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...

We report the far-infrared through visible photoinduced absorption (PA) spectra of polyaniline in the emeraldine-base and leucoemeraldine-base forms. The direct-absorption spectrum of emeraldine base (EB) has a broad absorption centered at 2 eV (the "exciton" band) and an absorption band at-3.6 eV (the m.-~* band gap). The PA spectra of EB for pumping into the "exciton" band and across the band gap are nearly identical, indicating the same types of charged defect states are created upon photoexcitation. The direct absorption spectrum of leucoemeraldine base (LB) shows only the~-m* band-gap absorption at-3.6 eV. For pumping into this absorption band, the PA spectrum of LB is very similar to that of EB, although important differences result from the lack of quinoid structures in this material. Based on our results, we propose a model for the photocreation of defect states in leucoemeraldine base and emeraldine base. The central roles of phenyl-ring rotations and of massive polarons are discussed.

No abstract

We present results of absorption, near-steady-state and long-time photoinduced absorption, and lightinduced electron-spin resonance (LESR) studies for the pernigraniline-base polymer (PNB), which has a Peierls ground state with multiple order parameters. Two kinds of defects have been observed and difterentiated by photoinduced-absorption spectroscopies in dift'erent time domains, namely polarons and solitons. Direct absorption of PNB shows the Peierls energy gap at 2.3 eV and~-transition energies at 3.8 and 4.3 eV. Pumping at 2.41 eV (into the Peierls gap), the near-steady-state photoinduced absorption spectrum at room temperature shows three induced absorption peaks at 1.0 eV (LE), 1.5 eV (ME), and 3.0 eV (HE); at a temperature of 10 K, the LE peak becomes two peaks at 1.0 eV (LE1) and 1.3 eV (LE2). With pump photon energy of 3.8 eV (into the mm* gap) and at 10 K, features similar to those observed by pumping into the Peierls gap were found, except the intensity of each of the features increased by a factor of 4. The LE features are short lived with a lifetime on the millisecond time scale and have associated short-lived infrared vibrations. The ME peak is very long lived; in addition, there are very long-lived, relatively weak infrared vibrations. The lifetime of the ME peak is measured to be greater than 24 h for temperatures below 200 K. The defect masses for long-lived and short-lived defects are estimated to be-300m, and-(5-10)m"respectively, using the amplitude-mode formalism. Light-induced ESR studies show the long-lived 1.5 eV peak has spin while the short-lived defect has no spin. We suggest that the LE peaks originate from charged soliton-antisoliton pairs (dominated primarily by the bond-length order parameter u), while the ME peak originates from massive ring-torsional polarons (with dominant ring-torsion-angle order parameters 6 and 0 p).

No abstract

No abstract

No abstract

No abstract