Femtosecond Measurements Of Size-Dependent Spin Crossover In

Subscriber access provided by CMU Libraries - http://library.cmich.edu

Letter

Femtosecond Measurements of Size-Dependent Spin Cross-Over in Fe(pyz)Pt(CN) Nanocrystals II

4

Dodderi M. Sagar, Frederick G. Baddour, Patrick E. Konold, Joel N. Ullom, Daniel A. Ruddy, Justin C. Johnson, and Ralph Jimenez J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02435 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 15, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Letters

Femtosecond Measurements Of Size-Dependent Spin Crossover In FeII(pyz)Pt(CN)4 Nanocrystals D. M. Sagar ¶, Frederick G. Baddour †, Patrick Konold ¶, Joel Ullom ‡, Daniel A. Ruddy†, Justin C. Johnson † and R. Jimenez¶,§

¶

JILA, University of Colorado at Boulder, 440 UCB, Boulder, CO 80309, USA

§

Department of Chemistry and Biochemistry, University of Colorado at Boulder, 215 UCB,

Boulder, CO 80309, USA ‡

National Institute of Standards and Technology (NIST), 325 Broadway, Boulder, CO 80305,

USA †

National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO 80401,

USA AUTHOR INFORMATION Corresponding Author *[email protected] [email protected] [email protected]

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 20

ABSTRACT: We report a femtosecond time-resolved spectroscopic study of size-dependent dynamics in nanocrystals (NCs) of Fe(pyz)Pt(CN)4. We observe that smaller NCs (123 nm or 78 nm cross section and 650 nm 25. The distinctly different kinetics of ground state bleach evolution among the NC samples suggests the SCO dynamics are size dependent. That such differences should exist is not immediately evident by inspection of their steady-state absorption spectra (Figure 1a), which would be distinct if the ground state electronic structure varied with size, as is the case for semiconductor quantum dots.26 For 78 nm NCs (Figure 2a), at ∆t < 0.5 ps, there is a narrow bleach feature near 460 nm, and a broad induced absorption band above 500 nm. At ∆t = 10 ps the bleach band has shifted and broadened beyond 500 nm. The induced absorption feature seen at 0.5 ps develops into a strong bleach signal that rises until about ∆t = 30 ps, after which the TA


6

Page 7 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spectrum is not time-dependent. The 375 nm NCs show a weaker signal and an overall shift in spectral features (Fig 2b). A weak excited state absorption grows from 0.1-30 ps in the range 450-500 nm. A bleach feature in the 500-600 nm region is present from ∆t = 0.1 ps, which increases in amplitude and shifts slightly to the red through ∆t = 30 ps. The transformation of induced absorption to bleach in the smaller NCs can be visualized by slicing the TA data at a probe wavelength of 535 nm (Figure 2c). The slowly developing bleach is evident for 78 and 123 nm NCs, but the bleach for 375 nm NCs rises instantaneously at this wavelength. Only the 375 nm sample shows a long-lived residual transient absorption in the range λ < 480 nm. The bleach signal has a lifetime of ~6 µs (Figure S3). The 5T2 to 1A1 recovery time was previously reported to be on a millisecond timescale24 in Fe(pyz)Pt(CN)4 NCs, but the SCO recovery time is known to depend on a variety of factors not investigated thoroughly here. To further explore the kinetic picture, we performed TA experiments at 100 K, where we expect the 1A1 state to dominate (T1/2≈200 K)3, 24. The time-dependent spectral maps measured at T=100 K for the 78 and 375 nm NCs are presented in Figure 3(a) and 3(b), respectively. For 78 nm NCs, the spectrum at 0.2 ps contains a bleach band at 495 nm, similar to that at room temperature, and a new bleach peak near 535 nm, reflecting the additional oscillator strength observed in the ground state upon the increase in LS state fraction (Figure S2).21 A broad excited state absorption can also be seen at early times (∆t < 1 ps). The 530-550 nm bleach grows in intensity with a time constant of ∼6 ps, whereas the photo-induced absorption decays with τ = 300 fs, and the 495 nm bleach band is static. For the 375 nm NC sample, bleach peaks at 500 nm and 550 nm appear at the earliest time delays. The peak near 500 nm decays concomitant with a slight red shift and rise of the 550-600 nm bleach. The kinetic traces near the peak of the redshifted bleach component (Fig 3c, reflecting LS population) show a size dependence. The rise is


