UV spectroscopic studies and charge transfer properties of azobenzene-containing cyclopentadienyliron complexes of arenes: A combined experimental and density functional theoretical study

Article abstract of DOI:10.1016/j.saa.2012.11.062

This study investigates the UV-Vis absorption and fluorescence emission of newly synthesized (η⁶-4-(4-nitrophenylazo) phenoxyl benzene) (η⁵-cyclopentadienyl) iron hexafluorophosphate (Fc-azo). Quantum chemical calculations of the orbital energy, geometrical structure, absorption spectra, and first hyperpolarizability (β) values of the Fc-azo were carried out using density functional (DFT/B3LYP and TD-DFT) methods. Comparisons between hydroxyl azobenzene compound and Fc-azo were made. Results showed that the observed spectra were in good agreement with the calculated values. The positive solvatochromism of the UV-Vis absorption of Fc-azo upon the increase in solvent polarity from the experiment and the calculated HOMO and LUMO energies showed that charge transfer occurred within the molecule. Moreover, the change in electron distribution led to the improved β for Fc-azo.

Full text of DOI:10.1016/j.saa.2012.11.062

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 287–291
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
UV spectroscopic studies and charge transfer properties of azobenzene-containing
cyclopentadienyliron complexes of arenes: A combined experimental and
density functional theoretical study
Guang Lei Li b, Ji Qiang Liu b, Bao Dong Zhao b, Tao Wang a,b,
⇑
a State Key Laboratory of Chemical Resource Engineering, College of Science, Beijing University of Chemical Technology, Beijing 100029, China
b Department of Organic Chemistry, College of Science, Beijing University of Chemical Technology, Beijing 100029, China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
Cyclopentadienyliron unit is a strong
electron-withdrawing group.
Fc-azo possesses positive
solvatochromic character.
Charge transfer occurs within the
molecule and leds to the improved b.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2012
Received in revised form 9 November 2012
Accepted 16 November 2012
Available online 5 December 2012
This study investigates the UV–Vis absorption and fluorescence emission of newly synthesized
(g g
6-4-(4-nitrophenylazo) phenoxyl benzene) ( 5-cyclopentadienyl) iron hexafluorophosphate (Fc-azo).
Quantum chemical calculations of the orbital energy, geometrical structure, absorption spectra, and first
hyperpolarizability (b) values of the Fc-azo were carried out using density functional (DFT/B3LYP and
TD-DFT) methods. Comparisons between hydroxyl azobenzene compound and Fc-azo were made. Results
showed that the observed spectra were in good agreement with the calculated values. The positive
solvatochromism of the UV–Vis absorption of Fc-azo upon the increase in solvent polarity from the
experiment and the calculated HOMO and LUMO energies showed that charge transfer occurred within
the molecule. Moreover, the change in electron distribution led to the improved b for Fc-azo.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Azobenzene
Cyclopentadienyliron complex of arene
Quantum chemical calculation
Charge transfer
Introduction
[4–7]. Ferrocenylazobenzene compounds have been reported.
Literature showed that these compounds had the reversible
Azobenzene compounds are versatile molecules that have re-
ceived a great deal of attention both in fundamental studies and
in applications [1–3]. Azo compounds containing a metallic atom
in their structure combine the properties of metal with those of
the azo group, achieving materials that can be processed with
interesting magnetic, electronic, optical, and anisotropic properties
trans–cis photoisomerization and electrical chemistry properties
[8–10].
When one cyclopentadienyl ring of ferrocene is substituted by
benzene ring, cationic
(g (g
6-arene) 5-cyclopentadienyl) iron
complexes ([arene-Fe-Cp]+) can be obtained. [Arene-Fe-Cp]+ has
some same properties as ferrocene, such as electrochemical and
photochemical activity [11–13]. However, there are still some dif-
ferent properties between ferrocene and [arene-Fe-Cp]+. For exam-
ple [arene-Fe-Cp]+ was found to be susceptible to attack by
nucleophiles, and the opposite ferrocene was easy to be attack by
⇑
Corresponding author. Address: Beijing University of Chemical Technology
144#, Beijing 100029, China. Tel.: +86 013 691179175; fax: +86 010 64445350.
