Influence of different amino substituents in position 1 and 4 on spectroscopic and acid base properties of 9,10-anthraquinone moiety

Article abstract of DOI:10.1016/j.saa.2013.01.085

A series of novel 1-amino and 1,4-diamino-9,10-anthraquinones, substituted with different alkyl groups, were synthesized as the result of alkylation with amino substituents. All the obtained aminoanthraquinone derivatives were characterized by NMR, IR spectroscopy and mass spectrometry. The spectroscopic properties of these compounds were determined by using UV-Vis spectroscopy in acetonitrile, and in the mixture of acetonitrile and methanol at different pH ranges. The effects of various substituents present in the newly developed anthraquinone derivatives and their ability to form hydrogen bonds between the carbonyl oxygen atom of anthraquinone moiety and nitrogen atom of N-H group in 1-aminoanthraquinone (1-AAQ) and 1,4-diaminoanthraquinone (1,4-DAAQ) were studied. Additionally, the effects of hydrogen bond formation between O-H group in hydroxyethylamino substituent and the carbonyl oxygen atom of anthraquinone were investigated. The spectroscopic behavior of the studied derivatives strongly depended on the solvent-solute interactions and the nature of solvent. The values of pK_a for the new anthraquinones were determined by the combined potentiometric and spectrophotometric titration methods.

Full text of DOI:10.1016/j.saa.2013.01.085

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 108 (2013) 82–88
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Influence of different amino substituents in position 1 and 4 on spectroscopic
and acid base properties of 9,10-anthraquinone moiety
⇑
_
´
Anna Wcisło , Paweł Niedziałkowski, Elzbieta Wnuk, Dorota Zarzeczanska, Tadeusz Ossowski
Faculty of Chemistry, University of Gdan´sk, Sobieskiego 18/19, 80-952 Gdan´sk, Poland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
"
"
"
The number of amino substituents
directly affects the basicity of the
examined compounds.
1,4-diamino-substituted compounds
have higher pKa value than
monosubstituted derivatives.
Two amino substituents present in
position 1 and 4 cause vibrational
overtones.
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel 1-amino and 1,4-diamino-9,10-anthraquinones, substituted with different alkyl groups,
were synthesized as the result of alkylation with amino substituents. All the obtained aminoanthraqui-
none derivatives were characterized by NMR, IR spectroscopy and mass spectrometry. The spectroscopic
properties of these compounds were determined by using UV–Vis spectroscopy in acetonitrile, and in the
mixture of acetonitrile and methanol at different pH ranges. The effects of various substituents present in
the newly developed anthraquinone derivatives and their ability to form hydrogen bonds between the
carbonyl oxygen atom of anthraquinone moiety and nitrogen atom of N–H group in 1-aminoanthraqui-
none (1-AAQ) and 1,4-diaminoanthraquinone (1,4-DAAQ) were studied. Additionally, the effects of
hydrogen bond formation between O–H group in hydroxyethylamino substituent and the carbonyl oxy-
gen atom of anthraquinone were investigated.
Received 13 November 2012
Received in revised form 18 January 2013
Accepted 23 January 2013
Available online 13 February 2013
Keywords:
Aminoanthraquinones
Acid–base properties
UV–Vis spectrophotometry
Spectrophotometric titrations
1,4-Diamino-9,10-anthraquinone
derivatives
The spectroscopic behavior of the studied derivatives strongly depended on the solvent–solute interac-
tions and the nature of solvent. The values of pKa for the new anthraquinones were determined by the
combined potentiometric and spectrophotometric titration methods.
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
ically, the differently substituted aminoanthraquinones are a sig-
nificant group of quinonoid dyes that are important for the
Amino- and hydroxyl-substituted anthraquinones are a very
important and interesting group of compounds that has been
extensively studied during the last decade. Both naturally-occur-
ring and synthetic anthraquinones have been used in the dye
industry, biological research, pharmacy and medicine [1,2]. Specif-
commercial dye industry [3].
Aminoanthraquinone derivatives are usually obtained by
substituting the hydrogen atom of anthraquinone skeleton by
amine or other groups [4]. The substitution of anthraquinone in
different ring positions changes the ground and excited state di-
pole moments and also their photophysical characteristics [5,6].
