Singlet and triplet exciton diffusion in a self-organizing porphyrin antenna layer

We have studied the photoinduced charge separation in a double layer consisting of a 50 nm thick, discotic antenna layer, i.e., free-base tetra-para-octylphenyl porphyrin (H2TOPP)a spin-coated onto a smooth layer of anatase TiO2 using the electrodeless "flash-photolysis time-resolved microwave conductivity" technique. (FP-TRMC): This method:enables the investigation of the relationship between the morphology of the antenna layer and the exciton diffusion dynamics. Photons absorbed by the porphyrin result in the formation of free mobile electrons in the conduction band of the TiO2. The freshly spin-coated double layer shows an incident-photon-to-charge-separation efficiency (IPCSE) of 10% on excitation at 430 nm. From the amplitude and the temporal shape of the photoconductivity transients we conclude that for the freshly spin-coated double layer, electron injection occurs both from the ringlet and the long-lived triplet excited state. These. triplet states can-travel by,(Dexter) energy transfer. over distances of at least 9.6,nm. After heating the; sample above the, crystalline-discotic-lamellar (C-D-L) phase-transition temperature of H2TOPP and subsequent-cooling, changes-in the optical absorption spectrum are observed, while the IPCSE drops to ca. 1%. These results are explained in terms of a transition of the morphology of the antenna layer from a predominantly random distribution of the porphyrin molecules just after spin-coating to. a lamellar arrangement upon annealing, consisting of domains of ordered porphyrin stacks. Since the porphyrin macrocycles are perpendicularly oriented with respect to. the plane of the lamellae, the center-to-center-distance between neighboring lamellae disables efficient triplet energy transfer, leaving only the lamella directly adjacent to the TiO2 layer photoactive.