Solvatochromic studies of fluorescent azo dyes: Kamlet-taft (π*, α and β) and Catalan (Spp, S A and SB) solvent scales approach

Article abstract of DOI:10.1007/s10895-011-0948-6

A new metal ion-responsive azo-based fluorescent probes have been synthesized and characterized by NMR spectral techniques. Steady-state fluorometric study has been used to analyze the spectroscopic and photophysical characteristics of dye derivatives in

Full text of DOI:10.1007/s10895-011-0948-6

J Fluoresc (2012) 22:213–221
DOI 10.1007/s10895-011-0948-6
ORIGINAL PAPER
Solvatochromic Studies of Fluorescent Azo Dyes:
Kamlet-Taft (π*, α and β) and Catalan
(Spp, SA and SB) Solvent Scales Approach
Jayaraman Jayabharathi & Venugopal Thanikachalam &
Marimuthu Venkatesh Perumal &
Karunamoorthy Jayamoorthy
Received: 4 May 2011 /Accepted: 3 August 2011 /Published online: 12 August 2011
#
Springer Science+Business Media, LLC 2011
Abstract A new metal ion-responsive azo-based fluorescent
probes have been synthesized and characterized by NMR
spectral techniques. Steady-state fluorometric study has been
used to analyze the spectroscopic and photophysical charac-
teristics of dye derivatives in various solvents. The fluores-
cence properties of these dyes are strongly solvent dependent,
the wavelength of maximum fluorescence emission shifts to
the red. The Kamlet-Taft and Catalan’s solvent scales were
found to be the most suitable for describing the solvatochro-
mic shifts of the absorption and fluorescence emission. The
hydroxy substituted azo dye formed complexes with several
metal ions (Co2+, Hg2+, Ni2+ and Cu2+) and fluorescence
quenching with metal ions reveal that it can be used as a new
fluorescence sensor to detect the Cu2+ ion.
in the view of both fundamental and applications [1–5].
Though atomic emission or mass spectroscopy (ICP-AES,
ICP-MS) used to detect heavy metals at low concentrations,
they are very expensive, therefore fluorescence sensors have
been developed [6–10]. Copper exists in human body, plants
and animals in trace amounts; however, high amount can
cause serious health problems such as nausea, vomiting and
diarrhea as well as damage to liver and kidneys [11].
Therefore, in the view of the biological and environmental
importance, considerable attention has been focused on
detection of Cu(II) ion [12, 13] by quenching of fluorescence
intensity of the employed sensing material [14–17].
Several factors contribute to the best laser perfor-
mance of dyes namely, low triplet-triplet absorption
capacity, a poor tendency to self-aggregate in organic
solvents and their high photo stability, which improves
the lifetime of the laser action [11–17]. The Stokes shift
modifies the lasing performance [18] in highly concentrated
solutions because it affects the reabsorption and reemission
phenomena, which shifts the emission band to longer wave-
length and reduces its efficiency. In the present paper, we
exploited the photophysical characterization of dye derivatives
1–4 in various solvents and the solvent effects were analyzed
using two different solvent scales namely Kamlet-Taft (π*, α
and β) and Catalan parameters (Spp, SA and SB) (multi-
component linear regression).
.
.
.
Keywords Azo dyes Kamlet-Taft Catalan solvent scales
.
Chemosensor Acidic solvents
Introduction
Measurements of concentration of analytically and biologi-
cally important ions by fluorescent chemosensors are indis-
pensable tool in numerous fields of modern medicine and
science. The design and development of fluorescent probes for
metal ion analysis remains an active research field. Azo
compounds are versatile molecules, forming stable complexes
with metal ions and have received much attention in research
Experimental
:
:
:
J. Jayabharathi (*) V. Thanikachalam M. V. Perumal
K. Jayamoorthy
Materials and Methods
Department of Chemistry, Annamalai University,
Annamalainagar 608 002 Tamilnadu, India
e-mail: jtchalam2005@yahoo.co.in
Aniline (Sigma-Aldrich Ltd.), Furan-2-carboxaldehyde,
Thiophene-2-carboxaldehyde, Pyridine-2-caroboxaldehyde,

