Four Dibutylamino Substituents Are Better Than Eight in Modulating the Electronic Structure and Third-Order Nonlinear-Optical Properties of Phthalocyanines

Article abstract of DOI:10.1021/acs.inorgchem.6b00100

2(3),9(10),16(17),23(24)-Tetrakis(dibutylamino)phthalocyanine compounds M{Pc[N(C₄H₉)₂]₄} (1-5; M = 2H, Mg, Ni, Cu, Zn) were prepared and characterized by a range of spectroscopic methods in addition to elemental analysis. Electrochemical and electronic absorption spectroscopic studies revealed the more effective conjugation of the nitrogen lone pair of electrons in the dibutylamino side chains with the central phthalocyanine π system in M{Pc[N(C₄H₉)₂]₄} than in M{Pc[N(C₄H₉)₂]₈}, which, in turn, results in superior third-order nonlinear-optical (NLO) properties of H₂{Pc[N(C₄H₉)₂]₄} (1) over H₂{Pc[N(C₄H₉)₂]₈}, as revealed by the obviously larger effective imaginary third-order molecular hyperpolarizability (Im{χ⁽³⁾}) of 6.5 × 10^-11 esu for the former species than for the latter one with a value of 3.4 × 10^-11 esu. This is well rationalized on the basis of both structural and theoretical calculation results. The present result seems to represent the first effort toward directly connecting the peripheral functional substituents, electronic structures, and NLO functionality together for phthalocyanine molecular materials, which will be helpful for the development of functional phthalocyanine materials via molecular design and synthesis even through only tuning of the peripheral functional groups.

Full text of DOI:10.1021/acs.inorgchem.6b00100

Article
pubs.acs.org/IC
Four Dibutylamino Substituents Are Better Than Eight in Modulating
the Electronic Structure and Third-Order Nonlinear-Optical
Properties of Phthalocyanines
Yuxiang Chen, Wei Cao, Chiming Wang, Dongdong Qi,* Kang Wang, and Jianzhuang Jiang*
Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, Department of Chemistry,
University of Science and Technology Beijing, Beijing 100083, China
S
* Supporting Information
ABSTRACT: 2(3),9(10),16(17),23(24)-Tetrakis(dibutylamino)-
phthalocyanine compounds M{Pc[N(C4H9)2]4} (1−5; M = 2H, Mg, Ni,
Cu, Zn) were prepared and characterized by a range of spectroscopic
methods in addition to elemental analysis. Electrochemical and electronic
absorption spectroscopic studies revealed the more effective conjugation of
the nitrogen lone pair of electrons in the dibutylamino side chains with the
central phthalocyanine π system in M{Pc[N(C4H9)2]4} than in M{Pc[N-
(C4H9)2]8}, which, in turn, results in superior third-order nonlinear-optical
(NLO) properties of H2{Pc[N(C4H9)2]4} (1) over H2{Pc[N(C4H9)2]8},
as revealed by the obviously larger effective imaginary third-order
molecular hyperpolarizability (Im{χ(3)}) of 6.5 × 10−11 esu for the former
species than for the latter one with a value of 3.4 × 10−11 esu. This is well
rationalized on the basis of both structural and theoretical calculation
results. The present result seems to represent the first effort toward
directly connecting the peripheral functional substituents, electronic structures, and NLO functionality together for
phthalocyanine molecular materials, which will be helpful for the development of functional phthalocyanine materials via
molecular design and synthesis even through only tuning of the peripheral functional groups.
octakis[4,5-bis(dimethylamino)-3,6-difluorophthalocyaninato]-
zinc,9 and tetrakis[4-chloro-5-(di-n-hexylamino)]-
phthalocyanines,10 to the best of our knowledge. For the
purpose of affording new members in this series toward
developing novel and improved functionalities, dibutylamino
groups have been introduced onto the phthalocyanine
periphery very recently.11 Cyclic tetramerization of 4,5-
bis(dibutylamino)phthalonitrile led to the successful isolation
of 2,3,9,10,16,17,23,24-octakis(dibutylamino)phthalocyanines.
This, however, is not exactly true for the case of
2(3),9(10),16(17),23(24)-tetrakis(dibutylamino)-
phthalocyanine counterparts with 4-(dibutylamino)-
phthalonitrile instead of 4,5- bis(dibutylamino)phthalonitrile
INTRODUCTION
■
Phthalocyanines have been an important class of dyes and
pigments with extensive applications in paint, printing, and
textile industries since their serendipitous discovery at the
beginning of the past century.1 More recently, their unique
spectroscopic and electrochemical properties in combination
with extraordinary chemical and thermal stabilities endow these
tetrapyrrole compounds great application potentials as
advanced functional materials in the fields of medicine,2 optical
storage,3 photocatalysis,4 and molecular-based nanoelectronic
devices.5 For the purpose of enhancing their application-related
functionalities, a wide range of different kinds of substituents
including the electron-withdrawing nitro, fluoro, chloro, bromo,
and iodo groups and the electron-donating alkyl, alkoxyl, ether,
and alkylthio groups have been incorporated onto their
peripheral and/or nonperipheral positions to modulate their
electronic structure and electrochemical and spectroscopic
properties.6 However, amino- and/or alkylamino-substituted
phthalocyanines have been relatively less studied despite their
significant role in tuning the electronic structure and, in turn,
the electrochemical and spectroscopic properties of phthalo-
cyanines expected from their strong electron-donating nature,
with corresponding examples limited to tetraaminophthylocya-
nines,7 2,3,9,10,16,17,23,24-octaaminophthalocyaninatonickel,8
tetrakis[4-chloro-5-(dimethylamino)phthalocyaninato]zinc,9
tetrakis[4-fluoro-5-(dimethylamino)phthalocyaninato]zinc,9
1
as the precursor because satisfactory H NMR spectra could
not be obtained for this series of compounds despite their right
mass spectroscopic (MS) results. Fortunately, the nature of the
ease of tetrakis(dibutylamino)phthalocyanine compounds
toward oxidation in comparison with their octakis-
(dibutylamino)phthalocyanine counterparts was finally recog-
nized because of their raised highest occupied molecular orbital
(HOMO) energy level associated with the more effective
conjugation between the dibutylamino nitrogen atoms and
phthalocyanine π system, resulting in the successful isolation
Received: January 14, 2016
© XXXX American Chemical Society
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Scheme 1. Synthesis and Schematic Molecular Structures of 2(3),9(10),16(17),23(24)-Tetrakis(dibutylamino)phthalocyanine
a
Compounds M{Pc[N(C4H9)2]4} (1−5; M = 2H, Mg, Ni, Cu, Zn) with Four Regioisomers
a
(i) Fe, HCl, CH3OH, reflux; (ii) first NaNO2, HCl, 0 °C; then CuCl, HCl, toluene, 80 °C; (iii) HN(C4H9)2, K2CO3, DMF, 140 °C; (iv) for M =
Mg, Mg, I2, n-C5H11OH, reflux; for M = 2H, MgPc, CF3COOH, rt; for M = Ni, Cu, and Zn, 1, M(OAc)2·H2O, DBU, n-C5H11OH, reflux.
