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Energy transfers in two kinds of peripheral light-harvesting complexes (LH2) of Rhodobacter sphaeroides (RS) 601 are studied by using femtosecond pump–probe spectroscopy with tunable laser wavelength at room temperature. These two complexes are native LH2 (RS601) and green carotenoid mutated LH2 (GM309). The obtained results demonstrate that, compared with spheroidenes with ten conjugated double bonds in native RS601, carotenoid in GM309 containing neurosporenes with nine conjugated double bonds can lead to a reduction in energy transfer rate in the B800-to-B850 band and the disturbance in the energy relaxation processes within the excitonic B850 band.

Excitation energy transfer in the isolated light-harvesting chlorophyll (Chl)-a/b protein complex of photosystem II (LHC II) was studied by the one-colour pump-probe technique with femtosecond time resolution. After exciting Chl-b by 638 nm beam, the dynamic behaviour shows that the ultrafast energy transfer from Chl-b at positions of B2, B3, and B5 to the corresponding Chl-a molecules in monomeric subunit of LHC II is in the time scale of 230 fs. While with the excitation of Chl-a at 678 nm, the energy transfer between excitons of Chl-a molecules has the lifetime of about 370 fs, and two other slow decay components are due to the energy transfer between different Chl-a molecules in a monomeric subunit of LHC II or in different subunits, or due to change of molecular conformation.

In recent years, azobenzene derivates have received much attention for their potential application in optical data storage, biophotonics, holographic memories and waveguide switches optical sensors, and sensitive optical components from laser damage in both civilian and military applications. Experimental and theoretical studies demonstrate clearly the effect of the sonor-pi-acceptor (D- -A) conjugation on the steady-state and time-resolved PL spectra of azobenzene derivate films in multifarious situations, but comparatively little is concerned about the two-photon absorption and refraction involved in a single benzene ring. Furthermore, the excitation laser source on the azobenzene derivates in some investigations is continuum laser or nanosecond pulsed laser, where it is hard to avoid thermal effect on nonlinear optical (NLO) process produced by these lasers. To explore the origin of the azobenzene derivates' D- -A conjugation-dependent NLO process is a challenging task and has great signicance in describing the molecular structures of these azobenzene nanostructures as well as improving the performance of azobenzene derivates' devices. The D- -A conjugation of azobenzene functional material can be modified by mixing the azobenzene derivates with metal nanoparticles, so it is convenient to study how the D- -A conjugation affects the NLO properties by using the azobenzene derivate-metal composites. In this letter, the D- -A conjugation-dependent NLO absorption and refraction of the two kinds of azobenzene derivates 4-((4'-hydroxybenzene) azo) benzyl acid(BN) and N-(3, 4, 5-octanoxyphnyl)-N'-4-[(4-hydroxyphenyl) azophenyl]1, 3, 4-oxadiazole (AOB-t8) are investigated by Z-scan technology using 32 ps laser pulse width at 532 nm. The azobenzene derivates' surface is modified using the D- -A conjugation control and overcoating Au nanoparticles on the azobenzene derivates; and the Au/AOB-t8 composites, BN and AOB-t8 are characterized by Z-scans and absorption/fluorescence spectrum, and also calculated based on plasma resonance. The third-order NLO susceptibility of AOB-t8 is enhanced as compared with BN due to the growing conjugate chain and the increasingly extended bond. However, the third-order NLO susceptibility of AOB-t8 is decreased in the composite(Au/AOB-t8) for the cooperation of the local field effect induced by the gold nanoparticles and the extended bond of organic molecules. This work may be helpful to the understanding of the physical mechanism of the surface states and the surface-related optical nonlinearity of semiconductor QDs.

The energy transfer pathways from the excited accessory bacteriochlorophylls (B*) within native and modified (pheophytin-exchanged) reaction centres isolated from Rhodobacter sphaeroides (RS601) have been investigated by using the femtosecond pump-probe technique. For native RS601, B* decays with a time constant of 240 fs, followed by a partial recovery of the ground-state bleaching with a timescale of about 2 ps. In modified RS601(Phe), however, B* decays with a time constant of 800 fs for magic angle (54.7°) polarization configuration. In addition to the general assumption of energy transfer pathway from B* to the special pair P, an alternative pathway for energy transfer probably operates as well.

The energy relaxation and kinetic evolution of transient spectra of bacteriochlorophylls (BChls) in light-harvesting complex LH2 from Rb. sphaeroides 601 were investigated using femtosecond pump-probe technique. Upon 783 nm excitation, the energy at B800 BChls experiences an intramolecular redistribution with 0.35 ps time constant before transferring to B850 BChls. With tuning the excitation wavelength, the dynamical evolution of excited BChls was clearly observed, which indicates an obvious competition between the ground state bleaching and excited state absorption (ESA) of BChls involved and an isosbestic point near 818 nm, and also demonstrates that from the lower electronic excited state of B800 BChls to the higher excitonic state of B850 BChls is an efficient routine for energy transfer. The excitation energy in higher excitonic states of B850 BChls relaxes rapidly to the next lowest excitonic state by interconversion, delocalization to adjacent molecular, populating the lowest excitonic state and the change of molecular conformation.