Highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) tuning is an important consideration in the development of organic-based semiconducting materials. A study of the specific effects and overall trends for the HOMO-LUMO tuning of a diverse series of 9-fluorenones by means of extended conjugation and substituent effects is described. Trends were explored in a range of compounds, beginning with structures having highly electron-withdrawing substituents and progressing to structures having highly electron-donating substituents. Compounds with an incremental increase in conjugation were also examined. Electrochemical and optical measurements were used to calculate the HOMO-LUMO levels and HOMO-LUMO bandgap (HLG) for each structure. Results from both methods were compared and correlated with the differences in molecular structure. Increasing the electron-donating character of the substituents was observed to decrease the HLG and increase the energy levels of the HOMO and the LUMO, whereas an increase in the electron-withdrawing character produced the opposite results. Increasing conjugation decreased the HLG, increased the HOMO energy level, but decreased the LUMO energy level. Spectroscopic evidence of substituent influence on the carbonyl suggests that substituents directly impact the HLG by influencing the availability of nonbonding electrons within the carbonyl, which impacts the probability of an nπ* transition. The data presented not only elaborate on the HOMO -LUMO tuning of 9-fluorenone systems but also enable the consideration of 9-fluorenones as analogous models for HOMO-LUMO tuning in other more complex polyaromatic systems such as bifluorenylidenes. These trends may provide insight into developing materials with specifically tuned HLGs and HOMO-LUMO levels for a variety of applications.

We report the synthesis of the fluorescent 2'-deoxycytidine analogue 5-methylpyrimidin-2-one nucleoside, its incorporation at three specified sites in a single 60-nucleotide DNA molecule, and the use of its total and polarized intrinsic fluorescence to characterize RecA-DNA complexes. [reaction: see text]

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A highly efficient total synthesis of (+/-)-yohimbenone and a formal synthesis of (+/-)-emetine is described. The key element of the synthesis consists of a conjugate addition-dipolar cycloaddition of 2,3-bis(phenylsulfonyl)-1,3-butadiene with an appropriate oxime. The resulting cycloadducts are cleaved reductively to provide azapolycyclic scaffolds with strategically placed functionality for further manipulation to the target compounds.

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