and characterization of this series of tetrakis(dibutylamino)-
phthalocyanine compounds by a series of spectroscopic
methods including H NMR spectroscopy with the help of
relationship for the alkylamino-substituted phthalocyanines.
Nevertheless, it must be pointed out that, despite the great
efforts paid toward revealing the direct relationship between the
peripheral substituents and photo/electric/magnetic function-
alities for phthalocyanine materials, most results in this field still
remain at the stage of realizing the connection of substituents
with functionality-related electronic/spectroscopic/electro-
chemical properties. As a consequence, the present result
seems to represent the first effort toward directly connecting
the peripheral functional substituents, electronic structure, and
NLO functionality together for phthalocyanine molecular
materials, which will surely be helpful for the future
development of functional phthalocyanine materials via
molecular design and synthesis even through only tuning of
the peripheral functional groups.
1
hydrazine hydrate in CD2Cl2.
In the present paper, we describe the synthesis and
spectroscopic and electrochemical properties of
2(3),9(10),16(17),23(24)-tetrakis(dibutylamino)-
phthalocyanine compounds M{Pc[N(C4H9)2]4} (1−5; M =
2H, Mg, Ni, Cu, Zn; Scheme 1). Electrochemical studies
revealed the significant shift in both the first oxidation and
reduction potentials in the negative direction, even relative to
those of M{Pc[N(C4H9)2]8}. Their lowest energy Q band also
takes an obvious red shift in comparison with their octakis-
(dibutylamino)phthalocyanine counterparts. This appears
strange at first glance but is well rationalized by the more
effective conjugation of the nitrogen lone pair of electrons in
the dibutylamino side chains with the central phthalocyanine π
system for the former species relative to the latter ones on the
basis of structural and theoretical calculation results. This, in
turn, results in superior nonlinear-optical (NLO) properties of
H2{Pc[N(C4H9)2]4} (1) over H2{Pc[N(C4H9)2]8}, as revealed
by the obviously larger effective imaginary third-order
molecular hyperpolarizability (Im{χ(3)}) of 6.5 × 10−11 esu
for the former species than the latter one with a value of 3.4 ×
10−11 esu.
At the end of this section, it is worth noting that some
phthalocyanine derivatives have been found to exhibit efficient
second-order and, in particular, third-order NLO effects owing
to their large polarizable π-conjugated system, showing
application potential in optical limiting, optical computing,
and optical communication.12 This is surely true for the present
phthalocyanine compounds. However, in the present case,
investigation of the application-related third-order molecular
hyperpolarizability of 1 in a comparative manner with that of
H2{Pc[N(C4H9)2]8} clearly reveals the structure−property
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization. The
key precursor for the synthesis of tetrakis(dibutylamino)-
phthalocyanine compounds, 4-(dibutylamino)phthalonitrile,
was prepared13 in three steps starting from 4-nitrophthalonitrile
with an overall yield of 28.7% (Scheme 1). Despite the
relatively low melting point for this compound associated with
its peripheral bulky and long dibutylamino side chain, 44−46
°C, single crystals of 4-(dibutylamino)phthalonitrile suitable for
X-ray diffraction analysis were fortunately obtained by
evaporating its methanol/dichloromethane (1:1) solution to
almost dryness at 0 °C. Interestingly, by means of a similar
method, single crystals of 4,5-bis(dibutylamino)phthalonitrile
suitable for X-ray diffraction analysis were also afforded by
evaporating its dichloromethane solution at 0 °C and rendering
X-ray diffraction analysis for this compound also possible for
the first time. Figure 1 shows the molecular structure of 4-
(dibutylamino)phthalonitrile. For comparative reasons, the
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dihedral angles between the benzene plane and the plane
determined by the carbon atom in the benzene moiety that
connects to the nitrogen atom, the nitrogen atom, and the
carbon atom neighboring to the nitrogen atom in the side
chain(s), Φ1 and Φ2, for 4-(dibutylamino)phthalonitrile, 3.98
and 7.98°, relative to those for 4,5-bis(dibutylamino)-
phthalonitrile, 18.42−18.95° and 55.08−55.88°, respectively
(Figure 1 and Table S4 in the Supporting Information (SI).
Cyclic tetramerization of 4-(dibutylamino)phthalonitrile in
refluxing n-pentanol in the presence of magnesium pentanoate
led to the isolation of Mg{Pc[N(C4H9)2]4} (2) in a yield of
28.1%. Further treatment of 2 yielded in situ with trifluoro-
acetic acid without the necessary isolation induced isolation of
the corresponding metal free phthalocyanine 1 in a yield of
19.8%. The reaction of 1 with M(OAc)2·H2O (M = Ni, Cu,
Zn) in refluxing n-pentanol and 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU) afforded the corresponding phthalocyaninatometal
complexes M{Pc[N(C4H9)2]4} (3−5; M = Ni, Cu, Zn) in
yields of 50.3−57.9%. Satisfactory elemental analysis results
were obtained for all of the newly prepared phthalocyanine
derivatives 1−5 after repeated column chromatography
followed by recrystallization. Their matrix-assisted laser
desorption ionization time-of-flight (MALDI-TOF) MS spectra
clearly show intense signals for the molecular ion M+, which
closely resemble the simulated ones, as shown in Figure S1 in
the SI. These newly prepared phthalocyanine derivatives have
also been characterized by other spectroscopic methods
1
including H NMR, electronic absorption, and IR spectros-
copies.
