Pressure dependence of singlet and triplet excitons in amorphous polymer semiconductors

We present optical studies of both singlet and triplet states of a ladder-type conjugated polymer as a function of hydrostatic pressure. The pressure coefficient of the triplet-triplet absorption is smaller compared to the pressure coefficient of the singlet excitation, highlighting the more localized nature of triplet excitons. The photoluminescence and phosphorescence energies red-shift at similar rates with increasing pressure, thus giving experimental evidence for the first time that the singlet-triplet splitting remains almost a constant under high pressure until 4GPa. The diffusion length of the triplet excitons decreases to a few hundred nm at high pressures, as compared with a few micrometers at atmospheric pressure. Copyright (C) EPLA, 2013

I. A study of the dynamics of triplet excitons in molecular crystals. II. An investigation of delayed light emission from Chlorella pyrenoidosa

I. PREAMBLE AND SCOPE

Brief introductory remarks, together with a definition of the scope of the material discussed in the thesis, are given.

II. A STUDY OF THE DYNAMICS OF TRIPLET EXCITONS IN MOLECULAR CRYSTALS

Phosphorescence spectra of pure crystalline naphthalene at room temperature and at 77˚ K are presented. The lifetime of the lowest triplet ³B_1u state of the crystal is determined from measurements of the time-dependence of the phosphorescence decay after termination of the excitation light. The fact that this lifetime is considerably shorter in the pure crystal at room temperature than in isotopic mixed crystals at 4.2˚ K is discussed, with special importance being attached to the mobility of triplet excitons in the pure crystal.

Excitation spectra of the delayed fluorescence and phosphorescence from crystalline naphthalene and anthracene are also presented. The equation governing the time- and spatial-dependence of the triplet exciton concentration in the crystal is discussed, along with several approximate equations obtained from the general equation under certain simplifying assumptions. The influence of triplet exciton diffusion on the observed excitation spectra and the possibility of using the latter to investigate the former is also considered. Calculations of the delayed fluorescence and phosphorescence excitation spectra of crystalline naphthalene are described.

A search for absorption of additional light quanta by triplet excitons in naphthalene and anthracene crystals failed to produce any evidence for the phenomenon. This apparent absence of triplet-triplet absorption in pure crystals is attributed to a low steady-state triplet concentration, due to processes like triplet-triplet annihilation, resulting in an absorption too weak to be detected with the apparatus used in the experiments. A comparison of triplet-triplet absorption by naphthalene in a glass at 77˚ K with that by naphthalene-h₈ in naphthalene-d₈ at 4.2˚ K is given. A broad absorption in the isotopic mixed crystal triplet-triplet spectrum has been tentatively interpreted in terms of coupling between the guest ³B_1u state and the conduction band and charge-transfer states of the host crystal.

III. AN INVESTIGATION OF DELAYED LIGHT EMISSION FROM Chlorella Pyrenoidosa

An apparatus capable of measuring emission lifetimes in the range 5 X 10^-9 sec to 6 X 10^-3 sec is described in detail. A cw argon ion laser beam, interrupted periodically by means of an electro-optic shutter, serves as the excitation source. Rapid sampling techniques coupled with signal averaging and digital data acquisition comprise the sensitive detection and readout portion of the apparatus. The capabilities of the equipment are adequately demonstrated by the results of a determination of the fluorescence lifetime of 5, 6, 11, 12-tetraphenyl-naphthacene in benzene solution at room temperature. Details of numerical methods used in the final data reduction are also described.

The results of preliminary measurements of delayed light emission from Chlorella Pyrenoidosa in the range 10^-3 sec to 1 sec are presented. Effects on the emission of an inhibitor and of variations in the excitation light intensity have been investigated. Kinetic analysis of the emission decay curves obtained under these various experimental conditions indicate that in the millisecond-to-second time interval the decay is adequately described by the sum of two first-order decay processes. The values of the time constants of these processes appear to be sensitive both to added inhibitor and to excitation light intensity.

Harvesting Excitons Via Two Parallel Channels for Efficient White Organic LEDs with Nearly 100% Internal Quantum Efficiency: Fabrication and Emission-Mechanism Analysis

By incorporating two phosphorescent dyes, namely, iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N,C-2']picolinate (Flrpic) for blue emission and bis(2-(9,9-diethyl-9H-fluoren-2-yl)-1-phenyl-1 H-benzoimidazol-N,C-3) iridium(acetylacetonate) ((fbi)(2)Ir(acac)) for orange emission, into a single-energy well-like emissive layer, an extremely high-efficiency white organic light-emitting diode (WOLED) with excellent color stability is demonstrated. This device can achieve a peak forward-viewing power efficiency of 42.5 lm W-1, corresponding to an external quantum efficiency (EQE) of 19.3% and a current efficiency of 52.8 cd A(-1). Systematic studies of the dopants, host and dopant-doped host films in terms of photophysical properties (including absorption, photoluminescence, and excitation spectra), transient photoluminescence, current density-voltage characteristics, and temperature-dependent electroluminescence spectra are subsequently performed, from which it is concluded that the emission natures of Flrpic and (fbi)(2)Ir(acac) are, respectively, host-guest energy transfer and a direct exciton formation process. These two parallel pathways serve to channel the overall excitons to both dopants, greatly reducing unfavorable energy losses.

