Fused pyrene-diporphyrins: Shifting near-infrared absorption to 1.5 μm and beyond

Article abstract of DOI:10.1002/anie.201002669

(Figure Presented) Sticking together: Direct fusion of pyrene rings with diporphyrins can be achieved without prior activation of aromatic rings. This simple method gives pyrene-diporphyrin hybrids (see picture, C (pyrene) red, C (porphyrin) dark blue, N light blue, Zn green) with a high near-infrared (NIR) absorption that reaches the wavelengths required for use in telecommunications.

Full text of DOI:10.1002/anie.201002669

Angewandte
Chemie
DOI: 10.1002/anie.201002669
Fused-Ring Systems
Fused Pyrene–Diporphyrins: Shifting Near-Infrared Absorption to
1.5 mm and Beyond**
Vyacheslav V. Diev, Kenneth Hanson, Jeramy D. Zimmerman, Stephen R. Forrest, and
Mark E. Thompson*
Porphyrins have been explored for a number of potential
optoelectronic applications that require strong absorption in
the near-infrared (NIR) spectral region; these applications
include organic electronics,[1,2] nonlinear optics,[3] and tele-
communication technologies.[4] Porphyrins have also been
investigated as active materials in photovoltaic cells[1] because
of their high efficiency of charge separation and transport,[5]
strong absorbance in the visible region, high chemical
stability, and the ease with which their optoelectronic proper-
ties can be tuned with chemical modification.[6] The absorp-
tion bands of porphyrins are not readily shifted into the deep-
red and NIR spectral regions, and also tend to be narrow, thus
minimizing their overlap with the solar spectrum. Triply
bridged, (b,meso,b), porphyrin tapes (Figure 1a, n = 0–22)
Figure 1. a) General structure of triply fused porphyrins. b)–d) Struc-
show marked red-shifts in the porphyrin absorption bands,
tures of diporphyrin hybrids calculated at B3LYP/6-31G with calculated
which extend deep into the NIR region.[7,8a] Triply fused
red-shifts of the lowest-energy transitions compared to the parent
diporphyrin.
porphyrins with n = 1,2 give absorbance in the mid-NIR
region (i.e., conventional wavelengths for telecommunica-
tions, ca. 1.5 mm), however, these porphyrins are difficult to
synthesize, have low solubility, and are isolated only in small
quantities.[8] Triply connected porphyrin dimers (Figure 1a,
n = 0) have a strong absorbance at l = 1050 nm, are photo-
and chemically stable, have a high solubility, and can be easily
prepared from monoporphyrins.[7] Development of new
organic dyes based on these accessible porphyrin dimers
with absorption at the wavelengths for telecommunications
(l = 1.5 mm) still remains a challenge.
extended through several modes of substitution involving the
meso, (b,b), (b,meso) and (b,meso,b) positions. For dipor-
phyrins substituted with two alkyne groups at the terminal
meso positions, the Q band is red-shifted by 130 nm (l =
1181 nm) relative to the parent dimer.[9] In contrast, extending
the conjugation in porphyrin dimers by benzannulating b,b-
pyrrolic positions red-shifts the Q band by only 18 nm, and
the resulting compounds have poor solubility.[8b] Recently, it
has been shown that anthracene rings can be fused to
porphyrin dimers through the (b,meso,b) mode, which leads
to a red-shift of the Q band to 1495 nm.[8c] However, the
anthracene-fused diporphyrin exhibits the same undesirable
difficulties found with higher porphyrin tapes, for example,
synthetic difficulty, low yields and low solubility.[8c] Moreover,
fusion of anthracene rings is limited only to alkoxy-substi-
tuted derivatives.
