Synthesis and nonlinear optical properties of fluorine-containing naphthalocyanines

Article abstract of DOI:10.1002/chem.200204683

The fluorine-containing metal naphthalocyanines [F₁₆NcGaCl] (7) and [(F₁₆NcGa)₂O] (8), which represent the first examples of peripherally fluorine substituted naphthalocyanines, were synthesized, and the nonlinear optical transmission was studied. Peripheral substitution by fluorine atoms enhances the solubility and photostability of the naphthalocyanines. In particular, for the axially μ-oxo-bridged naphthalocyanine dimer 8, practically no aggregation was observed in organic solvents and it has proved to be an efficient optical limiter when irradiated with laser light pulses at the wavelength of 532 nm, with pulse duration of 5 ns and repetition rate of 20 Hz.

Full text of DOI:10.1002/chem.200204683

FULL PAPER
Synthesis and Nonlinear Optical Properties of Fluorine-Containing
Naphthalocyanines
Guo Ying Yang,[a, b] Michael Hanack,*[a] Yiew Wang Lee,[b] Yu Chen,[a]
May Ka Yuen Lee,[b] and Danilo Dini[a]
Abstract: The fluorine-containing met-
al naphthalocyanines [F16NcGaCl] (7)
and [(F16NcGa)2O ] (8), which represent
the first examples of peripherally fluo-
rine substituted naphthalocyanines,
were synthesized, and the nonlinear
optical transmission was studied. Pe-
ripheral substitution by fluorine atoms
enhances the solubility and photostabil-
ity of the naphthalocyanines. In partic-
ular, for the axially m-oxo-bridged naph-
thalocyanine dimer 8, practically no
aggregation was observed in organic
solvents and it has proved to be an
efficient optical limiter when irradiated
with laser light pulses at the wavelength
of 532 nm, with pulse duration of 5 ns
and repetition rate of 20 Hz.
Keywords: fluorine
¥
gallium
¥
naphthalocyanine ¥ nonlinear optics
also have good thermal and photo stability, as well as good
processability. Nevertheless, few materials have been devel-
oped so far with satisfactory physical properties as well as
optical performances in protecting the human eye or an
optical sensor. Phthalocyanine-based materials, though
among the best materials, are still being investigated to
improve their optical-limiting properties and to expand their
optical-limiting window from visible to near IR range.[14]
Recently, we found that naphthalocyanine (Nc)-based
materials are also promising for optical-limiting applications
owing to their expanded ring structure and red-shifted
transmission window.[14] Their more extended p-electron
systems provide more pronounced nonlinear optical proper-
ties than that in the phthalocyanine (Pc) analogues, due to the
higher levels of electronic susceptibility.[15] Also, their
Q-bands are shifted to the near IR at about 800 nm, and the
hue of Nc solutions are thus determined by the B-band
absorption in the blue region at about 400 nm. We have
successfully synthesized a series of InIII Ncs in attempt to
meet the above requirements by increasing their solubility in
organic solvents, decreasing aggregation, and thus maintain-
ing their long lifetime of the excited state and good optical-
limiting properties.[16, 17] Attempts to obtain Ncs with im-
proved photostability are still on-going.
