Effect of Oligomer Length on Photophysical Properties of Platinum Acetylide Donor–Acceptor–Donor Oligomers

We report a systematic study that explores how the triplet excited state is influenced by conjugation length in a series of benzothiadiazole units containing donor–acceptor–donor (DAD)-type platinum acetylide oligomers and polymer. The singlet and triplet excited states for the series were characterized by an array of photophysical methods including steady-state luminescence spectroscopy and femtosecond–nanosecond transient absorption spectroscopy. In addition to the experimental work, a computational study using density functional theory was conducted to gain more information about the structure, composition, and energies of the frontier molecular orbitals. It is observed that both the singlet and triplet excited states are mainly localized on a single donor–acceptor–donor unit in the oligomers. Interestingly, it is discovered that the intersystem crossing efficiency increases dramatically in the longer oligomers. The effect is attributed to an enhanced contribution of the heavy metal platinum in the frontier orbitals (HOMO and LUMO), an effect that leads to enhanced spin–orbit coupling

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

Two sets of conjugated polyelectrolytes with different molecular weights (<i>M</i>_n) 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-<i>n</i>, where <i>n</i> = 7, 9, and 14) and methylene (-CH₂-) (P2-C-<i>n</i>, <i>n</i> = 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-<i>n</i> series increases with a decreasing molecular weight while the efficiency of the P2-C-<i>n</i> series remains constant regardless of the molecular weight. Steady state photophysical measurements and dynamic light scattering investigation prove that P1-O-<i>n</i> polymers aggregate in solution while P2-C-<i>n</i> series are in the monomeric state. In P1-O-<i>n</i> 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

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