Photophysical and thermal properties of novel solid state fluorescent benzoxazole based styryl dyes from a DFT study

Article abstract of DOI:10.1039/c4ra12908c

Novel benzoxazole styryl dyes 6a-6e with donor(D)-π(pi)-acceptor(A) were synthesized and characterized. The spectral (absorption and emission) and thermogravimetric properties of the dyes were investigated. They were found to possess red-shifted absorption with a high molar extinction coefficient when compared to their reported analogues. The compounds exhibited enhanced fluorescence emission in the solid state (540-603 nm). They showed excellent thermal and photochemical stability. The effect of the benzoxazole moiety on the photophysical properties is explained by comparing with reported analogues and density functional theory (DFT) calculations. Thus investigations of photophysical properties provide an important foundation for the molecular design and development of novel optoelectronic materials for OLED and NLO applications.

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Paper
Photophysical and thermal properties of novel solid state fluorescent
benzoxazole based styryl dyes with DFT study
a
a
a
Urmiladevi Narad Yadav , Haribhau Shantaram Kumbhar , Saurabh Satish Deshpande , Suban Kumar
b
a,
Sahoo , Ganapati Subray Shankarling *
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Novel benzoxazole styryl dyes 6aꢀ6e with donor(D)–π(pi)–acceptor(A) were synthesized and
characterized. The spectral (absorption and emission) and thermogravimetric properties of the
dyes were investigated. They were found to possess redꢀshifted absorption with high molar
extinction coefficient when compared to their reported analogues. Compounds exhibited enhanced
fluorescence emission in solid state (540ꢀ603 nm). They showed excellent thermal and
photochemical stability. The effect of benzoxazole moiety on photophysical properties is
explained by comparing with reported analogues and density functional theory (DFT)
calculations. Thus investigations of photophysical properties provide important foundation for the
molecular design and development of novel optoelectronic materials for OLED and NLO applications.
1
0
1
5
Introduction
study of the photophysical properties of synthesized dyes with
Fluorescent styryl dyes have attracted researchers all over the
globe for their applications in potentially interesting fields such
as organic lightꢀemitting diodes (OLEDs) , solar cells
5
0
other reported analogues highlighted the superiority in optical
properties. One of the added advantages of the system is the
combination of a weak donor (benzoxazole) and a very strong
acceptor (CN) end groups, which makes this type of compound
1
2
,
3
4
2
2
3
3
4
4
0
5
0
5
0
5
fluorescent viscosity sensors , molecular probes , nonꢀlinear
5
6
7
optics , laser dyes , fluorescent thermometer , etc. They have
great commercial demand since they are easily tailorꢀmade
depending on the requirements. Styryl dyes based on benzoxazole
have been exploited for their excellent luminescent property.
They have been known to exhibit strong photoluminescence (PL)
suitable for nonꢀlinear optical applications owing to nonꢀlinear
19
55
transparency
.
8
, 9
in the blue region . Styryl dyes with benzoxazole as acceptor
have been used as emitter material in organic electroluminescent
1
0
(
EL) devices . Intramolecular exciplexes based on benzoxazole
11
have been applied as fluorescent sensors for cations
.
Derivatives of 2ꢀhydroxy phenyl benzoxazole form borate
12
complexes (BODIPY) which are highly luminescent and useful
1
3
in monitoring of polymerization process . Benzoxazole is
widely employed as a key structure in many fields, i.e. ESIPT
1
4
15, 16
17
dyes ,
or metal cation indicators .
6
0
The reason for enhanced fluorescence on introduction of
benzoxazolyl group in stilbene can be attributed to the decreased
nonꢀradiative processes due to the higher resistance to rotation
offered by the bulky benzoxazolyl molecule while radiative
process suffer competition from wellꢀknown transꢀcis
18
isomerisation . Considering the effects of benzoxazole moiety
on enhanced fluorescence of stilbenes, novel styryl dyes
containing benzoxazole were synthesized (Scheme 1) and their
photophysical properties both in the solution as well as in solid
states were investigated. The effect of solvent polarity on the
absorption and emission spectra were evaluated. Thermal stability
of these synthesized dyes was also explored. The comparative
65
Scheme 1 Synthetic procedure for synthesis of benzoxazole
based styryl dyes 6a-6e
.
