Ultrafast Energy Migration in Platinum(II) Diimine Complexes Bearing Pyrenylacetylide Chromophores

Bis-alkynylphosphine Oxide Pt(II) Complexes: Aggregation-Induced Phosphorescence Enhancement and Mechanochromic Luminescence Properties

Four bis-alkynyl Pt(II) complexes [Pt(dtbpy)(C2-L-P(O)Ph2)2] with dtbpy = 4,4'-ditertbutyl-2,2'-bipyridine and alkynylphosphine oxide ligands (L = no linker, Pt0; phenyl, Pt1; biphenyl, Pt2; naphthyl, Pt3) have been synthesized and fully characterized by spectroscopic methods and single crystal XRD analysis. It has been found that the nature of the π-conjugated linker is a key factor in fine-tuning the emission energy of the complexes in solution and in achieving the aggregation-induced phosphorescence enhancement (AIPE) effect for complex Pt0 with the most compact linker. Phosphine oxide fragment, which can be involved in weak intermolecular interactions, promotes the existence of two solid forms with different luminescence properties. These two forms can be switched from one to another upon grinding, thus featuring distinct mechanochromic luminescence properties. TDDFT calculations are consistent with the experimental results and assign mixed 3MLCT and 3LL'CT solution emission character and 3MMLCT and 3LL'CT emission nature in supramolecular dimeric structures.

Long Excited-State Lifetimes in Three-Coordinate Copper(I) Complexes via Triplet-Triplet Energy Transfer to Pyrene-Decorated Isocyanides

There has been much effort to improve excited-state lifetimes in photosensitizers based on earth-abundant first-row transition metals. Copper(I) complexes have gained significant attention in this field, and in most cases, sterically driven approaches are used to optimize their lifetimes. This study presents a series of three-coordinate copper(I) complexes (Cu1-Cu3) where the excited-state lifetime is extended by triplet-triplet energy transfer. The heteroleptic compounds feature a cyclohexyl-substituted β-diketiminate (CyNacNacMe) paired with aryl isocyanide ligands, giving the general formula Cu(CyNacNacMe)(CN-Ar) (CN-dmp = 2,6-dimethylphenyl isocyanide for Cu1; CN-pyr = 1-pyrenyl isocyanide for Cu2; CN-dmp-pyr = 2,6-dimethyl-4-(1-pyrenyl)phenyl isocyanide for Cu3). The nature, energies, and dynamics of the low-energy triplet excited states are assessed with a combination of photoluminescence measurements at room temperature and 77 K, ultrafast transient absorption (UFTA) spectroscopy, and DFT calculations. The complexes with the pyrene-decorated isocyanides (Cu2 and Cu3) exhibit extended excited-state lifetimes resulting from triplet-triplet energy transfer (TTET) between the short-lived charge-transfer excited state (3CT) and the long-lived pyrene-centered triplet state (3pyr). This TTET process is irreversible in Cu3, producing exclusively the 3pyr state, and in Cu2, the 3CT and 3pyr states are nearly isoenergetic, enabling reversible TTET and long-lived 3CT luminescence. The improved photophysical properties in Cu2 and Cu3 result in improvements in activity for both photocatalytic stilbene E/Z isomerization via triplet energy transfer and photoredox transformations involving hydrodebromination and C-O bond activation. These results illustrate that the extended excited-state lifetimes achieved through TTET result in newly conceived photosynthetically relevant earth-abundant transition metal complexes.

Supramolecular assembly of bent dinuclear amphiphilic alkynylplatinum(ii) terpyridine complexes: diverse nanostructures through subtle tuning of the mode of molecular stacking

A new class of bent amphiphilic alkynylplatinum(ii) terpyridine complexes was found to adopt different modes of molecular stacking to give diverse nanostructures.

