HETEROCYCLES

Communication | Special issue | Vol. 79, No. 1, 2009, pp. 331-337
Received, 8th August, 2008, Accepted, 29th December, 2008, Published online, 29th December, 2008.
DOI: 10.3987/COM-08-S(D)9

■ High Binding Affinity of DABCO with Porphyrin in a Porphyrin-cis-Stilbene-Porphyrin Triad

Md. Wahadoszamen, Takashi Yamamura, Atsuya Momotake, Yoshinobu Nishimura, and Tatsuo Arai*

Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Ten-nodai, Tsukuba-shi, Ibaraki 305-0006, Japan

Abstract

Porphyrin-stilbene-porphyrin triad (mZnPst) where a central cis-stilbene unit is connected to zinc-tetraphenylporphyrin (ZnTPP) was synthesized and its binding ability with a selected guest ligand DABCO was investigated. The association constant was evaluated to be 2.47 x 108 M-1, from the iterative least squares fitting to a 1:1 binding model, which is larger than that of monomer ZnTPP/DABCO complex (3.50 x 105 M-1). In addition, when a toluene solution of mZnPst is mixed with 1 equivalent of DABCO, the color of the solution was changed abruptly to light purple from bright reddish, providing further a visual evidence of forming strong complex. Such a high association constant suggests mZnPst/DABCO to be a promising photoresponsive supramolecular system.

Nowadays, porphyrin appears to be a promising artificial host and/or receptor as its giant π surface to allow interacting efficiently with a suitable acceptor or ligand moiety give rise to a non-covalent complex. By taking the advantage of the ultimate rigidity of the porphyrin skeleton to which various functional groups can be attached at the periphery and the easier accessibility of incorporating various metals to the central part with varying Lews acidity, a variety of porphyrin based artificial receptors exhibiting strong binding affinity with ions,1 organic molecules2 and biological species,3 have been reported in recent years. Besides, in the pursuit of superbly stable porphyrin based supramolecular complex potential for photovoltaic applications, a variety of porphyrin dimers using different linkers such as alkyl or ether chain,4,5 1,2-polyphenylene,6 alkyne,7,8 photoresponsive azobenzene9 have also been developed and explored. The objective of inserting different spacer was nothing but to control as well as to optimize the electronic communication of host porphyrins with a guest acceptor or ligand.
Stilbene is well recognized photoresponsive material which, upon UV irradiation, undergoes reversible transformation between its two isomers (cis and trans) having completely different geometries (skewed and planer) as well as physical properties. We envisioned that, as like photoresponsive azobenzene,9 if the cis-stilbene be utilized as the spacer for porphyrin dimer its skewed geometry may allow the two peripheral porphyrin units to orient in a face-to-face fashion which may provide a flexible molecular pocket to a selective ligand or acceptor to interact simultaneously with the both porphyrin units and thereby resulting in a strong ground state host-guest complex. Motivated by this anticipation, in this study, we have synthesized a porphyrin-stilbene-porphyrin triad (mZnPst) where a central cis-stilbene unit is connected to the meta position of zinc-tetraphenylporphyrin (ZnTPP) and investigated its binding ability with the selected guest ligand DABCO. We wish to report here the extraordinarily high binding affinity of ZnTPP with DABCO in mZnPst.

The host compound mZnPst was synthesized in two steps starting from 5-(3-Hydroxyphenyl)-10,15,25-triphenylporphyrin10 and (Z)-4,4'-Bis(bromomethyl)stilbene11 by following the Scheme 1. The desired compound was separated by column chromatography with silica gel and its structure was characterized by 1H NMR and Maldi-Tof-Mass and UV-visible spectroscopies. The observed absorption spectrum of mZnPst is fairly the superposition of its components spectra, suggesting that electronic interaction among the triad chromophores is feeble in the ground state.
Figure 1 represents the results of titration of mZnPst with DABCO which demonstrates a blatant modification of the absorption spectrum of ZnTPP with increasing concentration of DABCO. As the proportion of DABCO in the mixture increases in succession, the absolute absorption intensity of both the Soret and Q bands reduces steadily and new absorption bands, the peaks of which are located at the wavelength region longer than those of the both bands, come up with the distinct signatures of isobestic points at 411, 421, 426.5, and 434.5 nm in the Soret band region and at 557.5, 582.5, and 590 nm in the Q bands region. The intensity of the newly formed absorption bands increases steadily with increasing

proportion of DABCO and finally reaches to the maximum when 1 equivalent of DABCO is added. The newly formed band is assigned to the absorption spectrum of the complex arising from the strong electronic interaction of mZnPst with DABCO in the ground state. The stoichiometry of the complex was estimated on the basis of Job’s plots to be 1:1 (Job plot is shown in the supporting information). The association constant was evaluated to be 2.47 x 108 M-1 from the iterative least squares fitting to a 1:1 binding model, which is certainly a large value exceeding apparently the upper limit of experimental determination for spectroscopic titration and found to be larger than that of monomer ZnTPP/DABCO complex (3.50 x 105 M-1). In addition, when a toluene solution of mZnPst is mixed with 1 equivalent of DABCO, the color of the solution was changed abruptly to light purple from bright reddish, providing further a visual evidence of forming strong complex. It seems that, due to strong binding between mZnPst and DABCO, the resulting complex likely to be appeared as a stable and distinct species having a distinct fluorescence of its own.

