A compound of the formula (I) wherein R1 and R2 independently of one another are hydrogen, C1-C22alkyl, -O, -OH, -CH2CN, C1-C18alkoxy, C2-C18alkoxy substituted by -OH; C5-C12cycloalkoxy, C3-C6alkenyl, C3-C6alkenyloxy, C7-C9phenylalkyl unsubstituted or substituted on the phenyl by 1, 2 or 3 C1-C4alkyl; or C1-C8acyl; and R3 and R4 independently of one another are C1-C22alkyl or a group of the formula (Ia) wherein R0 has one of the meanings of R1 and R2.本發明提供一種式(I)化合物:其中R1及R2彼此獨立地為氫、C1-C22烷基、-O、-OH、-CH2CN、C1-C18烷氧基、由-OH取代之C2-C18烷氧基;C5-C12環烷氧基、C3-C6烯基、C3-C6烯氧基、未經取代或在苯基上由1、2或3個C1-C4烷基取代之C7-C9苯基烷基;或C1-C8醯基;且R3及R4彼此獨立地為C1-C22烷基或式(Ia)之基團其中R0具有R1及R2之含義之一。