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Summary form only given. The X-band ESR, LESR, and ODMR of C/sub 70/ molecules isolated in a toluene/polystyrene (C/sub 70/:T/PS) glassy matrix and C/sub 70/ films is compared to the ODMR of C/sub 60/ glasses and films. As in C/sub 60/:T/PS, the ODMR of the C/sub 70/ glasses consists only of a broad triplet exciton powder pattern at g /spl tilde/ 2; the half field resonance due to the /spl Delta/ = 2 transitions of this triplet is unobservable. In contrast to C/sub 60/:T/PS, the triplet powder pattern of C/sub 70/:T/PS is strikingly asymmetric, apparently due to the lower symmetry of C/sub 70/. The ODMR of C/sub 70/ films is also similar to that of C/sub 60/ films, and includes a narrow PL-enhancing polaron resonance (/spl Delta/H/sub 1/2/ /spl tilde/ 6 G) at g /spl tilde/ 2.0025 and narrow and broad triplet exciton powder patterns at full- and half-fields. The narrow triplet pattern, attributed to a triplet exciton delocalized over the entire molecule, is slightly narrrower than in C/sub 60/, consistent with a slightly larger exciton. The broad pattern observed in films but not in the glasses is similar to that observed in /spl pi/-conjugated polymers and is suspected to result from a triplet localized on a pentagon or hexagon adjacent to a neighboring molecule. While the half-field resonances are comparably intense in C/sub 60/, in C/sub 70/ the broad pattern is much weaker than the narrow (delocalized) exciton pattern. The effects of UV excitation, as well as UV-induced photopolymerization, will also be discussed.

Summary form only given. We describe the formation and deactivation of excited singlet states in vapor deposited films of oligothienylenes, oligophenylenevinylenes and oligophenylenes as a function of the molecular chain-length, film thickness and molecular orientation in the film. The molecular order within the films is determined by angular resolved polarized UV/VIS absorption and fluorescence spectroscopy. The deactivation of excited states is investigated by means of photo- and electroluminescence, photoconductivity and singlet oxygen production. D.C. and A.C. impedance methods in the dark and under illumination are applied to the characterization of the interface between organic film and contacting metal. Vapor deposited oligothienylene films on polar substrates possess a high degree of molecular order which decreases with growing layer thickness and also upon introduction of bulky alkyl substituents. Concomitantly the fluorescence quantum yields increase from 10-5 to 10-3 while the photoconductivity decreases. No singlet oxygen production is observed upon irradiation, indicating that excited singlet states decay mainly by radiationless recombination via the singlet manifold or by charge carrier separation. Sexithiophene-aluminum junctions exhibit a diode-like behavior in the dark and a photovoltaic effect under illumination. Under forward bias, electroluminescence is observed with an onset voltage of 3V for unsubstituted and 10 V for alkylated sexithiophene.

Summary form only given. Steady-state photoluminescence (PL) and time-resolved PL of fullerene C/sub 60/ polycrystal and benzene solution are investigated. The PL of solid C/sub 60/ shoves a relatively strong peak at 1.71 eV and a weak broad band centers at 1.5 eV, that of C/sub 60/ in benzene showes two peakd at 1.82 eV and 1.73 eV excited under argon laser lines. Solid C/sub 60/ shows a dominatantly fast decay process of 140 picosecond and a slow process in I nanosecond regime, C/sub 60/ benzene solution shows a decay process of 1.4 nanosecond measured by a streak camera. The origins of PL are discussed based on these experiment results.

Summary form only given. Poly(p-phenylenevinylene) (C/sub 6/H/sub 4/CH=CH)/sub n/ (abbreviated as PPV) is a polymer with a prototypical nondegenerate ground-state, which shows high electrical conductivities upon doping. The mechanism of electrical conduction in conducting polymers has been discussed in terms of the self localized excitations (polarons and bipolarons in the case of PPV) created by doping. Recently, we have demonstrated that resonance Raman spectroscopy with visible and near-infrared excitations is useful for characterizing the self-localized excitations in doped conducting polymers. In the present study, we report the resonance Raman spectra of S0sub 3/-doped PPV (electrical conductivity, 10 S cm/sup -1/) excited with visible and near-infrared laser lines. The observed Raman spectra are analyzed by referring to the data of the radical cation and dication species of the model compounds CH/sub 3/(C/sub 6/H/sub 4/CH=CH)nC61-14CH3 (PVn, n=1-3), which well represent the spectroscopic properties of PPV. The radical cations and dications of the model compounds correspond, respectively, to positive polarons and positive bipolarons in the doped polymer. In the 632.8-nm excited resonance Raman spectrum of S0/sub 3/-doped PPV, positive polarons are observed. However, positive bipolarons and neutral parts are observed in the 1064-nm excited resonance Raman spectrum. These results are simillar to those of H/sub 2/S0/sub4/-treated PPV, suggesting that a polaron lattice (a regular array of polarons) may be formed in S0/sub 3/-doped PPV. The effect of doping level on the formation of self-localized excitations in S0/sub 3/-doped PPV will also be discussed.

