Photophysical Characterization of a Series of Platinum(II)-Containing Phenyl−Ethynyl Oligomers

Two-photon absorption characterization and comparison of Au(I) fluorenyl benzothiazole complexes in the visible wavelength regime

We use the two-photon excited fluorescence method to determine the two-photon absorption (2PA) cross sections of three series of (fluorenyl benzothiazole) gold(I) complexes in the visible wavelength range from 570 to 700 nm. We compare the effect of ancillary ligand substitutions on the 2PA magnitudes and find that the ancillary ligand does not drastically affect either the magnitude or the shape of 2PA. Even so, moderate 2PA cross sections were measured that ranged from 10 to 1000 s of GM (Göppert-Mayer, =10^-50cm⁴s/photon), making these types of complexes nonlinear optical materials for two-photon absorbing applications.

Efficient Deep Blue Platinum Acetylide Phosphors with Acyclic Diaminocarbene Ligands

Here we report five blue-phosphorescent platinum bis-phenylacetylide complexes with an investigation of their photophysical and electrochemical attributes. Three of the complexes (1-3) are of the general formula cis-Pt(CNR)₂ (C≡CPh)₂ , in which CNR is a variably substituted isocyanide and C≡CPh is phenylacetylide. These isocyanide complexes serve as precursors for complexes of the general formula cis-Pt(CNR)(ADC)(C≡CPh)₂ (4 and 5), in which ADC is an acyclic diaminocarbene installed by amine nucleophilic addition to one of the isocyanides. All of the complexes exhibit deep blue phosphorescence with λ_max ∼430 nm in poly(methyl methacrylate) (PMMA) thin films. Whereas isocyanide complexes 1-3 exhibit modest photoluminescence quantum yields (Φ_PL ), incorporation of one acyclic diaminocarbene ligand results in a three-fold to 16-fold increase in Φ_PL while still maintaining an identical deep blue color profile.

Nanoscale Organization of a Platinum(II) Acetylide Cholesteric Liquid Crystal Molecular Glass for Photonics Applications

The fabrication, molecular structure, and spectroscopy of a stable cholesteric liquid crystal platinum acetylide glass obtained from trans-Pt(PEt3)2(C≡C-C6H5-C≡N)(C≡C-C6H5-COO-Cholesterol), are described and designated as PE1-CN-Chol. Polarized optical microscopy, differential scanning calorimetry, and wide-angle X-ray scattering experiments show room temperature glassy/crystalline texture with crystal formation upon heating to 165 °C. Further heating results in conversion to cholesteric phase. Cooling to room temperature leads to the formation of a cholesteric liquid crystal glass. Scanning tunneling microscopy of a PE1-CN-Chol monolayer reveals self-assembly at the solid-liquid interface with an array of two molecules arranged in pairs, oriented head-to-head through the CN groups, giving rise to a lamella arrangement. The lamella structure obtained from molecular dynamics calculations shows a clear phase separation between the conjugated platinum acetylide and the hydrophobic cholesterol moiety with the lamellae separation distance being 4.0 nm. Ultrafast transient absorption and flash photolysis spectra of the glass show intersystem crossing to the triplet state occurring within 100 ps following excitation. The triplet decay time of the film compared to aerated and deoxygenated solutions is consistent with oxygen quenching at the film surface but not within the film. The high chromophore concentration, high glass thermal stability, and long triplet lifetime in air show that these materials have potential as nonlinear absorbing materials.

Triplet state structure-property relationships in a series of platinum acetylides: effect of chromophore length and end cap electronic properties

To develop quantitative structure-spectroscopic property relationships in platinum acetylides, we investigated the triplet state behavior of nominally centrosymmetric chromophores trans-Pt(PBu3)2(C[triple bond, length as m-dash]C-Phenyl-X)2, where X = diphenylamino, NH2, OCH3, t-Bu, CH3, H, F, benzothiazole, CF3, CN, and NO2. We measured ground state absorption, phosphorescence, excitation and triplet state absorption spectra and triplet lifetimes. By DFT we calculated the phosphorescence emission energy (ET), the spin density on the end cap (SD(X)), triplet state geometry and the distance between the triplet centroid and the central platinum atom (RS-Pt(X)). Compounds with electron-donating X have smaller triplet state lifetime, blue-shifted phosphorescence and larger triplet potential energy surface displacement associated with the C[triple bond, length as m-dash]C bond. Compounds with electron-withdrawing X have larger triplet lifetime, red-shifted phosphorescence and smaller triplet potential energy surface displacement associated with the C[triple bond, length as m-dash]C bond. The range of spin-orbit-coupling between the platinum atom and the triplet centroid was determined to be 6 Å. The quantity RS-Pt is shown to be a linear function of one-dimensional well length calculated from experimental ET. The multiple examples demonstrate RS-Pt is a useful descriptor for analyzing triplet state behavior.

