Spectroscopic investigations of a novel tricyanofuran dye for nonlinear optics

Article abstract of DOI:10.1016/j.saa.2007.11.010

A novel tricyanofuran dye was synthesized and the dye-in-polymer films were fabricated by spin-coating process. The spectroscopic properties of the dye in the solutions and polymer films were investigated by the absorption spectra and fluorescence emissio

Full text of DOI:10.1016/j.saa.2007.11.010

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 71 (2008) 86–89
Spectroscopic investigations of a novel tricyanofuran dye
for nonlinear optics
Likun Han, Yadong Jiang∗, Wei Li, Yuanxun Li, Peng Hao
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science
and Technology of China, Chengdu 610054, China
Received 14 September 2007; received in revised form 7 November 2007; accepted 15 November 2007
Abstract
A novel tricyanofuran dye was synthesized and the dye-in-polymer films were fabricated by spin-coating process. The spectroscopic properties
of the dye in the solutions and polymer films were investigated by the absorption spectra and fluorescence emission spectra. It is found that the
absorption and fluorescence maxima are largely red-shifted along with the increase of the solvent polarity. And the low values of fluorescence
quantum yield in higher polarity solvents suggest the presence of twisted intramolecular charge transfer states of the dye. Moreover, the second
order polarizability value of the novel dye was estimated based on the quantum-mechanical two-level model.
© 2008 Published by Elsevier B.V.
Keywords: Spectroscopic properties; Quantum yield; Stocks shift; Second order polarizability
1. Introduction
2. Experimental
Conjugated organic dyes with electron donor (D) and
acceptor (A) substituents have attracted considerable atten-
tion because of their remarkable intramolecular charge transfer
(ICT) properties and their related second order nonlinear opti-
cal (NLO) response, with potential applications for optical data
storage, optical signal processing and optical switches [1–3].
In this paper, a novel D-␲-A type organic dye (DCDHF-
2-V) containing a tricyanofuran acceptor was synthesized and
the spectroscopic properties of the dye in different solutions and
polymer films were investigated. The spectroscopic studies indi-
cate that the DCDHF-2-V molecule is sensitive to the polarity
of the medium and the polarity dependence may be explained
in terms of twisted intramolecular charge transfer (TICT) pro-
cesses. The second order polarizability (β) was calculated based
on the quantum-mechanical two-level model, which provides a
firm basis for the potential applications of the dye to nonlinear
optical devices.
2.1. Materials
All chemicals were purchased from Alfa Aesar, and poly
methylmethacrylate (PMMA) was purified by recrystallization
before use. All solvents used were of spectroscopic grade.
2.2. Synthesis and characterization
According to the method reported in Ref. [4], a new tri-
cyanofuran dye (DCDHF-2-V) was synthesized and its reaction
scheme is shown in Fig. 1. Crude products were recrystallized
from methanol/dichloromethane (1:1).
DCDHF-2-V: dark green, yield: 78%, mp: 243–245 ◦C. 1H-
NMR (CDCl3): δ: 1.25 (t, 6H), 1.74 (s, 6H), 3.49 (q, 4H), 6.70 (q,
2H), 7.53 (d, 2H), 7.60 (d, 2H). Anal. calcd for (C22H22N4O):
C 73.74, H 6.15, N 15.64; found: C 73.54, H 6.15, N 15.42;
IR (KBr disc): γ (cm−1) = 2924, 2378, 2222, 1560, 1522, 1375,
1261, 1190, 1149, 1105, 1078, 974.
2.3. Fabrication of DCDHF-2-V/PMMA films
∗
The mixture of DCDHF-2-V and PMMA was dissolved
in 1,2-dichloroethane. After the dye and polymer were dis-
Corresponding author. Tel.: +86 28 83207027; fax: +86 28 83206123.
E-mail address: jiangyd@uestc.edu.cn (Y.D. Jiang).
1386-1425/$ – see front matter © 2008 Published by Elsevier B.V.
doi:10.1016/j.saa.2007.11.010

