ARTICLE pubs.acs.org/JPCA
Ultrafast Dynamics of Isolated Fluorenone Juliane K€ohler,† Patrick Hemberger,†,^ Ingo Fischer,*,† Giovanni Piani,‡,§ and Lionel Poisson*,‡,§ †
Institut f€ur Physikalische und Theoretische Chemie, Universit€at W€urzburg, Am Hubland, D-97074 W€urzburg CNRS, IRAMIS, SPAM, Laboratoire Francis Perrin, URA 2453, F-91191 Gif-sur-Yvette, France § CEA, IRAMIS, SPAM, Laboratoire Francis Perrin, URA 2453, F-91191 Gif-sur-Yvette, France ‡
ABSTRACT: The ultrafast dynamics of isolated 9-fluorenone was studied by femtosecond time-resolved photoionization and photoelectron spectroscopy. The molecule was excited around 264266 nm into the S6 state. The experimental results indicate that the excitation is followed by a multistep deactivation. A time constant of 50 fs or less corresponds to a fast redistribution of energy within the initially excited manifold of states, i.e., a motion away from the FranckCondon region. Internal conversion to the S1 state then proceeds within 0.4 ps. The S1 state is long-lived, and only a lower bound of 20 ps can be derived. In addition, we computed excited state energies and oscillator strengths by TD-DFT theory, supporting the interpretation of the experimental data.
’ INTRODUCTION In this manuscript we describe recent work on the ultrafast dynamics of isolated fluorenone after UV excitation. The photophysics and photochemistry of this molecule is of interest because of its strong solvent dependence and the influence on the optoelectronic properties of polyfluorenes. The photochemical, photophysical, and spectroscopic properties of 9-fluorenone, depicted as an inset in Figure 1, were investigated in solution already for quite some time.1 Both fundamental and applied aspects have been addressed and different spectroscopic as well as theoretical methods have been applied to get insight into the excited state properties and interactions between the states.2,3 Like many systems containing a keto group, fluorenone undergoes intersystem crossing (ISC) to the lowest triplet state after photoexcitation.46 This process was the subject of many studies, and its dependence on molecular and solvent parameters was investigated widely.710 Thus 9-fluorenone serves as a model system for triplet-sensitized electron and proton transfer. Note that the strong solvent dependence of the photophysical properties motivated a large amount of the solution-phase work. In the time domain, the hydrogen-bond dynamics at 400 nm excitation has been studied in alcohols with picosecond time-resolution,10 and the vibrational dynamics in solution has been investigated using an IR probe.11 The photochemistry of fluorenone is also of relevance for optoelectronics. Polyfluorenes are promising materials for polymer light-emitting diodes (PLEDS).1215 Oxidative degeneration of single fluorene units to 9-fluorenone changes the emission properties of the polymer16,17 and limits its color stability, making the polymer less attractive for applications in color conversion systems.18,19 Furthermore, the molecule is a building block of the truxenone electron acceptor, which is embedded in donoracceptor systems20 that are also discussed for applications in nonlinear r 2011 American Chemical Society
Figure 1. Mass spectra of fluorenone recorded at zero time delay between pump and probe laser (bottom trace). A strong signal from the molecule (m/z = 180/181) as well as a small fragment peak (CO, m/z = 152) is detected. The one-color background signal is small for the pump-only case (top trace) and negligible for the probe-only case (center trace). The chemical structure of fluorenone is given as an inset.
