide (DMSO-d6). The chemical shifts are expressed in ppm values
referenced to tetramethysilane (TMS). ESI-MS measurements were made on Waters (SYNAPT
G2) with methanol as eluent.
Cyclic voltammetry (CV) data were determined in a N2-saturated three-electrode system,
where, the working, counter and reference electrodes are glassy carbon, platinum and Ag/Ag+,
respectively. The glassy carbon electrode was polished with 1.0μm alumina slurry and then
sonicated for 10 min in distilled water. The CSM solution (in chloroform) was drop casted on to
the glassy carbon disk and dried to get a uniform thin film of CSM, which was later dipped into
acetonitrile solution containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAH) as the
supporting electrolyte. Further, the electrochemical measurements were made at a scan rate of 50
mV/s. The Ag/Ag+ reference electrode was calibrated using a ferrocene/ ferrocenium (Fc/Fc+)
redox couples as an external standard.
4

ACCEPTED MANUSCRIPT
Scheme 1: Synthetic route for cyanopyridone (CP1-5) derivatives.
5

ACCEPTED MANUSCRIPT
Procedure for the preparation of cyanopyridone derivatives (CP1-5): A mixture of aromatic/
heteroaromatic aldehyde 1a-d (0.01 mol), acetyl aromatic/ heteroaromatic 2a-e (0.01 mol), ethyl
cyanoacetate (0.01 mol) and ammonium acetate (0.08 mol) in 1,4-dioxane (40 ml) was refluxed
for 20 h (Scheme 1). The obtained precipitate was filtered, washed successively with 1,4-
dioxane, followed by ethyl acetate and finally recrystallized from ethanol-chloroform mixture to
get the pure product (CP1-5).
2-Oxo-4,6-di-p-tolyl-1,2-dihydropyridine-3-carbonitrile (CP1): Pale yellow solid, yield 75 %,
1
M.P. > 300°C. FTIR (cm-1): 2887, 2214, 1622, 1599, 1467, 807. H NMR (400 MHz, DMSO-
d6) δ (ppm): 12.68 (s, 1H, NH on heterocyclic), 7.76 (d, J=8 Hz, 2H, Ar-H), 7.61 (d, J=8 Hz, 2H,
Ar-H), 7.35-7.29 (m, 4H, Ar-H), 6.73 (s, 1H, Ar-H (pyridine)), 2.35 (s, 6H, -CH3). 13C NMR
(100 MHz, DMSO-d6) δ (ppm): 162.5, 160.14, 160.12, 141.75, 140.81, 133.67, 129.92, 129.75,
128.61, 128.05, 117.10, 105.90, 21.36. MS (m/z) = 301 (M+H)+.
4,6-Bis(4-bromophenyl)-2-oxo-1,2-dihydropyridine-3-carbonitrile (CP2): Cream solid, yield 79
1
%, M.P. >300 °C. FTIR (cm-1): 3019, 2887, 2201, 1635, 1536, 807. H NMR (400 MHz,
CDCl3) δ (ppm): 7.87 (d, J=8 Hz, 2H, Ar-H), 7.72 (d, J=8 Hz, 2H, Ar-H), 7.64-7.61 (m, 4H, Ar-
H), 6.83 (s, 1H, Ar-H(pyridine)). 13C NMR (100 MHz, DMSO-d6) δ (ppm): 172.42, 165.38,
157.38, 153.36, 136.31, 134.25, 132.09, 130.78, 130.00, 124.64, 123.95, 117.82, 103.09, 96.75.
MS (m/z) = 431 (M+H) +.
4,6-Bis(4-nitrophenyl)-2-oxo-1,2-dihydropyridine-3-carbonitrile (CP3): Yellow solid, yield 77
%, M.P. >300 °C. FTIR (cm-1): 3230, 2887, 2217, 1653, 1596, 1519, 851. 1H NMR (400 MHz,
DMSO-d6) δ (ppm): 8.32 (d, J=8 Hz, 2H, Ar-H), 8.28-8.22 (m, 4H, Ar-H), 7.88 (d, J= 8 Hz, 2H,
13
Ar-H), 6.88 (s, 1H, Ar-H (pyridine)). C NMR (100 MHz, DMSO-d6) δ (ppm): 156.32, 153.93,
6

