Synthesis and nonlinear optical properties of tetrahedral octupolar phthalocyanine-based systems

Article abstract of DOI:10.1021/jp100827k

Two series of tetrahedral phthalocyanine-based systems presenting a central carbon or silicon atom have been synthesized and fully characterized. Ethynyl spacers connect the peripheral Pc units to the central core. Some of the structures contain four identical Pc moieties, whereas other ones bear either an electron-withdrawing or an electron releasing group in the fourth subunit. The synthetic strategy consisted in metal mediated coupling reactions between tri-tert-butylethynylphthalocyanine and the corresponding methane or silane derivatives. A second-order nonlinear optical (NLO) study, through hyper-Rayleigh scattering measurements, reveals that, by combining centrosymmetrical moieties that are not second-order NLO active by themselves, in an octupolar fashion, a large second-order NLO response is achieved, in contrast to classical octupolar combinations of donor-acceptor NLO active dipolar moieties. In particular, the C-centered tetramer exhibits a large ?2_HRS value, which is among the highest reported so far for octupolar Pc-based molecules. Interestingly, carbon-centered molecules show a better NLO response with respect to the silicon-centered ones, probably due to a different effective symmetry, largely T_d for the C-centered compounds and D_2d for the Si-centered systems. While other design strategies for second-order NLO effects have always fundamentally kept on relying on the old dipolar paradigm (even though the resulting molecular structure was octupolar - the most striking exponent of this is the octupolar 1,3,5-triamino-2,4,6-trinitrobenzene molecule, a simple octupolar expansion of the dipolar p-nitroaniline), we here present for the first time that the octupolar symmetry by itself, realized by four nondipolar moieties in a tetrahedral arrangement, results also in a large second-order nonlinear response. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp100827k

J. Phys. Chem. B 2010, 114, 6309–6315
6309
Synthesis and Nonlinear Optical Properties of Tetrahedral Octupolar
Phthalocyanine-Based Systems
Maurizio Quintiliani,† Javier Pe´rez-Moreno,‡ Inge Asselberghs,‡ Purificacio´n Va´zquez,†
Koen Clays,*,‡ and Toma´s Torres*,†
Departamento de Qu´ımica Orga´nica, Facultad de Ciencias, UniVersidad Auto´noma de Madrid, Cantoblanco,
E-28049 Madrid, Spain, and Department of Chemistry, UniVersity of LeuVen, B-3001 LeuVen, Belgium
ReceiVed: January 27, 2010
Two series of tetrahedral phthalocyanine-based systems presenting a central carbon or silicon atom have
been synthesized and fully characterized. Ethynyl spacers connect the peripheral Pc units to the central core.
Some of the structures contain four identical Pc moieties, whereas other ones bear either an electron-
withdrawing or an electron releasing group in the fourth subunit. The synthetic strategy consisted in metal
mediated coupling reactions between tri-tert-butylethynylphthalocyanine and the corresponding methane or
silane derivatives. A second-order nonlinear optical (NLO) study, through hyper-Rayleigh scattering
measurements, reveals that, by combining centrosymmetrical moieties that are not second-order NLO active
by themselves, in an octupolar fashion, a large second-order NLO response is achieved, in contrast to classical
octupolar combinations of donor-acceptor NLO active dipolar moieties. In particular, the C-centered tetramer
exhibits a large ꢀHRS value, which is among the highest reported so far for octupolar Pc-based molecules.
Interestingly, carbon-centered molecules show a better NLO response with respect to the silicon-centered
ones, probably due to a different effective symmetry, largely Td for the C-centered compounds and D2d for
the Si-centered systems. While other design strategies for second-order NLO effects have always fundamentally
kept on relying on the old dipolar paradigm (even though the resulting molecular structure was octupolar -
the most striking exponent of this is the octupolar 1,3,5-triamino-2,4,6-trinitrobenzene molecule, a simple
octupolar expansion of the dipolar p-nitroaniline), we here present for the first time that the octupolar symmetry
by itself, realized by four nondipolar moieties in a tetrahedral arrangement, results also in a large second-
order nonlinear response.