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 20

dominated by a 6 ps component for the 78 nm and 123 nm NC samples, and also exhibits a modulation on a roughly 30 ps time scale. For the 375 nm NCs, the majority of the rise occurs in 0.1 ps, followed by a weak modulation at later times. We employed singular value decomposition (SVD) to produce component spectra associated with a minimum number of exponential decay functions from the 3D set of TA data. A global fit to the primary SVD components produces a set of decay-associated spectra. The results are similar for the 78 and 123 nm (Figure 4b) NC sizes and only spectra for 78 and 375 nm NCs are shown. The spectra for 78 nm NCs (Figure 4a) reveal two dominant spectral components that can be assigned to 1MLCT excited state absorption and ground state bleach. The global fit to the kinetics of these components is best described by three time constants (400 fs, 8 ps, and >10 ns). The >10 ns spectrum for the final long-lived bleach (blue) closely matches the ground state absorption, thus underscoring the lack of strong state absorption from the HS state produced upon SCO. The 8 ps spectrum (red) represents the dynamics of the bleach, which shows a clear redshift compared with the >10 ns spectrum. The decay-associated spectra for the 375 nm NC samples are very different from those of smaller particles. The long-lived spectrum (blue) exhibits an excited state absorption (tentatively assigned to photoinduced absorption from 5T2) in the 450-500 nm range that distorts the bleach spectrum, forcing an imperfect match with the steady-state absorption. The spectrum associated with the 16 ps time constant (red) contains both positive and negative features, indicating a strong shift in the bleach, which results in a loss of intensity on the blue side and a gain of intensity on the red side. As is evident from the areas of the positive and negative regions, there is little or no net rise in the overall bleach, unlike for the smaller NCs for which the bleach strengthens over time.


8

Page 9 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In Figure 5a and 5b, the SVD analysis for d = 78 nm and d = 375 nm NCs at T = 100 K are presented. At 100 K, the bleach kinetics of the small NCs become are faster than at 300 K (5.5 ps vs. 8 ps) and the global fit spectra are strongly overlapped (Figure 5a and 5b). The >10 ns spectrum shows two bleach bands, but the 5.5 ps component spectrum contains only the strong band at 550 nm. The larger NCs show behavior similar to that at 300 K, with a bleach loss from 450-550 nm and a bleach gain beyond 600 nm. Prior time-resolved studies of similar types of SCO NCs hypothesize that several mechanisms can lead to photoinduced spectral changes on the fs to µs time scale27:

(i)

“Photoinduced switching”, in which 1A1 ground state population is photoexcited to the MLCT state, which then undergoes SCO to 5T2, with a time constant typically < 0.3 ps; (ii) “Elastic SCO”, in which volume expansion, typically on a ns time scale, brought about by ultrafast SCO and subsequent geometry changes, induces the LS→HS transition on nearby Fe centers; (iii) “Thermal SCO” in which heat diffusion causes equilibration of the lattice temperature, on a timescale approaching microseconds, inducing large scale SCO, or in effect, a phase transition; (iv) “Vibrational cooling”, in which excitation and subsequent nonradiative decay produces a “hot” 5T2 state, and subsequent evolution on the 5T2 potential energy surface causes spectral shifting on a ps timescale. These pathways are depicted in Figure 6 in an electronic level scheme consistent with the kinetic models reported for other SCO complexes14,16,18, along with specific modifications for the dynamics of Fe(pyz)Pt(CN)4 NCs discussed here. Process (i) is well-established in many types of Fe-based NCs, and in our samples will certainly occur at low temperature and likely occur even at room temperature if mixed spin domains are present that result from an energy landscape at the NC surface that reduces cooperativity.22 The lack of a clear signature of photoinduced population of the 5T2 state in all


