Article pubs.acs.org/JPCC
Probing Charge-Transfer Excited States in a Quasi-Nonluminescent Electron-Rich Fe(II)−Acetylide Complex by Femtosecond Optical Spectroscopy Ciro D’Amico,† Maciej Lorenc,*,† Eric Collet,† Katy A. Green,‡ Karine Costuas,‡ Olivier Mongin,§ Mireille Blanchard-Desce,§ and Frédéric Paul*,‡ †
Institut de Physique de Rennes, UMR CNRS 6251, Université de Rennes 1, Campus de Beaulieu 35042 Rennes, Cedex, France Sciences Chimiques de Rennes, UMR CNRS 6226, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes, Cedex, France § Chimie et Photonique Moléculaires, UMR CNRS 6510, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes, Cedex, France ‡
S Supporting Information *
ABSTRACT: Molecules with photoswitchable nonlinear optical (NLO) properties on the nanosecond time scale are attracting considerable attention as potential solid-state components of photonic devices, downsizeable at will, for ultrafast information encoding. In this context, we present here the study of an electron-rich Fe(II) 4-nitrophenylalkynyl complex which possesses a high hyperpolarizability in its singlet ground state and a negative (and presumably much decreased) NLO activity in its first excited MLCT state(s). On the basis of an ensemble of spectroscopic and time-resolved measurements we investigate the ultrafast dynamics for deactivation of the initially populated MLCT singlet state(s) of this particular organometallic complex, which are shown to decay into a metastable triplet state. The purported mechanism is rationalized with DFT calculations. We show that this triplet state, which should also exhibit a strongly diminished hyperpolarizability, is fully formed with a very high quantum yield within 15 ps.
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INTRODUCTION Quite early, “all optical” information processing has been envisioned as a means to speed up the (more classical) electric/ electronic-based treatment of information.1 While the advent of the first optical fibers constituted a tremendous advance toward this challenging goal in the 1980s, any further improvements now encounter the prospect of developing appropriate devices to encode or read logical information carried by light beams at ultrafast speed by switching material from paraelectric to ferroelectric,2 insulating to metal,3 or between different magnetic states.4 In this context, molecules or materials with specific nonlinear optical (NLO) properties have aroused considerable interest over the last decades, with special attention being paid to their lowest lying excited states,5 which are usually at the very origin of the NLO properties.6 Moreover, when short lived and quantitatively reverting to the ground state (GS), such states might be used to reversibly switch the optical properties of a given (molecular) material,7 providing a way to control propagation of light at much faster speeds than electrochemically driven switching operations.8 Thus, when photoswitching is envisioned as a means to control a given molecular property in the solid state, the decay of the lowest lying excited states of the molecule of interest has to be thoroughly studied in order to better delineate the time scale © 2012 American Chemical Society
and reversibility of this operation. In recent years, the paradigmatic possibility to exploit quantum-coherence effects to logically process information at ultrafast speed has further contributed to fostering the interest of the scientific community in the deactivation pathways undergone by the photoexcited states of various kinds of molecules.9 In this context, the Fe(II) nitro complex 1 exhibits a quite large hyperpolarizability for such a small molecule.10 This compound has a low-energy MLCT absorption band (FeCC → C6H4NO2) responsible for its purple color and certainly at the origin of its high hyperpolarizability.11,12 This excited state (1*(MLCT)), which might be sketched by the VB forms in Scheme 1 (inset), mostly determines the (ground state) NLO properties through their involvement in coherent spectroscopic processes. Starting from a conveniently poled solid sample of polymer containing 1,13 quantitative population transfer of the latter to its excited state 1*(MLCT) will result in an inverted charge distribution for the organometallic chromophore relative to the starting material and should permit manipulation of the nonlinear optical properties of this hybrid polymeric material. Received: September 4, 2011 Revised: December 20, 2011 Published: January 3, 2012 3719
dx.doi.org/10.1021/jp208522w | J. Phys. Chem. C 2012, 116, 3719−3727
The Journal of Physical Chemistry C
Article
pulse (hereafter called the pump) and probed by a second femtosecond laser pulse (hereafter called the probe). Two different detecting systems were used. The first is based on a lock-in amplifier17 (Stanford Research System SR 830), which detects the quantity ΔS = ⟨[S*(Δt) − S(Δt + T)]⟩, where S* is the signal transmitted by the sample in the presence of the pump and S is the signal transmitted by the sample in the absence of the pump during the successive laser shot. T = 1/f is the period between two successive laser shots in a laser system with a repetition rate equal to f (in our experiment f = 1 kHz). The repetition rate of the pump is one-half that of the probe, f. For every time delay (Δt) the signal is averaged over many laser shots in order to enhance the signal-to-noise ratio, which typically can reach 105. The second detecting system is composed of a spectrometer (Acton Research Corp., SPECTRAPro 2500i) and a CCD detector (Princeton Instruments, PIXIS 100). For every Δt, 10 000 spectra were collected, averaged, and analyzed using a double-reference method to improve the signal-to-noise ratio, which can reach 104 in this case. By means of this system we detect the ratio R(λ) = ⟨[S*(λ, Δt)/S(λ, Δt + T)]⟩, which is a spectral signature of the transmission experienced by the sample for a given Δt.18 Δt can be adjusted by controlling an optical delay line, the temporal resolution being limited only by the pulse duration (100 fs in our experiment). By monitoring the transmitted, or reflected, probe signal as a function of Δt it is possible to follow in real time the dynamics of electronic processes initiated in the sample by the pump pulse. The quantities ΔS (lock in) and R (spectrometer) are connected to the variation of the optical density (ΔOD) of the sample during the electronic transitions induced by the pump through the following formula (eq 1)
Scheme 1. Selected Complexes and Limiting Valence Bond (VB) Structures of the MLCT State (Inset)
Indeed, based on the widely different charge distribution of 1*(MLCT), a quite different hyperpolarizability is expected for this excited state. For instance, the oxidized Fe(III) counterpart 1[PF6],14 which resembles 1*(MLCT) by its cationic iron center, is almost NLO inactive at the second order, and exhibits a much smaller and negative hyperpolarizability.10 In order to gather data on 1* and on its various relaxation pathways, we decided to study this MLCT excited state by use of femtosecond pump−probe experiments conducted on crystalline samples of the complex dispersed in potassium bromide pellets. The solid-state structure of the complex has been solved, and its conformation and that of the neighboring species in the solid state are perfectly known.11 However, as a preliminary approach, we started by studying this first allowed excited state of 1 in solution by absorption and fluorescence spectroscopy.