Introduction
Since their invention in 1960s,[1] lasers are extensively inves-
tigated and used as a high-intensity light sources in optical
communication,[2] optical storage,[3] image processing,[4] envi-
ronmental monitoring,[5] surgery,[6] and military applications.[7]
The need for protection of optical sensors and human eyes
from accidental or hostile lasers has since then received
increasing attention.[7] In particular, it is of great importance
to develop passive optical limiters, which respond on the ns or
ps timescale, to protect sensors and human eyes from intense
laser pulses. In recent years, many materials have been
proposed and developed to meet this challenge,[8] including
porphyrins,[9] phthalocyanines,[10] fullerenes,[11] organometallic
compounds,[12] and indanthrone dyes.[13] Among them, phtha-
locyanines are especially attractive because of their great
reverse saturable absorption (RSA) properties, displaying
relatively low linear absorption (or high transmittance) for
light of low intensity, but high absorption (or low trans-
mittance) for intense light. However, in order to meet the
stringent requirement for sensors or human eye protectors, in
addition to be good optical limiters over the entire operating
wavelength band of the sensor system, the materials should
We report here on a novel approach for Nc modification,
namely introducing fluorine atoms on the peripheral positions
of the Nc ring. Fluorine substituents are known to enhance the
solubility of, for example, Pc,[18] in common organic solvents,
and, moreover, it is expected that the electron-withdrawing
character of fluorine substituents may improve the photo-
stability of the electronrich Nc rings against photoinduced
oxidation. Furthermore, our recent results have demonstrated
[a] Prof. Dr. Dr. h. c. M. Hanack, Dr. G. Y. Yang, Prof. Dr. Y. Chen,
Dr. D. Dini,
Institut f¸r Organische Chemie, Universit‰t T¸bingen
Auf der Morgenstelle 18, 72076 T¸bingen (Germany)
Fax : (‡49)7071-295268
E-mail: hanack@uni-tuebingen.de
[b] Dr. Y. W. Lee, M. K. Y. Lee, Dr. G. Y. Yang
DSONational Laboratories, 20 Science Park Drive
118230 Singapore (Singapore)
2758
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DOI: 10.1002/chem.200204683
Chem. Eur. J. 2003, 9, 2758 2762

2758 2762
that the solubility could be increased and the aggregation
could be prevented by introducing suitable peripheral as well
as axial substituents in Pc or Nc compounds, for example,
NMR signals for the fluorine atoms appear at d ˆ À145.0 (m,
2F) and À148.0 ppm (m, 2F).
The synthesis of [F16NcGaCl] (7) and [(F16NcGa)2O ] (8) is
shown in Scheme 2. Compound 7 was obtained by the
reaction of 6 with GaCl3 under N2 atmosphere at 2008C for
[20]
[tBu4PcGa(p-tmp)],[19] [(tBu4PcGa)2O], [(tBu4PcIn)2O],
[21]
[(tBu4NcGa)2O], and [{tBu4PcIn(tmed)}2].[22] All of these
materials have shown promising optical-limiting properties in
the visible region. Hence, hexadecafluorogalliumnaphthalo-
cyanine [F16NcGaCl] (7) was chosen as the target molecule,
and the m-oxo-bridged dimer [(F16NcGa)2O ] (8) was designed
to increase the solubility and block the aggregation further. To
our knowledge, these two molecules represent the first
examples of peripherally fluorine substituted naphthalocya-
nines.
Results and Discussion
As a precursor for the synthesis of [F16NcGaCl] (7), 1,2-
dicyano-4,5,6,7-tetrafluoronaphthalene (6) was chosen. The
synthetic route for 6 is shown in Scheme 1. 1,2,3,4-Tetrafluoro-
5,8-dihydro-5,8-N-methylaminonaphthalene (3) was obtained
Scheme 2. Synthesis of 7 and 8.
one hour. Compound 7 is slightly soluble in THF. The UV-
visible spectrum, exhibiting broad bands, shows that 7 is
heavily aggregated in THF (Figure 1). Compared to the
peripherally nonsubstituted NcGaCl,[26] which is practically
Scheme 1. Synthesis of precursor 6.
from a reaction that was slightly modified based on the
published method.[23] The intermediate, tetrafluorobenzyne
(2), generated from the reaction of n-butyllithium with
pentafluorochlorobenzene in diethyl ether, undergoes a
Diels Alder reaction with 1-methylpyrrole smoothly to
afford 3 in moderate yield, which is then converted to N-
methyl-4,5,6,7-tetrafluoroisoindole (4)[24] by reaction with 3,6-
di-(2-pyridyl)-1,2,4,5-tetrazine, leading to the loss of the
Figure 1. UV/Vis Spectra of 7 in THF (solid line) and 1-chloronaphthalene
(dashed line).