Two-Photon Spectroscopy of a Series of Platinum Acetylides: Conformation-Induced Ground-State Symmetry Breaking

With the goal of elucidating electronic and conformational effects on structure-spectroscopic property relationships in platinum acetylides, we synthesized a series of nominally centrosymmetric chromophores trans-Pt(PBu₃)₂(C≡C-Phenyl-X)₂, where X = diphenylamino (DPA), NH₂, OCH₃, t-Bu, CH₃, H, F, benzothiazole (BTH), CF₃, CN, and NO₂. We collected one- and two-photon absorption spectra and also performed density functional theory (DFT) and time-dependent (TD) DFT calculations on the ground- and excited-state properties of these compounds. The DFT calculations revealed facile rotation between the two ligands, suggesting that the compounds exhibit nonplanar ground-state conformations in solution. TDDFT calculation of the S₁ state energy and transition dipole moment for a nonplanar conformation gave good agreement with experiment. Two-photon absorption spectra obtained from these compounds allowed estimation of the change of permanent electric dipole moment upon vertical excitation from ground state to S₁ state. The values are small Δμ < 1.0 D for neutral substituents such as CH₃, H, and F but increase sharply to Δμ ≈ 11 D for electron-accepting NO₂. When in a nonplanar conformation, the corresponding calculated Δμ values showed good agreement with the experimental data indicating that the two-photon spectra result from nonplanar ground-state conformations. Previously studied related chromophores having extended conjugation ( Rebane, A.; Drobizhev, M.; Makarov, N. S.; Wicks, G.; Wnuk, P.; Stepanenko, Y.; Haley, J. E.; Krein, D. M.; Fore, J. L.; Burke, A. R.; Slagle, J. E.; McLean, D. G.; Cooper, T. M. J. Phys. Chem. A 2014 , 118 , 3749 - 3759 ) show similar dependence of Δμ on the substituents, which allows us to conclude that the excited-state properties of these floppy chromophores are a function of the electronic properties of the substituents, ligand size, and nonplanar molecular conformation.

Increasing phosphorescent quantum yields and lifetimes of platinum-alkynyl complexes with extended conjugation

Designing platinum-alkynyl complexes to access a metallo-cumulene resonance form increases the quantum yield via increased rigidity.

Nonlinear Optical Materials for the Smart Filtering of Optical Radiation

The control of luminous radiation has extremely important implications for modern and future technologies as well as in medicine. In this Review, we detail chemical structures and their relevant photophysical features for various groups of materials, including organic dyes such as metalloporphyrins and metallophthalocyanines (and derivatives), other common organic materials, mixed metal complexes and clusters, fullerenes, dendrimeric nanocomposites, polymeric materials (organic and/or inorganic), inorganic semiconductors, and other nanoscopic materials, utilized or potentially useful for the realization of devices able to filter in a smart way an external radiation. The concept of smart is referred to the characteristic of those materials that are capable to filter the radiation in a dynamic way without the need of an ancillary system for the activation of the required transmission change. In particular, this Review gives emphasis to the nonlinear optical properties of photoactive materials for the function of optical power limiting. All known mechanisms of optical limiting have been analyzed and discussed for the different types of materials.

Steric hindrance inhibits excited-state relaxation and lowers the extent of intramolecular charge transfer in two-photon absorbing dyes

Steric hindrance inhibits excited-state relaxation and intramolecular charge-transfer, which significantly alters the photophysical properties.

Functional Materials Based on Metal-Containing Polymers

Since the dawn of human civilization, there has been a demand for materials that include ceramics, metals, and polymers. Increasing demand as well as the need for enhanced performance has driven material scientists to research metal-containing polymers as complements of these materials. Consequently, metal-containing polymers that integrate the excellent thermal, electronic, optical, and magnetic properties of metals with the lightweight, low cost, and in some cases, the chemical stability of organic-based polymers have been designed, and used as catalysts, sensors, ceramic precursors, magnetic materials, and electrical conductors. This chapter provides an overview of some of these functional metal-containing polymers.