Introduction
tetrahedral organotin compounds,8 D2d biphenyl-based com-
pounds and copper(I) and zinc(II) complexes with 4,4′-bis(di-
The design and synthesis of new molecular systems for
nonlinear optics (NLO) have attracted much interest because
of their potential applications in the field of optoelectronics and
photonics.1 Organic molecules, due to their fast electronic
responses to external stimuli and flexibility of chemical design,
are predicted to enhance the performance of electro-optic devices
for high-speed optical communications, integrated optics, and
optical data processing/storage. For these materials, large
molecular hyperpolarizability (ꢀ) of the individual components
and their ordering into noncentrosymmetric arrangements are
still synthetic challenges.2 Recent investigations have revealed
that unconventional NLO chromophores with multiple donor
(D)-acceptor (A) substituents, ranging from dipolar “X-
shaped”3 and “Λ-shaped”4 to octupolar molecules,5 can afford
advantages over conventional one-dimensional conjugated
D-π-A dipolar chromophores, such as increased ꢀ responses,
without undesirable losses of transparency and improved phase-
matching.6
alkylaminostyryl)-[2,2′]bipyridine as ligands, and D2 symmetric
superstructures are among the most studied systems for such
purpose.9 It is interesting to observe that all the octupolar
chromophores studied so far all are indeed based on this truly
multidipolar concept, arranging two, three, or four individual
dipolar subunits in a D2d, D3(h), or Td octupolar fashion,
respectiVely, to take advantage of a multidimensional charge
transfer. The way to relate these symmetries to the generic
octupolar and dipolar archetypes has been explained in terms
of the “Guanidinium” route toward Td or D2d symmetry and in
terms of the “TATB” route toward D3(h) symmetry.9d It is also
interesting to observe that the central atom holding the three
(in D3(h)) or four (in Td symmetry) dipolar units has been
recognized to play a crucial role in the charge transfer.8a
Phthalocyanines (Pc’s),10 with their highly polarizable two-
dimensional 18 π-system, are well-known for their very good
second-order NLO properties when unsymmetrically substituted.
Several strategies have been pursued in order to break the
inherent symmetry of these macrocycle and to enhance the
quadratic NLO response of this family of macrocycles.11,12
Combination of dipolar and octupolar features in the same
molecule has already yielded some interesting NLO properties,
particularly in the field of Pc analogues such as subphthalo-
cyanines.13 Recently, we described the synthesis and preliminary
NLO studies of a series of pseudo-D3h symmetrical tris(phtha-
locyaninyl)benzenes and triazines.14 Notably, because of a strong
In particular, octupolar NLO chromophores are promising
candidates to overcome centrosymmetric ordering of the single
molecules in the bulk phase.7 Regarding the octupolar approach,
the tetrahedral design motif has been shown to give very efficient
NLO responses. Three-dimensional (3D) phosphonium ion and
* To whom correspondence should be addressed. E-mail: tomas.torres@
uam.es (T.T.); koen.clays@fys.kuleuven.be (K.C.).
† Universidad Auto´noma de Madrid.
‡ University of Leuven.
10.1021/jp100827k  2010 American Chemical Society
Published on Web 04/19/2010

6310 J. Phys. Chem. B, Vol. 114, No. 19, 2010
Quintiliani et al.
CHART 1: Tetrahedral Systems with Four (1-2) and
Three Pc’s (3-5)
standard local field correction factors were applied [((nD2 + 2)/
3)3, where n is the refractive index of the solvent at the sodium
D line].
Synthesis of 4-Tritylnitrobenzene (6). To a mixture of
4-tritylaniline (400 mg, 1.19 mmol) and Ti superoxide catalyst17
(100 mg, 25 wt %) in 1,2-dichloroethane (25 mL), 4 mL of
50% H2O2 (4.72 g, 0.138 mol) was added slowly under argon,
and the mixture was stirred for two days at 50 °C. The catalyst
was filtered off, and the solvent was removed. The residue was
dissolved in CHCl3 and washed with brine, with water and dried
over Na2SO4. The drying agent was filtered, and the solvent
was removed under reduced pressure. The crude was purified
by column chromatography on silica gel (eluent: hexane/dioxane
(5:1). The solid was washed with methanol affording 226.1 mg
(52%) of 6 as a pale yellow solid. Mp > 250 °C. 1H NMR (300
MHz, CDCl3): δ ) 8.15 (d, J ) 9.1 Hz, 2 H), 7.49 (d, J ) 9.1
Hz, 2 H), 7.21 (m, 15 H). 13C NMR (75.5 MHz, CDCl3): δ )
154.7 (C-1 nitro), 146.1 (C-NO2), 145.8, 132.1 (C-2 nitro),
131.1, 128.1, 126.7, 122.9 (C-3 nitro), 65.4 (central C). Fourier
transform infrared (FT-IR) (KBr): ν ) 2926, 2860, 1597, 1516,
1448, 1354, 1273, 1119, 1038, 858, 756, 702 cm-1. MS (FAB,
m-NBA): m/z 366.1 [M]+. Anal. Calcd for C25H19NO2: C, 82.17;
H, 5.24; N, 3.83 (%). Found: C, 82.01; H, 5.32; N, 3.72 (%).
octupolar effect and an extension of the π-conjugation, these
compounds exhibited an enhanced quadratic NLO response with
respect to unsymmetric Pc’s with dipolar character as expected.