ACCEPTED MANUSCRIPT
148.13, 148.03, 145.32, 145.24, 145.04, 130.09, 128.65, 124.03, 123.90, 104.60. MS (m/z) = 334
(M-NO)+.
6-(9H-fluoren-2-yl)-2-oxo-4-(thiophen-2-yl)-1,2-dihydropyridine-3-carbonitrile
(CP4):
Brownish yellow solid, yield 72 %, M.P. >300 °C. FTIR (cm-1): 2887, 2207, 1644, 1598, 1347,
830. 1H NMR (400 MHz, CDCl3) δ (ppm): 12.75 (s, 1H, NH on heterocyclic), 8.04-8.03 (m, 2H,
Ar-H), 7.94-7.85(m, 5H, Ar H) 7.31-7.29 (m, 3H, Ar-H), 7.24-7.22 (m, 1H, Ar-H), 3.30 (s, 2H,
Ar-H). 13C NMR (100 MHz, DMSO-d6) δ (ppm): 161.5, 158.43, 153.48, 148.81, 130.18, 129.99,
129.71, 124.28, 124.15, 124.09, 116.49, 113.89, 103.70, 27.63. MS (m/z) = 349.08 (M-OH)+.
2-Oxo-4,6-di(thiophen-2-yl)-1,2-dihydropyridine-3-carbonitrile (CP5): Lemon Yellow solid,
yield 82 %, M.P. >300 °C. FTIR (cm-1):3096,2887, 2209, 1630, 1593, 806.1H NMR (400 M Hz,
DMSO-d6) δ (ppm): 12.75 (s, 1H, NH on heterocyclic), 8.03 (d, J= 4 Hz, 1H, Ar-H), 7.96 (d, J=
4 Hz, 2H, Ar-H), 7.94 (s, 1H, Ar-H), 7.86 (d, J= 4 Hz, 1H, Ar-H), 7.29 (t, J= 4 Hz, 1H, Ar-H),
13
7.23 (t, J= 4 Hz, 1H, Ar-H). C NMR (100 MHz, DMSO-d6) δ (ppm): 152.42, 137.04, 131.85,
131.24, 131.22, 130.09, 130.07, 129.39, 129.06, 117.16. MS (m/z) = 285 (M+H) +.
Results and Discussion
Synthesis
Cyanopyridone derivatives (CP1-5) were synthesized in good yield (~72-82%) by a one-
pot method as shown in Scheme 1. The structures of synthesized compounds (CP1-5) were
1
characterized by IR, H NMR, 13C NMR and mass spectral analyses. Spectral data of all
synthesized compounds were in full agreement with the proposed structures. IR spectrum of
compound CP1 showed absorption bands at 2887, 2214, 1622, 1599, and 1467 cm-1 which were
due to the C–H, C≡N, C=O, C=N stretching’s, and C–H bend, respectively. In general, the
cyanopyridones can have the tendency to exhibit two tautomeric forms i.e. amidic and imidol
7

ACCEPTED MANUSCRIPT
form. Further, the observed C=O peak stretching in IR spectra of compounds CP1-5 clearly
confirms that the compounds are in amidic form. Thus, the predominant solid state form is
amidic form. In 1H-NMR spectra, all protons were seen according to the expected chemical shift
1
and integral values. The H NMR spectrum of CP1 showed a singlet at δ 11.39 corresponds to
N-H proton of pyridone. The observed peaks in the range of δ 7.68-7.33 were attributed to
aromatic protons. A singlet at δ 6.65 was due to pyridine 5H proton. Also, a singlet at δ 2.44 was
due to methyl proton. Further, the mass spectrum of CP1 showed molecular ion peak at m/z =
301 (M+H)+, which is in agreement with its molecular formula. Similarly, the spectral values for
all the compounds are given in the experimental part.
Photophysical Properties
The UV-visible absorption spectra of CP1-5 were recorded at the concentration of 10-5 M
in various solvents of increasing polarity to correlate the effect of solvent polarity on their
absorption parameters. For this purpose, solvents of increasing polarity were selected
(chloroform, ethylacetate, tetrahydrofuran (THF), methanol, dimethylformamide (DMF),
acetonitrile, and dimethyl sulfoxide (DMSO)) based on their dielectric constants. The values of
absorption maximum (
of CP1-5 in these solvents are given in Table 1. UV-visible
absorption spectra of CP1 in various solvents are depicted in Figure 1. For all the compounds,
the absorption bands were observed in the range of ~343-449 nm. The observed absorption
bands can be attributed to transition.
From the Table 1, it is clear that the absorption maximum of the CSM is affected by
solvent type. In case of CP1, CP2 and CP5, only a slight red shift (~16-23 nm) with the increase
in solvent polarity from chloroform to DMSO was seen. Whereas, a considerably increased red
shift (~62 nm) is observed in case of CP3 and a prominent red shift (~106 nm) is perceived in
8

ACCEPTED MANUSCRIPT
case of CP4 with the increase in solvent polarity. The spectral position of CSM’s in various
solvents further revealed interesting results. The absorption maximum (
) is lowest in the
case of chloroform, this is because, all other solvents used in this work are more polar than
chloroform and can engage more strongly in a solute-solvent type of interaction. However,
chloroform can interact with the CSM molecule in terms of dipole-dipole interactions, thereby,
resulting in a net stabilization of the ground state of the CSM molecule, and hence one can see a
hypsochromic shift in the spectrum. On the other hand, the absorption maximum (
) value is
shifted to lower energies i.e. bathochromic shift in highly polar solvents such as DMSO, because
of stabilization of the excited state or the specific interaction between the solvent and hydrogen
from NH group in the CSM molecule [38].
Figure 1: UV-visible absorption spectra of CP1 in various solvents.
9

