CONFORMATIONAL FOLDING INDUCED BY PI-PI INTERACTION IN A SERIES OF FLEXIBLE DYADS CONSISTING OF ISOMERIC MESOPORPHYRIN NITROBENZYL ESTERS

A series of isomeric (ortho, meta and para) nitrobenzyl mono- and di-e sters of mesoporphyrin, in which the nitrobenzyl group(s) is (are) mon o-linked through one or both propionate chains of the porphyrin, have been synthesized and their room-temperature conformations in a number of solvents have been investigated using H-1 NMR spectroscopy and theo retical calculations. Folding of these flexible dyads is consistent wi th the following observations: (1) H-1 NMR ring current upheld shift i n all solvents assayed [i.e., CDCl3, CDCl3-CD3OD (10:1) and C6D6] of 0 .5-2.5 ppm for the aryl protons; (2) these shifts are independent of t emperature over the range 25 to -55 degrees C; (3) the H-1 NMR resonan ce values of mesoporphyrin benzyl ester (lacking the nitro group) are not shifted, indicating a preferred, sterically less crowded, extended conformation. The presence of the nitro group on the phenyl ring(s) i s responsible for the folding of these flexible dyads, and the results can be interpreted in terms of pi-pi interactions (C. A. Hunter and J . K. M. Sanders, J. Am. Chem. Sec., 1990, 112, 5525-5534) between the two halves. In the theoretically calculated (molecular mechanics and s emiempirical calculations) minimum-energy folded conformations, each b enzyl group lies approximately parallel to the porphyrin pi-system wit h its centre slightly offset relative to the centre of the porphyrin; however, for each isomer, significant structural differences between t he ortho isomer and its meta and para counterparts are found. The impl ications of these differences for the photoinduced electron transfer e fficiencies in these dyads are discussed.