Figure 1. Molecular structures of 4-(dibutylamino)phthalonitrile (a)
and 4,5-bis(dibutylamino)phthalonitrile (b) with the dihedral angle
between the benzene plane and the plane determined by the carbon
atom in the benzene moiety that connects to the nitrogen atom, the
nitrogen atom, and the carbon atom neighboring to the nitrogen atom
in the side chain, Φ1 and Φ2 (c). All of the hydrogen atoms have been
omitted for clarity.
The 1H NMR spectra for compounds 1−3 and 5 containing
either no metal ion or diamagnetic metal ion were recorded in
CD2Cl2 in the presence of ca. 1% (v/v) hydrazine hydrate,
which was used to reduce the small fraction of paramagnetic
species to the neutral ones because of the easy oxidation nature
of these compounds, in particular in solution as detailed below.
Because of the paramagnetic nature of the divalent copper ion,
molecular structure of 4,5-bis(dibutylamino)phthalonitrile is
also displayed in the same figure. According to diffraction
analysis, the length of the C−N bond that links the benzene
moiety and dibutylamino group for 4-(dibutylamino)-
phthalonitrile amounts to 1.363 Å, which is significantly shorter
than those in 4,5-bis(dibutylamino)phthalonitrile with values of
1.392 and 1.395 Å, suggesting much stronger interaction
between the dibutylamino group and benzene moiety in the
former species due to the more effective p−π conjugation
between the nitrogen lone pair of electrons and the benzene
moiety than that in 4,5-bis(dibutylamino)phthalonitrile. This is
further clearly rationalized on the basis of the much smaller
1
the H NMR spectrum could not be recorded for the copper
complex 4 even with the help of hydrazine hydrate. As shown
in Figure S2 in the SI, the three groups of aromatic protons due
to the eight α protons and four β protons of the Pc ring in 1
give multiple signals at 8.97−8.85, 8.42−8.35, and 7.39−7.30
ppm with an integral ratio of 1:1:1, while the methylene,
methylene, methylene, and methyl protons in the peripheral
butyl groups give multiple signals at 3.75−3.72, 1.95−1.92,
1.64−1.62, and 1.20−1.14 ppm with an integral ratio of 2:2:2:3.
This is also true for the metal complexes M{Pc[N(C4H9)2]4}
(2, 3, and 5; M = Mg, Ni, Zn; Table S2 in the SI), except the
Table 1. Electronic Absorption Data for M{Pc[N(C4H9)2]4} (M = 2H, Mg, Ni, Cu, Zn), MPc (M = 2H, Zn), and
M{Pc[N(C4H9)2]8} (M = 2H, Zn) in CHCl3
compound
λmax, nm (log ε, M−1 cm−1
)
1
340 (4.86)
341 (4.94)
340 (4.99)
362 (5.04)
325 (4.99)
334 (4.98)
336 (4.91)
341 (5.00)
345 (4.52)
463 (4.54)
517 (4.47)
525 (4.64)
681 (4.56)
765 (5.08)
750 (5.17)
692 (5.22)
751 (5.24)
744 (5.15)
755 (5.23)
755 (5.23)
732 (5.37)
674 (5.13)
a
H2{Pc[N(C4H9)2]8}
725 (5.08)
656 (5.15)
b
H2Pc
2
3
4
5
462 (4.42)
459 (4.49)
459 (4.51)
463 (4.36)
513 (4.37)
508 (4.34)
515 (4.46)
513 (4.43)
522 (4.56)
672 (4.61)
664 (4.53)
672 (4.64)
674 (4.61)
657 (4.59)
609 (4.30)
a
Zn{Pc[N(C4H9)2]8}
b
ZnPc
a
b
Cited from ref 11. Cited from ref 15.
C
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lack of signals due to the inner isoindole protons. Because of
the effective p−π conjugation between the peripheral nitrogen
atoms and the central phthalocyanine chromophore, the ring
current of 1 becomes smaller than that in H2Pc(tBu)4, resulting
in the observation of two inner isoindole proton signals for the
former compound at a relatively lower field of 0.32−0.16 ppm
in comparison with that at −2.17 ppm for the latter species.14
At the end of this paragraph, it is worth noting that the mixture
nature of the series of 2(3),9(10),16(17),23(24)-tetrakis-
(dibutylamino)phthalocyanine compounds M{Pc[N(C4H9)2]4}
(1−3 and 5; M = 2H, Mg, Ni, Zn; each of which is composed
of four regioisomers, as shown in Scheme 1) results in the
multiplicity for all of the proton signals in their NMR spectra.
This is actually also responsible for the relatively broadened
absorption bands and redox peaks, as revealed in their
electronic absorption spectra and cyclic voltammetry (CV)
curves detailed below.
dibutylamino side chains with the central phthalocyanine π
system for the former species relative to the latter one.
Figure S7 in the SI displays the IR spectra of 1−5. As can be
found, in addition to the absorption bands that are contributed
from the central aromatic Pc macrocycle including the wagging
and torsion vibrations of the C−H groups, isoindole ring
stretching vibrations, and the CN aza group stretching
vibrations,16 the absorptions observed at ca. 1090−1024 cm−1
are attributed to the C−N stretching vibrations of the
−N(C4H9)2 groups. The intense absorption bands observed
at ca. 2954−2858 cm−1 in the IR spectra are the C−H
stretching vibrations of the −CH2− and −CH3 groups of the
side chains. In the IR spectrum of 1, a weak band at ca. 3291
cm−1 can be assigned to the N−H stretching vibrations of the
isoindole moieties, which disappear in the IR spectra of the
metal complexes M{Pc[N(C4H9)2]4} (2−5; M = Mg, Ni, Cu,
Zn).