L. Han et al. / Spectrochimica Acta Part A 71 (2008) 86–89
87
Fig. 1. Synthesis of DCDHF-2-V.
solved completely, the solution was filtered using 0.45 ␮m
3. Results and discussion
millipore filter. The polymeric thin films were prepared
by spin-coating method on polished glass substrates and
then heated at 70 ◦C overnight to remove any residual
solvent.
3.1. Spectroscopic properties of DCDHF-2-V in polymer
film
The absorption spectrum of the tricyanofuran dye-in-polymer
film is shown in Fig. 2. The dye has strong absorption in the vis-
ible region peaking at 554 nm. The absorption band is attributed
to ␲–␲* transition in the conjugation system formed by the ben-
zene ring, alkene group, diethylamino and tricyanofuran ring.
Fig. 3 shows the fluorescence spectrum of the tricyanofu-
ran dye-in-polymer film at room temperature. In this figure, in
addition to the main emission peak near 648 nm, an additional
emission peak appears around 833 nm, which should be assigned
to the photoisomerization process of DCDHF-2-V molecules in
a PMMA matrix. Most of the organic conjugated molecules are
expected to exist in a trans form in a rigid matrix, whereas the
illumination of the molecules causes the trans form to isomerize
to a cis form [7]. In a photostationary condition, it is possible to
see the emission from a cis and trans isomer. The 648 nm band
is attributed to the fluorescence from the excited state S1 of a
cis isomer and the 833 nm band is attributed to the fluorescence
from S1 state of a trans isomer [8].
2.4. Spectroscopic measurements
The absorption spectra were recorded using a Shimadzu
UV-1700 spectrophotometer. The fluorescence emission spectra
were recorded on a Hitachi F-4500 spectrofluorimeter at room
temperature and the excitation wavelength is 550 nm. The fluo-
rescence quantum yields Φf of DCDHF-2-V in organic solvents
were calculated from the corrected fluorescence spectra using
the following equation [5]:
ꢀ
ꢁ ꢀ
ꢁ
2
IdyeAref
IrefAdye
ndye
nref
Φf = Φref
(1)
where Idye and Iref are the integrated areas of the corrected
fluorescence spectra, Adye and Aref are the absorbances at the
excitation wavelength, ndye and nref are the refractive indices of
the solvents for the dye and reference, respectively. Rhodamine
B in ethanol (Φref = 0.97 [6]) was used as the reference for quan-
tum yield determination. The spectra of solutions were recorded
at a dye concentration of 1 × 10−6 g cm−3 using a 1 cm thick
cell.
3.2. Spectroscopic properties of DCDHF-2-V in different
solvents
The absorption and fluorescence spectra of DCDHF-2-V
in dioxane and dimethyl formamide (DMF) shown in Fig. 4
Fig. 2. Absorption spectrum of the spin-coated film.
Fig. 3. Fluorescence spectrum of the spin-coated film.

