The electronic structures of carbon-based ladder polymers and polynuclear aromatic hydrocarbons (PAHs) with acene- and phenanthrene-edge structures are studied with an approximate molecular-orbital method. The difference between polyacene and polyphenanthrene can be derived from detailed orbital interaction analyses of anthracene and phenanthrene. The fragment molecular-orbital (FMO) method successfully characterizes the distinct electronic structures of the two small PAHs with different types of edges. We shed light on the electronic structures of ladder polymers with the phenanthrene-edge structure, PPh(n), in which n is the number of cis-polyene chains included. With an increase in width of the ladder polymers, the bandgap of PPh(n) approaches zero with a behavior that has a periodicity of 3. The PPh(n) series are classified into three subgroups:  small-gap (metallic) PPh(3m + 1), large-gap PPh(3m + 2), and medium-gap PPh(3m), where m = 0, 1, 2, .... The oscillating behavior in the bandgap of the three subgroups is analyzed from the viewpoint of interchain interactions in the frontier crystal orbitals.

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The major pigments of both antenna and reaction centers (RCs) are chlorophylls (Chls) in higher plants and bacteriochlorophylls (BChls) in photosynthetic bacteria. Except for the metal-free pheophytin (Pheo) a, bacteriopheophytin (BPheo) a (see Fig. 1) and BPheo b acting as primary electron acceptors in RCs 1-3 , only Mg has been known as the central metal of Chls and BChls until we recently found a novel Zn-containing BChl a in an aerobic bacterium Acidiphilium rubrum growing at low pH (ca. 3) 4 and confirmed its structure to be identical with that of BChl a esterified with phytol.

Lithium shows some interesting properties, such as high chargeradius ratio and multicoordination. 1 These peculiar features of this simple element [whose number of electrons is only three with the electronic structure (1s) 2 (2s) 1 ] have attracted a great deal of interest, and the chemistry of lithium has been developed. 2 Recent fabrication of the rechargeable battery making use of lithium ions, that is, the Li-ion battery, has pushed forward recent research on Li-containing materials because this battery shows a high energy density as well as a high output voltage. 3,4 A typical Li-ion battery consists of a transition-metal oxide such as NiO 2 or CoO 2 as the cathode, carbonaceous material such as amorphous carbon or graphite as the anode, and a Li-salt solution such as LiPF 6 dissolved in ethylene carbonate as the electrolyte. Since lithium ions transport charge by moving from the anode to the cathode, the more an electrode contains lithium ions, the more the capacity increases. The capacity of a carbonous material, Q (mAh/g), relates to the [Li]/[C] atomic ratio by the following equation 5[Li]/[C] ϭ {3.6(12 ϩ x)Q}/96500[1] where x is [H]/[C] atomic ratio of a carbonous material. In addition, the difference in the chemical potential of Li ϩ ion and Li atom enables a high output voltage. Extensive research effort has been devoted to amorphous carbon materials which greatly contribute to the high capacity, high voltage, and cycle stability of Li-ion batteries. We should note that graphite and highly graphitic materials are also useful as the anode, but are not considered here because their capacity limit and Li storage mechanism are well understood. 6-8 Amorphous carbon materials can be classified into two groups with respect to the starting material and the heat-treatment temperature. 9,10 One is the hard carbon materials obtained from heating a nongraphitizable precursor at 1500 to 3000ЊC. They show reversible capacity up to ca. 600 mAh/g, larger than that of graphite (375 mAh/g). Their output voltage is also high, although it is not so constant as that of graphite. 9 From observed large Knight shifts (100 to 200 ppm) of their 7 Li nuclear magnetic resonance (NMR) spectra, it has been found that lithium metal clusters grown in the relatively large pores contribute to the capacity considerably. 11,12 Another group is the low-temperature pyrolyzed carbon materials which are prepared at 600 to 800ЊC from resins, polymers, and pitches. 5,9,[13][14][15][16] They contain a large amount of hydrogen atoms ([H]/[C] ϭ 0.2 to 0.4) owing to their low degree of carbonization. These carbons are very attractive as the anode because of their extraordinarily high rechargeable capacity much larger than that of graphite and even larger than those of hard carbons. 9 For instance, the polyacenic semiconductor (PAS) material prepared from phenol resin at 700ЊC shows a reversible capacity of 850 mAh/g. 17 However there are several disadvantages to this type of carbon. They show large hysteresis in the voltage-capacity profile durin...