β-Octamethoxy-Substituted 22π and 26π Stretched Porphycenes: Synthesis, Characterization, Photodynamics, and Nonlinear Optical Studies

Article abstract of DOI:10.1002/chem.201501299

Three meso-expanded tetrapyrrolic aromatic macrocycles, including 22π and 26π acetylene-cumulene bridged stretched octamethoxyporphycenes and octamethoxy[22]porphyrin-(2.2.2.2), are reported, for the first time, by modification of previously reported synthetic methods. This strategy led to an enhancement in the overall yield of their corresponding octaethyl analogues. The methoxy-substituted expanded porphycenes display slightly blueshifted absorption relative to their ethyl analogues, along with very weak fluorescence, probably due to efficient intramolecular charge transfer (ICT). Additionally, the two-photon absorption (TPA) cross sections of these macrocycles were evaluated; these are strongly related to core expansion of the porphyrin aromaticity through increased meso-bridging carbon atoms as well as conformational flexibility and substitution effects at the macrocyclic periphery. In particular, the octamethoxy stretched porphycenes display strong TPA compared with the octaethyl analogues due to the dominant ICT character of methoxy groups with a maximum TPA cross section of 830 GM at 1700 nm observed for 26π-octamethoxyacetylene-cumuleneporphycene.

Full text of DOI:10.1002/chem.201501299

DOI: 10.1002/chem.201501299
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&
Aromaticity
b-Octamethoxy-Substituted 22p and 26p Stretched Porphycenes:
Synthesis, Characterization, Photodynamics, and Nonlinear
Optical Studies
Anup Rana,[a] Sangsu Lee,[b] Dongho Kim,*[b] and Pradeepta K. Panda*[a]
Abstract: Three meso-expanded tetrapyrrolic aromatic mac-
rocycles, including 22p and 26p acetylene–cumulene
bridged stretched octamethoxyporphycenes and octame-
thoxy[22]porphyrin-(2.2.2.2), are reported, for the first time,
by modification of previously reported synthetic methods.
This strategy led to an enhancement in the overall yield of
their corresponding octaethyl analogues. The methoxy-sub-
stituted expanded porphycenes display slightly blueshifted
absorption relative to their ethyl analogues, along with very
weak fluorescence, probably due to efficient intramolecular
charge transfer (ICT). Additionally, the two-photon absorp-
tion (TPA) cross sections of these macrocycles were evaluat-
ed; these are strongly related to core expansion of the por-
phyrin aromaticity through increased meso-bridging carbon
atoms as well as conformational flexibility and substitution
effects at the macrocyclic periphery. In particular, the octa-
methoxy stretched porphycenes display strong TPA com-
pared with the octaethyl analogues due to the dominant ICT
character of methoxy groups with a maximum TPA cross sec-
tion of 830 GM at 1700 nm observed for 26p-octamethoxy-
acetylene–cumuleneporphycene.
Introduction
linear Csp2CspCspCsp2 structural units, the resultant macrocycles
are called acetylene–cumulene porphyrinoids, whereas the cor-
responding reduced product [22]porphyrin-(2.2.2.2)[4] is named
a stretched porphycene, owing to its closer relationship to por-
phycene. Although these macrocycles were anticipated to
have potential utility as an effective photosensitizer for photo-
dynamic therapy (PDT), previous reports showed limitations in
their application because of their low triplet energy gap, and
hence, poor singlet oxygen generation ability.[5] Also, porphy-
cenes have recently emerged as promising candidates for
third-order nonlinear optical (NLO) materials.[6] However, to the
best of our knowledge, there no reports have illustrated NLO
studies on their expanded analogues.
