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The semiempirical ZINDO/S CIS configuration interaction method has been used to study the ground- and excited-state absorption spectra of wild type and heterodimer M202HL reaction centers from purple bacterium Rhodobacter sphaeroidesas well as of peripheral LH2 and LH3 light harvesting complexes from purple bacterium Rhodopseudomonas acidophila. The calculations well reproduce the experimentally observed excited-state absorption spectra between 1000 and 17 000 cm−1, despite the necessarily limited number of chromophores and protein subunits involved in the calculations. The electron density analysis reveals that the charge transfer between adjacent chromophores dominates the excited-state absorption spectra. Clear spectroscopic differences observed between the wild type and heterodimer reaction centers as well as between the LH2 and LH3 antenna complexes arise from differences in the energy level manifolds of the complexes, particularly those of the charge transfer states. The calculations also imply that the lowest excited state of the bacterial reaction centers has charge transfer character that is related to charge transfer within the special pair and between the special pair and the accessory bacteriochlorophyll of the photosynthetically active electron transfer branch.

Time-resolved absorption spectroscopy was used to study the femtosecond and picosecond time scale electron injection from the excited singlet and triplet states of Ru(dcbpy)2(NCS)2 (RuN3) into titanium dioxide (TiO2) nanocrystalline particle film in acetonitrile. The fastest resolved time constant of ~30 fs was shown to reflect a sum of two parallel ultrafast processes, nonergodic electron transfer (ET) from the initially excited singlet state of RuN3 to the conduction band of TiO2 and intersystem crossing (ISC). The branching ratio of 1.5 between the two competing processes gives rate constants of 1/50 fs^-1 for ET and 1/75 fs^-1 for ISC. Following the ultrafast processes, a minor part of the electron injection (40%) occurs from the thermalized triplet state of RuN3 on the picosecond time scale. The kinetics of this slower phase of electron injection is nonexponential and can be fitted with time constants ranging from ~1 to ~60 ps.

Two color transient absorption measurements were performed on a LH2 complex from Rhodospirillum molischianum by using several excitation wavelengths (790, 800, 810, and 830 nm) and probing in the spectral region from 790 to 870 nm at room temperature. The observed energy transfer time of ~1.0 ps from B800 to B850 at room temperature is longer than the corresponding rates in Rhodopseudomonas acidophila and Rhodobacter sphaeroides. We observed variations (0.9−1.2 ps) of B800−850 energy transfer times at different B800 excitation wavelengths, the fastest time (0.9 ps) was obtained with 800 nm excitation. At 830 nm excitation the energy transfer to the B850 ring takes place within 0.5 ps. The measured kinetics, as well as steady-state absorption and CD spectra, are consistent with those calculated with the configuration interaction exciton method (CIEM) [Linnanto et al. J. Phys. Chem. B 1999, 103, 8739]. Fully excitonic simulation of the CD spectrum of the LH2 of Rs. molischianum is presented for the first time. The calculations put the E3 exciton states of B850 near the narrow excitonic B800 manifold and according to our model, these states provide the main route of energy transfer from the B800 ring to the B850 ring in the complex. The 1,2E2 states at 824 nm predicted by the calculations serve as an additional energy transfer channel as indicated by the observed fast transfer rate at 830 nm excitation.

The semiempirical PM3 method has been used to calculate fully optimized structures of bacteriochlorophylls a, b, c, d, e, f, g, and h, magnesium−chlorin, and magnesium−bacteriochlorin. Several configuration interaction (CI) methods, the PM3 (5,5) CIS and CISD, ZINDO/S CIS (n,n) with 2 < n <30, and ab initio CIS (5,5)/6-31G* methods, were tested for their predictive power in estimation of spectroscopic properties of bacteriochlorophylls. The ZINDO/S CIS (15,15) method turned out the best results for overall simulation of absorption spectra. Both the transition energies and relative intensities of the Qy, Qx, and Soret bands were correctly predicted. The effect of solvent coordination on the transition energies and oscillator strengths was also studied. The calculations predict solvent-induced spectroscopic shifts of the Qx and Soret bands, whereas the Qy transitions remain practically unshifted, in accord with experimental findings. The shifts are due to solvent-induced energy level shifts and charge redistribution of the magnesium atom in the excited states of bacteriochlorophylls. The results are discussed with reference to spectroscopic properties of bacteriochlorophylls in solution, in aggregates, and in photosynthetic light harvesting antenna.

Zinc-meso-tetrasulfonatophenyl-porphyrin (ZnTPPS^4-) spontaneously forms a complex with methyl viologen (MV²⁺) in aqueous solutions. Both the free porphyrin and its complex with viologen were studied by pump−probe spectroscopy. Excitation in the porphyrin Soret band with a short laser pulse (fwhm = 200 fs) populated the upper excited-state S2, and the subsequent processes were followed by the transient absorbance changes involved. For the free ZnTPPS,^4- the transient absorption spectrum of the upper excited-state S2 was different from that of the lowest excited-state S1. The S2 to S1 internal conversion could therefore be followed, and a S2 lifetime of 1.3 ± 0.2 ps was determined. In the ZnTPPS^4-−MV²⁺ complex, excitation in the Soret band produced spectral changes attributed to the charge separated state ZnTPPS^3-−MV⁺, with a rate limited by the laser pulse (rate constant >5 × 10¹² s^-1). This suggests that the photoinduced electron transfer from the excited porphyrin to the methyl viologen occurred directly from the S2 state of the porphyrin, before internal conversion to S1. The reverse electron transfer that followed regenerated the ground state reactants with a rate constant (1.3 ± 0.2) × 10¹² s^-1.