Formation and optical properties of self-organized pentameric porphyrin arrays

Abstract

Principles of formation, electronic absorption and fluorescence spectra are reported for self-organized pentameric arrays of tetrapyrrolic macrocycles. In these arrays two molecules of Zn-porphyrin dimers, Zn(II)l,4-bis [5-(10,15,20+tri-p-hexylphenylporphyrinyl)]-benzene ((ZnHTPP) 2) are bound via one molecule of a tetrapyridyl-substituted free base of porphyrin or tetrahydroporphyrin. The process of self-assembly is based on the twofold coordination of the central Zn ions !n the dimer with the nitrogen atoms of the pyridyl rings in the free base which is strong enough to make the complexes stable at room temperature. The formation of the complexes can be followed by changes in the absorption bands of (ZnHTPP) 2 characteristic of an axial extra-ligation of Zn-porphyrins with pyridine or pyridyl-substituted compounds. The spectral behavior of the free bases in the pentads is determined by a non-planar distortion of their macrocycle caused by the two-point binding with the dimers. The fluorescence intensity of the Zn-porphyrin dimer decreases essentially upon complexation with the tetrapyridyl-substituted free bases. This quenching effect is assigned to a singlet-silaglet energy transfer from the complexed Zn-porphyrin dimers to the free base subunit in the pentad.

Ligand Exchange Dynamics and Temperature Effects upon Formation of Nanocomposites Based on Semiconductor CdSе/ZnS Quantum Dots and Porphyrins: Ensemble and Single Object Measurements

“…However, an orientation of the porphyrin macrocycle nearly perpendicular to the QD surface would be easily possible for meta and para N positions in pyridyl substituted porphyrins (see Figure 1A). Like for porphyrin self-assembled triads, [40] the importance of the designed two-point interacting domain is also demonstrated upon formation of "QD-Porphyrin" nanocomposites. This process is based essentially on a "Lego-type" key-hole principle effectively controllable via steric factors, distance matching, optimization of relative orientations, and solvent composition.…”

“…Like in the case of multiporphyrin complexes, [15,18,20,39,40] the selective replacement of meso-phenyl rings in tetra-phenyl porphyrins by pyridyl rings, H 2 P(Pyr) n as well as CuP(Pyr) 4 , offers the possibility for a controllable formation of "QD-Porphyrin" nanocomposites via a coordination "key-hole" principle. [20,46,48,49] It is well-known from chemical background that the 3d transition metal Zn 2+ ion has empty 3d 10 orbital while heteroatom N-pyr of the porphyrin meso-pyridyl ring is a very good e-donor having an unshared electron pair.…”

“…Synthesis and spectral details of meso-pyridyl containing porphyrins have been reported earlier. [39,40] In ensemble experiments, porphyrin stock solutions were prepared in toluene under ultrasonic treatment at 40°C at concentrations in the range (3÷30)·10 -5 M. For single particle experiments, sufficiently high photostable pyridyl functionalized perylene diimide molecules were used. The synthesis of these compounds is described elsewhere.…”

“…[27][28][29][30] Correspondingly, based on ideas of the self-assembly discussed above, the combination of the two directions, that is the anchoring of functional organic molecules (including tetrapyrrolic compounds and other heteromacrocycles) or molecular complexes or even proteins to QDs, is of considerable scientific and a wide practical interest including material science and biomedical applications. [31][32][33][34][35][36][37][38] Inspired by our earlier work on self-assembled multiporphyrin arrays [39][40][41][42][43][44][45] we have elaborated the experimental approach in the direct labelling of trioctylphosphine oxide (TOPO)-and amino (AM) -capped semiconductor quantum dots (QD) CdSe/ZnS with functional ligands (dyes) of two types (pyridyl substituted porphyrins and heterocyclic pyridyl functionalized perylene diimide molecules) in liquid solutions and polymeric matrixes. [46][47][48][49][50][51][52][53][54][55] We have shown that depending on redox and electronic properties of interacting subunits as well as anchoring groups (connecting organic and inorganic counterparts) the formation of "QD-Dye" nanocomposites allows for a controlled realization of mutually relative (spatial) orientations and electronic energy scales in order to optimize intended photoinduced processes such as charge transfer, [56,57] fluorescence Foerster energy transfer (FRET) [20,46,47,50,52] or electron tunnelling in the conditions of quantum confinement (non-FRET process).…”

Multistep photoinduced electron transfer in self-organised nanoscale porphyrin triads

“…Apart from covalent linking the desired subunits the non-covalent self-assembly of various kinds has attracted a lot of interest [7]. In this relation, using the complexation of Zn-porphyrin chemical dimers by pyridyl-substituted tetrapyrrole extra-ligands (via two-point extra-coordination) we have succeeded to form self-organised nanosized multimolecular tetrapyrrole assemblies in solutions and ÿlms [8,9].…”