Expanded porphyrins endowed with a larger central core have
more than 16 atoms.[1] These synthetic analogues can be sche-
matically derived either by increasing the number of meso-me-
thine groups or by introducing additional heterocyclic rings
into the conjugation pathways. As a result of core expansion,
these expanded porphyrins often possess novel spectral and
electronic features, unprecedented capability to coordinate
both cations and anions, and unique structures with nonplanar
figure-of-eight motifs.[1] This success in pioneering a new area
of porphyrin chemistry motivated researchers to investigate
expanded porphycenes because the structures and photophys-
ical properties of porphycenes are more sensitive than those
of porphyrins.[2] In this direction, core expansion of porphy-
cenes by using an even number of bridging meso-carbon
atoms between the constituent pyrrole units was reported by
Vogel et al. in 1990.[3] Owing to the occurrence of pairwise
In this regard, we have synthesized two new acetylene–cu-
mulene porphyrinoids with 22p and 26p aromatic conjugation
pathways and a new [22]porphyrin-(2.2.2.2) derived from 3,4-
dimethoxypyrrole by modification of previously reported syn-
thetic methods, by which we could enhance the yields of their
ethyl analogues. Additionally, we have investigated their third-
order NLO properties along with substituent effects on their
structures, photophysical properties, and electrochemical prop-
erties.
[a] A. Rana, Dr. P. K. Panda
School of Chemistry and Advance Centre of Research in
High Energy Materials (ACRHEM)
University of Hyderabad, Hyderabad-500046 (India)
E-mail: pkpsc@uohyd.ernet.in
[b] S. Lee, Prof. Dr. D. Kim
Results and Discussion
Spectroscopy Laboratory for Functional p-Electronic
Systems and Department of Chemistry
Yonsei University, Seoul 120-749 (South Korea)
E-mail: dongho@yonsei.ac.kr
Synthesis of expanded porphycenes
Syntheses of 22p and 26p aromatic acetylene–cumulene por-
phyrinoids was achieved by McMurry coupling of the corre-
sponding mono- and diacetylenic bipyrroledialdehydes. They
were synthesized from mono- and diacetylene-bridged bipyr-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501299. It contains detailed experimen-
tal procedures, as well as NMR spectroscopy, photophysical, and electro-
chemical data.
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role dialdehyde precursors, which were synthesized by using
a modified method reported in the literature, starting from io-
dopyrrole aldehydes 7a (see the Supporting Information) and
7b[7] (Scheme 1). Briefly, Sonogashira coupling of 7a and 7b
with trimethylsilylacetylene in the presence of [PdCl2(PPh3)2]
and CuI led to trimethylsilylacetylinic pyrrole aldehydes 8a and
8b in 87 and 88% yield, respectively. Subsequent removal of
the trimethylsilyl group with TBAF provided acetylenic pyrrole
derivatives 9a and 9b in 97 and 99% yield, respectively, at
room temperature.[8] Sonogashira coupling of 9a and 9b led
to the successful synthesis of acetylene-bridged bipyrroledial-
dehydes 10a and 10b in 62 and 89% yield, respectively.[8] To
improve the yield of the desired product, we employed a modi-
fied Glaser–Hay coupling protocol, in which oxidative coupling
of 9a and 9b in the presence of a catalytic amount of
[PdCl2(PPh3)2] and CuI with a base under an oxygen atmos-
phere led to the desired diacetylene-bridged bipyrrolediald-
hydes 11 a and 11 b in 94 and 95% yield at room temperature,
respectively. McMurry coupling of 10b with Zn/TiCl4 in the
presence of CuCl under reflux conditions, followed by oxida-
tion in air gave desired b-octamethoxy 22p-acetylene–cumule-
neporphycene 4 in very poor yield with reduced 22p b-octa-
methoxy[22]porphyrin-(2.2.2.2) 6 as the major product.[3] Be-
cause the 3,4-dimethoxypyrrole analogue is a highly activated
system, we performed the McMurry coupling reaction by slow
addition of 10b at room temperature and this modification led
to the formation of the desired product 4 as the major product
and reduced products as a minor component. Although it was
difficult to purify the mixture of products by column chroma-
tography, we could obtain pure compound 4 in 26% yield
(Scheme 1), by simply washing with hexane and chloroform
(1:1). Interestingly, the McMurry coupling of 10a in the same
manner resulted in the exclusive formation of the desired 22p-
b-octaethylacetylene–cumuleneporphycene 1 without any ad-
ditional reduced products. The 27% yield in this reaction was
much improved compared with that reported by Vogel et al.
