Four new dimeric bis(BF(2))-2,2'-bidipyrrins (bisBODIPYs), and their corresponding BODIPY monomers, have been prepared and studied with respect to their structural and photophysical properties. The solid-state molecular structure of the dimers and the relative orientation of the subunits have been revealed by an X-ray diffraction study, which showed that the molecules contain two directly linked BODIPY chromophores in a conformationally fixed, almost orthogonal arrangement. Two of the fluorine atoms are in close contact with each other and the (19)F NMR spectra show a characteristic through-space coupling in solution. The new chromophores all exhibit a clear exciton splitting in the absorption spectra with maxima at about 490 and 560 nm, and are highly luminescent with an intense emission band at around 640 nm. The Stokes shift, which is the difference between the maximum of the lowest-energy absorption band and the maximum of the emission band, has a typical value of 5 to 15 nm for simple BODIPYs, whereas this value increases to 80 nm or more for the dimers, along with a slight decrease in fluorescence quantum yields and lifetimes. These properties indicate potential uses of these new fluorophoric materials as functional dyes in biomedical and materials applications and also in model compounds for BODIPY aggregates.

The photophysical and spectroscopic characterization of four new dimeric bis(BF 2 )-2,2 0 -bidipyrrins (BisBODIPYs) recently synthesized and characterized (Chem. Eur. J., 2008, 14, 2976-2983 has been undertaken along with that of the component BODIPY monomers. The monomers display the typical photophysical properties of this family; (i) narrow and intense single absorption band in the visible range at 530 nm; (ii) intense, solvent independent emission (F fl ca. 1); (iii) narrow emission band with a 200-400 cm À1 Stokes shifted emission, independent of solvent polarity; (iv) no absorption features for the singlet excited state; (v) very little triplet yield and no ability to sensitize singlet oxygen. The absorption spectra of the corresponding new dimers exhibit split band maxima in the visible range at about 490 and 560 nm, corresponding to an exciton splitting of ca. 2600 cm À1 . The luminescence in toluene is strong (F fl ca. 0.7, t =3.4 ns), broad and Stokes shifted by ca. 2200 cm À1 , but both luminescence yield and Stokes shift are solvent polarity dependent; F fl o 0.1, t o 1 ns and the Stokes shift is close to 2700 cm À1 in acetonitrile. Solvent viscosity does not appear to play an important role and freezing of the solvents to 77 K in a solid matrix cancels the differences in luminescence parameters. Singlet and triplet excited state absorbance was measured in toluene and acetonitrile and the ability of these new dyes to sensitize singlet oxygen was examined. The nature and dynamics of the excited state is discussed in comparison with the monomers properties and with some intra-or inter-molecular BODIPY dimers reported in the literature. Potential applications of these new dyes with respect to BODIPYs are pointed out on the basis of their spectroscopic and photophysical properties. Results and discussion Absorption in tolueneThe absorption spectra of some representative cases of the dimers studied in toluene solutions are reported in Fig. 1 with

The electrochemistry and the electrogenerated chemiluminescence (ECL) of the BODIPY species (2,6-diethyl-1,3,5,7-tetramethyl-8-n-pentyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene, i.e., B8-n-pentyl), an aromatic monomer (B8-phenyl), and a closely related dimer were examined. The B8-n-pentyl species with the n-pentyl donor chain in the meso position shows good electrochemical behavior in MeCN with the formation of stable radical ions. The ECL emission is characterized by long-wavelength emission during consecutive oxidation and reduction steps. This is attributed to aggregate formation on radical ion annihilation, the extent of which increased with increasing concentration of the compound. The B8-phenyl electrochemistry also shows stable radical ions. However, B8-phenyl with the meso phenyl group ECL emission does not show any long-wavelength emission. The synthesized dimer closely related to this species shows two electrochemical oxidations with peak separations of 0.5 V and two reductions with peak separations 0.2 V, which corresponds to a high degree of intermolecular interactions in the molecule. The growth of additional electrochemical peaks during the oxidation and reduction of B8-n-pentyl was not seen, and both radical ions were stable, suggesting that emitting secondary products were not formed.

Bulk heterojunction (BHJ) and dye-sensitized solar cells (DSSCs) have seen increased popularity over recent years and each technology has experienced tremendous improvements in power conversion efficiencies (PCEs), reaching 8 and 12%, respectively. The two technologies have been on independent improvement pathways, and this work establishes a link between them by using the archetypical hole conductor (poly-3-hexylthiophene, P3HT) in BHJs as a sensitizer on TiO(2) for DSSC applications. Three polymers were synthesized and examined as potential TiO(2) sensitizers in DSSCs under AM1.5 solar radiation. Using Grignard metathesis, regioregular P3HT was synthesized then functionalized with either one or two cyanoacrylic acid linker moieties to bind to the TiO(2) surface. End-group modification resulted in minimal changes to the optical and electronic properties as compared to pristine P3HT. Cyclic voltammetry (CV) experiments at anodic potentials of adsorbed sensitizer quantified the amount of alkylthiophene adsorbed on the TiO(2), whereas under reductive sweeps, cyanoacrylic acid end-group binding was determined. CVs of each polymer indicated that loading was drastically different as compared to pristine P3HT with the lowest loading on TiO(2) and monofunctionalized P3HT exhibited the highest loading. The DSSCs showed power conversion efficiencies (PCEs) of 0.1%, 0.2 and 2.2% for the polymer-sensitized TiO(2) of the unfunctionalized, monofunctionalized and difunctionalized polymers, respectively. DSSCs were then subjected to electrochemical impedance spectroscopy (EIS) in the dark and under monochromatic light radiation. The large variance in performance for the functionalized-P3HT sensitizers is attributed to differences in the adsorption modes of sensitizer on the TiO(2) surface, which in the difunctionalized case limits electrolyte recombination and favors forward charge transfer reactions.