Heteroleptic Tetrapyrrole-Fused Dimeric and Trimeric Skeletons with Unusual Non-Frustrated Fluorescence

Article abstract of DOI:10.1002/chem.201504837

Phthalocyanine (Pc) and porphyrin (Por) chromophores have been fused through the benzo[α]pyrazine moiety, resulting in unprecedented heteroleptic tetrapyrrole-fused dimers and trimers. The heteroleptic tetrapyrrole nature has been clearly revealed based o

Full text of DOI:10.1002/chem.201504837

DOI: 10.1002/chem.201504837
Full Paper
&
Porphyrins
Heteroleptic Tetrapyrrole-Fused Dimeric and Trimeric Skeletons
with Unusual Non-Frustrated Fluorescence
+
[a]
+ [b]
+ [a]
[a]
[a]
[b]
Yuehong Zhang , Juwon Oh , Kang Wang , Chao Chen, Wei Cao, Kyu Hyung Park,
[b]
[a]
Dongho Kim,* and Jianzhuang Jiang*
Abstract: Phthalocyanine (Pc) and porphyrin (Por) chromo-
phores have been fused through the benzo[a]pyrazine
moiety, resulting in unprecedented heteroleptic tetrapyrrole-
fused dimers and trimers. The heteroleptic tetrapyrrole
nature has been clearly revealed based on single-crystal X-
ray diffraction analysis of the zinc dimer. Electrochemical
analysis, theoretical calculations, and time-resolved spectro-
scopic results disclose that the two/three-tetrapyrrole-fused
skeletons behave as one totally p-conjugated system as
a result of the strong conjugative interaction between/
among the tetrapyrrole chromophores. In particular, the ef-
fectively extended p-electron system through the fused-
bridge induced strong electronic communication between
the Pc and Por moieties and large transition dipole moments
in the Pc–Por-fused systems, providing high fluorescence
quantum yields (>0.13) and relatively long excited state life-
times (>1.3 ns) in comparison with their homo-tetrapyrrole-
fused analogues.
Introduction
In this regard, fusing individual Por and Pc chromophores
into heteroleptic tetrapyrrole conjugated systems is a fascinat-
ing approach to produce new functional molecules. The heter-
oleptic nature leads to unique optical and electrochemical
properties in the new compounds. Moreover, their efficiently
extended p-conjugated systems can induce intriguing behav-
iors such as near-infrared (NIR) absorption, emission, and non-
linear optical properties, which provide the potential for use in
applications such as molecular sensors, NIR dyes, and PDT ma-
terials.
Porphyrins (Por) and phthalocyanines (Pc), with delocalized p-
electron systems and versatile physical and chemical proper-
ties, are promising molecular materials with a wide variety of
[
1]
applications. When these tetrapyrrole macrocycles are fused
into further extended aromatic skeletons, individual Pc/Por p-
systems conjugate together to result in characteristic optical
and redox features for this new class of compounds, rendering
them useful in various disciplines ranging from photosynthetic
antenna models to photodynamic therapy (PDT). However, at-
tention in this direction seems to focus just on the homo-tetra-
pyrrole conjugated systems including various kinds of homo-
Towards developing novel tetrapyrrole-based advanced mo-
lecular materials with diverse application potentials, in the
present case, we report the synthesis and characterization
of unprecedented heteroleptic tetrapyrrole-fused dimeric
and trimeric skeletons, {[H /ZnBDAPc(OC H ) ][(H /ZnTMP)]}
[
2,3]
Por-fused oligomers
and homo-Pc-fused dimers and trim-
[4]
ers. Heteroleptic tetrapyrrole oligomers that are constructed
by simultaneously fusing the Pc and Por skeletons together
have been rarely studied, with the very few preliminary reports
limited to the synthesis and some spectroscopic characteriza-
2
8
9 6
2
(H -1/Zn -1) and {[(H /ZnTMP)][cis-H /Zn(BDA) Pc(OC H ) ][(H /
4
2
2
2
2
8
9 4
2
ZnTMP)]}
(H -2/Zn -2)
[BDAPc(OC H ) =dianion
of
6
3
8
9 6
2,3,9,10,16,17-hexakis(2,6-dimethylphenoxy)-benzo-22,25-diaza-
[
5]
tions of only three Pc–Por dimers fused through the benzene,
phthalocyanine, cis-(BDA) Pc(OC H ) =dianion of 2,3,9,10-tetra-
2
8
9 4
[
6]
[7]
pyrazine, or naphthalene units (Scheme S1 in the Support-
ing Information).
kis(2,6-dimethylphenoxy)-15,18,22,25-bis(benzodiaza)phthalo-
cyanine, TMP=dianion of 5,10,15,20-tetrakis(2,4,6-trimethyl-
phenyl) porphyrin] (Scheme 1). Moreover, we have explored in
detail the ground-state and excited-state behavior of the
Pc–Por-fused dimers and trimers by steady-state and time-re-
solved spectroscopies and electrochemical analysis in combina-
tion with theoretical calculations.