The electronic absorption spectra of 1−5 were recorded in
CHCl3, and the data are summarized in Table 1. As shown in
Figure 2, 1 shows a typical nonaggregated molecular electronic
Electrochemical Properties. The electrochemical behav-
ior of 1−5 was investigated by CV in CH2Cl2 containing 0.1 M
[NBu4][ClO4]. The half-wave potentials are summarized in
Table 2. Figure 3 displays the cyclic voltammogram of
Table 2. Comparison in the Electrochemical Data between
1−5 and M{Pc[N(C4H9)2]8} (M = 2H, Mg, Cu, Zn)
compound
Oxd2
Oxd1
Red1
Red2
ref
1
+0.73
+0.26
+0.51
+0.10
+0.088
+0.20
+0.21
+0.29
+0.068
+0.10
−1.03
−0.89
−1.38
−1.36
−1.10
−1.13
−1.16
−1.35
−1.26
−1.40
−1.27
this work
11
H2{Pc[N(C4H9)2]8}
2
+0.56
this work
11
Mg{Pc[N(C4H9)2]8}
3
+0.78
+0.76
this work
this work
11
4
Cu{Pc[N(C4H9)2]8}
5
+0.53
this work
11
Zn{Pc[N(C4H9)2]8}
Cu{Pc[N(C4H9)2]4} (4) as a typical representative. As can
be seen, 4 exhibited two one-electron-like ligand-based
oxidations at 0.76 and 0.21 V and one one-electron-like
ligand-based reduction at −1.13 V with relatively broadened
peak width in comparison with that of the octakis-
(dibutylamino)phthalocyanine counterparts11 because of its
four-regioisomer-containing mixture nature. As can be seen in
Figure 3, linear convolution integral (LCI) analysis clearly gives
four individual peaks for the first oxidation process of 4 due to
the four regioisomers of this compound.17 This is surely also
true for the remaining redox processes of this compound and
remaining members of the whole series (Table SS2 in the SI).
In addition, a comparison of the corresponding electrochemical
properties between M{Pc[N(C4H9)2]4} and M{Pc[N-
(C4H9)2]8} indicates that the introduction of four dibutylamino
groups onto the peripheral positions of the phthalocyanine
chromophore induces a significant shift in both the first
oxidation and reduction potentials in the negative direction,
relative to those of octakis(dibutylamino)phthalocyanine
counterparts.11 This is truly strange at first glance but
fortunately well rationalized by the more effective conjugation
of the nitrogen lone pair of electrons in the dibutylamino side
chains with the central phthalocyanine π system for the former
species relative to the latter ones on the basis of structural and
theoretical calculation as detailed below. At the end of this
section, it is worth noting that the first oxidation potentials for
M{Pc[N(C4H9)2]4} (1−5; M = 2H, Mg, Ni, Cu, Zn) and
Figure 2. Comparison in the electronic absorption spectra between 1
(red) and H2{Pc[N(C4H9)2]8} (blue) as well as 5 (red) and
Zn{Pc[N(C4H9)2]8} (blue) in CHCl3.
absorption spectrum of the metal-free phthalocyanines with a
relatively broadened Soret absorption band appearing at 340
nm and a broad Q band at 765 nm. Interestingly, unlike the
case of H2Pc(tBu)4,14 the Q band for 1 does not get split
because of the effective p−π conjugation between the
peripheral nitrogen atoms and the central phthalocyanine
chromophore for the latter species, which, in turn, extends the
effective conjugation system for this compound. It is worth
noting that the band at 463−517 nm is due to the n → π*
transitions arising from the nitrogen lone pair of electrons.
Insertion of a metal ion into the phthalocyanine central core
induces an increase in the molecular symmetry from D2h for the
central tetrapyrrole core of 1 to D4h for M{Pc[N(C4H9)2]4}
(2−5; M = Mg, Cu, Ni, Zn), resulting in a significantly
narrowed Q band in the range of 744−755 nm for these four
metal complexes (Figure S6 in the SI and Table 1). In
particular, in a comparison with M{Pc[N(C4H9)2]8} (M = 2H,
Mg, Cu, Zn), the Q absorption band for 1−5 takes a significant
red shift (15−24 nm in the Q-band region), revealing the more
extended conjugation system of M{Pc[N(C4H9)2]4} than
M{Pc[N(C4H9)2]8}. This, in turn, suggests the more effective
conjugation of the nitrogen lone pair of electrons in the
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Figure 3. Cyclic voltammogram of 4 in CH2Cl2 containing 0.1 M [NBu4][ClO4] at a scan rate of 30 mV s−1 together with its LCI analysis.
Figure 4. Simulated molecular structures and LMO analysis for M{Pc[N(C4H9)2]4} and M{Pc[N(C4H9)2]8} (M = 2H, Zn).
M{Pc[N(C4H9)2]8} (M = 2H, Mg, Cu, Zn) locate in the ranges
of +0.068 to +0.26 and +0.088 to +0.51 eV, respectively,
indicating the easier oxidation of the four dibutylamino-
substituted phthalocyanines (or at least one of the four
regioisomers) than the eight dibutylamino-substituted counter-
parts. This, in turn, becomes responsible for the quite high
instability of the former species in particular in solutions. As
carbon/hydrogen/nitrogen and LanL2DZ for zinc, was
employed to describe the wave function.19 For the purposes
of comparison, those for M{Pc[N(C4H9)2]8} (M = 2H, Zn)
have also been calculated at the same level. It is worth
mentioning that, because of the lack of single-crystal structures
for M{Pc[N(C4H9)2]4}, the simulated ones were employed for
further theoretical studies. Fortunately, such a fact that the
simulated structures for M{Pc[N(C4H9)2]8} (M = 2H, Zn)
correspond well with their single-crystal molecular structures
ensures the validity of the present treatment. Additional
support for this point comes from the good accordance in
the two dihedral angles, Φ1 and Φ2, revealed by single-crystal
X-ray diffraction analysis for 4-(dibutylamino)phthalonitrile
with the calculated ones for M{Pc[N(C4H9)2]4} (M = 2H, Zn)
vide infra.
1
mentioned above, satisfied H NMR spectra for 1−3 and 5
could only be obtained with the help of hydrazine hydrate.
Density Functional Theory (DFT) Investigation. In
order to gain insight into the electronic structures and
electronic absorption spectra of these newly prepared
phthalocyanine compounds, DFT calculation over M{Pc[N-
(C4H9)2]4} (M = 2H, Zn) was carried out using B3LYP for
molecular optimization and τHCTHhyb for the simulation of
transition spectra.18 A mixed basis set, including 6-31G(d) for
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Figure 5. Molecular orbital energies of MPc, M{Pc[N(C4H9)2]4}, and M{Pc[N(C4H9)2]8} (M = 2H, Zn).
