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Two-photon absorption dye based on 2,5-bis(phenylacrylonitrile)thiophene with aggregration enhanced fluorescence

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This paper reports the synthesis and characterization of a 2,5-bis(phenylacrylonitrile)thiophene based two-photon dye, designed to show enhancement in fluorescence quantum yield in nanoaggregated form. Strong solvatochromism has been observed and explained by the favoritism of locally excited (LE) or internal charge transfer (ICT) state depending on the solvent polarity. Aqueous dispersions of nanoparticles have been prepared and investigated regarding their optical properties which were correlated to the LE and ICT state and the molecular structure of the aggregates.
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Two-photon absorption dye based on
2,5-bis(phenylacrylonitrile)thiophene
with aggregration enhanced fluorescence
Tobias Johann,1,2Kerstin Schmidt,1,2Prem Prabhakaran,2
Rudolf Zentel,1 and Kwang-Sup Lee2,
Abstract:
This paper reports the synthesis and characterization of a
2,5-bis(phenylacrylonitrile)thiophene based two-photon dye, designed to
show enhancement in fluorescence quantum yield in nanoaggregated form.
Strong solvatochromism has been observed and explained by the favoritism
of locally excited (LE) or internal charge transfer (ICT) state depending on
the solvent polarity. Aqueous dispersions of nanoparticles have been prepared
and investigated regarding their optical properties which were correlated to
the LE and ICT state and the molecular structure of the aggregates.
© 2016 Optical Society of America
OCIS codes: (160.2540) Fluorescent and luminescent materials; (160.4330) Nonlinear optical
materials; (160.48900) Organic materials.
References and links
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#256973
Received 7 Jan 2016; revised 18 Mar 2016; accepted 18 Mar 2016; published 23 Mar 2016
© 2016 OSA
1 Apr 2016 | Vol. 6, No. 4 | DOI:10.1364/OME.6.001296 | OPTICAL MATERIALS EXPRESS 1296
1Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, 55099 Mainz,
Germany
2Department of Advanced Materials, Hannam University, Daejeon 305-811, South Korea
kslee@hnu.kr
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1. Introduction
Two-photon absorption (2PA) is characterized by the simultaneous absorption of two photons
and therefore increases with the square of the light intensity. This nonlinear effect has substantial
advantages over conventional one-photon absorption and can be utilized for many applications
such as microfabrication, optical power limiting, three-dimensional optical data storage and
bioimaging [
1
]. Typically, near infrared light is used for excitation of organic molecules showing
2PA.
Biological tissues show high transmission at longer wavelengths enabling deeper imaging of
samples. The low foot print of two-photon fluorescence in the sample as well as the low energy
of wavelengths used to effect the two-photon absorption minimizes photodamage. This facilitates
in vivo monitoring of alive biological samples, an attribute much valued in translational cancer
research [
1
]. Though high laser intensities are required to induce two-photon fluorescence, the
nonlinear nature of this phenomenon limits it to the confines of the laser focus. The fluorescence
diminishes rapidly with decrease in intensity of exciting radiation at laser focus. This leads to
higher resolutions in two-photon fluorescence imaging [1].
The design and synthesis of efficient 2PA dyes with strong two-photon excited fluorescence
has been intensively investigated to develop tailor-made 2PA materials. A large 2PA cross-
section (TPACS) can be obtained by using
π
-electron donor-acceptor-donor (D-A-D) structures
with a highly conjugated
π
-system [
1
,
2
]. Additionally, high fluorescence under physiological
conditions is required for biophotonic applications. However, many 2PA molecules are only
soluble in organic solvents and suffer from fluorescence quenching in highly concentrated
solutions or when aggregating. Aggregation induced fluorescent dyes are a class of dyes which
shows high fluorescence on aggregation and thus overcoming the drawbacks of conventional
dyes [
2
] . Fluorescence behavior is affected by the solvent polarity and the local environment and
depends on the molecular structure of the dyes in solution and aggregated state [
3
]. Therefore,
molecules which are capable of specific interactions, such as J-aggregation or excimer formation,
tend to show aggregation enhanced fluorescence which is particularly interesting for gaining
strong signals in biophotonic applications.
