Solvatochromic effects in the absorption spectra of some azobenzene compounds

Article abstract of DOI:10.1016/j.molstruc.2008.01.053

Solvatochromic behavior of some 4,4′-substituted azoaromatic derivatives has been investigated in solvents of different polarities. The solvent dependent UV/vis spectral shifts, ν_max, were analyzed using some physical parameters such as refractive index, dielectric constant, Kamlet-Taft parameters α (hydrogen bond donating ability) and β (hydrogen bond accepting ability). The intermolecular interaction types in the azobenzene derivatives solutions were established on the basis of a multiple linear regression analysis. The fitting coefficients obtained from this analysis allowed us to estimate the contribution of each type of interactions to the total spectral shift in the studied solutions.

Full text of DOI:10.1016/j.molstruc.2008.01.053

Journal of Molecular Structure 887 (2008) 216–219
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Solvatochromic effects in the absorption spectra of some azobenzene compounds
a
Elena Rusu a, Dana-Ortansa Dorohoi b, , Anton Airinei
*
a ‘‘Petru Poni” Institute of Macromolecular Chemistry, 41 A Aleea Grigore Ghica Voda, Iaßsi 700487, Romania
b Department of Physics, Faculty of Physics, ‘‘Al. I. Cuza” University, 11 Carol I Bvd., Iaßsi 700056, Romania
a r t i c l e i n f o
a b s t r a c t
Solvatochromic behavior of some 4,40-substituted azoaromatic derivatives has been investigated in sol-
vents of different polarities. The solvent dependent UV/vis spectral shifts, mmax, were analyzed using some
physical parameters such as refractive index, dielectric constant, Kamlet–Taft parameters a (hydrogen
bond donating ability) and b (hydrogen bond accepting ability).
The intermolecular interaction types in the azobenzene derivatives solutions were established on the
basis of a multiple linear regression analysis. The fitting coefficients obtained from this analysis allowed
us to estimate the contribution of each type of interactions to the total spectral shift in the studied
solutions.
Article history:
Received 14 September 2007
Received in revised form 23 January 2008
Accepted 23 January 2008
Available online 20 March 2008
Keywords:
Azoaromatic compounds
Solvent–solute interactions
Electronic absorption spectra
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Azoaromatic compounds have attracted intensive attention due
to their widespread range of applications in dye stuff industry,
photoaligning substrates for liquid crystals, photorefractive media,
acid–base, redox and metallochrome indicators, optical actuators,
optical storage media, etc. [1–5]. The use of these compounds
emphasizes the importance of the understanding the solvent
effects in the electronic transitions occurring in these molecules.
Universal solvent–solute interactions [6,7] are determined by
electronic and nuclear solvent polarization and they can be de-
scribed by functions of the refractive index n, f(n) = (n2 ꢀ 1)/
(n2 + 1), or (2n2 ꢀ 1)/(n2+1) and the electric permittivity e, (e ꢀ 1)/
(e + 2), or e ꢀ 2)/(2e + 1). All these functions [6,7] can have different
influences on the electronic absorption spectra that manifest in
sign and magnitude, giving information on the intermolecular
interactions in solutions.
4-Methylaniline and 4-nitroaniline were obtained from Al-
drich Chemical Co. and used as received. The solvents used were
of spectrophotometric grade and purchased from Aldrich or
Fluka.
The 4,40-substituted azoaromatic derivatives AZ5–AZ9 were
synthesized by conventional diazotization–coupling reactions
starting from 4-methylaniline or 4-nitroaniline. The synthetic
route toward the target compounds involves the following steps:
diazotization of 4-methyl- or 4-nitroaniline, coupling reaction of
phenol with diazonium salts and esterification reaction with corre-
sponding acid chloride [10–12].
The procedure described for azoaromatic derivative AZ7 is
typical:
(i) Diazotization/coupling
Specific interactions can be present in solutions, the most
important ones being hydrogen bondings [6–10]. Their influence
on the absorption spectra is very important, depending on whether
the solvent molecules can act as donors or acceptors in the forma-
tion of hydrogen bondings with the solute molecules.
