Predicting Emission Wavelengths in Benzobisoxazole-Based OLEDs with Gradient Boosted Ensemble Models.

We demonstrate the use of gradient-boosted ensemble models that accurately predict emission wavelengths in benzobis[1,2-d:4,5-d']oxazole (BBO) based fluorescent emitters. We have curated a database of 50 molecules from previously published data by the Jeffries-EL group using density functional theory (DFT) computed ground and excited state features. We consider two machine learning (ML) models based on (i) whole cruciform molecules and (ii) their constituent fragment molecules. Both ML models provide accurate predictions with root-mean-square errors between 30 and 36 nm, competitive with state-of-the-art deep learning models trained on orders of magnitude more molecules, and this accuracy holds even when tested on four new BBO emitters unseen by the models. We also provide an interpretable feature importance analysis and discuss the relevant relationships between DFT and changes in predicted emission wavelength.

Evaluating the Role of Molecular Heredity in the Optical and Electronic Properties of Cross-Conjugated Benzo[1,2-d:4,5-d']bisoxazoles.

A series of eight benzo[1,2-d:4,5-d']bisoxazole (BBOs) were synthesized using the heredity principle as a design motif, whereby we investigated which characteristics of the linear parents were inherited by their cross-conjugated children. Four linear parents bearing 4-tert-butylbenzene (P) or 1,3-bis(4-tert-butylphenyl)benzene (M) at either the 2,6- or 4,8-position on the BBO and four cross-conjugated children bearing various combinations of the two isoelectronic aryl substituents were evaluated. Due to the bulky nature of the M substituent compared to that of the P substituent, the influence of steric hindrance along the BBO axes was explored theoretically and experimentally. The optical and electronic properties of each molecule were investigated in the solution and solid state using density functional theory (DFT) and time-dependent DFT (TD-DFT) and characterized using ultraviolet photoelectron spectroscopy (UPS), ultraviolet-visible (UV-vis) spectroscopy, and photoluminescence (PL) spectroscopy. The well-correlated theoretical and experimental results showed that the selective tuning of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels was possible through the strategic placement of substituents without impacting the H → L transition energy. Specifically, the theoretical results demonstrated that for the BBO children the HOMO and LUMO energy levels were inherited from the 4,8- and 2,6-parents, respectively. Each molecule was found to exhibit emission maxima ≤451 nm, making them ideal candidates for blue organic light-emitting diode (OLED) materials.

Evaluating the Impact of Fluorination on the Electro-optical Properties of Cross-Conjugated Benzobisoxazoles.

Six 2,4,6,8-tetrarylbenzo[1,2- d:4,5- d']bisoxazoles (BBOs) were synthesized: three bearing phenyl substituents at the 2- and 6-positions and three bearing perfluorophenyl groups at those positions. The influence of perfluoro-aryl group substitution on the physical, optical, and electronic properties of 2,4,6,8-tetrarylbenzo[1,2- d:4,5- d']bisoxazoles (BBO) was evaluated using both experimental and theoretical methods. The density functional theory (DFT) model was found to be well-matched to the experimental optical data, as evidenced by the UV-vis spectra. Both cyclic voltammetry (CV) and ultraviolet photoelectron spectroscopy (UPS) were used to determine the position of the HOMO with varying results. The values obtained by CV were deeper than those obtained via UPS and correlated well with the theoretical calculations. However, the UPS values were more consistent with the expected outcomes for a system with segregated frontier molecular orbitals (FMOs). The UPS results are also supported by the electrostatic potential maps, which indicate that the electron density within the LUMO and HOMO is nearly completely localized along the 2,6- or 4,8-axis, respectively. The summation of the results indicates that strongly electron-withdrawing groups can be used to selectively tune the LUMO level with minor perturbation of the HOMO, something that is challenging to accomplish in typical donor-acceptor systems.

A versatile and efficient synthesis of bithiophene-based dicarboxaldehydes from a common synthon.

Bithiophene dicarboxaldehydes are promising electron-rich building blocks for the development of arylene vinylene-based organic semiconductors, but their use has been limited due to their synthetic inaccessibility. To facilitate the facile synthesis of these compounds we have prepared a novel functional bithiophene, namely 2,2'-(3,3'-dibromo-[2,2'-bithiophene]-5,5'-diyl)bis(5,5-dimethyl-1,3-dioxane) in two high yielding steps from 3,3',5,5'-tetrabromo-2,2'-bithiophene. This synthon is readily transformed into variety of bithiophene-based dicarboxaldehydes, also in high yields. The use of these functional molecules in the synthesis of arylene vinylene-linked donor-acceptor copolymers is demonstrated by the synthesis of two copolymers with electron deficient benzobisazoles.

