Effect of Mesophase Structure on the Intermolecular Triplet Quenching of Photoexcited ZnTPP and CuTPP Mixtures

Time-resolved EPR detection of photoexcited Zn− and Cu−tetraphenylporphyrin (TPP) mixtures oriented in several thermotropic liquid crystals (LCs) is reported. Both the transient triplet EPR spectra of ZnTPP and the doublet ground state CW spectra of CuTPP were recorded and quantitatively analyzed via line shape analyses over a wide temperature range (130−370 K). It is shown that the photoexcited triplet state of ZnTPP is entirely quenched by the photoexcited CuTPP species exclusively in the fluid smectic phase of a multidomain LC (ZLI-1167), whereas in the other phases of this LC, as well as in other LCs (E-7 and ZLI-4389) there is no indication of any interaction between the different chromophores. The results are interpreted in terms of a ZnTPP−CuTPP transient complex formation, which depends on a favorable mutual orientation of the interacting chromophores, specifically imposed by the layers and multidomain structure of the smectic phase of ZLI-1167.