3-(para-Substituted phenylhydrazo)pentane-2,4-diones: Physicochemical and solvatochromic properties

Article abstract of DOI:10.1016/j.jphotochem.2011.02.006

New azoderivatives of pentane-2,4-dione, 3-(4-bromophenylhydrazo)pentane-2, 4-dione (HL₁) and 3-(4-acetylphenylhydrazo)pentane-2,4-dione (HL ₂), have been synthesized by reaction between pentane-2,4-dione and the corresponding aryldi

Full text of DOI:10.1016/j.jphotochem.2011.02.006

Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 159–165
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
3
-(para-Substituted phenylhydrazo)pentane-2,4-diones: Physicochemical and
solvatochromic properties
a,b,∗
b
b
b
Kamran T. Mahmudov
, Abel M. Maharramov , Rafiga A. Aliyeva , Ismayil A. Aliyev ,
b
a
a
a
Rizvan K. Askerov , Rasim Batmaz , Maximilian N. Kopylovich , Armando J.L. Pombeiro
Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, TU Lisbon, Av. Rovisco Pais, 1049–001 Lisbon, Portugal
a
b
Baku State University, Department of Chemistry, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan
a r t i c l e i n f o
a b s t r a c t
Article history:
1
New azoderivatives of pentane-2,4-dione, 3-(4-bromophenylhydrazo)pentane-2,4-dione (HL ) and 3-
Received 19 November 2010
Received in revised form 1 February 2011
Accepted 8 February 2011
(
4-acetylphenylhydrazo)pentane-2,4-dione (HL2), have been synthesized by reaction between pentane-
1
13
2
,4-dione and the corresponding aryldiazonium salts, and characterized by IR, UV–vis, H and C NMR
spectroscopies, potentiometry, X-ray diffraction analysis (for HL2) and thermogravimetry. The collected
data confirm that HL1–2 exist in DMSO solution and in solid phase exclusively in the hydrazo form, being
stabilized by an intramolecular hydrogen bonding. The thermodynamics of proton dissociation of HL1–2
and of the related known 3-(4-nitrophenylhydrazo)pentane-2,4-dione (HL3), in water–ethanol media,
were studied quantitatively and the corresponding functions were calculated, showing a dependence
on the inductive electron-acceptor character of the substituent in para-position of the aromatic part of
the molecule. The thermal decompositions of HL1–3 were also investigated showing their high thermal
stability with well-defined peaks of phase transition at 413–418 K. The wavelengths (or wavenumbers) of
maxima of absorption bands in the UV–vis spectra of HL1–3 and the corresponding absorption coefficients
are dependent on the para substituent, but no correlation was found with any particular property of the
solvent, such as polarity, H-bond donating or accepting ability.
Available online 15 February 2011
Keywords:
Azoderivatives of pentane-2,4-dione
Crystal structure
Hydrogen bonding
Thermodynamic parameters
Thermal analysis
Solvatochromism
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
[16,17], as effective ionophores for copper-selective electrodes [18]
or for the synthesis of metal complex catalysts for some oxidation
reactions [19].
Azo dyes constitute the largest group of colorants in terms of
their number and production volume. Such an abundance is driven
by the simplicity and versatility of their synthesis by diazotization
and azo coupling, as well as by their generally high specific light
absorption and fastness properties [1]. An important group of dis-
perse dyes concerns the azoderivatives of ␤-diketones (ABD, for
particular examples described in this paper see Scheme 1) which
were found to have interesting semiconducting [2], antineoplas-
tic [3,4], spectral [5], analgesic [6,7], antipyretic [6], antibacterial
Relevant properties of ABD dyes, such as colour, thermal sta-
bility and solubility, can significantly vary with the substituents
in the aromatic or ␤-diketone fragments, which also affect the
hydrazo ꢀ enol-azo tautomerism within the molecule [19]. This
tautomeric balance can also play an important role for the appli-
cation of ABD as bistate molecular switches [20–23] or regulate
the selectivity in analytical chemistry [16,17]. The equilibrium
between the mentioned tautomeric forms is dependent on sev-
eral factors, related to the structure of ABD or external influences
(e.g. solvent or temperature) [19]. Thus, it has been observed that
in ABD formed by unsymmetric ␤-diketones, a six-membered H-
bonded ring is generated at the more sterically favourable side of
the molecule [19,24] while the enol-azo tautomer starts to play
an essential role in solution [19]. Hence, an understanding of the
factors which influence the structure of ABD dyes in solution and
in solid state allows to evaluate their potential applications, e.g.
as induced molecular switches [20–23], optical recording media
[
8–10] and photoluminescent [11] properties, being also studied as
perspective optical recording media and spin-coating films [12,13]
or being applied for further organic synthesis [14,15]. ABD contain-
ing an hydroxy group in ortho-position of the aromatic part of the
molecule were also applied for spectrophotometric determinations
of copper(II) and iron(III) in various industrial and natural objects
∗ Corresponding author at: Centro de Química Estrutural, Complexo I, Instituto
Superior Técnico, TU Lisbon, Av. Rovisco Pais, 1049–001 Lisbon, Portugal.