À
ˆ
À
CH CH moiety. The monomer 6 is then obtained (yield:
66.7%) by reacting N-methyl-4,5,6,7-tetrafluoroisoindole (4)
with dicyanoacetylene,[25] followed by the addition of 3-chloro-
peroxybenzoic acid to remove the methylamino group
insoluble in common organic solvents, the solubility improve-
ment for 7, however, is quite significant, confirming that the
fluorine atoms do help to improve the solubility. The
aggregation in solution can be effectively suppressed when 7
is dissolved in 1-chloronaphthalene, as shown by the sharper
absorption bands in its UV-visible spectrum (Figure 1, dashed
line). In 1-chloronaphthalene, 7 displays the Q-band at
804.0 nm and the metal-to-ligand/ligand-to-metal charge
transfer (MLCT/LMCT) bands at 763.0, 717.0 nm. The MS-
À
( NMe) in a one-pot reaction. Compound 6 was fully
characterized by spectroscopic methods and elemental anal-
ysis. The molecular peak of 6 appears at m/z 250.0 in the MS
spectrum. In the IR spectrum, the absorption at 2236.0 cmÀ1 is
1
À
characteristic of the CN groups. The H NMR signal for the
two protons in 6 is located at d ˆ À8.58 ppm (s) and the 19F
Chem. Eur. J. 2003, 9, 2758 2762
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M. Hanack et al.
FULL PAPER
‡
FAB spectrum of 7 shows peaks at m/z ˆ 1103.8 [M ], 1068.9
‡
[M À Cl].
[(F16NcGa)2O ] (8) was obtained by the reaction of 7 with
concentrated H2SO4 at À10 to À208C, based on an approach
reported by us earlier.[20] The solubility of 8 in common organic
solvents is greatly improved when compared to that of 7. In
contrast to 7, in its UV-visible spectrum, compound 8 has a
sharp Q-band at 764.0 nm, the MLCT/LMCT bands at 725.0
and 686.0 nm, and the B-band at 339.0 nm even in THF
(Figure 2, solid line). The absorption pattern of 8 is character-
Figure 3. Changes of the UV/Vis absorption spectra of 8 before and after
exposure to indoor ambient light: a) before exposure (––), b) after one
week (- - - -).
Figure 2. UV/Vis Spectra of 8 in THF (solid line) and 1-chloronaphthalene
(dashed line).
Figure 4. Four-level model of reverse saturable absorption (sequential
two-photon absorption).
istic of Nc derivatives. The UV-visible spectrum is in agree-
ment with our earlier findings that the aggregation can be
effectively suppressed by axial substitution.[27] The diminished
aggregation of 8 is further demonstrated when its UV-visible
spectrum is recorded in 1-chloronaphthalene as solvent
(Figure 2, dashed line). The absorption pattern is retained,
with a general red-shift of all absorption bands; this is due to
the different solvent effect. The field desorption (FD) mass
spectrum of 8 shows a single peak at m/z ˆ 556.9 [M4‡‡H2O],
while under FAB mode, the spectrum gives a cluster of peaks
second photon; this leads the absorbing system to the excited
triplet state T2.
An efficient reverse saturable absorption (RSA) material
should have a high ratio of excited-state (T1 ! T2) to ground-
state (Sg ! S1) absorption cross section (sex/sg ꢀ 1), a rapid
intersystem crossing rate (tISC ꢁ t), a long internal conversion
lifetime (tIC ꢀ t), a high intersystem crossing quantum yield
(fS !T ꢂ 1), and a long triplet lifetime (tT ꢀ t).[31]
1
1
1
‡
centered at m/z ˆ 2155.2, which is assigned to [M ]. The
Due to the comparable low solubility of F16NcGaCl (7),
optical limiting was only determined for (F16NcGa)2O (8) in
THF (Figure 5). For comparison, optical-limiting measure-
ments were also conducted for C60, [tBu4NcGaCl],[21] and
1H NMR spectrum displays a signal at d ˆ À8.59 ppm and the
19F NMR signals for the fluorine atoms are found at d ˆ
À148.4 (m, 8F) and À156.5 ppm (m, 8F).
