Charge separation in mesoporous aluminosilicates

EPR spectroscopy has been used to investigate spontaneous and/or photo-induced electron transfer between adsorbed organic molecules and the mesoporous aluminosilicate MCM-41 host. Spontaneous electron transfer occurs from the host to electron acceptor molecules with sufficiently favourable reduction potentials (TCNE, TCNQ, 1,4-benzoquinone, 1,4-naphthaquinone and 1,4-anthraquinone), provided the MCM-41 contains aluminium and the radical anion yield correlates with the aluminium content of the host. The semiquinone radical anions are interacting strongly with exposed Al³⁺ sites, whereas the TCNE and TCNQ radical anions are loosely bound and can be washed from the host. Radical cation formation is observed when electron donor molecules with favourable oxidation potentials are adsorbed in MCM-41 containing aluminium, and the radical cations formed interact with exposed Al³⁺ sites. This work shows that aluminium-containing MCM-41 contains both electron donating and electron accepting sites which may intervene in intra-molecular charge separation processes in adsorbed organic molecules.