Tel.: +35 1934367357.
[
12,13] or solvatochromic colorants [1,25–27].
The intramolecular proton transfer related to the
hydrazo ꢀ enol-azo tautomerism is supported by the so-named
resonance-assisted hydrogen bonding (RAHB) which describes a
E-mail addresses: kamran chem@yahoo.com, kamran chem@mail.ru
(
K.T. Mahmudov), pombeiro@ist.utl.pt (A.J.L. Pombeiro).
1
010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.02.006

1
60
K.T. Mahmudov et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 159–165
Scheme 1. 3-(para-Substituted phenylhydrazo)pentane-2,4-diones considered in this work (both the hydrazo and enol-azo forms are presented).
synergistic reinforcement of the hydrogen bond by delocaliza-
tion of the ␲-conjugated chain connecting donor and acceptor
atoms [20–23,28]. Consequently, the study of the influence of
various factors on the RAHB is necessary for understanding how
to play with this tautomery. For example, the intramolecular
proton transfer is strongly influenced upon dissolution, when
intermolecular hydrogen and van-der-Waals interactions with
the solvent molecules change the distribution of the tautomers.
Thus, the solvent induces changes in the electronic transitions of
the colorant, which are dependent on the nature and extent of the
solute–solvent interactions (solvatochromism) [25–27]. However,
the presence of both tautomeric forms can hamper the interpreta-
tion of the spectroscopic data due to the bands overlapping [19],
while theoretical calculations sometimes allow to overcome these
difficulties by rationalizing the relation between the tautomeric
ratio and properties [20–23,29]. Thus, the synthesis of new ABD
performing solvatochromism and the search for correlations
between their physicochemical and spectral properties would be
an important contribution to the effective preparation of new dyes.
Taking in mind the above considerations, we focused this work
on the following aims: (i) to synthesize new ABD with bromo-
and acetyl- substituents in para-position of the aromatic ring
of the molecule (Scheme 1); (ii) to trace the influence of the
introduced substituents on the tautomeric balance and the physico-
chemical properties of ABD; (iii) to study solvatochromism of
the newly synthesized ABD as well as their known analog 3-(4-
spectrophotometer (Perkin-Elmer) in 1.00 cm quartz cells at room
temperature, with a concentration of HL1–3 of 3.50 × 10 M.
−
5
2
.2. Synthesis of HL1–2
The bromo- and acetyl-para-substituted azoderivatives of
pentane-2,4-dione were synthesized via the Japp–Klingemann
reaction [24,28,31] between the aromatic diazonium salts of the
corresponding 4-bromoaniline or 4-acetylaniline and pentane-2,4-
dione in water solution containing sodium hydroxide.
2
.2.1. Diazotization
-Bromoaniline or 4-acetylaniline (0.025 mol) was dissolved in
0 mL of water and then 1.00 g of crystalline NaOH was added.
4
5
The solution was cooled in an ice bath to 273 K and then 1.725 g
0.025 mol) of NaNO2 was added; 5.00 mL (33%) HCl were then
(
added in portions for 1 h. The temperature of the mixture should not
exceed 278 K. The resulting diazonium solution was used directly
in the following coupling procedure.
2.2.2. Coupling
NaOH (1.00 g, 0.025 mol) was added to a mixture of 2.55 mL
(
0.025 mol) of pentane-2,4-dione with 50 mL of water. The solu-
tion was cooled in an ice bath to ca. 273 K, and a suspension of
-substituted diazonium (see above) was added in three portions
under vigorous stirring for 1 h.
The identity of HL was demonstrated by element analysis, IR
nitrophenylhydrazo)pentane-2,4-dione (HL ), bearing the stronger
3
4
electron acceptor nitro substituent.
1–2
1
13
and H and C NMR spectroscopies.
