A physiochemical study of azo dyes: DFT based ESIPT process

Article abstract of DOI:10.1016/j.saa.2011.08.017

Azo linked dye derivatives were synthesized and characterized by NMR, mass and elemental analysis. An excited state intramolecular proton transfer (ESIPT) in hydroxy Schiff base has been analyzed, and found that two distinct ground state isomers of I and

Full text of DOI:10.1016/j.saa.2011.08.017

Spectrochimica Acta Part A 83 (2011) 200–206
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A physiochemical study of azo dyes: DFT based ESIPT process
Jayaraman Jayabharathi∗, Venugopal Thanikachalam, Marimuthu Venkatesh Perumal, Natesan Srinivasan
Department of Chemistry, Annamalai University, Annamalainagar 608 002 Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2011
Received in revised form 29 July 2011
Accepted 10 August 2011
Azo linked dye derivatives were synthesized and characterized by NMR, mass and elemental analysis.
An excited state intramolecular proton transfer (ESIPT) in hydroxy Schiff base has been analyzed, and
found that two distinct ground state isomers of I and II are responsible for the observed dual emission.
DFT calculation on energy, dipole moment, charge distribution of the rotamers in the ground and excited
states support the ESIPT process. PES calculation indicates that the energy barrier for the interconversion
of two rotamers is too high in the excited state than the ground state. By varying the addition of base
concentration to hydroxy Schiff base, two isobestic points were found which confirm the equilibrium
among the trans enol form, anion and the cis enol form. Fluorescence quenching with metal ions reveal
that hydroxy Schiff base can be used as a new fluorescence sensor to detect the Cu2+ ion.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Nuclear magnetic resonance
Ab initio calculations
Photoluminescence spectroscopy
Electrical properties
1. Introduction
by elemental, mass, IR and NMR analysis. Photophysical studies of
Schiff bases (1–4) were discussed in detail and the hydroxy Schiff
Azo compounds are versatile molecules and have received
much attention in research in the view of both fundamental
and applications [1,2]. Schiff base ligands are capable of forming
stable complexes with metal ions and show excellent catalytic
activity [3–5]. Atomic emission or mass spectroscopy (ICP-AES,
ICP-MS) has been widely used for the detection of heavy metals
even at very low concentrations. But these methods are relatively
expensive, therefore a series of fluorescence sensors have been
developed [6–10] and development of new fluorescent agents con-
taining chelating groups can help to detect heavy metals more
efficiently.
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 envi-
ronmental importance, considerable attention has been focused on
detection of Cu(II) ion [12,13] by quenching of fluorescence inten-
sity of the employed sensing material [14–17].
base was developed as a Cu+2 selective fluorescence sensor. DFT cal-
culations were carried out by using Gaussian-03 program [21,22]
to supplement the experimental results.
2. Experimental
2.1. Materials and methods
Aniline
(Sigma–Aldrich
Ltd.),
Furan-2-carboxaldehyde,
Thiophene-2-carboxaldehyde, pyridine-2-carboxaldehyde, 3-hy-
droxypyridin-2-carboxaldehyde (S.D. fine.) and all other reagents
used without further purification.
2.2. Optical measurements and computational details
NMR spectra were recorded on a Bruker 400 MHz instrument.
The ultraviolet–visible (UV–vis) spectra were measured on 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 spec-
trometer. MS spectra were recorded on a Varian Saturn 2200 GCMS
spectrometer. Quantum mechanical calculations were used to carry
out the optimized geometry, NLO, HOMO–LUMO, MEP and TD-
DFT with Guassian-03 program using the Becke3-Lee-Yang-Parr
(B3LYP) functional supplemented with the standard 6-31G(d,p)
basis set [21,22].
Metal chelates play an essential role in living organism and
a large number of metal proteins and other metal complexes of
biological importance have been studied [18,19]. Because of the
importance of azo-containing Schiff base compounds and in con-
tinuance of our previous studies [20], herein we report a series of
azo-linked Schiff bases (1–4) and their structures were confirmed
∗
Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.08.017

