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Time-resolved spectroscopic characterization of photo-induced valence tautomerism for a cobalt–dioxolene complex
Chemical Physics 314 (2005) 9–17 www.elsevier.com/locate/chemphys
Time-resolved spectroscopic characterization of photo-induced valence tautomerism for a cobalt–dioxolene complex Pier Luigi Gentili
b
a,*
, Laura Bussotti a, Roberto Righini Lapo Bogani b, Andrea Dei b
a,b,*
, Alessandra Beni b,
a LENS, UniversitaÕ di Firenze, Via Nello Carrara 1, 50019 Sesto Fiorentino, Firenze, Italy Dipartimento di Chimica, UniversitaÕ di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy
Received 24 November 2004; accepted 21 January 2005 Available online 16 February 2005
Abstract The valence tautomerism of low-spin CoIII(Cat-N-BQ)(Cat-N-SQ) (where Cat-N-BQ is 2-(2-hydroxy-3,5-di-tert-butylphenylimino)-4,6-di-tert-butylcyclohexa-3,5-dienone and Cat-N-SQ is the dianionic radical analogue) was investigated by means of UV–vis pump–probe transient absorption spectroscopy and 1H NMR technique in chloroform and dichloromethane. By exciting the CT transition of the complex at 480 nm, an intramolecular electron transfer process is selectively triggered. The photo-induced charge transfer is pursued by a cascade of two main molecular events characterized by the ultrafast transient absorption spectroscopy: the first gives rise to the metastable high-spin CoII(Cat-N-BQ)2 that, secondly, reaches the chemical equilibrium with the reactant species. The rate constant of back valence tautomerization estimated by measuring the lifetime of high-spin CoII(Cat-N-BQ)2 species and the equilibrium constant for the CoIII(Cat-N-BQ)(Cat-N-SQ) ¢ CoII(Cat-N-BQ)2 interconversion, is significantly large (on the order of 109 s1). It is interpreted under the point of view of the theory formulated by Jortner and Buhks et al. for nonadiabatic radiationless processes. 2005 Elsevier B.V. All rights reserved. Keywords: Valence tautomerism; Cobalt–dioxolene; Transient absorption
1. Introduction Molecular systems undergoing a reversible and controlled change of their physical properties following a change of an external parameter constitute an appealing perspective for the realization of molecular scale electronic devices [1,2]. The use of light for controlling the magnetic properties of molecular systems represents an intriguing tool for establishing potential applications * Corresponding authors. Tel.: +390554572485; fax: +390554572451. E-mail addresses: [email protected]fi.it (P.L. Gentili), righini@lens. unifi.it (R. Righini).
0301-0104/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2005.01.020
for information storage [3,4]. Examples of systems showing this behaviour are metal complexes undergoing spin-crossover and valence tautomeric interconversion [5–12]. Both these classes of metal complexes show interplay between charge mobility and magnetic properties and then their analogy with the spintronic devices is often emphasized. However, in addition to their possible technological interest, there is another important point deserving consideration. These metal derivatives offer the possibility of using molecular systems as ideal testing grounds for the observation of sophisticated physical properties. In particular, their investigation may provide new tools for understanding the mechanism of charge
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P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
transfer in the presence of magnetic fields including optical irradiation. This statement is particularly appropriate when cobalt–dioxolene complexes are considered. O-dioxolenes can bind to metal ions in three different redox states, corresponding to catecholate, Cat, semiquinone, SQ, and quinone, Q, respectively. The low covalent character of their bond to the metal ion in general gives rise to a well-defined charge distribution. In fact the metal dioxolene complex can often unambiguously be described, on the basis of their structural, magnetic and spectroscopic features, as one of the isomeric species [Mn+–Cat2](n 2)+, or [M(n 1)+–SQ](n 2)+, or [M(n 2)+–Q](n 2)+. Switchable molecules can be obtained when the coordinated dioxolene ligand to the metal ion, provide pairs of isomers having comparable energies. These systems have been called valence tautomers, the interconversion involving an intramolecular electron transfer between the metal acceptor and the coordinated redox active ligand [13,14]. The best investigated case is that of cobalt–dioxolene molecules, where the valence tautomeric interconversion involves an intramolecular electron transfer between a six-coordinated diamagnetic cobalt(III) metal ion ðt62g ; S ¼ 0Þ and a polyoxolene ligand yielding a high-spin cobalt(II) ðt52g e2g ; S ¼ 3=2Þ complex of the ligand with a less negative oxidation number. The possibility of this process is due to the localized electronic properties of the two interacting donor and acceptor moieties, i.e., a low covalent bonding character. This assumption in particular holds when the dioxolene is coordinated to a metal ion in low oxidation state, i.e., cobalt(II), whereas it can be considered only an operational approximation for metal ions in high oxidation state, like in the present case cobalt(III). Following this peculiar property, these complexes provide an important class of model compounds for understanding the details of the intramolecular electron transfer processes involving a transition metal ion and a coordinated redox active ligand. The first class of cobalt complexes showing valence tautomerism in condensed phase was constituted by molecular dioxolene complexes of general formula Co(N-N)(diox)2 (N-N = diazine ligand, diox = 3,5and 3,6-di-tert-butylcatecholato (DBCat) or the parent semiquinonato (DBSQ)). Pulsed laser photolysis studies carried out by Hendrickson and co-workers showed [15,16] that the transition
creased up to several hours. No details associated with the electronic mechanism associated with the transition were provided. Recently some of us have shown [19] that the time resolved femtosecond spectroscopic characterization of the CoII(Me4cyclam)(DBSQ)+ (Me4cyclam = 1,4,8,11tetramethyltetraazacyclotetradecane) was consistent with the involvement of at least three different electronic states in the optical interconversion process. Although it seems reasonable to believe that the kinetics of these processes can be largely modulated by the Franck– Condon reorganization energies and the energy differences between the ground and the metastable states, it is rather unclear the role of the electronic coupling between the ls-CoIII and hs-CoII states. Indeed, it is usually believed that differences in spin multiplicities may play a determinant role. Bearing this in mind, we have considered the cobalt complex formed by the monoanion of the tridentate ligand 2-(2-hydroxy-3,5-di-tert-butylphenylimino)-4,6-di-tert-butylcyclohexa-3,5-dienone (CatN-BQ (I)) as well as by its dianionic semiquinonato radical analogue (Cat-N-SQ) (II) (see Scheme 1). This complex was formulated as CoIII(Cat-NBQ)(Cat-N-SQ) at room temperature. The mixed ligand character of this complex was suggested on the basis of the structural parameters and was found consistent with the spectroscopic and magnetic properties of the solid compound [20]. However, the temperature dependence of the spectral and magnetic properties of this compound in the solid state [21] and in non-polar solvents [22] suggests the existence of the valence tautomeric equilibrium CoIII ðCat-N-BQÞðCat-N-SQÞ ¢ CoII ðCat-N-BQÞ2 ðS¼1=2Þ
ðS¼3=2Þ
The electronic spectrum of the CoIII(Cat-N-BQ)(Cat-NSQ) species is characterized by well-separated intraligand and charge transfer bands. In particular the CT transition at 526 nm, does not show any significant overlap with the absorption bands of the CoII(Cat-N-BQ)2 chromophore. This situation was not found for the previously investigated Co(N-N)(diox)2 complexes [15– 18].Therefore, this compound was believed to constitute a good molecular system for elucidating the electronic processes associated with the photo-induced valence
ls-CoIII ðN-NÞðDBSQÞðDBCatÞ ! hs-CoII ðN-NÞðDBSQÞ2 may be induced in solution by optical irradiation, the induced metastable states being characterized by some nanoseconds lifetimes. Some years later, Sato et al. [17] and Varret et al. [18] showed that the same transition can be optically induced in condensed phase at 5 K, the lifetime of the photo-induced product being in-
. N
N O
O
O
Cat-N-BQ (I)
O Cat-N-SQ (II)
Scheme 1.
P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
tautomeric transitions. Here, we wish to report the results of a transient absorption spectroscopic study using femtosecond pulsed laser photolysis.
