Comparison of FT Raman spectra of some 5-nitroquinoxalines and their electropolymers

Article abstract of DOI:10.1016/S0022-2860(01)00453-7

FT Raman spectroscopy was used to characterize a set of five newly synthesized 5-nitroquinoxaline derivatives 2,3-disubstituted with 2-pyrrolyl, 2-furyl, 2-thienyl, phenyl and 2-pyridyl groups. Afterwards the 5-nitroquinoxalines were electrochemically reduced to 5-aminoquinoxalines and electropolymerized as films on the Pt electrode coated with porous Au. Comparing the SERS spectra of polymers with the Raman spectra of monomers, a mechanism of polymerization has been suggested. Specific spectral and electrochemical properties of films based on pyrrole substituted 5-aminoquinoxaline were verified by electropolymerization of 2,3-dipyrrol-2′yl-5-nitroquinoxaline without a reduction step.

Full text of DOI:10.1016/S0022-2860(01)00453-7

Journal of Molecular Structure 565±566 (2001) 101±105
www.elsevier.nl/locate/molstruc
Comparison of FT Raman spectra of some 5-nitroquinoxalines
and their electropolymers
a
a,
a
a
a
b
J. Hrdlicka , P. Matejka , R. Volf , K. Volka , V. Kral , A.C. Try , J.L. Sessler
b
*
Ï
Ï
Â
a
Â
Department of Analytical Chemistry, Institute of Chemical Technology, Technicka 5, CZ-166 28 Prague 6, Czech Republic
bDepartment of Chemistry and Biochemistry, The University of Texas at Austin, Austin, TX 78712-1167, USA
Received 31 August 2000; revised 11 December 2000; accepted 11 December 2000
Abstract
FT Raman spectroscopy was used to characterize a set of ®ve newly synthesized 5-nitroquinoxaline derivatives 2,3-disub-
stituted with 2-pyrrolyl, 2-furyl, 2-thienyl, phenyl and 2-pyridyl groups. Afterwards the 5-nitroquinoxalines were electroche-
mically reduced to 5-aminoquinoxalines and electropolymerized as ®lms on the Pt electrode coated with porous Au. Comparing
the SERS spectra of polymers with the Raman spectra of monomers, a mechanism of polymerization has been suggested.
Speci®c spectral and electrochemical properties of ®lms based on pyrrole substituted 5-aminoquinoxaline were veri®ed by
electropolymerization of 2,3-dipyrrol-20-yl-5-nitroquinoxaline without a reduction step. q 2001 Elsevier Science B.V. All
rights reserved.
Keywords: FT-Raman spectroscopy; SERS; Electropolymerization; Surface ®lms
1. Introduction
Pt electrode coated with gold [3]) to form polymer
®lms. The surface ®lms were characterized by FT
SERS spectra prior to test potentiometric responses
to various anions [4].
A wide range of species with the ability to recog-
nize and sense anions was proposed in last few years
(e.g. sapphyrins, cyclodextrines) [1]. Black et al. [2]
have tested an ability of 2,3-dipyrrol-20-yl quinoxa-
line and its derivatives to coordinate F2, Cl2, H2PO42.
A set of ®ve 5-nitroquinoxaline derivatives 2,3-disub-
stituted with 2-pyrrolyl, 2-furyl, 2-thienyl, phenyl and
2-pyridyl groups (Fig. 1) has been newly synthesized
to test their capabilities of recognizing and sensing
anions. This study is focused on the FT Raman spec-
troscopic analysis of these compounds and their elec-
2. Experimental
2.1. Materials
The set of ®ve 2,3-disubstituted 5-nitroquinoxaline
derivatives was prepared according a procedure
described in Ref. [2]. All chemicals used were of
analytical grade.
trochemically 5-aminoquinoxaline
derivatives polymerized on a gilded Pt electrode (a
reduced
2.2. Instrumentation
FT Raman spectra were collected using a FT-
NIR spectrometer Equinox 55/S (Bruker) equipped
with Raman module FRA 106/S (Bruker). Samples
* Corresponding author. Tel.: 1420-2-2435-4281; fax: 1420-2-
311-2828.
Ï
E-mail address: pavel.matejka@vscht.cz (P. Matejka).
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(01)00453-7

