Ultrafast Intermolecular Hydrogen Bond Dynamics in the Excited State of Fluorenone

Abstract

Steady-state fluorescence and time-resolved absorption measurements in pico- and femtosecond time domain have been used to investigate the dynamics of hydrogen bond in the excited singlet (S(1)) state of fluorenone in alcoholic solvents. A comparison of the features of the steady-state fluorescence spectra of fluorenone in various kinds of media demonstrates that two spectroscopically distinct forms of fluorenone in the S(1) state, namely the non-hydrogen-bonded (or free) molecule as well as the hydrogen-bonded complex, are responsible for the dual-fluorescence behavior of fluorenone in solutions of normal alcoholic solvents at room temperature (298 K). However, in 2,2,2-trifluoroethanol (TFE), a strong hydrogen bond donating solvent, emission from only the hydrogen-bonded complex is observed. Significant differences have also been observed in the temporal evolution of the absorption spectroscopic properties of the S(1) state of fluorenone in protic and aprotic solvents following photoexcitation using 400 nm laser pulses. An ultrafast component representing the solvent-induced vibrational energy relaxation (VER) process has been associated with the dynamics of the S(1) state of fluorenone in all kinds of solvents. However, in protic solvents, in addition to the VER process, further evolution of the spectroscopic and dynamical properties of the S(1) state have been observed because of repositioning of the hydrogen bonds around the carbonyl group. In normal alcohols, two different kinds of hydrogen-bonded complex of the fluorenone-alcohol system with different orientations of the hydrogen bond with respect to the carbonyl group and the molecular plane of fluorenone have been predicted. On the other hand, in TFE, formation of only one kind of hydrogen-bonded complex has been observed. These observations have been supported by theoretical calculations of the geometries of the hydrogen-bonded complexes in the ground and the excited states of fluorenone. Linear correlation between the lifetimes of the equilibration process occurring because of repositioning of the hydrogen bonds and Debye or longitudinal relaxation times of the normal alcoholic solvents establish the fact that, in weakly hydrogen bond donating solvents, the hydrogen bond dynamics can be described as merely a solvation process. Whereas, in TFE, hydrogen bond dynamics is better described by a process of conversion between two distinct excited states, namely, the non-hydrogen-bonded form and the hydrogen-bonded complex.

The Role of Hydrogen‐Bonding Interactions in the Ultrafast Relaxation Dynamics of the Excited States of 3‐ and 4‐Aminofluoren‐9‐ones

“…However, recently quite a good number of theoretical and experimental studies have been devoted to understanding the dynamics of specific interaction as well as the structural and spectroscopic properties and the relaxation dynamics of the excited state of the hydrogenbonded complex formed via interaction of the excited states with the solvent. [6,7,31,[48][49][50][51][52][53][54][55][56][57][58][59][60][61][62] In these cases, the alcoholic solvents behave as both a dipolar liquid and a quencher of the excited state due to specific interaction. Recently, ultrafast time-resolved infrared spectroscopic techniques have been proved to be extremely powerful tools for investigating both the structural and temporal dynamics of the hydrogen-bonded complex in the excited state.…”

“…[50,51] The KCD or fluorenone molecule contains an aromatic carbonyl moiety and forms a hydrogen-bonded complex with alcoholic solvents in both the ground and excited states. [50,63] Ernsting and co-workers also recently applied the ultrafast visible pump-continuum probe transient absorption spectroscopic technique to studying the dynamics of proton-transfer reactions.…”

Hydrogen-bonding study of photoexcited 4-nitro-1,8-naphthalimide in hydrogen-donating solvents

“…Intermolecular hydrogen bonding, which is an important type of solute-solvent interaction, has been investigated extensively by many experimental and theoretical methods [8][9][10][11][12][13][14][15][16][17]. Some studies show that the solvent polarity can affect the photophysics of excited states [18][19][20][21][22][23][24][25][26].…”

Ultrafast Relaxation Dynamics of the Excited States of 1‐Amino‐ and 1‐(N,N‐Dimethylamino)‐fluoren‐9‐ones

“…[17,18] In a recent communication from our group, the dynamics of intermolecular hydrogen-bonding interaction in the excited singlet state of fluorenone and the protic solvents were the subject of detailed discussion. [19] In addition, the dynamics of hydrogen-bonding interaction in the ground electronic state of fluorenone in alcoholic solvents have also been investigated using a time-resolved IR pump-IR probe spectroscopic technique. [20] Zhao and Han used time-dependent density functional theory (TDDFT) to investigate the excited-state hydrogen-bonding dynamics of fluorenone as well as other kinds of probes in the hydrogenbond-donating solvent methanol.…”

“…[28] Relaxation processes in subpicosecond and picosecond time domains were measured with a femtosecond pump-probe transient absorption spectrometer, which has also been described in detail elsewhere. [19,28] It used a Ti:sapphire laser system (CDP, Russia) and provided pulses of about 50 fs and 300 mJ energy per pulse at a repetition rate of 1 kHz. Pump pulses of 400 nm were generated for excitation of the samples by frequency doubling of one part of the 800 nm output of the amplifier in a 0.5-mm-thick BBO (bbarium borate) crystal, and the other part of the amplifier output was used to generate the white-light continuum (470-1000 nm) probe in a 2-mm-thick sapphire plate.…”