Synthesis and photophysical investigations of C3-triazine based star-like conjugated molecules

Article abstract of DOI:10.1016/j.jphotochem.2018.04.016

Here, we reported two different C₃-triazine based star-like D-π-A compounds with a very good yield by Wittig-Horner reaction. Photophysical, aggregation enhanced emission and electrochemical properties were systematically studied. These compoun

Full text of DOI:10.1016/j.jphotochem.2018.04.016

Accepted Manuscript
Title: Synthesis and Photophysical Investigations of
C -Triazine Based Star-like Conjugated Molecules
3
Authors: Kumaraguru Duraimurugan, Vaithialingam
Dhanamoorthy, Jagannathan Madhavan, Ayyanar Siva
PII:
S1010-6030(18)30186-2
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2018.04.016
JPC 11231
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
9-2-2018
7-4-2018
8-4-2018
Please cite this article as: Kumaraguru Duraimurugan, Vaithialingam Dhanamoorthy,
Jagannathan Madhavan, Ayyanar Siva, Synthesis and Photophysical Investigations of
C3-Triazine Based Star-like Conjugated Molecules, Journal of Photochemistry and
Photobiology A: Chemistry https://doi.org/10.1016/j.jphotochem.2018.04.016
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Synthesis and Photophysical Investigations of C
3
-Triazine Based
Star-like Conjugated Molecules
a,b
a
b
Kumaraguru Duraimurugan , Vaithialingam Dhanamoorthy , Jagannathan Madhavan and
Ayyanar Siva a*
a*School of Chemistry, Madurai Kamaraj University, Madurai - 625 021, Tamilnadu, India.
b
Solar Energy Lab, Department of Chemistry, Thiruvalluvar University, Vellore, Tamilnadu,
India.
Corresponding author mail id: drasiva@gmail.com
Graphical abstract:
Highlights

C
3
-triazine based star-like D-π-A compounds (C8TR and C8PVT) with a very good
yield were synthesized.



Photophysical, aggregation enhanced emission and electrochemical behaviour was
studied.
These two compounds showed a large halochromic effects upon addition of acid and
base.
Abstract: Here, we reported two different C -triazine based star-like D-π-A compounds with
3
a very good yield by Wittig-Horner reaction. Photophysical, aggregation enhanced emission
and electrochemical properties were systematically studied. These compounds showed a large
halochromic effect on its emission color that changes from greenish-yellow to wine red upon
1
addition of trifluoroacetic acid. Further, the spectral changes are confirmed by H-NMR
studies.
Keywords: C
3
-triazine derivatives, Wittig-Horner, Photophysical properties, Aggregation
enhanced emission and Halochromic.
1
. Introduction
Organic smart molecules with donor-π-acceptor (D-π-A) system have attracted much
attention because of their potential applications in photonic and optoelectronics [1].
Generally, a fully conjugated architecture with an alternate double and triple bonds along a
chain have these kinds of properties. Many devices have been developed based on these type
of compounds. In particular, poly (p-phenylenevinylenes) linker is most widely used in light-
emitting/light-sensitive devices to increase their electro-optical properties depending on its
chain length and substituents [2]. D-π-A or D-π-A-π-D molecules exhibit fluorescence
properties with an intramolecular charge transfer (ICT) and act as chromophores [3]. The

