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.

Pseudorotation in pyrrolidine: rotational coherence spectroscopy and ab initio calculations of a large amplitude intramolecular motion.

Pseudorotation in the pyrrolidine molecule was studied by means of femtosecond degenerate four-wave mixing spectroscopy both in the gas cell at room temperature and under supersonic expansion. The experimental observations were reproduced by a fitted simulation based on a one-dimensional model for pseudorotation. Of the two conformers, axial and equatorial, the latter was found to be stabilized by about 29 +/- 10 cm(-1) relative to the former one. The barrier for pseudorotation was determined to be 220 +/- 20 cm(-1). In addition, quantum chemical calculations of the pseudorotational path of pyrrolidine were performed using the synchronous transit-guided quasi-Newton method at the MP2 and B3LYP levels of theory. Subsequent CCSD(T) calculations yield the energy preference of the equatorial conformer and the barrier for pseudorotation to be 17 and 284 cm(-1), respectively.

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.

Ultrafast photoinduced dynamics of pigment yellow 101: fluorescence, excited-state intramolecular proton transfer, and isomerization.

The ultrafast excited-state dynamics of the fluorescent pigment yellow 101 (P.Y.101) and the closely related 1,1'-naphthalazine, a nonfluorescent derivative that lacks the OH groups at the naphthyl rings, are studied combining femtosecond spectroscopy and high-level quantum chemical calculations. The observed ultrafast dynamics and the spectral signature of photoexcited 1,1'-naphthalzine can be consistently explained with a previously proposed mechanism, suggesting fluorescence quenching via an optically forbidden npi* state. In contrast, for a description of the excited-state dynamics of P.Y.101, the expected simple absorption/fluorescence model is not adequate. Instead, besides fluorescence as the main decay channel of the excited-state population, ultrafast excited-state intramolecular proton transfer (ESIPT) and isomerization processes have to be considered for a complete understanding of the complex subnanosecond dynamics. Combining experiment and theory, the following kinetic model is derived: upon photoexcitation a major part of the excited-state population decays via fluorescence from an enol-type isomer of P.Y.101, while a small part of the population undergoes ESIPT and fluoresces from a keto-type form. Furthermore, arguments are given that, to a minor extent, also trans-cis isomerization of the keto form takes place on the S1 surface leading probably to a long-lived cis-keto form in the ground state. The remarkable photostability of this organic pigment is thus achieved by the interplay of different ultrafast nondestructive decay channels.

Insights into the structure of cyclohexane from femtosecond degenerate four-wave mixing spectroscopy and ab initio calculations.

In this paper, we report the use of femtosecond time-resolved degenerate four-wave mixing rotationally resolved spectroscopy to obtain very accurate structural information on the symmetric top cyclohexane. Apart from highlighting the versatility of this method in determining accurate structures of large and complex molecules without dipole moment, the present study also details the comparison of the experimentally determined rotational constant B(0) with that obtained from high-level ab initio calculations. The theoretical calculations, which were carried out at both the second-order Møller-Plesset (MP2) and coupled-cluster with single, double, and perturbative triple substitutions [CCSD(T)] levels of theory, also take into account vibrational averaging effects. A detailed investigation of the vibrational averaging effects reveals that the corrections emerge from only the six highly symmetric A(1g) modes, a justification of which is provided by an analysis of these modes.