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DOI: 10.1039/C4RA12908C
Experimental Section
column chromatography using silica gel 100ꢀ200 mesh and
hexane:ethylacetate (9:1) as eluent system. Yield: 62%. FTꢀIR
6
6
7
7
8
8
9
9
0
5
0
5
0
5
0
5
Materials and methods
ꢀ1
1
(KBr, cm ): 2953 (CꢀH, Ar), 2217 (CN), 1560 (C=C). HꢀNMR
(
DMSOꢀd , 400 MHz): δ=1.03 (s, 6H, ꢀ2CH ), 2.58 (s, 2H, ꢀ
6
3
All common reagent grade chemicals were procured from SD
Fine Chemical Ltd. (Mumbai, India) and were used without
further purification. The reactions were monitored by thinꢀlayered
chromatography (TLC) using 0.25 mm EꢀMerck silica gel 60 F254
CH ), 2.64 (s, 2H, ꢀCH ), 6.98 (s, 1H, vinylic ꢀH), 7.38 (d, J =
2
2
5
0
5
0
5
0
5
0
5
0
5
1
6.4 Hz, 1H), 7.44 (t, J = 6.0, 11.2 Hz, 2H), 7.62 (d, J = 16.0 Hz,
1H), 7.82 (t, J = 8.8, 16.8 Hz, 2H), 7.94 (d, J = 8.4 Hz, 2H), 8.22
13
(
d, J = 8.4 Hz, 2H). CꢀNMR (CDCl 126 MHz): δ= 28.0, 32.0,
1
3,
precoated plates, which were visualized with UV light. H NMR
3
1
1
9.2, 42.9, 79.6, 110.6, 112.4, 113.2, 120.1, 124.5, 124.7, 125.4,
27.8, 127.9, 128.1, 130.8, 135.5, 138.5, 142.1, 150.7, 153.0,
spectra were recorded on a 400 MHz Varian mercury plus
spectrometer. Chemical shifts were expressed in δ ppm using
TMS as an internal standard. Mass spectra were obtained with a
micromassꢀQꢀTOF (YA105) spectrometer. All the DSCꢀTGA
measurements were performed out on a SDT Q600 v8.2 Build
1
1
2
2
3
3
4
4
5
5
+
62.3, 168.9. HRMS (m/z): calcd for C H N O ([M + H])
26
21
3
392.1763, observed 392.1721.
Synthesis of (E)-2-(3-((E)-4-(benzo[d]oxazol-2-yl)styryl)-5,5-
dimethylcyclohex-2-en-1-ylidene)-2-cyanoacetamide (6b)
1
00 model of TA instruments Waters (India) Pvt. Ltd. Absorption
spectra were recorded on a Perkin Elmer Lamda 25 UV–VIS
Spectrophotometer. Fluorescence spectra were recorded on a
Varian Cary Eclipse fluorescence Spectrophotometer using
freshly prepared solutions of 1 x 10 M. Absorption and
Emission spectra were performed using quartz cell of 1 cm path
length. All compounds were excited at their maximum absorption
values. All the experimental parameters were kept constant
throughout in order to have precision and accuracy. Absorption
and emission graphs were plotted using MATLAB R 2008a.Ink
software.
4
ꢀ(Benzo[d]oxazolꢀ2ꢀyl)benzaldehyde (4, 1.0 g, 4.5 mmol) and
(E)ꢀ2ꢀcyanoꢀ2ꢀ(3,5,5ꢀtrimethylcyclohexꢀ2ꢀenꢀ1ꢀ
ylidene)acetamide (5c, 0.92 g, 4.5 mmol) were dissolved in
absolute ethanol (15 mL). Piperidine (0.1 mL) was added and the
reaction mixture was refluxed for 14 h. The solvent was removed
under reduced pressure. The orange color dye 6b obtained was
purified by column chromatography using silica gel 100ꢀ200
ꢀ
5
mesh and hexane:ethylacetate (8:2) as eluent system. Yield: 67%.