2
. Experimental
HL : yield 85% (based on pentane-2,4-dione), orange powder
1
soluble in DMSO, methanol, ethanol and acetone, and insoluble in
2.1. Equipment and materials
water. Elemental analysis: C11H11BrN O (M = 283.1); C 46.58 (calc.
2
2
The 1H and 13C NMR spectra were recorded at ambient tem-
46.66); H 3.90 (3.92); N 9.87 (9.89)%. IR (KBr): 3448 ꢀ(NH), 1671
−
1 1
perature on a Bruker Avance II + 300 (UltraShieldTM Magnet)
ꢀ(C O), 1623 ꢀ(C O· · ·H), 1581 ꢀ(C N) cm
. H NMR in DMSO,
internal TMS, ı (ppm): 2.09 (s, 3H, free CH CO), 2.41 (s, 3H, CH CO
spectrometer operating at 300.130 and 75.468 MHz for proton
and carbon-13, respectively. The chemical shifts (ı) are reported
in ppm using tetramethylsilane as the internal reference. The
3
3
in H-bond), 7.15–7.29 (m, 2H, Ar–H), 7.52–7.55 (m, 2H, Ar–H), 10.22
1
(
s, 1H, N–H). 13C–{ H} NMR in DMSO, internal TMS, ı (ppm): 28.81
−1
(CH ), 30.70 (CH ), 116.85 (Ar–Br), 118.49 (Ar–H), 120.85 (Ar–H),
infrared spectrum (4000–400 cm ) was recorded on a Nicolet FT-
IR Nexus spectrophotometer on KBr pellets. Thermal properties
were analyzed with a Perkin-Elmer Instruction system (STA6000)
3 3
1
1
31.42 (Ar–H), 132.20 (Ar–H), 134.14 (C N), 142.48 (Ar–NH–N),
94.20 (C O), 196.23 (C O).
−1
HL : yield 79% (based on pentane-2,4-dione), orange powder
at a heating rate of 10 K min under a dinitrogen atmosphere. The
acidity of the solutions was measured using an I-130 potentiometer
with an ESL-43-07 glass electrode and an EVL-1M3.1 silver–silver
chloride electrode adjusted by standard buffer solutions. The
pH-metric titration was carried out in water–ethanol with consid-
eration of the Bates correction [30] at different temperatures of
2
soluble in DMSO, methanol, ethanol and acetone, and insoluble in
water. Elemental analysis: C13H14N O (M = 246.3); C 63.28 (calc.
2
3
6
3.40); H 5.69 (5.73); N 11.32 (11.38)%. IR (KBr): 3449 ꢀ(NH), 1675
−
1
1
ꢀ
(C O), 1642 ꢀ(C O), 1599 ꢀ(C O· · ·H), 1509 ꢀ(C N) cm
.
H
NMR in DMSO, internal TMS, ı (ppm): 1.79 (s, 3H, free CH CO),
3
2
.05 (s, 3H, free CH CO), 2.39 (s, 3H, CH CO in H-bond), 7.57
2
98 ± 0.5, 308 ± 0.5, and 318 ± 0.5 K. A constant temperature was
3
3
13
1
(m, 2H, Ar–H), 7.93 (m, 2H, Ar–H), 12.87 (s, 1H, N–H). C–{ H}
maintained within ± 0.5 K by using an ultrathermostat (Neslab 2
RTE 220). Solvents of spectrophotometric grade were purchased
from Sigma–Aldrich and used without extra purification. The
UV–vis absorption spectra in the 200–700 nm region were recorded
NMR in DMSO, internal TMS, ı (ppm): 26.51 (CH ), 27.03 (CH ),
3
3
3
0.69 (CH ), 115.91 (Ar–H), 122.96 (Ar–H), 129.62 (Ar–H), 129.98
3
(
Ar–H), 132.65 (Ar–COCH ), 135.58 (C N), 147.01 (Ar–NH–N),
3
−1
173.30 (C O), 196.40 (C O), 196.71 (C O).
