Enhanced optical limiting performance of substituted metallo- naphthalocyanines with wide optical limiting window

Article abstract of DOI:10.1016/j.dyepig.2014.05.020

Octa-(4-tert-butylphenoxy) substituted naphthalocyanines coordinated with different central metals (Ga, In) were synthesized and their photophysical and optical limiting properties were investigated. The naphthalocyanines substituted peripherally with bul

Full text of DOI:10.1016/j.dyepig.2014.05.020

Dyes and Pigments 109 (2014) 144e150
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Enhanced optical limiting performance of substituted metallo-
naphthalocyanines with wide optical limiting window
Jian Xu 1, Jun Chen 1, Li Chen, Rui Hu, Shuangqing Wang*, Shayu Li, Jin Shi Ma,
Guoqiang Yang*
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190,
PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Octa-(4-tert-butylphenoxy) substituted naphthalocyanines coordinated with different central metals
(Ga, In) were synthesized and their photophysical and optical limiting properties were investigated. The
naphthalocyanines substituted peripherally with bulky groups have good solubility in organic solvents
and the Q band absorption shows an obvious red shift to the near-IR region at about 800 nm. Studies on
transient absorption spectra, photophysical parameters and optical limiting properties indicate that
these compounds exhibit good optical limiting performance. The optical limiting thresholds of the
naphthalocyanines are 0.26, 0.15 J cmꢀ2 for Gallium naphthalocyanines and Indium naphthalocyanines,
Received 31 March 2014
Received in revised form
12 May 2014
Accepted 13 May 2014
Available online 22 May 2014
Keywords:
respectively. The naphthalocyanines possess higher triplet state quantum yields (ФT) and triplet-minus-
Naphthalocyanine
Phthalocyanine
Optical limiting
Photophysics
ground state extinction coefficients (
DεT) than other simple phthalocyanines, which results in better
optical limiting behaviour.
© 2014 Elsevier Ltd. All rights reserved.
Near-IR absorption
Transient absorption
1. Introduction
response time [4]. The occurrence of RSA requires that the excited
state absorption cross-section sex is greater than that of the ground
state sg [5]. For practical applications of optical limiting effect, the
reverse saturable absorbers are desirable, which allows high
transmission of light at low optical fields over a large spectral
window [6]. Introduction of substituent groups and central metals
can effectively change the photophysical properties and improve
optical limiting behaviour [7].
Up to now, phthalocyanine-based materials are still being
investigated to improve their optical limiting performance and
expand their optical limiting window from visible to near-IR range
in order to meet the requirements of practical applications. Naph-
Since the discovery of the laser in the 1960s, it has been
extensively investigated and widely used as high-intensity light
sources in many fields such as communication, image processing,
optical storage and other applications [1]. Laser techniques brings
enormous advantages for human society, while it also brings a
potential hazard for human eyes and optical sensors. Therefore,
there is a strong need to develop optical limiting materials for
protection of optical sensors and human eyes from hostile or
accidental intense laser pulses [2]. In the past years, some materials
have been studied to meet this challenge, including fullerenes,
porphyrins, phthalocyanines (Pcs) and other organicmetallic com-
pounds [3]. Among them, the Pcs with two-dimensional highly
thalocyanines have larger
p electron conjugation system than
phthalocyanines, which will result in different photophysical
properties and optical limiting behaviour and should be potential
compounds to meet this challenge [8]. As a kind of potential optical
limiting materials, different naphthalocyanines and metal naph-
thalocyanines have been studied in theoretical and experimental
aspects by some research groups [3i,9]. The results showed that the
optical limiting properties vary from different structures, even only
different substituents might change the optical limiting properties
dramatically. Our group had investigated the optical properties of a
series of peripherally substituted phthalocyanines with different
conjugated delocalized
p-electron system and metal-coordination
bond are outstanding optical limiting materials because of their
low linear absorption in the optical limiting wavelength window,
strong reverse saturable absorption (RSA) as well as an ultrafast
* Corresponding authors. Fax: þ86 10 82617315.
E-mail addresses: g1704@iccas.ac.cn (S. Wang), gqyang@iccas.ac.cn (G. Yang).
