Benzo-fused low symmetry metal-free tetraazaporphyrin and phthalocyanine analogs: Synthesis, spectroscopy, electro chemistry, and density functional theory calculations

Article abstract of DOI:10.1142/S1088424610002203

A whole series of low-symmetry, metal-free tetrapyrrole analogs 1-11 ranging from tetraazaporphyrin H₂TAP(tBu)₄ to napthalocyanine H₂Nc(tBu)₄ via H₂Pc(tBu) ₄ have been designed and prepared. Their electronic structures have been spectroscopically and electrochemically investigated. Through the preparation of a series of tetrapyrrole compounds with just the same number of four tert-butyl substituents at similar positions of the macrocyclic ring, the effect intrinsic to the enlargement of the ring system has been easily extracted and understood. Along with the increase in the size of central conjugated system, the half-wave potentials of the first oxidation for 1-11 are shifted significantly to the negative direction. This is also true for the half-wave potentials of the first reduction process of this series of tetrapyrrole derivatives but in a slight manner. These results indicate that along with the ring extension, the energy of the HOMO increases significantly while that of the LUMO only slightly increases. The effect of the ring size of these tetrapyrrole compounds on the electronic absorption and fluorescence spectra is also clear: both the Q band and fluorescence shifts to longer wavelength with the increase in the number of fused benzene rings but with a diminishing increase due to the extending of the central conjugated system. These results were reasonably explained by considering the energy levels of frontier molecular orbitals of the series of compounds obtained by the calculations using DFT method at the B3LYP/6-31G(d) level.

Full text of DOI:10.1142/S1088424610002203

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2010; 14: 421–437
DOI: 10.1142/S1088424610002203
Published at http://www.worldscinet.com/jpp/
Benzo-fused low symmetry metal-free tetraazaporphyrin
and phthalocyanine analogs: synthesis, spectroscopy,
electrochemistry, and density functional theory calculations
a,b
b
b,cž
b,c
Renjie Li , Dongdong Qi , Jianzhuang Jiang
and Yongzhong Bian*
a
College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China
Department of Chemistry, Shandong University, Jinan 250100, China
Department of Chemistry, University of Science and Technology Beijing, Beijing 100083, China
b
c
Received 1 September 2009
Accepted 30 October 2009
ABSTRACT: A whole series of low-symmetry, metal-free tetrapyrrole analogs 1–11 ranging from
tetraazaporphyrin H TAP(tBu) to napthalocyanine H Nc(tBu) via H Pc(tBu) have been designed and
2
4
2
4
2
4
prepared. Their electronic structures have been spectroscopically and electrochemically investigated.
Through the preparation of a series of tetrapyrrole compounds with just the same number of four tert-
butyl substituents at similar positions of the macrocyclic ring, the effect intrinsic to the enlargement of
the ring system has been easily extracted and understood. Along with the increase in the size of central
conjugated system, the half-wave potentials of the first oxidation for 1–11 are shifted significantly to
the negative direction. This is also true for the half-wave potentials of the first reduction process of
this series of tetrapyrrole derivatives but in a slight manner. These results indicate that along with the
ring extension, the energy of the HOMO increases significantly while that of the LUMO only slightly
increases. The effect of the ring size of these tetrapyrrole compounds on the electronic absorption and
fluorescence spectra is also clear: both the Q band and fluorescence shifts to longer wavelength with the
increase in the number of fused benzene rings but with a diminishing increase due to the extending of
the central conjugated system. These results were reasonably explained by considering the energy levels
of frontier molecular orbitals of the series of compounds obtained by the calculations using DFT method
at the B3LYP/6-31G(d) level.
KEYWORDS: tetraazaporphyrin, naphthalocyanine, electrochemistry, density functional theory
calculation.
agents in laser printers and photocopiers, and materials
INTRODUCTION
for optical storage. These macrocyclic molecular mate-
rials are expected to show potential applications in the
area of nonlinear optics [3], organic semiconductors for
OFETs [4], solar cell functional materials [5], liquid crys-
tals [6], gas sensors [7], photodynamic therapy agents [8],
antimycotic material [9], and corrosion inhibitors [10]. It
is worth pointing out that in a number of phthalocyanine
applications, the electrochemistry and, in particular, the
opticalspectroscopicchemistry,areofcriticalimportance.
However, it is impossible to find one phthalocyanine
compound that shows electronic spectroscopic property
with absorptions meeting all desired wavelengths. As a
result, structurally modified phthalocyanine derivatives
Porphyrin compounds have revealed significant role in
natural biological processes such as photosynthesis and
respiration [1]. Their most important artificial structural
analogs, phthalocyanines, have found widespread indus-
trial applications as dyes and pigments, as well as cata-
lysts for removing sulfur from crude oil since their first
synthesis at the beginning of the last century [1, 2]. In
recent years, they have been applied as photoconducting
ž
SPP full member in good standing
*
Correspondence to:Yongzhong Bian, email: yzbian@sas.ustb.
edu.cn, tel: +86 10-6233-4509
Copyright © 2010 World Scientific Publishing Company

4
22
R. LI ET AL.
including tetraazaporphyrins (TAPs), naphthalocyanines
tetrapyrrole analogs ranging from tetraazaporphyrin
(
Ncs), anthracocyanines (Acs), and their low symmetry
H TAP(tBu) to napthalocyanine H Nc(tBu) with just
2
4
2
4
analogs with different electronic spectroscopic properties
have received increasing interest from researchers very
recently.
the same number of four tert-butyl substituents at similar
positions of the macrocyclic ring, sheds more light on the
electronic and spectroscopic properties of low-symmetry
tetraazaporphyrin and phthalocyanine analogs.
Generally two methods have been employed to dimin-
ishthemolecularsymmetryofphthalocyaninederivatives.
Asymmetrical β-bonded substitution of the peripheral
protons has been revealed to exert only a marginal influ-
ence on the spectroscopy of the π-system, resulting in
slight shift in the wavelengths of the major absorption
centers [11]. In good contrast, reducing the molecular
symmetry by altering the ring size through asymmetri-
cal peripheral fusion of aromatic rings such as benzene
onto the TAP skeleton showed significant impact on the
spectroscopy of the π-system [12]. As early as in 1992,
Konami and co-workers successfully prepared and iso-
lated the whole series of low-symmetry phthalocyanine
analogs ranging from phthalocyanine to naphthalocya-
nine with one, two, and three peripheral fused benzene
ring(s) onto the Pc skeleton [13]. Later, Kobayashi,
Leznoff and Stillman studied their electronic structure
systematically with MCD and theoretical methods [14].
In 1997, Kenney and co-workers obtained a series of
similar phthalocyanine analogs with eight butyloxy sub-
stituents at the non-peripheral positions of the rings [15].
Quite recently, Kobayashi and co-workers completed the
whole series of low-symmetry tetraazaporphyrin analogs
ranging from tetraazaporphyrin to phthalocyanine with
one, two, and three peripheral fused benzene ring(s) onto
the TAP skeleton, in which different octyloxy and phenyl
groups are attached at the pyrrole and isoindole ring(s),
respectively [16].
EXPERIMENTAL
Measurements
1
H NMR spectra were recorded on a Bruker DPX 300
spectrometer (300 MHz) in CDCl using the residual sol-
3
vent resonance of CHCl at 7.26 ppm relative to SiMe as
3
4
internal reference. MALDI-TOF mass spectra were taken
on a Bruker Daltonics BIFLEX III™ MALDI-TOF mass
spectrometer with α-cyano-4-hydroxycinnamic acid as
matrix. Electronic absorption spectra were recorded on
a Hitachi U-4100 spectrophotometer. Fluorescence spec-
tra were recorded on a K2 ISIS spectrometer. IR spectra
were recorded as KBr pellets using a BIORAD FTS-165
-
1
spectrometer with 2 cm resolution. Elemental analyses
were performed by the Institute of Chemistry, Chinese
Academy of Sciences.
Electrochemical measurements were carried out with
a BAS CV-50W voltammetric analyzer. The cell com-
prised inlets for a glassy carbon disk working electrode of
3
.0 mm in diameter and a silver-wire counter electrode.
