H. Wachtel

PYRENE AS EMITTING CHROMOPHORE IN ORGANIC-INORGANIC LEAD HALIDE-BASED LAYERED PEROVSKITES WITH DIFFERENT HALIDES

Abstract The absorption, emission and excitation spectra of layered perovskites consisting of inorganic layers of lead halide and organic layers with the chromophore pyrene are investigated. The emitting species is the organic chromophore. By variation of the halide composition in the inorganic layer, one can shift the energy of the inorganic excitonic band in these compounds. This also has an influence on the chromophore phosphorescence in the organic layer. Its triplet lifetime and its excimeric emission in the presence of the different halides is discussed in terms of the heavy-atom effect, the crystal structure and an efficient energy transfer between the organic and inorganic layers.

Nanostructured organic materials: copper- and silver-TCNQ radical salts

Abstract Using a solid state diffusion technique, conducting tetracyanoquinodimethane (TCNQ) radical ion salt films have been prepared with Cu and Ag counterions on glass substrates. Some features of organic conductors, e.g. transport properties of reduced dimensionality, motivate to preserve the anisotropy in films, which additionally can be structured in dimensions below some hundred nanometers. This requires films consisting of crystallites with an average size of less than 50 nm as realized in vacuum deposited Cu- and Ag-TCNQ films. The processing of striplines of CuTCNQ is reported.

Spin dynamics in oriented lithium phthalocyanine thin films investigated by pulsed electron spin resonance

Thin films of monolithium phthalocyanine (PcLi) were prepared by vacuum deposition on cleaved mica. The highly anisotropic spin diffusion in these films was studied by electron spin resonance (ESR). Using averaging techniques, we succeeded in the detection of films as thin as 100 nm using pulsed ESR and 2 nm with cw ESR. The electronic relaxation is shown to depend on the substrate temperatures Ts during film deposition which determine the morphology of the films. We find relaxation rates of films which are faster by a factor of 20 (T1e−1) to 30 (T2e−1) than those of single crystals. At ambient temperature, a low‐dimensional spin exchange mechanism is proposed for the single crystal. At low temperatures, in single crystals the electronic relaxation rates T1e−1 and T2e−1 hint at a reduced exchange with increased dimensionality d≳1. This holds for the films at all temperatures and is explained by different crystal structures of single crystals and films. A generally applicable relaxation model is developed ...

Triplet optical line profiles of orientationally disordered single crystals: 2,3-dimethylnaphthalene and 2,3-dimethylanthracene

Abstract The excitonic T 1 triplet states in single crystals of 2,3-dimethylnaphthalene (DMN) and 2,3-dimethylanthracene (DMA) exhibit broadened (120 and 110 cm −1 , respectively) asymmetric absorption lines at helium temperature. The corresponding phosphorescence lines are narrower (40 and 50 cm −1 ), red-shifted and show an inverse asymmetry. The line broadening is attributed to the orientational disorder in DMN and DMA crystals. Based on the concept of dispersive energy transport, a statistical model is used to describe the emission line shift and line shape.

Transient/persistent switching of Cu-DCNQI by light and pressure

Abstract In Cu(2,5-DMe-DCNQI) 2 (DCNQI: N,N′-Dicyanoquinonediimine) crystals with metal-like conductivity, phase transitions to an insulating state can be induced by deuteration (internal pressure) and external pressure. A reentry to the metallic state at lower temperature is observed in alloyed Cu-DCNQI systems. The ESR-silence during metallic conductivity is lifted in the insulating state. The correlation of ESR and conductivity allows a quantitative detection of the gradual onset of the phase transition. Because of large hysteresis effects of σ- and ESR-measurements, switching by light and pressure is possible. These features point to prospective application for molecular electronic devices as cystalline (2,5-DMe-DCNQI) 2 Cu films represent, due to the crystal structure, conducting networks in molecular dimensions switchable from a 3-d to an 1-d conductivity behaviour. This might be realized spatially resolved in nanostructures if electron beam lithography is applied.