2. Experimental 2.1. Materials The CoIII(Cat-N-BQ)(Cat-N-SQ) complex was prepared according to the previously reported [23] procedure. The solvents dicholoromethane and chloroform from Fluka were purified according to the standard procedures. Chloroform-d1 (99.8%) from Merck and dichloromethane-d2 from Aldrich (99.5%) were used as received. 2.2. Spectroscopic measurements The experimental instrumentation and data processing for femtosecond time-resolved transient absorption spectroscopy have been described in detail in previous papers [24,25]. Briefly, ultrashort pulses (duration 100 fs at 800 nm, repetition rate 1 kHz, energy 700 lJ/pulse) are produced by a regenerative amplified Ti:Sapphire laser system. Tunable excitation pulses are obtained by means of a BBO-based optical parametric generator (OPG). In the present experiment, the excitation wavelength of 480 nm was obtained by sum frequency generation of fundamental (800 nm) and signal (1.2 lm). A small portion of the output of the amplifier, 2 lJ/pulse, was focused on a 2.5 mm thick calcium fluoride plate to generate the white-light continuum used for probing. The resulting spectrally broadened laser pulse spanned the entire visible region and extended in the near UV to roughly 350 nm. The white-light continuum was further split into two parts of equal intensity by a 50/50 fused-silica–Al beam splitter. One part, acting as a probe beam, was spatially overlapped with the excitation beam in the sample. The second part crossed the sample in a different position and provided a convenient reference signal. The probe and reference beams were spectrally dispersed in a flat-field 25 cm Czerny-Turner spectrometer, and detected by means of a back illuminated CCD camera with spectral response in the region 300–1000 nm. Two different configurations of the detection system were utilized to obtain the data sets herein presented, which are composed of both transient spectra (transmittance of the excited state vs. wavelength at a given pump–probe delay time) and kinetic plots (intensity of the probe vs. delay time at a fixed wavelength). In the transient spectra, the transmittance at a given delay time t and wavelength k, T(t, k), is defined as I(t, k)/I0(k), where I(t, k) and I0(k) are the intensities of the white-light continuum components reaching the
11
detector having or having not interacted with the pump pulse, respectively. Two measurements were performed: the first without the pump beam, thus acquiring the intensities of the probe (I0(k)) and reference pulses (IR(k)); the second with both excitation and probe beams, thus measuring I(t, k) and I 0R ðkÞ. The transient transmittance T(t, k) is then given by ½Iðt; kÞ=I 0R ðkÞ= ½I 0 ðkÞ=I R ðkÞ. Recording kinetic plots requires narrow bandwidth detection. The desired wavelengths were selected with 5 nm bandwidth interference filters. The intensity of the probe pulse was measured by a silicon difference photodiode and a lock-in amplifier synchronized to a chopper, switching the pump pulse on and off at half the repetition-rate of the laser system (500 Hz). In this way, the reading of the phase-locked amplifier gave the modulation of the probe pulse intensity due to the interaction with the pump pulse. In every experiment the relative pump–probe polarization was set to the magic angle (54.7) to discriminate against orientational dynamics. The sample solutions flew through a 1 mm thick calcium fluoride cell connected to a solution reservoir and a pump system. All the samples were prepared to have an optical density of approximately 1 at the excitation wavelength. Steady-state absorption spectra of the solutions were measured before and after the experiments to check for possible sample decomposition. All measurements were carried out at room temperature (22). 1 H NMR spectra were recorded on a Varian spectrometer operating at 300 MHz.
3. Results 3.1. Spectral properties The electronic absorption spectrum of CoIII(Cat-NBQ)(Cat-N-SQ) complex dissolved in chloroform is shown in Fig. 1. By comparing this spectrum with those of Zn(Cat-N-BQ)2 [22] and M(Cat-N-SQ)2 (where M = Ge and Sn) [26] as well as with that of Al(Cat-NBQ)(Cat-N-SQ) complex [22], it is evident that all the bands appearing in the near-UV, visible and near-IR regions, except that centered at 526 nm, are due to intraligand electronic transitions. The intense band at 526 nm is due to a charge-transfer transition. In order to trigger the intramolecular electron transfer between the ligand and the metal ion, CoIII(Cat-NBQ)(Cat-N-SQ) was excited at 480 nm. The wavelength of the pump pulse was chosen on the edge of the CT band, far from the maximum of a weak band centered at 538 nm and generated by an intraligand electronic transition [22]. By this selective photo-excitation only the CT was induced: the consequent spectral evolution, recorded in the near UV and visible regions in the time
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P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
of this spectral correction is shown in the inset of Fig. 2. After a few hundreds of femtoseconds the transient absorption band centered at 407 nm decreases in intensity whereby the bleaching components at 385, 436 and 550 nm become spectrally dominant. The disappearance of the first absorption band located in the blue portion of the visible region is accompanied by the appearance of a new absorption component located in the red portion of the spectrum that can be interpreted as the high frequency edge of a band situated in the far-red region. All the spectral features disappear in 1 ns from the excitation, suggesting that a complete recovery of the electronic ground state of the photo-sensitive CoIII(Cat-N-BQ)(Cat-N-SQ) is reached. Fig. 1. Steady-state absorption spectrum of CoIII(Cat-N-BQ)(Cat-NSQ) complex in chloroform. The down arrow shows the position of the excitation wavelength.
3.2. Kinetics
window between 0 and 1 ns, monitors the dynamics of valence tautomerism. The transient absorption spectra collected for CoIII(Cat-N-BQ)(Cat-N-SQ) in chloroform are portrayed in Fig. 2 at selected delay times. The interaction of the sample with the pump pulse instantaneously causes a transient absorbance increase centered at 407 nm that reaches its maximum within the pump–probe cross-correlation envelope (200 fs). The shape of this first transient absorption band is influenced by the spectral contribution of the bleaching of the CoIII(Cat-N-BQ)(Cat-N-SQ) electronic ground state absorption, producing two negative peaks at 385 and 436 nm, respectively. The absorption contribution can be separated from that of bleaching by adding to the transient spectrum at early delay time (DODTOT exp ðtÞ vs. k) the normalized steady-state absorption spectrum (ARif vs. k) of CoIII(Cat-N-BQ)(Cat-N-SQ). The result CoIII
To characterize quantitatively the evolution of CoIII(Cat-N-BQ)(Cat-N-SQ) after photo-excitation we recorded the intensities of the probe pulse at fixed wavelengths as functions of the pump–probe delay time. Two spectral regions are crucial to rationalize the relaxation dynamics: the blue portion of the spectrum covered by the first absorption band and the red portion where the second absorption component appears. The kinetics recorded at 409 nm and at 620 nm over the first 3 ps are shown in Figs. 3(a) and (b), respectively. The kinetics at 409 nm shows a fast rise followed by decay. The rise occurs on a time interval corresponding to the pump–probe cross-correlation function (200 fs), whereas the subsequent drop is characterized by a lifetime of s1 = (180 ± 20) fs. The kinetics at 620 nm shows an intensity growth well-described by a convolution of the response function with an exponential function having s1 = (180 ± 20) fs as lifetime, equivalent to that estimated from the kinetics at 409 nm. The numerical
Fig. 2. Transient absorption spectra of CoIII(Cat-N-BQ)(Cat-N-SQ) in chloroform, recorded at pump–probe delays of 200 fs, 600 fs, 1 ps and 1 ns. The excitation wavelength was 480 nm. In inset: corrected Rif transient absorption spectrum ðDODTOT exp ðtÞ þ ACoIII Þ measured at 200 fs delay time.
Fig. 3. Amplitude vs. time profiles of the transient signals recorded at (a) 409 nm and (b) 620 nm for CoIII(Cat-N-BQ)(Cat-N-SQ) in chloroform for short time delays.