+
The reference electrode was Ag/Ag (0.01 M), which was
connected to the solution by a Luggin capillary whose
tip was placed close to the working electrode. It was cor-
rected for junction potentials by being referenced inter-
+
nally to the ferrocenium/ferrocene (Fc /Fc) couple [E
1
/2
In the present paper, we describe the synthesis, spec-
troscopic, and electrochemical properties of a whole
series of low-symmetry, metal-free tetra-tert-butylated
tetraazaporphyrin (TAP) and phthalocyanine (Pc) analogs
ranging from tetraazaporphyrin H TAP(tBu) to naptha-
+
(
Fc /Fc) = 0.50 V vs. SCE]. Typically, a 0.1 M solution
of [Bu N][ClO ] in CH Cl containing 0.5 mM of sample
4
4
2
2
was purged with nitrogen for 10 min, then the voltammo-
grams were recorded at ambient temperature. The scan
rate was 20 and 10 mV.s for CV and DPV, respectively.
2
4
-1
locyanine H Nc(tBu) via H Pc(tBu) (Scheme 1). Intro-
2
4
2
4
duction of four tert-butyl groups (which give the weakest
effect to the π-system as a substituent) at similar posi-
tions of the macrocyclic ring for the whole series of com-
pounds renders it possible to extract the effect intrinsic to
the enlargement of the ring system through spectroscopic
and electrochemical methods. The low-symmetry TAP
and Pc analogs represent mixtures of all possible regioi-
somers due to the placement of the tert-butyl groups and
also possible tautomers due to placement of the inner
NH protons. Though the mixtures of regioisomers and
tautomers were unavoidable, they represented collective
characters. Most of the results were reasonably explained
by considering the energy levels of frontier molecular
orbitals of corresponding compounds obtained by the
calculations using DFT method at the B3LYP/6-31G(d)
level. The present work, representing a systemic study on
the synthesis, spectroscopic, and electrochemical prop-
erties of the whole series of low-symmetry, metal-free
Computational details
The structure model of 1–11 (Scheme 1) were obtained
by adding benzene rings onto different pyrrole rings of
tetraazaporphyrin and then attaching four methyl groups
(instead of the experimental tertiary butyl groups) onto
the peripheral positions. For the reason of calculation
economy, isomers caused by peripheral substituents were
not considered and the methyl groups were added onto
the same side of each pyrrole/isoindole positions. All the
possible isomers due to the relative positions of the inner
hydrogen atoms were involved, but only the most stable
isomers were used to compare with the experimental
results. The hybrid density functional B3LYP (Becke-
Lee-Young-Parr composite of exchange-correlation func-
tional) method was used for both geometry optimizations
and property calculations [17]. In all cases, the 6-31G(d)
basis set was used. The Berny algorithm using redundant
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 422–437

BENZO-FUSED, LOW SYMMETRY, METAL-FREE TETRAAZAPORPHYRIN AND PHTHALOCYANINE ANALOGS
423
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 423–437

4
24
R. LI ET AL.
+
internal coordinates was employed in energy minimiza-
tion and the default cutoffs were used throughout [18].
Using the energy-minimized structures generated in the
previous step, normal coordinate analyses were carried
out. Electronic absorption spectroscopic calculations
were made by TD-DFT method [19] on the basis of the
optimized structures. All calculations were carried out
using the Gaussian 03 program [20] in the IBM P690
system in Shandong Province High Performance Com-
puting Center.
ppm 5.86 (s, 1 H, H ), 1.27 (s, 9 H, tBu ). MS (EI ): m/z
a
+
+
[M - 57] (77), (-tBu); [M - 26] (51), (-CN). Anal. calcd.
(%) for C H N : C, 71.61; H, 7.51; N, 20.88; found: C,
8
10
2
71.32; H, 7.82; N, 20.45.
Preparation of tetra-tert-butylated tetraazapor-
phyrin H TAP(tBu) (1). A mixture of cis-1,2-dicyano-
2
4
3,3-dimethyl-1-butene (47 mg, 0.35 mmol) and lithium
(5 mg, 0.72 mmol) in n-pentanol (4 mL) was heated to
reflux under nitrogen for 4 h. After being cooled to room
temperature, the resulting green solution was poured into
methanol (100 mL) containing a few drops of concen-
trated HCl. The precipitate was collected by filtration and
chromatographed on a silica gel column using CHCl3/
hexane (1:1) as eluent. A violet band containing the tar-
get complex was developed which was collected and
rotary evaporated. Repeated chromatography followed
Chemicals
n-pentanol was distilled from sodium. Dichlo-
romethane for voltammetric studies was freshly distilled
from CaH under nitrogen. Column chromatography
2
was carried out on silica gel columns (Merck, Kieselgel
by recrystallization from CHCl and MeOH gave pure
3
6
0, 70–230 mesh) with the indicated eluents. Prepara-
target compound 1 as green powder (11 mg, 24%).
General procedure for the preparation of low sym-
metry tetraazaporphyrin analogs 2–5. A mixture of
cis-1,2-dicyano-3,3-dimethyl-1-butene and 4-tert-bu-
tylphthalonitrile in n-pentanol (4 mL) with a few drops
of DBU was heated to reflux under nitrogen for 4 h. After
being cooled to room temperature, the mixture was evap-
orated under reduced pressure and the residue was chro-
tive thin-layer chromatography (TLC) was performed
on glass plates (20 cm × 20 cm) with silica gel GF-254.
All other reagents and solvents were used as received.
Cis-1,2-dicyano-3,3-dimethyl-1-butene was prepared
with slightly improved procedure according to previous
report [21]. The compounds 4-tert-butylphthalonitrile
[
(
22], 6-tert-butyldicyanonaphthalene [23], H Pc(tBu)4
6) [24b,c], and H Nc(tBu) (11) [24b,c] were prepared
2
matographed on a silica gel column using CHCl /hexane
2
4
3
according to the published procedures.
(1:2) as eluent to give the first violet band and second
Preparation of cis-1,2-dibromo-3,3-dimethyl-1-
butene. To a mixture of 3,3-dimethyl-1-butyne (10 g,
blue band containing H TAP(tBu) (1) and H A3P1(tBu)
(2), respectively. After collecting these two bands, the
2
4
2
4
0
.12 mol) and benzoyl peroxide (0.05 g) as catalyst in
dry hexane (500 mL) at -10 °C, a solution of bromine
6.4 mL, 0.12 mol) in hexane (10 mL) was added drop-
column was further eluted with CHCl /hexane (2:3) to
3
give the third blue band containing H adj-A2P2(tBu) (3)
2
4
(
and H opp-A2P2(tBu) (4) following by the fourth blue
2
4
wise over 1 h. The mixture was stirred for 4 h at -10 °C
and another 0.5 h at room temperature with continuous
irradiation from a 450W tungsten lamp. The solvent was
evaporated under reduced pressure. Then, the residue
was purified by silica gel column chromatography with
band containing H A1P3(tBu) (5). Then pure CHCl
2 4
3
eluted the fifth light blue band containing H Pc(tBu)
2
4
(6). The third blue band containing H adj-A2P2(tBu) (3)
2
4
and H opp-A2P2(tBu) (4) was further separated using
2
4
thin-layer chromatography (TLC) with CHCl /hexane
3
toluene as eluent to give the target compound as a yellow
(4:7) as eluent, giving two bands containing H opp-
2
1
oil (12.42 g, 50%). H NMR (300 MHz, CDCl ): δ, ppm
A2P2(tBu) (4) and H adj-A2P2(tBu) (3), respectively.
3
4
2
4
6
.65 (s, 1 H, H ), 1.23 (s, 9 H, tBu). It is noteworthy that
Repeated chromatography followed by recrystallization
a
the second band of the eluent gives the trans-1,2-dibro-
mo-3,3-dimethyl-1-butene as solid. H NMR (300 MHz,
from CHCl and MeOH gave pure target metal-free tet-
3
1
raazaporphyrin analogs 2–5 together with H TAP(tBu)
2
4
CDCl ): δ, ppm 6.33 (s, 1 H, H ), 1.29 (s, 9 H, tBu).
(1) and H Pc(tBu) (6) as dark powders. H2A3P1(tBu)
3
a
2
4
4
Preparationofcis-1,2-dicyano-3,3-dimethyl-1-butene.
(2). By employing the general procedure described above
with cis-1,2-dicyano-3,3-dimethyl-1-butene (281 mg, 2.1
mmol) and 4-tert-butylphthalonitrile (64 mg, 0.35 mmol)
as precursors, 13 mg (12%) H A3P1(tBu) (2) was iso-
A mixture of cis-1,2-dibromo-3,3-dimethyl-1-butene
(
(
7.21 g, 0.03 mol) and CuCN (8.04 g, 0.09 mol) in DMF
100 mL) was heated to reflux for 8 h under nitrogen.
