Suppressing molecular motions for enhanced room-temperature phosphorescence of metal-free organic materials

Metal-free organic phosphorescent materials are attractive alternatives to the predominantly used organometallic phosphors but are generally dimmer and are relatively rare, as, without heavy-metal atoms, spin-orbit coupling is less efficient and phosphorescence usually cannot compete with radiationless relaxation processes. Here we present a general design rule and a method to effectively reduce radiationless transitions and hence greatly enhance phosphorescence efficiency of metal-free organic materials in a variety of amorphous polymer matrices, based on the restriction of molecular motions in the proximity of embedded phosphors. Covalent cross-linking between phosphors and polymer matrices via Diels-Alder click chemistry is devised as a method. A sharp increase in phosphorescence quantum efficiency is observed in a variety of polymer matrices with this method, which is ca. two to five times higher than that of phosphor-doped polymer systems having no such covalent linkage.ope

Exciton Diffusion in Polyfluorene Copolymer Thin Films: Kinetics, Energy Disorder and Thermally Assisted Hopping

A series of {(9,9-dioctylfluorene)(0.7-x)-(dibenzothiophene-S,S-dioxide)(0.3)-[4,7-b is(2-thienyl)-2,1,3-benzothiadiazole](x)} (PFS30-TBTx), where x represents the minor percentage of the red emitter 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT) randomly incorporated into the copolymer backbone, is investigated in order to follow the energy transfer from PFS30, to TBT moieties. The emission of the donor poly[(9,9-dioctylfluorene)(0.7)-(dibenzothiophene-S,S-dioxide)(0.3) identified by PFS30 and peaking at 450 nm, is clearly quenched by the presence of the red TBT chromophore emitting at 612 nm, with an isoemissive point observed when the spectra are collected as a function of temperature. A plot of the ratio between the TBT and PFS30, emissions as a function of the reciprocal of temperature gives a clear linear trend between 290 and 200 K, with an activation energy of 20 meV and showing a turn over to a non-activated regime below 200 K. Picosecond time-resolved fluorescence decays collected at the PFS30 and TBT emission wavelengths, show a decay of the PFS30, emission and a fast build-in, followed by a decay, of the TBT emission, confirming that the population of the TBT excited state occurs during the PFS30 lifetime(similar to 600 ps). The population of the TBT excited state occurs on a time regime around 150 ps at 290 K, showing an energy barrier of 20 meV that turns over to a non-activated regime below 200 K in clear agreement with the steady-state data. The origin of the activation barrier is attributed to the presence of physical and energetic disorder, affected by fast thermal fluctuations that dynamically change the energy landscape and control the exciton migration through the polymer density of states

Tuning the Intramolecular Charge Transfer Emission from Deep Blue to Green in Ambipolar Systems Based On Dibenzothiophene S S-Dioxide by Manipulation of Conjugation and Strength of the Electron Donor Units

The efficient synthesis and photophysical properties of a series of ambipolar donor−acceptor−donor systems is described where the acceptor is dibenzothiophene S,S-dioxide and the donor is fluorene, carbazole, or arylamine. The systems exhibit intramolecular charge transfer (ICT) states (of variable ICT character strengths) leading to fluorescence emission ranging from deep blue to green with moderate to high photoluminescence quantum yields. The emission properties can be effectively tuned by systematically changing the position of substitution on both donor and acceptor units (which affects the extent of conjugation) and the redox potentials of the donor units. The results are supported by cyclic voltammetric data and TD-DFT calculations

Electrical behavior of memory devices based on fluorene-containing organic thin films

We report on switching and negative differential resistance (NDR) behaviors of crossed bar electrode structures based on Al/organic layer/Al devices in which the organic layer was a spin-coated layer of 7-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}-9,9-dihexyl-N, N-diphenyl-fluoren-2-amine. The addition of gold nanoparticles (0.5 wt %) did not change the switching behavior of thicker film structures; however, devices incorporating the nanoparticles showed more reproducible characteristics. In most cases, a "forming" process, in which a large positive voltage was applied to the top Al electrode, was required before the NDR and conductivity switching were observed. Three different electrical conductivity mechanisms have been identified: Poole-Fretikel conductivity in unformed structures, linear current versus voltage characteristics for the ON state in formed devices, and superlinear current versus voltage behavior for the OFF state in formed devices. Models based on metallic filaments or on the injection and storage of charge do not explain all our experimental observations satisfactorily. Instead, an explanation based on the formation of nanocrystalline regions within the thin film is suggested. The devices can be used as two-terminal memory cells operating with unipolar voltage pulses. (C) 2008 American Institute of Physics