Synthesis, characterization, and spectroscopic investigation of benzoxazole conjugated Schiff bases

Article abstract of DOI:10.1021/jp206905f

Two Schiff bases were synthesized by reaction of 2-(4′-aminophenyl) benzoxazole derivatives with 4-N,N-diethylaminobenzaldehyde. UV - visible (UV - vis) and steady-state fluorescence in solution were applied in order to characterize its photophysical beha

Full text of DOI:10.1021/jp206905f

ARTICLE
pubs.acs.org/JPCA
Synthesis, Characterization, and Spectroscopic Investigation of
Benzoxazole Conjugated Schiff Bases
Fabiano S. Santos,†,‡ Tania M. H. Costa,† Valter Stefani,‡ Paulo F. B. Gonc-alves,§ Rodrigo R. Descalzo,§
Edilson V. Benvenutti,*,† and Fabiano S. Rodembusch*,‡
†Laboratꢀorio de Sꢀolidos e Superfícies, ‡Laboratꢀorio de Novos Materiais Org^anicos, and §Grupo de Química Teꢀorica e Computacional,
Universidade Federal do Rio Grande do Sul - Instituto de Química, Avenida Bento Gonc-alves, 9500, CP 15003, CEP 91501-970 Porto
Alegre-RS, Brazil
S Supporting Information
b
ABSTRACT: Two Schiff bases were synthesized by reaction of 2-(40-amino-
phenyl)benzoxazole derivatives with 4-N,N-diethylaminobenzaldehyde. UVꢀ
visible (UVꢀvis) and steady-state fluorescence in solution were applied in order
to characterize its photophysical behavior. The Schiff bases present absorption in
the UV region with fluorescence emission in the blue-green region, with a large
Stokes’ shift. The UVꢀvis data indicates that each dye behaves as two different
chromophores in solution in the ground state. The fluorescence emission spectra
of the dye 5a show that an intramolecular proton transfer (ESIPT) mechanism
takes place in the excited state, whereas a twisted internal charge transfer (TICT)
state is observed for the dye 5b. Theoretical calculations were performed in order
to study the conformation and polarity of the molecules at their ground and
excited electronic states. Using density functional theory (DFT) methods at
theoretical levels BLYP/Aug-SV(P) for geometry optimizations and B3LYP/
6-311++G(2d,p) for single-point energy evaluations, the calculations indicate that the lowest energy conformations are in all cases
nonplanar and that the dipole moments of the excited state relaxed structures are much larger than those of the ground state
structures, which corroborates the experimental UVꢀvis absorption results.
’ INTRODUCTION
any photochemical change. On the other hand, donorꢀacceptor
fluorescent molecules based on benzazoles have been described
in the literature,30ꢀ32 where an internal charge transfer (ICT)
state33ꢀ36 takes place on the excited state. Additionally, it is
desirable for a DSSC dye to present energy absorption in the
visible region, since in a Gr€atzel cell the semiconductor oxide film
TiO2 presents absorption in the UV region. In order to satisfy
this requirement using benzazole dyes, its conjugation must be
extended, as for instance in a Schiff base structure. In this way,
this work presents the synthesis, characterization, and photo-
physical study of novel Schiff bases with electronic structures
similar to those observed in DSSC dyes.
The growth of the world energy demand, as well as the search
for renewable energy sources that do not cause expressive
damage to the environment has led chemists to look for new
devices that can take advantage of solar radiation with larger effi-
ciency, increasing the contribution of renewable sources of
energy.1ꢀ3 Solar energy is highlighted due to its enormous
availability,4 as well as it being directly converted into electricity
by exploration of the photovoltaic effect.5ꢀ8 Photovoltaic de-
vices, particularly photoelectrochemical solar cells, such as the
Gr€atzel solar cell,9ꢀ13 take advantage of luminous energy to
promote oxireduction reactions using a dye-sensitized solar cell
(DSSC).14ꢀ16 Several photoactive structures have been used as
fluorescent sensitizers, such as coumarin and merocyanines.17ꢀ25
In this context, benzazole dyes can potentially be applied as
DSSC dyes, due to their well-know phototautomerism in the
excited state (ESIPT),26ꢀ28 which allows higher photochemical
and photothermal stability.29 In the ESIPT mechanism, the light
absorption by an enol conformer produces an excited enol, which
is quickly converted to an excited keto tautomer by an intramo-
lecular proton transfer. The excited keto tautomer decays,
emitting fluorescence to a keto tautomer in the ground state.