Summary form only given. Important information pertaining to electronic excited states and their relax ation processes in solid C60 can be obtained by the measurement of luminescence and photoconductivity. We have recently succeeded in obtaining a hither to unknown luminescence excitation spectrum of C60 thin film exhibiting well resolved vibronic structures In the optical edge region. These structures can be assigned and compared with a corresponding absorption spectrum showing less resolution. A mirror-image like relationship was established between peaks in the emission and excitation spectrum, indicating a 0-0 transition around 670nm. We also found that the excitation spectrum showed strong peaks at 440-500nm, well corresponding to the solid-state specific absorption structures in the same wavelength region. These peaks exceeded that arising from the allowed transition at 350nm. Another newly found solid-state effect was that an emission intensity vs. temperature curve In 77-450K showed a change of slope at the phase transition point (250-260K), below which luminescence increased more steeply, the first example of an optical property of solid C60 perturbed by the phase transition. Relationship between photoconductivity and luminescence in C60 thin film will also be discussed.

Summary form only given. By arc vaporization of graphite in the presence of such sources of nitrogen or sulfur as pyrrole or thiophene, fractions can be isolated which have heteroatoms substituted in the cage. As of this writing, no single pure compound has been isolated, but the fluorescence emission spectra of the fractions give strong evidence for the existence and interesting properties of cage doped fullerenes. Whereas C/sub 60/ and C/sub 70/ show very weak fluorescence, heteroatom substitution distorts the symmetry of the molecule and gives strong fluorescence. In the case of N substitution, the fluorescence from the excited states shows a broad band containing vibrational fine structures that are attributed to photo-separated charges in fragmented fullerenes and a shoulder on the low energy side that is related to intrinsic excitation in nitrogen doped species. Fluorescence results also suggest a band gap of 2.36 eV for the N doped fullerenes and the existence of intermediate excitonic transitions localized below the optical band gap. In the case of S substitution, an optical band gap of 2.5 eV results from the splitting of the hu degenerate state and a fluorescence lifetime of 2-7 ns is obtained.

Summary form only given. Applications of FTIR and Nd:YAG laser (1.064mm) excited Fourier Transforin Raman (NIR-FTR) spectroscopies to the characterization of fullerenes and products of interaction of C/sub 60/ with thermally generated xylylene radicals are described. It is shown that NIR-FTR spectra with high signal-to-noise ratios can be routinely obtained from pure C/sub 60/ and C/sub 70/ as well as from their mixtures at low (10-50 mW) excitation energies due to the fact that the fullerenes are-very strong Raman scatterers. Variants of the optical experiment and sampling techniques are discussed. A problem of correction of NIR-FTR spectra for the instrument response function is discussed in terms of reliability of bands intensity values obtainable. The quality of the corrected NIR-FT-Raman spectra seems to be adequate for a refinement of calculations of vibrational bands positions and intensities made for these closed-cage structures earlier. Vibrational spectra of vacuum deposited films of C/sub 60/ on quartz and spectra of free standing films of copolymers of C/sub 60/ with p-xylylene radicals are also reported. An apparent worsening of NIR-FTR spectra of these materials is discussed in terms of extensive structural disordering of the precursors. It is argued that the NIR-FTR spectra of these C/sub 60/ derivatives, similarly to those of doped conjugated polymers, often exhibiting electronic absorptions in the region from visible to NIR well may be affected by a resonance and/or multiphoton events, and should be treated accordingly.

