Dipole-dipole and exchange energy transfer in different types of porphyrin chemical dimers

Abstract

Experimental and theoretical studies of the singlet and triplet state deactivation in porphyrin and chlorin homo- (both free bases and metallocomplexes (Zn, Cu, Ni)) and heterodimers of various types in a wide temperature range have been carried out. Rigid dimers of cyclopentaneporphyrins as well as more flexible ethane-bisporphyrins and ethane-bischlorins with -CH₂-CH₂ — spacers have been investigated. First singlet/triplet states of the dimer subunits are involved in weak dipole-dipole/exchange interactions that result in pico-/nanosecond excitation energy transfer (ET). In homodimers at temperature lower than 77 K when essential spectral inhomogeneity exists and in heterodimers the ET is directed to the longer wave centers. Exchanged-π effects play the main role in excitation deactivation in heterodimers containing Cu ions. In heterodimers containing Ni ions, very fast energy transfer takes place to low-energyd-levels of Ni. Free bases of ethane-bisporphyrins have been shown to quench singlet oxygen by about an order of magnitude more effectively than the initial monomeric compounds. In the case of homo- and hetero-ethane-bisporphyrins containing metal ions, quenching efficiency depends both on the dimer composition and on the nature of the metal ions.