a b s t r a c tThe ground and excited state behaviors of a biologically important mitochondria selective dye, 5,5 0 ,6,6 0 -tetrachloro-1,1 0 ,3,3 0 -tetraethyl-benzimidazolo carbocyanine (TTBC), were investigated in order to give useful information related to the enhancement of its electronic and optical properties. DFT and TDDFT levels of theory with different functionals and basis sets were used. The results obtained by using PBEPBE/6-31+G(d,p)/C-PCM are in good agreement with the experiments. The potential energy surface sections of TTBC were explored and the geometry optimized in the first excited state, S 1 , to analyze the fluorescence spectrum of the dye. There are two distinct minima on the S 1 surface, one located near the ground state geometry and an energetically favorable twisted geometry, having a charge transfer character. The observation of similar ground and excited state dipoles as well as solvent-independent fluorescence and absorption wavelengths reveals that there is no competition between the two minima; the emission is dominantly coming from the more planar configuration.
a b s t r a c tThe effects of functional groups on the benzimidazole rings, length of the conjugated chain and alkyl groups bonded to the nitrogen atoms on the ground and excited state behaviors of the 1,1 0 ,3,3 0 -tetraethyl-5,50 ,6,6 0 -tetrachlorobenzimidazolocarbocyanine (TTBC or JC-1) have been analyzed via quantum chemical methods. DFT and TDDFT with B3LYP/6-31G(d,p) level of theory were used for the ground and excited state calculations, respectively. It has been found that TTBC has a very rigid geometry; no significant effect of functional groups has been predicted either as donor or acceptor on its optimum structure. However, the length of alkyl groups changes the structure of the molecule. It is possible to increase k max of TTBC based carbocyanine dye with NH 2 , butyl/propyl and increasing polymethine chain length.
ABSTRACT:The synthesis of donor-(D) and/or acceptor (A)-expanded [4]radialenes has been developed on the basis of readily available dibromoolefin (7), tetraethynylethene (10 and 20), and vinyl triflate (12) building blocks. The successful formation of D/A radialenes relies especially on (1) effective use of a series alkynyl protecting groups, (2) Sonogashira crosscoupling reactions, and (3) the development of ring closing reactions to form the desired macrocyclic products. The expanded [4]radialene products have been investigated by spectroscopic (UV−vis absorption and emission) and quantum chemical computational methods (density functional theory and time dependent DFT). The combined use of theory and experiment provides a basis to evaluate the extent of D/A interactions via the cross-conjugated radialene framework as well as an interpretation of the origin of D/A interactions at an orbital level. ■ INTRODUCTIONConjugated carbon-rich macrocycles are intriguing targets of study for both theoretical and experimental chemists because of their often symmetrical shape and aesthetically appealing structures. They are, however, useful molecules as well, with potential as the organic component for electronic, optical, and nonlinear optical applications.1−8 A specific subcategory of conjugated macrocycles are the [n]radialenes, which are cyclic, carbon-rich molecules with a general formula C 2n H 2n that contain n ring atoms and n exocyclic double bonds (1, Figure 1). "Expanded radialenes" are derivatives of radialenes that originate by formal insertion of an unsaturated spacer between each pair of exomethylene fragments of a radialene, giving rise to macrocycles such as 2 and 3 (Figure 1). 8−12 Work with expanded radialenes was pioneered by Diederich and co-workers 13−16 via the introduction of diacetylene moieties into the radialene framework to give derivatives with the general structure 3 17,18 as well as structurally related radiannulenes.19−27 More recently, expanded radialenes 2 composed of repeating enyne units have been realized and studied. 28−30The two-dimensionally (2D) conjugated structure of expanded [n]radialenes is rather special because it combines a number of linearly and cross-conjugated pathways placed on a nonbenzoid carbon framework.31−39 One key question concerning the properties of expanded [n]radialenes has been the role played by cross-conjugation 9,40,41 to the overall electronic makeup of these unique molecules. The influence of D−A or D/A interactions 42 via cross conjugation in acyclic systems has been explored by a number of groups. 9,10,43−46 To date, however, only a few examples of donor-or acceptor-expanded radialenes have been reported. 18,28 Finally, donor−acceptor-expanded radialenes remain unknown, and there have been no attempts to document cross-conjugated interactions in these derivatives using theory.Our group has recently reported a modular approach for the synthesis of perphenylated expanded [n]radialenes 29 and radiaannulenes. 22,29 This approach has been especially useful ...
a b s t r a c tIn this study, we investigated the effect of the second amino acid identity of hexapeptides on gas-phase structures and the proton affinities of N-terminal proline containing b 2 + ions produced from the fragmentation of b 6 + ions under low-energy collision-induced dissociation (CID) tandem mass spectrometry (MS/MS). It should be noted that, among all other fragments, the b 2 + and nominallywere mainly considered in this study. This is a unique example of consecutive cleavage of b 6 + ions which fragments to b 2 + and nominal b 4 + ions. All structural and proton affinity calculations for b 2 + ions were carried out with the B3LYP/6-31+G(d,p) level of theory. The study utilized C-terminal amidated model peptides consisting of PAAAAA-NH 2 and PXAAAA-NH 2 where X is phenylalanine (F), glutamic acid (E), tryptophan (W), and histidine (H) residue. Two main structural isomers of b 2 + ions, namely oxazolone and diketopiperazine, have been considered for the computations. The results demonstrated that the proton affinities of oxazolone isomers of PX are greater than its diketopiperazine isomers. Higher correlation coefficient is calculated if the structure of PX is considered as oxazolone rather than diketopiperazine isomer. Additionally, a linear fit is observed between intensity ratio (PX/AAAA) and calculated proton affinities of PX ions. Additionally, MS/MS results revealed that the relative intensities of b 2 + -PA, PF, and PE-ions are lower compared to the relative intensity of AAAA fragment ion. In contrast, b 2 + -PW and PH-ions have higher relative intensities compared to the AAAA ion. This behavior is explained by the proton affinities of fragment ions computationally.
Two methods involving the rhodium-catalyzed reaction of 2-en-4-yne acetates and the palladium-catalyzed reaction of 2-en-4-yne carbonates with organoboronic acids were investigated; both afforded exclusively the (E)-configured vinylallenes. The coordinative interaction of the rhodium with the acetate group promoted the δ-elimination of Rh(I)-OAc from the alkenylrhodium intermediate II in both syn and anti modes, with the syn-elimination being the major path. DFT calculations revealed that a conformer of this intermediate (II), which can lead to the (E)-configured vinylallene product via the syn-elimination mode, is energetically the most favorable conformer. The rhodium-catalyzed procedure is not applicable to reactions involving (E)-configured enyne acetates, because the geometry of the alkenylrhodium intermediate that is derived from the corresponding (E)-enyne acetate would not allow such coordinative interaction to occur. The palladium-catalyzed method, which proceeds through formation of the σ-vinylallenylpalladium intermediate, B, is suitable for both the (E)- and (Z)-configured enyne carbonates and appears to have a wider scope for both organoboronic acids and enyne substrates. The palladium-catalyzed reaction of an enantiomerically enriched enyne carbonate proceeded with racemization.
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