The photophysics of phenyl disubstituted polyacetylenes: picosecond photoluminescence and femtosecond photoinduced absorption studies of PDPA-n-Bu

We have obtained picosecond (ps) photoluminescence (PL) decays of phenyl disubstituted polyacetylenes (PDPA) with an attached n-butyl group (PDPA-n-Bu) in solution and as thin films. In solution, we observe only a single exponential decay at all emission wavelengths over the range from 2.15 to 2.50 eV. The PL lifetime in solution is 190 ps. The PL decays in thin films cannot be fit well using a single exponential, and are significantly longer in thin films than in solution. In addition, the PL decay times vary with emission wavelength in thin films with longer PL decay times observed at lower emission energies. We have also obtained the femtosecond (fs) photoinduced absorption (PA) spectra of PDPA-n-Bu in solution over the range from 1.70 to 2.95 eV. These data exhibit fast ground state bleaching at high energy, along with the growth of a broad PA feature at low energies that decays with a time constant consistent with the PL decay time in solution. At long times (>200 ps) we observe the growth of a long-lived PA feature peaked at 2.3 eV that persists for hundreds of nanoseconds. We also observe a broad feature in the infrared PA at 0.23 eV whose dynamics are the same as the band at 2.3 eV. We assign these two long-lived bands to polaron transitions.     

Probing of the 2Ag state in nanopolyacetylene on the time scale from femtoseconds to milliseconds

The dynamic properties of the 2A_g state in films of nanopolyacetylene have been measured using photoinduced spectroscopy and photoinduced polarimetry methods both in the transient and cw regime. The excitation density per polyacetylene nanoparticle was no more than one absorbed pump photon. Using femtosecond pulses, we evaluated the rise time (≤0.2 ps) and the decay time (≈1 ps) of the photoinduced response from the 2A_g state of trans-nanopolyacetylene. The longest relaxation time ≈7 μs was measured at room temperature. Using picosecond pulses and a novel photoinduced polarimetry method, we have done the first measurements of the transient polarization response of a conjugated material. The observed polarization signal is associated with a small photoinduced dichroism and birefringence in the 2A_g state. The decay time of the polarization signal was ≈0.3 ns.     

Long-lived photoexcitations in α-hexathiophene single crystals

We employed various steady-state spectroscopic techniques, such as absorption, photoluminescence, photoinduced absorption (PA) and PA-detected magnetic resonance (PADMR), to study long-lived photoexcitations in α-hexathiophene single crystals. The lowest lying absorption band appears at 2.4 eV and is polarized along the oligomer chains (ac-polarized), whereas the photoluminescence is largely unpolarized. The PA spectrum, recorded from ℏω=0.6 to 1.8 eV, is relatively weak and shows three bands at 0.75, 1.25 and 1.6 eV, respectively, where the 1.6 eV band is by far the most intense. All three PA bands are ac-polarized. Using PADMR we identify the 1.25 eV band as due to bipolarons, and show that the 0.75 eV band is due to spin 1/2 polarons. For the triplet exciton we found a broad PADMR band centered at 1.6 eV with 1 eV total width.     

Studies of photoexcitations in conducting polymers mixed with C60

We have studied photoexcitations in mixed compounds of C₆₀ with derivatives of poly (p-phenylenevinylene) and poly(phenyleneacetylene) using a variety of CW and picosecond (ps) transient spectroscopies. The CW techniques used include photoinduced absorption (PA), photoluminescence (PL) and optically detected PA (ADMR), whereas the ps transient techniques used were time-resolved PA, PL and stimulated emission (SE). Compared with the pristine polymers, all mixed C₆₀ compounds have shown enhancement of PA bands associated with charged photoexcitations. These PA bands have shown a single spin-half ADMR resonance, identified as due to spins on the polymer chains; no spin-half ADMR signal from photogenerated C₆₀⁻ has been observed. In the ps time domain we have observed a pronounced reduction of the transient PL and SE in the mixed compounds. Due to the similarity found between the ps transient and CW PA bands, we have identified the ps photoexcitations in the mixed compounds as polarons (P⁺−P⁻ ) on the polymer chains, which are indirectly photogenerated as a result of exciton dissociation in C₆₀⁻ related defect centers. The ultrafast charge transfer, claimed to exist between the polymer chains and C₆₀ molecules, has not been observed in the ps time domain.     

