Highlights of Customer Research: 2012

Ultrafast Systems is pleased to announce that our customers are carrying out exciting and innovative work with the spectrometric instruments developed in our manufacturing facility at Sarasota, Florida. From the many publications received from customers as a result of our recent call, we have selected a few to bring to the attention of our customer base. Brief accounts are presented below.

Comprehensive Photophysical Behavior of Ethynyl Fluorenes and Ethynyl Anthracenes Investigated by Fast and Ultrafast Time-Resolved Spectroscopy.
Benedetta Carlotti et al. ChemPhysChem 2012, 13, 724 – 735.

In research in the Elisei group at the University of Perugia, transient absorption and fluorescence spectrometries with nano- and femto-second time resolutions were carried out to characterize the natures of the lowest excited singlet and triplet states of three ethynyl fluorenes and three ethynyl anthracenes in solvents of different polarity. Upon increasing the solvent dielectric constant the fluorescence and triplet yields gradually decrease, behavior that has been attributed to an emitting state with charge transfer character, stabilized in polar solvents. This conclusion is supported by ultrafast transient absorption measurements in which two spectrally distinct species were observed and assigned to locally-excited (LE) and charge transfer (CT) states, the latter being the longer lived and fluorescent in more polar media. The experimental studies were supported by TDDFT calculations to determine the state order and nature of the lowest excited singlet and triplet states.

Synchronized energy and electron transfer processes in covalently linked CdSe-squaraine dye-TiO2 light harvesting assembly.
Choi, H.; Santra, P. K.; Kamat, P. V., ACS Nano 2012, 6, 5718–5726.

The manipulation of energy and electron transfer processes in a light harvesting assembly is an important criterion to mimic natural photosynthesis. Researchers in the Kamat group at the University of Notre Dame have succeeded in sequentially assembling CdSe quantum dot (QD) and squaraine dye (SQSH) on TiO2 films and thereby coupled energy and electron transfer processes to generate photocurrent in a hybrid solar cell. When attached separately, both CdSe QDs and SQSH inject electrons into TiO2 under visible–near-IR irradiation. However, CdSe QD, if linked to TiO2 via a SQSH linker, participates in an energy transfer process. The hybrid solar cells prepared in this way exhibited power conversion efficiency of 3.65% and good stability during illumination with global AM 1.5 solar conditions. Transient absorption spectrometry provided further insight into the energy transfer between excited CdSe QD and SQSH (rate constant of 6.7 × 1010 s–1) and interfacial electron transfer between excited SQSH and TiO2 (rate constant of 1.2 × 1011 s–1). The synergy of covalently linked semiconductor quantum dots and near-IR absorbing squaraine dye provides new opportunities to harvest photons from selective regions of the solar spectrum in an efficient manner.

The Role of Xanthophylls in Light Harvesting in Green Plants: A Spectroscopic Investigation of Mutant LHCII and Lhcb Pigment−Protein Complexes.
Marcel Fuciman et al. J. Phys. Chem. B 2012, 116, 3834−3849.

In research carried out in the group of Harry Frank at the University of Connecticut, the spectroscopic properties and energy transfer dynamics of protein-bound chlorophylls and xanthophylls in monomeric, major LHCII complexes, and minor LHCb complexes from genetically altered Arabidopsis thaliana plants have been investigated using both steady-state and time-resolved absorption and fluorescence spectrometric methods. The pigment−protein complexes that were studied contain Chl a, Chl b, and variable amounts of carotenoid entities, derived through mutations. The data reveal specific singlet energy transfer routes and excited state spectra and dynamics that depend on the xanthophyll present in the complex.

Ultrafast Studies of Excess Electrons in Liquid Acetonitrile: Revisiting the Solvated Electron/Solvent Dimer Anion Equilibrium.
Stephanie C. Doan and Benjamin J. Schwartz.  J. Phys. Chem. B, 2013, 117 (16), pp 4216–4221.

In the Schwartz group at the Department of Chemistry at UCLA, the researchers have re-examined the ultrafast relaxation dynamics of excess electrons injected into liquid acetonitrile using air- and water-free techniques, confirming the existence of two forms: a “traditional” solvated electron that absorbs in the near-IR, and a solvated molecular dimer anion that absorbs weakly in the visible. Excess electrons initially to localize as solvated electrons, but that there is a subsequent equilibration to form the dimer anion on an ∼80 ps time scale. The spectral signature of this inter-conversion between the two forms of the excess electron shows a clear isosbestic point. The data show that the molecular anion is favored by a factor of ∼4. It was also found that geminate recombination is essentially complete within the first 20 ps. The presence of small amounts of water in the acetonitrile can have a marked effect on the observed spectral dynamics, explaining the differences between our results and those in previously published work.