with a scan rate of 240 nm min
by using a Lambda 35 UV–vis

K.T. Mahmudov et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 159–165
161
−
2.3. Crystallography
σ
p
1
.4
−
2
σ
p
= 0.23δN−H – 2.16, r = 0.998
−NO
2
The crystals of HL2 suitable for X-ray structural analysis were
1.2
grown by slow evaporation at room temperature of its ethanol
solution. Intensity data were collected at room temperature on a
Rigaku Mercury CCD diffractometer with a graphite monochro-
mator using Mo-K␣ radiation (ꢁ = 0.71073 A˚ ). The structure was
−
C(=O)CH
3
1
0
0
0
.0
.8
.6
.4
−Br
solved by direct methods and Fourier’s difference, and refined by
least squares on F2 with anisotropic displacement parameters for
non-H atoms. All hydrogen atoms were located from difference
Fourier maps and refined isotropically. All calculations to solve the
structures, to refine the model proposed and to obtain the data
were carried out with SHELXS-97 and SHELXL-97 and SHELXTL/PC
computer programs [32–35]. The crystallographic and selected
structural details are listed in Tables S1 and S2, respectively. Crys-
tallographic data for the structure reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as sup-
0.2
0
1
0
11
12
13
14
15
δ
N−H/ ppm
−
Fig. 1. Correlation between the polar conjugation substituent constant ꢃp and ıN–H
for HL1–3.
3
. Results and discussion
3
.1. Synthesis of HL and their spectroscopic study
plementary publication, CCDC (764147) for compound HL . Copies
1−2
2
of this information may be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data
request/cif.
The new ABD compounds HL
(Scheme 1) were synthesized
1−2
by diazotation of the corresponding 4-substituted aniline and reac-
tion of the thus formed diazonium salt with pentane-2,4-dione,
following the Japp–Klingemann procedure [24,28,31]. Although
this reaction is usually performed in the presence of sodium acetate,
attempts to prepare the bromo- and acetyl-para-substituted ABD
using this base gave low yields of highly impure products. Thus, we
have modified the reported procedure and found that good yields
of those ABD can be obtained when the coupling is undertaken in
a sodium hydroxide solution [29]. For comparative purposes, we
2.4. Potentiometric measurements
The following mixtures (i) and (ii) were prepared and titrated
potentiometrically against standard 0.005 M KOH in 40/60% (v/v)
water–ethanol mixture at 298 K.
5
mL 0.001 M HCl + 5 mL 1 M KCl + 30 mL ethanol;
(i)
have also studied the known [39] analogous para-NO substituted
2
ADB (HL ).
3
5
mL 0.001 M HCl + 5 mL 1 M KCl + 25 mL ethanol
5 mL 0.001 M HL1
The IR spectra of the thus synthesized ABD reveal ␯(NH) at ca.
−
1
3
1
448–3449 cm . The carbonyl [ꢀ(C O) 1642–1675, ꢀ(C O· · ·H)
+
;
(ii)
−1
−1
−3
623–1599 cm ] and ꢀ(C N) (1509–1581 cm ) vibrations indi-
cate that in the solid state HL1 are stabilized in the H-bonded
−2
For each mixture, the volume was made up to 50.00 mL with bidis-
tilled water before the titration process. These titrations were also
repeated at temperatures of 308 and 318 K.
1
hydrazo form, what is also supported by X-ray data (see below). H
NMR spectra of HL1
temperature show a resonance at ı 10.22 (HL ), 12.87 (HL ) or
in dimethylsulfoxide-d6 solution at room
−3
1
2
1
4.54 (HL ) [39], which is assigned to the proton of the proto-
3
2.5. Calculation of the physico-chemical parameters of thermal
nated nitrogen atom adjacent to the aryl unit ( N–NH– hydrazo
form). In the 13C–{ H} NMR spectra, the two methyl groups of
the ␤-diketone moiety yield two resolved singlets, indicating their
non-equivalence, presumably due to formation of an intramolec-
ular six-membered H-bonded ring involving one of the carbonyl
groups and the NH of the hydrazo form (Scheme 2) similar to other
reported ABDs [20–23,28,29,39–42].
