Ferrocene-quinoxaline Y-shaped chromophores as fascinating second-order NLO building blocks for long lasting highly active SHG polymeric films

Article abstract of DOI:10.1039/c6dt01590e

The first example of a Y-shaped ferrocene quinoxaline derivative with a surprisingly high and stable second harmonic generation (SHG) response in composite polymeric films is reported. The interesting quadratic hyperpolarizability values of different substituted Y-shaped chromophores are also investigated in solution by the EFISH technique.

Full text of DOI:10.1039/c6dt01590e

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Ferrocene–quinoxaline Y-shaped chromophores
as fascinating second-order NLO building blocks
for long lasting highly active SHG polymeric films†
Cite this: Dalton Trans., 2016, 45,
11939
Received 24th April 2016,
Accepted 30th June 2016
a
a
b,c
Kabali Senthilkumar, Krishnan Thirumoorthy, Claudia Dragonetti,
b
b
b
d
DOI: 10.1039/c6dt01590e
Daniele Marinotto, Stefania Righetto, Alessia Colombo,* Matti Haukka and
a
www.rsc.org/dalton
Nallasamy Palanisami*
The first example of a Y-shaped ferrocene quinoxaline derivative
with a surprisingly high and stable second harmonic generation phore may act as auxiliary D or A and further improve the
SHG) response in composite polymeric films is reported. The optical nonlinearity of the chromophore. Quinoxalines/
Heteroaromatic moieties incorporated into an NLO chromo-
(
interesting quadratic hyperpolarizability values of different substi- pyrazines have been widely used in chromophore systems as a
tuted Y-shaped chromophores are also investigated in solution by strong electron acceptor because of their high electron
the EFISH technique.
deficiency originating from the two symmetric unsaturated
nitrogen atoms that lower the π* level of the conjugated
system. Limited information on the Y-shaped chromophores
Materials with second-order nonlinear optical (NLO) properties
are of great interest since they can be used for various impor-
tant applications such as optical communication, optical data
10
based on a quinoxaline moiety for NLO and dye-sensitized
11
solar cells (DSSCs) is available in the literature. In particular
1
processing and storage, and electro-optical devices.
Bureš et al., studied the structure–property relationship and
NLO properties for different substituted pyrazine push–pull
2
3
4
Organic chromophores with particular shapes (H, V, Y,
5
6
7
X,
T
and star type) are interesting for electro-optic appli-
5b,6,11b
chromophores with various π-linkers.
cations and their arrangement ensures efficient intramolecular
charge transfer (ICT) between the donor and acceptor moieties
and generates the push–pull system featuring low-energy and
In the field of NLO, ferrocene derivatives have been deeply
investigated and have played the role of an electron-donor in
charge transfer processes in chromophores where ferrocene is
8
intense CT absorption. Due to the ICT, the push–pull donor–
9b,12
linked to an acceptor moiety.
π-acceptor (D–π-A) molecules possess distinct nonlinear optical
Although ferrocene–quinoxaline based Y-shaped chromo-
phores, recently reported by Kumar et al., show interesting
(NLO) properties.
Coordination compounds are a growing class of second-
13
DSSC performance, their NLO properties have never been
order NLO chromophores that can offer additional flexibility,
when compared to organic compounds, due to the presence of
metal–ligand charge-transfer between the metal and the
ligands usually of high intensity and at relatively low energy,
tunable by nature, oxidation state and coordination sphere of
investigated. This type of organometallic based Y-shaped
chromophore structure allows good coupling between the d
orbitals of the metal and π* system of the quinoxaline moiety
that could afford a significant NLO response controlled by low-
energy metal ligand charge transfer (MLCT) transitions.
9
the metal center.