ꢀ1
FTꢀIR (KBr, cm ): 3461, 3334 (NH ), 3155, 2960, 2343 (CN),
2
1
1
662 (C=O), 1514 (CꢀO), 1401 (CꢀN). HꢀNMR (DMSOꢀd , 400
6
MHz): δ= 0.98 (s, 6H, ꢀ2CH ), 2.51 (s, 2H, ꢀCH ), 2.60 (s, 2H, ꢀ
3
2
Synthesis of the dyes
CH ), 6.84 (s, 1H), 7.11 (d, J = 16.4 Hz, 1H), 7.41 (d, J = 7.6 Hz,
2
2
H), 7.45 (d, J = 16.4 Hz, 1H), 7.67 (s, 1H), 7.79ꢀ7.84 (m, 4H,
General procedure for synthesis of 2-(p-tolyl)benzo[d]oxazole
13
ArH), 7.88 (d, J = 8.4 Hz, 2H), 8.18 (d, J = 8.4 Hz, 2H). Cꢀ
NMR (DMSOꢀd 100 MHz): δ= 27.8, 30.77, 31.06, 104.5, 110.8,
(3)
6
,
2ꢀAminophenol 1 (5 g, 0.0458 mol) and 4ꢀmethylbenzaldehyde
(5.5 g, 0.0458 mol) were added to DMSO (30 ml). The reaction
1
16.71, 119.7, 124.9, 125.27, 125.58, 125.8, 126.2, 127.5, 127.9,
2
128.17, 128.8, 131.9, 132.4, 139.8, 141.5, 148.4, 150.1, 157.5,
mass was heated to 80 °C for 5ꢀ6 h. Then, the reaction mixture
was cooled to room temperature and diluted with 50 ml cold
water. The precipitate obtained was filtered and dried after
repeated washing with water. Further, it was purified by
recrystallization in ethanol and dried in oven at 50°C. Yield = 7.1
g, 69.6%, Melting Point: 114°C.
+
1
4
61.9, 163.4. HRMS (m/z): calcd for C H N O ([M + H])
26 23 3 2
10.1869, observed 410.1861.
Synthesis of (E)-2-(1H-benzo[d]imidazol-2-yl)-2-(3-((E)-4-
(benzo[d]oxazol-2-yl)styryl)-5,5-dimethylcyclohex-2-en-1-
ylidene)acetonitrile (6c)
4
ꢀ(Benzo[d]oxazolꢀ2ꢀyl)benzaldehyde (4, 1.0 g, 4.5 mmol) and
E)ꢀethyl 2ꢀcyanoꢀ2ꢀ(3,5,5ꢀtrimethylcyclohexꢀ2ꢀenꢀ1ꢀ
ylidene)acetate (5d, 1.0 g, 4.5 mmol) were dissolved in absolute
General procedure for synthesis of 4-(benzo[d]oxazol-2-
yl)benzaldehyde (4)
(
Selenium dioxide (7.9 g, 0.0717 mol) was added to 1,4ꢀ
dioxane (30 ml). The reaction mass was heated to 50 °C. 2ꢀ(pꢀ
Tolyl)benzo[d]oxazole (3, 5 g, 0.0240 mol) was added to
reaction mass at 50 °C in portions. Then, the reaction temperature
was raised to 90°C and maintained for 12 h. Reaction was
monitored on TLC. After completion, reaction mass was filtered
through celite. The filtrate was concentrated under vacuum on
rotary evaporator. Solid obtained was purified by recrystallization
from ethanol. Yield = 1.6 g, 30%.
1
1
1
1
00 ethanol (15 mL). Piperidine (0.1 mL) was added and the reaction
mixture was refluxed for 14 h. The solvent was removed under
reduced pressure. The orange color dye 6c obtained was purified
by column chromatography using silica gel 100ꢀ200 mesh and
hexane:ethyl acetate (8:2) as eluent system. Yield: 48%. FTꢀIR
ꢀ1
05 (KBr, cm ): 2960, 2343, 2209 (CN), 1712(C=O), 1526 (C=C),
1
1
2
4
1
232 (CꢀO). HꢀNMR (DMSOꢀd , 400 MHz): δ= 1.03 (s, 6H, ꢀ
CH ), 1.28 (t, 3H, ꢀ CH ), 2.61 (s, 2H, ꢀCH ), 2.94 (s, 2H, ꢀCH ),
.25 (q, 2H, ꢀCH ), 6.96 (s, 1H, vinylic ꢀH), 7.26 (d, J = 16.0 Hz,
H), 7.36ꢀ7.45 (m, 3H, ArH), 7.52 (d, J = 16.0 Hz, 1H), 7.81 (t, J
6
3
3
2
2
2
Synthesis
dimethylcyclohex-2-en-1-ylidene)malononitrile (6a)
ꢀ(Benzo[d]oxazolꢀ2ꢀyl)benzaldehyde (4, 1.0 g, 4.5 mmol)
of
(E)-2-(3-(4-(benzo[d]oxazol-2-yl)styryl)-5,5-
10 = 8.8, 17.6 Hz, 2H), 7.92 (d, J = 8.4 Hz, 2H), 8.19 (d, J = 8.4
Hz, 2H). CꢀNMR (DMSOꢀd6, 100 MHz): δ= 13.9, 14.0, 27.6,
13
4
2
1
1
7.8, 30.6, 31.0, 31.4, 61.2,110.9, 115.9, 119.8, 124.6, 124.9,
25.6, 126.2, 126.6, 127.6, 128.2, 132.1, 132.4, 133.9, 134.1,
41.5, 150.2, 152.1, 152.9, 161.8, 162.2. HRMS (m/z): calcd for
and 2ꢀ(3,5,5ꢀtrimethylcyclohexꢀ2ꢀenꢀ1ꢀylidene)malononitrile (5a,
0.83g, 4.5 mmol) were dissolved in absolute ethanol (12 mL).