For these reasons, the UV–Vis measurement of novel anthraqui-
none derivatives is a very useful technique for determining their
properties.
⇑
Corresponding author. Tel.: +48 58 5235323; fax: +48 58 5235572.
E-mail address: anna.wcislo@chem.univ.gda.pl (A. Wcisło).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.01.085

A. Wcisło et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 108 (2013) 82–88
83
The chromophoric system of anthraquinones allows tracing the
interactions between aminoanthraquinone derivatives and differ-
ent molecules, and metal ions and anions by using spectral meth-
ods [7–10]. Aminoanthraquinones are usually used to create
supramolecular structures that are sensitive to pH and redox po-
tential changes or sensitive to the presence of selective organic
molecules [11,12].
Due to the presence of redox-active unit in the skeleton of ami-
noanthraquinone, the derivatives have also many practical applica-
tions as efficient electron acceptors. Therefore it is also possible to
observe many intermolecular interactions by means of electro-
chemical methods [13–15]. In this study we investigated six new
alkyl derivatives of aminoanthraquinone, namely, three derivatives
substituted in position 1,4 by alkylamino group and tosyl group,
and three substituted by two alkylamino groups in positions 1
and 4 (Scheme 1). The obtained results were compared to the mod-
el aminoanthraquinone derivatives, i.e. 1-aminoanthraquinone and
1,4-diaminoanthraquinone (Scheme 2). The pKa values of these
compounds have been determined. The results showed that the
number of substituents containing tertiary nitrogen affects the ba-
sicity of the compound.
Scheme 2. Model aminoanthraquinone derivatives, i.e. 1-aminoanthraquinone (1-
AAQ) and 1,4-diaminoanthraquinone (1,4-DAAQ).
were recorded on a Biflex III MALDI-TOF mass spectrometer. Ele-
mental analysis was conducted on a Carlo-Erba CHNS-O EA 1108
elemental analyzer. All spectrophotometric measurements were
performed on a Perkin Elmer Lambda 650.
Materials
All the reagents were commercially available and used without
further purification. Solvents were purified by standard proce-
dures. 1-aminoanthraquinone (1-AAQ) and 1,4-diaminoanthraqui-
none (1,4-DAAQ) were purchased from Sigma Aldrich and were
purified by column chromatography before use.
Experimental
Instruments
Synthesis
All reactions were magnetically stirred and monitored by thin-
layer chromatography on silica gel 60 F254 plates (Merck) with UV
detection. Flash column chromatography was performed on silica
gel 230–400 mesh (Merck). 1HNMR spectra were recorded on a
Varian, Mercury 400 MHz spectrometer in CDCl3 solution using
tetramethylsilane as the internal standard. IR spectra were re-
corded in KBr pellets on a Bruker IFS66 spectrometer. Mass spectra
1,4-bis-(4-p-toluenesulphonyloxy)-9,10-anthraquinone
(1)
used in the synthesis reactions as a substrate was prepared accord-
ing to the procedure published in literature [16,17]. Different
amines were used to synthesize the investigated compounds. In
the synthesis of disubstituted anthraquinones (2), (3), (4) 2.5 M ex-
cess of the amines in relation to 1,4-bis-(4-p-toluenesulphonyl-
oxy)-9,10-anthraquinone (1) was used. In the case of
Scheme 1. Synthetic route of target molecules.

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A. Wcisło et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 108 (2013) 82–88
monosubstituted aminoanthraquinones (5), (6), (7), a stoichiome-
tric amount of amines relative to 1,4-bis-(4-p-toluenesulphonyl-
oxy)-9,10-anthraquinone (1) was used (Scheme 1).
