Nonlinear transmission of a tetrabrominated naphthalocyaninato indium chloride

Article abstract of DOI:10.1021/jp0571776

The axially substituted complex chloro indium(III) 2-tetrabromo-3-tetra-(3, 5-di-terf-butylphenyloxy)naph-thalocyanine [Br₄(tBu ₂PhO)₄NcInCl (1); MW = 1996] has been synthesized for the first time, and its nonlinear transmission properties have been evaluated with the Z-scan technique in both open and closed aperture configurations at 532 nm for nanosecond pulsed radiation. The tetrabrominated complex 1 displayed a larger positive nonlinear absorption coefficient when compared to an analogous nonbrominated naphthalocyanine [(tBu₂PhO)₈NcInCl (2); MW - 2498]. The effect of the four Br atoms on the nonlinear optical behavior of 1 is evaluated, discussed, and compared with the nonlinear optical behavior of 2. It is shown that the bromination of the naphthalocyanine ring considerably improves the limiting properties of such a system when high-intensity radiations are produced by nanosecond laser pulses at 532 nm.

Full text of DOI:10.1021/jp0571776

12230
J. Phys. Chem. B 2006, 110, 12230-12239
Nonlinear Transmission of a Tetrabrominated Naphthalocyaninato Indium Chloride
Danilo Dini,*,† Mario J. F. Calvete,† Michael Hanack,† Richard G. S. Pong,‡
Steven R. Flom,‡ and James S. Shirk‡
Institute of Organic Chemistry, UniVersity of Tu¨bingen, Auf der Morgenstelle 18, D-72076 Tu¨bingen, Germany,
and Optical Sciences DiVision, NaVal Research Laboratory, Washington, D.C. 20375
ReceiVed: December 9, 2005; In Final Form: March 8, 2006
The axially substituted complex chloro indium(III) 2-tetrabromo-3-tetra-(3,5-di-tert-butylphenyloxy)naph-
thalocyanine [Br4(tBu2PhO)4NcInCl (1); MW ) 1996] has been synthesized for the first time, and its nonlinear
transmission properties have been evaluated with the Z-scan technique in both open and closed aperture
configurations at 532 nm for nanosecond pulsed radiation. The tetrabrominated complex 1 displayed a larger
positive nonlinear absorption coefficient when compared to an analogous nonbrominated naphthalocyanine
[(tBu2PhO)8NcInCl (2); MW ) 2498]. The effect of the four Br atoms on the nonlinear optical behavior of
1 is evaluated, discussed, and compared with the nonlinear optical behavior of 2. It is shown that the bromination
of the naphthalocyanine ring considerably improves the limiting properties of such a system when high-
intensity radiations are produced by nanosecond laser pulses at 532 nm.
Introduction
locyanine [Br4(tBu2PhO)4NcInCl, 1]} (Chart 1) to evaluate the
influence of bromination on the excited-state absorption proper-
ties of the resulting substituted complex. As a basis of
comparison the octasubstituted complex chloro indium(III) 2,3-
Phthalocyanines (Pcs) and naphthalocyanines (Ncs) are
classes of compounds with useful photophysical properties for
nonlinear transmission (NLT)1 applications, which require strong
attenuation of intense optical fields and the clear transmission
of low intensity fields. Several metallophthalo-2 and naphthalo-
cyanines2b,3 display an induced absorption with increasing
incident light fluence in the visible spectral region. This
nonlinear optical (NLO) effect has been termed reverse saturable
absorption (RSA).4 The effect takes place because pulsed
radiation with high intensity (in the order of several megawatts
per square centimeter), generates transient electronic excited
states of Pcs and Ncs that have sufficiently long lifetime for
the effective absorption of the remainder of the pulsed radiation.
The occurrence of RSA implies that the absorption cross section-
(s) of the excited state(s), σexc, of the Pc (or Nc) is(are) greater
than that of the ground state, σg, at the wavelength of excitation.5
For many practical applications of the NLT effect, it is
desirable to have reverse saturable absorbers, which allow the
complete transmission of light at low optical fields over a large
spectral window. Many Pc-based materials have been demon-
strated to fulfill these conditions in the mid-visible spectral
range.2 Expansion of the conjugated π-electron system of the
Pc skeleton to form naphthalocyanines results in a red-shifted
Q-band absorption and, consequently, a larger window of high
optical transmission between the Q- and B-bands when com-
pared to a Pc.6 This red-shifted broad transmission window has
inspired the synthesis of many different Nc-based structures,2b,3d,3e,7
the excited-state properties of which have been mainly studied
in the visible-NIR spectral range.
CHART 1. Chloro Indium(III) Tetrabrominated
Naphthalocyanine 1 (isomer with C4h symmetry), and
Chloro Indium(III) Octasubstituted Naphthalocyanine 2
In the present work we have undertaken the synthesis of a
new soluble tetrabrominated naphthalocyanine {chloro indium-
(III) 2-tetrabromo-3-tetra-(3,5-di-tert-butylphenyloxy)naphtha-
* To whom correspondence should be addressed. E-mail: danilo.dini@
uni-tuebingen.de.
† University of Tu¨bingen.
‡ Naval Research Laboratory.
10.1021/jp0571776 CCC: $33.50 © 2006 American Chemical Society
Published on Web 06/02/2006

NLT of Br4(tBu2PhO)4NcInCl and (tBu2PhO)8NcInCl
J. Phys. Chem. B, Vol. 110, No. 25, 2006 12231
octa-(3,5-di-tert-butylphenyloxy)naphthalocyanine [(tBu2PhO)8-
NcInCl, 2] (Chart 1) was also prepared. In naphthalocyanine 1,
the presence of four di-tert-butylphenoxy substituents makes
solubilization possible in common organic solvents even at high
concentrations (on the order of 10-2 M), thus allowing the
preparation of samples with high nonlinear optical response
concomitant with relatively weak molecular aggregation.2d In
the search for new structures with optimized NLT properties,8
we have taken advantage of the versatility of the synthetic
chemistry of Ncs, which allows the modification of the
peripheral substituents, central atoms, and axial ligands in a
controlled fashion with consequent production of a large number
of structural variations.9
The peripheral halogenation of Ncs examined here is expected
to bring about modifications of the electronic structure of the
substituted molecule10 due to the high electronegativity of an
halogen atom. Moreover, halogenation imparts a strong polar
character to the carbon-halogen bond and induces electron
deficiency on the halogenated carbon atom. Such features
considerably alter the redox,11 optical,12 electronic,13 photo-
physical,14 and photochemical15 properties of the halogenated
conjugated macrocycles by increasing their oxidation potential,
lowering the HOMO level, and shifting the optical absorption
to shorter wavelengths when compared to analogous unsubsti-
tuted systems.