214
J Fluoresc (2012) 22:213–221
Table 1 Photoluminescence spectral data of various solvents and solid emission spectra of picrate derivatives 1–4
Solvent
Absorptiona (l, nm)
Emissionb (l, nm)
Stokes shift (cm−1
)
1
2
3
4
1
2
3
4
1
2
3
4
n-Hexane
362
357
360
371
372
371
375
385
402
415
412
423
427
407
417
414
426
429
434
435
437
436
438
419
423
421
428
430
432
434
436
437
440
413
2749
3078
3201
3460
3608
3517
3608
3373
3519
3344
3441
3416
3623
3700
3790
3696
3537
3363
3174
3009
3911
3533
3644
3883
3698
3662
3414
3174
3158
3046
2741
2669
3314
2512
2555
2672
2262
1082
2221
3039
1,4-dioxane
Benzene
368
365
368
413
364
360
365
423, 480
414
Chloroform
Ethyl acetate
Dichloromethane
Butanol
369
368
367
370
372 (375)c
369
374 (380)c
371
373 (379)c
427
376 (381)c 428
418
370
377
378
361, 401
392, 420
370, 400
375
427
428
432
434
441
Ethanol
374
381
383
440
Methanol
375
383
384
439
Acetonitrile
379
387
388
424
a UV-vis absorption measured in solution concentration = 1×10−5
b Photoluminescence measured in solution concentration = 1×10−4
c 1abs calculated by Gaussion-03
M
M
3-hydroxypyridin-2-carboxaldehyde, (S.D. fine.) and all
other Reagents were used without further purification.
UV–vis spectrophotometer (Perkin Elmer, Lambda 35)
and corrected for background due to solvent absorption.
Photoluminescence (PL) spectra were recorded on a
(Perkin Elmer LS55) fluorescence spectrometer. MS
spectra were recorded on a Varian Saturn 2200 GCMS
spectrometer. Quantum mechanical calculations were
used to carry out the optimized geometry and TD-DFT
Optical Measurements and Computational Details
NMR spectra were recorded on a Bruker 400 MHz The
ultraviolet–visible (UV–vis) spectra were measured on
Fig. 1 Absorption spectra of
Schiff bases 1–4