Recombination studies in a polyfluorene copolymer for photovoltaic applications

We present detailed continuous wave (cw) and transient photoinduced absorption (PA) measurements in thin films of a novel alternating polyfluorene copolymer, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3-benzo-thiadiazole)] (DiO-PFDTBT), and its blends with the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in weight ratios of 1:0, 4:1 and 1:4. We measure the frequency, intensity and temperature dependence of the PA signal in the frequency domain, and compare with the results obtained from the transient PA decay measurements in the time domain. In all blends, the PA spectrum shows a broad high energy PA band ranging from ∼1 eV to 2 eV as well as a low energy band peaking at ∼0.35 eV. We attribute the low energy band to the P₁ transition of polarons and part of the high energy band to the correlated P₂ transition of polarons. Both frequency and time domain measurements show that the high energy band has two decay components, a faster component in the microsecond time regime and a slower component in the millisecond time regime. The slow component is strongly dispersive, whereas the fast component is practically non-dispersive.     

Magneto-optical studies of 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61; PCBM

In the quest for better power conversion efficiency of organic photovoltaic cells (OPVC) various architectures have been considered. Bulk heterojunction type OPVC proved to be the most successful, where the polymer as electron donor is blended with fullerene molecule as electron acceptor. With the goal of improving power conversion efficiency of OPVC we studied 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) as a film and dispersed in polystyrene matrix. We used a variety of continuous wave (cw) optical probes including photoinduced absorption (PA), photoluminescence (PL), and optically detected magnetic resonance (ODMR). The steady state photophysics of PCBM is dominated by triplet exciton. For both PCBM dispersed in polystyrene and PCBM film, the triplet exciton specie is characterized by a PA band at about 1.7 eV and spin 1 PA detected magnetic resonance (PADMR) powder pattern resonance around g ～ 2. However, in PCBM film the occurrence of a polaron PA band at 1.2 eV and spin 1/2 PADMR are also noticed.     

Picosecond time-resolved infrared absorption study on photoexcited dynamics of regioregular poly(3-hexylthiophene)

Picosecond time-resolved infrared absorption spectra of the photoexcited states of a spin-coated film of regioregular poly(3-hexylthiophene) (RR-P3HT) have been recorded at 77 K. The time-resolved photoinduced infrared absorptions due to electronic transitions at 2500 cm^?1 and vibrational transitions at 1575 cm^?1 have fast and slow decay components. By comparing the picosecond time-resolved infrared absorption spectrum of long-lived (slow decay component) transient species with the doping-induced infrared difference spectrum of RR-P3HT, the long-lived transient species are assigned to polarons. The short-lived transient species are assigned to singlet excitons and/or polaron pairs.     

Triplet state dynamics in poly(2,5-pyridine diyl)

Pulse radiolysis was used to determine the triplet state energy (2.3 eV) of Poly(2,5-pyridine diyl) (PPY) film which was found to be coincident with the emission energy. Detailed time-resolved photoluminescence (PL) and pump-probe measurements have been applied to study the photoexcited state relaxation dynamics. In films, a very large spectral red-shift (∼0.35 eV) for the PL occurred within the first 100 ps whereas no spectral red-shift was observed for the PPY in solution. This result shows clear evidence for the evolution of short-lived singlet emission (S₁→S₀) at ∼450 nm to long-lived triplet emission (T₁→S₀) at ∼520 nm for the PPY thin film. Streak camera measurement indicates the long live component has a decay time constant of several ns. The picosecond photo-induced triplet state absorption (T₁→T₂ transition) peaks at ∼600 nm as measured by pump-probe which is consistent with both the radiolysis and cw photo-induced absorption measurements. A triplet lifetime of ∼6 ns is measured which is again consistent with the streak camera measurement. These results lead us to believe that the long live component of the emission from PPY film is in fact phosphorescence. Furthermore, there is evidence that oxygen plays a very important role in the fast triplet radiative lifetime in PPY films.     

Time-resolved photoluminescence study on concentration quenching of a red emitting tetraphenylchlorin dye for organic electroluminescent devices

Time-resolved photoluminescence (PL) of a red emitting dye, tetraphenylchlorin (TPC), doped in poly(methylmethacrylate) (PMMA) at various concentrations was studied to clarify the mechanism of concentration quenching often observed in organic electroluminescence devices. At doping concentrations lower than 10⁻² mol/L, PL lifetimes of TPC were relatively constant (∼10 ns) and equal to that of TPC in dilute solution. At doping concentrations higher than 10⁻² mol/L, PL lifetime decreased rapidly with increasing concentration, down to 2 ns at 8 × 10⁻² mol/L. This decrease in PL lifetime was related to the concentration quenching of TPC monomer emission by the increasing number of TPC dimer sites formed at such higher concentrations. Our experimental results of concentration-dependence trapping agree fairly well with the theoretical model of trapping by dimers reported in literature.     