Co-first author.
1
http://dx.doi.org/10.1016/j.dyepig.2014.05.020
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

J. Xu et al. / Dyes and Pigments 109 (2014) 144e150
145
central metals [10]. The results indicated that the bulky substituted
groups could enhance the solubility and decrease the molecular
pep interaction, ultimately resulting in the improvement of the
optical limiting performance. For the purpose of providing useful
information for the practical applications in the field of optical
limiting materials, we synthesized octa-(4-tert-butylphenoxy)
substituted metallo-naphthalocyanines, extensively studied their
photophysical and optical limiting properties, and analysed the
difference of properties between naphthalocyanines and
phthalocyanines.
2.2.2. 1,2-Dibromo-4,5-bis(dibromomethyl)benzene (2)
A mixture of 1 (26.4 g, 0.1 mol), NBS (90 g, 0.5 mol) and 2,20-
azobisisobutyronitrile (AIBN, 0.1 g, 0.06 mmol) were added to CCl4
(200 mL), after stirring for 15 h at 95 ꢂC under a UV lamp irradia-
tion, the mixture was filtered while hot and the filtrate was evap-
orated to obtain an yellowish solid. The crude product was
recrystallized from CCl4 and dried at 45 ꢂC under vacuum to obtain
53.0 g of 2 (yield, 91%). Mp: 122 ꢂC; 1H NMR (DMSO-d6, 400 Hz):
8.09 (s, 2H), 7.62 (s, 2H) ppm; MS-EI (m/z): 579 (Mþ), 498 (MþꢀBr),
420 (Mþꢀ2Br), 339 (Mþꢀ3Br), 260 (Mþꢀ4Br).
2. Experimental
2.2.3. 2,3-Dicyano-6,7-dibromo-naphthalene (3)
A mixture of 2 (14.5 g, 0.025 mol), fumaronitrile (2.0 g, 0.03 mol)
and NaI (6.0 g, 0.04 mol) in DMF were stirred for 13 h at 80 ꢂC under
a nitrogen atmosphere. After the reaction mixture was cooled
overnight, it was poured into saturated NaHSO3 solution (400 mL),
the obtained yellowish precipitate was filtered, washed with water
and dried under vacuum at 80 ꢂC. The crude product was recrys-
tallized from CH2Cl2 to obtain 12.0 g white powder of 3 (yield, 71%).
Mp >250 ꢂC;1H NMR (DMSO-d6, 400 Hz): 8.91 (s, 2H), 8.75 (s, 2H)
ppm; MS-EI (m/z): 336 (Mþ), 255 (MþꢀBr), 1780 (Mþꢀ2Br).
2.1. General
All organic solvents were commercially available, dried and
distilled by appropriate methods before use. Phthalocyanines of 5b
and 6b were synthesized by a method based on our published
papers [10]. 1H NMR spectra were performed on a DPX400 Bruker
FT-NMR spectrometer with DMSO-d6 as solvent and tetrame-
thylsilane (TMS) as internal standard. The mass spectra were ob-
tained on a Biflex MALDI-TOF or Micromass GCT-MS spectrometer.
Elemental analyses were performed on a Carlo Erba-1106 elemental
analyzer. UVevis absorption spectra were recorded on a Hitachi U-
3010 spectrophotometer.
2.2.4. 2,3-Dicyano-6,7-di(4-tert-butyl)phenoxyl naphthalene (4)
A mixture of 3 (3.4 g, 10 mmol), 4-tert-butylphenol (3.3 g,
22 mmol) and anhydrous potassium carbonate (11.0 g, 80 mmol)
was added to dry DMF (30 mL) and stirred at 100 ꢂC for 12 h under
nitrogen condition. After the reaction mixture was cooled, it was
poured into cold water (100 mL), the obtained white-yellowish
precipitate was collected by suction filtration. The crude product
was crystallized from alcohol to give 4.3 g of white crystals 4 (yield,
90%). Mp: 205e208 ꢂC;1H NMR (DMSO-d6, 400 Hz): 8.29 (s, 2H),
8.07 (s, 2H), 7.48e7.51 (d, J ¼ 7.6 Hz, 4H), 7.07e7.10 (d, J ¼ 8.8 Hz,
4H), 1.38 (s, 18H) ppm; MS-EI (m/z): 474 (Mþ), 459 (MþꢀCN), 389
(MþꢀC6H12)), 326 (MþꢀtBuPhO-).