ACCEPTED MANUSCRIPT
However, anomalous behavior is observed in the absorption spectra of compounds CP1,
CP2 and CP5 in methanol solvent where a hypsochromic shift is observed possibly due to the
decrease of dipole movement upon excitation [39]. Cyanopyridone core has the ability to form
intermolecular hydrogen bonding with the polar protic solvents. The hydrogen bonding in polar
protic solvents can either increase in energy of ground state or decrease in the energy of exited
state or both causing hypsochromic shift [40]. Also, the anomalous behavior is perceived in the
absorption spectra of CP1-5 in acetonitrile, wherein, hypsochromic shift is observed compared to
that of DMF. This is due to the non-hydrogen donating property of acetonitrile where the
solvation of CSM occurs through dipole-dipole interaction which generally causes the
stabilization of the ground state [38]. Thus, this change in spectral position can be used as a
probe for various types of interactions between the solute and the solvent. The molar
absorptivity’s were observed in the range of 10 × 102 – 56 × 103 L mol -1 cm-1 and is tabulated in
Table 1.
Table 1: Optical properties of CP1-5 in various solvents.
CP1
Solvent
Molar absorptivity
(L mol-1 cm-1)
37×103
(nm)
(nm)
(eV)
Chloroform
Ethyl acetate
THF
Methanol
DMF
370
441
3.34
0.02
0.01
0.02
0.06
0.06
0.01
0.10
374
374
366
377
373
383
446
446
438
450
443
450
3.31
3.30
3.38
3.28
3.32
3.23
29×103
21×103
56×103
18×103
Acetonitrile
DMSO
55×103
37×103
CP2
Chloroform
Ethyl acetate
THF
Methanol
DMF
371
377
377
368
388
373
391
452
452
452
448
464
453
464
3.34
3.28
3.28
3.36
3.19
3.32
3.17
0.01
0.02
0.03
0.11
0.25
0.02
0.23
19×103
17×103
12×103
26×103
18×103
34×103
27×103
Acetonitrile
DMSO
10

ACCEPTED MANUSCRIPT
CP3
Chloroform
Ethyl acetate
386
391
475
486
3.21
3.17
0.05
0.05
70×102
80×102
90×102
12×103
60×102
10×103
70×102
THF
Methanol
DMF
Acetonitrile
DMSO
395
392
437
390
448
492
444
451
480
446
3.13
3.15
2.83
3.17
2.76
0.06
0.00
0.01
0.01
0.02
CP4
CP5
Chloroform
Ethyl acetate
THF
Methanol
DMF
385
394
343
394
440
389
449
474
484
492
438
457
486
437
3.22
3.14
3.61
3.14
2.83
3.19
2.76
0.03
0.03
0.03
0.01
0.01
0.02
0.03
15×103
20×103
24×103
80×102
50×102
70×102
40×102
Acetonitrile
DMSO
Chloroform
Ethyl acetate
THF
Methanol
DMF
397
401
404
390
407
397
414
469
470
472
452
464
460
468
3.12
3.08
3.06
3.17
3.05
3.11
3.00
0.40
0.56
0.76
0.49
0.11
0.32
0.61
30×102
20×102
10×102
40×102
20×102
40×102
60×102
Acetonitrile
DMSO
The fluorescence spectra of CP1-5 were also recorded at the concentration of 10-5 M in
various solvents of increasing solvent polarity (from chloroform to DMSO) and interesting
results were obtained. The fluorescence emission spectra is not dependent on the variation in
polarity of solvent. The fluorescence data is tabulated in Table 1. Fluorescence emission spectra
of CP1 in various solvents are shown in Figure 2. The compounds exhibit blue emission bands
in the range of ~438-492 nm with Stokes shift of around ~14-149 nm. Remarkably, an anti-
Stokes shift of 2 nm and 12 nm is observed from the compounds CP3 and CP4, respectively in
DMSO. It is important to note that, certain organic molecules where a π-conjugated bridge is
end-capped by an electron-donor and electron-withdrawing group can show upconverted photon
emission under specific conditions due to various processes [41]. In case of CP3, the acceptor
11

ACCEPTED MANUSCRIPT
cyanopyridone unit was linked by two para-nitrophenyl units at its position -4 and -6 to form π-
conjugated donor-acceptor structure. Whereas, compound CP4 composed of cyanopyridone
(acceptor), fluorenyl (donor) and thiophenyl (donor) units. In both the cases, observed anti-
Stokes shift in DMSO solvent can be attributed to the photon upconversion process caused by
the nature of molecular architecture, the substituent effect, and modulation of the electronic and
geometric parameters [41].
In case of CP1 and CP2 only a small red shift (~ 12-16 nm) in the emission spectra was
seen with the increase in solvent polarity from chloroform to DMSO. However, the largest
emission maximum
of 450 and 464 nm is observed in case of CP1 and CP2 respectively
in both DMF and DMSO. Whereas, in case of CP3, CP4 and CP5 the
(444, 438 and 452
nm) is the least in polar protic solvent i.e. methanol. Among the polar aprotic solvents, a greater
(492, 492 and 472 nm) is observed in case of CP3, CP4 and CP5 with THF. Further, the
fluorescence quantum yield (f) of CP1-5 in solution state was determined with reference to the
standard procedure using quinine sulphate in 0.1 M sulfuric acid as a standard (f = 0.54) [42-
44]. The CSM’s displayed a wide range of quantum yields (f = 0.01-0.76), wherein, the highest
quantum yield was exhibited by the compound CP5 in THF. Generally, the high quantum yield
is attributed to the extent of planar conformation in the molecule. In case of CP5, the positions -4
and -6 of central cyanopyridone core are connected with the two small sized five membered
thiophene ring. In addition, the steric hindrance caused by these five membered ring systems are
less compared to that of the six membered or polycyclic aromatic hydrocarbon ring systems (i.e.
substituted phenyl/ fluorenyl) connected to cyanopyridone at positions -4 and -6, respectively.
Thus, compound CP5 is possessing more planar conformation than that of the rest (CP1-4),
which further result in the observed high quantum yield.
12

