Third order nonlinear optical properties of stacked bacteriochlorophylls in bacterial photosynthetic light-harvesting proteins

Enhancement of the nonresonant second order molecular hyperpolarizabilities γ were observed in stacked macrocyclic molecular systems, previously in a μ-oxo silicon phthalocyanine (SiPcO) monomer, dimer and trimer series, and now in bacteriochlorophyll a (BChla) arrays of light harvesting (LH) proteins. Compared to monomeric BChla in a tetrahydrofuran (THF) solution, the 〈γ〉 for each macrocycle was enhanced in naturally occurring stacked macrocyclic molecular systems in the bacterial photosynthetic LH proteins where BChla's are arranged in tilted face-to-face arrays. In addition, the γ enhancement is more significant in B875 of LH1 than in B850 in LH2. Theoretical modeling of the nonresonant γ enhancement using simplified molecular orbitals for model SiPcO indicated that the energy level of the two photon state is crucial to the γ enhancement when a two photon process is involved. Additionally, charge transfer between the monomers may be important if this produces states which are close to one-photon resonance. The calculated results can be extended to γ enhancement in B875 and B850 arrays, suggesting that BChla in B875 are more strongly coupled than in B850. In addition, a 50-160 fold increase in 〈γ〉 for the S₁ excited state of relative to S₀ of bacteriochlorophyll in vivo was observed which provides an alternative method for probing excited state dynamics and a potential application for molecular switching.