Fluorescence spectra were recorded by a Hitachi F-4500 fluo-
rescence spectrophotometer. Fluorescence quantum yields (ФF) of
S1 were determined by the comparative method using zinc
phthalocyanine in 1-chloronaphthalene (ФF ¼ 0.30) as reference
standard [11]. The fluorescence lifetimes of these phthalocyanines
and naphthalocyanines were investigated with single-photon
counting technique with Edinburgh FL900 spectrophotometer.
Transient absorptions at nanosecond time scale were investi-
gated in argon-saturated THF solution at the concentration of
5 ꢁ 10ꢀ6 M. The excitation light was the harmonic of Nd:YAG laser
(Continuum Surelite II, 355 nm and 7 ns FWHM). The signals were
detected by Edinburgh LP900 and recorded on Tektronix TDS 3012B
oscilloscope and computer. The triplet-minus-ground state
2.2.5. Octa-(4-tert-butylphenoxyl) gallium naphthalocyanine (5a)
Compound 4 (2.4 g, 5 mmol) was added to dry 1-pentanol
(30 mL) containing 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU,
1.5 mL) as a catalyst. The mixture was stirred for 1 h at 100 ꢂC under
a nitrogen atmosphere and then anhydrous GaCl3 (0.26 g,1.5 mmol)
was added. The mixture was slowly brought to boiling over 1 h and
then refluxed for 36 h, after the reactant was cooled to room
temperature, methanol/water (1:1, 60 mL) mixture was added, the
obtained yellow-green precipitate was filtered, washed with hy-
drochloric acid (5%, 50 mL) and methanol (50 mL), and then puri-
fied by a silica-gel column with CH2Cl2/ethanol (10:1) as eluent. The
final product was dried at 50 ꢂC under vacuum overnight to give
1.7 g (yield: 67%) of 5a (C128H120N8O84GaCl). Mp >250 ꢂC; UVevis
(THF) lmax: 799, 340 nm; 1H NMR (DMSO-d6, 400 Hz): 9.05 (s, 8H),
8.29 (s, 8H), 7.51e7.53 (d, J ¼ 7.6 Hz, 16H), 7.36e7.39 (d, J ¼ 7.2 Hz,
16H), 1.38 (s, 72H) ppm; MALDI-TOF-MS (m/z): 2001.1(Mþ),
extinction coefficients (DεT) were calculated by the method of total
depletion or saturation [12]. The quantum yields of the triplet state
were determined by the comparative method [13], using unsub-
stituted ZnPc in 1-chloronaphthalene as reference standard
(
ФT ¼ 0.65). The triplet lifetimes were obtained by kinetic analysis
of the transient absorption.
The optical limiting properties were measured by the standard
setup of our previously reported method [10]. All samples were
dissolved in THF, placed in a 1.0 cm path length quartz cell and the
solutions were bubbled with pure argon for about 30min to remove
the dissolved O2. A 532 nm ns Nd:YAG laser (Continuum Surelite II,
7ns FWHM) was used as the laser source.
2.2. Synthesis
1966.2(MþꢀCl),
1817.1(MþeCleOR),
Elemental analysis (%), calculated for C128H120N8O84GaCl: C 76.73,
H 6.04, N 5.59; found C 76.73, H 5.76, N 5.96.
