Theoretical Estimation of Donor Strength of Common Conjugated Units for Organic Electronics.

Donor strength of commonly used conjugated building blocks in organic electronics were investigated with density functional theory. The donor molecules were coupled to thiophene-incorporated acceptor groups, and electronic structure calculations were performed for the energies of frontier orbitals, total charge on donors, and particle probability distribution. A novel approach is also developed to analyze the large set of data on frontier orbitals. The electron donating ability of a donor was determined by comparing the highest occupied molecular orbital energies of the calculated structures. The effect of selected acceptor group and chosen functional method were also investigated to accurately determine the donor strength of each conjugated building block. Ethylenedioxythiophene, propylenedioxythiophene, and triphenylamine derivatives were found to be best donors among the conjugated units investigated. Such a comparative analysis of donor strengths is believed to be useful for researchers in designing novel organic semiconductors for organic electronic applications.

Theoretical study of torsional disorder in poly(3-alkylthiophene) single chains: intramolecular charge-transfer character and implications for photovoltaic properties.

The role of polymer chain morphology on the optoelectronic properties of polythiophenes is an ongoing investigation with the promise of improving organic photovoltaic performance. Chain morphology is predominantly affected by torsional disorder, which causes localization of holes and electrons in the conjugated backbone. Using the model compound oligo(3-methylthiophene), torsionally disordered oligomers were created to compare with a trans-planar oligomer such as found in crystalline poly(3-hexylthiophene). Low lying electronic excitations are calculated using TD-HF and TD-DFT with various long-range corrected functionals. Probability densities of electron and hole were constructed from natural transition orbitals, giving insight into localization and electron-hole overlap. Overlap is found to be substantially higher in disordered oligomers, indicating a stronger Coulombic interaction between electron and hole. Results suggest that improved photovoltaic performance with increased crystallinity is partially explained by stronger light absorption in crystalline polymers and a higher barrier to charge separation in disordered polymers.

Evaluation of acceptor strength in thiophene coupled donor-acceptor chromophores for optimal design of organic photovoltaic materials.

A series of thiophene coupled acceptors were systematically investigated at the density functional theory level to reveal structure-property relationships for building blocks of materials used in organic photovoltaic applications. All of the acceptor groups studied in this work retain their aromaticity when coupled to thiophene groups as estimated from their aromatic stabilization energies. However, pure chains of acceptors may adopt quinoidal geometry along the conjugated backbone depending on the structure of interest. Spearman rank order correlation has been used to assess the relationships between the computed variables such as highest occupied molecular orbital, lowest unoccupied molecular orbital, E(g), oscillator strength, exciton binding energy, aromatic stabilization energy, etc. The relative acceptor strengths were plotted and electrostatic potential maps were generated to examine the charge distribution over the chromophores. It has been found that there is no correlation between acceptor strength and electron withdrawing ability of the acceptor. Electron rich and highly electronegative atoms within acceptor groups mainly affect the charge distribution over the acceptor geometry. Exciton binding energy increases with the increasing aromatic character of the acceptor group. The acceptor strength is inversely correlated with the oscillator strength for the lowest excited state transition.

An activated scheme for resonance energy transfer in conjugated materials.

Energy transfer mechanism in conjugated materials has been demonstrated with an activated expression, which is equivalent to Fermi's golden rule. Spectral overlap integrals obtained from simulated spectra of model chromophores agree very well with the results obtained with the activated formula. Although this approach works best for chromophores with spectral profiles resembling a Gaussian distribution, the activated expression formula also performs quite well for chromophores with vibronically resolved spectra. Activation energies for exciton hopping can also be predicted using a phonon coupled exciton relaxation scheme. The accuracy of predictions with this new approach is quite attractive and hence should allow practical applications.

Charge transport simulations in conjugated dendrimers.

We present here a theoretical methodology that exploits quantum mechanical calculations, molecular mechanics calculations, and Monte Carlo simulations to predict the time-of-flight measurement mobilities in films of phenyl-cored conjugated thiophene dendrimers. Our aim is to reveal structure-property relationships in amorphous films of organic pi-conjugated materials. The simulations show that both hole and electron mobilities increase with the size of dendrimer, and that the former is larger than latter in all dendrimers. Internal reorganization energies are inversely correlated with the mobilities. Our simulations also indicate that dendrimers have small density of states for energetic disorder (<60 meV), and both hole and electron mobilities possess weak electric field dependence. We examine the influence of external reorganization energy as well as the possible trap sites on charge transport in these materials.