(18%).[3] Syntheses of 26p-b-octasubstituted acetylene–cumule-
neporphycenes (2 and 5) were achieved by McMurry coupling
of diacetylene-bridged bipyrroledialdhydes 11 a and 11 b under
reflux conditions; this led to the formation of compounds 2
and 5 both in 17% yield. Again our method led to the forma-
tion of 2 in much higher yield than that reported by Vogel
et al. (9%).[5]
22p-Acetylene–cumuleneporphycenes (1 and 4) were re-
duced with Pd/C in the presence of 1 atm of H2 gas followed
by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ), which gave [22]porphyrins-(2.2.2.2) (3 and 6) in good
yield (Scheme 2). Octamethoxy-substituted acetylene–cumule-
neporphycene 4 undergoes a relatively slow reduction (48 h)
compared with the octaethyl analogue (20 h). Our method
shows an distinct advantage over the method reported by
Vogel et al., which used Lindlar catalyst (35% yield),[4] with
much improved yield of 3 (84%). A moderate yield for 6 may
be attributed to the longer reaction time. Furthermore, the
direct reductive McMurry coupling of 3,4-dimethoxypyrrole-
2,5-dialdehyde[9] led to 6 in 1.4% yield as the only non-poly-
mer product.[4]
Scheme 2. Synthesis of [22]porphyrins-(2,2,2,2).
1H NMR spectroscopy analysis of expanded porphycenes
The macrocycles revealed aromatic features in the 1H NMR
spectra because the outer meso protons were shifted down-
field and the inner NH protons were shifted upfield. The
1H NMR spectrum of 4 indicates that the meso protons are
marginally perturbed, whereas the NH protons are significantly
shifted upfield (d=0.86 ppm), in comparison to the octaethyl
analogue 1 (d=2.19 ppm). This may be attributed to increased
electron density in the aromatic core of 4 due to the increased
electron-donating ability of methoxy substituents at its periph-
ery. A similar trend is also observed for 26p-acetylene–cumule-
neporphycene analogues, in which the NH protons are shifted
Scheme 1. Synthesis of 22p and 26p expanded acetylene–cumuleneporphy-
cenes. Reagents and conditions: i) [PdCl2(PPh3)2] (4 mol%), CuI (8 mol%),
THF, 508C; ii) tetrabutylammonium fluoride (TBAF), THF, RT; iii) [PdCl2(PPh3)2]
(4 mol%), CuI (8 mol%), THF, reflux; iv) [PdCl2(PPh3)2] (5 mol%), CuI
(10 mol%), O2, THF, RT.
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upfield by 1.19 ppm for the methoxy analogue (d=0.77 ppm
for 5 and d=1.96 ppm for 2). Similar to octaethyl analogue 3,
octamethoxy[22]porphyrin-(2.2.2.2) 6 displays outer meso-Hs at
C-5, 11 and 12 that are shifted downfield, whereas the inner
meso-H at C-6 is shifted upfield. For example, in the case of 6,
the H-5 protons are shifted downfield by 0.21 ppm (d=
11.98 ppm for 6 and d=11.77 ppm for 3), the H-6 protons are
shifted upfield by 0.5 ppm (d=À7.97 ppm for 6 and d=
À7.47 ppm for 3), and the NH protons are shifted upfield by
0.5 ppm (d=0.83 ppm for 6 and d=1.33 ppm for 3); these re-
sults clearly indicate a stronger substituent effect for this class
of macrocycles than those of the acetylene–cumulene types.
Figure 2. Normalized UV/Vis absorption (c) and fluorescence spectra
(d) of compounds 1–6 in chloroform.
Structural analysis of expanded porphycenes
The molecular structures of 4 and 6 were characterized by X-
ray crystallography (Figure 1).[10] The crystal structure of ex-
panded porphycene 4 possesses a near-planar macrocyclic
core with a mean deviation of 0.05 Š for the nitrogen atoms
from the average macrocyclic plane. The structure of the acety-
lene–cumulene bridge Csp2CspCspCsp2 appears to be equivalent
on both sides of 4, which suggests that 4 exists in the form of
a resonance hybrid. The nitrogen atoms of 4 form a rectangular
core similar to that in 1, with N1···N2 and N2···N3 distances of
5.355 and 2.608 Š, respectively. X-ray crystal-structure analysis
of 6 confirms the cis, trans, cis, trans configuration of the com-
pound; however, the core of this macrocycle is slightly more
distorted (0.087 Š) than 3 (0.05 Š) with a mean deviation of
the nitrogen atoms from the mean ring plane.[4] The four nitro-
gen atoms of 6 define a near-parallelogram core geometry
(N1-N2-N3=77.98, N1-N4-N3=77.88).
in the UV/Vis absorption spectra. Both the split Soret and Q
bands are bathochromically shifted for 4 and 5 relative to
those of octamethoxyporphycene, due to the increased conju-
gation pathway (22p and 26p vs. the 18p-electron system).