2
4
After being cooled to room temperature, the mixture was
poured to ammonia (500 mL, 15 M) and blown with air
for 24 h. Then the solution was extracted with toluene
lated as the main target compound. H2adj-A2P2(tBu)4
(3) and H2opp-A2P2(tBu) (4). By using the general
4
procedure described above with cis-1,2-dicyano-3,3-
dimethyl-1-butene (47 mg, 0.35 mmol) and 4-tert-bu-
tylphthalonitrile (64 mg, 0.35 mmol) as precursors, 1.7
mg (1.5%) H adj-A2P2(tBu) (3) and 2.4 mg (2.2%)
(
3 × 100 mL). The combined extracts were washed with
water, dried over anhydrous MgSO , and evaporated. The
4
crude product was chromatographed over a silica gel col-
umn with petroleum ether/chloroform (1:1) as eluent. The
solvent was evaporated under reduced pressure, giving
2
4
H opp-A2P2(tBu) (4) were isolated as the main target
2
4
compounds, respectively. H2A1P3(tBu) (5). By utiliz-
4
white target compound of cis-1,2-dicyano-3,3-dimethyl-
ing the general procedure described above with cis-1,2-
dicyano-3,3-dimethyl-1-butene (47 mg, 0.35 mmol) and
1
1
-butene (1.82 g, 45%). H NMR (300 MHz, CDCl ): δ,
3
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 424–437

BENZO-FUSED, LOW SYMMETRY, METAL-FREE TETRAAZAPORPHYRIN AND PHTHALOCYANINE ANALOGS
425
4
-tert-butylphthalonitrile (387 mg, 2.1 mmol) as precur-
obtained by mixed cyclic tetramerization of two kinds
of ortho dinitrile precursors. Mixed cyclic tetramer-
ization of cis-1,2-dicyano-3,3-dimethyl-1-butene and
4-tert-butylphthalonitrile catalyzed with organic base
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in refluxing
n-pentanol led to the formation of a mixture of metal-free
compounds H TAP(tBu) (1), H A3P1(tBu) (2), H adj-
sors, 18 mg (16%) H A1P3(tBu) (5) was isolated as the
main target compound.
2
4
General procedure for the preparation of low sym-
metry phthalocyanine analogs 7–10. A mixture of
4
-tert-butylphthalonitrile and 6-tert-butyldicyanonaph-
thalene in n-pentanol (4 mL) with a few drops of DBU
was heated to 120 °C under nitrogen for 4 h. After being
cooled to room temperature, the mixture was evaporated
under reduced pressure and the residue was chromato-
2
4
2
4
2
A2P2(tBu) (3), H opp-A2P2(tBu) (4), H A1P3(tBu)
4
2
4
2
4
(5), and H Pc(tBu) (6). As expected, using different
2
4
ratio of the two dinitriles could change the product dis-
graphed on a silica gel column using CHCl /hexane (1:1)
tribution, giving H A3P1(tBu) (2), H adj-A2P2(tBu)
3
2
4
2
4
as eluent to give the first light blue band and second
and H opp-A2P2(tBu) (3 and 4), or H A1P3(tBu) (5)
2 4 2 4
light greenish blue band containing H Pc(tBu) (6) and
as the main product. When only cis-1,2-dicyano-3,3-
dimethyl-1-butene or 4-tert-butylphthalonitrile was used,
H TAP(tBu) (1) or H Pc(tBu) (6) was isolated as the
2
4
H P3N1(tBu) (7), respectively. After collecting these
2
4
two bands, the column was further eluted with CHCl3/
hexane (3:2) to give the third greenish blue band con-
taining H adj-P2N2(tBu) (8) and H opp-P2N2(tBu) (9).
2
4
2
4
sole target compound. It is noteworthy that purification of
general phthalocyanine compounds has been a challenge
for chemists, not to mention the separation of a series
of closely related and isomeric phthalocyanines. Fortu-
nately, in the present case, all the metal-free compounds
including the two opposite and adjacent isomers 3 and 4
could be separated and purified by general silica gel col-
umn chromatography with the help of thin-layer chroma-
tography (TLC). This is also true for the low-symmetry
phthalocyanine analogs ranging between H Pc(tBu) (6)
2
4
2
4
Then CHCl /hexane (2:1) led to the fourth green band
3
containing H P1N3(tBu) (10). It is worth noting that due
2
4
to the instability of H Nc(tBu) (11) in the weak acidic
2
4
silica gel column associated with its low potential of the
first oxidation process, H Nc(tBu) (11) was eluted with
2
4
CHCl /MeOH (95:5) as the fifth green band, which was
3
further purified by chromatography using neutral Al O
2
3
columnwithCHCl aseluent.Thethirdgreenishblueband
3
2
4
containing H adj-P2N2(tBu) (8) and H opp-P2N2(tBu)
and H Nc(tBu) (11) including H P3N1(tBu) (7), H adj-
2
4
2
4
2 4 2 4 2
(
9) was further separated using thin-layer chromatogra-
P2N2(tBu) (8), H opp-P2N2(tBu) (9), H P1N3(tBu)
4 2 4 2 4
phy (TLC) with CHCl /hexane (12:7) as eluent, giving
(10). Again to facilitate the purification, metal-free
tetra-tert-butylated naphthalocyanine H Nc(tBu) (11)
3
two bands containing H opp-P2N2(tBu) (9) and H adj-
2
4
2
2
4
P2N2(tBu) (8), respectively. Repeated chromatography
was synthesized from cyclic tetramerization of 6-tert-
butyldicyanonaphthalene.
4
followed by recrystallization from CHCl and MeOH
3
gave pure target metal-free phthalocyanine analogs 7–10
together with H Pc(tBu) (6) and H Nc(tBu) (11) as dark
It is worth noting that the separation and, in particu-
lar, identification of the adjacently and oppositely ben-
zo-fused tetraazaporphyrin analogs H adj-A2P2(tBu)
2
4
2
4
powders. H2P3N1(tBu) (7). By utilizing the abovede-
4
2
4
scribed with 4-tert-butylphthalonitrile (387 mg, 2.1 mmol)
and 6-tert-butyldicyanonaphthalene (82 mg, 0.35 mmol)
as starting material, 15 mg (14%) H P3N1(tBu) (7) was
(3) and H opp-A2P2(tBu) (4) are quite difficult
2
4
due to their identical molecular weight. However,
repeated TLC processes finally led to the isolation of
two fractions, with the first one being the oppositely
benzo-fused H opp-A2P2(tBu) (4) due to its rela-
2
4
isolated as the main target compound. H2adj-P2N2(tBu)4
8) and H2opp-P2N2(tBu) (9). By employing the general
(
4
2
4
procedure described above with 4-tert-butylphthalonitrile
tively smaller molecular polarity than the adjacently
(64 mg, 0.35 mmol) and 6-tert-butyldicyanonaphthalene
substituted analog, and the second one is the H adj-
2
(
82 mg, 0.35 mmol) as starting mateiral, 3 mg (2.1%) H adj-
A2P2(tBu) (3).
2
4
P2N2(tBu) (8) and 5 mg (3.4%) H opp-P2N2(tBu) (9)
were isolated as the main target compounds, respectively.
According to theoretical results using time-dependent
density functional theory (TDDFT) method at B3LYP/6–
4
2
4
H2P1N3(tBu) (10). By employing the general procedure
31G(D) level, the molecular polarity for H opp-A2P2,
4
2
described above with 4-tert-butylphthalonitrile (64 mg,
0.00 Debye, is smaller than that for H adj-A2P2, 2.30
2
0
2
.35 mmol) and 6-tert-butyldicyanonaphthalene (492 mg,
.1 mmol) as starting material, 12 mg (8.2%) H P1N3(tBu)
Debye. In addition, the fact that the simulated electronic
absorption spectra for H opp-A2P2 and H adj-A2P2 cor-
2
4
2
2
(
10) was isolated as the main target compound.
respond well with those of 4 and 3, respectively, confirms
such identification (it is noteworthy again that during the
calculations, the four tert-butyl groups are simplified
with methyl groups due to their similar and very weak
effect on either the molecular polarity or the electronic
absorption spectroscopy) [24]. This is also true for the
phthalocyanine analogs H adj-P2N2(tBu) (8) and H opp-
RESULTS AND DISCUSSION
Synthesis
2
4
2
All the low-symmetry, metal-free tetra-tert-butylated
P2N2(tBu) (9) with molecular polarity of 1.78 and 0.00
4
tetraazaporphyrin and phthalocyanine analogs were
Debye, respectively.