Excited-state dynamics in diketopyrrolopyrrole-based copolymer for organic photovoltaics investigated by transient optical spectroscopy

We investigate the photoexcited state dynamics in a donor-acceptor copolymer, poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]- pyrrole-1,4-dione-alt-naphthalene} (pDPP-TNT), by picosecond fluorescence and femtosecond transient absorption spectroscopies. Timeresolved fluorescence lifetime measurements of pDPP-TNT thin films reveal that the lifetime of the singlet excited state is 185 ± 5 ps and that singlet-singlet annihilation occurs at excitation photon densities above 6 × 1017 photons/cm3. From the results of singlet-singlet annihilation analysis, we estimate that the single-singlet annihilation rate constant is (6.0 ± 0.2) × 109cm3 s-1 and the singlet diffusion length is -7 nm. From the comparison of femtosecond transient absorption measurements and picosecond fluorescence measurements, it is found that the time profile of the photobleaching signal in the charge-transfer (CT) absorption band coincides with that of the fluorescence intensity and there is no indication of long-lived species, which clearly suggests that charged species, such as polaron pairs and triplet excitons, are not effectively photogenerated in the neat pDPP-TNT polymer.

Low-lying states of transverse substituted trans-polyacetylene and trans-polyacetylene: A comparative DMRG study

The density-matrix renormalization group (DMRG) method is used for a comparative study of low-lying excitations in trans-polyacetylene (t-PA) and transversely substituted t-PA (TS-t-PA). We have employed the Pariser-Parr-Pople model Hamiltonian which incorporates long-range electronic correlations to model these systems. We find some fundamental differences in the excited states of the t-PA and TS-t-PA. We find that the lowest two-photon allowed excited state in TS-t-PA is not made up of two triplet excitons and the gap to this state is nonzero even for undimerized chains in the thermodynamic limit. Contrary to earlier results for the Hubbard model, we find that the lowest two-photon state is always below the first optically allowed state in all the systems studied here making TS-t-PA systems only weakly fluorescent materials. Nonresonant tumbling averaged linear and third harmonic generation optic coefficients of TS-t-PA systems are also much smaller than that of t-PA.

High efficiency fluorescent white organic light-emitting diodes with red, green and blue separately monochromatic emission layers

Highly efficient fluorescent white organic light-emitting diodes (WOLEDs) have been fabricated by using three red, green and blue, separately monochromatic emission layers. The red and blue emissive layers are based on 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) doped N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) and p-bis(p-N,N-diphenyl-amino-styryl) benzene (DSA-ph) doped 2-methyl-9,10-di(2-naphthyl) anthracene (MADN), respectively; and the green emissive layer is based on tris(8-hydroxyquionline)aluminum(Alq(3)) doped with 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl- 1H,5H,1[H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-1]-one (C545T), which is sandwiched between the red and the blue emissive layers. It can be seen that the devices show stable white emission with Commission International de L'Eclairage coordinates of (0.41, 0.41) and color rendering index (CRI) of 84 in a wide range of bias voltages.

Achieving highly efficient white organic light-emitting diodes with reduced efficiency roll-off

A highly efficient and colour-stable three-wavelength white organic light-emitting diode with the structure of indium tin oxide (ITO)/MoO3/N,N'-diphenyl-N,N'-bis (1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB)/4,4'-N,N'-dicarbazole-biphenyl (CBP): bis(2,4-diphenylquinolyl-N,C-2') iridium( acetylacetonate) (PPQ)(2)Ir(acac)/NPB/p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph):2-methyl-9,10-di(2-naphthyl) anthracene (MADN)/tris (8-hydroxyquinoline) aluminum (AlQ): 10-(2-Benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-11-one (C545T)/AlQ/LiF/Al is fabricated and characterized. A current efficiency of 12.3 cdA(-1) at an illumination-relevant brightness of 1000 cd m(-2) is obtained, which rolls off slightly to 10.3 cdA(-1) at a rather high brightness of 10 000 cd m(-2). We attribute this great reduction in the efficiency roll-off to the wise management of singlet and triplet excitons between emissive layers as well as the superior charge injection and diffusion balance in the device.

