Charge-transfer complexations of 1,n-di(9-ethylcarbazol-3-yl)alkanes with tetracyanoethylene and tetranitromethane

Article abstract of DOI:10.1016/j.saa.2008.07.034

1,n-Di(9-ethylcarbazol-3-yl)alkanes, where n = 1-5, as the dichromophoric model compounds of poly-3-vinylcarbazoles were synthesized to examine their complexation behaviors with the electron acceptors tetracyanoethylene (TCNE) and tetranitromethane (TNM). 9,9′-Diethyl-3,3′-dicarbazolyl, di(3-ethylcarbazol-9-yl)methane, and three monomeric analogues were also included for comparison. In dichloromethane solution, the dicarbazoles formed stable 1:1 electron donor-acceptor complexes with TCNE having formation enthalpies around -3.5 kcal/mol. With TNM they formed more weakly bound complexes that showed little dependence on concentration and almost zero dependence on temperature changes having nearly 0 kcal/mol enthalpies of formation. The smaller gap between the two carbazole groups in 1,n-di(9-ethylcarbazol-3-yl)alkanes with n ≤ 2 affected complexation adversely, while such an effect was not observed in the dicarbazoles with n ≥ 3.

Full text of DOI:10.1016/j.saa.2008.07.034

Spectrochimica Acta Part A 71 (2009) 1973–1978
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Charge-transfer complexations of 1,n-di(9-ethylcarbazol-3-yl)
alkanes with tetracyanoethylene and tetranitromethane
Erol Askera,∗, John Masnovib
a Balıkesir University, Necatibey Faculty of Education, Department of Chemistry Education, 10100 Balıkesir, Turkey
b Cleveland State University, Department of Chemistry, Cleveland, OH 44115, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 August 2007
Accepted 23 July 2008
1,n-Di(9-ethylcarbazol-3-yl)alkanes, where n = 1–5, as the dichromophoric model compounds of poly-
3-vinylcarbazoles were synthesized to examine their complexation behaviors with the electron
acceptors tetracyanoethylene (TCNE) and tetranitromethane (TNM). 9,9ꢀ-Diethyl-3,3ꢀ-dicarbazolyl, di(3-
ethylcarbazol-9-yl)methane, and three monomeric analogues were also included for comparison. In
dichloromethane solution, the dicarbazoles formed stable 1:1 electron donor–acceptor complexes with
TCNE having formation enthalpies around −3.5 kcal/mol. With TNM they formed more weakly bound
complexes that showed little dependence on concentration and almost zero dependence on temperature
changes having nearly 0 kcal/mol enthalpies of formation. The smaller gap between the two carbazole
groups in 1,n-di(9-ethylcarbazol-3-yl)alkanes with n ≤ 2 affected complexation adversely, while such an
effect was not observed in the dicarbazoles with n ≥ 3.
Keywords:
1,n-Di(9-ethylcarbazol-3-yl)alkanes
Tetracyanoethylene
Tetranitromethane
CT complexes
Enthalpy and entropy of complexation
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
isomeric carbazole-containing photoconducting polymer that has
been applied to electrophotograpy [16,17].
As a tricyclic heteroaromatic compound with 14 ␲-electrons,
including the unshared electron pair of the nitrogen atom, car-
bazole has drawn much attention due to the photoconductivity
of its polymeric and lower weight systems. Poly-N-vinylcarbazole
(PVK) is among the most studied photoconducting polymers due to
its commercial application in electrophotography [1–4]. PVK itself
is not very efficient for electrophotographic purposes because of
its low sensitivity in the visible region of the light [5]. However,
this sensitivity is increased by the addition of a small amount of
certain dyes, such as triphenylmethane, xanthene, acridine, cya-
nine, merocyanine, or pyrylium dyes, or electron acceptors such
as tetracyanoethylene (TCNE), 2,4,7-trinitrofluorene-9-one (TNF),
and chloranils [6,7]. PVK comprised with TNF was the first organic
photoconductor commercially used for electrophotographic pur-
poses [8]. This photoconductive material is an EDA complex formed
between carbazole moieties and TNF molecules. It exhibits low
photosensitivity, toxicity due to TNF, and poor mechanical prop-
erties [9]. Therefore, other variations of EDA complexes of PVK
were investigated, as well as the dye sensitization of PVK [10–13].