The addition of two acetylene spacers to methoxy-substituted
bipyrrole resulted in a redshift of the lowest energy band from
l=636 to 758 nm, and further redshifted to l=884 nm upon
the addition of two more acetylene spacers. The UV/Vis ab-
sorption spectra of 4 and 5 show marginal changes in the ab-
sorption maxima compared with their ethyl analogues.[3,5]
However, there is a discernible difference in the spectral pat-
terns, in particular, in the Q-band region, where the methoxy
derivatives display the second band as the most intense one
of the three, whereas for the ethyl analogues, the lowest
energy band is the most intense one. Again the effect of sub-
stituents is more apparent in the case of [22]porphyrin-
(2.2.2.2), with the absorption spectrum of 6 showing the
lowest energy Q band blueshifted by about 17 nm compared
with that of 3.[4] We could not detect any fluorescence for 4
and 5. Also, compared with 3, the stretched porphycene 6 ex-
hibits weaker fluorescence at lmax =779 nm (ff =0.0095). These
results support that the methoxy-substituted macrocycles pos-
sess an efficient intramolecular charge-transfer (ICT) character-
istic. Additionally, we can assume that acetylene–cumulene
porphyrinoids have a stronger ICT character than that of b-oc-
tamethoxy[22]porphyrin-(2.2.2.2).
Two-photon absorption studies
Two-photon absorption (TPA) is a third-order NLO phenome-
non, the probability of occurrence of which is closely related
to p-electron behavior. Recent theoretical and experimental
studies have indicated that ICT character could lead to an en-
hancement of the TPA cross-sectional values.[11] In this context,
to investigate different degrees of ICT character between ethyl
and methoxy analogues, TPA measurements for 1–6 were con-
ducted by the Z-scan technique in toluene at l=1300–
1900 nm, at which the contribution from one-photon absorp-
tion (OPA) is negligible (Figure 3 and Figure S18 in the Sup-
porting Information). The TPA profiles revealed spectral fea-
tures that were in a good agreement with those of the lowest
Figure 1. X-ray crystal structures of a) 4 and b) 6 (top: front view, bottom:
side view) scaled to the 35% probability level. In the side views, the me-
thoxy groups are omitted for clarity. Color code: C, gray; N, blue; O, red; H,
white.
Absorption and emission properties
The UV/Vis absorption and fluorescence spectra of compounds
1–6 were recorded in chloroform at room temperature
(Figure 2 and Figures S16 and S17 in the Supporting Informa-
tion). The introduction of acetylene spacers into the bipyrrolic
units of octamethoxyporphycene[7] gives rise to a huge change
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Figure 3. a) OPA (c) and TPA spectra ( ) of 1 (top), 2 (middle), and 3 (bottom) in toluene. b) OPA (c) and TPA spectra ( ) of 4 (top), 5 (middle), and 6
(bottom) in toluene. The TPA spectra are plotted at lex/2 for comparison with the OPA spectra.
Femtosecond transient absorption studies
OPA band. The maximum TPA cross sections of 1, 2, and 3
were measured to be 580, 770, and 370 GM at l=1400, 1700,
and 1300 nm, respectively. Meanwhile, the maximum TPA cross
sections of 4, 5, and 6 were measured to be 640, 830, and
380 GM at l=1400, 1700, and 1500 nm, respectively. Based on
the observed trends, we can conclude that the TPA cross sec-
tions of the methoxy analogues are larger than those of the
ethyl analogues due to a greater ICT character. Furthermore,
we have observed higher TPA cross sections for 26p-acety-
lene–cumulene porphycenes compared with the correspond-
ing 22p-acetylene–cumulene porphycenes due to the more ex-
tended p-conjugated network for the former. Additionally, the
TPA cross sections of isoelectronic [22]porphyrins-(2.2.2.2) are
smaller than those of 22p-acetylene–cumulene porphyrinoids.
