Enhancement of two photon absorption properties by charge transfer in newly synthesized aza-boron-dipyrromethene compounds containing triphenylamine, 4-ethynyl-N,N-dimethylaniline and methoxy moieties

Article abstract of DOI:10.1016/j.jphotochem.2013.02.002

The effects of substitution and charge transfer on linear and nonlinear optical absorption (especially two photon absorption) properties of aza-BODIPY containing triphenylamine, 4-ethynyl-N,N-dimethylaniline and methoxy moieties were investigated by ultra

Full text of DOI:10.1016/j.jphotochem.2013.02.002

Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 23–28
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Enhancement of two photon absorption properties by charge transfer in newly
synthesized aza-boron-dipyrromethene compounds containing triphenylamine,
4-ethynyl-N,N-dimethylaniline and methoxy moieties
Sezen Tekina, Betül Küc¸ üköza, Halil Yılmazb, Gökhan Sevinc¸ b, Mustafa Hayvalıb,∗, H. Gul Yaglioglua,
Ayhan Elmalia,∗∗
a Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Bes¸evler, Ankara, Turkey
b Department of Chemistry, Faculty of Science, Ankara University, 06100 Bes¸evler, Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
The effects of substitution and charge transfer on linear and nonlinear optical absorption (especially two
photon absorption) properties of aza-BODIPY containing triphenylamine, 4-ethynyl-N,N-dimethylaniline
and methoxy moieties were investigated by ultrafast pump probe spectroscopy technique. Fluorescence
and ultrafast pump probe spectroscopy experiments revealed that aza-BODIPY compounds with good
electron donating moieties (triphenylamine and 4-ethynyl-N,N-dimethylaniline moieties) have charge
transfer from electron donating parts of the molecules to aza-BODIPY core. The two photon absorp-
tion cross sections increase with the electron donating strength. Maximum two photon absorption cross
section is obtained as 126 GM at telecommunication wavelength (1310 nm) for aza-BODIPY compound
containing triphenylamine, 4-ethynyl-N,N-dimethylaniline. Our results indicate that charge transfer
enhances two photon absorption properties.
Received 6 December 2012
Received in revised form 30 January 2013
Accepted 8 February 2013
Available online 18 February 2013
Keywords:
Aza-BODIPY
Charge transfer
Fluorescence
Ultrafast pump probe spectroscopy
technique
© 2013 Elsevier B.V. All rights reserved.
Two-photon absorption
1. Introduction
Boron-dipyrromethene (BODIPY) dyes are novel organic dyes with
high fluorescence quantum yields, excellent photo stability, good
Two-photon absorption (TPA) is an instantaneous nonlinear
process, in which a atoms or molecule absorbs two photons simul-
taneously. A particular property of the TPA is that the rate of the
absorption increases as the square of the incident light intensity
[1]. Such a quadratic dependence supplies the capability to localize
the excitation with high three dimensional spatial resolution in the
focus of laser beam. Molecules with large TPA coefficient are now in
great demand for a various applications; including optical limiting
applications [2,3], TPA imaging microscopy [4], three-dimensional
micro fabrication [5,6], optical data storage [7,8], photodynamic
therapy [9] and TPA up conversion lasing [10]. Therefore, the design
of efficient compounds with TPA response is one of the most
important challenges. Two photon active wavelengths of these
organic materials are mostly located around 700–1100 nm spec-
tral ranges [1]. There is an increasing need for devices presenting
TPA properties in near-infrared (NIR) spectral range for applications
such as signal processing in the telecommunication wavelengths.
solubility [11–13] and high two photon absorption cross sections
(TPCS) [14–16]. Although two photon absorption properties of
BODIPY compounds are widely studied in the literature [15–17],
there are only a few studies on TPA properties of aza-BODIPY
compounds [18,19]. Recently, Bouit et al. [18] reported that func-
tionalized aza-BODIPY dyes with electron donating substituent
exhibit strong TPA properties in the telecommunications spec-
tral range. In our previous work, we introduced various phenolic
and phenolate forms of aza-BODIPY molecules with different
electron-donating strength to study the effects of substitution and
intramolecular charge transfer (ICT) on TPA properties. Our pre-
vious results indicated that two photon absorption cross-section
values increase with electron donating strength and intramolecular
charge transfer enhances two photon absorption properties in the
telecommunications spectral range [19].
Based on our previous results, we synthesized new
aza-BODIPY compounds containing triphenylamine, 4-ethynyl-
N,N-dimethylaniline and methoxy moieties to enhance TPA
properties in this work. Triphenylamine and 4-ethynyl-N,N-
dimethylaniline moieties are known with their strong electron
donating strengths. Absorption spectrum of aza-BODIPY com-
pounds containing triphenylamine are greatly red shifted.
Electron-donating strength [19,20] and molecule conjugation
∗
Corresponding author. Fax: +90 312 3860824.
Corresponding author. Fax: +90 312 2127343.
E-mail addresses: hayvali@science.ankara.edu.tr (M. Hayvalı),
∗∗
elmali@eng.ankara.edu.tr, aelmali2010@gmail.com (A. Elmali).
1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2013.02.002

