Electrostatic self-organization of robust porphyrin-polyoxometalate films

Strategies to create thin films using layer-by-layer methods use oppositely charged polymeric polyelectrolytes for both or at least one component to beneficially exploit multitopic electrostatic interactions between the deposited layers with opposite charges. In contrast, the electrostatic deposition of tetracationic 5,10,15,20-tetrakis(1'-methyl-4'-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP(4+)) with tetraanionic polyoxometalates such as EuPW(11)O(39)(4-) or SiW(12)O(40)(4-) onto charged substrates, such as mica, or polar substrates, such as glass and indium-tin oxide (ITO), demonstrates that the use of polymeric components is not a priori necessary. The use of molecules in sequential dipping approaches requires a careful balance in the interaction energies between the oppositely charged molecules, as demonstrated by the observation that a tetraanionic porphyrin such as 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin does not form layers with TMPyP(4+). In the present case, these systems require several rounds of dipping to obtain films of uniform coverage and durability. The thin films deposited onto glass, quartz, ITO, and mica are surprisingly robust, since they are not removed by sonication in either organic solvents or 100 mM NaCl.