ACCEPTED MANUSCRIPT
Figure 2: Fluorescence emission spectra of CP1 in various solvents.
In the present work, cyanopyridone chromophore bearing various electron donating (i.e.
4-methyl phenyl, 2-fluorenyl and 2-thiophenyl) and electron accepting groups (i.e. 4-bromo
phenyl and 4-nitro phenyl) at positions -4 and -6 have been assembled to obtain D-A molecular
architecture (CP1-5) with tunable photophysical behavior. From the results, only a slight change
in photophysical properties were observed with change in the substituents. The bathochromic
shift of CP2 with respect to CP1 or CP3 with respect to CP2 can be explained by the electron
withdrawing characteristics of 4-bromo phenyl [45], or 4-nitro phenyl [46] group. In addition,
bathochromic shift in CP4 compared to CP3 is due to the enhanced resonance stabilization
caused by the fluorenyl ring [47] at position -4 and the electron donating capacity of thiophenyl
ring at position -6 of cyanopyridone. A pronounced bathochromic shift is observed in case of
CP5 owing to its planarity and the electron donating capacity of the thiophenyl [48] substituents.
As a whole, photophysical properties can be tuned and directed to the desired target by placing
13

ACCEPTED MANUSCRIPT
electron withdrawing and electron donating substituents at suitable positions on the π - rich and π
- deficient rings systems.
Lippert-Mataga plot (correlation between solvent polarity and Stokes shift)
The solvent induced spectral shifts caused by the general solvent effect is often
interpreted using the Lippert – Mataga plot [30-33] of Stokes shift ( ) versus orientation
polarizability ( f).
eqn 1
Figure 3: Lippert-Mataga plot for CP1. The straight line represents the best linear fit to 7 data
points.
The orientation polarizability (eqn 1) describes Stokes shift in terms of change in dipole
moment of the fluorophore and the dependence of the energy of the dipole on the dielectric
constant () and the refractive index of the solvent (n) [49]. The Lippert Mataga plot for CP1 is
shown in Figure 3 as representative example and rest of the plots can be found in the supporting
information. In all the cases, the plots were too scattered (r < 0.6) to allow to estimate the
14

ACCEPTED MANUSCRIPT
difference between ground and excited states. Further, Kamlet Taft-Catalán scale were employed
to account for the specific solvent interactions.
Kamlet Taft-Catalán scales
The specific solvent interactions are accounted most often using the Kamlet-Taft (eqn 2)
[50] and Catalán (eqn 3) [50] solvent scales which can be expressed using the multiple linear
equation.
eqn2
eqn 3
According to the Kamlet-Taft solvent scale the parameters and are the measure of
the specific interactions such as hydrogen bond accepting (acidity), hydrogen bond donating
(basicity) and dipolarity/ polarizability, respectively, of a given solvent [34, 51]. The
physiochemical parameters analyzed in the present work are the absorption maxima abs, the
emission maxima em and the Stokes shift = abs - em (all in cm-1). The solvents used, Kamlet-
Taft and Catalán parameters employed are tabulated in Table 2.
Table 2: Dielectric constant, refractive index, Kamlet- Taft ( and *) and Catalán (SA, SB,
SP and SdP) parameters for various organic solvents.
Solvent
η
π*
SA
SB
SP
SDP
β


Chloroform 04.8 1.45 0.20 0.10 0.58 0.047 0.071 0.783 0.614
Ethyl acetate 06.2 1.37 0.00 0.45 0.55 0.000 0.542 0.656 0.603
THF
Methanol
DMF
07.9 1.41 0.00 0.55 0.58 0.000 0.591 0.714 0.634
32.7 1.33 0.98 0.66 0.6 0.605 0.545 0.608 0.904
36.7 1.43 0.00 0.69 0.88 0.031 0.613 0.759 0.977
Acetonitrile 37.5 1.34 0.19 0.40 0.75 0.044 0.286 0.645 0.974
DMSO 46.7 1.47 0.00 0.76 1.00 0.072 0.647 0.830 1.000
The multi-linear regression analysis of the physiochemical parameters employed as a
function of the Kamlet – Taft solvent parameters were carried out on CP1-5 and the estimated
15

ACCEPTED MANUSCRIPT
coefficients y0, a, b, c* for CP1 are summarized in Table 3 (coefficient for CP2-5 can be
found in the supporting information). The multi-linear regression analysis of the data of
abs
CP1-5 in accordance to the Kamlet-Taft solvent scales provides a good linear fit measured by
the value of r ~0.819-0.989. The negative value of both b and c* (Table 3) indicates that there
is positive solvatochromism which points to bathochromic shift with increase in solvent
polarizability and solvent bond basicity. This suggests the stabilization of electronic state relative
to the ground state [52, 53]. Also, a considerably better regression value r ~0.837-0.989 is
observed with as a function of Kamlet-Taft parameters, the positive value of a specifies a
em
hypsochromic spectral shift with increasing hydrogen bond donating ability [53]. In case of as
a function of Kamlet-Taft parameter the regression value r ranges from ~ 0.850 - 0.968.
In case of Catalán scale, the parameters SA, SB, SP and SdP represents the acidity,
basicity, polarizability and dipolarity, respectively. From the Table 3, where the estimated
coefficients aSA, bSB, cSP and dSdP are summarized, it can be observed that cSP, dSdP have large
negative values. This explains the large bathochromic shift with increase in solvent polarity and
confirms the specific interaction of the excited state of molecule with the polar solvent [54].
Cyanopyridone derivatives owing to their tailor-made properties are of great interest for
application in the field of optoelectronics [21, 29]. The result obtained for CP1-5 indicates that it
is possible to manipulate their fluorescence properties by tuning the substitution, the arrangement
of functional groups and by the choice of appropriate solvents.
Table 3: Estimated coefficients (y0, a, b, c, aSA, bSB, cSP and dSdP), their standard errors and
correlation coefficients (r) for the multiple linear regression analysis of , abs, em of CP1 as a
function of the Kamlet-Taft (eqn 2) and Catalán (eqn 3) solvent scales. For Kamlet-Taft and
Catalán, seven solvents were used in the analysis of , abs, em.
16

