α-Octa(isopentoxy)anthralocyanine has been synthesized and is found to have an unprecedented low-energy Q-band absorption and a low first oxidation potential

Bedworth, Peter V.

Marder, Seth R.

Perry, Joseph W.

English

Peter V. Bedworth

Joseph W. Perry

Seth R. Marder

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The synthesis of a symmetrically substituted α-octa(isopentoxy)anthralocyanine

Caltech Authors - Main

N
H
O-isopentylisopentyl-O
N
O-isopentyl
O-isopentylN
H
N
isopentyl-O O-isopentyl
N
isopentyl-O
isopentyl-O N
N
N
1
OH
OHO
O O
O
O
O
Cl
Cl
OH
OH
CN
CN
OMe
OMe
CN
CN
vi
i, ii
iii
iv
v
The synthesis of a symmetrically substituted a-octa(isopentoxy)anthralocyanine
Peter V. Bedworth,a Joseph W. Perrya,b and Seth R. Marder*a,b
a Beckman Institute, 139-74 California Institute of Technology, Pasadena, CA 91125, USA
b Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
a-Octa(isopentoxy)anthralocyanine has been synthesized
and is found to have an unprecedented low-energy Q-band
absorption and a low first oxidation potential.
In an effort to synthesize dyes that may find use in optical
limiting devices1,2 we have been investigating phthalocyanine
complexes (Pcs). The Pcs have a region of relatively weak
absorption from ca. 450–650 nm which falls between their Q
and B bands. In addition, in this same spectral region they have
strong excited-state absorption in their singlet manifold and
even stronger absorption in their triplet manifold.1 Conse-
quently, in this spectral range the molecules are relatively
transparent under weak irradiation, but under intense irradia-
tion, such as from a pulsed laser source, it is possible to create
a significant excited-state population that will be relatively
opaque in the same spectral range. When the Pcs are
functionalized with heavy atoms it is also possible to enhance
the intersystem crossing rate so as to generate a large triplet
population. Since the excited-states of these molecules are
strongly absorbing, and it is possible to saturate the ground to
excited-state transition, they are referred to as reverse saturable
absorbers (RSAs) and are useful in optical limiting devices.1,3,4
For certain applications where it is important to protect sensors
from intense laser pulses in the 600–900 nm range, it is
necessary to move the transparency window (the region
between the high energy side of the Q band and the low energy
side of the B band) to lower energy. It has been demonstrated
that the transparency window of the phthalocyanines can be
tuned by a number of structural modifications.5 One approach is
to benzannulate the phthalocyanine molecules and another is to
substitute the rings with electron donors, thereby shifting the Q
and B bands and therefore the transparency window to lower
energy. We have also attempted to make these molecules
soluble in common organic solvents despite structural features
that would impart limited solubility. This is necessary in order
to make dyes that are useful for many device configurations.
Octaalkoxyphthalocyanines and octaalkoxynaphthalo-
cyanines,5 as well as a number of alkyl substituted anthralo-
cyanines,6–9 have been previously described. Here we describe
the synthesis, linear optical properties and electrochemistry of
a-octaisopentoxyanthralocyanine 1.
The synthetic pathway to octaalkoxyanthralocyanine 1 is
shown in Scheme 1. The method is analogous to the synthesis of
the octaalkoxynaphthalocyanines and tetra-tert-butylanthralo-
cyanine.9 The anthracene-1,4-dione was prepared by literature
methods.10,11 For 2,3-dichloroanthracene-1,4-dione, the
method of Langa was modified. Chlorine gas was added in a
vigorous stream to a solution of anthracene-1,4-dione in boiling
acetic acid, which was interrupted for 20 min every 5 min, until
the reaction was deemed complete as evidenced by 1H NMR
spectroscopy. The red solution with yellow crystalline precip-
itate was allowed to cool to ambient temperature and then
filtered. The yellow crystalline solid was dried under vacuum
(0.01 Torr) over KOH, to give the title compound in 75%
yield.10,11 2,3-Dicyano-1,4-dihydroxyanthracene was prepared
in moderate yield by treating 2,3-dichloroanthracene-1,4-dione
with an excess of KCN in hot EtOH.12 O-Methylation was
accomplished with dimethyl sulfate in the presence of K2CO3.
The synthesis of 1 from 2,3-dicyano-1,4-dimethoxyanthracene
was analogous to the synthesis of the corresponding
a-octaalkoxynaphthalocyanine to yield 15% of the desired
product after purification. As was consistent with the reactivity
of other alkoxy Pcs,13 we observed complete trans alkoxylation
of the methyl group by the isopentyl group from the solvent
(there were no observable singlet methyl resonances by 1H
NMR spectroscopy) under the reaction conditions.13 It is also
important to note that 1 is fairly sensitive to silica gel. Thus, the
isolated yield of 1 is significantly reduced if chromatography is
performed at conventional flow rates and accordingly, flash
Scheme 1 Reagents and conditions: i, NaBH4, MeOH; ii, H2O, HCl; iii, Cl2,
AcOH, reflux; iv, KCN, EtOH; v, dimethyl sulfate, K2CO3, acetone; vi,
lithium isopentoxide, isopentyl alcohol, reflux
Chem. Commun., 1997 1353
20
15
10
5
0
300 500 700 900 1100 1300
l / nm
e 
/ 1
04
 