Since the enol conformer is more stable than the keto tautomer
in the ground state, the initial enol form is regenerated without
’ EXPERIMENTAL SECTION
Materials and Methods. All the solvents and reagents were
used as received or purified using standard procedures. Spectro-
scopic-grade solvents (Merck) were used for fluorescence and
UVꢀvisible (UVꢀvis) measurements. Elemental analyses were
performed by a Perkin-Elmer model 240. Melting points were
Received: July 19, 2011
Revised:
October 6, 2011
Published: October 11, 2011
r
2011 American Chemical Society
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Figure 1. Synthesis of the Schiff bases 5a,b.
measured with a Gehaka PF 1000 apparatus and are uncorrected.
Infrared spectra were recorded on a Shimadzu Prestige 21 in ATR
mode. 1H and 13C NMR spectra were performed on a VARIAN
INOVA YH300 using tetramethylsilane (TMS) as the internal
standard and CDCl3 and DMSO-d6 (Aldrich) as the solvent at
room temperature. UVꢀvis absorption spectra were performed
on a Shimadzu UV-2450 spectrophotometer. Steady-state fluor-
escence spectra were measured with a Shimadzu spectrofluo-
rometer model RF-5301PC. Spectrum correction was performed
to enable measuring a true spectrum by eliminating instrumental
response such as wavelength characteristics of the monochro-
mator or detector using Rhodamine B as a standard (quantum
counter). All experiments were performed at room temperature
at a concentration of 10ꢀ5 M.
Theoretical Calculations. The theoretical calculations were
performed using Orca 2.8 software37 at the BLYP/Aug-SV(P)
level and also using Gaussian 03 software38 at the B3LYP/
6-311++G(2d,p) level of theory. The ground electronic state
conformation and properties of structures 5a(enol) and 5b were
obtained by first optimizing the geometry of the ground state of
each species using the BLYP pure functional (with RI-J ap-
proximation) together with the Aug-SV(P) basis set in Orca 2.8,
and subsequently performing a single-point energy calculation at
their optimized geometries using the B3LYP hybrid functional
together with the 6-311G(2d,p) basis set in Gaussian 03. The
excited electronic state conformation and properties of structures
5a(enol conformer), 5a(keto tautomer), and 5b were similarly
obtained. For each of the three species, the methodology was as
follows: The first step was to perform a pure time-dependent
density functional theory (TD-DFT) energy calculation of the
ground state optimized structure (as obtained above) in order to
determine which was the most probable excited state for the
current species; after that, the selected excited state had its
geometry optimized. Both steps were run using Orca at the level
of theory mentioned above, BLYP/Aug-SV(P). The final step
was to perform a hybrid TD-DFT energy calculation of the
optimized structure for the excited state obtained in the previous
step. The output data corresponding to the most probable
excited state as indicated by the oscillator strengths of this
final calculation, which was run using Gaussian at the B3LYP/
6-311++G(2d,p) level, was selected. Onsager radii were calcu-
lated using the polarizable continuum model as the self-consis-
tent reaction field method in Gaussian.
Figure 2. Normalized absorption spectra of the Schiff base 5a (a) and
5b (b). The precursors in dichloromethane are also presented for
comparison (dotted line).40
were prepared according to Figure 1. In a typical experiment, an
equimolar amount of the amino derivative 3a,b and 4-N,N-
diethylaminobenzaldehyde (4) was stirred under reflux for 4 h.
The product, which precipitates in the reaction media, was
filtered and washed with cold isopropanol several times and
dried at room temperature.
