Syntheses of 1,1′-Bicobaltocene Salts of Tetracyano-P-Quinodimethane,Tetrabromoquinone, and Tetrabromodiphenoquinone; and the Structure of 1,1′-Bicobaltocene[Co(III)Co(III)][TCNQ]3

Abstract

A general theory of exciton (and electron) transport in perfect molecular crystals is discussed. The emphasis is on qualitative results and the elucidation of the effect of exciton and phonon bandwidth on the scattering of excitons and on transport. Various microscopic models are discussed in this light.     

Migration and Annihilation of Triplet Excitation Energy in the Charge-Transfer Anthracene-Tetrachlorophtalic Anhydride Crystal

Abstract

The emission spectra of the charge-transfer crystal of anthracene-tetrachlorophtalic anhydride were studied in the 1.7-300 K temperature range. Delayed fluorescence was observed over the whole temperature range. At temperatures below ～40 K it results from heterogenous (mobile exciton-trapped exciton) triplet-triplet annihilation. At higher temperatures it is due to homogenous triplet-triaplet annihilation. The existence of mobile excitons is also proved by the character of the ESR lines which are very narrow Lorentzians. The phosphorescence, observed only in the range 1.7–40 K, originates from at least two different kinds of traps with energies ～6 cm^?1 and ～140 cm^?1, respectively. The vibrational structure of the phosphorescence is identical to that of anthracene, with 0—0 band blue-shifted by ～400cm^?1. A kinetic model of trapping and detrapping of triplet excitons as well as the nature of the traps are discussed.     

Time-resolved photoluminescence study of excited states in nanostructured melanin

Abstract

Time resolved emission spectra (TRES), 3D maps of photoluminescence excitation-emission (PLE maps), 3D maps of TRES, and kinetics of photoluminescence (PL) spectra of natural and synthetic melanin, as well as melanin composites with surfactant, sodium dodecyl benzenesulfonate, have been studied. The kinetics of PL and TRES for the melanin and the composites were investigated at low temperatures that allowed to discuss melanin’s intermolecular complexation, nano-aggregation and formation of charge transfer states. Particularly, the radiation of Frenkel excitons from fragments of different degrees of conjugation that are the parts of molecular nanodisks of melanin is observed. Furthermore, charge transfer excitons (CT-excitons) can be formed and observed at both room and helium temperatures. Some bands in the PL spectra of melanin, depending on temperature, behave like excimer radiation. The studies are aiming to understand the electronic properties of various melanins, particularly natural ones.     

Double Injection and Electroluminescence in p-Terphenyl Single Crystals

Abstract

Double injection currents in p-terphenyl are followed by light emission from decaying singlet excitons produced by the recombining electron-hole pairs. Only one singlet exciton is produced out of ～ 100 recombinations, the other recombinations are non-radiative. The current densities with gold and Na-Hg amalgam as hole and electron injecting contacts, respectively, approached the limit of volume-controlled currents, but were limited to well defined spots covering only 10 ³-10 ⁴ of the contacts area.

This work is a study of double injection currents and recombination radiation in p-terphenyl. Gold and Na-Hg amalgam contacts were used as hole and electron injectors, but efficient injection took place in only 10^?3-10^?4 of the contacts area. In that area the current densities approached the limit of volume-controlled currents. The recombination radiation has the same spectral distribution as the ordinary crystal fluorescence, both result from the radiative decay of singlet excitons. The major proportion of electron-hole recombinations is non-radiative. Only about one singlet exciton is produced per about 100 recombination events.     

Measurement of Singlet Exciton Diffusion Coefficient in the C′-Direction in Crystalline Naphthalene

Abstract

The diffusion coefficient of singlet excitons in the c¹ direction in crystalline naphthalene has been deduced from the temporal analysis of fluorescence quenching at the surface by an Al layer, D_C ¹ = 5 × 10^?5 cm² s^?l, corresponding to a diffusion length A_C ¹ = 230 Å.     