A new class of bent amphiphilic alkynylplatinum(ii) terpyridine complexes was found to adopt different modes of molecular stacking to give diverse nanostructures. In chlorinated solvents, the complexes aggregate in the presence of water droplets and assist in the formation of porous networks, while in DMSO solutions, they self-assemble to give fibrous nanostructures. The complexes would adopt a head-to-tail tetragonal stacking arrangement, as revealed by X-ray crystallographic studies, computational studies and powder X-ray diffraction (PXRD) studies. Their self-assembly follows a cooperative growth mechanism in DMSO and an isodesmic growth mechanism in DMSO–H₂O mixture. The balance between hydrophobic and hydrophilic components of the complex system, in conjunction with the nuclearity and the positioning of the substituents, are found to govern the mode of molecular stacking and the fabrication of precise functional nanostructures. This class of complexes serve as versatile building blocks to construct orderly packed molecular materials and functional materials in a well-controlled manner.

Directing energy transfer in Pt(bodipy)(mercaptopyrene) dyads

We report on the photophysical properties of three dyads that combine a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (bodipy, BDP) and a mercaptopyrene (SPyr) dye ligand at a Pt(PEt3)2 fragment. σ-Bonding of the dyes to the Pt ion promotes intersystem crossing (ISC) via the external heavy atom effect. The coupling of efficient ISC with charge-transfer from the electron-rich mercaptopyrene to the electron-accepting BDP ligand (PB-CT) gives rise to a multitude of (potentially) emissive states. This culminates in the presence of four different emissions for the mono- and dinuclear complexes BPtSPyr and BPtSPyrSPtB with an unsubstituted BDP ligand and either a terminal 1-mercaptopyrene or a bridging pyrene-1,6-dithiolate ligand. Thus, in fluid solution, near IR emission at 724 nm from the 3PB-CT state is observed with a quantum yield of up to 15%. Excitation into the BDP-based 1ππ* or the pyrene-based 1ππ* band additionally trigger fluorescence and phosphorescence emissions from the BDP-centred 1ππ* and 3ππ* states. In frozen solution, at 77 K, phosphorescence from the pyrene ligand becomes the prominent emission channel, while PB-CT emission is absent. Alkylation of the BDP ligand in KBPtSPyr funnels all excitation energy into fluorescence and phosphorescence emissions from the KBDP ligand. The assignments of the various excited states and the deactivation cascades were probed by absorption and emission spectroscopy, transient absorption spectroscopy, electrochemical and UV/Vis/NIR spectroelectrochemical measurements, and by quantum chemical calculations. Our conclusions are further corroborated with the aid of suitable reference compounds comprising of just one chromophore. All dyads are triplet sensitizers and are able to generate singlet oxygen.

Effect of Molecular Conformation Restriction on the Photophysical Properties of N^N Platinum(II) Bis(ethynylnaphthalimide) Complexes Showing Close-Lying 3MLCT and 3LE Excited States

Using naphthalimide (NI), complexes (Pt-PhNI and Pt-PhMeNI) based on the N^N platinum(II) bis(phenylacetylide) coordination framework were prepared, in which there are two close-lying triplet states, i.e., the metal-to-ligand-charge-transfer (³MLCT) and the NI localized emissive state (³LE). Pt-PhNI has better electronic communication between the Pt coordination center and the NI moiety, whereas in Pt-PhMeNI, they are more isolated by orthogonal geometry. For Pt-PhMeNI, the S₀ → ¹MLCT and S₀ → ¹LE absorption bands are separated by 5655 cm^-1, while they are more overlapped in Pt-PhNI. The ³MLCT → S₀ and ³LE → S₀ dual phosphorescence emissions were observed for both Pt-PhNI (in toluene) and Pt-PhMeNI (in benzonitrile). The molecular conformation tunes the ³MLCT/³LE state population ratio, and the orthogonal geometry makes the ³LE state in Pt-PhMeNI basically a dark state (in toluene). Switching of the relative energy levels of the ³MLCT/³LE states by variation of the solvent polarity and temperature was achieved. For Pt-PhMeNI, the energy level of ³MLCT state is higher in a polar solvent; thus, the ³MLCT emission decreases, while the phosphorescence lifetime is prolonged from 9.5 μs (in toluene) to 58 μs (in benzonitrile) because of the different equilibria with the nonemissive ³LE state. Conversely, increasing the temperature enhances the upward transition from the nonemissive ³LE state to the emissive ³MLCT state; as such, the phosphorescence of Pt-PhMeNI was intensified at higher temperature (which is unusual), and the phosphorescence lifetime decreased from 58 μs (298 K) to ca. 5 μs (348 K). The ultrafast intersystem crossing (ca. 0.5 ps) and intramolecular triplet-triplet energy transfer (3-11 ps) were studied by femtosecond transient absorption spectroscopy. These results are useful for an in-depth understanding of the photophysics of multichromophore transition-metal complexes and for the design of external stimuli-responsive sensing materials, for instance, temperature or microenvironment sensing materials.

