prov:value
| - A photochromic compound according to claim 1, wherein R1 is C1-4 -alkyl, phenyl or benzyl; R2 and R3 are each selected from C1-5 -alkyl groups, phenyl groups or are combined to form a cyclohexyl group; each of the R4 groups is selected from the group consisting of hydrogen, C1-2 -alkyl, chlorine, fluorine, bromine, iodine, C1-2 -trihaloalkyl and C1-5 -alkoxy; and R5 is a hydrogen atom, a C1-4 -alk
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