Photoluminescence quantum efficiency (PLQE) and PL decay characteristics of polymeric light emitting materials

We report photoluminescence quantum efficiency (PLQE) measurements on thin films of a variety of in-house synthesized conjugated polymers such as PPV, MEH-PPV and CN-PPV with improved techniques using integrating sphere and synchronous detection. Our method allows use of low level of excitation avoiding problems due to degradation during measurement. Time correlated single photon counting (TCSPC) has been used to study PL decay to obtain radiative lifetime controlling efficiency in these materials. The measured PL efficiency in PPV synthesized by xanthate precursor route is measured to be 0.07 ± 0.01, which is lower than reported for similar films owing to presence of intrinsic defects. The measured PL quantum efficiency of CN-PPV and MEH-PPV are 0.24 ± 0.07 and 0.17 ± 0.01, respectively. These values are comparable to the ones reported by others [N.C. Greenham, I.D. Samuel, G.R. Hayes, R.T. Phillips, Y.A.R.R. Kessener, S.C. Moratti, A.B. Holmes, R.H. Friend, Chem. Phys. Lett. 89 (1995) 241]. We suggest a reliable method of obtaining the radiative lifetime from PL decay curves using time domain spectroscopic technique.     

Multifunctional polyconjugated molecules with carbazolyl and pyrazolyl moieties for optoelectronic applications

Diethylene derivative possessing carbazolyl and pyrazolyl moieties and its analogue containing additional cyano groups were synthesized and investigated as potential multifunctional materials for organic light emitting diodes. The influence of the electron affinitive cyano groups on the ionization potential (I_p) of the films as well as on the photoluminescence (PL) spectrum, PL quantum yield (η) and PL decay time of the dilute solutions and thin films of the diethylene derivatives was studied. PL measurements revealed that highly luminescent (η = 0.80) cyano-free pyrazole–carbazole derivative in solution became weakly emissive (η = 0.04) by attaching cyano groups as a result of these groups-induced torsional deactivation. However in the solid state, the steric and electrostatic effects of the bulky and polar cyano groups prevented close packing of the molecules, thus significantly reducing migration-induced quenching of the excitons at the defects. Incorporation of the cyano groups resulted in the 3-fold enhancement of the PL quantum yield in the neat film of the polyconjugated derivative as compared to that of the cyano-free film. The I_p of 5.50 eV estimated for the cyano groups-containing compound was found to be higher as compared to the I_p of 5.35 eV for the cyano-free analogue, which in conjunction with the PL data for the films indicated increased electron affinity (by 0.40 eV) in the cyano groups-containing diethylene derivative.     

Polarization sensitive THz absorption spectroscopy for the evaluation of uniaxially deformed ultra-high molecular weight polyethylene

Mechanically extensional–deformation of ultra-high molecular weight polyethylene (UHMWPE) was examined by using polarization terahertz (THz) spectroscopy as well as X-ray diffraction (XRD) analysis. Due to the B_1u lattice translational mode, a THz absorption band was observed around 2.2 THz. The band intensity of the non-deformed UHMWPE is isotropic. For deformed UHMWPE, the dichroism appeared in the 2.2 THz band. The absorption intensity was smaller in spectra in which the THz wave electric polarization direction was parallel to the deformation direction than that in which the polarization was perpendicular. The intensities in each polarization direction are dependent on the amount of elongation strain. Based on the influences drawn from the THz and XRD results, THz non-destructive diagnosis of the tensile strain in deformed UHMWPE is proposed based on the dichroism of B_1u band intensities observed in polarization THz spectroscopy.     

Luminescent monomer of substituted tetrastyrylpyrene and poly(p-phenylenevinylene) derivative with pyrene segments: Synthesis and photophysics

1,3,6,8-Tetrabromopyrene reacted with 1-tert-butyl-4-vinylbenzene to afford the luminescent monomer M. In addition, the reaction of 1,6-dibromopyrene with 1,4-didodecyloxy-2,5-divinylbenzene yielded a poly(p-phenylenevinylene) (PPV) derivative P. M dissolved partially in chloroform and completely in THF. P showed an excellent solubility being readily soluble in common organic solvents. M emitted intense blue–green light in THF solution with photoluminescence (PL) maximum at 507 nm and green–yellow light in thin film with PL maximum at 570 nm. Polymer P displayed in THF a PL maximum at 464 nm and a shoulder at 513 nm. Thin film of this polymer exhibited PL maximum at 572 nm. Both M and P emitted from excimers in solid state. The PL emission quantum yields in THF were 0.35 for M and 0.32 for P. The emission maximum of M thin film was red shifted by 9 nm after annealing at 150 °C for 24 h as a result of conformational changes of the molecules with increase in temperature.     

