L. Zheng et al. / Tetrahedron Letters 57 (2016) 932–936
933
that of taurine. The optimal temperature for the syntheses of amin-
oderivatives (1–2)b was 50 °C, and that for 3b was about 45 °C.
Spectral properties of the aminoderivatives and their parents
The data of maximum UV–Vis absorption and emission wave-
lengths of the aminoderivatives and their parents in different sol-
vents are listed in Table 1. The maximum UV–Vis absorption and
emission wavelengths of the aminoderivatives in 586–760 nm
showed blue shifts compared to those of their parents in
Scheme 1. Structures of the parent dyes.
7
70–820 nm, but large Stokes shifts were exhibited. For instance,
the UV–Vis absorption and emission spectra of aminoderivative
c and its parent dye 1, could be visually seen from Figure 1. The
1
Results and discussion
maximum absorption wavelengths of aminoderivative 1c and its
parent dye 1 were 614 nm and 792 nm in methanol, and their
emission wavelengths were 733 nm and 818 nm in methanol,
respectively. The Stokes shift of aminoderivative 1c was 119 nm,
much larger than that of its parent dye 1 (26 nm).
Syntheses of the aminoderivatives
The synthetic route of the aminoderivatives and their dyes was
shown in Scheme 3. Intermediates 4–6 and bisaldehyde compound
condensed to form parent dyes 1–3,1
7,19,20
Compared to those of the parents, the shape and intensity of
absorption and emission spectra of the aminoderivatives changed
a lot. The absorption peaks of the aminoderivatives became
broader and the absorption intensity at the corresponding maxi-
mum wavelengths turned weaker than those of their parents in
methanol, such as 2a, 2b, 2c, and their parent dye 2 in Figure 2;
while the emission intensity at the corresponding maximum wave-
lengths of aminoderivatives were stronger than those of their par-
ents, such as 3a, 3b, 3c, and their parent dye 3 in Figure 3.
In different solvents, the aminoderivatives and their parents
exhibited different absorption and emission wavelengths (Table 1).
The wavelengths showed blue shifts with the increasing of polarity
of the protic solvents and exhibited negative solvatochromism, the
reason was that polarity of ground state molecule of the amin-
oderivatives and their parents was greater than that of the excited
state, the ground state molecule was better stabilized by solvation
than the molecule in the excited state, and made the energy band
between the ground state and excited state enlarge with the
7
and the dyes reacted
with benzylamine, butylamine, or taurine, respectively, for substi-
tution of nitrogen for chlorine to get nine novel aminoderivatives.
2
The products were purified by SiO column chromatography, and
1
characterized by H NMR and ESI-MS.
Synthetic conditions for the novel aminoderivatives were
adopted according to the Ref. 21–23. But in the references, DMF
was chosen as the solvent, so the post-treatment was complex
due to the high boiling point. In this Letter, we tried to use other
solvents for our syntheses, and found that ethanol could serve as
a good solvent for the aminoderivatives (1–2)a–c, and DCM for
3
a, 3b, and 3c, so they made the post-treatment simple and cost
less due to their lower boiling point and cheaper price. The syn-
thetic route of 1a, 1b, and 1c derived from dye 1 is shown in
Scheme 4.
Liquid benzylamine and butylamine are bases and good amino
reagents. In the synthetic process, they could act not only as strong
nucleophilic reagents to react with the dyes for the substitution of
the chlorine atoms, but also as deacid reagent to take away the
HCl formed in the reactions, and this made the syntheses of amin-
oderivatives 1a, 1c, 2a, 2c, 3a, and 3c easy. While the amino group
of taurine was in the form of an inner salt (ammonium salt) with
the sulfonic acid group, and this decreased its nucleophilicity, we
applied anhydrous sodium carbonate to accelerate the synthetic
reaction by taking up the HCl formed and neutralize the sulfonic
group to make the amino group free and the nucleophilicity
increase. Meanwhile, sodium carbonate also could work as a deacid
reagent to take up the HCl formed in the synthetic process of amin-
oderivatives 1b, 2b, and 3b. So, aminoderivatives (1–3)a,c could be
readily prepared at room temperature, aminoderivatives (1–3)b
needed to be synthesized under heating. Furthermore, the nucle-
ophilic activity of benzylamine and butylamine was higher than
2
4,25
increasing of the polarity of the solvents.
By the way, the
meso-nitrogen-atom in the molecules of the aminoderivatives with
a pair of lone electrons might form hydrogen-bonding with H-atom
2
4–26
in protic solvents,
and the hydrogen-bonding lowered the
energy level of the ground state, it could make the energy band
enlarge and the wavelength shift enhance.
Influence of different N-substituents on spectral properties of
the aminoderivatives
4
Compared to ACl, the N atom in ANHR had much more elec-
tron density, so substitution of ANHR group for ACl group at
4
the polymethine chain of the cyanines remarkably changed the
maximum UV–Vis absorption and emission wavelengths a lot.
4
Because the substitution of ANHR group for ACl altered the
energy of ground and excited states of the molecule, and made
the aminoderivatives possess larger energy bands and greater blue
2
5
shifts than those of their parents.
While an excited-state
intramolecular charge transfer took place between the donor and
acceptor in the aminoderivatives, the Stokes shift and fluorescence
1
8
intensity would increase further.
in ANHR group of the aminoderivatives also affected the
R
4
4
maximum absorption wavelengths due to their different elec-
tron-donating ability and space structures. Generally, stronger
electron-donating ability of the AR
of the maximum absorption wavelengths. Among them, the elec-
tron-donating ability of ACH group was least, so the corre-
4
group made greater blue shifts
2 6 5
C H
sponding aminoderivatives (1a, 2a, and 3a) possessed the least
Scheme 2. Structures of the aminoderivatives.
blue shifts.
Zheng, Lihui