P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
13
equivalence between the two s1 lifetimes unveiled that the disappearance of the band in the blue region was accompanied by the simultaneous appearance of the absorption band in the red region. From the kinetics at 620 nm, it was also possible to follow the progressive disappearance of the band located in the red portion of the spectrum occurring in some hundreds of picoseconds as timescale: it is reported in Fig. 4. The experimental data are well-fitted by a mono-exponential function with s2 = (410 ± 20) ps; no residual intensity is left after 1 ns. 3.3. Solvent effects In valence tautomeric complexes two or more electronic states lie close in energy. This involves significant vibronic interactions and then an appreciable sensitivity to the environment. For this reason the behaviour of CoIII(Cat-N-BQ)(Cat-N-SQ) was also investigated in dichloromethane and in the deuterated forms of chloroform (CDCl3) and dichloromethane (CD2Cl2). For all the solvents used, the spectral evolutions recorded after photo-excitation at 480 nm are qualitatively similar (see Section 3.1 for details). Furthermore, there are no quantitative discrepancies between the results obtained in CHCl3 and CDCl3 within the time resolution achievable with our instrumentation; similarly, no difference was observed between the data collected in CH2Cl2 and in CD2Cl2. Appreciable differences were instead revealed between the lifetimes determined for CoIII(Cat-NBQ)(Cat-N-SQ) in chloroform and those obtained in dichloromethane. In Fig. 5, the kinetics recorded at 620 nm in CHCl3 (solid curve) and in CH2Cl2 (dashed curve) are graphically compared. The rise of the tran-
Fig. 5. Amplitude vs. time profiles recorded at 620 nm for CoIII(CatN-BQ)(Cat-N-SQ) in CH2Cl2(dashed curves) and in CHCl3 (solid curve) for short delay times. The long delay time behaviour is shown in the inset.
sient band is clearly faster in dichloromethane than in chloroform. On the contrary, the intensity decay of the same band is slower in dichloromethane. In Table 1, the corresponding rise (s1) and decay (s2) times, obtained from an exponential fitting procedure of the data measured for the solvents, are collected. CoIII(Cat-N-BQ)(Cat-N-SQ) and its valence tautomer CoII(Cat-N-BQ)2 have energetically close-lying electronic states [20,21]. Spectroscopic, magnetic and NMR measurements [21] support the existence of a valence tautomeric equilibrium low-spin CoIII ðCat-N-BQÞðCat-N-SQÞ kf
¢ high-spin CoII ðCat-N-BQÞ2 kb
ð1Þ
in solution at room temperature. In analogy to the methodology pursued in [22], we determined the equilibrium constants values in chloroform and in dichloromethane at room temperature (298 K) by performing measurements of magnetic susceptibility and applying the Evans method [27]. Assuming a vT of 0.38 emu mol1 K for the CoIII(CatN-BQ)(Cat-N-SQ) reactant species (in agreement with the data obtained for the solid) and a vT of 3.21 emu mol1 K for the CoII(Cat-N-BQ)2 species (as extrapolated from solutions vT values), the Keq = kf/kb values of 0.7 and 0.3 were obtained at 298 K in chloroform and dichloromethane, respectively.
4. Discussion Fig. 4. Amplitude vs. time profile for the transient signal recorded at 620 nm for CoIII(Cat-N-BQ)(Cat-N-SQ) in chloroform in the 0–1000 ps time window.
In a previous work [22] where the valence tautomeric interconversion between the low-spin CoIII(Cat-NBQ)(Cat-N-SQ) and the high-spin CoII(Cat-N-BQ)2
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P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
Table 1 Experimental lifetimes of the two chemical components responsible for the entire spectral evolution for CoIII(Cat-N-BQ)(Cat-N-SQ) after excitation at 480 nm in chloroform and dichloromethane
s1 (fs) s2 (ps)
CHCl3
CH2Cl2
180 ± 20 410 ± 20
160 ± 20 490 ± 20
was induced by heating, it was shown that the process can be monitored following the changes of the intensities of the absorption bands at 800, 730, 533, 439 and 391 nm. In particular, the oxidation of the di-anionic semiquinonato ligand to the mono-anion catecholato analogue determines a significant increase of the absorbance in the 550–800 nm spectral region. The broad absorption band appearing above 550 nm in our transient spectra some hundreds femtoseconds after the excitation can then be ascribed to the formation of high-spin CoII(Cat-N-BQ)2, as supported by the here reported magnetic data and previously discussed ESR behaviour [21]. The spectral evolution described in the previous section shows that this molecular state is preceded by another state responsible of the transient band centered at 407 nm whose shape is reported in the inset of Fig. 2. This state is produced soon after the excitation at 480 nm and has a short lifetime of about 180 fs. By assuming that the dioxolene cobalt complex involved in the valence tautomerisation has an octahedral symmetry (Oh), the d-orbitals of the metal ion can be split into the familiar t2g and eg sets. Furthermore, the p* ligand orbitals involved in the electron transfer are empty in the Cat-N-BQ form, whereas they contain a single electron in the Cat-N-SQ oxidation state. Hence, two plausible mechanisms interpreting the experimental evidences and involving two electronic transitions can be formulated, as indicated in the following scheme: 6
0
1
0 1
6
1
0
ðt2g Þ ðeg Þ ðpSQ Þ ðpBQ Þ !ðt2g Þ ðeg Þ ðpBQ1 Þ ðpBQ2 Þ 2
5
2
0
!ðt2g Þ ðeg Þ ðpBQ1 Þ ðpBQ2 Þ 6
0
1
0
0 0 1
ð2aÞ 5
0
1
ðt2g Þ ðeg Þ ðpSQ Þ ðpBQ Þ !ðt2g Þ ðeg Þ ðpSQ1 Þ ðpSQ2 Þ 20
5
2
0
!ðt2g Þ ðeg Þ ðpBQ1 Þ ðpBQ2 Þ
0
0
1
ð2bÞ
In mechanism (2a) the first step, occurring in a time interval shorter than the experimental time resolution, involves an electron transfer from the pSQ orbital to the antibonding eg orbital of the CoIII metal ion, i.e., it is a LMCT transition; on the other hand, in mechanism (2b) it involves an electron transfer from one of the t2g orbitals of CoIII to the pBQ orbital of the CatN-BQ ligand, i.e., it is a MLCT transition. In the mechanism (2a), the low-spin CoII(Cat-N-BQ)2 state is produced in the first step. In less than 1 ps low-spin CoII(Cat-N-BQ)2 transforms to the high-spin CoII(Cat-
N-BQ)2 through an electronic redistribution localized in the CoII metal ion. On the contrary, if the photoreaction proceeds according to mechanism (2b), the first step gives rise to CoIV(Cat-N-SQ)2 state. In agreement with the experimental evidences, CoIV(Cat-N-SQ)2 produces high-spin CoII(Cat-N-BQ)2 state in a single step occurring in some hundreds femtoseconds. This process should entail a concerted bielectronic transfer from the two pSQ orbitals of the Cat-N-SQ ligands to the CoIV metal ion along with a structural rearrangement of the entire molecule. The choice between the two plausible mechanisms is not trivial. As far as we know, among the organometallic valence tautomers investigated up to now, there are not examples of processes like the second step of Scheme (2b), implying a simultaneous oxidation of two ligands and the bielectronic reduction of a metal ion. Furthermore, the measured conversion time of the CoIV(CatN-SQ)2 ! CoII(Cat-N-BQ)2 is surprisingly fast if compared to the relevant structural rearrangement required. The second step of mechanism (2a) is instead much more likely to be very fast, since it involves two states, the low-spin CoII and the high-spin CoII that according to DFT calculations [28] performed for similar o-quinone Co complexes, are expected to have almost the same energy. If we incline to (2a) mechanism, we should admit that the first step entails a ligand-tometal-charge-transfer. This is apparently in contradiction with the high extinction coefficient values determined for the CT band centered at 526 nm: since the LMCT is a symmetry-forbidden transition, involving two nearly orthogonal orbitals, it should give rise to a band of weak intensity, as it was revealed for other oquinone Co complexes in [28]. On the contrary, the alternative MLCT (first step of Scheme (2b)) is a symmetry-allowed transition and it can justify the observed high intensity of the CT band. However, DFT calculations [28] have revealed that in o-quinone Co complexes, while there are still localized electronic structural features reflecting the different metal and ligand oxidation states in the low-spin CoIII and high-spin CoIII tautomeric forms, appreciable covalent interactions exist between the cobalt ion and the ligands. These covalent interactions lead to appreciable mixing of orbitals and could influence the symmetry selection rules, rendering the LMCT a transition of appreciable intensity. As the high-spin CoII(Cat-N-BQ)2 is produced, it reaches the equilibrium with low-spin CoIII(Cat-NBQ)(Cat-N-SQ) within about 1 ns. The set-up of the equilibrium involves a reversible first-order reaction, where the experimentally determined lifetime ðs2 ¼ 1=k obs 2 Þ is correlated to the kinetic constants kf and kb characterizing the equilibrium by the relation s2 ¼
1 : kf þ kb
ð3Þ
P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