The photostability of 7 and 8 dissolved in THF was checked
by comparing the change of their UV-visible spectra before
and after light exposure. The intensities of the Q- and B-bands
for 7 decreased gradually with increased exposure to daylight
for one week without much change in its pattern. The change
for 8 under the same conditions is much less significant
(Figure 3), thus demonstrating that the dimer is quite stable.
Active materials for optical-limiting display low linear
absorption (or high transmittance) for light with low intensity,
and high absorption (or low transmittance) for very intense
light.[28] For reverse saturable absorption materials, such as
naphthalocyanines, this process can be explained by using a
simplified four-level model (Figure 4).[29, 30]
When the singlet ground state (Sg) of a molecule passes to
the first singlet excited state (S1) after absorption of a photon
emitted by a laser with a pulse width t, intersystem crossing
occurs and the absorbing system converts into the first triplet
state (T1). In the first triplet state T1, the system can absorb a
Figure 5. Comparison of the optical limiting performance for various Ncs
and [(F16NcGa)2O ] (8).
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Chem. Eur. J. 2003, 9, 2758 2762

Naphthalocyanines
2758 2762
[21]
[(tBu4NcGa)2O]. All of the samples in solution had the
molecular level, the cause of the enhanced optical-limiting
behavior in 8 is probably due to the involvement of the strong
electron-attracting nature of the fluorine substituents, im-
proved dipole moments and a higher effective sex/sg ratio.
linear transmission of 75% at 532 nm. It was found that the
optical-limiting performance for 8 is better than that for C60
and the nonfluorinated Nc, [(tBu4NcGa)2O]. The optical-
limiting threshold, which is defined as the average input
fluence at which the output fluence is 50% of what is
predicted by the linear transmission, for 8 and [(tBu4Nc-
Ga)2O] are 450 and 2150 mJcmÀ2, respectively (Table 1). As
Experimental Section
General: All syntheses were carried out under nitrogen atmosphere. FT-
IR: Perkin Elmer Spectrum 1000, UV/Vis: Shimadzu UV-365, MS: Varian
Mat 711 (FD: temperature of the ion source: 308C; FAB: temperature of
the ion source: 508C), 1H and 19F NMR: Bruker Avance DRX250
(235.334 MHz).
1,2,3,4-Tetrafluoro-5,8-dihydro-5,8-N-methylaminonaphthalene (3):[23] n-
Butyllithium (1.6m in hexane; 27.5 mL, 44.0 mmol) was added dropwise
to a magnetically stirred solution of 1 (5.2 mL, 40.0 mmol) in dry Et2O
(100 mL) under N2 at À788C. The solution was stirred for 1 h at À788C and
then treated dropwise with 1-methylpyrrole (14.4 mL, 80.0 mmol). The
mixture was further stirred for 1 h at that temperature and then allowed to
warm slowly to room temperature and stirred overnight. The brown
solution obtained was washed with 4n H2SO4 (3 Â 50 mL), and the aqueous
layer was basified with 4n NaOH and extracted with Et2O(3 Â 100 mL).
The organic layer was dried over NaSO4, evaporated to afford crude 3,
which was purified by crystallization from n-hexane: white crystals, 4.6 g,
yield: 50.2%, m.p. 74.5 76.08C (lit. [23] m.p. 75 768C).