To the best of our knowledge, there is only one previously
described example of a Pc molecule incorporated within an
octupolar tetrahedral framework, i.e. a series of phosphonium
salts that gave very good results when compared with the
corresponding nonoctupolar Pc.15 In this regard, we describe
herein the synthesis and an NLO study of five tetrahedral Pc-
based systems presenting a central atom (Chart 1). We have
varied the nature of this atom (either C or Si), the number of
Pc units (three or four), and the nature of the fourth subunit
(either nitrophenyl or anilyl or ethoxy) to investigate the relative
importance of these structural parameters. The design is based
on the original idea to induce the necessary noncentrosymmetry
directly and solely by the octupolar symmetry, without any
conjugation among the Pc moieties.
Synthesis of 4-[Tris(4-iodophenyl)methyl]nitrobenzene (7).
A suspension of 6 (152 mg, 0.416 mmol), bis(trifluoroacetoxy)-
iodobenzene (308.5 mg, 0.717 mmol), and iodine (166.3 mg,
0.655 mmol) in CCl4 (40 mL) was stirred at 50-60 °C for 2
days. The solvent was removed, and the residue was dissolved
in CHCl3 and washed with a saturated solution of Na2S2O3, with
water and dried over Na2SO4. After filtration of the drying agent,
the solvent was vacuum-evaporated. The brown crude was
subjected to column chromatography on silica gel using hexane/
dioxane (5:1) as eluent. The solid was suspended in hexane,
filtered, and washed with the same solvent and with methanol,
affording 123.6 mg (40%) of 7 as a white solid. Mp > 250 °C.
1H NMR (300 MHz, CDCl3): δ ) 8.12 (d, J ) 9.1 Hz, 2 H),
7.61 (d, J ) 8.7 Hz, 6 H), 7.36 (d, J ) 9.1 Hz, 2 H), 6.89 (d,
J ) 8.7 Hz, 6 H). 13C NMR (75.5 MHz, CDCl3): δ ) 153.4
(C-1 nitro), 145.1 (C-1 iodo), 144.7 (C-NO2), 137.4 (C-3 iodo),
132.8 (C-2 iodo), 130.7 (C-2 nitro), 123.2 (C-3 nitro), 92.8 (C-4
iodo), 64.9 (central C). FT-IR (KBr): ν ) 1599, 1518, 1464,
Experimental Section
General Considerations. Melting points (Mp’s) were de-
termined in Bu¨chi 504392-S equipment and are uncorrected.
UV/visible (UV/vis) spectra were recorded with a Hewlett-
Packard 8453 instrument. A Perkin-Elmer LS50B luminescence
spectrometer was employed for the fluorescence spectra. Fast
atom bombardment mass spectrometry (FAB-MS) spectra were
determined on a VG AutoSpec instrument. Matrix-assisted laser
desorption/ionization time of flight (MALDI-TOF) MS and high-
resoltion mass spectrometry (HRMS) spectra were recorded with
a Bruker Reflex III spectrometer. 1H NMR spectra were recorded
with BRUKER AC-300 and 500 instruments. Elemental analyses
were performed with Perkin-Elmer 2400 CHN equipment.
Column chromatography was carried out on silica gel Merck-
60 (230-400 mesh, 60 Å), and thin layer chromatography
(TLC) was performed on aluminum sheets precoated with silica
gel 60 F254 (E. Merck). Size-exclusion chromatography was
carried out using Bio-Beads S-X1 Beads (200-400 mesh) from
BIO-RAD. Chemicals were purchased from commercial sup-
pliers and used without further purification.
NLO Measurements. The experimental setup for the nano-
second hyper Rayleigh scattering experiments at 1064 and 800
nm has been described in detail.16 The chromophores were
dissolved in tetrahydrofuran (THF) and passed through 0.2-µm
filters. Crystal violet chloride (CV, 338 × 10-30 esu in CH3OH),
and p-nitroaniline (PNA, 21.6 × 10-30 esu in CH2Cl2) were
utilized as reference chromophores at 800 and 1064, respec-
tively. For these external references in different solvents,
1396, 1356, 1275, 1115, 1011, 852, 812, 758, 704, 677 cm-1
.
MS (FAB, m-NBA): m/z 743.9 [M]+ 618.0 [M-I]+, 494.9
[M-2I]+. Anal. Calcd for C25H16I3NO2: C, 40.41; H, 2.17; N,
1.88 (%). Found: C, 40.39; H, 2.05; N, 1.56 (%).
Synthesis of 4-[Tris(4-iodophenyl)methyl]aniline (8). Method
A: To a suspension of compound 7 (35 mg, 0.047 mmol) and
zinc dust (6 mg, 0.094 mmol) in MeOH (5 mL) and 1,2-
dichloroethane (3 mL), hydrazinium monoformate (0.1 mL) was
added, and the mixture was stirred under argon atmosphere for
24 h. The solvents were removed, and the residue was dissolved
in CH2Cl2 and washed with water and dried over Na2SO4. After
filtration of the drying agent, the solvent was vacuum-
evaporated. The yellow-brown crude obtained was subjected
to column chromatography on silica gel using CH2Cl2 as eluent.
