Synthesis, optical properties and ultrafast dynamics of aza-boron-dipyrromethene compounds containing methoxy and hydroxy groups and two-photon absorption cross-section

Article abstract of DOI:10.1016/j.jphotochem.2012.08.003

In this work, aza-boron-dipyrromethene (aza-BODIPY) compounds containing methoxy and hydroxy groups were synthesized. The effects of substitution and intramolecular charge transfer on linear and nonlinear optical absorptions (especially two photon absorpt

Full text of DOI:10.1016/j.jphotochem.2012.08.003

Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 24–29
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis, optical properties and ultrafast dynamics of
aza-boron-dipyrromethene compounds containing methoxy and hydroxy groups
and two-photon absorption cross-section
Betül Küc¸ üköza, Mustafa Hayvalıb,∗, Halil Yılmazb, Birhan Ug˘uza, Ulas¸ Kürüma, 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:
Received 21 May 2012
Received in revised form 27 July 2012
Accepted 7 August 2012
Available online 16 August 2012
In this work, aza-boron-dipyrromethene (aza-BODIPY) compounds containing methoxy and hydroxy
groups were synthesized. The effects of substitution and intramolecular charge transfer on linear and
nonlinear optical absorptions (especially two photon absorption) of these BODIPY derivatives were inves-
tigated. Drastic spectral changes of aza-BODIPY compounds containing hydroxy group were observed in
linear absorption spectra depending on basicity of solution. New bands in linear absorption spectra of
phenolate form of these compounds appeared bellow 500 nm and above 700 nm in THF solution. These
findings were caused by occurrence of the intramolecular charge transfer state in the phenolate form.
Ultrafast pump probe spectroscopy experiments revealed that phenolic form shows only bleaching sig-
nal around 600 nm. On the other hand, the phenolate form showed fast growing nonlinear absorption
signals bellow 500 nm and above 700 nm regions in addition to the bleaching signal around 600 nm.
Intramolecular charge transfer state occurs above 700 nm and the lifetime of this state is found on the
order of ultrafast time scales. Since the excitation of the fluorescence experiments is at 600 nm there is
no fluorescence signal due to the relaxation of the energy through intramolecular charge transfer state.
Aza-BODIPY compounds containing methoxy group did not show any two photon absorption proper-
ties due to weak electron-donating strength of methoxy group while aza-BODIPY compounds containing
hydroxyl showed two photon absorption properties in wide spectral range (1200–1450 nm). Our results
also indicated that intramolecular charge transfer enhances two photon absorption properties.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Aza-BODIPY
Intramolecular charge transfer
Fluerescence quenchimg
Z-scan
Ultrafast pump probe spectroscopy
Two-photon absorption technique
1. Introduction
Near-infrared (NIR) spectral range has been less studied although
there is an increasing need for devices presenting TPA properties
Materials with large two photon absorption (TPA) properties
have attracted increasing attention in relation with a wide range of
potential applications including optical limiting [1,2], TPA imaging
microscopy [3], three-dimensional microfabrication [4,5], optical
data storage [6,7], photodynamic therapy [8] and TPA upcon-
version lasing [9]. Therefore, design and synthesis of molecules
with large TPA cross-sections (TPCS) has received a lot of atten-
tion [10,11]. Many organic materials have been developed for
TPA applications [12]. However, emitting and two photon active
wavelengths of these organic materials are mostly located around
400–600 nm and 700–1100 nm spectral ranges, respectively [12].
in this spectral range such as signal processing in the telecom-
munication wavelengths (1.30–1.55 ␮m). Boron-dipyrromethene
(BODIPY) dyes are novel organic dyes with high fluorescence quan-
tum yields, excellent photo stability, good solubility [13–15] and
high TPCS [16–18]. It was found that linear absorption wavelength
of BODIPY dyes can be tuned to NIR (above 600 nm) by a deriva-
tization reaction with benzaldehyde derivatives [19–21]. One of
the various strategies to design and synthesize highly efficient two
photon active compounds is manipulating electron-donor and/or
acceptor properties and their incorporation in a molecular archi-
tecture [22–26]. In general, increasing electron-donor strength
leads to increasing TPCS [12]. Recently, Bouit et al. reported
that functionalized aza-BODIPY dyes exhibit strong TPA proper-
ties in the telecommunications spectral range [27]. Intramolecular
charge transfer (ICT) within BODIPY has also attracted considerable
attention due to its effect on linear absorption and fluorescence
properties [28].
∗
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 (A. Elmali).
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2012.08.003