J. Jayabharathi et al. / Spectrochimica Acta Part A 83 (2011) 200–206
201
Fig. 1. Absorption spectra of Schiff bases 1–4.
2.3. General procedure for the synthesis of Schiff bases
2.3.3.
(0E,4E)-4-(2-phenyldiazenyl)-N-(2-pyridylidene)benzenamine
The Schiff bases (1–4) were synthesized according to
the procedure reported in the literature [20]. A solution of
p-aminoazobenzene (1 mmol) and the corresponding heterocyclic
aldehydes (1.5 mmol) in 20 ml absolute ethanol were refluxed for
2 h. The resulting precipitate was filtered off and purified by column
chromatography.
(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, 19.57. MS: m/z 285.00,
calcd. 286.12. 1H 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.
2.3.1. (0E,4E)-4-(2-phenyldiazenyl)-N-(2-thiophenylidene)
benzenamine (1)
2.3.4. (0E,4E)-4-(2-phenyldiazenyl)-N-(o-hydroxypyridylidene)
benzenamine (4)
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, 13.75. MS: m/z 291.00,
calcd. 291.08. 1H 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.
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, 17.98. MS: m/z 301.00,
calcd. 302.12. 1H 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.
3. Results and discussion
3.1. Photophysical characterization of (1–4)
2.3.2.
(0E,4E)-4-(2-phenyldiazenyl)-N-(2-furfurylidene)benzenamine
(2)
The UV–vis spectra of the Schiff bases (1–4) were recorded in
protic and aprotic solvents in the region 200–500 nm, the band
appeared around 380.0 nm in dioxane is due to ␲–␲* transition
(Fig. 1). Since in aminobenzene dyes, n–␲* and ␲–␲* transitions are
in close proximity, the low intensity n–␲* transition is completely
overlaid by the intensive ␲–␲* transition [23,24]. The absorption
spectra largely depend on the solvent polarity and red shift was
observed in hydroxyl solvents. Larger red shift in the absorption
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. MS: m/z 274.00,
calcd. 275.11. 1H NMR: 8.44 (s, 1H), 8.10 (s, 1H), 7.84–7.97 (m, 5H),
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.

