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9,10-Dibromoanthracene under pressure: Phase transition and reduced excimer stability
Volume 151, number
3
CHEMICAL
PHYSICS LETTERS
14 October
19&8
9,10-DIBROMOANTHRACENE UNDER PRESSURE: PHASE TRANSITION AND REDUCED EXCIMER STABILITY A. BRILLANTE
‘, K. REIMANN and K. SYASSEN ftir Festkijrperforschung, D- 7WJ Stuttgart HO.Federal Republic qfGcrmany
Max-Plan&Instltut
Received 6 July 1988
Optical absorption,
fluorescence
and Raman spectra of 9, IO-dibromoanthracene
have been studied as a function
of pressure up
phase to 10 GPa. Raman-active lattice phonons show an abrupt change at about 1.5GPa which indicates a pressure-induced transition. Corresponding measurements of the opttcal absorption edge and excimer luminescence reveal a large decrease of the Stokes shift by about 2000 cm-’ at the same pressure. The reduced excimer stability is explained in terms of molecular reorientations of adjacent
face-to-face
molecules due to increased
1. Introduction Molecular solids crystallizing in stacked structures form an interesting class of materials in that their photochemical and photophysical properties depend on the quasi-one-dimensional character of the molecular arrays. Typical organic crystals of this kind are some anthracene derivatives whose crystal structures consist of stacks of translationally equivalent molecules with their molecular planes aligned faceto-face. A characteristic property is an electronic excited state where a pair of adjacent molecules is strongly bound and which yields the typical broad, structureless, Stokes-shifted excimer emission [ 11. The effect of pressure on the excimer states is particularly striking, since it affects the geometry of the molecular pair [ 2 1. At sufficiently high pressures the closer distances within the two-molecule system result in an increased short-range repulsive interaction between adjacent molecules. This effect is larger when substituents in the aromatic rings enhance the steric hindrance at contact distances [ 3 1. p-9,1 O-dichloroanthracene (DCIA) is a typical example, where a pressure-induced structural phase change possibly involving reorientations of the molecular units in the lattice [ 41 is associated with a less relaxed emitting ’ Also at: Dipartimento
di Chimica ersita, 40136 Bologna, Italy.
Fisica e Inorganica,
repulsive interactions
at the smaller intermolecular
separations.
state or, in other words, with a destabilization of the excimer. The aim of the present high pressure study of 9,l Odibromoanthracene (DBrA) has been to further test this view by increasing the size of the substituents in the 9 and 10 positions. Crystals of DBrA are triclinic, space group Pi or PI, with two translationally inequivalent molecules per unit cell [ 5 1. The interplanar separation along the stack axis a is 406 pm, compared to the 350 pm found in P-DClA [ 61. The temperature dependence of absorption and fluorescence spectra has been reported by Tanaka [ 71 and has not revealed any phase change down to 4.2 K. The present results on the pressure dependence of Raman-active lattice phonons of DBrA show a clear discontinuity at about 1.5 GPa which is attributed to a pressure-induced phase transition. Absorption and fluorescence spectra reveal a striking reduction of the Stokes shift by about 2000 cm-’ at this pressure, which we associate with a reorientation within the molecular pair giving rise to a more loosely bound configuration. Results are related to previous work on DClA and are interpreted in terms of destabilization of the excimer state produced by crystal constraints at high pressure.
Univ-
0 009-26 14/88/$ 03.50 0 Elsevier Science Publishers ( North-Holland Physics Publishing Division )
B.V.
243
Volume
151, number 3
CHEMICAL
\
2. Experimental Commercial DBrA (Aldrich) was purified by repeated crystallizations and sublimed twice. Optical spectra were measured in a gasketed diamond anvil mixture as prescell using a 4 : 1 methanol-ethanol sure medium. Pressures were determined by the ruby luminescence method [ 81. Raman spectra were measured in nearly backscattering configurations with red light excitation from an argon ion pumped dye laser (DCM dye, ~a=15450 cm-‘, 100 mW power, filtered by an additional 0.3 nm bandpass filter). The absorption edge and fluorescence spectra were recorded with a microoptical spectrometer similar to that described previously [ 9 1.
