Larry Lüer

Fluorescence and absorption spectra of oligophenylenevinylenes: Vibronic coupling, band shapes, and solvatochromism

Fluorescence emission and excitation spectra of para-phenylene vinylenes nPV with n=1–4 styryl units are investigated experimentally and theoretically as a function of the temperature and the polarizability of the solvent. At low temperatures, the vibronic structures of the S0↔S1 emission and excitation bands are mirror symmetrical with negligible 0–0 energy gaps. The frequencies of the prominent vibrational modes are assigned to the second longitudinal acoustic phonon modes of the entire molecules and to localized carbon–carbon stretching vibrations. The complete vibronic structures of the spectra are calculated at the ab initio Hartree–Fock (HF/6-311G*) and restricted configuration interaction singles (RCIS/6-311G*) levels of theory assuming planar C2h molecular symmetry. The theoretically predicted spectra are in good agreement with the experiments. At room temperature, a 0–0 energy gap between the first band maxima opens, and the mirror symmetry between absorption and emission is lost. The vibronic ba...

High-time-resolution pump-probe system with broadband detection for the study of time-domain vibrational dynamics.

We present an ultrafast transient absorption spectroscopy system in the visible combining high-sensitivity broadband detection with extreme temporal resolution. The instrument is based on an ultrabroadband sub-10 fs optical parametric amplifier coupled to an optical multichannel analyzer with fast electronics, enabling single-shot detection at 1 kHz repetition rate. For a given pump-probe delay tau, we achieve a differential transmission (DeltaTT) sensitivity of the order of 10(-4) over the lambda(pr)=490-720 nm probe wavelength range by averaging over 1000 shots, allowing the acquisition of complete two-dimensional DeltaTT (lambda(pr),Tau) maps within a few minute measurement time. We present application examples highlighting the capability of this instrument to observe ultrafast dynamical processes, follow impulsively excited vibrational motions with frequency as high as 3000 cm(-1) (11 fs period), and determine the probe wavelength dependence of amplitude and phase of the oscillations.

Ultrafast electron-hole dynamics in core/shell CdSe/CdS dot/rod nanocrystals.

Colloidal semiconductor nanocrystals are nanoscale materials whose optical, electronic and transport properties, due to their strong dependence on size and shape, can be finely tuned by advanced chemical synthesis approaches. Among various types of semiconductor nanocrystals, core-shell nanostructures comprised of a semiconductor core that is covered by a thin shell of another type of semiconductor material have been extensively investigated.

Ultrafast Excitation Energy Transfer in Small Semiconducting Carbon Nanotube Aggregates

We study excitation energy transfer in small aggregates of chirality enriched carbon nanotubes by transient absorption spectroscopy. Ground state photobleaching is used to monitor exciton population dynamics with sub-10 fs time resolution. Upon resonant excitation of the first exciton transition in (6,5) tubes, we find evidence for energy transfer to (7,5) tubes within our time resolution (<10 fs). Excitation in the visible spectral range, where the second excitonic transitions occur, is followed by fast intratube relaxation and subsequent energy transfer, in particular from the (8,4) tube toward other tubes, the latter process occurring in less than 10 fs.

Time-resolved methods in biophysics. 4. Broadband pump–probe spectroscopy system with sub-20 fs temporal resolution for the study of energy transfer processes in photosynthesis

In this paper we discuss how to push the temporal resolution limits of transient absorption spectroscopy in order to detect very fast processes (energy relaxation, energy or charge transfer, vibrational coherence) taking place in molecules of biological relevance. After reviewing the main principles of femtosecond pump-probe spectroscopy, we describe an experimental setup based on two synchronized non-collinear optical parametric amplifiers (NOPAs). Each NOPA can be independently configured to generate ultra-broadband sub-10 fs visible pulses, tunable 10-15 fs visible pulses, tunable 15-40 fs near-infrared pulses (900-1500 nm). This system enables to perform pump-probe experiments over nearly two octaves of spectrum with sub-20 fs temporal resolution. We then present an application example highlighting the capability of this instrument to track excited state dynamics in biomolecules on the sub-100 fs timescale: the study of carotenoid-bacteriochlorophyll energy transfer processes in peripheral light-harvesting complexes (LH2) from purple bacteria. We show that, by comparing excited-state dynamics of the carotenoids in organic solvents and inside the LH2 complexes, it is possible to visualize in the time domain the primary events in photosynthesis.

H‐Shaped Oligofluorenes for Highly Air‐Stable and Low‐Threshold Non‐Doped Deep Blue Lasing

H-shaped oligofluorenes as gain media exhibit excellent photo- (large robustness against oxidation) and thermal stabilities in ambient atmosphere for large σe and low-threshold (0.22 nJ pulse(-1) ) deep blue distributed feedback (DFB) lasers. Their amplified spontaneous emission (ASE) thresholds increase less than 3-fold and the emission spectra exhibit almost no shift with film samples annealed up to 200 °C in open air.