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 20

samples hinders our direct investigation of this process. However, the dominant kinetics we observe are slower and occur on a 5-20 ps time scale, which is incommensurate with previous ultrafast SCO reports. Some solution studies have reported a slowed electronic SCO process,28 and it is possible that a similar situation could occur in small NCs. Nonetheless, the increase in bleach strength in the LS→MLCT transition region by itself is not a signature of the kinetics of photoinduced switching, which conserves the total number of LS species. On the other hand, mechanisms (ii) and (iii) naturally occur more slowly and would increase the bleach strength, especially in the range of the strong LS→MLCT absorption near 550 nm (by promoting LS states directly to HS thermally). Even though the room temperature absorption is dominated by HS contributions at λ < 500 nm, LS→MLCT absorptions do contribute to the low energy tail, and the significant redshift in the bleach with ∆t is an indication of the overall shift in the HS/LS ratio on a ps time scale. The reported time scales for processes (ii) and (iii) are considerably slower (∼ns to µs) than we observe. Thermal transitions initiated by light pulses have been observed in µM-sized crystallites of [Fe(NH2-trz)3]Br2•3H2O,29 with a time constant of 44 ns, similar also to time constants derived by van der Veen et al. for large-scale structural changes in Fe(pyz)Pt(CN)4 NCs.24 In addition to the sub-ns dynamics, we also observe a weak ∼50 ns bleach rise when probing longer time scales (Fig S5), and it is possible that these slower changes align with previous reports that probed longer-range manifestations of SCO with ns time resolution. However, since our TA experiments are sensitive to the local LS/HS ratio changes (and not only longer-range structure), nonequilibrium propagation of energy, occurring with a few km/sec velocity,27 can stimulate the transition that gives rise to the bleach. For the thinnest particles (∼20 nm thick), the time needed to traverse the entire crystal is estimated to be 5-10 ps, changing


10

Page 11 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


linearly with thickness (∼20 ps for the largest NC size studied).

Power- and wavelength

dependent excitation conditions did not alter the rise kinetics of the bleach in our NCs (Figure S4), which may further suggest that the mechanism is more likely (ii) than (iii). However, the accessible range of powers is limited experimentally, and far different conditions may be required to observe significant kinetic deviations. In addition to the bleach kinetics, the amplitude of the bleach rise is revealing. The much larger amplitude of the bleach rise in smaller NCs compared with larger NCs (e.g., Figure 2c) indicates that smaller NCs have a larger residual LS fraction than large NCs, and thus a greater opportunity to promote the overall LS→HS transition that causes the bleach beyond 500 nm. This is expected if surface regions possess an anomalously high fraction of LS species. This enrichment in LS species at higher temperature has been previously inferred from magnetic susceptibility and Mossbauer spectroscopy measurements on similar NCs to those studied here.1 There, approximately twice the residual fraction of LS species at room temperature was observed for smaller particles compared with larger NCs.

Our own measurements of magnetic

susceptibility (Fig S6) also reveal a narrower hysteresis for smaller NCs and high-temperature saturation behavior at a lower value in small NCs than in large NCs. Although exact LS/HS fractions are not derived, these data align well with previous measurements on Fe(pyz)Pt(CN)4 NCs in which LS fractions of up to ∼17% exist at room temperature. At 100 K, which is well below T1/2, the overall LS/HS equilibrium has shifted considerably toward the LS state. The consequences are clearly seen for the small NCs, where we can resolve the dominant red-shifted and sharpened bleach bands of the LS state (near 550 nm) and the blue-shifted band also present at room temperature (near 500 nm). Unlike at room temperature, at low temperature we observe a nearly pure bleach rise around 550 nm, which is


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 20

expected for (ii) and (iii) if primarily LS to MLCT transitions are involved. However, the weak bleach feature near 500 nm is present immediately after excitation and does not increase with time delay. This static component may be due to a cancellation of the bleach rise with rising absorption features of the 5T2 state Overall, these features are commensurate with the initial process (i) being followed by processes (ii) and (iii) at low temperatures. Process (iv), also a subsequent process of photoinduced SCO, could play a role in some instances. In time-resolved Raman experiments on the SCO complex- [Fe(tren(6-R-py)3)](PF6)2 in solution, a decay component of ~5-10 ps has been assigned to vibrational cooling30. However, in the solid state one would expect a significant hastening of cooling due to the interconnected framework and many available degrees of freedom for cooling.