Piperidine (0.1 mL) was added and the reaction mixture was
refluxed for 9 h. The solvent was removed under reduced
pressure. The orange color dye 6a obtained was purified by
+
15 C H N O ([M + H]) 439.2022, observed 439.1988.
2
8
26
2
3
2
|
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Synthesis of 2-(1H-benzo[d]thiazol-2-yl)-3-(4-(benzo[d]oxazol-
dyes 6a-6c, bearing vinyl hydrogens display two characteristic
2
-yl)phenyl)acrylonitrile (6d)
doublets localized at chemical shifts in the range of 6ꢀ8 ppm.
4
ꢀ(Benzo[d]oxazolꢀ2ꢀyl)benzaldehyde (4, 1.0 g, 4.5 mmol) and 60 Based on the large coupling values (J) of 16ꢀ17 Hz for the
2
ꢀ(1Hꢀbenzo[d]thiazolꢀ2ꢀyl)acetonitrile (5e, 0.70 g, 4.5 mmol)
olefinic protons it is established that these dyes exist in transꢀ
5
were dissolved in absolute ethanol (15 mL). Piperidine (0.1 mL)
was added and the reaction mixture was refluxed for 7 h. The
solvent was removed under reduced pressure. The yellow color
dye 6d obtained was purified by column chromatography using
silica gel 100ꢀ200 mesh and hexane:ethylacetate (8:2) as eluent
conformation in the ground state.
UV-vis absorption and emission of dyes 6a-6e
65
The photophysical properties of the dye were investigated in
DMSO solvent (Figure 1 and Figure 2). All the dyes displayed an
intramolecular charge transfer (ICT) band arising due to the
presence of benzoxazole moiety as donor in the presence of
ꢀ
1
1
1
2
2
3
3
0
5
0
5
0
5
system. Yield: 59%. FTꢀIR (KBr, cm ): 3047 (CꢀH, Ar), 2343
1
(
CN), 1681 (C=O), 1450 (CꢀN), 1241, 1059, 759, 743. H NMR
(500 MHz, CDCl3) 7.36 – 7.44 (m, 2H), 7.46 (t, J = 7.1 Hz, 1H),
7
.56 (t, J = 8.2 Hz, 1H), 7.60 – 7.69 (m, 1H), 7.79 – 7.85 (m, 1H), 70 strong acceptor system (CN) which is in agreement with DFT
7
.94 (dd, J = 8.0, 0.5 Hz, 1H), 8.11 (dd, J = 8.2, 0.5 Hz, 1H), 8.18
study. Inspection of absorption spectra reveals that the intensity
and position of charge transfer (CT) band is dependent on the
type of acceptor heterocyclic system. Dyes 6a, 6b and 6c showed
an absorbance band at 415 nm, 387 nm and 407 nm respectively.
13
(d, J = 8.7 Hz, 2H), 8.29 (s, 1H), δ 8.39 (d, J = 8.5 Hz, 2H).
C
NMR (126 MHz, CDCl3) 106.94, 110.81, 116.24, 120.39,
1
1
21.74, 123.77, 124.95, 125.86, 126.26, 127.10, 128.18, 130.04,
30.76, 134.85, 135.08, 142.05, 145.22, 150.87, 153.59, 161.69, 75 The hypsochromic shift of 6b is due to weaker pulling strength of
+
δ 162.27. HRMS (m/z): calcd for C H N OS ([M + H])
acceptor cyanoꢀacetamide as compared with malononitrile (6a)
and ethylcyanoacetate (6c). Compounds 6d and 6e absorbed at
2
3
13
3
380.0858, observed 380.0826.
3
83 nm and 377 nm repectively. Thus a “blueꢀshift” is observed
Synthesis
benzo[d]oxazol-2-yl)phenyl)acrylonitrile (6e)
ꢀ(Benzo[d]oxazolꢀ2ꢀyl)benzaldehyde (4, 1.0 g, 4.5 mmol) and
of
2-(1H-benzo[d]imidazol-2-yl)-3-(4-
in absorption spectra of compounds on going from 6a-6e, due to
80 weaker pushꢀpull effect.