M
–
1  
cm
–
1
150010005000–500–1000–1500–2000
–3
–2
–1
0
1
2
3
4
I /
 µ
A
E / mV
chromatography techniques are necessary. All compounds gave
satisfactory spectroscopic and analytical data.†
The UV–VIS/NIR spectrum of 1 (Fig. 1) shows a small
splitting between the Qx and Qy bands and the bands are sharp,
characteristic of phthalocyanines. The Q bands for 1 occur at
980 and 954 nm. These bands are red shifted about 110 nm
compared to the analogous octaalkoxynaphthalocyanine and
red shifted 80 nm compared to the tetra-tert-butylanthralocya-
nine.6 The B bands of 1 are bathochromically shifted compared
to the B bands of the octabutoxynaphthalocyanine, giving
solutions of 1 a pleasant violet hue. The transparency window
has moved about 100 nm bathochromically compared to the
Npcs.6
The cyclic voltammogram of 1 (Fig. 2) shows four quasi-
reversible redox events; those at E1/2 = 21279 and21039 mV
we attribute to sequential one electron reductions and those at
35 and 366 mV we attribute to sequential one electron
oxidations. The separation of the inner-most redox events
correlates well with the energy of the Q band. We have shown
that through a systematic approach we have moved the
transparency window of a Pc to a region centred about 700 mm.
This will allow for the investigation of the utility of this
compound and derivatives thereof as the active elements for
optical limiters in a particularly important spectral range. 
We thank the Air Force Office of Scientific Research and the
US Wright Patterson Air Force Base for funding this effort.
P. V. B. thanks The James Irvine Foundation for a postdoctoral
fellowship.
Footnotes
* E-mail: srm@cco.caltech.edu
† 2,3-Dicyano-1,4-dimethoxyanthracene. To a slurry of 2,3-di-
chloroanthracene-1,4-dione (50 g, 185 mmol) in EtOH (1 l) at 60 °C under
argon was added a solution of KCN (61 g, 925 mmol) in water (240 ml). The
mixture was stirred at 60 °C for 2 h then filtered. The solution was then
cooled and neutralized with 3 m HCl (carefully in a hood with a sodium
hypochlorite trap). The brown solid precipitate was collected and dried
overnight over KOH under vacuum. To a mixture the crude 2,3-dicyano-
1,4-dihydroxyanthracene (34 g) and K2CO3 (76 g, 548 mmol) in acetone
(150 ml) was added dimethyl sulfate (40 ml, 411 mmol). The mixture was
heated to reflux for 3 h then poured into water. The brown solid residue was
further washed with water then dried under vacuum over P2O5. The bright
yellow product was collected by sublimation then recrystallized from
MeOH to give 5 g of the title product. 1H NMR (300 MHz, CD2Cl2): d 8.85
(s, 2 H), 8.14 (4 line m, 2 H), 7.69 (4 line m, 2 H), 4.39 (s, 6 H);  nmax(KBr)/
cm21 3062, 2949, 2226, 1452, 1351, 1029, 886, 749; lmax(CHCl3)/nm 404,
388, 370.
Compound 1. To a slurry of 2,3-dicyano-1,4-dimethoxyanthracene (2 g,
6.9 mmol) in anhydrous isopentyl alcohol (40 ml) at 25 °C under argon was
added lithium (5 mg). The mixture was heated to reflux for 1 h then allowed
to cool to room temperature and acetic acid (10 ml) was added and stirred
overnight at room temperature under argon. The mixture was neutralized
with deoxygenated saturated aqueus NaHCO3 and extracted with deoxy-
genated methylene CH2Cl2. The organic layer was washed with deoxygen-
ated water then dried with Na2SO4 (if MgSO4 was used a considerable
amount of AncMg was formed). The solvent was removed in vacuo and the
residue was quickly chromatographed (solvent flow rate was about four
times as fast as a standard flash column, if chromatography was run at a
conventional rate the yield was greatly attenuated) on a short (125 3 45
mm) silica gel column with deoxygenated solvent (1 : 1, cyclohexane–
CH2Cl2) to give 10% yield (0.29 g, 0.18 mmol) of the title compound; 1H
NMR (300 MHz, CD2Cl2): d 9.6 (br m, 8 H), 8.44 (br m, 8 H), 7.76 (br m,
8 H), 5.35 (m, 16 H), 2.19 (br q, J 7 Hz, 16 H), 2.26 (nonet, J 7 Hz, 4 H),
1.5 (br s, 24 H), 1.1 (br d, J 6 Hz, 36 H), 0.92 (br s, 12 H); 13C NMR (75
MHz, [2H6]THF): d 151.2, 133.4, 130.1, 129.9, 127.4, 124.8, 76.5, 40.5,
26.4, 23.4 (two quarternary carbons not observed); 13C NMR DEPT 90 (125
MHz, [2H6]THF): d 129.88, 127.36, 124.84, 26.43; FAB MS
C105H116N8O8: Calc., 1602.87. Found (MH+), 1603.9. Elemental analysis
C105H116N8O8: Calc., C, 77.94; H, 7.23; N, 6.92; Found, C, 78.00; H, 7.27;
N, 6.84%.
References
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Received in Corvallis, OR, USA, 21st January 1997; revised MS received
19th May 1997; 7/03454G
Fig. 1 The absorption spectrum of 1 in toluene
Fig. 2 Cyclic voltammogram of 1 in CH2Cl2, Ag/AgCl (aq.), 0.1 m
Bu4NPF6, 500 mV s21, Vinit. = 2600 mV. Four quasi-reversible redox
events at E1/2 = 21279, 21039, 35 and 366 mV are observed. The E1/2 of
ferrocene was found to be 514 mV under these conditions.
1354 Chem. Commun., 1997


Organic and Metal Containing Reverse Saturable Absorber for Optical Limiting,

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The synthesis of a symmetrically substituted α-octa(isopentoxy)anthralocyanine

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