for the rate in radiationless spin conversion processes in solution
Since Keq = kf/kb, the following equation results: kb ¼
1
s2 K eq þ 1
k obs 2
¼ : K eq þ 1
ð4Þ k¼ k obs 2
Therefore, from the experimental Keq and values, the kinetic constants of forward and backward reactions can be estimated. The numerical results in both solvents are reported in Table 2. First of all, it is worthwhile noticing that Keq parameter is sensitive to the polarity of the solvent: reminding that at 298 K in toluene Keq was estimated [22] to be 0.9, it is evident that the larger the solvent dipole moment (l(C6H5CH3)) < l(CHCl3) < l(CH2Cl2)), the higher is the percentage of low-spin CoIII(Cat-N-BQ)(Cat-N-SQ) respect to high-spin CoII(Cat-N-BQ)2 species, since the former has a larger intramolecular charge separation. Furthermore, it is worthwhile stressing the order of magnitude of the room-temperature kinetic constants of back valence tautomerization: they are larger between ten and hundred times than those estimated by Adams and Hendrickson [15] for other four cobalt bis-o-quinone complexes. In order to rationalize this numerical difference, it is necessary to elucidate the microscopic details of the back valence tautomerization process. A quantum mechanical theory of radiationless transitions was proposed by Buhks et al. [29] in order to explain the dynamics of spin crossover processes in solution. In this model the high spin ! low spin relaxation is described as a non-adiabatic radiationless process in the strong vibronic coupling limit occurring between two spin states in different nuclear configurations. The electronic energy of the initial state is transformed into the vibrational energy of the final state. The same parameters which modulate the rate of the spin crossover relaxation, i.e., DE (the energy separation between the high-spin and low-spin species), V (the electronic coupling matrix between initial and final states), DQ (the magnitude of the change in the reaction coordinate) and hx (the vibrational frequency of the mode involved in the reaction coordinate) can be assumed to characterize the valence tautomerization. It is then reasonable to describe the valence tautomeric interconversion within the framework proposed by Buhks et al. Starting from the Fermi Golden rule and invoking the Condon approximation, they derived the following expression
Table 2 Values of the equilibrium constants (Keq) and of the kinetic constants for the forward (kf) and backward (kb) reactions of the tautomerization process (Eq. (1)) in chloroform and dichloromethane at room temperature
Keq kf (s1) kb (s1)
15
CHCl3
CH2Cl2
0.7 9.8 · 108 1.4 · 109
0.3 4.5 · 108 1.5 · 109
2p g jV j2 G: h f
ð5Þ
Eq. (5) was first applied to valence tautomerization by Adams and Hendrickson [15]. The parameter gf of Eq. (5) is the electronic degeneracy of the final state (i.e., gls-CoIII ¼ 4 in our case); V represents the electronic coupling matrix element, and G is the thermally averaged nuclear Franck–Condon vibrational overlap factor. By assuming that only one internal mode with frequency x and with displacement Dr contributes to G and that the solvent is represented by very low frequency oscillators generating a continuous phonon spectrum, G is given [29] by 1 S G¼ exp½S coth x pxI p ; ð6Þ hx sinh x 2
measures the contribuwhere the parameter S ¼ mxðDrÞ 2 h tion of the change in the internal normal mode, x = hx/2kT is the reduced internal frequency and Ip is the modified Bessel function of order p where p¼
DE hx
ð7Þ
is the reduced electronic energy gap. Since the largest geometric changes accompanying the valence tautomeric process involve the metal–ligand bond length with ˚ [13,14,30,31], an initial assumption is that Dr 0.18 A the reaction coordinate will be approximately equivalent to the totally symmetric metal–ligand stretching normal coordinate of the complex whose frequency [32] is 350 cm1. Therefore, S = 30 and x = 0.85 at 298 K. As admitted by the authors [29], one shortcoming of the theory is its incomplete consideration of the reaction entropy. Spin multiplicities are fully taken into account, but entropy changes due to the solvent orientation are ignored. Therefore, it was suggested that the missing entropic contribution can be approximately accounted for by replacing DE with DG0 in Eq. (7) [29]. Since the value of the equilibrium constant for the tautomerization reaction is close to 1, the free energy of the reaction is approximately zero and consequently p = 0. From Eq. (6), we then obtain 2pGx ¼ k b g hjVxj2 ¼ 5 105 cm=s. By f introducing the experimental values of kb in Eq. (5), we can determine the |V| factor: it amounts to 495 and 510 cm1 in chloroform and dichloromethane, respectively. These values are definitely larger than those estimated by Adams and Hendrickson [15] for the cobalt bis-o-quinone complexes they studied. If the value of the reaction enthalpy [22] (DH0 = 42 kJ mol1) is used for DE in Eq. (7), an unrealistically large value of 9 · 104 cm1 for the electronic coupling coefficient is obtained. This appears as an implicit confirmation of the fundamental role played by the entropic
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P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
contribution in determining the kinetics of the valence tautomerization in these molecular systems. Therefore, we can draw the conclusion that the stronger electronic coupling between the high-spin CoII(CatN-BQ)2 and the low-spin CoIII(Cat-N-BQ)(Cat-N-SQ) species respect to those of the other Co bis-o-quinone complexes up to now analyzed, is responsible for the faster process of back valence tautomerization.
5. Concluding remarks This work provides insight into the mechanism of the thermally reversible valence tautomerism for a dioxolene cobalt complex. This process takes three distinct steps: a first step where the electron transfer between the ligand and the metal is induced by photo-excitation; a second one where there is an electron redistribution leading to the high-spin CoII(Cat-N-BQ)2 state, and a third one where the equilibrium between high-spin CoII (Cat-N-BQ)2 and the low-spin CoIII(Cat-N-BQ)(Cat-NSQ) species is thermally restored. All these events occur in less than two nanoseconds from the instant of the photo-excitation. The photo-induced molecular dynamics of dioxolene cobalt complex can be schematically illustrated with the aid of the mono-dimensional potential energy diagram shown in Fig. 6. In Fig. 6, A represents the low-spin CoIII(Cat-N-BQ)(Cat-N-SQ); A* is the intermediate electronic state reached from A with a vertical Franck–Condon transition. The experimental evidence and the additional arguments brought in the previous discussion are not conclusive enough to allow a resolutive assignment of this state. It can be attributed
Fig. 6. Schematic representation of the potential energy surfaces: A represents the low-spin CoIII(Cat-N-BQ)(Cat-N-SQ) state; A* is the first transient having a lifetime (in chloroform) of 180 fs; B is the highspin CoII(Cat-N-BQ)2 state with a lifetime of 410 ps. The corresponding lifetimes in dichloromethane are given in Table 1.
to the low-spin CoII(Cat-N-BQ)2 species having a single electron in the metal ion eg orbital or to a diradical adduct of the Co(IV) metal ion. B is the metastable high-spin CoII(Cat-N-BQ)2 state populated directly by a decay from A* in less than 1 ps. The internal coordinate involved in the valence tautomerism consists of the metal–ligand (Co–O) bond length and the internal o-quinone bond lengths: Co–O progressively increases passing from A to B as the eg r-antibonding orbitals are occupied. The changes of the internal o-quinone bond lengths are of lesser magnitude and often can be neglected. Finally, B reverts to A through a thermally driven non-adiabatic process whose large rate constant (1.5 · 109 s1) can be interpreted, by applying the theory developed by Buhks et al. [29], as due to a strong electronic coupling between high-spin CoII(Cat-N-BQ)2 and low-spin CoIII(Cat-N-BQ)(Cat-N-SQ) states.
Acknowledgements The authors are indebted to one of the referees for useful comments and suggestions. The present work was supported by the Italian Ministero dellÕIstruzione, Universita` e Ricerca (MIUR) and by the Commisiion of the European Communities under Contract No. RII3-CT-2003-506350.
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P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17 [18] F. Varret, M. Nogues, A. Goujon, in: J.S. Miller, M. Drillon (Eds.), Magnetism: Molecules to Materials, Wiley–VCH, Weinheim, 2001. [19] F.V.R. Neuwahl, R. Righini, A. Dei, Chem. Phys. Lett. 352 (2002) 408. [20] S.K. Larsen, C.G. Pierpont, J. Am. Chem. Soc. 110 (1988) 1827. [21] O. Cador, F. Chabre, A. Dei, C. Sangregorio, J. Van Slageren, M.G.F. Vaz, Inorg. Chem. 42 (2003) 6432. [22] A. Caneschi, A. Cornia, A. Dei, Inorg. Chem. 37 (1998) 3419. [23] A.Y. Girgis, A.L. Balch, Inorg. Chem. 14 (1975) 2724. [24] F.V.R. Neuwahl, L. Bussotti, P. Foggi, Res. Adv. Photochem. Photobiol. 1 (2000) 77. [25] P. Foggi, L. Bussotti, F.V.R. Neuwahl, Int. J. Photoenergy 3 (2001) 103.