Polymer Monoliths Containing Two-Photon Absorbing Phenylenevinylene Platinum(II) Acetylide Chromophores for Optical Power Limiting

A series of platinum(II) acetylide complexes containing p-phenylenevinylene and moieties end-capped with triphenylamine groups have been incorporated into poly(methyl methacrylate) (PMMA) monoliths for optical power limiting applications. The one- and two-photon photophysical properties were investigated and compared to the photophysical properties in THF. The absolute two-photon absorption cross-section values for the monolith samples were measured and are comparable to the values obtained in solution. In the PMMA monoliths, the complexes retained the important two-photon absorption and reverse saturable absorption properties necessary for optical power limiting via dual mode mechanism, and their strong nonlinear absorption property was demonstrated by the open-aperture Z-scan method. Photostability studies of the p-phenylenevinylene platinum(II) acetylide complexes showed two photodegradation processes: a trans-to-cis isomerization and a singlet-oxygen sensitized self-oxidative cleavage. The photostability of the least photostable complex TPV0 was increased upon incorporation into a PMMA matrix.

The effects of extended conjugation length of purely organic phosphors on their phosphorescence emission properties

Computational investigation and experimental data reveal that the intersystem crossing rate decreases with extended conjugation in bromobenzaldehyde derivatives.

Platinum carbon bond formation via Cu(i) catalyzed Stille-type transmetallation: reaction scope and spectroscopic study of platinum-arylene complexes

The preparation of Pt(ii) complexes of the type trans-L₂Pt(Ar)Cl, L₂Pt(Ar)₂, and L₂Pt(Ar)(Ar′) (L = PBu₃, Ar = arylene) by CuI catalyzed reaction of cis-(PBu₃)₂PtCl₂ with aryl-stannanes is reported.

Photophysical properties of trans-platinum acetylide complexes featuring N-heterocyclic carbene ligands

Platinum acetylide complexes featuring NHC-carbene ligands show enhanced photophysical properties relative to complexes with phosphine ligands.

High efficiency platinum acetylide nonlinear absorption chromophores covalently linked to poly(methyl methacrylate)

We report three platinum acetylide acrylate monomers containing known two-photon absorption (TPA) chromophores and their covalent incorporation into polymers via free radical polymerization with methyl methacrylate. The photophysical properties of the platinum acetylide monomers and resulting poly(methyl methacrylate) (PMMA) copolymers were investigated to determine if the one- and two-photon photophysical properties of the chromophores were maintained in the copolymers. The photophysical properties of the series of copolymers were studied in solution and solid state with minimum shifts exhibited in the ground state absorption, photoluminescence, and triplet-triplet transient absorption spectra. The polymer films displayed markedly stronger phosphorescence and longer triplet excited state lifetimes than the polymers in solution or the monomers. The incorporation of the platinum acetylide chromophores into the PMMA copolymers allows the materials to be cast as thin films or into free-standing monoliths. Films with ~3.6 μm in thickness and monoliths with 1 mm path length were fabricated and examined. The nonlinear absorption responses of the polymers in solution were measured via the nanosecond z-scan method, and the solid state polymer monoliths were measured via nonlinear transmittance. Both measurements indicate that the polymers exhibited strong transmittance attenuation at input pulse energies exceeding 100 μJ.

Effect of β-bromo substitution on the photophysical properties of meso-phenyl, meso-carbazole, and meso-triphenylamine porphyrins

We present results of an experimental photophysical study of a series of novel brominated and non-brominated porphyrins that contain phenyl, carbazole, or triphenylamine in the meso-position. In addition we have looked at the effects of incorporating a zinc metal into the porphyrin system relative to the free base. Structure-property relationships are established using various absorption and emission techniques including femtosecond pump probe transient absorption and nanosecond laser flash photolysis. With slightly increasing electron donating strength (phenyl < carbazole < triphenylamine) red shifts were observed in all data. The same effect was observed upon the addition of bromine in the beta position. Due to the heavy atom affect of the bromines both the singlet and triplet excited state lifetimes were significantly shorter in the brominated porphyrins. For the T1–Tn absorption data we observe a large absorption in the near infrared region with the brominated carbazole and triphenylamine. The largest effect of the addition of zinc was in the ground state absorption and emission where a blue shift in the data was observed. Some effects were also observed in the kinetic decays with zinc as the metal compared to the free base porphyrins.

