Halogen-bridged Mixed-valence Platinum Complexes Research Articles

Wavepacket Dynamics at Low Temperature in Localized Excitons of the Quasi-One-Dimensional Br-Bridged Pt Complex Studied by Femtosecond Luminescence Spectroscopy

We report on the femtosecond time-resolved luminescence spectroscopy in a quasi-one-dimensional halogen-bridged mixed valence platinum complex, [Pt(en) 2 ][Pt(en) 2 Br 2 ](ClO 4 ) 4 at low temperatures. From the temperature dependence of the lifetime of the self-trapped excitons (STEs), we estimated the potential barrier height in the direction to the non-radiative path to be 99±10 meV. The observed time-evolutions of the luminescence excited by 1.57 eV photons at 4 K show complicated behavior, which is considerably different from that at room temperature. From Fourier transform of the time domain data, a low frequency mode was found at 0.7 THz in addition to the main mode at 3.4 THz, which has been assigned to the motion of the halogen ions in the self-trapped excitons. Incorporating two modes into the model based on the theory of resonant secondary radiation, the experimental data are well reproduced. This result shows that the wavepacket moves on the two-dimensional adiabatic potential surface of the s...

A template-directed synthetic approach to halogen-bridged mixed-valence platinum complexes on artificial peptides in solution

A template-directed synthetic approach to halogen-bridged mixed-valence platinum complexes has been performed in organic media using, for instance, a synthetic peptide bearing two bis(ethylenediamine)-based Pt(IV) complexes with two axial bromide anionic ligands, [(Pt(IV)Br2(en))2](RSO3)4, and a [Pt(II)(en)2](RSO3)2 complex (R = (C12H25OCH2)2CHO(CH2)3-).

Ultrafast relaxation dynamics of neutral soliton pairs in a quasi-one-dimensional halogen-bridged mixed-valence platinum complex [Pt(en)2][Pt(en)2Br2](ClO4)4

Ultrafast optical response of photoexcited states is studied by femtosecond pump and probe spectroscopy in a quasi-one-dimensional halogen-bridged mixed-valence platinum complex, [Pt(en)2][Pt(en)2Br2](ClO4)4 (en=ethylenediamine). We observed two kinds of photo-induced absorption (PA) signals, and both of the PA signal intensities follow non-exponential decay profiles. The signals are ascribed to two kinds of neutral soliton pairs with different configurations, that is, a spatially confined soliton and antisoliton pair and a noncorrelated soliton and antisoliton pair. Their formations are found to be closely related with both the excitation energy and the excitation density.

Decay Processes of a Photo-Excited State in a Strong Electron-Phonon Coupling System Trans-[Pt(en)2Cl2](ClO4)2

A quasi one-dimensional compound of halogen-bridged mixed-valence platinum complexes [Pt(en)2][Pt(en)2X2] (ClO4)4 (X=Cl, Br, and I, and en denotes ethylenediamine, hereafter we abbreviate as Pt–X) is known to show a strong charge-transfer (CT) absorption. By the optical excitation of the CT band, one can make a CT exciton on a one-dimensional chain. The CT exciton relaxes, without a barrier, to localized ed exciton (STE) states and the relaxation process was observed as a series of Raman lines followed by a large broad luminescence band. Recent spectroscopic measurement of femtosecond time resolved luminescence by Tomimoto et al. revealed a wave-packet oscillation in an adiabatic potential well associated with a self-trapped exciton. 3) The time constants of cooling from the photo-excited states to the STE states were identified as 0.21 and 0.43 ps for Pt–Cl and Pt–Br, respectively. Recently, optical spectra were measured at a He temperature on a crystal of trans-[Pt(en)2Cl2](ClO4)2 which consists of the same chemical composition as the Pt–Cl. The absorption band observed at the lowest energy of 3.3 eV was due to the d-d transition on a platinum atom. 5) The luminescence spectrum had a Gaussianlike lineshape formed by the superposition of phonon side bands more than 20 series. The energy of phonons observed in the luminescence was quite close to that of a breathing mode of chlorines centered around the platinum ion (44.9meV) observed on the Raman spectrum. On the other hand, no phonon structures were observed on the luminescence of both one-dimensional Pt– Cl and Pt–Br. The electron phonon coupling strength (Huang-Rhys factor) of trans-[Pt(en)2Cl2](ClO4)2 was obtained to be 9.52. These results show that trans[Pt(en)2Cl2](ClO4)2 is a strong electron-phonon coupling system and that one of most suitable materials for the studies of the dynamical decay processes of photo-excited states in a strong electron-phonon coupling system. Special interest is at the point of comparison with onedimensional Pt–X. In this report, time resolved luminescence spectra of trans-[Pt(en)2Cl2](ClO4)2 are for various temperatures and the photo-excited state is shown to be cool-down on the adiabatic potential surface of the excited state at about 1 ns. self-trapp