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 425–437

4
26
R. LI ET AL.
a
Table 1. Analytical and mass spectroscopic data for compounds 1–5 and 7–10
+
b
Compound
M , m/z
Analysis, %
H
C
N
H TAP(tBu) (1)
538.8 (538.73)
588.9 (588.79)
638.6 (638.85)
638.7 (638.85)
689.1 (688.91)
788.7 (789.02)
838.7 (839.08)
838.6 (839.08)
888.3 (889.14)
71.71 (71.34)
73.21 (73.44)
75.10 (75.20)
74.96 (75.20)
76.56 (76.71)
79.23 (79.61)
79.98 (80.16)
80.32 (80.16)
81.40 (81.05)
7.65 (7.86)
7.68 (7.53)
7.58 (7.26)
7.38 (7.26)
7.25 (7.02)
6.80 (6.64)
6.50 (6.49)
6.31 (6.49)
6.26 (6.35)
20.45 (20.80)
19.12 (19.03)
17.43 (17.54)
17.79 (17.54)
16.51 (16.27)
14.56 (14.20)
13.62 (13.35)
13.18 (13.35)
12.47 (12.60)
2
4
H A3P1(tBu) (2)
2
4
H adj-A2P2(tBu) (3)
2
4
H opp-A2P2(tBu) (4)
2
4
H A1P3(tBu) (5)
2
4
H P3N1(tBu) (7)
2
4
H adj-P2N2(tBu) (8)
2
4
H opp-P2N2(tBu) (9)
2
4
H P1N3(tBu) (10)
2
4
a
b
Calculated values given in parentheses. By MALDI-TOF mass spectrometry.
Satisfactory elemental analysis results have been
obtained for all these metal-free tetra-tert-butylated tet-
raazaporphyrin and phthalocyanine analogs 2–5 and
investigated by cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) in CH Cl . All these metal-free
2
2
tetrapyrrole compounds display two one-electron oxida-
tions labeled as Oxd -Oxd and up to three one-electron
7
–10 including the metal-free tetraazaporphyrin 1 and
1
2
phthalocyanine 6, which have good solubility in common
reductions labeled as Red -Red within the electrochemi-
1 3
organic solvents such as CHCl , CH Cl , and toluene,
cal window of CH Cl under the present conditions. The
2 2
3
2
2
after repeated column chromatography and recrystalliza-
tion (Table 1). They have also been characterized by vari-
ous spectroscopic methods. The MALDI-TOF spectra of
separation between the reduction and oxidation peak
potentials for all the processes lies between 65 to 90 mV,
showing their reversible or quasi-reversible nature. All
these processes are attributed to successive removal from
or addition of one electron to the ring-based orbitals. The
half-wave redox potential values vs. SCE are summarized
in Table 2. Figure 1 displays the differential pulse volta-
mmogram for compound 10 as a typical representative.
The availability of the electrochemical data for
the whole series of metal-free tetra-tert-butylated tet-
raazaporphyrin and phthalocyanine analogs including
H TAP(tBu) (1), H Pc(tBu) (6), and H Nc(tBu) (11)
+
all these compounds showed the molecular ion [M] sig-
nals with correct isotopic pattern (Table 1). The isotopic
pattern closely resembles the simulated one as exempli-
fied by the spectrum of the compound 8 given in Fig. S2
(
Supporting information).
Electrochemistry
The electrochemical behavior of all the metal-free
tetraazaporphyrin and phthalocyanine analogs was
2
4
2
4
2
4
Table 2. Half-wave redox potentials of the metal-free tetraazaporphyrin and phthalocyanine derivatives 1–11
(
V vs. SCE) in CH Cl containing 0.1 M TBAP
2 2
o
a
1/2
Compound
Oxd2
Oxd1
Red1
Red2
Red3
∆E
b
1
+1.61
+1.58
+1.50
+1.42
+1.34
+1.27
+1.26
+1.25
+1.23
+1.16
+1.03
+1.31
+1.27
+1.14
+1.12
+1.07
+0.95
+0.95
+0.89
+0.90
+0.80
+0.65
-0.74
-0.74
-0.79
-0.73
-0.79
-0.80
-0.82
-0.84
-0.83
-0.86
-0.89
-1.18
-1.21
-1.22
-1.24
-1.20
-1.23
-1.15
-1.22
-1.15
-1.15
-1.17
-1.52
—
2.05
2.01
1.93
1.85
1.86
1.75
1.77
1.73
1.73
1.66
1.55
2
c
3
-1.55
—
4
5
—
b
c
c
H Pc(tBu) (6)
-1.47
-1.45
—
2
4
7
8
9
1
-1.49
—
0
b
H Nc(tBu) (11)
2
-1.44
4
a
o
∆E
is the potential difference between the first oxidation and first reduction processes, i.e. the HOMO-
/2
1
b
c
LUMO gap of corresponding molecules. Cited from reference 24c. Recorded by DPV.
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BENZO-FUSED, LOW SYMMETRY, METAL-FREE TETRAAZAPORPHYRIN AND PHTHALOCYANINE ANALOGS
427
energy of the HOMO increases significantly while that of
the LUMO only slightly increases. The half-wave poten-
tials of the second reduction for 1–11 seem independent
of the central ring size. This is also true for the half-wave
potentials of the third reduction of 1–11.
The trends can also be seen from the changes in poten-
tial differences across the whole series. The potential dif-
ference between Oxd and Red (∆E° ), which represents
1
1
1/2
the gap between the HOMO and LUMO, decreases grad-
ually from 2.05 V for 1 to 1.55 for 11 (Table 2). As the
first oxidation step and first reduction step, respectively,
involve the HOMO and the LUMO of the molecule, the
energy difference between these two redox processes for
compounds 1–11 corresponds to their electrochemical
molecular band gap. The value ∆E°1/2 thus should reflect
the energy necessary for the transition of an electron from
the HOMO to the LUMO of corresponding metal-free
tetrapyrrole derivatives and therefore should correlate
with the lowest energy optical transition in the electronic
absorption spectrum of these compounds. As detailed
below, the diminished trend observed for the ∆E°1/2 value
of metal-free tetrapyrrole derivatives in the order from
H TAP(tBu) (1) to H Nc(tBu) (11) is in good agreement
Fig. 1. Differential pulse voltammogram of compound 10 in
CH Cl2
2
with the same four tert-butyl substitutents at similar posi-
tions of ring renders it possible to compare the effect of
the ring size, actually the π-network on the electrochem-
istry of tetrapyrrole derivatives. The effect of the ring
size is clearly demonstrated by the fact that the half-wave
2
4
2
4
with the red shift of the Q absorption of these compounds
from 588 nm for 1 to 782 nm for 11 (Table 3). It is worth
noting that the Q band is determined as the center of the
split peaks, [(Qx0-0 + Qy0-0)/2] because of the orbital alter-
nation for LUMO and LUMO+1 in these low-symmetry,
metal-free analogs (see DFT and TDDFT calculations
section).
potentials of the first oxidation (Oxd ) and second oxida-
1
tion (Oxd ) processes for 1–11, both of which involve the
2
HOMO, are shifted significantly to the negative direction
along with the increase in the size of central conjugated
system (Table 2). Moreover, a linear correlation exists
between the potentials of the first oxidation and second
oxidation and the number of benzene rings fused onto the
TAP skeleton, as shown in Fig. 2.As can be seen, the half-
wave potentials of the first reduction (Red ) process of
1
Electronic absorption spectra
this series of tetrapyrrole derivatives also decrease along
with the central ring extension. However, the slope of this
line is significantly smaller in comparison with those of
the first oxidation and second oxidation processes. These
results indicate that along with the ring extension, the
As shown in Fig. S1 (Supporting information), along
with the increase in the number of fused benzene rings
in the order of 1 to 11, the solution color of the series of
compounds in CHCl systematically changes from violet
3
for 1 to green for 11 via blue, light blue, and greenish
blue for the analogs between them. Electronic absorption
spectra of the low-symmetry, metal-free tetra-tert-buty-
lated tetraazaporphyrin analogs 2–5 and phthalocyanine
analogs 7–10 were recorded in chloroform and the data
are summarized in Table 3. For the purpose of com-
parative studies, those for metal-free tetra-tert-butylated
tetraazaporphyrin H TAP(tBu) (1), phthalocyanine
2
4
H Pc(tBu) (6), and napthalocyanine H Nc(tBu) (11)
2
4
2
4
were also recorded and listed in Table 3. It is worth not-
ing that the Q bands of H Nc(tBu) do not show the split-
2
4
ting Q band characteristics of metal-free Pcs and TAPs.
