A series of "long-chain" 1-(2-methoxyphenyl)piperazine derivatives containing an environment-sensitive fluorescent moiety (4-amino-1,8-naphthalimide, 4-dimethylaminophthalimide, dansyl) was synthesized. The compounds displayed very high to moderate 5-HT(1A) receptor affinity and good fluorescence properties. 6-Amino-2-[5-[4-(2-methoxyphenyl)-1-piperazinyl]pentyl]-1H-benz[de]isoquinoline-1,3(2H)-dione (4) combined very high 5-HT(1A) receptor affinity (K(i) = 0.67 nM), high fluorescence emission in CHCl(3), and undetectable fluorescence emission in aqueous solution. It was evaluated for its ability to visualize 5-HT(1A) receptors overexpressed in CHO cells by fluorescence microscopy.