Graeme Copley

Lighting the way ahead with boron dipyrromethene (Bodipy) dyes

This review covers some recent advances made using boron dipyrromethene (Bodipy) compounds, highlighting aspects such as new sensing applications for reactive oxygen species and solvent rheology. The light-harvesting capabilities of the dye especially in the crystalline state are also discussed emphasising Bodipy derivatives as potential candidates for solid-state solar concentrators.

A Donor−Acceptor Molecular Dyad Showing Multiple Electronic Energy-Transfer Processes in Crystalline and Amorphous States

Singlet-singlet, singlet-triplet, and triplet-triplet energy transfer takes place within single crystals and amorphous solid-state solutions of a molecular dyad comprising boron dipyrromethene and oligo-thiophene subunits. The crystal and sublimed thin-films are strongly fluorescent.

Intramolecular excimer formation for covalently linked boron dipyrromethene dyes.

Photophysical properties have been recorded for a small series of covalently linked, symmetrical dimers formed around boron dipyrromethene (Bodipy) dyes. Within the series, a control dimer is unable to adopt a cofacial arrangement because of steric factors, while a second dimer possesses sufficient internal flexibility to form the cofacial geometry but with little overlap of the Bodipy units. The other three members of the series take up a cofacial arrangement with varying bite angles between the planes of the two Bodipy units. Fluorescence quantum yields and excited-state lifetimes indicate differing extents of electronic interaction between the two Bodipy head-groups, but only the compound with the smallest bite angle exhibits excimer emission in solution under ambient conditions. Time-resolved fluorescence studies show dual-exponential decay kinetics in each case, while temperature-dependent emission studies reveal reversible coupling between monomer and lower-energy excimer states. The latter is weakly fluorescent, at best, and is seen clearly only for dimers having small bite angles. The application of high pressure to dilute solutions of these dimers promotes excimer formation in certain cases and leads to loss of monomer-like fluorescence. Under high pressure, excimer emission is more evident, and the overall results can be discussed in terms of subtle structural rearrangements that favor excimer formation.

Exciplex Formation and Excited State Deactivation of Difluoroborondipyrromethene (Bodipy) Dyads

Two series of geometrically-related dyads are discussed based on the difluoroborondipyrromethene (Bodipy) unit, and incorporating covalently attached hydroquinone/quinone groups. These units are anchored directly, or via a phenylene spacer, to the Bodipy core at the meso position in one series (BD-MHQ, BD-MQ, BD-MPHQ, BD-MPQ), but for the second series the attachment site is the 2-position (BD-SHQ, BD-SQ, BD-SPHQ, BD-SPQ). The compounds show various levels of fluorescence depending on the oxidation state of the appended group and the substitution pattern. In non-polar solvents such as toluene, diethyl ether and dichlorobenzene, the S(1) state deactivation of the Bodipy unit in BD-SPQ and BD-MPQ is dominated by (1, 3)exciplex formation, which has not been reported for Bodipy derivatives so far. In the latter molecule, the decay of the exciplex is divided between population of the Bodipy triplet state (13 %-21 %) and ground state reformation. This partitioning is not seen for the side-on substituted derivative, BD-SPQ, and only ground state reformation is observed following decay of the exciplex. This difference in behavior is explained by the radical-pair inter-system-crossing mechanism, which more effectively operates in BD-MPQ because of the orthogonality of the donor-acceptor units. In the more polar solvent CH(3)CN all the quinone derivatives show fast formation of the charge-separated state (k(CS)) followed by slower charge recombination (k(CR)). The ratio k(CS)/k(CR)<or=80.

Thermoresponsive fluorescent polymers based on a quaterthiophene-containing boron dipyrromethene (Bodipy) dyad dispersed in silicone rubber

A quaterthiophene-based boron dipyrromethene (Bodipy) dyad when dispersed uniformally in silicone rubber produces a thermoresponsive fluorescent material. The yellow fluorescence colour observed at low temperatures <200 °C changes to green upon heating the polymer. In contrast, the phenylene-based Bodipy dyad in silicone rubber displays a partial on to off fluorescence change at around 70 °C.

Freezing and glass transition phenomena for 1,2-dichloroethane under high pressure as revealed by fluorescence spectroscopy

The viscosity of 1,2-dichloroethane increases steadily with increasing pressure, as does the density, refractive index and polarizability of this solvent. The pressure dependence for each of these properties can be monitored by a combination of absorption and fluorescence spectroscopy carried out in the presence of a fluorescent molecular rotor that responds to changes in the local environment. At 20 °C, dichloroethane freezes under an applied pressure of ca. 370 MPa, causing sudden extinction of the fluorescence of the molecular rotor due to the opaque nature of the frozen solvent. However, this same emission is enhanced dramatically if a small amount of inert polymer is present in the solution. The behaviour is interpreted in terms of the polymeric solute promoting establishment of a glassy matrix with reasonably good optical transparency for emission spectroscopy.