Doubly and triply linked porphyrin-perylene monoimides as near IR dyes with large dipole moments and high photostability

Article abstract of DOI:10.1021/jo1019046

Doubly and triply linked porphyrin-perylene monoimides 3 and 4, with extraordinary stability, large dipole moments, and strong near IR absorption, were prepared by means of one-pot oxidative cyclodehydrogenation promoted by FeCl₃.

Full text of DOI:10.1021/jo1019046

pubs.acs.org/joc
Doubly and Triply Linked Porphyrin-Perylene
Monoimides as Near IR Dyes with Large Dipole
Moments and High Photostability
Chongjun Jiao,† Kuo-Wei Huang,‡ Chunyan Chi,† 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 September 27, 2010
FIGURE 1. Structures of perylene-fused porphyrins 1-4.
cores5 has received much attention because these hybrid
molecules with π-extended cores and low symmetries inevi-
tably exhibit unusual optical and electronic properties includ-
ing NIR absorption, making them attractive for numerous
applications, such as NIR photodetectors,6 photovoltaics,5h
semiconductors,7 and two-photon absorption materials.5e
A general strategy used to obtain aromatic ring-fused
porphyrins is the intramolecular ring-closure reaction of
the singly linked porphyrin-aromatic ring dyads, which
normally can be prepared by Pd-promoted cross-coupling
reactions between appropriate derivatives of aromatic com-
pounds and porphyrin building blocks5f,g,i or by direct cross-
condensation of dipyrromethanes and the aldehydes.5e
Our group recently reported the syntheses of N-annulated
perylene-fused porphyrins (1 and 2 in Figure 1) with enhanced
NIR absorption and emission.8 Although 1 and 2 are stable
upon exposure to visible light, their solutions gradually
decompose upon UV irradiation. The moderate stability of 1
and 2can be attributed to the high electron density in these highly
conjugated systems, making them slowly oxidize by singlet
oxygen in the air upon irradiation with UV light. In our
previous work,9 we have found that the electron-withdrawing
Doubly and triply linked porphyrin-perylene mono-
imides 3 and 4, with extraordinary stability, large dipole
moments, and strong near IR absorption, were prepared
by means of one-pot oxidative cyclodehydrogenation
promoted by FeCl3.
Near Infrared (NIR) dyes1 are defined as compounds that
function (absorption and/or emission) in the spectral region
ranging from 700 to 2000 nm. Recently, studies on NIR dyes
have sparked considerable interest owing to their diverse appli-
cations, such as organic photovoltaics,2 nonlinear optics,3
and bioimaging.4 How to obtain stable NIR dyes, however,
remains a challenge. The fusion of aromatic rings to porphyrin
(5) (a) Lash, T. D.; Werner, T. M.; Thompson, M. L.; Manley, J. M.
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Noh, S. B.; Kim, D.; Osuka, A. Angew. Chem., Int. Ed. 2006, 45, 3944–3947.
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Chem. Commun. 2007, 2069–2071. (g) Davis, N. K. S.; Pawlicki, M.;
Anderson, H. L. Org. Lett. 2008, 10, 3945–3947. (h) Tokuji, S.; Takahashi,
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DOI: 10.1021/jo1019046
r
Published on Web 12/30/2010
J. Org. Chem. 2011, 76, 661–664 661
2010 American Chemical Society

Jiao et al.
JOCNote
SCHEME 1. Synthetic Route toward Compounds 3 and 4
FIGURE 2. UV-vis-NIR absorption spectra of 3 and 4 in toluene
(1.0 ꢀ 10-5 M).
because of the presence of an electron-deficient carboximide
group; thus other conditions for the cyclodehydrogenation
had to be used. Iron(III) chloride is known as a mild oxidant
for the cyclodehydrogenation of many branched oligophe-
nylenes into polycyclic aromatic hydrocarbons.12 Recently,
this reagent has been successfully applied for the preparation
of naphthyl-fused porphyrin carboxylic acid.5f Since the Zn-
porphyrin usually undergoes demetalation reaction upon
treatment with a strong Lewis acid including FeCl3, herein
Ni-containing porphyrin precursor 7 was used. Cyclization
of 7 with 10-fold excess FeCl3 at reflux conditions in anhy-
drous dichloromethane (DCM) generated doubly linked
porphyrin-perylene monoimide 3 in 38% yield, which had
a purple color. To our surprise, an unanticipated green com-
pound was also isolated in 25% yield, which has been proven
to be the triply linked porphyrin-perylene monoimide 4, as
confirmed by MALDI-TOF mass spectrometry and 1D and
2D 1H NMR characterization (see Supporting Information).