24
S. Tekin et al. / Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 23–28
Scheme 1. Structures of newly synthesized and investigated compounds.
length [1] enhance TPA properties i.e. ꢀ2 (TPCS) in wide spectral
range (1200–1450 nm). In this work, we investigated the effect of
different electron-donating strength of different moieties on TPA
properties.
were synthesized by reactions of 2a and 2b with ammonium
acetate in ethanol. After that, BF2-chelated-[5-(4-bromomphenyl)-
3-(4-methoxyphenyl)-1H-pyrrol-2-yl]-[5-(4-bromophenyl)-
3-(4-methoxyphenyl)
pyrrol-2-ylidene]
amine
(5a)
and
BF2-chelated-[5-(4-bromomphenyl)-3-(4-(N,N-diphenylamino)-
phenyl)-1H-pyrrol-2-yl]-[5-(4-bromophenyl)-3-(4-(N,N-
diphenylamino) phenyl) pyrrol-2-ylidene]amine (5b) were
synthesized by reactions of 3a and 3b with diisopropylethylamine
and boron trifluoride diethyl etherate in dry CH2Cl2.
2. Experimental
2.1. Synthesis of aza-BODIPY
The syntheses of compounds 5a, 5b, 6a and 6b
involved multistep approach as shown in Scheme 1. 1-(4-
Bromophenyl)-3-(4-methoxyphenyl)prop-2-en-1-one (1a) and
1-(4-bromophenyl)-3-(4-(N,N-diphenylamino)phenyl)prop-2-en-
1-one (1b) were synthesized from a reaction of the corresponding
benzaldehyde, acetophenone, and potassium phosphate. These
compounds were subsequently reacted with nitromethane
and diethylamine in dry ethanol to obtain respective 1-(4-
bromophenyl)-3-(4-methoxyphenyl)-4-nitrobutan-1-one (2a) and
1-(4-bromophenyl)-3-(4-(N,N-diphenylamino)phenyl)-4-nitro-
butan-1-one (2b). The [5-(4-bromophenyl)-3-(4-methoxyphenyl)-
1H-pyrrol-2-yl]-[5-(4-bromophenyl)-3-(4-methoxyphenyl)-
pyrrol-2-ylidene]amine (3a) and [5-(4-bromophenyl)-3-(4-(N,N-
diphenylamino)phenyl)-1H-pyrrol-2-yl]-[5-(4-bromophenyl)-3-
(4-(N,N-diphenylamino) phenyl) pyrrol-2-ylidene]amine (3b)
Table 1
Optical properties of compounds 5a, 5b, 6a and 6b in THF solutions.
d
Compound
ꢁa
(nm)
ꢁb
(nm)
ꢁc
(nm)
fluo
ꢀ2 (GM)
abs
fluo
5a
5b
6a
6b
675
660/770
700
710
–
–
710
450
410
450
–
37
25
83
680/770
–
a
The peak wavelength of absorption spectra.
The peak wavelength of one-photon fluorescence with excitation wavelength
b
ꢁabs
c
.
The peak wavelength of one-photon fluorescence with excitation wavelength
340 nm.
d
TPA cross section at wavelength of 1200 nm.