ACCEPTED MANUSCRIPT
CP1
Kamlet-Taft
y0 (103)
aα(103)
bβ(103)
cπ*(103)
r
ῡabs
ῡem
27.50±0.23 0.68±0.16 -0.42±0.30 -0.95±0.40
22.75±0.09 0.51±0.06 -0.46±0.11 -0.19±0.15
4.74±0.26 0.17±0.18 0.04±0.33 -0.75±0.44
0.974
0.989
0.853
r
∆ῡ
Catalán
ῡabs
y0(103)
aSA(103)
bSB(103)
cSP(103)
dSdP(103)
28.84±0.74 1.0±0.39 -0.7±0.34 -1.79±1.01 -0.68±0.41 0.967
23.43±0.23 0.70±0.12 -0.62±0.10 -0.73±0.31 -0.27±0.13 0.991
5.41±0.79 0.30±0.42 -0.16±0.37 -1.07±1.10 -0.41±0.44 0.827
ῡem
∆ῡ
Cyclic Voltammetry
Cyclic voltammetry (CV) is an effective tool to determine the molecular energy levels of
CSM’s, which is crucial for considering their abilities in terms of electron injection. CV
measurements for CP1-5 were carried out and the cyclic voltammetry curve of CP2 in
acetonitrile solution is shown in Figure 4 (cyclic voltammetry curve of CP1, CP3-5 are given in
supporting information). For all the CSM’s, reduction potential was observed, hence, LUMO of
the CSM’s were calculated using the formula
, where
are
the observed onset reduction potentials verses standard calomel electrode (ionization potential
+
for Fc/Fc measured was 0.43 V vs. Ag/AgCl). The
values obtained for CP1-5 are -0.55, -
0.57, -0.50, -0.59, and -0.55 eV, respectively. The LUMO energy levels for CP1-5 are tabulated
in Table 4. On the other hand, oxidation potential observed only for CP1, CP2 and CP5 (1.77,
1.14 and 1.72 eV, respectively). Since, oxidation potential was not obtained for the CP3 and
CP4, to maintain uniformity the HOMO of the CSM was calculated using the optical band gap
obtained from UV-visible spectra of CP1-5 using the formula
. The energy
levels and the electrochemical band gaps for the CSM’s are tabulated in Table 4.
17

ACCEPTED MANUSCRIPT
Table 4: Electrochemical properties of CP1-5.
Sample
ELUMO (eV) EHOMO (eV) Eg (eV)
-0.55
-0.57
-0.50
-0.59
-0.55
-3.81
-3.78
-3.86
-3.77
-3.81
-7.13
-7.07
-6.93
-6.90
-6.91
3.31
3.28
3.06
3.12
3.10
CP1
CP2
CP3
CP4
CP5
Figure 4: Cyclic voltammetry curve of CP2 in acetonitrile solution.
The sufficiently low HOMO energy levels of the CSM ensures its effective regeneration
and suppress the recapture of the injected electrons by the CSM cation radical [55]. This property
can be exploited in various application of CSM’s. The energy levels of the synthesized CSM’s
are in accordance with that of the standard N719 dye and hence can be used as a co-sensitizer in
preparation of dye sensitized solar cells.
18

ACCEPTED MANUSCRIPT
Conclusion
In precise, we have designed and developed five new cyanopyridone based conjugated
small molecules (CP1-5) featuring various donor and acceptor substituents at position -4 and -6
following appropriate synthetic routes. The reaction conditions were optimized and the
purification techniques for all the new compounds were established. The solvatochromic
behavior of the conjugated small molecules were investigated using seven solvents chosen based
on their increasing dielectric constant and interesting results were procured. The absorption
spectra revealed that all the compound showed a red shift with increasing solvent polarity while,
the emission spectra was not significantly altered by increasing solvent polarity. These
compounds were witnessed to be blue light emitting compounds. The observed Stokes shifts
were correlated with solvent orientation polarizability using the Lippert-Mataga equation but, the
plot were too scattered (r < 0.6) to allow the estimation of difference between ground state and
excited state. Hence, to account for the specific solvent interactions Kamlet-Taft and Catalan
solvent scales were employed and more linearly fit curve with better regression values (r ~ 0.8-
0.9) were observed, wherein, the dipolarity/ polarizability greatly influence the solute-solvent
interactions. Also, the result obtained for CP1-5 indicate that it is possible to manipulate their
fluorescence properties by tuning the substitution, the arrangement of functional groups and by
the choice of appropriate solvents. The band gap of these compounds were calculated using the
data obtained from the photophysical studies. The frontier orbital examined with cyclic
voltammetry revealed that the HOMO and LUMO of the conjugated molecule lie in the range of
-6.90 to -7.13 eV and -3.77 to -3.86 eV, respectively. The sufficiently low HOMO energy level
ensures the effective regeneration of the conjugated small molecules. All these results point out
19

