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

The third order hyperpolarizability, 〈γ〉, of the ground state S0 of bacteriochlorophyll a (BChla) has been determined using degenerate four wave mixing. The 〈γ〉 for BChl a in vivo (the B875 and B850 arrays of light harvesting proteins) is enhanced relative to that in vitro (THF). A significantly higher 〈γ〉 in B875 than that in B850 suggests stronger pigment interactions in the former resulting from superior overlap of adjacent macrocycles. In addition, a 50–160 fold increase in 〈γ〉 for the S1 excited state of relative to S0 of BChl a in vivo provides an alternative methods for probing excited state dynamics and a potential application for molecular switching.