Linear π-conjugated oligomers have been widely investigated, but the behavior of the corresponding cyclic oligomers is poorly understood, despite the recent synthesis of π-conjugated macrocycles such as [n]cycloparaphenylenes and cyclo[n]thiophenes. Here we present an efficient template-directed synthesis of a π-conjugated butadiyne-linked cyclic porphyrin hexamer directly from the monomer. Small-angle X-ray scattering data show that this nanoring is shape-persistent in solution, even without its template, whereas the linear porphyrin hexamer is relatively flexible. The crystal structure of the nanoring-template complex shows that most of the strain is localized in the acetylenes; the porphyrin units are slightly curved, but the zinc coordination sphere is undistorted. The electrochemistry, absorption, and fluorescence spectra indicate that the HOMO-LUMO gap of the nanoring is less than that of the linear hexamer and less than that of the corresponding polymer. The nanoring exhibits six one-electron reductions and six one-electron oxidations, most of which are well resolved. Ultrafast fluorescence anisotropy measurements show that absorption of light generates an excited state that is delocalized over the whole π-system within a time of less than 0.5 ps. The fluorescence spectrum is amazingly structured and red-shifted. A similar, but less dramatic, red-shift has been reported in the fluorescence spectra of cycloparaphenylenes and was attributed to a high exciton binding energy; however the exciton binding energy of the porphyrin nanoring is similar to those of linear oligomers. Quantum-chemical excited state calculations show that the fluorescence spectrum of the nanoring can be fully explained in terms of vibronic Herzberg-Teller (HT) intensity borrowing.

[structure: see text]. Halogen functional groups on pentacene can be used both as synthetic handles for further functionalization as well as to tune the pi-stacking in these systems. The halogenated pentacene derivatives described here (X = Br, X' = H, and X = X' = F) are all stable and soluble, with reduction potentials significantly lower than that of the parent functionalized pentacene (X = X' = H). The bromopentacenes could be further elucidated to pentacene nitriles, further decreasing the acene's reduction potential, while the charge-carrier mobility in the fluorinated systems was shown to scale with the degree of fluorine substitution.

Photochemical electron donor-acceptor triads having an aminopyrene primary donor (APy) and a p-diaminobenzene secondary donor (DAB) attached to either one or both imide nitrogen atoms of a perylene-3,4:9,10-bis(dicarboximide) (PDI) electron acceptor were prepared to give DAB-APy-PDI and DAB-APy-PDI-APy-DAB. In toluene, both triads are monomeric, but in methylcyclohexane, they self-assemble into ordered helical heptamers and hexamers, respectively, in which the PDI molecules are pi-stacked in a columnar fashion, as evidenced by small- and wide-angle X-ray scattering. Photoexcitation of these supramolecular assemblies results in rapid formation of DAB(+*)-PDI(-*) spin-polarized radical ion pairs having spin-spin dipolar interactions, which show that the average distance between the two radical ions is much larger in the assemblies (31 A) than it is in their monomeric building blocks (23 A). This work demonstrates that electron hopping through the pi-stacked PDI molecules is fast enough to compete effectively with charge recombination (40 ns) in these systems, making these materials of interest as photoactive assemblies for artificial photosynthesis and organic photovoltaics.

Herein we describe the synthesis of a new series of copolymers (PSeBx) containing selenopheno [3,4-b]selenophene and benzodiselenophene, which exhibited a high power conversion efficiency (PCE) of 6.87% in a bulk heterojunction (BHJ) solar cell device (PSeB2/ PC 71 BM). In comparison with its thiophene analogue, PTB9, the new polyselenopheno[3,4-b]selenophene-co-benzodiselenophene (PSeB2) showed a lower band gap and improved charge carrier mobility as high as 1.35 × 10 −3 cm 2 V −1 s −1 .