(
4
2ꢀ(benzo[d]imidazolꢀ2ꢀyl)acetonitrile (5e, 0.78g, 4.5 mmol) were
dissolved in absolute ethanol (15 mL). Piperidine (0.1 mL) was
added and the reaction mixture was refluxed for 6 h. The solvent
was removed under reduced pressure. The yellow color dye 6e
obtained was purified by column chromatography using silica gel
100ꢀ200 mesh and hexane:ethylacetate (9:1) as eluent
85
ꢀ
1
system.Yield: 64%. FTꢀIR (KBr, cm ): 3285 (NH), 3051 (CꢀH,
1
Ar), 2237 (CN), 1594, 1416 (CꢀN), 1314, 1243, 1057, 740. Hꢀ
NMR (DMSOꢀd , 400 MHz): δ= 7.27ꢀ7.32 (m, 2H), 7.37ꢀ7.45
90
6
(
m, 2H), 7.63ꢀ7.68 (m, 2H), 7.77ꢀ7.81 (m, 2H), 8.17 (d, 2H, J =
13
8.4 Hz), 8.38 (d, 2H, J = 8.4 Hz), 8.48 (s, 1H). CꢀNMR
DMSOꢀd6, 126 MHz): δ= 104.4, 111.5, 116.3, 120.5, 125.5,
(
1
1
26.5, 128.3, 128.9, 130.7, 136.0, 141.9, 144.3, 147.6, 150.8,
Fig. 1. Uvꢀvis absorption spectra of dyes 6a-6e in
+
61.8. HRMS (m/z): calcd for C H N O ([M + H]) 363.1203,
95
dimethylsulfoxide (DMSO) at 293 K.
2
3
14
4
observed 363.1181.
The fluorescence emissions of the compounds were studied
(Figure 2). Dye 6a exhibited good Stokes shift of 123 nm
followed by 6e (116nm), 6d (84 nm) while 6b and 6c gave
4
0
Results and discussion
Synthetic strategy of dyes 6a-6e
1
1
1
1
00 similar shift of 55 nm.
Dyes 6aꢀ6e were synthesized by classical Knovenaegel
condensation of 4ꢀ(benzo[d]oxazolꢀ2ꢀyl)benzaldehyde with
different acceptor chromophoric systems based on substituted
isophorone, benzthiazole and benzimidazole. (Benzo[d]oxazolꢀ2ꢀ
yl)benzaldehyde (4) was prepared by oxidation (selenium dioxide
in dioxane) of 2ꢀ(pꢀtolyl)benzo[d]oxazole (3), obtained by
condensation of orthoꢀaminophenol (1) with tolualdehyde (2).
The aldehyhe (4) after conventional condensation with different
active methylenes (5a-5e) in ethanolꢀpiperidine gave the target
styryl dyes (6a-6e). The structures and purities of the prepared
4
5
5
5
0
5
05
10
1
compounds were confirmed using FTIR, H NMR, ESIꢀMS
spectrometry and thin layer chromatography. The spectral data
are in good agreement with the expected structure of the
compounds and the data of analysis is provided in the
Fig. 2. Fluorescence spectra of dyes 6a-6e in
dimethylsulfoxide (DMSO) at 293 K.
15
1
experimental section. It is noteworthy that the H NMR spectra of
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The UVꢀvis absorption and emission of the compounds were
studied in solvents of varying polarity, dielectric constant and
refractive index as listed in Table 1. Molecules with πꢀelectron 40 chargeꢀtransfer band with increase in the polarity of the solvent
system exhibit different charge distribution in the electronic
ground state and in excited state and hence show
solvatochromism, which is veryꢀwell established experimentally.
All the dyes exhibited single broad absorption profile in the range
523 nm (DMSO) for dye 6d and 477 nm (toluene) to 494 nm
(DMSO) for dye 6e. The progressive bathochromic shift of
5
0
5
0
5
0
5
depicts that the fluorescence originates from a highly polar state.
Dye 6d was found to fluoresce intensely in DMSO indicating
greater stabilization of excited state. The increase in emission
intensity for dye 6d is tentatively assigned to decrease nonꢀ
1
1
2
2
3
3
of 300ꢀ600 nm. Thus, for the tested organic styryl dyes only a 45 radiative decay channel which is enhanced by high polarity.