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ΔOD = − log(ΔS /S + 1) = − log(R )
EXPERIMENTAL SECTION Complex 1 was obtained as previously described.11 Luminescence Measurements. Absorption and emission measurements in solution were performed with freshly prepared anaerobic solutions using deoxygenated solvents at room temperature (298 K). UV−vis absorption spectra were recorded on a Cary 5000 or on a Jasco V-570 (solutions) spectrophotometer. Luminescence measurements were performed on dilute solutions (ca. 10−6 M, optical density < 0.1) contained in 1 cm quartz sealable cells maintained in anaerobic conditions with a screw cap with a silicone rubber seal using an Edinburgh Instruments (FLS920) spectrometer equipped with a 450 W xenon lamp and a Peltier-cooled Hamamatsu R928P photomultiplier tube in photon-counting mode. Fully corrected emission spectra were obtained at λexc = λmaxabs with an optical density at λexc ≤ 0.1 to minimize internal absorption. Luminescence quantum yields were measured according to literature procedures.15,16 Solid-state absorption measurements were performed on freshly prepared KBr pellets containing 1 (ca. 1−2% in weight) on a Cary 5000 spectrometer equipped with a DRA 2500 integration sphere, while luminescence measurements were done with powdered samples of 1 on the Edinburgh Instruments with an integration sphere for powdered solid samples. Pump−Probe Measurements. Pump−probe measurements were performed on KBr pellets (see above) containing 1 at room temperature (298 K). In the pump−probe experiment the sample was irradiated by a femtosecond laser
(1)
In this paper we perform two types of measurements. In the first one the compound is pumped by a 100 fs laser pulse, coming from an optical parametric amplifier (OPA),19 with a central wavelength at 630 nm (Δλ ≈ 10 nm). The probe is a 100 fs laser pulse, coming from another OPA, and its central wavelength can be tuned across the entire absorption spectrum of the compound and thus probe it at different wavelengths. In this case the time resolution is limited by a pump−probe correlation function. A lock-in amplifier is used to detect a signal that is proportional to the optical density variation ΔOD as a function of Δt. In the second measurement, the compound is pumped by a 50 fs laser pulse coming from a third OPA, of which the central wavelength can also be tuned across the entire absorption spectrum of the compound, thereby exciting the sample at different wavelengths. A second femtosecond laser pulse, centered at 800 nm, is focused onto a fused silica (or CaF2) window in order to generate a white light supercontinuum, which is used to probe the change of the absorption spectrum in the visible region. In this case a spectrometer is used in order to disperse different wavelengths of supercontinuum, and a CCD camera reads out the ratio R(λ) between the transmitted signal on each wavelength in the presence of the pump and that transmitted in the absence of the pump. The broad bandwidth detection is of much help when the response of the sample at different wavelengths has to be measured simultaneously. The temporal resolution in such measurement is still pulse duration limited, but care has to be taken in defining time zero for each wavelength of the supercontinuum which acquires the temporal chirp in the silica 3720
dx.doi.org/10.1021/jp208522w | J. Phys. Chem. C 2012, 116, 3719−3727
The Journal of Physical Chemistry C
Article
window. The presence of the temporal chirp imposes frequency derivative under the temporal envelope of a polychromatic or quasi-monochromatic laser pulse; mathematically it is represented by a quadratic contribution to the temporal phase of the pulse. Because of the chirp in the present case, the blue edge of the spectrum lags behind the red edge by 2 ps. It should be noted that permanent changes, noticeable only on a time scale far longer than T, did not affect our results. Also, a multishot pump−probe experiment (the signal is averaged over a great number of laser shots) can be performed only if the electronic response has a lifetime significantly shorter than T in order to avoid accumulation effects. This is the case in our experiment. Otherwise single-shot measurements are required.
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RESULTS Lowest Energy Absorption and Emission of 1 in Solution. The absorption and emission spectra of 1 were recorded in various solvents of differing polarity by transmission and in KBr pellets by diffuse reflectance (Table 1). As Table 1. UV−Vis Absorption and Emission Data for the Fe(II) Complex 1 in Various Solvents compd/ solvent 1/pentane 1/Et2O 1/toluene 1/acetone 1/MeOH 1/CH3CN 1/CH2Cl2
1/KBr
absorption: λmax/ nm (10−3 ε in M−1 cm−1)a 305 (sh, 13.0), 505 (13.0) 318 (10.9), 538 (10.8) 320 (sh, 9.7), 549 (9.2)
