Energy Dissipation Processes of Singlet-excited 1-Hydroxyfluorenone and its Hydrogen-bonded Complex with N-methylimidazole¶

Abstract

Effects of solvent, pH and hydrogen bonding with N-methylimidazole (MIm) on the photophysical properties of 1-hydroxyfluorenone (1HOF) have been studied. Fluorescence lifetime, fluorescence quantum yield and triplet yield measurements demonstrated that intersystem crossing was the dominant process in apolar media and its rate constant significantly diminished with increasing solvent polarity. The acceleration of internal conversion in alcohols paralleled the strength of intermolecular hydrogen bonding. The faster energy dissipation from the singlet-excited state in cyclohexane was attributed to intramolecular hydrogen bonding. The pK(a) of 1HOF decreased from 10.06 to 5.0 on light absorption, and H(3)O(+) quenched the singlet-excited molecules in a practically diffusion-controlled reaction. On addition of MIm in toluene, dual fluorescence was observed, which was attributed to reversible formation of excited hydrogen-bonded ion pair. Rate constants for the various deactivation pathways were derived from the combined analysis of the steady-state and the time-resolved fluorescence results.

Proton Transfer Dynamics in the Aprotic Proton Accepting Solvent 1-Methylimidazole

“…In both of these studies, the third "associated" state was observed as a unique peak with an emission maximum between the protonated and deprotonated states. 23 This assignment is consistent with the idea that the fluorescence emission maximum is dependent on the amount of electron density in the photoacid π system. 24,25 In the results presented below, the entire fluorescence spectral range of the emission of HPTS is recorded as a function of time over the full time window of the dynamics.…”

“…Previous studies by Inoue and co-workers have also suggested this behavior for the photoacid 9-hydroxyfluorenone in mixtures of toluene and 1-methylimidazole. In both of these studies, the third “associated” state was observed as a unique peak with an emission maximum between the protonated and deprotonated states . This assignment is consistent with the idea that the fluorescence emission maximum is dependent on the amount of electron density in the photoacid π system. , …”

“…As mentioned above, evidence of a third peak in photoacid studies has been seen in previous studies and has been attributed to a state in which the proton is shared by the photoacid and the Lewis base, in this case the MeIm molecule. 22,23 Given the basicity of MeIm, the presence of somewhat shared proton states is plausible as has been previously suggested. 22,23 The protonated state of HPTS generally has a peak maximum of ∼440 nm, and the deprotonated peak has a maximum of ∼520 nm.…”

“…22,23 Given the basicity of MeIm, the presence of somewhat shared proton states is plausible as has been previously suggested. 22,23 The protonated state of HPTS generally has a peak maximum of ∼440 nm, and the deprotonated peak has a maximum of ∼520 nm. 42 For HPTS in MeIm, the three separated peaks shown in Figure 4B have maxima at 480, 505, and 520 nm; none of these correspond to the wavelength that is commonly identified as the protonated state emission maximum.…”

Solute–Solvent Hydrogen Bond Formation in the Excited State. Experimental and Theoretical Evidence

“…1-Hydroxyfluorenone allows us to study three types of H-bond: an intramolecular bond between the OH group and the carbonylic oxygen, an intermolecular bond of the OH with the protic solvents and an intermolecular H-bond between the hydroxylic hydrogen and an H-bond acceptor [106]. The fluorescence maximum is red-shifted on going from CHX to more polar solvents.…”