Tykwinski Rik R
Department of Chemistry , University of Alberta , Edmonton , Alberta T6G 2G2 , Canada.
Acc Chem Res. 2019 Aug 20;52(8):2056-2069. doi: 10.1021/acs.accounts.9b00216. Epub 2019 Jul 16.
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 (). 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. 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 (S) in one chromophore into a pair of triplets (T) shared with a neighboring chromophore. Pentacene derivatives are particularly well suited for this since (S) ≥ 2(T) 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.
并五苯具有独特的电子特性，长期以来一直受到重视和利用。在过去20年里，人们开发了新的合成方案来解决原始并五苯遇到的一些问题（如稳定性和溶解性），并五苯衍生物已成为有机半导体领域在有机发光二极管、有机场效应晶体管（OFET）和有机光伏等应用中的中流砥柱。在我们开展工作之初，绝大多数已知的并五苯衍生物都具有对称结构，这通常是依赖于对6,13 - 并五苯醌进行亲核加成的合成方案的结果。具有不对称骨架的并五苯的组装极具吸引力，但不存在通过逐步形成衍生物（其中特定功能可通过每个单独的加成步骤引入）的方法。本综述介绍了我们实验室和其他研究团队在不对称并五苯衍生物的合成与研究方面所做的贡献。通过依次向两个酮基中的每一个添加亲核试剂，为去对称化提供了一个理想的平台。加成可以通过一锅法完成，也可以通过单独的步骤完成，其中第一次加成的产物被分离出来，并用作一系列结构相关分子的发散合成中的前体。这种通用方法已被用于组装连接有炔基/芳基/烷基的并五苯衍生物、通过供体和/或受体基团取代形成的极化骨架以及由丁二炔基部分连接的共轭低聚物。逐步取代还为衍生物提供了显著的功能，包括并五苯 - 卟啉二元体系、悬挂的TEMPO自由基、氰基丙烯酸锚定基团（用于纳入染料敏化太阳能电池）以及具有双极性行为的OFET器件衍生物。分子内单线态裂变（iSF）的研究已成为不对称并五苯衍生物最有成效的应用之一。SF涉及一个发色团中光激发单线态（S）自发分裂为与相邻发色团共享的一对三线态（T）。并五苯衍生物特别适合于此，因为（S）≥2（T）满足SF的热力学要求，并且它们具有额外的特点，即两个发色团可以通过一个“间隔基团”连接在一起，这使得可以在稀溶液中进行iSF的光谱研究。从合成的角度来看，二聚体结构的主要优点是能够修饰间隔基团，从而可以控制两个并五苯基团之间的距离、几何关系和电子耦合。二聚体并五苯对于深入理解SF的分子机制至关重要，常常能提供在固态测量中无法实现的进展。
