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Two-Photon Spectra of Chlorophylls and Carotenoid–Tetrapyrrole Dyads
journal contribution
posted on 2017-10-24, 14:18 authored by Daniel
A. Gacek, Ana L. Moore, Thomas A. Moore, Peter Jomo WallaWe present a direct comparison of
two-photon spectra of various
carotenoid–tetrapyrrole dyads and phthalocyanines (Pc) as well
as chlorophylls (Chl) in the spectral range between 950 and 1360 nm,
corresponding to one-photon spectra between 475 and 680 nm. For carotenoids
(Car) with 8, 9, or 10 conjugated double bonds, the two-photon absorption
cross section of states below the optical allowed carotenoid S2 is at least about 3–10 times higher than that of Pc
or chlorophyll a and b at 550/1100
nm. A quantitative comparison of spectra from Pc with and without
carotenoids of eight and nine conjugated double bonds confirms energy
transfer from optically forbidden carotenoid states to Pc in these
dyads. When considering that less than 100% efficient energy transfer
reduces the two-photon contribution of the carotenoids in the spectra,
the actual Car two-photon cross sections relative to Chl/Pc are even
higher than a factor of 3–10. In addition, strong spectroscopic
two-photon signatures at energies below the optical allowed carotenoid
S2 state support the presence of additional optical forbidden
carotenoid states such as S*, Sx, or,
alternatively, contributions from higher vibronic or hot S1 states dominating two-photon spectra or energy transfer from the
carotenoids. The onset of these states is shifted about 1500–3500
cm–1 to lower energies in comparison to the S2 states. Our data provides evidence that two-photon excitation
of the carotenoid S*, Sx, or hot S1 states results in energy transfer to tetrapyrroles or chlorophylls
similar to that observed with the Car S1 two-photon excitation.