Luminous efficiency enhancement of PVK based OLEDs with fac-[ClRe(CO)(3)(bpy)]

The luminous efficiency of organic light-emitting diodes based on poly(N-vinylcarbazole), PVK, was improved by adding fac-[ClRe(CO)(3)(bpy)], bpy = 2,2`-bipyridine, to PVK host. Emissive layers with various Re(I) complex/host ratio were employed and optoelectronic properties were compared with the single PVK device. The single PVK device exhibits a characteristic electroluminescence with blue emission, lambda(max) 420 nm, assigned to the PVK excimer. On the other hand, the intense and broad band at lambda(max) 580 nm of the Re(I) complex/PVK OLEDs is ascribed to the metal-to-ligand charge transfer excited state emission of fac-[ClRe(CO)(3)(bpy)]. At 30 V, the device luminous efficiency increased from 16 mcd/A for the single PVK device to 211 mcd/A for the 11% (w/w) Re(I) complex/PVK OLED, in which fac-[ClRe(CO)(3)(bpy)] acts as an electron-trap in PVK films. The device current is space-charge limited and exhibits typical emissive layer thickness dependence. (C) 2011 Elsevier B.V. All rights reserved.

Simulationen zur Exzitonendiffusion in organischen Halbleitern

Die vorliegende Dissertation dient dazu, das Verständnis des Exzitonentransports in organischen Halbleitern, wie sie in Leuchtdioden oder Solarzellen eingesetzt werden, zu vertiefen. Mithilfe von Computersimulationen wurde der Transport von Exzitonen in amorphen und kristallinen organischen Materialien beschrieben, angefangen auf mikroskopischer Ebene, auf der quantenmechanische Prozesse ablaufen, bis hin zur makroskopischen Ebene, auf welcher physikalisch bestimmbare Größen wie der Diffusionskoeffizient extrahierbar werden. Die Modellbildung basiert auf dem inkohärenten elektronischen Energietransfer. In diesem Rahmen wird der Transport des Exzitons als Hüpfprozess aufgefasst, welcher mit kinetischen Monte-Carlo Methoden simuliert wurde. Die notwendigen quantenmechanischen Übergangsraten zwischen den Molekülen wurden anhand der molekularen Struktur fester Phasen berechnet. Die Übergangsraten lassen sich in ein elektronisches Kopplungselement und die Franck-Condon-gewichtete Zustandsdichte aufteilen. Der Fokus dieser Arbeit lag einerseits darauf die Methoden zu evaluieren, die zur Berechnung der Übergangsraten in Frage kommen und andererseits den Hüpftransport zu simulieren und eine atomistische Interpretation der makroskopischen Transporteigenschaften der Exzitonen zu liefern. rnrnVon den drei untersuchten organischen Systemen, diente Aluminium-tris-(8-hydroxychinolin) der umfassenden Prüfung des Verfahrens. Es wurde gezeigt, dass stark vereinfachte Modelle wie die Marcus-Theorie die Übergangsraten und damit das Transportverhalten der Exzitonen oftmals qualitativ korrekt wiedergeben. Die meist deutlich größeren Diffusionskonstanten von Singulett- im Vergleich zu Triplett-Exzitonen haben ihren Ursprung in der längeren Reichweite der Kopplungselemente der Singulett-Exzitonen, wodurch ein stärker verzweigtes Netzwerk gebildet wird. Der Verlauf des zeitabhängigen Diffusionskoeffizienten zeigt subdiffusives Verhalten für kurze Beobachtungszeiten. Für Singulett-Exzitonen wechselt dieses Verhalten meist innerhalb der Lebensdauer des Exzitons in ein normales Diffusionsregime, während Triplett-Exzitonen das normale Regime deutlich langsamer erreichen. Das stärker anomale Verhalten der Triplett-Exzitonen wird auf eine ungleichmäßige Verteilung der Übergangsraten zurückgeführt. Beim Vergleich mit experimentell bestimmten Diffusionskonstanten muss das anomale Verhalten der Exzitonen berücksichtigt werden. Insgesamt stimmten simulierte und experimentelle Diffusionskonstanten für das Testsystem gut überein. Das Modellierungsverfahren sollte sich somit zur Charakterisierung des Exzitonentransports in neuen organischen Halbleitermaterialien eignen.

The lowest singlet and triplet states of the oxyallyl diradical

Small S-T splitting : The photoelectron spectrum of the oxyallyl radical anion (see picture) reveals that the electronic ground state of oxyallyl is singlet, and the lowest triplet state is separated from the singlet state by only (55 ± 2) meV in adiabatic energy.

