DABCO-Induced Self-Assembly of a Trisporphyrin Double-Decker Cage:  Thermodynamic Characterization and Guest Recognition

Abstract

This paper describes the thermodynamic characterization of the self-assembly of a Zn trisporphyrin induced by coordination with 1,4-diazabicyclo[2.2.2]octane (DABCO) to form a stable 2:3 double-decker molecular coordination cage that recognizes benzene-1,3,5-tricarboxamides. The self-assembly process has been studied using UV-vis and (1)H NMR spectroscopy and quantitatively characterized in terms of a single stability constant that describes the strength of the individual coordination interactions and two effective molarities (EM) that describe the additional stability imparted by intramolecular cyclization. The EM values of the two consecutive cyclic intramolecular interactions are very similar. At micromolar concentrations, the formation of the fully assembled coordination cage is highly favored over the formation of intermediate species stabilized by fewer interactions, and so self-assembly is an all-or-nothing process. In contrast, at millimolar concentrations, the relative stability of intermediate species increases, leading to a stepwise self-assembly process, and a 2:2 intermediate can be clearly identified using (1)H NMR spectroscopy. The molecular recognition of benzene-1,3,5-tricarboxamides by the cage was investigated using (1)H NMR spectroscopy. The tricarboxamides bind inside the central cavity of the cage complex, and isothermal titration calorimetry (ITC) allowed the quantification of the stoichiometry and binding affinities.

Challenges and breakthroughs in recent research on self-assembly

“…The double-stranded ladder complex with 4,4 -bipyridyl exhibited an order of magnitude larger optical nonlinearity per macrocycle than the corresponding uncomplexed oligomers. DABCO-induced self-assembly of trisporphyrin double decker cage was also demonstraeted by Ballester, Hunter, and coworkers [392].…”

Trimer Porphyrin Star

“…The real equilibrium in the system of the molecule with three coordination sites with bidentate ligand is quite complex, so the Scheme 3 represents the simplified version of the reaction suggested on the basis of the reported analogous porphyrin coordination dimers with DABCO. [31] The detailed photophysical studies of supramolecular coordination assemblies of the porphyrin star are in progress. …”

Spectroscopic investigations and theoretical calculations of DABCO induced xanthene bridged self-assembled zinc(II) porphyrin dimer

“…Lower downfield shifts in the range of d = 1.5-3.5 ppm are observed when the DABCO molecule acts as an axial ligand for only one Zn(II) metal, as tends to be the case when more than 1.0 eq. of DABCO is added to a solution containing a Zn(II) porphyrin host dimer molecule [8,13]. A DFT geometry optimization at the B3LYP/6-31G(d) level of theory was carried out.…”

“…Upon titration with the guest-molecules, the spectroscopic properties of the Zn(II) porphyrin host changes significantly due to the "host-guest" and "hosthost" interactions. Generally, Zn(II) porphyrin dimers can be readily formed from porphyrin monomers through inter-molecular coordination upon addition of 0.0-1.0 eq guest molecules [7], or polymerized by the host-guest molecule interaction at higher concentrations [8]. It is noteworthy that porphyrin dimers with differently-sized bridging spacers exhibit significantly different electronic structure [9].…”