Figure 1. Jablonski diagram for the description of the mechanism of
reverse saturable absorption (RSA) upon short-pulse irradiation of Pcs
and analogues, in the visible spectrum. S0, S1(2), and T1(2) indicate the
ground singlet state, the first (second) excited singlet state, and the
first (second) excited triplet state, respectively. The cromophore absorbs
the first photon through the transition [S0 f S1] and, depending on the
dynamics of the irradiated system, will absorb sequentially a second
photon through either the [S1 f S2] or the [T1 f T2] transition. σS(T)
exc
and σg are the absorption cross sections from the excited singlet (triplet)
state and the ground state, respectively. Skewed and straight dotted
arrows indicate phosphorescence and fluorescence, respectively. ISC
stands for intersystem crossing [S1 f T1]. For the sake of clarity, only
the fundamental vibrational level of the various electronic levels has
been indicated.
The effects of fluorination and chlorination on the NLT effect
generated by substituted Pcs and analogues have been previously
examined by our group. From these studies it was concluded
that fluorinated Pcs and Ncs led to an improvement of the NLT
properties due to higher stability against photooxidation3d,3e,14a
and, in case of axially substituted complexes,16 to the increase
of the transition dipole moment associated with the excited-
state transition17 producing RSA. On the other hand, analogous
effects in perchlorinated Pc complexes14b could not be ef-
fectively evaluated since these systems formed highly aggregated
species,18 which result in excited states with relatively short
lifetimes and, therefore, ineffective NLT against nanosecond
pulsed radiation.19
Experimental Section
1,2-Dibromo-4,5-bis(dibromomethyl)benzene (4). To 26.4
g of 1,2-dibromo-4,5-dimethylbenzene25 (3, 0.1 mol) in CCl4
(250 mL), 90 g of NBS (0.5 mol) and 0.1 g of azobis-
(isobutyronitrile) (AIBN) were added. The mixture was stirred
and irradiated with a UV lamp for 8 h under reflux. The mixture
was hot filtered, and the remaining solid was washed with more
hot CCl4 (100 mL). The filtrate was concentrated to produce
an orange yellowish solid, which was washed portionwise with
750 mL n-hexane and dried at 40 °C under vacuum to give 50
g of 4 (85% yield). MS (EI, 70 eV): 579.5 [M+], 499.6 [M+ -
Br] 419.7 [M+ - 2 Br], 259.8 [M+ - 4 Br]. For the successive
synthesis of 6,7-dibromodicyanonaphthalene (5), the precursor
4 was used without any further purification.
The influence of bromination on the NLT effect generated
by poorly soluble tetra- and octabrominated Ncs such as 2,(3)-
tetrabromonaphthalocyaninato lead20 and 2,3-octabromonaph-
thalocyaninato zinc21 or octabrominated porphyrins22 has been
also studied. A conspicuous increase of the intersystem crossing
(ISC, see Figure 1) rate was generally observed, which increased
the yield of excited triplet state formation (tending toward unity)
presumably due to the internal heavy-atom effect23 of Br.4c
Moreover, the presence of Br also shortened the lifetime of the
excited triplet state22b through spin-orbit coupling23 and the
coupling between the atomic orbital of the halogen and the
molecular orbital of the ligand in the excited state.22b
6,7-Dibromodicyanonaphthalene (5). A mixture of 29 g of
1,2-di-bromo-4,5-bis(dibromomethyl)benzene (4) (0.05 mol), 4
g of fumaronitrile (0.06 mol), and 12 g of NaI (0.08 mol) in
200 mL of dry DMF was heated at 80 °C for 7 h and allowed
to stand at room temperature for an additional 4 h. The reaction
product was poured into 0.5 L of water, and then NaHSO3 (8
g) was added. The resulting suspension was filtered, washed
with water, and dried. The product was recrystallized from
toluene-hexane mixture (1:1) to give 11 g of 5 (65% yield).
MS (EI, 70 eV): 336.0 [M+], 256.31 [M+ - Br], H NMR
1
Similar results were found when the photophysical properties
of iodinated porphyrins were analyzed.22b,24 Besides the kinetic
effects related to ISC within the mechanism of sequential
multiphoton absorption (Figure 1), the role of Br as peripheral
substituent was not further considered in the analysis of the NLT
behavior of brominated conjugated complexes as far as the
excited-state absorption properties of these halogenated systems
were concerned. For this reason the preparation of a system
like Br4(tBu2PhO)4NcInCl (1) (Chart 1) has been accomplished,
in which the four Br atoms are expected to warrant fast and
efficient triplet excited-state formation due to heavy-atom effect,
without provoking a drastic reduction of the excited-state
lifetime.
(DMSO-d6, δ): 8.72 (s, 2H, H-4), 8.86 (s, 2H, H-2), 13C NMR
(DMSO-d6): 113.0 (CN), 115.4 (C-1), 121.1 (C-5), 126.0 (C-
3), 130.71 (C-2), 136.1 (C-4).
6-Bromo-7-[(3,5-di-tert-butyl)phenoxy]-2,3-dicyanonaph-
thalene (6). 3.36 g. of 6,7-dibromodicyanonaphthalene (5)26 (10
mmol), 2.27 g of 3,5-di-tert-butylphenol (11 mmol), and 9 g of
K2CO3 (66 mmol) were mixed together in DMF (50 mL) under
argon and heated at 100 °C for 3 h (the completion of the
reaction was followed with thin-layer chromatography using
dichloromethane as solvent). The mixture was cooled till room
temperature and then stirred upon addition of 0.4 L of ice water.
After 1 h of stirring the white-yellowish solid was collected
and washed again with water. The compound was dried in a

12232 J. Phys. Chem. B, Vol. 110, No. 25, 2006
Dini et al.
vacuum at 70 °C overnight. Recrystallization from ethanol gave
SCHEME 1. Preparation of
6,7-Dibromodicyanonaphthalene (5)a
4.5 g of 6 (97%). MS (EI, 70 eV): 461.3 [M+], 435.3 [M+
-
1
CN], 381.3 [M+ - Br]. H NMR (CDCl3, δ): 1.30 (s, 18H,
C(CH3)3), 6.66 (d, 2H, H-9), 7.38 (t, 1H, H-11), 7.69 (s, 1H,
H-5), 8.40 (s, 1H, H-7), 8.61 (s, 1H, H-3), 8.79 (s, 1H, H-2).
