University of Groningen Mechanisms in Ruthenium(II ... · Chapter 2 22 Figure 6. Raman spectrum of...
Transcript of University of Groningen Mechanisms in Ruthenium(II ... · Chapter 2 22 Figure 6. Raman spectrum of...
University of Groningen
Mechanisms in Ruthenium(II) photochemistry and Iron(III) catalyzed oxidationsUnjaroen, Duenpen
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.
Document VersionPublisher's PDF, also known as Version of record
Publication date:2017
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):Unjaroen, D. (2017). Mechanisms in Ruthenium(II) photochemistry and Iron(III) catalyzed oxidations:Photochemical, Electrochemical and Spectroscopic studies. [Groningen]: University of Groningen.
CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.
Download date: 04-06-2020
CHAPTER 2
Probing spin crossover phenomena [Fe(dppz)2(NCS)2]py in solution and in the solid state by Raman scattering techniques In this chapter the complexes [Fe(phen)2(NCS)2] (1) and [Fe(dppz)2(NCS)2]∙py (2) are studied to
explore the effect of temperature and environment on spin state using a combination of continuous
wave and pulsed excitation Raman spectroscopy. A key challenge discussed in the analysis of the
spin state of 2 in both of solution and in the solid state by using Raman spectroscopy is the non‐
innocence of the probe technique used. The transient excitation of 2 from it’s the singlet ground
state to the quintet excited state in solution is detected by “power‐resolved transient resonance
Raman” (TR2) spectroscopy at room temperature, which compliments the LIESST effect observed
for 2 in the solid state.
D. Unjaroen, A. Draksharapu, J. J. McGarvey and W. R. Browne, manuscript in preparation
Chapter 2
18
2.1 Introduction Combi’s discovery1of the first spin state interconversion in an Fe(III) complex in the early 1930s was
the first in a remarkable century of the development of our understanding of microstates in
transition metal complexes. The properties of a complex, including optical and physical, are
critically dependent on the complexes spin state and hence the observation of spin state switching
has opened up a wide field of applications in thermally and mechanically responsive materials.2
The spin crossover (SCO) behaviour of the complex [Fe(phen)2(NCS)2] (1) was first reported over
30 years later by Baker et al.3 and the spin cross over behaviour of Fe(II) complexes has seen
extensive interest for a range of applications, not least in memory materials4 and switching of
physical properties.5 The fundamental physics of the SCO phenomenon itself and especially the
dynamics of the crossover relaxation process remains a central fundamental point of interest,
whereas applications have focused predominantly on SCO behaviour in the solid state and
especially on hysteresis in SCO arising from cooperativity between molecules within the solid
matrix.6
Figure 1. Structures formula of [Fe(phen)2(NCS)2] (1) and [Fe(dppz)2(NCS)2]py (2).
The system discussed in this chapter, an Fe(II) complex with the dipyridophenazine ligand, dppz,
[Fe(dppz)2(NCS)2] (2), was described for the first time over 15 years ago where SCO involved a
singlet‐quintet, 1A to 5T, spin state change.7 In addition to SCO, the complex showed, in the solid
state, a wide thermal hysteresis of ca 40 K (Tc 123 K, Tc 163 K) accompanying the spin
equilibrium. The hysteresis is assigned to intra‐column π‐π stacking interactions and inter‐column
van der Waals interactions, which mean that a change in Fe‐N bond lengths accompanying the SCO
are transmitted via these interactions. A subsequent report by the same group demonstrated a
memory effect whereby upon rapid cooling to 5 K, a residual high spin fraction remained and only
relaxed to the LS state over extended periods of time.8
In 2012, Shepherd et al.9 showed pressure dependent bistability in the spin crossover (SCO)
between high spin (HS) and low spin (LS) states of [Fe(dppz)2(NCS)2] by single‐crystal X‐ray
diffraction and Raman scattering as probe the spin state. The change in the characteristic cyano (‐CN) mode from 2061 cm‐1 (HS) to 2105 cm‐1 (LS) upon increasing the pressure to 28.7 kbar at room
temperature indicated that full spin crossover from the HSLS state occurred. Interestingly, a
single geometrical change in the complex was responsible for both the largest negative linear
compression (NLC) and negative thermal expansion (NTE) behaviour yet observed in a molecular
material.
Recently, Paradis et al.10 investigated the interaction between the thermal decay of the low
temperature metastable HS state and the thermal hysteresis of the metal diluted spin‐crossover
[FexMn1‐x(dppz)2(NCS)2]py complexes. They showed that thermal decay temperature T(LIESST)
Light induced spin crossover in [Fe(dppz)2(NCS)2].py
19
could be shift into a quasi‐static hysteresis range by an increase in Mn content, increase of the
temperature ramp rate, and irradiation using visible light.