B. Küc¸ üköz et al. / Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 24–29
25
O N
O
2
O
O
O
R
2
R
R
2
2
K PO
3
H
CH NO (Et) NH
4
3
2
2
EtOH
Reflux MeOH
+
R
R
R
1
1
1
2a R = OCH , R = H
1a R = OCH , R = H
1
3
2
1
3
2
2b R = OCH , R = OCH
3
1b R = OCH , R = OCH
1
3
2
1
3
2
3
NH OAc
4
Reflux EtOH
R
R
R
1
3
R
1
1
R
R
1
1
R
R
2
R
2
R
2
R
R
2
2
2
2
N
N
N
BBr /CH Cl
2
3
2
BF .OEt
3
+
+
N
N
Diisopropylethylamine
CH Cl
N
N
NH
N
-
-
B
B
2
2
F
F
F
F
(MHABDP) R = OCH , R = H, R = OH
(DMABDP) R = OCH , R = H
3a R = OCH , R = H
1
3
2
3
1
3
2
1
3
2
3b R = OCH , R = OCH
3
(DHABDP) R = OH, R = H, R = OH
(TMABDP) R = OCH , R = OCH
3
1
3
2
1
2
3
1
3
2
(THABDP) R = OH, R = OH, R = OH
1
2
3
Scheme 1. Structures of newly synthesized methoxy (MABDP) and hydroxy (HABDP) substituted aza-BODIPY in the present study.
In this work, we investigate ICT properties of newly synthe-
(TMABDP) were synthesized by reactions of 3a and 3b with
diisopropylethylamine and boron trifluoride diethyl ether-
ate in dry CH2Cl2. Finally, hydroxy derivatives of aza-BODIPY,
BF2-chelated-[3-(4-hydroxyphenyl)-5-phenyl-1H-pyrrol-2-yl][3-
(4-methoxyphenyl)-5-phenylpyrrol-2-ylidene]amine (MHABDP),
BF2-chelated-[3-(4-hydroxyphenyl)-5-phenyl-1H-pyrrol-2-yl][3-
(4-hydroxyphenyl)-5-phenylpyrrol-2-ylidene]amine (DHABDP)
and BF2-chelated-[3-(3,4-dihydroxyphenyl)-5-phenyl-1H-pyrrol-
2-yl][3-(4-dihydroxyphenyl)-5-phenylpyrrol-2-ylidene]amine
(THABDP) were obtained from methoxy derivatives of correspond-
ing aza-BODIPY’s via hydrolyses with BBr3 than purified by flash
column chromatographic purification. The structural identification
sized aza-BODIPY compounds containing methoxy and hydroxy
groups and study the effect of ICT on the TPA properties in the
telecommunication spectral range. Therefore, we introduce vari-
ous phenolic and phenolate forms of aza-BODIPY molecules with
different electron-donating strength to study the effects of sub-
stitution and ICT on linear and nonlinear absorptions (especially
TPA).
2. Experimental
2.1. Synthesis of aza-BODIPY
of the newly synthesized compounds was established from 1H, 13
C
NMR, mass, and optical techniques (see supplementary material).
The syntheses of methoxy (MABDP), hydroxyl (HABDP)
aza-BODIPY and phenolate ABDPn− forms of aza-BODIPY com-
pounds involved a multistep approach as shown in Scheme 1,
and the details are given in the supplementary material.
3-(4-Methoxyphenyl)-1-phenylpropenone (1a) and 3-(3,4-
dimethoxyphenyl)-1-phenylpropenone (1b) were synthesized
2.2. Optical measurement
The UV–Vis absorption spectra of aza-BODIPY derivatives were
recorded by using a scanning spectrophotometer (Shimadzu UV-
1800). The fluorescence measurements, uncorrected emission and
corrected excitation spectra were obtained with Perkin Elmer
model LS 55 Fluorescence spectrometer.
Wavelength dependent pump probe measurements were
performed using Ti:Sapphire laser amplifier–optical parametric
amplifier system with 45 fs pulse duration and 1 kHz repetition rate
(Spectra Physics, Spitfire Pro XP, TOPAS). In order to investigate
intramolecular charge transfer nature of derivatives, pump probe
experimental setup (Spectra Physics, Helios) with white light con-
tinuum was used. Experiments were conducted with 600 nm pump
wavelength and 120 fs pulse duration inside pump probe setup.
Nonlinear optical absorption properties of aza-BODIPY deriva-
tives were investigated by using open aperture Z-scan technique
[29]. Laser source is mode-locked Ti:Sapphire laser amplifier sys-
tem with 45 fs pulse duration and 1 kHz repetition rate. Laser beam
from
a reaction of the corresponding benzaldehyde, ace-
tophenone, and potassium phosphate. These compounds were
subsequently reacted with nitromethane and diethylamine
in dry ethanol to obtain respective 3-(4-methoxyphenyl)-
4-nitro-1-phenylbutan-1-one
yphenyl)-4-nitro-1-phenylbutan-1-one
(2a)
and
(2b).
3-(3,4-dimethox-
[3-(4-Methox-
yphenyl)-5-phenyl-1H-pyrrol-2-yl][3-(4-methoxyphenyl)-5-
phenylpyrrol-2-ylidene]amine (3a) and [3-(3,4-dimethox-
yphenyl)-5-phenyl-1H-pyrrol-2-yl][3-(3,4-dimethoxyphenyl)-
5-phenylpyrrol-2-ylidene]amine (3b) were synthesized by
reactions of 2a and 2b with ammonium acetate in ethanol. Then,
BF2-chelated-[3-(4-methoxyphenyl)-5-phenyl-1H-pyrrol-2-yl][3-
(4-methoxyphenyl)-5-phenylpyrrol-2-ylidene]amine (DMABDP)
and BF2-chelated-[3-(3,4-dimethoxyphenyl)-5-phenyl-1H-pyrrol-
2-yl][3-(3,4-dimethoxyphenyl)-5-phenylpyrrol-2-ylidene]amine)

