Light harvesting in silicon(111) surfaces using covalently attached protoporphyrin IX dyes

We report the photosensitization of crystalline silicon via energy transfer using covalently attached protoporphyrin IX (PpIX) derivative molecules at different distances via changing the diol linker to the surface. The diol linker molecule chain length was varied from 2 carbon to 10 carbon lengths in order to change the distance of PpIX to the Si(111) surface between 6 A and 18 A. Fluorescence quenching as a function of the PpIX-Si surface distance showed a decrease in the fluorescence lifetime by almost two orders of magnitude at the closest separation. The experimental fluorescence lifetimes are explained theoretically by a classical Chance-Prock-Silbey model. At a separation below 2 nm, we observe for the first time, a Forster like dipole-dipole energy transfer with a characteristic distance of R-o = 2.7 nm

Twist-bend nematics, liquid crystal dimers, structure–property relations

<p>One of the current challenges in liquid crystal science is to understand the molecular factors leading to the formation of the intriguing twist-bend nematic phase (N_TB) and determine its properties. During our earlier hunt for the N_TB phase created on cooling directly from the isotropic phase and not the nematic phase, we had prepared 30 symmetric liquid crystal dimers. These had odd spacers and methylene links to the two mesogenic groups; desirable but clearly not essential features for the formation of the N_TB. Here, we report the phases that the dimers exhibit and their transition temperatures as functions of both the lengths of the spacer and the terminal chains. In addition we describe the transitional entropies, their optical textures, the X-ray scattering patterns and the ²H NMR spectra employed in characterising the phases. All of which may lead to important properties of the twist-bend nematic phase.</p

Tracking a photo-switchable surface-localised supramolecular interaction via refractive index

As supramolecular chemistry evolves, from the design of interactions in the solution and the solid state to applications at surfaces, there is a need for the development of analytical techniques capable of directly interrogating surface-localised supramolecular interactions. We present a proof-of-concept integrated optical Bragg grating sensor, capable of evanescently detecting small changes in refractive index at infrared wavelengths within a microfluidic system. The high spectral fidelity of the Bragg gratings combined with precise thermal compensation enables direct monitoring of the surface throughout the experiment, enabling the sensor to probe changes in situ and in real-time during surface preparation and chemical modification, and then to follow the progress of a dynamic surface-localised supramolecular interaction. In this study the sensor is assessed through the investigation of a photo-switchable inclusion complex between an azobenzene-functionalised surface and cyclodextrin in aqueous solution. The ability to investigate supramolecular interactions directly in real-time upon a planar surface via refractive index offers a valuable new tool in the understanding of complex dynamic supramolecular systems

Large surface photovoltages observed at methyl-terminated silicon surfaces synthesised through a two-step chlorination-alkylation method

Methylation of the silicon surface through a chlorination-alkylation method has been used to improve the electronic properties of silicon. Upon alkylation of the surface, an increase in the minority carrier recombination lifetime and the surface photovoltage is observed, in line with an increase in surface charge. A likely explanation of this unusually large band bending is charge accumulation during the removal of chlorine from the surface

The effect of preorganisation on the solid state behaviour of simple 'aromatic-cored' bis(guanidinium) sulfates

Single crystal X-ray diffraction structures of two simple bis(guanidinium) sulfates that have previously been shown to recognise oxoanions in solution are reported. Comparison of these solid state structures with previous data suggests a relationship between increasing ligand planarity (preorganisation) and internal sulfate complexation, the greatest degree of preorganisation resulting in formation of a discrete 1 : 1 host–guest complex

The potential of pyridine-2,6-dicarboxylic acid based organolanthanide complexes for efficient near-infrared emission

We present detailed spectroscopic data obtained from Nd pyridine-2,6-dicarboxylic acid based complexes in which the 4-position of the pyridine ring has been substituted with OH and Cl. In each case the ligands formed stable complexes with the Nd ion without the requirement for any additional 'neutral' ligand to satisfy the 8-9 coordination requirement of the lanthanide ion. Photoluminescence is observed from both the ligand (centered ∼700 nm) and the Nd ion (at ∼900 nm, 1064 nm, and 1320 nm due to the F → I, F → I, and F → I transitions respectively) following excitation in the low energy tail of the ligand π → π* absorption. The intensity of the ligand emission and sensitized Nd emission was found to be dependent on the substituted 4-position of the pyridine ring. The origin of the observed phenomena are discussed in relation to the energy transfer process from ligand to Nd ion and the non-radiative relaxation of the sensitized Nd ion. These results suggest that further modification of the ligand through complete halogenation and/or addition of other functional groups may provide an attractive route to obtaining an efficient near-infrared emitting organolanthanide complex

Modified dipicolinic acid ligands for sensitization of europium(III) luminescence

The effect of substitution of the 4- and 3,5-position in the pyridine ring of europium(III) pyridine-2,6-dicarboxylate complexes has been investigated with particular emphasis on sensitization of the Eu3+ ion. Sensitization of the Eu3+ 615nm emission was achieved through excitation of the ligands in which the 4-substituent was –H, -OH, and –Cl and the 3,5-position was -H. In these cases the ligand-to-Eu3+ ratio was confirmed as being 3:1. The sensitization was found to increase following substitution of the 4-position in the order Cl &gt; H &gt; OH. This is attributed to energy transfer occurring from the ligands into different Eu3+ intra-atomic energy levels, with spin selection rules governing the efficiency of this process. The Eu3+ luminescence lifetime was measured and found to vary from 1.16 to 2.90ms depending on excitation energy, ligand, and solvent. For the case of the 3,5-dibromo-4-hydroxy derivative no sensitization was observed and a ligand- to-Eu3+ ratio 1:1 was found. The solubility of these complexes in water and their long emission lifetime make them attractive for use as probes in biological systems