optics.21,22 Hence, the emission spectra of 9-fluorenone have received great attention and also triggered a large deal of theoretical work.2,16,17 Unfortunately there is a lack of studies on isolated fluorenone that can serve as benchmarks for the computations. The electronic spectroscopy was investigated in Received: August 2, 2011 Revised: November 2, 2011 Published: November 02, 2011 14249
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The Journal of Physical Chemistry A solid alkanes, but the spectra were strongly perturbed by the matrix.23 Only the lowest transition has been examined in the gas phase by laser-induced fluorescence, but no detailed analysis of the vibronic structure was reported.24 The aim of our studies is the investigation of the intrinsic molecular dynamics directly after excitation. Gas-phase experiments yield a kinetics that is not influenced by external parameters such as solvent motion. This is important to elucidate the intrinsic properties of a molecule, in particular when the photochemistry is strongly influenced by the solvent. As a method we chose femtosecond time-resolved photoelectron imaging. Detection of photoelectrons is known to provide a detailed insight into primary photophysical processes, beyond what can be achieved with mass spectrometry or optical detection methods.2529
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Table 1. List of the Seven Lowest Excited Singlet States and the Lowest Triplet, and Their Vertical Excitation Energies and Oscillator Strengths, Calculated with TD-DFT at the B3LYP/ 6-31G** Levela Eexc, eV (calcd)
oscillator strength
Eexc , eV (exp)
1
2 A1
3.12
0.0000
2.878b
1
1 B2
3.19
0.0044
c
2 1B2
4.09
0.0170
3.80d
3 1A1
4.53
0.0292
c
1 1A2
4.87
0.0000
3 1B2
5.08
0.8490
1
5.17 2.22
0.0398
state
4 A1 a 3B2 (T1) a
’ EXPERIMENTAL SECTION The details of the experimental setup are described elsewhere.28 9-Fluorenone (98%) was purchased from Aldrich and used without any further purification. Two different source setups were employed. In the first source, fluorenone was assembled with glass wool inside the sample container of a modified solenoid valve. After evaporation of the molecule by heating the source to approximately 100 C, 9-fluorenone was seeded in 1 bar of Ar. The mixture was then expanded through a 0.5 mm nozzle into the source chamber. In the second source setup, the sample was placed in a container mounted in front of a 1 mm nozzle. Here 1.5 bar of He was utilized as a carrier gas and the sample was not heated. After passing a 1-mm skimmer, the beam entered the main chamber. Here a time-of-flight mass spectrometer (TOF-MS) and a velocity map imaging (VMI) spectrometer30 were used to monitor either the ions or the electrons produced upon photoionization. For each delay between pump and probe pulse, the raw images of the photoelectrons were accumulated over several hundred laser shots. To obtain the electron kinetic energy distribution, the raw images were transformed by the pBASEXalgorithm31 which allows separation of the polarized and unpolarized parts of the image. After the image inversion, it is possible to study the evolution of the photoelectron spectrum as a function of the pumpprobe delay. A 20 Hz femtosecond Ti:Sa oscillator/amplifier laser system was used that has been described previously.28,32 The third harmonic (264266 nm, 9 μJ) and the second harmonic of the Ti:Sa (397 nm, 180 μJ) were applied to populate the excited states, which were then probed with the fundamental of the Ti:Sa (792 nm, 700 μJ) in a multiphoton process. The pulse energies were attenuated until one-color signals were minimized. Because the ionization potential of fluorenone is determined to be 8.29 eV,33 at least three probe photons are required to produce a pumpprobe signal. In time delay scans, the probe beam was delayed with respect to the pump via a delay line that was actuated by a computer-controlled stepper motor. The intervals between two data points were adjusted to the decay signal and were not equidistant. Before the rise of the signal and at longer delays, the time intervals were larger (between 100 to 500 fs) than around time zero where data points were taken about every 8 fs. Both beams were polarized horizontally within these experiments and overlapped in a small angle in the interaction region. Additionally experiments were carried out with the polarization of pump and probe beam perpendicular to each other, but results were identical. The cross-correlation was found to be around 70 fs. To minimize
4.84e 2.21f b
Selected experimental data are given for comparison. T0, gas phase.24 Band hidden under neighboring transition. d Heptane matrix.23 e Vertical transition, cyclohexane35 and 2-propanol.17 f Estimated from matrix data.23 c
the one-color background the intensity of both beams was adjusted. The pump beam was focused about 14 cm away from the interaction region, and the focus of the probe beam lay 5 cm away. All time-resolved traces were averaged over five or more scans.