Long-lived triplet excited state in a platinum(ii) perylene monoimide complex

We report the synthesis and solution based photophysical properties of a new Pt(ii)-terpyridine complex coupled to a perylene monoimide (PMI) chromophoric unit through an acetylene linkage. This structural arrangement resulted in quantitative quenching of the highly fluorescent PMI chromophore by introducing metal character into the lowest energy singlet state, thereby leading to the formation of a long-lived PMI-ligand localized triplet excited state (τ = 8.4 μs). Even though the phosphorescence from this triplet state was not observed, highly efficient quenching of this excited state by dissolved oxygen and the observation of singlet oxygen photoluminescence in the near-IR at 1270 nm initially pointed towards triplet excited state character. Additionally, the coincidence of the excited state absorbance difference spectra from the sensitized PMI ligand using a triplet donor and the Pt-PMI complex provided strong evidence for this triplet state assignment, which was further supported by TD-DFT calculations.

Unveiling excited state energy transfer and charge transfer in a host/guest coordination cage

Ultrafast excited-state dynamic processes, charge and energy transfer in a HGCT system are unveiled by using femtosecond transient absorption spectroscopy.

Chromophore-labelled, luminescent platinum complexes: syntheses, structures, and spectroscopic properties

Ligands based upon 4-carboxamide-2-phenylquinoline have been functionalised with naphthyl, anthracenyl and pyrenyl chromophores. The pyrene appended Pt(ii) complex shows excited state equilibration and a long emission lifetime.

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.

Altering molecular photophysics by merging organic and inorganic chromophores

Photofunctional molecules and assemblies lie at the heart of many important fundamental processes in nature, and researchers have generated multitudes of artificial chromophores intended to mimic these naturally occurring systems. As dynamic spectroscopic techniques are becoming more widely available, ultrafast techniques in particular, substantial insight continues to be gleaned from the initial photon stimulation event through internal conversion, structural rearrangements, intersystem crossing, energy migration, electron transfer events, and ultimately regeneration of the ground state chromophores in both naturally occurring and inspired chromophores. This Account details research endeavors motivated by the concept that merging organic and inorganic chromophores can lead to new molecules exhibiting novel excited state properties. Moreover, these excited state properties can be fundamentally understood using combinations of static and dynamic spectroscopic tools, yielding systematic improvements to molecules poised for application in diverse research areas including light-harvesting, lifetime engineering, photocatalysis, and photochemical upconversion. Initial explorations focused on utilizing Förster energy transfer processes in Ru(II)-based metal-organic chromophores for solar light-harvesting while maintaining long excited state lifetimes. This eventually led to molecules exhibiting triplet-triplet energy transfer between energetically proximate triplet states featuring thermally activated photoluminescence from the upper charge transfer excited state with markedly extended lifetimes. Interest in systematically producing long-lived excited states with concomitant large Stokes shifts inspired the development of numerous Pt(II) bipyridyl and terpyridyl acetylide charge transfer chromophores featuring ultrafast intramolecular energy migration, high quantum yield ligand-localized phosphorescence at room temperature, and synthetically tunable excited state absorption properties. This structural motif also made it possible to access the triplet excited states of perylenediimide chromophores, permitting quantitative examination of internal conversion and intersystem crossing processes in these complex molecules. The generation of new metal-organic structures featuring unique photophysics appears limitless and simply requires the continued ingenuity of researchers.

