The syntheses and crystal structures of sterically crowded mono-and bichromophoric BODIPY-based dyes are reported. The "monomeric" compound is weakly fluorescent in the liquid phase due to fast internal conversion associated with rotation of aryl rings at the borona tom. The side-byside "dimer"e xhibits weak excitonic coupling between the dipyrrin units and is much more emissive in fluid solution. Solid samples of both molecular entities are strongly fluorescent under near-UV illumination. Thus, the mono-chromophore exhibitsd ual fluorescencef rom what appears to be a mixture of crystallinea nd possibly amorphous (or interfacial regions) distributions. The bi-chromophore packs in the crystal as pairs of chromophores with each unit being provided by ad ifferent molecule. This leads to excitonic splitting and the formationo fastrong H-band in the absorption spectrum. Fluorescence occurs from the corresponding J-species and also from what appears to be an aggregateds tate associated with interfacial areas. Both bulk and interface-bound states show relativelyl ong-lived fluorescencew hile the crystal structures indicatet he likelihood for fast electronic energy migration between molecules.[a] Prof.

A new family of boron(III) chelates is introduced whereby molecular chirality, confirmed by circular dichroism, is imported during synthesis such that isolation of the diastereoisomers does not require separation procedures. The photophysical properties of two members of the family have been examined: the N,O,O‐salicylaldehyde‐based derivative shows pronounced intramolecular charge‐transfer character in fluid solution and is weakly fluorescent, with a large Stokes shift. The corresponding 2‐methylamino‐benzaldehyde‐derived N,N,O‐chelate absorbs and fluoresces in the visible region with a much smaller Stokes shift. Orange fluorescence is also observed for this compound as a cast film. Temperature‐dependence studies show that decay of the fluorescent state is weakly activated but emission is less than quantitative at 77 K. Quite rare for boron(III)‐based chelates, this derivative undergoes intersystem crossing to form a meta‐stable triplet‐excited state. X‐ray crystal structures are reported for both compounds, along with simulated ECD spectra.

A small series of boranil complexes has been studied by fluorescence spectroscopy. Weakly fluorescent in most organic solvents at room temperature, the target compounds display bright emission in the crystalline phase. X-ray diffraction patterns obtained for single crystals indicate a distorted tetrahedral geometry around the O–B–N center with the boron atom being displaced from the plane of the heterobicyclic ring. Consideration of the various bond lengths in comparison with those of reference compounds indicates that the ancillary phenyl ring, bearing different para-substituents, does not make a prominent contribution to the molecular dipole moment in the solid state. Absorption and fluorescence spectra recorded for the crystals remain remarkably similar to those for liquid solutions and display large Stokes shifts. Proximity broadening is observed in one case. The nitrophenyl derivative exhibits additional absorption and emission bands unique to the solid state and could be indicative of an intermolecular charge-transfer transition. The optical properties are discussed in terms of the crystal packing diagrams.

Many diverse natural systems use sunlight to drive critical chemical reactions. To harvest sufficient photon densities, natural organisms have developed highly sophisticated light absorbing antennae rather than rely on direct illumination of a single chromophore. Attempts to develop artificial analogues have resulted in the synthesis and spectroscopic characterisation of elaborate molecular assemblies and here we consider the case for using boron(III) chelates as the primary light absorbers. Such entities make attractive modules for the creation of multi-component arrays with individual units sited in a logical sequence for long-range electronic energy transfer. Alternatively, certain boron(III) chelates can be synthesised in high yield by simple strategies that avoid time-consuming purification. These latter materials are appealing as components for large-scale light harvesters. The use of photonic crystals avoids the need to position individual molecules at the catalyst but presents severe design challenges. Interrupting, or redirecting, the flow of excitons within the array requires the introduction of novel switches that can be activated by selective illumination. Protecting the array against adventitious photofading is a major objective that has yet to be achieved.