Andrew C. Benniston

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.

Pushing around electrons: towards 2-D and 3-D molecular switches.

In the new age of molecular electronics there has been a great deal of speculation about ways to control the passage of electrons through organic-based wires using electro- or photo-active switches. However, the next stage development is envisaged as the added sophistication of directionality so that electron migration can be switched into 2 and 3 dimensions. This short tutorial review will set out how this realisation may be achieved and highlight examples where the idea of directionality in electron transfer has been put into practice.

The photophysical properties of a julolidene-based molecular rotor

The photophysical properties of 9-dicyanovinyljulolidine are sensitive to solvent viscosity but are little affected by changes in polarity. In fluid solution, the lifetime of the first-excited singlet state is very short and triplet state formation cannot be detected by laser flash photolysis. Decay of the excited singlet state is strongly activated and weak phosphorescence can be observed in a glassy matrix at 77 K. Temperature dependent 1H NMR studies indicate that the molecule undergoes slow internal rotation in solution, for which the activation energy has a value of ca. 35 kJ mol(-1). This process is unlikely to account for the poor fluorescence quantum yield found in fluid solution. Instead, it is considered that the target compound undergoes rapid rotation around the dicyanovinyl double bond from the excited singlet state. The rate of rotation depends weakly on the viscosity of the solvent in a range of linear alcohols at room temperature. This might represent the fact that the rotor is relatively small and can pack into cavities in the solvent structure. In glycerol, the rate of rotation is more sensitive to viscosity effects but a quite complex temperature dependence is observed in ethanol. Here, the rate is almost activationless in a glassy matrix and in fluid solution at high temperature but strongly activated at intermediate temperatures.

The photophysical properties of a pyrene–thiophene–terpyridine conjugate and of its zinc(II) and ruthenium(II) complexes

A tripartite supermolecule, comprising a pyrene moiety tethered to a 2,2′:6′,2″-terpyridyl ligand via a 2,5-diethynylated thiophene linker, has been synthesized. This compound is highly fluorescent in solution due to the formation of an intramolecular charge-transfer (CT) state. From consideration of the electrochemical properties, it is concluded that the CT state arises because of charge transfer from pyrene to the central thiophene-based unit. Formation of the CT state involves an increase in dipole moment of ca. 18.5 D. Phosphorescence was not observed but the intermediate population of the triplet state was confirmed by laser flash photolysis. Addition of Zn2+ cations results in a drastic decrease in the fluorescence yield while the absorption spectrum exhibits a pronounced red shift. It appears that the dipole is extended upon cation binding, with the zinc terpyridine terminal acting as the electron acceptor. Again, no phosphorescence was apparent at 77 K. Coordination of a ruthenium(II) 2,2′;6′,2″-terpyridyl metallo-fragment to the vacant terpyridine terminal causes the appearance of weak phosphorescence in fluid solution at room temperature. The emitting species has a lifetime of 2.6 μs in deoxygenated acetonitrile at 20 °C. Luminescence, which shows a complex temperature dependence, is attributed to either the lowest-energy metal-to-ligand, charge-transfer triplet localised on the ruthenium(II) complex or to the intraligand CT state. In the later case, spin–orbit coupling effects induced by the ruthenium atom are responsible for promoting emission.

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.

Exploring Förster electronic energy transfer in a decoupled anthracenyl-based borondipyrromethene (bodipy) dyad

An anthracenyl-Bodipy dyad containing a triazole bridge, that acts to decouple the two units in the ground state, has been synthesised and structurally characterised. Efficient electronic energy transfer occurs from the anthracenyl-based unit to the Bodipy system in toluene in around 12 ps, and becomes faster in solvents of lower refractive index. The rate of electronic energy transfer is discussed in terms of Förster theory.

The effect of torsion angle on the rate of intramolecular triplet energy transfer

The magnitude of electronic coupling between the terminal chromophores shows a precise dependence on the dihedral angle around a bridging biphenyl group.

Artificial Photosynthesis: Mimicking Redox Asymmetry

Directional light-induced electron transfer takes place in the catenane shown schematically on the right. This catenane is similar to the photosynthetic reaction center: The two chemically identical electron acceptors (rectangles) bound to a ruthenium complex as donor have different reduction potentials because their environments are of different polarity. The electron transfer proceeds preferentially (85 %) to the external acceptor.

Effect on Charge Transfer and Charge Recombination by Insertion of a Naphthalene‐Based Bridge in Molecular Dyads Based on Borondipyrromethene (Bodipy)

The photophysical properties of two related dyads based on a N,N-dimethylaniline donor coupled to a fully-alkylated boron dipyrromethene (Bodipy) acceptor are described. In one dyad, BD1, the donor unit is attached directly to the Bodipy group, whereas in the second dyad, BD2, a naphthalene spacer separates the two units. Cyclic voltammograms recorded for the two dyads in deoxygenated MeCN containing a background electrolyte are consistent with the reversible one-electron oxidation of the N,N-dimethylaniline group and the reversible one-electron reduction of the Bodipy nucleus. There is a reasonable driving force (ΔG(CT)) for photoinduced charge transfer from the N,N-dimethylaniline to the Bodipy segment in MeCN. The charge-transfer state is formed for BD1 extremely fast (1.5 ps), but decays over 140 ps to partially restore the ground state. On the other hand, the charge-transfer state for BD2 is formed more slowly, but it decays extremely rapidly. Charge recombination for both dyads leads to a partial triplet formation on the Bodipy group. The naphthalene spacer group is extremely efficient at promoting back electron transfer.