Optical probes of excited states in poly(p-phenylenevinylene)

Abstract

We have studied electronic excited states in films of poly(p-phenylenevinylene) using picosecond transient and cw photomodulation, photoluminescence, and their excitation spectra, as well as electroabsorption spectroscopy. %'e have determined all the important energy levels of singlet excitons with odd and even parity, the onset of the continuum band, the two-electron (biexciton) states, and the two relevant triplet states, and show that good agreement exists with models involving electron correlation. PACS numbers: 78.47.+p, 72.20.3v, 78.55.Kz, 78.66.gn The photophysics and resonant nonlinear optical properties of conducting polymers are dominated by the locations and natures of the excited-state energy levels.These excited states include singlet excitons with odd (8") and even (As) parity, the continuum band (CB), two-electron (biexciton) states, and the triplet manifold [1,2]. Recent theoretical advances in the area of subgap third-order optical nonlinearity [3,4] provide information about a subset of the excited states, which include the lowest B"exciton (18"),a dominant As exciton (hereafter the mAs), and the CB threshold. The relative locations of the 18"and the lowest As (2As) excitons are determined by a sensitive interplay between electronelectron interaction and alternation (b) in the tr electron transfer integral along the polymer chain [5]. For realistic Coulomb interaction and small 8 [5], the optical gap Eg to the 1 B"exciton is relatively small, the 2Ag lies below the 18",and, due to the dipole forbidden character of the lowest singlet, photoluminescence (PL) is weak. Large b results in larger Es, state ordering E(2As) )E(18"), and consequently high PL efficiency with promising applications in displays [such as light emitting diodes (LED) [6]]. The benzene ring in the backbone structure of poly(p-phenylenevinylene) (PPV) yields an effective 8 for the extended n states that is large [2], and therefore PPV belongs to the latter category. Nevertheless, Coulomb interaction among the tr electrons in PPV leads to behavior qualitatively different from the predictions of single-particle Hiickel or SSH models. Recent subpicosecond PL [7] and site-selection PL [8] have demonstrated that the primary excitation in PPV is to an exciton, and that the associated lattice relaxation energy is small. This already suggests a subsidiary role of the electron-phonon interaction. The location of the mug exciton has been determined by two-photon luminescence [9], whereas long-lived triplet excitons have been found in thin films [10,11] and LEDs [12). In the present work, we present a more complete picture of the various photoexcitations and excited states in PPV, based on a variety of optical probes including picosecond transient and cw photomodulation (PM) and PL and their excita-tion dependence, and the electroabsorption (EA) technique. We have tentatively mapped the most relevant singlet and triplet electronic manifolds, including the CB threshold and the lowest biexciton, that are not seen in direct optical absorpt...

Linear and nonlinear optical properties of the conjugated polymers PPV and MEH-PPV

“…It has previously been reported that the EA response of MEH-PPV in this energy region was not matched by any feature in the derivative spectra. 11, 21 Liess et al 11 reported that similar features are present in the EA spectra of several luminescent conjugated polymers ͓͑dioctyloxy͒-PPV ͑DOO-PPV͒ and poly͑alkyl thiophene͒ ͑PAT͔͒ but absent from that of nonluminescent polymers ͑PTV, PDA͒ and speculated that it is due to a high-energy A g state, and that its prescence is characteristic of luminescent polymers. We see no evidence for such a feature in the EA spectrum of PPV, which is lumines- cent.…”

“…Both agree well with previous reports. 1,11,[21][22][23] The PPV spectrum consists of two strong peaks, labeled I and III, at 2.84 and 6.15 eV, and a third, weaker, peak II, at 4.77 eV. At around 3.66 eV the slope of peak I changes, suggesting that there are at least two overlapping components contributing to peak I.…”

Ultrafast Dynamics of Polarons in Conductive Polyaniline: Comparison of Primary and Secondary Doped Forms

“…[10][11][12][13][14][15][16][17][18][19][20][21] For example, in polydiacetylene, the primary excitation is a neutral exciton with strong exciton binding energy. 10,11 In contrast, the primary excitation in poly(p-phenylenevinylene) (PPV) and its derivatives is debated to be a neutral exciton [12][13][14][15][16][17] or a charged polaron. [18][19][20][21] By contrast, chemically doped conjugated polymers, which show high electrical conductivity at room temperature, have clearly charged defect (polaron) states that are extrinsically introduced in the bandgap by chemical treatment.…”

Photophysics of a poly(phenylenevinylene) with alternatingmeta-phenylene andpara-phenylene rings

“…This results in bleaching of the allowed absorption and the appearance of induced absorption bands in the EA spectrum. [13][14][15][16][17][18][19] Samples were mounted in a cold-finger liquid nitrogen cryostat equipped with optical and electrical access. A sinusoidal electric field modulated at 5 kHz was applied to the sample using the reference output of a lock-in amplifier driving a high voltage amplifier.…”