J Fluoresc (2012) 22:213–221
215
(0E,4E)-4-(2-Phenyldiazenyl)-N-(2-Pyridylidene)
Benzenamine (3)
Yield: 87%, m.p: 116 °C. Anal. calcd. for C18H14N4: C,
76.17; H, 5.43; N, 13.32. Found: C, 75.50; H, 4.93; N,
1
19.57. MS: m/z 285.00, calcd. 286.12. H NMR: 8.77 (bd,
1H), 8.69 (s, 1H), 8.04 (m, 1H), 7.41–7.45 (m, 3H), 7.49–
7.52 (m, 2H), 7.53–7.57 (m, 2H), 7.84–7.88 (m, 3H), 7.96
(m, 1H). 13C NMR: 161.41, 154.35, 153.31, 153.00,
152.75, 149.59, 145.59, 137.07, 136.76, 130.96, 129.86,
129.80, 129.11.
Fig. 2 Correlation of solvent shifts of absorption with Reichardt-
Dimroth solvent ET parameters for 1–4
(0E,4E)-4-(2-Phenyldiazenyl)-N-(o-Hydroxypyridylidene)
Benzenamine (4)
Yield: 83%, m.p: 123 °C. Anal. calcd. for C18H14N4O: C,
70.35; H, 4.67; N, 18.53. Found: C, 70.15; H, 4.23; N,
1
with Guassian-03 program using the Becke3-Lee-Yang-
Parr (B3LYP) functional supplemented with the standard
6–31 G(d,p) basis set [19].
17.98. MS: m/z 301.00, calcd. 302.12. H NMR: 8.51 (s,
1H), 8.01 (s, 1H), 7.45–7.93 (m, 5H), 7.45–7.53 (m, 4H),
7.39 (s, 1H), 6.97 (d, 1H), 4.91 (s, 1H). 13C NMR: 155.67,
152.32, 151.75, 140.12, 139.47, 131.32, 129.46, 127.53,
124.11, 122.79.
General Procedure for the Synthesis of Schiff Bases
The Schiff bases (1–4) were synthesized according to the
procedure reported in literature [20]. A solution of p-
aminoazobenzene (1 mmol) and the corresponding hetero-
cyclic aldehydes (1.5 mmol) in 20 ml absolute ethanol was
refluxed for 2 h. The resulting precipitate was filtered off
and purified by column chromatography.
Results and Discussion
Photophysical Characterization of (1–4)
UV-visible absorption and fluorescence emission spectra of
dye derivatives in a series of solvents of varying polarity
and their photophysical properties are compiled in Table 1.
The main absorption band attributed to the π-π* transition
since in aminobenzene dyes, n-π* and π-π* transitions are
in close proximity, the low intensity n-π* transition is
completely overlaid by the intensive π-π* transition [20,
21]. Theoretical calculations were carried out to study the
absorption transition in dichloromethane and the calculated
(0E,4E)-4-(2-Phenyldiazenyl)-N-(2-Thiophenylidene)
Benzenamine (1)
Yield: 78%, m.p: 142 °C. Anal. calcd. for C17H13N3S: C,
70.08; H, 4.50; N, 14.42. Found: C, 69.76; H, 4.17; N,
1
13.75. MS: m/z 291.00, calcd. 291.08. 160.24. H NMR:
8.66 (s, 1H), 8.01 (d, 2H), 7.96 (d, 2H), 7.86 (m, 1H),
7.49–7.59 (m, 2H), 7.43 (t, 1H), 7.38 (t, 1H), 7.29 (s, 1H),
7.19 (t, 1H), 6.77 (d, 1H). 13C NMR: 153.61, 153.00,
150.79, 149.58, 145.60, 142.66, 132.87, 130.85, 129.81,
129.11, 129.06, 129.01, 128.98, 125.13, 124.10, 122.81,
122.36, 121.77, 114.64.
(0E,4E)-4-(2-Phenyldiazenyl)-N-(2-Furfurylidene)
Benzenamine (2)
Yield: 78%, m.p: 137 °C. Anal. calcd. for C17H13N3O: C,
74.17; H, 4.76; N, 15.26. Found: C, 73.46; H, 4.43; N, 14.86.
1
MS: m/z 274.00, calcd. 275.11. H NMR: 8.44 (s, 1H), 8.10
(s, 1H), 7.84–7.97 (m, 3H), 7.53–7.45 (m, 4H), 7.29 (s, 1H),
6.77 (d, 1H). 13C NMR: 152.76, 129.34, 129.08, 128.97,
125.12, 124.78, 124.75, 124.03, 123.90, 123.79, 122.99,
117.01, 116.96, 116.56, 114.63.
Fig. 3 Correlation of solvent shifts of absorption with Marcus solvent
optical dielectric functions for 1–4

216
J Fluoresc (2012) 22:213–221
Fig. 4 Emission spectra of
Schiff bases 1–4
absorption maxima are in good agreement with the
experimental absorption maxima.
transfer takes place and causing colour changes. The
position of the longer-wavelength absorption and emission
bands in the spectra was determined in several protic and
aprotic solvents. Lippert-Mataga [23] plot (Fig. 5) was
Measurement of absorption solvatochromism (Fig. 1)
have been interpreted with Marcus and Reichardt-Dimroth
solvent functions to estimate the transition dipoles
associated with low lying excited state. The linear
correlation (Fig. 2) of solvent shift of absorption band
position of dyes (1–4) with Reichardt-Dimroth solvent ET
parameters is indicative of the fact that the dielectric
solute solvent interactions are responsible for the ob-
served solvatochromic shift for these dye derivatives. The
observed linear correlation (Fig. 3) of solvent shift of
absorption band positions of 1–4 with Marcus optical
ꢂꢀ
ꢁ ꢀ
ꢁꢃ
dielectric solvent function 1 ꢀ Dop = 2Dop þ 1 reveals
that transition dipoles associated with absorption and the
direction of excited dipole is opposite to that of the ground
state-dipole [22].
Solvatochromism of Dye Derivatives—Lippert-Mataga Plot
All these dye derivatives 1–4 show solvatochromism
(Fig. 4) i.e., changes in the polarity of the solvents, charge
Fig. 5 Lippert-Mataga plot of 1–4

