Flash-photolysis Time-resolved Microwave Conductivity Research Articles

Anisotropy of the optical absorption and photoconductivity of a zone-cast film of a discotic hexabenzocoronene

Polarized spectrophotometry and laser flash-photolysis time-resolved microwave conductivity (FP-TRMC) have been used to measure the anisotropy in the optical absorption and photoconductivity of a ca. 40nm thick film of hexakisdodecyl-hexa peri-hexabenzocoronene (HBC-C12) prepared by zone-casting onto a bare quartz substrate. At room temperature the film displays negligible optical anisotropy but an order of magnitude preference for conduction in the direction of zone casting. On heating above the K-to-Col transition temperature found for the bulk material the film becomes optically anisotropic with light polarized perpendicular to the direction of columnar alignment favored by a factor of ca. 3. This is ascribed to reorientation of the mesogens within the film from a close to 45° tilted columnar arrangement to closer to orthogonal stacking. A decrease in the photoconductivity of the film on entering the mesophase is observed and attributed to a decrease in the charge mobility on melting of the saturated hydrocarbon mantle.

Thermal switching of the optical anisotropy of a macroscopically aligned film of a discotic liquid crystal

We have studied the temperature dependence of anisotropy in the optical absorption and charge transport properties of an aligned film of hexakis-dodecyl-hexa-peri-hexabenzocoronene (HBC-C12) formed by zone-casting on a quartz substrate. At room temperature the film displays a large anisotropy in (photo)conductivity, as determined using the flash photolysis time-resolved microwave conductivity technique, with charge transport in the casting direction favoured by a factor of at least 10. The anisotropy in the optical absorption is however negligible. At the temperature corresponding to the transition from the crystalline solid to the liquid crystalline mesophase (c. 110°C), the optical anisotropy increases abruptly, with absorption of light polarized in the direction perpendicular to the alignment direction favoured by a factor of c. 3. On cooling, the dichroism reverts to its initial very low value with a hysteresis of c. 30°C. The results are explained in terms of a reversible change in the orientation of the molecules with respect to the axis of the aligned columnar stacks from tilted (at c. 45°) in the crystalline phase to close to orthogonal in the liquid crystalline phase.

Mobility and decay kinetics of charge carriers in photoexcited PCBM/PPV blends

The transient photoconductivity of blends of a highly soluble C60 derivative (PCBM) and a dialkoxy-phenylene-vinylene polymer (MDMO-PPV) has been studied using the electrodeless flash-photolysis time-resolved microwave conductivity technique (FP-TRMC). Films approximately 100 nm thick on a quartz substrate were prepared by spin-coating PCBM/PPV solutions with PCBM weight fractions ${(W}_{\mathrm{PCBM}})$ from 0.2 to 0.95. For all blends, the wavelength dependence of the photoconductivity in the range 420--700 nm closely resembled the photon attenuation spectrum, indicating that photoexcitation of both components contributes to mobile charge carrier formation. The product of the quantum yield for charge separation \ensuremath{\varphi} and the sum of the charge carrier mobilities \ensuremath{\Sigma}\ensuremath{\mu} was determined from the maximum (end-of-pulse) value of the transient photoconductivity. On excitation at 500 nm, \ensuremath{\varphi}\ensuremath{\Sigma}\ensuremath{\mu} remained almost constant in going from ${W}_{\mathrm{PCBM}}=0.2$ to 0.6 with an average value of $0.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{\mathrm{cm}}^{2}/\mathrm{V}\mathrm{}\mathrm{s}.$ Above ${W}_{\mathrm{PCBM}}=0.6,$ \ensuremath{\varphi}\ensuremath{\Sigma}\ensuremath{\mu} increased dramatically, reaching a maximum value of $83\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{\mathrm{cm}}^{2}/\mathrm{V}\mathrm{}\mathrm{s}$ for ${W}_{\mathrm{PCBM}}=0.85.$ This effect is attributed to the occurrence of phase separation above ${W}_{\mathrm{PCBM}}=0.6$ and to the resulting formation of highly mobile electrons within PCBM-rich aggregates. The much lower value of \ensuremath{\varphi}\ensuremath{\Sigma}\ensuremath{\mu} observed below ${W}_{\mathrm{PCBM}}=0.6$ is assigned mainly to mobile holes within the polymer component of the blend. Possible explanations for the decrease in \ensuremath{\varphi}\ensuremath{\Sigma}\ensuremath{\mu} with increasing light intensity, found for all blend compositions, are discussed.

