3,12-Dinitro-5,10,15,20-tetraphenylporphyrin – an example of a twisted intramolecular charge-transfer system

Abstract

The porphyrin ring in the title compound, 10,19-dinitro-2,7,12,17-tetraphenyl-21,22,23,24-tetraazapentacyclo[ 16.2.1.13'6.18'11 . 1 ~3'16]tetracosa-1,3,5,7,9,11 (23) S. DAHAL, M. NETHAJI AND V. KRISHNAN 315 12,14,16,18(21), 19-undecaene 0.5-dichloromethane solvate, C44H28N604.0.5CH2C12, adopts a saddle conformation with neighbouring pyrrole rings tilted with respect to each other. The two nitro groups are situated on alternate pyrrole rings and have their planes angled away from those of the pyrrole rings, thereby indicating that interaction between the porphyrin and nitro groups is slight. CommentNitro porphyrins offer interesting systems for the study of intramolecular charge-transfer states (Gust et al., 1990;Takahashi, Hase, Komura, Imanaga & Ohno, 1992;Rettig, 1986). Structure analysis of the title compound (I) was carried out to determine the orientation of the nitro groups with respect to the porphyrin plane. This structural feature is important to an understanding of the type and magnitude of charge transfer that takes place in the excited state of the system. Moreover, tetraphenyl porphyrins bearing substitutents at the pyrrole C atoms exhibit interesting conformational features 1991; Barkigia et al., 1990;Mandon et al., 1992). Details of the synthetic work will be published elsewhere (Dahal & Krishnan, 1993). The structure of (I) is found to be severely nonplanar with an average deviation of 0.36 A from the mean N 4 plane (the plane passing through the four N atoms of the pyrrole rings). Neighbouring pyrrole rings of the macrocycle are tilted by 26.5 (5), 27.4(5), 18.4(4) and 14.8 (4) ° with respect to each other, while the opposite pyrroles are tilted by 32.5 (4) and 25.1 (5) ° with respect to each other. The displacements of the meso C atoms, C5, C10, C15 and C20 from the mean N4 plane are -0.15 (1), -0.03 (1), -0.02 (1) and 0.21 (1)A, respectively. These distortions lead to a saddle conformation of the porphyrinoid skeleton similar to those observed for pyrrole-substituted tetraphenyl porphyrins (Bhyrappa, Nethaji & Krishnan, 1993;Mandon et al., 1992). The inner imino H atoms are positioned trans to each other on the pyrrole rings not bearing the nitro groups with N--H distances of 1.10 (9) and 0.96 (9) A (Silvers & Tulinsky, 1967;Chen & Tulinsky, 1971). The phenyl rings adjacent to the nitro groups are rotated by 36.5 (3) and 38.6 (4) ° from the mean N4 plane so as to avoid the steric influence of the nitro groups at positions 3 and 12 of the macrocycle. The other two phenyl rings are rotated by 88.6 (4) and 54.5 (4) ° from the mean N4 plane. The planes of the nitro groups are tilted by 42.2 (6) and 38.0 (9) ° with respect to those defined by the pyrrole rings to which they are attached, and deviate by 39.4 (8) and 42.1 (9) ° from the mean N4 plane of the porphyrin. The N atoms of the nitro groups are displaced by 0.27 (1) and 0.38 (1) A from the plane of the pyrrole rings to which they are attached. The nitro group rotations, coupled with their upward displacements, imply only slight ...

Non-planar porphyrins with mixed substituent pattern: bromination and formylation of ethyl-substituted tetraphenylporphyrins and tetraalkylporphyrins

“…The only compound with a similar substituent arrangement is 7,18dinitro-5,10,15,20-tetraphenylporphyrin, that also shows a saddle deformation. 24 We next turned our attention to the preparation of porphyrins in which all the free β positions of the porphyrins H 2 L 2 -H 2 L 5 would be substituted with bromine atoms. Such porphyrins should have a very similar overall degree of conformational distortion albeit with quite different redox potentials depending on the number of bromine atoms and present the first examples of "push-pull porphyrins" where ϩI and ϪI groups are located directly at the porphyrin ring system In addition, such porphyrins present excellent precursors for other dodecasubstituted porphyrins with mixed substituent pattern via Suzuki coupling.…”