Summary form only given. Relaxation of excited states as well as photocarrier generation and transport in C/sub 60/ polycrystalline films have been investigated by means of luminescence and steady-state photoconductivity. A tentative explanation of the relaxation of excited states and photocarrier generation in C/sub 60/ polycrystalline film has been developed, based on the following results. On one hand the shape of the luminescence emission spectrum is independent of the excitation energy (/spl lambda//sub exc/=330, 400, 550, 610, 640nm in the range (/spl lambda//sub em/=700-850nm). This result indicates that internal conversion takes place to the lowest localized electronic state. The maximum of the luminescence emission corresponds to 730nm (1.7eV). On the other hand the temperature dependence of the photoconductivity shows that the activation energy is constant Ea(/spl sigma/ph)=0.2eV for (/spl lambda//sub exc/=300 and 600nm. Carriers might be generated from the lowest excited state of Frenkel-type exciton carrying a substantial charge transfer character. In addition subsequent transport of carriers might happen in narrow conduction states by hopping from site to site (the conduction state has been estimated at 1.9eV). The steady-state photoconductivity action spectrum has been analyzed in term of the bimolecular recombination model [1]. This model shows that the photocarrier generation efficiency () is almost constant in the wavelength range 300/spl les//spl lambda//spl les/620nm. Moreover, the thickness dependence of the photoconductivity action spectrum can be also explained. Bimolecular recombination appears to be the most efficient process that shortens the free carrier life time.

Summary form only given. Earlier work has shown the ability of fullerene crystals to electroluminesce under DC conditions with a highly nonlinear current-intensity characteristic above a certain threshold voltage [1]. This is similar to the nonlinear phenomena found in the highly excited luminescence and photoconductivity, that are discussed in terms of many body effects [2]. To develop this theoretical model and to investigate the temporal behaviour of electron-hole recombination, measurements of the electroluminescent response of a silver-C60-silver device to triangle and rectangular voltage pulses of different frequencies were performed in the present case. The experiments show the occurence of a phase shift between the voltage pulse and the luminescence signal. This phase shift approaches asymptotically the value of /spl pi//2 with increasing frequency, corresponding to a time delay of about 100 /spl mu/s. In addition an accompanying drop of the luminescence intensity is observable. A theoretical analysis of these processes is proposed, taking into account the dependence of the accumulation and recombination time on the charge carrier concentration.

Summary form only given. We report detailed photoinduced absorption spectra of poly[2-methoxy, 5-(2'ethyl-hexyloxy)-p-phenylenevinylene], MEH-PPV, and its composite with C/sub 60/, from 0.01 eV (100 cm/sup -1/) to 1.3 eV (11,000 cm/sup -1/). The photoinduced absorption spectra of MEH-PPV are characterized by subgap electronic absorptions with a lower energy feature superposed with two peaks at 0.25 eV and 0.4 eV and a higher energy feature with onset at 1.1 eV together with infrared-active vibrational modes below the lower energy feature. The two features at 0.4 eV and 1.1 eV are assigned to the lower and upper energy bipolaron transitions, respectively. Another lower energy peak at 0.25 eV, which is absent in spectra of PPV, can be assigned to lower polaron transition. The magnitude of spectral response is smaller than that of PPV by a factor of 10. These results arise from the weak interchain coupling in MEH-PPV due to the large side group. As a result, interchain transport is inhibited by large interchain separation leading to the suppression of bipolaron formation, and the early time recombination is enhanced, resulting in weak spectral response compared with PPV. Upon addition of five percent of C/sub 60/ to MEH-PPV, the photoinduced absorption spectra are significantly enhanced in nearly two orders of magnitude. The photoinduced charge transfer from MEH-PPV onto C/sub 60/ enhances the quantum efficiency for photogeneration of charge carriers. As a result, the lifetime of the excited state is extended, thereby leading to the enhanced photoinduced respomm of MEH-PPV.

Summary form only given. We study the electronic states, including the dipole forbidden ones, of a directly synthesized trans-polyacetylene film. 1 ) A main peak of reflection spectra of this film shifts to the lower energy side by more than 0.3 eV and becomes narrower than that of an ordinary thermoisomerized film, as seen in Fig. 1. This fact indicates that the conjugation length has long average length and narrow distribution width. Resonance Raman spectra also support this conclusion. We are in the process of studying luminescence excited near absorption edge and electro-reflectance by using such a film.

Summary form only given. The photoluminescent properties of plasma benezen films are investigated. The photoluminescence is a fluoresconce rather than a process with a lifetime on the order of a few nanoseconds. Picosecond(ps) photoluminescence of samples indicated that the decay is a tri-exponential, with lifetime 0.4ns, 1.2ns and 2.3ns with excited by 532 nm laser. Again, the fluorescence is not produced by a few distinct chromophores , or is produced by different process such an excition complex or a band transition.