The reason is not yet elucidated; this effect may happen
if the splitting of the LUMO is small, and this is sup-
ported by the results of the DFT calculations described
later, also by the MCD spectra which have been reported
by Kobayashi and co-workers [24b]. Figure 3 shows the
electronic absorption spectra of tetraazaporphyrin ana-
logs 2–5 together with those of H TAP(tBu) (1) and
Fig. 2. Plot of half-wave potentials of redox processes of the
metal-free tetraazaporphyrin and phthalocyanine analogs 1–11
as a function of the number of benzene rings fused onto the
TAP skeleton
2
4
H Pc(tBu) (6). As can be seen, in addition to the general
2
4
Copyright © 2010 World Scientific Publishing Company
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4
28
R. LI ET AL.
Table 3. Electronic absorption spectroscopic data for the metal-free tetraazaporphyrin and phthalocyanine
derivatives 1–11 in chloroform
Compound
λmax, nm (log ε)
Qx0-0
B
336 (5.28)
Qy0-0
(Qx0-0+Qy0-0)/2
1
2
3
4
5
6
7
8
9
0
1
553 (5.11)
563 (5.13)
566 (5.13)
569 (5.12)
606 (5.17)
664 (5.17)
684 (5.20)
716 (5.20)
712 (5.17)
735 (5.22)
622 (5.23)
649 (5.19)
652 (5.14)
706 (5.20)
683 (5.21)
701 (5.23)
729 (5.21)
740 (5.24)
742 (5.19)
769 (5.10)
782 (5.28)
588
606
609
638
645
683
707
728
727
752
782
345 (5.15)
344 (5.20)
346 (5.19)
348 (5.15)
341 (4.96)
341 (5.01)
340 (4.96)
333 (4.89)
338 (4.91)
324 (5.23)
1
1
Soret band observed in the range of 336–348 nm for the
metal-free derivatives 1–6, intense Qy0-0 and Qx0-0 bands
are observed at 553 and 622 nm for 1 and 663 and 700
nm for 6, respectively. Also due to the similar molecular
symmetry, compounds 2 and 5 take the similar electronic
absorption feature with intense Qy0-0 and Qx0-0 bands at
5
63 and 649 nm for 2 and 606 and 683 nm for 5, respec-
tively. In line with the observation of Kobayashi and co-
workers on zinc complexes of tetraazaporphyrin analogs
[16a, 16d], similar electronic absorption spectroscopic
features to those of tetraazaporphyrin and phthalocya-
nine compounds, 1 and 6, are observed for the adjacently
benzo-fused, metal-free analog 3 despite its decreased
molecular symmetry, with two intense Qy0-0 and Qx0-0
bands at 566 and 652 nm, respectively. As for the oppo-
sitely benzo-fused metal free tetraazaporphyrin analog 4,
the largest splitting between the Qy0-0 band at 569 nm and
Qx0-0 absorption at 706 nm is observed among the whole
series of compounds 1–6. This result is well reproduced
by the theoretical calculations, as detailed below.
The electronic absorption spectra of phthalocyanine
analogs 7–10 as well as those of H Pc(tBu) (6) and
2
4
H Nc(tBu) (11) are shown in Fig. 4. Assignments of the
2
4
absorptions for 7 and 10 are similarly straightforward in
reference with those of metal-free phthalocyanine and
naphthalocyanine 6 and 11 as well as the tetrapyrrole
analogs (Table 3). In the case of dibenzo-fused phthalo-
cyanine analogs, the splitting between the Qy0-0 and Qx0-0
bands for the oppositely substituted compound 9 is larger
than that for the adjacently substituted 8 [16b]. Again,
this is reproduced by the calculation result, vide infra.
Inspection of the electronic spectra for the whole series
of metal-free tetraazaporphyrin and phthalocyanine ana-
logs ranging from H TAPc(tBu) (1) to H Nc(tBu) (11)
2
4
2
4
via H Pc(tBu) (6) clearly reveals that along with the
increase in the number of benzene rings fused onto the
TAP skeleton, the Q absorption (the position of which is
Fig. 3. Electronic absorption spectra of the low-symmetry,
metal-free tetraazaporphyrin analogs 2–5 together with those
for H TAP(tBu) (1) and H Pc(tBu) (6) for comparison
2
4
2
4
2
4
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BENZO-FUSED, LOW SYMMETRY, METAL-FREE TETRAAZAPORPHYRIN AND PHTHALOCYANINE ANALOGS
429
Fig. 5. Plot of the Q absorption wavenumbers of metal-free tet-
raazaporphyrin and phthalocyanine analogs 1–11 as a function
of the number of benzene rings fused onto the TAP skeleton
spectra of tetraazaporphyrin analogs with corresponding
phthalocyanine analogs reveals a smaller extent of split-
ting between the Qy0-0 and Qx0-0 bands for the latter series
due to their larger conjugated π-system [24b].
Fluorescence spectra
All the metal-free tetraazaporphyrin and phthalo-
cyanine derivatives show fluorescence at ambient tem-
perature. Figure 6 shows the fluorescence spectra of
tetraazaporphyrin analogs and phthalocyanine analogs.
As expected, the wavelength of the fluorescence of the
whole series of compounds generally takes a red shift
along with the enlargement of π-system, correspond-
ing well with the change of the Q band in the electronic
absorption spectra. However, associated with the longest
wavelength Qx0-0 band for the oppositely benzo-fused
metal free tetraazaporphyrin analog 4 in the series of
tetraazaporphyrin analogs including tetra-tert-butylated
tetraazaporphyrin H TAP(tBu) (1), and phthalocyanine
Fig. 4. Electronic absorption spectra of the low-symmetry,
metal-free phthalocyanine analogs 7–10 together with those for
H Pc(tBu) (6) and H Nc(tBu) (11) for comparison
2
4
2
4
2
4
defined as the center of the split peaks, (Qx0-0 + Qy0-0)/2)
takes a systematic shift to the lower-energy direction.
This is in line with the findings that along with the ring
enlargement of tetrapyrrole system from tetraazaporphy-
rin, phthalocyanine, to naphthalocyanine (and even to
anthracocyanine), their main Q absorption band gradu-
ally shifts to the lower-energy side [24b]. The effect of
number of the rings for the present tetrapyrrole com-
pounds on the electronic absorption spectra is clearly
illustrated in Fig. 5, which plots the wavenumbers of the
Q absorption band of 1–11 as a function of the number
of benzene rings fused onto the TAP skeleton. As can be
found, a relationship fitting the hyperbola equation exists
between the wavenumbers of the Q absorption band and
the number of benzene rings fused onto the TAP skeleton,
indicating that the Q band shifts to longer wavelength
with the increase in the number of fused benzene rings
but with a diminishing degree with increasing π-system.
Nevertheless, comparison in the electronic absorption
H Pc(tBu) (6), this compound presents the emission
2 4
band at the longest wavelength among 1–6. In addition,
the Stokes shift for all these compounds are relatively
small in the range of 3–9 nm except H Nc(tBu) (11),
2
4
suggesting the small structure relaxation during photo-
excitation and in turn the relatively rigid molecular struc-
ture of the whole series of compounds. Interestingly, the
fluorescence quantum yield (Φ ) of these compounds
F
increases from tetraazaporphyrin 1 to phthalocyanine 6,
firstly along with the enlargement of π-system, and then
decreases from phthalocyanine 6 to naphthalocyanine 11
along with the further enlargement of π-system, indicat-
ing the effect of the enlargement of the π-system on the
non-radiative decay and/or the intersystem crossing of
the excited state for these compounds. Consistent with
the electronic absorption spectroscopic result, the effect
of the ring size of these tetrapyrrole compounds on the
fluorescence spectra is also clear. Figure S3 (Supporting
information) plots the wavenumbers of the fluorescence
Copyright © 2010 World Scientific Publishing Company
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4
30
R. LI ET AL.
Fig. 6. Fluorescence spectra of the low-symmetry, metal-free tetraazaporphyrin analogs 2–5 together with those for H TAP(tBu) (1)
2
4
and H Pc(tBu) (6) and metal-free phthalocyanine analogs 7–10 together with those for H Pc(tBu) (6) and H Nc(tBu) (11)
2
4
2
4
2
4
of 1–11 as a function of the number of benzene rings
fused onto the TAP skeleton. Again, a relationship fitting
the hyperbola equation exists between the wavenumbers
of the fluorescence and the number of benzene rings
fused onto the TAP skeleton, revealing that the fluores-
cence shifts to longer wavelength with the increase in
the number of fused benzene rings but to a diminishing
extent with increasing π-system.