1669.0(MþeCle2OR);
2.2.1. 4,5-Dibromo-o-xylene (1)
Iodine (1.6 g, 6.3 mmol) was added to o-xylene (40 g, 0.38 mol)
and stirred under ice-water mixture, then bromine (124 g,
0.77 mol) was dropped slowly to keep the temperature at ꢀ5 to
2.2.6. Octa-(4-tert-butylphenoxyl) indium naphthalocyanine (6a)
Compound 6a was prepared by a similar method to compound
5a in 58% yield (C128H120N8O8InCl). Mp >250 ꢂC; UVevis (THF)
lmax: 802, 339 nm; 1H NMR (DMSO-d6, 400 Hz): 9.17 (s, 8H), 8.41 (s,
8H), 7.56e7.67 (d, J ¼ 8.8 Hz,16H), 7.37e7.45(d, J ¼ 8.8 Hz,16H),1.39
(s, 72H) ppm; MALDI-TOF-MS (m/z): 2046.2 (Mþ), 2010.6 (MþꢀCl),
1861.5(MþeCleOR); Elemental analysis (%) [14], calculated for
0
ꢂC. After stirring for 20 h, the mixture was dissolved in ether,
washed with aqueous potassium hydroxide and fresh water for
three times, dried with anhydrous MgSO4 and evaporated the sol-
vent to obtain a semi-solid crude product. After recrystallization
from alcohol, 61.5 g pure 4,5-dibromo-oxylene 1 was obtained
(yield, 62%). Mp: 85e88 ꢂC; 1H NMR (DMSO-d6, 400 Hz): 7.36 (s,
2H), 2.18 (s, 6H) ppm; MS-EI (m/z): 264 (Mþ), 185 (MþꢀBr), 104
(Mþꢀ2Br).
C128H120N8O8InCl: C 75.04, H 5.90, N 5.47; found C 75.72, H 5.74, N
5.76.

146
J. Xu et al. / Dyes and Pigments 109 (2014) 144e150
3. Results and discussion
3.1. Synthesis and characterization
Two different central metal (Ga and In) naphthalocyanines with
peripheral substituents of 4-tert-butylphenoxy at two -positions
g
were synthesized (shown in Scheme 1). o-Xylene was brominated
to give 1,2-dibromo-4,5-dimethylbenzene 1 [15],1 was successively
brominated with NBS on the two methyl groups under UV irradi-
ation using AIBN as catalyst in CCl4 to obtain 2 [16], then cyclization
of 2 with fumaronitrile to obtain dibrominated dinitriles 3 [17].
After alkyloxylation of 3 by 4-tert-butylphenol, the precursor of
newly substituted dinitrile 4 was obtained. The cyclization of the
dinitrile 4 with MCl3 (M ¼ Ga, In) resulted in the corresponding
naphthalocyanines 5a and 6a in 1-pentanol at 140 ꢂC using DBU as
catalyst under N2 atmosphere. This lower reaction temperature in
cyclization of dinitriles resulted in good yields [18]. The naph-
thalocyanines were purified by silica-gel column and then subli-
mated under vacuum to remove all of the residual impurity. All
naphthalocyanines were characterized by elemental analysis and
spectroscopic methods including UVevis, 1H NMR and MALDI-TOF-
MS, which were consistent with the proposed structures. It is worth
pointing out that these naphthalocyanines exhibit excellent solu-
bility in many organic solvents such as THF, chloroform, DMF and
DMSO, which is suitable for the investigation of photophysical and
optical limiting properties in solution.
3.2. Ground state absorption and fluorescence emission
Fig. 1. UVevis absorption spectra of naphthalocyanines of 5a, 6a and phthalocyanines
5b, 6b at the concentration of 5.0 ꢁ 10ꢀ5 M (a) and the absorption spectra of 6a in THF
at different concentrations, the insert is the value of OD vs the concentration (b).
Ground state absorptions of naphthalocyanines were investi-
gated (shown in Fig. 1). Naphthalocyanines 5a and 6a display the
Q bands at 799 and 802 nm, respectively. They are significantly
red-shifted compared with the corresponding phthalocyanines
with the same central metal 5b (714 nm) and 6b (722 nm),
states. Meanwhile, the B bands of the naphthalocyanines only
have slight red-shifts which results in wider optical limiting
windows. According to the wavelength at the intersection points
of the normalized absorption and fluorescence spectra, the energy
because of the larger
p electron conjugation system of naph-
thalocyanine reducing the energy gap between the S0 and S1
Scheme 1. Synthetic route of the naphthalocyanines 5a, 6a and the structures of the reference samples 5b and 6b. a) Br2, I2, ꢀ5 to 0 ꢂC, 16 h; b) NBS, AIBN, UV irradiation, CCl4, 8 h;
c) fumaronitrile, NaI, DMF, 80 ꢂC, 10 h; d) 4-tert-butylphenol, K2CO3, DMF, 100 ꢂC, 8 h; e) MCl3, DBU, 1-pentanol, 36 h.