In this chapter, light‐induced spin state changes in [Fe(dppz)2(NCS)2] in the solid state and in
methanol with selective excitation into the π‐π* transition of the dppz ligand and 1MLCT bands by
using Raman scattering is explored. Of particular interest is the non‐innocence of the excitation
laser used to obtain Raman spectra.
2.2 Results 2.2.1 Resonance Raman spectra of 2 in methanol.
The UV‐vis absorption spectrum of 2 in methanol (Figure 2, inset) indicates that the complex has a
singlet ground state in solution due to the strength of the 1MLCT transitions in the visible region
and that Raman spectra recorded at exc 355, 457, 473 and 532 nm (Figure 2) were under resonance
conditions. Excitation exc 355 nm into ligand‐centred π‐π* transition band, shows enhancement of
bands at 619, 721, 765, 1233, 1239, 1269, 1313, 1345, 1403, 1453, 1474, 1497, 1541, 1552, 1574
and 1601 cm‐1 (Figure 2a). Whereas irradiation into metal to the ligand charge transfer (1MLCT)
bands at exc 457, 473 and 532 nm showed resonance enhanced band at 579, 710, 847, 1045,
1145, 1184, 1207, 1228, 1339, 1357, 1448, 1469, 1492, 1547, and 1596 cm‐1 (vide infra).
Figure 2. (r)Raman spectra of 2 in methanol (0.1 mM) at exc (a) 355 nm, (b) 457 nm, (c) 473 nm and
(d) 532 nm and (inset) UV‐vis absorption spectrum of 2 in methanol.
Excitation of 2 in methanol at 355 nm at low temperature (77 K) shows the spectrum of the LS state
as observed at room temperature (Figure 3). However, the relative Raman intensities at 1496, 1451,
1359, 1344 and 1235 cm‐1 are increased together with the decrease of the band at 764 cm‐1 at 77
K. There is no change in the Raman spectrum between 294 and 77 K when excitation was into 1MLCT band at 532 nm.
Chapter 2
20
Figure 3. (r)Raman spectra of 2 in methanol at exc 355 nm (a) 294 K (b) 77 K. #oxygen peak,
*imperfect solvent subtraction.
2.2.2 Power‐resolved transient resonance Raman (TR2) of 2 in methanol.
Although excitation of the complex with a CW laser would be expected to access higher spin states
during relaxation, the steady state population of these higher states would be expected to be
negligible in solution even at 77 K owing to the absence of intermolecular cooperative interactions.
By contrast, excitation with short (6 ns) laser pulses allows for the transient change in population
in favour of the HS state to be observed using power‐resolved transient resonance Raman
spectroscopy, in which the excitation pulse both transiently excites and probes the excited spin
states as well as the ground state molecules.
Excitation at 354.5 nm of purely LS states using high energy laser pulses (33 mJ per pulse‐1)
results in the appearance of the HS marker band at 714 cm‐1 together with a weak resonantly
enhanced Raman bands in the region 1200‐1600 cm‐1 (Figure 4). Furthermore, the new resonantly
enhanced Raman bands at 1569, 1495, 1403, 1360, 1335 and 1299 cm‐1 are observed at high laser
power compared to CW Raman spectrum. Decreasing the pulse energy to 3.8 mJ pulse‐1 results in
a relative increase in the LS marker band at 723 cm‐1 together with relatively more intense of Raman
bands between 1200 to 1600 cm‐1 assignable to the LS ground state.
Excitation at 532 nm, within the 1MLCT band of the ground state of 2 in methanol, showed only
weak resonance Raman spectra of the LS state at 20 mJ pulse‐1 (Figure 5). In contrast, higher Raman
scattering intensity (relative to solvent) was observed when using lower laser powers (e.g., 1 mJ
pulse‐1). Unfortunately, irradiation at 532 nm is unable to detect the conversion of LSHS state,
due to the absence of resonance enhancement in the latter, however, the decrease of the Raman
intensity when using high laser powers indicated that the population of LS species decreases,
consistent with SCO.
Light induced spin crossover in [Fe(dppz)2(NCS)2].py
21
Figure 4. Power‐resolved transient resonance Raman spectra of 2 in methanol (0.1 mM) at exc =
354.5 nm at 294 K.
Figure 5. Power‐resolved transient resonance Raman spectra of 2 in methanol (0.1 mM) at exc =
532 nm at 294 K.