(Supporting information). As expected, except for com-
pounds of H TAP(tBu) (1) and H Pc(tBu) (6) with rela-
2
4
2
4
tively higher molecular symmetry in terms of the central
1
conjugated system (it is worth noting that satisfactory H
NMR spectrum for H Nc(tBu) (11) was not obtained
2
4
even after repeated trials), assignments of the spectra for
the remaining compounds can only be reached for aro-
matic protons, tert-butyl protons, and pyrrole protons
with the help of integration data (Table S1, Support-
ing information). Assignments to one-to-one matching
degree could not be reached even with the assistance of
1
H NMR spectroscopy
1
1
The NMR spectra for the whole series of tetrapyr-
role and phthalocyanine analogs 1–11 were recorded
the H- H COSY spectroscopic results. Significant over-
lapping was found for corresponding proton signals in the
analogs, in line with the fact that the systems are mixtures
in CDCl and the data are summarized in Table S1
3
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BENZO-FUSED, LOW SYMMETRY, METAL-FREE TETRAAZAPORPHYRIN AND PHTHALOCYANINE ANALOGS
431
symmetry of the central conjugated system. This is also
true for the phthalocyanine analog isomers 8 and 9.
As clearly shown in Fig. 8, a common relatively weak
peak for the whole series of metal free tetra-tert-butylated
-1
tetrapyrrole derivatives 1–11 observed at 3284–3296 cm
is due to the N-H stretching vibration, while the three
common intense absorptions at 2953–2959, 2896–2910,
-1
and 2861–2868 cm are contributed from the antisym-
metric C-H stretching and symmetric C-H stretching of
the tert-butyl groups, respectively. Another two common
-1
bands with medium intensity seen at 1249–1261 cm and
-1
with weak intensity at 3031–3099 cm , respectively, for
–11 are assigned to the skeleton vibration of the tert-
butyl groups and the aromatic C-H stretching on the con-
jugated systems of tetraazaporphyrin and phthalocyanine
analogs.
Comparison with the IR data for phthalocyanine and
naphthalocyanine compounds [26] reveals that in the IR
spectra of tetraazaporphyrin and phthalocyanine analogs
1
Fig. 7. H NMR spectrum of metal-free tetraazaporphyrin ana-
1
log H A1P3(tBu) (5) in CDCl . The signals due to residue
CHCl and water are denoted as * and #, respectively
2
4
3
3
of regioisomers regarding the placement of the t-butyl
group and NH tautomerism. Figure 7 shows the H NMR
spectrum of tetraazaporphyrin analog H A1P3(tBu) (5)
in CDCl for exemplification. The signals for the aromatic
protons of the conjugated ring for 5 are overlapped in the
region of δ = 7.96–9.40. The tert-butyl groups resonate
as two multiplets at δ = 2.25–2.32 and 1.74–1.87, respec-
tively. Several signals appearing in the high field region
at δ = -2.05 ~ -1.11 are obviously due to the pyrrole pro-
tons of this metal-free tetraazaporphyrin analog.
1
2
4
3
1
–11, generally four main IR absorptions correspond-
-1
ing to the C-H wagging at 742–755 cm , isoindole
breathing and aza stretching coupled at 1087–1119 cm
-1
-1
(see below), pyrrole stretching at 1317–1360 cm and
-1
isoindole stretching at 1479–1500 cm dominate their IR
spectra. In the spectral region from 400 to 1000 cm , the
-1
remaining weak vibrations have main contributions from
the dihedral plane deformation involving the aromatic
C-H groups (wagging, torsion and out-of-plane bending
vibrations). Aromatic in-plane C-H bending vibrations
are responsible for the IR bands of tetrapyrrole molecules
IR spectroscopy
IR spectroscopic characteristics of phthalocyanine and
naphthalocyanine in the sandwich-type complexes have
been systematically studied for the purpose of investigat-
ing the intrinsic properties of these phthalocyanine com-
pounds [25]. However, except the early efforts paid to
some phthalocyanine derivatives without any peripheral
substituents and the recent work on the calculated spectra
of several phthalocyanine compounds employing DFT
method, systematic investigation on the IR spectroscopy
of phthalocyanine compounds seems not yet conducted
thus far. Our compounds in the present work are therefore
expected to give representative data for the series of met-
al-free tetraazaporphyrin and phthalocyanine analogs.
It is worth noting that the characteristic pattern of
IR spectra for H TAP(tBu) (1), H Pc(tBu) (6), and
-1
in the region of 1000–1300 cm , with the exception of
-1
the band at 1087–1119 cm mainly contributed from
isoindole breathing together with aza stretching, and the
-1
band at 1035–1056 cm due to the coupling of isoindole
deformation and aza stretching. Vibrational frequencies
-1
in the range of 1300–1650 cm are derived from pyr-
role stretch, naphthalene stretch, isoindole ring stretch,
and the aza group stretch, for which the pyrrole stretch-
-1
ing vibrations appear at 1317–1360 cm , another pyrrole
-1
stretching-related peak vibrates at 1479–1500 cm cou-
pled with that of the aza stretching, and ortho-substituted
-1
benzene stretchings are observed at ca. 1580–1618 cm .
2
4
2
4
H Nc(tBu) (11) still remains relatively simpler than those
2
4
Density functional theory (DFT) and time-dependent
DFT (TDDFT) calculations
of 2–5 as well as 7–10, revealing the relatively higher
molecular symmetry of the central conjugated system for
the former compounds. The static molecular geometry of
the central conjugated system for compounds 1, 6, and 11
suggests a D point group for their molecules, and 2, 5,
To enhance the understanding of the effect of ring
fusing on the metal-free tetraazaporphyrin and phtha-
locyanine derivatives, we carried out DFT and TDDFT
calculations on these metal-free tetraazaporphyrin and
phthalocyanine analogs 1–11 within the framework
of the DFT method at the B3LYP/6–31G(d) level [17]
which has proved suitable in calculating MO and elec-
tronic absorption spectra of the phthalocyanine deriva-
tives [27].
2
h
7
, and 10, C , 3 and 8, C , and 4 and 9, D , respectively.
2v s 2h
This can be revealed by the larger number of vibrational
modes observed in the IR spectra of the latter series of
compounds, in particular, 2, 3, 5, 7, 8, and 10 compared
with those of 1, 6, and 11 (Fig. 8 and Table S2, Support-
ing information). The difference in the characteristic pat-
tern of IR spectra between the tetraazaporphyrin analog
isomers 3 and 4 is clearly due to their different molecular
Asdiscussedinthesectionofelectrochemistryof1–11,
the first oxidation potential shifts to negative direction
Copyright © 2010 World Scientific Publishing Company
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32
R. LI ET AL.
Fig. 8. IR spectra of the low-symmetry, metal-free tetraazaporphyrin analogs 2–5 and phthalocyanine analogs 7–10 together with
-1
those for H TAP(tBu) (1), H Pc(tBu) (6), and H Nc(tBu) (11) for comparison with 2 cm resolution
2
4
2
4
2
4
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BENZO-FUSED, LOW SYMMETRY, METAL-FREE TETRAAZAPORPHYRIN AND PHTHALOCYANINE ANALOGS
433
in the order from H TAP(tBu) (1) > H A3P1(tBu)
4
LUMO of 1–11 also diminishes along with the increase
in the size of π-conjugated system. As shown in Fig. 9,
the variation in the LUMO energy as well as the HOMO-
LUMO gap of 1–11 elegantly resembles the results seen
in their electrochemistry (Fig. 2).
2
4
2
(
>
2) > H adj-A2P2(tBu) (3) ≈ H opp-A2P2(tBu) (4)
2
4
2
4
H A1P3(tBu) (5) > H Pc(tBu) (6) > H P3N1(tBu)
2 4 2 4 2 4
(
7) > H adj-P2N2(tBu) (8) ≈ H opp-P2N2(tBu) (9) >
2
4
2
4
H P1N3(tBu) (10) > H Nc(tBu) (11) (Fig. 2). It is worth
2
4
2
4
noting that the first oxidation potential for the di(benzo)-
fused tetraazaporphyrin analog isomers of 3 and 4 is
almost the same, with a difference of only 0.02 V. This
is also true for the di(benzo)-fused phthalocyanine ana-
log isomers of 8 and 9, with only 0.01 V difference for
their first oxidation potential. These results indicate that
the first oxidation potential of the tetraazaporphyrin and
phthalocyanine analogs is dependent on the number of
benzene rings fused onto the TAP skeleton (actually the
size of π-conjugated system), which gradually shifts to
the negative direction along with the increase in the size
of π-conjugated system of the tetrapyrrole derivative,
indicating that their HOMO destabilizes along with the
expansion of the π-system. On the other hand, as can be
seen in Fig. 2, the first reduction potential of 1–11 also
shows dependence on the number of benzene rings fused
onto the TAP skeleton, which shifts to negative direc-
tion along with the same order but in a smaller gradient,
indicating that the LUMO of 1–11 also destabilizes along
with the expansion of the π-system.