In this work some azoaromatic derivatives (Fig. 1) were pre-
pared and ultraviolet–visible absorption spectra have been re-
corded in twenty four solvents in order to study the influence of
the solvents on their absorption spectra.
4-Hydroxy-40-methyl azobenzene: A cold solution of sodium ni-
trite (6.9 g, 0.1 mol) in 25 ml of water was added dropwise, under
stirring at 0 °C to a mixture of 4-methylaniline (10.71 g, 0.1 mol)
and 25 ml conc. HCl (0.3 mol) in 150 ml water. The obtained suspen-
sion was stirred at 0 °C for 1 h to produce the diazonium salt. The
mixture was filtered off and a cold solution of phenol (9.4 g
0.1 mol) and sodium hydroxide (4.0 g, 0.1 mol) in 50 ml water was
added dropwise to the filtrate while carefully maintaining the same
0 °C temperature for 5 min. The mixture was neutralized with so-
dium acetate and then the precipitate produced was collected and
washed with excess of water. After the removal of the water, the
product was purified by recrystallization from a mixture of DMF
and water (1:1).
* Corresponding author. Tel.: +40 2 3220 1182; fax: +40 2 3220 1150.
E-mail address: ddorohoi@uaic.ro (D.-O. Dorohoi).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.01.053

E. Rusu et al. / Journal of Molecular Structure 887 (2008) 216–219
217
2
1
R
R
N
N
Sample
AZ5
R1
R2
CH3 COO
NO2
AZ6
AZ7
AZ8
AZ9
CH3 – CH = CHCOO
CH2 = CHCOO
NO2
CH3
CH3
CH3
CH3 – CH = CHCOO
Ph – CH = CHCOO
Fig. 2. UV/vis absorption spectra of AZ5–AZ9 in dioxane.
Fig. 1. Structures of investigated azoaromatic compounds.
(ii) Esterification
were collected in Table 1. The electronic absorption spectra of
the studied azobenzene compounds in dioxane are drawn in
Fig. 2.
4-Methyl-40-oxymethacryloyl azobenzene: A mixture of 4-hy-
droxy-40-methyl azobenzene (2.12 g 0.01 mol) and acryloylchol-
ride (3 ml, excess) was dissolved in pyridine at room
temperature. Triethylamine (1.9 ml) was added into solution. The
resulting mixture was heated at 40 °C and kept stirring for 4 h.
Reaction was stopped by addition of acetone in excess. The crude
product was precipitated in cold water. The precipitate produced
was collected and washed with excess of water. After removal of
water, the product was crystallized twice with ether (70% yield).
Ultraviolet–visible (UV–vis) absorption spectra were recorded on
Specord M42 spectrophotometer (Carl Zeiss Jena) in 1 cm quartz
cells. No concentration dependence of the absorption maxima val-
ues of the azoaromatic compounds was observed in the studied con-
centration range. Dielectric constant, e refractive index, n and the
solvatochromic parameters, a and b were taken from the literature
[13–16].
These compounds mainly exhibit a strong absorption band lo-
cated at about 29850 cmꢀ1 (AZ8, dioxane). It is assigned to a p–p*
absorption band due to the conjugation between azo bridge and
the aromatic ring system [1,2]. The wave numbers of the p–p*
absorption maxima of azoaromatic compounds AZ5–AZ9
(Table 1) were measured in 24 solvents having a wide variety of
parameters including dielectric constant, refractive index and
hydrogen bonding abilities (Table 1).
Compared to nonpolar solvents, the maximum of the p–p* band
shows a bathochromic shift in polar solvents (Table 1). The solvent
effect for AZ5 (Table 1) reveals that the p–p* absorption maximum
ranges from 29670 cmꢀ1 in n-hexane to 29850 cmꢀ1 in dimethyl-
formamide corresponding to a difference of Dm = 555 cmꢀ1 stabil-
ization energy between these solvents having different polarities.
