2.33.2 NUCLEAR DECAY INDUCED EXCITED SPIN STATE TRAPPING (NIESST) 430
2.33.3 RELAXATION 430
2.33.4 CONCLUSIONS 433
2.33.5 REFERENCES 433
2.33.1 LIGHT-INDUCED EXCITED SPIN STATE TRAPPING (LIESST)
In general, coordination compounds of transition metal ions with ad5tod7electron configuration do not show any luminescence for ligand-field strengths in the vicinity of the crossover point between the high-spin (HS) and the low-spin (LS) states in the respective Tanabe–Sugano diagram. Low-lying ligand-field states form a perfect ladder for efficient nonradiative decay of higher excited states back to the ground state. This is particularly true for the spin-crossover compounds (see Chapter 2.32). Nevertheless, these compounds do possess interesting photophysical properties, to be discussed for iron(II)d6in thefollowing.
Figure1shows the potential well of the1A1(LS) state of iron(II), and at slightly higher energy theonefor the5T2(HS) state, which becomes thermally populated at elevated temperatures. In addition, the higher excited ligand-field states withSẳ0, 1, and 2 and possibleMLCT states are indicated.1–7In the case of ligands for which the weak ligand-field transitions are not obscured by strong MLCT bands, the former can be easily assigned. Figure2 shows absorption spectra of [Fe(ptz)6](BF4)2(ptzẳ1-propyltetrazole),8–10 a prototypefor theclass of compounds with tetra- zole and related ligands.11–13At 295 K, the one band in the near infrared corresponds to the spin- allowed 5T2!5E transition of the HS species. The thermal spin transition of this compound occurs at135 K,4–7 and as a consequence, the two absorption bands in the visible of the 20 K spectrum correspond to the spin-allowed transitions1A1!1T1and 1A1!1T2of theLS species.
Additionally, the weak spin-forbidden transitions1A1!3T1and 1A1!3T2can belocated in the near infrared. Another prototype for a spin-crossover system with pyridyl ligands is provided by [Fe(mepy)3tren]2ỵ, (me py)3trenẳtris{4-[(6-methyl)-2-pyrridyl]-3-aza-3-butenyl}amine.14,15 Figure3 gives its absorption spectra at 295 K and at 20 K doped into an inert host lattice.16–18Theintensity of the strong MLCT band at 550 nm is much higher for complexes in the LS state due to the
427
shorter metal–ligand bond length and the resulting larger overlap between metal-centered and ligand-centered orbitals.
Irradiation of spin-crossover compounds in the visible results in an efficient conversion of the complexes from the LS to the HS state. This was first realized by McGarvey et al.19,20 who determined the dynamics of the HS!LS relaxation of spin-crossover complexes in solution using
MLCT Nuclear decay
1T2
3T2
3T1
5T2
5E
1A1
1T1
LS
slow kHL
∆rHL
∆EHL0 HS fast
Figure 1 The electronic structure of iron(II) spin-crossover complexes. The mechanisms of LIESST, reverse- LIESST, and NIESST areindicated by curly arrows.
10,000 15,000 20,000 25,000 30,000
40
30
20
10
0
1A1
1A1
3T2
5T2 5E
3T1
1A1 1T1
1A1 1T2
x10
ε(1mol–1 cm–1)
Figure 2 Single-crystal absorption spectra (cm1) of [Fe(ptz)6](BF4)2: (– – –) at 295 K, (—) at 10 K, ( ) at 10 K following irradiation at 514.5 nm (thespectrum following irradiation at 980 nm is identical), and (- - -) at 10 K and irradiation at 820 nm. The spectra following irradiation at the different wavelengths provide
clear-cut evidence for the processes depicted inFigure 1.
laser flash photolysis. At low temperatures, the HS!LS relaxation slows down considerably, and below 50 K some systems can be trapped quantitatively in the HS state. Experimental evidence for such ‘‘Light-induced Excited Spin State Trapping (LIESST)’’ is provided by the absorption spectra at 20 K of [Fe(ptz)6](BF4)2 following irradiation into the 1A1!1T1 ligand-field band,4–10 and of [Zn1xFex(mepy)3tren] following irradiation into the MLCT band,16–18which areincluded inFigures 2and3, respectively. Since the discovery of LIESST in 1983, a large number of systems showing this effect have been found for a variety of spin-crossover systems such as in neat solids,21–24in mixed crystals,25,26in amorphous matrices27 and in Langmuir–Blodgett films.28
The mechanism for LIESST is sketched inFigure1: a double intersystem crossing step takes the excited complex to the HS state, where, as result of the energy barrier between the HS and the LS state due to the large difference in metal–ligand bond length and the small value of the zero- point energy difference, E0HL, it stays trapped at sufficiently low temperatures. The quantum efficiency of the double intersystem crossing step is close to unity.8–10Sub-picosecond experiments show that the process is very fast indeed and highly nonadiabatic.29,30 Thus, even though the triplet state does play an important role in the vibronic coupling between the singlet and the quintet manifolds, it cannot be considered a true intermediate state in the relaxation process.
The lifetime of the light-induced HS state even below 10 K is finite, but for some compounds it does reach values of 106s or more.4–10,16–18,21–28At somewhat higher temperatures, a thermally activated relaxation back to the LS ground state invariably sets in (see below). The scheme of Figure1suggests that the complexes may also be pumped back to the ground state optically by irradiating into the5T2!5Eband. This process is much less efficient than the forward process.
Indeed, it can only be observed for compounds which do not have intense MLCT bands with tails all the way into the near infrared. Even then, the reverse process is usually not quantitative.
Because of the spectral overlap of the spin-forbidden transitions of the LS species with the
5T2!5E band, a steady state-type situation results, with a LS population which depends upon thepreciseirradiation wavelength within the5T2!5Eband. This is demonstrated by the absorption spectrum of [Fe(ptz)6](BF4)2following irradiation at 930 nm as shown in Figure2. On theother hand, irradiation into thespin-forbidden1A1!3T1band on the low-energy side of the HS band again results in quantitative LIESST.4–10
In a limited number of systems which stay in the HS state down to low temperatures but sufficiently close to the crossover point, it is possible to populate the LS state as metastable state by irradiation in the near infrared.31–33 Cooperative effects can result in a true light-induced bistability for such systems, which can persist to quite high temperatures.31–33 In exchange- coupled binuclear iron spin-crossover systems, which are in the LS state at low temperatures, it is also possible to induce the HS state by irradiation. Although the individual iron centers are Figure 3 Single-crystal absorption spectra of [Zn1–xFex(mepy)3tren](PF6)2,xẳ0.05% at 295 K ( ), at
10 K (- - -), and at 10 K following irradiation at 514.5 nm (—).
Excited Spin State Trapping (LIESST, NIESST) 429
now in the HS state, the overall state of the binuclear complex is diamagnetic below 10 K due to antiferromagnetic coupling between the two metal centers.34,35
With respect to lifetimes of the light-induced HS states of the order of minutes to days, LIESST is restricted to iron(II) spin-crossover compounds with a small zero-point energy difference, E0HL, between the HS and the LS state. From the point of view of the double intersystem crossing step, LIESST is more general than that. It can also be observed for LS systems36 provided sufficiently fast excitation and detection methods are being used for monitoring the (in this case) much faster decay of the light-induced HS state (see below).