’ RESULTS AND DISCUSSION Absorption spectra of 9-fluorenone have been reported in various solvents.2,9 A detailed electronic analysis divided the spectrum into four absorption bands around 380, 310, 290, and 250 nm.2 While the three latter bands were described to be ππ* transition, the assignment of the lowest energy absorption was not straightforward. The assumption that this transition was of nπ* character34 was revised for polar solvents because of the observed solvent effects, such as hydrogen-bonding, and the relatively large extinction coefficient in solution. In polar solvents it was therefore regarded as another ππ* band superimposed with the low energy nπ* transition which shows very low or negligible intensity.2 However, the situation changes in the gas phase where no solvent interaction has to be considered. Here, ab initio computations can serve as a guideline for the assignment of absorption bands. We therefore carried out TD-DFT computations using the B3LYP functional and a 6-31G** basis set. The vertical excitation energies and oscillator strengths for transitions from the electronic ground state are summarized in Table 1. Experimental data are given for comparison where appropriate. The first transition from the S0 to the S1 state corresponds to the nπ* transition, which is optically forbidden, but nevertheless observed in isolated fluorenone24 due to interactions with the nearby 1B2 state. The third transition, 2 1B2 r X 1A1, has been observed in a heptane matrix, showing a sharp origin at 30667 cm1.23 However, perturbation by the matrix was evident. In the same experiments the T1 energy was derived from phosphorescence data. For the higher excited states, only solutionphase data are available. The state symmetries have been elucidated by circular dichroism and magnetic circular dichroism.35 As visible from Table 1, several excited singlet states lie within about 0.5 eV in the region around 5 eV. There is one excitation, namely the 3 1B2 r X 1A1 (S0S6) transition, which carries by far the highest oscillator strength. It is thus expected to dominate the absorption in the UV range. Only solution-phase data are available for this transition.17 Because the experimental results 14250
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Figure 2. Temporal profile of the m/z = 180 ion signal. Three time constants are used for the fit of the signal.
Figure 3. Unpolarized part of TR-PES of 9-fluorenone. A significant offset is visible at later delays, which is due to a deactivation process on the pico- to nanosecond time scale.
summarized in Table 1 indicate that the excitation energies are lower than simple theory predicts, one 266 nm photon (corresponding to 4.66 eV) should provide enough energy to reach the S6 state. The results are in good agreement with those from previous theoretical work using both semiempirical methods as well as density functional theory.16,17 However, the computed oscillator strengths differ.17 The time-of-flight mass spectra of 9-fluorenone obtained under various conditions are displayed in Figure 1. When only the pump laser is present (top trace), a small one-color twophoton signal appears, whereas the probe-only spectrum (center trace) reveals no multiphoton one-color signal. The pump probe spectrum at time zero (bottom trace) shows a two-color signal with a strong molecular peak at m/z =180 that is significantly more intense than the pump-only background. The time dependence of this mass signal, which is assigned to fluorenone, is depicted in Figure 2. The best fit included three time constants, a very fast sub-50 fs decay (τ1), a second constant τ2 = 400 fs (τ2), and a slower process with a time constant τ3 of at least 20 ps or more. It was not possible to obtain a reasonable fit to the time delay trace with only two time constants. Furthermore, the mass spectrum in Figure 1 shows the predicted 13C contribution of 14% at m/z = 181 as well as a fragment peak at m/z = 152. The latter originates from dissociative photoionization associated with loss of carbon monoxide and shows a similar time dependence as the peaks at m/z = 180 and 181. It is thus attributed to the ionization dynamics and not the excited state dynamics. Because no masses were identified that possibly perturb the dynamics, we predominately used time-resolved photoelectron spectroscopy (TR-PES) to get insight into the primary
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Figure 4. Electron kinetic energy distribution at maximum intensity and later delays for the 264 nm pump. Note the slight shift of the distribution to lower energies with time.