26
B. Küc¸ üköz et al. / Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 24–29
Table 1
Spectroscopic absorbance properties of aza-BODIPY with concentration 5 × 10−6
M
in THF.
Compound
R1
R2
R3
ꢀabs (max/nm)
FWHMabs (cm−1
)
DMABDP
TMABDP
MHABDP
DHABDP
THABDP
–OCH3
–OCH3
–OCH3
–OH
–H
–OCH3
–H
–H
–OH
–
–
–OH
–OH
–OH
664
662
664
660
654
1304
1604
1615
2251
3968
–OH
654 and 664 nm, depending on the substituent group. When the
electron-donating group is at the para position of the phenyl rings
(␣) attached to pyrrole in aza-BODIPY, significant bathochromic
shift is observed in the absorption spectra. On the other hand,
when the electron-donating group is at the ␤ position of pyrrole
ring, there is no considerable change in the absorption maxima as
observed in Fig. 1 [30]. Full width half maximum (FWHM) values
of the absorption bands and maximum absorption wavelengths are
given in Table 1.
Fig. 1. Absorption spectra of aza-BODIPY in THF solution.
In an attempt to introduce photoinduced ICT, we added NaH
to phenolic solution and obtained ionic phenolate form. In order
to check whether chemical degradation occurs upon addition of
base, we checked the reversibility of the compounds by adding
trifluoroacetic acid. We also checked the formation of the pheno-
late form by adding dropwise KOH solution (in THF). Depending
on the amount of KOH, a gradual change between phenol and phe-
nolate form was observed. Fig. 2a–c shows the absorption spectra
of the investigated phenolic/phenolate compounds in THF. Upon
increasing the amount of KOH solution for DHABDP, the most inten-
sive peak located at 666 nm decreased while new bands below
500 nm and above 700 nm appeared (Fig. 2e). New bands were
also observed in the literature [31,32]. Killoran et al. studied the
effect of pH changes on absorption spectrum of dimethylamino
was focused on solution in 1 mm thick cell by a lens with 20 cm focal
length. Concentrations of the solutions for two photon absorption
measurement are 0.056 M.
3. Results and discussion
3.1. Steady-state absorption and fluorescence measurement
UV–Vis absorption spectra of aza-BODIPY containing methoxy
and hydroxy groups in THF are shown in Fig. 1. All of the absorp-
tion spectra of studied aza-BODIPY molecules reveal strong So → S1
transition with maximum absorbance wavelength varying between
Fig. 2. (a–c) Normalized absorption spectrum of phenol and phenolate forms of aza-BODIPY, (d) fluorescence spectrum, (e) spectral changes of DHABDP derivative depending
on NaH saturated solution (in THF).

