At moderate temperatures in flowing gas, pentacene undergoes a disproportionation reaction to produce 6,13-dihydropentacene (DHP) and a series of polycondensed aromatic hydrocarbons, including the previously unknown peripentacene (PP). The process requires activation by heating to 320 degrees C and is possibly catalyzed by impurities such as DHP, 6,13-pentacenequinone (PQ), Al, or Fe found in the starting materials. These impurities also result in a decrease in the intrinsic field-effect mobility (FEM) of pentacene crystals. Subsequent purifications remove such impurities, thus inhibiting the formation of the disproportionation products and increasing the FEM of pentacene (2.2 cm(2)/Vs). These results clarify the importance of purification of semiconductive materials for measurements of intrinsic mobility and optimal device performance.

A new approach to the realization of the electrochemical DNA hybridization probe is described. It is based on the exchange of chloride ion between the polypyrrole layer and the buffer. The shape of the cyclic voltammogram is modulated by the negative charge density at this interface, resulting from the immobilized target DNA. The negative charge density increases when the complementary DNA hybridizes with the probe DNA. The hybridization event can be clearly seen in the cyclic voltammogram before and after the addition of the probe DNA. The immobilization is accomplished via the Mg2+ bridging complex between phosphonic acid groups of the poly[2,5-dithienyl-(N-3-phosphorylpropyl)pyrrole] grafted at the polypyrrole surface and the phosphate groups of the target DNA.

Housed within the 11-stranded β-barrel of the green fluorescent protein (GFP) is the arylideneimidazolidinone (AMI) chromophore, the component responsible for fluorescence. This class of small-molecule chromophore has drawn significant attention for its remarkable photophysical and photochemical properties, both within the intact protein and after its denaturation. All of the proteins so far isolated that have visible light fluorescence have been found to contain an AMI chromophore. These proteins comprise an extensive rainbow, ranging from GFP, which contains the simplest chromophore, p-hydroxybenzylideneimidazolidinone (p-HOBDI), to proteins having molecules with longer conjugation lengths and a variety of intraprotein interactions. The fluorescence invariably almost vanishes upon removal of the protective β-barrel. The role of the barrel in hindering internal conversion has been the subject of numerous studies, especially in our laboratories and those of our collaborators. A better understanding of these chromophores has been facilitated by the development of numerous synthetic protocols. These syntheses, which commonly use the Erlenmeyer azlactone method, have evolved in recent years with the development of a [2 + 3] cycloaddition exploited in our laboratory. The synthetic AMI chromophores have allowed delineation of the complex photophysics of GFP and its derivatives. Upon denaturation, AMI chromophores are marked by 4 orders of magnitude of diminution in emission quantum yield (EQY). This result is attributed to internal conversion resulting from conformational freedom in the released chromophore, which is not allowed within the restrictive β-barrel. To date, the photophysical properties of the AMI chromophore remain elusive and have been attributed to a variety of mechanisms, including cis-trans isomerization, triplet formation, hula twisting, and proton transfer. Advanced studies involving gas-phase behavior, solvent effects, and protonation states have significantly increased our understanding of the chromophore photophysics, but a comprehensive picture is only slowly emerging. Most importantly, mechanisms in structurally defined chromophores may provide clues as to the origin of the "blinking" behavior of the fluorescent proteins themselves. One approach to examining the effect of conformational freedom on rapid internal conversion of the chromophores is to restrict the molecules, both through structural modifications and through adjustments of the supramolecular systems. We thus include here a discussion of studies involving the crystalline state, inclusion within natural protein-binding pockets, complexation with metal ions, and sequestration within synthetic cavities; all of this research affirms the role of restricting conformational freedom in partially restoring the EQY. Additionally, new photochemistry is observed within these restricted systems. Many of the studies carried out in our laboratories show promise for these molecules to be adapted as molecular probes, wherein inclusion turns on the f...

To obtain insight into the effects of the environment on the photophysics and photochemistry of the green fluorescence protein (GFP), eight crystal structures of six synthetic aryl-substituted analogues (2-fluoro, 2-methyl, 3-hydroxy, 3-methoxy, 2,4-dimethyl and 2,5-dimethyl) of the GFP chromophore (4-hydroxy-benzylidenedimethylimidazolinone) were determined and correlated with their two-dimensional steady-state and time-resolved solid-state excitation-emission spectra. The stacking between the molecules greatly affected the emission energy and the lifetime of the emission of the chromophore, implying that pi-pi interactions could be critical for the photophysics of GFP. The reaction pathways were dependent on the excitation energy, resulting either in [2 + 2] photodimerization at the bridging double bond (UV excitation) or flipping of the imidazolone ring (visible excitation). The meta-hydroxy chromophore (3-HOBDI) was the only GFP-chromophore analogue that was obtained as more than one stable polymorph in the pure state thus far. Due to the asymmetric substitution with hydrogen bond donors and acceptors, 3-HOBDI is tetramorphic, the forms showing distinctly different structure and behavior: (1) while one of the polymorphs (3-HOBDI-A), having multilayer structure with alternating stereochemistry of linear hydrogen-bonded motifs, undergoes photodimerization under UV light, (2) another (3-HOBDI-C), which has dimeric head-to-tail structure, shows Z-to-E isomerization via tau-one-bond flip of the imidazolone ring by excitation in the visible region. X-ray diffraction analysis of a partially reacted single crystal of 3-HOBDI-C provided the first direct evidence of tau-one-bond flip occurring in a GFP-like compound. Moreover, the cooperative action of the photodimerization of 3-HOBDI-A appears as a photomechanical effect of unprecedented magnitude for a single crystalline specimen, where photoexcited single crystals bend to more than 90 degrees without breaking.