This fact suggests that the azoaromatic derivatives under study
are more polar in the excited states than in the ground state.
Plots of the p ? p* absorption band wavenumbers in function of
one-parameter proved satisfactory in some cases. In Fig. 3, for
example, we have obtained a good correlation between mmax and
3. Results and discussion
The electronic absorption spectra of the studied azoaromatic
derivatives were recorded in different solvents and relevant data
Table 1
UV/vis absorption maxima for azoaromatic derivatives and solvent parameters
No.
Solvent
e
n
b
a
mexp (cmꢀ1
)
AZ5
AZ6
AZ7
AZ8
AZ9
1
2
3
4
5
6
7
8
n-Hexane
1.89
2.02
2.22
2.24
2.38
4.27
4.81
6.08
9.08
10.42
15.13
17.84
17.93
18.56
20.18
20.80
21.01
25.30
33.00
38.25
37.50
41.40
37.78
47.24
1.3748
1.4266
1.4224
1.4601
1.4961
1.3526
1.4459
1.3723
1.4242
1.4448
1.4101
1.3993
1.3955
1.3788
1.3776
1.3855
1.3588
1.3611
1.3288
1.4305
1.3442
1.4318
1.4384
1.4770
0.00
0.00
0.37
0.00
0.11
0.47
0.00
0.45
0.00
0.00
0.92
0.88
0.84
0.48
0.95
0.85
0.48
0.77
0.62
0.69
0.31
0.52
0.76
0.76
0.00
0.00
0.00
0.00
0.00
0.00
0.44
0.00
0.30
0.00
0.70
0.79
0.79
0.06
0.76
0.78
0.08
0.83
0.93
0.00
0.19
0.90
0.00
0.00
29,630
29,460
29,390
29,370
29,150
29,500
29,330
29,440
29,370
29,200
29,110
29,390
29,590
29,370
29,370
29,500
29,460
29,460
29,500
28,170
29,760
29,240
29,110
29,070
29,240
29,110
29,070
28,990
28,820
29,240
28,900
29,280
29,070
28,940
28,930
28,570
28,820
29,150
29,280
29,030
29,280
28,860
29,460
28,820
29,460
27,660
28,940
28,860
30,490
30,300
29,990
30,170
29,810
30,350
29,900
30,120
30,080
29,900
30,080
30,080
30,300
29,850
29,810
30,120
29,720
29,940
30,210
29,460
30,350
29,630
29,720
29,590
30,210
30,120
29,830
29,850
29,630
30,030
29,760
29,940
29,880
29,760
29,760
29,850
29,940
29,670
29,990
29,880
29,720
29,910
29,990
29,410
29,990
29,150
29,540
29,370
30,490
30,330
29,940
29,940
29,900
30,330
30,030
30,350
30,300
30,120
30,400
30,400
31,060
29,850
30,490
30,860
29,760
30,490
30,630
30,030
30,490
30,300
30,030
29,900
Cyclohexane
1,4-Dioxane
CCl4
Toluene
Diethyl ether
Chloroform
Ethyl acetate
Dichloromethane
1,2-Dichloroethane
1-Pentanol
1-Butanol
iso-Butanol
2-Butanone
2-Propanol
1-Propanol
Acetone
Ethanol
Methanol
DMF
Acetonitrile
Ethylene glycol
Dimethylacetamide
DMSO
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24

218
E. Rusu et al. / Journal of Molecular Structure 887 (2008) 216–219
Consequently, a multiparametric correlation [17] of spectral
0.3
0.28
0.26
0.24
0.22
0.2
data (mmax) with some independent physically justifiable parame-
ters of the medium could provide information on different interac-
tion in solution.