ACCEPTED MANUSCRIPT
that the synthesized cyanopyridone derivatives can be probed for application in the wide field of
optoelectronics.
Acknowledgments
This research was supported by the Science & Engineering Research Board
(SERB) under Young Scientists Scheme (Project File no.: YSS/2014/000835), Govt. of
India, New Delhi. The author acknowledges the Mysore University, Mysore for the
instrumental analysis.
References
[1] K. Namba, A. Osawa, A. Nakayama, A. Mera, F. Tano, Y. Chuman, E. Sakuda, T.
Taketsugu, K. Sakaguchi, N. Kitamura, K. Tanino, Synthesis of yellow and red fluorescent
1,3a,6a-triazapentalenes and the theoretical investigation of their optical properties, Chem. Sci. 6
(2015) 1083-1093.
[2] J. Shinar, V. Savvateev, Introduction to organic light-emitting devices, in: Organic Light-
Emitting Devices, Springer, 2004, pp. 1-41.
[3] B. Valeur, Molecular Fluorescence Wiley, VCH Weinheim, 2002.
[4] D.R. Kanis, M.A. Ratner, T.J. Marks, Design and construction of molecular assemblies with
large second-order optical nonlinearities. Quantum chemical aspects, Chem. Rev. 94 (1994) 195-
242.
[5] M.M.M. Raposo, A.M. Sousa, G. Kirsch, P. Cardoso, M. Belsley, E. de Matos Gomes,
A.M.C. Fonseca, Synthesis and characterization of dicyanovinyl-substituted thienylpyrroles as
new nonlinear optical chromophores, Org. lett. 8 (2006) 3681-3684.
20

ACCEPTED MANUSCRIPT
[6] I. Bylińska, M. Wierzbicka, C. Czaplewski, W. Wiczk, Solvatochromic studies of pull–push
molecules containing dimethylaniline and aromatic hydrocarbon linked by an acetylene unit,
RSC Adv. 4 (2014) 48783-48795.
[7] M. Kivala, F. Diederich, Acetylene-Derived Strong Organic Acceptors for Planar and
Nonplanar Push−Pull Chromophores, Acc. Chem. Res. 42 (2009) 235-248.
[8] F. Artizzu, F. Quochi, A. Serpe, E. Sessini, P. Deplano, Tailoring functionality through
synthetic strategy in heterolanthanide assemblies, Inorg. Chem. Front. 2 (2015) 213-222.
[9] T. Goodson, W. Li, A. Gharavi, L. Yu, Oligophenylenevinylenes for light‐emitting diodes,
Adv. Mater. 9 (1997) 639-643.
[10] O. Ershov, S. Fedoseev, M.Y. Belikov, M.Y. Ievlev, Domino-synthesis and fluorescence
properties of 4-cyano-2-oxo-1, 2-dihydropyridine-3-carboxamides and 2-oxo-1, 2-
dihydropyridine-3, 4-dicarbonitriles, RSC Adv. 5 (2015) 34191-34198.
[11] A. Facchetti, Π-conjugated polymers for organic electronics and photovoltaic cell
applications†, Chem. Mater. 23 (2010) 733-758.
[12] T.-Y. Chu, J. Lu, S. Beaupré, Y. Zhang, J.-R. Pouliot, S. Wakim, J. Zhou, M. Leclerc, Z. Li,
J. Ding, Y. Tao, Bulk Heterojunction Solar Cells Using Thieno[3,4-c]pyrrole-4,6-dione and
Dithieno[3,2-b:2 ,3 -d]silole Copolymer with a Power Conversion Efficiency of 7.3 , J. Am.
Chem. Soc. 133 (2011) 4250-4253.
[13] B. Carlotti, E. Benassi, V. Barone, G. Consiglio, F. Elisei, A. Mazzoli, A. Spalletti, Effect of
the π Bridge and Acceptor on Intramolecular Charge Transfer in Push–Pull Cationic
Chromophores: An Ultrafast Spectroscopic and TD-DFT Computational Study, ChemPhysChem
16 (2015) 1440-1450.
21