The solid was suspended in hexane, filtered, and washed with
the same solvent and with methanol, affording 7.3 mg (22%)
of 8 as a white solid. Method B: To a solution of compound 7
(30 mg, 0.040 mmol) in 1,2-dichloroethane (3 mL) and
[bmim][BF4] (0.5 mL), stannous chloride (45 mg, 0.199 mmol)
was added, and the mixture was stirred for 24 h. The reaction
mixture was neutralized with sodium bicarbonate and extracted
with CH2Cl2. The combined organic extracts were dried with
Na2SO4, filtered, and concentrated in Vacuo. The yellow-brown

Tetrahedral Octupolar Phthalocyanine-Based Systems
J. Phys. Chem. B, Vol. 114, No. 19, 2010 6311
SCHEME 1: Synthesis of tetrahedral methane derivatives 6-8. (i) 50% H2O2, Ti(IV)OO•-, 1,2-dichloroethane, 50 °C,
(52%). (ii) I2, PIFA, CCl4, 60°C, (40%). (iii) Zn, H2N-NH2•HCOOH, MeOH, 50 °C, (22%). (iv) SnCl2•2H2O, [bmim][BF4],
1,2-dichloroethane, rt. (40%)
residue was purified by silica gel column chromatography
(eluent: CH2Cl2). The solid was suspended in hexane, filtered,
and washed with the same solvent and with methanol, affording
vis (THF): λmax (log ε) ) 686 (5.50), 672 (5.45), 351 (5.24)
nm. MS (MALDI-TOF, dithranol): m/z 2669.7 [M+H]+. HRMS
(MALDI-TOF): calc. for C163H133N25O2Zn3, 2663.8948. Found,
2664.8987. 4: Yield: (14.3 mg, 77%), green solid. Mp > 250
1
11.4 mg (40%) of 8 as a white solid. Mp > 250 °C. H NMR
1
(300 MHz, CDCl3): δ ) 7.55 (d, J ) 8.6 Hz, 6 H), 6.93 (d, J
) 8.6 Hz, 6 H), 6.89 (d, J ) 8.6 Hz, 2 H), 6.56 (d, J ) 8.6 Hz,
2 H), 3.64 (s, NH). 13C NMR (75.5 MHz, CDCl3): δ ) 146.8
(C-1 iodo), 146.3 (C-4 amino), 136.8 (C-3 iodo), 135.9 (C-1
amino), 133.2 (C-2 iodo), 132.0 (C-2 amino), 114.5 (C-3 amino),
92.0 (C-4 iodo), 64.0 (central C). FT-IR (KBr): ν 1631, 1456,
1389, 1281, 1192, 1138, 1003, 876, 808, 768, 673 cm-1. MS
(FAB, m-NBA): m/z 714.3 [M]+ 586.9 [M-I]+, 509.9
[M-C6H5I]+, 494.8 [M-2I]+, 384.0 [M-2(C6H5I)]+. Anal. Calcd
for C25H18I3N: C, 42.11; H, 2.54; N, 1.96 (%). Found: C, 42.07;
H, 2.66; N, 1.88 (%).
°C. H NMR (300 MHz, [D8]THF): δ ) 9.49-7.59 (br, 52H,
arom. H), 3.7 (br, 1H, NH), 1.82 (br, 81H; C(CH3)3). FT-IR
(KBr): ν ) 2962, 2908, 2868, 1612, 1487, 1393, 1339, 1258,
1153, 1099, 1045, 932, 831, 752, 702 cm-1. UV/vis (THF):
λmax (log ε) ) 685 (5.52), 675 (5.51), 348 (5.34) nm. MS
(MALDI-TOF, dithranol): m/z 2640.0 [M+H]+. HRMS (MALDI-
TOF): calc. for C163H135N25Zn3, 2633.9207. Found, 2635.9128.
5: Yield: (13.5 mg, 74%), green solid. Mp > 250 °C. 1H NMR
(300 MHz, [D8]THF): δ ) 9.41-8.05 (br, 48H, arom. H), 3.82
(q, 2H), 1.86 (br, 81H; C(CH3)3), 1.21 (t, 3H). FT-IR (KBr) ν:
2957, 2903, 2876, 1620, 1481, 1400, 1319, 1265, 1190, 1151,
1097, 1057, 932, 829, 752, 698 cm-1. UV/vis (THF): λmax (log
ε) ) 688 (5.43), 672 (5.39), 351 (5.20) nm. MS (MALDI-TOF,
dithranol): m/z 2609.0 [M+H]+. HRMS (MALDI-TOF): calc.
for C158H134N24OSiZn3, 2602.8816. Found, 2602.8726.
