Predicting the electronic structure of extended organic molecules constitutes an important fundamental task of modern chemistry. Studies of electronic excitations, charge-transfer, energy-transfer, and isomerization of conjugated systems form the basis for our understanding of the photophysics and photochemistry of complex molecules1-3 as well as organic nanostructures and supramolecular assemblies.4,5 Photosynthesis and other photochemical biological processes that constitute the basis of life on Earth involve assemblies of conjugated chromophores such as porphyrins, chlorophylls, and carotenoids.6-8 Apart from the fundamental interest, these studies are also closely connected to numerous important technological applications.9 Conjugated polymers are primary candidates for new organic optical materials with large nonlinear polarizabilities.10-19 Potential applications include electroluminescence, light emitting diodes, ultrafast switches, photodetectors, biosensors, and optical limiting materials.20-27 Optical spectroscopy which allows chemists and physicists to probe the dynamics of vibrations and electronic excitations of molecules and solids is a powerful tool for the study of molecular electronic structure. The theoretical techniques used for describing spectra of isolated small molecules are usually quite different from those of molecular crystals, and many intermediate size systems, such as clusters and polymers, are not readily described by the methods developed for either of these limiting cases.28

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Density Matrix Analysis and Simulation of Electronic Excitations in Conjugated and Aggregated Molecules

DFT schemes based on conventional and less conventional exchange-correlation (XC) functionals have been employed to determine the polarizability and second hyperpolarizability of π-conjugated polyacetylene chains. These functionals fail in one or more of several ways: (i) the correlation correction to α is either much too small or in the wrong direction, leading to an overestimate; (ii) γ is significantly overestimated; (iii) the chain length dependence is excessively large, particularly for γ and for the more alternant system; and (iv) the bond length alternation effects on γ are either underestimated or qualitatively incorrect. The poor results with the asymptotically correct van Leeuwen–Baerends XC potential show that the overestimations are not related to the asymptotic behavior of the potential. These failures are described in terms of the separate effects of the exchange and the correlation parts of the XC functionals. They are related to the short-sightedness of the XC potentials which are relatively insensitive to the polarization charge induced by the external electric field at the chain ends.

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Assessment of Conventional Density Functional Schemes for Computing the Polarizabilities and Hyperpolarizabilities of Conjugated Oligomers: An Ab Initio Investigation of Polyacetylene Chains

Reaction of the butadiynyl complex (η5-C5Me5)Re(NO)(PPh3)(C⋮CC⋮CH) (ReC4H) with Cu(OAc)2 (pyridine, 80 °C) gives the μ-octatetraynediyl complex ReC8Re (70%). Analogous cross-coupling of ReC4H and ReC2H gives (after chromatography) ReC4Re (14%), ReC6Re (44%), and ReC8Re (15%). Longer sp carbon chains are accessed by reactions of ReC4H with n-BuLi and CuI, which give ReC4Cu. This isolable species is treated in situ with BrC⋮CSiEt3 or BrC⋮CC⋮CSiMe3 (excess EtNH2, THF) to give ReC6SiEt3 or ReC8SiMe3 (84−77%). Desilylations (wet n-Bu4N+F-) yield ReC6H or ReC8H (88−73%). Then Cu(OAc)2 (pyridine, 50 °C) gives ReC12Re or ReC16Re (71−67%). The former is also available from ReC4Cu and BrC⋮CC⋮CBr (45%), and ReC10Re can be accessed by cross-coupling. ReC6H and ReC8H are similarly converted to ReC10SiR3 (R = Me, Et; 51−26%) and ReC12SiMe3 (43%). Desilylation of ReC10SiR3 gives labile ReC10H, but only black powder is obtained from ReC12SiMe3. In situ coupling of ReC10H gives ReC20Re (52−34%), which unlike lower homolog...