MO calculation results are displayed in Fig. 9. The
HOMO energy of 1–11 increases with increasing the size
of π-conjugated system. This is consistent with the elec-
trochemical results. On the other hand, the calculation
results also indicate the destabilization of the LUMO and
LUMO + 1 of the series of compounds along with the
increase in the size of π-conjugated system although in a
decreased gradient than that for the HOMO. As a conse-
quence, the difference in energy between the HOMO and
To understand the effect of the ring fusion on the elec-
tronic absorption spectra, the behavior of the MO ener-
gies in particular those of the LUMO and LUMO + 1 is
discussed in terms of the electron distribution of MOs
hereafter.As can be seen from Fig. 10, the orbital distribu-
tion of both LUMO and LUMO + 1 orbitals are different
for the pyrrole rings at x- and y-axis directions (defined as
the direction from left to right and from bottom to top in
Fig. 10, respectively), resulting in different effect of the
fused benzene rings on the orbital energy in x- and y-axis
directions. Generally, in the case that C atoms of pyrrole
β
rings in the y-axis direction have more contribution to
the LUMO or LUMO + 1 of tetraazaporphyrin, the fused
benzene ring(s) onto these pyrrole ring(s) in the y-axis
direction will exert more effect in increasing the energy of
LUMO or LUMO + 1 than fused benzene ring(s) onto
the pyrrole ring(s) in the x-axis direction. For example, the
LUMO of H TAP(tBu) (1) is mainly distributed on the
2
4
C and C atoms of pyrrole rings in the y-axis direction,
α
β
aza nitrogen atoms, and C and nitrogen atoms of pyrrole
α
rings in the x-axis direction, while the C atoms of pyrrole
β
rings in the x-axis direction have no contribution to this
orbital. As a result, benzene ring fusion onto the pyrrole
rings in the y-axis direction will have more influence in
increasing the energy of LUMO than at the x-axis direc-
tion. As shown in Fig. 10, for compounds 1–5, 7, and 9,
the molecular orbital distribution of LUMO in the y-axis
direction is larger than that in the x-axis direction, while
the orbital distribution of LUMO + 1 in the x-axis direc-
tion of these compounds is larger
than that in the y-axis direction.
In contrast, the LUMO of 6, 8, 10,
and 11 has more distribution in the
x-axis direction than in the y-axis
direction while the LUMO + 1 has
less distribution in the x-axis direc-
tion than in the y-axis direction.
When benzene rings are fused
onto the pyrrole rings of the TAP
skeleton, the HOMO is destabi-
lized due to the large distribution
of HOMO on the C atoms. How-
β
ever, the orbital distribution of the
HOMO on the four pyrrole rings of
1–11 is almost the same despite the
low molecular symmetry resulting
from the inner two hydrogen atoms
and the fused benzene rings. As a
total result, the HOMO of 1–11 is
independent on the position of fused
pyrrole ring(s) but dependent on the
size of the π-conjugated system.
Fig. 9. Energy levels (in eV) of the frontier orbitals for the low-symmetry, metal-free
tetraazaporphyrin and phthalocyanine analogs from 1 to 11 obtained by DFT calcula-
tions at B3LYP/6–31G(D) level
Copyright © 2010 World Scientific Publishing Company
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34
R. LI ET AL.
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BENZO-FUSED, LOW SYMMETRY, METAL-FREE TETRAAZAPORPHYRIN AND PHTHALOCYANINE ANALOGS
435
Fig. 12. Absorption wavelengths and oscillator strengths
obtained by TD-DFT calculation on the low-symmetry, met-
al-free phthalocyanine analogs 7–10 together with those of
Fig. 11. Absorption wavelengths and oscillator strengths
obtained by TDDFT calculations on the low-symmetry, met-
al-free tetraazaporphyrin analogs 2–5 together with those of
H TAP(tBu) (1) and H Pc(tBu) (6) for comparison
H Pc(tBu) (6) and H Nc(tBu) (11) for comparison
2
4
2
4
the basis of our calculation results (Fig. 12). This seems
contrary to the experimental result. However, this is in
2
4
2
4
line with the calculation result on either H Nc(tBu) (11)
Towards clearly understanding the electronic absorp-
2
4
or H Ac(tBu) of Kobayashi and co-workers using PPP
tion spectra of 1–11 in a more quantitative manner,
TDDFT calculations were carried out. The calculated
electronic absorption spectra are shown in Figs. 11 and
2
4
Hamiltonian that the splitting of the Q band for these two
compounds is not small [24b]. As expected, the calcula-
tion results reveal that there are too many transitions in
the Soret band region for 1–11.
1
2, respectively, and the calculation results including
the direction of transition dipoles and configurations
are summarized in Table S3 (Supporting information).
On the basis of calculation results, the Q band for the
whole series of tetrapyrrole derivatives 1–11 is com-
posed of two degenerate transitions. The energy split-
ting between the two Q bands for the tetraazaporphyrin
analogs including metal-free phthalocyanine 6 decreases
in the order 4 > 2 > 5 > 1 > 3 > 6, while for the phthalo-
cyanine analogs excluding metal-free naphthalocyanine
CONCLUSION
In summary, a whole series of low-symmetry, met-
al-free tetraazaporphyrin and phthalocyanine analogs
ranging between tetraazaporphyrin H TAP(tBu) and
2
4
napthalocyanine H Nc(tBu) with different central con-
2
4
jugated system through fusing different number of
benzo rings onto the TAP skeleton have been prepared
and fully characterized. With such a series of tetrapyr-
role compounds having the same number of tert-butyl
groups at similar positions of the macrocyclic ring, the
effect intrinsic to the enlargement of the central conju-
gated system has been easily extracted and understood on
the basis of systematic investigation over their spectro-
scopic and electrochemical properties, which are further
supported by considering the energy levels of frontier
molecular orbitals of these compounds obtained by cal-
culations using TDDFT method. Due to various practical
1
1 in the order 9 > 7 > 8 > 6 = 10. These trends in the Q
band regions well reproduce the experimental findings as
detailed above. In particular, the calculated energy split-
ting between the two Q bands for H opp-A2P2(tBu) (4)
2
4
is significantly larger than that for H adj-A2P2(tBu) (3),
2
4
confirming the nature of compounds 3 and 4 with corre-
sponding electronic absorption feature separated experi-
mentally. This is also true for H opp-P2N2(tBu) (9) and
2
4
H adj-P2N2(tBu) (8). It is worth noting that the energy
2
4
splitting of the Q band for H Nc(tBu) (11) is not small,
2
4
which is even a bit larger than that for H Pc(tBu) (6), on
2
4
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 435–437

4
36
R. LI ET AL.
applications of phthalocyanine derivatives, the present
results are believed useful for researchers in both scien-
tific and industrial field.
pp 341–392. b) Guillaud G, Simon J and Germain
JP. Coord. Chem. Rev. 1998; 180: 1433–1484.
8. a) Phthalocyanines – Chemistry and Functions,
Shirai H and Kobayashi N. (Eds.) IPC: Tokyo,
1997. b) Hasrar H and van Lier JE. Chem. Rev.
Acknowledgements
1
999; 99: 2379–2450. c) Zimcik P, Miletin M,
Financial support from the Natural Science Foun-
dation of China and Ministry of Education of China is
gratefully acknowledged.
Ponec J, Kostka M and Fielder Z. J. Photochem.
Photobiol. A 2003; 155: 127–131. d) Mitzel F,
FitzGerald S, Beeby A and Faust R. Chem. Eur. J.
2
003; 9: 1233–1241.
Supporting information
9
. Cosomelli B, Roncuccin G, Dei D, Fantetti L, Fer-
roin F, Ricci M and Spinelli D. Tetrahedron 2003;
59: 10025–10030.