Ultrafast charge transfer dynamics in supramolecular Pt(ii) donor–bridge–acceptor assemblies: the effect of vibronic coupling

Thanks to major advances in laser technologies, recent investigations of the ultrafast coupling of nuclear and electronic degrees of freedom (vibronic coupling) have revealed that such coupling plays a crucial role in a wide range of photoinduced reactions in condensed phase supramolecular systems. This paper investigates several new donor–bridge–acceptor charge-transfer molecular assemblies built on a trans-Pt(ii) acetylide core. We also investigate how targeted vibrational excitation with low-energy IR light post electronic excitation can perturb vibronic coupling and affect the efficiency of electron transfer (ET) in solution phase. We compare and contrast properties of a range of donor–bridge–acceptor Pt(ii) trans-acetylide assemblies, where IR excitation of bridge vibrations during UV-initiated charge separation in some cases alters the yields of light-induced product states. We show that branching to multiple product states from a transition state with appropriate energetics is the most rigid condition for the type of vibronic control we demonstrate in our study.

Triplet state formation in homo- and heterometallic diketopyrrolopyrrole chromophores

The synthesis, structural characterization, and excited-state dynamics of series of diketopyrrolopyrrole (DPP) bridged homodinuclear Ir(III) and heterodinuclear Ir(III)/Pt(II) complexes is described. Steady-state and time-resolved photoluminescence along with transient absorption measurements were used to probe the nature of the emissive and long-lived excited states. Upon excitation into the (1)DPP ligand-localized excited state in the presence of coordinated Ir(III) or Pt(II) metal centers, the intersystem crossing is enhanced, leading to a quenching of the (1)DPP fluorescence and the formation of the long-lived (τ ≈ 30-40 μs) (3)DPP excited state in all instances.

Exploring excited-state tunability in luminescent tris-cyclometalated platinum(IV) complexes: synthesis of heteroleptic derivatives and computational calculations

The synthesis, structure, electrochemistry, and photophysical properties of a series of heteroleptic tris- cyclometalated Pt(IV) complexes are reported. The complexes mer-[Pt(C^N)2 (C'^N')]OTf, with C^N=C-deprotonated 2-(2,4-difluorophenyl)pyridine (dfppy) or 2-phenylpyridine (ppy), and C'^N'=C-deprotonated 2-(2-thienyl)pyridine (thpy) or 1-phenylisoquinoline (piq), were obtained by reacting bis- cyclometalated precursors [Pt(C^N)2 Cl2] with AgOTf (2 equiv) and an excess of the N'^C'H pro-ligand. The complex mer-[Pt(dfppy)2 (ppy)]OTf was obtained analogously and photoisomerized to its fac counterpart. The new complexes display long-lived luminescence at room temperature in the blue to orange color range. The emitting states involve electronic transitions almost exclusively localized on the ligand with the lowest π-π* energy gap and have very little metal character. DFT and time-dependent DFT (TD-DFT) calculations on mer-[Pt(ppy)2 (C'^N')](+) (C'^N'=thpy, piq) and mer/fac-[Pt(ppy)3](+) support this assignment and provide a basis for the understanding of the luminescence of tris-cyclometalated Pt(IV) complexes. Excited states of LMCT character may become thermally accessible from the emitting state in the mer isomers containing dfppy or ppy as chromophoric ligands, leading to strong nonradiative deactivation. This effect does not operate in the fac isomers or the mer complexes containing thpy or piq, for which nonradiative deactivation originates mainly from vibrational coupling to the ground state.

Ultrafast photoinduced charge transport in Pt(ii) donor–acceptor assembly bearing naphthalimide electron acceptor and phenothiazine electron donor

The Pt(ii)-based molecular triad engages in step-wise photoinduced charge-separation; the charge recombination occurs through-space via two distinct pathways.

Photoinduced charge separation in a PtII acetylide donor–acceptor triad based on 2-(1-pyrazole)-pyridine modified with naphthalene mono-imide electron acceptor