13C NMR (CDCl3): 29.7, 29.9 (CH3), 35.3, (C-tBu), 106.0,
106.3 (C-1), 110.0 (CBr), 113.2 (CN), 115.4 (C-5), 116.0 (C-
11), 124.7, 126.0 (C-6), 127.4 (C-7), 136.0 (C-9), 138.7 (C-3),
148.4 (C-4), 151.0 (C-8), 156.0 (C-10).
Chloro Indium(III) 2-Tetrabromo-3-tetra-(3,5-di-tert-bu-
tylphenyloxy)naphthalocyanine [Br4(tBu2PhO)4NcInCl] (1).
6-Bromo-7-[(3,5-di-tert-butyl)phenoxy]-2,3-dicyanonaphtha-
lene (6) (1.4 g, 3 mmol) and InCl3 (220.0 mg, 1 mmol) were
mixed in 1-chloronaphthalene (1 mL) and heated at 195 °C for
3.5 h. After the reaction mixture cooled, it was poured into
methanol (100 mL) and stirred. The precipitate was collected
and washed with hot methanol to give 1 as a dark yellow-green
solid (0.40 g, yield: 52%). MS-FAB (m/z): 1995.1 (M+), 1015.2
(M+ - Br). 1H NMR (THF-d8, δ): 1.33 (s, br, 72H, CH3), 7.06,
7.39 (br, 12H, phenoxy), 8.17, 8.28 (br, 8H on C-2), 10.81 (br,
8H on C-4). 13C NMR (THF-d8): 30.7 (CH3), 34.9 (C-CH3),
114.6 (br, C-8, C-10), 118.9 (br, C-2,C-4), 134.2 (br, C-3, C-5),
135.3 (br, C-6), 153.3 (br, C-7), 155.5 (br, C-1).
6,7-Di[(3,5-di-tert-butyl)phenoxy]-2,3-dicyanonaphtha-
lene (7). 3.36 g of 526 (10 mmol), 4.55 g of 3,5-di-tert-
butylphenol (22 mmol), and 11 g of K2CO3 (80 mmol) were
mixed together in DMF (60 mL) under argon and heated at
100 °C for 7 h (the completion of the reaction was followed
with thin-layer chromatography using dichloromethane as
solvent). The mixture was cooled till room temperature and then
stirred upon addition of 0.4 L of ice water. After 1 h of stirring,
the white-yellowish solid was collected and washed again with
water. The compound was dried in a vacuum at 70 °C overnight.
Recrystallization from ethanol gave 5.5 g of 7 (94%). MS (EI,
70 eV): 586.8 [M+], 561.8 [M+ - CN]. 1H NMR (CDCl3, δ):
1.30 (br, 36H, C(CH3)3), 6.74 (d, 4H, H-7), 7.30 (t, 2H, H-9),
7.72 (s, 2H, H-4), 8.55 (s, 2H, H-2). 13C NMR (CDCl3): 29.7,
29.9 (CH3), 35.3, (C-tBu), 105.1, (C-1), 113.4 (CN), 120.0 (C-
4), 121.2.0 (C-3), 124.7, (C-2), 127.4 (C-7), 138.0 (C-5), 148.7
(C-8), 153.0 (C-6).
a Reagents and conditions: (a) Br2, I2, from -5 till room temperature,
16 h; (b) NBS, AIBN, UV irradiation, CCl4 (reflux), 8 h; (c)
fumaronitrile, NaI, DMF, 80 °C, 7 h.
was 10 Hz in all Z-scan experiments. The energy of the incident
beam was varied with suitable half waveplate/polarizer com-
binations. f/5 focusing optics were used for excitation in all
experiments whereas the optics of collection in the open aperture
configuration was f/0.8. In the Z-scan experiments, the sample
was a thin layer of a solution of 1 in deaerated toluene, which
was tightly sandwiched between two CaF2 optical windows. The
CaF2 optical windows were separated by a thin flexible
polymeric membrane with rest thickness of approximately 30
µm. The cell thickness was evaluated interferometrically. The
sample was translated along the beam axis and through the focal
plane by means of a computer controlled translation stage. The
beam radius was measured by a knife-edge technique to be 1.5
µm. In the closed-aperture configuration the measured energy
corresponds to 40% of the total energy transmitted by the sample
due to the insertion of a spatial filter in front of the energy
detector.
Nonlinear Transmission Measurements. Deaerated solu-
tions of 1 and 2 in toluene at different concentrations were put
in quartz cuvettes with 2 mm path for the determination of
nonlinear transmission measurement. The focusing optics were
f/5 with the focus placed approximately at the center of the
solution. The experimental setup was similar to the one
described in ref 28 with the use of a doubled, Q-switched Nd:
YAG laser producing 5 ns long pulses.
Chloro Indium(III) 2,3-Octa-(3,5-di-tert-butylphenyloxy)-
naphthalocyanine [(tBu2PhO)8NcInCl] (2). 6,7-Di[(3,5-di-tert-
butyl)phenoxy]-2,3-dicyanonaphthalene (7) (2.35 g, 4 mmol)
and InCl3 (300.0 mg, 1.4 mmol) were mixed in 1-chloronaph-
thalene (1.5 mL) and heated to 195 °C for 3.5 h. After the
reaction mixture cooled, it was poured into methanol (100 mL)
and stirred. The precipitate was collected and washed with hot
methanol to give 2 as a dark yellow-green solid (1.2 g, yield:
48%). MS-FD (m/z): 2497.6 (M+). UV-Vis (nm, in toluene):
Results and Discussion
Synthesis of Naphthalocyanines 1 and 2. Naphthalocyanines
Br4(tBu2PhO)4NcInCl (1) and (tBu2PhO)8NcInCl (2) are directly
obtained from the template reaction between the corresponding
dinitriles (6 and 7 for 1 and 2, respectively) and indium
thrichloride at high temperature (approximately 200 °C). The
formation of dinitriles 6 and 7 requires the preparation and the
isolation of 6,7-dibromodicyanonaphthalene (5) as precursor.
The multistep procedure for the synthesis of precursor 5 starts
with the bromination of o-xylene to give 1,2-dibromo-4,5-
dimethylbenzene (3) (step a in Scheme 1).
1
804 (λmax), 742, 711, 371. H NMR (THF-d8, δ) 1.33 (s, br,
144H, CH3), 7.03, 7.23, 7.44 (3s, 24H, phenoxy), 7.89, (s, 8H
on C-2), 9.55 (br, 8H on C-4). 13C NMR (THF-d8): 30.9 (CH3),
34.9 (C-CH3), 113.8, 114.2 (br, C-8, C-10), 118.9, 121.7 (br,
C-2,C-4), 131.3, 133.3 (br, C-3, C-5), 134.3 (br, C-6), 152.9
(br, C-7), 156.0 (br, C-1).