photophysical processes in 9-fluorenone. TR-PES is known to be a particularly powerful method to follow intramolecular dynamics, because it is sensitive to the partial ionization crosssection rather than the integral one.25,36 Figure 3 shows a typical TR-PES contour plot obtained upon 264 nm excitation. The photoelectron intensity for the P0 term is displayed as a function of the electron kinetic energy (eKE, vertical axis) and of the time delay (horizontal axis). The spectrum reveals a broad electron energy distribution shortly after excitation. In agreement with the ion signal, the electron signal decreases quickly, indicating a fast deactivation. However, a residual signal is visible even after a few picoseconds, which shifts to lower kinetic energies with time. Measurements monitoring the dynamics up to about 35 ps after excitation show that the signal intensity does not decay to zero at later time delays. Because the P2 term yields the same time constants and shows the same dynamics, it is not presented in the further analysis. No dependence of the dynamics on the relative polarization of pump and probe pulse was observed. Therefore, an influence of molecular rotation on the observed dynamics can be excluded. The kinetic energy distribution at three different time delays is displayed in Figure 4. The spectrum at early times features a broad, rather unstructured band with a maximum at 0.64 eV that extends to energies beyond 2.0 eV. Because the ionization energy (IE) was determined to be 8.29 eV, both [1 + 30 ] and [1 + 40 ] (i.e., a one-photon pump and a three- or four-photon probe) processes contribute to the pumpprobe signal. The maximum possible kinetic energies of 0.951.12 eV for a [1 + 30 ] and 2.442.69 eV for a [1 + 40 ] process are indicated in Figure 4. As visible higher-order process are of minor importance. The maximum around 0.64 eV is most likely caused by an accidental intermediate resonance in the probe step, i.e., a high-n Rydberg state. Such resonances have been observed previously in multiphoton processes using short-pulse lasers.37 Furthermore, a sharp peak around zero kinetic energy is observed. These electrons either originate from autoionizing states or from the excited D1 state that extends from 9.15 to 9.47 eV in the static photoelectron spectrum.33 Although the signal decreases at all energies at later delay times, a shift to lower kinetic energies is evident in the spectrum recorded at 1 ps as compared to the one at t0. The electron signal above 1 eV has decreased to almost zero, whereas the signal below 0.2 eV still shows significant intensity. At late delay times, ionization seems to be associated preferentially with electrons of low kinetic energy. To fit the data, we again use a model that assumes a fast biexponential decay of the signal to a long-lived state that decays with a long time constant. No satisfactory fit to the decay was possible with one or two time constants only. The decay was convoluted with a 14251
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Figure 5. Temporal profile of the photoelectron intensity integrated over all kinetic energies.
Figure 6. Excited states of 9-fluorenone taken from the TD-DFT calculations (cf., Table 1). The initial excitation into the S6 state is most likely because it holds by far the largest oscillator strength.
Gaussian-shaped instrument response function (IRF) with a FWHM of 70 fs (cross-correlation of pump and probe pulse). We analyzed the data in three different ways: (1) Parts of the photoelectron spectrum (i.e., the signal at zero kinetic energy, the band between 0.4 eV and 0.95 eV kinetic energies above 0.95 eV) were fitted separately, (2) the signal was integrated over all electron energies and subsequently fitted, and (3) a global fit was carried out using the Glotaran program.38 All approaches yielded the same time constants within the error bars, including the band at zero kinetic electron energy. Only the amplitudes associated with the time constants differed. Hence, Figure 5 depicts the temporal profile of the signal integrated over all electron kinetic energies (eKE), which gives the best signal/noise ratio. The time constants of 20 ps are very similar to those extracted from the ion signal. With the aid of the computed electronic excitation energies summarized in Table 1 and shown in Figure 6, we can now interpret the various time constants. The excited states are taken from the TD-DFT calculations at the B3LYP/6-31G** level. Because of its large oscillator strength it is assumed that the S6 state is initially prepared. As this state is energetically close to several others excited singlet states, they are presumably strongly coupled. This means that the pump pulse will produce a wave packet that can be described as a superposition of several vibronic states. The motion of this wave packet out of the Franck Condon region and the accompanying geometric reorganization explains the first 50 fs time constant. Because the first step proceeds on the same time scale as was found for the given IRF, the constant τ1 can only be approximated to lie between 20 and 50 fs. The second decay shows a time constant of τ2 = 400 fs. We
