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J. Phys. Chem. 1996, 100, 3414-3417
Infrared-Induced Processes in a Disordered Crystal of a Tutton Salt Suli Fei and Herbert L. Strauss* Department of Chemistry, UniVersity of California, Berkeley, California 94720-l460 ReceiVed: September 26, 1995; In Final Form: NoVember 27, 1995X
Hole burning of the four N-D stretching bands of NH3D+ in a mixed Tutton salt [(NH4)2Co0.25Ni0.75(H2O)6(SO4)2], dilute in NH3D+ and HDO, has been investigated. The burning changes the orientational distribution of the NH3D+ ions. Relaxation in the dark of the altered distribution is accompanied by slower changes of the HDO molecules. The motion of the HDO molecules demonstrates long-range coupling among the orientational coordinates of the molecules in the crystal and the propagation of strain through the crystal.
I. Introduction The technique of spectral hole burning has been uncovering ever more subtle changes in both ordered and disordered solids. These changes are of interest for elucidating the mechanisms of vibrational and electronic energy transfer and of mechanical coupling in the solids. The possibility of making optical storage devices by hole-burning systems of dye molecules in amorphous polymer matrices has driven many of these investigations.1 However, crystalline hostssperhaps containing a dopantshave more easily definable sites and are more suitable for characterizing the spectral changes in terms of the motion of specific molecules from specific site to specific site. Most of these studies have used hole burning of an electronic transition in the visible to produce a change in the crystal, for example, irradiation of the pentacene visible absorption bands in the system: pentacene as a substitutional dopant in crystalline benzoic acid induces changes in the hydrogen-bonded arrangements of the benzoic acid dimers. The changes in the benzoic acid in turn shift the absorption spectrum of the pentacene. There are many characterizable changes, and they relax into one another sequentially in periods of hours.2 Another way to approach the interaction of the chromophore and its surroundings is to use infrared radiation to alter the surroundings (matrix) directly. This approach has been successful for the system of a phthalocyanine and water both doped into a disordered polymer, poly(methyl methacrylate) (PMMA).3 Infrared irradiation of the vibrational water bands broadens the spectral hole induced by visible irradiation of the phthalocyanine electronic absorption band.4 Infrared excitation puts much less energy into the system, and by using infrared, we have been able to hole burn Tutton salts containing a small amount of NH3D+. The Tutton salts, X2M(H2O)6(SO4)2, where X is an alkali metal or ammonium and M is a divalent transition-metal ion, form a well-characterized series of simple isomorphous crystals. The hole burning can be achieved by irradiating any of four N-D stretching bands and results in well-characterized structural changes.5-7 These changes are rotations of the NH3D+ ion as evidenced by a decrease in the intensity of the irradiated band and a corresponding increase in the intensities of the other three bands. When the irradiating source is removed, the N-D bands eventually return to their original intensity distribution. At the low temperature at which we have done these experiments, about 7 K, the reequilibration occurs entirely by tunneling. The various Tutton salts are isomorphous, and even the mixed * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, February 1, 1996.
