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

Abstract

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.

Keywords: DFT/TDDFT calculations; carotenoids; ultrafast laser spectroscopy.

Figure 1

Structural formulae of 13,13′-diphenylalkyl-β-carotenes. Compound 1: 13,13′-diphenylpropyl-β-carotene (n = 3). Compound 2: 13,13′-diphenyl-β-carotene (n = 0). Compound 3: β-carotene (methyl groups in the positions 13 and 13′ instead of the two phenylalkyl substituents).

Figure 2

Steady-state absorption spectra of compound 1 in n-hexane (red solid line) and n-hexadecane (black dashed line) and of compounds 2 and 3 in n-hexane (green and blue solid lines, respectively). (a) Complete spectra for the spectral range 235–670 nm. (b) Magnification for the wavelength range 410–520 nm. The spectra of compounds 2 and 3 were reproduced from Refs. [11,17] with permission from the PCCP Owner Societies.

Figure 3

Ultrafast transient absorption experiments of compound 1 in n-hexane. (a) Contour plot of the experimental transient absorption spectra (note the logarithmic time axis). (b–d) Selected transient spectra at the pump–probe time delays indicated, covering times up to 0.1 ps, 1.5 ps and 30 ps, respectively. The inverted steady-state absorption spectrum is shown as a magenta dotted line. (e,f) Selected kinetics covering time scales up to 2 ps and 45 ps, respectively, for the four probe wavelengths indicated. (g) Contour plot of the results from the global kinetic analysis using the kinetic scheme of Figure 4. (h) Species-associated spectra for the electronic species involved (S₂: green, S₁^*: red, S₁: blue, and S₀: black). (i,j) Simulation of the transient absorption spectra (open circles) at 0.3 and 2 ps, respectively, including the total fit (cyan line) and the individual contributions of the different electronic species (same color coding as in panel h). (k,l) Two representative fits (cyan lines) of the experimental kinetics (open circles) at 450 and 560 nm, respectively, and the individual contributions of the different electronic species (same color coding as in panel h).

Figure 4

Kinetic scheme used for modeling the decay of compound 1 after photoexcitation.

Figure 5

Same as in Figure 3, but for the solvent n-hexadecane.

Figure 6

Zoom-in of the S₁→S_n ESA band of compounds 1 (red), 2 (green) and 3 (blue) at a pump–probe delay time of 1.5 ps. Each transient spectrum was normalized at the same amplitude of the S₀→S₂ GSB (not shown).

Figure 7

(a) Transient absorption spectrum of compound 1 in n-hexadecane averaged over the time range 30–40 ps (black) compared with the inverted S₀→S_n steady-state absorption spectrum (magenta). Pump wavelength: 500 nm, pump beam power: 0.3 mW. (b) Same as in panel a, but excited at a pump wavelength of 400 nm (pump beam power: 1.3 mW). Here, transient spectra are averaged over the time ranges 30–40 ps (black) and 50–70 ps (green).

Figure 8

Three different views (a–c) of the molecular structure of compound 1 from DFT calculations at the B3LYP/6-311G(d,p) level of theory.

Figure 9

(a) Detachment electronic density (red) and attachment electron density (blue) for the five lowest excited singlet states of compound 1 from TDDFT/TDA calculations employing the BLYP functional and the 6-31+G(d) basis set. (b) Zoom-in of the detachment and attachment electron densities of the S₂ state highlighting the changes in the bond alternation and the “spectator role” of the phenyl rings during the electronic transition.