Other carbazole-containing polymers have also been prepared
for the same purpose [14,15]. Poly-3-vinylcarbazole (P3VK) is an
In studies conducted to understand the nature of the
charge-transfer (CT) complex formation with certain electron
acceptors, researchers tend to prefer working on the model
compounds of polymers to avoid dealing with the complex
spectroscopic properties and other disadvantages of polymers,
such as purification, polydispersity, and difficulty in working
at microscale levels. For the same reasons, a series of 1,n-
di(9-carbazolyl)alkanes (DKA) as the model compounds of PVK
and 1,n-di(9-anthryl)alkanes (DAA) as the model compounds of
poly-9-vinylanthracene were synthesized, and their physical and
chemical properties were widely investigated [18,19]. We report
here the results of a study aimed at investigating thermodynamic
properties and stoichiometries of complexation of 1,n-di(9-
ethylcarbazol-3-yl)alkanes (D1–D5), 9,9ꢀ-diethyl-3,3ꢀ-dicarbazolyl
(D0), di(3-ethylcarbazol-9-yl)methane (D1-9), 9-ethylcarbazole
(M1), 9-ethyl-3-methylcarbazole (M2), and 3,9-diethylcarbazole
(M3) with TCNE and TNM in dichloromethane (Scheme 1).
2. Experimental
2.1. Instrumentation
All absorbance measurements were recorded on a Hewlett-
Packard HP8452A diode array spectrophotometer in 1 cm optical
Pyrex sample cells using dichloromethane as the solvent.
∗
Corresponding author. Tel.: +90 266 241 2762x245; fax: +90 266 249 50 05.
E-mail addresses: asker@balikesir.edu.tr, erolasker@yahoo.com (E. Asker).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.07.034

1974
E. Asker, J. Masnovi / Spectrochimica Acta Part A 71 (2009) 1973–1978
Scheme 1. Molecular structures of the compounds discussed in this paper.
2.2. Materials
of 3,6-disubstituted carbazoles was modified for the prepara-
tion of 1,4-Di(9-ethylcarbazol-3-yl)butane (D4). The procedure
involves cross coupling of 3-bromo-9-ethylcarbazole with the
di-Grignard reagent of 1,4-dibromobutane in the presence of
NiCl2(dppp) catalyst [dppp = l,3-bis(diphenylphosphino)propane].
1,5-Di(9-ethylcarbazol-3-yl)pentane (D5) was prepared via
the catalytic hydrogenation (Pd-C) of 1,5-bis(9-ethylcarbazol-
3-yl)-1,3-pentadiene, which was prepared according to
9-Ethylcarbazole (M1) (Aldrich) was purified by column chro-
matography (80–200 mesh alumina) eluting fractionally with
hexane/dichloromethane and by recrystallization from ethanol. 9-
Ethyl-3-methylcarbazole (M2) and 3,9-diethylcarbazole (M3) were
prepared via the reduction of 9-ethylcarbazole-3-carboxaldehyde
(Aldrich) and 9-ethyl-3-acetylcarbazole using AlCl3/LiAlH4 as the
reducing agent in anhydrous ether. 9,9ꢀ-Diethyl-3,3ꢀ-dicarbazolyl
(D0) was prepared according to the literature procedure [20]
via oxidation of 9-ethylcarbazole by FeCl3. Di(9-ethylcarbazol-3-
yl)methane (D1) was prepared via acid-catalyzed condensation
of 9-alkylcarbazoles with formaldehyde [21]. Di(3-ethylcarbazol-
9-yl)methane (D1-9) was prepared through the reaction between
dibromomethane and potassium salt of 3-ethylcarbazole accord-
ing to the literature [22]. 1,2-Di(9-ethylcarbazol-3-yl)ethane (D2)
was prepared via catalytic (Pd-C) hydrogenation of 1,2-di(9-
ethylcarbazol-3-yl)ethane, which was prepared by the reductive
coupling of 9-ethylcarbazole-3-caboxaldehyde according to the
literature procedure [23]. 1,3-Di(9-ethylcarbazol-3-yl)propane
(D3) was prepared via the reduction of 1,3-di(9-ethylcarbazol-3-
yl)propenone, which was prepared through a base catalyzed aldol
condensation of 9-ethyl-3-acetylcarbazole with 9-ethylcarbazole-
3-aldehyde [24]. Literature procedure [25] for the preparation
literature [26], through
1,3-bis(triphenylphosphonium)propane
a
bis-Wittig reaction between
dibromide and 9-
ethylcarbazol-3-carboxaldehyde. TCNE (Aldrich) was purified by
successive sublimation and crystallization from dichloromethane.