10. Beltrán HI, Esquive R, Sosa-Sánchez A, Sosa-Sán-
chez JL, Höpfl H, Barba V, Farfán N, García MG,
Olivares-Xometl O and Zamudio-Rivera LS. Inorg.
Chem. 2004; 43: 3555–3557.
Figs S1–S3 and Tables S1–S3 are given in the sup-
plementary material. This material is available free-of-
charge at http://www.worldscinet.com/jpp/jpp.shtml.
REFERENCES
1
. The Porphyrin Handbook, Vols. 1–20, Kadish KM,
Smith KM and Guilard R. (Eds.) Academic Press:
San Diego, 2000 and 2003.
11. a) Kobayashi N, Kondo R, Nakajima S and Osa T.
J. Am. Chem. Soc. 1990; 112: 9640–9641. b) Weit-
emeyer A, Kliesch H and Wöhrle D. J. Org. Chem.
1995; 60: 4900–4904. c) Sastre A, Del Rey B and
Torres T. J. Org. Chem. 1996; 61: 8591–8597.
d) Sheng N, Zhang Y, Xu H, Bao M, Sun X and
Jiang J. Eur. J. Inorg. Chem. 2007; 20: 3268–3275.
e) Sheng N, Li R, Choi CF, Su W, Ng DKP, Cui X,
Yoshida K, Kobayashi N and Jiang J. Inorg. Chem.
2006; 45: 3794–3802.
12. a) Kobayashi N, Kondo R, Nakajima S and Osa
T. J. Am. Chem. Soc. 1999; 112: 9640–9641.
b) Kobayashi N, Ishizaki T, Ishii K and Konami H.
J. Am. Chem. Soc. 1999; 121: 9096–9110.
13. Ikeda Y, Konami H, Hatano M and Mochizuki K.
Chem. Lett. 1992; 5: 763–766.
14. Mack J, Kobayashi N, Leznoff CC and Stillman MJ.
Inorg. Chem. 1997; 36: 5624–5634.
15. Aoudia M, Cheng G, Kennedy VO, Kenney ME
and Rodgers MAJ. J. Am. Chem. Soc. 1997; 119:
6029–6039.
16. a) Miwa H, Kazuyuki I and Kobayashi N. Chem.
Eur. J. 2004; 10: 4422–4435. b) Kobayashi N, Miwa
H and Nemykin VN. J. Am. Chem. Soc. 2002; 124:
8007–8020. c) Kobayashi N, Mack J, Ishii K and
Stillman MJ. Inorg. Chem. 2002; 41: 5350–5363.
d) Kobayashi N and Fukuda T. J. Am. Chem. Soc.
2002; 124: 8021–8034. e) Ishii K, Itoya H, Miwa
H, Fujitsuka M, Ito O and Kobayashi N. J. Phys.
Chem. A 2005; 109: 5781–5787.
2
. a) Phthalocyanine: Properties and Applications,
Vols. 1–4, Lever ABP and Leznoff CC. (Eds.) VCH:
New York, 1989–1996. b) Phthalocyanines Materi-
als: Synthesis, Structure and Function, McKeown
NB. (Ed.) Cambridge University Press: New York,
1998.
3
. a) de la Torre G, Torres T and Agulló-Lópz F. Adv.
Mater. 1997; 9: 265–269. b) de la Torre G, Vázquez
P, Agulló-Lópz F and Torres T. J. Mater. Chem.
1
998; 8: 1671–1683.
4
. See for example: a) Bao Z, Lovinger AJ and Doda-
balapur A. Appl. Phys. Lett. 1996; 69: 3066–3068.
b) Bao Z, Lovinger AJ and Brown J. J. Am. Chem.
Soc. 1998; 120: 207–208. c) Zhang J, Wang J, Wang
H and Yan D. Appl. Phy. Lett. 2004; 84: 142–144.
d) Zeis R, Siegrist T and Kloc Ch. Appl. Phys. Lett.
2
005; 86: 22103/.1–3. e) Su W, Jiang J, Xiao K,
Chen Y, Zhao Q, Yu G and Liu Y. Langmuir 2005;
1: 6527–6531. f) Chen Y, Su W, Bai M, Jiang J, Li
X, Liu Y, Wang L and Wang S. J. Am. Chem. Soc.
005; 127: 15700–15701. g) Jiang J and Ng DKP.
2
2
Acc. Chem. Res. 2009; 42: 79–88.
5
. a) Wöhrle D and Meissner D. Adv. Mater. 1991;
3
: 129–138. b) Morandeira A, López-Duarte I,
Matínez-Díza MV, O’Regan B, Shuttle C, Haji-Zai-
nulabidin NA, Torres T, Palomares E and Durrant
JR. J. Am. Chem. Soc. 2007; 129: 9250–9251.
6
. a) Van der Pol JF, Neeleman E, Zwikker JW, Nolte
RJM, Drenth W, Aerts J, Visser R and Picken S.
J. Liq. Cryst. 1989; 6: 577–592. b) Simon J and
Sirlin C. Pure Appl. Chem. 1989; 61: 1625–1629.
c) Iino H, Hanna J, Bushby RJ, Movaghar B, Whi-
taker BJ and Cook MJ. Appl. Phys. Lett. 2005; 87:
17. a) Lee C, Yang W and Parr RG. Phys. Rev. B 1988;
37: 785–789. b) Becke AD. J. Chem. Phys. 1993;
98: 5648–5652. c) Becke AD. J. Chem. Phys. 1996;
104: 1040–1046.
18. Peng C, Ayala PY, Schlegel HB and Frisch MJ.
J. Comput. Chem. 1996; 17: 49–56.
1
32102.
19. a) Stratmann RE, Scuseria GE and Frisch MJ.
J. Chem. Phys. 1998; 109: 8218–8224. b) Bauern-
schmitt R and Ahlrichs R. Chem. Phys. Lett. 1996;
256: 454–464. c) Casida ME, Jamorski C, Casida
7
. a) Snow AW and Barger WR. In Phthalocyanine.
Principles and Properties, Vol. 1, Leznoff CC and
Lever ABP (Eds.) VCH: NewYork, 1989; chapter 5,
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 436–437

BENZO-FUSED, LOW SYMMETRY, METAL-FREE TETRAAZAPORPHYRIN AND PHTHALOCYANINE ANALOGS
437
KC and Salahub DR. J. Chem. Phys. 1998; 108:
439–4449.
0. Frisch MJ, Trucks GW, Schlegel HB et al. Gaussian
J. 2004; 10: 6294–6312. c) Li R, Zhang X, Zhu P,
Ng DKP, Kobayashi N and Jiang J. Inorg. Chem.
2006; 45: 2327–2334.
4
2
2
2
03, Revision B05, Gaussian, Inc., Pittsburgh PA,
25. Jiang J, Bao M, Rintoul L and Arnold DP. Coord.
Chem. Rev. 2006; 250: 424–448 and references
therein.
26. Aroca R, Clavijo RE, Jennings CA, Kovacs GA,
Duff JM and Loutfy RO. Spectrochimica Acta 1989;
45A: 957–962.
27. a) ZhangY, Cai X, Yao P, Xu H, BianY and Jiang J.
Chem. Eur. J. 2007; 13: 9503–9514. b) ZhangY, Cai
X, Zhou Y, Zhang X, Xu H, Liu Z, Li X and Jiang
J. J. Phys. Chem. A 2007; 111: 392–400. c) Mack
J, Asano Y, Kobayashi N and Stillman MJ. J. Am.
Chem. Soc. 2005; 127: 17697–17711. d) Keizer SP,
Mack J, Bench BA, Gorun SM and Stillman MJ.
J. Am. Chem. Soc. 2003; 125: 7067–7085.
2003.
1. Kopranenkov VN, Goncharova LS and Luk’yanets
EA. J. Gen. Chem. USSR 1977; 47: 1954–1958
(
Chem. Abstr. 1960; 24333f).
2. a) Marinina LE, Mikhalenko SA and Luk’yanets
EA. Zh. Obshch. Khim. 1973; 43: 2025–2029. b)
Kobayashi N, Nakajima N and Osa T. Inorg. Chim.
Acta 1993; 210: 131–133.
2
3. Kovshev EI, Punchanova VA and Luk’yanets EA.
J. Gen. Chem. USSR 1977; 7: 364–369.
2
4. a) Kobayashi N, Ashida T, Osa T and Konami H.
Inorg. Chem. 1994; 33: 1735–1740. b) Kobayashi
N, Nakajima S, Ogata H and Fukuda T. Chem. Eur.
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