Influence of conjugation axis on the optical and electronic properties of aryl-substituted benzobisoxazoles.

Six different 2,6-diethyl-4,8-diarylbenzo[1,2-d:4,5-d']bis(oxazoles) and four different 2,4,6,8-tetraarylbenzobisoxazoles were synthesized in two steps: a Lewis acid catalyzed orthoester cyclization followed by a Suzuki or Stille cross-coupling with various arenes. The influence of aryl group substitution and/or conjugation axis variation on the optical and electronic properties of these benzobis(oxazole) (BBO) compounds was evaluated. Structural modifications could be used to alter the HOMO, LUMO, and band gap over a range of 1.0, 0.5, and 0.5 eV, respectively. However, depending on the location and identity of the substituent, the HOMO level can be altered without significantly impacting the LUMO level. This is supported by the calculated frontier molecular orbitals. Our results indicate that the FMOs and band gaps of benzobisoxazoles can be readily modified either jointly or individually.

Synthesis of 3,7-diiodo-2,6-di(thiophen-2-yl)benzo[1,2-b:4,5-b']difurans: functional building blocks for the design of new conjugated polymers.

3,7-Diiodo-2,6-di(thiophen-2-yl)benzo[1,2-b:4,5-b']difurans are efficiently prepared by an iodine-promoted double cyclization. This new heterocyclic core is readily modified by the attachment of alkyl chains for improved solubility. The use of these compounds for the synthesis of new conjugated polymers is also reported.

Tuning the optical and electronic properties of 4,8-disubstituted benzobisoxazoles via alkyne substitution.

In an effort to design new electron-deficient building blocks for the synthesis of conjugated materials, a series of new trans-benzobisoxazoles bearing halogen or alkynyl substituents at the 4,8-positions was synthesized. Additionally, the impact of these modifications on the optical and electronic properties was investigated. Theoretical calculations predicted that the incorporation of various alkynes can be used to tune the energy levels and band gaps of these small molecules. The targeted 4,8-disubstituted benzobisoxazoles were easily prepared in good yields using a two-step reaction sequence: Lewis acid catalyzed orthoester cyclization followed by Sonogashira cross-coupling. The experimentally determined HOMO values for these 4,8-disubstituted benzobisoxazoles ranged from -4.97 to -6.20 eV and showed reasonable correlation to the theoretically predicted values, with a percent deviation that ranged from 2.4-12.8%. However, the deviation between actual and predicted HOMO values was reduced to less than 3.5% when the theoretical values were extrapolated to the long-chain limit and compared to copolymers containing the 4,8-disubstituted benzobisoxazoles. Collectively, these results indicate that these 4,8-disubstituted trans-benzobisoxazoles can be used for the synthesis of new conjugated materials with electronic properties that are variable and predictable.

Synthesis, characterization and photovoltaic properties of poly(thiophenevinylene-alt-benzobisoxazole)s.

Herein we report the synthesis of two solution processible, conjugated polymers containing the benzobisoxazole moiety. The polymers were characterized using (1)H NMR, UV-Vis and fluorescence spectroscopy. Thermal gravimetric analysis shows that the polymers do not exhibit significant weight loss until approximately 300 °C under nitrogen. Cyclic voltammetry shows that the polymers have reversible reduction waves with estimated LUMO levels at -3.02 and -3.10 eV relative to vacuum and optical bandgaps of 2.04-2.17 eV. Devices based on blends of the copolymers and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) exhibited modest power conversion efficiencies. Theoretical models reveal that there is poor electron delocalization along the polymer backbone, leading to poor performance. However, the energy levels of these polymers indicate that the incorporation of benzobisoxazoles into the polymer backbone is a promising strategy for the synthesis of new materials.

Facile synthesis of 2,6-disubstituted benzobisthiazoles: functional monomers for the design of organic semiconductors.

