No abstract

We have measured the temperature-dependent photoluminescence quantum yields (PLQYs) of poly(9, 9-dioctylfluorene) (PFO) films with four morphologies, namely as-spin-coated (SC) glass, quenched nematic glass, crystalline, and vapour-treated SC glass containing a fraction of 21 helix conformation (β-phase) chains. We find that the room temperature PLQYs of the as-SC, crystalline, and quenched films all increase as the temperature is reduced. However, the PLQY of the film containing β-phase chains decreases at temperatures below 150 K. Via temperature-dependent photoinduced absorption measurements, we show that the polaron population in films containing β-phase PFO chains grows as the temperature is reduced, and is significantly larger than in films with any of the other morphologies. Because of the smaller HOMO–LUMO (highest occupied molecular orbital–lowest unoccupied molecular orbital) energy gap of the β-phase chains compared to chains in the surrounding glassy PFO matrix, they act as recombination sites for excitons, and as traps for polarons. Hence at low temperatures, the polarons become strongly localized on these chains, where they quench the singlet excitons and reduce the PLQY.

No abstract

The origin of electrophosphorescence from a doped polymer light emitting diode ͑LED͒ has been investigated. A luminescent polymer host, poly͑9,9-dioctylfluorene͒ ͑PFO͒, was doped with a red phosphorescent dye, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum͑II͒ ͑PtOEP͒. The maximum external quantum efficiency of 3.5% was obtained at a concentration of 4% PtOEP by weight. Energy transfer mechanisms between PFO and PtOEP were studied by absorption, photoluminescence, and photoinduced absorption spectroscopy. Even though electroluminescence spectra were dominated by PtOEP at a concentration of only 0.2 wt % PtOEP, Förster transfer of singlet excitons was weak and there was no evidence for Dexter transfer of triplet excitons. We conclude that the dominant emission mechanism in doped LED's is charge trapping followed by recombination on PtOEP molecules.

We have used absorption and electroabsorption spectroscopy to investigate the electronic structure of poly͑para-phenylene vinylene͒ ͑PPV͒ and poly "2-methoxy, 5-(2Ј-͑ethyl͒hexyloxy͒-p-phenylene vinylene… ͑MEH-PPV͒. In particular we examine the often used assumption that the electronic structure of PPV and its dialkoxy substituted derivatives are essentially the same. The absorption spectrum of PPV consists of three peaks, while that of MEH-PPV has four peaks. We discuss the controversial origin of the extra peak as well as evidence for Davydov splitting effects in the absorption spectrum of PPV. The analysis of the nonlinear spectra shows further differences between the two materials. First, the binding energy of the 1B u exciton for PPV is some 0.1 eV higher than for MEH-PPV. Second, the peak value of Im͕ (3) (Ϫ ;0,0, )͖ for PPV is approximately 40 times higher than that of MEH-PPV. We also found that the sum-over-states modeling of the electroabsorption spectra indicates that the transition dipole moment between the mA g and nB u states is of opposite sign in the two polymers. ͓S0163-1829͑99͒02523-0͔

Two novel dioxolane-substituted pentacene derivatives, namely, 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (TP-5) and 2,2,10,10-tetraethyl-6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (EtTP-5), have been synthesized and spectroscopically characterized. Here, we examine the steady-state and time-resolved photoluminescence (PL) of solid-state composite films containing these pentacene derivatives dispersed in tris(quinolin-8-olato)aluminum(III) (Alq(3)). The films show narrow red emission and high absolute photoluminescence quantum yields (phi(PL) = 59% and 76% for films containing approximately 0.25 mol % TP-5 and EtTP-5, respectively). The Förster transfer radius for both guest-host systems is estimated to be approximately 33 A. The TP-5/Alq(3) thin films show a marked decrease in phi(PL) with increasing guest molecule concentrations, accompanied by dramatic changes in the PL spectra, suggesting that intermolecular interactions between pentacene molecules result in the formation of weakly radiative aggregates. In contrast, a lesser degree of fluorescence quenching is observed for EtTP-5/Alq(3) films. The measured fluorescence lifetimes of TP-5 and EtTP-5 are similar (approximately 18 ns) at low concentrations but deviate at higher concentrations as aggregation begins to play a role in the TP-5/Alq(3) films. The onset of aggregation in EtTP-5/Alq(3) films occurs at higher guest molecule concentrations (>1.00 mol %). The addition of ethyl groups on the terminal dioxolane rings leads to an increase in the intermolecular spacing in the solid, thereby reducing the tendency for pi-pi molecular stacking and aggregation.

