Simon G. King

Dipolar Stabilization of Emissive Singlet Charge Transfer Excited States in Polyfluorene Copolymers

The singlet excited-state dynamics in poly[(9,9-dioctylfluorene)-(dibenzothiophene-S,S-dioxide)] (PFSx ) random copolymers with different contents of dibenzothiophene-S,S-dioxide (S) units have been studied by steady-state and time resolved fluorescence spectroscopies. Emission from PFSx copolymers shows a pronounced solvatochromism in polar chloroform, relative to the less polar toluene. An excited intramolecular charge transfer state (ICT) is stabilized by dipole-dipole interactions with the polar solvent cage, and possibly accompanied by conformational rearrangement of the molecular structure, in complete analogy with their small oligomer counterparts. The spectral dynamics clearly show that the ICT stabilization is strongly affected by the surrounding medium. In the solid state, emission from PFSx copolymers depends on the content of S units, showing an increase of inhomogeneous broadening and a red shift of the optical transitions. This observation is consistent with stabilization of the emissive ICT state, by the local reorientation of the surrounding molecules at the location of the excited chromophore, which results in favorable dipole-dipole interactions driven by the increase in the dielectric constant of the bulk polymer matrix with increasing S content, in analogy to what happens in polar solvent studies. Furthermore, in clear agreement with the interpretation described above, a strong increase in the emission quantum efficiency is observed in the solid state by decreasing the temperature and freezing out the molecular torsions and dipole-dipole interactions necessary to stabilize the ICT state.

Oligo(fluorenyl)pyridine ligands and their tris-cyclometalated iridium(III) complexes: synthesis, photophysical properties and electrophosphorescent devices

The new extended tri- and penta-fluorenylpyridine ligands Fl3Py 2 and Fl5Py 3 and their tris-cyclometalated iridium(III) complexes Ir[Fl3Py]35 and Ir[Fl5Py]36 have been synthesised and their properties compared with the known iridium(III) complex Ir[Fl1Py]34. The lowest energy (emissive) excited states of the complexes 4–6 are dominated by ligand centred (LC) π→π* triplet states, as observed for their uncomplexed ligands 1–3. The emission maximum of complex 4 is ∼546 nm with a triplet lifetime of 2.8 µs. For complexes 5 and 6 the emission maxima are both ∼566 nm with triplet lifetimes of 7.4 µs and 7.8 µs, respectively. Devices made from poly(9,9′-spirobifluorene) (PSF) as the host and doped with complexes 4–6 show good stability; the EL spectra are unchanged after repeated operation over several days. Devices containing complexes 5 and 6 exhibit higher external quantum efficiency (EQE) values. Turn-on voltages of ∼3 V, giving an EQE of 2.8% at a current density of 30 mA cm−2, with a power efficiency of 4.3 lm W−1 and electroluminescence (EL) intensity of 25 000 cd m−2 at 550 mA cm−2 were observed for ITO/PEDOT ∶ PSS/PSF ∶ 6/Ca/Al devices.

Triplet build in and decay of isolated polyspirobifluorene chains in dilute solution

The triplet kinetics of a conjugated polymer, polyspirobifluorene, have been studied using time resolved photoinduced absorption spectroscopy and gated emission delayed fluorescence. Working on isolated polymer chains in dilute solution, we pay particular attention to the buildup and decay of the triplet states following intersystem crossing from the excited singlet state. Confirmation of intersystem crossing as a monomolecular cold process has been made. At high excitation powers an initial fast decay of the triplet has been observed; this is attributed to intrachain triplet-triplet annihilation. From this observation we estimate the lower bound of the intersystem crossing yield as 1.2%. We also calculate the intrachain annihilation constant to be (2.9+/-0.1)x 10(8) cm(3) s(-1).