The Raman spectra of polyacetylene films doped with electron acceptors (I/sub 2/, BF/sub 3/, SO/sub 3/, etc.) at several doping levels have been measured at room temperature with excitation wavelengths between 457.9 and 1064 nm on a polychromator equipped with a multichannel detector or a Fourier transform spectrophotometer. The observed Raman spectra have been compared with those of the polyacetylene film doped heavily with electron donor (Na). The observed Raman bands of heavily doped samples are attributed to positively charged domains created by acceptor-doping. On the other hand, at low or medium dopant contents, the observed bands are ascribed to the neutral parts remaining the doped films. Since the cross sections of the Raman bands arising from the charged domains are much smaller than those arising, from the neutral polyene parts, the Raman bands of charged domains are observed only at heavy doping levels. The Raman bands of the charged domains are classified into four regions: v/sub 1/, 1560-1470 cm/sup -1/; v/sub 2/, 1300 cm/sup -1/; v/sub 3/, 1180-1100 cm /sup -1/; v4, 990 cm/sup -1/. The v/sub 1/ bands are assigned to the C=C stretching vibrations. The high wavenumbers of the v/sub 1/ bands suggest that bond alternation persists in the charged domains. Significant differences are observed between acceptor-doping and donor-doping. For example, in the 1064-nm excited Fourier-transform Raman spectrum of a heavily iodine-doped polyacetylene film, the wavenumber of the v/sub 1/ band, 1490 cm/sub -1/ , is about 20 cm/sub -1/ lower than that of the Na-doped polyacetylene, the v/sub 2/ wavenumber, 1295 cm/sub -1/, is about 40 cm/sub -1/ higher than that of the Na-doped one, the peak at about 1130 cm/sub -1/ and the shoulder at about 1175 cm/sub -1/ are observed though two peaks are observed at 1165 and 1113 cm/sub -1/ for Na-doping. These observations indicate that geometrical chan-es induced by acceptor-doping are different from those induced by donor-doping.

Summary form only given. The nonlinear two-photon absorption spectrum of conjugated polymers is theoretically studied in the Pariser-Parr-Pople model by taking into account conhiguration interaction among all the single and double excitations on the Hartree-Fock basis using the cyclic boundary condition. It is shown that, for relatively long (-40 sites) chains and moderate electron-electron interactions, the spectrum consists of two absorption bands (here a band means a group of peaks): a low-energy band involving mainly single electron-hole excitations, and a high energy band with contributions mainly from double excited states. The intensity of the latter band is at least one order of magnitude greater than that of the former. These results are in contrast to the previously studied case of small chains with very strong interactions [1], where the single and double excitations are fairly mixed and the distinction between the two bands is lost In increasing either the interaction strength or the bond alternation, the magnitude of optical nonlinearity decreases rapidly, implying that conjugated polymers with weak electron-electron and electron-phonon interactions are better candidates for optically nonlinear systems

Summary form only given. The absorption, photoluminesence (PL), and X-band (9.35 GHz) ESR and ODMR spectra of PDPAA derivative films is described and discussed. The absorption spectra of solutions of the polymers peak at 390 - 415 nm, and of the films at 415 - 422 nm. The intense emission of the solutions peaks at 420 - 450 nm, and of the films at 450 - 490 nm. The ODMR excited at 254 /spl les/ /spl lambda//sub ex/ /spl les/ 400 nm includes the following familiar features: (i) a narrow resonance at g /spl sp/ 2.002 attributed to the effects of polaron recombination on the radiative singlet exciton decay; (ii) full- and half-field triplet powder patterns, attributed to such effects of triplet excitons. However, the narrow resonance and full-field triplet powder pattern, which is /spl ap/1500 G wide, suggest that this polymer is highly disordered, with a broad distribution of polaron and triplet exciton trapping sites. The results will also be discussed in relation to the nature of the polaron and triplet exciton states and the absorption edge in conjugated polymers.

Summary form only given. The title compounds, asymmetric tripyrrane-containing 22/spl pi/ electrons pentaazadentate aromatic macrocyclic metal complexes, which have zero phonon line appearing on near IR region /spl lambda//sub max/ 760nm, were synthesized by an improved procedure. The nonlinear excited state absorptions at 532nm in the cadmium texaphyrin solution have been studied using 8ns and 23ps laser pules. Third-order nonlinear optical susceptibilities for the solutions of two asymmetric porphyrin-like metal complexes in methanol at 1064nm were measured by the degenerate four wave mixing frequency and obtained the off-resonant third-order hyperpolarizabilities as 1.2 x 10/sup -31/ esu and 1. 5 x 10/sup -31/ esu for asymmetric cadmium and gadoliaium texaphyrin respectively. The optical limiting of CW laser through thermal self-defoucsing and reverse saturable absorption for the title compounds have been demonstrated and an improved optical limiter was showed.