+
+
[
a] Y. Zhang, Dr. K. Wang, C. Chen, Dr. W. Cao, Prof. Dr. J. 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 (P.R. China)
E-mail: jianzhuang@ustb.edu.cn
+
[
b] J. Oh, K. H. Park, Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-Electronic Systems and
Department of Chemistry, Yonsei University, Seoul 120-749 (Korea)
E-mail: dongho@yonsei.ac.kr
Results and Discussion
Synthesis and characterization
+
[
] These authors contributed equally to this work.
The key precursor for the synthesis of the Pc–Por-fused oligo-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504837.
mers H -1 and H -2, 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)-
4
6
Chem. Eur. J. 2016, 22, 4492 – 4499
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Scheme 1. Molecular structures of Pc–Por-fused dimers 1 and trimers 2 in-
volved in the present work together with their monomeric phthalocyanine
and porphyrin references 3 and 4.
Scheme 2. Synthesis of Pc–Por-fused dimers (H
4
-1 and Zn
-2) as well as the reference phthalocyanines (H
11OH, reflux, 2 h; CH COOH, CH OH, 1 h. (II) Zn(OAc)
2
-1) and trimers
-3 and Zn-3).
·2H O, DMF,
(
(
H
6
-2 and Zn
I) Li, n-C
08C, 4 h.
3
2
5
H
3
3
2
2
9
2
’,3’-di-cyanopyrazino[2,3-b]porphyrin (5), was prepared by
heating at reflux in trichlorobenzene with metal-free
5
,10,15,20-tetrakis(2,4,6-trimethylphenyl)porphyrin (H TMP; H -
ther characterized with a range of spectroscopic techniques in-
cluding NMR, IR, electronic absorption, and fluorescence spec-
troscopy.
2
2
4
) and 2,3-bis(bromomethyl)pyrazine as the starting materials
according to the literature procedure to give 5 in approximate-
[
8]
1
ly 10.0% yield. It is worth noting that for the purpose of en-
hancing the solubility and impeding the aggregation of the
target Pc–Por-fused oligomeric compounds, 4,5-bis(2,6-dime-
thylphenoxy)phthalonitrile with two bulky substituents was
chosen as the second precursor. As a result, mixed cyclic tetra-
merization of 5 and 4,5-bis(2,6-dimethylphenoxy)phthalonitrile
in a 1:10 molar ratio in the presence of lithium pentanolate in
n-pentanol heated at reflux followed by treatment with acetic
acid led to the isolation of the target metal-free Pc–Por-fused
dimer H -1 (9.8%) and the first Pc–Por-fused trimer H -2 (2.6%)
Satisfactory H NMR spectra were obtained for the dimer
H -1 in CDCl (Figure S2 in the Supporting Information). With
4
3
1
1
the help of the H- H COSY spectrum (Figure S3 in the Sup-
porting Information), all the signals could be unambiguously
assigned with the results detailed in Table S2 (in the Support-
1
ing Information). The H NMR spectrum of H -1 exhibited three
4
singlets at d=8.92, 8.51, and 8.42 ppm from the Pc a protons
and one singlet at d=8.06 ppm from the protons on the ben-
zo[a]pyrazine moiety owing to the C2v molecular symmetry.
The two doublets at d=8.72 and 8.58 ppm, which were corre-
4
6
1
1
with quite good solubility in common organic solvents in
addition to a large quantity of metal-free 2,3,9,10,16,17,23,24-
lated with each other in the H- H COSY spectrum, were as-
signed to the two kinds of b protons on the Por moiety. The
other b proton signal on the Por moiety was observed at d=
8.92 ppm, which is overlapped by one Pc a proton singlet. The
two singlets observed at d=7.56 and 7.30 ppm were attribut-
ed to the protons of the meso-attached aryl groups on the Por
moiety. Signals for the aromatic protons of 2,6-dimethylphe-
noxyl substituents exhibited one singlet at d=7.50 ppm and
multiplets at d=7.44–7.37 ppm with an integral ratio of 1:2.
octakis(2,6-dimethylphenoxy)phthalocyanine
H Pc(OC H )
2 8 9 8
(
Scheme 2).
For
reference,
unsymmetrical
metal-free
2
,3,9,10,16,17-hexakis(2,6-dimethylphenoxy)-22,25-diaza-phtha-
locyanine H -3 was also synthesized and isolated according to
the literature procedure. Further reaction of the metal-free
2
[9]
tetrapyrrole species, including the dimer H -1, trimer H -2, and
monomer H -3, with Zn(OAc) ·2H O in DMF at 908C led to the
isolation of the zinc complexes Zn -1, Zn -2, and Zn-3.
All these newly prepared compounds gave satisfactory ele-
mental analysis data (Table S1 in the Supporting Information).