The optimized structures of M{Pc[N(C4H9)2]4} including
M{Pc[N(C4H9)2]4-C4}, M{Pc[N(C4H9)2]4-C1}, M{Pc[N-
(C4H9)2]4-Cs}, and M{Pc[N(C4H9)2]4-C2v} together with
M{Pc[N(C4H9)2]8} (M = 2H, Zn) are shown in Figure 4. As
can be found, the distorted degree between the peripheral
−N(C4H9)2 substituents and the central Pc macrocycle can be
measured by two key dihedral angles, Φ1 and Φ2 (Figure 4 and
Table S5 in the SI). For M{Pc[N(C4H9)2]4} (M = 2H, Zn),
DFT calculation results reveal that Φ1 and Φ2 locate in the
ranges of 1−7° and 3−25°, respectively. These data are in good
accordance with those found in 4-(dibutylamino)phthalonitrile
by X-ray diffraction analysis (Figure 1 and Table S3 in the SI).
However, the steric hindrance between the two dibutylamino
groups at the neighboring peripheral positions of the
phthalocyanine skeleton for M{Pc[N(C4H9)2]8} (M = 2H,
Zn) leads to an increase in both Φ1 and Φ2 to 23−26° and 56−
58°, which are in line with the experimental findings for both
4,5-bis(dibutylamino)phthalonitrile and M{Pc[N(C4H9)2]8}
(M = 2H, Zn).11 These results obviously suggest the more
effective p−π conjugation between the dibutylamino nitrogen
atoms and the central phthalocyanine chromophore in the
former tetrakis(dibutylamino)phthalocyanines in comparison
with that in the latter octakis(dibutylamino)phthalocyanine
counterparts, which is further revealed by the local molecular
orbital (LMO) analysis20 (Figure 4). For the purpose of further
disclosing the p−π conjugating strength in a quantitative
manner, the Mayer bond order analysis21 for both M{Pc[N-
(C4H9)2]4} and M{Pc[N(C4H9)2]8} (M = 2H, Zn) is carried
out. According to the calculating result, the total π(N−C) bond
order between the peripheral −N(C4H9)2 substituents and the
central Pc macrocycle amounts to 0.449 for M{Pc[N-
(C4H9)2]4}, which is only 0.155 for M{Pc[N(C4H9)2]8}.
In order to differentiate the effect of four peripheral
dibutylamino substituents from eight at the phthalocyanine
periphery, the molecular orbitals for MPc, M{Pc[N(C4H9)2]4},
and M{Pc[N(C4H9)2]8} (M = 2H, Zn) have been calculated in
a comparative manner. As can be clearly seen in Figure 5 and
Table S9 in the SI, incorporation of the electron-donating
dibutylamino substituents onto the peripheral positions of
phthalocyanine leads to an obvious increase in the HOMO
from −5.19 eV for H2Pc to (−4.21)−(−4.16) eV for 1 and
−4.25 eV for H2{Pc[N(C4H9)2]8} and the lowest unoccupied
molecular orbital (LUMO) energy level from −3.20 eV for
H2Pc to (−2.42)−(−2.49) eV for 1 and −2.5 eV for
H2{Pc[N(C4H9)2]8}. Interestingly, four peripheral dibutylami-
no substituents introduced are more effective than eight in
elevating the HOMO and LUMO energy levels of the
phthalocyanine system because of the much more effective
p−π conjugation in 1 than in H2{Pc[N(C4H9)2]8}. This is also
true for the phthalocyaninatozinc complexes (Figure 5). These
results appear to be in line with the experimental data, as
revealed by the electrochemical method detailed above.
In order to clarify the effect of phthalocyanine peripheral
dibutylamino substituents on the electronic absorption spectra,
the electron transition properties for M{Pc[N(C4H9)2]4} and
M{Pc[N(C4H9)2]8} (M = Zn, 2H) are carried out using time-
dependent DFT calculation. As can be found in Tables S7 and
S8 in the SI, the electronic absorption spectra in the range of
400−800 nm can be divided into two regions including region I
(600−800 nm) and region II (400−600 nm) based on the
different electron transition models. For 1, the absorption in
region I (600−800 nm) originates from the transition from
HOMO to LUMO/LUMO+1. The energy gap between
HOMO and LUMO/LUMO+1 for H2Pc is 1.99/2.08 eV
(Figure 5). However, the introduction of four electron-
donating −N(C4H9)2 substituents results in a significant
decrease in the HOMO and LUMO/LUMO+1 energy gap to
1.73/1.85, 1.75/1.84, 1.72/1.89, and 1.74/1.84 eV for H2{Pc-
[N(C4H9)2]4-C1}, H2{Pc[N(C4H9)2]4-Cs}, H2{Pc[N(C4H9)2]4-
C2v}, and H2{Pc[N(C4H9)2]4-C4}, respectively, mainly because
of the effective p−π conjugation between the peripheral
−N(C4H9)2 substituents and the central Pc macrocycle. This,
in turn, induces a significant red shift in the phthalocyanine Q
bands of 1 relative to those of H2Pc, with 718/681 nm for
H2{Pc[N(C4H9)2]4-C1}, 701/695 nm for H2{Pc[N(C4H9)2]4-
Cs}, 729/672 nm for H2{Pc[N(C4H9)2]4-C2v}, and 704/690 nm
for H2{Pc[N(C4H9)2]4-C4} to 598/594 nm for H2Pc. It is
worth mentioning that overlapping of the Q bands for the four
isomers leads to a relatively broadened Q band as a whole, as
observed for H2{Pc[N(C4H9)2]4 by UV−vis spectrometry. This
F
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Article
Figure 6. Open-aperture Z-scan traces of 1 (a), H2{Pc[N(C4H9)2]8} (b), H2[Pc(OC5H11)8] (c), and H2Pc(tBu)4 (d) in toluene with theoretical
fitting curves.
is surely also true for Zn{Pc[N(C4H9)2]4} (5; Table S8 in the
SI). Because of the lack of effective p−π conjugation between
the peripheral −N(C4H9)2 substituents and the central Pc
macrocycle in the octakis(dibutylamino)phthalocyanine com-
pounds as detailed above, the incorporation of eight
dibutylamino groups onto the phthalocyanine periphery leads
to a lesser degree of decrease in the HOMO and LUMO/
LUMO+1 energy gap to 1.99/2.08 eV, which, in turn, results in
a lesser degree of red-shifted Q bands from 598/594 nm for
H2Pc to 689/678 nm for H2{Pc[N(C4H9)2]8} (Tables S6 and
S7 in the SI). These results are in line with the experimental
findings.