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J. Jayabharathi et al. / Spectrochimica Acta Part A 83 (2011) 200–206
Table 1
Photoluminescence spectral data of various solvents and solid emission spectra of picrate derivatives 1–4.
Solvent
Absorptiona (ꢀ, nm)
Emissionb (ꢀ, nm)
Stokes shift (cm−1
)
1
2
3
4
1
2
3
4
1
2
3
4
n-Hexane
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
Dichloromethane
Butanol
Ethanol
Methanol
Acetonitrile
362
368
364
369
370
372
370
374
375
379
357
365
360
368
369
374
377
381
383
387
360
368
365
367
371
373
378
383
384
388
371
372
371
375
385
376
361, 401
392, 420
370, 400
375.6
402
415
412
423
427
428
427
428
432
434
407
417
414
426
429
434
435
437
436
438
419
423
421
428
430
432
434
436
437
440
413
413
423, 480
414
427
418
441
440
439
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
424
a
UV–Vis absorption measured in solution concentration = 1 × 10−5 M.
Photoluminescence measured in solution concentration = 1 × 10−4 M.
b
band (Table 1) may be due to the stabilizing interaction between
the hydroxyl group of the alcohol and the hetero atoms (N, S, O) of
the Schiff bases.
transitions from bulk to vacuum energy level of the compounds
and these compounds have high electrical conductivity as well as
fluorescence property. The higher Stokes’s shift value (Table 1) sup-
plies very low background signals and resultantly allows the usage
of the material in construction of a fluorescence sensor [26]. As seen
in Table 1 in comparison with the other solvents, benzene solution
of 4 has larger Stokes shift value than other solvents, indicates that
the usage of 4 as an ion selective sensor in benzene solution.
The changes in the fluorescence properties of 4 caused by differ-
ent metal ions such as Co2+, Hg2+, Ni2+ and Cu2+ were measured in
ethanol. The fluorescence of 4 quenched markedly with the grad-
ual addition of Cu2+ but the fluorescence properties of 4 were
influenced by other metal ions (Fig. 4a). The fluorescence inten-
sity of 4 was linearly reduced with increase in concentration of
Cu2+ (Fig. 4b). According to these spectra when the Cu2+ concentra-
tions are 5 × 10−3 and 5 × 10−5 mol/L the relative intensity changes
(I0 − I/I0) are 0.88 and 0.30, respectively (Fig. 5). 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. S1) [25,27]. The possible reason for the fluorescence quenching
is the formation of a ground state non-fluorescent complex and the
enhancement of spin–orbit coupling [28] for 4–Cu2+ is presumed
resulting in the fluorescence quenching and the observed linear
regression equation is, (I0 − I/I0) = −0.163 − 0.196 log[Cu2+] where,
I0 is the emission intensity of Cu2+ free Schiff base, I is the fluores-
cence intensity of the Schiff base (4) in the presence of metal ion
Cu2+, [Cu2+] is the concentration of metal ion and the correlation
co-efficient is 0.992. According to the obtained results, Schiff base
(4) can be used as a new fluorescence sensor to detect the quan-
tity of Cu2+ ion in any sample solution depending on the relative
intensity change.
The absorption spectra for the yellowish colour solution of
hydroxy Schiff base (4) in neutral ethanol show band at 392 nm
along with a shoulder at 420 nm. Addition of different concentra-
tions of NaOH [0–0.017 M] to the ethanol solutions of 4 resulted
in a single band at 400 nm at the expense of both 392 and 420 nm
bands with two isobestic points around 360 and 410 nm. In hexane,
the absorption spectrum of 4 shows a single band at 371 nm. There-
fore, the absorption band at 392 nm can be safely assigned to the
intramolecularly hydrogen bonded enol form, 400 nm is assigned
to the anion and 420 nm is due to the cis enol form of 4, respectively,
and the isobestic points confirm the equilibrium among the trans
enol form, anion and the cis enol form (Fig. 2).
All Schiff bases (1–4) fluoresce strongly at room temperature
(Table 1), the emission spectrum (Fig. 3) for the yellowish colour
solution of 4 in neutral ethanol shows a band at 445 nm. The emis-
sion is switched ‘On’ with decreasing pH, because at low pH the
hydroxyl oxygen is protonated, its oxidation potential raised which
switches fluorescence ‘On’ and at high pH, the intensity of fluores-
cence emission is low, i.e., switched ‘Off’ due to anion formation
which quenches fluorescence.
The solid state UV–vis spectra of Schiff bases 1–4 exhibit
absorbance (ꢀmax) at 544, 550, 568 and 560. The optical band gap
value of Schiff bases 1–4 are found to be 2.27, 2.25, 2.18 and 2.21 eV,
respectively, and these are smaller than the previously reported
Schiff base [25]. The smaller band gap indicates easy electronic
3.2. Excited state intramolecular proton transfer (ESIPT) process
PES studies
Hydroxy Schiff base (4) exists as intramolecular hydrogen
bonded isomers namely trans enol I and cis enol II in aprotic sol-
vents (Fig. S2(a)). Excitation of the trans enol isomer I should lead
to the formation of the keto-isomer III due to ESIPT (Fig. S2(b)). But,
we found out that the fluorescence spectrum in dioxane contains
two emission bands at shorter wavelength (423 nm) are assigned to
isomer I and the small shoulder peak at higher wavelength 480 nm
reveals that only the isomer II of 4. However in alcoholic solvent,
a short wavelength emission band appears which may be due to
the presence of intermolecular hydrogen bonding with solvent
molecule leading to the stabilization of solvated isomer IV in which
ESIPT is impossible [29].
Fig. 2. Absorption spectrum of Schiff base (4) in ethanol in the presence of sodium
hydroxide ([NaOH] = 0–0.017 M).