Extending the size of p conjugation in porphyrin systems
results in most cases in a bathochromic (red) shift of the
absorption.[7,8b,c] The conjugation of porphyrin dimers can be
[*] Dr. V. V. Diev, Dr. K. Hanson, Prof. M. E. Thompson
Department of Chemistry, University of Southern California
Los Angeles, CA 99089 (USA)
Fax: (+1) 213-740-8594
E-mail: met@usc.edu
Homepage: http//met.usc.edu
The effects of aromatic ring fusion to porphyrin tapes in a
(meso,b) mode have not been explored. We have analyzed
the structures of the diporphyin core (Figure 1b),
a
Dr. J. D. Zimmerman, Prof. S. R. Forrest
(b,meso,b) triply fused aromatic system (Figure 1c), and a
(b,meso) doubly fused molecule (Figure 1d) using standard
DFT methods. Significant bathochromic shifts of the lowest-
energy transition are expected in all cases. Unlike the case of
anthracene-fused porphyrins and porphyrin tapes, in which
the planarity causes aggregation and low solubility, the
pyrene–(b,meso)-fused diporphyrin displays out-of-plane
distortion that is known to improve solubility and processi-
bility in conjugated aromatics.[10] By taking into account the
predicted bathochromic shift, distortion from planarity, and
ease of synthesis, the (b,meso)-fused pyrene diporphyrin from
Department of Electrical Engineering and Computer Science
and Department of Physics
University of Michigan, Ann Arbor, MI 48109(USA)
[**] We thank the Defense Advanced Research Projects Agency HARDI
Program and Universal Display Corp. for their partial financial
support. The views, opinions, and/or findings contained in this
article/presentation are those of the author/presenter and should
not be interpreted as representing the official views or policies,
either expressed or implied, of the Defense Advanced Research
Projects Agency or the Department of Defense.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002669.
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this molecular class has an optimal design for numerous
3). To the best of our knowledge, this reaction represents the
first example of direct fusion of aromatic rings to porphyrins
without the need to activate porphyrin rings with nickel, or
the aromatic rings with alkoxy groups. The formation of only
one isomer of doubly fused diporphyrins was observed
applications. We report herein the synthesis and properties of
a new class of diporphyrin–pyrene hybrid compounds (Fig-
ure 1d) with relatively simple synthesis and very good
solubility.
The key step in the synthesis of pyrene-fused diporphyrins
is the oxidative ring closure of the pyrene with the porphyrin
core. Although there are several reports of the direct fusion of
polycyclic aromatic rings with monoporphyrins, these proce-
dures employ electron-rich aromatic rings (e.g., rings that are
activated by several alkoxy groups,[8c,11a,c] or appropriately
arranged azulene rings[11d]), and/or nickel(II) porphyrins to
direct fusion.[11a,b,d] Attempts to fuse unsubstituted pyrene
rings with porphyrins were unsuccessful.[11a]
1
(confirmed by H NMR spectroscopy and TLC). Based on
previously reported data on the selectivity of oxidative
coupling of porphyrins,[11d,12] the formation of anti-regioiso-
meric porphyrins is proposed.
Among other possible metalloporphyrins, the Q bands of
the PbII derivatives exhibit strong red-shifts.[13] We found that
the free-base derivative of the pyrene–diporphyrin hybrid 4a
can be metalated with Pb(OAc)2 to give 4c. Taking into
account the size of their extended p-conjugation systems, the
fully fused products 4a, 4b, and especially 4c display
surprisingly good solubility in organic solvents.
The pyrene-substituted porphyrin dimer 3 was prepared in
two steps (Suzuki coupling and Osukaꢀs oxidative fusion of
porphyrin rings) starting from the disubstituted porphyrin
building block 1 (Scheme 1; detailed synthetic procedures and
characterization of 1–4 are given in the Supporting Informa-
The absorption spectra of the pyrene dimer 3 and
metalated fused pyrene diporphyrins 4b and 4c in dichloro-
methane containing 1% pyridine are shown in Figure 2. The
*
Figure 2. UV/Vis–NIR absorption spectra of 2 (c), 3 (*), 4a ( ),
*
^
4b ( ), and 4c ( ) in dichloromethane/1% pyridine. The e value for 2
has been multiplied by 0.2 in order to present all the spectra in the
same plot.
