2,3-Diarylindenes are ligands for the estrogen receptor which display intrinsic fluorescence. In order to optimize the receptor binding affinity of these compounds while preserving their desirable fluorescence properties, a series of torsionally modified analogues were prepared. A fluorine or methyl group was introduced on either of the two phenyl substituents ortho to their attachment site to the indene nucleus, in order to increase the out-of-plane twist of the appended rings. The analogues were prepared by the benzylation of appropriate deoxybenzoins, followed by Friedel-Crafts cyclic alkylation-dehydration. Comparison of the X-ray crystal structure of one analogue with unsubstituted analogues confirms the torsional perturbation effected by the ortho substituent. The torsional disposition of the C-2 aryl group in the substituted diphenylindenes is further investigated by UV (absorbance maxima and molar absorptivities), fluorescence (Stokes' shift), and NMR (chemical shifts). These spectroscopic measurements indicate increasing twisting between the C-2 aryl substituent and the indene system according to the following order: 3-ring o-Me-indene 9f less than diphenylindene 15 = 20 degrees less than 3-ring o-F-indene 9c less than 1-Me-indene 16 less than 2-ring o-F-indene 9b less than 2-ring o-Me-indene 9e = 63 degrees. The binding affinity of these analogues to the estrogen receptor was evaluated by a competitive radiometric receptor binding assay. While o-fluoro or o-methyl substitution on the 3-ring increases binding only slightly, binding of the o-fluoro 2-ring analogue is increased ca. 6-fold and the o-methyl analogue 11-fold, giving, in the latter case, a compound with an affinity equivalent to that of estradiol. The increase in binding affinity afforded by ortho substitution correlates with the increase in the torsion angle of the C-2 aryl ring. A thermodynamic evaluation of the receptor fit (Andrews, P. R.; Craik, D. J.; Martin, J. L. J. Med. Chem. 1984, 27, 1648) indicates that, for the o-methyl 2-ring analogue, the effect of the ortho substitution on increasing receptor binding appears to be a combination of increased surface area due to the substituent itself, together with a change in surface area of the ligand that results from the increased torsion of the two aryl rings. An o-fluoro substituent on the 2-ring provides a compromise between the relative planarity required for high fluorescence intensity and the molecular shape needed for increased estrogen receptor binding affinity.(ABSTRACT TRUNCATED AT 400 WORDS)