2.2.3 Raman spectra of 2 in solid state at ex 785 nm.
The Raman spectrum at exc 785 nm of [Fe(dppz)2(NCS)2] (2) in the solid state showed the bands at
257, 419, 710, 1045, 1316, 1356, 1399, 1440, 1489, 1536, 1588, 1597 and 2060 cm‐1 (Figure 6). The
characteristic cyano stretching (‐CN) modes of thiocyanate ligands have been used to assign the
spin state of complex 2;9 the spin marker bands at 2060 cm‐1 and 2117 cm‐1 for the HS and LS states,
respectively. Despite that the solid material is deep purple in colour due to absorption by the LS
complex, at 294 K, 2 shows a maker band at 2060 cm‐1 which corresponds to the HS state and only
a small shoulder at 721 cm‐1 which is a marker band of LS state appeared.
Chapter 2
22
Figure 6. Raman spectrum of 2 in a solid state at exc 785 nm at room temperature (294 K).
2.2.4 Raman spectra of solid 2 dispersed in KCl at ex 355 nm.
Excitation into the ligand‐centred π‐π* transitions of 2 is achieved at 355 nm. The Raman spectrum
of 2, as a solid dispersed in KCl at room temperature, shows resonance enhance bands at 618, 710,
720, 765, 842, 1004, 1047, 1184, 1236, 1274, 1316, 1340, 1383, 1401, 1440, 1468, 1492, 1537,
1577 and 1592 cm‐1 (Figure 7a). At 77 K, the band at 720 cm‐1 is absent together with a change in
the relative intensity of the Raman bands in region 1100−1600 cm‐1 (Figure 7b). The Raman bands
at 710 and 721 cm‐1 were previously reported to the spin marker bands of HS and low LS states,
respectively.11
The data indicated that a mixture of HS and LS species is present in a solid state at room
temperature while only the HS state is observed at low temperature, which is counter intuitive
given that low temperatures favour the low spin state. The result can be understood by considering
the Light Induced Spin State Trapping (LIESST) effect when excitation at 355 nm, which is resonant
for both LS and HS states, and hence a photostationary state will be reached between the states as
the thermal recovery of the LS state is inhibited at 77 K.
Figure 7. Resonance Raman spectra (exc = 355 nm, CW, 10 mW) of solid 2 dispersed in KCl at (a)
294 K and (b) 77 K.
Light induced spin crossover in [Fe(dppz)2(NCS)2].py
23
2.2.5 Raman spectra of solid 2 dispersed in KCl at ex 532 nm.
Excitation of 2 in the solid state (dispersed in KCl) into a metal to ligand charge transfer (1MLCT)
band at room temperature shows a weak HS marker band at 2065 cm‐1 together with Raman
resonance enhanced bands of the LS species at 579, 617, 709, 720, 765, 835, 1044, 1183, 1264,
1314, 1340, 1358, 1400, 1468, 1493, 1536, 1551, 1571 and 1598 cm‐1 (Figure 8a). These data are
consistent with the bulk of the material being in the HS state. The observation of intense LS complex
bands is due to the increase in Raman scattering because of resonance with the 1MLCT absorption
band of the complex.
At 77 K, the LS state is favoured in the bulk, however, excitation at 532 nm would be expected
to lead to an increase in the population of the HS state (LIESST), and the observation of marker
bands of the HS state (which are not resonantly enhanced) indicate that the extent of switching is
substantial. The HS marker band at 2065 cm‐1 is the most obvious feature in the spectra, however,
the intensity of the Raman bands at 1598, 1536, 1400 and 720 cm‐1 are decreased together with
the increase of the bands at 1571, 1551, 1493 and 710 cm‐1 also (vide infra).
Figure 8. Resonance Raman spectra (exc = 532 nm, CW, 200 mW) of solid 2 dispersed in KCl at (a)
294 K and (b) 77 K.
2.2.6 Power‐resolved Transient resonance Raman (TR2) of solid 2 dispersed in KCl.
Excitation at 532 nm (30 mJ pulse‐1) of 2 in the solid state resulted in a broadening of the band of
HS state at 2060 cm‐1, which indicated that the sample heating, with a mixture of primary bands of
the LS state as well as from the HS state (Figure 9a). The Raman bands narrow with decreasing laser
power together with a relative decrease in the intensity of the Raman bands at 1402, 1339 and 710
cm‐1. As noted above, excitation at 532 nm is in resonance with an absorption of the LS state. Hence,
although the spectrum is dominated by contributions from the LS state, bands which are not
enhanced, such as the ‐CN mode are not observed as in solution. Hence, although LS features are
observed, the sample is majoritively in the HS state at room temperature in the solid state.
