Precisely nano-sized aluminosilicate particles (average diameter size 50 to 80 nm) with tailor-made internal surface polarity are suitable as host material for photochromic dyes of the chromene type in order to produce optical transparent photoswitchable polymer films. The sophisticated host material does accomplish two important requirements for a technical application, appropriate internal space for the light induced reversible switching process of the adsorbed dye and suppression of disturbing chemical reactions. The adjustment of the internal surface polarity of the host material was achieved by silane reagents which could be observed by means of specific solvatochromic probes

The synthesis of new 3H-naphtho[2,1-b]pyran dyes substituted at position 8 of the naphthalene ring by either a stilbazolium or an 4-ethenyl-quinolinium group and their photochemical properties are described. In non-polar media the naphthopyran-stilbazolium conjugates do not show any photochromic response while the formation of the ring opened species is promoted in strong polar media. Additional investigations of the new dyes within the pores of mesoporous MCM 41 particles showed a significant enhancement of the photochromism compared with that in solution. The generation of the ring-opened species upon irradiation with UV-light is accompanied by a decrease of the fluorescence intensity, which could be recovered by illumination with visible light

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