S. Tekin et al. / Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 23–28
25
Fig. 1. Absorption spectrum of (a) compounds 5b and 6b, (b) compounds 5a and 6a in THF solution.
BF2-chelated-[5-(4-bromomphenyl)-3-(4-methoxyphenyl)-
1H-pyrrol-2-yl]-[5-(4-N,N-dimeyhlamino) ethenylphenyl-3-(4-
methoxyphenyl) pyrrol-2-ylidene]amine (6a) were obtained
from compound 5a and [5-(4-bromophenyl)-3-(4-(N,N-
diphenylamino)phenyl)-1H-pyrrol-2-yl]-[5-(4-N,N-dimeyhl-
amino)ethenylphenyl)-3-(4-(N,N-diphenylamino)phenyl) pyrrol-
2-ylidene]amine (4b) were synthesized from corresponding boron
free ligands of these aza-BODIPY (3b). After obtaining this free li-
gand 4b than BF2 complexation were done and obtained compound
6b. Experimental details were given in supporting information.
absorption spectra of studied aza-BODIPY molecules reveal strong
S0 → S1 transition with maximum absorbance wavelength vary-
ing between 600 and 700 nm (Table 1), depending on the electron
donating moieties. The absorption peak around 770 nm in Fig. 1a
may be attributed to charge transfer state as observed in our
previous study [19]. Our previous results revealed that there is
no fluorescence signal when charge transfer occurs due to elec-
tron donor property of phenolate form of hydroxyl groups in
compounds. Therefore, we have performed fluorescence measure-
ment at various excitation wavelengths to investigate relationship
between fluorescence quenching and charge transfer.
In the literature, there are many aza-BODIPY structures which
have high fluorescence quantum yield and high extinction coeffi-
cient [11–13]. However, fluorescence signal was not observed for
studied compounds except 5a when aza-BODIPY core was excited
due to charge transfer mechanism (Fig. 3 and Table 1). Fluorescence
property and charge transfer relation will be discussed in detail
Section 3.2. On the other hand, absorption bands of triphenyla-
mine moieties are around 350 nm. In attempt to clarify whether the
fluorescence signal comes from aza-BODIPY core or not when tri-
phenylamine is excited, we chose 340 nm as excitation wavelength.
The results of measurements show that the fluorescence signal was
not observed from aza-BODIPY instead signal came from triphen-
ylamine moiety itself. There is strong fluorescence signal around
450 nm for 5b, 6a and 6b upon excitation of triphenylamine and
4-ethynyl-N,N-dimethylaniline moieties with 340 nm wavelength
(Fig. 2). This fluorescence signal was not observed for 5a. Com-
pound 5a has single fluorescence signal at 710 nm for both 340 nm
and 675 nm excitation wavelengths (Fig. 2). As seen in Fig. 3 for
compound 5a, we observed fluorescence signal from singlet state
of aza-BODIPY core when aza-BODIPY core (675 nm excitation) or
methoxy group (340 nm excitation) was excited.
2.2. Optical measurement
UV–vis absorption spectra of aza-BODIPY derivatives were
recorded by using a scanning spectrophotometer (Shimadzu UV-
1800). Uncorrected emission and corrected excitation spectra in
the fluorescence measurements were obtained with Perkin Elmer
model LS 55 Fluorescence spectrometer.
Nonlinear optical absorption properties of aza-BODIPY deriva-
tives were investigated by using open aperture (OA) Z-scan
technique [21]. Laser source is mode-locked Ti:Sapphire laser
amplifier system with 1200 nm and 1310 nm wavelength, 45 fs
pulse duration and 1 kHz repetition rate (Spectra Physics, Spitfire
Pro XP, TOPAS). Laser beam was focused on solution in 1 mm thick
cell by a lens with 20 cm focal length. Concentrations of the solu-
tions for two photon absorption measurements were 0.01 M.
Wavelength dependent ultrafast pump probe spectroscopy
measurements were performed by using Ti:Sapphire laser
amplifier-optical parametric amplifier system (Spectra Physics,
Spitfire Pro XP, TOPAS) and commercial setup (Spectra Physics,
Helios). Pulse duration was measured as 100 fs. Wavelengths of the
pump beam were chosen according to the peak of the absorption
spectra as 700 nm for 5b, 6b and 650 nm for 5a, 6a. White light
continuum was used as a probe beam.
In an attempt to obtain deep insight on charge transfer mech-
anism and to investigate relationship between charge transfer
and excited state lifetime for aza-BODIPY compounds containing
triphenylamine and 4-ethynyl-N,N-dimethylaniline and methoxy
moiety we have performed ultrafast pump probe spectroscopy
experiments.
3. Results and discussion
3.1. Absorption and fluorescence measurement
UV–vis absorption and fluorescence spectra of aza-BODIPY
compounds (in THF) containing triphenylamine, 4-ethynyl-N,N-
dimethylaniline and methoxy moieties are shown in Figs. 1 and 2.
Optical parameters were summarized in Table 1. When 4-ethynyl-
N,N-dimethylaniline moiety is at position 5 of aza-BODIPY core,
bathochromic shift (about 20 nm) is observed in the absorption
spectra as observed previously in the literature [18]. All of the
3.2. Ultrafast pump probe spectroscopy experiments
Ultrafast pump probe spectroscopy with white light continuum
experiment was used to investigate the optical dynamics and decay
processes of excited states of aza-BODIPY derivatives containing
triphenylamine, 4-ethynyl-N,N-dimethylaniline and methoxy moi-
eties. The aza-BODIPY core in the absorption spectra (Fig. 1) has an