The Wannier-Mott exciton’s dynamic properties without limitations on the magnitudes of the wave vectors of carriers

Abstract

Most often, the excitation of the Wannier-Mott excitons’ type is generated by electromagnetic action when the electron from the valence band passes into the conduction band. The influence of a magnitude wave vector for the exciton dynamic on the wave vector of the dynamics of the internal relative motion of the carriers is considered. It is shown that due to the effect of the field of intra-crystalline stresses on the exciton movement, a substantial displacement of the exciton spectrum may occur. Estimates of the possible magnitude of such a displacement were made.     

Excitation Photon Energy Dependent Relaxation Processes of the Photoexcited States in Wide Gap MX-Chain Complexes

Abstract

Photo-induced absorption (PA) spectra of [Pt (en)₂] [Pt (en)₂X₂] (SO₂)₂. 6H₂O (X=Cl and Br) have been measured. Large excitation photon energy dependent changes were found in the features of the PA spectra. The excitation photon energy dependences are considered to be due to the difference between the relaxation processes of the charge transfer excitons and those of the higher energy excited states. Several new PA bands have been found.     

Ultrafast Optical Response in Quasi-One Dimensional Halogen-Bridged Mixed-Valence Complexes [Pt(en)2][PtBr2(en)2](ClO4)4 and [Pd(en)2][PdCl2(en)2](ClO4)4

Abstract

Ultrafast optical response in two quasi-one-dimensional halogen-bridged mixed-valence complexs [Pt(en)₂][PtBr₂(en)₂](ClO₄)₄ (en=ethylenediamine) (abbreviated as PtBr) and [Pt(en)₂][PtBr₂(en)₂](ClO₄)₄ (as PdCl) has been investigated by femtosecond absorption spectroscopy at room temperature by pump-probe spectroscopy. The photo-induced absorption around 1.3 eV and the bleaching from 1.5 eV to 2.7 eV were observed in PtBr. Both consist of a fast-decay component due to STEs and a slow-decay component due probably to polaron pairs. The former decays exponentially with the time constant of 1.4 ± 0.2 ps. The latter decays as erf(t ^β) with β = -0.22 ± 0.02, indicating the geminate recombination of an electron polaron and a hole polaron after moving freely along the chain. The deviation of β from the ideal random-walk model (β = -0.5) is explained by introducing the effect of potential barrier between the polarons hindering the recombination. A pump-probe absorption spectrum of PdCl is obtained from the reflection spectrum by the Kramers-Kronig relations. The time dependence of the transient photoinduced absorption around 1.7 eV and the bleaching from 1.9 eV to 2.5 eV were calculated to be described with three components. They correspond to free excitons with lifetime of about 800 fs, self-trapped excitons with lifetime of about 3 ps, and polaron pairs which hardly relax within 100 ps.     

Spectral Characteristics of C60-Conducting Polymer Junctions: Various Molecular D-A Type Photocells

Abstract

We have found recently that thin film layered heterojunction between C₆₀ and conducting polymer, like poly(3-alkylthiophene) (PAT) shows a photovoltaic effect due to photoinduced charge transfer at the interface. Here we describe two other examples of such photocells in RO-PPV/C₆₀ and PPP/C₆₀ heterojunctions and study their spectral characteristics. Contrary to conventional inorganic semiconductor p-n junction photocells, the C₆₀-PAT junction can be rather viewed as molecular donor-acceptor (D-A) type photocell in which the processes of photogeneration and separation of charge carriers are quite distinct. In a p-n junction free electron-hole pairs are known to be primarily photogenerated at interband transition and then separated in the internal electric field of the barrier, while in D-A molecular photocell mainly neutral excitons are first created by light, with charges being primarily separated at the narrow interface region due to D-A type intermolecular charge transfer interactions. We analyze how this processes depend on the wavelength of the pumping light and on the polarity and magnitude of the applied voltage.     

