Photoinduced Dynamics of 13,13'-Diphenylpropyl-β-carotene

Carotenoids are ubiquitous pigment systems in nature which are relevant to a range of processes, such as photosynthesis, but the detailed influence of substitutions at the polyene backbone on their photophysics is still underexplored. Here, we present a detailed experimental and theoretical investigation of the carotenoid 13,13'-diphenylpropyl-β-carotene using ultrafast transient absorption spectroscopy and steady-state absorption experiments in n-hexane and n-hexadecane, complemented by DFT/TDDFT calculations. In spite of their bulkiness and their potential capability to "fold back" onto the polyene system, which could result in π-stacking effects, the phenylpropyl residues have only a minor impact on the photophysical properties compared with the parent compound β-carotene. Ultrafast spectroscopy finds lifetimes of 200-300 fs for the S₂ state and 8.3-9.5 ps for the S₁ state. Intramolecular vibrational redistribution with time constants in the range 0.6-1.4 ps is observed in terms of a spectral narrowing of the S₁ spectrum over time. We also find clear indications of the presence of vibrationally hot molecules in the ground electronic state (S₀*). The DFT/TDDFT calculations confirm that the propyl spacer electronically decouples the phenyl and polyene π-systems and that the substituents in the 13 and 13' positions point away from the polyene system.