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While structure-property relationships are commonly developed in applications of physical organic chemistry to real-world problems at the graduate level, they have not been generally emphasized in the undergraduate chemistry curriculum. For instance, the ability to modify the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in p-conjugated small molecules and polymers is of particular interest in the fields of polymer chemistry and materials science, where small-gap systems can function as semiconductors in electronic devices. In order to demonstrate a general strategy for tuning the HOMO-LUMO energy gap for p-conjugated materials, a new experiment was designed for the undergraduate organic laboratory course that combines synthesis with theoretical calculations for p-conjugated biphenyl systems with differing electronic properties to generate a simple structure-property study of functional group substitution on the biphenyl core. The synthesis of electron-rich and electron-poor biphenyl compounds was accomplished by students with a modified procedure from the literature that uses green Suzuki conditions to yield 4'-(tert-butyl)-[1,1'-biphenyl]-4-ol (1) and methyl 4'-(tert-butyl)-[1,1'-biphenyl]-4-carboxylate (2) with average yields of ~50% and ~60%, respectively. The students characterized their products with TLC, NMR, IR, and UV-vis (from a representative sample) spectroscopies. A comparison of the UV-vis absorption spectra from density functional theory calculations and experiment showed how a p-conjugated system's HOMO-LUMO gap was tunable with structural modifications to the core, which illustrated how physical organic principles could be used to understand the electronic properties of a p-conjugated system. [ABSTRACT FROM AUTHOR]

The molecular recognition properties of the nucleobases instruct the formation of complex three-dimensional architectures in natural and synthetic systems; relatively unexplored is their use as building blocks for π-conjugated materials where they might mutually tune electronic and supramolecular structures. Toward this goal, an introductory set (1a-d and 2a-d) of six purine-terminated and two pyrimidine-terminated π-conjugated oligomers has been synthesized and used to develop experimental electronic and photophysical structure-property trends. Unlike 2,2':5',2"-terthiophene (TTT) derivatives 2a-d, intramolecular charge transfer dominates oligomers 1a-d bearing a 4,7-bisthienylbenzothiadiazole (TBT) spacer due to the strong electron-accepting ability of its benzothiadiazole (BTD) ring. The resulting donor-acceptor-donor systems feature lower HOMO-LUMO gaps than the terthiophene-linked nucleobases (ΔEg ~ 1.8 eV vs 2.4 eV based on electrochemical measurements), and the lowest so far for π-conjugated molecules that include nucleobases within the π-framework. Experiments reveal a dependence of photophysical and electronic structure on the nature of the nucleobase and are in good agreement with theoretical calculations performed at the B3LYP/6-31+G** level. Overall, the results show how nucleobase heterocycles can be installed within π-systems to tune optical and electronic properties. Future work will evaluate the consequences of these information-rich components on supramolecular π-conjugated structure. [ABSTRACT FROM AUTHOR]

"Push-pull" purines have been synthesized by the introduction of electron-accepting functional groups (A = CN, CO2Me, and CONHR) to the heterocyclic C(8) position to complement typical electron- donating substituents at C(2) (D¹) and C(6) (D²). The donor-acceptor purines show significantly altered, and overall improved photophysical properties relative to their acceptor-free precursors (A = H); these include red-shifted (20-50 nm) absorption maxima, highly solvatochromic emission profiles (em λmax from 355-466 nm depending on substitution pattern and solvent) with excellent linear correlations between emission energy and solvent polarity (ETN), improved photochemical stability upon continuous irradiation, and enhanced (up to 2500%) fluorescence quantum yields. Comprehensive structure-property studies show how the absorption/emission maxima and quantum yields depend on donor and acceptor structure, relative donor position (C(2) or C(6)), and solvent (1 ,4-dioxane, dichloromethane, acetonitrile, methanol, and in some cases water). Further insight regarding electronic structure comes from a quantitative treatment of the solvent-dependent emission data (that provides Δμge values ranging from 1.9 to 3.4 D) and DFT (B3LYP/6-31 1 ++G**) electronic structure calculations. X-ray crystal structures of several derivatives showcase the molecular recognition capabilities of the donor-acceptor chromophores that overall have photophysical and structural properties suitable for applications in biosensing and materials. [ABSTRACT FROM AUTHOR]

The synthesis and unique reactivity of a series of arylsulfonate-based nucleophile assisting leaving groups (NALG) containing oligomeric ether units (including crown ethers) attached to the arylsulfonyl ring in the ortho orientation are described. The reactions of a variety of these ether-containing alkyl sulfonates with metal halides proceeded at substantially greater rates than electronically similar sulfonates. These ether-containing leaving groups also displayed marked selectivity for lithium halides relative to the corresponding sodium and potassium salts in nucleophilic displacement reactions. [ABSTRACT FROM AUTHOR]

A series of arylsulfonate nucleophile assisting leaving groups (NALGs) were prepared in which the metal chelating unit is attached to the aryl ring via an ether linker. These NALGs exhibited significant rate enhancements in halogenation reactions using metal halides. Studies with a NALG containing a macrocyclic ether unit suggest that rate enhancements of these nucleophilic halogenation reactions are facilitated by stabilization of charge in the transition state rather than through strong precomplexation with metal cation. In several cases, a primary substrate containing one of the new leaving groups rivaled or surpassed the reactivity of triflates when exposed to nucleophile but was otherwise highly stable and isolable. These and previously disclosed chelating leaving groups were used in 18F-fluorination reactions using no-carrier-added [18F]fluoride ion (t1/2 = 109.7 min β+ = 97%) in CH3CN. Under microwave irradiation and without the assistance of a cryptand, such as K2.2.2, primary substrates with select NALGs led to a substantial improvement (2-3-fold) in radiofluorination yields over traditional leaving groups. [ABSTRACT FROM AUTHOR]

A study of the role of base in the isomerization of manganese-coordinated conjugated alkynyl carbonyls to the corresponding allenyl carbonyls is described. The use of phosphine additives indicates that manganese requires a ligand prior to isomerization with amine bases. A series of amine bases were also examined for their efficacy in this isomerization reaction revealing a strong dependence on pKa. By contrast, potassium tert-butoxide led to rapid isomerization in the absence of added manganese ligand. [ABSTRACT FROM AUTHOR]

Arylsulfonates of hindered secondary alcohols are converted to the corresponding alkyl chlorides very rapidly and in good yields in the presence of titanium tetrachloride at low temperatures. These reactions proceed with exclusive retention of configuration. [ABSTRACT FROM AUTHOR]