To the best of our knowledge, this is the first example to
demonstrate that not only the peri-positions but also the meta-
position of perylene monoimide can be involved in the
cyclodehydrogenation to form a highly π-conjugated system.
Such an unusual multiple C-C bond formation could account
for the high reactivity of the Ni-porphyrin at the β-positions13
and the refluxing reaction conditions. Moreover, no cyclized
product was obtained when the reaction was carried out at
room temperature.
Compounds 3 and 4 have good solubility in common
organic solvents. As shown in Figure 2, both 3 and 4 show
broad absorption spectra that cover the entire visible and
a part of the NIR spectral regions. The absorption maximum
of 3 was found at 803 nm, while a further red-shift of 94 nm
was observed for 4 compared with 3, owing to the higher
conjugation in 4. Noteworthy is that perylene monoimide-
fused porphyrins 3 and 4 exhibit remarkably intense NIR
absorption, with molar extinction coefficients ε = 91 000
and 59 000 M-1 cm-1 at long-wavelength maximum, respec-
tively. Compound 3, in particular, displays the strongest Q
bands among aromatic ring-fused monoporphyrin hybrid
molecules.5 Unlike N-annulated perylene-fused porphyrins
1 and 2, compounds 3 and 4, possessing electron-deficient
perylene monoimide as well as electron-donating porphyrin,
(a) Pd(PPh3)4, Cs2CO3, toluene/DMF; (b) Ni(acac)2, toluene; 59%
for two steps; (c) FeCl3, nitromethane, DCM, 38% for 3 and 25% for 4.
dicarboxylic imide groups can significantly lower the high-
lying HOMO energy level of the respective N-annulated
perylene and quarterrylene, which are unstable upon long-term
exposure to air and light. Perylene monoimide, possessing a
strong electron-withdrawing imide group, was thus consid-
ered to be a rational building block to replace the relatively
electron-rich N-annulated perylene unit in the perylene-fused
porphyrin compounds. Herein, we present an unprecedented
one-pot synthesis of doubly and triply linked porphyrin-
perylene monoimides 3 and 4 (Figure 1), which are expected to
possess improved stability compared to the electron-rich dyes 1
and 2. In the meanwhile, a “push-pull” structure is constructed
in 3 and 4 that can result in further red-shift of the absorption
spectra. More importantly, such a push-pull structure can
facilitate a fast electron injection from the excited dye to the
conduction band of TiO2 in dye-sensitized solar cells,2 thus
qualifying 3 and 4 as promising light-harvesting NIR dyes
after replacement of the 2,6-diisopropylphenyl groups with
an anchoring group.10 The connection between the perylene
and porphyrin unit can also be modified by chemistry, with
doubly peri-meso; peri-β linkage in 3 and triply peri-β; peri-
meso; meta-β0 linkage in 4, and this difference should also
result in tunable optical and electronic properties. Given the
electron-withdrawing character of carboximide and the dif-
ficulty in fusing the inactive aromatic compound to the
porphyrin core, a new cyclization method will have to be
developed.
As shown in Scheme 1, porphyrin monobromide 58 was
chosen as the key intermediate with the aim of enhancing the
solubility of the target molecules. Precursor 7 was synthesized
by Suzuki coupling of 5 with 611 followed by metalation. Ring
closure of 7 using Sc(OTf)3/DDQ herein could not afford the
cyclized compound of the perylene monoimide fused porphyrin
€
(10) (a) Edvinsson, T.; Li, C.; Pschirer, N.; Schoneboom, J.; Eickemeyer,
F.; Sens, R.; Boschloo, G.; Herrmann, A.; Mullen, K.; Hagfeldt, A. J. Phys.
