The progress of pentiptycene chemistry is reviewed. Pentiptycene belongs to the iptycene family and possesses a rigid, aromatic, and H-shaped scaffold. An important feature for pentiptycene vs. triptycene is the presence of a 'sterically shielded' central benzene ring. Such a feature has led to the use of pentiptycene as a conformational regulator and in the formation of functional molecules, including fluorescent chemosensors, molecular machines, low dielectric constant materials, and porous solids. The synthesis of these materials relies on central-ring prefunctionalized pentiptycene building blocks. A useful approach toward the preparation of these building blocks is the derivatization of pentiptycene quinone.

Conjugated polymers (CPs) have attracted substantial attention because of their high potential as materials for optoelectronic devices such as light-emitting diodes and photovoltaic cells. [1][2][3] To engineer systems with specific electronic properties, a detailed understanding of the structure-property relationships of CPs is essential. However, the poorly defined chain length, chain conformation, and structural defects, as well as the multiple-chromophoric nature [4] of CPs often complicate data analysis and thus make any further investigation difficult. In this context, complementary evidence from the corresponding monodisperse single-chromophorebased conjugated oligomers might offer a solution. We report herein an intriguing example of the concept of torsioninduced excitation localization.Torsion along the conjugated backbone of CPs is known to have a dramatic impact on optical bandgap and electron mobility because of a decrease in conjugation interactions. [5, 6] It is also widely accepted that a flexible CP chain can be considered as an ensemble of chromophores of different lengths because of the conformational disorder of large torsion angles, [7] and, consequently, excitation delocalization is dominated by resonance energy transfer (ET; Förster-type energy transfer or hopping) among the chromophores. [8] Recent quantum chemical calculations further revealed that a medium torsion angle ( 508) is sufficient to confine the excitation within a short subunit of CPs.[9] Nevertheless, the concept of torsion-induced localization of excitation and the resulting intrachain ET have not yet been verified with structurally well-defined conjugated oligomers, presumably because of the difficulties in conformational control [10] and in prevention of excited-state torsional relaxation [11] for the twisted conformers.Our recent work [12,13] on the pentiptycene-derived oligo(p-phenyleneethynylene)s (OPEs) 1 and 2 have revealed that these systems are excellent candidates for investigation of the concept of torsion-induced localization of excitation. Two distinct fluorescing states, which correspond to excited conformers of small and large torsion angles, have been identified for both 1 and 2. To simplify the discussion, we refer to the two fluorescing states as the planar (P) and the twisted (T) excited states, respectively. The planar excited state can be generated by selective excitation of the planar conformers, which dominate the longer-wavelength region of the absorption profile, or through torsional relaxation of a twisted Franck-Condon excited state at room temperature. The twisted excited state can be generated and captured by selective excitation of the twisted conformers with shortwavelength light (e.g., 302 nm) in a 2-methyltetrahydrofuran (MTHF) glass at 80 K. This procedure relies not only on the neighboring noncovalent pentiptycene-pentiptycene interactions that favor the twisted form at low temperatures, but also on the large resistance to rotation of the pentiptycene groups in a frozen glass. One partic...

The synthesis of central-ring halogenated pentiptycene phenols 12-14 and dihalogenated pentiptycenes 15-17 is reported. Their utilities as building blocks for preparing new pentiptycene-derived pi-conjugated systems through the Sonogashira, Heck, and Suzuki reactions are also demonstrated.

The nucleophilic aromatic substitution (S(N)Ar) reactions of the nitro-substituted pentiptycene triflate 15 with LiBr and LiI and the resulting halides 18 and 19 with N(3)(-), CN(-), and ArS(-) in DMF provide an efficient route toward pentiptycene halides and dihalides and other new pentiptycene building blocks. The reactivity of diaminopentiptycene in Pd-catalyzed C-N coupling reactions is also demonstrated.