Herein, we note that the TPA cross section is closely related to
conformational flexibility in the p-conjugated network.[12] We
can see that acetylene–cumulene porphyrinoids are much rigid
and tend to be more effectively conjugated than [22]porphy-
rins-(2.2.2.2) with a vinylinic linker, which is known to undergo
To examine the excited-state dynamics of 1–6, we measured
femtosecond transient absorption (TA) spectra of these com-
pounds (Figure 4). The TA spectra exhibited ground-state
bleaching signals, the spectral features of which corresponded
to their ground-state absorption bands. The fitted time com-
ponents of the ethyl analogues are 630, 36, and 2000 ps for 1,
2, and 3, respectively (Figure S19 in the Supporting Informa-
tion). These results are in good agreement with those reported
previously.[5] Also, the decay times of the methoxy analogues
are 11, 7.3, and 370 ps for 4, 5, and 6, respectively (Figure S19
in the Supporting Information). Compared with the ethyl ana-
logues, the methoxy analogues exhibit significantly shorter ex-
cited-state lifetimes. This feature could be attributed to the ICT
character from the methoxy substituent to the porphyrinoid
core. Because the time resolution of our TA setup is about
150 fs, we can assume that the timescale of the ICT process
lies within a few hundred femtoseconds, which hinders the
an isodynamic transformation involving rotation around trans direct observation of the ICT process in the TA spectra.
ÀCH=CHÀ bonds[4] and lead to a relatively low TPA cross sec-
tion. Interestingly, expansion of the aromaticity could be clear-
Electrochemical studies
ly observed for all expanded porphycenes as evident from
their higher TPA cross section values compared with their por-
phycene analogues.[6e]
The redox potentials of all expanded porphycenes were ana-
lyzed by cyclic voltammetry and differential pulse voltammetry
(DPV) in dichloromethane (Figure 5). Similar to b-octamethoxy-
porphycene,[7] all expanded porphycenes show two reversible
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Figure 4. a) TA spectra of 1 (top), 2 (middle), and 3 (bottom) in toluene obtained with photoexcitation at l=670 (top), 830 (middle), and 670 nm (bottom), re-
spectively. b) TA spectra of 4 (top), 5 (middle), and 6 (bottom) in toluene obtained with photoexcitation at l=670 (top), 830 (middle), and 670 nm (bottom),
respectively.
reductions and two reversible or quasi-reversible oxidations.
Oxidation and reduction potentials referenced versus Ag/AgCl
for all expanded porphycenes are summarized in Table 1. The
first oxidation potentials for octamethoxy-substituted expand-
ed porphycenes appeared at +0.74, +0.68, and +0.62 V for 4,
5, and 6, respectively, and the first reduction potentials for 4,
5, and 6 were observed at À0.58, À0.33, and À0.89 V, respec-
tively. Due to the presence of electron-rich methoxy groups at
their periphery, the first oxidation and first reduction potentials
are less positive and more negative, respectively, for these ex-
panded porphycenes, which indicate the more electron-rich
nature of these macrocycles compared with ethyl-substituted
analogues. Interestingly, changes in the oxidation and reduc-
tion potentials are marginal, as observed in the corresponding
porphycenes (b-ethyl and b-methoxy analogues),[7,14] probably
owing to the absence of distortion between adjacent pyrrolic
moieties of bipyrrole units (arising from van der Waals repul-
sion between their inner b substituents), by inserting acetylinic
Table 1. Comparative oxidation and reduction potentials (in V vs. Ag/
AgCl) for expanded porphycenes.
Reduction
Oxidation
HOMO/
LUMO [V]
1[a]
2[a]
3[a]
4
5
6
À0.88, À0.57
À0.49, À0.30
À1.02, À0.87
À0.76, À0.58
À0.46[b], À0.33[b]
À1.05, À0.89
+0.80, +1.09
+0.72, +0.98
+0.67, –
+0.74[b], +1.10[b]
+0.68[b], +0.92[b]
+0.62, +1.02
1.37
1.02
1.54
1.32
1.01
1.51
Figure 5. Cyclic voltammograms of 4, 5, and 6 in dichloromethane at 258C
[a] Taken from Ref. [13]. [b] Measured by DPV.
(scan rate: 50 mVsÀ1).
