Ultrafast fluorescence upconversion measurements were carried out on the peripheral (LH2) light harvesting antenna complex of Rhodobacter sphaeroides isolated in the detergents N-octyl--D-glucopyranoside and lauryl dimethylamine oxide. The B800 and B850 bands were excited in separate experiments, and the B850 emission was detected in each case. We make use of the recently determined crystal structure of a purple bacterial LH2 complex to simulate our data and to calculate the exciton level structure of the B850 aggregate. The B800 to B850 excitation transfer occurs with a 650 fs time constant. The depolarization of B850 emission follows a wavelength dependent, biexponential decay with time constants 50-90 and 400-500 fs. We can reproduce the non-exponentiality of the depolarization by assuming incoherent hopping between dimeric sites with a 250 cm -1 full width at half-maximum (fwhm) Gaussian distribution of site energies (inhomogeneity). We calculate the homogeneous hopping time between dimers in B850 to be ∼100 fs. The exciton calculations including pigment energetic disorder demonstrate the validity of a hopping picture of excitation transfer within the B850 band at room temperature.

One- and two-dimensional random walk processes and their effects on EPR (electron spin paramagnetic resonance) line shapes are examined. Discrepancies between the coarse continuum model and the discrete hopping model are discussed. Closed-form formulas are prescribed for the distribution function, correlation (memory) function and the EPR line shapes, covering the entire range between the slow and the fast hopping limits. Deviation from Gaussian or Lorentzian line shape is shown for intermediate hopping rates. Differences between the cases of a closed loop and an open chain are illustrated. Time-domain analysis of line shape measurements can provide information about the hopping rate constants, site numbers in an open or closed chain, and the activation energy.

A thermally cross-linkable, second-order nonlinear optical (NLO) polymer containing ethynyl groups has been synthesized. The ethynyl groups in the polymer backbone can undergo thermal intramolecular or intermolecular reactions with amide groups to form heterocyclic structures or cross linkages. The thermal curing of the polymer can be simultaneously effected during electric poling and second-harmonic generation measurements indicated that the cured polymers indeed showed high stability in dipole orientation; no obvious decay at room temperature for more than 1000 h. Long-term stability of second-harmonic coefficients was observed even at 90 °C, at least for 500 h.

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