In this paper, we report the synthesis and characterization of a 2PA dye based on a 3,3-
(thiophene-2,5-diyl)bis(2-(4-bromphenyl)acrylonitrile) (Tp-bis(PhBr-ACN)) core using triph-
enylamine (TPA) as electron-donor moiety. Symmetrical D-A-D structures similar to our 2PA
dye are known to show a large two photon absorption cross section due to the highly extended
#256973
Received 7 Jan 2016; revised 18 Mar 2016; accepted 18 Mar 2016; published 23 Mar 2016
© 2016 OSA
1 Apr 2016 | Vol. 6, No. 4 | DOI:10.1364/OME.6.001296 | OPTICAL MATERIALS EXPRESS 1297
π
-conjugation and the likelihood of an internal charge transfer (ICT) [
4
]. Linear and non-linear
optical properties, solvatochromism and aggregation effects of Tp-bis(StACN-TPA) have been
investigated.
2. Experimental
2.1. Materials
All starting materials were purchased from Sigma-Aldrich or Tokyo Chemical Industry. All
reagents purchased commercially were used without any further purification if not otherwise
mentioned. Tp-bis(PhBr-ACN) was kindly provided by Prof. H.-K. Shim’s lab in KAIST. All
solvents used for column chromatography were purchased from Samchun Pure Chemical.
2.2. Instruments
1
H and
13
C nuclear magnetic resonance (NMR) spectra and COSY and HSQC spectra were
recorded using a Bruker AVANCE II spectrometer operated at 500 MHz (125 MHz respectively
for
13
C).
1
H NMR spectra at 300 MHz were recorded using a Varian Mercury NMR spectrometer.
For internal reference tetramethylsilane was used. UV-visible spectra and photoluminescence
spectra were measured using a Shimadzu UV-3600 UV-Vis-NIR spectrophotometer and a Scinco
FluoroMate FS-2 fluorescence spectrometer, respectively. IR measurements were carried out
using a Shimadzu IR-Affinity-1S FT-IR spectrophotometer using an ATR-Unit. Matrix-assisted
laser desorption/ionization time of flight (MALDI-ToF) spectra were recorded using a Voyager-
DETM STR Biospectrometry Workstation model using a dithranol matrix. The two-photon
absorption cross-section (TPACS) was obtained by two-photon induced fluorescence method
using a Ti-sapphire 100 femto-second pulse laser with a repetition rate of 90 MHz.
Rhodamine
6G was used as reference fluorophore for the TPACS measurement.
2.3. Synthesis of 4-(diphenylamino)benzaldehyde (TPA-CHO)
TPA-CHO was synthesized by Vielsmeyer-Haack reaction according to literature [
5
]. A dry
50 mL Schlenk-Flask under N
2
atmosphere was charged with 1.77 mL (1.68 g, 23.0 mmol,
2.8 eq) N,N-dimethylformamide (DMF). While cooling with an ice-water bath, 2.12 mL (3.56 g,
23.2 mol, 2.8 eq) freshly distilled POCl
3
were added dropwise. After that, the water-ice bath
was removed and 2.00 g (8.2 mol, 1 eq) triphenylamine were added as a solution in 25 mL
DMF. The flask was sealed using a septum and N
2
balloon and heated for 34 h at 40
°C
. The
solution was cooled to room temperature and poured into 100 mL water. After cooling the
mixture for 5 h at -18
°C
the yellow precipitate was filtrated using vacuum filtration. The product
was purified by column chromatography using n-hexane and ethyl acetate (19:1) as eluent to
obtain 1.50 g (5.5 mmol, 67 %) of a yellowish solid.
Rf: 0.35 (n-hexane:ethyl acetate = 19:1).
IR-ATR: ν
[cm
1
] = 3007 w, 1681 m (C=O), 1581 m (C=C), 1487 m (C=C), 1328 m, 1274 s,
1261 s, 1219 m, 1155 m, 1074 w, 823 m, 750 s, 694 m.
