How the terminal methyl group affects the S0, S1 and T1 electronic structures of a conjugated aldehyde chain: A comparison between all-trans-β-apo-12′-carotenal and its analogue by NMR and Raman spectroscopy

Article abstract of DOI:10.1016/S1386-1425(97)83030-2

An analogue of all-trans-β-apo-12′-carotenal, with a methyl group being shifted from the 13′ to the 14′ position, has been synthesized. The electronic absorption, the ¹H and ¹³C NMR, and the S₀-, S₁- and T₁-state Raman spectra of β-apo-12′-carotenal (C₂₅ aldehyde) and the analogue (C_25′ aldehyde) were recorded for comparison. No clear differences were found for the A_g → B_u absorption, but systematic differences were found in the ¹³C and ¹H chemical shifts in the vicinity of the methyl group; the differences were explained in terms of the inductive effect and van der Waals interactions. The frequency of the in-phase C=C stretching Raman line (a measure of the overall carbon-carbon bond order of the conjugated chain) was almost the same in the S₀ state, but the C=C stretching frequency of C_25′ aldehyde was lower than that of C₂₅ aldehyde in both the S₁ and T₁ states. Thus, the methyl position affects only the local electronic structure in the S₀ state, but it can affect the electronic structure of the entire conjugated backbone in the excited states.