SELF-ASSEMBLY OF ZINC PORPHYRINS AROUND THE PERIPHERY OF HYDROGEN-BONDED AGGREGATES THAT BEAR IMIDAZOLE GROUPS

This paper describes the preparation and characterization of four aggr egates that are based an the rosette of derivatives of isocyanuric aci d (CA) and melamine (M). These aggregates comprise a trismelamine, hub (M-Im)(3), that presents imidazole groups around its periphery; these imidazoles organize zinc tetraphenyl porphyrin (ZnTPP) by coordination of the imidazole to the zinc center. Aggregate 1 forms a single roset te upon mixing 1 equiv of hub(M-Im)(3) and 3 equiv of CA. Adding 3 equ iv of ZnTPP yields 2. Aggregate 3 forms as a stacked bisrosette upon m ixing 2 equiv of hub(M-Im)(3) and 3 equiv of bisCA. Adding 6 equiv of ZnTPP yields 4. The stoichiometries of aggregates 1-4 were obtained by titrating the trismelamines with CA and by titrating the aggregates w ith ZnTPP. The stoichiometry is defined as the ratio at which addition al CA remains insoluble or additional ZnTPP appears as free ZnTPP in t he H-1 NMR spectrum. Electrospray ionization mass spectrometry (ESI-MS ) is compatible with the measured stoichiometries, The structures of t hese aggregates were determined using variable-temperature H-1 NMR spe ctroscopy;analogous structures were inferred for 5 and 6, the tert-but yl analogues of 1 and 2. The shapes of the traces from gel permeation chromatography (GPC) suggest that imidazole groups destabilize the agg regates when they are not involved in coordination to zinc; that is, t he stability seems to be 6 approximate to 4 > 3 and 5 approximate to 2 > 1. A direct comparison of the relative stability of 1, 2, and 5 con firms the results of the GPC analysis: mixing 1 (hub(M-IM)(3).3CA) wit h the trismelamine component of 5 (hub(M)(3)) leads to the formation o f a 3:2 mixture of 5:1. Adding ZnTPP to this solution leads to a 3:2 m ixture of 5:2 with free trismelamines remaining in solution: 1 is not observed, The results of UV/vis spectroscopy are consistent with the o ther spectroscopic and chromatographic results and indicate that 3 equ iv of ZnTPP are organized around the periphery of 2 and at least 4 equ iv around the periphery of 4.