26
S. Tekin et al. / Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 23–28
Fig. 2. Fluorescence spectrum of (a) compounds 5b and 6b, (b) compounds 5a and 6a in THF solution (1 × 10−5 M). The excitation wavelengths are given in figure legends.
The figure inset indicates small fluorescence intensity scale between 0 and 5 for compounds 5b and 6b with excitation wavelength 773 nm.
intense maximum at approximately 700 nm for 5b, 6b and 650 nm
for 5a, 6a. Therefore, these wavelengths were used for the pump
beam to populate the excited states (S1) of aza-BODIPY.
wavelength were given in Fig. 5a. Compounds 5b and 6b have sim-
ilar decay processes. We have observed two decay times in Fig. 5a:
the fast decay time (less than 3 ps) and the slow decay time (around
couple hundred ps).
Compound 5a shows bleach signal around 660 nm (Fig. 6) which
corresponds to S0 → S1 transition in absorption spectra. Besides,
three nonlinear absorption signals can be observed around 440 nm,
565 nm and 780 nm wavelengths for compound 5a. These signals
represent excited state absorptions to higher energy levels (Fig. 3a).
Compounds 5a and 6a have similar transient absorption data with
30 nm shifts on wavelength scale (Fig. 6). This shift is compatible
with the absorption spectra of 5a and 6a.
Transient absorption data versus wavelength for various time
delays are shown in Fig. 4 for aza-BODIPY derivatives containing tri-
phenylamine moieties. 5b and 6b compounds show positive signal
bellow 575 nm and negative bleach signal above 575 nm in tran-
sient absorption spectra (Fig. 4). The positive signal is attributed to
electron transition from S1 state of aza-BODIPY to singlet state of
the moieties. Compound 5b has a single and broad bleach signal
between 600 and 800 nm and single and broad nonlinear absorp-
tion signal around 500 nm. Since linear absorption spectrum of this
compound is broadened, the observed broad nonlinear bleach sig-
nal corresponds to singlet state of moieties as seen in Fig. 3. Time
evolution of the nonlinear absorption of 5b and 6b at 490 nm probe
Two aza-BODIPY compounds containing methoxy groups have
different excited state lifetimes (Fig. 5b). While compound 5a has a
slow decay process (around thousand ps), compound 6a has faster
Fig. 3. Energy level diagrams for compounds (a) 5a and (b) 6a.