Keeping this in mind, we have synthesized Y-shaped
chromophores by condensation of a 1,6-bis-ferrocenyl-hexa-
1
,5-diene-3,4-dione with a substituted 1,2-diamino compound;
a
Department of Chemistry, School of Advanced Sciences, VIT University, Vellore
this is one of the most versatile methods and was used for the
construction of quinoxaline derivatives, YQ1, YQ2, YQ5, and
YQ6; for YQ3 and YQ4, a different synthetic strategy was used
as shown in Scheme S1.†
6
b
32014, Tamilnadu, India. E-mail: palanisami.n@gmail.com; Tel: +91 98426 39776
Department of Chemistry and Centro CIMAINA, University of Milan,
INSTM-Research Unit, Via C. Golgi 19, 20133 Milan, Italy.
E-mail: alessia.colombo@unimi.it
c
ISTM-CNR via Golgi 19 Milan, Italy
The Y-shaped substituted quinoxaline derivatives YQ1–YQ6
d
1
Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä,
were characterized by using FT-IR, H-NMR, and mass and
Finland
single crystal X-ray diffraction techniques (ESI†). The single
crystal X-ray structure of YQ1 and YQ2 (Fig. 1) shows a
Y-shaped structure and the ferrocene moieties are facing the
antennae type. The structural parameters obtained from X-ray
†
Electronic supplementary information (ESI) available: X-ray crystallographic
data, theoretical details, cartesian coordinates, and all experimental details.
CCDC 1440374 and 1440375. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c6dt01590e
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UV region which can safely be ascribed to a high energy
ligand-centred π–π* electronic transition (ICT transition). The
other bands in the visible region can be assigned to other loca-
lized excitations with a lower energy produced either by two
nearly degenerate transitions, a Fe(II) d–d transition (assigned
1
1
14
to E1g ← A1g) or by an MLCT process (dπ–π*). By increasing
the strength of R as an electron withdrawing group on the
quinoxaline scaffold, a bathochromic shift is observed. The
detailed absorption bands and their energy gap are given in
Table 1.
To gain insight into the structural and electronic properties
of chromophores YQ1–YQ6, we performed DFT and time-
dependent DFT (TD-DFT) calculations using Gaussian 09 soft-
1
5
ware. The main optimized geometrical parameters and calcu-
Fig. 1 Chemical structures of Y-shaped ferrocene-based chromo- lated highest occupied molecular orbital (HOMO) and lowest
phores YQ1–YQ6 and single crystal X-ray structures of chromophores
YQ1 and YQ2 (30% probability ellipsoids). Hydrogen atoms are omitted
for clarity.
unoccupied molecular orbital (LUMO) energies, absorption
maxima (λmax), are reported in Table 1 (ESI†). The lowest
energy electronic transitions for all the quinoxaline chromo-
phores originate from the HOMO to LUMO. The HOMO–
LUMO energy gap decreases by increasing the strength of the
analysis closely matches with the optimized structures
acceptor, and the experimental results confirm this trend as
obtained from density functional theory (DFT) studies at
evidenced by a red shift of the MLCT band. The density plots
B3LYP/6-31+G** level of theory (ESI†) except that there is a
of HOMO and LUMO levels and their energy gaps are influ-
slight difference in the values of the dihedral angles with
enced by the electron donor, ferrocene, π-spacer and acceptor
3
respect to the substituents (H and CF ). This may be due to
quinoxaline derivative strength. The HOMO is mainly localized
on the ferrocene moiety and π-spacer double bond, whereas
the electron density in the LUMO is mainly localized on the
substituted quinoxaline moiety (Fig. 3). The HOMO energies of
chromophores YQ1–YQ6 are quite similar (−5.6 to −5.3 eV)
while their LUMO energies are influenced by virtue of the
nature of R substituents. The biggest destabilizations are
observed when R is an electron-donor substituent (YQ5 and
YQ6) reaching values ranging from −2.80 to 2.24 eV. Therefore,
the increased HOMO–LUMO gap calculated for all the chromo-
phores is essentially related to LUMO destabilization. The cal-
culated absorption maxima of the chromophores originate by
single HOMO–LUMO transitions (at 3.12 to 2.64 eV/397 to
the solid state packing and the associated intermolecular
hydrogen bonding (ESI†).