17
[26] S. Bruni, A. Caneschi, F. Cariati, C. Delfs, A. Dei, D. Gatteschi, J. Am. Chem. Soc. 116 (1994) 1388. [27] D.F. Evans, J. Chem. Soc. (1959) 2003. [28] D.M. Adams, L. Noodleman, D.N. Hendrickson, Inorg. Chem. 36 (1997) 3966. [29] E. Buhks, G. Navon, M. Bixon, J. Jortner, J. Am. Chem. Soc. 102 (1980) 2918. [30] D.M. Adams, A. Dei, A.L. Rheingold, D.N. Hendrickson, J. Am. Chem. Soc. 115 (1993) 8221. [31] A. Bencini, A. Caneschi, C. Carbonera, A. Dei, D. Gatteschi, R. Righini, C. Sangregorio, J. Van Slageren, J. Mol. Struct. 656 (2003) 141. [32] E. Buhks, M. Bixon, J. Jortner, J. Phys. Chem. 85 (1981) 3763.
Time-resolved spectroscopic characterization of photo-induced valence tautomerism for a cobalt–dioxolene complex Pier Luigi Gentili
b
a,*
, Laura Bussotti a, Roberto Righini Lapo Bogani b, Andrea Dei b
a,b,*
, Alessandra Beni b,
a LENS, UniversitaÕ di Firenze, Via Nello Carrara 1, 50019 Sesto Fiorentino, Firenze, Italy Dipartimento di Chimica, UniversitaÕ di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy
Received 24 November 2004; accepted 21 January 2005 Available online 16 February 2005
Abstract The valence tautomerism of low-spin CoIII(Cat-N-BQ)(Cat-N-SQ) (where Cat-N-BQ is 2-(2-hydroxy-3,5-di-tert-butylphenylimino)-4,6-di-tert-butylcyclohexa-3,5-dienone and Cat-N-SQ is the dianionic radical analogue) was investigated by means of UV–vis pump–probe transient absorption spectroscopy and 1H NMR technique in chloroform and dichloromethane. By exciting the CT transition of the complex at 480 nm, an intramolecular electron transfer process is selectively triggered. The photo-induced charge transfer is pursued by a cascade of two main molecular events characterized by the ultrafast transient absorption spectroscopy: the first gives rise to the metastable high-spin CoII(Cat-N-BQ)2 that, secondly, reaches the chemical equilibrium with the reactant species. The rate constant of back valence tautomerization estimated by measuring the lifetime of high-spin CoII(Cat-N-BQ)2 species and the equilibrium constant for the CoIII(Cat-N-BQ)(Cat-N-SQ) ¢ CoII(Cat-N-BQ)2 interconversion, is significantly large (on the order of 109 s1). It is interpreted under the point of view of the theory formulated by Jortner and Buhks et al. for nonadiabatic radiationless processes. 2005 Elsevier B.V. All rights reserved. Keywords: Valence tautomerism; Cobalt–dioxolene; Transient absorption
1. Introduction Molecular systems undergoing a reversible and controlled change of their physical properties following a change of an external parameter constitute an appealing perspective for the realization of molecular scale electronic devices [1,2]. The use of light for controlling the magnetic properties of molecular systems represents an intriguing tool for establishing potential applications * Corresponding authors. Tel.: +390554572485; fax: +390554572451. E-mail addresses: [email protected]fi.it (P.L. Gentili), righini@lens. unifi.it (R. Righini).
0301-0104/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2005.01.020
for information storage [3,4]. Examples of systems showing this behaviour are metal complexes undergoing spin-crossover and valence tautomeric interconversion [5–12]. Both these classes of metal complexes show interplay between charge mobility and magnetic properties and then their analogy with the spintronic devices is often emphasized. However, in addition to their possible technological interest, there is another important point deserving consideration. These metal derivatives offer the possibility of using molecular systems as ideal testing grounds for the observation of sophisticated physical properties. In particular, their investigation may provide new tools for understanding the mechanism of charge
10
P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
transfer in the presence of magnetic fields including optical irradiation. This statement is particularly appropriate when cobalt–dioxolene complexes are considered. O-dioxolenes can bind to metal ions in three different redox states, corresponding to catecholate, Cat, semiquinone, SQ, and quinone, Q, respectively. The low covalent character of their bond to the metal ion in general gives rise to a well-defined charge distribution. In fact the metal dioxolene complex can often unambiguously be described, on the basis of their structural, magnetic and spectroscopic features, as one of the isomeric species [Mn+–Cat2](n 2)+, or [M(n 1)+–SQ](n 2)+, or [M(n 2)+–Q](n 2)+. Switchable molecules can be obtained when the coordinated dioxolene ligand to the metal ion, provide pairs of isomers having comparable energies. These systems have been called valence tautomers, the interconversion involving an intramolecular electron transfer between the metal acceptor and the coordinated redox active ligand [13,14]. The best investigated case is that of cobalt–dioxolene molecules, where the valence tautomeric interconversion involves an intramolecular electron transfer between a six-coordinated diamagnetic cobalt(III) metal ion ðt62g ; S ¼ 0Þ and a polyoxolene ligand yielding a high-spin cobalt(II) ðt52g e2g ; S ¼ 3=2Þ complex of the ligand with a less negative oxidation number. The possibility of this process is due to the localized electronic properties of the two interacting donor and acceptor moieties, i.e., a low covalent bonding character. This assumption in particular holds when the dioxolene is coordinated to a metal ion in low oxidation state, i.e., cobalt(II), whereas it can be considered only an operational approximation for metal ions in high oxidation state, like in the present case cobalt(III). Following this peculiar property, these complexes provide an important class of model compounds for understanding the details of the intramolecular electron transfer processes involving a transition metal ion and a coordinated redox active ligand. The first class of cobalt complexes showing valence tautomerism in condensed phase was constituted by molecular dioxolene complexes of general formula Co(N-N)(diox)2 (N-N = diazine ligand, diox = 3,5and 3,6-di-tert-butylcatecholato (DBCat) or the parent semiquinonato (DBSQ)). Pulsed laser photolysis studies carried out by Hendrickson and co-workers showed [15,16] that the transition
creased up to several hours. No details associated with the electronic mechanism associated with the transition were provided. Recently some of us have shown [19] that the time resolved femtosecond spectroscopic characterization of the CoII(Me4cyclam)(DBSQ)+ (Me4cyclam = 1,4,8,11tetramethyltetraazacyclotetradecane) was consistent with the involvement of at least three different electronic states in the optical interconversion process. Although it seems reasonable to believe that the kinetics of these processes can be largely modulated by the Franck– Condon reorganization energies and the energy differences between the ground and the metastable states, it is rather unclear the role of the electronic coupling between the ls-CoIII and hs-CoII states. Indeed, it is usually believed that differences in spin multiplicities may play a determinant role. Bearing this in mind, we have considered the cobalt complex formed by the monoanion of the tridentate ligand 2-(2-hydroxy-3,5-di-tert-butylphenylimino)-4,6-di-tert-butylcyclohexa-3,5-dienone (CatN-BQ (I)) as well as by its dianionic semiquinonato radical analogue (Cat-N-SQ) (II) (see Scheme 1). This complex was formulated as CoIII(Cat-NBQ)(Cat-N-SQ) at room temperature. The mixed ligand character of this complex was suggested on the basis of the structural parameters and was found consistent with the spectroscopic and magnetic properties of the solid compound [20]. However, the temperature dependence of the spectral and magnetic properties of this compound in the solid state [21] and in non-polar solvents [22] suggests the existence of the valence tautomeric equilibrium CoIII ðCat-N-BQÞðCat-N-SQÞ ¢ CoII ðCat-N-BQÞ2 ðS¼1=2Þ
ðS¼3=2Þ
The electronic spectrum of the CoIII(Cat-N-BQ)(Cat-NSQ) species is characterized by well-separated intraligand and charge transfer bands. In particular the CT transition at 526 nm, does not show any significant overlap with the absorption bands of the CoII(Cat-N-BQ)2 chromophore. This situation was not found for the previously investigated Co(N-N)(diox)2 complexes [15– 18].Therefore, this compound was believed to constitute a good molecular system for elucidating the electronic processes associated with the photo-induced valence
ls-CoIII ðN-NÞðDBSQÞðDBCatÞ ! hs-CoII ðN-NÞðDBSQÞ2 may be induced in solution by optical irradiation, the induced metastable states being characterized by some nanoseconds lifetimes. Some years later, Sato et al. [17] and Varret et al. [18] showed that the same transition can be optically induced in condensed phase at 5 K, the lifetime of the photo-induced product being in-
. N
N O
O
O
Cat-N-BQ (I)
O Cat-N-SQ (II)
Scheme 1.
