Optimizing Simultaneous Two-Photon Absorption and Transient Triplet−Triplet Absorption in Platinum Acetylide Chromophores

Controlling the triplet states and their application in external stimuli-responsive triplet-triplet-annihilation photon upconversion: from the perspective of excited state photochemistry

The property of organic light-responsive materials is determined by their electronic excited states to a large extent, for instance, the radiative decay rate constants, redox potentials, and lifetimes. Tuning the excited state properties with external stimuli will lead to versatile functional materials; a representative example is the fluorescence molecular probes, in which the singlet excited states are controlled by the external stimuli, i.e., by interaction with the analytes. In comparison, controlling the triplet excited state with external stimuli has been rarely reported, although it is also crucial for the development of novel materials for targeted photodynamic therapy (PDT) reagents and phosphorescent molecular probes. The reported results show that the principles used in singlet excited state tuning are unable to be simply applied to the triplet excited state. In this review article, we summarized the recent results on controlling the triplet excited states by the external stimuli (chemical or light), and the application of the triplet state tuning in the chemical/light controllable triplet-triplet-annihilation upconversion (TTA UC). We discussed the methods for the control of the triplet states, as well as singlet excited state, for the purpose of controlling the TTA UC. Both successful and unsuccessful methods are discussed. This information is helpful for understanding the photophysical processes in which the triplet excited state is involved, and the development of novel external stimuli-responsive triplet photosensitizers.

Stereochemical Effects on Platinum Acetylide Two-Photon Chromophores

A series of cis-platinum(II) acetylide complexes containing two-photon-absorbing chromophores have been synthesized and characterized to explore the effects of stereochemistry on the nonlinear absorption properties. The molecules feature 4-(phenylethynyl)phenylethynylene (PE2), diphenylaminofluorene (DPAF), and benzothiazolylfluorene (BTF) ligands. The photophysical properties were investigated under one- and two-photon conditions and compared to the known trans analogues via UV-visible absorption, photoluminescence, femtosecond and nanosecond transient absorption (TA), nanosecond z-scan, and femtosecond two-photon absorption (2PA). The bent cis complexes exhibit blue shifts in the absorption, emission, femtosecond, and nanosecond TA spectra along with lower molar extinction coefficients and lower phosphorescence yields relative to the trans complexes suggesting less efficient Pt-induced spin-orbit coupling and intersystem crossing in the cis configuration. The cis chromophores are noncentrosymmetric and therefore show dipolar behavior with a pronounced 2PA in the 0-0 transition of the S₀ → S₁ band, while the trans complexes show quadrupolar behavior with a forbidden 0-0 transition. In the S₀ → S_n region, both cis and trans complexes show intense two-photon-absorption bands (up to 3700 GM by the peak cross section for cis-BTF) which contain a significant contribution from the excited state absorption (S₁ → S_n). All six complexes exhibit comparable nonlinear absorption response with a significant contribution from triplet-triplet absorption that slightly favors trans complexes but is more strongly dependent upon the structure of the π-conjugated chromophore.

An Optical Power Limiting and Ultrafast Photophysics Investigation of a Series of Multi-Branched Heavy Atom Substituted Fluorene Molecules

A common molecular design paradigm for optical power limiting (OPL) applications is to introduce heavy atoms that promote intersystem crossing and triplet excited states. In order to investigate this effect, three multi-branched fluorene molecules were prepared where the central moiety was either an organic benzene unit, para-dibromobenzene, or a platinum(II)–alkynyl unit. All three molecules showed good nanosecond OPL performance in solution. However, only the dibromobenzene and Pt–alkynyl compounds showed strong microsecond triplet excited state absorption (ESA). To investigate the photophysical cause of the OPL, especially for the fully organic molecule, photokinetic measurements including ultrafast pump–probe spectroscopy were performed. At nanosecond timescales, the ESA of the organic molecule was larger than the two with intersystem crossing (ISC) promoters, explaining its good OPL performance. This points to a design strategy where the singlet-state ESA is balanced with the ISC rate to increase OPL performance at the beginning of a nanosecond pulse.

Rise of Conjugated Poly-ynes and Poly(Metalla-ynes): From Design Through Synthesis to Structure-Property Relationships and Applications

Conjugated poly-ynes and poly(metalla-ynes) constitute an important class of new materials with potential application in various domains of science. The key factors responsible for the diverse usage of these materials is their intriguing and tunable chemical and photophysical properties. This review highlights fascinating advances made in the field of conjugated organic poly-ynes and poly(metalla-ynes) incorporating group 4-11 metals. This includes several important aspects of conjugated poly-ynes viz. synthetic protocols, bonding, electronic structure, nature of luminescence, structure-property relationships, diverse applications, and concluding remarks. Furthermore, we delineated the future directions and challenges in this particular area of research.

