Two-Photon Spectra of Chlorophylls and Carotenoid-Tetrapyrrole Dyads

We 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∗, S_x, 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∗, S_x, or hot S₁ states results in energy transfer to tetrapyrroles or chlorophylls similar to that observed with the Car S₁ two-photon excitation.