Photoelectron spectroscopy of HCCN- and HCNC- reveals the quasilinear triplet carbenes, HCCN and HCNC

Negative ion photoelectron spectroscopy has been used to study the HCCN- and HCNC- ions. The electron affinities (EA) of cyanocarbene have been measured to be EA(HCCN (X) over tilde (3)Sigma(-)=2.003+/-0.014 eV and EA(DCCN (X) over tilde (3)Sigma(-))=2.009+/-0.020 eV. Photodetachment of HCCN- to HCCN (X) over tilde (3)Sigma(-) shows a 0.4 eV long vibrational progression in nu(5), the H-CCN bending mode; the HCCN- photoelectron spectra reveal excitations up to 10 quanta in nu(5). The term energies for the excited singlet state are found to be T-0(HCCN (a) over tilde (1)A('))=0.515+/-0.016 eV and T-0(DCCN (a) over tilde (1)A('))=0.518+/-0.027 eV. For the isocyanocarbene, the two lowest states switch and HCNC has a singlet ground state and an excited triplet state. The electron affinities are EA(HCNC (X) over tilde (1)A('))=1.883+/-0.013 eV and EA((X) over tilde (1)A(') DCNC)=1.877+/-0.010 eV. The term energy for the excited triplet state is T-0(HCNC (a) over tilde (3)A("))=0.050+/-0.028 eV and T-0(DCNC (a) over tilde (3)A("))=0.063+/-0.030 eV. Proton transfer kinetics in a flowing afterglow apparatus were used to re-measure the enthalpy of deprotonation of CH3NC to be Delta(acid)H(298)(CH3NC)=383.6+/-0.6 kcal mol(-1). The acidity/EA thermodynamic cycle was used to deduce D-0(H-CHCN)=104+/-2 kcal mol(-1) [Delta(f)H(0)(HCCN)=110+/-4 kcal mol(-1)] and D-0(H-CHNC)=106+/-4 kcal mol(-1) [Delta(f)H(0)(HCNC)=133+/-5 kcal mol(-1)]. (C) 2002 American Institute of Physics.

Singlet and triplet molecular interaction in excimers

A perturbative approach is developed to determine, term by term, the contributions of the various forces to the excimer potentials of the singlet and triplet excimers. The results show that the singlet excimer of naphthalene is more stable than the corresponding triplet excimer primarily due to large contributions of the exciton-resonance and the dispersion energy terms. The variation of the various terms with the conformations of the excimers suggests that the singlet and triplet excimers of naphthalene cannot have identical structure.

Triplet-state Photophysics and Transient Photochemistry of Cyclic Enethiones A Laser Flash Photolysis Study

The triplets of four cyclic enethiones, including thiocoumarin, have been investigated by nanosecond laser flash photolysis. Data are presented for transient spectra and kinetics associated with triplets, quantum yields of intersystem crossing and singlet oxygen photosensitization. The quenching of the thiocoumarin triplet (A:, = 485 nm, E:,, = 8.8 x lo3 dm3 mol-' cm-'in benzene) by several olefins, amines and hydrogen donors occurs with rate constants of 107-5 x lo9 dm3 mol-' s-'; the lower limits of quantum yields ( c#+~) for the related photoreactions, estimated from ground-state depletion, are generally small (0.0-0.1 1 in benzene, except for good hydrogen donors, namely, p-methoxythiophenol and tri-n-butylstannane) . The radical anion of thiocoumarin (A,,, = 405-435 nm) is formed in two stages upon triplet quenching by triethylamine in acetonitrile; the fast component is the result of direct electron transfer to the triplet and the slower component is assigned to secondary photoreduction of the thione ground state by the a-aminoalkyl radical derived from the triethylamine radical-cation.

Oxidations of thio ketones by singlet and triplet oxygen

Oxidation of di-tert-butyl thioketone (1) and 2,2,4,4-tetramethylcyclobutylth ioketone (2) by singlet oxygen yields the corresponding sulfine and ketone; in the case of 1 the sulfine is the major product, whereas in 2 it is the ketone. 1,2,3-Dioxathietane has been suggested as the precursor for the ketones, and the zwitterionic/diradid peroxide is believed to be a common primary intermediate for both sulfine and ketone. Steric influence is felt both during primary interaction between singlet oxygen and thioketone and during the partitioning of the peroxide intermediate. Steric interaction is suggested as the reason for variations in the product distribution between 1 and 2. Singlet oxygen is also generated through energy transfer from the triplet state of thioketones. These excited states also directly react with oxygen to yield ketone.

Self-Induced Transparency in Excitons

The interaction of intense coherent light with Frenkel excitons has been studied for investigating the self-induced transparency. Some nonlinear effects neglected before have been included. It is found that the frequency spectrum consistent with the pulse propagation is wider by two orders of magnitude compared with the previous result.