Design, synthesis and evaluation of fluorescent molecules for optical sensing applications

Results on two separate projects will be discussed. The first project has resulted in the identification of two fluorescent photochromic spiropyrans which reversibly bind certain metal ions in solution. This represents an improvement upon other fluorescent metal ion sensors which are not conveniently reversible. These spiropyrans display complex thermochromism, photochromism, and metal ion-dependent changes in fluorescence emission properties. In the presence of certain Group IIb and alkaline earth metal ions, the fluorescence emission of these molecules increases up to 20-fold upon binding. Reversible detection of nanomolar levels of Zn$\sp{2+}$ and Hg$\sp{2+}$ has been achieved using these spiropyran-based fluorescent sensors and, to our knowledge, one of these spiropyrans is the first example of a photoreversible fluorescent metal ion sensor. Second, in an effort to improve upon the spectroscopic characteristics of donor-acceptor tetrahydrochrysene systems that were previously prepared as fluorescent ligands for the estrogen receptor, we have synthesized two aza analogs as receptor ligand prototypes and characterized their absorbance and fluorescence spectra. The UV spectra of these heterocycles were quite insensitive to solvent polarity, but showed marked red shifts under acidic or basic conditions. These heterocycles displayed complex fluorescence spectra, with emission bands around 400 nm in aprotic medium that shifted to longer wavelength in protic solvents. In many cases, acid and base caused strong red shifts in the emission, with large alterations in quantum yield; in some cases emission bands were shifted as far as 600 nm. Thus, these two azatetrahydrochrysenes show greater environmental sensitivity than do the previously reported donor-acceptor fluorescent ligands