Also, vibrational cooling

generally produces spectral narrowing, which we do not observe. The shifting and broadening of the bleach that occurs for 375 nm NCs may be related to the transfer of energy within NCs, but it lacks the features typically associated with vibrational cooling. Further investigation is necessary to fully understand this behavior. In conclusion, we present the first femtosecond time-resolved spectroscopic study of size and temperature-dependent photoinduced spin crossover in NCs of Fe(pyz)Pt(CN)4. A 5-10 ps rise in bleach strength at the position of the 1A1→MLCT band in small NCs is indicative of a self-propagating SCO process driven by the spreading of excess energy away from a photoexcitation event. The process is faster and more efficient in NCs with thickness less than 20 nm compared to those with ∼45 nm thickness, most likely due to the increased residual LS fractions in small particles that support small domains and exhibit weakened cooperativity These results have important implications for efforts toward manipulating the overall spin state of Febased complexes using light or heat on varying time scales. Further work comparing the size


12

Page 13 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


effects in different types of SCO NCs, including 2D coordination polymers, should enable more detailed mechanistic understanding that will pave the way towards tuning the energetics and kinetics of SCO via nanoscale engineering.

Figure 1: (a) Absorption spectra of Fe(pyz)Pt(CN)4 NCs at T = 300 K. Inset: Molecular structure of the crystal3. (b) Representative TEM images of NCs.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 20

Figure 2: Femtosecond TA spectra vs. ∆t for (a) 78 nm NCs and (b) 375 nm NCs at T = 300K. (c) Kinetic traces for 78, 123, and 375 nm NCs at probe wavelength 535 nm (vertical line in (a) and (b)).


14

Page 15 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3: TA spectra vs. ∆t for (a) 78 nm and (b) 375 nm NCs at T=100 K. λpump=400 nm. (c) Kinetics trace for λ = 550 nm.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 20

Figure 4: Global fit-derived spectra for (a) 78 nm, and (b) 375 nm NCs at T=300 K, all at λpump=400 nm. Positive (negative) features represent induced absorption (bleach) for the infinite and sub-ps components and bleach loss (gain) for the ps components.


16

Page 17 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5: Global fit-derived spectra for (a) 78 nm, (b) 375 nm NCs at T=100 K. λpump=400 nm


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 20

Figure 6. A schematic model of SCO displaying processes (i) (blue), (ii)-(iii) (green), and (iv) (red). 3MLCT and 3T states are omitted for clarity. Weaker photoinduced processes arising from 5

T2 are also not shown. Straight arrows designate photoinitiated transitions while wavy arrows

indicate nonradiative (thermal) transitions.

ASSOCIATED CONTENT Supporting_Information Sample synthesis and size histograms, additional spectroscopic data and fits, magnetic susceptibility data. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT Supported by the NIST Innovations in Measurement Science program. JCJ acknowledges the DOE, Office of Science, Division of Chemical Sciences, Biosciences, and Geoscience (contract no DE-AC36-08GO28308 with NREL). We thank Prof. Niels Damrauer and Prof. Jose Real for useful discussions and Joe Ryerson and Tom Silva for assistance with absorption and magnetic susceptibility measurements.

REFERENCES (1). Boldog, I.; Gaspar, A. B.; Martínez, V.; Pardo-Ibañez, P.; Ksenofontov, V.; Bhattacharjee, A.; Gütlich, P.; Real, J. A., Spin-Crossover Nanocrystals with Magnetic, Optical, and Structural Bistability near Room Temperature. Angew. Chem. Int. Ed. 2008, 47, 6433-6437. (2). Molnár, G.; Niel, V.; Real, J.-A.; Dubrovinsky, L.; Bousseksou, A.; McGarvey, J. J., Raman Spectroscopic Study of Pressure Effects on the Spin-Crossover Coordination Polymers Fe(Pyrazine)[M(Cn)4]·2H2O (M = Ni, Pd, Pt). First Observation of a Piezo-Hysteresis Loop at Room Temperature. J. Phys. Chem. B 2003, 107, 3149-3155. (3). Tayagaki, T.; Galet, A.; Molnár, G.; Muñoz, M. C.; Zwick, A.; Tanaka, K.; Real, J.-A.; Bousseksou, A., Metal Dilution Effects on the Spin-Crossover Properties of the ThreeDimensional Coordination Polymer Fe(Pyrazine)[Pt(Cn)4]. J. Phys. Chem. B 2005, 109, 1485914867. (4). Tissot, A., Photoswitchable Spin Crossover Nanoparticles. New J. Chem. 2014, 38, 18401845. (5). Cobo, S.; Ostrovskii, D.; Bonhommeau, S.; Vendier, L.; Molnár, G.; Salmon, L.; Tanaka, K.; Bousseksou, A., Single-Laser-Shot-Induced Complete Bidirectional Spin Transition at Room