€
Chem. C 2007, 111, 15137–15140. (b) Li, C.; Yum, J.-H.; Moon, S.-J.;
€
Herrmann, A.; Eickemeyer, F.; Pschirer, N. G.; Erk, P.; Schoneboom, J.;
Mullen, K.; Gratzel, M.; Nazeeruddin, M. K. ChemSusChem. 2008, 1, 615–
€
€
€
618. (c) Li, C.; Liu, Z.; Schoneboom, J.; Eickemeyer, F.; Pschirer, N. G.; Erk,
P.; Herrmann, A.; Mullen, K. J. Mater. Chem. 2009, 19, 5405–5415.
€
(11) Weil, T.; Reuther, E.; Beer, C.; Mullen, K. Chem.;Eur. J. 2004, 10,
€
€
(12) Wu, J.; Pisula, W.; Mullen, K. Chem. Rev. 2007, 107, 718–747.
(13) Tsuda, A.; Furuta, H.; Osuka, A. J. Am. Chem. Soc. 2001, 123,
1398–1414.
10304–10321.
662 J. Org. Chem. Vol. 76, No. 2, 2011

Jiao et al.
JOCNote
FIGURE 4. Cyclic voltammograms of compounds 3 and 4 in DCM
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 of 20 mV/s. Fcþ/Fc was used as external reference.
FIGURE 3. Optimized molecular structures, dipole moments (indi-
cated by arrows), and frontier molecular orbital profiles of mole-
cules 3 and 4 (the branched aliphatic chains are replaced by ethyl
during the TD-DFT calculations).
revealing the extremely strong electron-accepting ability of 3
and 4 because of the imide group. Chemical oxidation titra-
tions of compounds 3 and 4 were conducted in DCM by
using SbCl5 as an oxidant, and the process was followed by
UV-vis-NIR absorption spectroscopy (Figure S9 in Sup-
porting Information). Both compounds can be oxidized by
SbCl5 into stable radical cations with the appearance of new
characteristic absorption bands in the shorter and longer
wavelengths around the original NIR absorption band. The
oxidized species can also be reversibly reduced into the neu-
tral state upon adding Zn dust to the solution containing the
oxidized species (Figure S10 in the Supporting Information).
It is worth noting that upon exposure to sunlight, air-
saturated solutions of 3 and 4 show no significant changes in
their absorption spectra for months. Even under irradiation
of a 4 W UV lamp (emitting at 254 nm) for 48 h, 95% of their
initial optical density remains unchanged. The extraordinary
stabilities of 3 and 4, to our knowledge, are comparable to
the most stable arylene compounds with NIR absorptions.15
In contrast, the half-lives of 1 and 2 were estimated as 244
and 547 min, respectively, under the same UV irradiation
conditions (Figure S11 in the Supporting Information) due
to their relatively high-lying HOMO levels.
In summary, a facile one-pot synthesis of doubly and triply
linked porphyrin-perylene monoimides 3 and 4 was estab-
lished. It is worth noting that both meta- and peri-positions
of perylene can be simultaneously involved in ring-closure
reaction during the synthesis of the triply linked compound
4. The introduction of an electron-withdrawing imide group
effectively stabilizes the hybrid molecules. In addition, dyes 3
and 4, with an intramolecular “push-pull” structure, display
intensified NIR absorptions, large dipole moments, and
high photostabilities, qualifying them as good NIR dyes
for dye-sensitized solar cells. This requires further function-
alization of the imide unit with anchoring groups, and the
relevant work is underway in our laboratories.
exhibit only very weak fluorescence,14 presumably due to the
intramolecular charge transfer as well as the presence of Ni2þ
ion, both of which quench fluorescence.
Time-dependent density function theory (TDDFT at
B3LYP/6-31G**) calculations were conducted for 3 and 4
to further understand their geometric and electronic struc-
tures. Their optimized molecular structures, dipole moments,
and the frontier molecular orbital profiles are shown in
Figure 3. The perylene moiety in 3 slightly deviates from
the porphyrin plane due to the steric hindrance between the
β-proton of porphyrin and the meta-proton of the perylene
core, while the entire molecule 4 turns out to be bowl shaped
due to the fusion of the five-membered ring. The HOMO is
predominantly localized on the porphyrin core, while the
LUMO is mainly localized on the perylene monoimide unit
for both 3 and 4, and this asymmetry obviously gives rise to
different dipole moments, which are calculated as 9.88 and
11.54 D, respectively. Such orbital partitioning and large
dipole moments are beneficial to sensitizers for dye-sensitized
solar cells.2a TDDFT calculations also predict that each
compound exhibits three major absorption bands, and the
longest absorption maxima for 3 and 4 are located at 756.9
and 976.7 nm. These data agree well with our experimental
results (Figure 2).
