Azobenzene undergoes trans→cis isomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis→trans isomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.

Two new fluorescent sensors for Zn(2+) that utilize fluorescein as a reporting group, Zinpyr-1 and Zinpyr-2, have been synthesized and characterized. Zinpyr-1 is prepared in one step via a Mannich reaction, and Zinpyr-2 is obtained in a multistep synthesis that utilizes 4',5'-fluorescein dicarboxaldehyde as a key intermediate. Both Zinpyr sensors have excitation and emission wavelengths in the visible range ( approximately 500 nm), dissociation constants (K(d1)) for Zn(2+) of <1 nM, quantum yields approaching unity (Phi = approximately 0.9), and cell permeability, making them well-suited for intracellular applications. A 3- to 5-fold fluorescent enhancement is observed under simulated physiological conditions corresponding to the binding of the Zn(2+) cation to the sensor, which inhibits a photoinduced electron transfer (PET) quenching pathway. The X-ray crystal structure of a 2:1 Zn(2+):Zinpyr-1 complex has also been solved, and is the first structurally characterized example of a complex of fluorescein substituted with metal binding ligands.

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