1H-NMR:
(300 MHz, CDCl
3
)
δ
[ppm] = 9.787 (s, 1H), 7.656 (dt, J=8.4 Hz, 2H),
7.323 (t, J=7.5 Hz, 4H), 7.159 (m, 6H), 6.994 (dt, J=8.7 Hz, 2H).
2.4. Synthesis of N’N-diphenyl-4-vinylaniline (TPA-C=C)
TPA-C=C was synthesized by Wittig olefination of TPA-CHO [
6
,
7
]. A 25 mL Schlenk flask was
freed of water and oxygen using Schlenk technique. While maintaining a constant N
2
stream
696 mg (1.95 mmol, 2.7 eq) methyltriphenylphosphoniumbromide (CH
3
PPh
3
Br) was added to
5 mL THF (freshly dried and distilled over sodium). The flask was cooled to 0
°C
using an ice
#256973
Received 7 Jan 2016; revised 18 Mar 2016; accepted 18 Mar 2016; published 23 Mar 2016
© 2016 OSA
1 Apr 2016 | Vol. 6, No. 4 | DOI:10.1364/OME.6.001296 | OPTICAL MATERIALS EXPRESS 1298
water bath and 175.2 mg (7.3 mmol, 10 eq) sodium hydride were added slowly. The cooling bath
was removed and the mixture was stirred until the white suspension turned green (ca. 30 min
at RT). After that 200 mg (0.73 mmol, 1 eq) TPA-CHO were added to the greenish suspension
which turned instantly grayish. The flask was sealed using a septum and a N
2
balloon and stirred
for 24 h at room temperature. The reaction mixture was poured slowly into 100 mL water-ice
(1:1 ratio) and extracted three times with 100 mL methylene chloride. The combined organic
phase was washed using 25 mL brine and dried over MgSO
4
. After evaporation of the solvent
the crude product was purified using column chromatography (n-hexane:ethyl acetate = 19:1) to
obtain 178.8 mg (0.66 mmol, 90 %) as a yellow solid showing blue PL when excited at 365 nm
UV light.
Rf: 0.56 (n-hexane:ethyl acetate = 19:1).
IR-ATR: ν
[cm
1
] = 3032 vw, 2960 vw, 1625 w, 1589 s, 1506 s, 1485 s, 1327 m, 1282 s,
1265 s, 1176 m, 1074 w, 1028 w, 989 m, 889 m, 839 s, 756 s, 742 m, 696 s.
1H-NMR:
(300 MHz, CDCl
3
)
δ
[ppm] = 7.28 - 7.20 (m, 6H), 7.09 - 7.05 (m, 4H), 7.01 -
6.85 (m, 4H), 6.639 (dd, J=11.1 Hz, 17.7 Hz, 1H), 5.616 (dd, J=1.2 Hz, 17.7 Hz, 1H),
5.133 (dd, J=1.2 Hz, 11.0 Hz, 1H).
2.5.
Synthesis of (2Z,2’Z)-3,3’-(thiophene-2,5-diyl)bis(2-(4-((E)-4-(diphenylamino)styryl)
phenyl)acrylonitrile) (Tp-bis(StACN-TPA))
Tp-bis(StACN-TPA) was synthesized by Heck coupling of TPA-C=C with (2Z,2’Z)-
3,3’-(thiophene-2,5-diyl)bis(2-(4-bromophenyl)acrylonitrile) (Tp-bis(BrPhACN)) [
8
].
While maintaining a constant N
2
stream 54.8 mg (0.201 mmol, 2.1 eq) TPA-C=C,
47.8 mg (0.096 mmol, 1 eq) Tp-bis(BrPhACN), 10.5 mg (0.035 mmol, 0.35 eq) tri(o-
tolyl)phosphine and 1.8 mg (0.008 mmol, 0.08 eq) Pd(OAc)
2
were added to 2.1 mL DMF
(freshly dried and distilled over CaH
2
) and 0.24 mL trimethylamine (freshly dried and distilled
over CaH
2
) in a dried 25 mL Schlenk flask. The mixture was degassed three times using
freeze-pump-thaw cycling and heated to 140
°C
under static vacuum. After the color changed
from yellow to dark-red (ca. 1 h) the mixture was heated for further 48 h and after that cooled
to room temperature. The mixture was poured into 200 mL water and extracted 4 times with
100 mL methylene chloride (MC). The combined organic phase was washed with 50 mL brine
and dried over MgSO
4
. The crude product was purified using gradient column chromatography
starting with n-hexane:MC = 7:3 slowly increasing the amount of MC until pure MC was used
for elution. The product TP-bis(StACN-TPA) was obtained as a dark red shiny solid at a yield of
44 %, 37 mg (0.042 mmol).
