Design of integrated fluorescent estrogens: The second donor effect on absorption, fluorescence, and ground-state molecular orbital properties of trans-4,4′-methoxynitrostilbene systems

In the design of inherently fluorescent ligands for the estrogen receptor, biological constraints limit to hydroxyl the functional groups that can be utilized as electron donors. In an attempt to optimize the fluorescent properties of such probes, we have studied the effect that a second alkoxy donor has on the fluorescence properties of two systems, peripherally located in a nitro-substituted diarylindene, and centrally located in a nitrostilbene. The second donor causes bathochromic shift in the absorbance spectrum (stilbene greater than diarylindene); however, the Stokes shift and fluorescence solvatochromism are less in the dual donor systems compared to the corresponding monodonor system. The fluorescence solvatochromism is investigated by using a model for solvent dispersive and dipole interactions. The relevant parameters are evaluated as follows: molecular geometry is obtained from X-ray crystallography, molecular mechanics, or semiempirical MO methods; molecular volumes are determined by molecular graphics; ground-state dipole moments calculated by the AM1 method; the noncollinearity of the ground- and excited-state dipoles is considered. This analysis rationalizes the effect of the second donor on reducing the fluorescence solvatochromism and demonstrates that the magnitude of this reduction depends on the conjugative contact of the second donor, being greater in the stilbene system than the diarylindene. Such an approach may be useful in the design of other fluorescent spectroscopic probes.