The synthesis of several synthetically useful 2,6-disubstituted benzobisthiazoles is described. The method is based on the Lewis acid-catalyzed ring-closing reaction between substituted orthoesters and diamino benzene dithiol. The resulting benzobisthiazoles are obtained cleanly and in good yields. These materials are of interest for the development of new organic semiconductors.

An efficient synthesis of 2,6-disubstituted benzobisoxazoles: new building blocks for organic semiconductors.

2,6-Disubstituted benzobisoxazoles have been synthesized by a highly efficient reaction of diaminobenzene diols with various orthoesters. The scope of this new reaction for the synthesis of substituted benzobisoxazoles has been investigated using four different orthoesters. The utility of these compounds as building blocks for the synthesis of conjugated polymers is demonstrated.

Organic-inorganic nanocomposites via directly grafting conjugated polymers onto quantum dots.

Nanocomposites of poly(3-hexylthiophene)-cadmium selenide (P3HT-CdSe) were synthesized by directly grafting vinyl-terminated P3HT onto [(4-bromophenyl)methyl]dioctylphosphine oxide (DOPO-Br)-functionalized CdSe quantum dot (QD) surfaces via a mild palladium-catalyzed Heck coupling, thereby dispensing with the need for ligand exchange chemistry. The resulting P3HT-CdSe nanocomposites possess a well-defined interface, thus significantly promoting the dispersion of CdSe within the P3HT matrix and facilitating the electronic interaction between these two components. The photophysical properties of nanocomposites were found to differ from the conventional composites in which P3HT and CdSe QDs were physically mixed. Solid-state emission spectra of nanocomposites suggested the charge transfer from P3HT to CdSe QDs, while the energy transfer from 3.5 nm CdSe QD to P3HT was implicated in the P3HT/CdSe composites. A faster decay in lifetime further confirmed the occurrence of charge transfer in P3HT-CdSe nanocomposites.

Self-assembled monolayers of polythiophene conductive polymers improve biocompatibility and electrical impedance of neural electrodes.

There is continued interest in the development of conductive polymer coatings to improve the electrical properties and biocompatibility of electrodes for neural prostheses. We present here a new type of coating, based on mixed self-assembled monolayers (SAMs) of thiolated poly(alkylthiophene)s and functionalized alkanethiols. When assembled as a SAM on electrodes designed for in vitro electrophysiology, these polymers are able to significantly lower electrode impedance at 1 kHz. The same mixed formulation is able to promote the outgrowth of neurites from primary mouse cortical neurons when the alkanethiol component is functionalized with a neural cell adhesion molecule (NCAM) binding antibody. Atomic force microscopy of the SAMs shows that they exert their effect through the well-known mechanism of increasing electrode surface area. These new covalently bound films have the potential to be more robust and are more controllable in their composition than existing electrodeposited conductive polymer coatings.

Volatile organic compound detection using nanostructured copolymers.

Regioregular polythiophene-based conductive copolymers with highly crystalline nanostructures are shown to hold considerable promise as the active layer in volatile organic compound (VOC) chemresistor sensors. While the regioregular polythiophene polymer chain provides a charge conduction path, its chemical sensing selectivity and sensitivity can be altered either by incorporating a second polymer to form a block copolymer or by making a random copolymer of polythiophene with different alkyl side chains. The copolymers were exposed to a variety of VOC vapors, and the electrical conductivity of these copolymers increased or decreased depending upon the polymer composition and the specific analytes. Measurements were made at room temperature, and the responses were found to be fast and appeared to be completely reversible. Using various copolymers of polythiophene in a sensor array can provide much better discrimination to various analytes than existing solid state sensors. Our data strongly indicate that several sensing mechanisms are at play simultaneously, and we briefly discuss some of them.

Nanostructure dependence of field-effect mobility in regioregular poly(3-hexylthiophene) thin film field effect transistors.

Low polydispersity regioregular polythiophenes with number average molecular weights ranging from 2 to 13 kDa were cast under the same conditions from solution to form a series of field effect transistors (FETs). Tapping mode AFM and grazing incidence small-angle X-ray scattering revealed that in all cases the polymers formed regular nanofibrillar morphologies with the width of nanofibrils proportional to the weight average contour length of polymer chains, indicating that conjugated backbones were oriented perpendicular to the nanofibril axes. FET charge carrier mobilities exhibited exponential dependence on nanofibril width, pointing to the decisive role of extended conjugated pathways in charge transport.