Excited State Characterization of Carborane-Containing Poly(dihexyl fluorene)s

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.