P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
tautomeric transitions. Here, we wish to report the results of a transient absorption spectroscopic study using femtosecond pulsed laser photolysis.
2. Experimental 2.1. Materials The CoIII(Cat-N-BQ)(Cat-N-SQ) complex was prepared according to the previously reported [23] procedure. The solvents dicholoromethane and chloroform from Fluka were purified according to the standard procedures. Chloroform-d1 (99.8%) from Merck and dichloromethane-d2 from Aldrich (99.5%) were used as received. 2.2. Spectroscopic measurements The experimental instrumentation and data processing for femtosecond time-resolved transient absorption spectroscopy have been described in detail in previous papers [24,25]. Briefly, ultrashort pulses (duration 100 fs at 800 nm, repetition rate 1 kHz, energy 700 lJ/pulse) are produced by a regenerative amplified Ti:Sapphire laser system. Tunable excitation pulses are obtained by means of a BBO-based optical parametric generator (OPG). In the present experiment, the excitation wavelength of 480 nm was obtained by sum frequency generation of fundamental (800 nm) and signal (1.2 lm). A small portion of the output of the amplifier, 2 lJ/pulse, was focused on a 2.5 mm thick calcium fluoride plate to generate the white-light continuum used for probing. The resulting spectrally broadened laser pulse spanned the entire visible region and extended in the near UV to roughly 350 nm. The white-light continuum was further split into two parts of equal intensity by a 50/50 fused-silica–Al beam splitter. One part, acting as a probe beam, was spatially overlapped with the excitation beam in the sample. The second part crossed the sample in a different position and provided a convenient reference signal. The probe and reference beams were spectrally dispersed in a flat-field 25 cm Czerny-Turner spectrometer, and detected by means of a back illuminated CCD camera with spectral response in the region 300–1000 nm. Two different configurations of the detection system were utilized to obtain the data sets herein presented, which are composed of both transient spectra (transmittance of the excited state vs. wavelength at a given pump–probe delay time) and kinetic plots (intensity of the probe vs. delay time at a fixed wavelength). In the transient spectra, the transmittance at a given delay time t and wavelength k, T(t, k), is defined as I(t, k)/I0(k), where I(t, k) and I0(k) are the intensities of the white-light continuum components reaching the
11
detector having or having not interacted with the pump pulse, respectively. Two measurements were performed: the first without the pump beam, thus acquiring the intensities of the probe (I0(k)) and reference pulses (IR(k)); the second with both excitation and probe beams, thus measuring I(t, k) and I 0R ðkÞ. The transient transmittance T(t, k) is then given by ½Iðt; kÞ=I 0R ðkÞ= ½I 0 ðkÞ=I R ðkÞ. Recording kinetic plots requires narrow bandwidth detection. The desired wavelengths were selected with 5 nm bandwidth interference filters. The intensity of the probe pulse was measured by a silicon difference photodiode and a lock-in amplifier synchronized to a chopper, switching the pump pulse on and off at half the repetition-rate of the laser system (500 Hz). In this way, the reading of the phase-locked amplifier gave the modulation of the probe pulse intensity due to the interaction with the pump pulse. In every experiment the relative pump–probe polarization was set to the magic angle (54.7) to discriminate against orientational dynamics. The sample solutions flew through a 1 mm thick calcium fluoride cell connected to a solution reservoir and a pump system. All the samples were prepared to have an optical density of approximately 1 at the excitation wavelength. Steady-state absorption spectra of the solutions were measured before and after the experiments to check for possible sample decomposition. All measurements were carried out at room temperature (22). 1 H NMR spectra were recorded on a Varian spectrometer operating at 300 MHz.
3. Results 3.1. Spectral properties The electronic absorption spectrum of CoIII(Cat-NBQ)(Cat-N-SQ) complex dissolved in chloroform is shown in Fig. 1. By comparing this spectrum with those of Zn(Cat-N-BQ)2 [22] and M(Cat-N-SQ)2 (where M = Ge and Sn) [26] as well as with that of Al(Cat-NBQ)(Cat-N-SQ) complex [22], it is evident that all the bands appearing in the near-UV, visible and near-IR regions, except that centered at 526 nm, are due to intraligand electronic transitions. The intense band at 526 nm is due to a charge-transfer transition. In order to trigger the intramolecular electron transfer between the ligand and the metal ion, CoIII(Cat-NBQ)(Cat-N-SQ) was excited at 480 nm. The wavelength of the pump pulse was chosen on the edge of the CT band, far from the maximum of a weak band centered at 538 nm and generated by an intraligand electronic transition [22]. By this selective photo-excitation only the CT was induced: the consequent spectral evolution, recorded in the near UV and visible regions in the time
12
P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
of this spectral correction is shown in the inset of Fig. 2. After a few hundreds of femtoseconds the transient absorption band centered at 407 nm decreases in intensity whereby the bleaching components at 385, 436 and 550 nm become spectrally dominant. The disappearance of the first absorption band located in the blue portion of the visible region is accompanied by the appearance of a new absorption component located in the red portion of the spectrum that can be interpreted as the high frequency edge of a band situated in the far-red region. All the spectral features disappear in 1 ns from the excitation, suggesting that a complete recovery of the electronic ground state of the photo-sensitive CoIII(Cat-N-BQ)(Cat-N-SQ) is reached. Fig. 1. Steady-state absorption spectrum of CoIII(Cat-N-BQ)(Cat-NSQ) complex in chloroform. The down arrow shows the position of the excitation wavelength.
3.2. Kinetics
window between 0 and 1 ns, monitors the dynamics of valence tautomerism. The transient absorption spectra collected for CoIII(Cat-N-BQ)(Cat-N-SQ) in chloroform are portrayed in Fig. 2 at selected delay times. The interaction of the sample with the pump pulse instantaneously causes a transient absorbance increase centered at 407 nm that reaches its maximum within the pump–probe cross-correlation envelope (200 fs). The shape of this first transient absorption band is influenced by the spectral contribution of the bleaching of the CoIII(Cat-N-BQ)(Cat-N-SQ) electronic ground state absorption, producing two negative peaks at 385 and 436 nm, respectively. The absorption contribution can be separated from that of bleaching by adding to the transient spectrum at early delay time (DODTOT exp ðtÞ vs. k) the normalized steady-state absorption spectrum (ARif vs. k) of CoIII(Cat-N-BQ)(Cat-N-SQ). The result CoIII
To characterize quantitatively the evolution of CoIII(Cat-N-BQ)(Cat-N-SQ) after photo-excitation we recorded the intensities of the probe pulse at fixed wavelengths as functions of the pump–probe delay time. Two spectral regions are crucial to rationalize the relaxation dynamics: the blue portion of the spectrum covered by the first absorption band and the red portion where the second absorption component appears. The kinetics recorded at 409 nm and at 620 nm over the first 3 ps are shown in Figs. 3(a) and (b), respectively. The kinetics at 409 nm shows a fast rise followed by decay. The rise occurs on a time interval corresponding to the pump–probe cross-correlation function (200 fs), whereas the subsequent drop is characterized by a lifetime of s1 = (180 ± 20) fs. The kinetics at 620 nm shows an intensity growth well-described by a convolution of the response function with an exponential function having s1 = (180 ± 20) fs as lifetime, equivalent to that estimated from the kinetics at 409 nm. The numerical
Fig. 2. Transient absorption spectra of CoIII(Cat-N-BQ)(Cat-N-SQ) in chloroform, recorded at pump–probe delays of 200 fs, 600 fs, 1 ps and 1 ns. The excitation wavelength was 480 nm. In inset: corrected Rif transient absorption spectrum ðDODTOT exp ðtÞ þ ACoIII Þ measured at 200 fs delay time.
Fig. 3. Amplitude vs. time profiles of the transient signals recorded at (a) 409 nm and (b) 620 nm for CoIII(Cat-N-BQ)(Cat-N-SQ) in chloroform for short time delays.
