Porphyrin dimer carbocations with strong near infrared absorption and third-order optical nonlinearity

Article abstract of DOI:10.1002/anie.200802687

Colored carbocations beyond the visible! A porphyrin dimer carbocation is synthesized with effective delocalization over 18 conjugated bonds. Placing a carbocation at the center of a conjugated porphyrin dimer shifts its absorption into the infrared and results in strong ultrafast nonlinear optical behavior. (Graph Presented)

Full text of DOI:10.1002/anie.200802687

Angewandte
Chemie
DOI: 10.1002/anie.200802687
Nonlinear Optics
Porphyrin Dimer Carbocations with Strong Near Infrared Absorption
and Third-Order Optical Nonlinearity**
Karl J. Thorley, Joel M. Hales, Harry L. Anderson,* and Joseph W. Perry*
Materials with large ultrafast third-order nonlinearities at
near-infrared (NIR) wavelengths are required for all-optical
signal processing.[1] Telecommunications systems operate in
this spectral region, and all-optical switching[2] and wave-
length conversion[3] devices will lead to faster signal process-
ing than can be achieved by current electronic methods, thus
providing greater bandwidths and data-transfer rates. Dyes
with large optical nonlinearities will be used in a new
generation of devices requiring modest drive powers and
short path lengths,[4] and in ultrafast image-correlation
security systems.[5]
achieving efficient electronic delocalization in long chromo-
phores. The idea behind this work is to synthesize a cyanine-
like dye with two-dimensionally delocalized porphyrin mac-
rocycles as terminals (Figure 1), thus accessing greater
Herein we present a family of porphyrin dimer carboca-
tions 1a–c which show intense NIR absorption maxima at
1100–1340 nmand large ultrafast third-order nonlinearities at
1550 nm. It is well known that carbocations tend to be more
electronically delocalized than neutral conjugated hydro-
carbons, and many common dyes, such as cyanines and
rhodamines, can be regarded as resonance-stabilized carbo-
cations. Cyanines are interesting in that they have large
negative third-order nonlinearities at energies below the S0–S1
gap.[6,7] Furthermore, the optical nonlinearity of cyanine dyes
scales with a high-order power of the conjugation length,
suggesting that extended cyanines could provide extremely
large nonlinearities.[7] However, extended cyanines can
undergo a symmetry-breaking process which reduces elec-
tronic delocalization and nonlinearity.[8] Hales et al. recently
reported that extended bis-(dioxaborine) polymethine 2
sustains delocalization at lengths of approximately 14 con-
jugated bonds, beyond that of conventional cyanines.[9] The
nature of the terminal groups evidently plays a key role in
Figure 1. The concept of porphyrin dimer carbocations as cyanine dye
analogues.
effective delocalization lengths. The conjugated bridge of
our “porphocyanines” differs fromthose in standard cyanine
dyes in that we utilize triple rather than double bonds;
alkynes provide better electronic coupling to porphyrins
because they cannot twist out of conjugation.[10]
[*] K. J. Thorley, Prof. H. L. Anderson
Department of Chemistry, Oxford University, Chemistry Research
Laboratory
Neutral conjugated porphyrin oligomers[11] and poly-
mers[12] have been shown to exhibit large third-order optical
nonlinearities. Porphyrin-stabilized carbocations have
scarcely been investigated,[13] although Smith and co-workers
reported formation of cationic porphyrin dimer 3 with NIR
absorption (lmax = 1030 nm; e = 41500mÀ1 cmÀ1),[14] indicating
significant charge delocalization.
Porphocyanine 1a was synthesized as shown in Scheme 1.
Deprotonation of the terminal acetylene 4 and reaction with
0.5 equivalents of methyl benzoate afforded the precursor 5a
in 50% yield. Addition of trifluoracetic acid (TFA, 2% by
volume) to 5a generated carbocation 1a, as revealed by
Mansfield Road, Oxford, OX1 3TA (UK)
Fax: (+44)1865-285-002
E-mail: harry.anderson@chem.ox.ac.uk
Homepage: users.ox.ac.uk/~hlagroup
Dr. J. M. Hales, Prof. J. W. Perry
School of Chemistry and Biochemistry, Georgia Institute of
Technology
Atlanta, GA 30332-0400 (USA)
E-mail: joe.perry@gatech.edu
Homepage: www.chemistry.gatech.edu/faculty/Perry
[**] This work was supported in part by the STC Program of the National
Science Foundation under Agreement Number DMR-0120967 and
the DARPA MORPH Program and ONR (N00014-06-1-1013 and
N00014-06-1-0897). We thank Prof. S. R. Marder and Dr. S. Barlow
for valuable discussion and the EPSRC Mass Spectrometry Service
(Swansea) for mass spectra.
dramatic changes in the UV/Vis/NIR spectrum (1a: lmax
=
1243 nm; e = 170000mÀ1 cmÀ1) and by NMR spectroscopy.
The NIR bands are substantially red-shifted in comparison to
neutral porphyrin oligomers. In addition, the absorption is
even red-shifted in comparison to the previously reported b-
linked dimer 3, as a result of the more efficient conjugation
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802687.
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A reference compound (6), containing just one porphyrin,
was synthesized by reaction of acetylene porphyrin 4 with
benzophenone (Scheme 1). Upon formation of cation 7 by
addition of TFA, a significant shift in the absorption spectrum
was again observed (Figure 2b. 7: lmax = 824 nm; e =