E-mail address: wangtwj2000@163.com (T. Wang).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.11.062

288
G.L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 287–291
electrophiles. So, ferrocene is usually used as an electron donor;
[arene-Fe-Cp]+ is used as an electron acceptor [14–18]. Recently,
some new azobenzene-containing [arene-Fe-Cp]+ complexes were
synthesized in our laboratory and reported in the precious papers
[19,20]. These compounds are different from common [arene-Fe-
Cp]+ complexes. They showed photostability and were found to
be difficult to photolysis.
Fluorescence quantum yields
Fluorescence quantum yields were calculated using the follow-
ing equation: Ux ¼ ðAs ꢁ Fx ꢁ n2x ꢁ UsÞ=ðAs ꢁ Fs ꢁ ns2Þ, where A is the
absorbance at the excitation wavelength, F is the area under the
fluorescence curve, and n is the refraction index. Subscripts s and
x refer to the standard and the sample of the unknown quantum
yield, respectively. Fluorescence quantum yields were measured
In the current study, the UV–Vis absorption and fluorescence
emission of newly synthesized (
g
6-4-(4-nitrophenylazo) phenoxyl
in CH3CN using anthracene in ethanol (U = 0.27) as standard [21].
benzene) (
g
5-cyclopentadienyl) iron hexafluorophosphate (Fc-azo)
were investigated. Quantum chemical calculations of the orbital
energy, geometrical structure, absorption spectra, and first hyper-
polarizability (b) values of Fc-azo were carried out using density
functional (DFT/B3LYP and TD-DFT) methods.
Computational details
Quantum chemical calculations were carried out using the
Gaussian-09 software package. A Pentium IV personal computer
(CPU at 3.20 GHz), with Windows XP operating system, was used
to calculate quantum chemistry. The molecular structures of
H-azo and Fc-azo in the ground state were optimized based on
density function theory (DFT) at the Becke 3-Lee–Yang–Parr
(B3LYP)/Genecp (Fe with Lanl2dz basis set and C, H, N, and O with
6-31Gꢂꢂ basic set) and via visual inspection using the Gaussview
program (Version 5.0). The absorption spectra of Fc-azo were
calculated using the time-dependent density functional theory
(TD-DFT) method from the gas phase optimized geometries.
Experimental
Reagent and apparatus
All chemicals used were of analytical reagent grade. The melting
points of the compounds were determined using an XT-4 micro-
scopic melting point apparatus. The 1H NMR spectra were recorded
on a Bruker AV500 unity spectrometer operated at 500 MHz using
acetone-d6 as deuterated solvent. FTIR spectra were recorded on a
Nicolet 5700 instrument (Thermo Electron Corporation, Waltham,
MA). Mass spectrometry was performed with a Nermag R10-10C
spectrometer. UV–Vis absorption spectra were recorded on a Hit-
achi U2500 UV–Vis spectrophotometer (Hitachi High-Technologies
Corporation, Tokyo, Japan). Fluorescence spectra were obtained on
a Hitachi F-4500 spectrophotometer at room temperature.
DFT was used to calculate the dipole moment (
l), mean polar-
izability ( ), and total first static hyperpolarizability (b) of Fc-azo in
a
terms of x, y, z components, which are given by the following
equations:
1=2
l
a
¼ ðl2x
¼ ðaxx
þ
þ
ly2
þ
lz2
Þ
ð1Þ
ð2Þ
ð3Þ
ayy
þ
azzÞ=3
Fc-azo synthesis
1=2
b ¼ ðb2x þ b2y þ b2z Þ
The experimental procedure used in the current study was
based on a previous study [19,20]. The synthetic route of Fc-azo
is shown in Supplementary material. Fc-azo was synthesized from
X
bj ¼ bjjj
þ
ðbjii þ 2bijjÞ=3 ði–j; i; j ¼ x; y; zÞ
ð4Þ
(g (g
6-chlorobenzene 5-cyclopentadienyl) iron hexafluorophos-
phate (Fc-Cl) and 40-hydroxyl-4-nitroazobenzene (H-azo). SNAr
reactions of Fc-Cl with H-azo were carried out in the presence of
potassium carbonate as a base in DMF at 120 °C. Termination of
the SNAr reaction was monitored by observing the disappearance
of Fc-Cl via TLC. The reaction mixture was transferred into water
after reacting thoroughly, and a granular precipitate was formed.