Apparatus for Z-scan Experiments. The description of the
experimental setup for the nanosecond nonlinear transmission
experiments with the Z-scan technique27 has been previously
reported by us.2d In the Z-scan experiments a doubled Nd:YAG
laser was used for generating the nanosecond pulsed beam at
532 nm (pulse width: 8 ns). The experimental beam profile
for Z-scan experiments fit a Gaussian beam profile with a
correlation higher than 95%. A spatial filter transmitting only
the 10% of the full beam is placed in front of the sample before
focusing the input laser beam on the sample, which provides a
top-hat intensity profile. The frequency of sample irradiation
Compound 3 is successively brominated with NBS on the
two adjacent methyl groups under UV irradiation and in the
presence of the radical generator AIBN with resulting formation
of 1,2-dibromo-4,5-bis(dibromomethyl)benzene (4) (step b in
Scheme 1). 6,7-Dibromodicyanonaphthalene (5) is obtained
upon addition of fumaronitrile to the two aliphatic carbon atoms
of 4 and the simultaneous elimination of four bromine atoms

NLT of Br4(tBu2PhO)4NcInCl and (tBu2PhO)8NcInCl
J. Phys. Chem. B, Vol. 110, No. 25, 2006 12233
as HBr and IBr from the dibromomethyl groups of 4 (step c in
Scheme 1).
In the dilute solution complex 1 displays Q-band absorption
at 806 nm with the group of vibronic satellite bands peaked at
764 and 716 nm, whereas B-band absorption is centered at 339
nm (full line spectrum in Figure 2). The largest variations of ꢀ
with concentration are observed at wavelengths near the Q-band
for Br4(tBu2PhO)4NcInCl (1) (Figure 2). The maximum value
of ꢀQ-band is 1.6 × 105 L mol-1 cm-1 within the concentration
range 1.4 × 10-5 < C < 1.5 × 10-4 M. When the concentration
Dinitriles 6 and 7 are produced in basic conditions upon
replacement of one and two bromine atoms in 6,7-dibromodi-
cyanonaphthalene (5), respectively, with the 3,5-di-tert-bu-
tylphenoxy group generated from the deprotonation of 3,5-di-
tert-butylphenol (Scheme 2).
SCHEME 2. Preparation of Dinitriles 6 and 7a
of 1 in toluene increases from 1.5 × 10-4 to 2.0 × 10-2
M
ꢀQ-band steadily decreases from the maximum value to 4.5 ×
104 L mol-1 cm-1. A partially resolved, broad band centered at
about 760 nm (gray and dashed line spectra in Figure 2) appears
when C > 1.5 × 10-4 M. The molar extinction coefficient for
this new additional band reaches a maximum value 6 × 104 L
mol-1 cm-1 when C ) 7 × 10-4 M (gray line spectrum in
Figure 2). Upon further increase of concentration of 1, ꢀ grad-
ually decreases at 760 nm (dashed line spectrum in Figure 2).
The intensity of the 760 nm band increases at the expense of
the Q-band transition. Such a spectral evolution with concentra-
tion is thought to arise from the onset of optical absorption at
about 750 nm from aggregated forms of Br4(tBu2PhO)4NcInCl
(1).18a In this context, the energy levels of Br4(tBu2PhO)4NcInCl
aggregates can be considered as perturbed levels of 1 in the
nonaggregated state.29 From the observed changes of the
absorptive properties of 1 with concentration, the aggregates
are expected to be dimers or higher order oligomers with
predominantly cofacial arrangement.18b,30 In fact, exciton-
coupling models account for the allowance of a transition
resulting into a blue-shifted Q-band for cofacially aggregated
complexes of Pcs (or Ncs).29,31
a Reagents and conditions: (a) 1 equiv of 5, 1.1 equiv of 3,5-di-
tert-butylphenol, 6 equiv of K2CO3, 100 °C, 4 h; (b) 1 equiv of 5, 2.2
equiv of 3,5-di-tert-butylphenol, 8 equiv of K2CO3, 100 °C, 7 h.
The occurrence of absorption in the range 800-1000 nm in
the spectra of Br4(tBu2PhO)4NcInCl (1) (Figure 2) is probably
associated with electronic transitions involving the partial or
full exchange of charge between the naphthalocyaninato ligand
and the central metal indium.32 A relatively weak dependence
on concentration was observed for the NIR absorption of 1.
Dinitriles 6 and 7 can be selectively formed under similar
conditions upon variation of the number of equivalents of 3,5-
di-tert-butylphenol with respect to a fixed amount of 6,7-
dibromodicyanonaphthalene (5) (Scheme 2). When the molar
ratio of 5 to 3,5-di-tert-butylphenol is one precursor 6 is
preferentially formed (step a in Scheme 2). If the molar ratio
of 5 to 3,5-di-tert-butylphenol is two the formation of precursor
7 is then favored (step b in Scheme 2).
Linear Optical Properties. The optical spectra of Br4(tBu2-
PhO)4NcInCl (1) in toluene at different concentration (C) values
are presented in Figure 2 as variations of the molar extinction
coefficient (ꢀ) with the wavelength (λ).
Nonlinear Optical Properties. The nonlinear optical absorp-
tion and refraction of Br4(tBu2PhO)4NcInCl (1) have been
studied with the Z-scan technique27 using nanosecond laser
pulses at 532 nm. In Z-scan experiments, the transmittance (T)
of the sample is measured as a function of the sample position
along the optical axis. For Figure 3, the transmitted radiation is
collected at the detector through an open aperture while Figure
4 represents the ratio of the light transmitted through 40%
aperture divided by the light transmitted by the open-aperture
experiment.
In the profiles of Figures 3 and 4, the zero position
corresponds to the location of the focal plane. The sample was
a thin layer (thickness, L ) 30 µm) of a highly concentrated
solution of 1 (C ) 2.0 × 10-2 M) in toluene. Such a
configuration is required since the sample thickness has to be
smaller than the Rayleigh range ZR of the Gaussian beam in
Z-scan experiments (L and ZR are 30 and about 50 µm,
respectively). This allows the generation of uniform optical
fields within the sample when this is positioned in the proximity
of the focal plane of the Gaussian beam.27,33 A highly
concentrated solution of Br4(tBu2PhO)4NcInCl (1) was chosen
for these experiments in order to maximize the nonlinear optical
signal, which is proportional to the amount of molecular material
in the sample.2d,8c Moreover, the decrease of the molar extinction
coefficient of 1 with concentration (see Figure 2) allows the
realization of an optical attenuator with relatively lower absorp-
tion in the linear optical regime as required for some
applications.3d,3e,34
Figure 2. Variation of the molar extinction coefficient (ꢀ) with the
wavelength (λ) at different concentrations (C) of Br4(tBu2PhO)4NcInCl
(1) in toluene. C ) 1.4 × 10-5 M (cuvette thickness d ) 10 mm, solid
black line); C ) 7.0 × 10-4 M (cuvette thickness d ) 0.2 mm, solid
gray line); C ) 2 × 10-2 M (d ) 30 µm, dashed line).

