C O M M U N I C A T I O N S
a
Table 1. TPA and TPEF Enhancement in Porphyrin Macrocycles
porphyrin
dimer number total δ (GM) δ/dimer (GM)
η
ηδ/dimer (GM)
dimer
1
11
13
17
19
572
572
0.342
0.026
0.034
0.044
0.017
196
969
1342
1763
894
5
4
4
4
4
n ) 1 (4R)
n ) 3 (4R)
n ) 7 (4R)
n ) 9 (4R)
4.1 × 10 3.7 × 10
5
5.1 × 10 3.9 × 10
5
6.8 × 10 4.0 × 10
6
1.0 × 10 5.3 × 10
a
GM ) 10-50 cm s/photon.
4
absorption and emission properties of this system were investigated
by the TPEF method with ∼100 fs pulses as outlined elsewhere in
1
,2
our previous reports. The dimer in this investigation shows only
a small 2P response when considered in isolation. However, upon
cyclization and subsequent covalent linkage, a 2 order of magnitude
2
increase in δ and an order of magnitude increase in the two-photon
Figure 2. Two-photon cross section spectra per dimer unit for the porphyrin
macrocycles studied. The dimer is shown at 15 times it is actual value for
contrast.
fluorescence signal was observed. Though the acetylinic bridging
between dimer units is not assumed to contribute to the two-photon
cross section of the macrocycle implicitly, they are assumed to
prevent dissociation of the imidazolyl linkages and make the
assemblies more viable for use in a wider assortment of environments.
macrocycles can be generated and separated by size. An increase
of 2 orders of magnitude for the δ per dimer unit was observed in
2
these porphyrin macromolecules. The macrocycles have greater
promise for two-photon applications in multiple solvents and at
wider pH ranges than aggregates which lack covalent bonds. These
materials provide greater 2P response than linear counterparts,
showing potential for use in both TPA and TPEF applications and
providing a new avenue from which to probe porphyrin dimer-dimer
interactions and flexible covalent bridges in imidazolyl-Zn(II)
porphyrin systems.
2
The δ for the cyclized systems is given in Figure 2. For these
systems it was found that the maximum two-photon cross sections
occurred near 830 nm. It was also found that for cyclized porphyrin
macromolecules the increased ring size leads to an increased dimer
cross section, making the 19-mer a preferred candidate for TPA
applications. This is one of the largest cross sections reported for
3-5
porphyrin systems on the femtosecond time scale.
The maximum
TPEF is attributed to the macrocycle with the lowest strain at it is
linkage points, the 17-mer, and is also one of the largest values
Acknowledgment. Support for this project was provided by
NSF-DMR-Polymers.
3
reported for porphyrin system at a δ
2
× η value of 1.8 × 10 GM
per dimer.
Supporting InformationDetailed experimental procedures and
associated spectrometric data (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
References
(
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Figure 1. Steady-state absorption and emission data for porphyrin
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porphyrin macrocycles when compared to the dimer. Only selected spectra
are presented for clarity.
1
(
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2
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imidazolyl group to the neighboring porphyrin’s Zn(II) enhances
3,6
the local dipole-dipole interaction in the macrocycle. In forming
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7
between bound chromophores. This binding in effect leads to lower
dihedral angle disruption between the porphyrin units and fluorene
5
d
and higher dimer symmetry. All of these properties have been
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(
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2
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1
In this study we show that by an elegant combination of self-
assembly and a metathesis ring closing reaction, large porphyrin
JA804415B
J. AM. CHEM. SOC. 9 VOL. 130, NO. 51, 2008 17213