4
comparatively less solvent dependence is observed in UV–vis
absorption spectra, while considerable effect is seen over
fluorescence spectra. For dye 6a (Figure S1) with an isophoroneꢀ
malononitrile acceptor system, a single broad absorption maxima
The high molar extinction coefficients values (2.48ꢀ6.93 x 10
ꢀ
1
ꢀ1
L mol cm ) of dyes 6a-6e is due to the S → CT band
0
transitions. The intramolecular charge transfer takes place
through DꢀπꢀA system involving the electron flow from
at 407 nm in toluene was observed which varied to 415 nm in 50 benzoxazole moiety towards CN groups resulting in decrease of
DMSO. Furthermore, a redꢀshift was observed in fluorescence
spectra with maxima changing from 502 nm in toluene to 530 nm
in DMSO. Thus, a remarkable solvatofluorism was observed for
dye 6a with weak emission intensity which is attributed to the
energy gap between HOMO and LUMO. All the dyes exhibited
3
3
ꢀ1
large Stokes shift in the range of 3.25 x 10 ꢀ6.58 x 10 cm . The
large bathochromic shifts of the emission bands with the solvent
polarity indicate greater stabilization of the excited singlet state in
stabilization of excited state by solvent molecules. On excitation, 55 polar solvents. It is the characteristic for molecules which have
intra molecular charge transfer (ICT) occurs, and the excited state
gets much more dipolar than ground state. Stabilization of this
dipolar state by differential solvation results in lowering of band
enlarged dipoles and charge transfer (CT) characters in their
21
excited singlet states . The relative fluorescence quantum yields
of the dyes 6a-6e are listed in Table 2. Anthracene was used as
standard for the measurement of the fluorescence quantum yield
2
0
gap giving rise to redꢀshifted emission .
For dyes 6b and 6c bearing an isophoroneꢀcyanoacetamide and 60 (Φ = 0.27 in ethanol). Dyes 6d and 6e displayed high quantum
f
isophoroneꢀethylcyanoacetate based acceptor system respectively
Figure S2 and Figure S3), different absorption maxima were
yield value. In addition, benzoxazolyl containing styryl dyes have
higher quantum yield as compare to previously reported
(
22
observed. In case of dye 6b, a hypsochromic shift was observed
in both absorption and emission spectra. The absorption shifted
from 396 nm in toluene to 387 nm in DMSO whereas emission 65 absorption and emission wavelengths of these dyes are
value varied from 461 nm to 447 nm in DMSO. Dye 6c showed
compound . A comparative study of synthesized dye molecules
with reported analogues was done. It is interesting to note that the
significantly redꢀshifted when compared to reported analogues as
listed in Table 3. The redꢀshifted emission is attributed to the
decreased nonꢀradiative decay processes in the excited state. The
thermal stability of these compounds was remarkably higher as
solvatofluorism with increase in solvent polarity from toluene
(461 nm) to DMSO (486 nm).
Dyes 6d and 6e (Figure S4 and Figure S5) with benzthiazole
and benzimidazole as acceptors, were found to show positive 70 compared to their analogues.
solvatofluorism. The emission changed from 460 nm (toluene) to
Table 1. Photoꢀphysical properties of the dyes 6a-6e in different solvents
.
6
c
6
a
6b
Solvents
λabs, nm
λems
nm
,
λexct
nm
,
ꢁλ,
cm
λabs, nm4
(ε x 10 )
λems
nm
,
λexct
nm
,
ꢁλ,
cm
λabs, nm4
(ε x 10 )
λems
nm
,
λexct
nm
,
ꢁλ,
cm
4
ꢀ1
ꢀ1
ꢀ1
(
ε x 10 )
Toluene 407 (6.26)
502 406
396 (3.37)
398
395
388
401 (5.05) 461 405
4
650
461
485
3561
4762
3246
EDC
THF
408 (6.27)
405 (6.43)
515 409 5092 394 (3.92)
403 (5.32) 495 404 4612
520 411
527 404
465 409
525 407
538 412
530 418
388 (3.70)
383 (2.84)
382 (3.69)
382 (2.84)
383 (2.39)
387 (3.63)
397 (5.53) 450 397
395 (4.81) 464 398
396 (5.63) 457 398
394 (5.50) 500 395
402 (5.12) 508 402
407 (5.26) 486 409
5
5
3
5
5
5
461
839
247
828
863
193
453
456
450
446
463
447
3698
4180
3956
3756
4511
3568
2967
3765
3371
5381
5191
3994
Acetone 403 (5.90)
385
383
384
385
387
Ethanol
ACN
404 (6.02)
402 (6.53)
409 (5.33)
415 (4.73)
DMF
DMSO
Molar absorptivity (ε) in Mꢀ1 cmꢀ1. The concentration of dyes used for recording absorbance and emission spectra was 1×10−5 mol L−1.