18

Page 19 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Temperature in Single Crystals of (Feii(Pyrazine)(Pt(Cn)4)). J. Am. Chem. Soc. 2008, 130, 90199024. (6). Chergui, M., Ultrafast Studies of the Light-Induced Spin Change in Fe(Ii)-Polypyridine Complexes. In Spin-Crossover Materials, John Wiley & Sons Ltd: 2013; pp 405-424. (7). Cannizzo, A.; Milne, C. J.; Consani, C.; Gawelda, W.; Bressler, C.; van Mourik, F.; Chergui, M., Light-Induced Spin Crossover in Fe(Ii)-Based Complexes: The Full Photocycle Unraveled by Ultrafast Optical and X-Ray Spectroscopies. Coord. Chem. Rev. 2010, 254, 26772686. (8). Auböck, G.; Chergui, M., Sub-50-Fs Photoinduced Spin Crossover in [Fe(Bpy)3]2+. Nat. Chem. 2015, 7, 629-633. http://www.nature.com/nchem/journal/v7/n8/abs/nchem.2305.html#supplementary-information. (9). Bressler, C., et al., Femtosecond Xanes Study of the Light-Induced Spin Crossover Dynamics in an Iron(Ii) Complex. Science 2009, 323, 489-492. (10). McCusker, J. K., Femtosecond Absorption Spectroscopy of Transition Metal ChargeTransfer Complexes. Acc. Chem. Res.2003, 36, 876-887. (11). Gawelda, W., et al., Structural Determination of a Short-Lived Excited Iron(Ii) Complex by Picosecond X-Ray Absorption Spectroscopy. Phys. Rev. Lett. 2007, 98, 057401. (12). Chang, J.; Fedro, A.; van Veenendaal, M., Ultrafast Cascading Theory of Intersystem Crossings in Transition-Metal Complexes. Phys. Rev. B 2010, 82, 075124. (13). Hauser, A., Reversibility of Light-Induced Excited Spin State Trapping in the Fe(Ptz)6(Bf4)2, and the Zn1−Xfex(Ptz)6(Bf4)2 Spin-Crossover Systems. Chem. Phys. Lett. 1986, 124, 543-548. (14). Hauser, A., Intersystem Crossing in the [Fe(Ptz)6](Bf4)2 Spin Crossover System (Ptz=1‐ Propyltetrazole). J. Chem. Phys. 1991, 94, 2741-2748. (15). Byrnes, M. J.; Chisholm, M. H.; Gallucci, J. A.; Liu, Y.; Ramnauth, R.; Turro, C., Observation of 1MLCT and 3MLCT Excited States in Quadruply Bonded Mo2 and W2 Complexes. J. Am. Chem. Soc. 2005, 127, 17343-17352. (16). Monat, J. E.; McCusker, J. K., Femtosecond Excited-State Dynamics of an Iron(Ii) Polypyridyl Solar Cell Sensitizer Model. J. Am. Chem. Soc. 2000, 122, 4092-4097. (17). Martínez, V.; Boldog, I.; Gaspar, A. B.; Ksenofontov, V.; Bhattacharjee, A.; Gütlich, P.; Real, J. A., Spin Crossover Phenomenon in Nanocrystals and Nanoparticles of [Fe(3Fpy)2m(Cn)4] (Mii = Ni, Pd, Pt) Two-Dimensional Coordination Polymers. Chem. Mater. 2010, 22, 4271-4281. (18). Bertoni, R.; Lorenc, M.; Tissot, A.; Servol, M.; Boillot, M.-L.; Collet, E., Femtosecond Spin-State Photoswitching of Molecular Nanocrystals Evidenced by Optical Spectroscopy. Angew. Chem. Int. Ed. 2012, 51, 7485-7489. (19). Félix, G.; Nicolazzi, W.; Salmon, L.; Molnár, G.; Perrier, M.; Maurin, G.; Larionova, J.; Long, J.; Guari, Y.; Bousseksou, A., Enhanced Cooperative Interactions at the Nanoscale in Spin-Crossover Materials with a First-Order Phase Transition. Phys. Rev. Lett. 2013, 110, 235701. (20). Mikolasek, M.; Felix, G.; Nicolazzi, W.; Molnar, G.; Salmon, L.; Bousseksou, A., Finite Size Effects in Molecular Spin Crossover Materials. New J. Chem. 2014, 38, 1834-1839. (21). Delgado, T.; Tissot, A.; Besnard, C.; Guénée, L.; Pattison, P.; Hauser, A., Structural Investigation of the High Spin→Low Spin Relaxation Dynamics of the Porous Coordination Network [Fe(Pz)Pt(Cn)4]⋅2.6 H2o. Chemistry – A European Journal 2015, 21, 3664-3670.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 20