12234 J. Phys. Chem. B, Vol. 110, No. 25, 2006
Dini et al.
implies that Br4(tBu2PhO)4NcInCl (1) has a positive nonlinear
absorption coefficient,27,35 and behaves as a reverse saturable
absorber for nanosecond pulses at 532 nm. These features
indicate that concentrated solutions of Br4(tBu2PhO)4NcInCl (1)
can act as optical attenuators1a,1b,36 for nanosecond pulses at
532 nm.
The refractive component of the nonlinear response derived
from the closed-aperture Z-scans profiles have been also
determined for 1 (Figure 4) at the same values of incident
energies that were used for the determination of the open-
aperture Z-scans in Figure 3. Similar to what we previously
observed for a side-by-side dimeric binuclear phthalocyanine8c
Br4(tBu2PhO)4NcInCl (1) also displayed a series of unsym-
metrical profiles of the Z-scans with respect to the focal plane
at Z ) 0 for the different values of the incident energy. This
feature of the closed-aperture transmission curves does not make
possible a quantitative analysis of the nonlinear refractive
properties of 1 at 532 nm. However, the peak-valley shape of
the closed-aperture profiles in Figure 4 demonstrates an increase
of transmittance at prefocal sample positions (Z < 0) due to a
focusing effect of the sample. When the sample traverses the
path comprised between the focus and the closed aperture (i.e.,
for Z > 0), a defocusing effect is observed instead. This trend
is indicative of a negative variation of the refractive index upon
increase of the incident energy in the nonlinear optical regime
of Br4(tBu2PhO)4NcInCl (1) in toluene solutions.35 Such a
negative contribution clearly has a thermal component, but the
asymmetry of the data indicates the coexistence of an excited-
state contribution also of negative sign to the nonlinear variation
of the refractive index of 1. The transmission of 1 at the focal
plane as a function of incident energy Ein is shown in Figure 5
by plotting the minima of transmission of 1 from the open-
aperture profiles of Figure 3.
Figure 3. Z-scan profiles of the transmission of Br4(tBu2PhO)4NcInCl
(1) in toluene at 532 nm (C ) 2 × 10-2 M; d ) 30 µm) in the open-
aperture configuration. On X-axis the distance between the sample and
the focus (located at Z ) 0) is reported. The value of the incident energy
is specified in nJ for each profile on the upper part of the right side.
The data shows that the threshold for NLT, defined as the
incident energy at which the transmission is half the linear
transmission, is ca. 140 nJ (in our experimental conditions this
energy value corresponds to Fin ≈ 2.3 J cm-2) (Figure 5). This
threshold is larger than that observed for Pcs, which is not
surprising given that 532 nm is near the edge of the nonlinear
absorption band.
Figure 4. Normalized Z-scan profiles of the refractive part of the
relative transmission of Br4(tBu2PhO)4NcInCl (1) in toluene at 532 nm
(C ) 2 × 10-2 M; d ) 30 µm). Data are the ratio of the closed-aperture
transmission divided by the open-aperture transmission. On the X-axis
the distance between the sample and the focus (located at Z ) 0) is
reported. The value of the incident energy is specified in nJ for each
profile on the upper part of the right side.
For the evaluation of σexc (excited-state absorption cross
section) and RNL (nonlinear absorption coefficient) of Br4(tBu2-
PhO)4NcInCl (1) at 532 nm, it is more convenient to plot the
reciprocal of the normalized nonlinear optical transmission at
The linear transmittance (T0) of the sample for nonlinear
optical measurements at 532 nm was 0.61. The laser wavelength
is approximately at the center of the highly transmissive window
of Br4(tBu2PhO)4NcInCl (1) in which ꢀ < 104 L mol-1 cm-1
.
This corresponds to the spectral region ranging from about 480
nm to 675 nm in the high concentration spectrum of 1 (dashed
line in Figure 2). The wavelength of analysis (532 nm) is a
standard one for the evaluation of the NLT properties of
materials,1a,b and, in the case of Ncs, such a value does not
correspond to the wavelength of their maximum nonlinear
absorption (approximately 620 nm).3a The energy of the incident
radiation (Ein) was varied over the range 0.2 < Ein <550 nJ.
This interval corresponds to the range of peak fluence (Fin) 3
× 10-3 < Fin <10 J cm-2, which is experienced by the sample
(the area of the spot at the beam focus is approximately 6 ×
10-8 cm2 being the beam waist 1.5 µm at the focus).
The Z-scan profiles of 1 in toluene in the open-aperture
configuration are shown in Figure 3. At each of the excitation
energies, the concentrated solution of tetrabrominated NcInCl
1 in toluene shows a decreasing transmission as the focal
position is approached and incident fluence increases. This
Figure 5. Variation of the nonlinear optical transmission at 532 nm
for Br4(tBu2PhO)4NcInCl (1) in toluene (C ) 2 × 10-2 M; d ) 30
µm) when the thin-layer sample is located on the focal plane of the
Gaussian beam. Points are extracted from the corrected minima of the
Z-scan profiles determined in the open-aperture configuration.

NLT of Br4(tBu2PhO)4NcInCl and (tBu2PhO)8NcInCl
J. Phys. Chem. B, Vol. 110, No. 25, 2006 12235
532 nm versus incident fluence. This is because sequential two-
photon absorbers such as Br4(tBu2PhO)4NcInCl (1) display a
variation of transmittance through the sample which is governed
by the relationship:1a,2d,37
T
0 ) 1 + RNLLeffFin
(1)
T
when the sample is an optically thin specimen. In eq 1, Leff
represents the effective length of the optical path (in µm), which
is defined as [1 - exp(-R0L)]/R0 (R0 and L are the linear
absorption coefficient at the given wavelength and the actual
length of the optical path in µm, respectively). For simple three
level systems, where RNL is a constant, the excited-state
absorption cross section σexc can be directly determined via the
equation:1a,2d,8c,37
Figure 7. Variation of the reciprocal of the normalized nonlinear
optical transmission at 532 nm (T0/T)532 for Br4(tBu2PhO)4NcInCl (1)
in toluene (C ) 2 × 10-2 M; d ) 30 µm) and best linear fit as a
function of the incident fluence Fin at the onset of the nonlinear optical
behavior.