The obtained precipitate was filtrated, and the obtained solid
was dried at room temperature. The rough product was finally
purified by column chromatography and further recrystallized
using acetone/ether (1:5).
The b components of Gaussian output are reported on atomic
units and therefore the calculated b components have been
converted into electrostatic units btot (esu) (1 a.u. = 8.6393 ꢁ
10ꢀ33 esu) [22].
Results and discussion
Molecular geometry of Fc-azo
The geometric structure of Fc-azo was optimized at DFT level
with the B3lyp/Genecp(6-31Gꢂꢂ/Lanl2dz) basis set, and is shown
in Fig. 1. The optimized structure parameters are listed in Table 1
based on the atom numbering scheme given in Fig. 1. The optimal
structure of Fc-azo revealed that the azo framework contains two
benzene rings and that the N@N linkage is nearly planar in the opti-
Mp: 232–234 °C. 1H NMR (500 MHz, acetone-d6): d = 8.50 (d,
2H, J = 8.70), 8.21 (d, 2H, J = 8.71), 8.19 (d, 2H, J = 8.60), 7.63 (d,
2H, J = 8.51), 6.60 (t, 4H, J = 7.04), 6.41 (t, 1H, J = 5.43), 5.35 (s,
5H). FT-IR spectra
m
(cmꢀ1): 3112.74 (CAH, aromatic), 1591.68,
1515.39, 1491.14, 1457.17(AC@CA), 1419.01 (AN@NA), 1139.85
(OAC), 822.67 (PAF). ESI–MS Calcd (found): m/z = 439.97 [cation+].
mized structures of the compounds, thus enabling the
p-electron
Fig. 1. Molecular structure and atom numbering scheme for Fc-azo.

G.L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 287–291
289
Table 1
Selected bond distance, bond angle and torsinal angle of optimized Fc-azo.
2.0
DMSO
DMF
CH2Cl2
Parameters
B3LYP/Genecp Fc- Experimental XRDa
azo
azobenzene
1.5
1.0
0.5
0.0
CH3CN
Bond lengths (Å)
N(33)AN(45)
C(35)AN(45)
C(2)AO(32)
1.284
1.425
1.365
1.257
1.431
Fc-azo
Bond angles (°)
C(35)AN(45)AN(33)
N(45)AN(33)AC(29)
C(22)AO(32)AC(2)
115.4
114.7
122.1
Dihedral angles (°)
C(23)AC(22)AO(32)AC(2) ꢀ127.7
C(24)AC(22)AO(32)AC(2) 57.9
a
X-ray data taken from Ref. [23].
300
350
400
450
500
550
Wavelength(nm)
Fig. 3. UV–Vis absorption spectra of Fc-azo in the four different polarity solvents
(5 ꢁ 10ꢀ5 M).
2.0
H-azo
Fc-azo
H-azo
Fc-azo
A
B
Calculation
Experimental
Table 3
1.5
1.0
0.5
0.0
Spectra properties of Fc-azo in four solvents with pꢂ values by Kamlet and Taft.
Solvent (pꢂ
)
DMSO
(1.00)
DMF
(0.88)
CH2Cl2
(0.82)
CH3CN
(0.75)
kmax (nm)
348.1
345.3
338.1
336.2
emax
31,200
27,600
24,800
28,600
(L molꢀ1 cmꢀ1
)
cloud delocalized over the AN@NA and aryl rings in the HOMO
orbital of Fc-azo, whereas the electron cloud delocalized over the
cyclopentadienyliron cation and coordinated with arene in their
LUMO orbitals. This result suggests that electron transfer occurred
from HOMO to LUMO in Fc-azo. However, the quantum chemical
calculation of H-azo showed that its electron clouds in the LUMO
and HOMO were delocalized on the whole molecule, and that no
obvious intramolecular electron transfer was observed.
200
300
400
500
600
200
300
400
500
Wavelength(nm)
Fig. 2. Comparison of the experimental UV–Vis spectra (A) with calculated UV–Vis
spectra (B).