starting porphyrin dimer 3 have a strong Soret band
absorption near l = 420 nm, which is similar to that in
unperturbed monoporphyrin 2, and a strongly red-shifted
Soret band near l = 580 nm.[7b] Upon pyrene ring fusion, both
Soret bands are red-shifted and appear as one broad band at l
ꢀ 600 nm. The most significant differences in absorption
spectra are observed for the Q bands. The increase in
conjugation of the porphyrin dimer by adding two fused
pyrene rings results in a large bathochromic shift of the
Q band from l = 1141 nm in the pyrene dimer 3 to l =
1323 nm in the fused pyrene dimer 4b. Upon metalation
with PbII, the Q band is shifted to l = 1459 nm. Thus, the
overall effect of ring fusion and metal substitution (4c) is a
red-shift of 318 nm compared to 3. In the case of the PbII
derivative 4c the Q band is quite broad, and covers the NIR
region from approximately l = 1150 to 1530 nm. Substitution
with two singly connected pyrene rings in compound 3 does
not change the energy of the Q band, but causes significant
enhancement of its intensity (2.7 times) compared to the
parent (tBu)2Ph-substituted porphyrin dimer.[14] This effect is
even more pronounced for the fused products 4a–4c with the
Q band almost five times as intense than for the reference di-
Scheme 1. Synthesis of fully fused pyrene diporphyrin hybrids (4a–c,
Ar=3,5-di-tert-butylphenyl). a) NBS, CH2Cl2, pyridine, À108C;
b) 4,4,5,5-tetramethyl-2-(pyren-1-yl)-1,3,2-dioxaborolane, [Pd(PPh3)4],
Cs2CO3, toluene, 1108C; c) Sc(OTf)3, DDQ, toluene, 1108C; d) FeCl3,
CH2Cl2, 208C, then aq HCl (M=2H); Zn(OAc)2, MeOH, CH2Cl2
(M=Zn); Pb(OAc)2, pyridine, CH2Cl2 (M=Pb). DDQ=2,3-dichloro-
5,6-dicyano-1,4-benzoquinone, NBS=N-bromosuccinimide, Tf=tri-
fluoromethanesulfonyl.
tion).[7] After examining several different reaction conditions,
we found that the double fusion of two pyrene rings with the
diporphyrin moiety in 3 can be achieved by using anhydrous
FeCl3 in dichloromethane to give 4a. The crude product of
this reaction can be readily metalated with Zn(OAc)2 to give
the fully fused porphyrin hybrid 4b (68–77% yields based on
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tert-butyl porphyrin dimer,[14] and is comparable to the
intensity of the Q bands in porphyrin trimers.[15]
The electrochemical properties of zinc porphyrins 2, 3,
and 4b,c have been studied by cyclic voltammetry (versus
ferrocene/ferrocenium (Fc/Fc+), Table 1). Consistent with
Table 1: Redox potentials of pyrene-functionalized porphyrins.[a]
DEox–red
ox,3
ox,2
ox,1
red,1
red,2
Compound
E1/2
E1/2
E1/2
E1/2
E1/2
2
3
4b
4c
1.26
0.89
–
0.81
0.35
0.18
0.15
0.52
À0.01
À0.13
À0.18
À1.78
À1.10
À0.97
À0.95
À2.18
À1.27
À1.14
À1.18
2.3
1.09
0.84
0.77
&
Figure 3. UV/Vis–NIR absorption spectra of thin films of 4a ( ),
–
~
*
4b ( ), and 4c ( ) on glass. Samples were spin-cast from toluene/5%
[a] Values recorded in dichloromethane and reported in V versus Fc/Fc+.
pyridine solutions.
previously reported data,[7,14,15] the triply connected porphyrin
dimers show a significant decrease in the separation between
E1/2ox,1 and E1/2red,1 potentials (DEox–red) relative to the monop-
orphyrin values, for example, DEox–red(2) = 2.3 V, and DEox–
red(3) = 1.09 V. DEox–red decreases further to 0.84 V for the
fused pyrene diporphyrin 4b; this value is close to that of
triply connected porphyrin trimers (Figure 1a, n = 1).[15]
Theoretical calculations (B3LYP/6-31G) performed on the
model free-base diporphyrins also predict a smaller energy
gap for the fused pyrene–diporphyrin compared to porphyrin
dimer with significant contribution from the fused pyrene
rings to both the highest occupied and lowest unoccupied
molecular orbitals (HOMO and LUMO, respectively, see the
Supporting Information).