Triplet excitons in diphenylbutadiene and diphenylhexatriene single crystals by zero-field delayed fluorescence ODMR

Abstract The triplet T 1 states in diphenylbutadiene and diphenylhexatriene single crystals have been investigated by delayed fluorescence detected magnetic resonance at zero-field between 1.2 and 2.1 K and T > 30 K. The signals are identified as being due to metastable triplet excitons and to X-traps. Their | D| and | E | values are discussed with respect to delocalization of the triplet spins.

Conductivity without spin-signal, spin signal without conductivity-switchable radical ion salts of deuterated Cu(DCNQI)2

Abstract Cu(DCNQI) 2 can be switched from a metallic state to an insulating one by external pressure or internal (chemical) pressure in a very abrupt manner. We have used the pronounced anticoincidence of the ESR-intensity and the conductivity to study the processes at the phase transitions in detail by measuring simultaneously the ESR and conductivity as a function of T and under pressure. We have a break down of the conductivity if 20 % of the total volume is switched into ESR-active areas. This is interpreted by a percolation limit between the still conducting (ESR-silent) domains. Including individual internal pressure values for differently deuterated Cu(DCNQI) 2 salts and alloys between deuterated and undeuterated DCNQl, a general phase diagram can be given.

Pressure and light effects on the phase transition of deuterated Cu(2,5‐dimethyl‐dicyanoquinonediimine)2

The deuterated dicyanoquinonediimine salt Cu(2‐CD3,5‐CD3‐DCNQI)2, (d6), a highly conductive organic metal, exhibits a phase transition from the metallic to the insulating state at Tc≊60 K. In a wide temperature range we observe an anticoincidence of high conductivity and electron spin resonance (ESR). These experiments were performed simultaneously in our ESR apparatus equipped for application of He pressure up to 200 bar. Near Tc there is a coexistence of conducting (ESR‐silent) and insulating (ESR‐detectable) domains. This correlation of ESR and conductivity σ points to a percolation limited conductivity. Upper and lower limits of the size of the domains are given. Light shifts Tc to lower temperatures, pressure to higher ones. This could be due to shrinking of the lattice which induces the phase transition when critical structural parameters are approached. This is most probably explained by a simultaneous development of a charge density wave (CDW) and a spin pairing within the DCNQI stack together wit...

Light, pressure, and stress induced phase transitions of deuterated Cu(DCNQI)2 radical ion salts

Abstract The extremely sharp metal-insulator phase transition at Tc = 60 K of 6-fold deuterated Cu(DCNQI)2 could be shifted to lower temperature T by illumination with light and by mechanical tension, both acting contrarily to the application of pressure. By proper selection of T, using pressure cycles, the samples could be switched to a persistent insulating state. Applying light (of different wavelengths) we could reestablish the conductivity. Correlating the temperature dependence of the crystal; length (interferometric, resolution Δ1/1

Conductive Cu(2,5‐dimethyl‐dicyanoquinonediimine)2 radical ion salts: Systems with none, one, or two phase transitions

Phase transitions in the radical ion salts of Cu(2,5‐dimethyl‐dicyanoquinonediimine)2 (Cu(DCNQI)2) can be achieved either by ‘‘external’’ pressure or by ‘‘internal’’ pressure. The latter is introduced by chemical modifications at the DCNQI‐molecules (e.g., deuteration) or by alloying deuterated Cu(2,5‐(CD3)2‐DCNQI)2 (d6) with undeuterated Cu(2,5‐(CH3)2‐DCNQI)2 (h8), giving the mixture (h8/d6) in different ratios. In this work we present simultaneous conductivity (σ) and electron spin resonance (ESR) experiments on differently deuterated Cu(DCNQI)2‐systems and on alloys (h8/d6) under external pressure. The anticoincidence of σ and ESR allows the determination of phase transition temperatures even in the absence of electrical contacts. For each system an individual phase diagram is established. Introducing an effective pressure peff=p0+p with p0 being the ‘‘internal’’ (chemical) pressure, a general phase diagram could be constructed by determining the individual p0 value for all systems. For the alloys (h8/...