Simultaneous Two-Photon Excited Fluorescence and One-Photon Excited

Simultaneous Two-Photon Excited Fluorescence and One-Photon Excited

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
Simultaneous Two-Photon Excited Fluorescence and One-Photon Excited Phosphorescence from Single Molecules of an Organometallic Complex Ir(ppy) 3 Yohei Koide, Susumu Takahashi, and Martin Vacha* Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama 2-12-1-S8, Meguro-ku, Tokyo 152-8552, Japan Received March 9, 2006; E-mail: vacha@op.titech.ac.jp Transition metal complexes are an important class of materials in many areas of science and technology. Recently, various iridium or platinum complexes have attracted attention as emitters in organic light emitting diodes (OLED). 1 A common feature of many of these complexes is strong phosphorescence observable even at room tem- perature. For the most widely studied iridium complex, fac- tris(2- phenylpyridine)iridium (Ir(ppy) 3 ), it has been shown that triplet metal-to-ligand charge transfer state ( 3 MLCT) is the lowest-lying emissive state. 2 Quantum efficiency and lifetime of the correspond- ing MLCT phosphorescence vary depending on the solvent used; in a polystyrene matrix, phosphorescence quantum efficiency of 0.92 and a lifetime of 1.2 μ s have been reported. 3 The emission spectrum is dominated by a phosphorescence band centered around 510 nm. A weaker band at 450 nm has also been reported 4,5 and ascribed to ligand-centered 3 π - π * phosphorescence 4 or to 1 MLCT fluorescence. 5 The high emission quantum yield and short luminescence lifetime enabled the study of several organometallic complexes on a single- molecule level. 6 - 9 In this work, we used a time-resolved technique to measure excited-state lifetime of Ir(ppy) 3 molecules on ensemble and a single-molecule level. The results reveal that at low concen- trations strong two-photon absorption gives rise to fast nanosecond transitions coexisting with the slow microsecond phosphorescence. We interpret the fast decay as fluorescence corresponding to the 450 nm emission band. Ir(ppy) 3 was dispersed in thin films of poly(methyl methacrylate) and measured in nitrogen atmosphere on a scanning confocal microscope with picosecond laser diode excitation (438 nm) and avalanche photodiode detection. Emission lifetimes are obtained using time-correlated single-photon counting detection scheme. Figure 1 insets show logarithmic plots of emission decay profiles at various concentrations. The bulk concentration sample (5 × 10 - 4 M) decays, as expected, exponentially with the lifetime of 1.1 μ s. In lower concentration samples, the microsecond decay is accompanied by a fast component ofa3ns lifetime. On a single- molecule level, the slow component is barely discernible (but still present) and the fast component dominates the decay profile. We have verified that the fast component is not due to residual scattering
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 2

Simultaneous Two-Photon Excited Fluorescence and One-Photon Excited

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online