J1 = 8.4 Hz); 7.95–7.97 (dd, 1H, H5–Ar, J1 = 1.6 Hz, J2 = 7.6 Hz);
8.13–8.15 (dd, 1H, H8–Ar, J1 = 1.6 Hz, J2 = 7.6 Hz);
IR (KBr, cmꢁ1): 2928, 1672, 1593, 1376, 1261, 21176, 967, 775,
725;
General procedure for the synthesis of compounds described below
Maldi–Tof: m/z 422.4 [M + H]+; 444.3 [M + Na]+; (MW = 421.46);
1,4-bis(dimethylamino)-9,10-anthraquinone
(103.30 g, 2.23 mmol) was added into a stirred solution of 1,4-
bis-(4-p-toluenesulphonyloxy)-9,10-anthraquinone (500.00 mg,
(2). Dimethylamine
1-(diethylamino)-4-(4-p-toluenesulphonyloxy)-9,10-anthraquinone
(6). Yield: (167.84 mg) 41%. Dark red powder;
0.911 mmol) and potassium carbonate (307.70 g, 2.23 mmol) in
toluene (20 ml). The reaction mixture was vigorously stirred at
80 °C for 6 days. After the reaction mixture was cooled to room
temperature, it was diluted with dichloromethane (50 ml) and fil-
tered. The solvent was evaporated under reduced pressure, dis-
solved in dichloromethane (150 ml) and washed with water
(3 ꢀ 200 ml). The organic layer was dried over MgSO4 and evapo-
rated. The crude product was purified by flash chromatography
on silica gel by eluting with mixture dichloromethane:methanol
(50:1, v/v) mixture to obtain the targeted compound (185.11 mg,
78%) in the form of a dark blue solid.
TLC (SiO2):CH2Cl2:MeOH (5:0.5); Rf = 0.4;
1H NMR (400 MHz, CDCl3) d (ppm): 1.25–1.29 (t, 6H, N–CH2–
CH3, J1 = 6.8 Hz, J1 = 7.2 Hz, J2 = 7.0 Hz); 2.41 (s, 3H, CH3–OTs);
3.54–3.59 (q, 4H, N–CH2–CH3, J = 6.8 Hz); 6.23–6.25 (d, 2H, Ar–
OTs, J1 = 9.6 Hz); 7.23–7.25 (d, 1H, H2–Ar, J1 = 7.8 Hz); 7.48–7.51
(d, 1H, H3–Ar, J1 = 8.4 Hz); 7.64–7.70 (m, 3H, H6–Ar, Ar–OTs,);
7.74–7.78 (t, 1H, H7–Ar, J1 = 6.8 Hz, J1 = 7.6 Hz, J2 = 7.2 Hz); 8.33–
8.36 (dd, 1H, H5–Ar, J1 = 1.2 Hz, J2 = 8.2 Hz); 8.36–8.38 (dd, 1H,
H8–Ar, J1 = 1.2 Hz, J2 = 8.0 Hz);
IR (KBr, cmꢁ1): 2980, 1547, 1461, 1293, 1160, 770;
Maldi–Tof: m/z 450.2 [M + H]+, (MW = 449.13);
TLC (SiO2):CH2Cl2:MeOH (5:0.2); Rf = 0.2;
1H NMR (400 MHz, CDCl3) d (ppm): 2.96 (s, 12H, Ar–N–(CH3)2);
7.15–7.17 (d, 1H, H2–Ar, J1 = 7.6 Hz); 7.32–7.34 (d, 1H, H3–Ar,
J1 = 7.4 Hz); 7.,61–7.71 (m, 2H, H6–Ar, H7–Ar); 8.17–8.20 (d, 1H,
H5–Ar, J1 = 8.0 Hz); 8.20–8.23 (d, 1H, H8–Ar, J1 = 8.6 Hz);
Maldi–Tof: m/z 295.2 [M + H]+; (MW = 294.34);
1-((2-hydroxyethyl)(methyl)amino)-4-(4-p-toluenesulphonyloxy)-
9,10-anthraquinone (7). Yield: (300.40 mg) 73%. Dark red powder;
TLC (SiO2):CH2Cl2:MeOH (5:0.5); Rf = 0.25;
1H NMR (400 MHz, CDCl3) d (ppm): 2.32 (s, 3H, CH3–OTs); 2.87
(s, 3H, CH3–N–); 3.64–3.72 (t, 2H, CH3–N–CH2–CH2–OH,
IR (KBr, cmꢁ1): 2940, 1627, 1589, 1565, 1260, 725;
J1 = J2 = 4.8 Hz);
3.79–3.80
(t,
2H,
CH3–N–CH2–CH2–OH,
J1 = 4.4 Hz, J1 = 4.8 Hz, J2 = 4.6 Hz); 7.22–7.25 (d, 2H, Ar–OTs,
