Polyoxometalates (POMs) are an interesting class of macromolecular materials with excellent catalytic and electrochemical properties. Although several studies have focused on POMs, literature on their optical properties is limited owing to their negligible absorption in the visible region. Our research aim is to increase the optical activity of POMs via interacting them with chromophores. Meso-tetra(4-N-methylpyridyl)porphyrin (TMPyP) is a cationic dye exhibiting interesting photophysical properties such as visible absorption and fluorescence. As it is positively charged, it makes a good candidate to interact with the negatively charged POMs. Three different POMs, [H2W12O40]6- , [PW12O40]3- and [SiW12O40]4- were chosen for the study. Although all three investigated POMs share the same Keggin structure, they differ in the heteroatom that they incorporate, representing a range in redox potentials and also have different charge and thus they constitute a great choice to study the interaction with the TMPyP dye. The interaction of TMPyP with these three POMs was studied using steady-state and time-resolved optical measurements. Optical absorption measurements have shown a shift in absorption of TMPyP upon binding to the POMs, indicating ground state electronic interaction. Steady-state fluorescence measurements have shown the quenching of fluorescence of TMPyP, indicating excited state interaction. As the charges are different in POMs, different stoichiometric complexes are observed. Femtosecond transient absorption measurements were carried out on TMPyP with different POMs and interesting early time spectral and kinetic profiles were observed. By changing the pH of solutions, the electrostatic interaction of TMPyP with POM was disrupted leading to the retrieval of TMPyP in solution features. Observed differences in transient absorption features were ascribed to charge-transfer interactions between TMPyP and POMs. Three POMs have shown differing kinetic decay profiles indicating that the redox potentials of POMs govern the interactions.

Ultrafast excited state dynamics in Keggin polyoxometalate-cationic porphyrin architectures

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