<p>The invention relates to compounds of formula: (I) or a salt or solvate thereof, wherein: R<1> is -<11>CH2R<5> or [<18>F]-C1-4 fluoroalkyl wherein R<5> is hydrogen or C1-4 alkyl; R2 is hydrogen or C1-4 alkyl; R<3> is halo; and R<4> is halo, C1-4 alkylthio, or C1-4 alkyl; and their use for imaging central nervous system (CNS) receptors.</p>