electron-rich nature of the donor moiety increases the energy of the highest occupied
molecular orbital (HOMO), while the electron-poor nature of the acceptor concurrently
lowers the lowest unoccupied molecular orbital (LUMO). This leads to a very narrow energy
gap for small chromophores [4]. Therefore, the N-containing aromatic ring is an excellent
candidate to be incorporated in such constructions. In addition, this heterocycle is reported to
have a high electron-withdrawing character, significant aromaticity that leads to highly
conjugated molecules with basic and potential luminescent properties. These properties help
in modulating the optoelectronic properties of molecules as reported in earlier studies [5].
During the past, an intensive research has been contributed towards the synthesis of
N-based heterocycles such as pyrimidine, pyridazine and pyrazine heterocycles with π-
conjugated materials [6]. In specific, various shape based structures with electron-
withdrawing group display intense emission, halochromic behavior and optical properties [7].
Among these, triazine has a highly π-deficient character and an easy functionalizing 1,3,5-
positions, which makes it an appropriate electron withdrawing heteroaromatic for the
construction of D-π-A architectures [8]. In this context, a variety of donor-substituted triazine
derivatives, in which the electron-donating unit is linked either directly or via a π-spacer,
have been synthesized by several research groups [9]. Likewise, another study on
triarylamine and 1,3,5-triazine-based TPA materials synthesized along with their electrical
and optical properties were also reported [10]. Raquel et al., have reported on the synthesis of
C
3
-symmetrical system 2,4,6-tris (triazolyl)-1,3,5- triazine for n-type organic semiconductors
[
8]. Several reports on 1,3,5-triazine based conjugated molecules exhibits increased nonlinear
optical and two-photon absorption properties depending on the intramolecular charge transfer
properties [11]. Very recently, s-triazine-based conjugated molecules showed excellent
columnar mesomorphism and n-type semiconductor applications were reported.[12].

Inspired by the available literature, we have attempted the following objectives in the
present study. (i) to synthesize two different C -triazine based D-π-A compounds with
3
conjugation and an extended π-conjugation along with tris-alkoxy at the end of periphery, (ii)
to test the two compounds for photophysical, aggregation enhanced emission, halochromic
and electrochemical behaviors, and finally; (iii) to characterize all the final and intermediate
1
13
compounds by H-NMR, C-NMR, MALDI-TOF mass and FT-IR spectroscopy.
2
2
. Experimental section
.1. Materials and methods
Methyl gallate, lithium aluminium hydride (LAH), methyltriphenylphosphonium
bromide, pyridinium chlorochromate (PCC), tetrabutylammonium bromide (TBAB), N-
bromosuccinimide (NBS), 4-bromobenzaldehyde, p-tolunitrile, triethylphosphite, potassium
t
tert-butoxide (K OBu) and palladium(II) acetate were acquired from Sigma-Aldrich.
1
-bromooctane, trifluoromethanesulfonic acid, trifluroacetic acid and triethylamine were
purchased from Avra. Solvents used for chromatography were distilled before use.
2
.2. Characterization
Bruker Avance (300 MHz) was used to record H-NMR and 13C-NMR spectra.
1
Deuterated solvent was obtained from Aldrich chemicals. Chemical shifts (δ) is indicated in
ppm relative to residual solvent peaks or TMS, and the coupling constants are reported in Hz.
For TLC analysis, silica gel-G plates (Merck) were used with a mixture of n-hexane and ethyl
acetate as an eluent. Silica gel (60-120 mesh) was used for column chromatography using
hexane and ethyl acetate acting as an eluent. JASCO V-630 spectroscopy was used to
measure absorbance, Agilent 8000 was employed to measure emission spectra and JASCO
FT/IR-410 spectrometer was used to record IR stretchings. The quantum yield (Φ ) of C8TR
F