The functional groups in para position of the aromatic ring
of the molecule influence the NH chemical shift (ıN–H) of HL1–3
and possible relations between the Hammett’s [ꢃp = 0.23 (–Br),
decomposition of HL1–2
1
The thermodynamic parameters of decomposition of the com-
plexes, namely the activation energy (E*), enthalpy (ꢂH*), entropy
(
ꢂS*) and free energy of decomposition (ꢂG*), as well as the
pre-exponential factor (A), were evaluated graphically using the
Coats–Redfern relationship [36–38]:
ꢀ
ꢁ
ꢂ
ꢃ
∗
−
ln(1 − ˛)
E
AR
ln
= −
+ ln
,
2
∗
T
RT
ϕE
0
.51 (–C( O)CH ), 0.78 (–NO )] or related [inductive ꢃI = 0.47
3 2
where ˛ is the fraction of sample decomposed at temperature T, A is
the pre-exponential factor, ϕ is the heating rate, E* is the activation
o
(
–Br), 0.29 (–C( O)CH ), 0.63 (–NO ); Taft’s ꢃ = 0.26 (–Br), 0.46
3
2
p
−
–C( O)CH ), 0.83 (–NO ); polar conjugation ꢃp = 0.25 (–Br),
3 2
(
2
energy, R is the gas constant. A plot of ln{[−ln(1 − ˛)]/T } against 1/T
0
.84 (–C( O)CH ), 1.27 (–NO )] substituent constants [43–45] and
3 2
gives a slope from which E* was calculated and A was determined
from the intercept. The entropy of activation (ꢂS*), enthalpy of
activation (ꢂH*) and the free energy change of activation (ꢂG*)
were calculated using the following equations:
ıN–H were analyzed. The best correlation was found for the polar
−
conjugation ꢃp substituent constant (Fig. 1) (for comparison:
2
o
2
ꢃp = 1.25ıN–H −1.06, r = 0.986; ꢃ = 1.27 ıN–H −1.07, r = 0.912),
p
but there is no reasonable relation with the inductive ꢃI substituent
ꢄ
ꢅ
2
Ah
constant (ꢃ = 0.027ı
+ 0.12, r = 0.124).
∗
I
N–H
ꢂS = 2.303R log
kTs
3.2. Crystal structure of HL2
∗
∗
ꢂH = E − RTs
The molecule is quasi planar with small deviations around the
C(1)–C(7), C(9)–C(10) and C(9)–C(12) bonds (Fig. 2). As expected,
the bond lengths of free C(12)–O(3) 1.209(2) A˚ and H-bonded
C(10)–O(2) 1.217(2) Å carbonyl groups are not equal. The molec-
ular structure of HL2 involves an intramolecular six-membered
ꢂG = ꢂH − Ts ꢂS∗
∗
∗
where k, h and Ts are the Boltzmann constant, Planck constant and
the DTG peak temperature, respectively.

1
62
K.T. Mahmudov et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 159–165
Scheme 2. Possible tautomeric equilibria in HL1–3.
Table 1
Distances/Å and angles/ of hydrogen bond interactions in HL2.
◦
D–H•••A
Symmetry
Distances
D–H
Angles
D· · ·A
H· · ·A
D–H· · ·A
N(1)–H(1 N)· · · O(2)
N(1)–H(1 N)· · · O(1)
C(2)–H(2)· · · O(3)
C(5)–H(5)· · · O(1)
x, y, z
0.8387(14)
0.8387(14)
0.9300(13)
0.9300(15)
2.588(18)
3.432(23)
3.369(25)
3.248(21)
1.983(14)
2.689(14)
2.705(13)
2.390(13)
128.30(12)
148.42(12)
129.00(11)
153.34(12)
x, y,−1 + z
−x, 1 − y,−z
x + 0.5, −y + 1.5, z − 0.5
hydrogen-bonded
ring,
H(1 N)–N(1)–N(2)–C(9)–C(10)–O(2),
seem to favour the equalization of O(1) and N(2) proton affinities,
lengthening of N· · ·O hydrogen bond distance, and the heterodienic
delocalization. Finally, the intermolecular interactions through the
O1 atom of the para-acetyl group stabilize the overall structure
of HL2 (Table 1). In the overall, the above mentioned factors cre-
ate a multi-state switchable pattern induced by a proton transfer
and an interchange of the single and double bonds within the res-
onant spacer. Hence, some applications of the compound in the
field of functional materials can be envisaged. For that purpose, the
dissociation process connected to the described H-bonds must be
quantified and further discussed.
