Singlet and Triplet Exciton Diffusion in a Self‐Organizing Porphyrin Antenna Layer

Abstract

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), 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 singlet 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–DL) 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.

Metal‐Free Porphyrin‐Sensitized Mesoporous Titania Films For Visible‐Light Indoor Air Oxidation

“…Kroeze et al reported that free electrons in the conduction band (CB) of the TiO 2 are generated as a result of the photon absorption of an antenna layer (porphyrin dyes). [39] According to the above data analysis, the following mechanism is suggested. Within this 3D TPPS/TiO 2 framework, TPPS absorbs visible light to induce a p-p* transition; the excited-state electrons from the p*-orbital are then readily injected into the d-orbital conduction band (CB) of TiO 2 and subsequently transferred to the TiO 2 surface where they react with molecular oxygen to yield O 2 ·À radicals, which will then be involved in the overall oxidation of CH 3 CHO (Figure 6 and [Equations (1-5)]).…”

Photosensitization and the Photocurrent Switching Effect in Nanocrystalline Titanium Dioxide Functionalized with Iron(II) Complexes: A Comparative Study

“…Keywords: electrochemistry · electron transfer · photochemistry · semiconductors · titanium dioxide case diverse organic dyes (porphyrins, [36][37][38][39][40] phthalocyanines, [41][42][43][44][45] thiacarbocyanine dyes, [46,47] various natural dyes [48][49][50][51][52][53][54][55][56] ) and metal complexes [49] undergo photoexcitation with visible light and inject electrons into the conduction band ( Figure 1b). Chemisorption of chromogenic molecules onto the TiO 2 surface results in formation of ligand-to-metal or metal-to-metal charge transfer (LMCT and MMCT, respectively; more precisely referred to as ligand-to-particle or metal-to-particle charge transfer; LPCT and MPCT) involving surface Ti IV ions and surface-bound ligands or transition-metal complexes, respectively.…”

Redox‐Controlled Photosensitization of Nanocrystalline Titanium Dioxide

“…[28,37,38] An inspiration for light-harvesting molecules and antenna systems capable of titania photosensitization comes from various biosystems. [39] There are numerous trials of solar-cell construction which are based on biomolecules and supramolecular systems; for instance, porphyrins, [40][41][42][43][44] phtalocyanines, [45] thiocarbocyanine dyes, [46,47] natural dyes, [48][49][50][51][52][53][54] etc. In every case, the redox-active molecules supply electrons from their excited states.…”