Cyclic voltammetry was employed to investigate the redox
behavior of 3 and 4 (Figure 4 and Table S1). Compound 3 in
dry DCM exhibits two reversible oxidation waves with half-
wave potentials (Eoxn) at 0.37 and 0.77 V (vs Fcþ/Fc), while
three reversible oxidative waves were observed with Eoxn at
0.20, 0.43, and 0.77 V for 4. It is obvious that the triply linked
compound 4 has a larger extended pi-system, which can sta-
bilize the multiple charges. Furthermore, both compounds 3
and 4 show only one reversible reduction wave, with Ered at
-1.15 and -1.15 V, respectively. The first reduction poten-
tials of 3 and 4 are much less negative than those of 1 and 2,8
Experimental Section
General Procedures. All reagents were purchased from com-
mercial suppliers and used as received without further purifica-
tion. Anhydrous DCM and N,N-dimethylformaldehyde (DMF)
were distilled from CaH2. Toluene and THF were distilled from
(14) Compound 3 emits very weak fluorescence in solution with a max-
imum at 828 nm, and the fluorescence quantum yield was less than 0.3%
(Figure S2 in Supporting Information). For compound 4, it is hard to
determine any fluorescence under the same conditions. The low fluorescence
quantum yields can account for the intramolecular charge transfer and the
presence of Ni in the porphyrin units. The Ni-porphyrins usually have a low
fluorescent quantum yield. For comparison, we also attempted to replace the
Ni metal center with Zn. However, treatment of the dyes 3 and 4 with strong
acid, such as CF3COOH or H2SO4, led to significant decomposition perhaps
due to the relatively low HOMO energy levels of these compounds.
(15) (a) Geerts, Y.; Quante, H.; Platz, H.; Mahrt, R.; Hopmeier, M.;
€
Bohm, A.; Mullen, K. J. Mater. Chem. 1998, 8, 2357–2369. (b) Pschirer,
N. G.; Kohl, C.; Nolde, F.; Qu, J.; Mullen, K. Angew. Chem., Int. Ed. 2006,
€
€
45, 1401–1404.
J. Org. Chem. Vol. 76, No. 2, 2011 663

Jiao et al.
JOCNote
sodium benzophenone immediately prior to use. The 1H NMR
and 13C NMR spectra were recorded in CDCl3 or C6D6 solution
on NMR spectrometers with tetramethylsilane (TMS) as the
internal standard. The chemical shift was recorded in ppm, and
the following abbreviations were used to explain the multi-
plicities: s = singlet, d = doublet, t = triplet, m = multiplet,
br=broad. Mass spectra were recorded with an EI ionization
source. MALDI-TOF mass spectra were measured by using
1,8,9-trihydroxyanthracene as a matrix. Elemental analyses
were performed only for C, H, and N elements. UV-vis absorp-
tion and fluorescence spectra were recorded in HPLC pure
solvents. IR spectra were recorded by blending 1 wt % sample
together with anhydrous KBr. The electrochemical measure-
ments were carried out in anhydrous DCM with 0.1 M tetra-
butylammonium hexafluorophosphate (Bu4NPF6) as the sup-
porting electrolyte at a scan rate of 0.02 V/s at room temperature
under the protection of nitrogen. A gold disk was used as work-
ing electrode, platinum wire was used as counting electrode, and
Ag/AgCl (3 M KCl solution) was used as reference electrode.