Femtosecond dynamics of self-trapped excitons in quasi-one-dimensional halogen-bridged Pt complexes

Dynamics of the self-trapped excitons (STE) in quasi-one-dimensional halogen-bridged mixed-valence platinum complexes [Pt(en) 2][Pt(en) 2X 2](ClO 4) 4 (en=ethylenediamine, X=Cl or Br) has been studied by luminescence up-conversion technique. Time evolution of the luminescence from STE has been studied from 0.79 to 2.1 eV in the X=Cl system with a time resolution of 180 fs. The cooling time of hot STEs was determined as 210 fs, and the formation time of STE was found to be shorter than 140 fs, which is limited by our time resolution. In the X=Br system, we achieved a time resolution of 90 fs and succeeded in observing the wave packet oscillation in the adiabatic potential well of STE by luminescence for the first time. The period of the local mode oscillation is 290 fs (97 cm −1) and the damping time constant is 430 fs. Usefulness of the up-conversion spectroscopy for wave packet observation is demonstrated.

Supramolecular Assemblies Comprised of One-Dimensional Mixed Valence Platinum Complex and Anionic Amphiphiles in Organic Media

Abstract A novel class of one-dimensional supramolecular assembly dispersed in organic media was prepared from halogen-bridged mixed valence platinum complex and anionic amphiphiles. Their intervalence absorption characteristics and nanostructures varied depending on the chemical structure of amphiphiles.

Electrocrystallization of a halogen-bridged mixed-valence platinum complex

Electrocrystallization of [Pt(en) 2][PtCl 2(en) 2](ClO 4) 4 (1), which is a halogen-bridged mixed-valence platinum complex having Pt II ··· X − Pt IV − X ··· chains, is reported. Electrochemical reduction of trans-[PtCl 2(en) 2]Cl 2 in an aqueous solution of perchloric acid at a potential of −1.5 to −1.6 V vs Pt counter electrode gave red needle crystals of (1) which are several mm long. The crystals appeared not only on the electrode but also away from it. On the other hand, electrochemical oxidation of a Pt II complex gave no crystals of (1) and the reason for this result is discussed.

Excitation energy dependence of the photoluminescence from self-trapped excitons in halogen-bridged mixed-valence platinum complexes

The excitation energy dependence of the photoluminescence (PL) of self-trapped excitons has been investigated in [Pt(en) 2][Pt(en) 2Cl 2(1− x) Br 2 x ](ClO 4) 4 ( x = 0, 0.08, 0.15) single crystals. The peak energy of the PL shifts to higher energy with increasing excitation photon energy from 2.4 to 2.6 eV. When the excitation energy is above 2.6 eV, the position of the peak energy is independent of the excitation energy and becomes almost constant. The magnitude of the peak energy shift increases with the molar fraction of bromine. These results are well understood by the distribution of the electron-hole separation which may depend on the excitation energy.

The Electron-Lattice Coupling in Quasi-One-Dimensional Halogen-Bridged Mixed-Valence Platinum Complexes (HMPC's)11The project supported by National Natural Science Foundation of China.

The effect of site-diagonal and site-off-diagonal electron-lattice coupling on polaronic binding energy in quasi-one-dimensional halogen-bridged mixed-valence platinum complexes (HMPC's) is investigated. It shows that the site-diagonal part is determinative in this kind of material, and agreeable with experiments.