A class of molecular electron transfer cascades—those based on PtII complexes of 2-(1-pyrazole)-pyridine (pzpy) ligands—are reported. The synthesis of a new electron-acceptor imide-modified pzpy ligands is reported, and their application to transition-metal chemistry demonstrated by the synthesis of the PtII chloride and acetylide complexes. These donor–acceptor assemblies are promising models for investigation of photoinduced charge separation. Accordingly, picosecond time-resolved infrared (TRIR) and femtosecond transient absorption (TA) studies have been undertaken to elucidate the nature and dynamics of the lowest excited states in Pt(NAP-pyr-pyrazole)(–CC–Ph–C7H15)2. It has been established that the initial population of an MLL'CT excited state in the chromophoric [Pt(pyridine-pyrazole)(acetylide)] core is followed by an electron transfer to the naphthalimide (NAP) acceptor, forming a charge-separated state. This state is characterized by a large shift in ν(CO) vibrations of the NAP acceptor, as well as by a very intense and broad [×10 times in comparison to ν(CO)] asymmetric acetylide stretch which incorporates –CC–Pt–CC– framework and occurs at approximately 300 cm–1 lower in energy than its ground-state counterpart. In CH2Cl2 at room temperature, the charge-separated state with the lifetime of 150 ps collapses into an almost isoenergetic NAP-localized triplet state; the rate of this transformation changes upon decreasing the temperature to 263 K. This final excited state, 3NAP-(pyr-pyrazole)Pt(–CC–Ph–C7H15)2, has an unusually long, for PtII complexes, excited-state lifetime of tens of microseconds. The work demonstrates the possibility of tuning excited-state properties in this new class of PtII chromophores designed for electron-transfer cascades.

Excited-state properties of heteroleptic iridium(III) complexes bearing aromatic hydrocarbons with extended cores

The synthesis, complete structural characterization, electrochemistry, and excited-state dynamics of a series of four bis-heteroleptic iridium(III) charge-transfer complexes composed of a single acac-functionalized and two ortho-metalated 2-phenylpyridine ligands. The formed iodophenyl complex (2) was used as a metallosynthon to introduce extended-core ethynyltolyl (3), ethynylpyrene (4), and ethynylperylene (5) residues into these structures projecting from the acac ancillary ligand. Static and dynamic photoluminescence along with ultrafast and conventional transient absorption measurements in conjunction with cyclic voltammetry were employed to elucidate the nature of the intramolecular energy-transfer processes occurring in the excited states of polychromophores 4 and 5 and are directly compared with those of model complexes 2 and 3. Upon charge-transfer excitation of these molecules, the long-lived triplet-state metal-to-ligand charge-transfer ((3)MLCT)-based photoluminescence readily observed in 2 and 3 (τ = 1 μs) is nearly quantitatively quenched, resulting from production of the associated triplet intraligand ((3)IL) excited states in 4 and 5 through intramolecular triplet-triplet energy transfer. The respective formation of the extended-core (3)*pyrenyl and (3)*perylenyl-localized excited states in 4 and 5 is confirmed by their ultrafast excited-state evolution, which ultimately generates features associated with these (3)IL excited states and their greatly extended excited-state lifetimes with respect to the parent complexes 2 and 3.

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.

Coordination mode dependent excited state behavior in group 8 phosphino(terthiophene) complexes

The ground and excited state behavior of four Ru(II) and Os(II) bipyridyl complexes containing the 3'-(diphenylphosphino)-2,2':5',2''-terthiophene (PT(3)) ligand in two different coordination modes (P,S and P,C) is reported. The complexes are generally stable under extended photoirradiation, except for [Ru(bpy)(2)PT(3)-P,S](PF(6))(2) which decomposes. Emission lifetimes and transient absorption spectra and lifetimes have been obtained for all the complexes. These data support a PT(3) ligand based lowest excited state in the case of both P,S bound complexes, and a charge separated lowest excited state in both P,C bound complexes, conclusions supported by Density Functional Theory (DFT) calculations.

Naphthalimide phosphorescence finally exposed in a platinum(II) diimine complex

Room temperature (RT) phosphorescence is observed from a naphthalimide species for the first time in the square-planar chromophore Pt(dbbpy)(C[triple bond]C-NI)(2), where NI = N-butyl-4-ethynylnaphthalimide and dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine. The combination of static and time-resolved absorption and photoluminescence data is uniformly consistent with triplet-state photophysics localized on an appended C[triple bond]C-NI unit following excitation into the low-energy absorption bands. This molecule features rather impressive long-lifetime, high-quantum-efficiency NI-based RT phosphorescence (tau = 124 micros; Phi = 0.215) centered at 621 nm, exemplifying how the platinum acetylide linkage strongly promotes intersystem crossing in the NI subunit, representative of a class of molecules whose excited states are typically dominated by singlet fluorescence.