Their MALDI-TOF mass spectra clearly show intense signals for
4
6
2
2
2
1
1
Also, on the basis of the analysis of the H- H COSY spectrum
of this compound, the aliphatic proton signals can be unam-
biguously assigned, with the signals appearing at d=2.95,
2.65, and 1.92 ppm, which are correlated with the singlets at
d=7.56 and 7.30 ppm, belonging to the methyl protons on
the Por moiety. The signals appearing at d=2.54, 2.48, and
2.46 ppm, correlated with the signals for the aromatic protons
of the 2,6-dimethylphenoxyl substituents, were assigned to the
2
3
+
the corresponding molecular ion [M] . The isotopic pattern
closely resembles the simulated one, as exemplified by the
spectrum of the Pc–Por-fused dimer H -1 (Figure S1 in the Sup-
porting Information). Nevertheless, these compounds were fur-
4
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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methyl protons on the Pc moiety. In addition, the two singlets
at d=À0.51 and À2.17 ppm were assigned to the inner isoin-
dole/pyrrole protons. As can be expected, except for the ab-
sence of the inner isoindole/pyrrole proton signals, the zinc
Structural study
Single crystals of the zinc complex Zn -1 suitable for X-ray dif-
fraction analysis were obtained by slow diffusion of methanol
2
complex Zn -1 gives similar signals in its NMR spectrum, which
into the CHCl solution of this compound with the help of 1,4-
2
3
are therefore assigned in a similar manner (Figure S4 in the
Supporting Information). This was also true for the two analo-
gous trimers H -2 and Zn -2 (Figures S5–S7 in the Supporting
diazabicyclo[2.2.2]octane (DABCO). Zn -1 crystallizes in the tri-
clinic space group P 1¯ with two dimeric molecules and two
2
DABCO molecules per unit cell. The crystal structure of
6
3
Information).
Zn -1 unambiguously reveals its planar structure (Table S3 and
2
It is noteworthy that two possible structural isomers with
cis- and trans-configuration might exist for the Pc–Por-fused
trimer H -2. However, cis-H -2 was revealed as the sole isomer
Figure S9 in the Supporting Information). However, despite
great efforts thus far, repeated trials still failed to afford single
crystals of the Pc–Por-fused trimer 2 suitable for X-ray diffrac-
tion analysis. The geometrical optimizations of H -1, H -2, Zn -
6
6
isolated from the reaction mixture on the basis of the NMR
spectroscopic results. All efforts thus far have failed to afford
the trans-isomer of the Pc–Por-fused trimer. This seems strange
at first glance, but can be easily rationalized on the basis of
the theoretical analysis of the phthalocyanine formation mech-
anism with the half-phthalocyanine as the key intermedi-
4
6
2
1, and Zn -2 have been carried out based on the crystal struc-
3
ture of Zn -1 (Figure S10 in the Supporting Information). The
2
optimized structure of Zn -1 adopts an almost coplanar molec-
2
ular structure with a dihedral angle between the mean planes
of the 25-atom ZnPor moiety and 41-atom ZnPc moiety as
small as 2.158. Similarly, H -1, H -2, and Zn -2 displayed copla-
[10,11]
ate.
Nevertheless, this is also in good agreement with the
4
6
3
previous isolation of the cis-isomer, rather than the trans-
nar geometries with the dihedral angles of 5.648, 18.348, and
17.908, respectively. These features indicate the efficiently p-
conjugated heteroleptic tetrapyrrole nature, which extends
over the whole Pc–Por-fused dimer or trimer through their
benzo[a]pyrazine bridges.
isomer, of pyrene-fused unsymmetrical phthalocyanine deriva-
[9]
tive Zn[Pc(Pz-pyrene) (OC H ) ]. As exhibited in Figure S5 (in
2
8
9 4
the Supporting Information), observation of the two Pc
a proton singlets at d=9.34 and 9.03 ppm, and two singlets
for the protons on the benzo[a]pyrazine rings at d=8.78 and
1
8
.30 ppm in the H NMR spectrum of H -2 clearly reveals the
6
Electrochemical properties
C2v molecular symmetry of this trimer with a cis-configuration
rather than D2h symmetry with a trans-configuration. This was
further supported by the observation of four signals resulting
from the b protons on the Por moieties at d=8.74, 8.64, 8.61,
and 8.53 ppm. Signals for the protons in the meso-attached
aryl moieties on the Por side appear at d=7.82, 7.62, and
The electrochemical behavior of H -1 and H -2 as well as their
4
6
zinc complexes Zn -1 and Zn -2 was investigated by differen-
2
3
tial pulse voltammetry (DPV) in CH Cl and the half-wave redox
2
2
potentials versus saturated calomel electrode (SCE) are tabulat-
ed in Table S4 (in the Supporting Information). For a compara-
7
.32 ppm, whereas the aromatic protons of the 2,6-dimethyl-
tive study, the electrochemistry of reference compounds H -3
2
phenoxyl substituents exhibit one singlet at d=7.51 ppm and
and H -4 as well as Zn-3 and Zn-4 was also studied. Figure S11
(in the Supporting Information) shows the differential pulse
2
1
1
multiplets at d=7.48–7.41 ppm. According to the H- H COSY
spectrum (Figure S6 in the Supporting Information), the ali-
voltammograms of compounds H -1, H -2, H -3, and H -4. As
4
6
2
2
phatic proton signals in the NMR spectrum of trimer H -2 can
can be seen, the Pc–Por-fused dimer H -1 displayed four reduc-
6
4
also be unambiguously assigned in a similar way to the dimer
tion potentials at À0.61, À0.93, À1.30, and À1.55 V, and three
oxidation potentials at +1.18, +1.04, and +0.84 V. Observa-
tion of three oxidation processes at +0.84, +1.04, and
H -1 as tabulated in Table S1 (in the Supporting Information).