2(hν)RNL
σexc ) σ0 +
(2)
NTσ0
a slope value of 0.27 ( 0.01 cm2 J-1. The resulting value of
the nonlinear absorption coefficient of Br4(tBu2PhO)4NcInCl
(1) is RNL ) 114 cm J-1 and, from eq 2, σexc ) 1.35 × 10-17
cm2 for 1 at 532 nm.
where NT, σ0, and hν represent the total density of the molecular
absorber (in cm-3), the ground-state absorption cross section
(in cm-2) at the wavelength of analysis, and the energy of the
single photon (in J) at the analyzed wavelength, respectively.
Figure 6 shows the entire range of the reciprocal transmission
as a function of excitation fluence and demonstrates that 1 is
not well-described as a simple three-level system since the slope
of the plot varies with increasing fluence. Nevertheless, analysis
of the different fluence regions allows one to extract effective
cross sections for the multilevel system.
The variation of the slope of nonlinear optical transmission
of the high concentrated solution of Br4(tBu2PhO)4NcInCl (1)
is ascribed to the involvement of distinct excited states of 1
having different absorption properties, which are generated with
increasing doses of irradiation.38 In the present case, both excited
states of Br4(tBu2PhO)4NcInCl (1) present absorption cross
section values at 532 nm which are larger than the respective
values in the ground state, thus giving rise to the effect of RSA
at 532 nm over the whole range of incident fluence. A qualitative
description of the mechanism of multiphoton absorption of
Br4(tBu2PhO)4NcInCl (1) at high concentration is presented in
Figure 9.
Application of eq 1 gives the slope value 2 ( 0.14 cm2 J-1
when only the experimental points comprised in the range 0 <
Fin < 100 mJ cm-2 are fitted (Figure 7). In this range the
resulting value of the nonlinear absorption coefficient of
Br4(tBu2PhO)4NcInCl (1) is RNL ) 844.4 cm J-1 since R0 )
156 cm-1 at 532 nm, L ) 30 µm, and Leff ) 23.7 µm in the
adopted experimental conditions. For the 2 × 10-2 M solution
of 1 in toluene, the linear transmittance T0 is 0.63 at 532 nm,
which corresponds to the absorbance value 0.2. From eq 2, it
is found that the value of the effective excited-state absorption
cross section of Br4(tBu2PhO)4NcInCl (1) at 532 nm is 1.75 ×
10-17 cm2 at the onset of the nonlinear optical behavior being
NT ) 1.2 × 1019 cm-3 and σ0 ) 1.3 × 10-17 cm2 at the photon
energy hν ) 3.74 × 10-19 J.
At lower incident fluences (Fin < 0.1 J cm-2) the system
initially absorbs a photon from the ground state with σ0 ) 1.3
× 10-17 cm2 and, successively, a second photon from a first
excited singlet or triplet state with an effective cross section,
σ
exc(1) ) 1.75 × 10-17 cm2. At higher incident fluences (Fin >
0.1 J cm-2) the system is able to pump a third excited state
formed during the pulse and with a sufficiently long lifetime
(longer than the pulse duration) for the further absorption of
the third photon with σexc(2) ) 1.35 × 10-17 cm2 (Figure 9).
Over the incident fluence range 0.1 < Fin < 10 J cm-2 the
linear fitting of the experimental data (Figure 8) with eq 1 gives
Figure 6. Variation of the reciprocal of the normalized nonlinear
optical transmission at 532 nm (T0/T)532 for Br4(tBu2PhO)4NcInCl (1)
in toluene (C ) 2 × 10-2 M; d ) 30 µm) as a function of the incident
fluence Fin.
Figure 8. Variation of the reciprocal of the normalized nonlinear
optical transmission at 532 nm (T0/T)532 for Br4(tBu2PhO)4NcInCl (1)
in toluene (C ) 2 × 10-2 M; d ) 30 µm) and best linear fit as a
function of the incident fluence in the range 0.1 < Fin < 10 J cm-2
.

12236 J. Phys. Chem. B, Vol. 110, No. 25, 2006
Dini et al.
Figure 9. Qualitative description of the mechanism of multiphoton
absorption in the case of high concetrated solutions of Br4(tBu2-
PhO)4NcInCl (1) (C > 1 × 10-3 M) in toluene. Decay processes and
spin multiplicity for the various levels are not indicated. The symbols
σ
exc(1) and σexc(2) indicate the excited-state absorption cross sections
of 1 when the latter is irradiated with lower (Fin < 1 J cm-2) and higher
(Fin > 1 J cm-2) fluence, respectively. From the fits in Figures 7 and
8, it results that σexc(1) > σexc(2) and σexc(1), σexc(2) > σo.
An evaluation of the lifetimes and the spectral properties of
the distinct excited states of 1 goes beyond the scopes of the
present work.
The mechanism of three-photon absorption depicted in Figure
9 operates for the nonlinear transmission effect of Br4(tBu2-
PhO)4NcInCl (1) at 532 nm in the whole range of concentrations
of 1. In fact, slope variations of the curves (T0/T)532 versus Ein
could be found also when the nonlinear optical behavior of
Br4(tBu2PhO)4NcInCl (1) was determined at different values
of concentration in toluene solutions with d ) 2 mm within the
energy range 0 < Ein < 10 µJ (Figure 10a). In Figure 10b, the
variations of (T0/T)532 versus Ein for 1 are presented for the larger
incident energy range 0 < Ein < 200 µJ.
Figure 10. Variations of the inverse of the normalized optical
transmission at 532 nm for Br4(tBu2PhO)4NcInCl (1) samples in toluene
with different values of linear transmittance (as indicated in the legend),
within small (a) and large (b) ranges of incident energy. The 2 mm
thick sample is located on the focal plane of a Gaussian beam with f/5
optics of incidence. The transmitted radiation is collected through an
open aperture (iris-less configuration). The corresponding concentration
values for the various solutions of 1 are 2.5 × 10-3, 1.3 × 10-3, 0.9 ×
10-3, and 0.2 × 10-3 M in increasing order of linear transmittance.
For an analysis of the variation of the nonlinear absorp-
tion properties of 1 with concentration and incident energy we
have considered the linear slopes (S) of the curves in Figure 10
(Table 1).