S. Tekin et al. / Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 23–28
27
Fig. 4. Transient absorption data versus wavelength for various time delays (a) compound 5b and (b) compound 6b in THF solution.
decay process (on the order of ultrafast time scales). This fast transi-
tion can be attributed to charge transfer from S1 state of aza-BODIPY
core to S0 state of 4-ethynyl-N,N-dimethylaniline group [19,22].
This leads to fluorescence quenching for compound 6a.
3.3. Two photon absorption properties
TPA properties were enhanced upon increasing charge transfer
for BODIPY and aza-BODIPY compounds [16,18,19]. We investi-
gated TPA properties of these compounds by OA Z-scan technique
[16] using mode-locked Ti: Sapphire laser amplifier system with
45 fs pulse duration and 1 kHz repetition rate. In order to calculate
TPA coefficient (ˇ) and TPCS (ꢀ2), Eq. (1) was used [20].
Nonlinear transmittance T is given in terms of laser intensity I0,
1
T(I0) =
(1)
1 + I0ˇl
is
where
l
optical
path
)
length.
TPCS
value
ꢀ2
(1 GM = 10−50 cm4 s photon−1
equation,
is obtained from the following
Fig. 5. Time evolution of the nonlinear absorption of (a) compounds 5b and 6b at a
probe wavelength of 490 nm and (b) compounds 5a and 6a at a probe wavelength
of 455 nm. The figure inset indicates slow decay time between 0 and 150 ps time
scales for compounds 5a and 6a.
hvˇ
NAd0 × 10−3
ꢀ2
=
(2)
where NA is the Avogadro number and d0 is molar concentration of
the solution.
OA Z-scan experimental results at 1200 nm wavelength and
6 × 1015 W/m2 peak intensity are shown in Fig. 7. It is known that
absorption spectrum can be shifted into NIR and nonlinear opti-
cal properties can be enhanced by using electron donating moiety
as a functional group for BODIPY structure [20]. Triphenylamine is
a strong electron donor moiety and absorption spectrum of com-
pounds containing triphenylamine is greatly red shifted. Thus, TPA
properties can be seen in wide spectral range (1200–1450 nm) and
ꢀ2 increases with increasing electron donating strength [19,20] and
molecule conjugation length [1]. Compounds 5b and 6b have bet-
ter nonlinear optical absorption (Fig. 7 and Table 1) and higher
TPCS (ꢀ2) values than compounds 5a and 6a at the same pulse
intensities due to increasing electron donating strength. TPCS (ꢀ2)
values of compound 5b for 1200 nm and 1310 nm wavelengths are
37 GM and 83 GM respectively. In order to enhance TPCS value,
functional groups were introduced with strong electron donat-
ing property and long conjugation length. Therefore, by using
4-ethynyl-N,N-dimethylaniline group as a functional group, we
obtained maximum ꢀ2 values for 6b as 83 GM and 126 GM at
1200 nm and 1310 nm wavelengths respectively. Similar effect was
observed for aza-BODIPY containing methoxy groups as well. It is
Fig. 6. Transient absorption data versus wavelength for compounds 5a and 6a at
1.76 ps.

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S. Tekin et al. / Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 23–28
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.jphotochem.
2013.02.002.
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In summary, we synthesized new aza-BODIPY compounds
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laser spectroscopic techniques. The two photon absorption cross
sections are obtained as 25, 37 and 83 GM at 1200 nm for 6a,
5b and 6b, respectively. Maximum two photon absorption cross
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Acknowledgement
We gratefully acknowledge the financial support by Scientific
and Technical Research Council of Turkey (TUBITAK) (No. 110T630).