Laren M. Tolbert

A technique to compare polythiophene solid-state dye sensitized TiO2 solar cells to liquid junction devices

In this communication, we report on a technique to fabricate solid-state polythiophenebased dye sensitized solar cells (DSSCs) that can be directly compared to analogous liquid junction devices. The device configuration is based on non-porous TiO2 thin films and one of the three undoped polythiophene hole conductors: poly[3-(11 diethylphosphorylundecyl) thiophene], P3PUT, poly(4-undecyl-2,2 0 -bithiophene), P4UBT, or poly(3-undecyl-2,2 0 -bithiophene), P3UBT. These polymers were spin coated and cast from organic solutions onto the TiO2 films. The dense TiO2 thin films (ca. 30 nm) were deposited on conductive glass via facile spray pyrolysis and sol–gel techniques. After that, cis-(SCN)2 Bis(2,2 0 bipyridyl-4,4 0 dicarboxylate) ruthenium(II) (a.k.a. Ru N3 dye) was adsorbed on the TiO2 surface, and the polythiophenes were utilized as hole conductors in a simplified solar cell geometry. The results were compared to the control DSSC device made with dense TiO2 and a liquid electrolyte, or

Topochemistry and photomechanical effects in crystals of green fluorescent protein-like chromophores: effects of hydrogen bonding and crystal packing.

To obtain insight into the effects of the environment on the photophysics and photochemistry of the green fluorescence protein (GFP), eight crystal structures of six synthetic aryl-substituted analogues (2-fluoro, 2-methyl, 3-hydroxy, 3-methoxy, 2,4-dimethyl and 2,5-dimethyl) of the GFP chromophore (4-hydroxy-benzylidenedimethylimidazolinone) were determined and correlated with their two-dimensional steady-state and time-resolved solid-state excitation-emission spectra. The stacking between the molecules greatly affected the emission energy and the lifetime of the emission of the chromophore, implying that pi-pi interactions could be critical for the photophysics of GFP. The reaction pathways were dependent on the excitation energy, resulting either in [2 + 2] photodimerization at the bridging double bond (UV excitation) or flipping of the imidazolone ring (visible excitation). The meta-hydroxy chromophore (3-HOBDI) was the only GFP-chromophore analogue that was obtained as more than one stable polymorph in the pure state thus far. Due to the asymmetric substitution with hydrogen bond donors and acceptors, 3-HOBDI is tetramorphic, the forms showing distinctly different structure and behavior: (1) while one of the polymorphs (3-HOBDI-A), having multilayer structure with alternating stereochemistry of linear hydrogen-bonded motifs, undergoes photodimerization under UV light, (2) another (3-HOBDI-C), which has dimeric head-to-tail structure, shows Z-to-E isomerization via tau-one-bond flip of the imidazolone ring by excitation in the visible region. X-ray diffraction analysis of a partially reacted single crystal of 3-HOBDI-C provided the first direct evidence of tau-one-bond flip occurring in a GFP-like compound. Moreover, the cooperative action of the photodimerization of 3-HOBDI-A appears as a photomechanical effect of unprecedented magnitude for a single crystalline specimen, where photoexcited single crystals bend to more than 90 degrees without breaking.

Fluorescence Self-Quenching of a Mannosylated Poly(p-phenyleneethynylene) Induced by Concanavalin A

We report that static quenching of a mannosylated conjugated polymer (sugar-PPE) by Concanavalin A is positively dependent upon sugar-PPE concentration, that is, the recorded Stern-Volmer constants increase with increasing sugar-PPE concentration. Comparison with data obtained from isothermal titration calorimetry (ITC) display the increased sensitivity of the quenching method when compared to ITC. The proposed mechanism suggests the interaction of two or more chains of PPE with one Con A molecule leading to a quenched sugar-PPE-Con A construct.