and C8PVT were calculated using quinine sulfate as standard (Φ
F
= 0.54) in 0.1 M H
2
SO .
4
The different water fractions of THF/water mixtures were prepared by slowly adding distilled
water into THF solution of different aliquot and ultra-sonicated at room temperature. CHI
6
80 electrochemical workstation was used to measure CV at room temperature with a scan
rate of 100 mV/s. Scanning electron microscopy image of C8TR and C8PVT were studied
by VEGA 3 TESCAN instrument at 15 Kv.
2
.3. Preparation of (E)-4-(tris(octyloxy)styryl)benzaldehyde (6)
4
-Bromo benzaldehyde (0.68 g, 3.6 mmol) and styrene 5 [13] (1.5 g, 3.06 mmol) was
taken in mixture of toluene/DMF (1:1) (20 mL) and nitrogen gas was purged for 15 mins, to
this TBAB (0.98 g, 3.06 mmol) and triethylamine (1.23 mL) were added one by one. Finally,
°
1
0 mol % of palladium acetate was added. The reaction mass was agitated at 80 - 90 C and
maintained for 12 hrs. Then the reaction was monitored by TLC. The completed reaction
mixture was poured into methanol and the solid was separated. The separated compound was
washed with methanol and then dried. The crude product was purified by column
1
chromatography. The yield is 1.7 g (93 %). H-NMR (300 MHz, CDCl
3
): δ = 9.98 (s, 1H),
7
.88-7.85 (d, J=9Hz, 2H), 7.65-7.62 (d, J=9Hz, 2H), 7.19-7.14 (d, J=15Hz, 1H), 7.03-6.95 (d,
J=24Hz, 1H), 6.74 (s, 2H), 4.05-3.96 (m, 6H), 1.8-1.78 (m, 6H), 1.49-1.47 (m, 6H), 1.29(m,
1
3
3
1
0H), 0.90-0.86 (t, J=12Hz, 9H); C NMR (75 MHz, CDCl ) δ = 191.8, 153.9, 143.9, 133.1,
3
31.9, 130.5, 127.0, 126.6, 106.0, 74.0, 69.9, 32.2, 29.7, 29.6, 26.5, 23.0, 14.4.
2
.4. Preparation of C8TR (11)
Phosphonate salt (0.25 g, 0.32 mmol) and compound 4 (0.52 g, 1.05 mmol) were
t
taken in dry THF under a nitrogen atmosphere; K OBu (0.23 g, 1.92 mmol) was added all at
once. Then the reaction mixture was stirred at room temperature. After the completion of the
reaction, the mass was quenched by 1.5 N HCl solution. The mass was extracted with DCM

and washed with NaCl solution and concentrated it. The crude product was purified by
column chromatography using 10% ethyl acetate in hexane. The yield is 0.59 g (96%).
1
Yellow sticky solid was obtained. H-NMR (300 MHz, CDCl
3
); δ = 8.78-8.75 (d, J=9Hz,
6
H), 7.71-7.68 (d, J=9Hz, 6H), 7.22-7.17 (d, J=15Hz, 3H), 7.11-7.06 (d, J=15Hz, 3H), 6.78
(s, 6H), 4.08-3.98 (m, 18H), 1.90-1.73 (m, 18H), 1.51-1.49 (m, 18H), 1.33-1.25 (m, 90H),
1
3
0
1
3
1
.90-0.88 (t, J=6Hz, 27H); C NMR (75 MHz, CDCl
3
) δ = 171.5, 153.7, 142.0, 139.1, 135.5,
32.6, 131.4, 129.7, 127.3, 126.9, 124.8, 124.2, 116.4, 116.2, 74.2, 69.6, 32.3, 32.2, 31.8,
0.7, 30.5, 29.9, 29.8, 29.7, 26.5, 23.1, 14.5; MALDI-TOF mass for C117
769.3518, found: 1769.4460.
H
177
N
3
9
O (m/z) is
2
.5. Preparation of C8PVT (12)
The procedure was followed same as above, yield is 0.87 g (94%). Yellow fluffy solid
1
was obtained. H-NMR (300 MHz, CDCl
3
): δ = 8.80-8.78 (d, J=6Hz, 6H), 7.75-7.72 (d,
J=9Hz, 6H), 7.60-7.52(d, J=24Hz, 12H), 7.38-7.36 (d, J=6Hz, 3H), 7.27-7.23 (d, J=12Hz,
3
1
2
1
2
H), 7.18-7.14 (d, J=12Hz, 3H), 7.12-7.04 (d, J=24Hz, 6H), 7.02 (s, 6H), 4.06-3.96 (m,
8H), 2.08-1.81 (m, 18H), 1.50-1.48 (m, 18H), 1.33-1.30 (m, 90H), 0.90-0.86 (t, J=12Hz,
1
3
7H). C-NMR (75 MHz, CDCl
3
)
δ = 171.2, 153.7, 153.6, 141.8, 138.7, 137.6, 136.4,
35.8, 133.0, 127.5, 127.1, 118.3, 115.2, 105.5, 74.4, 69.5, 32.3, 32.2, 30.7, 30.0, 29.8, 29.7,
6.5, 23.1, 14.5; MALDI-TOF mass for C141
H N
195 3
9
O (m/z) is 2076.1230, found: 2077.6255.
3
3
. Results and discussion
.1. Synthesis
The synthetic way to the preparation of aldehydes (4 and 6) is shown in scheme 1.
Methyl gallate reacts with 1-bromooctane to give the product 2 with a very good yield.
Further, the ester gets reduced with LAH to give compound 3. The compound 3 was oxidized
by PCC under the inert atmosphere to give aldehyde 4. The compound 4 is confirmed by the