4
The singlets at d=0.44 and À2.07 ppm with an integral ratio
of 1:2 can be assigned to the inner isoindole and pyrrole pro-
tons, respectively.
+1.18 V for the Pc–Por-fused dimer H -1, which cannot be
4
simply assigned to those of the individual moieties (at +1.02
and +1.35 V for H -3 and +0.97 and +1.38 V for H -4), indi-
The IR spectra of H -1 and H -2 as well as Zn -1 and Zn -2
4
6
2
3
2
2
are shown in Figure S8 (in the Supporting Information). In ad-
dition to the bands associated with the aromatic Pc and Por
moieties, such as the CÀH wagging and torsion vibrations, and
the isoindole ring and the C=N aza group stretching vibra-
cates the strong electronic communication between the
HOMOs of the two constituents in the fused dimer system,
H -1. Moreover, H -1 showed a small potential difference be-
4
4
o
tween the first oxidation and reduction processes (DE 1
=
/2
[
12]
tions,
the bands at 2966–2972, 2918–2920, and 2848–
+1.45 V) in comparison with +1.59 V for H -3 and +2.22 V for
2
À1
2
850 cm are assigned to the asymmetric and symmetric CÀH
H -4. These results support the effective conjugation interac-
2
stretching vibrations of the -CH groups, whereas those at
tion through the benzo[a]pyrazine unit in the Pc–Por-fused
3
À1
1
263–1284 and 1093–1095 cm are ascribed to the asymmet-
system. In the reduction processes, the first and second reduc-
ric and symmetric CÀOÀC stretching vibrations of the 2,6-di-
tion processes of H -1 can be said to occur mainly on the Pc
moiety owing to the very close reduction potentials of the ref-
4
[13]
methylphenoxy groups in the Pc moiety. In the IR spectra of
H -1 and H -2, two weak bands from the asymmetric NÀH
erence Pc compound, H -3, at À0.57 and À0.91 V, whereas the
4
6
2
stretching vibrations of the isoindole and pyrrole moieties
third and fourth reduction processes of H -1 seem to mainly
4
À1 [14]
were observed at approximately 3300 cm , which disappear
correspond to the Por moiety owing to the reduction process-
in the IR spectra of the zinc complexes Zn -1 and Zn -2.
es with similar potentials at À1.25 and À1.68 V for the refer-
2
3
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ence Por compound, H -4. These results can be ascribed to the
fused bridge (Figure 1), suggesting that the fused dimeric com-
2
large energy gap between the LUMOs of the Pc and Por moiet-
ies compared with that of the HOMOs, which results in less ef-
fective electronic communication between the Pc and Por moi-
eties, leading to the different features between the oxidation
pound H -1 behaves as a single, totally p-conjugated molecu-
4
lar system. In the LUMO/LUMO+1 and LUMO+2/LUMO+3,
the molecular orbitals were partially delocalized over the Pc
and Por moieties, respectively. This feature is in accordance
and reduction processes of H -1 (see below). Similar features
with the electrochemical analysis of H -1, which result from
4
4
were observed for the dimeric zinc complex Zn -1.
the large energy gap between the LUMOs of the Pc and Por
2
For the trimer, H -2, six reduction (at À0.39, À0.57, À0.86,
moieties. This is also true for the Pc–Por-fused trimer H -2 and
6
6
À1.02, À1.31, and À1.73 V) and three oxidation (at +0.85,
the zinc complexes Zn -1 and Zn -2 (Figures S12–S14 in the
2
3
+
1.05, and +1.22 V) processes were revealed in the voltam-
Supporting Information). Here, in the effectively p-conjugated
systems, these partially delocalized molecular orbital distribu-
tions allude to the intramolecular charge-transfer (CT) charac-
ter in the Pc–Por-fused systems.
o
1
mogram. In addition, the DE
of +1.24 V was estimated for
/2
H -2, which is smaller than that for H -1. In the comparative
6
4
analysis with H -1, H -3, and H -4, these results indicate that
4
2
2
the expansion of the fused system intensifies the electronic
communication over the three tetrapyrrole chromophores in
H -2. The zinc complex analog, Zn -2, showed similar features
Optical properties
6
3
in the voltammograms.
The electronic absorption spectra of H -1 and H -2 as well as
4
6
their zinc complexes Zn -1 and Zn -2 were recorded in toluene
2
3
and toluene/pyridine (100:1), respectively. The data are sum-
marized in Table S5 (in the Supporting Information). Figure 2
compares the electronic absorption spectra of H -1 and H -2
Electronic structures
4
6
To gain insight into the electronic structures of the heteroleptic
tetrapyrrole-fused dimer and trimer, density functional theory
with those of the individual tetrapyrrole reference compounds,
H -3 and H -4. In the electronic absorption spectra, H -3 shows
2
2
2
(
DFT) calculations were carried out at the B3LYP/LANL2DZ
level. Despite the slightly overestimated vertical transition en-
ergies, the simulated electronic absorption spectra of H -1,
4
H -2, H -3, and H -4 together with their zinc complexes Zn -1,
6
2
2
2
Zn -2, Zn-3, and Zn-4 are in good agreement with the experi-
3
mental results (details in the Supporting Information, and see
below). Figure 1 compares the molecular orbital distribution
and energy levels of H -1 with monomeric references H -3 and
4
2
H -4 to clarify the interactions between the two tetrapyrrole
2
cores in the Pc–Por-fused dimer H -1.