The HOMO and LUMO energies for Fc-azo, based on the opti-
mized structure, were computed at ꢀ8.82 eV and ꢀ5.38 eV, respec-
tively. The calculated Egap for Fc-azo was 3.44 eV. The HOMO and
LUMO energies for H-azo, based on the optimized structure, were
computed at ꢀ6.97 eV and ꢀ3.62 eV, respectively, which were
higher than those of the Fc-azo. The energy gap (Egap) of H-azo
(3.17 eV) was a little smaller than that of Fc-azo (3.44 eV). These
results indicate that azobenzene derivatives containing cationic
cyclopentadienyliron moiety have lower chemical reactivity and
kinetic stability than their corresponding azobenzene compounds.
Table 2
Leading electronic transitions of H-azo and Fc-azo by TD-DFT method.
Molecule
Fc-azo
E (eV)
k (nm)
Main contribution
fos
2.8065
3.5839
441.7
345.9
0.0166
0.8695
HOMO ꢀ 1 ? LUMO
d–d
MLCT
p–
pꢂ
HOMO ꢀ 2 ? LUMO + 2
H-azo
3.118
397.6
0.8290
HOMO ? LUMO
conjugation to extend throughout the two aryl rings and N@N.
However, the benzene ring complexing with iron atom and the ben-
zene rings linking AN@NA were not on the same planar; they have
about 120° dihedral angles. Table 1 shows that the bond lengths
and valence angles for Fc-azo agree with the values in azobenzene
and their derivatives. The length of the N@N bonds in Fc-azo was
estimated as 1.284 Å, which is longer than that of diazene (HNANH)
(1.252 Å) [24] because of the conjugation with phenyl rings. The
longer N@N bond length and shorter N(azo)AC length (slightly
shorter than that of C(sp3)AC(sp2) [25]) indicate an increased con-
jugation of the phenyl through the azo link.
UV–Vis spectra
The absorption spectra of H-azo and Fc-azo in CH3CN were mea-
sured (Fig. 2A). Fig. 2A shows that the maximum absorption (kmax
)
in the visible light of Fc-azo had an obvious blue shift relative to
the corresponding H-azo, and the emax of Fc-azo in this area also
depressed relative to Fc-azo. The blue shift in kmax of Fc-azo means
that cationic cyclopentadienyliron moiety is a strong electron-
accepting group.
The electron absorption spectra of H-azo and Fc-azo were calcu-
lated at the TD-DFT/Genecp (Fe atom with Lanl2dz basic set and C,
H, O, and N atom with 6-31Gꢂꢂ basic set) to rationalize the
observed spectral properties (Fig. 2B). The first 50 spin-allowed
singlet–singlet excitations for Fc-azo were calculated using the
TD-DFT approach. TD-DFT calculations started from the optimized
geometry using the same level of theory and performed for gas
HOMO and LUMO analyses
The HOMO and LUMO energy level diagrams of Fc-azo were cal-
culated to check the transfer of electron density when this com-
pound was excited. The calculations showed that the electron

290
G.L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 287–291
Table 4
phase to calculate excitation energies. The wavelength (k), oscilla-
tor strength (fos), and major contributions of the calculated
transitions are given in Table 2. The calculated results indicate
MLCT transition for Fc-azo and agree reasonably well with the
experimentally measured spectral data mentioned in Table 2. For
example, the calculated gas phase transitions for Fc-azo appeared
at 345.9 nm, which is in good agreement with the experimental
data obtained from the UV–Vis result, viz. 336.2 nm in acetonitrile
solution. The calculated kmax of Fc-azo showed blue shift relative to
the kmax of H-azo, which is also in good agreement with the previ-
ous experimental result.
The UV–Vis bands in the electronic spectra of Fc-azo in four or-
ganic solvents with different polarity, including DMF, DMSO,
dichloromethane, and acetonitrile, were recorded to investigate
the solvatochromic behavior. The UV–Vis spectra are shown in
Fig. 3, and the data are listed in Table 3. The polarity of the four sol-
vents followed the order of DMSO > DMF > CH2Cl2 > CH3CN, as
determined by Kamlet et al. [26]. The results showed that Fc-azo
possesses positive solvatochromic (bathochromic shift) character
upon the increase in solvent polarity. This observed behavior sug-
gests that Fc-azo in the ground and excitation states indicates dif-
ferent polarities and has strong intramolecule charge transfer
ability.