At low temperature (77 K), the complex is primarily in the LS state and upon irradiation with
high peak powers the shape of ‐CN band at 2060 cm‐1 is observed due to the light‐induced spin
state from LSHS (vide infra). The HS bands at 1340 and 710 cm‐1 are more intense at 30 mJ pulse
energy than at 14 mJ due to the decrease in the residual LS state in the sample (incomplete LIESST).
It should be noted however that although the spectrum is dominated by bands of the LS state,
Chapter 2
24
these are resonantly enhanced and the absence of the LS C‐N stretching mode confirms that in fact,
the LS state is present only at low concentrations in the sample after excitation.
Figure 9. Power‐resolved transient resonance Raman spectra (at exc 532 nm) of 2 in the solid state
dispersed in KCl at (a) 294 K and (b) 77 K at various pulse energies (indicated).
2.2.7 Raman spectra of solid 1 dispersed in KCl at ex 532 nm.
Upon excitation at 532 nm of [Fe(phen)2(NCS)2]2+ (1) in the solid state at room temperature, Raman
bands assigned to the HS state are observed at 559, 723, 1051, 1101, 1143, 1209, 1209, 1251,
1338, 1301, 1338, 1417, 1455, 1513, 1580, 1632 and 2075 cm‐1 (Figure 10a). At low temperature
(77 K), the HS marker band at 2075 cm‐1 appears due to the LIESST effect, i.e. light‐induced spin
state switching from LSHS occurs, however, the bands of the residual LS present are resonantly
enhanced and are apparent by comparison with the spectrum obtained at RT (Raman bands of LS
state at 1580, 1455, 1251, 877 and 739 cm‐1). The observation of mixture of HS and LS bands is
consistent with a limiting PSS between high and low spins states.
Light induced spin crossover in [Fe(dppz)2(NCS)2].py
25
Figure 10. Raman spectra (exc 532 nm, CW, 200 mW) of 1 dispersed in KCl at (a) 294 K and (b) 77
K.
2.2.8 Power‐resolved Transient resonance Raman (TR2) of 1 dispersed in KCl.
As for 2, broadening of bands from the HS species 1 was observed at high laser pulse energies (30
mJ pulse‐1) at exc 532 nm ascribed to sample heating. At lower powers bands assignable to the LS
complex is observed at 1251, 876 and 738 cm‐1 together with those of the HS species (Figure 11a).
Furthermore, the shift of the band at 1625 cm‐1 to 1629 cm‐1 observed along with the appearance
of the LS bands at 1576, 1512 and 1499 cm‐1 indicated that the LS species on the surface of the
microcrystals provide resonance enhancement of the Raman scattering at 532 nm.
At 77 K, the LIESST effect is observed for 1 as well as in 2, with visible light converting between
the HS and LS states manifested in a decrease in the enhanced bands of the LS state as the laser
pulse power is increased. At low pulse powers, the conversion of LSHS is reduced manifested in
an increase of the LS (band at 738 cm‐1) concomitant with a decrease of HS (band at 1578 cm‐1).
Chapter 2
26
Figure 11. Power‐resolved Transient resonance Raman spectra at exc 532 nm of 1 dispersed in KCl
at (a) 294 K and (b) 77 K.
2.3 Discussion The spin state of 1 and 2 in the solid state was shown earlier to be temperature dependent and
subject to the LIESST effect at low temperatures. In the present study Raman spectroscopy under
resonant conditions is shown to be affected by both the LIESST effect and resonance enhancement
of bands of the complexes in the low spin state. In solution, both complexes 1 and 2 are in the low
spin state at all temperatures due to the absence of matrix effects. Nevertheless, the single colour
TR2 technique was shown to allow for pumping of the ground state (1A) and detection of the excited
state (5T) of 2 in methanol within both the ‐* transition of ligand (354.5 nm) and the 1MLCT
transition (532 nm) of the complex.
At a probe wavelength (354.5 nm) resonant with the ‐* transition of the ligand, vibration modes of both LS and HS states were enhanced, with the HS band at 714 cm‐1 decreasing
concomitant with an increase of LS band at 723 cm‐1 upon a decrease in pulse energy from 33 mJ
to 3.8 mJ per pulse. Furthermore, the relative Raman intensity bands in the region 1200‐1600 cm‐
1 of LS species decrease with increasing laser power which indicated a fast relaxation of excited
states (5T). Excitation at 532 nm did not allow for signals of the excited state (5T) complex to be
observed due to the absence of resonance enhancement. Hence excitation into the 1MLCT (at 532
Light induced spin crossover in [Fe(dppz)2(NCS)2].py
27
nm) transition resulted in a decrease in the intensity (relative to solvent) of the signals of the low
spin state which consistent to the lower population of the ground state (LS).