Design of Hydrogen Bond Network in Halogen-Bridged Mixed-Valence Platinum Complexes by Substitution of Counter Ion

Abstract Sulfates in one-dimensional halogen-bridged mixed-valence platinum complexes were synthesized in order to elucidate contributions of hydrogen bonds between in-plane ligand amines and counter ions to the mixed-valence state. Their intervalence charge-transfer bands measured by using the single crystals and their structures determined by single crystal X-ray diffraction analyses show that the energy difference of dz 2 orbitals between the PtII and the PtIV becomes larger with decreasing linkages between the Pt units by the hydrogen bonds, comparing with perchlorates and hydrogensulfates.

PHOTO-INDUCED ABSORPTION BAND IN QUASI-ONE-DIMENSIONAL HALOGEN-BRIDGED MIXED-VALENCE PLATINUM COMPLEXES

Photo-induced absorption states in quasi-one-dimensional halogen-bridged mixed valence platinum complex is investigated. Based on the experimental data, a mechanism for photo-induced absorption is presented. Photo-induced absorption spectra both deformations of alternative and polaron induced for halogen ions are calculated. The comparison with experiments is made.

Nonlinear optical study of quasi-one-dimensional platinum complexes: Two-photon excitonic resonance effect

Third-harmonic generation measurements have been made for the first time on oriented films of halogen-bridged mixed-valence platinum complexes for the pump wavelength region from 0.6 to 1.0 μm. The ‖χ(3)(−3ω;ω,ω,ω)‖ spectra obtained by the Maker’s fringe method show a distinct peak at about 1.8 eV with the peak value of about 4×10−11 esu. From comparison with the electroabsorption data, this peak has been identified with the two-photon resonance peak with the forbidden exciton in the Pt-halogen chains. One-dimensional exciton states in this compound take an important role in the third-order optical nonlinearity, similar to the case in conjugated polymers.

Optical response of excitons in one-dimensional semiconductors

Exciton effect on interband absorption in a one-dimensional semiconductor with a dimerization gap is studied theoretically. An important role of the delocalization length is pointed out, and the peculiarity of the lowest singlet exciton state is elucidated by calculating the one- and the two-photon spectra. Results are compared with experiments in polydiacetylene, polysilane, and halogen-bridged mixed-valence platinum complexes.

Polaronic states induced by photo-excitation and halogen-doping in quasi-one-dimensional, mixed-valence platinum complexes -drift mobilities of electron and hole polarons-

The drift mobilities of the quasi-one-dimensional halogen-bridged mixed-valence platinum complexes, were measured on both the pure and halogen-doped samples by the pulsed photoconductivity technique. The results show that the hopping mechanism dominates the electric conduction and that the halogen dopants produce hole polarons on chains and the ionized dopants entered between the chains become a hopping center.

Luminescence study of the self-trapped exciton polarons of mixed crystals of quasi-one-dimensional mixed-valence metal complexes

Mixed crystals of quasi-one-dimensional halogen-bridged mixed-valence platinum complexes [Pt 3−ϱ (en) 2] [Pt 3+ ϱ (Cl 1−xBr x) 2(en)] (ClO 4) 4 (0<×<1, 0<<ϱ≤1) were studied by luminescence measurements. The self-trapped states of the charge transfer excitons were of the “amalgamation” type as well as the free states. By comparing the line shapes with the calculated profiles using a model, the polaron size of the self-trapped exciton was estimated to be larger than four molecular units of [-Pt t− ϱ -x −-Pt 3+ ϱ -x −-] (X = Cl,Br). This result is consistent with our previous estimates from the lifetime and the quantum efficiency of the luminescence.

DOPING AND ELECTRICAL CONDUCTIVITY IN HALOGEN-BRIDGED MIXED-VALENCE PLATINUM COMPLEX

A model Hamiltonian is presented to describe halogen-bridged mixed-valence platinum complex (HMPC), the Green function is used to calculate the electrical conductivity of doping HMPC, the dope induced defect states are discussed in detail, the results are in agreement with experiments satisfactorily.

