Photophysics and spectroscopic properties of3-benzoxazol-2-yl-chromen-2-one
Antonio Eduardo da Hora Machado a,�, Jacques Antonio de Miranda a,Silvana Guilardi a, David Ernest Nicodem b, Divinomar Severino b
a Universidade Federal de Uberlandia �/ Instituto de Quımica, P.O. Box 593,
Laboratorio de Fotoquımica/GFQL 38400-089 Uberlandia, Minas Gerais, Brazilb Universidade Federal do Rio de Janeiro �/ Instituto de Quımica, Laboratorio de Espectroscopia Resolvida no Tempo,
CEP 21949-900 Rio de Janeiro, RJ, Brazil
Received 11 February 2002; received in revised form 22 April 2002; accepted 23 April 2002
Abstract
The photophysics of 3-benzoxazol-2-yl-chromen-2-one was studied in different solvents. High molar absorptivities,
between 14 800 and 22 900 dm3/mol cm, were observed for the absorption peak related to the S00/S1 transition which
suggests a p0/p�character. This compound presents a limited solvatochromism, attributed to the benzoxazole group,
and high fluorescence quantum yields, Ff. The fluorescence quantum yield is lowered with the increase of solvent
polarity, favouring the participation of internal conversion as deactivation path of the S1 state. The Stokes shift shows
that the excited state is stabilised with increasing solvent polarity. The dipole moment was estimated by ab initio
calculations as being between 5.28 and 5.62 Debye for S1, and 4.75 Debye for S0. Phosphorescence was not observed. A
small but not negligible quantum yield of singlet oxygen generation (FD�/0.15) was measured in chloroform.
The geometric parameters obtained by semi-empirical calculation (PM3) are in good agreement with crystallographic
data, showing a r.m.s. deviation of 0.153 A for the superposition of both structures. The predicted structure is all
planar, while the crystallographic data reveal a dihedral angle of 6.58, between the coumarin and benzoxazole rings. The
theoretical description of the electronic spectra, obtained from a PM3 CI calculation, shows excellent agreement with
the experimental data. Deviations lower than 2% are observed in the predicted absorption maxima, with best results
when solvation is considered. For electronic states calculation, ZINDO/S gave a better prediction of excited state
energies, with a deviation lower than 7% for the S1 energy. The most probable sequence for the first four excited states
is: T1(np�)B/T2(pp�)B/S1(pp�)B/S2(np�).
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Photophysics; Coumarin derivative; Molecular structure; Electronic structure; PM3; ZINDO/S; ab initio
1. Introduction
Coumarins have been extensively studied due to
their commercial importance in several fields [1,2].
Optical brightners, laser dyes, sensitizers in photo-
� Corresponding author. Tel.: �/55-34-3239-4143; fax: �/55-
34-3239-4208
E-mail address: [email protected] (A.E. da Hora Machado).
Spectrochimica Acta Part A 59 (2003) 345�/355
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PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 1 6 4 - 6
therapy, etc. are some of the uses of this class ofcompounds. Considerable attention has been given
to the study of the luminescent properties in
different media of new coumarin derivatives [3�/
6]. Solute/solvent interactions exert an important
role in the deactivation of their excited states,
reflecting directly on their photophysical and
spectroscopic properties [3,7�/10]. Substitution in
the coumarin ring has been shown to be of primaryimportance for the spectroscopic behaviour of
these compounds [1,3,4,11]. Knowledge of the
state ordering of excited states, energy, their
nature and the possibility of occurrence of mixing
of states are fundamental for a good understand-
ing of the photophysics and photochemistry of
coumarins.
In this study we evaluate the photophysics andspectroscopic properties of the coumarin deriva-
tive 3-benzoxazol-2-yl-chromen-2-one (Fig. 1), by
comparing experimental and theoretical results
[12]. This is a coumarin derivative which presents
high fluorescence quantum yields (Ff) in solvents
of different polarities, and generates singlet oxygen
with a relatively low quantum efficiency. Due to
these characteristics and its good photochemicalstability, this compound has a good potential as a
laser dye in the range between 438 and 450 nm.