4
Along with the electrochemical results, the delocalized mo-
lecular orbital over the entire molecule in the HOMO of
H -1 clearly describes the strong conjugative interaction be-
4
tween the Pc and Por moieties through the benzo[a]pyrazine-
Figure 2. Electronic absorption (black line) and fluorescence spectra (blue
line) of H -4, H -3, H -1, and H -2 in toluene.
2
2
4
6
an intense B (or Soret) band at 356 nm, an n–p* band associat-
ed with the lone electron pairs on the oxygen atoms at
[
15]
4
30 nm,
and two intense Q bands at 662 and 683 nm,
whereas H -4 displays a strong Soret band at 420 nm and four
2
[
16]
weak Q bands at 515, 547, 592, and 648 nm. The Pc–Por-
fused dimer H -1 shows two Soret-like bands at 361 and
4
4
7
20 nm, together with the split Q-like bands at 701 and
26 nm with two shoulders at 633 and 668 nm. In comparison
with the reference compounds H -3 and H -4, significant
2
2
broadening and redshift take place in the absorption bands of
Figure 1. Frontier molecular orbitals of H
of Por (H -4) and Pc (H -3). The HOMOs and LUMOs are represented as H
and L, respectively.
4
-1 with the corresponding orbitals
H -1, indicating the effective p-conjugation extension between
4
2
2
the fused Pc and Por chromophores. These features are also
Chem. Eur. J. 2016, 22, 4492 – 4499
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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observed in the Pc–Por-fused systems, H -2, Zn -1, and Zn -2
absorption spectral features, these results suggest that either
the Pc or Por subunit in the Pc–Por-fused oligomers cannot act
as an independent chromophore as in non-conjugated Pc–Por
6
2
3
in Figure 2 and Figure S15 (in the Supporting Information).
The fluorescence spectra of H -1, H -2, Zn -1, and Zn -2 as
4
6
2
3
[
18]
well as their reference monomeric compounds were measured
in toluene and toluene/pyridine (100:1) at room temperature.
The maxima for the fluorescence bands were observed at 735,
oligomers.
To further clarify the nature of the excited-state dynamics of
the Pc–Por-fused systems, femtosecond transient absorption
(fs-TA) measurements were carried out. For comparative stud-
ies, the TA spectra were recorded with photoexcitation at 530/
720 and 530/760 nm for H -1 and H -2 and at 550/720 and
7
80, 744, and 776 nm for H -1, H -2, Zn -1, and Zn -2, respec-
4 6 2 3
tively, which were clearly redshifted compared with those of
references H -3, H -4, Zn-3, and Zn-4. The fluorescence quan-
2
2
4
6
tum yields were estimated as 0.41 for H -1, 0.27 for H -2, 0.15
550/760 nm for Zn -1, and Zn -2, respectively (Figure 4 and
2 3
4
6
for Zn -1, and 0.13 for Zn -2 (Table 1). Moreover, the fluores-
Figure S17 in the Supporting Information). The TA spectra of
2
3
Table 1. Fluorescence data, quantum yields (F
diative (k ) and nonradiative (knr) decay rates for the metal-free and the
corresponding zinc compounds for 1–4.
f f
) and lifetimes (t ), and ra-
r
[
a]
[
r
b]
7
À1
[b]
7
À1
Compound
l
max [nm]
F
f
t
f
[ns]
k
[10 s
]
knr [10 s ]
H
H
H
H
Zn
Zn
4
6
2
2
-1
-2
-3
-4
735
780
688
651
744
776
685
645
0.41
0.27
0.54
0.06
0.15
0.13
0.24
0.045
5.3
2.5
6.0
10.8
1.9
1.3
2.6
2.4
7.7
10.8
9.0
0.6
7.9
10.0
9.2
1.9
11.2
29.2
7.6
8.7
2
-1
-2
44.7
66.9
29.2
39.8
3
Zn-3
Zn-4
Figure 4. Femtosecond transient absorption spectra and decay profiles
[
a] The metal-free compounds for 1–4 were measured in toluene and the
zinc compounds for 1–4 were measured in toluene/pyridine(v/v=100:1).
b] Radiative and nonradiative decay rates can be calculated by using the
observed fluorescence quantum yields and fluorescence lifetimes based
on the equations: F =k /(k +knr) and t =1/(k +knr).
(
inset) of (a) H
toluene, and (c) Zn
in toluene/pyridine (100:1) at room temperature.