62000mÀ1 cmÀ1). As the NIR band for porphocyanine dimer
1a is red-shifted by over 400 nm( DE ꢀ 0.50 eV) in compar-
ison with monomer carbocation 7, the positive charge must be
delocalized over both porphyrin units in 1a.
The linear optical properties of the porphocyanines show
similarities with highly delocalized cyanine-like structures.
The full widths at half maxima (FWHM) for the NIR bands
are small: 920 cmÀ1 for 1a compared to 1640 cmÀ1 for the
Soret band of 5a, indicating a small geometry change upon
excitation. The large extinction coefficients of the NIR bands
also indicate fully allowed transitions, as in standard cyanine
dyes.
13C NMR spectroscopy provides a direct probe for charge-
delocalization in porphocyanine 1a.[15] The most informative
resonances are those of the quaternary carbons near the
center of the molecule. These signals are difficult to assign by
C–H correlation experiments, so it proved necessary to
synthesize 13C-labeled variants of compounds 5a and 6
(fromPh 13CO2Me and Ph213CO respectively). The central
Scheme 1. Synthesis of cations 1a (X=H), 1b (X=NO2), 1c
(X=OMe) and 7. Ar=3,5-(tBu)2C6H3. a) XC6H4CO2Me (0.5 equiv);
LiN(SiMe3)2 (4 equiv), THF; b) Ph2CO (1.5 equiv), LiN(SiMe3)2
(4 equiv), THF; c) F3CCO2H (TFA, 2% by vol.), CHCl3.
1
carbon atomC in the resulting porphyrin-stabilized cations
1a and 7 carry less positive charge than C1 in the correspond-
ing aryl-stabilized cations 8 and 9[16] (Table 1), as revealed by
through the meso positions. Cation formation is reversible;
washing with water regenerates alcohol 5a in 92% yield, as
shown by absorption spectroscopy.
The wavelength of the NIR absorption band of 1a can be
tuned by placing electron-donating or electron-withdrawing
substituents on the central phenyl ring to stabilize or
destabilize the carbocation and thus to control the degree of
delocalization onto the porphyrins. Thus a para-nitro sub-
stituent shifts the absorption to longer wavelength (Figure 2a.
1b: lmax = 1348 nm; e = 120000mÀ1 cmÀ1) whereas a para-
Table 1: Selected 13C NMR spectroscopic shifts (dC) and changes in
chemical shift on cation formation from alcohol precursors (DdC) of
porphyrin cations 1a and 7, measured at 298 K in CDCl3/2% d-TFA, in
comparison to phenyl stabilized analogues 8 and 9.[16]
methoxy substituent shifts it to shorter wavelength (1c: lmax
=
dC1 (DdC1)
dC2 (DdC2)
dC3 (DdC3)
1176 nm; e = 110000mÀ1 cmÀ1).
1a
7
8
133.9 (66.3)
160.8 (85.0)
162.1 (96.4)
186.6 (112.1)
118.9 (22.8)
113.0 (14.3)
106.5 (17.4)
105.9 (14.4)
136.1 (49.5)
163.6 (75.4)
145.5 (60.6)
159.1 (72.5)
9
an upfield shift of 20–30 ppmin its chemical shift, dC1. The
chemical shift difference, between the C1 atomof dimer 1a
and that of its reference monomer 7 is 27 ppm, confirming
that the second porphyrin increases the delocalization of the
positive charge.
The molecular third-order polarizability, g, of carbocation
1a and its alcohol precursor 5a were determined by femto-
second-pulsed Z-scan measurements at 1550 nm. Represen-
tative results for both the open-aperture (sensitive to Im(g))
and closed-aperture experiments (sensitive to Re(g)) are
shown in Figure 3 (Im(g) and Re(g) are the imaginary and
real components of the molecular third-order polarizability).
Despite the nearly five-fold decrease in irradiance, the
magnitudes of the signals (proportional to the nonlinearity)
for the carbocation solution dwarf those of the precursor. By
Figure 2. a) Absorption spectra of porphocyanines 1a (bold), 1b
(dotted) and 1c (dashed) in CHCl3/2% TFA, in comparison with
precursor alcohol 5a (plain) in CHCl3. Alcohols 5b and 5c gave
essentially the same absorption spectra as 5a. b) Absorption spectrum
of porphyrin monomer carbocation 7 (dashed) in CHCl3/2% TFA, and
its precursor alcohol 6 (plain) in CHCl3.
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The uniqueness of cation 1a lies both in the large ratio of
Re(g)/Im(g), suggesting that an acceptable 2PA figure of
merit (FOM)[17] could be achieved with this material, and in
the magnitudes for each component of g, which represent
some of the largest off-resonant nonlinearities reported in this
spectral region.[18] These attributes, coupled with the ultrafast
response of the nonlinearity, imply that this system has the
critical characteristics for all-optical switching. Furthermore,
despite this large ratio, Im(g) is also quite sizeable; it
corresponds to a 2PA cross-section of ca. 3100 GM (at
1550 nm), which is even larger (on a per-macrocycle-basis)
than those of conjugated porphyrin polymers with sizable
values in this spectral region.[19,20]
In summary, a porphyrin dimer carbocation has been
synthesized and was found to have effective delocalization
over approximately 18 conjugated bonds, significantly longer
than traditional cyanine dyes. The resulting large third-order
nonlinearities at 1550 nmand two-photon absorption figure
of merit suggest that these dyes are promising materials for
all-optical switching and dynamic noise suppression.
Received: June 7, 2008
Published online: July 30, 2008
Keywords: carbocations · dyes/pigments · materials science ·
.
nonlinear optics · porphyrinoids
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Communications
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