Complexation and Interchromophoric Interactions in Self-Organized Porphyrin and Chlorin Triads

Abstract

Spectral properties and equilibrium constants of multimolecular complexes (triads) formed by 2-fold coordination of dipyridyl-substituted free bases of porphyrin, chlorin, or tetrahydroporphyrin with Zn-porphyrin and Zn-chlorin dimers bridged by either of two different spacers between the monomeric entities have been studied in methylcyclohexane at room temperature. The ability of the dimers bridged by a -CH 2 -CH 2 -spacer to form complexes with the free bases is found to be much lower than that for the dimers with a phenyl spacer having complexation constants up to K C ) 5 × 10 7 M -1 . The complexation equilibrium is also affected strongly by the chemical nature of the free base and the position of the pyridyl substituents. Spectral effects occurring upon complexation in absorption spectra of the dimers are mainly due to ligation effects. Strong quenching of the dimer fluorescence in the complexes is attributed to effective singletsinglet energy transfer (ET) to the ligands.

Formation and optical properties of self-organized pentameric porphyrin arrays

“…Similar to the results of our study on triadic systems [23,25], titration of a (ZnHTPP) 2 solution with a solution of a tetrapyridyl-substituted free base was expected to result in the formation of complexes containing two dimer molecules both of which are attached to one molecule of free base via two-point coordination bonds as shown in Fig. 1.…”

“…2) matches the N-N distance between active sites in the free bases l (l = 10.0-15.1 ~, Fig. 2) as well as to the fact that their geometry is suitable for two-point coordination [25]. Additionally, the structure of the dimers and the free bases is flexible enough to compensate for small mismatches (of the order of 2 • ~) existing for some of the dimer-ligand pairs [25].…”

“…Synthesis and identification of the Znporphyrin dimer, Zn(II)l,4-bis [5-(10,15,20-tri-phexylphenylporphyrinyl)]benzene or (ZnHTPP)2 are described in Ref. [25]. All the pyridyl-containing compounds (H2THP(mPyr)4, H2P(pPyr)4 and H2P(mPyr) 4) used for complexation of (ZnHTPP) 2 have been synthesized and purified according to known methods [26][27][28][29][30].…”

“…Recently we have applied a key-hole mechanism which is based on a modification of the abovementioned coordination method [22] and provides high complexation constants (= 107 M -l) for the construction of self-organized heterogeneous trimeric porphyrin assemblies [23][24][25]. In these aggregates Zn-porphyrin or Zn-chlorin dimers are coordinated at two points (i.e.…”

“…In these aggregates Zn-porphyrin or Zn-chlorin dimers are coordinated at two points (i.e. both the Zn ions in the dimer subunits are coordinated simultaneously by the same free base) by the nitrogen atoms of two pyridyl substituents of tetrapyrrolic molecules [23][24][25]. Phenyl-bridged chemical dimers of Zn-porphyrins have been shown to facilitate the complex formation (complexation constants up to 5 × 10 7 M-~).…”