formed by the NH-moiety and one of the carbonyl groups, simi-
larly to the other described ABD [20–23,28,29,39–42]. This cycle
is a distorted hexahedron with mutual angles N(1)–N(2)–C(9),
C(9)–C(10)–O(2), O(2)–H(1 N)–N(1) and N(2)–C(9)–C(10) equal
◦
◦
◦
◦
to 122.81(13) , 118.97(14) , 128.30 and 123.62(14) , respec-
tively (Table S2). The N–H· · ·O bond with the N· · ·O distance of
2
.588(18) A˚ falls in the N· · ·O distances range, 2.538–2.603 A˚ ,
observed for other ABD [20–23,28,29,39–42], and indicates a
low delocalization within the heterodienic moiety. The N–H· · ·O
◦
angle of 128.30 (Table 1) significantly differs from the average
◦
O–H· · ·O angle (149 [46]) of ␤-diketone enols involved in similar
intramolecular RAHB. No H-bond-induced lengthening of the
3.3. Thermodynamics of HL1–2 dissociation
N–H bond distance is observed. In contrast, the N–H distance
of 0.8387 A˚ is even shorter than the average distance in such
The dissociation constants of HL1–2 were determined by pH-
metric titration using known equations [49] (see Section 2), in
water–ethanol media (Table 2). The ionic strength was maintained
constant (I = 0.1 M) by adding a calculated amount of KCl. The
ability to abstract a proton from various groups in tautomeric
forms of related ligands was evaluated previously [19,50] and it
was considered that pK concerns the proton abstraction from the
hydrazo group ( N–NH–) (See Scheme 2, hydrazo form). The ther-
modynamic parameters of the dissociation process of HL1–2 were
determined using the dependence of the dissociation constants
from temperature and calculated by the following known equations
moieties unperturbed by H-bonding. This bond shortening can be
explained by a combination of the RAHB effect and proton affinity
equalization of the atoms involved, which, in its turn, is tuned by
the electronic properties of the substituents at the phenyl rings
bonded to the nitrogen atom [46–48].
Thus, the para-C( O)CH3 substituent (with ꢃp = 0.51) causes
a significant shortening of the N–H bond and a slight elonga-
tion of the N· · ·O distance. These discrepancies can be explained
by the electron-withdrawing properties of this substituent which
[
51]:
◦
−R(pK(T3) − pK(T1))
◦
◦
ꢂG = 2.303RTpK; ꢂH
=
;
ꢂS
(
1/T ) − (1/T )
3
1
◦
◦
ꢂH − ꢂG
=
.
T
The data thus obtained (Table 2) testify that, with an increase
of temperature and of the inductive electron-withdrawing prop-
erties of the functional groups in para-position of the aromatic
ring [ꢃI = 0.29 (–C( O)CH ), 0.47 (–Br), 0.63 (–NO )] the acidity of
3
2
HL1–3 increases (pK values decrease) and hence the deprotonation
is endothermic. On the other hand, with an increase of the induc-
◦
◦
tive effect of the para-functional groups, ꢂS and ꢂH decrease,
the dissociation processes of HL1–3 thus being mainly contributed
by the enthalpy factor. Those data are consistent with the facts that
Fig. 2. Molecular structure of HL2.

K.T. Mahmudov et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 159–165
163
Table 2
Thermodynamic characteristics of dissociation of HL1–3 in water-ethanol solution.
◦
−1
◦
−1
◦
−1 −1
HL1–3
HL1
T, K
pK
ꢂG , kJ mol
ꢂH , kJ mol
ꢂS , J mol K
298 ± 0.5
8.43 ± 0.03
8.21 ± 0.03
7.98 ± 0.06
48.09 ± 0.12
48.50 ± 0.10
46.22 ± 0.11
−24.16 ± 2.29
−13.35 ± 2.42
−51.40 ± 1.81
3
3
08 ± 0.5
18 ± 0.5
40.89 ± 2.17
44.52 ± 2.32
30.90 ± 1.70
HL2
298 ± 0.5
8.50 ± 0.01
8.26 ± 0.02
8.01 ± 0.03
3
3
08 ± 0.5
18 ± 0.5
HL3 [29]
298 ± 0.5
8.10 ± 0.02
7.95 ± 0.03
7.76 ± 0.05
3
3
08 ± 0.5
18 ± 0.5
with an increase of temperature the solvent molecules can induce
new conformations of HL1–3 (growing of entropy) but at the cost of
breaking a part of the hydrogen bonds among them (dropping of
enthalpy). Hence, the solvent and temperature are essential factors
which potentially allow to shift the tautomeric balance and conse-
quently the dependent properties of the compounds such as their
colour, transparence, conductivity, etc. The influence of the solvents
on the colour of the dyes is described in the following section.
trast to the molar absorption (Table 3 and Fig. 3). The third band
(363–383 nm) can be associated [25,52–54] to the n → ␲* transi-
tion or electron transitions of the hydrazo to the enol-azo form.
A bathochromic shift is also observed for the compound HL3 in
comparison with HL and HL . Under excitation, the ADB dyes can
differently shift between the hydrazo and the enol-azo tautomeric
forms, depending on the substituent and on the bonding acceptor-
donating ability (˛ or ˇ) of the solvent. The obtained data confirm
that εmax depends on the solvent and on the nature of ABD, but no
any clear correlation was found. One can only speculate that due
to different polarizabilities of the ground and excited states, ABD
with non-polarized ground state are better polarized in protic sol-
vents. In this case the excited state is lowered while the ground
1
2
3.4. UV–vis spectra of ABD
As mentioned above, the influences of specific interactions in
solutions, mainly hydrogen bonds, on the absorption spectra is
important and depend on whether the solute molecules act as
donors or acceptors upon interaction with the solvent [25,52,53].