Synthesis of Tetrakis{4-[2-(phthalocyaninyl)ethynyl]phenyl}-
methane (1), Tetrakis{4-[2-(phthalocyaninyl)ethynyl]phenyl}
silane (2), 4-Tris{4-[2-(phthalocyaninyl)ethynyl]phenylmethyl}-
nitrobenzene (3), 4-Tris{4-[2-(phthalocyaninyl)ethynyl]-
phenylmethyl}aniline (4) and Ethoxytris{4-[2-(phthalocyaninyl)-
ethynyl]phenyl}silane (5). General Procedure: A mixture of
tritert-butylethynylphthalocyaninatozinc(II) (9)11b (54 mg, 0.07
mmol), the corresponding methane and silane derivative 7-8,
10-12 (0.007 mmol), Pd2(dba)3 (4 mg, 0.004 mmol), AsPh3 (4
mg, 0.015 mmol), and triethylamine (0.5 mL) in dry THF (6
mL) was stirred at room temperature under argon for 48 h. The
solvent was removed under reduced pressure, and the crude
product was extracted with CHCl3, dried over Na2SO4 and
purified first by silica gel column chromatography (hexane/THF
2:1) and then by Bio-Beads size-exclusion column chromatog-
raphy (THF). The green solid obtained was then washed with
methanol and hexane and dried under vacuum. 1: Yield: (20.5
Results and Discussion
Synthesis and Characterization. The synthesis of com-
pounds 1-5 was carried out by 4-fold or 3-fold Sonogashira
cross-coupling between tri-tert-butylethynylphthalocyanina-
tozinc(II) (9)11b and the corresponding substituted methanes 10,18
7 and 8 or silanes 1119 and 12.20 4-Tritylnitrobenzene (6) was
synthesized by means of catalytic oxidation of 4-tritylaniline
according to a described procedure,17 consisting in the use of
50% H2O2 and an exceptionally stable Ti superoxide radical
(Scheme 1).
Alternatively to the described protocol, it was observed that
the oxidation of 4-tritylaniline did not evolve in methanol at
25 °C, and it was necessary to use 1,2-dichloroethane as solvent
and to increase the temperature to 50 °C, yielding 52% of 6.
Compound 7 was generated in 40% yield by iodination of
4-tritylnitrobenzene (6) in the presence of iodine and phe-
nyliodine(III)bis(trifluoracetate) (PIFA).21 Several methods were
examined in order to prepare 8. First of all, the iodination of
4-tritylaniline in the presence of I2 and PIFA was attempted,
but unexpectedly a mixture of colored compounds was obtained,
probably as a result of oxidations on the aniline ring. It was
therefore decided to reduce the nitro group of 7. Treatment of
compound 7 with hydrazinium monoformate solution and zinc22
in a mixture of methanol and 1,2-dichloroethane afforded 8 in
22% yield. On the other hand, compound 8 could be prepared
by reduction of 7, previously dissolved in 1,2-dichloroethane,
with stannous chloride in [bmim][BF4],23 yielding 40% of the
desired product. The first tetrahedral system synthesized was
compound 1 with four Pc moieties. Tri-tert-butylethynylphtha-
locyaninatozinc(II) (9)11b was reacted with tetrakis(4-iodophe-
nyl)methane (10)19 as depicted in Scheme 2.
1
mg, 86%), green solid. Mp > 250 °C. H NMR (300 MHz,
[D8]THF): δ ) 9.38-7.88 (br, 64H, arom. H), 1.85 (br, 108H,
C(CH3)3). FT-IR (KBr): ν ) 2966, 2872, 1616, 1489, 1394,
1327, 1259, 1155, 1101, 1047, 924, 835, 760, 692 cm-1. UV/
vis (THF): λmax (log ε) ) 688 (5.70), 672 (5.63), 352 (5.41)
nm. MS (MALDI-TOF, dithranol): m/z 3394.3 [M+H]+. HRMS
(MALDI-TOF): calc. for C209H172N32Zn4, 3385.1609. Found,
3385.1719. 2: Yield: (22.2 mg, 88%), green solid. Mp > 250
1
°C. H NMR (300 MHz, [D8]THF): δ ) 9.41-8.11 (br, 64H,
arom. H), 1.85 (br, 108H; C(CH3)3). FT-IR (KBr): ν ) 2957,
2916, 2876, 1618, 1493, 1398, 1331, 1263, 1105, 1051, 928,
839, 764, 683 cm-1. UV/vis (THF): λmax (log ε) ) 687 (5.58),
673 (5.54), 350 (5.37) nm. MS (MALDI-TOF, dithranol): m/z
3410.2 [M+H]+. HRMS (MALDI-TOF): calc. for
C208H172N32SiZn4, 3401.1378. Found, 3402.1465. 3: Yield: (14.6
1
mg, 78%), green solid. Mp > 250 °C. H NMR (300 MHz,
[D8]THF): δ ) 9.41-7.73 (br, 52H, arom. H), 1.85 (br, 81H;
C(CH3)3). FT-IR (KBr): ν ) 2928, 2860, 1616, 1495, 1468,
1373, 1265, 1157, 1090, 1049, 928, 833, 752, 698 cm-1. UV/

6312 J. Phys. Chem. B, Vol. 114, No. 19, 2010
Quintiliani et al.