The fluorescence quantum yields were measured by optical
10 equiv) in nitromethane (0.5 mL). The reaction mixture was
refluxed for 12 h and quenched by addition of a saturated
NaHCO3 solution. The organic layer was washed with saturated
brine and dried over anhydrous Na2SO4. The solvent was re-
moved under vacuum, and the residue was purified by prepara-
tive TLC (DCM/hexane, 3:2) to give a purple solid product
(15 mg, 38%). 1H NMR (C6D6, 500 MHz) δ: 9.29 (s, 1H), 9.20
(d, J = 4.2 Hz, 1H), 9.05 (d, J = 5.1 Hz, 1H), 9.00 (d, J = 4.8 Hz,
1H), 8.95 (d, J = 5.1 Hz, 1H), 8.87-8.89 (m, 4H), 8.24 (br, 1H),
8.11 (br, 3H), 8.00-8.02 (m, 2H), 7.90 (m, 1H), 7.74 (br, 1H),
7.61-7.63 (m, 1H), 7.49-7.52 (m, 1H), 7.41-7.44 (m, 2H),
4.43-4.48 (m, 1H), 4.29 (m, 1H), 4.09-4.14 (m, 1H), 3.95
(m, 1H), 3.21-3.25 (m, 2H), 2.01-2.07 (m, 2H), 0.63-1.39
(m, 62H). 13C NMR (CDCl3, 125 MHz), δ: 164.04, 164.01,
149.5, 146.0, 144.1, 143.9, 142.8, 142.5, 141.6, 141.3, 140.3,
139.4, 138.5, 137.1, 136.9, 136.2, 133.9, 133.0, 132.6, 131.9,
131.8, 131.5, 130.7, 130.3, 129.5, 129.4, 128.4, 128.1, 126.3,
125.2, 124.4, 124.0, 123.9, 123.4, 121.5, 120.5, 120.0, 118.8,
40.8, 40.6, 39.2, 39.0, 38.8, 38.0, 37.8, 35.1, 31.7, 29.7, 29.3,
27.97, 27.95, 25.0, 24.1, 22.6, 22.4, 20.1. Anal. Calcd for
C86H91N5NiO2: C, 80.36; H, 7.14; N, 5.45; Ni, 4.57; O, 2.49.
Found: C, 80.59; H, 7.27; N, 5.69. (MALDI-TOF): m/z =
1284.231 (M þ H)þ; calcd for C86H91N5NiO2, 1283.653. IR
(KBr): ν = 2954, 2923, 2855, 1702, 1662, 1574, 1459, 1243, 1078,
dilute method16 (A < 0.05) using Cardio-Green (λabs,max
780 nm, Φ = 0.13 in DMSO) as reference.
=
Compound 7. Porphyrin 5 (91 mg, 1.1 equiv), 6 (61 mg, 1 equiv),
Pd(PPh3)4 (6 mg, 0.05 equiv), and Cs2CO3 (65 mg, 2 equiv) were
dried under vacuum and then purged with argon. To this were
added degassed toluene (10 mL) and DMF (4 mL), and the
mixture was stirred for 36 h at 96 °C. After cooling, water was added
and the product was extracted with ethyl acetate (3 ꢀ 10 mL). The
organic layer was washed with saturated brine and dried over
anhydrous Na2SO4. The solvent was removed under vacuum, and
the residue was purified by column chromatography (silica gel,
DCM/hexane, 1:1) to give a red, waxy solid product. Immediately,
thissolid was dissolvedin toluene (10 mL), andNi(acac)2 (100 mg)
was added slowly. The mixture was stirred for 40 h at 100 °C.