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J. Hrdlicka et al. / Journal of Molecular Structure 565±566 (2001) 101±105
102
NO2
R1=R2=
N
N
N
H
S
O
N
R2
R1
A
B
C
D
E
Fig. 1. Structures of 5-nitroquinoxaline derivatives 2,3-disubstituted with: (A) 2-pyrrolyl; (B) 2-furyl; (C) 2-thienyl; (D) phenyl; and (E) 2-
pyridyl groups.
of 5-nitroquinoxaline derivatives and electropoly-
merized ®lms were irradiated by the focused beam
of Nd-YAG laser (1064 nm, Coherent) with power
80 and 15 mW, respectively. The scattered light
was collected in backscattering geometry. Accu-
mulated 256 and 512 interferograms were
processed to obtain Raman spectra with 2 and
4 cm21 resolutions of 5-nitroquinoxaline deriva-
tives and of polymerized ®lms, respectively. Two
spectra were measured for every compound; ®ve
spectra were collected in different parts of any
polymeric ®lm.
Table 1
Wavenumbers (cm21) of selected bands (characteristic bands of individual 2-,3-substituents are given in italics, bands assigned to stretching
vibrations of NO2 [5,8] are underlined) observed in the FT Raman spectra of 2,3-disubstituted 5-nitroquinoxalines in ranges 3200±2950, 1650±
1300 and 1300±800 cm21
Pyrrolyl
Thienyl
Furyl
Pyridyl
Phenyl
Average valuea
Average valuea of some 5-
substituted quinoxalines [7]
3141 w
3133 m
3119 m
3103 m
3154 m
3139 w
3115 w
3111 m
3085 m
3073 w
3062 w
3085 w
3070 sh
3064 m
3044 w
3028 w
1617 w
1598 s
3073 m
3062 m
3053 sh
3037 w
1615 w
1592 s
3064 m
3068 m
3064
1612
1615 w
1571 w
1549 m-s
1556 sh
1536 m
1524 m
1615 w
1596 w
1614
1568 s
1571 m-s
1554 m
1530 w
1581 w
1554 w
1543 m
1554 w
1537 m
1524 m
1541 sh
1535 m
1552
1536
1484 s
1498 m
1475 m
1442 s
1424 m
1428 s
1418 s
1393 vs
1348 m
1331 m
1243 w
1148 w
1065 w
862 m
1390 vs
1400 vs
1381 w
1338 m
1244 sh
1148 w
1069 w
854 w
1394 vs
1397 vs
1395
1383
1343 m
1249 m
1153 w
1072 w
852 w
1341 m
1245 m
1152 w
1065 w
871 w
1331 m
1245 m
1147 w
1067 w
858 w
1337
1245
1150
1068
859
1240
1153
1068
863
826 w
828 w
821 w
837 w
838 w
830
828
808 m
806 m
804 m
815 m
811 m
809
817
a
Average values are arithmetic means of band positions for all derivatives studied either in this study or in Ref. [7].

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J. Hrdlicka et al. / Journal of Molecular Structure 565±566 (2001) 101±105
103
2.3. Electropolymerization
analogous intensity and shape is at 1596 cm21. A
weak band observed at 1554 cm21 in all spectra
with the exception of the furyl derivative
(1541 cm21) could be a ring-stretching mode of the
quinoxaline skeleton [7]. The range 1500±1400 cm21
contains both characteristic bands of ring substituents
(Table 1) and some weak common features (e.g. band
in the range 1465±1454 cm21) attributed to the
quinoxaline skeleton [7]. The most intense common
band is located at ca. 1395 cm21 that is even higher
than the interval for previously studied 5-substituted
quinoxalines (1391±1375 cm21) [7]. Many common
bands are observed for all compounds close to the
average values published for a set of 5-substituted
quinoxalines [7] (Table 1). An exception is the band
at 817 cm21 (825±813 cm21) [7]. We observe a band
in ranges 815±811 and 808±804 cm21 for derivatives
with 6- and 5-membered rings, respectively. Other
well-resolved common bands occur at ca. 650 and
205 cm21. In addition, characteristic bands of NO2
group [5,8] are observed in all the spectra (Table 1).
In conclusion, the spectra exhibit both typical features
of 5-nitroquinoxaline skeleton and characteristic
bands of cyclic substituents con®rming the structures
proposed (Fig. 1).
A polarographic analyzer PA 2 (Laboratory
Devices Prague) with cyclic voltammetric adapter
was used for electropolymerization. Polymerization
was performed analogously to previously described
procedure [3] in a three-electrode cell by repeated
cyclic scanning the working electrode potential from
20.3 to 11.25 V at 50 mV/s. The Ag/AgCl saturated
electrode and Pt plate were used as a reference elec-
trode and an auxiliary one, respectively. The working
electrode was a gilded Pt plate (Pt plate diameter
7.0 mm, thickness 0.3 mm, coated with porous gold
layer Ð thickness of Au layer ca. 2 mm [3]). Fifteen
milligrams of individual 5-nitroquinoxaline species
were dissolved in 5 ml of glacial acetic acid and
then 0.25 ml of aqueous H2SO4 (1.0 mol l21) was
added to increase the conductivity. Firstly, 5-nitroqui-
noxalines were electrochemically reduced to 5-
aminoquinoxalines under the potential 21.5 V (vs.
Ag/AgCl) during 1 h with exception of a particular
experiment with 2,3-dipyrrolyl-5-nitroquinoxaline.
Working solutions were purged by nitrogen 10 min
before polymerization. The formed electrodes were
thoroughly washed with distilled water and stored in
distilled water.
3.2. Spectroscopic data of electropolymer ®lms
FT SERS spectra of electropolymerized ®lms were
compared with each other and with the data of mono-
mers. The only band in the range 3200±2950 cm21
reliably observed is at 3063 cm21 in the spectrum of
phenyl derivative corresponding to the most intense
band of the monomer in this range (3064 cm21). The
spectra of polymers in the range 1650±1300 cm21
exhibit quite broad bands and differ from the spectra
of monomers. Nevertheless, the substituent speci®c
bands can be distinguished with exception of the
data of pyrrolyl derivative. The 1598, 1497 cm21
bands of the phenyl derivative polymer correspond
to the 1598, 1498 cm21 of the initial species. Analo-
gously, the 1589, 1561, 1471, 1433 cm21 bands and
1592, 1571, 1475, 1442 cm21 bands are the character-
istic features of the pyridyl-substituted polymer and
monomer, respectively. The 1524, 1424, 1416 cm21
bands of the thienyl substituted polymer correspond to
the 1524, 1428, 1418 cm21 bands of the initial
species. The two speci®c bands of furyl group
3. Results and discussion
3.1. Spectroscopic data of monomer compounds
FT Raman spectra of 5-nitroquinoxalines were
mutually compared in the range 3200±2950 and
1650±175 cm21 (Table 1). The range 3200±
2950 cm21 is dominated by characteristic bands of
n(C±H) of individual substituents [5,6]. The assign-
ment of some band to n(C±H) of quinoxaline moiety
is dif®cult, because there is no band of analogous
intensity in all spectra. Nevertheless, a common
band of varying intensity is observed in the range
3070±3060 cm21. The speci®ed range is not typical
for 5-membered rings, i.e. the band can be attributed
to n(C±H) of quinoxaline moiety. All the substituted
quinoxalines previously studied [7] exhibit a band in
the range 1625±1600 cm21; a weak band at ca.
1615 cm21 is observed in all measured spectra with
exception of the furyl derivative where a band of