Rf: 0.68 (n-hexane:MC = 3:7).
IR-ATR: ν
[cm
1
] = 3035 w, 2924 vw, 2208 m (CN), 1587 s (C=C), 1485 s (C=C), 1417 w,
1329 m, 1315 m, 1284 m, 1270 m, 1239 m, 1173 m, 1110 w, 1076 w, 1028 w, 975 s,
893 m, 874 w, 835 s, 802 m, 732 m, 694 s, 637 w, 621 m.
1H-NMR:
(500 MHz, CDCl
3
)
δ
[ppm] = 7.882 (s, 2H), 7.655 (d, J=8.5 Hz, 4H), 7.641 (s, 2H),
7.561 (d, J=8.5 Hz, 4H), 7.405 (d, J=8.5 Hz, 4H), 7.275 (dd, J=8.5 Hz, 8.5 Hz, 8H),
7.141 (d, J=16.0 Hz, 2H), 7.123 (d, J=8.5 Hz, 8H), 7.060 (d, J=8.5 Hz, 4H),
7.052 (t, J=8.5 Hz, 4H), 6.998 (d, J=16.0 Hz, 2H).
13
C-NMR: (125 MHz, CDCl
3
)
δ
[ppm] = 147.82, 147.82, 141.12, 139.03, 132.02, 131.88,
131.44, 130.77, 129.78, 129.32, 127.57, 126.94, 126.08, 125.54, 124.68, 123.22.
Mass: 876.43 (100 %), 877.42 (88 %), 878.44 (44 %), 879.43 (14 %), 880.42 (3 %).
#256973
Received 7 Jan 2016; revised 18 Mar 2016; accepted 18 Mar 2016; published 23 Mar 2016
© 2016 OSA
1 Apr 2016 | Vol. 6, No. 4 | DOI:10.1364/OME.6.001296 | OPTICAL MATERIALS EXPRESS 1299
2.6. Preparation of Nanoaggregates
Nanoaggregates were prepared by a simple precipitation method using 50
µ
L of a 0.1 mg/mL
concentrated stock solution of Tp-bis(StACN-TPA) in THF. The aliquots were diluted to a total
volume of 4 mL starting with THF and slowly adding water. Solutions from 10 to 100 % ratio
of THF with a final concentration of 6.25
µ
g/mL Tp-bis(StACN-TPA) were produced by this
method.
3. Results and Discussion
3.1. Synthesis
The synthetic route for Tp-bis(StACN-TPA) is shown in
Fig. 1
. For the detailed procedure
please refer to experimental part. In the final symmetrical product the triphenylamine-groups act
as electron-donor (D) moieties while the thiophene-bis-acrylnitrile-group acts as an electron-
acceptor (A) moiety forming a symmetrical D-A-D structure. The highly aromatic dye shows
good solubility in low to medium polarity solvents like toluene, diethyl ether, ethyl acetate,
methylene chloride and acetone. It is also soluble in THF which can be used for the formation
of nanoaggregates by water addition. In polar solvents like DMSO or DMF the solubility was
limited.
