Tuning the spectroscopic properties of aryl carotenoids by slight changes in structure

Two carotenoids with aryl rings were studied by femtosecond transient absorption spectroscopy and theoretical computational methods, and the results were compared with those obtained from their nonaryl counterpart, β-carotene. Although isorenieratene has more conjugated C═C bonds than β-carotene, its effective conjugation length, Neff, is shorter than of β-carotene. This is evidenced by a longer S1 lifetime and higher S1 energy of isorenieratene compared to the values for β-carotene. On the other hand, although isorenieratene and renierapurpurin have the same π-electron conjugated chain structure, Neff is different for these two carotenoids. The S1 lifetime of renierapurpurin is shorter than that of isorenieratene, indicating a longer Neff for renierapurpurin. This conclusion is also consistent with a lower S1 energy of renierapurpurin compared to those of the other carotenoids. Density functional theory (DFT) was used to calculate equilibrium geometries of ground and excited states of all studied carotenoids. The terminal ring torsion in the ground state of isorenieratene (41°) is very close to that of β-carotene (45°), but equilibration of the bond lengths within the aryl rings indicates that the each aryl ring forms its own conjugated system. This results in partial detachment of the aryl rings from the overall conjugation making Neff of isorenieratene shorter than that of β-carotene. The different position of the methyl group at the aryl ring of renierapurpurin diminishes the aryl ring torsion to ∼20°. This planarization results in a longer Neff than that of isorenieratene, rationalizing the observed differences in spectroscopic properties.