TNM prepared according to the literature procedure [27] and
purified by steam distillation followed by repeated extraction with
distilled water and freeze–thaw degassing to remove traces of
NO2. Dichloromethane (Quantum Chemical Co.) was purified by
distillation over P2O5 under argon atmosphere prior to using in
the spectrophotometric measurements.
2.3. Absorption measurements
The Benesi–Hildebrand technique [28] was used for the deter-
mination of the equilibrium constants and charge-transfer molar
extinction coefficients (ε) for the formation of EDA complexes of

E. Asker, J. Masnovi / Spectrochimica Acta Part A 71 (2009) 1973–1978
1975
the dicarbazolylalkanes with TCNE and TNM. For the carbazole-
TCNE CT measurements, in a 1.0 cm UV cuvette 1.0 mL of 0.06 M
TCNE:0.75 mM carbazole solution was placed. This was diluted 10
times by addition of increments of 0.1 mL and 10 times by addi-
tion of increments of 0.2 mL of the 0.75 mM carbazole solutions, to
make total of 20 dilutions. The TCNE–carbazole concentration ratios
were varied from about 80:1 to 20:1 through a series of 20 dilu-
tions with stock solutions of carbazoles. Thus, the concentration of
the electron donor (carbazole unit) was kept constant whereas the
concentration of the electron acceptor (TCNE) decreased through-
out the experiment. For the carbazole–TNM CT measurements, the
initial concentrations of 4.64 mM carbazole and 0.6 M TNM concen-
tration were used. The carbazole–TNM concentration ratios were
varied from about 129:1 to 30:1 by diluting initial solution with
5.0 mM carbazole solutions. Absorbance measurements of the sam-
ples containing TNM had to be collected in dim red light due to the
rapid photochemical reactions with carbazoles. D0 readily reacts
with TNM to give nitration products, even in dark. Hence, it was
excluded from the complexation studies with TNM.
Fig. 1. Benesi–Hildebrand plots of carbazole–TCNE EDA complexes.
Absorbance changes were monitored after each dilution by mea-
suring the optical density (O.D.) at the wavelength of the interest.
Average of three runs was taken to minimize the experimental
errors. The wavelengths of the maximum absorbance were deter-
mined from the UV–vis spectra of the complexation.
Allcarbazoles formedblue EDAcomplexeswithTCNEandbrown
complexes with TNM in dichloromethane according to following
hypothetical equation.
K
D + AꢀD, A
The equilibrium constant K for the above reaction can be written
as
[D, A]
K =
(1)
([A]0 − [D, A])([D]0 − [D, A])
The equilibrium constants (K) for EDA complex formation
are related to the maximum absorption (A) of the complex,
the molar extinction coefficient (ε), and the initial concentra-
tions of donor ([D]0) and acceptor ([A]0). Replacing [D,A] with
(A/ε) from Beer–Lambert law and ignoring [D,A] concentra-
tion in ([A]0 − [D,A]) term since [A]0 ꢁ [D]0, Eq. (1) yields the
Benesi–Hildebrand equation.
Fig. 2. Benesi–Hildebrand plots of carbazole–TNM EDA complexes.
1 cm path length and equilibrated in water or ice-water baths of
the desired temperatures for 15–20 min. The UV cell was dried
off with paper wipes prior to every scan and average of three
scans were taken for the calculations. The volume changes were
measured for these temperatures and concentration changes were
taken into account in calculating the thermodynamic constants.
ꢀ
ꢁ
[D]0
A
1
[A]0
= (Kε)−1
+ (ε)−1
(2)
Ploting [D]0/A vs. (1/[A]0), (Kε)−1 as the slope and (ε)−1
as the intercept were obtained. Benesi–Hildebrand plots of
carbazole–TCNE and carbazole–TNM EDA complexes are given in
Figs. 1 and 2, respectively.