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J. Hrdlicka et al. / Journal of Molecular Structure 565±566 (2001) 101±105
104
R
R
N
N
N
N
R = unaffected substituents
n
N
N
R
R
A
Y
N
N
N
N
Y = NH2, NO2
n
N
N
N
N
Y
B
Fig. 2. Proposed structures of electropolymers of 5-aminoquinoxaline derivatives 2,3-disubstituted with (A) 2-furyl, 2-thienyl, phenyl and 2-
pyridyl groups and (B) of 2,3-dipyrrol-20-yl-quinoxaline derivatives.
(1568, 1484 cm21) are not shifted after electropoly-
merization. No such analogy is observed for the
pyrrolyl derivative. New bands at ca. 1562 and
1425 cm21 are observed in the spectra of all polymers
excepting the pyrrolyl derivative. The most intense
common band of monomers (1400±1390 cm21) is
located in the same range for phenyl, pyridyl and
thienyl derivatives. The analogous 1400 cm21 band
in the spectra of furyl monomer and the furyl speci®c
band at ca. 1380 cm21 are overlapped in the spectrum
polymer and monomer can be found below 1300 cm21
excepting the pyrrolyl derivative, e.g. 1031, 1001 cm21
for the phenyl derivative; 1045, 995 cm21 for the
pyridyl species; 1085, 1055 cm21 for the thienyl deri-
vative; and 1090, 1020 cm21 for the furyl species. No
such precise analogy is observed for the pyrrolyl deri-
vative. In conclusion, we suggest the electropolymeri-
zation does not affect phenyl, 2-pyridyl, 2-thienyl, 2-
furyl substituents. Nevertheless the polymerization
proceeds, that is indicated by observation of new
bands. We propose that the amino groups are primarily
involved in polymerization mechanism and that the
structure of 5-aminoquinoxaline skeleton is changed
via rearrangement of conjugated bonds. The proposed
structure of polymer chain (Fig. 2A) is analogous for all
derivatives except the 2-pyrrolyl derivative.
of polymer ®lm showing a broad band at 1384 cm21
.
The spectrum of pyrrolyl derivative polymer differs
completely both from spectra of other polymers and
from the spectrum of initial pyrrolyl substituted
compound. The most intense band is extremely
broad and shifted; two maxima at ca. 1366 and
1355 cm21 can be resolved. Other weak bands
(1494, 1560 cm21) have no counterparts in the spec-
trum of the monomer (2,3-dipyrrol-20-yl-5-nitroqui-
noxaline), but could be compared with the 1488 and
1577 cm21 bands of a polypyrrole ®lm [9].
The FT Raman data show that the whole skeleton of
2,3-dipyrrol-20-yl-5-aminoquinoxaline is involved in
electropolymerization process. Some analogy with
polypyrrole ®lms [9] can be found. The non-reduced
2,3-dipyrrole-20-yl-5-nitroquinoxaline was electro-
polymerized to con®rm that the pyrrolyl groups are
Some other corresponding bands of particular