0022-3654/96/20100-3414$12.00/0
Tutton salts crystallize into the same structure.8 The mixed salts thus provide a series of crystals with calibrated amounts of disorder. We have studied the ammonium (Co, Ni) mixed salt extensively. The mixed Tutton salts show faster reequilibration kinetics than the neat salts and thus haveson the averageslower barriers to reorientation of the ammonium ions. We found a surprising result: the reequilibration of the NH3D+ ions is accompanied by a change in the distribution of the HDO molecules. In this work, we report on the results of irradiation of each of the four N-D bands in turn. We find that irradiation of any of the four bands results in a change in the HDO distribution. The change in HDO distribution proceeds considerably more slowly than the reequilibration of the orientation of the NH3D+ ions, and both the relative rates and the structure of the crystal imply mechanisms involving propagation of strain slowly through the crystal. II. Experimental Section We have expanded the capability of our previous apparatus7 by adding a number of new laser sources. The basic apparatus now consists of an FTIR spectrometer equipped with a number of flip mirrors. The mirrors can be set to allow irradiation of the sample by a laser source or to allow the acquisition of a normal FTIR spectrum. We have tried both pulsed and CW laser sources. The pulsed source uses a LiIO3 crystal as a difference-frequency generator (DFG). It produces the difference frequency between the 532nm radiation and that generated by a dye laser (Spectra Physics PDL-3). The 532-nm radiation is the doubled output of a NdYAG laser (Spectra Physics GCR-150, 500 mJ at 1064 nm in a 5-ns pulse, 30 pps). The 532-nm power is split by a dichroic mirror so that 20% of the power goes directly to the LiIO3 and the remainder pumps the dye laser.9 The crystal generates about 800 µJ/pulse at 2500 cm-1 and 160 µJ/pulse at 2260 cm-1 in the infrared. We have new CW diode lasers (MDS 2020 lead salt diodes) (Boston Electronics Corp.), which tune over a considerably wider range and are more stable than our previous diodes. The new diodes allow us to burn each of the N-D bands. Both of the lasers first collimated and then focused onto the sample with a 6-in. (l5-cm) focal length parabolic lens. The diameter of the collimated beam was estimated for the DFG laser by burning a film and for the diode laser by determining the diameter of an iris that just limited the energy. These diameters are both about 8 mm. The diffraction limited image of the lasers is then about 0.2 mm. Both of the laser beams © 1996 American Chemical Society
Infrared-Induced Processes in a Tutton Salt
Figure 1. Infrared spectra of the cobalt Tutton salt in the N-D and O-D stretching region. The crystalline Tutton salt (ammonium cobalt sulfate hydrate) is doped with about 5% deuterium. The lower spectrum is that of the original sample at 7 K; the upper spectrum is the difference between the spectrum after 30 min of laser irradiation and the original. The laser frequency was set to the peak of ND I.
(and the FTIR beam) are focused through a 1 mm hole in a mask at the sample. The flux density is then about 1.4 × 104 W/cm2, taking the average power of the laser to be about 10 mW. At some frequencies, the power of the dipole laser was only one-tenth of this. We burned holes with both the pulsed DFG system and the CW diode laser system. The detection was with a FTIR on a time scale of about 1 min. Each of the four N-D bands could be burned using either the diode lasers or the DFG. We tried burning the OD bands with the DFG system but were not successful. The sample was a freshly made mull of ACNS II, (NH4)2Co0.25Ni0.75(H2O)6(SO4)2. As before, the mull was put on a CaF2 plate held in a closed-cycle refrigerator at temperatures down to about 7 K. III. Results The infrared spectrum of the pure Tutton salts such as the ammonium cobalt Tutton salt, doped with a small amount of deuterium, has been studied a number of times. The deuterium forms some NH3D+ ions and some HDO molecules. The N-D, O-D stretching region shows four N-D bands, one for each orientation of the NH3D+, and six O-D bands, one for each distinguishable O-D bond. The 6 HDO molecules are related by an inversion center at the transition-metal site, so only 6 of the possible 12 O-D positions are distinguishable. The four N-D and six O-D bands are shown in Figure 1. We previously showed that each of the N-D bands can be burned with a diode laser.6 The result of one such a burn at about 2260 cm-1 (band N-D I) is shown in Figure 1. Burning position I (the infrared band and, concomitantly, molecules in orientation I) increases the intensity of bands II, III, and IV. Note that the O-D bands do not change in intensity. The process by which the excitation of a given N-D vibration results in a rotation of the NH3D+ to the other possible position is complicated and not entirely understood. We do know that the quantum efficiencies of rotation to various final positions can
J. Phys. Chem., Vol. 100, No. 9, 1996 3415
Figure 2. Spectral holes and antiholes of a mixed (Co,Ni) Tutton salt immediately after irradiating with a diode laser for 5 min at 7 K. The irradiation is of the different ND bands, specifically at 2261, 2290, 2329, and 2366 cm-1, respectively. The OD bands do not appear changed during the burning process.