The stoichiometries of the complexation of carbazoles with
TCNE were determined using Job plots [29] (method of continuous
variation). In 5.0 mL volumetric flasks, 10 mM of carbazole donor
and 10 mM of TCNE in dichloromethane were prepared separately.
These solutions were mixed in a 1.0 mL volumetric flask in which
the mole fraction of the components differed from 0.1 to 0.9. The
average absorptions of four different scans of the EDA complexes
on each dilution were recorded at the maximum CT wavelength.
Job plots of the carbazole–TCNE complexes are given in Fig. 3.
Temperature dependencies of complexations were determined
using van’t Hoff equation and Beer–Lambert law by measuring
absorption spectra of the complexes at six different temperatures,
0 ◦C, 7 ◦C, 14 ◦C, 21 ◦C, 28 ◦C, and 35 ◦C ( 1 ◦C), at ꢀCT. In a 10 mL
volumetric flask, a solution containing 5.0 mM TCNE and 2.5 mM
carbazole unit (and 0.4 M TNM and 5 mM carbazole unit) at 21.0 ◦C
was prepared. Then the solution was placed into UV a cell with
Fig. 3. Job plots of carbazole–TCNE EDA complexes.

1976
E. Asker, J. Masnovi / Spectrochimica Acta Part A 71 (2009) 1973–1978
Fig. 4. Van’t Hoff plots of carbazole–TCNE EDA complexes (2.5 mM donor-group,
5 mM TCNE).
The integrated form of the van’t Hoff equation is
ꢂ
ꢃ
ꢀ
ꢁ
ꢀ
ꢁ
ꢁH
R
ꢁS
R
A/ε
−
T−1
+
= ln
(3)
Scheme 2. Representation of CT process between HOMO’s of an electron donor and
LUMO of an electron acceptor molecule.
([D]0 − A/ε)([A]0 − A/ε)
The value of the molar extinction coefficient, ε, was calculated as
1260 M−1 cm−1 for carbazole–TCNE complexes and 380 M−1 cm−1
for carbazole–TNM complexes from the Benesi–Hildebrand plots.
A plot of ln K vs. 1/T in Eq. (3) would yield –ꢁH/R as the slope and
ꢁS/R as the intercept. The van’t Hoff plots of carbazoles with TCNE
and TNM are given in Figs. 4 and 5, respectively.
nents are, to a large extent, preserved. When the ground state of
an EDA complex absorbs light that matches the excitation energy
of the complex (hvCT), the excited state is formed. In this excited
state, an electron is transferred from one of the higher occupied
molecular orbitals of the excited donor (D*) to the lowest unoccu-
pied molecular orbital (LUMO) of the acceptor (A) to form a pair of
ions (D•+, A•−).
3. Results and discussion
The Benesi–Hildebrand technique was used to determine the
molar extinction coefficient, ε, and equilibrium constant, K, for the
weak donor–acceptor complexes.
Formation of a CT complex results from the association of an
electron donor with an electron acceptor. The ionization potential
IP of a donor and the electron affinity Ea of an acceptor are the major
factors in determining the strength of the complexation. Carbazoles
are good electron donors having low IP’s around 7.6–8.0 eV [30,31]
and TCNE is a good electron acceptor with Ea = 3.17 eV [32]. TNM
forms more weakly bound complexes with carbazoles compared to
TCNE. The tetrahedral structure of TNM, a ␴-acceptor, may prevent
a stronger association with planar donor compounds when com-
pared with planar TCNE molecule whose four –CN groups are in
conjugation with each other. A CT absorption occurs when one elec-
tron from the HOMO (or the lower energy orbitals) of the donor is
promoted to the LUMO of the acceptor (Scheme 2). EDA complexes
absorb light in a manner different than either donor or acceptor
molecules, although the physical properties of the original compo-
The average molar extinction coefficient of carbazole–TCNE
complexes was determined to be 1260 M−1 cm−1, where that
of carbazole–TNM complexes found to be 380 M−1 cm−1. These
results are consistent with the results of the earlier studies [19].