3. Results and discussion For a molecular crystal possessing two molecules per unit cell nine zone-center modes are expected in the lattice phonon region. X-ray structural data [ 61 do not discriminate between Pi and P 1 as the space group of DBrA. Spectroscopic measurements, however, would be able to distinguish the symmetry of the crystal structure since for the non-centrosymmetric group all nine lattice phonons would be expected in both infrared (IR) and Raman spectra. Six Raman and three IR active lattice phonons should be observed in the case of the centrosymmetric space group Pi. Low-frequency Raman spectra of DBrA at ambient pressure (T= 300 K) show five lattice modes of strong and medium intensity at 23,29, 37, 63 and 78 cm-‘. Preliminary measurements in the far-infrared show three bands at 44, 59 and 73 cm-‘. Assuming that there is only one band missing in the Raman spectrum, we can draw as a first conclusion that DBrA definitely belongs to the centrosymmetric space group pi. Raman spectra at two different pressures arc shown in fig. 1 and the pressure dependence of the frequencies of the observed Raman-active lattice modes in the region O-10 GPa is shown in fig. 2. The relative intensities depend on the orientation of the single crystal in the high pressure cell with respect to the polarization of the incoming laser beam. The data reported in figs. 1 and 2 refer to the backscattering 244
14 October 1988
PHYSICS LETTERS
I
,
,
,
,
1
,
,
,
I
m Er
Br
tn c t 3 D z
P=tlGPa
RAMAN
SHIFT
(cm-l)
Fig. 1. Raman spectra of DBrA at two different pressures before (P= 1.15 CPA) and after (P=2 GPa) the phase transition.
configuration where the laser beam is polarized perpendicular to the stack axis. This configuration is the most favorable for a high intensity of the observed
0
2
4
PRESSURE
6
6
10
12
IGPa)
Fig. 2. Pressure dependence of Raman-active lattice modes of DBrA. Different symbols refer to different samples. Full symbols and crc~sses for increasing pressure and open symbols for decreasing pressure.
Volume 15 1, number 3
CHEMICAL
PHYSICS LETTERS
bands. From inspection of fig. 2 a discontinuity in the pressure dependence of mode frequencies is evident at about 1.5 GPa. Since Raman spectra in the low-frequency lattice region are very sensitive to phase transitions [lo], we infer that a pressure-induced phase change occurs in DBrA at 1.S GPa. Thus, the spectra in fig. 1 refer to pressures immediately before and after the phase transition. As expected, the spectral profiles of DBrA are markedly different in the two phases (which we denote as phase I and II). By supposing that the center of symmetry is maintained in phase II with the number of molecules per cell unchanged, the Raman spectrum is complete, showing a total of six active modes. The possibility of observing all allowed lattice modes at high pressure is enhanced due to the greater spectra1 spread of the Raman lines compared to ambient pressure. Pressure-induced changes in the Raman spectra are fully reproducible for different samples and are perfectly reversible on releasing the pressure from about 10 GPa. Luminescence spectra of DBrA also cover the range up to 10 GPa. The fluorescence of DBrA was excited by the 457.9 nm argon ion laser line, i.e. just at the edge of the absorption at 300 K [ 71. Excimer emission profiles at different pressures are shown in fig. 3. A broad emission peaked at about 23000 cm-’ is observed at ambient conditions. Its peak position, shape and polarization are in agreement with previous results of Tanaka [ 71. The overall spectra1 profiles do not change significantly with increasing pressure. The maxima of the excimer emission undergo the expected shift to lower energies [ 11 1.