TABLE 1: Values of the Linear Slopes S(1) and S(2) of the
Curves in Figure 10 for 1 in the Ranges Ein < Ein′ and Ein
>
Ein′, Respectivelya
have been also fitted with two linear trends, and the relative
fitting parameters have been listed in Table 1. The linear fit for
the low energy range, Ein < Ein′, is characterized by a general
decrease of the value of σexc*(1) with increasing concentration
of the photoactive material. Such a decrease of σexc*(1) could
be a direct consequence of the reduction of the actual fraction
of 1, which populates the level 2 (Figure 9) due to occurrence
of molecular aggregation. When the linear fit within the range
Ein > Ein′ is considered, it is found that σexc*(2) tends to increase
with C (Table 1) [σexc*(2) is related to the excited-state transition
between levels 3 and 4 of 1, Figure 9]. The direct proportionality
between the values of σexc*(2) and C may imply that molecular
aggregation favors the occurrence of the second excited-state
transition of Br4(tBu2PhO)4NcInCl (1). Such a finding could
indicate the occurrence of charge-transfer events between
Br4(tBu2PhO)4NcInCl molecules in the excited state as a
potential origin of the second transition when Ein > Ein′. This
is because charge transfer between equal molecules would occur
with higher probability at increasing molecular concentration.39
S(2)/
σ
exc*(1)/
J-1
σ
exc*(2)/
J-1
C/M
Leff × NTσ0 S(1)/J-1 J-1
Ein′/µJ
0.2 × 10-3
0.9 × 10-3
1.3 × 10-3
2.5 × 10-3
0.05
0.35
0.55
0.90
0.20
0.55
0.80
1.20
0.02
0.13
0.27
0.60
3.80
1.60
1.45
1.30
0.40
0.37
0.50
0.67
3.3
3.1
3.5
3.6
a σexc*(n) values are obtained from the relationship: σexc*(n) ) S(n)/
LeffNTσ0 ) S(n)/1 - T0. In the last column, the values of the incident
energies (Ein′) at which the slope change of the curves in Figure 10 is
verified are reported.
If the parameter S is divided by Leff and NTσ0 the resulting
factor, here indicated with σexc* (Table 1), is directly propor-
tional to the difference between the absorption cross section of
the more energetic excited state of 1 involved in the multiphoton
absorption process and σ0. Therefore, in the present context σexc
might be considered as a good representation of the difference
*
2d,5
σexc - σ0
when Ein replaces Fin in eq 1. This allows a
comparative evaluation of the nonlinear transmission efficiency
of the system under investigation when the sample thickness
(d ) 2 mm in the experiments of Figure 10) is larger than the
Rayleigh range of the incident Gaussian beam, and the area of
the beam is not constant through the sample. For the experiments
of Figure 10 the beam focus was approximately located at half
thickness of the sample (see Experimental Section).
Similar to the procedure adopted for the analysis of the
nonlinear transmission curves of Br4(tBu2PhO)4NcInCl (1) for
the thin layer sample (Figures 7 and 8), the curves in Figure 10
The curves of nonlinear transmission reported as the recipro-
cal of normalized transmittance vs incident energy in Figure
10 for different concentrations of Br4(tBu2PhO)4NcInCl (1)
reveal that the variations of nonlinear optical transmission are
dependent on the concentration of 1 when the optical path is
the same for all solutions and T0 varies accordingly.40 It is found
that the nonlinear transmission effectiveness of Br4(tBu2-
PhO)4NcInCl (1) increases with increasing concentration

NLT of Br4(tBu2PhO)4NcInCl and (tBu2PhO)8NcInCl
J. Phys. Chem. B, Vol. 110, No. 25, 2006 12237
(or decreasing linear trasmittance) when this is directly evaluated
through the ratio [(T0 - Tmin)/T0] (Table 2), Tmin being the
minimum value of transmittance reached by the system in
nonlinear optical regimes without irreversible damage.36 The
analysis of the trend of the sole difference T0 - Tmin gives the
opposite trend with the concentration of Br4(tBu2PhO)4NcInCl
(1) (Table 2).
The value of σexc*(1) is generally lower for (tBu2PhO)8NcInCl
(2) with respect to Br4(tBu2PhO)4NcInCl (1) (Table 1), whereas
σexc*(2) values are similar for the two chloro-indium complexes
1 and 2. Since σexc*(1) as well as σexc*(2) represent a time-
averaged value of excited-state absorption cross section, the
lowest value found in the case of (tBu2PhO)8NcInCl (2) can be
also a consequence of the lower quantum yield of formation
(ΦT1) of the first triplet excited state (level 2 in Figure 9), in
comparison to the brominated complex 1, which is expected to
produce a larger value of ΦT1 for the presence of an heavy atom
like Br.23 The variation of σexc*(1) with C is less pronounced
for complex 2 with respect to 1. This is ascribed to the weaker
tendency of (tBu2PhO)8NcInCl (2) with respect to Br4(tBu2-
PhO)4NcInCl (1) to form aggregates since a larger number of
aryloxy substituents acting as molecular spacers is present in
the formula of 2 (Chart 1). In contrast to tetrabrominated 1, the
octasubstituted complex (tBu2PhO)8NcInCl (2) does not display
a decrease of σexc*(1) with the increase of C (Table 3). Since
molecular aggregation is expected to influence at a lower extent
the photophysical behavior of 2 with respect to 1, the small
increase of σexc*(1) with C could be due to the lower percentage
of degraded material in the more concentrated solution of 2,
which is obtained upon photoinitiated reaction of 2 with the
nearly constant concentration of dissolved oxygen in toluene.
The variation of the nonlinear optical transmission of two
solutions of the tetrabrominated complex 1 and (tBu2PhO)8NcInCl
(2) having the same linear optical density at 532 nm has been
measured and compared (Figure 12).
TABLE 2: Values of the Various Parameters Related with
the Minimum Transmittance (Tmin) of the System for the
Evaluation of the Nonlinear Transmission Effectiveness of 1
at Different Concentration Values
C/M
T0
Tmin
T0 - Tmin
T0 - Tmin/T0
0.2 × 10-3
0.9 × 10-3
1.3 × 10-3
2.5 × 10-3
0.95
0.65
0.45
0.10
0.180
0.020
0.010
0.001
0.77
0.52
0.44
0.099
0.812
0.956
0.977
0.99
For sake of comparison we prepared the octasubstituted
complex (tBu2PhO)8NcInCl (2), which does not present any
bromine atom as peripheral substituent in the structure (Chart
1). Curves of normalized nonlinear transmission for 2 in the
configuration of thick film (d ) 2 mm) have been also
determined when it had different values of concentration in
toluene solutions (Figure 11).
