Department of Chemistry, University College London, London WC1H 0AJ, U.K.
Department of Physical-Chemistry, University of Málaga, Campus de Teatinos, Málaga, 29071 Málaga, Spain.
J Am Chem Soc. 2023 Feb 15;145(6):3507-3514. doi: 10.1021/jacs.2c12008. Epub 2023 Feb 3.
Narrow bandgap conjugated polymers are a heavily studied class of organic semiconductors, but their excited states usually have a very short lifetime, limiting their scope for applications. One approach to overcome the short lifetime is to populate long-lived triplet states for which relaxation to the ground state is forbidden. However, the triplet lifetime of narrow bandgap polymer films is typically limited to a few microseconds. Here, we investigated the effect of film morphology on triplet dynamics in red-emitting conjugated polymers based on the classic benzodithiophene monomer unit with the solubilizing alkyl side chains C and CC and then used Pd porphyrin sensitization as a further strategy to change the triplet dynamics. Using transient absorption spectroscopy, we demonstrated a 0.45 ms triplet lifetime for the more crystalline nonsensitized polymer CC, 2-3 orders of magnitude longer than typically reported, while the amorphous C had only a 5 μs lifetime. The increase is partly due to delaying bimolecular electron-hole recombination in the more crystalline CC where a higher energy barrier for charge recombination is expected. A triplet lifetime of 0.4 ms was also achieved by covalently incorporating 5% of Pd porphyrin into the C polymer, which introduced extra energy transfer steps between the polymer and porphyrin that delayed triplet dynamics and increased the polymer triplet yield by 7.9 times. This work demonstrates two synthetic approaches to generate the longest-lived triplet excited states in narrow bandgap conjugated polymers, which is of necessity in a wide range of fields that range from organic electronics to sensors and bioapplications.
窄带隙共轭聚合物是一类研究较多的有机半导体，但它们的激发态寿命通常很短，限制了其应用范围。克服短寿命的一种方法是填充长寿命的三重态，其中弛豫到基态是被禁止的。然而，窄带隙聚合物薄膜的三重态寿命通常限制在几微秒内。在这里，我们研究了薄膜形态对基于经典苯并二噻吩单体单元的红色发射共轭聚合物中三重态动力学的影响，该单体单元带有可溶的烷基侧链 C 和 CC，然后使用钯卟啉敏化作为进一步改变三重态动力学的策略。通过瞬态吸收光谱，我们证明了更结晶的非敏化聚合物 CC 的三重态寿命为 0.45 毫秒，比通常报道的长 2-3 个数量级，而无定形的 C 只有 5 微秒的寿命。这种增加部分是由于在更结晶的 CC 中延迟双分子电子-空穴复合，预计在那里电荷复合的能量势垒更高。通过将 5%的钯卟啉共价结合到 C 聚合物中，也实现了 0.4 毫秒的三重态寿命，这引入了聚合物和卟啉之间的额外能量转移步骤，延迟了三重态动力学，并将聚合物三重态产率提高了 7.9 倍。这项工作展示了两种在窄带隙共轭聚合物中产生最长寿命三重态激发态的合成方法，这在从有机电子学到传感器和生物应用等广泛领域都是必要的。