14 October
1988
These values are plotted in fig. 4 (broken line). The major change on going to phase II is the large increase of the slope of the energy shift. An additional feature is the small hysteresis on returning to phase I. An interesting point to note is that by returning to ambient pressure after a pressure cycle exceeding 10 GPa the excimer emission is completely recovered, unlike other anthracene derivatives previously studied [2,12]. This appears to be an indication that the bromine substituents act as spacers which suppress the reactivity usually encountered in unsaturated aromatic hydrocarbons at pressures approaching IO GPa [13]. The absorption edge of the first X*+X singlet transition of DBrA has been recorded for samples of approximately 30 pm thickness. Optical densities have been measured by taking the ratio of the intensity transmitted through an empty part of the pressure cell next to the sample to that transmitted through a small portion of the crystal. The absorption edge energies as a function of pressure are also shown in fig. 4 (full line). The energies given correspond to an optical density of two. A small hysteresis is observed in the pressure dependence of the absorption edge on
Br
23000
21000 7 E 0 20000
a mw x 19000 1 5
18000
7
z ‘,
17000
,fjOOO 1 -1,‘L 0 1
2
3
4
5
PRESSURE 16000
IO
Fig. 3. Excimer emission (T=300
K).
20000
18000
WAVENUMBER
22000
(cm-11
profiles of DBrA at different
pressures
Fig. 4. Pressure
6
‘\
7
k
8
9
1 10
(GPO)
dependence of the absorption edge (full line)
and excimer luminescence (broken lint) of DBrA. Full and open symbols refer to mcreasmg and decreasmg pressure, respectively. The energy difference between the two curves yields the value of the Stokes shift.
245
Volume 15 1, number
3
CHEMICAL
PHYSICS LETTERS
returning from phase II to phase I. The surprising result of the absorption edge shift in the stability range of phase I shows an opposite trend with respect to that of the luminescence maxima. The redshift is now much larger in phase I, while a pressure behaviour almost similar to that of the excimer maxima is observed over the whole phase II range. An estimate of the relaxation of the excimer state is given by the energy difference between the absorption and luminescence maxima. This value, the Stokes shift, is directly related to the lattice deformation accompanying excimer formation [ 1] and is indicative of the strength of interaction of the molecular pair involved in the excimer emission [ 31. An obvious observation from the data in fig. 4 is that the Stokes shift of DBrA undergoes a striking reduction from the initial value of some 3000 cm-’ to about 1000 cm-’ in phase II, where it remains approximately constant up to 10 GPa. The loss in Stokes shift of about 2000 cm-’ on passing from phase I to phase II is a clear indication of the destabilization of the excimer state in the high pressure phase. This reduced stability of the excimer state of DBrA above 1.5 GPa can be related to the effect of repulsive interactions at the smaller intermolecular separations under pressure. Steric hindrance in an anthracene skeleton due to substituents in both 9 and 10 positions is a crucial requirement in producing this high pressure instability. Indeed, a similar observation is made for the pressure-induced phase transition of P-DClA at about 3 GPa [ 41, whereas anthracenes substituted in the 9 position only, in the cases studied so far [ 3,12 1, have shown cxcimer state stability up to 10 GPa. The analogy between excimer destabilisation in DBrA and B-DClA is very significant in this context: the transition pressure is lower for the molecule whose substituents have larger size. Thus, the pressure marking a reduced excimer stability of structurally similar compounds can be related to the steric factors determined by the substituents in the aromatic rings. When intermolecular distances between adjacent face-to-face molecules are sufficiently small the molecular pair presumably relaxed in a rotated or tilted configuration where mutual overlap is reduced. Excimer emission would then occur at higher energies and, in a limiting case, totally disappear giving rise to ex246
14October
1988
citon-like monomer emission. ln conclusion, we have performed Raman, optical absorption, and luminescence experiments on DBrA at pressures up to 10 GPa. Based on vibrational spectra in the low-frequency lattice phonon region we conclusively assign the space group of DBrA as Pi. The pressure dependence of Raman spectra reveals the occurrence of a phase change at about 1.5 GPa. This hitherto unknown phase transition is further discussed on the basis of the complementary optical absorption and luminescence measurements. The large dccrcase of the Stokes shift at the transition pressure implies orientational rearrangements involving a smaller overlap between neighbouring molecules in the same stack. The consequence is an excimer emission from a less relaxed state, i.e. a loss of stability of the excimer state at high pressure.