J. Jayabharathi et al. / Spectrochimica Acta Part A 83 (2011) 200–206
203
Fig. 3. Emission spectra of Schiff bases 1–4.
Table 2
Relative energiesa (kcal/mol), electron density and dipole moments (D) in the ground and excited state for rotamers I, II and III of Schiff base (4).
Compound
Rotamers
Electron density
N(24)
Ground state
Excited state
O(36)
ꢁ (D)
E (kcal/mol)
E (kcal/mol)
ꢁ (D)
I
II
III
−0.462 (−0.523)b −0.531 (−0.501)b 4.16
1.10 (0.00)
2.21 (0.82)
16.23 (8.60)
108.00 (91.26)
112.28 (100.57)
96.08 (350.82)
9.02
8.26
5.92
–
–
3.30
5.80
4
0.431
−0.592
a
Relative energies are calculated with respect to the ground state minimum energy form in ethanol. Values in the parenthesis are recorded in ethanol.
Excited state isomer I.
b
DFT calculation was carried out for better understanding the
and decrease at O(36) atom resulting in ESIPT and formation of the
excited keto isomer in excited state which emits luminescence and
returns to the ground state keto form and is characterized by a large
positive charge at the N(24) atom and negative charge at the O(36)
ESIPT mechanism. The keto and enol isomers in the ground and
the excited states (Table 2) reveal that excitation of trans enol iso-
mer (I) leads to an increase in the electron density at N(24) atom
Fig. 4. (a) Fluorescence spectra of Schiff base (4) in the presence of various metal ions. (b) Fluorescence chemosensors blocking by metal ion binding.

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J. Jayabharathi et al. / Spectrochimica Acta Part A 83 (2011) 200–206
molecule. Table 3 implies that Schiff bases are highly polar having
non-zero dipole moment and hence have well microscopic NLO
behaviour [30]. To determine the transference region and hence
to know the suitability of 1–4 for microscopic nonlinear optical
applications, the UV–vis spectral studies have been taken for dis-
cussion. The increased transparency in the visible region (␲–␲*
transition) might enable the microscopic NLO behaviour with non-
zero values [31]. The ˇ values (Table 3) computed here might
be correlated with UV–vis spectroscopic data in order to under-
stand the molecular structure and NLO relationship in view of a
future optimization of the microscopic NLO properties. The band
at around 370.0 nm exhibits a solvatochromic shift, characteristic
of a large dipole moment (Table 3) and frequently suggestive of
a large hyperpolarizability (Table 3). These compounds show red
shift in absorption with increasing solvent polarity, accompanied
with the upward shifts non-zero values in the ˇ-components have
large hyperpolarizabilities and hence have rather well microscopic
NLO behaviour [32].
Fig. 5. Plot of normalized intensity (I0 − I/I0) against concentration for Cu2+ ion.
atom. As a result, a reverse process occurs in the ground state of
the molecule producing an initial molecule in the enol form.
The ESIPT process was explained by large number of theoret-
ical calculations [30–32]. The ground, excited state energies (CIS)
and dipole moment of the three species I, II and III were calculated
using DFT/6-31G(d,p) method. The barrier for interconversion of
I and II in the ground state is 4.81 kJ/mol (Isolated molecule) and
4.64 kJ/mol (EtOH) and for the excited state 17.89 kJ/mol (Isolated
molecule) and 14.92 kJ/mol (EtOH), respectively. The barriers for
interconversion in the excited state are much higher than that in
the ground state (Table 2).
3.4. Conformation of Schiff bases 1–4
Comparison of the chemical shifts of azomethine proton in the
Schiff bases 1–4 with that of aldehydic proton in their correspond-
ing substituted benzaldehydes reveals that considerable shielding
has been observed due to the conversion of aldehydes to Schiff
bases. Generally in Schiff bases, the bulky group adopts trans ori-
entation. Therefore, it is inferred that the azomethine proton is syn
to diazenophenyl benzenamine ring in Schiff bases 1–4. Therefore,
the favoured conformation of the Schiff bases 1–4 is predicted to
be the anti conformation, i.e., the CH N and N N groups are anti
to each other.
3.3. Hyperpolarizability of Schiff bases 1–4 by DFT method
Generally in the Schiff base derived from p-aminoazobenzene,
two possible conformations (Fig. 7) with respect to the CH
N
The HOMO–LUMO energy gap (Fig. 6) 7.61 (1), 7.65 (2), 7.70 (3)
and 7.76 (4) kcal/mol was calculated by B3LYP/6-31G(d,p), which
confirmed the eventual charge transfer interactions within the
and N N groups are shown. In the planar anti conformation,
the CH N bond and N N bond are trans to each other whereas
in the planar syn conformation, the CH N bond and N N bond
Fig. 6. HOMO–LUMO molecular orbitals and MEP surfaces of 1–4.

