Martin K Lindsey, Krishnamurthy Aditi, Strahan John, Young Elizabeth R, Carter Kenneth R
Department of Polymer Science and Engineering , University of Massachusetts-Amherst , 120 Governors Drive , Amherst , Massachusetts 01003 , United States.
Department of Chemistry , Amherst College , P.O. Box 5000, Amherst , Massachusetts 01002 , United States.
J Phys Chem A. 2019 Mar 7;123(9):1701-1709. doi: 10.1021/acs.jpca.8b07955. Epub 2019 Feb 25.
Carborane-containing poly(dihexylfluorene)s experience drastic solvatochromism in both the solution and solid states, a characteristic that is advantageous for use in environmental and biological sensing applications. Understanding the intrinsic decay mechanisms that give rise to such sensitive emission properties is important for designing responsive sensors. The solution-state photophysical properties of homopolymer, poly(9,9-dihexyl(bisfluorenyl)carborane) (PFCY), and alternating copolymer, poly(9,9-dihexyl-2,7-fluorene- alt-9,9-dihexyl(bisfluorenyl)carborane) (PFCS), were deciphered using steady-state, electrochemical, spectroelectrochemical, and time-resolved spectroscopic methods. From these techniques, it was discovered that following excitation the conjugated fluorene local excited state (LES) donates an electron to the carborane molecule, forming an intramolecular charge transfer (ICT) state between a radical cation on the fluorene moiety and a radical anion on the carborane moiety. From the global analysis of transient absorption data, it was discovered that the rate of electron transfer from the fluorene to the carborane is heavily influenced by solvent polarity and is significantly faster in more polar solvents. Once formed, the ICT state can decay through radiative or nonradiative mechanisms and is more likely to undergo radiative decay in nonpolar solvents, due to an intramolecular restriction of the polar ICT state. This study elucidates the effects that polarity has on the excited-state formation and subsequent decay mechanisms of fluorene-carborane systems, conclusively explaining the solvatochromism and steady-state emission properties exhibited by this system.
含碳硼烷的聚（二己基芴）在溶液和固态中都表现出显著的溶剂化显色现象，这一特性有利于用于环境和生物传感应用。了解导致这种敏感发射特性的内在衰减机制对于设计响应式传感器很重要。使用稳态、电化学、光谱电化学和时间分辨光谱方法解析了均聚物聚（9,9-二己基（双芴基）碳硼烷）（PFCY）和交替共聚物聚（9,9-二己基-2,7-芴-alt-9,9-二己基（双芴基）碳硼烷）（PFCS）的溶液态光物理性质。通过这些技术发现，激发后，共轭芴局部激发态（LES）将一个电子转移给碳硼烷分子，在芴部分的自由基阳离子和碳硼烷部分的自由基阴离子之间形成分子内电荷转移（ICT）态。通过对瞬态吸收数据的全局分析发现，从芴到碳硼烷的电子转移速率受溶剂极性的严重影响，在极性更强的溶剂中明显更快。一旦形成，ICT态可以通过辐射或非辐射机制衰减，并且由于极性ICT态的分子内限制，在非极性溶剂中更有可能发生辐射衰减。这项研究阐明了极性对芴-碳硼烷体系激发态形成和随后衰减机制的影响，最终解释了该体系表现出的溶剂化显色现象和稳态发射特性。
