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C. Hamciuc et al. / Reactive and Functional Polymers 103 (2016) 17–25
Table 3
The fluorescence quantum yield (QY, %) values for polymer 4a, in tetrahydrofuran solution, under excitation with different wavelength.
Sample
4a
QY, % at λex = 320 nm
QY, % at λex = 334 nm
QY, % at λex = 350 nm
QY, % at λex = 360 nm
QY, % at λex = 365 nm
QY, % at λex = 375 nm
8.98
8.73
7.43
7.91
6.45
4.23
from NMP and DMSO solvents. Also, from spectral data (Table 2) we
observed that the wavelengths of maximum emission of all samples
were red shifted depending on the solvent characteristics. The solvent
polarity exerted strongly effect (emission spectra showed a remarkable
bathochromic shift with increasing solvent polarity, Δλmax = 1–55 nm)
on its excited species compared to its ground-state electronic transition
(only minor shifts of the absorption band of these compounds occurred
from changing the solvent polarity, Δλmax = 2–9 nm). Hence,
solvatochromism was more pronounced in the fluorescence spectra.
The remarkable shifts of the emission maxima with increasing of the
medium polarity reflect the fact that the excited state of investigated
molecules is more polar than its ground state and also suggest an
increase in dipole moment of excited state compared to ground state.
Thus, the solvent (medium) with higher polarity will stabilize the excited
state more preferably with respect to the ground state [28].
To obtain detailed information about optical characteristics of
studied compounds their fluorescence excitation spectra were mea-
sured. Fig. 7 exemplified the excitation spectra of all analyzed samples
in DMSO solution. These spectra were characterized by two main
bands in the 250–400 nm region. The position of the excitation maxima
wavelength of the Band I of the studied samples in all used solvents ap-
peared at around 272 nm. These excitation maxima were shifted to the
long wavelength (~272 nm) compared to the first peak from the
absorption spectra (~247 nm, see Table 2) and indicates that the ab-
sorbing species of Band I give a weak fluorescence signal. Instead, the
second excitation maxima for all samples occurred at wavelength
values very close (the small red-shifts to the absorption maxima,
Δλmax b 6 nm) to those of the corresponding absorption maxima ones,
indicating that the excited emission species can be considered as iden-
tical to the absorbing ones [29].
Also, for polyimide 4c, the Fig. 9 illustrates the CIE-1931 chromaticity
diagram of this compound in different media. The values of the chroma-
ticity coordinates were represented functions of solvent media. Thus,
the emission color changes from THF (x — 0.1523; y — 0.0514,
λem = 419 nm), DCM (x — 0.1530; y — 0.398, λem = 414 nm),
NMP (x — 0.1536; y — 0.1752, λem = 454 nm) to DMSO (x —
0.1735; y — 0.2420, λem = 469 nm).
The calculated Stokes shift values were relatively large, ranging from
80 to 123 nm (3), 81–115 nm (4a), 81–127 nm (4b), 78–126 nm (4c),
79–121 nm (4d) and 79–138 nm (4e) (see Table 2).
Fluorescence quantum yields (QY) of the investigated samples have
also been measured in THF and DMSO solutions and the obtained values
were summarized in Table 2. The QY values were sensitive to solvent
polarity and to excitation wavelength values (Table 3). Thus, the quan-
tum yields of sample 4a in THF decreased with increasing excitation
wavelength from 320 to 375 nm (see Table 3). This is an exception
from Kasha's rule and Vavilov's law, which was attributed either to pho-
todecomposition or to the enhancement of intramolecular energy trans-
fer process competing with internal conversion between singlet states.
Also, it can be observed that for samples 4b and 4c high quantum
yield values were obtained in DMSO (polar solvent) which may be
due to the strong overlap of the electronic transitions of species from
system [28].
The pH effect on fluorescence characteristics of 3 and 4a samples in
THF was investigated by adding various amounts of HCl 1N (Fig. 10). For
compound 3 (see Fig. 10a) the maximum of emission intensity from
416 nm was gradually reduced and red shifted to 452 nm after 500 μL
HCl 1 N were added. This red-shift is attributable to protonation of nitro-
gen atoms from molecular structure of compound 3. In the case of sam-
ple 4a, when low amounts of HCl (1–10 μL, see Fig. 10b) were added, the
emission intensity of system was slightly enhanced, probably in this
context the effect of heavy-atom was reduced [30], instead further in-
creased HCl concentrations induce a gradual emission quenching and
maxima of emission intensity were found red-shifted by ≈15 nm
upon increasing the concentration of HCl. These changes in emission
characteristics upon adding acid are due to protonation of the nitrogen
atoms from investigated molecular structure.
4. Conclusions
The
incorporation
of
pendant
2-(4-phenoxy)-5-(4-
dimethylaminophenyl)-1,3,4-oxadiazole units in the macromolecular
chains of some aromatic polyimides containing flexible aromatic
ether, isopropylidene, hexafluoroisopropylidene or fluorene groups
leads to polymers having high thermal stability and good solubility in
organic solvents. The optical properties including UV–visible absorption
spectra, fluorescence excitation and emission spectra, Stokes shift
values and the relative fluorescence quantum yield were measured in
four different solvents. The fluorescence characteristics consist in a
blue emission band for all samples, large and solvent dependent Stokes
shifts values, as well as solvent dependent quantum yields values.
Namely, the fluorescence quantum yield of 4a in THF decreases as the
Fig. 10. Fluorescence spectra of model compound 3 (a) and polymer 4a (b) in
tetrahydrofuran solution, before and after the addition of various amounts of HCl.