As brought to the attention of the community by Hendon et al. and noted by previous workers, the π orbitals of the equilibrium geometry odd-carbon (even number of double bonds = n) [n]cumulenes may be written in either rectilinear or helical form. We trace the origins and detailed composition of the helical orbitals of cumulenes, which emerge in the simplest Hückel model and are not much modified in advanced computations. For the α,ω-disubstituted even [n]cumulenes, the helical representation is obligatory as the symmetry is reduced from D2d to C2. A relationship is apparent between these helical orbitals of the even [n]cumulenes, seen as a Herges coarctate system, and the corresponding Möbius cyclic polyene orbitals. The twist of the orbitals varies in interesting ways along the helix, and so does the contribution of the component atomic orbitals. Though the electronic structures of even [n]cumulenes and Möbius cyclopolyenes are closely related, they differ for higher n in intriguing ways; these are linked to the constrained rotation of the basis orbitals along the helical twist itinerary. Relations are constructed between the level patterns of the π-systems of even [n]cumulenes and ideas of Hückel and Möbius aromaticity.

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The performance of the six second order linear response methods RPA(D), SOPPA, SOPPA(CCSD), CIS(D), CC2, and CCSD, which include either noniterative or iterative doubles contributions, has been studied in calculations of vertical excitation energies. The benchmark set consisted of 39 valence and 76 Rydberg states of benzene and five polycyclic aromatic hydrocarbons. As reference values we have used the results of the corresponding calculations with the third order method CCSDR(3), which includes noniterative triples contributions. In addition we have also carried out equivalent calculations at the level of the random phase approximation as well as with the configuration interaction singles and multireference configuration interaction singles and doubles methods.

In this contribution, we develop and investigate a general 2D hopping model for the photovoltaic action in polymer-based thin films. The model takes a microscopic origin and accounts for the molecular photonic and electronic processes by a simple kinetic scheme that eventually leads a linearized master equation for the time evolution of the photovoltaic system. With an emphasis on the topology of blends of donor/acceptor functionalized polymers, we attempt to characterize the dependence of the short-circuit current, internal quantum efficiency, IV characteristics, and fill factors on the morphology of the blend. Several different morphologies for the polymer film are considered, and they show quite different transport and efficiency behavior (e.g., for so-called double cable structures, nearly quantitative conversion efficiencies are computed, and for other structures similar efficiencies may be found, but with short-circuit currents orders of magnitude lower). The model neglects effects such as exciton migration, the built-in potential, and interaction in the third dimension. Nontheless, significant conclusions can be drawn: in particular, we demonstrate that a viable photovoltaic system driven only by concentration gradients of charge carriers (no built-in field) is possible.