No 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.

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.

On the basis or experimems Ihe oscillalor model has been cslnblished Tar individual NH vu~omers or porphyrins with asymmetrical subslirution. CNDO/Z calculations rxplain the inversion of' Q,,(O.O) and Q_,.(O.O) electronic transition intensities in NH !auIomers as a consequence 01 Ihe inversion or LUMO coefficicnu c, and c2 [or fixed x and _v molecular oscillators.

Supramolecular ensembles stable at room temperature (complexation constant and activation energy range from 5 106 1 to 5. i07 NI' and from 0.5 to 1.0 eV correspondingly) containing up to five macrocyclic fragments have been constructed using two-fold ligation of Zn-porphyrin and Zn-chlorin chemical dimers by pyridyl substituted porphyrin or related molecules. Spectral, photophysical and thermodynamic properties oftriadic and pentadic arrays have been studied in a temperature range from 140 to 360 K. Kinetic behavior of the complexes s investigated using a fluorescent picosecond laser setup (zt30 ps) with 2-D (velengthlifetime) registration. Observed spectral properties are explained in terms ofextra-ligation (red shift ofall electronic bands 550 cm 1) and excitomc splitting (AE<1900 cni1). Noriradiative for-and backrd excitation energy transfer (K>1010 c'), electron transfer and d-it interactions are discussed as the main paths ofelectronic excitation deactivation in the complexes.