Synthesis of Unsymmetrical Derivatives of Pentacene for Materials Applications

Pentacene shows unique electronic properties that have long been appreciated and exploited. Over the past 20 years, new synthetic schemes have been developed to address some of the problems encountered with pristine pentacene (e.g., stability and solubility), and pentacene derivatives have become a mainstay in the realm of organic semiconductors in applications such as organic light-emitting diodes, organic field-effect transistors (OFETs), and organic photovoltaics. At the onset of our work, the vast majority of known pentacene derivatives featured a symmetrical structure, often as the result of synthetic protocols that rely on nucleophilic additions to 6,13-pentacenequinone (PQ). The assembly of pentacenes featuring an unsymmetrical framework held great appeal, but the stepwise formation of derivatives, in which a specific function might be incorporated through each individual addition step, did not exist. This Account presents contributions from our lab and others to the synthesis and study of unsymmetrical pentacene derivatives. PQ offers an ideal platform for desymmetrization through the sequential addition of nucleophiles to each of the two ketone groups. Addition can be completed in a one-pot protocol, or through individual steps in which the product of the first addition is isolated and used as a precursor in the divergent synthesis of a series of structurally related molecules. This general approach has been used to assemble pentacene derivatives appended with alkynyl/aryl/alkyl groups, polarized frameworks via substitution with donor and/or acceptor groups, and conjugated oligomers linked by butadiynyl moieties. Stepwise substitution also provides derivatives with remarkable functionality, including pentacene-porphyrin dyads, pendent TEMPO free radicals, cyanoacrylic acid anchor groups (for incorporation into dye-sensitized solar cells), and derivatives with ambipolar behavior for OFET devices. The study of intramolecular singlet fission (iSF) has emerged as one of the most fruitful applications of unsymmetrical pentacene derivatives. SF involves the spontaneous splitting of a photoexcited singlet state (S₁) in one chromophore into a pair of triplets (T₁) shared with a neighboring chromophore. Pentacene derivatives are particularly well suited for this since E(S₁) ≥ 2E(T₁) satisfies the thermodynamic requirements for SF, and they have the additional feature that two chromophores can be tethered together by a "spacer" that allows spectroscopic studies of iSF to be done in dilute solution. From a synthetic perspective, the major advantage of the dimeric structure is the ability to modify the spacer, which allows for control over the distance, geometric relationship, and electronic coupling between the two pentacene groups. Dimeric pentacenes are central to providing an in-depth understanding of the molecular mechanism of SF, often providing advances not possible from measurements in the solid state.