J. Jayabharathi et al. / Spectrochimica Acta Part A 83 (2011) 200–206
205
Table 3
Electric dipole moment (D), polarizability (˛) and hyperpolarisability (ˇtotal) of 1–4 calculated by B3LYP/6-31G(d,p).
Parameter
1
2
3
4
Dipole moment (D)
ꢁx
ꢁy
ꢁz
ꢁtot
−0.6457
0.0316
0.1474
−0.4667
−1.2857
−0.1463
−0.1487
−1.5807
0.1837
0.1812
−0.0019
0.2884
0.4702
−0.2680
0.6879
0.6011
Polarizability (˛)
˛
5.6528
7.2352
4.9652
5.6354
7.2654
9.9605
2.2658
6.2568
xx
˛
xy
˛
1.9651
1.9632
4.7011
2.3698
yy
˛
˛
˛
˛
−2.1235
−3.0231
2.9856
−2.8652
−4.1256
1.3215
−3.8453
−6.9998
5.3716
−1.2568
−3.2365
2.1258
xz
yz
zz
tot × 10−25
5.2381
4.0754
8.5650
3.3401
Hyperpolarisability (ˇ)
ˇxxx
ˇxxy
ˇxyy
ˇyyy
ˇxxz
ˇxyz
ˇyyz
ˇxzz
ˇyzz
ˇzzz
ˇtot × 10−33
−0.1390
0.0666
−0.0600
0.0142
0.2438
−0.2210
0.3513
0.2673
0.2948
1.2109
15.947
−0.2142
0.0938
−0.0566
0.0164
−0.1891
−0.0818
−0.0706
0.0074
0.2412
0.2377
−0.0768
−0.0242
−0.0102
−0.0884
0.0799
−0.0176
0.0043
0.2672
−0.0924
0.2269
0.3576
0.2604
−0.0203
−0.2385
−0.2479
0.6093
0.0600
−0.3042
0.3030
4.3421
1.2436
10.1006
16.2522
are cis to each other. Therefore, these two conformations are
labelled as anti and syn conformations, respectively. The favoured
conformation of all the Schiff bases (1–4) were predicted as
anti by DFT/B3LYP-631G(d,p) method.
The MEP map shows that oxygen and nitrogen atoms represent
the most negative potential region but the nitrogen atom seems
to exert comparatively small negative potential as compared to
oxygen atom. The hydrogen atom attached to the six and five mem-
bered ring bear the maximum positive charge. The Mulliken charge
distribution (Fig. 8) reveals that when compared to nitrogen atoms
N12, N13 and N24, N24 is considered as more basic site, which
also support the proton migration from hydroxy group (4) to N24
atom (ESIPT). The charge distribution shows that the more negative
charge is concentrated on oxygen atom whereas the partial positive
charge resides at hydrogens.
Fig. 8. Mulliken atomic charges for Schiff bases 1–4.
4. Conclusion
Photophysical aspects of Schiff bases derived from heterocyclic
aldehydes and ESIPT process in hydroxy Schiff base have been stud-
ied in detail. The fluorescence quenching studies of hydroxy Schiff
base implies that it can be used as a new fluorescence sensor to
detect the Cu2+ ion. PES calculation reveals the barrier for inter-
conversion in the excited state is much higher than that in the
ground state. The non-zero dipole moment values show that these
compounds are highly polar molecules and have microscopic NLO
behaviour. Its electric dipole moments, polarizability, hyperpolar-
isability, HOMO–LUMO energies and MEP studies were determined
by DFT calculation.
Acknowledgments
One of the author Dr. J. Jayabharathi, Associate Professor in
Chemistry, Annamalai University is thankful to Department of Sci-
ence and Technology [No. SR/S1/IC-73/2010] and University Grants
commission (F. No. 36-21/2008 (SR)) for providing funding to this
research work.
Fig. 7. Syn and anti conformers of Schiff base (4).

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J. Jayabharathi et al. / Spectrochimica Acta Part A 83 (2011) 200–206
Appendix A. Supplementary data
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