(E) N,N-Diethyl-4-[40-(benzoxazolyl)-30-hydroxyphenyl]-
benzylideneimine (5a). Yield: 87%; mp: > 200 °C. IR (ATR,
cmꢀ1): 3081 and 3055 (νCH arom.), 2970, 2925, and 2866
(νCH aliphatic), 1633 (νCdC arom.), 1585 (νCdN imino),
1487 (νCdC arom.). 1H NMR (300 MHz, CDCl3): δ (ppm) =
11.53 (broad s, 1H, OH), 8.31 (s, 1H, CH imine), 7.98 (d, Jo =
8.3 Hz, 1H, HA), 7.75 (d, Jo = 8.6 Hz, 2H, HB), 7.67ꢀ7.70 (m,
1H, H7), 7.56ꢀ7.58 (m, 1H, H1), 7.33ꢀ7.36 (m, 2H, H5 and
H6), 6.84 (dd, Jo = 8.3 Hz, Jm = 1.76 Hz, 1H, HB), 6.69 (d, Jo = 8.6
Hz, 2H, HB1), 6.68 (s, 1H, HX), 3.42 (q, 4H, CH2), 1.20 (t, 6H,
CH3). 13C NMR (75.4 MHz, CDCl3): δ (ppm) = 163.1; 160.9;
159.6; 158.1; 150.4; 148.9; 140.1; 131.1; 127.7; 124.9; 124.7;
123.0; 118.8; 113.8; 110.9; 110.4; 108.6; 107.1; 44.5; 12.5. Anal.
Calcd. for C24H23N3O2 (385.46 g molꢀ1): C: 74.78; H: 6.01; N:
3
10.90. Found: C: 72.97; H: 5.89; N: 10.37. EIMS: m/z (%) =
385.20 [M+] (60), 370.15 (100).
(E) N,N-Diethyl-4-[40-(benzoxazolyl)]benzylideneimine (5b).
Yield: 88%; mp: > 200 °C. IR (ATR, cmꢀ1): 3075 and 3020
(νCH arom.), 2967, 2931, and 2888 (νCH aliphatic), 1621
(νCdC arom.), 1602 (νCdN imino), 1486 (νCdC arom.). 1H
NMR (300 MHz, CDCl3): δ (ppm) = 8.34 (s, 1H, CH imine),
8.25 (d, Jo = 8.6 Hz, 2H, HB2), 7.77 (d, Jo = 8.9 Hz, 2H, HA1),
7.74ꢀ7.76 (m, 1H, H1), 7.58ꢀ7.55 (m, 1H, H1), 7.35ꢀ7.32 (m,
4H, H2, H3 and HA2), 6.69 (d, Jo = 8.9 Hz, 2H, HB1), 3.43 (q, 4H,
CH2), 1.21 (t, 6H, CH3). 13C NMR (75.4 MHz, CDCl3): δ
(ppm) = 163.2, 160.8, 155.9, 150, 142.2, 131, 128.6, 124.7, 124.4,
123, 121.5, 119.6, 114.5, 110.9, 110.4, 44.5, 12.5. Anal. Calcd. for
Synthesis of the Schiff Bases. The dyes 3a,b were prepared
using a methodology previously described (spectroscopic data
available in the Supporting Information).39 The Schiff bases 5a,b
C24H23N3O2 (369.46 g molꢀ1): C; 78.02; H: 6.27; N: 11.37.
3
Found: C: 76.71; H: 6.49; N: 10.78. EIMS: m/z (%) = 369.12
[M+] (61), 354.15 (100).
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Table 1. Relevant UVꢀVis Data of the Schiff Bases in Different Solvents
dye
solvent
LW (nm/eV)
ε ꢁ 104 (l molꢀ1 cmꢀ1
)
SW (nm/eV)
ε ꢁ 104 (l molꢀ1 cmꢀ1
)
3
3
3
3
5a
1,4-dioxane
387/3.20
392/3.16
389/3.19
390/3.18
383/3.24
388/3.20
389/3.19
387/3.20
5.6
3.0
1.9
3.0
5.0
5.0
2.8
2.4
338/3.67
345/3.59
348/3.56
342/3.63
340/3.65
351/3.53
351/3.53
344/3.60
7.2
3.7
3.2
5.6
3.2
4.5
3.9
3.0
dichloromethane
absolute ethanol
acetonitrile
5b
1,4-dioxane
dichloromethane
absolute ethanol
acetonitrile
Figure 3. Proposed chromophores in nonplanar Schiff bases, where X =
OH or H.
’ RESULTS
UVꢀVis Absorption. In Figure 2a,b are presented the normal-
ized UVꢀvis absorption spectra of the Schiff bases 5a-b, respec-
tively in 1,4-dioxane, dichloromethane, absolute ethanol, and
acetonitrile. The relevant UVꢀvis data are summarized in
Table 1.