Figure 4. Spectral changes of the NIR absorption of thin films of
*
4b/[C61]-PCBM mixtures. Equivalents of PCBM added: 0 (c), 0.5 ( ,
~
!
^
*
gray line), 1.0 ( ), 2.0 ( ), 2.5 ( ), 3.0 (3), 4.0 ( , black line).
The 1H NMR spectra of 4a–4c exhibited only broad
signals at temperatures ranging from À408C to 708C in a
variety of solvents. Such behavior is common for larger
porphyrin tapes[7,8] and is usually attributed to aggregation of
oligoporphyrin molecules by strong p–p interactions in
The red-shift (45 nm) is saturated at approximately l =
1375 nm for the film containing fused porphyrin 4b and
PCBM (1:3 ratio). Fused porphyrins 4a and 4b exhibit similar
behavior in the thin films with the maximum absorption of the
Pb dimer 4c at l = 1566 nm (40 nm shift). These shifts in
absorption are attributed to electronic interactions between
the porphyrins and the fullerene.
Pyrene-fused dimers 4a–4c have been used as active
materials in NIR photodetectors, and give external quantum
efficiencies (EQEs) of up to 6.5% at l = 1350 nm for 4b.[18] To
the best of our knowledge, this efficiency is among the highest
EQE values reported for NIR organic dyes.
In summary, fusion of the two pyrene units with dipor-
phyrin tape in the (meso,b) fashion can be accomplished with
high efficiency by using a FeCl3-mediated oxidative ring-
closure reaction. This is the first example of direct fusion of
aromatic rings with porphyrins which does not require
activation of porphyrins or aromatic rings. The fused
pyrene–diporphyrin hybrid structures 4a–c resemble porphy-
rin trimers in their absorption and electrochemical properties.
However, pyrene-fused dimers have the advantages of a
simple preparation, high solubility, and film proccesibility.
This method represents a straightforward route to obtain NIR
dyes with high absorption. The two pyrene rings of the
products are suited for both intermolecular interactions and
supramolecular contacts with fullerene acceptors.
1
solution. A well-resolved H NMR spectrum of 4b could be
obtained in benzene at 708C if a small amount of pyridine was
added. The pyridine apparently prevents aggregation by
coordinating to ZnII (see the Supporting Information).
Aggregation in the solid state is expected to be even more
pronounced than in solution. The fused pyrene moieties of
4a–c are not shielded and are therefore suited for intermo-
lecular attractive pyrene-pyrene interactions.[16] The thin-film
absorption spectra of fused porphyrins 4a–c exhibit a
considerable red-shift of the Q band, for example, for 4c to
1527 nm at peak maximum (ca. 70 nm shift) compared to l =
1459 nm in solution. Thin films of the PbII diporphyrin 4c
have a measurable absorption that extends to l = 1.9 mm
(Figure 3).
Selective noncovalent interactions between porphyrins
and p-conjugated acceptors, such as fullerenes, have previ-
ously been exploited to prepare extended assemblies with
applications in surface engineering and organic electronics.[17]
To investigate this type of interaction, the spectral changes for
thin films comprised of fused porphyrins 4a–c with different
ratios of porphyrin to PCBM were examined (PCBM = [6,6]-
phenyl-C61-butyric acid methyl ester; Figure 4). An increase
in the concentration of PCBM results in a shift of the peak
position of the Q band further into the NIR region (Figure 4).
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Received: May 3, 2010
Published online: July 2, 2010
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