SCHEME 2: Synthesis of Pc-Based Tetrahedral Systems
1-5. (i) Pd2(dba)3, AsPh3, Et3N, THF r.t., 48 h
Figure 1. Absorption spectra of 1-5 in THF (C ) 2 × 10-6 M).
TABLE 1: Second-Order NLO Properties of 1-5: First
Hyperpolarizability ꢀHRS and Depolarization Ratio G at
Indicated Measurement Wavelength
compound
ꢀHRS (800 nm)
F (800 nm)
ꢀHRS (1064 nm)
1
2
3
4
5
270 ( 15
200 ( 15
200 ( 10
175 ( 10
170 ( 10
1.93 ( 0.05
1.94 ( 0.05
1.86 ( 0.05
1.89 ( 0.05
2.00 ( 0.05
495 ( 75
290 ( 40
250 ( 40
270 ( 30
230 ( 30
Figure 2. Fluorescence spectra of 1-5 in THF, excited at 675 nm (C
) 5 × 10-7).
octupolar tetrahedral arrangement of independent chromophoric
moieties, as there is no change in shape or position of the
spectral features. This can be explained by the presence of the
linkage by an sp3-hybridized C or Si atom that does not allow
conjugation, and the fourth branch does not have any influence.
As a consequence, there is no possibility for through-bond
charge transfer or for more extended electronic delocalization
over a larger conjugated system. Because of the weak electron
donating or withdrawing capacity of the individual moieties and
the inefficient relative orientation of the moieties, through-space
charge transfer is not observed. The same effect of orientation
was previously observed for 1,3,5-tris(phthalocyaninyl)benzenes
with ethynyl linkages.14 While ethenyl linkages allow a coplanar
conformation of Pc moieties and central benzene core, and
hence, an octupolar enhancement factor of 3 (for D3h symmetry),
the ethynyl linkage did not show such an effect (factor of 1)
because of the orthogonal orientation of the Pc ring with respect
to the central benzene core.
NLO Properties. For these octupolar chromophores, only
hyper-Rayleigh scattering can be used to determine the molec-
ular first hyperpolarizability ꢀ.16a,25 The ꢀHRS values are reported
in Table 1, along with the depolarization ratio F.
Only the ꢀHRS values obtained with our frequency-resolved
approach with femtosecond pulses at 800 nm have been
considered for quantitative analysis. The ꢀHRS values have also
been determined at 1064 nm with our generic nanosecond HRS
setup,16a,25 but these values could not be guaranteed to be free
of multiphoton fluorescence contribution,26 although an earlier
study did not report multiphoton fluorescence contribution to
the HRS signal.11c From the fluorescence spectra shown in
Figure 2, the possibility of luminescence at the second-harmonic
wavelength (532 nm) can be expected.
In order to minimize the formation of the bisphthalocyanine
derivative from 9 via oxidative homocoupling,11b the reaction
was performed by using tris(dibenzylidenacetone)dipalladium(0)
([Pd2(dba)3]) as the palladium catalyst and triphenylarsine as
the ligand.24 In spite of this, the Pc dimer butadiynyl-bridged
bisphthalocyaninato was formed at the very first steps of the
mechanism to generate the active catalytic species [Pd(0)]. To
avoid the formation of an undesired mixture of Pc-adducts, a
strong excess of Pc 9 (10 equivalents) was necessary. Tetramer
1 was separated by column chromatography, yielding 86% of
1. The same conditions of reaction and purification adopted for
1 were employed for the synthesis of 2-5, by cross-coupling
of ethynyl-Pc 9 with 11, 7, 8, 12, in 88% 2, 78% 3, 77% 4, and
74% 5 yields, respectively. Compounds 1-5 are insoluble in
most of the solvents such as chloroform, dichloromethane,
dimethyl sulfoxide (DMSO), acetone, or methanol, but very
soluble in THF or 1,4-dioxane. Due to the existence of several
positional regioisomers, the 1H NMR spectra of 1-5 presented
broad features for the aromatic protons. MALDI-TOF MS
experiments revealed the molecular ion of 1-5 as the most
prominent peak, displaying an isotopic pattern that compares
properly with the simulated one.
