The boron dipyrrin (Bodipy) chromophore was combined with either a free-base or a Zn porphyrin moiety (H(2)P and ZnP respectively), via an easy synthesis involving a cyanuric chloride bridging unit, yielding dyads Bodipy-H(2)P (4) and Bodipy-ZnP (5). The photophysical properties of Bodipy-H(2)P (4) and Bodipy-ZnP (5) were investigated by UV-Vis absorption and emission spectroscopy, cyclic voltammetry, and femtosecond transient absorption spectroscopy. The comparison of the absorption spectra and cyclic voltammograms of dyads Bodipy-H(2)P (4) and Bodipy-ZnP (5) with those of their model compounds Bodipy, H(2)P, and ZnP shows that the spectroscopic and electrochemical properties of the constituent chromophores are essentially retained in the dyads indicating negligible interaction between them in the ground state. In addition, luminescence and transient absorption experiments show that excitation of the Bodipy unit in Bodipy-H(2)P (4) and Bodipy-ZnP (5) into its first singlet excited state results in rapid Bodipy to porphyrin energy transfer-k(4) = 2.9 × 10(10) s(-1) and k(5) = 2.2 × 10(10) s(-1) for Bodipy-H(2)P (4) and Bodipy-ZnP (5), respectively-generating the first porphyrin-based singlet excited state. The porphyrin-based singlet excited states give rise to fluorescence or undergo intersystem crossing to the corresponding triplet excited states. The title complexes could also be used as precursors for further substitution on the third chlorine atom on the cyanuric acid moiety.
Relative kinetic data were determined for the 5-endo-trig cyclization of radical 12 compared to hydrogen abstraction from (TMS)(3)SiH in the temperature range of 344-430 K, which allows for the estimation of a rate constant of 2 x 10(4) s(-)(1) at 298 K with an activation energy of ca. 9 kcal/mol for the cyclization process. The 5-endo-trig cyclization of a variety of radicals that afford five-membered nitrogen-containing heterocycles was addressed computationally at the UB3LYP/6-31G level. The 5-endo vs 4-exo mode of cyclication and the effect of delocalization of the unpaired electron in the transition state were investigated. Because the ring formed during cyclization contains five sp(2) centers, electrocyclization via a pentadienyl-like resonance form was also considered. For comparison, similar calculations were performed for 4-penten-1-yl and related radicals. The factors that affect the activation energies of homolytic 5-endo-trig cyclization were determined. In the absence of steric or conformational effects, the endo cyclization to form the five-membered ring was strongly favored over exo cyclization to form the four-membered ring not only on thermodynamic grounds but also kinetically. When a substituent on the double bond was able to delocalize the unpaired electron in the transition state of the 4-exo path, the two modes of cyclization became kinetically comparable. These results have an important bearing on the generalization of the Baldwin-Beckwith rules, which classified the 5-endo-trig radical cyclization as a "disfavored" process.
Various structural possibilities for the Si2C4 and Si4C2 clusters are investigated by employing a basis set of triple-zeta plus polarization quality; electron correlation is generally accounted for by second-order Mo/ller–Plesset and, in certain instances, by higher-order perturbation (CASPT2) approaches. The building-up principle recently suggested from an analysis of Si3C3 clusters is found to be fully operative for Si2C4 and Si4C2 clusters. A comparison of the structure and stability of various geometrical arrangements in the series C6, Si2C4, Si3C3, Si4C2, and Si6 shows that linear and planar structures become rapidly less stable if carbons are replaced by silicons and that the three-dimensional bipyramidal forms become less favorable as soon as silicons are exchanged by carbons in the parent Si6 structure. The effects can be rationalized in qualitative terms based on differences in silicon and carbon bonding.
Abstract. Various structural possibilities for Si 3 C 3 clusters are investigated by ab initio calculations employing basis sets of double-and triple-zeta quality augmented by d polarization functions. Correlation effects are included by a second-order Moeller Piesset perturbation treatment. For the two lowest-lying structures higher-order correlation corrections and multi-reference effects are also included. Bonding features are investigated by two different types of population analyses to obtain insight into the nature of chemical bonding. A total of 17 stationary points were investigated, 14 of which correspond to local minima and three being transition states. The energetically lowest-lying structures are: A "pyramidlike" structure with various multicenter bonds, followed by a es symmetric isomer closely related to the ground state Si 6 structure. Planar structures, favoured in small carbon clusters, lie higher in energy and are transition states. The lowest-lying triplet system is found to be the linear nonsymmetric Si -C-C-C-Si -Si structure, which is calculated to lie about 38 kcalfmole above the singlet ground state. A building-up principle based on bonding criteria is suggested for the occurence of the various structural possibilities.
he results obtained by a recently pmposed empirical potential for silicon which includes four-body re-arc -pared with the resulis of quantum-mechanical tight-binding calmlations. In paniculu, the ground-state energy and ~fmcmre of the Sin duster were canputed by both methods. By perfoming an equivalent calculation using d y up to three-body interactions we demonstrate that the four-body tem is abnollIlely necessary in order to achisve gmd agreement with the quantum method. Biswas R and Haman D R 1987 Pkys. Rev. B 36 6434 Tenoff J 1988 Pkys. Rev. B 38 9902 Mistriotis A D. Flytzanis N and Faranms S C 1989 Phys. Rev. B 39 1212 Chelikowsky J R, Glassford K M and Phillips J C 1991 Phys. Rev. B 44 1538 Menm M and Allen R E 1986 Pkys. Rev. B 33 7099 Menon M and Allen R E 1988 Pkys. Rev. B 38 6196 Mistriotis A D. Froudakis G E. Vendras P and Flytzanis N 1993 Pkys. Rev. B 47 10648 Harrison W A 1980 Elecrrom'c Snwfure nnd the Properria of S o l a (San Francisco, C A W H Freeman) Harrison W A 1981 Pkys. Rev. B U 5835 Fmudakis G E and Menon M unpublished Tmanek D and Schluter M 1987 Phys. Rev. B 36 1208 Kaxiras E 1990 Phyr. Rev. Led. 64 551 Laassonen K and Nieminen M 1990 J. Pkys. C: SoIidSkm Phys. 2 1509 Ioner R 0 and Gunnarssm 0 1989 Rev. Mod. Pkys. 61 689 JelsE D A, Swift B L. Rantala T T, Xia X and Genrge T F 1991 J . Ckem. Pkys. 95 8552
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