2. Experimental
All solvents were of spectroscopic grade. The
absorption spectra were recorded on a HACHDR-4000U spectrophotometer. The fluorescence
measurements were obtained using a HITACHI F-4500 spectrofluorimeter equipped with a low-
temperature accessory. The fluorescence spectra
were obtained, using the right angle configuration,
by exciting the sample at the wavelength of
maximum excitation. The fluorescence quantum
yields were estimated from the corrected fluores-
cence spectra using 9,10-diphenylantracene in
cyclohexane (FF�/0.90 at 293 K) as standard,employing the methodology proposed by Eaton
[13]. Solutions were prepared with absorbance
values lower than 0.100 at the excitation wave-
length, to minimise self-absorption effects.
Low-temperature measurements were carried
out at 77 K, under liquid nitrogen, for deaerated
solutions, using methyl cyclohexane as solvent.
Time resolved measurements were done on aCD-900 Edinburgh Analytical Instruments time
resolved spectrophotometer, operating with a
hydrogen filled nanosecond flash lamp at 30 kHz
pulse frequence. All lifetime values reported reflect
data taken with at least 1000 counts in the
maximum channel, and 0.900B/x2B/1.100 during
the measurements. The measurements were taken
with the sample in a front-face configuration.The quantum yield of singlet oxygen generation
was measured in chloroform, using solutions with
an absorbance of 0.300 at 355 nm. Before its use,
the solvent was treated with activated silica for 12
h. The measurement was carried out with a LP 900
Edinburgh Analytical Instruments time-resolved
system, with 355 nm laser pulses (5 ns) furnished
by a Nd-YAG Continuum Surelite II (Q-switcheddelay 200 ms). The laser power was varied from 0
to 8 mJ. A North Coast EO-817 detector was used
for the detection of singlet oxygen phosphores-
cence at 1270 nm. Phenalenone (fD,s�/0.97 at 293
K), ALDRICH, was used as standard in these
measurements, using the methodology proposed
by Schmidt et al. [14], based on the following
equation,
fD;a�Ia
Is
fD;s
where Ia is the emission intensity of the sample, Is
is the emission intensity of the standard and fD,a is
the quantum efficiency of singlet oxygen genera-Fig. 1. Representation of the compound 3-benzoxazol-2-yl-
chromen-2-one.
A.E. da Hora Machado et al. / Spectrochimica Acta Part A 59 (2003) 345�/355346
tion by the sample. The ratio Ia/Is was calculatedfrom the slopes of the Signal Intensity vs. Laser
Power plot. The standard (phenalenone) and
samples were excited at different laser powers.
This permitted a more accurate evaluation of Ia/Is.
This value was obtained taking the ratio between
the slope of the plot of the signal due to singlet
oxygen at 1270 nm, at t�/0 s, expressed in mV,
versus the laser power, expressed in mJ, for thesample and for the standard,
Ia
Is
:
�DS
DP
�a�DS
DP
�s
�(DS)a
(DS)s
(DP )a�/(DP )s, considering that the experiments
were done under the same conditions.
The ground state geometries of the moleculewere firstly optimized using the PM3 semi-empiri-
cal Hamiltonian (HYPERCHEM 5.11, Polak-
Ribiere optimizer, Gradient norm 0.10 kcal/
mol A). This geometry was also optimized using
some facilities from AMPAC 6.56 PC. These
results were compared to crystallographic data
[15].
The electronic spectra for the isolated andsolvated molecule (in methanol/water and in
chloroform) were predicted using CI calculations,
considering 25 occupied and 25 unoccupied mole-
cular orbitals (PM3, Hyper), totalizing 1251 con-
figurations. In the theoretical electronic spectrum,
only the lines with oscillator strengths higher than0.001 were considered. For the solvated molecule,
a facility implemented in the Hyperchem 5.11
software package was used [16]. Forty-five solvent
molecules were distributed in the neighbour of the
compound under study (for simulation of metha-
nolic solution, 25 molecules of water and 20 of
methanol were randomically distributed). For
excited electronic states calculations, 3 occupiedand 4 unoccupied MO were used, totalizing 1225
configurations. The minimal distance between
solvent molecules and the substrate was 2.3 A
[16]. The set of molecules was firstly optimized
before single point calculations.
Dipole moments of ground and S1 states were
calculated using ab initio calculations at the
Hartree-Fock level for the ground state, andConfiguration Interaction for the S1 state, using
3-21G basis-functions. Gaussian 94W was used to
perform these calculations [17].
3. Results and discussion
3.1. Structure
Fig. 2 shows superimposition of the optimised
and experimental structures, which show only
small differences between both representations,
A r.m.s. deviation of 0.153 A was estimated for
the superposition of the structures. The experi-
Fig. 2. Superimposition of the structures obtained by X-ray crystallography and semi-empirical calculations (PM3).
