Intramolecular Hydrogen-Bond-Assisted Planarization of Asymmetrically Functionalized Alternating Phenylene–Pyridinylene Copolymers Muthalagu Vetrichelvan [a] and Suresh Valiyaveettil* [a, b] Introduction Conjugated polymers have received much attention in the past two decades owing to their potential applications in light-emitting diodes, [1–7] thin-film transistors, [1–2] chemical sensors, [8] as well as electronic and photonic devices. [9–11] The optoelectronic properties of conjugated polymers vary sig- nificantly depending on the degree of extended conjugation between the consecutive repeating units and the inherent electron densities on the polymer backbone. [12–14] Among them, conjugated pyridine-incorporated polymers have at- tracted much attention owing to the possibility of fine- tuning the optical properties by means of 1) the electron-ac- cepting ability, 2) N-protonation, 3) N-oxidation, 4) N-alky- lation, and 5) metal complexation. [15–29] p-Conjugated poly- mers that incorporate aryl heterocyclic units, such as pyri- dine, pyrrole, and thiophene rings, have been synthesized and employed in industrial applications. [30–34] The ladder- type polymers incorporated with pyrazines, [35] benzothiad- azoles, [36] pyrroloimides, [37] pyridobisimidazoles, [38] and benzo- thiazole [39] that contain intramolecular hydrogen bonds along the polymer backbone are also known in the litera- ture. One of the most important features of the pyridine-in- corporated polymers is the high electron affinity of pyridine compared to phenylene-based polymers. [40] Furthermore, the use of a donor–acceptor system [41] and planarization of the polymer backbone by means of weak interactions (e.g., in- trachain hydrogen bonds) allow us to fine-tune the optical and conducting properties of the polymers. [21–22] Recently, a few reports have focused on the incorporation of pyridine into a conjugated polymer backbone. [27–29, 42–46] Chemical sensors based on conjugated polymers are inter- esting owing to their high sensitivity. [8] The synthesis of con- jugated polymers with bipyridine/phenanthroline moieties or crown ethers or a combination of both groups have al- ready been reported in the literature. [8, 15, 46] Our research ef- Abstract: We report on the synthesis and characterization of a series of asymmetrically functionalized amphi- philic polymers with alternating p- donor units (e.g., substituted benzene) and p-acceptor units (e.g., pyridine) along the polymer backbone. The pur- pose of our present work involves in- corporation of functional groups along the main chain to form intrachain hy- drogen bonds, which promote planari- zation of the polymer backbone, and to fine-tune the optical properties. The structure–property relationship of poly- mers P1–P6 was investigated by means of analytical methods, such as FTIR spectroscopy, 1 H and 13 C NMR spec- troscopy, UV/Vis spectroscopy, fluores- cence spectroscopy, gel permeation chromatography, thermogravimetric analysis, cyclic voltammetry, and X-ray powder diffraction. All polymers were soluble in common organic solvents, and the optical and fluorescence spec- tra of the polymers showed significant changes according to the formation (P4, P5) or absence (P6) of intramo- lecular hydrogen bonding along the polymer backbone. Moreover, the 2,6- or 3,5-linkage of the pyridine rings in P5 and P6, respectively, reduced the conjugation along the polymer back- bone and this is reflected in their opti- cal properties. The optical properties of the polymers were influenced by the addition of acid (P1–P6), base (P4– P6), and metal ions (e.g., Cu 2 + , Fe 3 + , Ag + , Ni 2 + , Pd 2 + , Mn 2 + , Zn 2 + , Mg 2 + , and Pr 3 + ). Such polymers could be used in various applications, including sensors and stimuli-responsive displays. Keywords: optical tuning · planari- zation · poly(paraphenylene) · pyri- dine · sensors · transition metals [a] Dr. M. Vetrichelvan, Prof. S. Valiyaveettil Singapore-MIT Alliance, 4 Engineering Drive 3 National University of Singapore—117 576 (Singapore) E-mail: chmsv@nus.edu.sg [b] Prof. S. Valiyaveettil Department of Chemistry, 3 Science Drive 3 National University of Singapore—117 543 (Singapore) Fax: (+ 65) 6779-1691 Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author. Chem. Eur. J. 0000, 00,0–0 DOI: 10.1002/chem.200500078  2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 FULL PAPER