An absorption band maxima, the so-called long wavelength
(LW), located around 389 nm with molar extinction coefficient
values (ε) in agreement with πꢀπ* transitions, can be observed
for the dye 5a (Figure 2a). A more intense blue-shifted band, the
so-called short wavelength (SW), can be observed located at
343 nm, and is also ascribed to πꢀπ* transitions. A positive
solvatochromic effect can be observed for this dye (5ꢀ10 nm).
The difference in the intensities of the absorption bands can be
related to a difference of planarity of the different chromophores
present in the dye,28,41 where a nonplanar structure does not
allow a more effective electronic delocalization among the three
π systems. In this way, for these structures it can be discussed in
terms of Schiff, benzoxazolic, and diethylamino chromophores,
denoted as C, Ca, and Cb, respectively (Figure 3). The Schiff
chromophores (C) would be responsible for the LW absorption
bands, and the benzoxazolic chromophore (Ca) is related to the
SW bands. The observed difference between the molecular
planarity of this dye is confirmed taking into account the molar
extinction coefficient values. A less planar structure usually
presents a lower probability for the πꢀπ* transition, as observed
in this dye. Additionally, as already observed with similar ESIPT
dyes,28,42,43 the benzoxazolyl moiety can present a conforma-
tional equilibrium in solution in the ground state, confirmed by
the solvatochromic effect presented in the UVꢀvis spectra.
For the dye 5b (Figure 2b), both absorption bands (LW and
SW) can be observed. However, for this dye, the absorption band
intensities seem to be tailored by the solvent polarity. In less
Figure 4. Normalized fluorescence emission of 5a using (a) LW as the
excitation wavelength (387ꢀ392 nm) and (b) SW as the excitation
wavelength (338ꢀ348 nm).
polar media (1,4-dioxane and dichloromethane), the LW band,
located around 387 nm, is more intense. However, in more polar
solvents (absolute ethanol and acetonitrile), the more intense
band is the SW one located at 346 nm. The molar extinction
coefficient values for both bands are in agreement with πꢀπ*
transitions. Concerning the UVꢀvis absorption band intensities
from the two dyes, it can be observed that the hydroxyl group in
the benzoxazolyl chromophore 5a plays a fundamental role in the
benzoxazolic chromophores (Ca) through its intramolecular
hydrogen bond. Once a hydrogen bond is present in the dye
structure, the benzoxazolyl chromophore will be more planar
than the whole molecule in solution in the ground state. For the
dye 5b, the planarity seems to be dependent on the solvent
polarity. Additionally, a more intense positive solvatochromic
effect can be observed for this dye (10ꢀ12 nm) in relation to 5a,
indicating that the absence of the hydroxyl group allows a more
effective electronic delocalization, where a higher pushꢀpull
character can be observed between the benzoxazolic ring and
the imino moiety, respectively, in the ground state. The electro-
static potential surfaces of these dyes (Figure 9a and Supporting
Information Figure SI 25) obtained by theoretical calculations
corroborates with this result and will be discussed later.
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Table 2. Relevant Data from the Fluorescence Emission Curves of the Dye 5aa
solvent
LW (nm/eV)
(λeenmol) (nm/eV)
(λkemeto) (nm/eV)
(ΔλST
)enol (nm/eV)
(ΔλST
)keto (nm/eV)
1,4-dioxane
387/3.20
392/3.16
389/3.19
390/3.18
484/2.56
500/2.48
497/2.08
515/2.41
97/0.64
dichloromethane
absolute ethanol
acetonitrile
108/0.68
108/1.11
125/0.77
solvent
SW (nm/eV)
(λeenmol) (nm/eV)
(λkemeto) (nm/eV)
(ΔλST
)enol (nm/eV)
(ΔλST)keto (nm/eV)
1,4-dioxane
338/3.67
345/3.59
348/3.56
342/3.63
373/3.32
470/2.64
492/2.52
480/2.58
512/2.42
35/0.35
132/1.03
147/1.07
132/0.98
170/1.21
dichloromethane
absolute ethanol
acetonitrile
381/3.25
378/3.21
33/0.31
36/0.42
a The first and second sets of data were obtained using LW and SW from the UVꢀvis curves as excitation wavelengths, respectively.
