Exceptional Two- to Five-Photon Absorption at Mono/Bis(Donor)-Porphyrin-Mono/Bis(Donor) Triads and Pentads

Porphyrins are key components of biologically-active molecules involved in oxygen transport, light harvesting, and electron transfer; their photophysical properties are consequently of considerable fundamental interest. We herein report that certain donor-porphyrin-donor triads and bis(donor)-porphyrin-bis(donor) pentads exhibit nonlinear optical activity toward femtosecond pulsed radiation over the spectral range 800-2150 nm. We show that these triads and pentads exhibit unusually strong molecular quadratic optical nonlinearity, two-photon absorption, and three-photon absorption, and represent a new molecular motif exhibiting four-photon absorption. We also report the first porphyrins, and therefore a new class of molecule, to exhibit five-photon absorption. The two-, three-, four-, and five-photon absorption maxima are found close to the corresponding multiples of intense linear absorption bands that time-dependent density functional theory assigns as admixtures of porphyrin-localized π* ← π and donors-to-porphyrin charge-transfer transitions in the case of the donor-porphyrin-donor triads.