A.E. da Hora Machado et al. / Spectrochimica Acta Part A 59 (2003) 345�/355 347
mental dihedral angle between the coumarin and
benzoxazole rings is 6.5(3)8, while the structurefurnished by PM3 is totally planar. The geometric
parameters are gathered in Tables 1�/3. The
observed differences between the two representa-
tions are attributed to intermolecular polar inter-
actions present in the crystal packing [15].
3.2. Absorption spectra
Fig. 3 shows a typical UV/Vis spectrum in
methanol for the compound under study.In the 200�/500 nm range, the spectrum presents
two typical absorption bands (206 and 352 nm).
Both are attributed to p0/p� transitions, consider-
ing their high molar absorptivity, and the nature of
the molecular orbitals. The first (206 nm) and
second (352 nm) band should be attributed,
respectively, to the ethylenic, and benzenoid tran-
Table 1
Bond lengths (A) estimated by X-ray crystallography and semi-
empirical calculation, for 3-benzoxazol-2-yl-chromen-2-one
Atoms Bond length, A
X-ray PM3
O1�/C1 1.374(2) 1.372
O1�/C2 1.381(3) 1.383
O2�/C10 1.364(2) 1.393
O2�/C12 1.380(2) 1.387
O3�/C2 1.203(2) 1.209
N�/C10 1.296(3) 1.334
N�/C11 1.395(3) 1.422
C1�/C8 1.380(3) 1.405
C1�/C9 1.392(3) 1.405
C2�/C3 1.461(3) 1.476
C3�/C4 1.356(3) 1.358
C3�/C10 1.461(3) 1.454
C4�/C9 1.421(3) 1.442
C5�/C6 1.385(3) 1.383
C5�/C9 1.395(3) 1.404
C6�/C7 1.381(3) 1.399
C7�/C8 1.373(3) 1.384
C11�/C16 1.384(3) 1.393
C11�/C12 1.387(3) 1.413
C12�/C13 1.371(3) 1.391
C13�/C14 1.376(3) 1.389
C14�/C15 1.393(3) 1.403
C15�/C16 1.384(3) 1.389
Table 2
Angles (8) estimated by X-ray crystallography and semi-
empirical calculation, for 3-benzoxazol-2-yl-chromen-2-one
Atoms Angle (8)
X-ray PM3
C1�/O1�/C2 123.4(2) 118.9
C10�/O2�/C12 104.3(2) 105.4
C10�/N�/C11 104.3(2) 107.0
O1�/C1�/C8 117.6(2) 116.5
O1�/C1�/C9 120.4(2) 122.3
C8�/C1�/C9 122.0(2) 121.2
O3�/C2�/O1 116.1(2) 107.5
O3�/C2�/C3 127.7(2) 131.7
O1�/C2�/C3 116.2(2) 120.8
C4�/C3�/C2 120.1(2) 119.0
C4�/C3�/C10 118.8(2) 120.3
C2�/C3�/C10 121.1(2) 120.7
C3�/C4�/C9 121.9(2) 119.8
C6�/C5�/C9 119.7(2) 119.9
C7�/C6�/C5 120.2(2) 120.5
C8�/C7�/C6 121.1(2) 120.8
C7�/C8�/C1 118.5(2) 118.6
C1�/C9�/C5 118.4(2) 118.9
C1�/C9�/C4 117.8(2) 119.2
C5�/C9�/C4 123.7(2) 121.9
N�/C10�/O2 115.3(2) 112.3
N�/C10�/C3 124.4(2) 127.0
O2�/C10�/C3 120.3(2) 120.7
C16�/C11�/C12 119.9(2) 120.8
C16�/C11�/N 131.2(2) 132.8
C12�/C11�/N 108.9(2) 106.4
C13�/C12�/O2 129.1(2) 128.3
C13�/C12�/C11 123.7(2) 122.8
O2�/C12�/C11 107.2(2) 108.9
C12�/C13�/C14 115.9(2) 115.7
C13�/C14�/C15 121.9(2) 122.1
C16�/C15�/C14 121.3(2) 122.1
C15�/C16�/C11 117.3(2) 116.6
Table 3
Dihedral angles (8), estimated by X-ray crystallography and
semi-empirical calculation, for 3-benzoxazol-2-yl-chromen-2-
one
Atoms Dihedral angle (8)
X-ray PM3
C4�/C3�/C10�/N 6.5(3) 0
C4�/C3�/C10�/O2 �/173.7(2) �/180
C2�/C3�/C10�/N �/173.5(2) �/180
C2�/C3�/C10�/O2 6.4(3) 0
A.E. da Hora Machado et al. / Spectrochimica Acta Part A 59 (2003) 345�/355348
sition [18]. The band at 352 nm corresponds to the
S00/S1 transition.