J Fluoresc (2012) 22:213–221
217
constructed for the normal fluorescence spectra of dye
derivatives (1–4) using the following equation.
aprotic solvents with excellent correlation coefficient. Large
stokes shifted fluorescence band suggests that this emission
has originated from the species which is not present in the S0
state and large geometrical changes have takes place in the
species when excited to S1 state.
The higher bathochromic shifts observed in the fluores-
cence band of the dye derivatives can be interpreted by the
Brunings-Corwin effect [25]. Because the distortion of the
geometry in the excited state, the fluorescence band is
bathochromically shifted to a higher extent than the
absorption band. Moreover, the loss of planarity in the
excited state of the dye derivatives explained the lower
fluorescence quantum yield in apolar solvents owing to an
increase in the non-radiative processes [26, 27].
2
3
ꢄ
ꢅ
2
2 me ꢀ mg
6
4
7
5
nꢀss ¼ naꢀb ꢀ nfꢀl ¼ const þ
f ðD; nÞ
ð1Þ
hca3
where f ðD; nÞ ¼ ðD ꢀ 1Þ=ð2D þ 1Þ ꢀ ðn2 ꢀ 1Þ=ð2n2 þ 1Þ,
indicates the orientation polarizability and depicts polarity
parameter of the solvent [24], n is refractive index, D is
dielectric constant, μe and μg are dipole moments of the
species in S1 and S0 states, respectively, h, Planck’s constant;
c, velocity of light and a, Onsager’s cavity radius. The
Lippert-Mataga plot is linear for the non-polar and polar/
Table 2 Adjusted Coefficients ((υx)0, ca, cb and cc) and Correlation
Coefficients (r) for the Multilinear Regression Analysis of the
Absorption υab and Fluorescence υfl Wavenumbers and Stokes Shift
(Δυss) of Schiff base derivatives 1–4 with the Solvent Polarity/
Polarizability, and the Acid and Base Capacity Using the Taft (π*, α
and β) and the Catalan (SPPN, SA and SB) Scales
1
2
3
4
Kamlet-Taft
(υx)0cm−1
(π*)
c
c
r
α
β
1ab
(2.74 0.02)×104
(2.36 0.01)×104
(0.37 0.01)×104
(υx)0cm−1
(2.73 0.03)×104
(2.36 0.02)×104
(0.37 0.01)×104
(υx)0cm−1
(2.73 0.01)×104
(2.42 0.01)×104
(0.31 0.)×104
(υx)0cm−1
