V.D. Gupta et al. / Dyes and Pigments 88 (2011) 378e384
383
methylene compounds 4a and 4b as shown in Scheme 1. In the first
3.3. Thermal stability of dyes
step, the 9H-carbazole 1 was N-ethylated using diethyl sulphate
which was further mono- and bis-formylated at 3- and 3,6-position
by VilsmeiereHaack formylation reaction to obtain compounds 3a
and 3b respectively. Finally aldehyde 3a or 3b and suitable active
methylene compound 4aeb were refluxed in absolute ethanol
containing a catalytic amount of piperidine to yield desired styryl
carbazole dyes 5aeb and 6aeb. The structures of the dyes were
confirmed by FT-IR, 1H NMR, 13C NMR, Elemental analysis and Mass
spectra. The 1H NMR spectra of dye 5a showed doublet peaks at
In order to give more insight into the dyes 5a, 5b, 6a and 6b the
thermal studies of the compounds have been carried out using
thermal gravimetric techniques (DSCeTGA). The thermogravi-
metric studies have been carried out in the temperature range
50e600 ꢀC under nitrogen gas at a heating rate of 10 ꢀC minꢁ1. The
DSCeTGA results indicated that the due to presence of carbazole
frame work of the synthesized dyes are stable up to 300 ꢀC. TGA
revealed the onset decomposition temperature (Td) of compounds
5a, 5b, 6a and 6b at 306 ꢀC, 300 ꢀC, 340 ꢀC and 320 ꢀC, respectively
as shown in Fig. 6. Above 300 ꢀC the thermogravimetric curve of the
d
7.07 and 7.67 having trans vicinal coupling (J ¼ 15.2 Hz) which
indicates the ethylenic protons at styryl group are in trans form. The
elemental analysis of these samples showed that they are pure and
values were consistent with the calculated values.
synthesized compounds show
a major loss in weight. The
comparisons of the Td (decomposition temperature) show that the
thermal stability of the 5a, 5b, 6a and 6b decreases in the order
6a > 6b > 5a > 5b. The results showed that synthesized dyes have
good thermal stability. Dye 5a, 5b showed sharp decomposition at
306 and 300 ꢀC respectively and completely decomposed beyond
550 ꢀC. However dyes 6a and 6b showed two step decomposition
behaviors and completely decomposed beyond 600 ꢀC. Differential
scanning calorimetric (DSC) analysis of target molecules show
exothermal peak at 387 ꢀC, 376 ꢀC, 383 ꢀC and 372 ꢀC, respectively
as shown in Fig. 7. Except dye 5a, other dyes 5a, 6a, 6b showed
melting point behavior.
3.2. Photo-physical properties
The UVeVis absorption and fluorescence emission spectra of
1 ꢂ 10ꢁ6 mol Lꢁ1 solution of dyes 5aeb and 6aeb were measured
in different solvent of varying polarity, absorption and emission
maxima are also reported in the Table 1. These dyes with Ae
peD
(5a and 6a) and Ae eDe eA (5b and 6b) structure consist of an
p
p
electron-donating N-ethyl carbazolyl unit and electron-with-
drawing cyano or carboethoxy group. The results showed that
these dyes exhibited strong solvatofluoric properties. The intro-
duction of electron accepting groups in the positions induced
intramolecular charge transfer and mesomeric dipole moment.
Depending upon the electron affinity of acceptor groups, the CT
band was red shifted as shown in Table 1. The absorption and
emission spectra of dye 5a, in several solvents having different
polarities shown in Figs. 1 and 2.
The absorption maxima showed red shift with the increase in
the solvent polarity which extended from 444 nm to 457 nm for
dye 5a, 426 nm to 430 nm for dye 5b, 465 nm to 484 nm for dye 6a
and 447 nm to 459 nm for dye 6b (Table 1). The only dye 5b showed
less shift in the absorption maxima.
4. Conclusions
In summary, we have developed an efficient and simple protocol
for the synthesis of fluorescent extended mono- and bis-styrylated
chromophores containing both carbazole and electron with-
drawing cyano/carboethoxy moieties. The synthesized compounds
were confirmed by FT-IR, 1H NMR, 13C NMR, Elemental analysis and
Mass spectra. From the emissive properties, it was concluded that
the electronic coupling between D and A was sufficient to allow
charge transfer (CT) in their molecules. The ICT maximal emission
displayed a large wavelength shifted and Stokes shift increased in
response to the increase of the solvent polarity. The DSCeTGA
results show that they are having good thermal stability and hence
these dyes may have various applications in nonlinear optics (NLO),
electronic and photonic devices, organic light-emitting diodes.
The fluorescence emission spectra also exhibited a strong
solvent effect. The emission maxima showed shift with solvent
polarity which extended from 530 nm to 574 nm for dye 5a, 522 nm
to 560 nm for dye 5b, 526 nm to 584 nm for dye 6a and 524 nm to
574 nm for dye 6b.
In highly polar solvent such as DMF the emissive S1 state of
intramolecular charge transfer (ICT) character is strongly solvated
and its energy is hence dramatically lowered. As a consequence, the
Acknowledgements
The authors are greatly thankful to I.I.T. Mumbai for recording
the 1H and 13C NMR and Mass spectra.
energy gap DE (S1, S2) is enlarged so that the coupling of the S1 state
directly to the ground state stays opened and the intersystem
crossing from S1 to T state is enhanced. The high electron-donating
character of carbazole moiety leads to a red shift (lower energy) of
the emission relative to the absorption is caused by energy losses
due to dissipation of vibrational energy during the decay and is
influenced by interaction between the fluorophore and the solvent
molecules around the excited dipole, hydrogen bonding and
formation of charge complexes.
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Gupta, Vinod D.