Although there is a significative shift between the absorption
maxima from precursor 3b and the SW bands from dye 5b
(Figure 2b), we believe that these bands located at 340ꢀ351 nm
can be ascribed as the chromophore Ca in the dye 5b. For this
statement, we are assuming that (i) a comparison can be made
between isolated precursors with chromophores inside the Schiff
base structure, which are moderately similar to each other (3a/5a
and 3b/5b); (ii) the isolated precursors present different absorption
maxima (Tables SI 1 and SI 2), since the absence of the hydroxyl
group in an aromatic ring can blue shift the absorption maxima up to
60 nm;44,45 (iii) after conjugation (precursor’s derivatization), the
chromophores present very similar absorption regions, which are
red-shifted from the precursors, as expected.46,47 The obtained data
indicates that the precursor’s derivatization changes the electronic
distribution of the benzoxazole chromophore (Ca) showing that
now the hydroxyl group does not play a fundamental role in the
electronic distribution in the chromophore (contrary to what was
observed for the precursors), since 5a and 5b present SW bands in
very similar regions. In this way, considering that the SW bands from
5a and 5b are in similar regions (due to the respective chromo-
phores), and that the precursor 3b presents a blue-shifted absorp-
tion maxima (in spite of the precursor 3a), it was expected to
Figure 5. Normalized fluorescence emission of 5b using (a) LW as the
excitation wavelength (383ꢀ389 nm) and (b) SW as the excitation
wavelengths (340ꢀ351 nm).
observe a higher shift between 3b and 5b than 3a and 5a.
Fluorescence Emission and Excitation Spectra. Different
organic solvents with a wide range of dielectric constants were
used in the photophysical characterization of the Schiff bases
(Figure 4a,b). The emission spectra were obtained using the LW
maxima (387ꢀ392 nm), as well as the SW maxima (338ꢀ
348 nm), respectively. In Table 2 are summarized the relevant
data from this photophysical study. In Figure 4a, a main emission
band located around 500 nm can be observed, with a relevant
solvatochromic effect (∼31 nm), which probably indicates that
the dipole moment of the dyes are higher in the excited state than
in the ground state. An excitation at the LW means that the fully
planar dye is responsible for the absorption followed by fluores-
cence emission with large Stokes shift. The molecular structure of
the dye, as well as the large Stokes shift (∼100 nm) probably
indicates that a phototautomerism (ESIPT) takes place in the
excited state. The Supporting Information depicts a general
scheme (Figure SI 15) of the ESIPT mechanism in the dye 5a,
where an excited keto tautomer (K*), which arises from a totally
planar enol conformer (EI) in the excited state, is responsible for
the fluorescence emission with large Stokes shift.27,28
wavelength. One main fluorescence emission band can be ob-
served in dichloromethane under excitation at 345 nm, which
leads to the stabilization of the more planar structure (higher
conjugation) in the excited sate. The data presented in the
literature indicate that the precursor 3a presents an emission
ascribed to the ESIPT mechanism located at lower wavelengths
in comparison to their Schiff base 5a.28 In this way, the fluor-
escence emission observed for this dye in all studied solvents is
due to a more planar structure, the so-called Schiff chromophore
(C), which excludes the additional chromophores (Figure 3). On
the other hand, a particular photophysical behavior can be
observed, characterized by a dual fluorescence emission in 1,4-
dioxane, absolute ethanol, and acetonitrile. In these solvents, a
red-shifted band is present (470ꢀ512 nm), ascribed to the
ESIPT band and a blue-shifted one (373ꢀ381 nm) related to
the normal emisson.28 Usually, the dual fluorescence emission
presents a band at higher wavelengths attributed to the excited
keto tautomer (K), and a blue-shifted one due to the conforma-
tional forms, which are stabilized in solution and present normal
In Figure 4b are depicted the emission curves to the dye
5a using the SW maxima (338ꢀ348 nm) as the excitation
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Table 3. Relevant Data from the Fluorescence Emission Curves of the Dye 5ba
solvent
LW(nm/eV)
(λLemE) (nm/eV)
(λIeCmT) (nm/eV)
(ΔλST LE
)
(nm/eV)
(ΔλST ICT
)
(nm/eV)
1,4-dioxane
383/3.24
388/3.20
389/3.19
387/3.20
462/2.68
491/2.53
495/2.50
507/2.45
78/0.58
dichloromethane
absolute ethanol
acetonitrile
103/0.67
106/0.69
120/0.75
solvent
SW(nm/eV)
(λLemE) (nm/eV)
(λIeCmT) (nm/eV)
(ΔλST
)
LE (nm/eV)
(ΔλST)ICT (nm/eV)
1,4-dioxane
340/3.65
351/3.53
351/3.53
344/3.60
368/3.37
374/3.32
381/3.25
375/3.31
28/0.28
23/0.21
30/0.28
31/0.29
dichloromethane
absolute ethanol
acetonitrile
494/2.51
143/1.02
a The first and second set of data was obtained using LW and SW from the UVꢀvis curves as excitation wavelengths, respectively.