-1
appearance of the carbonyl stretching frequency at 1695 cm in FT-IR. Further, the aldehyde
1
is treated with Wittig salt to give the styrene 5. In the H-NMR spectrum, alkene proton
shows doublet of doublet at 5.58 and 5.18 ppm. On further treatment of compound 5 [13]
with 4-bromo benzaldehyde in the presence of palladium acetate (Heck reaction) give rises to
product 6. The formation of compound 6 was confirmed by the appearance of υC=O stretching
-1
frequency at 1731 cm . Further, triazine precursor was synthesized, according to previously
°
reported earlier [14]. This intermediate was heated with triethyl phosphite at 130 C for 12 hrs
to give phosphonate salt as a colorless solid with a good yield (Scheme. 2).
°
Reagent and conditions: (a). 1-bromo octane, K
2
CO
3
, DMF, rt, (b). LAH, THF, 0 C, rt, (c).
°
t
°
PCC, DCM, 0 C, rt, (d). Wittig salt, K OBu, DCM, 0 C, rt, (e). Pd(OAc)
2
, TBAB,
°
triethylamine, toluene/DMF, 90 C, 12 hrs.
Scheme 1. Synthetic way of intermediates 4 and 6.

°
Reagent and conditions: (a) Tf
2
O, CHCl
3
, 0 C, rt, (b) NBS, BPO, benzene, reflux, (c) triethyl
°
t
°
phosphite, 120 C, 12 hrs, (d) 4 or 6, K OBu, DCM, 0 C, rt.
Scheme 2. Synthesis of intermediate 10 and target molecules 11 and 12.
3
.2. Photophysical properties
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
a
b
C8TR solution
C8TR film
C8PVT solution
C8PVT film
C8TR solution
C8TR film
C8PVT solution
C8PVT film
300
350
400
450
500
550
450
500
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
Fig. 1. Absorption (a) and emission spectra (b) of C8TR and C8PVT in solution (THF) and
thin film.

Table 1. Absorption, emission and quantum yield values of C8TR and C8PVT in solution
(THF) and thin film.
Solution
Thin film
Φ
F
Stoke’s
shift (cm )
Compounds
-1
λ
abs(nm)
λemi(nm)
λ
abs(nm)
λemi(nm)
(solution)
0.37
(solution)
C8TR
376
396
532
563
380
400
537
568
7799
7490
C8PVT
0.43
The absorption and emission spectra of C8TR and C8PVT were measured in solution
(THF) and thin film. The absorption spectra show a major absorption band near 375-400 nm,
which corresponds to intramolecular charge transfer transition between donor and acceptor
moiety (Fig. 1a). The emission spectra were also obtained by exciting C8TR and C8PVT
compounds at its absorption maximum and their corresponding emission values are tabulated
as 532, 563 nm in solution and 537, 568 nm in thin film respectively (Table 1). The
absorption and emission value of both the compounds in the solid state are red shifted in
comparison with that in solution due to π-π stacking interaction [15,16]. The solvent effect
investigation on the photophysical behavior of these compounds was measured to their
absorption and emission spectra in different solvents varying from non-polar to polar aprotic
solvents such as hexane, benzene, chloroform and ethyl acetate in a sequence of their
dielectric constant (Figure. 1-2, ESI).
The absorption spectra of C8TR and C8PVT show negligible solvatochromic
changes. (Table. 1, ESI) Whereas, the wavelength in the emission spectra of C8TR seems to
be increased showing large Stokes shifts with increasing the solvent polarity. This behavior
indicates that the intramolecular charge transfer (ICT) is greater in the excited state than the
ground state. Meanwhile, the emission value of C8PVT shows a bathochromic shift