J1 = 8.0 Hz); 7.32–7.35 (d, 1H, H2–Ar, J1 = 9.2 Hz); 7.40–7.42 (d,
1H, H3–Ar, J1 = 9.6 Hz); 7.65–7.70 (dt, 1H, H6–Ar, J1 = 1.6 Hz,
J2 = J3 = 7.2 Hz); 7.70–7.74 (dt, 1H, H7–Ar, J1 = 2.0 Hz, J2 = 7.2 Hz,
J2 = 7.6 Hz, J3 = 7.4 Hz); 7.79–7.82 (d, 2H, Ar–OTs, J1 = 8.4 Hz);
7.99–8.01 (dd, 1H, H5–Ar, J1 = 1.6 Hz, J2 = 7.2 Hz); 8.15–8.17 (dd,
1H, H8–Ar, J1 = 1.2 Hz, J2 = 7.6 Hz);
1,4-bis(diethylamino)-9,10-anthraquinone (3). Yield: (255.53 mg)
80%. Dark blue powder;
TLC (SiO2):CH2Cl2:MeOH (5:0.5); Rf = 0.55;
1H NMR (400 MHz, CDCl3) d (ppm): 1.10–1.14 (t, 6H, N–CH2–
CH3, J1 = 6.8 Hz, J1 = 7.2 Hz, J2 = 7.0 Hz); 1.39–1.43 (t, 6H, N–CH2–
CH3, J1 = J2 = 7.2 Hz); 3.20–3.25 (q, 4H, N–CH2–CH3, J1 = 6.8 Hz,
J1 = 7.2 Hz, J2 = 7.0 Hz, J3 = 7.0 Hz); 3.43–3.48 (q, 4H, N–CH2–CH3,
J1 = 6.0 Hz, J1 = 7.6 Hz, J2 = J3 = 6.8 Hz); 7.22–7.24 (d, 1H, H2–Ar,
J1 = 9.6 Hz); 7.68–7.70 (dd, 1H, H3–Ar, J1 = 1.6 Hz, J2 = 6.8 Hz);
7.71–7.75 (dt, 1H, H7–Ar, J1 = 1.6 Hz, J2 = 7.6 Hz, J2 = 7.2 Hz,
J3 = 7.4 Hz); 7.78–7.82 (dt, 1H, H6–Ar, J1 = 1.6 Hz, J2 = 7.6 Hz,
J2 = 7.2 Hz, J3 = 7.4 Hz); 8.33–8.35 (dd, 1H, H5–Ar, J1 = 1.2 Hz,
J2 = 7.6 Hz); 8.35–8.37 (dd, 1H, H8–Ar, J1 = 1.2 Hz, J2 = 7.6 Hz);
IR (KBr, cmꢁ1): 2972, 1577, 1490, 1287, 727;
IR (KBr, cmꢁ1): 3452, 2920, 1668, 1353, 1203, 1027, 713.
Maldi–Tof: m/z 452.4 [M + H]+, (MW = 451.49);
Spectroscopic measurements
All measurements were carried out in acetonitrile and an aceto-
nitrile:methanol mixture. The band separation in the absorption
spectra and the determination of the band positions were per-
formed by using Spectra DataLab software [18]. The concentrations
of compounds measured spectrophotometrically were around
10ꢁ4–10ꢁ5 M. All measurements were performed at the tempera-
ture of 298 K. For all compounds, the molar absorption coefficient
values were determined.
Maldi–Tof:
m/z
351.3
[M + H]+;
373.5
[M + Na]+;
(MW = 350.45);
1,4-bis((2-hydroxyethyl)(methyl)amino)-9,10-anthraquinone
(4). Yield: (174.43 mg) 54%. Dark blue powder;
TLC (SiO2):CH2Cl2; Rf = 0.43;
Acid–base titrations were carried out in acetonitrile:methanol
(9:1) mixture. The pH value was obtained by potentiometric titra-
tions using microtitration automatic system Cerko-Lab. In acid-
base titrations the compound was dissolved in methanesulfonic
acid (CH4SO3) solution (in acetonitrile) and titrated by sodium/tet-
rabutylammonium hydroxide (in methanol).
In the case of the compound containing two tertiary nitrogen
groups, another method was used. The compounds were dissolved
in acetonirile and titrated with 0.1 M methanesulfonic acid solu-
tion (in acetonitrile) or sodium/tetrabutylammonium hydroxide
(in methanol). The pKa values were calculated by using Origin
Lab software.