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interpret it as the time constant for internal conversion to the lowest excited singlet state likely through a conical intersection. Because the excess electronic energy is converted to internal energy, this state is highly vibrationally excited. Thus, reasonably large FC-factors exist only for transitions to high-lying vibrational levels of the cationic ground state, which are associated with comparably slow electrons. Therefore, the signal at low eKE decreases less than the one at high eKE. The superposition of three- and four-photon ionization dilutes this effect to some extent. The assignment of the final state of the second step of the nonradiative deactivation to the S1 state of fluorenone is in line with previous work. It is known that the lowest singlet state can deactivate by intersystem crossing to the first excited triplet state T1. Time constants of several hundreds of picoseconds in nonpolar solvents up to more than 10 ns in polar solvents were determined for ISC in fluorenone.1 This also shows that neither τ1 nor τ2 can be assigned to ISC, because they are much too short. The third time constant can only be estimated to be at least 20 ps or more, because the maximum time window of the present experiment was insufficient to follow this last deactivation process. Thus, conclusions on the mechanism and time for repopulation of the ground state can only be drawn from comparison with literature data. Because efficient ISC is known to occur in unpolar solvents, where the lowest excited state is of nπ * character and the absorption spectrum in the gas phase resembles the spectra found in nonpolar solvents, we anticipate triplet formation to be an important pathway for deactivation of the S1 state. Matsushita et al. note that the emission yield of excited fluorenone in the gas phase is low.24 However, the fact that fluorescence is observed from the lowest excited state in the gas phase shows that ISC is not the only important deactivation pathway. In this context, it is interesting to note that in the structurally very similar molecule benzophenone ISC is much faster9 due to the smaller S1/T1 energy gap ΔEST of only 2000 cm1. For comparison, our DFT computations yield ΔEST = 0.9 eV (=7300 cm1) for fluorenone. In our case, the high vibrational excitation of the S1 state (at least 1.4 eV) likely favors the vibronic coupling with the S0 state which competes also with ISC. Previous work reported a decrease of ISC with increasing vibrational energy.9 In light of the earlier data, we therefore attribute the >20 ps time decay to the lifetime of the S1 state which might be reduced by the relaxation to S0 or simultaneously by ISC. In a further experiment, the excitation wavelength was set to 400 nm (3.15 eV), which is in resonance with the S0S1 transition (cf., Table 1).24 The observed pumpprobe signal for the TRPES was very weak, and the pump-only background could not be suppressed. In order to get a reasonably good signalto-noise ratio, the spectrum had to be averaged over seven scans at least. The data were analyzed in the same way as described above. The time constants are on the same order of magnitude as those obtained for 264266 nm excitation, and the data can be interpreted within the same three-step deactivation model. Therefore, we assume that we did not excite the S1 state but rather populated the UV bands in a two-photon pump step, exciting states around 200 nm. This interpretation agrees with the low oscillator strengths computed for the S0 f S1 transition and with the assignment of this band to a nπ* transition. It also suggests that fluorenone is a good two-photon absorber. Therefore, oxidatively damaged polyfluorenes might also absorb two photons efficiently. A high two-photon cross-section is also 14252
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The Journal of Physical Chemistry A relevant for the interpretation of solution-phase experiments conducted at 400 nm.
’ SUMMARY AND CONCLUSION The excited state dynamics of 9-fluorenone were investigated by time-resolved photoelectron spectroscopy. The molecule was excited in the UV range at 264266 nm to a tier of higher singlet state. Most of the oscillator strengths are carried by the 3 1B2 (S6) state, described as a ππ* transition. After the initial promotion to the excited states, a very fast sub-50 fs relaxation within the UV bands was observed, corresponding to motion out of the FranckCondon region and fast redistribution of energy within the tier of electronic states. In a second step, the first excited singlet state is populated by internal conversion within 400 fs, most likely through a conical intersection. This state is of nπ* character in the isolated molecule. In unpolar solvents, intersystem crossing to the lowest triplet is known to be an efficient pathway for deactivation and assumed to be important in the isolated molecule as well, although in competition with radiative decay. Due to the limited time window of this experiment only a lower bound of 20 ps can be given for this last deactivation step. The results on the isolated molecule do also give insight into the photochemistry of the molecule occurring in unpolar solvents. Furthermore gas-phase data provide a benchmark for high level quantum chemistry computations that aim at describing charge trapping in polyfluorenes. ’ AUTHOR INFORMATION Corresponding Author
*E-mail: ingo.fi
[email protected] (I.F.); lionel.poisson@ cea.fr (L.P.). Present Addresses ^
Paul Scherrer Institute, CH-5232 Villigen, Switzerland.
’ ACKNOWLEDGMENT This paper is dedicated to Prof. G. Bringmann on the occasion of his 60th birthday. We acknowledge financial support from the graduate research school GRK 1221 and from the German Science Foundation (DFG, FI 575/9-1). Travel support was provided by Laserlab Europe (grant agreement no. 228334, EC’s Seventh Framework Programme), and the DAAD, Egide (Procope). G.P. thanks the RTRA “Triangle de la physique” for support under the contract 2008-062T “DYNANEX”. L.P. thanks the ANR for support through the contract ANR-09-JCJC0090-01 “CHROMADYNE”. Furthermore, we thank the CEA/ SLIC staff, in particular Olivier Gobert and Michel Perdrix, for technical support, and Beno^it Soep for valuable discussions.
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