Toward Metal-Capped One-Dimensional Carbon Allotropes: Wirelike C6−C20 Polyynediyl Chains That Span Two Redox-Active (η5-C5Me5)Re(NO)(PPh3) Endgroups

The authors gratefully acknowledge
the financial support of the FNRS-FRFC and the “Loterie
Nationale” for Convention No. 2.4578.02, and of the
FUNDP.

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Assessment of the efficiency of long-range corrected functionals for some properties of large compounds.

Publisher Summary This chapter surveys the rapidly growing field comprising the chemistry of transition metal complexes containing ligands which are made up only of carbon atoms within their coordination spheres. The chapter describes metal complexes which contain linear or cyclic ligands of composition C n (n n ligands, in order of increasing carbon content of the ligand have been discussed. This is followed by a discussion on complexes of cyclic all-carbon ligands, such as [C 3 {Fe(CO) 2 Cp} 3 ] + . Complexes containing C n ligands associated with binuclear metal–metal bonded systems are followed by an account of metal cluster complexes containing C n ligands. It also includes hybrid complexes where a cluster supports a C n ligand also attached to a mononuclear metal–ligand fragment. The study is broadly based on a group-by-group approach and includes salient details of synthesis, properties, chemistry and electronic structure.

Transition Metal Complexes Containing All-Carbon Ligands

Ab initio finite oligomer method for nonlinear optical properties of conjugated polymers. Effect of electron correlation on the static longitudinal hyperpolarizability of polyacetylene

The evolution in the static longitudinal polarizability and second hyperpolarizability of the C4H2 through C54H2 oligomers of polyyne is studied at the Hartree–Fock level of theory. We find that the calculated values are highly sensitive to the adopted geometry. Using improved geometries, new extrapolation procedures and careful finite field determinations we extend earlier work by others so as to provide reliable estimates for the above properties in the infinite polymer limit.

Hartree-Fock static longitudinal (hyper)polarizability of polyyne

A comparative study of the effect of electron correlation in the hyperpolarizability of polyyne, polyacetylene and polypyrrole oligomers

The longitudinal polarizability, αzz, and second order hyperpolarizability, γzzzz, of the (C4H3N)RH2 oligopyrroles, where R=1−9, are determined at the Hartree–Fock (HF) level of theory using a 6‐31G basis set. The effect of electron correlation is taken into account using Moller–Plesset perturbation theory through second order for the R=1−5 oligopyrroles. Methods for extrapolation to the infinite polymer values are explored. The convergence behavior of the correlated results as a ratio with the corresponding HF values may indicate that a scaled HF procedure could be appropriate for finding correlated estimates to the infinite polymer property values. We also propose a generalized finite field recipe which can easily be used in conjunction with popular black‐box electronic structure programs for determining molecular polarizabilities through second order. The αzz and γzzzz values obtained from using this procedure on a variety of polyacetylene and polypyrrole oligomers are in excellent agreement with those...

Abinitio polarizability study of polypyrrole

Axial components of the polarizability, hyperpolarizability, and second order hyperpolarizability of retinal model molecules are calculated using the ab initio coupled‐perturbed Hartree–Fock theory. Conformational rearrangements are shown to affect some components in a systematic fashion. We also find that the general trend in the hyperpolarizability behavior as a function of protonation and charge transfer is in agreement with available experimental data. The second hyperpolarizability of the protonated Schiff base is found to be highly dependent on the adopted geometry optimization. We explore a possible explanation for this as well as present a brief discussion with regard to future experimental and theoretical developments towards the construction of molecularly based nonlinear optical devices.

Teressa Tangredi Toto

Papers

Ab initio finite oligomer method for nonlinear optical properties of conjugated polymers. Effect of electron correlation on the static longitudinal hyperpolarizability of polyacetylene

Hartree-Fock static longitudinal (hyper)polarizability of polyyne

A comparative study of the effect of electron correlation in the hyperpolarizability of polyyne, polyacetylene and polypyrrole oligomers

Abinitio polarizability study of polypyrrole

Ab initio studies of the polarizabilities of retinal analogs