4
-1 with lpump =530 nm and (b) H
6
-2 with lpump =530 nm in
-1 with lpump =550 nm and (d) Zn
3
-2 with lpump =550 nm
[
2
f
r
r
f
r
H -1 displayed intense ground-state bleaching for the Q-like
4
band in the range 615–750 nm and broad excited-state ab-
sorption bands in the range 450–610 nm. The TA decay profile
cence decay profiles were fitted with the time constants of 5.3,
.5, 1.9, and 1.3 ns for H -1, H -2, Zn -1, and Zn -2, respectively,
2
of H -1 was fitted with the time constant of 5.3 ns, which is
4
6
2
3
4
as shown in Figure 3. Here, the expansion of the fused system,
well-matched with its fluorescence decay. Similar features were
from H -3 to H -1 and H -2, induced a decrease in fluorescence
observed in the TA spectra and decay profiles of H -2, Zn -1,
2
4
6
6
2
quantum yield, from 0.54 to 0.27, and lifetime, from 6.0 to
.5 ns (Figure S16 in the Supporting Information), indicating
the acceleration of the nonradiative decay rate along with the
and Zn -2. Here, even with the investigation for the fast excit-
3
2
ed-state dynamics in the range of approximately 2 ps (Figur-
es S17–S20 in the Supporting Information), no significant spec-
tral features for energy flow from the Por moiety to Pc moiety
was observed. Along with the calculation results and electro-
chemical analysis of H -1, H -2, Zn -1, and Zn -2, these TA data
[4a,17]
extension of the p-electron system.
A similar tendency was
observed for Zn-3, Zn -1, and Zn -2. In accordance with their
2
3
4
6
2
3
strongly suggest that the effective conjugative interaction
through the benzo[a]pyrazine-fused bridge induces the Pc–
Por-fused oligomers to behave as one-quantum molecular sys-
tems.
Fusion of either the Por or Pc moieties into the homo-tetra-
pyrrole dimers and trimers provides them with distinct physi-
cochemical and optical features induced by the extended con-
jugated electronic structures, such as extremely redshifted ab-
sorption bands to the NIR region. This feature in turn results
from the decreased HOMO–LUMO energy gap and the more
extended vibrational level structures, which lead to the acceler-
[
4a,19]
ation of the nonradiative decay rate.
As a result, the
homo-Pc-fused systems typically show extremely low fluores-
cence yield and short excited-state lifetimes in comparison
Figure 3. Nanosecond time-resolved fluorescence decay profiles for
a) H -1 and (b) H -2 in toluene and (c) Zn -1 and (d) Zn -2 in toluene/pyri-
dine (100:1).
[
3,4]
with their monomers,
as exemplified by the significantly de-
(
4
6
2
3
creased fluorescence quantum yield and lifetime of homo-bi-
Chem. Eur. J. 2016, 22, 4492 – 4499
www.chemeurj.org
4496
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
nuclear/trinuclear Pcs (0.04/0.8 ns and 0.02/0.4 ns, respectively)
through the benzo[a]pyrazine-fused bridge leads to very fast
charge separation and recombination processes in the CT state
in these systems.
[
4a]
in comparison with 0.33/6.2 ns for the Pc monomer. In the
case of homo-Por-fused systems, although such extension of
the p-conjugation decreases the S -state lifetime, the perturba-
1
tion of the electronic structure of Por increases the oscillator
strength of the forbidden Q-band transitions, leading to an en-
Conclusion
[19–20]
hancement of fluorescence quantum yield.
Interestingly,
The Pc–Por-fused skeleton has been extended into the trimer.
In particular, compared with homo-tetrapyrrole-fused analogs,
both Pc–Por-fused dimers and trimers exhibit high fluores-
cence quantum yields (>0.13) and relatively long excited-state
lifetimes (>1.3 ns) mainly as a result of the large transition
dipole moments originating from the effective p-conjugation
extension in the Pc–Por-fused systems. These results will be
helpful for the design and synthesis of tetrapyrrole-fused oligo-
meric skeletons and even corresponding two-dimensional
nanostructures with well-defined compositions, rich with nitro-
gen atoms, and tunable metal ions and, therefore, with a wide
range of potential applications ranging from photonic and
electronic nanodevices to catalysis.
the Pc–Por-fused dimers and trimers still exhibit quite high
fluorescence quantum yield (>0.13) and long excited-state life-
times (>1.3 ns) in comparison with the homo-tetrapyrrole-
fused dimers and trimers. Such intense fluorescence in the NIR
region for the Pc–Por-fused systems can be attributable to the
effective conjugative interaction between the Pc and Por moi-
eties through the benzo[a]pyrazine-fused bridge. In the molec-
ular orbitals of the Pc–Por-fused dimers and trimers, the strong
electronic interaction between the HOMOs of the Pc and Por
moieties results in the delocalization of the HOMO over the
entire molecular skeleton. With their large extinction coeffi-
cient at Q-like bands, these results reflect the fact that the ef-
fective delocalization of the p-electron density along the long
axis of the fused systems induces large transition dipole mo-
ments, leading to the high radiative decay rate and increased Experimental Section
fluorescence quantum yield upon expansion of the fused sys-
General remarks
[
19–20]
tems (Table 1).
Moreover, the effective electronic commu-
1
,2,4-Trichlorobenzene (TCB) was freshly distilled from CaH under
2
nication through the benzo[a]pyrazine-fused bridge induces
the perturbation of electronic structures of the Pc and Por moi-
eties. For homo-Por-fused systems, it is expected that the per-
turbation intensifies the transition to the lowest excited state,
resulting in the enhancement of fluorescence quantum
nitrogen. n-Pentanol was distilled from sodium. Column chroma-
tography was carried out on silica gel (Merck, Kieselgel 60, 70–230
mesh) and biobead (BIORAD S-X1, 200–400 mesh) columns with
the indicated eluents. All other reagents and solvents were used as
received. The compounds 4,5-bis(2,6-dimethylphenoxy)phthaloni-
[
19–20]
yield.