We have applied a variety of ps transient and cw optical techniques to elucidate the dynamics, absorption, and emission properties of excitons in soluble derivatives of poly( p-phenylene vinylene) neat films and dilute solutions. We found that the photogenerated singlet excitons in both films and solutions are characterized by strong stimulated emission and two photoinduced absorption bands. We demonstrate that these bands can be used to form an ultrafast optical switch in the near IR spectral range with variable switching times. [S0031-9007(97)03244-4] The most striking optical property of many pconjugated conducting polymers is a bright photoluminescence (PL) band with high quantum efficiency, which can be chemically tuned to cover the complete visible spectral range [1]. Their application in optical emission devices such as light emitting diodes and, more recently, laser-active media has led to intensive investigations of conducting polymers, such as poly(p-phenylene-vinylene) (PPV) and its derivatives [2]. Attention has been focused on the properties of photogenerated excitons in neat films of these materials [3,4]. However, the exact spectral signatures of photoexcitations in PPV, i.e., singlet excitons, triplet excitons, and polaron pairs, have remained unclear so far because of their strong overlapping spectral features in the visible to near IR (NIR) spectral range [3].In this Letter we have elucidated the dynamics, absorption, and emission properties of excitons in neat films and dilute solutions of PPV derivatives, using transient and cw photomodulation (PM) and PL measurements, photoinduced absorption (PA) detected magnetic resonance (PADMR) and electroabsorption (EA) spectroscopies. We show that the photogenerated singlet excitons are the primary excitations in both neat films and solutions. By applying a novel transient experimental technique involving three ps pulsed beams, we prove that the singlet excitons are characterized by a stimulated emission (SE) band at 2.2 eV accompanied by phonon replicas, and two PA bands at 0.9 and 1.5 eV, respectively. Making use of the fact that both the SE and PA bands are related to the same excitons, we demonstrate an ultrafast optical excitonic switch in the NIR spectral range.Transient PM measurements were performed using the pump-and-probe correlation technique employing three lasers synchronously pumped by a mode-locked Nd:YAG laser: two dye lasers and a color center laser [5]. The pump photon energy was fixed at 2.2 eV, whereas the probe photon energy was varied between 0.76 and 0.86 eV, using the color center laser, and from 1.25 to 2.2 eV, using one of the dye lasers. We measured the changes DT in the probe transmission T using a fast acousto-optic modulation scheme, Si and Ge detectors, and a fast lock-in amplifier. The time resolution of the transient PM apparatus was 5-10 ps, as determined by a measurement of pump-probe cross correlation. The transient PL decays were measured with a streak camera having 10 ps time resolution in a synchroscan mode. The...

Organic light‐emitting diodes (OLEDs) containing red‐light‐emitting dioxolane‐substituted pentacene derivatives are fabricated and characterized. The OLEDs feature guest–host emitting layers consisting of either 6,14‐bis(triisopropylsilylethynyl)‐1,3,9,11‐tetraoxa‐dicyclopenta[b,m]pentacene (TP‐5) or 2,2,10,10‐tetraethyl‐6,14‐bis(triisopropylsilylethynyl)‐1,3,9,11‐tetraoxa‐dicyclopenta[b,m]pentacene (EtTP‐5) dispersed in tris(quinolin‐8‐olato) aluminum(III) (Alq3). High external electroluminescence (EL) quantum efficiency (ηEL = 3.3 %), not far from the theoretical limit, is observed for an OLED device based on a dilute EtTP‐5:Alq3 emitting layer (0.25 mol % EtTP‐5). The proposed EL mechanism is a combination of Förster energy transfer and direct electron–hole recombination on the guest pentacene molecules, as inferred by changes in the EL versus photoluminescence spectra and the positions of the highest occupied molecular orbital and lowest unoccupied molecular orbital gap of the guest within that of the host (estimated via cyclic voltammetry). Further evidence of charge trapping is provided by increased operational voltages at increased guest‐molecule concentration.