88
L. Han et al. / Spectrochimica Acta Part A 71 (2008) 86–89
Fig. 5. Plot of Stocks shift ꢀv of DCDHF-2-V against the solvent polarity
function ꢀf.
Fig. 4. Absorption (1, 2) and fluorescence (3, 4) spectra of DCDHF-2-V in
dioxane and DMF.
reveal a bathochromic shift (positive solvatochromism) when
going from low polar dioxane to high polar DMF. The max-
imum values of bathochromic shifts are 1361 cm−1 for the
absorption spectra and 814 cm−1 for the fluorescence ones
(Fig. 4 and Table 1). This means that the dipole moment
of the DCDHF-2-V molecule is higher in the excited state
than in the ground state. The high intensity broad absorp-
tion band is assigned to transition from the ground state S0
to the first excited singlet state S1. The solvent dependency
of the maximum of red-shifted emission band is the char-
acteristics of the excited state charge transfer process. This
type of excited state behavior is explained as intramolecular
charge transfer (ICT) accompanied by a structural reorganiza-
tion [9,10].
mental error. The slope k of the resulting straight line is equal to
the term 2(μe − μg)2/hcr3 in Eq. (2). The value of k is obtained
as 3397 cm−1. As a result of these calculations, it is found that
ꢀμ = μe − μg = 33.09 × 10−30 C m. Such a large change in the
excited state dipole moment of the molecule seems to suggest
corresponding large changes in the excited state configuration.
One possible cause could be the geometrical distortion of the
molecule in the excited state [13].
Fluorescence quantum yields (Φf) of DCDHF-2-V in differ-
ent solvents at room temperature are listed in Table 1. It is seen
that the fluorescence quantum yields are lower in high polar sol-
vents as compared to low polar solvent like dioxane, which is
assigned to the participation of non-emissive TICT states of the
dye in higher polarity solvents [14]. According to this model,
on electronic excitation, the molecule initially forms a mod-
erately non-polar state with a geometry similar to that in the
ground state. The transfer of an electron from an electron donor
to an electron accepter group results in a twisted configuration
of the molecule, in which the donor and accepter groups are
oriented almost perpendicular to each other [13,15]. The sta-
bilization of the excited state in polar solvents indicates that
large extent of twisted intramolecular charge transfer is involved
which results in the rapid radiationless decay of the excited
state.
To understand spectral shifts with solvent polarity, it is
important to correlate Stocks’ shifts (ꢀvSt = vambasx − vmf ax) with
solvent parameter (ꢀf), which are expected to follow linear
Lippert–Mataga relationship as [11,12]
2(μe − μg)2
ꢀvSt = ꢀv0 +
ε − 1
ꢀf
(2)
(3)
hcr3
n2 − 1
ꢀf =
−
2ε + 1 2n2 + 1
where μe and μg are the excited and ground state dipole
moments, respectively, h is the Planck’s constant, c is the veloc-
ity of light and r is the radius of the Onsager interaction sphere.
ε and n are the dielectric constant and the refractive index of
the solvent. Fig. 5 shows the ꢀvSt versus ꢀf plot for the dye
in different polarity solvents. The plot is linear within experi-
However, the quantum yield values in chlorinated solvents
are significantly large, which may be due to the restriction in the
formation of the TICT state, because of some specific interaction
[16].
Table 1
The solvent parameters and spectral characteristics of DCDHF-2-V in different solvents
abs
abs
1/2h
Solvent
ε
n
ꢀf
λ
(nm)
εmax (L mol−1 cm−1
)
ꢀv
(cm−1
)
f
λfmax (nm)
Φf
β (m4/V, 1.9 ␮m)
max
(mean value)
8.045 × 104
6.805 × 104
8.683 × 104
7.442 × 104
14.211 × 104
12.758 × 104
11.553 × 104
3312
2682
2457
3106
2956
2797
2992
1.2253
0.7689
0.9811
0.8409
1.606
611
630
622
627
642
643
635
0.065
0.165
0.113
0.057
0.052
0.027
0.024
908.9 × 10−40
Dioxane
2.22 1.4224
5.62 1.5248
4.81 1.4467
7.58 1.4073
0.0213
0.1429
0.1479
0.2095
0.2478
0.2764
0.2843
547
586
588
572
583
591
578
Chlorobenzene
Chloroform
THF
Cyclohexanone
DMF
18.2
38.3
20.7
1.4507
1.4269
1.3588
1.3054
1.4416
Acetone

L. Han et al. / Spectrochimica Acta Part A 71 (2008) 86–89
89
3.3. Evaluation of the second order polarizability β
in the fluorescence quantum yield, which is explained by TICT.
The second order polarizability β is calculated according to the
two-level model and the high value of β allows to consider
DCDHF-2-V as a promising material for NLO applications.
On the basis of the data obtained, one can estimate the second
order polarizability β of DCDHF-2-V, which is an important
microcosmic data for NLO chromophores. The value of β can
be derived from a two-level model [17]:
Acknowledgement
3e2h¯ 2
2m
W
This work was supported by the National Science Fund
for Distinguished Young Scholars of China under Grant No.
60425101.
β =
fꢀμ
(4)
[W2 − (2h¯ ω)2][W2 − (h¯ ω)2]
Here e is the electron charge, h¯ is the Planck’s constant, m is the
electron mass, W (W = h¯ ωeg) is the energy difference between
the ground and the first excited states of the dye, ω is the laser
radiation frequency, f is the oscillator strength and ꢀμ is the
difference between the dipole moment of the excited and the
ground states. f is related to the intensity of the transition and
can be gotten from the area under the band by means of Eq. (5)
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In this paper, we have synthesized a novel tricyanofuran dye
(DCDHF-2-V) and investigated the absorption spectra and flu-
orescence emission spectra of the dye in different solutions and
a polymer matrix. A large bathochromic shift in the absorption
spectra and emission spectra was observed when increasing sol-
vent polarity, which is related to a difference between the dipole
moments of a molecule in the ground and the excited states.
Moreover, an increase in the solvent polarity leads to a decrease