Rigid trisporphyrin with 1,3,5-triazine core: Synthesis, spectroscopy, and DABCO-induced self-assembly properties

Abstract

A rigid Zn–trisporphyrin (1), the Zinc complex of 2,4,6-tris(5,10,15-triphenylporphyrinatozinc)-1,3,5-triazine (2) with three porphyrin moieties bridged by a rigid 1,3,5-triazine group, was synthesized and characterized by a series of spectroscopic methods including mass, 1H NMR, electronic absorption, and IR spectroscopy in addition to elemental analysis. The UV-vis spectrophotometric titration results revealed that along with the addition of the 1,4-diazabicyclo[2.2.2]octane (DABCO) form 0 to 3 equiv, 1 could form a 2:3 sandwich-type coordination cage in chloroform, which then transformed into a 1:3 open complex when the DABCO concentration was more than 3 equiv. Such DABCO-induced self- and disassembly processes were also confirmed by 1H NMR spectrophotometric titrations. In particular, it was demonstrated by comparative studies that the synergistic effect of the three porphyrin moieties linked together by the rigid 1,3,5-triazine group in 1 promoted the formation and stabilization of the 2:3 sandwich-type coordination cage. This result will be helpful towards the fabrication of various self-assembly structures based on metalloporphyrins with application potential in diverse areas such as molecular recognition and separation.

Spectrophotometric Study of Bridging N-Donor Ligand-Induced Supramolecular Assembly of Conjugated Zn-Trisporphyrin with a Triphenylamine Core

“…To validate our results, we employed DABCO, a common bridging ligand, previously used to construct double-decker complexes [ 31 , 32 , 33 , 36 ]. Titrimetric addition of DABCO to a toluene solution of 1 afforded a series of isosbestic points (439, 578, 598, and 632 nm) along with a shift in the Soret band (439 to 446 nm), as well as in the Q-bands (570 to 582 nm and 627 to 645 nm).…”

“…In addition, a series of isosbestic points (441, 581, 599, and 634 nm) along with a shift in the Q-bands (570 to 583 nm and 627 to 647 nm) were also observed during the titration process. Based on the fitting of the titration data to the three species ( 1 , 3:2 double-decker coordination complex, and 3:1 open complex) and the binding model described in Scheme 3 , binding constants K 32 and K 31 were calculated to be (9.26 ± 0.07) × 10 27 M −4 and (3.1 ± 0.13) × 10 15 M −3 , respectively, using global multivariate factor analysis [ 31 , 32 , 33 , 35 , 36 , 37 ]. On the other hand, the BIPY titration of a toluene solution of monomeric 2 resulted in a two-phase absorbance change, which appeared stable even after the addition of more than 3 equiv.…”

“…This implies that rigid BIPY forms a more robust self-assembled double-decker complex than flexible DPYP. Using the above stability constants of 1 ∙BIPY and 1 ∙DPYP, and the SPECFIT program, we simulated the speciation profile considering only three species ( 1 , 3:2 sandwiched complex, and 3:1 open structure) according to the reported methods [ 32 , 33 ], as shown in Figure 5 . It should be mentioned that for simplicity, we neglected other species (~2:2 or 4:2 model structures mentioned in [ 32 ]), due to their negligible content at micromolar concentration.…”

“…After each addition, a new UV-vis spectrum was obtained. The data obtained from the UV-vis spectrophotometric titrations were analyzed by fitting the entire spectral series at 1 nm intervals using the SPECFIT software [ 39 ], as mentioned in earlier reports [ 32 , 33 ]. To determine the binding constants, the titration results were fitted using the nonlinear regression method within the Origin 9 software and generated the following equation:…”

“…However, covalent metalloporphyrins, such as dendrimers, double-stranded conjugated ladder porphyrins, porphyrin arrays, double-decker porphyrins, cyclic porphyrins, and tetra-porphyrins, readily self-assemble without any external stimuli and form a larger aggregation of porphyrin arrays in solution [ 25 , 26 , 27 , 28 , 29 , 30 ]. Among the various supramolecular architectures, tris-metalloporphyrins assembled with N -donor ligands, such as 1,4-diazabicyclo[2.2.2]octane (DABCO), form sandwich-type coordination cages that are attractive for supramolecular catalysis applications and artificial molecular receptor synthesis [ 31 , 32 , 33 ].…”