Linear Spectroscopy. The absorption and fluorescence
spectra of 1-5 are depicted in Figures 1 and 2, respectively.
From the similarity with reported Pc absorption spectra (strong
“Q” band in the visible, somewhat weaker “B”or “Soret” band
in the UV, and no additional features indicative of new charge-
transfer bands, neither in the transmission window between the
B and Q-band, nor beyond the Q-band),10 it is concluded that
the individual branches retain their apolar nature. The linear
optical properties also show that compounds 1-5 behave as an

Tetrahedral Octupolar Phthalocyanine-Based Systems
J. Phys. Chem. B, Vol. 114, No. 19, 2010 6313
hyperpolarizability that is observed for systems with multiple
absorption bands (Pc’s and porphyrins) has been reported to be
unusual.1c In addition, for the sake of comparing hyperpolar-
izability values for compounds with essentially identical absorp-
tion spectra, identical resonance factors may be assumed, which
cancel out in the comparison.
Concerning the Pc-methane derivatives, the ratio of the ꢀHRS
value for compound 1, over that for the compounds 3 or 4 is
4:3, indicating that the individual units behave independently.
The second-order NLO property is reported in terms of ꢀHRS
,
since from the depolarization ratio it is not clear what the major
hyperpolarizability tensor component should be. From this ratio
in ꢀHRS value for compound 1 to compounds 3 or 4, again it is
clear that the NLO response comes from the octupolar orienta-
tion of the individual subunits and not from dipolar contributions
and conjugation from donor to acceptor. On the other hand,
the Pc-silane derivatives 2 and 5 exhibit lower ꢀHRS values,
which do not show the ratio of 4:3 observed for the C-centered
compounds. Within experimental error, the ꢀHRS values for
compounds 2 and 5 are identical. This can confirm the
importance of the inefficient relative orientation for extended
delocalization in this tetrahedral arrangement here, since Si is
expected to be more efficient in charge delocalization, through
its larger size and its d-orbitals, when the geometrical arrange-
ment would allow it.29 From the observation that the linear
optical properties (shape and position of UV/vis absorption
spectra) for all compounds are very similar, it follows that the
lower value for the second-order NLO response of compounds
2 and 5 must have to do with a different effective symmetry
for these two compounds. From the results reported by Dos
Santos et al. based on geometry optimization with the semiem-
pirical AM1 Hartree-Fock method,29 D2d symmetry can be
expected, rather than Td symmetry. A single silicon atom as
spacer between two conjugated parts results in coplanarity for
two arms. In the present case, the four arms could result in two
perpendicular planes that intersect in the central Si atom (D2d
symmetry). These symmetries, again, are derived without taking
into account conformational flexibility around the ethynyl
linkage nor positional isomers. Thus, the actual symmetry can
be approximately Td for C-centered compounds and D2d for the
Si-centered systems.
Figure 3. First hyperpolarizability values at 800 nm in THF for 1-5
as a function of modulation frequency (1: squares, 2: triangles up, 3:
diamonds, 4: circles, 5: triangles down).
Luminescence at 400 nm is already much less likely for these
chromophores, but the absence of a multiphoton fluorescence
contribution at 400 nm has been experimentally proven by the
constant value for ꢀHRS as a function of amplitude modulation
frequency (see Figure 3), and by the zero phase difference
between signal and pure scatterer at all modulation frequencies.
Any fluorescence contribution, when present, is demodulated
at higher modulation frequencies, resulting in an apparent
decreasing of ꢀHRS for increasing frequency. Moreover, the time
delay between immediate scattering and delayed fluorescence
translates into a growing phase shift for increasing frequencies.16b,27
This approach is the Fourier transform equivalent in the
frequency domain of the corresponding approach in the time
domain.28 It also exhibits the discriminating power between
fluorescence and scattering that the approach in the spectral
domain is lacking. Indeed, two- and three-photon fluorescence
can spectrally overlap with the second harmonic. However, both
signatures for fluorescence in our frequency domain approach,
i.e., demodulation of the apparent hyperpolarizability and
growing phase shift for higher frequencies, are not observed
for HRS at 800 nm. The reference for the HRS measurements
at 800 nm is the octupolar ꢀxxx,800 value of 338 × 10-30 esu for
crystal violet in methanol.28 Please also note that crystal violet,
a frequently used reference chromophore for HRS experiments,
is another example of a multidipolar molecule derived along
the “TATB” route from the octupolar archetype, with the central
carbocation holding the three dipolar units playing a crucial role
in the charge transfer from the strong dimethylamino donor.5e,9d
Local field solvent correction factors have been applied, with a
refractive index of 1.328 for methanol and 1.407 for the solvent,
THF. For the purely octupolar crystal violet, a depolarization
ratio of 1.28 was measured. This value is lower than the
theoretical value of 1.5 that should be obtained for infinitely
small numerical aperture. For the realistic aperture, a somewhat
smaller value is obtained. Therefore, also the values for
compounds 1 to 5 are lower estimates. The HRS measurements
were performed at concentrations of 5 × 10-6 M and lower
and in cells with 0.2 cm path length to avoid reabsorption of
the second-harmonic.