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column chromatography with methanol/chloroform (1:99) to pro-
vide a mixture of 4 with acetylenic dihydro and tetrahydro 6 re-
duced products. The mixture of products was washed with chloro-
form/hexane (1:1), filtered and washed with the same solution
until the filtrate became colorless, to give 4 as a blue crystalline
solid (47 mg, 26%). The filtrate was evaporated to dryness under
reduced pressure, which could be reduced to tetrahydro product 6
in good yield by using 20 mol% Pd/C at 1 atm H2 pressure. M.p.
and vinylinic spacers. Upon extension of the conjugation path-
way from 22p to 26p, the changes in the first oxidation poten-
tials were less clear than those of the first reduction potential;
this may be attributed to minimal perturbation in the energy
levels of the HOMO rather than the LUMO, for these molecules,
which indicates that the extension of conjugation leads to
greater stabilization of the LUMO.[13] The HOMO–LUMO energy
gaps of all methoxy-substituted expanded porphycenes (DE=
Eox1ÀEred1) were similar to those of ethyl analogues, which was
further supported by minimal changes observed in the lowest-
energy Q bands in the absorption spectra.
>3008C; IR (KBr): n˜ =2066 cmÀ1 1H NMR (500 MHz, CDCl3): d=
;
10.05 (s, 4H), 5.34 (s, 12H), 4.92 (s, 12H), 0.86 ppm (s, 2H); UV/Vis
(CHCl3): lmax (loge)=411 (4.97), 445 (4.65), 672 (4.29), 714 (4.54),
758 nm (4.41); HRMS (ESI+): m/z calcd for C32H31N4O8 [M+H+]:
599.2136; found: 599.2142.
Conclusion
Synthesis of 2,3,8,9,14,15,20,21-octaethyl-5,6,17,18-tetrade-
hydro[22]porphyrin-(2.2.2.2) (1)
We synthesized b-octamethoxy acetylene–cumulene porphy-
cenes and [22]porphyrin-(2.2.2.2) along with their octaethyl
congeners by employing a modified synthetic protocol that
gave higher yields. The substituent effect was more severe in
the case of [22]porphyrin-(2.2.2.2) than acetylene–cumulene-
bridged porphycenes, as observed from their absorption spec-
tra. The dominant ICT character of methoxy groups could be
observed through enhanced TPA cross sections for the me-
thoxy-substituted macrocycles compared with their ethyl ana-
logues. From this study, we can conclude that expansion of
the p-conjugation network by the introduction of meso-me-
thine groups or pyrrole moieties results in a similar NLO re-
sponse. The NLO properties of the expanded porphycenes
studied herein were dominated by the p-conjugated network,
but conformation flexibility also played an important role. The
ICT property of the methoxy groups led to much shorter excit-
ed-state lifetimes for methoxy analogues. Electrochemical stud-
ies revealed that the first oxidation and reduction potentials
became less positive and more negative, respectively; this
complied with the electron-rich character of these octame-
thoxy-expanded porphycenes.
The same procedure as that described for 4, but with aldehyde
10a (200 mg, 0.62 mmol), was used. The crude reaction mixture
was purified by column chromatography on silica gel with chloro-
form/hexane (1:1) as the eluent to give 1 as a blue crystalline solid
1
(48 mg, 27%). H NMR (400 MHz, CDCl3): d=10.00 (s, 4H), 4.46 (q,
8H, J=7.6 Hz), 4.12 (q, 8H, J=7.6 Hz), 2.25 (t, 12H, J=7.6 Hz), 2.19
(s, 2H), 1.94 ppm (t, 12H, J=7.6 Hz); UV/Vis (CHCl3): lmax (loge)=
407 (5.29), 440 (4.90), 679 (4.60), 726 (4.88), 768 nm (4.91).
Synthesis of 2,3,10,11,16,17,24,25-octamethoxy-
5,6,7,8,19,20,21,22-tetradehydro[26]porphyrin-(2.4.2.4) (5)
A slurry of activated zinc (1.46 g) and copper(I)chloride (221 mg,
2.24 mmol) was taken in THF (100 mL) under nitrogen and TiCl4
(1.23 mL, 11.2 mmol) was added slowly. The reaction mixture was
then heated at reflux for 3 h with vigorous stirring. Aldehyde 11 b
(200 mg, 0.56 mmol) in THF (100 mL) was added slowly over 2 h
with vigorous stirring. The reaction mixture was heated under
reflux for an additional 2 h and then hydrolyzed by slow addition
of a 10% aqueous solution of sodium carbonate (ca. 100 mL) to
the ice-cooled reaction mixture. Subsequently, chloroform (ca.
