Femtosecond Time-Resolved Optical and Raman Spectroscopy of Photoinduced Spin Crossover: Temporal Resolution of Low-to-High Spin Optical Switching

Abstract

Understanding the mechanisms by which molecules interconvert among their distinct electronic and/or geometric configurations in response to external perturbations constitutes an important step toward the development of molecular-based materials.1 Low-spin Fe II complexes comprise an intriguing class of compounds in this regard due to their potential utility in solar energy conversion strategies 2 as well as the basis for magneto-optical devices. 3 It is well-known that photoexcitation of such compounds results in the eventual formation of the high-spin form of the molecule; the net two-quantum spin conversion coincides with an exceptionally large structural reorganization (∆r Fe-L ∼0.2 Å, ∆V ∼25 cm 3 mol -1 ), significant attenuation of the compound's optical density (∆ ∼10, and a dramatic change in the compound's magnetic properties (S ) 0 f S ) 2). 4 Several groups have exploited various spectroscopic probes in an effort to quantify the dynamics of this spin-state conversion but thus far have only been able to define an upper limit for the time constant associated with formation of the high-spin state.5 In this report, we present femtosecond optical and stimulated Raman scattering data on a prototypical low-spin Fe II charge-transfer complex that, for the first time, kinetically resolves the formation of the high-spin state and thus defines the time scale for optical switching in this class of compounds.The compound under study is 3 )] 2+ (R ) H (1), CH 3 (2)), which is illustrated in Chart 1 along with a simplified energy level diagram showing the low-lying ligand-field excited states of a d 6 transition metal ion. As shown by Drago and coworkers, 6 this compound sits very close to the so-called spincrossover point wherein the energies of the low-spin ( 1 A 1 ) and highspin ( 5 T 2 ) forms are in close proximity. Compound 1 possesses a low-spin ground state, whereas the steric constraints imposed by the introduction of the CH 3 groups in compound 2 require an elongation of the Fe-N bond, resulting in stabilization of the highspin 5 T 2 term as its ground state. This system thus has the unique characteristic of having reciprocal ground-and lowest-energy excited states; i.e., the lowest-energy excited state of one corresponds to the ground state of the other. We have previously exploited this feature to probe the ultrafast electronic absorption spectroscopy of [Fe(tren(py) 3 )] 2+ in the visible region and have shown that charge-transfer excitation of this compound results in the formation of the 5 T 2 state in <1 ps. 5aOwing to the nature of the absorption spectra of the low-spin and high-spin forms of the molecule, more diagnostic features can be found in the ultraviolet region of the spectrum. Specifically, there exist two isosbestic points between the 1 A 1 and 5 T 2 states at 285 ( 5 nm and 320 ( 5 nm as indicated by the calculated differential absorption spectrum between compounds 1 and 2 as well as data from nanosecond time-resolved absorption spectroscopy (Figure 1). 7These observations constitute imp...

Photo-Induced Spin-State Conversion in Solvated Transition Metal Complexes Probed via Time-Resolved Soft X-ray Spectroscopy

“…24 These efforts were extended into the ultraviolet region by Smeigh and co-workers which served to define the time scale for formation of the known ground-state/excited-state isosbestic point in the compound's electronic absorption spectrum. 34 Significantly, this report also detailed the use of femtosecond stimulated Raman scattering to examine changes in the vibrational structure of the compound: the confluence of data established a time constant of τ = 190 ± 50 fs for the formation of the 5 T 2 state in solution. Studies by Chergui and co-workers on [Fe(bpy) 3 ] 2+ (where bpy is 2,2'-bipyridine) yielded similar conclusions in terms of extremely rapid formation of the high-spin excited state, 35,36 suggesting that sub-picosecond, formally ∆S = 2 spin-state conversion might be a common feature following charge-transfer excitation of low-spin iron polypyridyl complexes.…”

“…We expect the Fe L 2,3 -edges to respond on a time scale well below 1 ps, similar to the transient changes observed in optical spectroscopy measurements. 24,34 Thus, the rise-time of the transients shown in Figure 4A,C is determined by the 70 ps x-ray probe pulses. We model the data with an instantaneous step-like molecular response convolved with a Gaussian instrument response of 70 ps FWHM such that ∆A(E,t) = ∆A 0 (E)/2 · [Erf(t)+1].…”

“…Solvation dynamics consist of distinct processes that can be differentiated with respect to the time scale on which they occur. In the sub-picosecond domain where the formation of the 5 T 2 state occurs, 34 inertial dynamics are dominant (the inertial response time for the solvent CH 3 CN is ~60 fs). 56,57 While such sub-picosecond dynamics have been shown to influence the excited-state evolution of chargetransfer states, 58 the ~50 ps resolution of our experiment infers that there should be negligible contribution from such processes to the results we have described herein.…”

Computational approach to the study of thermal spin crossover phenomena

“…3 However, the interest in SCO materials increased when it was discovered that conversion between the two spin states can be controlled by light irradiation, thus opening the possibility of using such materials as optically switchable devices. [4][5][6] This phenomenon, called Light-Induced Excited Spin State Trapping (LIESST), initially found in Fe(II) complexes [7][8][9][10][11] and later also observed in systems containing Fe(III), [12][13][14][15] and Ni(II) [16][17][18] has been intensively studied in the last years in order to unravel its mechanism both with experimental techniques 11,[19][20][21][22][23][24][25] and by means of theoretical calculations. [26][27][28][29][30][31][32][33] The most numerous and most studied family of SCO systems involves octahedral Fe(II) complexes in the solid state or in solution.…”

Homoleptic complexes of a porphyrinatozinc(ii)-2,2′:6′,2′′-terpyridine ligand

“…The MLCT band is completely lost at the end of the experiment, leaving only Q bands at 577 and 617 nm indicating that the ligand is irreversibly chemically modified. 39,40 Hence, it is reasonable to assume that the resulting triplet-triplet spectrum is solely due to absorption by the Q-band. …”