J. Xu et al. / Dyes and Pigments 109 (2014) 144e150
147
Table 1
Table 2
Photophysical parameters of naphthalocyanines and phthalocyanines.
Parameters of triplet state for naphthalocyanines 5a, 6a and phthalocyanines 5b, 6b.
l/nm
l
/nm ε(Q)/Mꢀ1 cmꢀ1 Emission Emission ФF
tF(S1)/ E (S1)/
Compounds lmax
/
ε0/Mꢀ1 cmꢀ1
D
εT/Mꢀ1 cmꢀ1 tT
/m
s
kST/sꢀ1
ФT
(Q)
(B)
(Q)/nm
(B)/nm
(S1)
ns
ev
nm (Tn)
5a 799 340 2.1 ꢁ 105
6a 802 339 1.7 ꢁ 105
5b 714 331 1.5 ꢁ 105
6b 722 335 1.1 ꢁ 105
809
813
723
735
411
411
406
403
0.066 2.51
0.028 0.71
0.429 3.66
0.030 0.76
1.55
1.54
1.73
1.71
5a
6a
5b
6b
610
620
580
590
3.0 ꢁ 103
2.6 ꢁ 103
1.2 ꢁ 103
6.1 ꢁ 102
12.8 ꢁ 104
12.6 ꢁ 104
6.4 ꢁ 104
7.2 ꢁ 104
71.4 4.2 ꢁ 109 0.70
8.3 2.1 ꢁ 1010 0.72
114.2 3.5 ꢁ 108 0.51
24.1 2.7 ꢁ 109 0.62
level of S1 state is about 1.5ev for naphthalocyanines and about
1.7ev for phthalocyanines, respectively, and they are summarized
in Table 1. UVevis absorptions of 6a in THF at different concen-
trations were investigated (Fig. 1b), which follows the Beer's Law
when the concentration is below 1.0 ꢁ 10ꢀ4 M. This result is in
agreement with the assumption that the aggregation can be
effectively suppressed by introducing the peripheral substituents
and the axial chloride atom.
Fluorescence emission spectra were obtained as shown in Fig. 2.
For naphthalocyanines 5a and 6a, the fluorescence emissions
display shorter Stokes shifts (7 nm) than that of phthalocyanines 5b
and 6b (15 nm). However, the lower energy gap between S1 and S0
states of naphthalocyanines 5a, 6a results in lower fluorescence
quantum yields than phthalocyanines 5b and 6b, as summarized in
Table 1.
phthalocyanines of 5b and 6b at 580 nm and 590 nm, which should
be caused by a smaller energy gap between T1 and Tn states of the
naphthalocyanines than that of phthalocyanines.
The triplet state quantum yields (
crossing (kST) were evaluated [10] and summarized in Table 2. The
values of T are 0.70 and 0.72 for 5a and 6a, which are higher than
ФT) and the rate of intersystem
Ф
that of 0.51 and 0.62 for 5b and 6b, respectively. The kST values of
naphthalocyanines are one order of magnitude larger than corre-
sponding phthalocyanines. The values of ФT and kST in Table 2
suggest that naphthalocyanines have lower energy gap between
S1 and T1 states, leading to higher probability of intersystem
crossing from S1 to T1 states. In addition, the triplet-minus-ground
state extinction coefficient (
efficient of ground state absorption (ε0) at 532 nm are evaluated
and summarized in Table 2. The values of εT are higher for
DεT) [10] and the molar extinction co-
D
naphthalocyanines which would result in a stronger T1-Tn ab-
sorption and finer optical limiting effect.
3.3. Transient absorption
Decays of triplet state were investigated as shown in Fig. 4(b).