Therefore, the perturbation should also be responsi-
[21]
[22]
[23]
[24]
trile,
2,3-di-cyanopyrazine,
H TMP (H -4),
ZnTMP (Zn-4),
2
2
ble for the quite high fluorescence quantum yield for the Pc–
Por-fused systems.
5,10,15,20-tetrakis(2,4,6-trimethylphenyl)-2’,3’-di-cyanopyrazino[2,3-
b]porphyrin were prepared according to the published proce-
[25]
dures.
Motivated by the localized LUMO and LUMO+1 of H -1,
4
H -2, Zn -1, and Zn -2, we measured the electronic absorption
6
2
3
and fluorescence spectra for the two dimers and two trimers
in a highly polar solvent, benzonitrile. In benzonitrile, although
the absorption spectral features were similar to those in tolu-
ene, weak fluorescence spectra were also observed (Figure S21
and Table S6 in the Supporting Information). Furthermore, the
TA results displayed that their singlet-excited-state lifetimes
were significantly decreased to 160, 60, 100, and 40 ps for H4-
Preparation of Pc–Por-fused dimer H
-1 and trimer H -2
4 6
A mixture of 4,5-bis(2,6-dimethylphenoxy)phthalonitrile (220.8 mg,
0
.60 mmol), 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)-2’,3’-dicyano-
pyrazino[2,3-b]porphyrin (56.1 mg, 0.060 mmol), and lith-
5
ium(10.5 mg, 1.5 mmol) in n-pentanol (3.0 mL) was heated to
reflux under nitrogen for 2 h. After cooling to room temperature,
the resulting green solution was poured into methanol (300 mL)
containing 3.0 mL of CH COOH. The precipitate was collected by
3
1
, H -2, Zn -1, and Zn -2, respectively, where the slow decay
6 2 3
filtration and purified by chromatography on a silica gel column
using CH Cl as the eluent. The target products were further puri-
components with the time constant longer than 10 ns were as-
signed to their triplet states (Figure S22 in the Supporting In-
2
2
fied by biobead column chromatography with CHCl as the eluent,
3
formation). As seen in the molecular orbital structures of H -1,
4
giving the trimer complex H -2 as the first fraction, followed by the
6
H -2, Zn -1, and Zn -2, these results demonstrate the intramo-
6
2
3
dimer complex H -1. Repeated chromatography followed by recrys-
4
lecular CT character of the Pc–Por-fused systems. Whereas the
nonpolar environment in toluene frustrates the localization of
electron density on the Pc moiety, the strong dipole moment
of the polar solvent induces a stable charge-separated state in-
volving the localized electron density on the Pc moiety, result-
ing in the fast decay in the excited-state dynamics (Scheme S2
in the Supporting Information). Here, the formation of the
charge-separated state in all compounds was not observed
even in the few ps range for the investigation of the fast dy-
namics. In conjunction with the TA results in toluene, this
result suggests that the effectively extended p-conjugation
tallization from CHCl and CH OH gave both H -1 (12.0 mg, 9.8%)
3
3
4
and H -2 (4.0 mg, 2.6%) as green powders.
6
1
H -1: H NMR (CDCl , 400 MHz): d=8.92 (br, 4H), 8.72 (d, 2H, J=
4
3
4.00 Hz), 8.58 (d, 2H, J=4.00 Hz), 8.51 (s, 2H), 8.42 (s, 2H), 8.06 (s,
H), 7.56 (s, 4H), 7.50 (s, 6H), 7.44–7.37 (m, 12H), 7.30 (s, 4H), 2.95
2
(
(
(
s, 6H), 2.65 (s, 6H), 2.54 (s, 12H), 2.48 (s, 12H), 2.46 (s, 12H), 1.92
s, 24H), À0.51 (s, 2H), À2.17 ppm (s, 2H); UV/Vis (toluene): l
max
log e)=361 (4.82), 420 (5.21), 633 (4.61), 668 (4.65), 701 (5.15),
7
2
26 nm (5.13); MALDI-TOF MS: an isotopic cluster peaking at m/z=
+
042.0 (calcd for C136H116N O6 [M] =2041.9); elemental analysis
14
calcd (%) for C136H116N O ·1.5CHCl ·2.5CH OH: C 73.06, H 5.58, N
14
6
3
3
8.51; found: C 73.41, H 5.97, N 8.48.