75
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6
d
6
e
Solvents
λabs, nm
λems
nm
,
λexct
nm
,
ꢁλ,
cm
4101
λabs, nm4
(ε x 10 )
386 (2.36)
λems
nm
,
λexct
nm
,
ꢁλ,
cm
4
ꢀ1
ꢀ1
(
ε x 10 )
3
87 (4.04)
460
506
450
464
452
480
472
523
388
385
378
378
377
379
385
385
477
500
477
479
477
482
490
494
388
412
378
372
365
375
380
378
4942
4272
5631
5861
6584
5991
6047
6282
Toluene
EDC
3
3
3
3
3
3
3
83 (4.50)
77 (3.19)
76 (2.98)
77 (3.74)
77 (2.57)
84 (4.16)
83 (1.63)
6347
4303
5044
4401
5692
4855
6989
412 (1.97)
376 (3.19)
374 (3.62)
363 (3.08)
374 (4.14)
378 (3.72)
377 (3.04)
THF
Acetone
Ethanol
ACN
DMF
DMSO
Molar absorptivity (ε) in Mꢀ1 cmꢀ1. The concentration of dyes used for recording absorbance and emission spectra was 1×10−5 mol L−1.
Table 2. Relative quantum yield of dyes 6a-6e.
Solvent 6a (Φf)
6b Φf)
6c (Φf)
6d (Φf)
6e (Φf)
THF
EDC
0.0444
0.0242
0.0436
0.0303
0.0783
0.0285
0.0426
0.2400
0.1704
0.1152
0.2149
0.02102 0.0177
0.0153 0.0115
Ethanol 0.0977
DMF
0.02154 0.01755 0.01097 0.1036
The concentration of dye (C) used for calculation of quantum yield is 1×10−5 mol L−1.
5
Table 3. Comparative study with reported analogues.
Sr.no.
Reported Dyes
Synthesized dye
Ref.
NC
NC
CN
CN
O
N
1
.
E)ꢀ2ꢀ(5,5ꢀdimethylꢀ3ꢀstyrylcyclohexꢀ
[23,24]
2
ꢀenꢀ1ꢀylidene)malononitrile (S)
λabs : 393 nm, λems : 480 nm
λabs : 404 nm, λems : 542 nm
(Ethanol)
(
Ethanol)
ꢀ
1
ꢀ1
ꢁ
λ = 4872 cm , M.P. 167ꢀ169°C
ꢁλ : 6364cm
M.P. >200 °C
Dyes: 6a-6e
O
N
CN
2
.
[25]
λabs : 325ꢀ420 nm, λems : 413ꢀ540 nm
λabs : 329 nm, λems : 403 nm
Solid state fluorescence
15 Cary Eclipse Fluorescence Spectrophotometer. Dyes 6a-6e
exhibited yellow fluorescence in solid state and their emission
maxima are shown in figure 3. The results suggest that these solid
state yellow luminescent dyes could possibly be used in
electroluminescence devices, as phosphor and in organic light
It was found that the dyes 6a-6e are highly fluorescent in solid
state (Figure 3). Intense yellowish emissions were observed upon
illumination of the solid dye under UV lamp (365 nm). Materials
showing fluorescence in solid state have potential application as
1
0
electroluminescence devices. The solid state fluorescence 20 emitting diodes.
emission maxima for the compounds were recorded on Varian
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1
1
2
2
0
5
0
5
The functional used was B3LYP which combines Becke’s three
parameter exchange functional (B3) with the nonlocal correlation
functional by Lee, Yang, and Parr (LYP). The basis set used for
all atoms was 6ꢀ31G(d,p). Figure 4 gives clear elucidation of
electron density distribution on HOMO and LUMO. HOMO of
the dyes is mainly located on the benzoxazole derivative while
the LUMO of the dyes are contributed by strong electron
withdrawing cyano group. This clearly denotes an efficient
charge migration from the donor unit to the acceptor segment on
HOMO to LUMO electronic excitation. Variation in dipole
moment explains the electron withdrawing capabilities of
functional group attached to benzoxazole moieties. The vertical
excitation energies at the ground state equilibrium geometries
were calculated with TDꢀDFT. Table 4 signifies the close
agreement between observed and theoretical values of HOMOꢀ
LUMO and band gap. It also represents theoretical vertical
excitation values determined by using TDꢀB3LYP/6.31G(d,p) in
gas phase.