(2.74 0.01)×104
(2.45 0.02)×104
(0.29 0.013)×104
(υx)0cm−1
(2.73 0.04)×104
(2.38 0.03)×104
(0.35 0.01)×104
(υx)0cm−1
(2.75 0.03)×104
(2.40 0.02)×104
(0.35 0.01)×104
(υx)0cm−1
(2.68 0.03)×104
(2.41 0.01)×104
(0.26 0.01)×104
(υx)0cm−1
(9.44 3.17)×103
(5.48 2.31)×103
(16.27 96.22)×103
(10.08 7.02)×103
(6.14 6.15)×103
−(7.71 7.27)×103
−(5.27 5.306)×103
−(2.44 4.651)×103
0.93
0.88
0.86
r
1fl
Δυss = υab–υfl
−(3.96 2.02)×103
N
Catalan
cSPP
cSPP
cSPP
cSPP
cSA
cSB
1ab
(14.96 11.18)×103
(12.65 6.01)×103
−(2.31 5.83)×103
(π*)
(5.49 2.69)×103
(7.09 5.66)×103
(51.79 47.14)×103
(46.62 25.35)×103
(5.72 24.59)×103
−(50.20 48.95)×103
−(45.63 26.32)×103
−(4.57 25.54)×103
0.53
0.61
0.40
r
1fl
Δυss = υab–υfl
Kamlet-Taft
c
c
α
β
1ab
(9.86 8.18)×103
(12.70 17.20)×103
−(2.84 10.22)×103
−(4.56 6.19)×103
0.82
0.68
0.48
r
1fl
−(6.42 13.00)×103
Δυss = υab–υfl
−(15.92 3.36)×103
(1.86 7.73)×103
N
Catalan
cSA
cSB
1ab
(6.07 2.75)×103
(8.73 4.42)×103
−(2.66 2.19)×103
(π*)
(0.47 6.99)×103
(0.054 5.91)×103
(10.86 8.25)×103
(15.57 13.24)×103
−(4.70 6.57)×103
−(5.11 6.21)×103
0.88
0.88
0.85
r
1fl
−(7.97 9.77)×103
Δυss = υab–υfl
(2.87 4.95)×103
Kamlet-Taft
c
c
α
β
1ab
−(3.46 22.77)×103
(3.37 19.25)×103
(0.075 8.96)×103
−(3.20 17.76)×103
−(2.71 15.01)×103
−(0.50 6.99)×103
0.31
0.32
0.61
r
1fl
Δυss = υab–υfl
−(0.04 2.75)×103
N
Catalan
cSA
cSB
1ab
(22.22 11.18)×103
(20.92 9.32)×103
−(1.30 5.42)×103
(π*)
(13.64 2.61)×103
(0.31 1.30)×103
(81.54 47.13)×103
(76.11 39.26)×103
(5.43 22.82)×103
−(80.21 48.94)×103
−(73.14 40.77)×103
−(7.07 23.70)×103
0.61
0.62
0.61
r
1fl
Δυss = υab–υfl
Kamlet-Taft
c
c
α
β
1ab
−(55.69 8.51)×103
−(7.24 4.24)×103
−(48.45 6.44)×103
(46.91 6.64)×103
(5.71 3.31)×103
−(41.20 5.02)×103
0.92
0.97
0.95
r
1fl
Δυss = υab–υfl
Catalan
−(13.33 1.98)×103
N
cSA
cSB
1ab
(2.66 0.03)×104
(2.39 0.02)×104
(0.27 0.04)×104
(6.98 13.46)×103
(0.70 6.96)×103
−(6.28 14.38)×103
−(23.72 56.73)×103
−(20.32 29.34)×103
−(3.39 60.62)×103
(12.81 58.92)×103
(26.66 30.47)×103
−(13.85 62.95)×103
0.62
0.80
0.45
1fl
Δυss = υab–υfl