compared to C8TR in the same solvent (around 510 to 530 nm) which indicates the extended
conjugation of a donor-acceptor moiety. Further, the increased emission wavelength was
observed from non-polar to polar aprotic solvents, This might be due to charge separation
upon excitation, which is confirmed by Dimroth-Reichardt solvent polarity correlations [17]
(Figure 3, ESI). The quantum yield value of C8TR and C8PVT are measured in THF
solution with respect to quinine sulfate as standard, which has quantum values of 0.54 in
0
.1M sulfuric acid solution (Table 1).
3
.3. Aggregation enhanced emission
C8TR
C8TR
1
60
140
20
0%
0
.10
0%
10%
1
0%
a
20%
30%
40%
2
3
0%
0%
b
0.08
0.06
0.04
0.02
0.00
40%
5
0%
1
50%
60%
70%
60%
70%
100
80
0
.04
0
3
7
%
0%
0%
0.02
60
4
2
0
0
0
0.00
4
50
Wavelength(nm)
300
350
400
450
500
550
400
450
500
550
600
650
700
Wavelength(nm)
Wavelength (nm)
Fig. 2. (a) Absorption (Insect picture shows the differential spectra of C8TR in THF and
increasing water fraction) and emission spectra (b) of C8TR in THF: H
picture shows C8TR in THF and at 70 % water fraction).
2
O mixtures. (Insect
To study the Aggregation properties of the compounds, the spectral behavior of the
diluted THF-water mixtures with different water fractions were examined. Both the
molecules were found to be soluble in THF and insoluble in H
2
O. On increasing the
percentage of H O, the existing form of C8TR in pure THF has changed to aggregated form.
2
This aggregated form is believed to enhance the emission behavior due to the restriction of
intramolecular rotation. The absorption result shows a gradual rise in the intensity and moved

to longer wavelength region. A level of a tail in the visible region was observed, which may
due to the formation of nanoparticle suspensions in the aggregated state. The change in
absorbance with increasing water fraction as shown in Fig.2a (insect spectra). The redshift in
the absorption is due to the weak π-π stacking interaction of the solute molecule in its
aggregated state and attributed to the J-aggregation formation [18-19].
The emission spectra of C8TR are found to exhibit very weak emission in pure THF
solution, whereas on increasing water fraction from 0 to 50 %, the emission intensity is
gradually increased. On further increase of water fraction to 60 %, an enhanced twofold
intensity is achieved. The plot of [(I/Io)-1] Vs f
with and without water in the THF-water mixtures) displayed the emitting change process of
C8TR in the THF- H O mixed solvent (Figure 5, ESI). This enhanced emission intensity
w
, (where I and Io are the emission intensities
2
could be due to the AEE effect caused by the formation of molecular aggregates in the
presence of a large amount of non-solvent. This concept is explained by the enhanced
radiative transition led by the limitation of the rotation of the molecules at the aggregation
state (Fig.2b). The formation of nano-aggregates was confirmed by Scanning Electron
Microscopy (Figure 6, ESI).
On the other hand, the emission spectra of C8PVT differ slightly from C8TR. On
increasing the water content from 0 to 20 %, there is a slight increase in the emission
intensity. On further increase up to 40-60 %, the emission gets red shifted by 25 nm with
two-fold enhancements. Upon increasing the water fraction, the emission wavelength showed
a remarkable blue shift to 575 nm [20] (Figure 4, ESI), a further increase of the water fraction
resulted in a decreased emission intensity. Two reasons are commonly cited for this kind of
spectral behavior. Firstly, a restricted the rotation of intramolecular aromatic rings at the
aggregation state which could have to enhance the light emission. Second, the aggregation of
the solute molecule as nanoparticle suspensions might have led to a reduced intensity [21].