The absorption spectra in CH Cl of chromophores YQ1–
2 2
YQ6 show variably intense CT absorption bands as reported in
Fig. 2. All molecules depict prominent absorption bands in the
469 nm) based on the DFT calculations. The theoretically
observed trend is comparable with the experimental values.
The absolute values vary by the range of 100 nm with experi-
mental values which are due to the limitation in the basis set
used for calculations. The calculated data and main transitions
2 2
Fig. 2 Absorption spectra of YQ1–YQ6 in CH Cl at 298 K.
Table 1 Photophysical data and second order NLO properties of the investigated chromophores YQ1–YQ6
HE
a
3
LE
a
3
b
b
b
μb,d
(×10
e
λ
max [nm (eV)]/ε (×10 )
λ
max [nm (eV)]/ε (×10 )
HOMO
(eV)
LUMO
(eV)
λ
max
µβ
−
1
−1
−1
−1
−18
−48
Sample
[M cm
]
[M cm
]
[nm (eV)]
esu)
(×10
esu)
YQ1
YQ2
YQ3
YQ4
YQ5
YQ6
349 (3.55)/17.5
356 (3.48)/22.1
389 (3.18)/18.7
379 (3.27)/12.8
350 (3.54)/20.1
352 (3.52)/19.8
498 (2.48)/0.55
522 (2.37)/0.68
558 (2.22)/0.57
536 (2.31)/0.39
501 (2.47)/0.61c
499 (2.48)/0.64
−5.40
−5.37
−5.45
−5.63
−5.37
−5.35
−2.29
−2.37
−2.80
−2.74
−2.25
−2.24
397 (3.12)
413 (2.99)
469 (2.64)
430 (2.88)
397 (3.11)
398 (3.11)
3.4
5.9
6.3
5.5
1.1
1.3
−790
−960
−820
−560
−580
−430
a
b
c
d
Experimental data (HE = high energy, LE = low energy). Theoretical data calculated with B3LYP/6-31+G** theory. Shoulder. For compounds
YQ2, YQ3 and YQ4, the values were also determined experimentally in CHCl
3 3
(3.3, 3.4 and 2.1 × 10− esu, respectively). In anhydrous CHCl
18
e
estimated uncertainty in EFISH measurements ± 10%.
11940 | Dalton Trans., 2016, 45, 11939–11943
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The investigated chromophores show
a
“push–pull”
Y-shaped structure which is the origin of a non- zero dipole
moment and it is directed from the symmetrical ferrocenyl
moiety as a positive pole to quinoxaline moiety as a negative
pole (ESI†).
The substituents are responsible for the different ground
state dipole moments: on going from YQ1 (R = H) to YQ2–YQ4
(
R = electron withdrawing group) there is an increase of the
dipole moment, whereas on going from YQ1 (R = H) to YQ5–
YQ6 (R = electron donor group) a decrease of the dipole
moment occurs (Table 1). These data show that the origin of
the high µβ values of YQ2 and YQ3 is a particularly high
dipole moment. Surprisingly, although second order NLO pro-
perties both in solution and in the solid-state of many ferro-
Fig. 3 Schematic representation of the energy levels of chromophores
YQ1–YQ6 at B3LYP/6-31+G** level of theory. Isodensity surface plots of
HOMO and LUMO molecular orbitals are also shown.
9b
cene derivatives have been investigated, only a few works of
thin films of pure ferrocene or ferrocene derivatives have been
1
9
published.
Host–guest polymethylmethacrylate (PMMA)
2
0
films with ferrocene were reported but the films were studied
for their luminescence properties only. To our knowledge, no
SHG measurements of PMMA films with ferrocene guests have
been reported. When dipolar chromophores are introduced
into polymeric systems as dopants, a highly polar push–pull
structure usually leads to centrosymmetric alignment, due to
the strong electronic interactions. However, only those
materials lacking centrosymmetry could exhibit nonzero
macroscopic second-order susceptibility. A convenient method
to achieve the non-centrosymmetric alignment of chromo-
phore moieties with high µ is to heat the film to a temperature
involved in the principle orbitals are given as density plots in
the ESI.†
Here we report the second-order NLO properties in CHCl
solution of all the chromophores, using the electric field
3
1
6
induced second harmonic (EFISH) generation technique.
