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

Enhancement of the nonresonant second order molecular hyperpolarizabilities gamma were observed in stacked macrocyclic molecular systems, previously in a mu-oxo silicon phthalocyanine (SiPcO) monomer, dimer and trimer series, and now bacteriochlorophyll a (BChla) arrays of light harvesting (LH) proteins. Compared to monomeric BChla in a tetrahydrofuran (THF) solution, the <gamma> 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 gamma is more significant in B875 of LH1 than in B850 in LH2. Theoretical modeling of the nonresonant gamma enhancement using simplified molecular orbitals for model SiPcO indicated that the energy level of the two photon state is crucial to the gamma 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 gamma 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 <gamma> for the S-1 excited state of relative to S-0 of bacteriochlorophyll in vivo was observed which provides an alternative method for probing excited state dynamics and a potential application for molecular switching.