ACCEPTED MANUSCRIPT
[14] T. Aotake, H. Tanimoto, H. Hotta, D. Kuzuhara, T. Okujima, H. Uno, H. Yamada, In situ
preparation of highly fluorescent pyrene-dyes from non-luminous precursors upon
photoirradiation, Chem. Commun. 49 (2013) 3661-3663.
[15] S.-W. Yang, A. Elangovan, T.-I. Ho, Donor-substituted phenyl-[small pi]-chromones:
electrochemiluminescence and intriguing electronic properties, Photochem. Photobiol. Sci. 4
(2005) 327-332.
[16] V. Rao, A. KY, Jen, KY Wong and KJ Drost, Tetrahedron Lett. 34 (1993) 1747-1750.
[17] P.R. Varanasi, A.K.-Y. Jen, J. Chandrasekhar, I. Namboothiri, A. Rathna, The important
role of heteroaromatics in the design of efficient second-order nonlinear optical molecules:
theoretical investigation on push-pull heteroaromatic stilbenes, J. Am. Chem. Soc. 118 (1996)
12443-12448.
[18] I.D. Albert, T.J. Marks, M.A. Ratner, Large molecular hyperpolarizabilities. Quantitative
analysis of aromaticity and auxiliary donor-acceptor effects, J. Am. Chem. Soc. 119 (1997)
6575-6582.
[19] A. Hovhannisyan, L. Aristakesyan, R. Hakobyan, G. Melikyan, Synthesis of New 3-
Cyanopyridine-2 (1H)-Ones with Unsaturated Substituents at C-4, Chem. Biol. (2015) 9-13.
[20] E.F. Scriven, R. Murugan, Pyridine and pyridine derivatives, Kirk-Othmer Encyclopedia of
Chemical Technology, (2005).
[21] M. Hagimori, Y. Shigemitsu, R. Murakami, K. Yokota, Y. Nishimura, N. Mizuyama, B.-C.
Wang, C.-K. Tai, S.-L. Wang, T.-L. Shih, K.-D. Wu, Z.-S. Huang, S.-C. Tseng, J.-W. Lu, H.-H.
Wei, J. Nagaoka, T. Mukai, S. Kawashima, K. Kawashima, Y. Tominaga, 2-Pyridone-based
fluorophores containing 4-dialkylamino-phenyl group: Synthesis and fluorescence properties in
solutions and in solid state, Dyes Pigments 124 (2016) 196-202.
22

ACCEPTED MANUSCRIPT
[22] A. TN, Synthesis, Characterization, and Mesomorphic Properties of New Pyridine
Derivatives, ChemistryOpen 4 (2015) 786-791.
[23] X.-L. Zhao, T.C.W. Mak, Silver cages with encapsulated acetylenediide as building blocks
for hydrothermal synthesis of supramolecular complexes with n-cyanopyridine and pyridine-n-
carboxamide (n = 3, 4), Dalton Trans. (2004) 3212-3217.
[24] K. Meena, K. Muthu, V. Meenatchi, M. Rajasekar, G. Bhagavannarayana, S.
Meenakshisundaram, Growth, crystalline perfection, spectral, thermal and theoretical studies on
imidazolium l-tartrate crystals, Spectrochim. Acta Mol. Biomol. Spectrosc. 124 (2014) 663-669.
[25] T. Ahipa, A.V. Adhikari, Synthesis and mesomorphism of new 2-methoxy-3-cyanopyridine
mesogens, in: SPIE OPTO, International Society for Optics and Photonics, 2012, pp. 827915-
827915-827910.
[26] T. Ahipa, V. Kumar, A.V. Adhikari, New columnar liquid crystal materials based on
luminescent 2-methoxy-3-cyanopyridines, Struct. Chem. 25 (2014) 1165-1174.
[27] T.N. Ahipa, P.R. Kamath, V. Kumar, A.V. Adhikari, New luminescent 2-methoxy-6-(4-
methoxy-phenyl)-4-p-tolyl-nicotinonitrile: Synthesis, crystal structure, DFT and photophysical
studies, Spectrochim. Acta Mol. Biomol. Spectrosc. 124 (2014) 230-236.
[28] A. Albert, J.N. Phillips, 264. Ionization constants of heterocyclic substances. Part II.
Hydroxy-derivatives of nitrogenous six-membered ring-compounds, J. Chem. Soc. (1956) 1294-
1304.
[29] O.V. Ershov, S.V. Fedoseev, M.Y. Ievlev, M.Y. Belikov, 2-Pyridone-based fluorophores:
Synthesis and fluorescent properties of pyrrolo[3,4-c]pyridine derivatives, Dyes Pigments 134
(2016) 459-464.
23

ACCEPTED MANUSCRIPT
[30] R.L. Joseph, R. Lakowicz, Principles of fluorescence spectroscopy, in, Kluwer
Academic/Plenum Publishers, New York, 1999.
[31] E. Lippert, Spectroscopic determination of the dipole moment of aromatic compounds in the
first excited singlet state, Z. Elektrochem. 61 (1957) 962-975.
[32] N. Mataga, Y. Kaifu, M. Koizumi, Solvent Effects upon Fluorescence Spectra and the
Dipolemoments of Excited Molecules, Bull. Chem. Soc. Jpn. 29 (1956) 465-470.
[33] E. Perochon, A. Lopez, J. Tocanne, Fluorescence properties of methyl 8-(2-anthroyl)
octanoate, a solvatochromic lipophilic probe, Chem. Phys. Lipids 59 (1991) 17-28.
[34] Y. Lan, M.G. Corradini, X. Liu, T.E. May, F. Borondics, R.G. Weiss, M.A. Rogers,
Comparing and correlating solubility parameters governing the self-assembly of molecular gels
using 1,3:2,4-dibenzylidene sorbitol as the gelator, Langmuir 30 (2014) 14128-14142.
[35] M.J. Kamlet, R.W. Taft, The solvatochromic comparison method. I. The .beta.-scale of
solvent hydrogen-bond acceptor (HBA) basicities, J. Am. Chem. Soc. 98 (1976) 377-383.
[36] R. Taft, M.J. Kamlet, The solvatochromic comparison method. 2. The. alpha.-scale of
solvent hydrogen-bond donor (HBD) acidities, J. Am. Chem. Soc. 98 (1976) 2886-2894.
[37] M.J. Kamlet, J.L. Abboud, R. Taft, The solvatochromic comparison method. 6. The. pi.*
scale of solvent polarities, J. Am. Chem. Soc. 99 (1977) 6027-6038.
[38] M.A. Rauf, A.A. Soliman, M. Khattab, Solvent effect on the spectral properties of Neutral
Red, Chem. Cent. J. 2 (2008) 19.
[39] J.B. Prieto, F.L. Arbeloa, Martı, amp, x, V.M. nez, amp, x, nez, T.A. López, F. Amat-
Guerri, M. Liras, I.L. Arbeloa, Photophysical properties of a new 8-phenyl analogue of the laser
dye PM567 in different solvents: internal conversion mechanisms, Chem. Phys. Lett. 385 (2004)
29-35.
24