Since polarities of the ground and excited state of a chromophore
are typically different, a change in the solvent polarity will lead to
different stabilizations of the ground and excited states, and thus, to
a change in the energy gap between them. Consequently, the posi-
tion, intensity, and shape of the absorption spectra are driven by
the specific interactions between the solute and solvent molecules.
The investigated ABD dyes show colour dependence on the para-
substituent and on the protic properties of the solvents (Fig. S1).
a
1
1
1
0
0
0
0
.4
.2
.0
.8
.6
.4
.2
HL
1
HL3
HL2
Thus, HL (ABD with –Br substituent) is yellow in acetone, acetoni-
0
1
2
10
260
310
360
410
460
trile, DMF, DMSO, methanol and ethanol (Fig. S1f), while HL (ABD
2
Wavelength, nm
with –C( O)CH substituent) is green in acetone, acetonitrile, DMF,
3
DMSO and yellow in methanol and ethanol (Fig. S1g). On the other
b
1.4
.2
.0
hand, HL (ABD with –NO substituent) is pink in acetone, acetoni-
3
2
1
trile, DMF, DMSO and yellow in methanol and ethanol (Fig. S1h).
Thus, in acetone, acetonitrile, DMF and DMSO the studied ABD
exhibit the following colours, depending on the para-substituent:
yellow (–Br), light green (–C( O)CH ) or pink (–NO ) (Fig. S1a–e),
HL2
1
0.8
0.6
0.4
0.2
0
HL
1
3
2
what can be used for, e.g. qualitative determination of such aprotic
solvents in protic ones, e.g. methanol.
HL
3
The quantitative parameters of the solvatochromic effect are
given in Table 3 and Fig. 3. All the investigated ABD show two or
three (this depends on the solvent used and on the substituent
within the molecule of the colorant) absorption maxima in the
UV-vis spectra. The first band at 236–245 nm can be assigned
2
20
270
320
370
420
Wavelength, nm
c
1.8
1
.6
.4
[
25,52–54] to the excitation of the ␲-electrons within the C O
1
bonds of the pentane-2,4-dione. The position of this band as well
as the molar absorption (εmax) are dependent on the electron-
1
.2
HL
2
withdrawing properties of the para-substituent [ꢃI = 0.63 (–NO ),
1.0
0.8
0.6
2
0
.47 (–Br), 0.29 (–C( O)CH )] (Table 3). The ꢁmax values of ABD do
3
HL
1
not correlate with any particular property of the solvent, such as
polarity, H-bond donor or acceptor ability or electric permittivity.
Thus, neither the electrostatic solvation nor specific solute–solvent
interactions can solely explain the observed solvatochromism.
The second UV absorption bands around 268–276 nm can be
assigned [25,52–54] to the ␲ → ␲* transition of the C N and aro-
matic ring of ABD. Apparently, in this case the para-substituents
and the nature of solvents do not strongly influence ꢁmax, in con-
HL
3
0
0
.4
.2
0
265
350
435
520
605
Wavelength, nm
Fig. 3. Absorption spectra of HL1–3 in CH3OH (a), CH3CN (b) and DMF (c).

1
64
K.T. Mahmudov et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 159–165
Table 3
Data from UV–vis spectra of HL1–3 in different solvents at 298 K.
HL1–3
Solvent
εa
˛b [52]
ˇc [54]
Peak 1
Peak 2
Peak 3
ꢁmax, nm
εmaxd
ꢁmax, nm
εmaxd
ꢁmax, nm
εmaxd
HL1
HL2
HL3
CHCl3
5
0.44
0.00
–e
–e
–e
–e
371
372
383
27492
24789
24786
243
9014
274
7324
e
e
e
e
–
–
–
–
HL2
HL2
HL2
THF
8
21
30
33
0.00
0.08
0.83
0.93
0.54
0.48
0.77
0.62
240
–e
9014
–e
269
–e
4788
–e
369
367
369
23098
6760
Acetone
Ethanol
Methanol
239
9039
272
6780
25423
HL1
HL2
HL3
–f
–f
268
273
270
36363
5650
33142
365
368
375
18787
22599
33714
239
7345
f
f
–
–
HL1
HL2
HL3
CH3CN
DMF
37
38
47
0.19
0.00
0.00
0.31
0.76
0.76
245
237
236
9970
14370
7122
–g
274
278
–g
11540
4273
363
366
373
18127
36901
15954
e
e
HL1
HL2
HL3g
–
–
274
275
276
49576
9019
37628
365
368
381
16928
30440
6841
e
e
–
–
e
e
–
–
HL2
a
DMSO
–e
–e
276
25352
370
42243
Electric permittivity of solvent.
b
c
d
e
f
H-bond donating ability of solvent.
