The influence of the chemical composition and silylation of mesoporous MCM-41 materials on the photochromic
behaviour of adsorbed spiropyran (BIPS) and 6-nitrospiropyran was studied. Upon incorporation, the spiropyrans
underwent ring opening to form either zwitterionic merocyanine or its corresponding O-protonated form. In all
silica MCM-41 or in the MCM-41 containing aluminium, the O-protonated merocyanine was predominantly
formed. In the case of MCM-41 modified by silylation of the OH groups, a mixture of zwitterionic merocyanine
and spiropyran was present. The photochromic response was studied by means of steady-state irradiation and by
laser flash photolysis. Steady-state irradiation (λ > 450 nm) of the solid samples gives rise in all cases to an intensity
decrease of the absorption bands corresponding to either the protonated or the unprotonated merocyanine form
(reverse photochromism). In contrast, laser flash photolysis at 308 nm of spiropyrans supported on silylated
MCM-41 allows observation of the photochemical ring opening of residual spiropyran to the corresponding
zwitterionic form (normal photochromism).