Fluorescence properties of H-azo and Fc-azo.
kex (nm)
kem (nm)
Stoke’s shift (nm)
UF
H-azo
Fc-azo
278.0
269.1
326.1
415.7
48.1
146.6
0.039
0.103
Table 5
Total static dipole moment
hyperpolarizability b of H-azo and Fc-azo.
l, the mean polarizability a0 and the mean first
l
(a.u.) (a.u.)
a
b (a.u.)
btot ꢁ 10ꢀ30 esu
Fc-azo
H-azo
24.04
7.88
ꢀ142.42
1784.65
446.93
14.94
3.86
101.49
Fluorescence quantum yield (UF) was examined for H-azo and
Fc-azo to gain a better understanding of the deexcitation processes
involved. UF values were calculated relative to a standard solution
of anthracene in EtOH (U = 0.27). The UF values of H-azo and Fc-azo
are listed in Table 4, showing that both H-azo and Fc-azo have very
low UF values. The low UF values of the azo compounds indicate
that they can carry out photochemical reaction, such as photo Z-
E isomerization.
Fluorescence spectra
First hyperpolarizability
The fluorescence emission and excitation spectra of H-azo and
Fc-azo in the CH3CN solution are shown in Fig. 4 and the data
are listed in Table 4. The fluorescence emission of H-azo was ob-
served in the 250–450 nm region, with pronounced maxima at
326.1 nm. The fluorescence emission and excitation spectra of Fc-
azo were observed in the 350–500 nm region, with pronounced
maxima at 415.7 nm. The fluorescence intensity of Fc-azo obvi-
ously decreased compared with that of H-azo.
Stokes shift is a parameter that determines how strongly the
secondary inner filter effect affects the fluorescence output and
its spectrum and indicates differences in the properties and struc-
ture of fluorophores between the ground state and the first excited
state. Fc-azo showed larger Stokes shifts of about 150 nm, than H-
azo of about 55 nm. The large shift was assumed to result from a
substantial change in the dipole moment from the ground state
to the first excited state. The phenomenon indicates that a strong
ITC from the N@N and benzene rings linking AN@NA to cationic
cyclopentadienyl iron moiety exist when the Fc-azo molecule
was excited by light.
The b values of H-azo and Fc-azo calculated by the DFT-B3LYP
functional with the 6-31Gꢂꢂ basis set, are given in Table 5. A large
value of the first hyperpolarizability is a prerequisite to behave as a
good NLO material, and the important parameters influencing b
generally are nature of substituents, conjugated system, and the
influence of planarity. The stronger accepted properties of cationic
cyclopentadienyliron moiety should increase the b value of Fc-azo.
The results showed that the b value of Fc-azo is larger than that of
H-azo. The value of btot obtained by Sun et al. using the B3LYP/6-
31G(d) method for urea was 0.372 ꢁ 10ꢀ30 esu and is generally
used for the comparison [27]. The btot of Fc-azo was about forty
times larger than that of urea. Therefore, Fc-azo is a good candidate
for NLO materials.
Conclusions
In this study, an azobenzene-containing cyclopentadienyliron
complex of arene was investigated by experimental and theoretical
methods. The optimized molecular structure and energy levels of
frontier molecular orbitals were yielded by the density functional
theory. UV–Vis absorption spectra were analyzed via experimental
method and theoretical calculation (TD-DFT). The theoretical cal-
culation revealed that the cyclopentadienyliron unit is a strong
electron-withdrawing group and that the intramolecule charge
transfer occurred from the HOMO to the LUMO in Fc-azo. Intramo-
lecular electron transfer subsequently led to the improved NLO re-
sponse of Fc-azo.
5000
4000
3000
2000
1000
0
200
Fc-Azo
em
H-Azo
ex
em
ex
100
Acknowledgement
The authors wish to thank for financial support of National Nat-
ural Science Foundation of China (Project Grant No. 21176016).
0
200
300
400
500
200
300
400
Appendix A. Supplementary material
Wavelength(nm)
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.11.062.
Fig. 4. Fluorescence emission and excitation spectra of H-azo and Fc-azo in CH3CN
solution (5 ꢁ 10ꢀ5 M).

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