The values of equilibrium constant, K, ranged between 3.34 and
8.98 M−1 for the carbazole–TCNE complexes and between 0.199
and 0.431 M−1 for the carbazole–TNM complexes (Table 1). In dicar-
bazolylalkanes, D1-9 has the smallest K values for its complexations
with both TCNE and TNM. D0 (with TCNE) and D1 have the smallest
K values among the dicarbazoles that are bridged through carbazole
benzene rings. This result is attributable to the higher electron den-
sity at nitrogens, which are not allowed to associate freely with the
donor molecules in D1-9 due to the proximity of the carbazole rings.
Comparably, in D0, where the two carbazole groups are directly
attached to each other at the para position to the nitrogen atom, and
D1, where the two carbazole groups are separated with a methylene
group, effect of proximity is far less than it is in D1-9. The presence
of electron donating ethyl groups on the carbazole rings of D1-9 had
little effect on the complexation compared to its unsubstituted ana-
logue (K = 3.13 M−1; Ref. [19]). The K values of D0 and D1 found to
be higher than D1-9 but expectedly lower than M2, M3, and D2–D5.
Nearness of carbazole groups in D0 and D1 prevented complexation
of each carbazole group independently. The equilibrium constants
for M2, M3, and D3–D5 were similar. This indicates that for the
dicarbazolyl alkanes where n ≥ 3 there is little effect of the car-
bazole rings on each other to prevent formation of complexes with
TCNE. It can be concluded that the length of methylene chain has
a positive effect on the complexation for the dicarbazoles D1–D3,
however, this effect was diminished in the dicarbazoles with n ≥ 3.
The temperature dependences of complexations were deter-
mined by calculating the enthalpy and entropy from Eq. (3). The
Fig. 5. Van’t Hoff plots of carbazole–TNM EDA complexes (5 mM donor group, 0.4 M
TNM).

E. Asker, J. Masnovi / Spectrochimica Acta Part A 71 (2009) 1973–1978
1977
Table 1
Thermodynamic properties of EDA complexes of selected carbazoles with TNM in CH2Cl2
b
c
TCNE
ꢀa
ꢀCT
KεCT (M−2 cm−1
)
Kd (M−1
)
ꢁH (kcal/mol)
ꢁS (eu)
M1
M2
M3
D0
D1-9
D1
D2
D3
D4
D5
347
352
352
347
341
352
352
352
352
352
596
610
614
594
598
613
611
612
613
611
7,235 69
12,489 51
11,200 74
8,079 77
4,209 30
8,377 61
8,926 78
10,829 70
10,376 80
11,318 81
5.74
9.91
8.89
6.41
3.34
6.65
7.09
8.59
8.24
8.98
−3.12 0.03
−3.27 0.13
−3.48 0.07
−3.35 0.05
−2.85 0.07
−3.33 0.07
−3.54 0.07
−3.70 0.07
−3.76 0.13
−3.77 0.04
−7.08 0.11
−6.76 0.46
−7.56 0.23
−6.31 0.19
−6.31 0.23
−6.06 0.23
−6.56 0.25
−6.82 0.24
−6.99 0.45
−6.92 0.15
b
c
TNM
ꢀa
ꢀCT
KεCT (M−2 cm−1
)
Ke (M−1
)
ꢁH (kcal/mol)
ꢁS (eu)
M1
M2
M3
D1-9
D1
D2
D3
D4
D5
347
352
352
341
352
352
352
352
352
534
547
550
536
549
547
548
549
548
89.3 0.22
105.7 0.64
79.6 0.34
95.8 0.42
140.1 0.26
163.8 0.36
154.3 0.34
144.4 0.37
143.9 0.32
0.234
0.275
0.199
0.251
0.187
0.215
0.203
0.190
0.189
−0.19 0.06
−0.27 0.07
−0.06 0.02
−0.04 0.03
−0.09 0.03
0.13 0.07
−0.23 0.04
0.00 0.03
0.02 0.04
−3.36 0.20
−3.35 0.25
−3.14 0.08
−2.67 0.11
−1.76 0.10
−0.99 0.23
−2.08 0.12
−1.35 0.11
−1.35 0.14
a
Lowest energy absorption maximum of uncomplexed carbazole (nm).
Lowest energy CT maximum (nm).