1H NMR (400 MHz, CDCl3) d (ppm): 2.35 (s, 3H, CH3–N–); 2.70
(s, 3H, CH3–N–); 3.10–3.13 (t, 4H, CH3–N–CH2–CH2–OH,
J1 = 4.8 Hz, J1 = 5.2 Hz, J2 = 5.0 Hz); 3.90–3.93 (t, 4H, CH3–N–CH2–
CH2–OH, J1 = J2 = 4.8 Hz); 7.32–7.35 (d, 2H, H2–Ar, H3–Ar
J1 = 8.0 Hz); 7.45–7.60 (m, 2H, H6–Ar, H7–Ar); 7.71–7.73 (d, 2H,
H5–Ar, H8–Ar, J1 = 8.0 Hz);
IR (KBr, cmꢁ1): 3389, 2926, 1654, 1454, 1185, 1034, 814;
Maldi–Tof:
m/z:
355.0
[M + H]+,
357.3
[M + Na]+,
(MW = 345.39);
1-(dimethylamino)-4-(4-p-toluenesulphonyloxy)-9,10-anthraquinone
(5). Yield: (176.70 mg) 46%. Dark red powder;
TLC (SiO2):CH2Cl2:MeOH (5:0.2); Rf = 0.4;
Potentiometric titrations
1H NMR (400 MHz, CDCl3) d (ppm): 2.29 (s, 3H, CH3–OTs); 3.03
(s, 6H, Ar–N–(CH3)2); 7.19–7.22 (d, 2H, Ar–OTs, J1 = 9.6 Hz); 7.24–
7.25 (d, 1H, H2–Ar, J1 = 4.8 Hz); 7.34–7.36 (d, 1H, H3–Ar,
J1 = 9.6 Hz); 7.62–7.66 (dt, 1H, H6–Ar, J1 = 1.6 Hz, J2 = 7.2 Hz,
J2 = 7.6 Hz, J3 = 7.4 Hz); 7. 68–7.72 (dt, 1H, H7–Ar, J1 = 1.6 Hz,
J2 = 7.2 Hz, J2 = 7.6 Hz, J3 = 7.4 Hz); 7.77–7.79 (d, 2H, Ar–OTs,
Potentiometric titrations were performed by means of a microt-
itration automatic Cerko-Lab system equipped with a Schott Blue
line N16 pH electrode and by using a 0.5 ml Hamilton syringe.
The pH glass-electrode was calibrated with acetonitrile:methanol
(9:1) mixture by applying the method based on 2,6-dinitrophenol

A. Wcisło et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 108 (2013) 82–88
85
and its tetrabutylammonium salt [19]. For the evaluation of elec-
trode parameters, the STOICHIO version CVEQUID software was
used [20]. The resolution of the voltage measurements
was <0.1 mV.
Theoretical calculations
Electrode parameters were calculated using a STOICHIO com-
puter program based on the non–linear least-squares Gauss–New-
ton–Marquardt algorithm [21]. The values of equilibrium constant
were calculated by means of Origin Lab software; the Henderson–
Hasselbach equation was applied which is based on the change in
absorption as a function of pH of the solution [22].
The acid–base equilibrium constant Ka can be described by the
following equations:
BH22þ ¢ BHþ þ Hþ
½BHþꢂ½Hþꢂ
ð1Þ
ð2Þ
Ka1
¼
½BH22þꢂ
BHþ ¢ B þ H2
½Bꢂ½Hþꢂ
Ka2
¼
½BHþꢂ
where B is the compound (anthraquinone derivative) in non-ion-
ized form, and BH+, BH22þ is the compound in protonated form.
Deconvolution of the UV–Vis spectra bands was carried out
with the ‘Spectra Data Lab’ computer program developed by Dor-
oshenko [18]; the program uses an iterative non-linear least-
squares method based on the Fletcher–Pauell algorithm.