The transient absorption of naphthalocyanines 5a and 6a was
investigated at the concentration of 5.0 ꢁ 10ꢀ5 M with excited
wavelength at 355 nm (Fig. 3). All compounds exhibit strong and
broad T1eTn absorption at about 600 nm with clear bleaching at Q
and B bands. The lmax of transient absorptions of 5a and 5b at
610 nm and 620 nm respectively, are red-shift compared with the
The lifetimes of T1 state, tT, are 71.4, 8.3
ms for 5a and 6a, and 114.2,
24.1 s for 5b and 6b, respectively. Compared with the phthalocy-
m
anines, the naphthalocyanines possess shorter triplet state life-
times. However, for the nanosecond pulse laser, the lifetimes are
long enough to achieve the reverse saturable absorption (RSA) and
optical limiting behaviour.
Fig. 2. a) Fluorescence emission spectra of naphthalocyanines 5a, 6a and phthalocyanines 5b, 6b. b) Decay profiles of naphthalocyanine 5a and phthalocyanine 5b.
Fig. 3. Transient absorption spectra of naphthalocyanines 5a and 6a (excited at 355 nm).

148
J. Xu et al. / Dyes and Pigments 109 (2014) 144e150
Fig. 4. (a) Transient absorption spectra of 5a, 6a and 5b, 6b at the time when
DOD reached to highest value after excitation. (b) Decay profiles of triplet state for naphthalocyanines
and phthalocyanines.
3.4. Optical limiting properties
loxy]-naphthalocyaninato lead) is as low as 0.10 J cmꢀ2 [3i], which
maybe result from additional heavy atom effect. Dini and Hanack
Fig. 5 shows the optical limiting performance in 70% linear
transmittances TLin at 532 nm of 5a and 6a in argon-saturated THF.
It is obvious that naphthalocyanines 5a and 6a exhibit better optical
limiting behaviour than corresponding phthalocyanines 5b and 6b.
The optical limiting thresholds are 0.26, 0.15 J cmꢀ2 for naph-
thalocyanines 5a and 6a, and 0.46, 0.22 J cmꢀ2 for phthalocyanines
5b and 6b, respectively. In addition, the nonlinear attenuation
factors (NAF) are 10.4, 16 for 5a and 6a, and 8.8, 14 for 5b and 6b,
respectively. All of the parameters suggest that naphthalocyanines
possess better optical limiting behaviour than the corresponding
phthalocyanines. The Sun group has published a similar naph-
thalocyanine indium chloride (2,3-octa(3,5-di-tert-butylphenoxy)-
2,3-naphthalocyaninato indium chloride) [9a], in nanosecond op-
tical limiting studies in 70% linear transmittances TLin at 532 nm,
the optical limiting threshold is 0.27 J cmꢀ2. This optical limiting
threshold value is comparable to those in our studies, but the op-
tical limiting threshold of lead naphthalocyanine with bromo
substituents (2,(3)-tetrabromo-3,(2)-tetra[(3,5-di-tert-butyl)pheny
published the same naphthalocyanines indium chloride as the Sun
group reported, and also resulted that the naphthalocyanines dis-
played good optical limiting properties [9b].
As previously discussed for the naphthalocyanine, the enlarged
p
electron conjugation system results in a lower energy of S1 state
and higher rate of intersystem crossing (kST), which leads to higher
probability of intersystem crossing from S1 to T1 states. Corre-
spondingly, higher values of
Meanwhile, naphthalocyanines possess longer wavelength of TeT
absorption and higher value of εT. Generally, high values of T and
εT may result in good optical limiting properties for the reverse
ФT is observed for naphthalocyanines.
D
Ф
D
saturable absorption (RSA) mechanism, which has been discussed
in some published papers [4,10]. All of these parameters indicate
that the naphthalocyanines have stronger reverse saturable ab-
sorption (RSA) and better optical limiting behaviour, and which is
capable to meet the challenge in practical applications for optical
limiting materials, especially in wide wavelength range of optical
limiting window.
Fig. 5. Optical limiting behaviour of naphthalocyanines 5a, 6a compared to phthalocyanines 5b and 6b.

J. Xu et al. / Dyes and Pigments 109 (2014) 144e150
149
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