Contactless Determination of the Photoconductivity Action Spectrum, Exciton Diffusion Length, and Charge Separation Efficiency in Polythiophene-Sensitized TiO2 Bilayers

The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to study photoinduced charge separation in bilayers consisting of a smooth, transparent, 80 nm thick layer of anatase TiO2 onto which poly(3-hexylthiophene) (P3HT) sensitizer layers have been spin-coated. Interfacial charge separation, resulting from excitation of the polymer in the visible, is found to persist well into the millisecond time domain. Photoconductivity action spectra have been measured between 420 and 700 nm for P3HT layer thicknesses, L, from ∼2 to 200 nm. Using this electrodeless technique, the bilayers could be irradiated from either the polymer (“front”) or semiconductor (“back”) side. On front-side irradiation at 540 nm (close to the absorption maximum of the polymer), the efficiency of charge separation per incident photon (IPCSE) initially increased to a maximum value of 0.8% for L ≈ 10 nm. For thicker layers the IPCSE gradually decreased, eventually to 0.1% for L ≈ 170 nm. On back-side irradi...

Photo-Induced Charge Separation in a Blend of Perylenediimide and Hexabenzocoronene Derivatives Studied by FP-TRMC

Photo-induced charge separation has been monitored in a spin-coated blend of hexabenzocoronene and perylene-diimide derivatives using the electrodeless flash-photolysis time-resolved microwave conductivity technique (FP-TRMC). Evidence is found for a direct charge-transfer transition between the components at longer wavelengths.

Contactless determination of the efficiency of photo-induced charge separation in a porphyrin–TiO 2 bilayer

We have studied photo-induced charge separation in a bilayer consisting of a 60 nm thick layer of a free-base porphyrin spin-coated onto a smooth, 80 nm thick layer of anatase TiO 2 using the (electrodeless) flash-photolysis time-resolved microwave conductivity (FP-TRMC) technique. After correction for direct sub-bandgap excitation, the wavelength dependence of the interfacial incident-photon-to-charge-separation efficiency (IPCSE) closely follows the photon attenuation spectrum of the porphyrin layer from 420 to 700 nm with a maximum value of ca. 0.6% at the Soret band maximum (430 nm). Only the first ca. 1 nm of the porphyrin layer is active in charge separation, i.e. exciton diffusion plays no significant role. Conduction band electrons formed by direct bandgap (300 nm) or sub-bandgap (>400 nm) excitation within the TiO 2 layer decay much more rapidly than those formed by electron injection from the porphyrin layer. In the latter case, the lifetime towards interfacial charge recombination extends well into the millisecond time regime.

Symmetry breaking in the relaxed S(1) excited state of bianthryl derivatives in weakly polar solvents.

The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to investigate the nature of the relaxed S(1) state of 9,9'-bianthryl (AA), 10-cyano-9,9'-bianthryl (CAA), and 10,10'-dicyano-9,9'-bianthryl (CAAC). Changes in both the real, Deltaepsilon' (dielectric constant), and imaginary, Deltaepsilon' ' (dielectric loss), components of the complex permittivity have been measured. The dielectric loss transients conclusively demonstrate the dipolar nature of S(1) for all three compounds in the pseudopolar solvents benzene and 1,4-dioxane, and even in the nonpolar solvents n-hexane and cyclohexane. The required symmetry breaking is considered to result from density and structural fluctuations in the solvent environment. The dipole relaxation times for AA (CAAC) are ca. 2 ps for the alkanes and 7.9 (5.3) and 14 (14) ps for benzene and dioxane, respectively. The time scale of dipole relaxation for the symmetrical compounds is much shorter than that for rotational diffusion and is attributed to intramolecular, flip-flop dipole reversal via a neutral excitonic state. The dipole moment of the transient dipolar state is estimated to be ca. 8 D, that is much lower than the value of ca. 20 D determined from the solvatochromic shifts in the fluorescence in intermediate to highly polar solvents which corresponds to close to complete charge separation. For the asymmetric compound, CAA, a dipole moment close to 20 D is found in all solvents, including n-hexane. Dipole relaxation in this case occurs on a time scale of several hundred picoseconds and is controlled mainly by diffusional rotation of the molecules. The mechanism and kinetics of formation of the dipolar excited states are discussed in the light of these results.

