Isophlorins and its analogues bridge the structural features between porphyrins and annulenes. Invariably they are known to be unstable but can be stabilized by appropriate substituents in the core and periphery of a macrocycle. Solid-state characterization of these 20pi systems displays planarity for each macrocycle and even in their supramolecular arrangement due to C-H...F-C interactions.
A series of alkenylboranes were prepared by 1,1-carboboration routes and used as Lewis acid components for the generation of frustrated Lewis pairs (FLPs). The reactions of 1-alkynes with B(C6F5)3 gave the RCHC(C6F5)B(C6F5)2 systems 4a (R = n-C3H7), 4b (R = n-C4H9), 4c (R = Ph), and 4d (R = t-C4H9), respectively. The alkenylborane/tBu3P FLPs derived from compounds 4a–d reacted rapidly with dihydrogen (2.5 bar) at ambient temperature. The bulky system 4d left the CC double bond of the alkenylborane unsaturated and gave the dihydrogen cleavage product [tBu3PH][tBuCHC(C6F5)BH(C6F5)2] (10d). In contrast, the less bulky systems 4a/tBu3P and 4b/tBu3P split dihydrogen under these conditions and had their CC double bonds cleanly reduced to yield the corresponding 1-pentafluorophenylalkyl hydridoborate salts [tBu3PH][RCH2CH(C6F5)BH(C6F5)2] 9a (R = n-C3H7) and 9b (R = n-C4H9), respectively. The 4c/tBu3P FLP gave a mixture of both product types (9c/10c). 1,1-Carboboration of symmetrical internal alkynes gave the alkenylboranes R2CC(C6F5)B(C6F5)2 4e (R = C2H5), 4f (R = n-C3H7), 4g (R = Ph), and 4h (R = p-MeC6H4), respectively. The 4e–h/tBu3P FLPs cleaved dihydrogen under mild conditions but retained their CC double bonds to give the respective [tBu3PH][R2CC(C6F5)BH(C6F5)2] products (10e–h). Selected examples of these alkenylboranes undergo FLP reactions acting as catalysts for the hydrogenation of imines. Perhaps most remarkably, some of these alkenylboranes retain the CC double bonds under FLP/H2 reaction conditions and heterolytically split dihydrogen in the presence of the Lewis base DABCO and catalyze the hydrogenation of the electron-poor CC double bonds of diaryl-substituted enones.
Acid-catalyzed condensation of furan and pentafluorobenzaldehyde yielded conjugated macrocycles with six and eight furan rings. They represent systems similar to annulenes with 30pi and 40pi electrons. From their structural analyses, it was found that furan rings in both the molecules were inverted in an alternative fashion and displayed nontwisted conformations. [structure: see text].
b S Supporting Information S olution processable organic semiconductors have generated tremendous interest because of their applicability in organic microelectronics, light-emitting diodes, and photovoltaics. 1-6 Both p-type and n-type materials, with commensurate mobilities, are required for effective functioning of many of these devices. Significant progress has been made in the development of materials possessing high charge mobilities with materials based on heteroaromatic molecules, especially thiophenes. Whereas thiophene-based materials are often p-type materials, 7,8 it has been shown that the incorporation of fluorinated substituents as terminal functionalities causes these materials to exhibit n-type characteristics. 9-15 This behavior has often been attributed to the lowering of LUMO energy level, making electron injection easier and hence imparting n-type character to the molecule. 11,13 However, it is not clear whether the factors underlying stabilization of frontier orbital energy levels have correlations to the transport of the injected charges. We were interested in testing this issue with a molecular scaffold that satisfies the following design criteria: (i) changing the hydrocarbon substituent to a fluorocarbon substituent should have minimal effect on the frontier orbital energy level and (ii) the scaffold allows for tuning the frontier orbital energy level independent of the fluorocarbon substituent. For this purpose, we report here the design, syntheses, and charge transport characteristics of solution-processable systems based on cyclopenta[2,1-b:3,4-b 0 ]dithiophene.The cyclopenta[2,1-b:3,4-b 0 ]dithiophene, referred here as simply cyclopentadithiophene (CPD), unit can be said to be a combination of structural motifs found in fluorenes and oligothiophenes. The rigid fused ring structure in this molecule lowers the reorganization energy, a parameter that strongly affects the rate of intermolecular charge hopping and therefore the charge carrier mobility in organic semiconductors. 16 The CPD unit has two locations where substituents can be incorporated: (a) the R-positions in the thiophenes, where hydrocarbon or fluorocarbon substituents can be incorporated, and (b) the bridgehead position, where substituents can be incorporated to alter significantly the frontier orbital energy levels. In fact, it has been shown previously that variations at these positions in such fused thiophene structures can be used to tune the frontier orbital energy levels. [17][18][19] The molecules that satisfy our design criteria and potentially address the issue in hand are shown in Chart 1. The CPD core is functionalized at the bridgehead position with the electronwithdrawing carbonyl or dicyanomethylene functionality, whereas the R-positions are substituted with either phenyl or pentafluorophenyl moieties. Installation of electron-withdrawing functionalities, such as carbonyl group, at the bridgehead position of the CPD leads to lowering of bandgap, which has been attributed to the stabilization of the quinoid fo...
Ever since the discovery of the trityl radical, isolation of a stable and neutral organic radical has been a synthetic challenge. A (4n+1)π open-shell configuration is one such possible neutral radical but an unusual state between aromatic (4n+2)π and antiaromatic (4n)π electronic circuits. The synthesis and characterization of an air- and water-stable neutral 25π pentathiophene macrocyclic radical is now described. It undergoes reversible one-electron oxidation to a 24π antiaromatic cation and reduction to a 26π aromatic anion, thus confirming its amphoteric behavior. Structural determination by single-crystal X-ray diffraction studies revealed a planar configuration for the neutral radical, antiaromatic cation, and aromatic anion. In the solution state, the cation shows the highest upfield chemical shift ever observed for a 4nπ system, while the anion adhered to aromatic nature. Computational studies revealed the delocalized nature of the unpaired electron as confirmed by EPR spectroscopy.
pi-Conjugated macrocycles consisting of thiophenes and benzenes exhibit benzenoid features for 4npi macrocycles, whereas (4n + 2)pi macrocycles are annulenoid due to rapid interconversion between quinoid and Kekule canonical forms in the benzene units.
Synthesis and structural diversity of novel aromatic expanded isophlorins are described. Expanded isophlorins are higher analogues of 20pi isophlorin; 30pi isophlorins are the simplest examples of expanded isophlorins. They are synthesized from easy to make precursors. Owing to the sp(2) carbons along the conjugated network, they represent higher analogues of annulenes which are not realized until date. In contrast to the parent isophlorin 2, expanded isophlorins 7-9 and 11-13 are aromatic (4n + 2)pi systems and also differ in their optical properties and structural features. They exhibit ring inverted structures and the number of ring inversions is dependent on the nature of the heteroatoms present in the core of the macrocycle. This was confirmed by both (1)H NMR spectroscopy and single crystal-ray diffraction analysis. The upfield chemical shifts for the protons of the inverted rings confirmed the diatropic ring current effects as expected for (4n + 2)pi aromatic systems. Solid state analysis revealed a planar conformation for 7 and near planar conformations for other macrocycles. They also show strong intermolecular interactions through C(sp(2))-H...F-C(sp(2)) hydrogen bonds.
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