Intrinsic Photophysics Research Articles

Interplay of device structure and intrinsic polymer photophysics and its effects on the ITO/PEDOT:PSS/MEH-PPV/Al photocurrent spectra

The dc photocurrent spectra of the indium tin oxide/poly(3,4-ethylene- dioxythiophene):poly(styrenesulfonate)/poly (2-methoxy-5-(2´-ethylhexyloxy)-1,4-phenylene- vinylene)Al (ITO/PEDOT:PSS/MEH-PPV/Al) device are measured for three different MEH-PPV thin-film thicknesses in a wide range of bias voltages and for several incident light intensities. The device operation is modeled based on a singlet exciton diffusion and hole polaron drift and diffusion. The Poole-Frenkel transport and the bimolecular Langevine recombination for hole polarons are assumed. The extrinsic photocarrier generation through singlet exciton dissociation on the electrodes and charge transfer in bulk of the polymer film is discussed. We considered three intrinsic charge-carrier photogeneration mechanisms: exciton-exciton annihilation, hot-exciton dissociation, and field dissociation. It is shown that the photocurrent is dominated by the field dissociation of singlet excitons. Experimental results compared to model predictions indicate that absorption coefficient of the MEH-PPV film is thickness dependent. This surprising result is experimentally investigated and confirmed. Excellent agreement between theory and experiment for all the measured photocurrent spectra is achieved.

Photophysics of 9,10-Anthracenediol and a Bifunctional Sacrificial Template in Solution and Xerogels

Site selectively templated and tagged xerogels (SSTTX) represent a new sensing platform. Although this platform has several attractive features, the template formation process is not fully understood. To address this issue we have explored the photophysics of two model compounds (9,10-anthracenediol and a bifunctional sacrificial template (BST)) when dissolved in solution and when sequestered within a xerogel. The solution experiments show that the carbamate tethers on the BST (which are eventually cleaved to form the analyte responsive sites that make up the SSTTX) do not alter the anthracene residue's intrinsic photophysics. In contrast, 9,10-anthracenediol and BST molecules sequestered within a xerogel sense and report from a distribution of microenvironments. The distribution mean values are very similar, but the variance is statistically greater for the BST-doped xerogel in comparison to the 9,10-anthracenediol-doped xerogel. The most likely causes of this behavior are heterogeneity and electron and energy transfer processes that are controlled by differences in the position/orientation of the anthracene moiety at the pore surface in the Class I (9,10-anthracenediol) and Class II (BST) xerogels. These results also suggest that the initial template sites produced during the SSTTX formation process are not discrete; they are intrinsically more diverse (maybe 30%) in comparison to the types of template sites created by traditional molecular imprinting strategies. However, our previously reported SSTTX binding studies do not reveal any evidence for a distribution of analyte-to-SSTTX binding. This apparently anomalous behavior may result because the relative standard deviation of the binding process is intrinsically small and/or one or more of the steps that follow template site formation attenuate the final template site distribution within the SSTTX.

Simulations of the Absorption and Fluorescence of Indole in Aqueous Solution and at a Nonpolar/Polar Interface

Theoretical results are presented on the absorption and fluorescence of indole in aqueous solution as well as at the air/water surface. We use a combined quantum chemical statistical mechanical model with explicit solvent. An approximate ab initio complete active space self-consistent field description of the indole molecule is used, coupled to a discrete polarizable water medium. From the bulk simulations, strong support is found for the interchange mechanism, which explains the unusual solvent shift of the fluorescence of indole or tryptophan in a polar surrounding by a solvent induced switch of the fluorescing state. Two mechanisms are given to explain the different shifts for indole at the interface. First, a dielectric depletion effect, which is expected from the reduction of the amount of polar media. Second, an interface-specific effect, which derives from the stronger hydrogen bond formation at the surface. The latter effect acts to increase the shift for both absorption and emission at the surface as compared to the bulk. From these results, the intrinsic probe photophysics of tryptophan in proteins is discussed in terms of the properties of the protein/solvent interface and the orientation of the amino acid.

Intrinsic photophysics of semiconductor nanocrystals in dielectric media: Formation of surface states

The photoluminescence of CdSe/ZnS nanocrystals in toluene at room temperature shows inhomogeneous emission and excitation spectra. This is accompanied by a non-exponential photoluminescence decay on time scales between 1 and 80 ns. The inhomogeneous behaviour is due to a remaining size distribution, which can be partly photoselected by a combination of excitation and emission wavelengths, which reveals the contribution of at least 4 different electronic states. The identification of these states became possible by the application of a decay time distribution analysis of the photoluminescence decay. In addition to the size-related CdSe exciton emission, two trap or surface states with decay components close to 10 and 80 ns have been identified. The 80 ns component is shifted to the red by about 100 meV from the exciton peak, whereas the 10 ns component cannot be discriminated spectrally from the exciton band.

Gas-Phase Studies of Chromophore Molecules in an Electrostatic Storage Ring

The use of an electrostatic storage ring for the study of chromophore ions in the gas phase is demonstrated. In this way the intrinsic photophysics and photochemistry of “naked” chromophore ions without the “disturbance” of a solvation shell is elucidated. The ring is combined with an electrospray ion source which enables the production of large molecular ions. Experiments described here involve absorption spectroscopy of protein chromophores and measurements of lifetimes for dissociation processes and excited states of nucleotide and protoporphyrin ions. Comparisons with solution phase experiments allow us to address the perturbation of a solvent or a protein environment on the electronic structure of chromophores.