Multiple Exciton Generation and Dissociation in PbS Quantum Dot-Electron Acceptor Complexes.
Ye Yang et al. Nano Lett. 2012, 12, 4235−4241.

Multiple exciton generation (MEG) in quantum dots (QDs) is a process by which one absorbed photon generates multiple electron−hole pairs and thereby provides exciting possibilities for improving the energy conversion efficiency of photovoltaic and photocatalytic devices. However, implementing MEG in practical devices requires the extraction of multiple charge carriers before exciton−exciton annihilation and the development of new materials with improved MEG efficiency. The research group of Tianquan Lian at Emory University using PbS QD/methylene blue complexes as a QD/electron acceptor model system have demonstrated that the presence of electron acceptors does not affect the MEG efficiency of QDs and all generated excitons can be dissociated by electron transfer to the acceptor, achieving MEG and multiple exciton dissociation efficiencies of 112%. We further demonstrate that these efficiencies are not affected by the charging of QDs.

Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation.
J. D. Megiatto, Jr. et al. PNAS 2012, 109, 15578–15583.

In the photosynthetic photosystem II, electrons are transferred from a manganese-containing, oxygen-evolving complex (OEC) to the oxidized primary electron-donor chlorophyll P680•+ by a proton-coupled electron transfer process involving a tyrosinehistidine pair. Proton transfer from the tyrosine phenolic group to a histidine nitrogen positions the redox potential of the tyrosine between those of P680•+ and the OEC. In this publication, the Gust-Moores’ group at ASU report the synthesis and time-resolved spectroscopic study of a molecular triad that models this electron transfer. The triad consists of a high-potential porphyrin bearing two pentafluorophenyl groups (PF10), a tetracyanoporphyrin electron acceptor (TCNP), and a benzimidazole-phenol secondary electron-donor (Bi-PhOH). Excitation of PF10 in benzonitrile is followed by singlet energy transfer to TCNP (τ = 41 ps), whose excited state decays by photo-induced electron transfer (τ = 830 ps) to yield Bi-PhOH-PF10•+-TCNP•-. A second electron transfer reaction follows (τ < 12 ps), giving a final state postulated as BiH+-PhO-PF10-TCNP•-, in which the phenolic proton now resides on benzimidazole. This final state decays with a time constant of 3.8 μs. The triad thus functionally mimics the electron transfers involving the tyrosine-histidine pair in PSII. The final charge-separated state is thermodynamically capable of water oxidation, and its long lifetime suggests the possibility of coupling systems such as this system to water oxidation catalysts for use in artificial photosynthetic fuel production.

Corrole-Porphyrin Conjugates with Interchangeable Metal Centers.
T. H. Ngo et al. Eur. J. Org. Chem., 2012, 5605-5617.

In the Campagna group at the University of Messina oligoporphyrinoid materials composed of two or three tetrapyrrolic macrocycles were synthesized by means of either mono- or di-substitution (SNAr) of a phenolic Zn-AB3-porphyrin on a meso-dichloropyrimidinyl-substituted Cu-AB2-corrole. Selective metallation/demetallation sequences were carried out on these mixed corrole-porphyrin conjugates, affording multichromophoric systems with variable metal centers. The absorption spectra of the free-base corrole-porphyrin systems were essentially additive, demonstrating that only weak intercomponent interaction takes place in these assemblies, which can therefore be largely regarded as supramolecular systems. Photophysical studies of the free-base corrole–free-base porphyrin conjugates showed that these species are highly fluorescent, with fluorescence occurring from the lowest-energy singlet state of the porphyrin subunit(s), which is (are) the lowest energy state(s) of the assemblies. Pump-probe transient absorption spectroscopy experiments demonstrated that very efficient (>95%) corrole-to-porphyrin singlet-singlet energy transfer takes place in these pyrimidinyl-bridged multichromophoric systems, with rate constants on the picosecond timescale and operating through a coulombic mechanism.

Evidence for a Through-Space Pathway for Electron Transfer from Quantum Dots to Carboxylate-Functionalized Viologens.
Adam J. Morris-Cohen et al.  J. Phys. Chem. Lett. 2012, 3, 2840−2844.

This letter from the Weiss group at the Department of Chemistry at Northwestern University concludes that ultrafast transient absorption measurements reveals that the rate constant for photo-induced electron transfer (PET) from colloidal CdS quantum dots (QDs) to alkylcarboxylate-functionalized viologens is independent of the number of methylene groups in the alkyl chain (n). The rate constant for PET is (1.2 ± 0.3) × 1010 s1 for n = 1, 2, and 3, and for n = 0 (methyl viologen). The insensitivity of the electron transfer rate constant to the length of the functional groups on the viologen suggests that a “through-space” pathway, where the electron by-passes the alkylcarboxylate and tunnels instead through only the orbitals of the QD and of the bipyridinium core, is the dominant PET pathway.