ACCEPTED MANUSCRIPT
[40] M.J. Sawicka, J.A. Soroka, M. Gąsiorowska, E.K. Wróblewska, The Spectroscopic
Behavior of Two New 5-(5-R-6-Hydroxynaphthyl-2)-7,7-dimethyl-7H-indolo[1,2-a]quinolinium
Merocyanines in Various Solvents, J. Solution Chem. 41 (2012) 25-35.
[41] A. Abbotto, L. Beverina, R. Bozio, S. Bradamante, C. Ferrante, G.A. Pagani, R. Signorini,
Push–Pull Organic Chromophores for Frequency-Upconverted Lasing, Adv. Mater. 12 (2000)
1963-1967.
[42] D.F. Eaton, Reference materials for fluorescence measurement, Pure Appl. Chem. 60 (1988)
1107-1114.
[43] H.J. Yvon, A guide to recording Fluorescence Quantum Yields, HORIBA, Jobin Yvon Ltd.,
Stanmore, Middlesex, UK, (2012).
[44] J. Drobnik, E. Yeargers, On the use of quinine sulfate as a fluorescence standard, J. Mol.
Spectrosc. 19 (1966) 454-455.
[45] R.P. Sabatini, T.M. McCormick, T. Lazarides, K.C. Wilson, R. Eisenberg, D.W.
McCamant, Intersystem Crossing in Halogenated Bodipy Chromophores Used for Solar
Hydrogen Production, J. Phys. Chem. Lett. 2 (2011) 223-227.
[46] J.A. Mikroyannidis, P. Suresh, M.S. Roy, G.D. Sharma, New photosensitizer with
phenylenebisthiophene central unit and cyanovinylene 4-nitrophenyl terminal units for dye-
sensitized solar cells, Electrochim. Acta 56 (2011) 5616-5623.
[47] A. Yella, R. Humphry-Baker, B.F.E. Curchod, N. Ashari Astani, J. Teuscher, L.E. Polander,
S. Mathew, J.-E. Moser, I. Tavernelli, U. Rothlisberger, M. Grätzel, M.K. Nazeeruddin, J. Frey,
Molecular Engineering of a Fluorene Donor for Dye-Sensitized Solar Cells, Chem. Mater. 25
(2013) 2733-2739.
25

ACCEPTED MANUSCRIPT
[48] X. Zhang, H. Yu, Y. Xiao, Replacing Phenyl Ring with Thiophene: An Approach to Longer
Wavelength Aza-dipyrromethene Boron Difluoride (Aza-BODIPY) Dyes, J. Org. Chem. 77
(2012) 669-673.
[49] S. Fery-Forgues, J.-P. Fayet, A. Lopez, Drastic changes in the fluorescence properties of
NBD probes with the polarity of the medium: involvement of a TICT state?, J. Photochem.
Photobiol. A Chem. 70 (1993) 229-243.
[50] A.T. N, A.V. Adhikari, New cyanopyridone based luminescent liquid crystalline materials:
synthesis and characterization, Photochem. Photobiol. Sci. 13 (2014) 1496-1508.
[51] J. Catalán, Toward a generalized treatment of the solvent effect based on four empirical
scales: dipolarity (SdP, a new scale), polarizability (SP), acidity (SA), and basicity (SB) of the
medium, J. Phys. Chem. B 113 (2009) 5951-5960.
[52] Solvatochromic Behaviour of Formazans and Contribution of, Chem. Sci. Trans. 3 (2014)
919-928.
[53] M. Rauf, S. Hisaindee, Studies on solvatochromic behavior of dyes using spectral
techniques, J. Mol. Struct. 1042 (2013) 45-56.
[54] H. Singh, J. Sindhu, J.M. Khurana, Determination of dipole moment, solvatochromic studies
and application as turn off fluorescence chemosensor of new 3-(4-(dimethylamino)phenyl)-1-(5-
methyl-1-(naphthalen-1-yl)-1H-1,2,3-triazol-4-yl)prop-2-en-1-one, Sens. Actuator B-Chem. 192
(2014) 536-542.
[55] K.R. Justin Thomas, Y.-C. Hsu, J.T. Lin, K.-M. Lee, K.-C. Ho, C.-H. Lai, Y.-M. Cheng, P.-
T. Chou, 2,3-Disubstituted Thiophene-Based Organic Dyes for Solar Cells, Chem. Mater. 20
(2008) 1830-1840.
26

ACCEPTED MANUSCRIPT
Graphical Abstract
27

ACCEPTED MANUSCRIPT
Highlights
 Versatile conjugated small molecules bearing cyanopyridone core were synthesized.
 Solvatochromic studies were conducted in seven organic solvents.
 Lippert-Mataga, Kamlet-Taft and Catalan solvent scales were used for determination of
solute-solvent interactions.
 Synthesized conjugated small molecules possesses low lying HOMO energy levels.
28