Spectral Properties and Chi(3) of Metallo Meso-Tetra-Substituted Tetrabenzporphyrins
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Figure 1.
Metallo-meso-Tetraphenyltetrabenzporphyrin 179
Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
RESULTS AND DISCUSSION The benzporphyrins were synthesized in-house. The Chi(3) was determined by the use of the degenerate four-wave mixing technique Visible spectra were obtained (DFWM) with a 532 nm 17 ps laser'. In in a usual manner at room temperature in tetrahydrofuran. Figure 2, a typical visible spectrum of a metallo-TPTBP is shown.
I 0.8 Ei 0.6 0)
0. w
0.4 zM 0ý
0.2
0
Ul)
M
0
300
400
500
600
700
WAVELENGTH (nm)
Figure 2. Electronic Spectrum of Zn meso-Tetra (p-methoxyphenyl) TBP The spectral data of Zn meso-tetra-substituted phenyl TBP are summarized in Table I together with Chi(3) values. Table I.
Spectral Peaks and Chi(3) Values of Zn meso-Tetra-Substituted TBPs
Meso Substituent Q Band Peak (nm) B Band Peak (nm) Q/B Height Ratio Chi(3) x 103 esu 1.5 0.33 434 625 H 0.4 -0.9 0.3 438 639 C6 F5 m-F-C 6H4
651
461.5
0.19
2.5-5.0
C6 H5
649
462
0.18
3
p.CIC 6H4
650
464
0.19
5.3-6.6
p-CH 3O-C6H4
650
465
0.16
3.5-4.0
p-HO.C 6H4
650.5
465.5
0.15
3.0- 4.0
p-(CH 3)2NC6H4
652
475
0.14
5.0-9.5
180
The compounds are arranged in increasing order of the B peak The Table shows that B peak shifts are accompanied by wavelength. smaller Q peak shifts and by decreasing peak height ratios Q/B. These are characteristics observed for other porphyrin spectra2 . example of a systematic parametric study However, this is the first The made on the effect of meso substituents on any porphyrin. variation in the value of Chi(3) with the substituent may be seen in Figure 3.
U)
5
x
U]
C,,
0
I
430
435
440
Ii
445
i
450
I
455
I
460
I
465
i
470
475
B Band Peak Wavelength (nm) Figure 3.
Chi(3)
[532 nm] vs. B Band Peak Wavelength for Various ZnTBPs
The points do not show the uncertainty range for the sake of clarity. The figure shows the tendency of Chi(3) to increase with the B band peak wavelength. The observed conjugation effect suggests that the phenyl ring A molecular is not exactly perpendicular to the porphyrin ring. modeling calculation was done to estimate the angle between the The programs used were Insight II and Discover (both from rings. Inc). They were used to compare the Biosym Technologies, The programs structural difference between ZnTBP and ZnTPTBP. calculated coordinates of each atom in the molecule by minimizing Since the constant valance force field the molecular energy. (CVFF) program did not have parameters for Zn, we substituted Si parameters in place of Zn parameters for expediency. A striking contrast between the calculated structure of the two molecules is shown in Figure 4; ZnTBP had a planar structure, while the TBP portion of ZnTPTBP had a saddle-shape structure. Such similar results were obtained for Zn octaethylporphyrin and Zn octaethyltetraphenylPorphyrin by Shelnutt, et al. using more This saddle structure forces the sophisticated calculations . plane of the phenyl ring and the plane consisting of the meso ca
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