Singlet fission in heterogeneous lycopene aggregates

Abstract

We have prepared lycopene aggregates with low scattering in an acetone-water suspension. The aggregates exhibit highly distorted absorption, extending from the UV up to 568 nm, as a result of strong excitonic interactions. We have investigated the structural organization of these aggregates by resonance Raman and TEM, revealing that the lycopene aggregates are not homogeneous, containing at least four different aggregate species. Transient absorption measurements upon excitation at 355, 515, and 570 nm, to sub-select these different species, reveal significant differences in dynamics between each of the aggregate types. The strong excitonic interactions produce extremely distorted transient electronic signatures, which do not allow an unequivocal identification of the excited states at times shorter than 60 ps. However, these experiments demonstrate that all the lycopene aggregated species form long-living triplets via singlet fission.

Keywords: Resonance Raman; Self-assembled lycopene; Singlet exciton fission; Transient absorption.

Fig. 1

Room temperature absorption spectra of monomeric lycopene in n-hexane (solid blue line), in CS₂ (dotted blue line), and lycopene aggregates in acetone: water 1:1 suspension (solid red line).

Fig. 2

Resonance Raman spectra at 77 K of lycopene monomer in diethyl ether (a), and lycopene crystalloids excited at 363.8, 488.0, 501.7, 514.5 and 577.0 nm—asterisks mark acetone solvent bands (b).

Fig. 3

TEM of lycopene aggregates at two different magnifications: scale bar (a) 500 nm, and (b) 1 µm.

Fig. 4

Femtosecond-to-nanosecond transient absorption data of lycopene aggregates upon excitation at 355 nm. (A) contour plot of the dataset. (B) kinetic traces measured at different wavelengths (indicated on the graphs) along with the results of the global fit to the data (solid lines). Horizontal dashed lines in panel A indicate the wavelengths at which the kinetic traces were taken. Note that the signals measured above 450 nm were multiplied by a factor of 3 to aid viewing.

Fig. 5

Dispersion-corrected time-gated pump-probe spectra of lycopene aggregates excited at 355 nm (A), 515 nm (B) and 570 nm (C). The times at which the spectra were taken are indicated on the graphs. Panels (D, E and F) show the EADS for the same datasets along with time constants, associated with different evolution steps, as per the legends.

Fig. 6

Nanosecond-to-microsecond transient absorption data of lycopene aggregates at 355, 515 and 570 nm excitation: (a, d, g) dataset overview; (b, e, h) EADS estimated by global fitting, using 3-component evolutionary model; (c, f, i) selected kinetics and fitting.