TABLE 1: Hole Depth (Ab - Ao)a band (burned)
ND Ib
ND IIb
ND IIIb ND IVb OD II,IIIc OD Vc
ND I -0.0205 0.0018 0.0036 0.0027 ND II 0.0006 -0.0014 0.0021 0.0018 ND III 0.00077 0.0010 -0.0100 0.0019 ND IV 0.0031 0.0021 0.0009 -0.0090
0.0220 0.0021 0.0160 0.0070
-0.0230 -0.0020 -0.0190 -0.0080
a A ) absorbance after laser irradiation. A ) absorbance before b o laser irradiation. b Immediately after a 5-min burn. c Change in absorption many hours after the burn. Bands OD II and OD III are difficult to distinguish.
differ from one another10 and that the quantum efficiencies are temperature independent at low temperatures.10,11 The mixed Tutton salts, such as the Co-Ni salt we investigated here, have very similar spectra to that of the neat salt but with somewhat broader bands.7 The bands are inhomogeneously broadened due to the random distribution of the Co and Ni atoms.7 In this paper, we concentrate on the Co0.25N0.75 salt because it provides the clearest example of the change in the OD bands. As the NH3D+ relax back to their original distribution, HDO molecules change their orientational distribution, and this only happens in the mixed salts. In these experiments, we have systematically burned each of the N-D bands in turn. The resulting changes in the other N-D bands are illustrated by the spectra in Figure 2. Burning any of the bands results in an increase in the intensity of each of the other bands (increases that can be considered antiholes). Each band was irradiated for 5 min, and this resulted in a change of a few thousandths to a few hundredths of an absorption unit (Figure 2 and Table 1). The figure and table make clear that following the excitation of a given band, the ammonium ions rotate into the different positions with different probabilities. The N-D bands are broader in the mixed salt than in the neat salts, and they are relatively weak since the concentration of D is only 4%. The changes caused by the hole burning are correspondingly small, and we were not able to find a consistent way to translate the changes of intensity into population changes. The various holes and antiholes decay. Typical decay curves are shown in Figures 3 and 4. Figure 3 shows the decay of a holesthe intensity of the burned band increases toward its unburned value; Figure 4 shows the decay of an antihole. The
3416 J. Phys. Chem., Vol. 100, No. 9, 1996
Fei and Strauss
Figure 3. Recovery of the hole in band ND III of the mixed salt after irradiation of this band. The half-time is 117 min. The overshoot on the recovery may be real and has been seen in the neat salt.6,12 The solid line is a single-exponential fit.
Figure 4. Recovery of the antihole in band ND II after burning of band ND III for the mixed salt. The half-time is 108 min.
TABLE 2: Half-Times for Decay of Holes and Antiholesa in Minutes ND
OD
band burned
I
II
III
IV
II,III
V
I II III IV
16.3 16.6 39 59
36.3 89 108 26b
60 9b 117 26b
62 20b 21 34
465 570 1370 660
630 1060 1970 679
a Half-times, which for an exponential fit, are ln 2/k. b Very small spectral changes with a noisy time dependence.
decay of the holes and antiholes occurs by the tunneling between orientational positions, and the tunneling can be sequential from site to site. The equations describing such a sequence of firstorder rates can be solved6 and predict that the holes should decay as a sum of a number of exponential decay terms. We fit the observed decay curves to sums of exponential functions. In Table 2, we list the results in terms of a time to half decay, regardless of the exact form of the decay. We used the halftime because the signal-to-noise was not good enough to support
Figure 5. Change of OD band II after burning band ND I in the mixed salt. The half-time is 465 min, much longer than the processes illustrated in Figures 3 and 4.
reliable multiterm fits. Figure 3 shows the “overshoot” that appears in some casesswe have still fit this type of curve with a single exponential.12 The ND holes decay with half-times of 16-110 min. Some time latersin a time characterized by half times of 400-2000 min, the OD bands change. Although the times for the OD relaxation seem long, they are in the range of the relaxation times for the NH3D+ orientations of the neat salts.6 Examination of Table 2 shows that the overall relaxations of both the ND bands and the OD bands are characterized by many different rate constants. Our crystal is disordered, and since the holes and antiholes and other changes are small and represent the change of only a small proportion of the ammonium ions, the different experiments likely pick out ammonium and water molecules in different groups of sites. For pentacene in benzoic acid, Trommsdorf et al. have suggested that the complicated kinetics among the many sites may be modeled as tunneling of the system through a multidimensional surface containing many shallow wells.2 For this to be possible in a crystalline system, the chromophore must be affected by the position of many of the surrounding molecules. For the Tutton salts, the ammonium ions contact (via hydrogen bonds) only sulfate ions. These, in turn, hydrogen bond to the water molecules. Thus, the changes in the water molecules are driven by the relatively indirect motion of the ammonium molecules, presumably via small displacements of the sulfate ions. HDO molecules can be moved from orientation II/III to orientation V by raising the temperature to 70 K or higher.7 The reverse change can be effected by the process started by absorption of an infrared photon at the frequency of the N-D stretches. We reiterate that the OD bands do not change in an unirradiated sample at 7 K. Neither do they change in a neat Tutton salt which has been subjected to irradiation of the N-D bands. Why might the water molecule rearrange in Tutton salts? In a series of very detailed X-ray studies, Burgi et al. have explored the relationship between thermal atomic displacements and force fields.13 In most cases, the atomic displacements fit the predictions of the internal force field of the molecules for the internal displacements. To this must be added a term for the translation-rotation of the molecule as a whole.