Characterization of nanocrystalline and thin film TiO2 solar cells with poly(3-undecyl-2,2′-bithiophene) as a sensitizer and hole conductor

We report on the use of poly(3-undecyl-2,2′-bithiophene) and three different types of TiO2 film layers to determine the dependence of photovoltaic performance on the morphology of the TiO2 films. It was observed that the TiO2 film morphology plays an important role in the performance of photovoltaic solar cells with polythiophene used as both sensitizer and hole conductor. The polymer was tested on a flat TiO2 layer made from a sol–gel process, a larger surface area TiO2 layer derived from an aqueous TiF4 solution, and a nanocrystalline thin TiO2 film. We observe over twice the improvement in short circuit current density in the nanocrystalline cells (jsc=233.6 μA cm−2) over that of the flat titania cells (89.8 μA cm−2). The best photocurrent density (448 μA cm−2) performance was from the TiF4 derived titania based cells when using the polymer. It was noted that soaking the nanocrystalline and TiF4 cells in the polymer for at least 24 h improves their overall performance, while no noticeable improvement is evident for the flat sol–gel derived cells. We explain our results by proposing that pore filling is likely to be easier in the TiF4 derived TiO2 due to larger pore sizes. Multiple reflections of light may also play an important role in the TiF4 titania films by enhancing the amount of light absorbed by the polymer. These observations suggest that the relative size of the sensitizer molecule and the pores of nanocrystalline films may be a critical factor to consider in designing photovoltaic devices such as solar cells based on nanoporous materials.

Poly(4-undecyl-2,2'-bithiophene) as a hole conductor in solid state dye sensitized titanium dioxide solar cells

Reference LPI-ARTICLE-2001-007doi:10.1016/S0379-6779(00)00650-0View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Activation and Tuning of Green Fluorescent Protein Chromophore Emission by Alkyl Substituent-Mediated Crystal Packing

O-Alkyl synthetic analogues of the green fluorescent protein chromophore are nonfluorescent in solution but demonstrate a bright emission in the crystalline state. Three-dimensional steady-state and time-resolved emission spectroscopies in the solid state, as well as single-crystal X-ray diffraction, reveal the nature of complex emission in the crystals. A hypsochromic emission shift with an increase of the alkyl size is determined by the monomer-aggregate emission ratio.

Meta and para effects in the ultrafast excited-state dynamics of the green fluorescent protein chromophores

Femtosecond transient absorption and fluorescence upconversion experiments have been performed to investigate the photoinduced dynamics of the meta isomer of the green fluorescent protein chromophore, m-HBDI, and its O-methylated derivative, m-MeOBDI, in various solvent mixtures at neutral, acidic, and basic pH. The para isomer, p-HBDI, and its O- and N-methylated derivatives, p-MeOBDI and p-HBDIMe(+), were also studied for comparison. In all cases, fast quenching of the excited S1 state by internal conversion (IC) to the ground state was observed. In the para compounds, IC, presumably promoted by the internal twisting, arises in <1 ps. A similar process takes place in the meta compounds in nonaqueous solvents but with notably slower kinetics. In aqueous solutions, the meta compounds undergo ultrafast intermolecular excited-state proton transfer that competes with isomerization.

Isomerization in Fluorescent Protein Chromophores Involves Addition/Elimination

The green fluorescent protein (GFP) chromophore undergoes both photochemical and thermal isomerizations. Typically, the Z form is more stable and undergoes photochemical conversion to the E form followed by thermal reversion over a period of seconds or minutes. Although the mechanism of the thermal reversion has been the subject of some investigations, the surprisingly low activation energy for this process has not sparked any controversy. We now show that the chromophore is surprisingly stable in both E and Z forms and that the facile thermal reversion is the result of a novel nucleophilic addition/elimination mechanism. This observation may have implications for the intervention of such processes, as well as blinking and kindling, in fluorescent proteins.

Recapture of GFP Chromophore Fluorescence in a Protein Host

When encapsulated by human serum albumin (HSA), certain derivatives of the green fluorescent protein (GFP) chromophore recover their fluorescence due to inhibition of torsional motion. These derivatives show remarkable sensitivity and selectivity as well as favorable spectroscopic properties toward HSA, thus providing selective probes for this and similar proteins and demonstrating the use of GFP chromophores as topological fluorophores.

Fluorescence imaging using synthetic GFP chromophores

Green fluorescent protein and related proteins carry chromophores formed within the protein from their own amino acids. Corresponding synthetic compounds are non-fluorescent in solution due to photoinduced isomerization of the benzylideneimidiazolidinone core. Restriction of this internal rotation by binding to host molecules leads to pronounced, up to three orders of magnitude, increase of fluorescence intensity. This property allows using GFP chromophore analogs as fluorogenic dyes to detect metal ions, proteins, nucleic acids, and other hosts. For example, RNA aptamer named Spinach, which binds to and activates fluorescence of some GFP chromophores, was proved to be a unique label for live-cell imaging of specific RNAs, endogenous metabolites and target proteins. Chemically locked GFP chromophores are brightly fluorescent and represent potentially useful dyes due to their small size and high water solubility.