Results and discussion
Changes in spectroscopic and acid–base properties of the
anthraquinone moiety depend on its closest environment and elec-
tronic state. UV–Vis spectra of the investigated monoamine and
diamine compounds were compared to 1-aminoanthraquinone
(1-AAQ) and 1,4-diaminoanthraquinone (1,4-DAAQ), respectively
(Fig. 1 and Fig. 2). The substitution of the amino group in position
1 of anthraquinone moiety (compound (5)–(7)) alters the molar
absorption coefficient; it also shifts the position of the band in vis-
ible region, as shown in Fig. 2a. Moreover, there is a noticeable
change in the value of band intensity for the visible region in com-
parison to that for UV region. This finding is consistent with the
structural changes in the molecule. The N–H group present in
Fig. 2. UV–Vis spectra recorded for acetonitrile solution of: (a) 1-AAQ and
compound (7) and (b) 1,4-DAAQ and compound (4).
anthraquinone moiety forms a hydrogen bond with the carbonyl
oxygen atom and therefore creates resonance forms [23].
Our studies show that the position and number of the substitu-
ent in anthraquinone moiety is crucial. For only one electron-
donating substituent present in the molecule (compounds (5)–
(7)), a single band is observed in the visible region as shown in
Figs. 1 and 2a. When two substituents are present in the molecule
(compounds (2)–(4)) and their positions are 1 and 4, a two-headed
band is observed (Figs. 1 and 2b). These changes are most likely
consistent with the formation of a hydrogen bond, as shown in
Fig. 3. It is known that the hydrogen bonds are present in 1,4-
diaminoanthraquinone (1,4-DAAQ) and its derivatives [24]. Similar
Fig. 1. UV–Vis spectra recorded for acetonitrile solution of 1-AAQ and 1,4-DAAQ.
Fig. 3. Hydrogen bond formation in aminoanthraquinone molecules.

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A. Wcisło et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 108 (2013) 82–88
Table 1
changes in spectral characteristics have also been noticed in the
case of our compound (4), where a N-disubstituted nitrogen atom
is present. The formation of a hydrogen bond between the carbonyl
oxygen atom and O–H group in hydroxyethylamino substituent in
positions 1 and 4 causes this type of a two-headed band. For the
compounds (2) and (3), where the hydroxyl group is not present,
only weak vibrational overtones associated with the electron flow
are observed (Fig. 4 a). It is possible that this effect is a result of the
formation of tautomeric forms, as has already been discovered by
Ya Fain [25].
All compounds containing only one tertiary nitrogen group
(compounds (5) –(7)) have similar molar absorption coefficients
(Table 1). The position and shape of the visible bands of these com-
pounds are also alike. In comparison to1-aminoanthraquinone (1-
AAQ), the intensity of the visible band is lower and its position is
batochromically shifted (from 466 nm to 500–580 nm). For the
The values of molar absorption coefficient for anthraquinone derivatives (2)–(7), 1-
aminoanthraquinone, 1,4-diaminoanthraquinone and 1-NEt2AQ (for the visible region
band) in acetonitrile.
a
b
Compound
kmax
emax
(2)
(3)
(4)
(5)
(6)
(7)
1-AAQ
1,4-DAAQ
1-NEt2AQ
580
582
570
502
512
504
466
544
512
6990
5840
10960
4510
3790
4500
6230
11220
3790
a
[nm].
b
[dm3 molꢁ1 cmꢁ1].
UV band, a batochromic shift (Dk = 30 nm) and an increase in
intensity is observed. A tosyl group has no influence on the spectral
characteristic of the compound in the visible range (Table 1). Com-
paring the UV–Vis spectra of compound (6) and a similar derivative
without a tosyl group (1-NEt2AQ), no changes in the visible range
have been found (Fig. 5).
For 1- and 4-substituted compounds (2)–(4), the more complex
changes are observed. Compound (4) is characterized by similar
molar absorption coefficient, shape and position of the visible band
compared to 1,4-diaminoanthraquinone (1,4-DAAQ), whereas the
compounds (2) and (3) show lower intensity of this band and its
position is shifted by about 10 nm. The vibrational overtones are
Fig. 5. UV–Vis spectra of compounds 1-NEt2AQ and (6).
also present, but they are not that intense as in compound (4)
and 1,4-diaminoanthraquinone (1,4-DAAQ).
Our studies show that N-substituted aminoanthraquinones are
bases. The compounds containing two nitrogen atoms (compounds
(2)–(4)) should be described by two dissociation constants,
whereas the monosubstituted derivatives (compounds (5)–(7))
by one. In A vs. pH curves two equilibria for diamino- and one
for monoaminoanthraquinones are observed. This finding is con-
sistent with the presence of isosbestic points (Figs. 6 and 7).