Acceptor in excess.
b
c
ε = 1260 M−1 cm−1 at 21 1 ◦C.
d
e
ε = 380 M−1 cm−1 at 21 1 ◦C.
formation enthalpies around −3.5 kcal/mol shows that carbazoles
form weakly associated complexes with TCNE. The effects of the
alkyl substituents on the carbazole rings and the length of the
alkylene separator can be judged from the data (Table 1). It was
found in the studies of Haderski et al. [19] that 1,n-dicarbazol-
N-ylalkanes form weak complexes with TCNE having formation
enthalpies around −3 kcal/mol. The slightly more favorable ꢁH
values in this study may be due to the presence of an extra elec-
tron donating group (–ethyl) on each carbazole ring in addition to
the alkylene chain. The effect of alkyl substituents on the forma-
tion enthalpies can be seen more clearly when the ꢁH values of
monocarbazoles compared where M2 and M3 have more negative
enthalpies compared to M1. The formation enthalpies of D0, D1,
and D1-9 were the least favorable among the dichromophores.
The near-zero enthalpies of formation for the compounds stud-
ied indicate that there is almost no temperature effect on the
complexations of carbazoles with TNM. There was no noticeable
effect attributable to the substituents or to the number of –CH2–
groups (n) in the spacer connecting the two carbazolyl groups.
The effect of the alkyl substituent on the formation entropies
of the TCNE complexes depends on the size of the alkyl group.
The methyl group in M2 (−6.76 0.46 eu) has no noticeable
effect whereas the ethyl group in M3 (−7.56 0.23 eu) signifi-
cantly changed the entropy, unfavorably, compared to that of M1
(−7.08 0.11 eu). The chain length in dicarbazoles appears to have a
negative effect on the formation entropies. The complex formation
for D0, D1, and D1-9 is entropically more favorable compared to
D2–D5. However, this effect disappeared among D3–D5, in which
the carbazolyl groups are separated with longer alkylene chains.
Entropies of formation for the monocarbazole–TNM complexes
were found to be more negative (i.e. more unfavorable) than those
for the dicarbazolylalkanes. For the TNM–dicarbazolylalkene com-
plexes the ꢁS values of D1–D5 are considerably larger than those
of M1–M3 and D1-9. It appears that the two carbazolyl groups
complex essentially independently for D1–D5, but not for D1-9.
Independent behavior of the two carbazolyl groups would increase
the entropy of complexation in D1–D5 by R ln 2 (or about 1.4 eu)
compared to the monocarbazole analogues. As seen in Table 1, the
experimental values differ by close to this amount. The difference in
the complex formation behavior of D1-9 and D1 can be ascribed to
the more open geometry of D1, which places the carbazolyl groups
farther apart so that acceptors experience less steric interactions
with the second carbazolyl group.
4. Conclusion
1,n-Di(9-ethylcarbazol-3-yl)alkanes and selected monocar-
bazoles form weak EDA complexes with TCNE and even weaker
EDA complexes with TNM as indicated by the thermodynamic con-
stants ꢁH and ꢁS. The K values of D0–D2 were found to increase
in the same order as the length of their methylene chain, while
the K values for D3–D5 were about the same. Relatively larger K
values for the complexation of M2 and M3 with TNM and TCNE
compared to M1 indicate that electron-donating alkyl substituents
on the benzylic rings of carbazoles also favor the complexation.
All the compounds form 1:1 carbazole group–TCNE complexes at
the concentrations studied. The electron-donating methyl and ethyl
substituents in M2 and M3 make the enthalpies more negative (i.e.
more favorable for the complex formation) compared to M1. In the
dicarbazoles, the complex formation is favored for the compounds
with n ≥ 2. The entropies of formation for the carbazole–TCNE com-
plexes also become more negative (less favorable) in M2 and M3
and D2–D5. Similar trends were observed in carbazole–TNM com-
plexes except that their enthalpies of formation were found to be
between −0.27 and +0.13 kcal/mol. The entropies of formation for
complexation with TNM are found to be less negative (less unfavor-
able) than those for TCNE, consistent with their “contact” in nature
behavior.
Acknowledgements
The authors are thankful to CSU College of Graduate Studies and
the Turkish Ministry of Education for partial support of this work.

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E. Asker, J. Masnovi / Spectrochimica Acta Part A 71 (2009) 1973–1978
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