H-bonding acceptor ability of solvent.
L mol cm
Due to high absorbance by solvent, the peaks were overlapped.
Peak not observed.
A new peak is observed at 537 nm with εmax = 9122 L mol cm .
−1
−1
.
g
−1
−1
state is hardly affected, and hence the energy difference between
the ground and excited states decreases. Thus, H1–3 exhibit specific
solvent-dependent absorptions at 236–537 nm that made them
efficient solvatochromophores.
3.5. Thermal behaviour of HL1–3
Potentially ADB can be good candidates for optical recording
media [12,13] or optical sensors [1]. For such and similar applica-
tions, the thermal behaviour of the material is an important issue
[
55–57]. Thus, we investigated the thermal decomposition of the
dyes by simultaneous thermogravimetry–differential calorimetry,
and the corresponding physicochemical parameters of this process
were calculated (Table 4).
Differential thermogravimetric (DTG) curves of HL1–3 show
phase transition peaks at 413 (–NO ), 415 (–Br) and 418 K
2
(
–C( O)CH ) without significant mass loss (Fig. 4, for HL ). As the
Fig. 4. Thermal decomposition of HL1.
3
1
temperature increases, TG and DTG curves exhibit a sharp mass loss
in the interval 435–588 K, obviously related to decomposition of the
compounds. The kinetic parameters of this process were evaluated
negative entropy of activation suggests that the thermal decompo-
sition reaction is slow [55]. The values of the pre-exponential factor
in the Arrhenius equation (A) are within 10 –10
to establish the reaction molecularity [55–57]. The found parame-
ters with a sharp thermal decomposition threshold and high mass
loss rate are characteristic for optical storage materials forming a
small and sharp recording mark edge [58].
(
Table 4) and show that the activation enthalpy of decomposi-
8
10 −1
s , not allowing
tion (and hence the rate of decomposition) lay in the following
order: HL > HL > HL , which is in accord with the Hammett’s and
3
2
1
related substituent constants of the functional groups. The val-
−
1
ues of E* and ꢂH* differ by a small value of 4.24–4.39 kJ mol
meaning that within experimental error they are equivalent. The
Table 4
Parameters of thermal decomposition of HL1–3.
A (s−1)
E* (kJ mol
−1
)
ꢂS* (J K mol
−1
−1
)
ꢂH* (kJ mol
−1
)
−1
ꢂG* (kJ mol )
Dye
Temperature of
phase
transition (K)
Temperature of
decomposition
(maximum) (K)
Weight loss, %
8
1
1
HL1
HL2
HL3
415
418
413
435–587 (528)
448–583 (514)
453–588 (513)
44.95
45.56
33.48
1.5 × 10
1.2 × 10
8.5 × 10
93.53
112.24
120.55
−93.12
−56.50
−40.21
89.14
108.00
116.29
138.35
137.05
95.66
0
0

K.T. Mahmudov et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 159–165
165
4
. Conclusions
New para-Br (HL ) and –C( O)CH (HL ) substituted phenylhy-
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1
3
2
drazo derivatives of pentane-2,4-dione were synthesized and fully
characterized. It was found that HL1–2 are stabilized in the hydrazo
form in solution and in the solid state by an intramolecular six-
membered, hydrogen-bonded ring. A higher inductive effect of the
substituent [–C( O)CH < –Br < –NO ] leads to decreases of pK and
of ꢂG , ꢂH and ꢂS for the proton dissociation process of HL1–3.
The maxima and intensity of ␲ → ␲* and n → ␲* absorption bands
of the studied compounds (ꢁmax and εmax) depend on their struc-
ture and solvent, but no correlation was found with any particular
property of the solvent such as polarity, H-bond donating and H-
bond accepting ability. However, the investigated dyes can be used
for qualitative determinations of aprotic solvents in protic ones. On
the other hand, their absorption in the blue part of the spectrum
in combination with their thermal stability, high weight loss rate
and sharp thermal decomposition threshold make them potential
high-density optical recording materials.
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