Infrared and Raman studies of the phase transition in the organic conductor (TTM-TTP)I3

We report the polarized IR reflectance as well as Raman scattering investigations of the organic charge transfer salt (TTM-TTP)I₃ as a function of temperature, below and above the metal–insulator phase transition at T = 160 K. The IR reflectance was measured in the frequency region from 600 to 10,000 cm⁻¹, for the electrical vector of the polarized light parallel and perpendicular to the TTM-TTP stacking axis. For the polarization parallel to stacks the IR spectra are typical for semiconducting charge transfer salts. The electronic part of IR spectra was analysed in terms of a Lorentz model and temperature dependence of the optical transport parameters was determined. For the polarization perpendicular to the stacks we observed two electronic bands at about 5000 and 8000 cm⁻¹. The phase transition at 160 K has nearly no influence on the IR spectrum. The Raman scattering for different excitations (λ = 514.5, 632.6 and 785 nm) was mainly studied within the region of CC stretching vibrations. In this frequency range, three Raman lines at 1426, 1453 and 1486 cm⁻¹ attributed to TTM-TTP molecules are observed. Below 160 K a splitting of the band 1486 cm⁻¹ into two peaks at about 1488 and 1498 cm⁻¹ is found. The intensity and temperature behaviour of the split band at 1498 cm⁻¹ is strongly dependent on sample. The observed spectral modifications are related to an asymmetric deformation of TTM-TTP. Taking into account temperature dependence of bands attributed to the CH stretching and SCH₃ bending vibrations, we suggest that the TTM-TTP deformation can exist also above the phase transition temperature. Above 160 K molecules fluctuate between distorted and symmetrical state forming non-stable domains (pre-transitional effects), but below 160 K the molecular distortion and domains are stable. The existence of electronic band at 5000 cm⁻¹ for the polarization perpendicular to TTM-TTP supports this picture.     

Low temperature solution-synthesis and photoluminescence properties of ZnO nanowires

A general and simple approach has been developed to prepare zinc oxide (ZnO) nanowires from commercially available zinc acetate precursors using a solution-phase reaction. The influence of post-annealing temperature on the morphology of the zinc oxides nanowires was also investigated. Using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies, we confirm that the ZnO nanowires are highly crystalline in nature with preferential orientation in the c-axis direction, with nanowire diameters of around 30–60 nm. The photoluminescence (PL) studies demonstrate a strong UV luminescence band around 382 nm, and three weak, broad bands at 454, 468, and 510 nm, respectively, in the blue and green range. After being annealed under an air atmosphere, the products show an apparent red-shift in the UV emission band. The intensity of the defect-related emission bands was significantly suppressed.     

Exciplex emission and energy transfer in white light-emitting organic electroluminescent device

We report a white light organic electroluminescent (EL) device achieved through exciplex formation between organic materials as a green color light source. Exciplex formation at 500 nm originated from poly(N-vinylcarbazole) (PVK) and 2,5-bis(5-tert-bytyl-2-benzoxazolyl)thiophene (BBOT), exciplex formation at 550 nm between from N,N′-diphenyl-N,N′-bis(3-metylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) and BBOT was assumed to correspond to the complex formation covering the green color region. In the mixed emitting materials, the PL decay lifetimes of BBOT and poly(3-hexylthiophene) (P3HT) were increased as much as 35–68 and 1.5–19 ns, respectively. This indicates that the energy transfer occurred from PVK to BBOT and P3HT, enhancing the quantum efficiency of these materials. We achieved white light emission with brightness as great as 12.3 μW/cm² at 20 V, corresponding to CIE coordinates of x=0.29 and y=0.353. The EL spectrum of the device was changed with a function of the applied voltage.     