Two-photon spectroscopy of tungsten(0) arylisocyanides using nanosecond-pulsed excitation

The two-photon absorption (TPA) cross sections (δ) for tungsten(0) arylisocyanides (W(CNAr)₆) were determined in the 800-1000 nm region using two-photon luminescence (TPL) spectroscopy. The complexes have high TPA cross sections, in the range 1000-2000 GM at 811.8 nm. In comparison, the cross section at 811.8 nm for tris-(2,2'-bipyridine)ruthenium(ii), [Ru(bpy)₃]²⁺, is 7 GM. All measurements were performed using a nanosecond-pulsed laser system.

Mechanical Activation of Platinum-Acetylide Complex for Olefin Hydrosilylation

Harnessing mechanical forces to activate latent catalysts has emerged as a novel approach to control the catalytic reactions in organic syntheses and polymerization processes. However, using polymer mechanochemistry to activate platinum-based catalysts, a class of important organometallic catalysts in industry, has not been demonstrated so far. Here we show that the platinum-acetylide complex is mechanoresponsive and can be incorporated into a polymer backbone to form a new mechanophore. The mechanically induced chain scission was demonstrated to be able to release catalytically active platinum species which could catalyze the olefin hydrosilylation process. Various control experiments were conducted to confirm that the chain scission and catalytic reaction were originated from the ultrasound-induced dissociation of platinum-acetylide complex. This work further exemplifies the utilization of organometallic complexes in design and synthesis of latent catalysts for mechanocatalysis and development of self-healing materials based on silicone polymers.

Our Expedition in Linear Neutral Platinum-Acetylide Complexes: The Preparation of Micro/nanostructure Materials, Complicated Topologies, and Dye-Sensitized Solar Cells

During the past few decades, the construction of various kinds of platinum-acetylide complexes has attracted considerable attention, because of their wide applications in photovoltaic cells, non-linear optics, and bio-imaging materials. Among these platinum-acetylide complexes, the linear neutral platinum-acetylide complexes, due to their attractive properties, such as well-defined linear geometry, synthetic accessibility, and intriguing photoproperties, have emerged as a rising star in this field. In this personal account, we will discuss how we entered the field of linear neutral platinum-acetylide chemistry and what we found in this field. The preparation of various types of linear neutral platinum-acetylide complexes and their applications in the areas of micro/nanostructure materials, complicated topologies, and dye-sensitized solar cells will be summarized in this account.

Ligand effects on cooperative supramolecular polymerization of platinum(ii) acetylide complexes

Ligand substitutes exert significant impacts on supramolecular polymerization and macroscopic gelation behaviors of platinum(ii) acetylide monomers.

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.

Triangular platinum(II) metallacycles: syntheses, photophysics, and nonlinear optics

Three triangular platinum(II) diimine metallacycles incorporating large cyclic oligo(phenylene-ethynylene) (OPE) bisacetylide ligands are synthesized, and their photophysical properties are studied. Two types of triplet excited states with ligand/metal-to-ligand charge-transfer and acetylide-ligand-centered characteristics respectively, are exhibited by these complexes depending on the size (conjugation length) and electronic features of the cyclic OPE ligands. When the energy levels of the two excited states are close to each other, the lowest triplet state is found to switch between the two in varied solvents, resulting from their relative energy inversion induced by solvent polarity change. Density functional theory and time-dependent density functional theory calculations provide corroborative evidence for such experimental conclusions. More importantly, the designed metallacycles show impressive two-photon absorption (2PA) and two-photon excitation phosphorescing abilities, and the 2PA cross section reaches 1020 GM at 680 nm and 670 GM at 1040 nm by two different metallacycles. Additionally, pronounced reverse saturable absorptions are observed with these metallacycles by virtue of their strong transient triplet-state absorptions.

Self-assembly of benzothiadiazole-functionalized dinuclear platinum acetylide bolaamphiphiles for bio-imaging application

Benzothiadiazole-functionalized dinuclear platinum(ii) acetylide bolaamphiphiles have been demonstrated to form nanoparticles in protic solvents, serving as novel fluorescent labels for bio-imaging applications with good biocompatibility and sufficient stability.

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.

Extending the bandwidth of reverse saturable absorption in platinum complexes using two-photon-initiated excited-state absorption

Pt(II) complexes bearing 4-(7-(benzothiazol-2'-yl)-9,9-diethylfluoren-2-yl)-2,2':6',2″-terpyridine or 4-(7-(benzothiazol-2'-yl)-9,9-diethylfluoren-2-yl)ethynyl-2,2':6',2″-terpyridine ligand exhibit strong reverse saturable absorption in the visible spectral region and large two-photon initiated excited-state absorption in the near-IR region. They are promising broadband nonlinear absorbing materials from the visible to the near-IR region. The extended π-conjugation in complex 2 that has a C≡C linker between the terpyridine ligand and the 4-(7-(benzothiazol-2'-yl)-9,9-diethylfluoren-2-yl) substituent significantly increases the two-photon absorption cross sections (σ(2)), making it among the strongest of two-photon absorbing Pt(II) complexes.

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.