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-esters of mesoporphyrin, in which the nitrobenzyl group(s) is (are) mono-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 theoretical calculations. Folding of these flexible dyads is consistent with the following observations: (1) H-1 NMR ring current upheld shift in 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 temperature over the range 25 to -55 degrees C; (3) the H-1 NMR resonance 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) is 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 semiempirical calculations) minimum-energy folded conformations, each benzyl group lies approximately parallel to the porphyrin pi-system with its centre slightly offset relative to the centre of the porphyrin; however, for each isomer, significant structural differences between the ortho isomer and its meta and para counterparts are found. The implications of these differences for the photoinduced electron transfer efficiencies in these dyads are discussed.