Exciton migration in conjugated dendrimers: a joint experimental and theoretical study.

We report a joint experimental and theoretical investigation of exciton diffusion in phenyl-cored thiophene dendrimers. Experimental exciton diffusion lengths of the dendrimers vary between 8 and 17 nm, increasing with the size of the dendrimer. A theoretical methodology is developed to estimate exciton diffusion lengths for conjugated small molecules in a simulated amorphous film. The theoretical approach exploits Fermi's Golden Rule to estimate the energy transfer rates for a large ensemble of bimolecular complexes in random relative orientations. Utilization of Poisson's equation in the evaluation of the Coulomb integral leads to very efficient calculation of excitonic couplings between the donor and the acceptor chromophores. Electronic coupling calculations with delocalized transition densities revealed efficient coupling pathways in the bulk of the material, but do not result in strong couplings between the chromophores which are calculated for more localized transition densities. The molecular structures of dendrimers seem to be playing a significant role in the magnitude of electronic coupling between chromophores. Simulated diffusion lengths correlate well with the experimental data. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors in determining the size of the exciton diffusion length in amorphous films of conjugated materials.

Pyrophosphate Sensor Based on Principal Component Analysis of Conjugated Polyelectrolyte Fluorescence.

The pyrophosphate anion (PPi) plays an important role in biochemical processes. Therefore, a simple but reliable analytical technique is essential for selective detection of PPi in biochemical systems. Here, we present a principal component analysis (PCA) method for analytical determination of PPi concentration using a fluorescent conjugated polyelectrolyte (CPE) combined with a polyamine modifier. The CPE has anionic side chains and dissolves molecularly in water, as indicated by its structured fluorescence emission spectrum. However, addition of tris(3-aminoethyl)amine (tetraamine or N4) quenches the CPE fluorescence emission. Tetraamine, which is a polycation at neutral pH, binds multiple anionic CPE chains, leading to aggregate formation, resulting in aggregation-induced fluorescence quenching. Addition of PPi to the polymer-amine aggregate reverses the process, resulting in fluorescence recovery. The relatively higher concentration of PPi compared to that of the polymer allows it to effectively compete to bind the amine, thus releasing molecularly dissolved polymer chains. Fluorescence correlation spectroscopy of the P1/N4 complex and of P1/N4/PPi confirms the change in size of the CPE aggregates that occurs upon reversible aggregation. Application of PCA to the fluorescence emission data set of standard samples yields two principal components, which are used to create a predictive model for PPi analysis. The PCA method is able to directly determine the concentration of PPi with approximately 95% accuracy within the concentration range from 100 µM to 3 mM, without the need for a reference state as is typically needed for ratiometric fluorescence assays.

Intramolecular triplet energy transfer in anthracene-based platinum acetylide oligomers.

Platinum acetylide oligomers that contain an anthracene moiety have been synthesized and subjected to photophysical characterization. Spectroscopic measurement and DFT calculations reveal that both the singlet and triplet energy levels of the anthracene segment are lower than those of the platinum acetylide segment. Thus, the platinum acetylide segment acts as a sensitizer to populate the triplet state of the anthrancene segment via intramolecular triplet-triplet energy transfer. The objective of this work is to understand the mechanisms of energy-transfer dynamics in these systems. Fluorescence quenching and the dominant triplet absorption that arises from the anthracene segment in the transient absorption spectrum of Pt4An give clear evidence that energy transfer adopts an indirect mechanism, which begins with singlet-triplet energy transfer from the anthracene segment to the platinum acetylide segment followed by triplet-triplet energy transfer to the anthracene segment.

Effective solubilization of single-walled carbon nanotubes in THF using PEGylated corannulene dispersant.

PEG-derivatized corannulene compound has been found to be very effective in solubilizing single-walled carbon nanotubes in tetrahydrofuran. Solubilizing efficiency is close to the commonly used anionic surfactant, sodium dodecyl sulfate (SDS). Corannulene derivative has also been found to have a tendency to disperse metallic nanotubes more effectively than the SDS counterpart. Theoretical calculations predict higher dispersion interactions of corannulene backbone with the convex surface of nanotubes in comparison to those calculated with other commonly used polyaromatic hydrocarbon derivatives.