The bands of region II (400−600 nm) for M{Pc[N-
(C4H9)2]4} (M = Zn, 2H) mainly come from the transition
from HOMO−4 to LUMO/LUMO+1. As can be found in
Table S8 in the SI, the starting orbital HOMO−4 distributes at
both the peripheral −N(C4H9)2 substituents and the central Pc
macrocycle, revealing the coupled orbital nature of this orbital
due to the p[−N(C4H9)2]−π(Pc) conjugation. On the basis of
the photon-induced electron-transfer analysis22 (Table S8 in
the SI), the broad absorption band in region II (400−600 nm)
for 1 is induced by the electron density movements from the
peripheral substituent to the central macrocycle along with the
electron pathway constructed by p−π conjugation, indicating
the origination of this new absorption band associated with the
introduction of the peripheral conjugated system. This is also
true for 5 (Table S8 in the SI).
NLO Properties. The third-order NLO properties of 1 and
H2{Pc[N(C4H9)2]8} in toluene at a concentration of 1.0 × 10−4
mol L−1 were studied using the Z-scan technique with a
Nd:YAG laser as the light source (a pulse width of 8 ns and a
wavelength of 532 nm).23 For comparative purposes, those of
H2Pc(tBu)4 and H2[Pc(OC5H11)8] have also been investigated.
In addition, measurement over pure toluene was also
performed underexactly the same experimental conditions to
verify the origination of the third-order NLO properties from
the phthalocyanine material rather than from the solvent.
The NLO absorption components were obtained by the Z-
scan experiment under an open-aperture configuration. As
shown in Figure 6, both 1 and H2{Pc[N(C4H9)2]8} in toluene
display reverse saturation absorption under an open-aperture
configuration. According to the equations employed previously
to well describe the third-order NLO absorption process,24 the
effective imaginary third-order molecular hyperpolarizability
(Im{χ(3)}) for 1 was calculated to be 6.5 × 10−11 esu, which is
significantly larger than that for H2{Pc[N(C4H9)2]8}, 3.4 ×
10−11 esu, because of the more extended conjugation system for
the former species, as detailed above. In addition, even the
effective imaginary third-order molecular hyperpolarizability
(Im{χ(3)}) for H2{Pc[N(C4H9)2]8} was found to still be
obviously larger than that of H2[Pc(OC5H11)8] and H2Pc(tBu)4
with values of 2.2 × 10−11 and 8.2 × 10−12 esu, respectively,
revealing the more electron-donating property of the −N-
t
(C4H9)2 group than the OC5H11 and Bu groups.
G
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Inorganic Chemistry
Article
temperatures below 0 °C. Single crystals of 4-(dibutylamino)-
phthalonitrile suitable for X-ray diffraction analysis were obtained by
evaporating its methanol/dichloromethane (1:1) solution to almost
dryness at 0 °C. Mp: 44−46 °C. 1H NMR (CDCl3, 400 MHz): δ 7.48
(d, J = 9.00 Hz, 1 H), 6.83 (d, J = 2.20 Hz, 1 H), 6.74 (q, J = 3.76 Hz,
1 H), 3.31 (t, J = 7.74 Hz, 4 H), 1.56 (m, J = 7.63 Hz, 4 H), 1.37 (m, J
= 7.49 Hz, 4 H), 0.97 (t, J = 7.30 Hz, 6 H).
CONCLUSION
■
In summary, 2(3),9(10),16(17),23(24)-tetrakis(dibutylamino)-
phthalocyanine compounds have been prepared and spectro-
scopically characterized for the first time. Electrochemical
measurement reveals the significantly raised HOMO and
LUMO energy levels due to the effective p−π conjugation
between the peripheral nitrogen atoms and phthalocyanine
chromophore in comparison with their 2,3,9,10,16,17,23,24-
octakis(dibutylamino)phthalocyanine counterparts. This, in
turn, results in superior third-order NLO properties of 1 over
H2{Pc[N(C4H9)2]8}. This result directly connects the periph-
eral functional substituents, electronic structures, and NLO
functionalities of phthalocyanine molecular materials together
for the first time, opening a new way toward developing novel
functional phthalocyanine materials via molecular design by
relying upon the peripheral functional groups.
Synthesis of 2(3),9(10),16(17),23(24)-Tetrakis(dibutylamino)-
phthalocyaninatomagnesium Mg{Pc[N(C4H9)2]4} (2). A mixture of
magnesium turnings (24.6 mg, 1.03 mmol) and a small amount of
iodine in anhydrous n-pentanol (5 mL) were refluxed for 1 h under a
slow steam of nitrogen. Then 4-(dibutylamino)phthalonitrile (522 mg,
2.04 mmol) was added. The resulting mixture was refluxed for another
24 h. After being cooled, the solvent was evaporated. The crude
product was chromatographed on an alkaline alumina column with
dichloromethane/methanol (100:2) as the eluent. Repeated column
chromatography followed by recrystallization from THF and methanol
gave a dark-green solid with a yield of 28.1% (150 mg). Elem anal.
Calcd for C64H84N12Mg·0.75CH2Cl2·0.125C4H8O: C, 70.07; H, 7.79;
N, 15.03. Found: C, 69.99; H, 7.86; N, 15.09. 1H NMR (CD2Cl2, 400
MHz): δ 9.12−8.93 (m, 4 H), 8.59−8.52 (m, 4 H), 7.42−7.36 (m, 4
H), 3.78−3.74 (m, 16 H), 1.98−1.91 (m, 16 H), 1.68−1.59 (m, 16 H),
1.17−1.13 (m, 24 H). MS (MALDI-TOF). Calcd for C64H84N12Mg
[(M)+]: m/z 1044.7. Found: m/z 1044.1. UV−vis [CHCl3; λ, nm (log
ε, ×10−5 M−1 cm−1)]: 751 (1.74), 672 (0.404), 513 (0.237), 462
(0.263), 362 (1.09).