Acknowledgement We acknowledge useful discussions with Professor D.P. Craig. We thank Mrs. R. Hempel for technical assistance.
References 1V. Yakhot, M.D. Cohen and Z. Ludmer. Advan. Photothem.
11 (1979)
489.
1A. Brillante, K. Strijssner and K. Syassen. Chem. Phys. Letters 134 (1987)
331.
‘I A. Brillante, R.G. Della Valle, K. Striissner and K. Syassen, J. Luminescence40/41 (1988) 278. A. Brillante, R.G. Della Valle and K. Syassen, J. Chem. Phys., to he published. J. Trotter, Acta Cryst. 11 (1958) 564, 583. R. Krauss, H. Schulz, R. Nesper and K.H. Tlemann, Acta Ctyst.B35(1979) 1419. ‘I J. Tanaka and M. Shihate, Bull. Chem. Sot. Japan 41 ( 1968) 34. ‘1G.J. Piermarini, S. Block, J.P. Barnett and R.A. Forman, J. Appl. Phys. 46 (1975) 2774. [ 91 K. Syassen and R. Sonnenschein, Rev. Sci. lnstr. 53 ( 1982) 644. [IO] B.A. Weinstein and R. Zallen, in: Light scattering in solids, Vol. 4, eds. M. Cardona and G. Gtinterodt (Springer, Berlin, 1984). ] H.G. Dnckamer and C.W. Franck, Electronic transitions and the high pressure chemistry of solids (Chapman and Hall, London, 1974). ] A. Arillante, M. Hanfland and K. Syassen, Chem. Phys. Letters 119 (1985) 42. ] M. Ntcoland G.Z. Yin, J. Phys. (Paris) C8-45 (1984) 163.
3
CHEMICAL
PHYSICS LETTERS
14 October
19&8
9,10-DIBROMOANTHRACENE UNDER PRESSURE: PHASE TRANSITION AND REDUCED EXCIMER STABILITY A. BRILLANTE
‘, K. REIMANN and K. SYASSEN ftir Festkijrperforschung, D- 7WJ Stuttgart HO.Federal Republic qfGcrmany
Max-Plan&Instltut
Received 6 July 1988
Optical absorption,
fluorescence
and Raman spectra of 9, IO-dibromoanthracene
have been studied as a function
of pressure up
phase to 10 GPa. Raman-active lattice phonons show an abrupt change at about 1.5GPa which indicates a pressure-induced transition. Corresponding measurements of the opttcal absorption edge and excimer luminescence reveal a large decrease of the Stokes shift by about 2000 cm-’ at the same pressure. The reduced excimer stability is explained in terms of molecular reorientations of adjacent
face-to-face
molecules due to increased
1. Introduction Molecular solids crystallizing in stacked structures form an interesting class of materials in that their photochemical and photophysical properties depend on the quasi-one-dimensional character of the molecular arrays. Typical organic crystals of this kind are some anthracene derivatives whose crystal structures consist of stacks of translationally equivalent molecules with their molecular planes aligned faceto-face. A characteristic property is an electronic excited state where a pair of adjacent molecules is strongly bound and which yields the typical broad, structureless, Stokes-shifted excimer emission [ 11. The effect of pressure on the excimer states is particularly striking, since it affects the geometry of the molecular pair [ 2 1. At sufficiently high pressures the closer distances within the two-molecule system result in an increased short-range repulsive interaction between adjacent molecules. This effect is larger when substituents in the aromatic rings enhance the steric hindrance at contact distances [ 3 1. p-9,1 O-dichloroanthracene (DCIA) is a typical example, where a pressure-induced structural phase change possibly involving reorientations of the molecular units in the lattice [ 41 is associated with a less relaxed emitting ’ Also at: Dipartimento
di Chimica ersita, 40136 Bologna, Italy.
