Ultrafast dynamics of differently aligned COOH-DTE-BODIPY conjugates linked to the surface of TiO2.

The photoinduced dynamics of two DTE-BODIPY conjugates A, B with carboxylic acid anchoring groups coupled to the surface of TiO2 were studied by ultrafast transient absorption spectroscopy. For compound A, with an orthogonal orientation of the BODIPY chromophore and the photoswitchable DTE unit, a charge separated state could not be reliably detected. Nevertheless, besides the energy transfer from the BODIPY to the ring-closed DTE-c, indications for an electron transfer reaction were found by analyzing fluorescence quenching on TiO2 in steady state fluorescence measurements. For compound B with a parallel orientation of chromophore and photoswitch, a charge separated state was conclusively identified for the coupled dyad (TiO2) via the observation of a positive absorption signal (at λ pr > 610 nm) at later delay times. An electron transfer rate of 7 × 1010 s-1 can be extracted, indicating slower processes in the dyads in comparison to previously published electron transfer reactions of DTE compounds coupled to TiO2.

Light-harvesting chlorophyll protein (LHCII) drives electron transfer in semiconductor nanocrystals.

Type-II quantum dots (QDs) are capable of light-driven charge separation between their core and the shell structures; however, their light absorption is limited in the longer-wavelength range. Biological light-harvesting complex II (LHCII) efficiently absorbs in the blue and red spectral domains. Therefore, hybrid complexes of these two structures may be promising candidates for photovoltaic applications. Previous measurements had shown that LHCII bound to QD can transfer its excitation energy to the latter, as indicated by the fluorescence emissions of LHCII and QD being quenched and sensitized, respectively. In the presence of methyl viologen (MV), both fluorescence emissions are quenched, indicating an additional electron transfer process from QDs to MV. Transient absorption spectroscopy confirmed this notion and showed that electron transfer from QDs to MV is much faster than fluorescence energy transfer between LHCII and QD. The action spectrum of MV reduction by LHCII-QD complexes reflected the LHCII absorption spectrum, showing that light absorbed by LHCII and transferred to QDs increased the efficiency of MV reduction by QDs. Under continuous illumination, at least 28 turnovers were observed for the MV reduction. Presumably, the holes in QD cores were filled by a reducing agent in the reaction solution or by the dihydrolipoic-acid coating of the QDs. The LHCII-QD construct can be viewed as a simple model of a photosystem with the QD component acting as reaction center.

Highly efficient modulation of FRET in an orthogonally arranged BODIPY-DTE dyad.

The photoswitchable boron-dipyrromethene-dithienylethene molecular dyad is introduced as a prototype for the efficient fluorescence intensity modulation on the molecular level. The functionality of the system is based on the photochromism of the dithienylethene, which facilitates an efficient on- and off-switching of a Förster-type intramolecular energy transfer between the photoexcited BODIPY donor and the dithienylethene acceptor moiety. The switching behavior and dynamics of the molecular dyad are monitored by steady state and time-resolved spectroscopic methods. A quenching efficiency of up to 96% in the off-state is observed and explained by a drastically accelerated decay of the boron-dipyrromethene excited state due to the efficient energy transfer despite the orthogonal arrangement of donor and acceptor. An energy transfer time orders of magnitude shorter than the lifetime of the boron-dipyrromethene in the open state is determined.

Vibrational coherence transfer in an electronically decoupled molecular dyad.

The ring opening of a dithienylethene photoswitch incorporated in a bridged boron-dipyrromethene - dithienylethene molecular dyad was investigated with ultrafast spectroscopy. Coherent vibrations in the electronic ground state of the boron-dipyrromethene are triggered after selective photoexcitation of the closed dithienylethene indicating vibrational coupling although the two moieties are electronically isolated. A distribution of short-lived modes and a long-lived mode at 143 cm(-1) are observed. Analysis of the theoretical frequency spectrum indicates two modes at 97 cm(-1) and 147 cm(-1) which strongly modulate the electronic transition energy. Both modes exhibit a characteristic displacement of the bridge suggesting that the mechanical momentum of the initial geometry change after photoexcitation of the dithienylethene is transduced to the boron-dipyrromethene. The relaxation to the dithienylethene electronic ground state is accompanied by significant heat dissipation into the surrounding medium. In the investigated dyad, the boron-dipyrromethene acts as probe for the ultrafast photophysical processes in the dithienylethene.

Discrimination between FRET and non-FRET quenching in a photochromic CdSe quantum dot/dithienylethene dye system.

A photochromic Förster resonance energy transfer (FRET) system was employed to disentangle the fluorescence quenching mechanisms in quantum dot/photochromic dye hybrids. In the off-state of the dye the main quenching mechanism is FRET whereas the moderate quenching in the on-state is due to non-FRET pathways opened up upon assembly.