100 mL) was added to the reaction mixture and allowed to stir at
room temperature in air for an additional 2 h. The reaction mixture
was filtered through Celite, the organic layer was separated, and
then the Celite portion was taken in a beaker and extracted with
chloroform repeatedly until the solution became colorless. The
combined organic layer was passed through anhydrous Na2SO4
and evaporated to dryness under reduced pressure. The crude re-
action mixture was purified by column chromatography on silica
gel by using methanol/chloroform (1:99) as the eluent to provide
a mixture of 5 with some impurities. The mixture of product was
washed with chloroform/hexane (1:1), filtered, and washed with
the same solution until the filtrate became colorless, to give 5
(30 mg, 17%) as a shiny green crystalline solid. M.p. >3008C; IR
Experimental Section
Synthesis of 2,3,8,9,14,15,20,21-octamethoxy-5,6,17,18-tet-
radehydro[22]porphyrin-(2.2.2.2) (4)
A slurry of activated zinc (1.57 g) and copper(I)chloride (237 mg,
2.4 mmol) were taken in THF (100 mL) under nitrogen and TiCl4
(1.32 mL, 12 mmol) was added slowly. The reaction mixture was
then heated at reflux for 3 h with vigorous stirring and the slurry
was allowed to come to room temperature. Aldehyde 10b
(200 mg, 0.60 mmol) in THF (100 mL) was added slowly over 5 h at
room temperature with vigorous stirring. The reaction mixture was
stirred at room temperature for an additional 2 h and then hydro-
lyzed by slow addition of a 10% aqueous solution of sodium car-
bonate (ca. 100 mL) to the ice-cooled reaction mixture. Subse-
quently, chloroform (ca. 100 mL) was added to the reaction mixture
and allowed to stir at room temperature in air for an additional
2 h. The reaction mixture was filtered through Celite, the organic
layer was separated, and then the Celite portion was taken in
a beaker and extracted with chloroform repeatedly until the solu-
tion became colorless. The combined organic layer was passed
through anhydrous Na2SO4 and evaporated to dryness under re-
duced pressure. The crude reaction mixture was purified by
(KBr): n˜ =2016 cmÀ1 1H NMR (500 MHz, CDCl3): d=10.15 (s, 4H),
;
5.42 (s, 12H), 4.96 (s, 12H), 0.77 ppm (s, 2H); UV/Vis (CHCl3): lmax
(loge): 456 (5.10), 500 (4.87), 832 (4.86), 884 nm (4.76); HRMS
(ESI+): m/z calcd for C36H31N4O8 [M+H+]: 647.2136; found:
647.2145.
Synthesis of 2,3,10,11,16,17,24,25-octaethyl-
5,6,7,8,19,20,21,22-tetradehydro[26]porphyrin-(2.4.2.4) (2)
The same procedure as that described for 5, but with aldehyde
11 a (200 mg, 0.57 mmol), was used. The crude reaction mixture
was purified by column chromatography on silica gel with chloro-
form/hexane (1:1) as the eluent to give 2 as a blue crystalline solid
Chem. Eur. J. 2015, 21, 12129 – 12135
www.chemeurj.org
12134
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
1
(30 mg, 17%). H NMR (400 MHz, CDCl3): d=10.09 (s, 4H), 4.57 (q,
anhydrous Na2SO4 and evaporated to dryness under reduced pres-
sure. The crude reaction mixture was purified by column chroma-
tography on silica gel by using chloroform as the eluent to provide
a mixture of 6 as a blue crystalline solid (2.4 mg, 1.4%).
8H, J=7.6 Hz), 4.15 (q, 8H, J=7.6 Hz), 2.30 (t, 12H, J=7.6 Hz), 1.98
(t, 12H, J=7.6 Hz), 1.96 ppm (s, 2H); UV/Vis (CHCl3): lmax (loge):
449 (5.18), 496 (5.01), 841 (5.00), 889 nm (5.03).