N N N
DMF, POCl3
Vilsmeyer-Haack reaction
TPA-CHO
67%
CH3PPh3Br
NaH, THF
Wittig reaction
O
TPA-C=C
90%
S
NC
CN
NN
TPA-C=C
Tri-(o-tolyl)-phosphine
Pd(OAc)2
Triethylamine
DMF
S
NCCN
Br Br
Heck reaction
Triphenylamine
TPA
4-(Diphenylamino)benzaldehyde N,N-Diphenyl-4-vinylaniline
(2Z,2'Z)-3,3'-(Thiophene-2,5-diyl)bis(2-(4-
bromophenyl)acrylonitrile)
Tp-bis(BrPhACN) Tp-bis(StACN-TPA)
Thiophene-bis(styreneacrylnitrile-triphenylamine)
44%
34h, 40°C 24h, RT
48h, 140°C
Fig. 1. Synthetic scheme of Tp-bis(StACN-TPA).
3.2. Linear optical properties
The linear optical properties of Tp-bis(StACN-TPA) were investigated using UV-Vis absorption
and photoluminescence (PL) measurements in solvents with different polarities at various
concentrations as listed in Table 1.
An example UV-Vis and PL spectrum in THF of a 6.25
µ
g/mL concentrated solution is shown
in
Fig. 2
. Tp-bis(StACN-TPA) shows three absorbance maxima in THF at 490 nm, 360 nm and
300 nm. The extinction coefficient in THF was determined as 7.2
±0.1·104
L
·
mol
1·
cm
1
using
9 samples with concentrations from 0.05 mg/mL to 1.95
·104
mg/L. For other solvents only
one concentration was used and therefore these values are afflicted with a higher error. Due to
the limited solubility of Tp-bis(StACN-TPA) in 2-methyl-2-butanol the calculated values need
to be considered with caution [
9
]. For all PL measurements excitation was performed at 490 nm.
Quantum yields were determined by using Rhodamin 6G as a reference. The maximum of the
PL intensity can be seen at 678 nm for the solution in THF. Fluorescence quenching occurs at
concentrations of 12.5 µg/mL (1.43 µmol/L) or higher.
#256973
Received 7 Jan 2016; revised 18 Mar 2016; accepted 18 Mar 2016; published 23 Mar 2016
© 2016 OSA
1 Apr 2016 | Vol. 6, No. 4 | DOI:10.1364/OME.6.001296 | OPTICAL MATERIALS EXPRESS 1300
Table 1. One-photon photo-physical properties of Tp-bis(StACN-TPA).
Solvent f(1)λabs
max (2)ε(3)λf lu
max (4)φf(5)
Toluene 0.014 495 7.2 591 49.1
Diethyl ether 0.166 480 2.2 624 7.8
2-Methyl-2-butanol 0.187 490 3.0 646 1.2
Ethyl acetate 0.201 482 6.1 674 1.7
Tetrahydrofurane 0.209 489 7.2 678 0.6
Methylene chloride 0.218 492 6.7 681 0.2
N,N-Dimethylformamide 0.275 494 6.0 -()0.0
(1)Polarity parameter as calculated by Eq. (1). (2)Absorption maximum wavelength in nm.
(3)Molar extinction coefficient (104L·mol1·cm1). (4)Fluorescence maximum wavelength in nm.
(5)Fluorescence quantum yield determined relative to Rhodamin 6G in ethanol in percent.
()Not detectable in our measurement system.
There is only a small spectral overlap within the UV-Vis and PL spectra, therefore the chance
of quenching due to F
¨
orster resonance energy transfer is low [
3
]. The calculated and observed
absorption maxima of Tp-bis(StACN-TPA) (see
Table 2
in section 3.3) are closer to each other
compared to the calculated and observed emission maxima. In solid state high fluorescence
intensity can be observed which can be explained by stacking-induced planarization. This might
diminish the non-radiative decay pathway due to molecular movements. Furthermore, it is
proposed that even in solid state a certain amount of distortion is maintained. This explains
the high PL intensity in solid state as usually intermolecular quenching effects occur in close
packings [4].
300 400 500 600 700 800 900
0.000
0.125
0.250
0.375
0.500
Absorbance (a.u.)
Wavelength (nm)
UV
(a)
0
200
400
600
800
1000
PL
PL Intensity (a.u.)
0.00 0.02 0.04 0.06 0.08 0.10
0.0
5.0x10
4
1.0x10
5
1.5x10
5
Integrated PL intensity (a.u.)