Figure 11. Variation of the inverse of the normalized optical
transmission at 532 nm for (tBu2PhO)8NcInCl (2) samples in toluene
with different values of linear transmittance as indicated in the panel.
The 2 mm thick sample is located on the focal plane of a Gaussian
beam with f/5 optics of incidence. The transmitted radiation is collected
through an open aperture (iris-less configuration). The corresponding
concentration values for the various solutions of 2 are 4.0 × 10-3 and
2.0 × 10-3 M in increasing order of linear transmittance.
Figure 12. Variations of (T0/T)532 for the THF solutions of Br4(tBu2-
PhO)4NcInCl (1) and (tBu2PhO)8NcInCl (2) with the same value of T0
() 0.10) at 532 nm. Sample thickness and optical setup are the same
as in Figure 10. Concentration values are 1.6 × 10-3 and 1.8 × 10-3
M for the toluene solutions of 1 and 2, respectively.
Similar to the tetrabrominated complex 1 (Figure 10), the
octaphenoxy substituted system 2 presents variations of non-
linear transmission versus incident energy, which can be
considered as a succession of two linear fits. The analysis of
the linear fits of the two curves in Figure 11 has led to the
determination of the various fitting parameters reported in
Table 3.
The latter condition of equal linear optical densities warrants
the meaningfulness of the comparison between the nonlinear
transmission properties of different systems since the nonlinear
transmission effectiveness at a fixed wavelength is evaluated
by means of the difference (T0 - Tmin) or the ratio [(T0 - Tmin)/
T0] (Table 4).36
TABLE 4: Values of the Various Parameters Related with
the Comparative Evaluation of the Nonlinear Transmission
Effectiveness of 1 and 2 Having Same Linear Transmittance
(T0 ) 0.10) in Toluene
TABLE 3: Values of the Linear Slopes S(1) and S(2) of the
Curves in Figure 11 for 2 in the ranges Ein < Ein′ and Ein
>
Ein′, Respectivelya
S(1)/ S(2)/
σ
exc*(1)/
σ
exc*(2)/
T0 - Tmin T0 - Tmin/T0 DRa/10-6
J
C/M
Leff × NTσ0 J-1
J-1
J-1
J-1
Ein′/µJ
compd
C/M
Tmin
1.6 × 10-3 0.0011 0.0988
0.988
0.915
150
30
0.2 × 10-3
0.70
0.80
0.61 0.24
0.87
1.38
0.34
0.65
2.4
3.6
1
2
1.8 × 10-3 0.0085 0.0915
4.0 × 10-3
1.1
0.52
a DR, dynamic range.
a σexc*(n) values are obtained from the relationship: σexc*(n) ) S(n)/
LeffNTσ0 ) S(n)/1 - T0. In the last column, the values of the incident
energies Ein′ at which the slope change of the curves in Figure 11 occurs
are reported.
From the data of Figure 12 the nonbrominated system 2
presents a considerably smaller variation of the inverse of the

12238 J. Phys. Chem. B, Vol. 110, No. 25, 2006
Dini et al.
normalized transmittance at 532 nm when compared to the
tetrabrominated complex 1. This is equivalent to saying that
the nonlinear transmission effect produced by Br4(tBu2-
PhO)4NcInCl (1) is stronger than that observed with (tBu2-
PhO)8NcInCl (2). Moreover, the tetrabrominated system 1 shows
a considerably wider dynamic range with respect to 2, as shown
by the observation of reversible changes of transmittance for 1
with incident energy values as high as 150 µJ, whereas for
nonbrominated 2 the transmittance reversibly varies with only
to a maximum value of 30 µJ under the same experimental
conditions. The differences in dynamic ranges of Br4(tBu2-
PhO)4NcInCl (1) and (tBu2PhO)8NcInCl (2) are associated to
the higher photostability of 1, which is induced by the presence
of electronegative atoms such as Br similar to what is found in
the case of fluorinated complexes.3d,3e,14a
Acknowledgment. Financial support from EU (Contracts
HPRN-CT-2000-00020 and HPRN-CT-2002-00323) is grate-
fully acknowledged.
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Conclusions
The soluble peripherally substituted naphthalocyanineschloro
indium(III) 2-tetrabromo-3-tetra-(3,5-di-tert-butylphenyloxy)-
naphthalocyanine [Br4(tBu2PhO)4NcInCl,1] and chloro indium-
(III) 2,3-octa-(3,5-di-tert-butylphenyloxy)naphthalocyanine [(tBu2-
PhO)8NcInCl, 2] have been synthesized, and their nonlinear
transmission properties at 532 nm for ns laser pulses have been
evaluated and analyzed. A variable slope trend for the nonlinear
optical transmission of both systems with low values of fluence
of the incident radiation has been found. Such a behavior has
been modeled through the modification of a simpler model
taking into account additional excited-state transitions that take
place at higher light intensities. These additional transitions were
associated to the possible occurrence of a charge transfer process
between excited molecules. Partial bromination of a naphtha-
locyaninato chloro-indium complex in the peripheral positions
of the macrocycle leads to the general improvement of the
nonlinear transmission properties of such a system in comparison
to a nonhalogenated chloro-indium naphthalocyanine when high
fluence radiations are produced by nanosecond laser pulses at
532 nm. The observed changes in the nonlinear transmission
effect generated by the two differently substituted systems,
Br4(tBu2PhO)4NcInCl (1) and (tBu2PhO)8NcInCl (2), are as-
cribed to the modification of several concomitant factors and
properties like:
(i) the ground-state absorption cross sections with consequent
use of different concentrations for the attainment of equal linear
transmission;
(ii) the quantum yield of formation of the absorbing excited
state;
(iii) the stability of the photoactive molecule in the conditions
of irradiation
which all might intervene to some extent. The differences
between the nonlinear optical behavior of tetrabrominated and
nonhalogenated Ncs have to be ascribed also to other factors,
the influence of which has not been directly evaluated in the
present work. These factors include the profile of the excited-
state spectrum, the transition dipole moments involved in the
transitions between excited states, and the lifetime of the
absorbing excited state.
The main reasons for the improved nonlinear properties of
the brominated system 1 are found in the higher photostability
(compare Figure 12) imparted by the presence of electronegative
Br, higher yield of excited-state formation due to heavy-atom
effect. Larger variations of the transition dipole moments
associated with the excited-state transitions of the tetrabromi-
nated naphthalocyaninato complex when compared to nonha-
logenated system are also expected.
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