Structural and Electronic Properties of Photoexcited TiO₂ Nanoparticles from First Principles

The structure and energetics of excitons and individual electron and hole polarons in a model anatase TiO₂ nanoparticle (NP) are investigated by means of Density Functional Theory (DFT) and Time Dependent (TD)-DFT calculations. The effect of the Hartree–Fock exchange (HF-exc) contribution in the description of TiO₂ NPs with unpaired electrons is examined by comparing the results from semilocal and hybrid DFT functionals with different HF-exc percentages, including a long-range corrected hybrid functional. The performances of TD-DFT and ground state (SCF) DFT approaches in the description of the photoexcited polaron states in TiO₂ NPs are also analyzed. Our results confirm that the HF-exc contribution is essential to properly describe the self-trapping of the charge carriers. They also suggest that long-range corrected functionals are needed to properly describe excited state relaxation in TiO₂ NPs. TD-DFT geometry optimization of the lowest excited singlet and triplet states deliver photoluminescence values in close agreement with the experimental data

Structural Effects of the Donor Moiety on Reduction Kinetics of Oxidized Dye in Dye-Sensitized Solar Cells

One of the major factors influencing the regeneration rate of the oxidized dye in dye-sensitized solar cells (DSSCs) is the energy difference (Δ<i>G</i>) between the levels of the dye’s HOMO and redox couple in the electrolyte. To investigate other factors that influence this process, we examined the effect of structural differences of donor moieties on the reduction rate of the oxidized dye of organic dyes that is composed of an acceptor unit, a π-conjugated linker unit, and a donor unit, including carbazole dye (<b>MK-1</b>), triphenylamine dye (<b>MK-88</b>), and coumarin dye (<b>MK-31</b>). The DSSCs using <b>MK-88</b> showed the fastest regeneration rate even though the Δ<i>G</i> was not the largest among the dye structures evaluated. The regeneration rates of all of the dyes were enhanced by reducing the number of adsorbed dyes. On the basis of the results, we attribute the fast regeneration of <b>MK-88</b> to the large collision cross section of the oxidized dye, that is, the increased reduction rate to the larger exposure of the HOMO of the dyes to the redox species. The effect of the exposed surface area on the reduction rate was as large as the free-energy difference, suggesting a new design strategy for efficient sensitizers

Template Synthesis of Decaphyrin without <i>Meso</i>-Bridges: Cyclo[10]pyrrole

An acenaphthylene-fused cyclo[10]­pyrrole <b>1b</b> was selectively synthesized via an oxidative coupling reaction of the corresponding 2,2′-bipyrrole with the appropriate dianion template, croconate anion. The structure of <b>1b</b> as the isolated largest cyclo­[<i>n</i>]­pyrrole was elucidated by X-ray crystallographic analysis. The absorption spectrum exhibited a markedly red-shifted, intensified L band at 1982 nm, which was interpreted by application of Michl’s perimeter and Gouterman’s 4-orbital models, supported by magnetic circular dichroism (MCD) data and theoretical calculations

Template Synthesis of Decaphyrin without <i>Meso</i>-Bridges: Cyclo[10]pyrrole

An acenaphthylene-fused cyclo[10]­pyrrole <b>1b</b> was selectively synthesized via an oxidative coupling reaction of the corresponding 2,2′-bipyrrole with the appropriate dianion template, croconate anion. The structure of <b>1b</b> as the isolated largest cyclo­[<i>n</i>]­pyrrole was elucidated by X-ray crystallographic analysis. The absorption spectrum exhibited a markedly red-shifted, intensified L band at 1982 nm, which was interpreted by application of Michl’s perimeter and Gouterman’s 4-orbital models, supported by magnetic circular dichroism (MCD) data and theoretical calculations

Template Synthesis of Decaphyrin without <i>Meso</i>-Bridges: Cyclo[10]pyrrole

An acenaphthylene-fused cyclo[10]­pyrrole <b>1b</b> was selectively synthesized via an oxidative coupling reaction of the corresponding 2,2′-bipyrrole with the appropriate dianion template, croconate anion. The structure of <b>1b</b> as the isolated largest cyclo­[<i>n</i>]­pyrrole was elucidated by X-ray crystallographic analysis. The absorption spectrum exhibited a markedly red-shifted, intensified L band at 1982 nm, which was interpreted by application of Michl’s perimeter and Gouterman’s 4-orbital models, supported by magnetic circular dichroism (MCD) data and theoretical calculations