N-annulated perylene fused porphyrins with enhanced near-IR absorption and emission

Article abstract of DOI:10.1021/ol1016383

N-Annulated perylene fused porphyrins 1 and 2 were synthesized by oxidative dehydrogenation using a Sc(OTf)₃/DDQ system. These newly synthesized hybrid molecules are highly soluble in organic solvents and exhibit remarkably intense near-IR absorption, as well as detectable photoluminescence quantum yields, all of which are comparable to or even exceed those of either meso-β doubly linked porphyrin dimer/trimer or bis/tri-N-annulated rylenes.

Full text of DOI:10.1021/ol1016383

ORGANIC
LETTERS
2010
Vol. 12, No. 18
4046-4049
N-Annulated Perylene Fused Porphyrins
with Enhanced Near-IR Absorption and
Emission
Chongjun Jiao,† Kuo-Wei Huang,‡ Zhenping Guan,† Qing-Hua Xu,† and
Jishan Wu*,†
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3,
117543 Singapore, and KAUST Catalysis Center and DiVision of Chemical and Life
Sciences and Engineering, 4700 King Abdullah UniVersity of Science and Technology,
Thuwal 23955-6900, Kingdom of Saudi Arabia
chmwuj@nus.edu.sg
Received July 15, 2010
ABSTRACT
N-Annulated perylene fused porphyrins 1 and 2 were synthesized by oxidative dehydrogenation using a Sc(OTf)3/DDQ system. These newly
synthesized hybrid molecules are highly soluble in organic solvents and exhibit remarkably intense near-IR absorption, as well as detectable
photoluminescence quantum yields, all of which are comparable to or even exceed those of either meso-ꢀ doubly linked porphyrin dimer/
trimer or bis/tri-N-annulated rylenes.
Large π-systems with near-infrared (NIR) absorption/emis-
sion1 have attracted considerable attention in light of their
increasing applications in the fields of solar cells,2 nonlinear
optics,3 and bioimaging.4 Stimulated by the promising
potential, scientists have been devoted to developing facile
methods for the preparation of polycyclic aromatics that do
function in the NIR spectral region. Porphyrins and metal-
loporphyrins generally possess an intense absorption in the
400-450 nm region called the Soret band and relative weak
absorptions in comparison to the Soret band in the 500-700
nm region named Q bands. In order to achieve NIR
absorption and simultaneously enhance the Q-band absorp-
tion, numerous efforts have been made toward the design
and synthesis of π-extended porphyrins with low symmetry,
which include fused porphyrin arrays5 and aromatic ring-
fused porphyrins.6 On the other hand, perylene and its
derivatives,7 key chromophores in dye chemistry, have been
intensively studied not only owing to their remarkable
physical properties (large extinction coefficients and high
fluorescence quantum yields), outstanding chemical, thermal,
and photochemical inertness, nontoxicity, and low cost but
† National University of Singapore.
‡ King Abdullah University of Science and Technology.
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10.1021/ol1016383  2010 American Chemical Society
Published on Web 08/24/2010

also due to the availability of their active sites (peri- or bay-
positions) that allow further chemical modifications. A recent
rising interest is to synthesize rylene bisimides, namely,
terrylene,8 quaterrylene,9 pentarylene,10 and hexarylene bi-
simides,10 or to construct push-pull structures11 based on
the perylene core, both of which can shift the absorption
into the NIR spectral region.
ring-closure reaction. Herein, we report the first class of
perylene-fused porphyrins 1 and 2 (Scheme 1) with varied
optical and electronic properties.
Scheme 1
Since both the perylene unit and the porphyrin core are
attractive and intriguing, we envisaged that incorporation of
perylene to the porphyrin skeleton could lead to a NIR
absorption with unusual properties including high molar
extinction coefficient and desirable quantum yield as a result
of the outstanding physical properties of perylene itself and
loss of symmetry of the porphyrin core. Although the
perylene unit had been covalently attached to the porphyrin
core through a single C-C bond,12 the fusion of perylene
moiety to the porphyrin core, to the best of our knowledge,
has never been reported, presumably because of the lack of
appropriate active building blocks and the difficulty in the
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The major challenge for the synthesis of perylene-fused
porphyrins is the intramolecular ring cyclization of the singly
linked porphyrin-perylene dyads, which usually can be
prepared by Pd-catalyzed coupling reactions between ap-
propriate perylene and porphyrin building blocks. It has been
demonstrated that the ring fusion of electron-rich metal-
loporphyrins (e.g., Zn porphyrin) requires the second com-
ponent to be also electron-rich, and Sc(OTf)3-DDQ system
is usually used as the oxidant.5,6 We therefore chose electron-
rich N-annulated perylene as one of the building blocks in
compounds 1 and 2 because it has been proven that
N-annulated perylenes can undergo self-fusion reactions to
give higher order quaterrylene and hexarylene derivatives
upon treatment of Sc(OTf)3/DDQ.13
Bulky 4-tert-butylphenyl, 3,5-di-tert-butylphenyl, 3,5-di-
tert-butylbenzyl, 2,6-diisopropylphenyl, and branched ali-
phatic chains were introduced to these molecules to surmount
the solubility problem and to suppress the aggregation of
the chromophores in solution. Scheme 1 outlines the synthetic
route for compounds 1 and 2. The monobrominated por-
phyrin 4 was first synthesized as the key porphyrin building
block (see Supporting Information for details). Suzuki
coupling between 4 and N-annulated perylene boronic ester
3 (see Supporting Information) and subsequent Zn-metalation
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Org. Lett., Vol. 12, No. 18, 2010
4047