Synthesis of 2,3,8,9,14,15,20,21-octamethoxy[22]porphyrin-
(2.2.2.2) (6)
Acknowledgements
Compound 4 (10 mg, 0.017 mmol) and 5% Pd/C (7 mg) were taken
in THF (30 mL) under H2 (1 atm, balloon). Progress of the reaction
was monitored with UV/Vis spectroscopy by taking a small aliquot
from the reaction mixture. The reaction mixture was quenched
with chloroform and removed from the hydrogen atmosphere
after 48 h. Subsequently, DDQ (12 mg, 0.051 mmol) was added and
the solution was stirred for another 30 min. The reaction mixture
was passed through Celite and washed with chloroform. The com-
bined organic layer was evaporated to dryness under reduced
pressure and purified by column chromatography on silica gel by
using chloroform as the eluent to afford desired product 6 as
This work is supported by the Department of Science and
Technology (DST), India (SR/S1/IC-56/2012 to P.K.P). A.R. thanks
the Council of Scientific and Industrial Research (CSIR), India,
for a senior research fellowship and the Defence Research and
Development Organisation (DRDO), India, for financial assis-
tance. This work at Yonsei University was financially supported
by the Mid-career Researcher Program (2005-0093839) adminis-
tered through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and Tech-
nology (MEST).
a
blue crystalline solid (4 mg, 39%). M.p. 260.98C; 1H NMR
(400 MHz, CDCl3): d=11.98 (d, 2H, J=14.8 Hz), 9.90 (d, 2H, J=
11.2 Hz), 9.84 (d, 2H, J=11.2 Hz), 5.05 (s, 6H), 4.93 (s, 6H), 4.87 (s,
6H), 4.82 (s, 6H), 0.83 (s, 2H), À7.97 ppm (d, 2H, J=14.8 Hz);
13C NMR (100 MHz, CDCl3): d=145.9, 145.0, 143.1, 142.9, 139.4,
138.8, 132.0, 131.7, 114.5, 112.8, 109.3, 106.6, 63.61, 63.55, 63.5,
62.1 ppm; UV/Vis (CHCl3): lmax (loge): 442 (5.26), 467 (4.96), 665
(4.50), 708 (4.10), 774 nm (4.57); fluorescence (CHCl3): 779 nm
(lexe =442 nm); fluorescence quantum yield (CHCl3): 0.0095; HRMS
(ESI+): m/z calcd for C32H35N4O8 [M+H+]: 603.2449; found:
603.2454.
Keywords: absorption
porphycenes · porphyrinoids
· charge transfer · conjugation ·
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on silica gel by using chloroform/hexane (1:1) as the eluent to
afford desired product 3 as a blue crystalline solid (17 mg, 84%).
1H NMR (400 MHz, CDCl3): d=11.77 (d, 2H, J=15.2 Hz), 9.93 (d, 2H,
J=11.2 Hz), 9.88 (d, 2H, J=11.2 Hz), 4.68 (q, 4H, J=7.6 Hz), 4.24 (q,
4H, J=7.6 Hz), 4.14 (q, 4H, J=7.6 Hz), 4.09 (q, 4H, J=7.6 Hz), 2.27
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1.91 (t, 6H, J=7.6 Hz), 1.33 (s, 2H), À7.47 ppm (d, 2H, J=15.2 Hz);
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Synthesis of 6 by McMurry coupling of 3,4-dimethoxypyr-
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A slurry of activated zinc (3.59 g) and copper(I)chloride (544 mg,
5.5 mmol) was taken in THF (100 mL) under nitrogen and TiCl4
(1.23 mL, 11.2 mmol) was added slowly. The reaction mixture was
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pyrrole-2,5-dialdhyde (200 mg, 1.09 mmol) in THF (50 mL) was
added slowly over 2 h with vigorous stirring under reflux condi-
tions. The reaction mixture was heated under reflux for an addi-
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solution of sodium carbonate (ca. 100 mL) to the ice-cooled reac-
tion mixture. The reaction mixture was allowed to stir at room
temperature in air for an additional 2 h. The reaction mixture was
filtered through Celite and washed with chloroform. The organic
layer was separated and the aqueous layer was extracted twice
with chloroform. The combined organic layer was passed through
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Received: April 2, 2015
Published online on July 6, 2015
Chem. Eur. J. 2015, 21, 12129 – 12135
www.chemeurj.org
12135
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim