C.E.A. de Melo et al.
Dyes and Pigments 184 (2021) 108757
Compound synthesized from 5–nitro–2–thiophenecarboxaldehyde and
all constants were obtained through fitting of least–square curves using
the program ORIGIN 8.5.
◦
p–anisidine. Bright red–orange solid (yield: 87%). M. p.: 155.6 C. IR
(KBr) νmax/cmꢀ 1: 3422 (intramolecular hydrogen bond of the N from the
All theoretical calculations were performed with the ORCA 4.2.1
software [36].
–
–
–
C
N); 3096 (C–H); 1609 (C N); 1535 and 1493 (N–O); 1329 (CH );
3
–
1289 and 1244 (C–O); 1191 (C–S). 1H NMR (200 MHz, DMSO–d6)
δ/ppm: 8.71 (s, 1H); 7.96 (d, 1H, J = 4.3 Hz); 7.47 (d, 1H, J = 4.3 Hz);
7.34 (d, 2H, J = 9.0 Hz); 6.99 (d, 2H, J = 9.0 Hz); 3.82 (s, 1H). 13C NMR
(50 MHz, DMSO–d6) δ/ppm: 159.1; 150.5; 149.3; 141.9; 131.0; 130.5;
123.3; 114.6; 55.4; 18.5. HRMS (ESI, TOF) m/z: 263.0487 [M+H]+,
calculated for C12H10N2O3S, 263.0485.
The
geometry
of
dye
7
was
optimized
at
the
Tao–Perdew–Staroverov–Scuseria (TPSS) density–functional level [37],
employing the triple–ξ Karlsruhe basis set (def2–TZVP) [38]. Dispersion
forces were accounted for with Grimme’s D3 correction [39]. Fre-
quencies were calculated for all molecules to ensure energy minima.
Solvent effects were mimicked with the conductor–like polarizable
continuum model (CPCM) approach [40]. Transition energies in each
solvent were then obtained from time–dependent density functional
theory (TD/DFT) calculations on the optimized geometries at the same
level of theory, with the pure functional (PBE) [41], the range–separated
4–Methoxy–N–[(1E)–(5–nitro–2–furanyl)methylene]aniline
(10). Compound synthesized from 5–nitro–2–furaldehyde and
p–anisidine. Bright yellow solid (yield: 80%). M. p.: 123.2 ◦C. IR (KBr)
νmax/cmꢀ 1: 3424 (intramolecular hydrogen bond of the N from the
–
–
–
–
C
N); 3157 (C–H); 1619 (C N); 1529 and 1501 (N–O); 1354 (CH );
hybrid functional ω–B97X–D3 [42], and the def2–TZVP basis set [38].
1297 and 1244 (C–O). 1H NMR (200 MHz, DMSO–d6) δ/ppm: 8.64 3(s,
1H); 7.81 (d, 1H, J = 3.9 Hz); 7.42 (d, 2H, J = 8.8 Hz); 7.35 (d, 1H, J =
3.9 Hz); 7.01 (d, 2H, J = 8.8 Hz); 3.79 (s, 1H). 13C NMR (50 MHz,
DMSO–d6) δ/ppm: 159.2; 153.3; 152.1; 145.0; 142.4; 123.2; 116.9;
114.6; 114.3; 55.4. HRMS (ESI, TOF) m/z: 247.0710 [M+H]+, calcu-
lated for C12H10N2O4, 247.0713.
4. Results and discussion
4.1. Synthesis and characterization
Compounds 7–12 were synthesized following the methodology
adapted from de Melo et al. 25. The corresponding Schiff bases were
obtained through the condensation reaction of the corresponding amine
with 5–nitro–2–thiophenecarboxaldehyde or 5–nitro–2–furaldehyde in
the presence of dry ethanol and glacial acetic acid, with the consequent
elimination of water (Scheme 1). Novel compounds 7–12 were fully
characterized using IR, 1H NMR, 13C NMR, HRMS, and DSC techniques
(Figs. S2–S31) and the products were obtained with yields ranging from
73 to 87%. The results show that all compounds were obtained with the
required purity for the UV–vis spectrophotometric studies to be carried
out.
4–Methylthio–N–[(1E)–(5–nitro–2–thienyl)methylene]aniline
(11). Compound synthesized from 5–nitro–2–thiophenecarboxaldehyde
and 4–(methylthio)aniline. Bright red solid (yield: 77%). M. p.: 116.7
◦C. IR (KBr) νmax/cmꢀ 1: 3424 (intramolecular hydrogen bond of the N
–
–
–
from the C N); 1605 (C N); 1537 e 1509 (N–O); 1336 (CH ); 1195
–
(C–S). 1H NMR (400 MHz, CDCl3) δ/ppm: 8.55 (s, 1H); 7.90 (d,31H, J =
4.2 Hz); 7.35 (d, 1H, J = 4.2 Hz); 7.28 (dd, 2H, J1 = 2.0 Hz e J2 = 8.7 Hz);
7.23 (dd, 2H, J1 = 2.2 Hz e J2 = 8.7 Hz); 2.51 (s, 3H). 13C NMR (50 MHz,
DMSO–d6) δ/ppm: 152.2; 148.8; 145.9; 138.0; 131.6; 130.4; 126.5;
122.3; 14.7. HRMS (ESI, TOF) m/z: 279.0259 [M+H]+, calculated for
C12H10N2O2S2, 279.0256.