218
J Fluoresc (2012) 22:213–221
Fig. 6 a Stabilizing interaction
of 1–3 with basic solvents. b
ESIPT process in Schiffs base 4
The solvent effect on νabs, νem, and Δνss can also be
described on the basis of a multi-linear expression (Eq. 2):
dependency of the physicochemical property (y) in a given
solvent on the various (A, B, C) solvent parameters. At least
two different sets of solvent scales can be found in the
literature to characterize these solvent properties. In the
present analysis, the polarity/polarizability, the acidity and
the basicity of the solvent are considered. Kamlet and Taft
[28] put forward the π*, α and β parameters to character-
y ¼ y0 þ aA þ bB þ cC
ð2Þ
in which y0 stands for the physicochemical property of
interest in the absence of solvent (i.e., in the gas phase); a,
b, and c are adjustable coefficients that reflect the
Fig. 7 a Correlation between
the experimental absorption
wavenumber with the predicted
values obtained by a multicom-
ponent linear regression using
the π*, α and β-scale (Taft)
solvent parameters for 1–4. b
Correlation between the experi-
mental fluorescence wavenum-
ber with the predicted values
obtained by a multicomponent
linear regression using the π*, α
and β-scale (Taft) solvent
parameters for 1–4

J Fluoresc (2012) 22:213–221
Fig. 7 (continued)
219
ize, respectively, the polarity/polarizability, the acidity, and
the basicity of a solvent (Eq. 2). Conversely, Catalan et al.
proposed an empirical solvent scales for polarity/polariz-
ability [29–32] (SPP), acidity [30] (SA) and basicity [30,
31] (SB) to describe the respective properties of a given
solvent (Eq. 3).
The dominant coefficient affecting the absorption and
fluorescence band of dye derivatives 1–4 is the polarity/
polarizability of the solvent [Cπ* or CSPP N] having a
positive value, corroborating the solvatochromic shifts
with the solvent polarity. For 1–3, the adjusted coeffi-
cient representing the electron releasing ability or basicity
of the solvent, Cβ or CSB has the large negative values
(Table 2), suggesting solvent basicity play an important
role in absorption and fluorescence displacements which
can be due to the existence of positive charge on the
imine carbon so that the basic solvent stabilize the
»
y ¼ y0 þ aaa þ bbb þ cp»p
ðKamlet ꢀ TaftÞ
ð2Þ
y ¼ y0 þ aSASA þ bSBSB þ cSPPSPP
ðCatalanÞ
ð3Þ
Fig. 8 (a) Fluorescence spectra of Schiffs base (4) in the presence of various metal ions. b Fluorescence chemisensors blocking by metal ion binding

220
J Fluoresc (2012) 22:213–221
Fig. 9 Possible structure of
Schiffs base (4)-Cu2+ complex
structures and cause solvatochromic shifts (Fig. 6a). In
the case of 4, the coefficient controlling the H-donor
capacity or acidity of the solvent, Cα or CSA, has the
negative values (Table 2), suggesting that the absorption
and fluorescence bands shifted to lower energies with the
increasing acidity of the solvent. This effect can be due to
the existence of keto-enol tautomers of 4 due to ESIPT
process (Fig. 6b). The existence of good correlations
(Fig. 7a and b) between the absorption and fluorescence
wavenumbers calculated by the multicomponent linear
regression employing the Taft-proposed solvent parame-
ters and the obtained correlation coefficients are tabulated
in Table 2. The two sets of solvent parameters gives
qualitatively similar results.
The changes in the fluorescence properties of 4
caused by different metal ions such as Co2+, Hg2+, Ni2+
and Cu2+ were measured in ethanol. The fluorescence of 4
quenched markedly with the gradual addition of Cu2+
(Fig. 8). The quenching in fluorescence intensity of Schiff
base by the addition of Cu2+ cation indicates the
complexation of Schiff base with Cu2+, the Schiff base
has bidentate sites and forming the expected Cu2+
complex (Fig. 9). The possible reason for the fluorescence
quenching is the formation of a ground state non-
fluorescent complex 4-Cu2+. According to the obtained
results, Schiff base (4) can be used as a new fluorescence
sensor to detect the quantity of Cu2+ ion in any sample
solution depending on the relative intensity change.
We have developed a new fluorescent chemisensor for
transition metal ions Hg2+, Pb2+ and Cu2+ ions Co2+.
Acknowledgment One of the authors Dr. J. Jayabharathi, Associate
professor Department of Chemistry, Annamalai University is thankful
to Department of Science and Technology [No. SR/S1/IC-73/2010]
and University Grants commission (F. No. 36-21/2008 (SR)) for
providing fund to this research work.
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Conclusion
The present study reports the solvatochromic effect on
the fluorometric behaviour of Schiff base derivative in
different solvents with varying polarity. The fluores-
cence property of the molecule is very much sensitive
to the polarity and the protic character of the solvent.
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