This technique offers a valuable alternative to Hyper-Rayleigh
scattering (HRS) which suffers from the limitation of possible
overestimation of values of the quadratic hyperpolarizability
due to multiphoton fluorescence. EFISH can provide direct
information on the intrinsic molecular NLO properties
through eqn (1):
near the glass transition temperature of the polymer (T ) in the
g
presence of an electric field, leading to the poling-induced
2
1
non-centrosymmetric alignment of chromophores.
γEFISH ¼ ðμβλ=5kTÞ þ γðꢀ2ω; ω; ω; 0Þ
ð1Þ
The high quadratic hyperpolarizability value of chromo-
phore YQ2 in solution prompted us to investigate its potential
where µβEFISH/5kT is the dipolar orientational contribution to as a molecular building block for composite films with SHG
the molecular nonlinearity, and γ(−2ω, ω, ω, 0), the third order properties. It was dispersed both in PMMA and polystyrene
polarizability at frequency ω of the incident light, is a purely (PS) matrices (5 wt% of chromophore with respect to the
electronic cubic contribution to γEFISH which can usually be matrix) and oriented by poling (ESI†). The corona wire poling
neglected when studying the second-order NLO properties of dynamics of the SHG behavior of polystyrene and the PMMA
dipolar compounds. Chromophores YQ1–YQ6 are character- composite film are reported in Fig. 4. The UV-Vis absorption
ized by good to excellent values of µβEFISH (−430 to −960 × spectra after and before poling and the SEM images of YQ2 in
−
−
48
3
1
0
esu), see Table 1, working in CHCl at a concentration of polystyrene and in PMMA films are reported in Fig. S1 and
3
1
0
M with a non-resonant incident wavelength of 1.907 µm, S3,† respectively. The second order NLO coefficient d33 for
pressure by using a poled films was obtained by following the standard Maker
obtained by Raman-shifting under high H
Q-switched, mode-locked Nd :YAG laser. The largest µβ
2
3
+
fringe technique (see the ESI†).
EFISH
values are observed for CF
3
(YQ2) and NO
2
(YQ3) derivatives.
We found excellent d33 values both in PS and in PMMA (d33
In order to obtain the projection along the dipole moment axis values of 2.96 pm V− and 5.27 pm V , respectively. For d33
1
−1
of a vectorial component of the tensor of the quadratic hyper- calculation, see the ESI†); remarkably to our knowledge, the
polarizability (βEFISH), it is necessary to know the dipole
d33 value in PMMA is the highest ever reported for a host/guest
moment, μ. So in this work, we have calculated the theoretical system based on organometallic chromophores. Surprisingly,
dipole moments (B3LYP/6-31+G**) of all the chromophores. the SHG signal remains unchanged also when the electric
For comparison, we have also experimentally determined the field is switched off. Its stability is remarkable, after four
for compounds YQ2, YQ3, and YQ4 months, we found a d33 value of 1.72 pm V− , with a stability
1
dipole moments in CHCl
(
3
1
7
Table 1) using the Guggenheim’s method. Experimental of 33%. It is worth pointing out that the PMMA system
values follow the same trend of theoretical ones but are lower gives an SHG response higher than polystyrene, but a higher
than the calculated dipole moments, in accordance with the stability (50%) is achieved using polystyrene as a matrix (d33 of
1
8
−1
literature.
1.47 pm V ).
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5
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KSK thanks DST for the Inspire fellowship (IF-110138). KSK
and NP gratefully acknowledge SIF DST-VIT-FIST, VIT Univer-
sity, Vellore for providing NMR, and IR data, and CDRI-
Lucknow and IIT-Madras for HR-mass spectral analysis. We
thank Dr Daniela Meroni for the SEM images.
9
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