The strong absorptions in the ultraviolet range of the studied systems are almost all resulted from electronic promotions between frontier molecular orbitals (FMOs) HOMO-1/HOMO → LUMO/LUMO+1, as shown in Table . All of them correspond to typical π-π* excitations. Due to the symmetry of the carbon chains H-(C≡C) n -H (n = 3-9, 15), the HOMO-1/HOMO and LUMO/LUMO+1 of the molecules are mutually degenerate in energy, respectively. The energy levels of occupied FMOs (HOMO-1/HOMO) and unoccupied FMOs (LUMO/LUMO+1) respectively increase and decrease as the chain length increases in Fig. (b), resulting in a significant decrease in the FMO energy gap of the molecules. The decreasing gap reflects the increased electron correlation in longer carbon chains and the same behavior has been observed for other π-conjugation oligomers . Other MO levels can be expected to change in the same way as those of these FMOs, which rationalizes the significant red-shift of the absorption band as the carbon chain grows. It is worth noting that with the growth of chain length, the contribution of the HOMO-1/HOMO → LUMO/LUMO+1 electronic transition to the maximum absorption gradually decreases, indicating that there are more other orbitals in longer carbon chains participating in the 1 1 Σ u + ← X 1 Σ g + transition. This is the inevitable result that the energy levels of FMOs become denser as the chain length increases. Incidentally, rather constant red-shifts relative to the gas-phase wavelength are usually observed in neon matrix and solutions for carbon chains without a significant dependence on the size of the chain . Discussion on electronic transition in non-vacuum is beyond the scope of this work.