relaxation (EIIꢀIII).27,28 Although it is well known that the lower
wavelength emissions are due to normal enol conformers, it is
worth mentioning that the blue-shifted bands are related to the
twisted Schiff bases (benzoxazole chromophore), since the emis-
sion values must be higher than the absorption ones (Stoke’s law).
In order to confirm the presence of conformers in solution in
the ground state, the fluorescence excitation spectra of the dyes
were recorded at their enol and/or keto fluorescence emission
wavelengths (Figure SI 16). It could be observed for the dye 5a in
1,4-dioxane, absolute ethanol, and acetonitrile that the excita-
tions at the enol band and the ESIPT band give two distinct
spectra. This indicates that the two fluorescence bands originate
from at least two different conformers in the ground state, and
the absorption spectra of this dye is a mixture of these conformers
in the ground state. In dichloromethane, only one band can be
observed, probably due to the conformer EI, which corroborates
the main emission band in the fluorescence emission spectra.
The same photophysical study was performed for the dye 5b,
where the fluorescence emission spectra were obtained using
the LW maxima (383ꢀ389 nm), as well as the SW band (340ꢀ
351 nm), respectively, and are presented in Figure 5a,b. Table 3
summarizes the relevant data from this photophysical study. One
main fluorescence emission band can be observed in all solvents
under excitation at LW, which leads to the stabilization of the Schiff
chromophore (higher conjugation) in the excited sate. Increasing
the solvent polarity, the fluorescence maxima shift from 462 up
to 507 nm with a relevant solvatochromic effect (∼45 nm)
(Figure 5a). Such solvatochromic shifts in the emission maxima
are usually associated with a drastic change in the excited state
charge distribution as compared to that in the ground state. The
excited state often has ICT or twisted ICT (TICT) character.35,49
In a more polar solvent, the species with charge separation (ICT
state) may become the lowest energy state. In a nonpolar solvent,
the species without charge separation, the so-called locally excited
(LE) state, may have the lowest energy, which can be ascribed to
the benzoxazole chromophore. In this way, the role of the solvent
polarity is not only to lower the energy of the excited state due to
general solvent effects, but also to govern which state has the lowest
energy.50 It is worth observing that the fluorescence emission curve
in the less polar solvent 1,4-dioxane seems broader in relation to the
additional curves in Figure 9, probably indicating that the LE
(∼420 nm) and the ICT (484 nm) states are present in this
solvent.35
Figure 6. Solvent effect on the spectral position of the absorption
maximum (top), and of the two fluorescence bands, SW (middle) and
LW (bottom) for the Schiff bases 5a,b. The LippertꢀMataga solvent
polarity function Δf is given in eq 4.
dichloromethane can a clear dual fluorescence emission be ob-
served with a main band located at 494 nm ascribed to the ICT
state and the blue-shifted one related to an LE state (345 nm).
In 1,4-dioxane, absolute ethanol, and acetonitrile, the emission
band in the SW region mainly originates from the LE state. This
result probably indicates that an inadequate geometry (twisted
geometry) is present in these solvents, which are not able to
present the ICT state.
The non-ESIPT dye 5b demonstrates evidence of internal
charge transfer (TICT or ICT) in the excited state. In order to
correlate for mixed solvents the solvatochromic shifts with the
solvent polarities, the dielectric constant (εmix) and the refractive
index (nmix) were calculated as already presented in the
literature:50,52
εmix ¼ fAεA þ fBεB
n2mix ¼ fAn2A þ fBnB2
ð1Þ
ð2Þ
where fA and fB are the volumetric fractions of the two solvents.
