Energetic requirements of iridium(III) complex based photosensitisers in photocatalytic hydrogen generation.

A new family of Ir(III) complexes were synthesised and employed as light-induced hydrogen-production photosensitisers in aqueous systems, where hydrogen evolution was observed only when the PS* was reduced by the sacrificial agent, NEt3, signifying that a minimum potential difference of >0.2 V between E(PS*/PS(-)) and E(NEt3(+)/NEt3) is required for efficient hydrogen production [i.e., E(PS*/PS(-)) >1.19 V versus NHE]. The analytical method developed here is demonstrated to be useful for screening new photosensitisers for light-driven hydrogen generation.

Spin-orbit coupling in phosphorescent iridium(III) complexes.

We study the excited states of two iridium(III) complexes with potential applications in organic light-emitting diodes: fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)(3)] and fac-tris(1-methyl-5-phenyl-3-n-propyl-[1,2,4]triazolyl)iridium(III) [Ir(ptz)(3)]. Herein we report calculations of the excited states of these complexes from time-dependent density functional theory (TDDFT) with the zeroth-order regular approximation (ZORA). We show that results from the one-component formulation of ZORA, with spin-orbit coupling included perturbatively, accurately reproduce both the results of the two-component calculations and previously published experimental absorption spectra of the complexes. We are able to trace the effects of both scalar relativistic correction and spin-orbit coupling on the low-energy excitations and radiative lifetimes of these complexes. In particular, we show that there is an indirect relativistic stabilisation of the metal-to-ligand charge transfer (MLCT) states. This is important because it means that indirect relativistic effects increase the degree to which SOC can hybridise singlet and triplet states and hence plays an important role in determining the optical properties of these complexes. We find that these two compounds are remarkably similar in these respects, despite Ir(ppy)(3) and Ir(ptz)(3) emitting green and blue light respectively. However, we predict that these two complexes will show marked differences in their magnetic circular dichroism (MCD) spectra.

Multilayer nanostructured porphyrin arrays constructed by layer-by-layer self-assembly.

UV-vis absorption, atomic force microscopy (AFM), contact angle, and X-ray reflectivity experiments were performed on thin films deposited on crystalline silicon substrates as alternating layers of a porphyrin with anionic functionality, tetra-5,10,15,20-(4-sulfonatophenyl)porphine (TSPP) or the metalated version, Cu(II)TSPP, and the cationic polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA). The films were made by dipping in alternating aqueous solutions containing film components (layer-by-layer deposition). Modeling of the X-ray reflectivity data revealed differences in the films' thickness depending on the method of film deposition. An unusual decrease in film thickness after each polyelectrolyte dip was also observed for films using TSSP. UV-vis measurements revealed that a similar amount of TSSP was included within films despite the method of formation. UV-vis measurements also revealed the presence of free-base, H-aggregate, and J-aggregate forms of the porphyrin after TSPP dipping, and the subsequent disappearance of the J-aggregate after dipping in the PDDA solution. A model of film formation was proposed on the basis of the concept of two different types of porphyrin aggregates being present after dipping in porphyrin solution. A layer of porphyrin molecules initially attach to the Si surface such that the planar molecules are arranged side by side as H-aggregates with an excess of J-aggregated material on top. The J-aggregate is then removed and replaced by a layer of PDDA. A change in contact angle of 14 degrees was observed between porphyrin and polyelectrolyte layers due to the more hydrophobic nature of the polymer. The presence of the J-aggregate was confirmed in AFM images obtained from the porphyrin layer. Exposure of the films to solutions of alternating pHs of 10 and 1.8 resulted in reproducible switching of the UV-vis spectra, indicating a possible sensing application.

Solid-state dendrimer sensors: probing the diffusion of an explosive analogue using neutron reflectometry.

Determining how analytes are sequestered into thin films is important for solid-state sensors that detect the presence of the analyte by oxidative luminescence quenching. We show that thin (230 +/- 30 A) and thick (750 +/- 50 A) films of a first-generation dendrimer comprised of 2-ethylhexyloxy surface groups, biphenyl-based dendrons, and a 9,9,9',9'-tetra-n-propyl-2,2'-bifluorene core, can rapidly and reversibly detect p-nitrotoluene by oxidative luminescence quenching. For both the thin and thick films the photoluminescence (PL) is quenched by p-nitrotoluene by approximately 90% in 4 s, which is much faster than that reported for luminescent polymer films. Combined PL and neutron reflectometry measurements on pristine and analyte-saturated films gave important insight into the analyte adsorption process. It was found that during the adsorption process the films swelled, being on average 4% thicker for both the thin and thick dendrimer films. At the same time the PL was completely quenched. On removal of the analyte the films returned to their original thickness and scattering length density, and the PL was restored, showing that the sensing process was fully reversible.