Femtosecond Dynamics Within Nanotubes and Nanocavities of Mesoporous and Zeolite Materials

The steady-state, picosecond and femtosecond time-resolved emission of Sudan I within MX (M = Na+, Li+, Mg2+), NaY zeolites, and MCM-41 mesoporous silica have been studied in the presence of solvents. The results show that the rate constant of the initial excited-state intramolecular proton-transfer (ESIPT) reaction in the trans-azoenol (trans-AZO) structure to give the trans-ketohydrazone (trans-HYZ) form of Sudan I remains ultrafast (tau < 50 fs) as in pure solution. However, within these nanohosts, the time necessary for a trans-cis isomerization in the formed trans-HYZ tautomer to yield the cis-HYZ structure increases. The largest effect was obtained when the MX hosts contained polar and H-bonding solvents (water, methanol). The result is explained in terms of a stronger solvation of the metal cation enhancing its shielding and therefore lowering its interaction with the included dye. The use of different metal cations of MX zeolite shows that nanosolvation of cations (M+) increases the measured times for isomerization and relaxation of the excited structures of Sudan I. Furthermore, the observation of similar times for NaX, NaY, and MCM-41 (hosts different in polarity and diameter of the pore/cavity) suggests that that the trans-cis isomerization occurs by an inversion mechanism. The slowing of the dynamics within the nanopores is reminiscent of that occurring in biological media, such as within proteins, and it should be relevant to photonic devices using this kind of nanomaterials.