Study of Mixed-Valence State in Halogen-Bridged Mixed-Valence Platinum(II,IV) Complexes by Temperature Dependence of Intervalence Charge-Transfer Spectra

Abstract The low-energy absorption edge due to the intervalence charge-transfer (IVCT) transition from PtII to PtIV has been investigated at 4.2 K and room temperature in single crystals of [Pt(en)2][PtBr2(en)2]Y4 {Y=ClO4(PBC), HSO4(PBHS)}, [Pt(en)2][PtI2(en)2](HSO4)4·4H2O (PIHS) and [Pt(en)2][PtX2(en)2](SO4)2·6H2O {X=Cl(PCSW), Br(PBSW), I(PISW)}. The IVCT absorption edges in PCSW, PBSW, and PISW shifted to the lower-energy side (i.e. red shift), while those in PBC, PBHS, and PIHS shifted to the higher-energy side (i.e. blue shift) with an increase in temperature. We consider that the thermal effect of the valence fluctuation of the Pt mixed-valence state causes a red shift of the IVCT absorption edge and that the thermal effect of the hydrogen bond between the PtII complex ion and the PtIV complex ion causes a blue shift of the IVCT absorption edge. These two effects coexist and compete with each other in one complex. The dominant effect of the two effects determines the direction of the shift of the IVCT absorption edge, namely, the temperature dependence of the Pt mixed-valence state.

Comparison between Optical Properties of Sulfates and Hydrogensulfates in Halogen-Bridged Mixed-Valence Platinum(II, IV) Complexes

Abstract Syntheses and optical properties of some new halogen-bridged mixed-valence platinum complexes analogous to Wolffram’s Red Salt, [Pt(en)2][PtBr2(en)2](HSO4)4 (PBHS), [Pt(en)2][PtI2(en)2](HSO4)4·4H2O (PIHS), and [Pt(en)2][PtX2](en)2](SO4)2·6H2O {X=Br(PBSW), I(PISW)}, are reported. The powder X-ray diffraction pattern shows that PBHS is isomorphous with [Pt(en)2][PtCl2(en)2](HSO4)4 (PCHS), and that PBSW and PISW are isomorphous. The hydrogensulfates PBHS and PIHS crystallize from an aqueous sulfuric acid solution, whereas the sulfates PBSW and PISW crystallize from an aqueous solution. The single crystal absorption spectra of PBHS, PIHS, PBSW, and PISW have been investigated at 4.2 K. The intense optical absorption due to intervalence charge-transfer transition from Pt(II) to Pt(IV) has been observed for the light polarized parallel to the Pt chain axis (E//chain). From the comparison of the energy of the intervalence charge-transfer absorption edges in PBHS, PBSW, and PISW, one can conclude that the influence of the substitution of the counterions HSO4− with SO42− on the Pt(II)–Pt(IV) mixed-valence state is no less better than that of the substitution of the bridging halogen. The role of the counterion on the mixed-valence state is explained by the hydrogen bond network (Pt–NH···counterion···HN–Pt) on individual chains.

Effects of Impurity-Induced Interchain Coupling on Polarons in Quasi-One-Dimensional Solids

When an impurity is placed between chains in quasi-one-dimensional solids, it can cause transfer of electrons between chains through itself. The effects of this impurity induced interchain coupling on polarons are studied by means of the continuum version of the Holstein model. It is found that when the coupling strength exceeds a certain critical value, the polaron wave function extends itself over plural chains surrounding the impurity, rather than being localized on a single chain. Higher conductivity in directions perpendicular to the chains than in the parallel direction observed quite recently in doped halogen-bridged mixed-valence platinum complexes lends support to the above-mentioned picture.

Polarons and soliton pairs (bipolarons) in halogen-doped quasi-one-dimensional mixed-valence platinum complexes.

The first experimental studies on halogen doping in quasi-one-dimensional, halogen-bridged, mixed-valence platinum complexes (HMPC) are presented. The electrical conductivity of the iodine compound increases over 7 orders of magnitude with iodine doping. The absorbance of the two optical-absorption bands near the midgap and the ESR intensity in dilute doped HMPC increase with the dopant concentration. In the dilute regime, halogen doping yields the same effects on optical absorption and ESR as does photoexcitation at an energy greater than or equal to the energy of the charge-transfer absorption band. The doping-induced states are located on single chains and have charge and spin. These results strongly suggest polaron formation by halogen doping. At high doping levels, a new doping-induced absorption appears at an energy of 1.55 eV at 77 K, and the ESR decreases in intensity as the doping proceeds. It appears that upon halogen doping, singly charged polarons are produced and the formation of confined charged soliton pairs (bipolarons) via the process ${P}^{+}$+${P}^{+}$\ensuremath{\rightarrow}${S}^{+}$+${S}^{+}$ (or ${B}^{2+}$) is favored.