The invention provides a compound of formula (I); or a salt or solvate thereof, wherein: R1 is hydrogen or C1-4 alkyl; R2 and R4 are each independently selected from C1-4 alkyl, [11C]C1-4 alkyl, and [18F]C1-4 fluoroalkyl provided that at least one of R2 and R4 is [11C]C1-4 alkyl or [18F]C1-4 fluoroalkyl; and R3 is halo. Such compounds having use for imaging central nervous system receptors.
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