The absorption maxima corresponding to these
transitions, estimated by semi-empirical PM3-CI
calculations, agree very well with the experimental
values (Table 4, Fig. 4), with discrepancies be-
tween experimental and theoretical data lower
than 2%. The inclusion of solvation in the
calculation results in an improvement in the
agreement. A similar trend has been observed for
another coumarin analogue [8].
In all cases, the theoretical spectrum presents
spurious signals between 250 and 275 nm, which
can be attributed to the approximations and
parameterisations of the PM3 method.
Table 5 presents experimental data related to the
coumarin. The spectra show considerable Stokes
shifts. Although these shifts are large, they do not
vary significantly with increasing solvent polarity
indicating the limited solvatochromism for the
compound.
Fig. 3. Absorption and emission (lexc�/352 nm; lem�/440
nm). Spectra of 3-benzoxazol-2-yl-chromen-2-one in methanol.
The bands at 206 and 352 nm are attributed to p0/p�transitions.
Table 4
Absorption lmax for S00/S1 and S00/S2 transitions, predicted
(PM3) for isolated and solvated molecule, compared with the
corresponding experimental values
lmax (PM3), nm lmax(exp.), nm D%
Isolated 217; 389 �/ �/
Methanol 208; 359 206; 352 1.0; 2.0
Chloroform 230; 360 356 1.0
Fig. 4. UV/VIS spectra predicted by semi-empirical PM3-CI
calculation: (a) isolated molecule; (b) in methanol; (c) in
chloroform.
A.E. da Hora Machado et al. / Spectrochimica Acta Part A 59 (2003) 345�/355 349
3.3. Fluorescence
Fig. 3 shows a typical fluorescence spectrum. It
is approximately the mirror image of the absorp-
tion spectrum in all studied solvents. This beha-
viour can be seen as an indication that the
geometry of the S1 state is closely similar to that
of S0 state [19,20]. This is confirmed by the
theoretical predictions, which show no significant
changes in geometry for the S1 state, when
compared with the ground state, other than an
increase in structural rigidity, indicated by a
decrease in the bond length between the benzox-
azole and coumarin rings. The length of this bond
in the S0 state, estimated by semi-empirical calcu-
lations, is 1.454 A, whereas the value estimated
from X-ray diffraction is 1.461 A. The value in the
S1 state, calculated, using ab initio calculations, is
1.382 A.
Fig. 5 shows an approximately inverse correla-
tion, between FF and solvent polarity, represented
by the parameter orientation polarizability, Df .
This implies that internal conversion becomes
important as a deactivation route as solvent
polarity increases, showing that polar interactions,
most probably with the benzoxazole group, act as
a bridge for non-radiative deactivation. The results
based on the optimisation of the molecule when
considering solvent molecules show that polar
interactions between the solvent and the benzox-
azole group promote considerable changes in the
planarity of the molecule. For the two situations
studied (methanol/water and chloroform), the
predicted dihedral angle between the rings changes
from 180.08, for the isolated molecule, to respec-
tively 174.78 and 170.48. The mobility of these
rings due to polar interactions with the solvent
explains the increased contribution of internal
conversion as a deactivation pathway. Gao et al.
have suggested that torsion of a substituent at the
3-position has a significant effect on the decay of
the singlet state of ketocoumarin derivatives [3].
Fig. 6 shows the fluorescence spectrum for the
compound at 77 K. The spectrum shows fine
structure, which permitted a good estimate of the
S1 energy.
In contrast to coumarin derivatives possessing
an electron-donor group at position 7 of the
coumarin ring, the fluorescence quantum yield
does not show a temperature dependence for this
molecule [9,23]. The values of FF, estimated at 77
K (FF�/0.97) and 298 K (FF�/0.94) do not differ
significantly.
