Solvatochromism and excited-state characteristics of fluorenone-1-carboxylic acid

Abstract

The solvatochromism of the fluorenone derivative 9-fluorenone-1-carboxylic acid (9F1C) was examined in binary cyclohexane-tetrahydrofuran mixtures, along with a theoretical investigation of possible excited state decay pathways. This fluorenone derivative differs from many previously investigated derivatives in that it possesses an electron withdrawing substituent and possibly undergoes intramolecular hydrogen bonding and proton transfer. A conventional Lippert-Mataga analysis was applied to the measured steady-state absorption and emission spectra to estimate the dipole moment difference between ground and emitting excited states. The estimated difference of ~2.8 D was found to be slightly larger than the average value of previously estimated differences for unsubstituted fluorenone (~2.4 D). The 9F1C derivative also differed from unsubstituted fluorenone in that fluorescence emission decreased, rather than increased, with increasing solvent polarity. Interpreting the possible cause of this difference prompted time-dependent density functional theory (TD-DFT) calculations of the relative energies of non-hydrogen bonded and intra-molecular hydrogen bonded excited states. These calculations suggested that an explanation for the decrease in emission for non-hydrogen bonded species could be enhancement of an intersystem-crossing (ISC) pathway. However, these same calculations suggested this explanation could not account for possible decreased emission from intra-molecular hydrogen-bonded species.