Energy Band Structure of CuInS2 Crystals

In this work the reflectivity spectra and wave-length derivative reflectivity (WDR) spectra of CuInS₂ crystals have been investigated in the region E ≥ E_g. The n = 1, n = 2 and n = 3 excitonic states are determined and contours of exciton lines n = 1 are calculated. The parameters of excitons and bands have been determined for the region of band gap minimum. The main band gaps are determined for Γ-, N- and T-points of the Brillouin zone.     

EPR Studies of Electron Spin Polarization of Triplet Exciton and its Transfer into Triplet Traps of Naphthalene-TCNQ and Naphthalene-pChloranil Guest Complexes in the Naphthalene-TCNB Host

The EPR spectra of triplet excitons in single crystals of Naphthalene-TCNB (N-TCNB) charge-transfer (CT) complexes show optical electron spin polarization (OEP) which is accounted for by a spin selective populating process from the excited CT singlet into an excited triplet state localized on the acceptor. OEP is also observed for triplet traps in N-TCNB crystals doped with TCNQ and pChloranil. The analysis of the orientation dependence of spin polarization allows one to establish that also selective decay from the trap spin sublevels is important for the onset of the observed OEP features.     

Photoluminescence properties of erbium-doped structures of silicon nanocrystals in silicon dioxide matrix

We present a study of the photoluminescence (PL) of structures of Si nanocrystals (nc-Si) in erbium-doped amorphous silicon dioxide. It is shown that the energy of excitons confined in nc-Si of 2–5 nm sizes is efficiently transferred to Er³⁺ ions in surrounding SiO₂ and a strong PL line at 1.5 μm appears. At high excitation intensity the population inversion of Er³⁺ ion states is achieved. These results demonstrate good perspectives of Er-doped nc-Si/SiO₂ structures for possible applications in Si-based optical amplifiers and lasers.     

Photoluminescence studies of localized excitons in GaSe1−xSx solid solutions

We have measured the photoluminescence spectra of indirect excitons localized by alloy fluctuation in GaSe_1?xS_x solid solutions. The presence of the mobility edge for the localized exciton states is essential for explaining the observed luminescence spectra.     

Growth and Optical Characterization of CdI2 Single Crystals

Single crystals of CdI₂ were grown by employing the horizontal zone refining method. Emission spectra of these crystals were recorded at room temperature and liquid nitrogen temperature. Various emission parameters such as optical gain coefficient, lifetime, gain, stimulated emission cross section were evaluated for the various emission components. The results show high gain at low temperature whereas very low value at rmm temperature. This has been explained by invoking the existence of self trapped excitons in the crystal.     

Impurity-trapped excitons: Experimental evidence and theoretical concept

The paper summarizes experimental evidence of anomalous luminescence in RE-doped dielectric materials. Special attention is paid to Pr³⁺-doped LiNbO₃ and LiTaO₃, and Eu²⁺-doped XF_2, where X = Ba, Sr. Luminescence, luminescence excitation spectra and luminescence kinetics results are discussed obtained at ambient and high hydrostatic pressure at various temperatures. Hydrostatic pressure has been shown to cause a red shift of anomalous luminescence. The experimental data show the existence of temperature- and pressure-induced spectral transformation where anomalous luminescence is replaced with normal d–f emission in Eu²⁺ centers. The model predicts strong electron-lattice coupling of bound excitons and relaxation in the system’s excited states, as well as the pressure effect of spectral transformation from anomalous to normal emission.     

Electron-paramagnetic resonance and photoluminescence study of Si nanocrystals-photosensitizers of singlet oxygen molecules

Si nanocrystals formed by electrochemical porosifying of c-Si wafers are investigated by means of the electron-paramagnetic resonance (EPR) and photoluminescence (PL) techniques. The PL spectra and transients give evidence of the photosensitization of singlet oxygen molecules by the energy transfer from excitons confined in Si nanocrystals to oxygen molecules adsorbed on the nanocrystal surfaces. The EPR experiments show that the singlet oxygen generation is accompanied by a slowing down of the spin-spin relaxation time of Si dangling bonds on the nanocrystal surfaces. This effect is explained by the transition of a large part of the adsorbed O₂ molecules in their ground (triplet) states to the excited (singlet) ones. The EPR data allow us to estimate the concentration of the photosensitized singlet oxygen molecules to be on the order of 10¹⁸ cm⁻³.     