Fisica e Inorganica,
repulsive interactions
at the smaller intermolecular
separations.
state or, in other words, with a destabilization of the excimer. The aim of the present high pressure study of 9,l Odibromoanthracene (DBrA) has been to further test this view by increasing the size of the substituents in the 9 and 10 positions. Crystals of DBrA are triclinic, space group Pi or PI, with two translationally inequivalent molecules per unit cell [ 5 1. The interplanar separation along the stack axis a is 406 pm, compared to the 350 pm found in P-DClA [ 61. The temperature dependence of absorption and fluorescence spectra has been reported by Tanaka [ 71 and has not revealed any phase change down to 4.2 K. The present results on the pressure dependence of Raman-active lattice phonons of DBrA show a clear discontinuity at about 1.5 GPa which is attributed to a pressure-induced phase transition. Absorption and fluorescence spectra reveal a striking reduction of the Stokes shift by about 2000 cm-’ at this pressure, which we associate with a reorientation within the molecular pair giving rise to a more loosely bound configuration. Results are related to previous work on DClA and are interpreted in terms of destabilization of the excimer state produced by crystal constraints at high pressure.
Univ-
0 009-26 14/88/$ 03.50 0 Elsevier Science Publishers ( North-Holland Physics Publishing Division )
B.V.
243
Volume
151, number 3
CHEMICAL
\
2. Experimental Commercial DBrA (Aldrich) was purified by repeated crystallizations and sublimed twice. Optical spectra were measured in a gasketed diamond anvil mixture as prescell using a 4 : 1 methanol-ethanol sure medium. Pressures were determined by the ruby luminescence method [ 81. Raman spectra were measured in nearly backscattering configurations with red light excitation from an argon ion pumped dye laser (DCM dye, ~a=15450 cm-‘, 100 mW power, filtered by an additional 0.3 nm bandpass filter). The absorption edge and fluorescence spectra were recorded with a microoptical spectrometer similar to that described previously [ 9 1.
3. Results and discussion For a molecular crystal possessing two molecules per unit cell nine zone-center modes are expected in the lattice phonon region. X-ray structural data [ 61 do not discriminate between Pi and P 1 as the space group of DBrA. Spectroscopic measurements, however, would be able to distinguish the symmetry of the crystal structure since for the non-centrosymmetric group all nine lattice phonons would be expected in both infrared (IR) and Raman spectra. Six Raman and three IR active lattice phonons should be observed in the case of the centrosymmetric space group Pi. Low-frequency Raman spectra of DBrA at ambient pressure (T= 300 K) show five lattice modes of strong and medium intensity at 23,29, 37, 63 and 78 cm-‘. Preliminary measurements in the far-infrared show three bands at 44, 59 and 73 cm-‘. Assuming that there is only one band missing in the Raman spectrum, we can draw as a first conclusion that DBrA definitely belongs to the centrosymmetric space group pi. Raman spectra at two different pressures arc shown in fig. 1 and the pressure dependence of the frequencies of the observed Raman-active lattice modes in the region O-10 GPa is shown in fig. 2. The relative intensities depend on the orientation of the single crystal in the high pressure cell with respect to the polarization of the incoming laser beam. The data reported in figs. 1 and 2 refer to the backscattering 244
14 October 1988
PHYSICS LETTERS
I
,
,
,
,
1
,
,
,
I
m Er
Br
tn c t 3 D z
P=tlGPa
RAMAN
SHIFT
(cm-l)
Fig. 1. Raman spectra of DBrA at two different pressures before (P= 1.15 CPA) and after (P=2 GPa) the phase transition.
configuration where the laser beam is polarized perpendicular to the stack axis. This configuration is the most favorable for a high intensity of the observed
0
2
4
PRESSURE
6
6
10
12
IGPa)
Fig. 2. Pressure dependence of Raman-active lattice modes of DBrA. Different symbols refer to different samples. Full symbols and crc~sses for increasing pressure and open symbols for decreasing pressure.