B. Küc¸ üköz et al. / Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 24–29
27
Fig. 3. Femtosecond transient absorption spectra at different time intervals: (a) DHABDP, (b) ABDP2−. The figure inset indicates fast decay of transient bands corresponding
to intramolecular charge transfer state.
derivatives of aza-BODIPY [31]. It was found that by decreasing
the pH of dimethylamino derivatives of aza-BODIPY, absorption
band around 800 nm also decreased due to decreasing electron-
donating properties of amine group [31]. Very recently, Amin
et al. investigated electrochemistry and photodynamics of ferro-
azadipyrromethane (ADP) donor–acceptor dyads and triads [32].
They observed spectral changes due to formation of charge sep-
aration during the first reduction of aza-BODIPY compound ADP
in benzonitrile using a thin layer spectroelectrochemical cell. Dur-
ing the course of reduction, the peak located at 658 nm diminished
in intensity with concurrent appearance of new bands at 446 and
820 nm. Observed new bands were assigned to aza-BODIPY radical
anion (ADP*−) and confirmed charge separation [32].
3.2. Ultrafast pump probe spectroscopy studies
Ultrafast wavelength-dependent pump probe spectroscopy
technique was used to investigate photodynamics and nonlinear
optical properties of DHABDP (phenol) and ABDP2− (phenolate).
Femtosecond transient absorption spectra for phenol/phenolate
forms show different characteristics as shown in Fig. 3. While
phenol form shows only bleaching signal around 600 nm, fast grow-
ing nonlinear absorption signals appear bellow 500 nm and above
700 nm regions in phenolate form in addition to the bleach sig-
nal around 600 nm. The signals at upper and lower wavelengths
correspond to ICT states of aza-BODIPY as observed in Fig. 2e.
Upon excitation of the deprotonated molecules the aza-BODIPY
band shifts (now the S2 state) slightly hypsochromically as can be
seen best for DHABDP while at the same time there appears another
band above 700 nm corresponding to the direct excitation of this
ICT state (the S1 state) (Fig. 2). As seen in Fig. 3, the excited state
absorption (positive signal) is formed immediately after excitation
and it decays at the same rate as the ground state depletion (bands
around 600 and 675 nm). There is actually a isosbestic point close
to 530 nm indicating that we have only two species: The extremely
While methoxy derivatives of aza-BODIPY showed sharp
absorption band, hydroxy derivatives of aza-BODIPY showed
broader absorption band (Figs. 1 and 2). Oxy form of aza-
BODIPY especially di- and tetra-oxy derivatives (phenolate
ABDPn−
(Fig. 2).
)
showed the most broadened absorption band
It was previously shown that aza-BODIPY structures showed
high extinction coefficients and fluorescence quantum yields
[13–15]. Phenolic form of MHABDP and DHABDP revealed a
single fluorescence band at 730 nm (Fig. 2d). However, steady-
state fluorescence was not observed for the phenolate form of
MHABDP and DHABDP. Besides, there is no fluorescence sig-
nal for the phenol form of THABDP as well. THABDP compound
is more ionic than MHABDP and DHABDP. Therefore, charge
transfer may occur in the phenol form of THABDP. Recently, phe-
nol/phenolate dependent fluorescence quenching was observed
for BODIPY [33–36]. Fluorescence quenching was attributed to
photoinduced electron transfer between phenolate and BOD-
IPY subunit. It was also found that significant fluorescence
quenching in BODIPY occurs by intramolecular energy trans-
fer [37]. This property was used as highly sensitive fluorescent
probe.
Phenolate form of aza-BODIPY including high electron-donating
moiety that is –O− (oxy) results in ICT. Observed new bands above
700 nm in absorption spectra (Fig. 2e) during phenol/phenolate
transition correspond to ICT state. Presence of nonradiative decay
of such state can be very fast and hence fluorescence will not be able
to compete efficiently with the nonradiative decay. In an attempt to
understand the relationship between fluorescence properties and
ICT state we have performed ultrafast pump probe spectroscopy
experiments.
Fig. 4. Effects of different number of phenol substituent on transient absorption
spectra at 681 nm probe wavelength. The figure inset indicates slow decay time
between 2 and 3 ns time scales.

