Poly(fluorene)-type materials are widely used in polymer-based light emitting devices. In their pristine state, they emit in the deep blue spectral region. During operation there appears, however, an additional emission peak at around 2.3 eV. This observation has usually been attributed to aggregate or excimer formation. Recently, it has been shown that photo- and/or electro-oxidation of poly(fluorene) chains resulting in ketonic defects (i.e., formation of fluorenone groups) can also be held responsible for emission in that spectral region. In this contribution, we apply quantum-chemical techniques to gain a detailed understanding of the optical properties of poly(fluorene)s containing ketonic defects. In particular, we compare model systems for poly(fluorene) with their ketone-containing counterparts, focusing on the influence of excited-state localization effects. The results of the theoretical calculations are confirmed by experimental investigations on statistical copolymers of fluorene and 9-fluorenone.

Green emission from poly(fluorene)s: The role of oxidation

In order to clarify mechanisms of excited state interactions in hydrogen-bonded pairs, we have studied the kinetics of dynamic quenching of singlet and triplet fluorenone by a series of alcohols, phenols, and related compounds, in which hydrogen-bonding power, redox potential, and acidity are systematically varied. In addition, effects of solvent basicity or polarity and deuteration help identify the role of hydrogen-bonding in physical or chemical quenching processes. Alcohols and weak acids, with high oxidation potentials, do not quench the triplet, but quench the singlet at rates which parallel hydrogen-bonding power. This is attributed to a physical mechanism, involving vibronic coupling to the ground state via the hydrogen bond. This is much stronger in the excited state than in the ground state, and provides efficient energy dissipation in the radiationless transition. Phenols, with hydrogen-bonding power comparable to that of the alcohols but with much lower oxidation potentials, quench both single...

Quenching Processes in Hydrogen-Bonded Pairs: Interactions of Excited Fluorenone with Alcohols and Phenols

The transient absorption spectra of a number of compounds in solution at room temperature have been measured with picosecond‐scale time resolution and unprecedented accuracy and reliability. It is found that the spectra of benzophenone, xanthone, 9‐fluorenone, p‐benzoquinone, acridine, quinoxaline, t‐stilbene all show significant evolution following excitation with the 353 nm third harmonic of a mode‐locked Nd:glass laser. These spectral changes provide evidence for previously unrecognized photophysical behavior in the systems studied, including vibrationally nonthermalized triplet states in benzophenone which persist for tens of picoseconds, and formerly unobserved singlet absorptions in acridine, quinoxaline, fluorenone, and benzoquinone. The inadequacy of single wavelength probing techniques in deducing the dynamics of complex molecular systems such as these is demonstrated by the large extent to which spectra of different states overlap. In many cases a reinterpretation of previous single wavelength results is required because of the presence of vibrationally and electronically unrelaxed species. The apparatus used here to obtain the high quality visible transient spectra is described and its performance characterized.

Picosecond transient spectroscopy of molecules in solution

The efficiency of aromatic ketones as singlet-oxygen (1O2(1Δg)) sensitizers can vary considerably with the electronic configuration of their lowest triplet state and the solvent used. Near-infrared measurements of tie luminescence of singlet oxygen have shown that the quantum yield of singlet-oxygen production (ΦΔ) by 1H-phenalen-1-one (1) is close to unity in both polar (ΦΔ = 0.97±0.03 in methanol) and non-polar solvents (ΦΔ = 0.93±0.04 in benzene). Analysis of the absorption spectra of the ground state and phosphorescence measurements show that the lowest singlet and triplet states have dominant π, π* electronic configurations. The quantum yield of intersystem crossing (ΦISC) of 1, determined by laser flash photolysis (partial-saturation method), is equal to unity. In comparison with other aromatic ketones, these parameters are important for the discussion of the surprisingly high ΦISC of 1 and the efficient energy transfer from its triplet state to molecular oxygen. The 1H-phenalen-1-one (1), being one of the most efficient singlet-oxygen sensitizers in both polar and non-polar media, could be used as a reference sensitizer, in particular in the area of relatively high energies of excitation.

1H‐Phenalen‐1‐one: Photophysical Properties and Singlet‐Oxygen Production

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.

T. Kobayashi

Papers

Picosecond time-resolved spectroscopy and the intersystem crossing rates of anthrone and fluorenone