(22). Raza, Y., et al., Matrix-Dependent Cooperativity in Spin Crossover Fe(Pyrazine)Pt(Cn)4 Nanoparticles. Chem. Comm. 2011, 47, 11501-11503. (23). Lara, F. J. M.; Gaspar, A. B.; Aravena, D.; Ruiz, E.; Munoz, M. C.; Ohba, M.; Ohtani, R.; Kitagawa, S.; Real, J. A., Enhanced Bistability by Guest Inclusion in Fe(Ii) Spin Crossover Porous Coordination Polymers. Chem. Comm. 2012, 48, 4686-4688. (24). van der Veen, R. M.; Kwon, O.-H.; Tissot, A.; Hauser, A.; Zewail, A. H., SingleNanoparticle Phase Transitions Visualized by Four-Dimensional Electron Microscopy. Nat. Chem. 2013, 5, 395-402. (25). Laisney, J.; Tissot, A.; Molnar, G.; Rechignat, L.; Riviere, E.; Brisset, F.; Bousseksou, A.; Boillot, M. L., Nanocrystals of Fe(Phen)2(Ncs)2 and the Size-Dependent Spin-Crossover Characteristics. Dalton Trans. 2015, 44, 17302-17311. (26). Nozik, A. J.; Beard, M. C.; Luther, J. M.; Law, M.; Ellingson, R. J.; Johnson, J. C., Semiconductor Quantum Dots and Quantum Dot Arrays and Applications of Multiple Exciton Generation to Third-Generation Photovoltaic Solar Cells. Chem. Rev. 2010, 110, 6873-6890. (27). Lorenc, M., et al., Cascading Photoinduced, Elastic, and Thermal Switching of Spin States Triggered by a Femtosecond Laser Pulse in an Fe(Iii) Molecular Crystal. Phys. Rev. B 2012, 85. (28). Liu, Y. Z., et al., Towards Longer-Lived Metal-to-Ligand Charge Transfer States of Iron(Ii) Complexes: An N-Heterocyclic Carbene Approach. Chem. Comm.s 2013, 49, 64126414. (29). Fouche, O.; Degert, J.; Jonusauskas, G.; Daro, N.; Letard, J. F.; Freysz, E., Mechanism for Optical Switching of the Spin Crossover [Fe(Nh2-Trz)(3)](Br)(2)Center Dot 3h(2)O Compound at Room Temperature. Phys. Chem. Chem. Phys. 2010, 12, 3044-3052. (30). Smeigh, A. L.; Creelman, M.; Mathies, R. A.; McCusker, J. K., Femtosecond TimeResolved Optical and Raman Spectroscopy of Photoinduced Spin Crossover: Temporal Resolution of Low-to-High Spin Optical Switching. J. Am. Chem. Soc. 2008, 130, 14105-14107.


20

Femtosecond Measurements Of Size-Dependent Spin Crossover In

Recommend Documents