EXPERIMENTAL SECTION
■
General Procedures. 4-Nitrophthalonitrile was purchased from
J&K Scientific Ltd. n-Pentanol was freshly distilled from sodium. N,N-
Dimethylformamide (DMF) was freshly distilled from CaH2. Column
chromatography was carried out on silica gel (200−300 mesh) or
alkaline alumina (200−300 mesh) with the indicated eluents. The
electrolyte [NBu4][ClO4] was recrystallized twice from tetrahydrofur-
an (THF). All other reagents and solvents were used as received.
1H NMR spectra were recorded on a Bruker DPX 400 spectrometer
in CDCl3 or CD2Cl2/80% hydrazine hydrate (100:1, v/v). Spectra
were referenced internally using the residual solvent resonance (7.26
ppm in CDCl3 or 5.32 ppm in CD2Cl2 for 1H NMR) relative to SiMe4.
Electronic absorption spectra were recorded on a Hitachi U-2910
spectrophotometer. IR spectra were recorded as KBr pellets using a
Bruker Tensor 37 spectrometer with 2 cm−1 resolution. MALDI-TOF
MS spectra were taken on a Bruker BIFLEX III ultrahigh-resolution
Fourier transform ion cyclotron resonance mass spectrometer with α-
cyano-4-hydroxycinnamic acid as the matrix. Elemental analyses were
performed on an Elementar Vavio El III elemental analyzer.
Electrochemical measurements were carried out with a BAS CV-
50W voltammetric analyzer. The cell comprised inlets for a glassy-
carbon-disk working electrode with a diameter of 2.0 mm and a silver-
wire counter electrode. The reference electrode was Ag+/Ag (a
solution of 0.01 M AgNO3 and 0.1 M TBAP in acetonitrile), which
was connected to the solution by a Luggin capillary, whose tip was
placed close to the working electrode. It was corrected for junction
potentials by being referenced internally to the ferrocenium/ferrocene
(Fc+/Fc) couple [E1/2(Fc+/Fc) = 0.501 V vs SCE]. Typically, a 0.1 M
solution of [NBu4][ClO4] in CH2Cl2 containing a 1 mM sample was
purged with nitrogen for 10 min, and then the voltammograms were
recorded at ambient temperature. Crystal data for 4-(dibutylamino)-
phthalonitrile and 4,5- bis(dibutylamino)phthalonitrile were deter-
mined by X-ray diffraction analysis at 298 K using an Oxford
Diffraction Gemini E system with Mo Kα radiation (λ = 0.71073 Å),
and details of the structure refinement are given in Table S3 in the SI.
CCDC 1442281 and 1442282 containing the supplementary crystallo-
graphic data for this paper can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Synthesis of 2(3),9(10),16(17),23(24)-Tetrakis(dibutylamino)-
phthalocyanine H2{Pc[N(C4H9)2]4} (1). Compound 2 (117 mg, 0.112
mmol) was dissolved in CF3COOH (3 mL) and stirred under a slow
steam of nitrogen for 15 min. The reaction mixture was then poured
into cold water and neutralized with an ammonia solution. The
precipitate collected was washed several times with CH3OH and then
applied on an alkaline alumina column with dichloromethane as the
eluent. Repeated chromatography followed by recrystallization from
THF and methanol afforded the target compound as a dark-green
solid with a yield of 70.5% (80.7 mg). Elem anal. Calcd for C64H86N12·
0.125CH2Cl2·0.25H2O: C, 74.16; H, 8.42; N, 16.18. Found: C, 74.11;
1
H, 8.24; N, 16.21. H NMR (CD2Cl2, 400 MHz): δ 8.97−8.85 (m, 4
H), 8.42−8.35 (m, 4 H), 7.39−7.30 (m, 4 H), 3.75−3.72 (m, 16 H),
1.95−1.92 (m, 16 H), 1.64−1.62 (m, 16 H), 1.20−1.14 (m, 24 H),
0.32−0.16 (m, 2 H). MS (MALDI-TOF). Calcd for C64H86N12
[(M)+]: m/z 1022.7. Found: m/z 1022.9. UV−vis [CHCl3; λ, nm
(log ε, ×10−5 M−1 cm−1)]: 765 (1.20), 681 (0.366), 517 (0.296), 463
(0.343), 340 (0.728).
Synthesis of 2(3),9(10),16(17),23(24)-Tetrakis(dibutylamino)-
phthalocyaninatonickel Ni{Pc[N(C4H9)2]4} (3). A mixture of com-
pound 1 (80.5 mg, 0.0787 mmol), Ni(OAc)2·4H2O (78.3 mg, 0.315
mmol), and DBU (0.5 mL) in anhydrous n-pentanol (3 mL) was
heated to reflux for 10 h under nitrogen. After being cooled to room
temperature, the mixture was evaporated to dryness under reduced
pressure, and the residue was chromatographed on an alkaline alumina
column using dichloromethane/methanol (200:1) as the eluent.
Repeated chromatography followed by recrystallization from THF
and methanol gave a dark-green sample with a yield of 50.6% (43.0
mg). Elem anal. Calcd for C64H84N12Ni·0.75CH2Cl2: C, 67.99; H,
7.53; N, 14.69. Found: C, 67.83; H, 7.58; N, 14.64. 1H NMR (CD2Cl2,
400 MHz): δ 8.64−8.33 (m, 4 H), 8.11−7.86 (m, 4 H), 7.20−6.99 (m,
4 H), 3.65−3.57 (m, 16 H), 1.91−1.89 (m, 16 H), 1.64−1.61 (m, 16
H), 1.18−1.16 (m, 24 H). MS (MALDI-TOF). Calcd for
C64H84N12Ni [(M)+]: m/z 1078.6. Found: m/z 1078.5. UV−vis
[CHCl3; λ, nm (log ε, ×10−5 M−1 cm−1)]: 744 (1.42), 664 (0.342),
508 (0.220), 459 (0.312), 325 (0.977).