of the as-formed porphyrin-perylene dyad gave the key
intermediate 5. The combination of Sc(OTf)3 and DDQ was
then applied to promote the ring closure of 5 and eventually
led to the desired compound 1. Extension of π-conjugation
length of 1 along the long molecular axis by fusion two
porphyrin units with one N-annulated perylene is supposed
to lead to a more red-shifted and intense NIR absorption.
Thus a precursor such as 8 was prepared by Suzuki coupling
reaction between porphyrin boronic ester 6 and dibrominated
N-annulated perylene 7 (see Supporting Information), and
this was followed by oxidative dehydrogenation by Sc(OTf)3/
DDQ. However, when the branched alkyl chains (i.e., R1
and R2) were used as the substituents, a complicated mixture
was obtained after cyclodehydrogenation and the separation
of this mixture and characterization of final product turned
out to be very difficult because of the strong aggregation
tendency of the obtained perylene-fused porphyrins. There-
fore, bulky 3,5-di-tert-butylphenyl (R3) and 3,5-di-tert-
butylbenzyl (R4) groups were used, and pure hybrid molecule
2 was successfully obtained in 74% yield. The final
compound 2 did not show obvious aggregation in solution
and allowed us to separate and characterize it more conve-
niently.
coordination of pyridine to Zn(II), which is effective to
suppress the aggregation of perylene-fused porphyrins.
Perylene-fused porphyrins 1 and 2 exhibit remarkably intense
NIR absorptions in toluene with molar extinction coefficients
ε ) 39,980 and 119,380 M-1 cm-1 at long wavelength
maximum, respectively. Such intense NIR absorptions, espe-
cially the molar extinction coefficient of 2, have seldom been
reported among aromatic compound-fused porphyrin ana-
logues.6 Also noteworthy is that compound 1 shows NIR
photoluminescence with emission maximum at 800 nm (Figure
S1 in Supporting Information), and the photoluminescence
quantum yield was measured as 5.6%. Considering the relatively
low photoluminescence quantum yield of porphyrin tapes5g,14
in NIR region, this value is acceptable. In addition, the
absorption maximum of 2 is comparable to that of meso-ꢀ
doubly linked Zn(II) porphyrin trimers5g but much longer
13d
than that of tri-N-annulated hexarylene.
Different from
porphyrin trimers,5g the bis-porphyrin fused N-annulated
perylene 2 shows detectable fluorescence with emission
maximum at 982 nm in toluene containing 1% pyridine, and
a photoluminescent quantum yield of 0.8% was determined
(Figure S2 in Supporting Information). Such an enhancement
in the NIR absorption/emission of 1 and 2 is remarkable,
and the reason is still under investigation in our laboratory.
Because of their low emission quantum yields and expected
short excited state lifetimes, their excited state lifetimes were
measured by using femtosecond pump probe experiments
(Figure S3 in Supporting Information). Their average excited
state lifetimes were determined to be 245 and 221 ps for 1
and 2, respectively, which is consistent with their emission
quantum yield data.
Compounds 1 and 2 appear as carmine and brown waxy
solids, respectively, and are highly soluble in common
organic solvents. The absorption spectra of the N-annulated
perylene-fused porphyrin 1 and bis-porphyrin fused N-
annulated perylene 2 in toluene demonstrate a significant
bathochromic shift of the absorption maximum with respect
to those of their precursors 5 and 8 (Figure 1 and the data
Time-dependent density function theory (TDDFT at
B3LYP/6-31G**) calculations were conducted for 1 and 2
to further understand their geometric and electronic struc-
tures. Their optimized molecular structures, dipole moments,
and frontier molecular orbital profiles are shown in Figure
2. The perylene moieties in 1 and 2 slightly deviate from
Figure 1. UV-vis-NIR absorption spectra of 1, 2, 5, and 8 in
toluene (1.0 × 10-5 M). The molar extinction coefficients of 5 and
8 were divided by 2 because of their strong absorption around 450
nm.
are collected in Table S1 in Supporting Information). The
absorption maximum of 1 was found at 775 nm, and
extension of π-conjugation length along the long molecular
axis in 2 apparently leads to a bathochromic shift with
absorption maximum at 952 nm. Addition of a small amount
of pyridine to the toluene solution of 2 further promotes the
absorption maximum to 981 nm, probably due to the
Figure 2. Optimized molecular structures, dipole moments (indi-
cated by arrow), and frontier molecular orbital profiles of molecules
1 and 2 (only branched aliphatic chains are replaced by ethyl during
the TDDFT calculations).
4048
Org. Lett., Vol. 12, No. 18, 2010