Summary form only given. We present the polarized electric field modulation spectra of dilute blends of MEH-PPV oriented in ultra-high molecular weight polyethylene, PE. The polarized electroabsorption spectra of the oriented blend demonstratesa field-induced nonlinear refractive index anisotropy in excess of 150:1, with preferred polarization parallel to the draw axis. Because of the orientation, the enhanced order and the dilution of the MEH-PPV conjugated polymer in the polyethylene matrix, the MEH-PPV/PE blend is an ideal system in which to investigate the intrinsic linear and nonlinear optical properties of oriented conjugated polymer chains. The absorption is directly proportional to the imaginary part of X/sup (10)/(w), and the electroabsorption is directly proportional to the imaginary part of X/sup (3)/(w, 0, 0) of the ordered MEH-PPV /spl pi/-electron system. The optical data obtained from the oriented blends is therefore directly relevant to existing theoretical models which do not account for disorder. Using a model for the off-axis transition dipole moment of the conjugated polymer, which is based on the measured off-axis transition dipole moment of transstilbene and the geometric structure of MEH-PPV, we demonstrate that the observed anisotropy of 150:1 in the field-induced absorption requires that the instantaneous excited state wavefunctions are delocalized over a minimum of 50 unit cells (400 /spl Aring/).

Summary form only given. The intrinsic dynamics of photoexcited carriers in oxygen-free C/sub 60/ film and their remarkable evolution as the film is exposed to oxygen are revealed by transient and steady-state photoconductivity measurements at various temperatures, light intensities and photon energies, as well as by transient photoinduced absorption in the picosecond time regime. Exposure of C/sub 60/ film to oxygen creates deep traps which reduce drastically the carrier lifetime and consequently the room temperature steady-state photoconductivity by three to six orders of magnitude. Oxygen affects the steady-state photoconductivity excitation spectrum in qualitative similar way as decreasing the ambient temperature; in both cases the photoconductivity decreases faster when carriers are excited below the band edges. The transient and steady-state PC of C/sub 60/ film fully exposed to oxygen become temperature independent. While exposing the sample to oxygen changes dramatically the photoconductivity, it does not affect significantly the optical absorption as well as the transient photoinduced absorption in the picosecond time regime.

Summary form only given. We propose a concept of a localized charge-transfer exciton, based on the fact that the real samples of conducting polymer are composed of neutral finite-length. fragments of the polymer chains [1]. The states of such exciton involve the excited state of one fragment and the states where one of its electons is transferred to neighbor fragments. These states appear when one photon with energy h/spl nu/ /spl cong/ /splk utri/E /spl cong/ 2 eV is absorbed, where /spl utri/E is the energy of the intense maximum in the absorption spectrum. The following energy relation provides a means for such a transfer: I - A - P - G /spl cong/ /spl utri/E, where I /spl cong/ 6 eV and A /spl cong/ 1 eV is the ionization potential and the electron affinity of neutral fragment, (-G) /spl les/ 1 eV is the Coulomb interaction energy of two charged fragments, and P /spl cong/ 3 eV is the energy of electronic polarization of polymer fragments in the Co ulomb field of two charged fragments. The photoexcitation spectra, which are nothing but the absorption spectra owing to transi tions between the states of the exciton, are simulated numerically for polyacetylene and polyaniline in accord with experiment.

Summary form only given. The UV-ODMR of poly(p-phenylenevinylenes) (PPVs), poly(p-phenylene-acetylenes) (PPAs), poly(3-alkylthiophenes) (P3ATs), and poly(p-diphenylene-aniline-acetylene) (PDPAA) derivatives is described and compared to the ODMR observed by visible excitation. Visible excitation of PPVs, PPAs, and P3ATs yields a narrow photolurninescence (PL)-enhancing polaron resonance and full- and half-field PL-enhancing triplet exciton resonances. However, excitation of PPVs at /spl lambda//sub ex//spl les/ 353 nm and P3ATs at /spl lambda//sub ex//spl les/ 308 nm weakens the PL intensity and yields an intense narrow PL-quenching resonance which is observed in addition to the polaron resonance. In the relatively highly disordered PPAs and the higher-gap PDPAAs no such quenching resonance is observed. It is tentatively suggested that the polaron levels are stabilized by defects, the fundamental absorption edge is due to a rigid singlet exciton, and that the narrow PL-quenching ODMR is due to the dynamics of electrons and holes in the continuum bands.