Notably, the colour of 2 in the solid state is purple, which indicates that at least the surface of
the microcrystals is in the low spin state even at room temperature. This is confirmed by Raman
spectroscopy at exc 785 nm of 2 in the solid state at room temperature in which a small shoulder
band at 721 cm‐1 corresponding to a marker band of LS state is observed. These data indicated that
2 in the solid state is not a pure HS material but rather a mixture with some of LS component. Given
that the bulk material is in the HS state but not the material near the crystal surfaces, it is expected
that thin films of the complex may prefer to be in the low spins state.
At room temperature (294 K), 2 in KCl with excitation into the ‐* transition of the ligand (355 nm) and the 1MLCT bands (532 nm) shows a mixture of resonantly enhanced Raman bands of both
HS and LS species. Excitation at 355 nm at 294 K mostly shows the HS bands together with some
bands of LS state whereas excitation at 532 nm shows a weak band of CN vibrational mode of HS
state at 2065 cm‐1 along with other HS bands and combined with the enhanced bands of the LS
species. The LS state absorbs at 532 nm which can become a resonance Raman enhancement even
a very small amount of the LS state presence. Thus this effect will swamp the HS signal in the Raman
spectrum.
2.4 Conclusions The results discussed in this chapter demonstrate that light‐induced spin crossover of the complex
[Fe(dppz)2(NCS)2]∙py (2) in both solution and the solid state occurs even at room temperature.
Excitation of 2 in methanol at 355 nm followed by the power resolved TR2 technique allows for
observation of the metastable HS state together with a decrease of the group state (LS) population
at higher laser powers. Whereas the LIESST effect of 2 in solid state is always observed at low
temperature in both excitations into ‐* and 1MLCT bands. Notably powders of complex 2
contains both complexes in the HS and LS states, which is manifested in enhanced bands of LS state
appearing in the spectrum upon excitation at 532 nm (at 294 K) and thermal induced spin state
from LSHS states at high laser power.
2.5 Acknowledgements Prof. John J. McGarvey is acknowledged for discussion and Dr. Apparao Draksharapu is thanked
for assistance with TR2 experiments.
Chapter 2
28
2.6 References
(1) (a) Cambi, L.; Cagnasso, A. Atti Accad. Naz. Lincei 1931, 13, 809. (b) Cambi, L.; Szego, L.; Cagnasso, A.
Atti R. Accad. Naz. Lincei 1932, 15, 329.
(2) Brady, C.; McGarvey, J. J.; McCusker, J. K.; Toftlund, H.; Hendrickson, D. N. Time‐Resolved Relaxation Studies of Spin Crossover Systems in Solution. Topics in Current Chemistry 2004, 235, 1−20.
(3) Baker, W. A.; Bobonich, H. M. Inorg. Chem. 1964, 3, 1184−1188.
(4) (a) Gütlich, P.; Garcia, Y.; Goodwin, A. H. Chem. Soc. Rev. 2000, 29, 419−427. (b) Gütlich, P.; Gaspar, B.
A.; Garcia, Y. Beilstein J. Org. Chem. 2013, 9, 342−391. (c) Gütlich P. Z. Anorg. Allg. Chem. 2012, 638, 15–
43.
(5) Real, J. A.; Gaspara, A. B.; Munozb, M. C. Dalton. Trans. 2005, 2062−2079.
(6) Real, J. A.; Gaspar, A. B.; Niel, V.; Munoz, M. C. Coord. Chem. Rev. 2003, 236, 121−141.
(7) Zhong, J. Z.; Tao, J. Q.; Yu, Z.; Duan, C. Y.; Liu Y. J.; You, X. Z. J. Chem. Soc., Dalton Trans. 1998, 327−328.
(8) Yu, Z.; Liu, K.; Tao, J. Q.; Zhong, Z. J.; You, X. Z.; Siu, G. G. Appl. Phys. Lett. 1999, 74, 4029−4031.
(9) Shepherd, H. J.; Palamarciuc, T.; Rosa, P.; Guionneau, P.; Molnµr, G.; Letard, J.‐F.; Bousseksou, A. Angew.
Chem., Int. Ed. 2012, 51, 3910−3914.
(10) Paradis, N.; Chastanet, G.; Palamarciuc, T.; Rosa, P.; Varret, F.; Boukheddaden, K.; Létard, J‐F. J. Phys.
Chem. C 2015, 119, 20039−20050. (11) Kate Louise Ronayne. Ph.D. Thesis, The Queen’s University of Belfast, (2004).