As can be seen from Table 5, the fluorescence
quantum yields are usually high. This occurs due
to the electron-withdrawing character of the
Table 5
Absorption and emission maxima, molar absorptivities, Stokes shift, fluorescence quantum yields, and other photophysical data for 3-
benzoxazol-2-yl-chromen-2-one
Solvent lmax, nm log o lem, nm DStokes, 1/cm Df Ff texp, ns kf, 1/s knr, 1/s kf/knr
Methyl cyclohexane 356 4.36 438 5259 0.000 0.94
Carbon tetrachloride 360 4.32 444 5255 0.012 0.92 3.17 2.90�/108 2.52�/107 11.51
Toluene 360 4.23 445 5306 0.013 0.83
1,4-dioxane 357 4.29 443 5438 0.022 0.84
Chloroform 356 4.29 443 5517 0.148 0.87
Ethyl acetate 352 4.17 443 5836 0.197 0.86
Tetrahydrofuran 354 4.31 444 5726 0.212 0.77
Dimethyl sulfoxide 354 4.27 453 6174 0.267 0.77
2-butanol 353 4.29 445 5857 0.264 0.80
Dimethyl formamide 352 4.29 445 5937 0.275 0.72
Ethylene glycol 354 4.34 447 5877 0.276 0.68 3.78 1.80�/108 8.47�/107 2.13
2-propanol 354 4.29 444 5726 0.273 0.76
Propanone 352 4.18 444 5887 0.285 0.76
Acetonitrile 352 4.30 443 5836 0.305 0.72
Methanol 352 4.33 440 5682 0.313 0.67 3.47 1.93�/108 9.51�/107 1.74
A.E. da Hora Machado et al. / Spectrochimica Acta Part A 59 (2003) 345�/355350
Fig. 5. Dependence between FF and Df : (a) methyl cyclohexane, (b) carbon tetrachloride, (c) toluene, (d) 1,4-dioxane, (e) chloroform,
(f) ethyl acetate, (g) tetrahydrofuran, (h) 2-butanol, (i) dimethyl sulfoxide, (j) 2-propanol, (k) dimethyl formamide, (l) ethylene glycol,
(m) propanone, (n) acetonitrile, (o) methanol.
Fig. 6. Fluorescence spectrum for 3-benzoxazol-2-yl-chromen-2-one in methyl cyclohexane, at 77 K.
A.E. da Hora Machado et al. / Spectrochimica Acta Part A 59 (2003) 345�/355 351
benzoxazole group at 3 position, of the chromen-
2-one ring [3,7,9]. Despite the polar interactions
with solvent, which reduces the importance of
fluorescence as a deactivation route, this group is
responsible for the increase of electron delocaliza-
tion over the molecule.
Fig. 7 shows a plot of the Stokes shift versus the
parameter orientation polarizability.
The Stokes shift tends to increase with solvent
polarity, indicating S1 is more polar than the
ground state, which is confirmed by the calculated
dipole moments for the S0 to S1 states. For the
isolated molecules, the dipole moments are, re-
spectively, 4.754 and 5.622 Debye, giving a value
of 0.868 Debye for Dm . The calculated Dm value,
based on the Lippert equation [19], considering the
Onsager radius as being 40% of the maximum size
of the molecule [21], is 5.28 Debye. The value of
the Onsager radius was estimated as being equal to
5.18 A. This value, certainly, must be overesti-
mated, considering the limitations of the Lippert
theory [19]. The value of Dm using the ET(N)
polarity scale, which is more suitable to describe
polar interactions [22], is 2.07 Debye [23]. This can
be an indication that the real value for the S1
dipole moment could be between 0.87 and 2.07
Debye higher than the one found for S0.
As can be seen from the kf/knr ratio, as solvent
polarity increases the non-radiative process be-
come more pronounced. For example, for carbon
tetrachloride, with Df�/0.012, where no significant
polar interaction between the S1 state and the
solvent is expected, this ratio is 11.51, whereas for
ethylene glycol, with Df�/0.276, kf/knr�/2.13.
However, the competition is less effective than
that observed for coumarin derivatives possessing
electron-donor groups in position 7 of the cou-
marin group [8,9]. In general, it can be concluded
that the deactivation of the S1 state is predomi-
nantly fluorescent. Although, some intersystem
crossing can occur, despite the absence of detect-
able phosphorescence at 77 K. A small quantum
efficiency of singlet oxygen generation (FD�/0.15)
was measured for this coumarin in chloroform.