Infrared-Induced Processes in a Tutton Salt However, Burgi et al. have investigated one Tutton salt, the potassium-ruthenium salt, that does not follow the general rule.14 Instead, to fit the observed thermal parameters, the Ru-O stretching coordinate must be coupled directly to the rigid ion displacements. We may extrapolate from this study to assume that the metal hexaaquo ion in our samples in turn displaces the sulfate and the ammonium ions and that this chain of interactions will be effective in the opposite direction. That is, an extension of the N-D bond caused by infrared excitation will move the ammonium, the sulfate, the hexaaquo metal ion, and then the internal coordinates of the water molecules. IV. Summary and Conclusions Isotopically labeled water molecules (HDO) in crystalline Tutton salts reorient at low temperatures. This reorientation can be effected by the absorption of an infrared photon by any of the N-D stretching modes. In the dark, the NH3D+ relax back to their original distribution andsmore slowlysthe HDO molecules reorient. The potential energy which governs the orientations of the label molecules must depend on many different coordinatessthat is, the reorientation of one water or ammonium must affect many others. The resulting potential energy surface must contain many shallow minima, which the system traverses by tunneling at low temperatures. It is remarkable that this complex chain of events can be set off by absorption of a 2300-cm-1 photon. By contrast, the complex changes set off by hole burning in the visible are triggered by photons with 7 or more times greater energy. In both the
J. Phys. Chem., Vol. 100, No. 9, 1996 3417 infrared and visible hole burning, the triggering photon sets off changes that use directly only a small portion of the triggering energy. Acknowledgment. We are pleased to acknowledge support by the National Science Foundation (CHE 9220908) and enlightening conversations with H. P. Trommsdorf and H. B. Burgi. References and Notes (1) Horic, K., Tani, T., Wild, U. P., Eds. Luminescence 1995, 64 (16). Moerner, W. E., Small, G. J., Eds. J. Opt. Soc. Am. 1992, 9 (5). (2) Trommsdorff, H. P.; Corval, A.; von Laue, L. Pure Appl. Chem. 1995, 67, 191. (3) Most samples of PMMA contain some water, unless great care is taken to exclude it. (4) Barth, K.; Richter, W. J. Luminesc. 1995, 64, 63. (5) Trapani, A. P.; Strauss, H. L. J. Chem. Phys. 1987, 87, 1899. (6) Trapani, A. P.; Strauss, H. L. J. Am. Chem. Soc. 1989, 111, 910. (7) Fei, S.; Strauss, H. L. J. Phys. Chem. 1995, 99, 2256. (8) Cotton, F. A.; Daniels, L. M.; Murillo, C. M.; Quesada, J. E. Inorg. Chem. 1993, 32, 4861. (9) Kung, A.; Fei, S.; Strauss, H. L. Appl. Spectrosc., in press. (10) Weier, J.; Strauss, H. L. J. Chem. Phys. 1992, 96, 8799. (11) Cho, H. G.; Strauss, H. L. J. Chem. Phys. 1993, 99, 5661. (12) Recovery curves with overshoot are one possible solution of the sequential first-order rate equations, and we have seen such curves before (ref 6). (13) Hummel, W.; Raselli, A.; Burgi, H. B. Acta Crystallogr. 1990, B46, 683. (14) Burgi, H. B.; Raselli, A. Struct. Chem. 1993, 4, 23.
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