Lash Miller Chemical Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada.
J Am Chem Soc. 2012 Jan 11;134(1):539-47. doi: 10.1021/ja208917m. Epub 2011 Dec 19.
We have synthesized a series of cyclopentadithiophene-benzochalcogenodiazole donor-acceptor (D-A) copolymers, wherein a single atom in the benzochalcogenodiazole unit is varied from sulfur to selenium to tellurium, which allows us to explicitly study sulfur to selenium to tellurium substitution in D-A copolymers for the first time. The synthesis of S- and Se-containing polymers is straightforward; however, Te-containing polymers must be prepared by postpolymerization single atom substitution. All of the polymers have the representative dual-band optical absorption profile, consisting of both a low- and high-energy optical transition. Optical spectroscopy reveals that heavy atom substitution leads to a red-shift in the low-energy transition, while the high-energy band remains relatively constant in energy. The red-shift in the low-energy transition leads to optical band gap values of 1.59, 1.46, and 1.06 eV for the S-, Se-, and Te-containing polymers, respectively. Additionally, the strength of the low-energy band decreases, while the high-energy band remains constant. These trends cannot be explained by the present D and A theory where optical properties are governed exclusively by the strength of D and A units. A series of optical spectroscopy experiments, solvatochromism studies, density functional theory (DFT) calculations, and time-dependent DFT calculations are used to understand these trends. The red-shift in low-energy absorption is likely due to both a decrease in ionization potential and an increase in bond length and decrease in acceptor aromaticity. The loss of intensity of the low-energy band is likely the result of a loss of electronegativity and the acceptor unit's ability to separate charge. Overall, in addition to the established theory that difference in electron density of the D and A units controls the band gap, single atom substitution at key positions can be used to control the band gap of D-A copolymers.
我们合成了一系列环戊二噻吩-苯并杂二唑给体-受体（D-A）共聚物，其中苯并杂二唑单元中的单个原子从硫变为硒再变为碲，这使我们能够首次明确研究 D-A 共聚物中的硫到硒到碲取代。含 S 和 Se 的聚合物的合成很直接；然而，含 Te 的聚合物必须通过聚合后单原子取代来制备。所有聚合物都具有代表性的双带光吸收谱，由低能和高能光学跃迁组成。光学光谱表明，重原子取代会导致低能跃迁红移，而高能带在能量上保持相对恒定。低能跃迁的红移导致 S、Se 和 Te 含量聚合物的光学带隙值分别为 1.59、1.46 和 1.06 eV。此外，低能带的强度减弱，而高能带保持不变。这些趋势不能用目前的 D 和 A 理论来解释，该理论认为光学性质仅由 D 和 A 单元的强度决定。一系列光学光谱实验、溶剂化变色研究、密度泛函理论（DFT）计算和时间相关 DFT 计算用于理解这些趋势。低能吸收的红移可能是由于电离势降低、键长增加和受体芳香性降低所致。低能带强度的降低可能是由于电负性的丧失和受体单元分离电荷的能力丧失所致。总的来说，除了 D 和 A 单元的电子密度差控制带隙的既定理论外，关键位置的单原子取代也可用于控制 D-A 共聚物的带隙。