Synthesis and light-emitting properties of polyfluorene copolymers containing a hydrazone derivative as a comonomer

Poly[9,9-bis(2′-ethylhexyl)fluorene-2,7-diyl] (PBEHF) and a series of fluorene copolymers containing a hydrazone comonomer, poly{9,9-bis(2′-ethylhexyl)fluorene-2,7-diyl-co-[aza(2,7-fluorene-9-ylidene)methyl]diphenylamine} [poly(BEHF-co-FDPA)], were synthesized through Ni(0)-mediated polymerization. The synthesized polymers were characterized and their photophysical, electrochemical, and electroluminescent properties were investigated. Interestingly, the copolymer containing only 1% FDPA, poly(99BEHF-co-1FDPA), was found to exhibit photoluminescence (PL) emission that is significantly red-shifted with respect to that of the PBEHF homopolymer. All the copolymers exhibit PL emission maxima in the yellow region of the spectrum in the range from 530 to 540 nm, whereas the PBEHF homopolymer film produces maximum PL emission in the deep blue region at 424 nm. Light-emitting devices were fabricated in an ITO/PEDOT:PSS (50 nm)/polymers (80 nm)/Ca (50 nm)/Al (200 nm) configuration. The electroluminescence (EL) spectra of these devices consist of similar wavelengths to those found in the PL results; however, the EL devices constructed from the copolymers exhibited device performances that were significantly better than that of the device using the PBEHF homopolymer. This enhancement of performance is due to a more efficient charge carrier balance in the devices, which arises because of the effective exciton confinement (or charge carrier trapping) in the FDPA groups.     

Organic light-emitting diodes and organic light-emitting electrochemical cells based on silole–fluorene derivatives

Silole groups are known to present a high electron affinity. Initially, copolymerization of siloles with fluorene was aimed at improving electron injection into the polymer layer and so improving the electroluminescent properties of organic light-emitting diodes (OLED's) made from fluorene. But it also provides the ability to turn the light emission colour to the green part of the spectrum and to stop the well-known spectral shift degradation occurring in fluorene-based materials. In this paper we report the synthesis and the characterisation of 1,1-dimethyl-2,5-bis(fluoren-2-yl)-3,4-diphenylsilole 4, and of two soluble conjugated random copolymers derived from 9,9-ditetradecylfluorene and 1,1-dialkyl-2,5-diphenylsilole, where the alkyl group is either methyl 11a or n-hexyl 11b. Silole 4 crystallizes in the triclinic P-1 space group with a = 9.8771(8), b = 10.6240(10), c = 16.585(2) Å, α = 95.775(8), β = 97.025(7), and γ = 111.738(8)°. The results obtained with this molecule, operating in a single-layer OLED (luminance ≈450 Cd/m² at 12 V; η_max = 0.2 Cd/A), give evidences for the complementarity of the silole and the fluorenyl moieties in the improvement of the charge injection processes when compared with 1,1-dimethyl-2,3,4,5-tetraphenylsilole. The results obtained from organic light-emitting electrochemical cells (LEC's) made from silole–fluorene copolymers 11a, 11b and molten salts show an improvement of both the device lifetime and the spectral stability when compared with polyfluorene. To explain devices performances electrical characterisation data and atomic force microscope (AFM) imaging were combined.     

Sub-10 fs time resolved study of excited state relaxation in all-trans-β-carotene

The ultrafast internal conversion S₂→S₁ following photoexcitation in all-trans-β-carotene is studied with unprecedented time resolution. Sub-10 fs optical pulses are used to excite and probe the optical dynamics of the molecule dissolved in solution. We observed the ultrafast decay of the stimulated emission from S₂ and formation of the absorption band assigned to S₁→S_n transitions within 500 fs. Periodic oscillations of the amplitude are superimposed to the transient transmission difference and assigned to ground state nuclear wavepacket motion excited via impulsive Raman scattering.     

New amorphous electron-transporting materials based on Tris-benzimidazoles for all wet-process OLED devices

New amorphous electron-transporting materials 1,3,5-tris[1-(phenoxy phenyl)-1H-benzimidazol-2-yl]benzene (6a and 6b) were synthesized by dehydration reaction of corresponding triamides and fully characterized. Compounds 6a and 6b showed high thermal stability (T_g = 108 to 110 °C) and good solubility in common organic solvents, such as acetone, 2-methoxyetanol, tetrahydrofuran, chloroform, and 2-butanone. Pinhole-free transparent films were obtained by spin-casting the 2-butanone solution. 6a and 6b were fabricated as electron-transporting layer by spin-casting on a hole-transporting layer that was deposited by spin-cast beforehand and was insoluble in 2-butanone. Both devices showed no emission, but carrier transporting property was observed (400 mA/cm²). The device showed a blue emission (35 cd/m²) from 9,10-diphenylanthracene (DPA) when 10 wt.% DPA was mixed into the 6b layer.