Ultrafast dynamics of dithienylethenes differently linked to the surface of TiO2 nanoparticles.

The photoinduced dynamics of a dithienylethene chromophore coupled to the surface of TiO(2) by either a tripodal linker or a carboxyl group was investigated with ultrafast transient absorption spectroscopy. The absence of electron transfer from the photoexcited tripodal dithienylethene chromophore demonstrates that the tripod efficiently uncouples the electronic systems of dithienylethene and TiO(2). Contrary to this situation, photoinduced electron transfer can compete with ultrafast intramolecular relaxation in the COOH-dithienylethene/TiO(2) coupled system. An electron transfer rate of 1.1 × 10(12) s(-1) can be extracted, which is considerably slower than the intramolecular relaxation rate of the dithienylethene (3.7 × 10(12) s(-1)). Consequently, the electron transfer reaction exhibits a low efficiency.

Ultrafast electron transfer from photoexcited CdSe quantum dots to methylviologen.

The ultrafast charge separation at the quantum dot (QD)/molecular acceptor interface was investigated in terms of acceptor concentration and the size of the QD. Time-resolved experiments revealed that the electron transfer (ET) from the photoexcited QD to the molecular acceptor methylviologen (MV(2+)) occurs on the fs time scale for large acceptor concentrations and that the ET rate is strongly reduced for low concentrations. The increase in the acceptor concentration is accompanied with a growth in the overlap of donor and acceptor wavefunctions, resulting in a faster reaction until the MV(2+) concentration reaches a saturation limit of 0.3-0.4 MV(2+) nm(-2). Moreover, we found significant QD size dependence of the ET reaction, which is explained by a change of the free energy (ΔG).

Donor/Acceptor adsorbates on the surface of metal oxide nanoporous films: a spectroscopic probe for different electron transfer pathways.

A composite model system which consists of a molecular donor/acceptor pair coupled to the surface of metal oxide nanoporous films is proposed to study the contribution of the surface trap states and their influence on the interfacial ET as well as charge trapping and spatial diffusion processes in films. The photophysics of this donor/acceptor system is investigated by time-resolved transient absorbance spectroscopy in the UV/Vis (347-675 nm) spectral region. Variation of the band gap allows one to disentangle the ET pathways. Adsorption of the donor/acceptor pair on the nonreactive Al(2)O(3) surface shows that coupling to the surface assists electron transfer between adsorbed donor and acceptor molecules, resulting in ultrafast intermolecular electron transfer of approximately 100 fs. On the other hand, a competition between interfacial and intermolecular electron transfer is observed for the donor/acceptor pair coupled to a reactive TiO(2) surface. The subsequent transfer of the conduction band electron to the electron acceptor is examined by monitoring the free charge carrier absorption in the mid-IR (approximately 5 microm) spectral region.

Ultrafast charge separation in multiexcited CdSe quantum dots mediated by adsorbed electron acceptors.

Ultrafast ET with a characteristic time constant of approximately 70 fs between CdSe QDs (mean radii of 1.4 nm) photoexcited in the lowest 1S electron state (lambda(exc) = 539 nm), and the molecular electron acceptor MV(2+) adsorbed on the QD surface was observed. The photophysics of such a system was investigated by time-resolved transient absorbance spectroscopy in the UV-visible spectral region. Our studies for the coupled system as a function of excitation intensity at lambda(exc) = 387 nm show that the ET processes compete efficiently with Auger recombination in CdSe QDs and at least 4 e-h pairs can be separated by ET to the electron acceptor MV(2+).

Ultrafast photoinduced processes in alizarin-sensitized metal oxide mesoporous films.

Close to the edge: Photoexcitation of alizarin coupled to the surface of mesoporous TiO(2) films leads to ultrafast electron transfer to the TiO(2) conduction band (see picture). Complex kinetics after photoexcitation depend on the excitation energy, and indicate a position of the alizarin excited state close to the TiO(2) conduction band edge, where the density of acceptor states is reduced. The photoinduced dynamics in Al(2)O(3) and TiO(2) mesoporous films sensitized by the strongly coupled alizarin dye is investigated by femtosecond transient absorption spectroscopy in the spectral range from UV to mid-IR. Alizarin/Al(2)O(3) acts as a nonreactive reference system, in which no electron transfer is observed. For comparison, the photoexcitation of the alizarin dye coupled to the surface of TiO(2) films leads to ultrafast electron transfer from the dye to the TiO(2) conduction band on the sub-100-fs timescale. We observe a fast relaxation of the alizarin excited state as well as a fast recombination of injected electrons with the alizarin cation on the picosecond timescale, which gives rise to very complex kinetics at short delay times. The infrared measurements clearly indicate that trapping of injected electrons is the main mechanism responsible for the observed long-lived charge separation in TiO(2) mesoporous films. The experimental findings can be explained by a position of the dye excited state close to the conduction band edge.