Photophysical Properties of a Nonlinear Dye in PMMA at High Concentration

The effects of high concentration on the photophysical properties of a nonlinear material have been of interest for some time in our group. It is well known in the literature that for a nonlinear absorbing dye to be the most effective, high concentrations are needed. The problem is that most photophysical studies in solution are done at low concentration. These low concentration studies are important for understanding inherent materials properties but it is also important to understand what happens in a material at high concentration. In addition to this, efforts have been made to study the effects of incorporating a dye into a solid matrix environment to better understand the constraints this environment has to a given material. Preliminary results for a PMMA system reveal the formation of excimers (excited state dimers) with an increase in concentration. Excimers are forming from the triplet excited state of the E1-BTF. A rate constant for this formation is 2.3 x 106 M-1 s-1. While rather slow, at high concentration the excimer is readily formed. This must be considered when making nonlinear absorption measurements since the excimer will certainly contribute to the overall nonlinearity.

Organoplatinum chromophores for application in high-performance nonlinear absorption materials

Chromophores and materials that exhibit nonlinear absorption over a broad spectrum and with high temporal dynamic range are of interest for application in materials engineering and biology. Recent work by a number of research groups has led to the development of a new family of organometallic chromophores and materials featuring interesting and useful nonlinear absorption properties. These systems contain the platinum acetylide moiety as a fundamental molecular unit, combined with delocalized, π-conjugated electron systems. These organometallic chromophores provide a unique combination of properties, such as negligible ground state absorption in the visible region, large spin-orbit coupling giving rise to high triplet excited state yield, triplet lifetime in the microsecond domain, high two-photon cross-section in the visible and near-infrared regions, and high triplet-triplet absorption cross-section in the visible and near-infrared region. This Spotlight on Application highlights recent developments in this area, combining background and review on nonlinear absorption in platinum acetylide chromophores and describing significant recent results from our own laboratory.

Negative polaron and triplet exciton diffusion in organometallic "molecular wires"

The dynamics of negative polaron and triplet exciton transport within a series of monodisperse platinum (Pt) acetylide oligomers is reported. The oligomers consist of Pt-acetylide repeats, [PtL(2)-C≡C-Ph-C≡C-](n) (where L = PBu(3) and Ph = 1,4-phenylene, n = 2, 3, 6, and 10), capped with naphthalene diimide (NDI) end groups. The Pt-acetylide segments are electro- and photoactive, and they serve as conduits for transport of electrons (negative polaron) and triplet excitons. The NDI end groups are relatively strong acceptors, serving as traps for the carriers. Negative polaron transport is studied by using pulse radiolysis/transient absorption at the Brookhaven National Laboratory Laser-Electron Accelerator Facility (LEAF). Electrons are rapidly attached to the oligomers, with some fraction initially residing upon the Pt-acetylide chains. The dynamics of transport are resolved by monitoring the spectral changes associated with transfer of electrons from the chain to the NDI end group. Triplet exciton transport is studied by femtosecond-picosecond transient absorption spectroscopy. Near-UV excitation leads to rapid production of triplet excitons localized on the Pt-acetylide chains. The excitons transport to the chain ends, where they are annihilated by charge separation with the NDI end group. The dynamics of triplet transport are resolved by transient absorption spectroscopy, taking advantage of the changes in spectra associated with decay of the triplet exciton and rise of the charge-separated state. The results indicate that negative polarons and excitons are transported rapidly, on average moving distances of ~3 nm in less than 200 ps. Analysis of the dynamics suggests diffusive transport by a site-to-site hopping mechanism with hopping times of ~27 ps for triplets and <10 ps for electrons.

Organometallic acetylides of Pt(II), Au(I) and Hg(II) as new generation optical power limiting materials

Within the scope of nonlinear optics, optical power limiting (OPL) materials are commonly regarded as an important class of compounds which can protect the delicate optical sensors or human eyes from sudden exposure to damaging intense laser beams. Recent efforts have been devoted to developing organometallic acetylide complexes, dendrimers and polymers as high performance OPL materials of the next generation which can favorably optimize the optical limiting/transparency trade-off issue. These metallated materials offer a new avenue towards a new family of highly transparent homo- and heterometallic optical limiters with good solution processability which outperform those of current state-of-the-art visible-light-absorbing competitors such as fullerenes, metalloporphyrins and metallophthalocyanines. This critical review aims to provide a detailed account on the recent advances of these novel OPL chromophores. Their OPL activity was shown to depend strongly on the electronic characters of the aryleneethynylene ligand and transition metal moieties as well as the conjugation chain length of the compounds. Strategies including copolymerization with other transition metals, change of structural geometry, use of a dendritic platform and variation of the type and content of transition metal ions would strongly govern their photophysical behavior and improve the resulting OPL responses. Special emphasis is placed on the structure-OPL response relationships of these organometallic acetylide materials. The research endeavors for realizing practical OPL devices based on these materials have also been presented. This article concludes with perspectives on the current status of the field, as well as opportunities that lie just beyond its frontier (106 references).