Glen P. Miller

Substituent Effects in Pentacenes: Gaining Control over HOMO−LUMO Gaps and Photooxidative Resistances

A combined experimental and computational study of a series of substituted pentacenes including halogenated, phenylated, silylethynylated and thiolated derivatives is presented. Experimental studies include the synthesis and characterization of six new and six known pentacene derivatives and a kinetic study of each derivative under identical photooxidative conditions. Structures, HOMO-LUMO energies and associated gaps were calculated at the B3LYP/6-311+G**//PM3 level while optical and electrochemical HOMO-LUMO gaps were measured experimentally. The combined results provide for the first time a quantitative assessment of HOMO-LUMO gaps and photooxidative resistances for a large series of pentacene derivatives as a function of substituents. The persistence of each pentacene derivative is impacted by a combination of steric resistance and electronic effects as well as the positional location of each substituent. Silylethynyl-substituted pentacenes like TIPS-pentacene possess small HOMO-LUMO gaps but are not the longest lived species under photooxidative conditions, contrary to popular perception. A pentacene derivative with both chlorine substituents in the 2,3,9,10 positions and o-alkylphenyl substituents in the 6,13 positions is longer lived than TIPS-pentacene. Of all the derivatives studied, alkylthio- and arylthio-substituted pentacenes are most resistant to photooxidation, possess relatively small HOMO-LUMO gaps and are highly soluble in a variety of organic solvents. These results have broad implications for the field of organic molecular electronics where OFET, OLED, and other applications can benefit from highly persistent, solution processable pentacene derivatives.

Design, Synthesis, and Characterization of a Persistent Nonacene Derivative

A significant technical barrier (i.e., facile oxidative degradation) that has prevented the preparation of large acenes has now been breached. Using a combination of experimentally and theoretically derived substituent effects, the design, synthesis, isolation, and characterization of the first persistent nonacene derivative is described. The molecular design strategy includes placement of arylthio (or alkylthio) substituents on the terminal rings of the nonacene skeleton, effectively converting an open-shell singlet diradical into a closed-shell system. These powerful substituent effects appear to be suitable for the synthesis of other persistent, soluble, large acene derivatives required for advanced thin-film organic semiconductor applications.

Exploiting Substituent Effects for the Synthesis of a Photooxidatively Resistant Heptacene Derivative

Substituent effects have been exploited to produce an unusually persistent heptacene derivative. In total, four new heptacene derivatives with varying levels of photooxidative resistance (1 < 2 < 3 < 4) have been synthesized. A combination of p-(t-butyl)thiophenyl substituents at positions 7 and 16 (i.e., arylthio substituents attached to the most reactive ring) and o-dimethylphenyl substituents at positions 5, 9, 14, and 18 (i.e., steric resistance on neighboring rings) make heptacene derivative 4 especially resistant to photooxidation. It persists for weeks as a solid, for 1-2 days in solution if shielded from light, and for several hours in solution when directly exposed to both light and air. Heptacene derivative 4 has been fully characterized. It possesses a small HOMO-LUMO gap of 1.37 eV.

Self-assembly of long chain alkanes and their derivatives on graphite

We combine scanning tunneling microscopy (STM) measurements with ab initio calculations to study the self-assembly of long chain alkanes and related alcohol and carboxylic acid molecules on graphite. For each system, we identify the optimum adsorption geometry and explain the energetic origin of the domain formation observed in the STM images. Our results for the hierarchy of adsorbate-adsorbate and adsorbate-substrate interactions provide a quantitative basis to understand the ordering of long chain alkanes in self-assembled monolayers and ways to modify it using alcohol and acid functional groups.

Hydrogenation of Single-Wall Carbon Nanotubes Using Polyamine Reagents : Combined Experimental and Theoretical Study

We combine experimental observations with ab initio calculations to study the reversible hydrogenation of single-wall carbon nanotubes using high boiling polyamines as hydrogenation reagents. Our calculations characterize the nature of the adsorption bond and identify preferential adsorption geometries at different coverages. We find the barrier for sigmatropic rearrangement of chemisorbed hydrogen atoms to be approximately 1 eV, thus facilitating surface diffusion and formation of energetically favored, axially aligned adsorbate chains. Chemisorbed hydrogen modifies the structure and stability of nanotubes significantly and increases the inter-tube distance, thus explaining the improved dispersability in solvents like methanol, ethanol, chloroform, and benzene.

Open-Shell Singlet Character of Stable Derivatives of Nonacene, Hexacene and Teranthene

The electronic ground states of the recently synthesized stable nonacene derivatives (J. Am. Chem. Soc. 2010, 132, 1261) are open-shell singlets with a polyradical nature instead of closed-shell singlets as originally assumed, according to the unrestricted broken spin-symmetry density functional theory (UBS-DFT) computations (at B3LYP/6-31G*). It is the bulky protecting groups, not the transfer from the open-shell singlet to closed-shell singlet ground state, that stabilizes these longest characterized acenes. Similar analyses also confirmed the open-shell singlet character of the hexacene and teranthene derivatives.

Synthesis and Characterization of a C60-Pentacene Monoadduct

Abstract C60 reacts with 1 equivalent of pentacene to give a C60-pentacene monoadduct in 59% yield. lH and 13C NMR data confirm a C2V symmetric structure for the monoadduct consistent with C60 cycloaddition across central carbon atoms 6 and 13 of pentacene. Seven bispentacene adducts are formed in 13% overall yield.

An Improved Synthesis of Pentacene: Rapid Access to a Benchmark Organic Semiconductor

Pentacene is an organic semiconductor used in a variety of thin-film organic electronic devices. Although at least six separate syntheses of pentacene are known (two from dihydropentacenes, two from 6,13-pentacenedione and two from 6,13-dihydro-6,13-dihydroxypentacene), none is ideal and several utilize elevated temperatures that may facilitate the oxidation of pentacene as it is produced. Here, we present a fast (~2 min of reaction time), simple, high-yielding (≥90%), low temperature synthesis of pentacene from readily available 6,13-dihydro-6,13-dihydroxypentacene. Further, we discuss the mechanism of this highly efficient reaction. With this improved synthesis, researchers gain rapid, affordable access to high purity pentacene in excellent yield and without the need for a time consuming sublimation.

Hydrogen-protected acenes

The first systematic study concerning the hydrogenation of acenes and acenequinones is presented. Phenyl substituted acenes and acenequinones are hydrogenated in excellent yield and with complete regioselectivity using HI–AcOH. The resulting H-protected acenes bear alternating aromatic and non-aromatic rings and are stable, soluble molecules that may be stored indefinitely and then deprotected to afford the parent acenes. In this manner, H-protected acenes have been utilized in the syntheses of several [60]fullerene-acene adducts. Buckminsterfullerene also hydrogenates in HI–AcOH yielding C3v symmetric C60H18.

Fulleranes produced via efficient polyamine hydrogenations of [60]fullerene, [70]fullerene and giant fullerenes

All fullerenes including [60]fullerene, [70]fullerene and giant fullerenes are hydrogenated in excellent yield using polyamines like diethylenetriamine (DET), triethylenetetramine (TET), tetraethylenepentamine (TEP) or pentaethylenehexamine (PEH) at elevated temperatures. The resulting hydrogenated fullerenes or fulleranes have been characterized by a combination of NMR spectroscopy and mass spectrometry. The addition of elemental cobalt promotes formation of more highly hydrogenated fulleranes. Hydrogenation reaction times can be reduced to minutes using a microwave reactor. The mechanism of polyamine hydrogenation is discussed in terms of successive electron transfer–protonation cycles as evidenced by deuterium labeling studies.