Pyridine-terminated low gap π-conjugated oligomers: design, synthesis, and photophysical response to protonation and metalation

Pyridine terminated donor–acceptor π-conjugated oligomers have been prepared and their absorption and emission properties have been evaluated with respect to protonation and metalation.

Self-Assembled Bilayers on Nanocrystalline Metal Oxides: Exploring the Non-Innocent Nature of the Linking Ions

Self-assembled bilayers on nanocrystalline metal oxide films are an increasingly popular strategy for modulating electron and energy transfer at dye-semiconductor interfaces. A majority of the work to date has relied on Zr^II and Zn^IV linking ions to assemble the films. In this report, we demonstrate that several different cations (Cd^II, Cu^II, Fe^II, La^III, Mn^II, and Sn^IV) are not only effective in generating the bilayer assemblies but also have a profound influence on the stability and photophysical properties of the films. Bilayer films with Zr^IV ions exhibited the highest photostability on both TiO₂ and ZrO₂. Despite the metal ions having a minimal influence on the absorption/emission energies and oxidation potentials of the dye, bilayers composed of Cu^II, Fe^II, and Mn^II exhibit significant excited-state quenching. The excited-state quenching decreases the electron injection yield but also, for Cu^II and Mn^II bilayers, significantly slows the back electron transfer kinetics.

Triplet excited state properties in variable gap π-conjugated donor-acceptor-donor chromophores

A series of variable band-gap donor-acceptor-donor (DAD) chromophores capped with platinum(ii) acetylide units has been synthesized and fully characterized by electrochemical and photophysical methods, with particular emphasis placed on probing triplet excited state properties. A counter-intuitive trend of increasing fluorescence quantum efficiency and lifetime with decreasing excited state energy (optical gap) is observed across the series of DAD chromophores. Careful study of the excited state dynamics, including triplet yields (as inferred from singlet oxygen sensitization), reveals that the underlying origin of the unusual trend in the fluorescence parameters is that the singlet-triplet intersystem crossing rate and yield decrease with decreasing optical gap. It is concluded that the rate of intersystem crossing decreases as the LUMO is increasingly localized on the acceptor unit in the DAD chromophore, and this result is interpreted as arising because the extent of spin-orbit coupling induced by the platinum heavy metal centers decreases as the LUMO is more localized on the acceptor. In addition to the trend in intersystem crossing, the results show that the triplet decay rates follow the Energy Gap Law correlation over a 1.8 eV range of triplet energy and 1000-fold range of triplet decay rates. Finally, femtosecond transient absorption studies for the DAD chromophores reveals a strong absorption in the near-infrared region which is attributed to the singlet excited state. This spectral band appears to be general for DAD chromophores, and may be a signature of the charge transfer (CT) singlet excited state.

Effect of Selenium Substitution on Intersystem Crossing in π-Conjugated Donor-Acceptor-Donor Chromophores: The LUMO Matters the Most

This study explores the effect of substitution of selenium (Se) for sulfur (S) on the photophysical properties of a series of π-conjugated donor-acceptor-donor chromophores based on 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT). The effect of Se substitution is studied systematically, where the substitution is in the thiophene donors only, the benzothiadiazole acceptor only, and in all of the positions. The fluorescence quantum yield decreases with an increase in Se substitution. Nanosecond-microsecond transient absorption and singlet oxygen sensitization experiments show that the effect of Se is due to an increase in the rate and efficiency of intersystem crossing with increased Se substitution. The relationship between intersystem crossing efficiency and heteroatom substitution pattern shows that the effects are largest when the heavy atom Se is in the acceptor benzothiadiazole unit. DFT calculations support the hypothesis that the effect arises because the LUMO is concentrated in the acceptor moiety, enhancing the spin-orbit coupling effect imparted by the Se atom.

Conjugated Polyelectrolyte-Sensitized TiO2 Solar Cells: Effects of Chain Length and Aggregation on Efficiency

Two sets of conjugated polyelectrolytes with different molecular weights (Mn) in each set were synthesized. All polymers feature the same conjugated backbone with alternating (1,4-phenylene) and (2,5-thienylene ethynylene) repeating units, but different linkages between the backbone and side chains, namely, oxy-methylene (-O-CH2-) (P1-O-n, where n = 7, 9, and 14) and methylene (-CH2-) (P2-C-n, n = 7, 12, and 18). They all bear carboxylic acid moieties as side chains, which bind strongly to titanium dioxide (TiO2) nanoparticles. The two sets of polymers were used as light-harvesting materials in dye-sensitized solar cells. Despite the difference in molecular weight, polymers within each set have very similar light absorption properties. Interestingly, under the same working conditions, the overall cell efficiency of the P1-O-n series increases with a decreasing molecular weight while the efficiency of the P2-C-n series remains constant regardless of the molecular weight. Steady state photophysical measurements and dynamic light scattering investigation prove that P1-O-n polymers aggregate in solution while P2-C-n series are in the monomeric state. In P1-O-n series, a higher-molecular weight polymer results in a larger aggregate, which reduces the amount of polymers that are adsorbed onto TiO2 films and overall cell efficiency.

Effect of isomerism and chain length on electronic structure, photophysics, and sensitizer efficiency in quadrupolar (donor)₂-acceptor systems for application in dye-sensitized solar cells

We report on quadrupolar (donor)2-acceptor sensitizers for dye-sensitized solar cells (DSSCs). The acceptor units are based on dithieno[2,3-a:3',2'-c]phenazine and dithieno[3,2-a:2',3'-c]phenazine coupled to thiophene donors. The optoelectronic and photophysical properties of two sets of isomers reveal a rigid structure for linear isomers and an efficient nonradiative decay for branched isomers. These sensitizers were integrated into DSSCs, and the quadrupolar structure is an operational design, as the IPCE reached up to 38% from 400 nm to 600 nm. The lengthening of the donor chain increases the efficiency, demonstrating the appeal of these oligomeric dyes for DSSCs.