Change in electronic structure of polyenes due to interaction with polyacenes and with graphitic strips
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Hartree-Fock calculations are first presented when a polyene is brought down parallel to the plane of some polyacenes and also, for one case, in a perpendicular orientation. Attention is given to the variation of (a) bond lengths and (b) the HOMO-LUMO energy gap. Following these numerical investigations on finite systems, some tight-binding models (Hckel-like) are worked out to illustrate the effects of different environments on the -electron assembly in polyenes. Again the focal points are whether bond alternation is enhanced, or suppressed, and the consequences for the HOMO-LUMO energy gap. Some analytic progress proves possible for an infinite polyene chain interacting with (a) an infinite polyacene and (b) an infinite polyphenacene, when a "hopping" energy is used to couple the polyene with its "environment". In the former model (a), with no HOMO-LUMO gap in the isolated infinite polyacene, a -electron polyene gap is opened up. In model (b), a similar variation of the energy gap is found. For future studies, reference is finally made to experiments on the (polyene) chromophore in the retinal protein bacteriorhodopsin. The observed red shift needs mechanisms additional to those considered in the present study, and some suggestions are made here.