P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
13
equivalence between the two s1 lifetimes unveiled that the disappearance of the band in the blue region was accompanied by the simultaneous appearance of the absorption band in the red region. From the kinetics at 620 nm, it was also possible to follow the progressive disappearance of the band located in the red portion of the spectrum occurring in some hundreds of picoseconds as timescale: it is reported in Fig. 4. The experimental data are well-fitted by a mono-exponential function with s2 = (410 ± 20) ps; no residual intensity is left after 1 ns. 3.3. Solvent effects In valence tautomeric complexes two or more electronic states lie close in energy. This involves significant vibronic interactions and then an appreciable sensitivity to the environment. For this reason the behaviour of CoIII(Cat-N-BQ)(Cat-N-SQ) was also investigated in dichloromethane and in the deuterated forms of chloroform (CDCl3) and dichloromethane (CD2Cl2). For all the solvents used, the spectral evolutions recorded after photo-excitation at 480 nm are qualitatively similar (see Section 3.1 for details). Furthermore, there are no quantitative discrepancies between the results obtained in CHCl3 and CDCl3 within the time resolution achievable with our instrumentation; similarly, no difference was observed between the data collected in CH2Cl2 and in CD2Cl2. Appreciable differences were instead revealed between the lifetimes determined for CoIII(Cat-NBQ)(Cat-N-SQ) in chloroform and those obtained in dichloromethane. In Fig. 5, the kinetics recorded at 620 nm in CHCl3 (solid curve) and in CH2Cl2 (dashed curve) are graphically compared. The rise of the tran-
Fig. 5. Amplitude vs. time profiles recorded at 620 nm for CoIII(CatN-BQ)(Cat-N-SQ) in CH2Cl2(dashed curves) and in CHCl3 (solid curve) for short delay times. The long delay time behaviour is shown in the inset.
sient band is clearly faster in dichloromethane than in chloroform. On the contrary, the intensity decay of the same band is slower in dichloromethane. In Table 1, the corresponding rise (s1) and decay (s2) times, obtained from an exponential fitting procedure of the data measured for the solvents, are collected. CoIII(Cat-N-BQ)(Cat-N-SQ) and its valence tautomer CoII(Cat-N-BQ)2 have energetically close-lying electronic states [20,21]. Spectroscopic, magnetic and NMR measurements [21] support the existence of a valence tautomeric equilibrium low-spin CoIII ðCat-N-BQÞðCat-N-SQÞ kf
¢ high-spin CoII ðCat-N-BQÞ2 kb
ð1Þ
in solution at room temperature. In analogy to the methodology pursued in [22], we determined the equilibrium constants values in chloroform and in dichloromethane at room temperature (298 K) by performing measurements of magnetic susceptibility and applying the Evans method [27]. Assuming a vT of 0.38 emu mol1 K for the CoIII(CatN-BQ)(Cat-N-SQ) reactant species (in agreement with the data obtained for the solid) and a vT of 3.21 emu mol1 K for the CoII(Cat-N-BQ)2 species (as extrapolated from solutions vT values), the Keq = kf/kb values of 0.7 and 0.3 were obtained at 298 K in chloroform and dichloromethane, respectively.
4. Discussion Fig. 4. Amplitude vs. time profile for the transient signal recorded at 620 nm for CoIII(Cat-N-BQ)(Cat-N-SQ) in chloroform in the 0–1000 ps time window.
In a previous work [22] where the valence tautomeric interconversion between the low-spin CoIII(Cat-NBQ)(Cat-N-SQ) and the high-spin CoII(Cat-N-BQ)2
14
P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
Table 1 Experimental lifetimes of the two chemical components responsible for the entire spectral evolution for CoIII(Cat-N-BQ)(Cat-N-SQ) after excitation at 480 nm in chloroform and dichloromethane
s1 (fs) s2 (ps)
CHCl3
CH2Cl2
180 ± 20 410 ± 20
160 ± 20 490 ± 20
was induced by heating, it was shown that the process can be monitored following the changes of the intensities of the absorption bands at 800, 730, 533, 439 and 391 nm. In particular, the oxidation of the di-anionic semiquinonato ligand to the mono-anion catecholato analogue determines a significant increase of the absorbance in the 550–800 nm spectral region. The broad absorption band appearing above 550 nm in our transient spectra some hundreds femtoseconds after the excitation can then be ascribed to the formation of high-spin CoII(Cat-N-BQ)2, as supported by the here reported magnetic data and previously discussed ESR behaviour [21]. The spectral evolution described in the previous section shows that this molecular state is preceded by another state responsible of the transient band centered at 407 nm whose shape is reported in the inset of Fig. 2. This state is produced soon after the excitation at 480 nm and has a short lifetime of about 180 fs. By assuming that the dioxolene cobalt complex involved in the valence tautomerisation has an octahedral symmetry (Oh), the d-orbitals of the metal ion can be split into the familiar t2g and eg sets. Furthermore, the p* ligand orbitals involved in the electron transfer are empty in the Cat-N-BQ form, whereas they contain a single electron in the Cat-N-SQ oxidation state. Hence, two plausible mechanisms interpreting the experimental evidences and involving two electronic transitions can be formulated, as indicated in the following scheme: 6
0
1
0 1
6
1
0
ðt2g Þ ðeg Þ ðpSQ Þ ðpBQ Þ !ðt2g Þ ðeg Þ ðpBQ1 Þ ðpBQ2 Þ 2
5
2
0
!ðt2g Þ ðeg Þ ðpBQ1 Þ ðpBQ2 Þ 6
0
1
0
0 0 1
ð2aÞ 5
0
1
ðt2g Þ ðeg Þ ðpSQ Þ ðpBQ Þ !ðt2g Þ ðeg Þ ðpSQ1 Þ ðpSQ2 Þ 20
5
2
0
!ðt2g Þ ðeg Þ ðpBQ1 Þ ðpBQ2 Þ
0
0
1
ð2bÞ
In mechanism (2a) the first step, occurring in a time interval shorter than the experimental time resolution, involves an electron transfer from the pSQ orbital to the antibonding eg orbital of the CoIII metal ion, i.e., it is a LMCT transition; on the other hand, in mechanism (2b) it involves an electron transfer from one of the t2g orbitals of CoIII to the pBQ orbital of the CatN-BQ ligand, i.e., it is a MLCT transition. In the mechanism (2a), the low-spin CoII(Cat-N-BQ)2 state is produced in the first step. In less than 1 ps low-spin CoII(Cat-N-BQ)2 transforms to the high-spin CoII(Cat-
N-BQ)2 through an electronic redistribution localized in the CoII metal ion. On the contrary, if the photoreaction proceeds according to mechanism (2b), the first step gives rise to CoIV(Cat-N-SQ)2 state. In agreement with the experimental evidences, CoIV(Cat-N-SQ)2 produces high-spin CoII(Cat-N-BQ)2 state in a single step occurring in some hundreds femtoseconds. This process should entail a concerted bielectronic transfer from the two pSQ orbitals of the Cat-N-SQ ligands to the CoIV metal ion along with a structural rearrangement of the entire molecule. The choice between the two plausible mechanisms is not trivial. As far as we know, among the organometallic valence tautomers investigated up to now, there are not examples of processes like the second step of Scheme (2b), implying a simultaneous oxidation of two ligands and the bielectronic reduction of a metal ion. Furthermore, the measured conversion time of the CoIV(CatN-SQ)2 ! CoII(Cat-N-BQ)2 is surprisingly fast if compared to the relevant structural rearrangement required. The second step of mechanism (2a) is instead much more likely to be very fast, since it involves two states, the low-spin CoII and the high-spin CoII that according to DFT calculations [28] performed for similar o-quinone Co complexes, are expected to have almost the same energy. If we incline to (2a) mechanism, we should admit that the first step entails a ligand-tometal-charge-transfer. This is apparently in contradiction with the high extinction coefficient values determined for the CT band centered at 526 nm: since the LMCT is a symmetry-forbidden transition, involving two nearly orthogonal orbitals, it should give rise to a band of weak intensity, as it was revealed for other oquinone Co complexes in [28]. On the contrary, the alternative MLCT (first step of Scheme (2b)) is a symmetry-allowed transition and it can justify the observed high intensity of the CT band. However, DFT calculations [28] have revealed that in o-quinone Co complexes, while there are still localized electronic structural features reflecting the different metal and ligand oxidation states in the low-spin CoIII and high-spin CoIII tautomeric forms, appreciable covalent interactions exist between the cobalt ion and the ligands. These covalent interactions lead to appreciable mixing of orbitals and could influence the symmetry selection rules, rendering the LMCT a transition of appreciable intensity. As the high-spin CoII(Cat-N-BQ)2 is produced, it reaches the equilibrium with low-spin CoIII(Cat-NBQ)(Cat-N-SQ) within about 1 ns. The set-up of the equilibrium involves a reversible first-order reaction, where the experimentally determined lifetime ðs2 ¼ 1=k obs 2 Þ is correlated to the kinetic constants kf and kb characterizing the equilibrium by the relation s2 ¼
