Addition of 4,4'-bipyridyl to a solution of a meso-meso butadiyne-linked conjugated zinc porphyrin polymer in chloroform results in self-assembly of a double-strand ladder complex. Excess ligand causes this duplex to dissociate into single strands. These binding processes were elucidated by near-IR and NMR titrations, as well as by gel permeation chromatography and small-angle neutron scattering. Ladder-making and -breaking are highly cooperative, with Hill coefficients of 3.0 and 3.7, respectively. Self-assembly of the ladder holds the pi-system in a planar conformation, enhancing the conjugation, resulting in a red-shift and intensification of the Q-band. Both the real and imaginary parts of the third-order susceptibility per macrocycle are amplified by ladder formation, as revealed by degenerate four-wave mixing measurements at 1064 nm. At this wavelength, the double-strand polymer complex has |chi(3)xyyx| = 6.0 x 10-17 m2 V-2 per macrocycle, compared with 6.6 x 10-18 m2 V-2 for the single-strand polymer under the same conditions.

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Using low intensity femtosecond duration laser pulses at 708 nm, we have observed absorption transients associated with electron transfer through the primary electron acceptor A 0 in the photosystem I (PSI) reaction center from spinach under nonreducing conditions. At this wavelength the electron donor P 700 is excited directly, although some antenna chlorophylls are also excited. Using a nanosecond duration preflash of 690 nm to oxidize P 700 , and then measuring the absorption transients from the antenna alone, it is possible by subtraction to isolate the absorption transients arising from electron transfer. We discuss this method critically. The spectrum of A 0 --A 0 does not appear promptly but takes ∼3 ps to reach maximum intensity and resembles those spectra previously obtained from higher plants, with a maximum bleaching at 685 ( 2 nm and a shoulder in the region 670-675 nm. The decay time of the primary radical pair P 700 + A 0is calculated as 20 ps. Analysis of absorption transients indicates that the intrinsic rate constant forming the primary radical pair P 700 + A 0cannot be measured directly because energy migration in the antenna is fast and quenching is approaching "trap limited" behavior. With use of a detailed model of the antenna energy migration based on the X-ray structure, the intrinsic rate constant for electron transfer is estimated as k 1 ∼ 0.7 ps -1 . The implications of these findings on energy and electron transfer are discussed.

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