4–Methylthio–N–[(1E)–(5–nitro–2–furanyl)methylene]aniline
(12). Compound synthesized from 5–nitro–2–furaldehyde and 4–
(methylthio)aniline. Orange solid (yield: 75%). M. p.: 102.0 ◦C. IR (KBr)
νmax/cmꢀ 1: 3424 (intramolecular hydrogen bond of the N from the
4.2. Solvatochromic studies
Fig. 3 shows solutions and UV–vis spectra for compounds 7 (A) and 8
(B) in solvents of different polarities, which show that these dyes are
solvatochromic, presenting different solution colors in solvents with
different polarity. By comparison, compounds 9–12 exhibit a weaker
solvatochromism (Figs. S32 and S33). The solvatochromic bands
observed in the visible region of the spectrum of compounds 7–12 are
–
–
–
–
C
N); 3073 (C–H); 1619 (C N); 1515 and 1493 (N–O); 1352 (CH );
1258 (C–S). 1H NMR (400 MHz, CDCl3) δ/ppm: 8.41 (s, 1H); 7.42 (d, 13H,
J = 3.8 Hz); 7.28 (d, 2H, J = 9.1 Hz); 7.22 (d, 2H, J = 9.1 Hz); 7.18 (d,
1H, J = 3.8 Hz); 2.51 (s, 3H). 13C NMR (50 MHz, DMSO–d6) δ/ppm:
153.0; 152.2; 146.6; 146.4; 138.1; 126.6; 122.2; 117.6; 114.6; 14.7.
HRMS (ESI, TOF) m/z: 263.0483 [M+H]+, calculated for C12H10N2O3S,
263.0485.
the result of electronic transitions of the
π
–
π
* type, with a charge
transfer from the electron–donor moiety (N,N–dimethylamino, methoxy
or
methylthio
groups)
to
the
electron–acceptor moiety
(2–nitrothiophenyl or 2–nitrofuranyl). Taking compound 7 as an
example, the λmax of the solvatochromic band measured in n–hexane
2.3. UV–vis measurements
appears at 464 nm, while in diethyl ether, the band is displaced at λ
max
= 476 nm (Δλmax = +12 nm). In ethanol and DMSO, the λmax is verified
A stock solution with concentration of 7.0 × 10ꢀ 3 mol Lꢀ 1 for each
at 487 nm and 511 nm, respectively (Δλ
= +24 nm). Compounds 8
and 11 show similar behavior, while themoatxher compounds show only a
very small variation in maximum absorption wavelengths in solvents
with different polarities.
compound was prepared in acetone. 24 μL of this stock solution were
transferred to 5 mL volumetric flasks and after the evaporation of
acetone the compound was dissolved in pure solvents. The final con-
centration of the resulting solutions was 4.0 × 10ꢀ 3 mol Lꢀ 1. Then, the
UV–vis spectra were collected at 25 ◦C. The maxima of the UV–vis
spectra were calculated from the first derivative of the absorption
spectrum, with a precision of ±0.5 nm, and the reproducibility was
verified through the determination of two spectra for each dye in each
pure solvent. The maximum wavelength (λmax) values thus obtained
were used to determine the ET (dye) values in each solvent, given with a
precision of ±0.1 kcal molꢀ 1, through Eq. (1) [1,5].
The UV–vis spectra for each compound in various solvents were used
to obtain, through the first derivative, the λmax values of the sol-
vatochromic bands, which were used to calculate the corresponding ET
(dye) values in kcal molꢀ 1, using Eq. (1) (Table 1).
Fig. 4 shows the plots of the ET (dye) obtained for compounds 7–12 in
each of the solvents studied, as a function of ET (30). Compounds 7, 8,
and 11 exhibit a positive solvatochromic behavior. Taking compound 7
as an example, the solvatochromic band has a maximum at 464 nm [ET
(7) = 61.6 kcal molꢀ 1] in n–hexane, while in DMSO the λmax = 511 nm
[ET (7) = 56.0 kcal molꢀ 1]. Thus, a reduction in the ET (7) value by 5.6
kcal molꢀ 1 occurs following an increase in the polarity of the medium,
leading to a bathochromic change of Δλmax = +47 nm, which is char-
acteristic of positive solvatochromism. For compound 8, considering the
λmax values in n–hexane (444 nm) and in benzyl alcohol (496 nm), a
Δλmax = +52 nm was verified.
ET (dye) / (kcal m olꢀ 1) = 28590/λm ax (nm )
(1)
3. Calculation methods
For the regression analyses of the experimental transition energies,
3