4497 ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2016, 22, 4492 – 4499
www.chemeurj.org

Full Paper
1
H -2: H NMR (CDCl , 400 MHz): d=9.34 (s, 2H), 9.03 (s, 2H), 8.78
(
8
8
(
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San Diego, 2000 and 2003; d) Advances in Functional Phthalocyanine
Materials, Structure and Bonding (Ed.: J. Jiang), Springer, Heidelberg,
6
3
s, 2H), 8.74 (br, 4H), 8.64 (d, 2H, J=8.00 Hz), 8.61 (d, 2H, J=
.00 Hz), 8.53 (s, 4H), 8.30 (s, 2H), 7.82 (s, 4H), 7.62 (s, 4H), 7.32 (s,
H), 7.48–7.41 (m, 6H), 7.51 (s, 6H), 3.21 (s, 6H), 2.98 (s, 6H), 2.66
s, 12H), 2.07 (s, 12H), 1.96 (s, 36H), 2.57 (s, 12H), 2.51 (s, 12H),
(log e)=426 (5.61),
À2.07 ppm (s, 4H); UV/Vis (toluene): l
max
7
2
58 nm (5.48); MALDI-TOF MS: an isotopic cluster peaking at m/z=
2010; e) P.-C. Lo, X. Leng, D. K. P. Ng, Coord. Chem. Rev. 2007, 251,
+
608.2 (calcd for C176H150N O [M] =2608.2); elemental analysis
20
4
2334–2353.
calcd (%) for C176H150N O ·1.75CHCl ·3CH OH·2H O: C 73.61, H
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20
4
3
3
2
6
.12, N 9.53; found: C 73.58, H 5.73, N 9.50.
1
29, 10080–10081; c) H. Uoyama, K. S. Kim, K. Kuroki, J.-Y. Shin, T.
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Preparation of dimeric zinc complex Zn -1
2
2
A
mixture of H -1 (8.0 mg, 0.004 mmol) and Zn(OAc) ·2H O
4 2 2
[
(
5.15 mg, 0.024 mmol) in DMF (1 mL) was heated at 908C for 4 h
2
under nitrogen. After cooling to room temperature, the mixture
was evaporated under reduced pressure and the residue was puri-
fied by chromatography on a silica gel column using CHCl as the
eluent. The first pale-green band containing the target compound
Zn -1 was collected. Repeated chromatography followed by recrys-
3
2
tallization from CHCl and n-hexane gave Zn -1 as a green powder
3
2
1
(
6.0 mg, 70.4% yield). H NMR (CDCl , 400 MHz): d=9.05 (s, 2H),
3
8
.78 (s, 2H), 8.65 (d, 2H, J=4.00 Hz), 8.50 (d, 2H, J=4.00 Hz), 8.46
(
s, 2H), 8.27 (s, 2H), 8.15 (s, 2H), 7.50 (s, 4H), 7.44 (s, 6H), 7.35 (br,
[
4] a) S. Makarov, C. Litwinski, E. A. Ermilov, O. Suvorova, B. Rçder, D.
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1
3
6H), 2.91 (s, 6H), 2.52 (s, 18H), 2.45 (s, 12H), 2.43 (s, 12H),
.84 ppm (s, 24H); UV/Vis (toluene/pyridine=100:1): lmax (log e)=
69 (5.07), 422 (5.35), 718 nm (5.36); MALDI-TOF MS: an isotopic
+
cluster peaking at m/z=2168.8 (calcd for C136H112N O Zn [M] =
14
6
2
2
168.7);
elemental
analysis
calcd
(%)
for
C136H112N O Zn ·1.75CHCl : C 69.57, H 4.82, N 8.25; found: C 69.84,
H 4.86, N 8.30.
14
6
2
3
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Preparation of trimeric zinc complex Zn -2
3
By employing the procedure for the preparation for Zn -1 with H -
2
6
2
instead of H -1 as the starting material, Zn -2 was isolated in
4 3
1
5
8
8
8.1% yield. H NMR (CDCl , 400 MHz): d=9.56 (s, 2H), 9.25 (s, 2H),
.81 (s, 2H), 8.75 (br, 4H), 7.52–7.41 (m, 12H), 8.61–8.56 (m, 8H),
.30 (s, 2H), 7.82 (s, 4H), 7.60 (s, 4H), 7.31 (s, 8H), 3.23 (s, 6H), 2.97
3
[
(
s, 6H), 2.66 (s, 12H), 2.58 (s, 12H), 2.52 (s, 12H), 2.07 (s, 12H),
1
4
.95 ppm (s, 36H); UV/Vis (toluene/pyridine=100:1): lmax (log e)=
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23 (5.61), 567 (4.95), 765 nm (5.45); MALDI-TOF MS: an isotopic
+
cluster peaking at m/z=2799.1 (calcd for C176H144N O Zn [M] =
2
6
20
4
3
1103–1107.
799.0); elemental analysis calcd (%) for C176H144N O Zn ·3CHCl : C
20 4 3 3
8.09, H 4.69, N 8.87; found: C 67.96, H 4.64, N 9.23.
[
[
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3
[
Financial support for the work at University of Science and
Technology Beijing was from the National Key Basic Research
Program of China (Grant Nos. 2013CB933402, 2012CB224801),
National Natural Science Foundation of China (21401009 and
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2
1290174), and Beijing Municipal Commission of Education.
The work at Yonsei University has been financially supported
by
the
Global
Research
Laboratory
Program
(
2013K1A1A2A02050183).
[
[
Keywords: fluorescence
·
fused systems
·
oligomers ·
phthalocyanine · porphyrins
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Received: December 2, 2015
Published online on February 16, 2016
Chem. Eur. J. 2016, 22, 4492 – 4499
www.chemeurj.org
4499
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim