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: aeduardo@ufu.br (A.E. da Hora Machado).

Spectrochimica Acta Part A 59 (2003) 345�/355

www.elsevier.com/locate/saa

1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

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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