Concentration (g/L)
(b)
Fig. 2.
(a)
: UV-Vis and PL spectra of Tp-bis(StACN-TPA) (
c=6.25 µ
g/mL).
(b): Integrated PL intensity versus concentration of Tp-bis(StACN-TPA) in THF.
Solvatochromic effects were studied using the Lippert-Mataga model as shown in
Fig. 3
.
The solvent polarities were calculated using the Lippert-Mataga equation Eq. (1) with
n
as the
refractive index and εas the dielectric constant [3, 10].
f=ε1
2ε+1n21
2n2+1(1)
No solvatochromism is noticeable in the absorbance data, therefore it seems that the ground state,
which usually has a lower dipole moment compared to the excited state, is scarcely affected by
solvent polarity in the case of Tp-bis(StACN-TPA). However, the emission spectra are strongly
#256973
Received 7 Jan 2016; revised 18 Mar 2016; accepted 18 Mar 2016; published 23 Mar 2016
© 2016 OSA
1 Apr 2016 | Vol. 6, No. 4 | DOI:10.1364/OME.6.001296 | OPTICAL MATERIALS EXPRESS 1301
influenced by the solvent polarity. In nonpolar solvents like toluene a high quantum yield of
49 % and an emission maximum at 591 nm are observable. With increasing solvent polarity the
emission maximum is red-shifted and the quantum yield decreases linearly Fig. 3(a).
This behavior is in line with the Lippert-Mataga model until 2-methyl-2-butanol with a solvent
polarity parameter of 0.187 is used for the measurements. At this point a bimodal emission
spectrum as shown in
Fig. 3(b)
can be measured with maxima at 632 nm and 678 nm. For
solvents with higher polarity only a strong red shifted maximum is found and the quantum yield
decreases below 1 %. The observed bathochromic shift exceeds the expected behavior described
by the Lippert-Mataga model Fig. 3(c).
500 600 700 800 900
0.00
0.25
0.50
0.75
1.00
Normalized PL intensity (a.u.)
Wavelength (nm)
Diethyl ether
THF
2-Methyl-2-butanol
(b)
0.00 0.05 0.10 0.15 0.20 0.25 0.30
15000
16000
17000
18000
19000
20000
21000
Absorbance
Fluorescence
(cm
-1
)
Polarity parameter (
f
)
ICT
LE
(c)
Fig. 3.
(a)
:Dependence of the quantum yield of the solvent polarity and different excitation
states.
(b)
:PL spectra of Tp-bis(StACN-TPA) in different solvents.
(c)
:Solvatochromism
according to Lippert-Mataga model.
It should be noted that the Lippert equation is only an approximation for solvatochromism and
the interpretation of solvent-dependent emission spectra is a very complex topic, as the emission
is not only affected by solvent polarity, but also by various factors such as fluorophore-solvent
and probe-probe interactions, solvent viscosity and the formation of ICT states [3].
The bimodal emission spectrum of Tp-bis(StACN-TPA) in 2-methyl-2-butanol can be in-
terpreted as superposition of the spectra of ICT and locally excited (LE) state. The ICT state
is stabilized in polar solvents and therefore the emission maximum is shifted to longer wave-
lengths. Furthermore, due to the strong interactions between the ICT state dipole and the solvent
molecules, the lifetime of the ICT state might be increased, which could explain the low quan-
tum yields below 1 % for Tp-bis(StACN-TPA) in solvents with high polarity [
11
]. In nonpolar
solvents the LE state, showing no charge separation, represents the energetically lowest excited
state [3]. Therefore, high quantum yields and emission at shorter wavelengths can be observed.
#256973
Received 7 Jan 2016; revised 18 Mar 2016; accepted 18 Mar 2016; published 23 Mar 2016
© 2016 OSA
1 Apr 2016 | Vol. 6, No. 4 | DOI:10.1364/OME.6.001296 | OPTICAL MATERIALS EXPRESS 1302
3.3. Quantum chemical calculations
To correlate the optical properties to the chemical structure of Tp-bis(StACN-TPA) quantum
chemical calculations using DFT methods were carried out. The DFT-B3LYP hybrid functional
with 6-31G** as basic set was used on the Firefly (PC GAMESS) program package to obtain
an optimized structure for the ground state [
12
,
13
]. The dihedral angle between the central
acrylonitrile-thiophene group and the stilbene group is
20
. Overall, between both triphenylamine
groups a twist of less then 5was calculated.