This value is higher than the observed for other
coumarin derivatives [8,9]. This result may be
associated with the polar interactions, which
must change the characteristics of the excited
Fig. 7. Stokes shift versus orientation polarizability (Df ): (a) methyl cyclohexane; (b) carbon tetrachloride; (c) toluene; (d) 1,4-dioxane;
(e) chloroform; (f) ethyl acetate; (g) tetrahydrofuran; (h) 2-butanol; (i) dimethyl sulfoxide; (j) 2-propanol; (k) dimethyl formamide; (l)
ethylene glycol; (m) propanone; (n) acetonitrile; (o) methanol, Dn�/2015.55 Df�/5302.97 (R�/0.88).
A.E. da Hora Machado et al. / Spectrochimica Acta Part A 59 (2003) 345�/355352
electronic states, viabilizing the mixing of states.
This has been noted for PM3 CI calculations for
another coumarin derivative [8].
3.4. Electronic states
The prediction of the electronic states was done
for the isolated molecule and for the solvated
molecule in methanol/water 5:4, using ZINDO/S
and PM3. The results are shown in Table 6.
The result furnished by ZINDO/S for the S1
energy shows a better agreement with the experi-
mental data (302.6 kJ/mol in methyl cyclohexane)
measured at 77 K, being further improved when
solvation is considered. On the other hand, the
predictions made by ZINDO/S for the electronic
spectra are not compatible with the experimental
data. Both CI calculations were done based on the
PM3 optimised structure. As ZINDO/S is notparameterised for chlorine, the electronic structure
calculations were done only for methanolic solu-
tions for both methods.
The calculated electronic structure shows that
the T2 and S1 states must be very close. McCarthy
and Blanchard have proposed, in a study involving
molecular orbital calculations based on the AM1
Hamiltonian, of the electronic structure of several
coumarins, that the electronic states near S1 must
be considerably close [24]. A similar trend was
observed by Seixas de Melo et al. [7]. However,
this close proximity between electronic states does
not necessarily imply in efficient population of the
T1 state from S1. Considering the low rate of
singlet oxygen generation and the absence of
phosphorescence as a deactivation route of the
excited state, and taking in account the multi-
plicity of the first electronic states, as predicted by
semi-empirical CI calculations (Scheme 1), the
most probable energy sequence for the electronic
states is: T1(np�)B/T2(pp�)B/S1(pp�)B/S2(np�).
This sequence agrees with the selection rules for
intersystem crossing [20,26], and explains the low
probability of intersystem crossing and phosphor-
escence for this coumarin derivative and some of
its analogues [8,9]. The energy differences D(T2,
T1) and D(S2, S1) should make mixing of states
unlikely. This would favour intersystem crossing,
and is in contrast to the situation proposed for
some psoralen analogues that have been studied
[25].
Scheme 1. Jablonskii diagram of the first electronic states for 3-benzoxazol-2-yl-chromen-2-one, estimated by PM3 CI calculations.
A.E. da Hora Machado et al. / Spectrochimica Acta Part A 59 (2003) 345�/355 353
However, as can also be seen from Table 6, theinclusion of solvent interactions can reduce the
energy difference between these states, which can
favour state mixing, and so increase the probabil-
ities of intersystem crossing. This can explains the
experimental result obtained in chloroform.
Acknowledgements
CNPq, CAPES and FAPEMIG foundations are
acknowledged for research grants. To Dr Ana
Maria F. Oliveira-Campos, University of Minho,
Portugal, who kindly furnished the compound
under study. To LERT/IQ/UFRJ for the use of
the laboratorial infrastructure.
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Table 6
Energy of S1, T1 and T2 states, predicted by ZINDO/S and PM3: the percentage deviation from experimental data (D%) is presented
for S1 state
State ZINDO/S, kJ/mol PM3, kJ/mol
Isolated Solvated (methanol/water) Isolated Solvated (methanol/water)
T1 229.8 233.7 269.7 268.9
T2 319.4 320.9 334.7 328.8a
S1 322.8 (D%B/7) 323.7 (D%�/7) 350.8 (D%�/15.9) 359.5 (D%�/18.8)
S2 414.2 413.9 422.3 418.9
a A third triplet state was predicted: T3�/355 kJ/mol; Experimental S1 energy: 302.6 kJ/mol.
A.E. da Hora Machado et al. / Spectrochimica Acta Part A 59 (2003) 345�/355354
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A.E. da Hora Machado et al. / Spectrochimica Acta Part A 59 (2003) 345�/355 355
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