Anabinitioinvestigation of the charge‐transfer complexes of alkali atoms with oligo (α,α′) thiophenes and oligoparaphenylenes: A model calculation on polaronic and bipolaronic defect structures

Excited-State Electronic Asymmetry Prevents Photoswitching in Terthiophene Compounds

The diarylethene moiety is one of the most extensively used switches in the field of molecular electronics. Here we report on spectroscopic and quantum chemical studies of two diarylethene-based compounds with a non- C₃-symmetric triethynyl terthiophene core symmetrically substituted with RuCp*(dppe) or trimethylsilyl termini. The ethynyl linkers are strong IR markers that we use in time-resolved vibrational spectroscopic studies to get insight into the character and dynamics of the electronically excited states of these compounds on the picosecond to nanosecond time scale. In combination with electronic transient absorption studies and DFT calculations, our studies show that the conjugation of the non- C₃-symmetric triethynyl terthiophene system in the excited state strongly affects one of the thiophene rings involved in the ring closure. As a result, cyclization of the otherwise photochromic 3,3″-dimethyl-2,2':3',2″-terthiophene core is inhibited. Instead, the photoexcited compounds undergo intersystem crossing to a long-lived triplet excited state from which they convert back to the ground state.

Electronic structure and band gaps in cationic heterocyclic oligomers. Multidimensional analysis of the interplay of heteroatoms, substituents, molecular length, and charge on redox and transparency characteristics

Oxidative doping of extended pi-conjugated polymers and oligomers produces dramatic changes in optical and electrical properties, arising from polaron and soliton-derived midgap states. Despite the great importance of such changes for materials properties, far less is known about the cationic polaron states than about the neutral, semiconducting or insulating, undoped materials. The systematic, multifactor computational analysis of oligoheterocycles such as oligothiophenes, oligofurans, and oligopyrroles presented here affords qualitative and quantitative understanding of the interplay among skeletal substitution pattern, electronic structure, and the effective band gap reduction on p-doping. A simple linear relation is derived for predicting p-doped oligomer and polymer effective band gaps based on those of the neutral oligomers; this relationship confirms the effectiveness of a "fixed band" approximation and explains the counterintuitive increase of the effective band gap on p-doping of many small band gap oligomers. The present analysis also suggests new candidates for transparent conductive polymers and predicts limiting behavior of ionization potential, electron affinity, and other properties for various polyheterocyclic systems. The results yield insight into materials constraints in electrochromic polymers as well as on p- and n-type conductors and semiconductors.

Electronic structure of Li-intercalated oligopyridines: A comparative study by photoelectron spectroscopy

The role of nitrogen in the charge transfer and storage capacity of lithium-intercalated heterocyclic oligophenylenes was investigated using photoelectron spectroscopy. The development of new occupied states at low binding energies in the valence band region, as well as core level chemical shifts at both carbon and nitrogen sites, demonstrates partial charge transfer from lithium atoms to the organic component during formation of the intercalated compound. In small compounds, i.e., biphenyl and bipyridine derivatives, the position of the nitrogen heteroatom significantly affects the spacing between gap states in the Li-intercalated film; yet it has minimal effects on the charge storage capacity. In larger, branched systems, the presence of nitrogen in the aromatic system significantly enhances the charge storage capacity while the Li-N bond strength at high intercalation levels is significantly weakened relative to the nitrogen-free derivative. These observations have strong implications towards improved deintercalation processes in organic electrodes in lithium-ion batteries.

Li−Biphenyl−1,2-Dimethoxyethane Solution: Calculation and Its Application

Metallic lithium reacts with biphenyl in 1,2-dimethoxyethane (DME) solvent at room temperature. This reaction has been studied using density functional theory (DFT) at the B3LYP level together with the 6-311++G (d,p) basis set. From the energy results of the corresponding optimized geometries for intermediate complexes, the reaction can be interpreted as a charge-transfer process between lithium and biphenyl followed by Li+ coordination with ether oxygens in DME. In addition, the experimentally observed vibrational bands can be unambiguously assigned and interpreted according to the normal modes calculated for the biphenyl-Li-DME complex. This organic complex solution has been demonstrated as a very effective chemical lithiation agent. V2O5 can be lithiated up to 1.45 lithium ions per formula. The lithiated V2O5 shows a high Li-extraction capacity of 173 mAh/g as cathode material for lithium ion batteries.

Reliability of the density functional approximation to describe the charge transfer and electrostatic complexes involved in the modeling of organic conducting polymers

Both the intermolecular interaction energies and the geometries for M... thiophene, M... pyrrole, M(n+)... thiophene, and M(n+)... pyrrole (with M = Li , Na, K, Ca, and Mg; and M(n+) = Li+, Na+, K+, Ca2+, and Mg2+) have been estimated using four commonly used density functional theory (DFT) methods: B3LYP, B3PW91, PBE, and MPW1PW91. Results have been compared to those provided by HF, MP2, and MP4 conventional ab initio methods. The PBE and MPW1PW91 are the only DFT methods able to provide a reasonable description of the M...pi complexes. Regarding M(n+)...pi complexes, the four DFT methods have been proven to be adequate in the prediction of these electrostatically stabilized systems, even though they tend to overestimate the interaction energies.