Synthesis of 2(3),9(10),16(17),23(24)-Tetrakis(dibutylamino)-
phthalocyaninatocopper Cu{Pc[N(C4H9)2]4} (4). When the procedure
described above using Cu(OAc)2·H2O (62.8 mg, 0.315 mmol) instead
of Ni(OAc)2·4H2O as the starting material was employed, compound
4 was obtained after repeated chromatography with dichloromethane/
methanol (200:1) as the eluent, followed by recrystallization from
THF and methanol, which gave a dark-green sample with a yield of
50.3% (43.0 mg). Elem anal. Calcd for C64H84N12Cu·0.25CH2Cl2·
Synthesis of 4-(Dibutylamino)phthalonitrile. To a mixture of 4-
chlorophthalonitrile (6.70 g, 41.2 mmol) and K2CO3 (11.4 g, 82.5
mmol) in DMF (70 mL) at 140 °C was added dibutylamine (15.9 g,
124 mmol). The resulting system was heated at the same temperature
for 18 h. After being cooled to room temperature, the reaction mixture
was poured into water (200 mL), extracted with petroleum ether,
dried over Na2SO4, and filtered. After the solvent was removed by
evaporation under reduced pressure, the crude product was chromato-
graphed on a silica gel column using dichloromethane/petroleum
ether (1:1) as the eluent, giving the target compound as light-pink oil
with a yield of 66.2% (6.97 g), which becomes a pink solid at
H
DOI: 10.1021/acs.inorgchem.6b00100
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Article
0.125H2O·0.5CH3OH: C, 69.16; H, 7.78; N, 14.95. Found: C, 69.13;
H, 7.58; N, 14.89. MS (MALDI-TOF). Calcd for C64H84N12Cu
[(M)+]: m/z 1083.6. Found: m/z 1083.4. UV−vis [CHCl3; λ, nm (log
ε, ×10−5 M−1 cm−1)]: 755 (1.70), 672 (0.432), 508 (0.220), 459
(0.323), 334 (0.966).
(c) Kadish, K. M., Smith, K. M., Guilard, R., Eds. The Porphyrin
Handbook; Academic Press: San Diego, CA, 2000 and 2003; Vols. 1−
20. (d) Mingos, D. M. P., Jiang, J., Eds. Functional Phthalocyanine
Molecular Materials, Structure and Bonding; Springer-Verlag: Heidel-
berg, Germany, 2010; Vol. 135.
Synthesis of 2(3),9(10),16(17),23(24)-Tetrakis(dibutylamino)-
phthalocyaninatozinc Zn{Pc[N(C4H9)2]4} (5). When the above-
described procedure used to prepare Ni{Pc[N(C4H9)2]4} (3) with
Zn(OAc)2·2H2O (69.1 mg, 0.315 mmol) instead of Ni(OAc)2·4H2O
as the starting material was employed, pure 5 was isolated as a dark-
green sample with a yield of 57.9% (49.5 mg). Elem anal. Calcd for
C64H84N12Zn·0.5CH2Cl2: C, 68.60; H, 7.59; N, 14.88. Found: C,
68.44; H, 7.58; N, 14.83. 1H NMR (CD2Cl2, 400 MHz): δ 9.13−9.09
(t, 4 H), 8.60−8.59 (d, 4 H), 7.44−7.43 (d, 4 H), 3.78−3.75 (t, 16 H),
1.98−1.90 (m, 16 H), 1.66−1.61 (m, 16 H), 1.17−1.14 (t, 24 H). MS
(MALDI-TOF). Calcd for C64H84N12Zn [(M)+]: m/z 1084.6. Found:
m/z 1084.5. UV−vis [CHCl3; λ, nm (log ε, ×10−5 M−1 cm−1)]: 755
(1.68), 674 (0.407), 513 (0.269), 463 (0.230), 336 (0.815).
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ASSOCIATED CONTENT
* Supporting Information
■
S
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Phys. Chem. C 2015, 119, 21992−22000.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00100.
Validity of the functional τHCTHhyb and a systematic
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M{Pc[N(C4H9)2]4} and M{Pc[N(C4H9)2]8}, experi-
mental and simulated isotopic patterns for 1−5, 1H
NMR spectrum of 1−3 and 5, electronic absorption and
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analytical and MS data for 1−5, H NMR data (δ) for
1−3 and 5, crystal data and structure refinements for 4-
(dibutylamino)phthalonitrile and 4,5-bis(dibutylamino)-
phthalonitrile, two dihedral angles, Φ1 and Φ2, for 4-
(dibutylamino)phthalonitrile, 4,5-bis(dibutylamino)-
phthalonitrile, M{Pc[N(C4H9)2]4} (M = 2H, Zn), and
M{Pc[N(C4H9)2]8} (M = 2H, Zn), electron density
difference plots of electron transitions and molecular
orbital maps for MPc (M = 2H, Zn), M{Pc[N(C4H9)2]8}
(M = 2H, Zn), and M{Pc[N(C4H9)2]4} (M = 2H, Zn)
(PDF)
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X-ray crystallographica data in CIF format (CIF)
X-ray crystallographica data in CIF format (CIF)
(9) Venkatramaiah, N.; Rocha, D. M. G. C.; Srikanth, P.; Almeida
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: qdd@ustb.edu.cn (D.Q.).
*E-mail: jianzhuang@ustb.edu.cn (J.J.).
Notes
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(11) Chen, Y.; Cao, W.; Wang, K.; Jiang, J. Inorg. Chem. 2015, 54,
9962−9967.
(12) (a) Claessens, C. G.; Gonzalez-Rodriguez, D.; Torres, T. Chem.
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C. G.; Gonzalez-Rodriguez, D.; Rodriguez-Morgade, M. S.; Medina,
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Phthalocyanines 2014, 18, 305−315. (e) Qi, D.; Jiang, J.
ChemPhysChem 2015, 16, 1889−1897.
(13) (a) D’Souza, S.; Antunes, E.; Litwinski, C.; Nyokong, T. J.
Photochem. Photobiol., A 2011, 220, 11−19. (b) Han, M.; Zhang, X.;
Zhang, X.; Liao, C.; Zhu, B.; Li, Q. Polyhedron 2015, 85, 864−873.
(c) Patrick, D. A.; Bakunov, S. A.; Bakunova, S. M.; Kumar, E. V. K. S.;
Lombardy, R. J.; Jones, S. K.; Bridges, A. S.; Zhirnov, O.; Hall, J. E.;
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Key Basic Research
Program of China (Grants 2012CB224801 and
2013CB933402), National Natural Science Foundation of
China (Grants 21290174, 21301017, and 21401009), and
Beijing Municipal Commission of Education is gratefully
acknowledged.
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