N-Methyl-4,5,6,7-tetrafluoroisoindole (4):[24] 3,6-Bis(2-pyridyl)-1,2,4,5 -tet-
razine (472.0 mg, 2.0 mmol) was added in one portion to a magnetically
stirred solution of 3 (458.0 mg, 2.0 mmol) in CH2Cl2 (40 mL). The resulting
magenta solution was stirred for 3 h at room temperature. The solution was
then evaporated and purified by chromatography on neutral Al2O3 with
CH2Cl2 as eluent to give 4: 340.0 mg, 83.7%, m.p. 176 177.58C (lit. [24]
m.p. 1788C).
Table 1. Optical limiting thresholds for the tested materials.
Compounds
Optical limiting threshold [mJcmÀ2
]
8
450
C60
1248
2150
3945
[(tBu4NcGa)2O]
[tBu4NcGaCl]
these two compounds share the basic Nc structure and the
same metal (gallium), it is highly possible that they have the
similar intersystem crossing rates, internal conversion life-
times, intersystem crossing quantum yields, and triplet life-
times. However, due to the electron-withdrawing effect of the
fluorine atoms, the Q-bands of 8 are significantly blue-shifted
[32]
relative to those of [(tBu4NcGa)2O].
Apparently, their
difference in the ground state absorption, and probably the
excited state absorption too, leads to a different ratio of their
excited-state (T1 ! T2) to ground-state (Sg ! S1) absorption
cross section (sex/sg). Based on the optical-limiting measure-
ment results, the ratio of effective sex/sg for 8 is higher than
that for compound [(tBu4NcGa)2O]. Comparable results were
also found with fluorinated metal Pcs, which also showed
increased effective sex/sg ratio in comparison with the non-
fluorinated Pcs.[33] The improvement was attributed to the
augmentation of the difference of the dipole moment between
the excited states associated with the optical transition
responsible for the optical-limiting effect.
1,2-Dicyano-4,5,6,7-tetrafluoronaphthalene
(6):
Dicyanoacetylene[25]
(114.0 mg, 1.5 mmol) was added slowly to a magnetically stirred solution
of 4 (305.0 mg, 1.5 mmol) in CHCl3 (20 mL) at 08C. The reaction mixture
was warmed to room temperature and stirred for 1 h. 3-Chloroperoxyben-
zoic acid (77% max; 656.5 mg, ꢂ3.0 mmol) was added in one portion, and
the mixture was stirred at room temperature overnight. The solution was
washed with aqueous NaOH solution (2n, 50 mL Â 3, to remove 3-chlor-
obenzoic acid) and water (50 mL Â 3), and then dried over Na2SO4. After
evaporation, the residue obtained was subjected to flash chromatography
on silica gel with CHCl3 as eluent to yield 6: 250.0 mg, 66.7%, m.p. 215
‡
216.58C; MS (m/z): 250.0 [M ]; 1H NMR (CDCl3): d ˆ 8.58 ppm (s); 19F
NMR (CDCl3, C6H5CF3 as reference): d ˆ À145.0 (m, 2F), À148.0 ppm
(m, 2F); IR (KBr): nÄ ˆ 2236.0 cmÀ1 ( CN); elemental analysis calcd (%)
À
for C12H2N2F4: C 57.62, H 0.81, N 11.20, F 30.38; found: C 57.82, H 0.61, N
11.30, F 30.80.
Conclusion
[F16NcGaCl] (7): Compound 6 (100.0 mg, 0.4 mmol) and GaCl3 (176.0 mg,
1.0 mmol) were mixed and heated to 2008C for 1 h. The reaction mixture
was dissolved in 1-chloronaphthalene and added dropwise into methanol
(100 mL). The precipitate was collected and washed with methanol to give
In conclusion, we have synthesized and characterized the
hexadecafluoronaphthalocyanine materials, [F16NcGaCl] (7)
and [(F16NcGa)2O ] 8(), which are the first examples of
peripherally fluorine-substituted Ncs. Our results have shown
that the solubility and photostability of these new Ncs can be
significantly improved by the introduction of fluorine atoms
on the peripheral positions of the Nc ring. The high photo-
stability of 7 and 8 is attributed to the electron-withdrawing
property of the fluorine atoms, which reduce the electron
density in the Nc ring and make its oxidation more difficult
when irradiated. We have again demonstrated that the axial
dimeric Nc system, represented here by compound 8, is
effective in improving the solubility and suppressing aggre-
gation. In fact, no observable aggregation was found for 8 in
THF. The high solubility of 8 in THF allowed the measure-
ment of its optical limiting property at 532 nm in solution.
