Optimizing of 2,3-Diarylindenes As Fluorescent Estrogens: Variation of the Acceptor Group, Ortho Substitution of the 2-Ring, and C-1 Methylation

In an attempt to elucidate steric and electronic factors that affect the fluorescence and estrogen receptor binding properties of 2,3-diarylindenes, we have prepared and examined the behavior of 11 analogues bearing substituents on the 1-position or on the 2-aryl ring. These compounds were synthesized by alkylation of a 1,2-diarylethanone with 3-methoxybenzyl chloride, followed by cyclodehydration to the indene. The electronic spectra of those compounds without π-electron accepting groups on the 2-aryl ring display the absorbance and fluorescence of a hindered stilbene system; those with nitro and cyano substituents on the 2-aryl ring show charge-transfer character, having a more bathochromic absorption and fluorescence. One bisphenolic nitroindene, in particular, shows a strong, long-wavelength absorption and an intense emission, with a large Stokes' shift that is highly sensitive to solvent polarity. Estrogen receptor binding affinity measurements on these compounds indicate that substituents that twist the pendant aryl rings (such as a 1-methyl group, or an o-methyl or trifluoromethyl group on the 2-phenyl ring) increase binding affinity. Bulky (4-bromo) or electron-withdrawing groups (3- and 4-nitro, 4-cyano) on the 2-phenyl group, or its replacement with a 3-pyridyl group, greatly reduce binding affinity, suggesting that the complementary region of the receptor is relatively intolerant of bulk and may have specific hydrogen-bonding requirements. This investigation of the concurrent effects of substituents on the fluorescence properties and receptor binding affinity of 2,3-diarylindenes should assist in the development of effective, inherently fluorescent ligands for the estrogen receptor.