The dihedral angle between the N-phenyl rings and the 4-styrylaniline is
80
. From the
calculated data the molecular distortion is rather low, therefore extended conjugation can be
expected. However, the geminal N-phenyl groups are highly twisted and are located out of the
conjugation plane. These may prevent closed stacking in solid state. Tranisition energy and
molecular levels were calculated using time dependend DFT with B3LYP/6-31G** as basic set.
As shown in
Fig. 4
the HOMO is evenly delocalized throughout the entire molecule except for
the N-phenyl rings. The LUMO is localized on the core of the molecule. These orbitals show the
internal charge transfer (ICT) characteristics of a D-A-D type molecule. The
π
-electrons are
redistributed from the donor end-groups to the acceptor center.
HOMO
LUMO
Fig. 4. Illustration of molecular orbitals of Tp-bis(StACN-TPA).
Table 2. Calculated optical properties of Tp-bis(StACN-TPA).
State λabs
max (nm) λf lu
max (nm) EHOMOLUMO (eV)
gas 490 760 2.10
THF 511 847 1.91
3.4. Non-linear optical properties
The two-photon cross section of Tp-bis(StACN-TPA) in THF was measured using a Ti-sapphire
mode-locked femtosecond laser. As this method depends on the available laser sources, only the
range from 720 nm to 890 nm was measurable with the available instruments. These wavelengths
#256973
Received 7 Jan 2016; revised 18 Mar 2016; accepted 18 Mar 2016; published 23 Mar 2016
© 2016 OSA
1 Apr 2016 | Vol. 6, No. 4 | DOI:10.1364/OME.6.001296 | OPTICAL MATERIALS EXPRESS 1303
correspond to 360 nm to 445 nm of one-photon absorbance. Unfortunately these wavelengths
represent a local minimum in the absorbance spectrum of Tp-bis(StACN-TPA). For the TPACS
at 890 nm a value of 675±225 GM was measured.
It is well established that for centrosymmetric molecules like the one studied here, that the
first two-photon allowed state is located at a higher energy than the first one-photon allowed
state [
1
,
14
]. We therefore expect this molecule to show maximum 2PA in solution state at
a wavelength close to but lesser than double the one-photon maximum at 490 nm in THF.
Despite this the TPACS values of 675
±
225 GM is on the higher side of this type of dyes [
15
].
Furthermore, the symmetry considerations mentioned above strongly suggest the probability of
higher values of TPACS as the excitation wavelength approaches 980 nm.
3.5. Aggregation enhanced fluorescence
Aggregation enhanced fluorescence was investigated using a simple precipitation method. THF
water solutions of Tp-bis(StACN-TPA) with a concentration of 6.25
µ
g/mL were measured
regarding their absorbance and PL. As shown in
Fig. 5(a)
, the PL intensity decreases with the
addition of only a small amount of water to the THF solution.
0.0 0.2 0.4 0.6 0.8 1.0
0.0
4.0x10
4
8.0x10
4
1.2x10
5
1.6x10
5
2.0x10
5
Integrated PL intensity (a.u.)
Fraction of THF
(a)
550 600 650 700 750 800 850
0.00
0.25
0.50
0.75
1.00
Normalized PL intensity (a.u.)
Wavelength (nm)
THF:H
2
O = 2:8
THF:H
2
O = 1:0
(b)
Fig. 5.
(a)
: PL intensity depending in the fraction of THF from 10 to 100 %.
(b)
: PL spectra
of pure Tp-bis(StACN-TPA) in pure THF compared to THF:water = 2:8.
