− , Figure a and b shows the relationships between the π-linker torsion angle Φ 3 and the HOMO–LUMO energy gap with λ max for the ( A–D ) 1 (R = H) dyes, respectively. Comparing the visible-light-absorbing series, a notable trend was the further red-shifted absorption observed in series C and D , resulting from a larger configuration coefficient associated with the least energetic HOMO → LUMO electronic transition, as well as the narrowing of the HOMO–LUMO gap.
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