On the basis of the assumption that a point dipole is situated
at the center of the spherical cavity and neglecting the mean
solute polarizability (α) in the states involved in the transition
Figure 5b depicts the emission curves to the dye 5b using
SW (340ꢀ351 nm) as the excitation wavelength. Only in
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Figure 7. Fluorescence emission spectra for the dyes 5a (left) and 5b (right) in mineral oil Nujol. The two sets of curves are in the same Y axis scale.
Table 4. Calculated Properties of Schiff Bases in Ground and
Excited Electronic States
(α = αe = αg = 0, where e and g stand for excited and ground
states, respectively), one obtains48,51
hcνmax ¼ hcνmaxð0Þ ꢀ ½2μeðμe ꢀ μgÞ=a3ꢂ Δf
ð3Þ
ð4Þ
conformers
3
calculated parameters
5a(enol)
5a(keto)
5b
Δf ¼ fðεÞ ꢀ ð1=2ÞfðnÞ
Ground State
6.8531
dipole moment (debye)
torsion anglea (degree)
Onsager radius (ångstr€om)
7.8923
39.8063
5.30
fðεÞ ¼ ðε ꢀ 1Þ=ð2ε þ 1Þ; fðnÞ ¼ ðn2 ꢀ 1Þð2n2 þ 1Þ ð5Þ
42.3464
5.34
where μg is the dipole moment of the solute in the ground state,
νmax is the solvent-equilibrated fluorescence maxima, νmax(0) is the
value of spectral position of the fluorescence maxima extrapolated
to the gas phase, and a is the radius of the cavity in which the dye
resides. It is worth mentioning that alcohols are usually excluded
from these correlations to avoid specific soluteꢀsolvent interac-
tions. Since charge transfer states can be more stabilized, increasing
the solvent polarity, a linear relation of the fluorescence maxima
(νmax) versus the solvent polarity function (Δf) indicates the
occurrence of the ICT state. It can be observed in Figure 6 that
only the ICT/ESIPT bands show a linear fit (r2 = 0.99 and 0.98 to
the 5a,b, respectively) of the fluorescence maxima versus the
solvent polarity function, which can be related to the ICT state.
Regarding the molecular structure of the dyes 5a,b, the observed
linearity seems to arise from a different mechanism in the excited
state. For the dye 5b, the ICT state arises from the pushꢀpull
character of the end groups presented in this structure.49 On the
other hand, the charge transfer character observed in ESIPT dyes
generates a zwitterionic species in the excited state, as already
discussed in similar dyes.53ꢀ56 In this way, we believe that the ICT/
ESIPT graphs are presenting the same interpretation: charge
Excited State
26.4531
dipole moment (debye)
torsion anglea (degree)
17.1239
30.7233
24.8199
36.8968
40.7939
a Dihedral angle between the atoms indicated in structure “a” of Figure 8.
transfer, but from different pathways: electron delocalization
(5b) and ESIPT mechanism (5a). The same linearity could not
be observed for the absorption and LE bands in relation to the sol-
vent polarity function, since these species are less polar than the
ICT/ESIPT species.
Although there seems to be a lack of agreement in the need of
the twisting to reach some ICT states, a simple experiment could
lead us to observe this state in these compounds. It is well-known
that viscosity can have a dramatic effect on the emission intensity
of the dye. An example can be observed in the trans-stilbene,
where the decrease of the fluorescence intensity can be inter-
preted as due to rotation about the central ethylene double bond
in the excited state, which is thought to affect the emission of
many other dyes.57ꢀ60 Increase in local viscosity contributes to
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Figure 8. Structure of the Schiff base 5a, where structure “a” is the enol
conformer in the ground state and “b” and “c” are the enol conformer
and the keto tautomer in the excited state, respectively.