Volume 15 1, number 3
CHEMICAL
PHYSICS LETTERS
bands. From inspection of fig. 2 a discontinuity in the pressure dependence of mode frequencies is evident at about 1.5 GPa. Since Raman spectra in the low-frequency lattice region are very sensitive to phase transitions [lo], we infer that a pressure-induced phase change occurs in DBrA at 1.S GPa. Thus, the spectra in fig. 1 refer to pressures immediately before and after the phase transition. As expected, the spectral profiles of DBrA are markedly different in the two phases (which we denote as phase I and II). By supposing that the center of symmetry is maintained in phase II with the number of molecules per cell unchanged, the Raman spectrum is complete, showing a total of six active modes. The possibility of observing all allowed lattice modes at high pressure is enhanced due to the greater spectra1 spread of the Raman lines compared to ambient pressure. Pressure-induced changes in the Raman spectra are fully reproducible for different samples and are perfectly reversible on releasing the pressure from about 10 GPa. Luminescence spectra of DBrA also cover the range up to 10 GPa. The fluorescence of DBrA was excited by the 457.9 nm argon ion laser line, i.e. just at the edge of the absorption at 300 K [ 71. Excimer emission profiles at different pressures are shown in fig. 3. A broad emission peaked at about 23000 cm-’ is observed at ambient conditions. Its peak position, shape and polarization are in agreement with previous results of Tanaka [ 71. The overall spectra1 profiles do not change significantly with increasing pressure. The maxima of the excimer emission undergo the expected shift to lower energies [ 11 1.
14 October
1988
These values are plotted in fig. 4 (broken line). The major change on going to phase II is the large increase of the slope of the energy shift. An additional feature is the small hysteresis on returning to phase I. An interesting point to note is that by returning to ambient pressure after a pressure cycle exceeding 10 GPa the excimer emission is completely recovered, unlike other anthracene derivatives previously studied [2,12]. This appears to be an indication that the bromine substituents act as spacers which suppress the reactivity usually encountered in unsaturated aromatic hydrocarbons at pressures approaching IO GPa [13]. The absorption edge of the first X*+X singlet transition of DBrA has been recorded for samples of approximately 30 pm thickness. Optical densities have been measured by taking the ratio of the intensity transmitted through an empty part of the pressure cell next to the sample to that transmitted through a small portion of the crystal. The absorption edge energies as a function of pressure are also shown in fig. 4 (full line). The energies given correspond to an optical density of two. A small hysteresis is observed in the pressure dependence of the absorption edge on
Br
23000
21000 7 E 0 20000
a mw x 19000 1 5
18000
7
z ‘,
17000
,fjOOO 1 -1,‘L 0 1
2
3
4
5
PRESSURE 16000
IO
Fig. 3. Excimer emission (T=300
K).
20000
18000
WAVENUMBER
22000
(cm-11
profiles of DBrA at different
pressures
Fig. 4. Pressure
6
‘\
7
k
8
9
1 10
(GPO)
dependence of the absorption edge (full line)
and excimer luminescence (broken lint) of DBrA. Full and open symbols refer to mcreasmg and decreasmg pressure, respectively. The energy difference between the two curves yields the value of the Stokes shift.
