Selective triplet-state formation during charge recombination in a fullerene/Bodipy molecular dyad (Bodipy=borondipyrromethene).

A conformationally restricted molecular dyad has been synthesized and subjected to detailed photophysical examination. The dyad comprises a borondipyrromethene (Bodipy) dye covalently linked to a buckminsterfullerene C60 residue, and is equipped with hexadecyne units at the boron centre in order to assist solubility. The linkage consists of a diphenyltolane, attached at the meso position of the Bodipy core and through an N-methylpyrrolidine ring at the C60 surface. Triplet states localised on the two terminals are essentially isoenergetic. Cyclic voltammetry indicates that light-induced electron transfer from Bodipy to C60 is thermodynamically favourable and could compete with intramolecular energy transfer in the same direction. The driving force for light-induced electron abstraction from Bodipy by the singlet excited state of C60 depends critically on the solvent polarity. Thus, in non-polar solvents, light-induced electron transfer is thermodynamically uphill, but fast excitation energy transfer occurs from Bodipy to C60 and is followed by intersystem crossing and subsequent equilibration of the two triplet excited states. Moving to a polar solvent switches on light-induced electron transfer. Now, in benzonitrile, the charge-transfer state (CTS) is positioned slightly below the triplet levels, such that charge recombination restores the ground state. However, in CH2Cl2 or methyltetrahydrofuran, the CTS is slightly higher in energy than the triplet levels, and decays, in part, to form the triplet state localized on the C60 residue. This step is highly specific and does not result in direct formation of the triplet excited state localized on the Bodipy unit. Subsequent equilibration of the two triplets takes place on a relatively slow timescale.

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.

On the conjugation length for oligo(ethynylnaphthalene)-based molecular rods.

The synthesis is described for a small series of oligomers built from (2, 3, 4 or 6) ethynyl-naphthalene repeat units and end-capped with solubilising 1,2,3-tris-dodecyloxy-benzene groups. These compounds absorb in the near-UV region and exhibit strong fluorescence in both fluid solution and a glassy matrix at 77 K. The spectral profiles are fully consistent with a structurally heterogeneous ground state becoming more planar upon excitation and with the low-temperature glass further stabilising the co-planar orientation. The absorption and fluorescence maxima move towards lower energy with increasing number of repeat units and there is a corresponding increase in the Huang-Rhys factor for the radiative process. The non-radiative rate constants also depend on molecular length and are well explained in terms of the energy-gap law. In contrast, very weak phosphorescence is observed at 77 K for which the peak maximum and lifetime remain insensitive to the number of naphthalene units. The triplet lifetimes recorded at ambient temperature are also independent of the molecular length but the triplet-triplet absorption spectra change throughout the series. These results are discussed in terms of the degree of electronic coupling between adjacent repeat units for each of the relevant excited states. During these studies it was noted that the rate of intersystem crossing to the triplet manifold is but weakly affected by heavy-atom perturbers. A non-fluorescent complex is formed between iodoethane and the molecular rod but the corresponding bimolecular process occurs at well below the diffusion-controlled limit. This behaviour is considered in terms of spin-orbit coupling between the excited states and takes account of the differing conjugation lengths.

Simultaneous fluorescence and redox modulation in an irreversible photochrome based on a strained dibenzo-acridinium cation.

A highly strained non-luminescent dibenzo-acridinium cationic compound is identified that undergoes in acetonitrile light-induced ring closure to create a highly fluorescent, planar, eight-ringed cationic anti-aromatic dye.