We tried to establish a correlation between experimental spec-
tral values (mmax) with the four solvatochromic variables of the
polarity function f(2) = (e + 1)/e + 2), electronic polarizability
f(n) = (n2 ꢀ 1)/(n2 + 2), H-bonding donor ability (a) and H-bonding
acceptor ability (b) in order to evaluate the respective contribu-
tions to the solute-solvent interactions of the azoaromatic
compounds.
In this case, the position of the absorption band, mmax, in a given
solvent has been expressed as a linear function of solvent parame-
ters described above, as follows:
m
max: ¼ C0 þ C1 ꢁ fðnÞ þ C2 ꢁ fðeÞ þ C3 ꢁ b þ C4 ꢁ a
ð1Þ
28,000
28,500
29,000
exp (cm-1)
29,500
30,000
ν
where C0 signifies the extrapolated value of the wavenumber in gas-
eous state and the coefficients Ci i = 1,2,3,4 reveals the relative con-
tributions of the considered solvatochromic parameters. The
coefficients Ci were determined by multiple linear regression tech-
nique [17].
The second term in relation (1) describes the contribution of the
orientation induction interactions to the frequency shift and the
third term describes the contribution of the dispersion–polariza-
tion forces to the total spectral shift.
The results of the multiple correlation analysis are summarized
in Table 2. As one can see from the data of this table, the parame-
ters Ci i = 1,2,3,4 depend on the structural features of the spectrally
active molecules.
The magnitude and size of the coefficients Ci can show the de-
gree of different solute–solvent interactions. The parameters C1
and C2 have the highest absolute values which means that the
spectral shifts in the p–p* transition are controlled mainly by both
the refractive index and solvent dielectric constant (dispersion and
dipolar interactions).
Fig. 3. Wavenumbers in the p–p* band of AZ5 azobenzene vs dispersion function.
f(n) for AZ5, suggesting the important role of solvent electronic
polarizability in determinating the spectral shifts.
From the Fig. 4 it results that the wavenumber in the maximum
of the p ? p* absorption band of azobenzene compounds does not
depend linearly on the dielectric function f(e). Some deviations
were observed in the aprotic dipolar solvents. A study of the
dependence of the wavenumber in the maximum of the same band
on the H-bonding donating ability a of the solvents (Fig. 5) also
shows us that the choose of this parameter for linearization is
not enough.
1
0.8
0.6
0.4
0.2
Table 2
Regression parameters in relation (1) for the studied azoaromatic compounds
Compound C0
(cmꢀ1
C1
C2
C3
C4
R
SD CP
)
(cmꢀ1
)
(cmꢀ1
)
(cmꢀ1
)
(cmꢀ1
)
AZ5
AZ6
AZ7
AZ8
AZ9
30860
ꢀ299.1
6.1
ꢀ771.8
ꢀ319.9
285
ꢀ5177.8 ꢀ100.2
ꢀ6196.8 ꢀ102.7
69.0
ꢀ149.2
238.7
268.3
257.9
0.98 32
5
30670
31830
31860
31970
0.98 34 4.9
0.99 44 4.9
0.98 43 5.1
0.99 36 5.1
0
ꢀ5121.9
ꢀ54.6
29,000 29,200 29,400 29,600 29,800 30,000 30,200 30,400
ꢀ7087.1 ꢀ243.5
-1
ꢀ6143.9
ꢀ59.7
νexp (cm )
Fig. 4. Wavenumbers in the p–p* band of AZ8 azobenzene vs orientation function.
29,400
29,200
29,000
28,800
1.10
0.90
0.70
ν
0.50
α
0.30
0.10
27,500
28,000
28,500
29,000
29,500
30,000
-0.10
27,500
νexp (cm-1)
28,000
28,500
29,000
29,500
30,000
ν
exp (cm-1)
Fig. 6. Relationship between calculated mcalc.and experimental mexp. wavenumbers in
the maximum of p–p* absorption band of AZ6 compound.
Fig. 5. Wavenumbers in the p–p* band of AZ6 azobenzene vs a.