According to Eqs. (1) and (2) (in Theoretical calculations), two
pKa values are possible to calculate for the derivatives containing
two amino groups and one for monosubstituted anthraquinones
(Table 2). The pKa2 values for the investigated compounds do not
change substantially (pKa2 range 17.76–18.53), whereas for pKa1
values a significant variation has been observed (pKa1 range
4.82–13.56). When changing the substituent from methyl to ethyl
group, the respective pKa1 values change by about six units (from
pKa1 = 13.56 to pKa1 = 7.84). The second dissociation constant does
not change. Small decrease in the second constant is observed
when hydroxyl group is present (pKa2 = 17.76), but the pKa1 value
is much more acidic than for compounds (2) and (3)
(pKa1 = 4.82). In comparison to 1,4-DAAQ, the acid dissociation
constant of compound (4) is much lower. The monosubstituted
derivatives are described by higher acid dissociation constant
(pKa ꢃ 10) than 1-AAQ (pKa = 8.01). Despite low basicity of 1-
AAQ, due to the hydrogen bond formation between N–H group
and the carbonyl oxygen atom, the 1,4-DAAQ exhibits an addi-
tional interaction stabilizing the deprotonated form, which lowers
Fig. 4. UV–Vis spectra of compounds: (a) (2), (3), (4) and (b) (5), (6), (7).

A. Wcisło et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 108 (2013) 82–88
87
Table 2
pKa values (with uncertainty) for anthraquinone derivatives (2)–(7), 1-aminoanthra-
quinone and 1,4-diaminoanthraquinone in acetonitrile (or acetonitrile:methanol).
Compound
pKa1
pKa2
(2)
(3)
(4)
(5)
(6)
(7)
1-AAQ
1,4-DAAQ
13.56 0.02
7.84 0.04
4.82 0.89
9.07 0.02
10.54 0.03
10.10 0.01
8.01 0.01
6.06 0.01
18.41 0.04
18.53 0.06
17.76 0.61
–
–
–
–
–
the pKa value (down to pKa = 6.06). Similar phenomena are ob-
served for compound (4).
The high values of pKa are consistent with the basicity of the
solvent and its capability of solvating the compounds [26]. The
pKa values suggest that the compounds containing two tertiary
nitrogen atoms (compounds (2)–(4)) display higher value of the
variable pKa2 than those with one such substituent (compounds
(5)–(7); by about seven units). The basicity of such compound is
higher. The number of substituents containing tertiary nitrogen af-
fects the basicity of the compound.
Conclusions
Amino substituents present in anthraquinone ring are responsi-
ble for the hue of these compounds. They change the absorption
spectra of the molecule. The number and position of these groups
plays a significant role in changing the spectra. The presence of two
substituents does not result is doubling of the monosubstituted
compound’s spectrum but rather gives rise to vibrational overtones
evident in the visible region of UV–Vis spectra. This phenomenon is
caused by the hydrogen bond between N–H group (or O–H group
in hydroxyethylamino group) of the substituent and the carbonyl
oxygen atom of anthraquinone moiety. The disubstituted com-
pound in comparison to the monosubstituted one has higher pKa
values. In conclusion, the substituent in positions 1 and 4 of
9,10-anthraquinone provides many changes in the spectral behav-
ior of the molecule. Even in the case when no possibility of the
hydrogen bond formation has occurred, weak vibrational over-
tones are still present. The formation of a tautomeric form can be
possibly responsible for this phenomenon. The presented investi-
gation allowed us to determine the role of the aforementioned sub-
stituents, and therefore made it possible to synthesize more
complicated derivatives which, hopefully, will have practical use
during the formation of molecular recognition systems.
Fig. 6. Titration spectra for compound (2) in acetonitrile:methanol (9:1) mixture
for the pH range 7.9–19.7: (a) full titration curve and (b) UV–Vis spectra for
different pH values]. For titration conditions, see in Spectroscopic measurements.
Arrows indicate the change in absorbance during the titration.
Acknowledgements
This study was financed by the State Funds for Scientific Re-
search via the National Center for Science Grant No. (NN204 122
640), and by the University of Gdansk within the project support-
ing young scientists and PhD students (Grant No. 538-8210-1028-
12).
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