Fig. 3. Normalized solid state fluorescence plots for dyes 6a-6e.
Inset showing photograph of solid dyes under UV light and
daylight.
5
Theoretical results
The ground state geometry of the compounds 6aꢀ6e was
optimized in vacuum using Density Functional Theory (DFT).
30
Fig. 4. DFT computed HOMO and LUMO diagrams of the dyes 6a-6e at the B3LYP/6ꢀ31G(d,p) level.
Table 4. TDꢀB3LYP/6ꢀ31G(d,p) in the gas phase
35
Dipole
moment
Excitation
energy
(eV)
2.7645
3.3969
1.7361
0.2697
ELUMO
EHOMO
;
a
λabs, nm
λem, nm
Dyes
ꢀE (eV)
(Debye)
b
ꢀ
0.11255; ꢀ
449 (1.54)
b
6
6
a
0.10584
0.10904
7.98
495 (1.70)
0
.21839
0.10014;
0.20918
365 (0.42)
435 (1.74)
357 (0.27)
ꢀ
b
2.67
487 (1.90)
ꢀ
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ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
0.10264
0.21073
0.10404
0.21995
0.09995
0.21296
440 (1.73)
360 (0.27)
423 (1.53)
377 (0.03)
434 (1.31)
390 (0.14)
2.8185
3.4408
2.9347
3.2875
2.8541
3.1739
6
c
0.10809
4.32
2.26
0.93
492 (1.90)
6
d
0.11591
0.11301
485 (1.58)
508 (1.08)
6
e
a
ꢀ
E = ELUMO ꢀ EHOMO
b
Oscillatory strength (f)
Thermal and photochemical stability study
0
0
0
.7
.6
.5
S
5
The thermal degradation temperatures (T ) of dyes 6a–6e were
d
recorded by thermogravimetric analysis (Figure 5). The dyes
were heated at the rate of 10°C/min from 30ꢀ600 °C under inert
atmosphere of nitrogen gas. These novel colorants showed
excellent thermal stability. The comparisons of the Td
(decomposition temperature) at 95% weight of the sample, show
that the thermal stability of dyes decreases in the order
6a
6b
6
c
0.4
1
0
6d
0.3
0.2
0.1
0
6
e
6
d(336°C)>6e(327°C)>6c(293°C)>6a(291°C)>6b(252°C).
0
1
2
3
4
5
6
Time of irradiation (in hours)
30
Fig. 6. Photochemical stability study for dyes 6a-6e.
Conclusions
Novel dyes synthesized exhibited good thermal stability and
solid state luminescence property. Their photophysical properties
reveal that they can be potential candidates for OLED, NLO and
polymeric security application. Emission spectra of these dyes
were red shifted in solvents of varying polarity showing positive
solvatofluorism. Comparison with reported analogues showed the
advantage of synthesized dye in terms of photophysical and
thermal properties.
35
1
5
Fig. 5. TGA overlay graph for dyes 6a-6e.
Photochemical stability was next investigated by irradiating
the solution of dyes, in acetonitrile at room temperature in UV
cabinets (at 254 nm, 0.15 mW/cm ) for a period of 6 h . In the
photostability study, we have tried to discuss about the effect of
UV irradiation on photostability of the dye molecules in terms of
40
2
26
Acknowledgements
Authors are thankful to UGCꢀCAS for providing financial
2
0
assistance and to the Institute of Chemical Technology (ICT),
the absorption. No changes in absorbance intensity of the dyes 45 SAIF IITꢀBombay for recording FTꢀIR, HRꢀMS and TIFR
1
13
were observed on plotting the absorbance versus duration of
irradiation, which inferred that the dyes are extremely photoꢀ
chemically stable (Figure 6, S6). The photochemical stability was
compared by carrying out a control experiment with similar
molecule (S). It was observed that the absorbance intensity of S
was comparable with synthesized molecule 6a (Figure 6).
Mumbai for H and CꢀNMR.
Notes and references
2
5
a
Dyestuff Technology Department, Institute of Chemical Technology,
Matunga, Mumbai 400 019, India. Fax: +91 22 33611020; Tel: +91
2233612708; Eꢀmail:gsshankarling@gmail.com,
gs.shankarling@ictmumbai.edu.in
50
b
Department of Applied Chemistry, SV National Institute of Technology
(SVNIT), Suratꢀ395007, India. Eꢀmail:suban_sahoo@rediffmail.com
5
5
† Electronic Supplementary Information (ESI) available: [ Spectrocopic
1
13
Data, HRMS,
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