Theoretical studies on conjugated phenyl-cored thiophene dendrimers for photovoltaic applications.

Pi-conjugated dendrimers are an important class of materials for optoelectronic devices, especially for light-harvesting systems. We report here a theoretical investigation of the optical response and of the excited-state properties of three-arm and four-arm phenyl-cored dendrimers for photovoltaic applications. A variety of theoretical methods are used and evaluated against each other to calculate vertical transition energies, absorption and excitation spectra with vibronic structure, charge transport, and excitonic behavior upon photoexcitation and photoemission processes. Photophysical phenomena in these dendrimers are, in general, better explained with ab initio methods rather than with semiempirical techniques. Calculated reorganization energies were found to correlate well with the device photocurrent data where available. The excitons formed during photoexcitation are calculated to be more delocalized than the ones formed after vibrational relaxation in the excited states for fluorescence emission. The localization of excitons in emission processes is a result of geometrical changes in the excited state coupled with vibronic modes. Correlated electron-hole pair diagrams illustrate breaking of pi-conjugation in three-arm dendrimers due to meta linkage of arms with the core, whereas four-arm dendrimers are not affected by such breaking due to presence of ortho and para branching. Yet, ortho branching causes large twist angles between the core and the arms that are detrimental to pi-electron system delocalization over the structure.

Photophysics of platinum-acetylide substituted hexa-peri-hexabenzocoronenes.

This manuscript reports the synthesis and photophysical investigation of two hexa-peri-hexabenzocoronenes (HBCs) that are functionalized with platinum(II) acetylide units of the type trans-(Ar-CC-)2Pt(PBu3)2. In one complex, the platinum is directly linked to the HBC chromophore by an ethynyl spacer, whereas in the second, the platinum is separated from the HBC via a 1,4-phenylene ethynylene spacer. The Pt-acetylide units introduce strong spin-orbit coupling into the HBC chromophore, giving rise to high yields of the triplet excited state along with moderately intense phosphorescence at ambient temperature. On the basis of emission spectroscopy, the triplet state of the HBC chromophore is located at 2.14 eV and the S-T splitting is 0.6 eV. The triplet-triplet absorption and radical cation absorption spectra of the Pt-HBCs are determined by laser flash photolysis. Aggregation of the Pt-HBCs in a poor solvent such as hexane leads to quenching of the triplet state, but spectroscopy provides no evidence for the formation of a triplet excimer, even under conditions where the molecules are strongly aggregated.

Morphology and oxygen sensor response of luminescent Ir-labeled poly(dimethylsiloxane)/polystyrene polymer blend films.

Polymer films consisting of a linear poly(dimethylsiloxane) end-functionalized with a luminescent Ir(III) complex (Ir-PDMS), blended with polystyrene (PS), function as optical oxygen sensors. The sensor response arises by quenching of the luminescence from the Ir(III) chromophore by oxygen that permeates into the polymer film. The morphology and luminescence oxygen sensor properties of blend films consisting of Ir-PDMS and PS have been characterized by fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. The investigations demonstrate that microscale phase segregation occurs in the films. In blends that contain a relatively small amount of Ir-PDMS in PS (ca. 10 wt %), the Ir-PDMS exists as circular domains, with diameters ranging from 2 to 5 mum, surrounded by the majority PS phase. For larger weight fractions of Ir-PDMS in the blends, the film morphology becomes bicontinuous. A novel epifluorescence microscopy method is applied that allows the construction of Stern-Volmer quenching images that quantify the oxygen sensor response of the blend films with micrometer spatial resolution. These images provide a map of the oxygen permeability of the polymer blend films with a spatial resolution of ca. 1 mum. The results of this investigation show that the micrometer-sized Ir-PMDS domains display a 2-3-fold higher oxygen sensor response compared to the surrounding PS matrix. This result is consistent with the fact that PDMS is considerably more gas permeable compared to PS. The relationship of the microscale morphology of the blends to their performance as macroscale optical oxygen sensors is discussed.