This behavior indicates that AEE properties are mainly based on intramolecular rotation in
solution can minimize the emission efficiency. But in the aggregated state, such rotations
effectively can be restricted that would lead to blue shifted emission with distorted
conformation [11c-d, 13].
3
.4. Trifluoroacetic acid titration
120
C8TR
C8TR
0
0
0
0
.6
.4
.2
.0
TFA
b
-6
a
-6
100
80
10 M
TFA
10
M
-5.5
TFA
1
1
1
1
1
0
0
0
0
0
M
-5
TEA
M
-1
0 M
-4.5
1
M
-4
M
-3.5
M
-3
60
max= 485nm
10 M
-2.5
10
M
-2
10
M
-
1.5
40
10
M
1
-
10 M
20
0
3
00
350
400
450
500
550
600
650
4
50
500
550
600
650
700
Wavelength(nm)
Wavelength (nm)
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
C8TR
Without TFA
With TFA
5
58 nm 589 nm
c
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 3. Absorption (a) and emission spectra (b) of C8TR in DCM upon addition of TFA at rt.
c) Normalised emission spectra of C8TR with and without TFA addition in DCM at
(
excitation wavelength of 385 nm (Insect picture shows reversible color change of C8TR
towards addition of TFA and TEA).
The nitrogen atoms present in C8TR and C8PVT are basic in nature that can be
easily protonated. Thus, the effect of protonation on C8TR and C8PVT in DCM solutions
was studied [7, 15a]. The DCM solution of these compounds underwent a significant color

-1
change in the presence of trifluoroacetic acid (TFA) (10 M). This color change was fully
reversible by neutralization with a base like triethylamine (TEA) or potassium tert-butoxide
t
(
K OBu).
The spectral change in the absorption and emission of C8TR upon addition of TFA
acid is illustrated in Fig. 3. The absorption spectra were recorded with increasing amount of
trifluoroacetic acid with C8TR in DCM solution. On observation, new and more intense red-
shifted bands are observed at around 485 nm. This may be due to the presence of protonated
species. The bathochromic shifts of the absorption can be explained by an increased charge
-1
transfer from the donor to the triazine moiety. For increasing additions of TFA up to 10 M,
we observed an increase in the absorbance band at 485 nm with a concomitant decrease in the
absorbance band at 376 nm. The spectral shift between the absorbance maximum of C8TR
before and after addition of TFA is 110 nm. Such a remarkable shift could be readily visible
by naked-eye (wine red) at ambient temperature (Fig. 3a) [15a,22]. The observation of
isosbestic points (325 and 420 nm) reveals that the presence of the neutral and protonated
forms are in equilibrium and is more predominant at higher concentrations of TFA. Based on
the above results, it is inferred that the compound exhibits acid-base equilibria [23].
In the emission spectra, the fluorescence spectral changes of C8TR upon the addition
of TFA are shown in Fig. 3b. It was clearly indicated that the maximum emission band at 558
nm in DCM solution. The fluorescence intensity was gradually decreased and quenched by
various additions of TFA with a slight red shift. A further increase in TFA, the solution emits
a wine red fluorescence with new red-shifted peak appeared at ca. 590 nm [Table 2]. The
yellow emission of C8TR could be quenched significantly by TFA. Therefore, C8TR could
be used as a probe to detect TFA by the naked eye. The spectral change in optical properties
induced by TFA could be recovered by the addition of triethylamine (TEA) (Fig. 4). A
similar spectral change also observed for C8PVT too (Figure 7, ESI).

