Robert Westlund

Click Chemistry for Photonic Applications : Triazole-Functionalized Platinum(II) Acetylides for Optical Power Limiting

Three different triazole-containing platinum(II) acetylide compounds were synthesized by click chemistry and evaluated for their use in optical power limiting (OPL) applications. The triazole unit was incorporated at three different positions within, or at the end of, the conjugation path of the chromophore. The aim is to explore the possibilities of using click chemistry to prepare dendronized chromophores, and to evaluate how the triazole structure affects the photophysical properties and the optical power limiting abilities of these acetylide compounds. It is shown that the concept of click chemistry can be used to attach branched monomer units to ethynyl-phenyl arms by Huisgen 1,3-dipolar cycloaddition, forming triazole units within the chromophore. Photophysical characterization of these triazole-containing materials shows an absorption maximum within the UV-A region and emission through both fluorescence and phosphorescence. Bright phosphorescence was emitted from argon purged samples, and decay measurements thereof showed triplet lifetimes of up to 100 µs. The results from the photophysical characterization suggest that the triazole does break the conjugation path, and in order to gain maximum optical limiting the triazole needs to be placed at the end of the conjugation. All three investigated triazole-containing platinum(II) acetylides show good optical power limiting at 532 nm (10 ns pulse, f/5 set-up, 2 mm cells). The most efficient compound, with the triazole positioned at the end of the conjugation, reaches a defined clamping level of 2.5 µJ for a sample with a concentration of 50 mM in THF and a linear transmission above 80% at 532 nm. These data can be compared to the OPL properties of Zn-based porphyrins or derivatized thiophenes, reaching clamping levels of 6–15 µJ.

Multi-functionalized platinum(II) acetylides for optical power limiting

Preliminary results on the optical power limiting properties of platinum(II) acetylides containing triazole units are presented. It is shown that the triazole units give a positive contribution to the limiting abilities of the platinum(II) acetylide and that this modified chromophore could have potential use in sensor protection devices. Moreover, this paper discusses how the versatile building block 2,2-bis(methylol)propionic acid (bis-MPA) can be used advantageously to functionalize nonlinear optical (NLO) platinum(II) acetylides. The bis-MPA units can be used to prepare dendritic substituents offering site isolation to the chromophore leading to improved clamping. The bis-MPA functionalization also improves the solubility of the platinum(II) acetylides in many organic solvents. The preparation of solid-state optical power limiters, where the NLO chromophore is inserted in an optically transparent matrix, is addressed. Again, the bis-MPA unit can be employed to increase the number of accessible end-groups to which matrix-compatible species can be attached. It is concluded that the hydroxy-functional platinum(II) acetylides can be modified to fit almost any matrix, organic or inorganic. Finally, depending on functionalization, it is possible to prepare doped glasses where the chromophore is either embedded in the matrix, or covalently bonded to the matrix.

Photo-physical properties and triplet-triplet absorption of platinum(II) acetylides in solid PMMA matrices

Because of their strong nonlinear optical properties, Platinum(II) acetylides are investigated as potential chromophores for optical power limiting (OPL) applications. The strong excited state absorption and efficient intersystem crossing to the triplet states in these materials are desired properties for good OPL performance. We recently reported on OPL and photo-physical properties of Pt(II)-acetylide chromophores in solution, modified with thiophenyl or triazole groups. [R. Westlund et al. J. Mater. Chem. 18, 166 (2008); E. Glimsdal et al. Proc. SPIE 6740, 67400M (2007)] The chromophores were later incorporated into poly(methyl-methacrylate) (PMMA) glasses. A variety of doped organic solids were prepared, reaching concentrations of up to 13 wt% of the guest molecule. Raman spectra of the doped solid devices proved that the chemical structure of the nonlinear dyes remains intact upon the polymerization of the solid matrix. Luminescence spectra confirm that the basic photo-physical properties (absorption, emission and inter-system crossing) observed for the solute molecules in THF are maintained also in the solid state. In particular, the phosphorescence lifetime stays in the order of μs to ms, just as in the oxygen evacuated liquid samples. Also, the wavelength dependence and time-dynamics of the triplet absorption spectra of the dyes, dissolved in THF solution and dispersed in solid PMMA matrices, were investigated and compared. Ground state UV absorption spectra between 300 and 420 nm have corresponding broad band visible triplet-triplet absorption between 400 and 800 nm. The triplet state extinction coefficients were determined to be in the order of 104 M-1cm-1.

Photo-physical properties and OPL of some new longer thiophenyl-containing arylalkynyl Pt(II) compounds

The photo-physical and optical power limiting (OPL) properties of several new thiophenyl-containing arylalkynyl Pt(II) complexes with longer arylalkynyl groups, named ATP3, ATP4, ATP6 and ATP7 were studied. Thiophene units were introduced into the structure as an attempt to modify photo-physical properties and OPL capability. The new compounds have their thiophene rings either close to the Pt-atom (ATP7), in the middle of the chain (ATP3), or at the terminal end (ATP4). The measurement results were compared with those of the earlier studied PE3 compound. ATP6 is similar to ATP4, but with methoxy groups in the arylalkynyl ligands. Just as PE3, all thiophenyl derivatives showed large intersystem crossing capabilities and triplet phosphorescence, thus having the potential of large nonlinear absorption and good OPL performance. All compounds are characterized by absorption and emission spectra, quantum yield, luminescence decay (fluorescence and phosphorescence) and two-photon absorption capabilities at 780 nm, and compared to the properties of the PE3 compound. Also analogous triazole-containing compounds, abbreviated Z1, Z2 and Z3, were studied in the same way, and compared to the earlier studied Pt1-G1 compound. The OPL performance of all compounds were measured, and clamping levels of approximately 2.5 to 5 μJ pulse energy from 30 mM (ATP) and 50 mM (triazole) concentration samples were found. All compounds possess high transmission in the visible region and fluorescence quantum yields in the order of 10−2 (ATP) and 10−3 (triazoles).