Compound 8 displayed a better optical-limiting performance
than C60 and the nonfluorinated [(tBu4NcGa)2O]. At a
‡
a dark blue product (40.0 mg, 36.2%). MS-FAB (m/z): 1103.8 [M ], 1068.9
‡
[M À Cl]; IR (KBr): nÄ 1655.0 (m), 1606.0 (m), 1498.0 (vs), 1377.0 (vs),
1129.0 (s), 1093.0 (s), 1068.0 (w), 996.0 (vs), 972.0, (vs) 893.0 (w), 862.0 (w),
809.0 (w), 760.0 (w), 742.0 (w), 681.0 (m), 639.0 (w), 512.0 (w), 494.0 (w),
440.0 cmÀ1 (w); UV/Vis (THF): lmax ˆ 765.0, 718.0, 338.0 nm; UV/Vis (1-
chloronaphthalene): lmax ˆ 804.0, 763.0, 717.0 nm; elemental analysis calcd
(%) for C48H8ClGaN8F16: C 52.16, H 0.73, N 10.14, Cl 3.21; found C 49.62,
H 0.90, N 9.61, Cl 3.23.
[(F16NcGa)2O] (8): Compound 7 (40.0 mg, 0.018 mmol) was dissolved in
conc. H2SO4 (20 mL) at À10 to À208C and stirred for 2 h. The dark blue
reaction mixture was quenched with ice-water (200 mL), filtered, and dried
to obtain a dark green solid (18.0 mg, 46.0%). MS-FD (m/z): 556.9
‡
[M4‡‡H2O]; MS-FAB (m/z): 2155.2 [M ]; IR (KBr): nÄ ˆ 1655.0 (m), 1604.0
(m), 1496.0 (vs), 1377.0 (vs), 1189.0 (w), 1130.0 (s), 1094.0 (s), 1068.0 (w),
996.0 (vs), 977.0 (vs), 897.0 (w), 864.0 (w), 810.0 (w), 763.0 (w), 745.0 (w),
682.0 (m), 640.0 (w), 516.0 (w), 439.0 cmÀ1 (w); UV/Vis (THF): lmax ˆ 764.0,
725.0, 686.0, 339.0 nm; UV/Vis (1-chloronaphthalene): lmax ˆ 805.0, 763.0,
716.0, 339.0 nm; 1H NMR ([D8]THF): d ˆ 8.59 ppm (s); 19F NMR
Chem. Eur. J. 2003, 9, 2758 2762
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¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Hanack et al.
FULL PAPER
([D8]THF, C6H5CF3 as reference): d ˆ À148.4 (m, 8F), À156.5 ppm (m,
8F). elemental analysis calcd (%) for C96H16Ga2F32N16O: C 53.50, H 0.75, N
10.40; found: C 51.02, H 0.94, N 9.26.
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conducted by using a laser light with wavelength of 532 nm, a pulse
duration of 5 ns, and a repetition rate of 20 Hz. The pulses were generated
by a frequency-doubled Q-switched Nd/YAG laser. A small part of the
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Acknowledgement
We acknowledge the financial support for this work by DSONational
Laboratories (Singapore), Directorate of Research and Development
(Singapore), and Deutsche Forschungsgemeinschaft (Ha 280/165-1).
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Received: December 18, 2002 [F4683]
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