Figure 9. Electrostatic potential surfaces of the Schiff base 5a, where
structure “a” is the enol conformer in the ground state, and “b” and “c”
are the enol conformer and the keto tautomer in the excited state,
respectively. The most negative (electron-rich) potential is colored red,
and the most positive (electron-poor) is colored blue.
the increased intensities displayed by many dyes when bound to
biomolecules. However, when a twisting motion is needed to
achieve an ICT state, the increase of the viscosity leads to a
decrease of the fluorescence intensity, since the TICT produces a
fluorescence quantum yield dependent on the surrounding
environment.35 In this way, additional fluorescence emission
spectra were performed for two solutions of the dyes 5a,b, in the
same concentration, in mineral oil (Nujol) (viscosity 211 cP)
(Figure 7).61,62 It can be observed that only the fluorescence
intensity from the dye 5b is affected by the increase of the
viscosity, as expected when a TICT state takes place in the
excited state. This evidence leads us to conclude that the TICT
state is formed upon the LE state of the dye 5b.
increase in charge separation when these molecules are electro-
nically excited. It is worth mentioning that the dye 5a shows
ESIPT in the excited state and also a linear plot of the
fluorescence maxima versus the solvent polarity, indicating an
unusually polar structure related to the ESIPT. Once again, the
calculations reveal a charge separation in the excited state for the
keto tautomer, as expected for this dye. A similar result could be
obtained for 5b in the ground and excited states (Figures SI 25
and 26 in the Supporting Information).
Theoretical Calculations. The theoretical calculations indi-
cate particular results concerning the planarity and electronic
distribution of the Schiff bases. The most stable conformation for
each species, both in the ground and excited state, is a nonplanar
one, with torsion between the phenylbenzazolic chromophore
and the diethylamino ring ranging from 30° to 42° (Table 4).
These results are consistent with the experimental observation
from the UVꢀvis spectra, where a less planar chromophore
(Ca in Figure 3) absorbs radiation at the lower wavelengths.
Figure 8 presents the optimized geometries corresponding to the
Schiff base 5a. Although the dye 5b does not present the ESIPT
mechanism, which excludes a keto tautomer in the excited state, a
similar result could be obtained for the ground and excited states
(Figures SI 23ꢀ24 in the Supporting Information).
’ CONCLUSIONS
Two Schiff bases (5a,b) were synthesized by reaction of 2-
(40-aminophenyl)benzoxazoles derivatives with 4-N,N-diethyla-
minobenzaldehyde and fully characterized by means of infrared,
13C and 1H NMR, and elemental analysis. UVꢀvis and steady-
state fluorescence in solution were also applied in order to
characterize its photophysical behavior. These Schiff bases pre-
sent absorption in the UV region (∼388 nm) with fluorescence
emission in the blue-green region, with a large Stokes shift. The
equilibrium between conformers in solution in the ground state,
confirmed by the moderate solvatochromic effect due to two
different conjugation lengths, reflects the dual fluorescence
emission presented by these dyes. The fluorescence emission
from the Schiff bases arises from different decay pathways. For
the dye 5a, an intramolecular proton transfer mechanism
(ESIPT) mechanism may take place, where the emission at
LW is related to the ESIPT mechanism, and the blue-shifted ones
are due to conformational forms with a normal relaxation. On the
other hand, a TICT is observed for the dye 5b. Theoretical
calculations show, consistently with experiment, that the most
stable conformation of each species, both in the ground and
The dipole moments of each species could also be calculated,
where the structures in the excited state present a higher dipole
moment in comparison to the ground state. These results are
once again consistent with the experimental observation, since
these dyes presented a positive solvatochromic effect when
the polarity of the solvents is increased. Figure 9 depicts the
electrostatic potential surfaces of Schiff base 5a in the ground and
excited states, which provides a means of visualizing this large
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excited states, is nonplanar, and that the charge separation is
much larger in the excited state when compared to the
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’ ASSOCIATED CONTENT
S
Supporting Information. Original data from the spectro-
b
scopic characterization (FTIR, 1H and 13C NMR, and EI-MS), a
general scheme of the ESIPT mechanism, the photophysical
behavior of the precursors, and additional figures from the
theoretical calculations. This material is available free of charge
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’ AUTHOR INFORMATION
(31) Fayed, T. A.; Etaiw, S. E.-D. H.; Khatab, H. M. J. Photochem.
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Corresponding Author
*Phone: +55 51 3308 7176. Fax: +55 51 3308 7304. E-mails
rodembusch@iq.ufrgs.br (F.S.R.); benvenutti@iq.ufrgs.br (E.V.B.).
’ ACKNOWLEDGMENT
(35) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. Rev. 2003,
103, 3899–4031.
We are grateful for financial support and scholarships from the
Brazilian agencies CNPq and CAPES and Instituto Nacional de
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