A local minimum at a fraction of 40 % water can be observed, followed by increasing PL
intensities with increasing amount of water. At a fraction of 80 % water the PL intensity is
200 % of the initial value of a THF solution. This effect can be correlated with the observed
solvatochromism behavior. Low fractions of water seem to increase the solvent polarity and
therefore decrease the quantum yield by favoring the ICT state, while the solubility of the dye is
still sufficient. At about 40-60 % fraction of water the formation of bulk precipitates visible by
eye occurs slowly within 10 to 20 minutes after preparation of the samples. The measured PL
intensities of these THF water mixtures are low at the given sample concentration of 6.25
µ
g/mL.
This might be due to the low surface to volume ratio of the bulk precipitates. With further water
addition the solubility of Tp-bis(StACN-TPA) in the THF water mixtures is further reduced
and therefore the formation of aggregates occurs much faster. Similar behavior was reported by
Prasad et al. [
4
]. However, it should be noticed that a higher concentrated sample (c=0.05 mg/ml)
shows 5 times the PL intensity compared to a THF solution when bulk precipitation occurs at
a water fraction of 50 % . This increase can be explained as the THF solutions show intense
quenching effects at concentrations higher than 12.5
µ
g/mL. This relativises the seen effect.
At a fraction of 70 to 90 % water light scattering can be observed due to the formation of
nanoaggregates. These small aggregates show a locally nonpolar environment that favors the LE
state, hence increasing the PL intensity while showing a high surface to volume ratio. Moreover,
#256973
Received 7 Jan 2016; revised 18 Mar 2016; accepted 18 Mar 2016; published 23 Mar 2016
© 2016 OSA
1 Apr 2016 | Vol. 6, No. 4 | DOI:10.1364/OME.6.001296 | OPTICAL MATERIALS EXPRESS 1304
a behavior similar to solid state can be expected. As shown in
Fig. 5(b)
a blue shift of the
PL maximum compared to a THF solution can be observed. Furthermore, the shoulder peak
increases to form a separated peak showing the LE state. At higher amounts of water the PL
intensity surprisingly decreases again. This behavior was described by Tang et al. [16].
At a fraction of 90 % water Tp-bis(StACN-TPA) seems to agglomerate too fast, thus forming
amorphous particles. The aggregation enhanced fluorescence seems to be dependent on concen-
tration and water to THF ratio and therefore might be a function of the size of the aggregates
and their structure.
There are a number of well-established techniques to deliver dyes showing aggregation
enhanced fluorescence into cells. A popular approach involves the use of hydrophilic micelles
with or without incorporated targeting functionalities encapsulating the nanoaggregates of the
dyes [
17
,
18
]. Alternatively, dyes have been encapsulated into silica nanoparticles to deliver
them into cells [
19
,
20
]. These methods also provides the flexibility to work around the varying
solubility of the dye in water. In our opinion both methods are suited for delivering Tp-bis(StACN-
TPA) into cells for in vitro studies. Studies investigating Tp-bis(StACN-TPA) encapsulated in
micelles are currently in progress and will be reported soon in another communication.
4. Conclusion
A new 2,5-bis(phenylacrylonitrile)thiophene based 2PA-dye has been synthesized and quantum
chemical calculations have been performed. From the experimental results it has been shown
that solvent polarity has a crucial influence on the excited states of Tp-bis(StACN-TPA). The
formation of aggregates can be used as a viable method to overcome the low quantum yields in
polar solvents. Further investigation regarding the size and structure of the aggregates needs to
be done to gather insights about the parameters necessary for creating optimal conditions for the
practical use of aggregation enhanced fluorescence.
Acknowledgments
This work was supported by the Active Polymer Center for Patterned Integration (ERC R 11-
2007-050-01002-0) of the National Research Foundation of Korea. We also thank Juhyoung Jung
for carrying out the PL measurements and Jinsun Park for performing the TPACS measurement.
#256973
Received 7 Jan 2016; revised 18 Mar 2016; accepted 18 Mar 2016; published 23 Mar 2016
© 2016 OSA
1 Apr 2016 | Vol. 6, No. 4 | DOI:10.1364/OME.6.001296 | OPTICAL MATERIALS EXPRESS 1305
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