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J. Hrdlicka et al. / Journal of Molecular Structure 565±566 (2001) 101±105
105
involved in polymerization. Many similarities of the
spectra of the two polymer ®lms show the analogous
way of polymerization. The above-mentioned bands
(1366, 1494, 1560 cm21) have their counterparts at
1371, 1495, 1561 cm21 for the non-reduced polymer.
Nevertheless, a new band observed at 1518 cm21 and
three bands at 888, 844, 806 cm21 similar to the initial
noxaline derivatives shows both evident common
features of 5-nitroquinoxaline skeleton and character-
istic bands of individual cyclic substituents. The
analogy of the type of electropolymerization for 2-
furyl, 2-thienyl, 2-pyridyl and phenyl derivatives is
demonstrated. These cyclic substituents are not
involved in the polymerization. The other type of
electropolymerization proceeds for the pyrrolyl deri-
vatives where the structure of whole molecule is
changed. This type of polymerization is practically
unaffected by the presence of nitro and/or amino
group on the quinoxaline skeleton suggesting the
primary role of pyrrolyl rings in the polymerization
mechanism.
5-nitroquinoxaline derivative (886, 852, 808 cm21
)
indicate the presence of nitro group in the structure
of the non-reduced polymer ®lm. To conclude, both 5-
amino- and 5-nitro- 2,3-(20-pyrrolyl)-quinoxalines are
primarily polymerized via pyrrolyl groups, but also
the system of conjugated bonds of quinoxaline
moieties is changed (Fig. 2B). The presence of
amino group is not important for polymerization of
2,3-dipyrrol-20-yl-5-aminoquinoxaline.
Acknowledgements
3.3. Cyclic voltammetry
The cyclic voltammograms are analogous for poly-
merization 0 of both reduced and non-reduced 2,3-
dipyrrole-2 -yl-5-nitroquinoxalines but differ signi®-
cantly from voltammograms of all other polymeriza-
Financial support from Czech Grant Agency (No.
203/97/P062 to P.M.) and from the Ministry of Educa-
tion of Czech republic (grant VS 97135 to V.K.) is
gratefully acknowledged.
tion
suggesting
different
mechanism
of
electropolymerization of 2,3-dipyrrol-20-yl-quinoxa-
line derivatives compared to 2-furyl, 2-thienyl, 2-
phenyl and 2-pyridyl substituted quinoxalines. Poly-
merization of both pyrrolyl derivatives is character-
ized by an evident peak at ca. 0.7 V (vs. Ag/AgCl
sat.), which is growing and shifting to higher potential
during reaction. The analogy of this peak indicates
that the way of electropolymerization does not depend
on the presence of amino group in the monomer. To
summarize, an interpretation of cyclic voltammo-
grams agrees with the main result given by Raman
spectroscopy of polymer ®lms, i.e. two different
types of electropolymerization (Fig. 2). A complete
voltammetric and potentiometric study will be
published later [4].
References
[1] F.P. Schmidtchen, in: A. Bianchi, K. Bowman-James, E.
Â
Garcõa-EspanÄa (Eds.), Supramolecular Chemistry of Anions,
Wiley-VCH, New York, 1997 (chap. 4).
[2] C.B. Black, B. Andrioletti, A.C. Try, C. Ruiperez, J.L. Sessler,
J. Am. Chem. Soc. 121 (1999) 10438.
Ï Â Â
[3] R. Volf, T.V. Shishkanova, P. Matejka, M. Hamplova, V. Kral,
Anal. Chim. Acta 381 (1999) 197.
Ï
Ï Â
[4] J. Hrdlicka, R. Volf, L. Placek, T.V. Shishkanova, V. Kral, P.
Ï
Matejka, A.C. Try, J.L. Sessler, in preparation.
[5] N.P.G. Roeges, A Guide to the Complete Interpretation of
Infrared Spectra of Organic Structure, Wiley, Chichester, UK,
1994.
Ï
[6] P. Matejka, unpublished data.
[7] G.W.E. Cheeseman, A.R. Katritzky, B.J. Ridgewell, J. Chem.
Soc. (1963) 3764.
[8] J. Renault, M. Baron, P. Mailliet, S. Giorgi-Renault, C. Paoletti,
S. Cros, Eur. J. Med. Chem. 16 (1981) 545.
[9] C.M. Jenden, R.G. Davidson, T.G. Turner, Polymer 34 (1993)
1649.
4. Conclusions
Comparison of FT Raman spectra of 5-nitroqui-