Time-resolved photoluminescence of implanted SiO2:Si+ films

In this paper we present results of a low-temperature time-resolved photoluminescence (PL) investigation of thin SiO₂ films implanted with silicon ions. In addition to the luminescence of well-known ODCs, some other bands are present in the low-energy region of PL spectra that are attributed to silicon nanoclusters (quantum dots – SiQDs), excitons and hydrogen-related species (HRS). Specific features of SiQD and HRS bands are the nanosecond kinetics and unusual “stepped” PL excitation spectrum in the 3.5–7.5 eV range. The possible origin of discovered phenomena is discussed. The obtained results are interpreted taking into account the interference of exciting radiation and dimensional quantization effects.     

Spectroscopy of Molecular Crystals A Bibliography for 1984

The 1984 Bibliography continues the series of annual issues of Bibliographies Spectroscopy of Molecular Crystals. The full list of these issues from 1957–1968 may be found in Molecular Crystals and Liquid Crystals, 6, 175(1969). The issues for 1969, 1970, 1971, 1972, 1973, 1974, 1975, 1976, 1977, 1978, 1979, 1980, 1981, 1982 and 1983 are published ibid. 14, 329(1971), 17,55(1972), 19,331(1973), 26,87(1974), 30,239(1975), 33,261(1976), 39,259(1977), 46,25(1978), 54, 60(1979), 61,251(1980), 73, 105(1981), 91, 197(1983), 104, 1(1984), and 135, 129(1986).     

Synthetically tuned structural variations in coordination polymers based on dicyanamide toward diverse physical properties

Three new two-dimensional coordination polymers (CPs), {Ni[N(CN)₂]₂(H₂O)₂·2DMAC}_n for 1, {Ni[N(CN)₂]₂(DMAC)₂}_n for 2, and {Co[N(CN)₂]₂(DMSO)₂}_n for 3, have been synthesized with different synthetic methods. The crystallographic analysis indicates that CPs 1 and 3 crystallize in the monoclinic system with space group of P2₁/c, while CP 2 crystallizes in the orthorhombic system with space group of Pnma. CPs 1 and 3 exhibit the antiferromagnetic properties, while CP 2 shows the weak ferromagnetic property. The solid-state luminescence properties of CPs 1–3 have also been investigated. Compared with the dicyanamide, 1 and 2 show the blue shift with emission maximum around 310?nm, while CP 3 shows the band at 335?nm.     

Inclusion Compounds of the Host 9-(4-Methylphenyl)-9<Emphasis Type="Italic">H</Emphasis>-xanthen-9-ol with a Series of Organic Guests

Abstract  The host 9-(4-methylphenyl)-9H-xanthen-9-ol (H) forms inclusion compounds with the guests cyclohexane (H · ?CHEX), benzene (H · ?BENZ), 1,4-dioxane (H · ?DIOX), cyclohexanone (H · CYONE), N,N-dimethylformamide (H · DMF) and N,N-dimethylacetamide (H · DMA). For those guest molecules possessing electron acceptor atoms/functionalities hydrogen bonding with the host is the prominent mode of interaction and presents itself in the form of (Host)–OH···O(Guest) linkages. The cyclohexane and benzene guests however, occupy voids created by neighbouring host molecules which form hydrogen bonded dimers of the form (Host)–OH···O(Host). The structures of the inclusion compounds were investigated and their thermal stabilities determined. Selectivity experiments were also conducted. Graphical Abstract  The host 9-(4-methylphenyl)-9H-xanthen-9-ol (H) forms inclusion compounds with the guests cyclohexane (H · ?CHEX), benzene (H · ?BENZ), 1,4-dioxane (H · ?DIOX), cyclohexanone (H · CYONE), N,N-dimethylformamide (H · DMF) and N,N-dimethylacetamide (H · DMA). Crystal structures and thermal stabilities were determined. The host selectivity was also investigated.