The effect of oxygen induced degradation on charge carrier dynamics in P3HT:PCBM and Si-PCPDTBT:PCBM thin films and solar cells

Due to their light weight, transparency and flexibility, organic photovoltaic (OPV) devices are ideal for building integration. As this application requires solar cell life times of more than twenty years and oxygen ingress cannot be avoided at competitive cost on this time scale, OPV modules must be intrinsically stabilized against photo-oxidation. To this end, the mechanism of rapid performance loss of OSCs due to oxygen-induced degradation must be understood. Here, we combine transient absorption experiments with electrical studies in P3HT:PCBM and Si-PCPDTBT:PCBM thin films and solar cells after controlled photo-oxidation, studying charge carrier dynamics on the femtosecond to millisecond time scale. We find that oxygen-induced degradation does not significantly influence charge generation, while its influence on charge recombination is strong in both materials. A dramatic retardation of charge recombination already at low levels of oxygen-induced degradation is attributed to a substantial reduction of charge mobilities. We also observe a significant increase of the background concentration of charge carriers with the level of degradation, which leads to a crossover from second order towards pseudo-first order recombination behaviour. Extraction is shown to be retarded even more strongly than recombination, possibly by a reduction of the extraction field by the background carriers. Overall, the recombination yield is increased with degradation, explaining the strong performance loss already at low degradation levels.

Excited State Features and Dynamics in a Distyrylbenzene-Based Mixed Stack Donor-Acceptor Cocrystal with Luminescent Charge Transfer Characteristics.

Combined structural, photophysical, and quantum-chemical studies at the quantum mechanics/molecular mechanics (QM/MM) level precisely reveal the structure-property relationships in a mixed-stack donor-acceptor cocrystal, which displays vibronically structured fluorescence, strongly red-shifted against the spectra of the parent donor and acceptor, with high quantum yield despite the pronounced CT character of the emitting state. The study elucidates the reasons for this unusual combination, quantifies the ordering and nature of the collective excited singlet and triplet state manifold, and details the deactivation pathways of the initially created Franck-Condon state.

Tracking energy transfer between light harvesting complex 2 and 1 in photosynthetic membranes grown under high and low illumination

Energy transfer (ET) between B850 and B875 molecules in light harvesting complexes LH2 and LH1/RC (reaction center) complexes has been investigated in membranes of Rhodopseudomonas palustris grown under high- and low-light conditions. In these bacteria, illumination intensity during growth strongly affects the type of LH2 complexes synthesized, their optical spectra, and their amount of energetic disorder. We used a specially built femtosecond spectrometer, combining tunable narrowband pump with broadband white-light probe pulses, together with an analytical method based on derivative spectroscopy for disentangling the congested transient absorption spectra of LH1 and LH2 complexes. This procedure allows real-time tracking of the forward (LH2 → LH1) and backward (LH2←LH1) ET processes and unambiguous determination of the corresponding rate constants. In low-light grown samples, we measured lower ET rates in both directions with respect to high-light ones, which is explained by reduced spectral overlap between B850 and B875 due to partial redistribution of oscillator strength into a higher energetic exciton transition. We find that the low-light adaptation in R. palustris leads to a reduced elementary backward ET rate, in accordance with the low probability of two simultaneous excitations reaching the same LH1/RC complex under weak illumination. Our study suggests that backward ET is not just an inevitable consequence of vectorial ET with small energetic offsets, but is in fact actively managed by photosynthetic bacteria.

Primary photo-events in a metastable photomerocyanine of spirooxazines

We report on the ultrafast excited-state relaxation dynamics of the metastable photo-merocyanine (open-form) isomer of spiro-phenantroxazine, measured by pump and probe spectroscopy with sub-40-fs temporal resolution. We found that the photo-induced yield for ring-closure is negligible, and that the excited-state lifetime is only on the order of 300 fs. Relaxation leads to the non-adiabatic formation of a hot ground state (HGS). In this state, a coherent oscillation with 45 cm−1 frequency is present, showing strong anharmonicity. We attribute it to the motion (torsion/bending) of the molecular backbone attempting geometric relaxation to the close form. The strength of the coherent oscillation and the subsequent spectral relaxation in the HGS, together with the ultrashort lifetime, points to a crossing through a conical intersection (CI). We conclude that excited-states on the merocyanine form pass through a CI that is different from the one that would lead to ring-closure. We discuss design rules for the spiro-oxazine class, allowing for bidirectional switching avoiding this parasitic CI.