the porphyrin plane due to the steric hindrance between the
ꢀ-proton of porphyrin and the meta-proton of the perylene
core. The asymmetry in 1 and 2 obviously gives rise to
different dipole moments, which are calculated as 3.1439
and 1.8457 D, respectively. TDDFT calculations predict that
each compound exhibits three major absorption bands (see
Supporting Information), which were also observed in Figure
1. The longest absorption maxima for 1 and 2 are calculated
to be 706.6 and 895.4 nm, respectively, and such tendency
also agrees well with our experimental results.
reversible reduction wave, with Ered at -1.42 and -1.31 V
for 1 and 2, respectively. Chemical oxidation titrations of
compounds 1 and 2 were conducted in DCM by using SbCl5
as oxidant and the process was followed by UV-vis-NIR
absorption spectroscopy (Figure S11 in Supporting Informa-
tion). Both compounds can be oxidized by SbCl5 into stable
radical cation with appearance of new characteristic absorp-
tion bands in the shorter and longer wavelengths around the
original NIR absorption band. It is worth noting that despite
extremely narrow band gaps and the high electron density,
solutions of 1 and 2 are stable upon exposure to visible light,
whereas the bis-N-annulated quarterrylene can be readily
oxdized by singlet oxygen in the air.13c This observation
clearly demonstrates that the porphyrin unit is able to stabilize
the highly conjugated system with an electron-rich character.
In summary, the electron-rich N-annulated perylene was
successfully fused to the porphyrin core for the first time to
form compounds 1 and 2 with an intense NIR absorption/
emission and different electrochemical behavior. The por-
phyrin unit turns out be effective to stabilize a highly
electron-rich π-system. In particular, dye 1 shows an
acceptable fluorescence quantum yield, which is typically
difficult to achieve for porphyrin-based materials. Further
extension of the conjugation from 1 to 2 results in intense
NIR absorption that is comparable to that of a porphyrin
trimer. However, N-annulated perylene fused porphyrin 2
has the advantages of high solubility, ease of preparation,
and detectable photoluminescence quantum yield. Further
studies on the nonlinear optical properties (e.g., two-photon
absorption) of these dyes for potential optical limiting
applications as well as the synthesis of their higher homo-
logues with more extended conjugation are currently under-
way in our laboratories.
The electrochemical properties of compounds 1 and 2 were
investigated by cyclic voltammetry (CV) in dry DCM (Figure
3 and Table S1 in Supporting Information). The cyclic
Figure 3. Cyclic voltammograms of compounds 1 and 2 in
dichloromethane with 0.1 M Bu4NPF6 as supporting electrolyte,
AgCl/Ag as reference electrode, Au disk as working electrode, Pt
wire as counter electrode, and scan rate at 20 mV/s. Fc/Fc+ was
used as internal reference.
Acknowledgment. This work was financially supported
by Singapore DSTA DIRP Project (DSTA-NUS-DIRP/2008/
03), the NRF Competitive Research Program (R-143-000-
360-281), and NUS Young Investigator Award (R-143-000-
356-101) to J.W. and a KAUST baseline funding to K.-W.H.
voltammogram of 1 exhibits two reversible oxidation waves
with half-wave potentials (Eoxn) at 0.12 and 0.47 V (vs Fc+/
Fc), whereas four reversible oxidative waves with Eox at
-0.08, 0.40, 0.65, 0.81 V were measured for 2. It is obvious
that the perylene-fused porphyrin tape 2 has a larger extended
π-system and can stabilize multiple charges. The lower first
oxidation potential observed for 2 can be explained by the
extended π-conjugation between the three fused electron-
rich units. Furthermore, both compounds show one quasi-
n
Supporting Information Available: Experimental details
and characterization data of all new compounds, theoretical
calculation data, and chemical oxidation titration details. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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