After removal of the solvent, the crude product was purified by
column chromatography (silica gel, hexane/DCM, 1:1) to give a
red solid product, 7 (80 mg, 59% in two steps). 1H NMR (CDCl3,
500 MHz) δ: 9.32 (d, J = 5.1 Hz, 2H), 9.26 (d, J = 4.4 Hz, 2H),
8.88 (d, J = 5.1 Hz, 2H), 8.83-8.86 (m, 2H), 8.76 (d, J = 7.6 Hz,
2H), 8.61-8.63 (m, 2H), 8.57 (d, J = 8.2 Hz, 1H), 8.46-8.47
(m, 2H), 7.78-7.91 (m, 3H), 7.53-7.56 (m, 1H), 7.40-7.43
(m, 2H), 7,17-7.22 (m, 1H), 7,07 (m, 1H), 4.76-4.82 (m, 2H),
4.39-4.46 (m, 2H), 2.86-2.91 (m, 2H), 2.15 (m, 2H), 0.71-1.60
(m, 62H). 13C NMR (CDCl3, 125 MHz) δ: 164.09, 164.07, 149.1,
145.8, 142.9, 142.6, 142.5, 141.6, 141.5, 141.45, 141.4, 139.7, 137.9,
137.7, 133.1, 132.3, 132.2, 132.0, 131.1, 130.9, 130.8, 130.7, 130.3,
130.2, 129.5, 129.4, 129.1, 128.8, 128.7, 127.6, 127.2, 127.0, 124.5,
124.0, 123.97, 123.7, 122.2, 121.3, 121.2, 120.6, 120.5, 119.1, 117.2,
114.1, 40.9, 39.4, 39.2, 39.1, 37.9, 37.8, 35.0, 33.8, 31.9, 31.7, 31.4,
30.2, 29.7, 27.9, 25.0, 24.1, 22.7, 22.6, 22.5, 20.0, 14.1. Anal. Calcd
for C86H93N5NiO2: C, 80.23; H, 7.28; N, 5.44; Ni, 4.56; O, 2.49.
Found: C, 80.04; H, 7.17; N, 5.51. (MALDI-TOF): m/z =
1286.567 (M þ H)þ; calcd for C86H93N5NiO2, 1285.668.
1015, 791 cm-1
.
Compound 4. Apart from the formation of 3 using the con-
ditions mentioned above, 4 can be separated in 25% yield as a
green solid. 1H NMR (C6D6, 500 MHz) δ: 8.91 (d, J = 7.8 Hz,
1H), 8.87 (d, J = 6.2 Hz, 1H), 8.52 (br, 1H), 8.36-8.41 (br, 4H),
8.24 (br, 1H), 7.98 (br, 3H), 7.87 (br, 1H), 7.73 (br, 1H), 7.56
(br, 1H), 7.36-7.48 (m, 4H), 3.92 (br, 2H), 3.54 (br, 2H), 3.38
(br, 2H), 2.17-2.21 (m, 2H), 0.57-1.78 (m, 62H). 13C NMR
(C6D6, 70 °C, 125 MHz), δ: 164.4, 154.3, 153.3, 151.3, 147.6,
147.5, 147.1, 146.5, 145.4, 144.3, 143.3, 140.6, 136.8, 133.4,
133.3, 132.5, 127.5, 126.0, 124.4, 123.8, 123.4, 123.0, 121.7,
121.2, 118.6, 39.55, 39.49, 39.3, 35.8, 35.4, 31.5, 30.2, 30.0,
29.6, 28.4, 28.2, 25.6, 25.4, 24.6, 24.5, 23.0, 22.8, 22.7, 20.5,
20.0. Anal. Calcd for C86H89N5NiO2: C, 80.49; H, 6.99; N, 5.46;
Ni, 4.57; O, 2.49. Found: C, 80.72; H, 7.15; N, 5.23. (MALDI-
TOF): m/z = 1282.621 (M þ H)þ; calcd for C86H89N5NiO2,
1281.637. IR (KBr): ν = 2958, 2925, 2854, 1697, 1627, 1461,
1357, 1261, 1094, 803 cm-1
.
Acknowledgment. J.W. acknowledges the financial support
from Singapore DSTA DIRP Project (DSTA-NUS-DIRP/
2008/03), NRF Competitive Research Program (R-143-000-
360-281), and NUS Young Investigator Award (R-143-000-
356-101). K.-W.H. acknowledges the financial support from
KAUST.
Supporting Information Available: Characterization data of
all new compounds, absorption and emission spectra and data,
electrochemcial data, TDDFT calculation details, chemcial oxi-
dation titration experiments, and photostability test. This ma-
terial is available free of charge via the Internet at http://pubs.
acs.org.
Compound 3. To a solution of 7 (40 mg, 1 equiv) in degassed
anhydrous DCM (20 mL) was added a solution of FeCl3 (50 mg,
(16) Licha, K.; Riefke, B.; Ntziachristos, V.; Becker, A.; Chance, B.;
Semmler, W. Photochem. Photobiol. 2000, 72, 392–398.
664 J. Org. Chem. Vol. 76, No. 2, 2011