From the values for the depolarization ratio F, all in a narrow
range between 1.86 and 2.00, it is possible to deduce that the
effective symmetry of the nonlinear scatterers is very similar
(D2d or Td). The depolarization ratio indicates a small dipolar
contribution to a largely octupolar symmetry. In agreement with
the observed independent behavior of the subunits, fluctuations
in conformation, together with positional regioisomers, give rise
to partial dipolar character. Interestingly, no significant differ-
ence in depolarization ratio is observed between compound 1
and compounds 3 and 4, or between compounds 2 and 5. This
may give an indication about the importance of the fluctuations
that are expected for these ethynyl-linked polarizable moieties.
Only for identical conjugation through the four ethynyl bridges
(identical angles between inner phenyl rings and outer Pc’s) is
purely octupolar symmetry expected. In addition, each Pc
subunit exists in different positional isomers. When the conjuga-
tion or substitution pattern is different for different arms, a
dipolar contribution originates. A similar dipolar contribution
(similar depolarization ratio) originates whether a dipolar subunit
(anilyl or nitrophenyl) is present or not. This leads to the
conclusion that the presence or absence of a dipolar subunit is
irrelevant for the effective symmetry. The small dipolar
contribution observed is therefore induced by the difference in
We did not attempt to apply a dispersion model to our data.
With accurate fluorescence-free hyperpolarizability values at
only one wavelength, the validity of any wavelength-dependent
model could not be checked. Furthermore, the dispersion of the

6314 J. Phys. Chem. B, Vol. 114, No. 19, 2010
Quintiliani et al.
conjugation through the four ethynyl linkage and the presence
of regioisomers.
Acknowledgment. This work was supported by the Spanish
MEC (CTQ-2008-00418/BQU and CONSOLIDER INGENIO
2010, CSD2007-00010), the Comunidad de Madrid (MADRISO-
LAR-2, S2009/PPQ/1533), the ESF-MEC (MAT2006-28180-
E, SOHYDS), and the Flemish Fund for Scientific Research
(FWO G.0297.04 and G.0312.08). We would like also to thank
Dr. G. Hennrich. I.A. and J.P.M. are postdoctoral fellows of
the FWO.
The main result, however, is that by the mere octupolar (Td
or D2d) arrangement of symmetrical Pc groups (i.e., without
electron donor or acceptor group), a value for the first hyper-
polarizability ꢀ can be obtained that is equivalent to, or even
surpasses, the values obtained in Pc’s asymmetrically substituted
with multiple strong donor and acceptor groups. As a case in
point, we compare the strongly dipolar (dipole moment of 20.6
D) Pc-derivative with two n-alkoxy donor groups and two
nitrophenylethynyl acceptor groups (ꢀHRS at 1064 nm )530 ×
10-30 esu)11c with our octupolar compound 1 without any donor
or acceptor substituents (ꢀHRS at 1064 nm ) 495 ( 75 × 10-30
esu). Dipolar Pc-derived compounds with three substituents
(either two alkoxy donors with a single nitrophenylethynyl
acceptor group, ꢀHRS at 1064 nm ) 220 × 10-30 esu,11c or a
single dimethylaminophenylethynyl donor group with two
propylsulfonyl acceptor groups, ꢀHRS at 1064 nm ) 114 × 10-30
esu)11c can not provide the level of second-order NLO response
achieved by our original strategy of purely octupolar arrange-
ment of the symmetrical unsubstituted Pc-moiety. Please note
that the substituents on the unsymmetrical Pc-moiety had already
been selected to promote the electronic push-pull effect with
respect to earlier dipolar Pc-derivatives,11c while our Pc-moiety
only has the very weakly donating phenylethynyl linker group
(lacking the strong nitro donor or dimethylamino acceptor
group). Finally, it is striking that up to 12 2,2,2-trifluoroethoxy
donor groups have been used in conjunction with a nitro
acceptor group to induce nonzero second-harmonic generation
from push-pull Pc’s in thin film format30 in sharp contrast with
the large hyperpolarizability from our octupolar arrangement
of unsubstituted Pc’s.
Supporting Information Available: Characterization data
for new compounds 1-8 (1H NMR, 13C NMR, IR, MS, and
HR-MS spectra). This material is available free of charge via
the Internet at http://pubs.acs.org.
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In summary, we have prepared two series of 3D octupolar
compounds presenting a central carbon or silicon atom. The
analysis of the combined linear and second-order NLO results
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octupolar fashion (largely Td symmetry for the C-centered
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