1 : kf þ kb
ð3Þ
P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
for the rate in radiationless spin conversion processes in solution
Since Keq = kf/kb, the following equation results: kb ¼
1
s2 K eq þ 1
k obs 2
¼ : K eq þ 1
ð4Þ k¼ k obs 2
Therefore, from the experimental Keq and values, the kinetic constants of forward and backward reactions can be estimated. The numerical results in both solvents are reported in Table 2. First of all, it is worthwhile noticing that Keq parameter is sensitive to the polarity of the solvent: reminding that at 298 K in toluene Keq was estimated [22] to be 0.9, it is evident that the larger the solvent dipole moment (l(C6H5CH3)) < l(CHCl3) < l(CH2Cl2)), the higher is the percentage of low-spin CoIII(Cat-N-BQ)(Cat-N-SQ) respect to high-spin CoII(Cat-N-BQ)2 species, since the former has a larger intramolecular charge separation. Furthermore, it is worthwhile stressing the order of magnitude of the room-temperature kinetic constants of back valence tautomerization: they are larger between ten and hundred times than those estimated by Adams and Hendrickson [15] for other four cobalt bis-o-quinone complexes. In order to rationalize this numerical difference, it is necessary to elucidate the microscopic details of the back valence tautomerization process. A quantum mechanical theory of radiationless transitions was proposed by Buhks et al. [29] in order to explain the dynamics of spin crossover processes in solution. In this model the high spin ! low spin relaxation is described as a non-adiabatic radiationless process in the strong vibronic coupling limit occurring between two spin states in different nuclear configurations. The electronic energy of the initial state is transformed into the vibrational energy of the final state. The same parameters which modulate the rate of the spin crossover relaxation, i.e., DE (the energy separation between the high-spin and low-spin species), V (the electronic coupling matrix between initial and final states), DQ (the magnitude of the change in the reaction coordinate) and hx (the vibrational frequency of the mode involved in the reaction coordinate) can be assumed to characterize the valence tautomerization. It is then reasonable to describe the valence tautomeric interconversion within the framework proposed by Buhks et al. Starting from the Fermi Golden rule and invoking the Condon approximation, they derived the following expression
Table 2 Values of the equilibrium constants (Keq) and of the kinetic constants for the forward (kf) and backward (kb) reactions of the tautomerization process (Eq. (1)) in chloroform and dichloromethane at room temperature
Keq kf (s1) kb (s1)
15
CHCl3
CH2Cl2
0.7 9.8 · 108 1.4 · 109
0.3 4.5 · 108 1.5 · 109
2p g jV j2 G: h f
ð5Þ
Eq. (5) was first applied to valence tautomerization by Adams and Hendrickson [15]. The parameter gf of Eq. (5) is the electronic degeneracy of the final state (i.e., gls-CoIII ¼ 4 in our case); V represents the electronic coupling matrix element, and G is the thermally averaged nuclear Franck–Condon vibrational overlap factor. By assuming that only one internal mode with frequency x and with displacement Dr contributes to G and that the solvent is represented by very low frequency oscillators generating a continuous phonon spectrum, G is given [29] by 1 S G¼ exp½S coth x pxI p ; ð6Þ hx sinh x 2
measures the contribuwhere the parameter S ¼ mxðDrÞ 2 h tion of the change in the internal normal mode, x = hx/2kT is the reduced internal frequency and Ip is the modified Bessel function of order p where p¼
DE hx
ð7Þ
is the reduced electronic energy gap. Since the largest geometric changes accompanying the valence tautomeric process involve the metal–ligand bond length with ˚ [13,14,30,31], an initial assumption is that Dr 0.18 A the reaction coordinate will be approximately equivalent to the totally symmetric metal–ligand stretching normal coordinate of the complex whose frequency [32] is 350 cm1. Therefore, S = 30 and x = 0.85 at 298 K. As admitted by the authors [29], one shortcoming of the theory is its incomplete consideration of the reaction entropy. Spin multiplicities are fully taken into account, but entropy changes due to the solvent orientation are ignored. Therefore, it was suggested that the missing entropic contribution can be approximately accounted for by replacing DE with DG0 in Eq. (7) [29]. Since the value of the equilibrium constant for the tautomerization reaction is close to 1, the free energy of the reaction is approximately zero and consequently p = 0. From Eq. (6), we then obtain 2pGx ¼ k b g hjVxj2 ¼ 5 105 cm=s. By f introducing the experimental values of kb in Eq. (5), we can determine the |V| factor: it amounts to 495 and 510 cm1 in chloroform and dichloromethane, respectively. These values are definitely larger than those estimated by Adams and Hendrickson [15] for the cobalt bis-o-quinone complexes they studied. If the value of the reaction enthalpy [22] (DH0 = 42 kJ mol1) is used for DE in Eq. (7), an unrealistically large value of 9 · 104 cm1 for the electronic coupling coefficient is obtained. This appears as an implicit confirmation of the fundamental role played by the entropic
16
P.L. Gentili et al. / Chemical Physics 314 (2005) 9–17
contribution in determining the kinetics of the valence tautomerization in these molecular systems. Therefore, we can draw the conclusion that the stronger electronic coupling between the high-spin CoII(CatN-BQ)2 and the low-spin CoIII(Cat-N-BQ)(Cat-N-SQ) species respect to those of the other Co bis-o-quinone complexes up to now analyzed, is responsible for the faster process of back valence tautomerization.
5. Concluding remarks This work provides insight into the mechanism of the thermally reversible valence tautomerism for a dioxolene cobalt complex. This process takes three distinct steps: a first step where the electron transfer between the ligand and the metal is induced by photo-excitation; a second one where there is an electron redistribution leading to the high-spin CoII(Cat-N-BQ)2 state, and a third one where the equilibrium between high-spin CoII (Cat-N-BQ)2 and the low-spin CoIII(Cat-N-BQ)(Cat-NSQ) species is thermally restored. All these events occur in less than two nanoseconds from the instant of the photo-excitation. The photo-induced molecular dynamics of dioxolene cobalt complex can be schematically illustrated with the aid of the mono-dimensional potential energy diagram shown in Fig. 6. In Fig. 6, A represents the low-spin CoIII(Cat-N-BQ)(Cat-N-SQ); A* is the intermediate electronic state reached from A with a vertical Franck–Condon transition. The experimental evidence and the additional arguments brought in the previous discussion are not conclusive enough to allow a resolutive assignment of this state. It can be attributed
Fig. 6. Schematic representation of the potential energy surfaces: A represents the low-spin CoIII(Cat-N-BQ)(Cat-N-SQ) state; A* is the first transient having a lifetime (in chloroform) of 180 fs; B is the highspin CoII(Cat-N-BQ)2 state with a lifetime of 410 ps. The corresponding lifetimes in dichloromethane are given in Table 1.
to the low-spin CoII(Cat-N-BQ)2 species having a single electron in the metal ion eg orbital or to a diradical adduct of the Co(IV) metal ion. B is the metastable high-spin CoII(Cat-N-BQ)2 state populated directly by a decay from A* in less than 1 ps. The internal coordinate involved in the valence tautomerism consists of the metal–ligand (Co–O) bond length and the internal o-quinone bond lengths: Co–O progressively increases passing from A to B as the eg r-antibonding orbitals are occupied. The changes of the internal o-quinone bond lengths are of lesser magnitude and often can be neglected. Finally, B reverts to A through a thermally driven non-adiabatic process whose large rate constant (1.5 · 109 s1) can be interpreted, by applying the theory developed by Buhks et al. [29], as due to a strong electronic coupling between high-spin CoII(Cat-N-BQ)2 and low-spin CoIII(Cat-N-BQ)(Cat-N-SQ) states.
Acknowledgements The authors are indebted to one of the referees for useful comments and suggestions. The present work was supported by the Italian Ministero dellÕIstruzione, Universita` e Ricerca (MIUR) and by the Commisiion of the European Communities under Contract No. RII3-CT-2003-506350.
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