We infer that the spectral behavior exhibited in C8TR and C8PVT aroused from
predominant stepwise protonation of the molecules, because of the π-electron donation from
the tris-alkoxy peripheral groups. It is possible that the triazine nitrogen is protonated first,
causing the red shifts in the absorption and emission. Protonation of the acceptor triazine will
greatly enhance its electron withdrawing ability and forming a strong charge-transfer system
which leads to red-shifted emission. The possible explanation behind the decreased emission
might be due to the twisting of the aromatic system that would lead to non-emissive TICT
state which is confirmed by the formation of locally excited state (LE) at 444 nm in C8TR
and 470 nm in C8PVT indicating twisted conformation that would interrupt charge transfer
process [6-7, 22-25] (Fig. 3c.).
-1
Table 2. Summary of absorption and emission value changes after addition of TFA (10 M).
Abs λmax
Emi λmax
Compd
∆ λabs
∆ λemi
(nm)
(nm)
a
b
C8TR
376 (485 )
109
558 (589 )
31
396 (520a)
124
590 (620b)
30
C8PVT
aAbsorption value changes after addition of TFA
b
Emission value changes after addition of TFA

Fig. 4. Protonation and deprotonation of C8TR in the addition of TFA and triethylamine.
3
.5. NMR studies
1
To validate the above hypothesis, we also carried out H-NMR titration of C8TR with
1
TFA. H-NMR spectral changes of C8TR with the various addition of TFA was monitored in
CDCl at room temperature. C8TR shows a sharp doublet peak at 8.781 and 7.711 ppm
corresponding to aromatic proton adjacent to triazine as indicated (a and b) in Fig. 5
3
-1
respectively. After 20 µL addition of TFA (10 M), there is a slight upfield shift from 8.781
to 8.689 ppm and 7.711 ppm that has slightly moved to 7.740 ppm downfield shift is
observed [15a]. On further addition of TFA, we observed upfield shift from 8.689 to 8.596
and downfield from 7.740 to 7.765 ppm and a doublet of doublet peak at 7.112 ppm which
corresponds to -CH=CH- moving towards the downfield at 7.251 ppm and multiplets peak at
4
.080 ppm become well resolved and simplified with addition of TFA which indicates that
the aromatic system gets twisted. A similar result also was observed for C8PVT too (Figure
, ESI).
8

1
Fig. 5. H-NMR spectra of C8TR in CDCl (blue) and 20 µL (black) and 40 µL (red)
3
-1
addition of TFA (10 M).
3
.6. Electrochemical studies
Cyclic voltammetry was used to investigate the electrochemical properties of the
C8TR and C8PVT and to estimate their HOMO and LUMO energy levels in 0.1 M solution
of (Bu N)PF in DCM using Ag/AgCl as a reference electrode (Figure 8, ESI). Both the
4
6
molecules showed a similar trend of the irreversible oxidation process. Electrochemical band
gap (Eg) of the compounds was calculated from the difference between EHOMO and ELUMO.
The HOMO levels of molecules are calculated as - 4.99 and - 5.18 eV with the onset
oxidation potential starting at 0.232 V for C8TR and 0.424 V for C8PVT. Likewise, the
band gap of the compounds is estimated as 2.64 and 2.78 eV respectively. We have observed
the increased LUMO energy values from 2.34 to 2.39 eV of C8TR to C8PVT [12]. This
might be due to the existence of extended conjugation present in C8PVT. From the band gap
and HOMO-LUMO values, it can be inferred that the synthesized C8TR and C8PVT could
be effectively used as green emitters in fluorescent OLED applications.

4
. Conclusion
We have synthesized C -triazine based donor-acceptor functionalized π-conjugated
3
fluorescent molecules with extended conjugation by Wittig-Horner reaction. These two
molecules showed a significant photophysical and aggregation enhanced emission properties
and also compounds show an intense and reversible color change upon addition of TFA as a
result of the protonation of the nitrogen in the triazine ring. These finding reveals that a
design of the molecules with the heteroaromatic ring as an acceptor in D-π-A system and
understanding their optical properties could aid in the development of promising colorimetric
and pH sensor applications.
Acknowledgments
The authors acknowledge the financial support of the Department of Science and
Technology, SERB, Extramural Major Research Project (Grant No. EMR/2015/000969),
Council of Scientific and Industrial Research (CSIR), HRDG, No. 01(2901)/17/EMR-II, and
0
1(2540)/11/EMR-II), New Delhi, Department of Science and Technology
DST/TM/CERI/C130(G), New Delhi, India.
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