Flash-photolysis time-resolved microwave conductivity of a poly(3-hexylthiophene) thin film

The transient conductivity of a spincoated poly(3-hexylthiophene) film upon laser excitation is probed using an electrodeless microwave conductivity technique which circumvents the drawbacks of electrode contacts, space charge and polarization in conventional DC methods. The dependence of the kinetics and the peak values of the conductivity transients is investigated as a function of excitation wavelength and intensity. From these results, insight into the mechanism of charge carrier generation, mobility and decay can be obtained.

Photoexcitations of Covalently Bridged Zinc Porphyrin Oligomers: Frenkel versus Wannier−Mott Type Excitons

The effect of increasing chain length on the excited-state properties of two series of covalently linked zinc porphyrin oligomers has been investigated. One series has a 1,4-phenylene bridge, (PB)n with n = 1, 2, 3, and 5, and the other series has a 1,4-butadiynylene bridge, (yPy)n with n = 1−6. The two series differ dramatically in the nature of the porphyrin−porphyrin interaction. Flash-photolysis time-resolved microwave conductivity measurements (FP-TRMC) show that the excited singlet state (S1) of the (PB)n oligomers has a very small excess polarizability (<20 Å3) which is characteristic of a tightly bound, Frenkel type exciton. In agreement with this, the optical absorption and emission spectra indicate that only electrostatic interactions occur between the transition dipoles of the porphyrin moieties. In contrast, very large excess polarizabilities are found for the S1 state of the (yPy)n series, which indicates that a high degree of electron exchange occurs between the porphyrin moieties. This is confirmed by the marked changes also observed in the optical absorption and emission spectra. The polarizability increases with increasing length of the oligomers up to a maximum of 960 Å3 indicative of a loosely bound, Wannier−Mott type exciton. In contrast, the triplet excited state of the (yPy)n series has an excess polarizability of <50 Å3, indicating that it is much more localized than the S1 state.

Excitonic Interactions in the Singlet and Triplet Excited States of Covalently Linked Zinc Porphyrin Dimers

The excess polarizability volumes, ΔVp, of the relaxed S1 and T1 excited states of several covalently bridged zinc porphyrin dimers and their corresponding monomers have been measured using the flash photolysis time-resolved microwave conductivity technique. ΔVp(S1) increases from close to zero for a diaryl-substituted monomer up to a maximum value of 590 Å3 for a dimer coupled by a 9,10-diethynylanthracene bridge, yPyAyPy. The particularly large excess polarizability of yPyAyPy is attributed to strong electronic coupling between the porphyrin moieties resulting from stabilization of the cumulenic quinoidal resonance structure of the bridge. The strength of the electronic interaction, as indicated by the magnitude of ΔVp(S1), increases in the following order of bridging units: 1,4-phenylene < single σ-bond < 1,4-diethynylbenzene < 2,5-diethynylthiophene ≈ butadiyne < 9,10-diethynylanthracene. The results provide an example of an inverse distance effect whereby the electronic interaction between porphyrin moieties actually increases with increasing length of the intervening bridge. The product of the intersystem crossing efficiency and the excess polarizability volume of the triplet state, φiscΔVp(T1), is more than an order of magnitude less than ΔVp(S1), indicating a much smaller degree of exciton delocalization in T1 than in S1. The microwave results are compared with results on the optical absorption and emission spectra which provide additional information on both the electronic and Coulombic excitonic interactions.

Contactless determination of the efficiency of interfacial charge separation in a TiO 2/phenylene vinylene polymer junction

We have used the flash-photolysis time-resolved microwave conductivity technique (TRMC) to monitor charge separation between a smooth film of anatase TiO 2 (ca 80 nm) and spin-coated layers of a dialkoxyphenylene-vinylene polymer (20–140 nm thick) on nanosecond flash-photolysis in the polymer absorption band at 544 nm. The quantum efficiency of electron injection into the TiO 2 from photons absorbed in the polymer layer can be calculated by comparing the conductivity resulting from 544 nm irradiation with that from direct band gap excitation of the anatase layer at 308 nm. The maximum efficiency per incident photon, is ca 6% for thicknesses in the range of 30–50 nm of the polymer layer. Charge separation persists well into the microsecond region.

Opto-Electric Molecules Studied by Time-Resolved Microwave Conductivity (Trmc)

Abstract Charge separation in the singlet and triplet excited states of donor-spacer-acceptor, DSA, molecules can be studied using the flash-photolysis time-resolved microwave conductivity (TRMC) technique. Highly dipolar, DSA states with microsecond lifetimes can result from Intersystem crossing and through-bond coupling for π-bond conjugated and sigma-bond separated donoracceptor compounds respectively.