This research was undertaken to obtain new "BODIPY" dyes that fluoresce at relatively long wavelengths. The title compounds 1a-e were prepared via a divergent route involving Suzuki couplings of arylboronic acids to N-tert-butoxycarbonyl-4-bromopyrrole 2, condensation of the products with an acid chloride, and incorporation of the boron difluoride entity. Two alkyl-substituted systems 7a and 7b were also prepared for comparison; the critical difference between structures 1 and 7 is that the former have an aryl group attached to each pyrrole nucleus whereas the latter only have alkyl substituents on that same ring. UV absorption and fluorescence emission data were compared for compounds 1 and 7. Absorption and fluorescence emission maxima for compounds 1 occur at higher wavelengths than for compounds 7, and the Stokes shifts for the aryl-substituted compounds 1 are larger than for the alkyl-substituted compounds 7. Fluorescence quantum yields measured for compounds 1 are less than for compounds 7, and possible reasons for this are outlined. Other physical data for the compounds were also collected. Oxidation and reduction potentials of the systems were obtained from cyclic voltammetry experiments, and a single-crystal X-ray structure determination was performed for the bisnaphthyl-substituted compound 1b.

cOver the last 15 years electron transfer activation has emerged as a valuable concept for accomplishing novel reactions and has €ound widespread application in the selective transformation of increasingly complex molecules. This review presents in a conceptualized manner the vast number of \ radical cation reactions obtained after chemical, electrochemical, and photoinduced one-electron oxidation. To familiarize the reader with the concepts of electron transfer a simple and straightforward classification of radical cation reactions has been devised to allow the presentation of a manifold . of reactions in a readily understandable manner. Whenever possible, thermochemical and kinetic data are provided.

Compounds based on the 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (BODIPY) framework are excellent fluorescent markers. When BODIPY dyes of this type are conjugated to functionalities that absorb at relatively short wavelengths, those functionalities can, in some molecules, transmit the absorbed energy to the BODIPY which then fluoresces. In such cases the BODIPY fragment acts as an acceptor while the other group serves as a donor. Energy transfer efficiencies in such donor-acceptor cassette systems must vary with the relative orientation of the two components, and with the structure of the linkers that attach them. This study was designed to probe these issues for the special case in which the linkers between the donor and acceptor fragments are conjugated. To do this, the cassettes 3-10 were prepared. Electrochemical studies were performed to provide insight into the degree of donor-acceptor conjugation in these systems. X-ray Crystallographic studies on single crystals of compounds 7 and 9 revealed the favored conformations of the donor and acceptor fragments in the solid state. Absorption, fluorescence, and time-resolved fluorescence spectra of the compounds were recorded, and quantum yields for the cassettes excited at the donor lambda(max) were measured. Fluorescence steady-state anisotropy data were determined for cassettes 3 and 9 to provide information about the mutual direction of the transition dipole moments.

Five new, constrained, aryl-substituted 4,4-difluoro-4-bora-3a, 4a-diaza-s-indacene (BODIPY) dyes (3f,g and 4h-j) were prepared and investigated to see if they have more favorable fluorescence characteristics than the unconstrained systems 2 that were prepared in previous studies. Dye types 3 and 4 have relatively rigid conformations caused by the heteroatom (3f and 3g) or ethylene bridge (4h-j) linkers that preclude free rotation of the substituted-benzene molecular fragments. In the event, the new dye types 3 and 4 have longer lambda(max abs) (620-660 nm) and lambda(max)(fluor) (630-680 nm) values than compounds 2. They also exhibit higher extinction coefficients (>100 000 M(-1) cm(-1), except for 3g). Their fluorescent quantum yields are high (up to 0.72 for 4j), with the exception of compound 3g, which has a quantum yield of only 0.05. The redox properties of dyes 3 and 4 have also been examined.

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