To diagnose the structural details as well as the dynamics of the complexation, NMR of spectrum of mZnPst was taken in the absence and in the presence of 1 equivalent of DABCO. The NMR spectrum of mZnPst devoid of DABCO (Figure 2a) exhibits multriplets at all the regions specified as A (β-pyrrole-H), B (orthophenyl-H), C (substituted orthophenyl-H), D (meta-, para-phenyl-H) and E (substituted meta-phenyl-H). The multriplet structure in the NMR spectrum probably results from the several conformational isomers of mZnPst arising from the flexible spacer. Upon addition of 1 equivalent of DABCO, the bands A around 9.0 ppm were shifted and appeared around 8.50-8.70 ppm as two broadening peaks (Figure 2b). Simultaneously, peaks B-E (Figure 2a) were shifted to give broaden signals as in Figure 2b. Noteworthy modification of the NMR signals especially of A and B and the concomitant shifting upon adding DABCO provides another sheer evidence of a strong association of DABCO with ZnTPP in existing molecular framework of mZnPst. In such a circumstance, it will not be an unusual consideration that DABCO is incorporated between the two ZnTPP units of mZnPst and thereby resulting in a stable 1:1 sandwich type complex (Figure 3). The estimated extremely high association constant of mZnPst/DABCO complex in comparison with monomer ZnTPP/ DABCO complex also provides a reasonable support for such a conclusion.

On UV irradiation at room temperature in toluene, mZnPst underwent photoisomerization to trans-mZnPst, which was revealed by the change in the absorption spectra. The absorption changes for irradiation of mZnPst are shown in Figure 4a. The initial absorbance of the cis-stilbene moiety around 320 nm increased with irradiation time to give a new band. Figure 4b shows the differential spectrum between the samples before and after irradiation of mZnPst, which is similar to the absorption spectra of model compound, trans-4,4’-dimethylstilbene. The result indicates that the new band shown in Figure 4a is due to the trans-stilbene chromophore (Figure 5). During the photoirradiation, the absorption spectrum of porphyrin moiety did not change, probably because the electronic structure of trans-mZnPst is similar to that of original cis-isomer except stilbene moiety.
In conclusion, we have synthesized a porphyrin-stilbene triad (mZnPst) and explored its binding affinity with DABCO. DABCO is found to interact strongly with both ZnTPP units in mZnPst yielding a superbly stable 1:1 complex. The association constant of the complex is estimated to be 2.47 x 108 M-1. Such an extremely high association constant suggests mZnPst/DABCO to be a promising supramolecular system and thus a full investigation of its photophysics is warranted indeed Finally, we demonstrated the photochemical isomerization to give trans-isomer of mZnPst.

ACKNOWLEDGEMENTS
This work was supported by a Grant-in-Aid for Science Research in a Priority Area "New Frontiers in Photochromism (No. 471)" from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

References

1. Y.-H. Kim and J.-I. Hong, Chem. Commun., 2002, 512; CrossRef A. V. Gusev and M. A. J. Rodgers, J. Phys. Chem., 2002, 106, 1985; CrossRef H. Shinmori, Y. Yasuda, and A. Osuka, Eur. J. Org. Chem., 2002, 67, 1197; CrossRef M. Ikeda, T. Tanida, M. Takeuchi, and S. Shinkai, Org. Lett., 2002, 2, 1803. CrossRef
2. C. A. Hunter and R. Tregoning, Tetrahedron, 2002, 58, 691; CrossRef F. D’Souza and G. R. Deviprasad, J. Org. Chem., 2001, 66, 4601; CrossRef P. Wallimann, T. Marti, A. Fuerer, and F. Diederich, Chem. Rev., 1997, 97, 1567. CrossRef
3. M. Sirish, V. A. Chartkov, and H.–J. Schneider, Chem. Eur. J., 2002, 8, 1181; CrossRef T. Hayashi, T. Aya, M. Nonoguchi, T. Mizutani, Y. Hiseada, S. Kitagawa, and H. Ogoshi, Tetrahedron, 2002, 58, 2803; CrossRef T. Mizutani, K. Wada, and S. Kitagawa, J. Org. Chem., 2002, 65, 6097. CrossRef
4. R. Yang, K. Wang, L. Long, D. Xao, X. Yang, and W. Tan, Anal. Chem., 2002, 74, 1088. CrossRef
5. D. Monti, M. Venanzi, G. Mancini, F. Marotti, L. La Monica, and T. Boschi, Eur. J. Org. Chem., 1999, 64, 1901. CrossRef
6. A. Osuka, S. Nakajima, T. Nagata, K. Maruyama, and K. Toriomi, Angw. Chem., Int. Ed. Engl., 1991, 30, 582. CrossRef
7. J. Seth, V. Palaniappan, T. E. Johnson, S. Prathapan, J. S. Linsey, and D. F. Bocian, J. Am. Chem. Soc., 1994, 116, 10578. CrossRef
8. J. Seth, V. Palaniappan, R. W. Wagner, T. E. Johnson, J. S. Linsey, and D. F. Bocian, J. Am. Chem. Soc., 1996, 118, 11194. CrossRef
9. T. Yamamura, A. Momotake, and T. Arai, Tetrahedron Lett., 2004, 45, 9219. CrossRef
10. S. Banfi, E. Caruso, L. Buccafurni, R. Murano, E. Monti, M. Gariboldi, E. Papa, and P. Gramatica, J. Med. Chem., 2006, 49, 3293. CrossRef
11. J. C. Rosa, D. Galanakis, C. R. Ganellin, and P. M. Dunn, J. Med. Chem., 1996, 39, 4247. CrossRef