245
Volume 15 1, number
3
CHEMICAL
PHYSICS LETTERS
returning from phase II to phase I. The surprising result of the absorption edge shift in the stability range of phase I shows an opposite trend with respect to that of the luminescence maxima. The redshift is now much larger in phase I, while a pressure behaviour almost similar to that of the excimer maxima is observed over the whole phase II range. An estimate of the relaxation of the excimer state is given by the energy difference between the absorption and luminescence maxima. This value, the Stokes shift, is directly related to the lattice deformation accompanying excimer formation [ 1] and is indicative of the strength of interaction of the molecular pair involved in the excimer emission [ 31. An obvious observation from the data in fig. 4 is that the Stokes shift of DBrA undergoes a striking reduction from the initial value of some 3000 cm-’ to about 1000 cm-’ in phase II, where it remains approximately constant up to 10 GPa. The loss in Stokes shift of about 2000 cm-’ on passing from phase I to phase II is a clear indication of the destabilization of the excimer state in the high pressure phase. This reduced stability of the excimer state of DBrA above 1.5 GPa can be related to the effect of repulsive interactions at the smaller intermolecular separations under pressure. Steric hindrance in an anthracene skeleton due to substituents in both 9 and 10 positions is a crucial requirement in producing this high pressure instability. Indeed, a similar observation is made for the pressure-induced phase transition of P-DClA at about 3 GPa [ 41, whereas anthracenes substituted in the 9 position only, in the cases studied so far [ 3,12 1, have shown cxcimer state stability up to 10 GPa. The analogy between excimer destabilisation in DBrA and B-DClA is very significant in this context: the transition pressure is lower for the molecule whose substituents have larger size. Thus, the pressure marking a reduced excimer stability of structurally similar compounds can be related to the steric factors determined by the substituents in the aromatic rings. When intermolecular distances between adjacent face-to-face molecules are sufficiently small the molecular pair presumably relaxed in a rotated or tilted configuration where mutual overlap is reduced. Excimer emission would then occur at higher energies and, in a limiting case, totally disappear giving rise to ex246
14October
1988
citon-like monomer emission. ln conclusion, we have performed Raman, optical absorption, and luminescence experiments on DBrA at pressures up to 10 GPa. Based on vibrational spectra in the low-frequency lattice phonon region we conclusively assign the space group of DBrA as Pi. The pressure dependence of Raman spectra reveals the occurrence of a phase change at about 1.5 GPa. This hitherto unknown phase transition is further discussed on the basis of the complementary optical absorption and luminescence measurements. The large dccrcase of the Stokes shift at the transition pressure implies orientational rearrangements involving a smaller overlap between neighbouring molecules in the same stack. The consequence is an excimer emission from a less relaxed state, i.e. a loss of stability of the excimer state at high pressure.
Acknowledgement We acknowledge useful discussions with Professor D.P. Craig. We thank Mrs. R. Hempel for technical assistance.
References 1V. Yakhot, M.D. Cohen and Z. Ludmer. Advan. Photothem.
11 (1979)
489.
1A. Brillante, K. Strijssner and K. Syassen. Chem. Phys. Letters 134 (1987)
331.
‘I A. Brillante, R.G. Della Valle, K. Striissner and K. Syassen, J. Luminescence40/41 (1988) 278. A. Brillante, R.G. Della Valle and K. Syassen, J. Chem. Phys., to he published. J. Trotter, Acta Cryst. 11 (1958) 564, 583. R. Krauss, H. Schulz, R. Nesper and K.H. Tlemann, Acta Ctyst.B35(1979) 1419. ‘I J. Tanaka and M. Shihate, Bull. Chem. Sot. Japan 41 ( 1968) 34. ‘1G.J. Piermarini, S. Block, J.P. Barnett and R.A. Forman, J. Appl. Phys. 46 (1975) 2774. [ 91 K. Syassen and R. Sonnenschein, Rev. Sci. lnstr. 53 ( 1982) 644. [IO] B.A. Weinstein and R. Zallen, in: Light scattering in solids, Vol. 4, eds. M. Cardona and G. Gtinterodt (Springer, Berlin, 1984). ] H.G. Dnckamer and C.W. Franck, Electronic transitions and the high pressure chemistry of solids (Chapman and Hall, London, 1974). ] A. Arillante, M. Hanfland and K. Syassen, Chem. Phys. Letters 119 (1985) 42. ] M. Ntcoland G.Z. Yin, J. Phys. (Paris) C8-45 (1984) 163.