28
B. Küc¸ üköz et al. / Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 24–29
Fig. 5. (a) Experimental results (filled circle) and theoretical fits (solid line) of open aperture Z-scan experiments for MHABDP, ABDP−, DHABDP and ABDP2− in THF, using
1200 nm femtosecond pulses. (b) TPCS ␴ comparison for phenol and phenolate form of ABDP.
2
fast (by internal conversion following excitation in the S2 state)
populated ICT state decays directly to the ground sate on a ps
time scale (inset of Fig. 3b). Since the excitation of the fluorescence
experiments is at 600 nm there is no fluorescence signal (Fig. 2b)
due to the relaxation of the energy through ICT state above 700 nm.
In general, BODIPY [38] and aza-BODIPY [39] structures have
singlet–triplet transition with intersystem crossing (ISC) mecha-
nism. Adarsh et al. measured the triplet lifetimes in the order of
microseconds with nanosecond laser flash photolysis study [39].
We have observed three decay times in Fig. 4: The fast decay (less
than 1 ps), the middle decay (around couple hundred ps) and slow
decay time (on the order of ␮s). The measured long lifetime may
be related to the lifetime of the triplet state in phenol form for
MHABDP, DHABDP, THABDP in Fig. 4. Upon increasing electron-
donating property, relative lifetime shortened as observed in the
literature [32].
to weak electron-donating strength of methoxy group. OA Z-scan
experimental results at 1200 nm wavelength and 2.61 × 1015 W/m2
peak intensity are shown in Fig. 5a and b. MHABDP, DHABDP
and THABDP samples show TPA properties in wide spectral
range (1200–1450 nm) and ꢁ2 increases with increasing electron-
donating strength (Fig. 5b). Fig. 5a and b also shows the effect of
ICT on the TPA properties in the phenolate form. MHABDP and
ABDP− have approximately the same ꢁ2 values (Fig. 5b). On the
other hand, ABDP2− has larger ꢁ2 than DHABDP. Zhang et al. studied
effects of various groups such as phenyl, carbazole and tripheny-
lamino on TPCS [41]. They found that ꢁ2 increases with the strength
of electron-donating properties. Formation of the phenolate form
of THABDP causes broadening of linear absorption peak, thus we
could not observe nonlinear optical absorption at investigated
wavelength range. We obtained maximum ꢁ2 value for THABDP
derivative with 12.8 GM at 1200 nm wavelength due to strength of
electron-donating property.
BODIPY and aza-BODIPY compounds exhibit strong TPA behav-
ior [18,27]. Therefore, it is interesting to investigate TPA properties
and the effect of ICT.
4. Conclusion
3.3. Two photon absorption properties
In summary, we synthesized new aza-BODIPY compounds con-
taining methoxy and hydroxy group. Obtaining phenolate forms
of aza-BODIPY containing hydroxyl group caused drastic changes
in linear absorption spectra. New bands in phenolate form of
these compounds appeared bellow 500 nm and above 700 nm in
linear absorption spectra in THF solution. No fluorescence sig-
nals were observed with 600 nm excitation for phenolate forms.
Ultrafast pump probe spectroscopy experiments confirmed occur-
rence of intramolecular charge transfer state above 700 nm with
ultrafast lifetime. Therefore, 600 nm excitation in steady-state flu-
orescence experiments did not reveal any fluorescence signal due to
the relaxation of the energy through intramolecular charge trans-
fer state above 700 nm. The investigated aza-BODIPY compounds
containing hydroxy group (HABDP) reveal two photon absorption
properties in 1200–1450 nm spectral range. Two photon absorption
cross-section values increase with increasing electron-donating
strength. Our results also indicated that intramolecular charge
transfer enhances two photon absorption properties. These find-
ings may open the path to synthesize new BODIPY compounds
with good two photon absorption properties in telecommunication
wavelength range.
TPA properties were investigated by open aperture (OA) Z-scan
technique [18,29] 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), we used the following
equations [40]:
Nonlinear transmittance T is given in terms of laser intensity I0,
1
T(I0) =
(1)
1 + I0bl
where l is optical path length. ꢁ2 (1 GM = 10−50 cm4 s photon−1) is
obtained from the following equation,
hvˇ
NAd0 × 10−3
ꢁ2
=
(2)
where NA Avagadro number and d0 is molar concentration of the
solution.
By using electron-donating moiety as a functional group for
BODIPY structure the absorption spectrum can be shifted into NIR
and nonlinear optical properties can be enhanced [40]. DMABDP
and TMABDP compounds did not show any TPA properties due

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29
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