E. Rusu et al. / Journal of Molecular Structure 887 (2008) 216–219
219
30,400
30,000
29,600
29,200
4. Conclusions
Universal and specific interactions determine the shifts of the
p–p* band in azoaromatic derivatives solutions. Coefficients C1
and C2 determine the strength of the universal forces, while C3
and C4 coefficients characterize the strength of the specific interac-
tions of the analyzed compounds.
The spectral shifts of p–p* band of azoaromatic compounds in
solutions are mainly controlled by the universal interactions.
References
[1] H. Rau, in: J.F. Rabek (Ed.), Photochemistry and Photophysics, vol. 2, CRC Press,
Boca Raton, 1990.
[2] G.S. Kumar, Azo Functional Polymers, Technomic Publ., Lancaster, 1992.
[3] O. Yaroshchuk, T. Sergan, J. Lindau, S.N. Leve, L.C. Chien, J. Chem. Phys. 114
(2001) 5330.
28,800
29,200
29,600
νexp (cm-1)
30,000
30,400
Fig. 7. Relationship between calculated mcalc. and experimental mexp. wavenumbers
in the maximum of p–p* absorption band of AZ8 compound.
[4] M.S. Masoud, A.E. Ali, M.A. Shaker, M.A. Ghani, Spectrochim. Acta 60A (2004)
3155–3159.
[5] T. Nakamura, T. Shibusawa, Textile Res. J. 70 (2000) 801–809.
[6] N.G. Bakhshiev, Spektroskopija mezhmolekuljarnyh vzaimodestvii, Leningrad,
Izd., Nauka, 1972.
[7] A. Airinei, E. Rusu, D. Dorohoi, Spectroscopy Lett. 34 (2001) 65–74.
[8] E. Bunce, R. Rajagopal, Acc. Chem. Res. 23 (1990) 226.
[9] A.S. Klymchenko, A.P. Demchenko, Phys. Chem. Chem. Phys. 5 (2003) 461–468.
[10] O. Pytela, Coll. Czech. Chem. Commun. 53 (1988) 1333.
[11] A. Airinei, E. Rusu, V. Barboiu, E. Comanita, Rev. Roum. Chim. 40 (1995) 779–789.
[12] A. Sanielevici, F. Urseanu, Sinteze de intermediary aromatici, Ed. Tehnic,
Bucuresti, vol.2, 1983.
[13] D.R. Lide (Ed.), Handbook of Chemistry and Physics, 76th ed., CRC Press, Boca
Raton, Florida, 1995.
[14] M.J. Kamlet, J.L. Abboud, M.H. Abraham, R.W. Taft, J. Org. Chem. 48 (1983)
2877–2887.
[15] Y. Marcus, Chem. Soc. Rev. 22 (1993) 409–416.
[16] C. Reichardt, Solvents and Solvent Effect in Organic Chemistry, Wiley-VCH,
Weinheim, 2003.
[17] G. Henrion, A. Henrion, R. Henrion, Beispiele zur Datenanalyse mit BASIC
Programmen VEB D.V.W., Berlin, 1988.
Also, it is worth observing that C3 coefficient is significantly
smaller than the C4 coefficient for AZ6, AZ7 and AZ9. This fact dem-
onstrates the H-bonding donating ability is weaker than the ability
to accept the H-bonding.
The negative sign of the C2 coefficient reflects the mentioned
bathochromic spectral shifts in all studied solutions of azoaromatic
derivatives [18].
Figs. 6 and 7 display the plots of the calculated absorption max-
ima using relation (1) as a function of the corresponding experi-
mental values for AZ6 and AZ8 in the selected solvents. As one
can see, the calculated values by using relation (1), are in good
accordance with the experimental data, excepting the value corre-
sponding to ethylene glycol.
[18] M.G. Hutchings, P. Gregory, J.S. Campbell, A. Strong, J.P. Zamy, A. Lepre, A.
Mills, Chem. Eur. J. 3 (1997) 1719–1727.