Effect of hydrostatic pressure on excitons in α-perylene crystals

121

Chemical Physics 111 (1987) 121-128 North-Holland, Amsterdam

EFFEm

OF HYDROSTATIC

PRESSURE

ON EXCITONS

IN a-PERYLENE

CRYSTALS

Atsuo MATSUI, Takeshi OHNO, Ken-i&i MIZUNO Department

of Physics, Konan University, Okamoto, Kobe 6.58, Japan

Teruo YOKOYAMA’

and Michihiro KOBAYASHI

Faculty of Engineering Science, Osaka University,

Dedicated

Toyonaka, Osaka 560, Japan

to Professor Yutaka Toyozawa in honor of his 60th birthday

Received 9 April 1986; in final form 3 October 1986

The absorption and luminescence spectra of cu-perylene crystals have been measured under pressure in the range from ambient pressure (0 kbar) to 40 kbar. At 77 K, the free-exciton luminescence band shifts toward lower energy with pressure, at a rate of - 58 cm-r/kbar, while the luminescence band arising from self-trapped excitons shifts toward higher energy at a rate of + 55 cm- ‘/kbar, which is indicative of the instability of self-trapped excitons. The change in the exciton-phonon coupling strength from a strong- (g > 1) to weak-coupling (g < 1) regime occurs at about 6 kbar. At both room and liquid-nitrogen temperatures, the Y state turns out to be the most stable state at high pressure.

1. Introduction The exciton-phonon coupling constant, g, is an important parameter in describing gross features of exciton dynamics, and is defined as

g = E,,/B,

0)

where E,, stands for the lattice-relaxation energy and B the exciton-band half-width. In some cases it is more convenient to introduce the self-trap depth, J&, instead of g, which is given as E STE=B-ELR.

(2)

For g>l, EsTE appears to be negative and g -C1, Es, appears to be positive. Aromatic hydrocarbon crystals consisting identical molecules provide simple examples study exciton dynamics. The exciton-phonon teraction in aromatic hydrocarbon crystals pends in general on the chemical identity of material and the molecular arrangement in

for

’ Present address: Fujitsu Ltd., Kawasaki, Japan.

of to indethe the

crystal. To extract information on exciton dynamics, independent of the chemical species, comparative study of materials that occur in multiple modifications is useful. As examples, IX- and pperylene or (Y- and P-dichloroanthracene can be studied. Along the same line, high-pressure experiments are very powerful. Experiments performed under high pressure are equivalent to the study of many modifications, with variety of exciton-phonon coupling constants, because by applying high pressure the exciton-phonon coupling constant of the crystal can be reduced or increased artificially over a wide range. The pressure-induced change in the exciton-phonon coupling constant has been confirmed for anthracene, in which the coupling constant g = 0.85 at ambient pressure becomes g > 1.6 at 22 kbar [l]. Reported high-pressure data on crystals, such as naphthalene [2] and phenanthrene [3], also suggest that the exciton-phonon coupling constant becomes larger with increasing pressure. In crystals such as anthracene, naphthalene and phenanthrene, the intermolecular distance be-

0301-0104/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

122

A. Matsui et al. / a-perylene under high pressure

tween nearest-neighbor molecules is about 6 A, which is fairly large; accordingly, upon applying high pressure, the intermolecular distance becomes favorable to the effective exciton-phonon coupling strength. The main subject of this paper is to investigate if this is true for crystals in which the intermolecular distance between the nearestneighbor molecules is already as short as 4 A at ambient pressure. The intermolecular distance in ol-perylene is 3.86 A (calculated using X-ray data in ref. [4]). We found that the exciton-phonon coupling strength in ol-perylene decreases as pressure increases, in contrast to the case of anthracene, where the coupling strength increases with pressure. The present investigation also provides information on the relative energy of the so-called Y state [5], which was found to be the most stable exciton state at high pressures.

2. Experimental 2.1. Room-temperature

The light transmitted through the specimen was dispersed by the monochromator and detected by a photomultiplier whose output was processed by a microcomputer to convert the transmission spectra into absorption spectra, referring to the blank (without specimen) transmission spectra. 2.2. Low-temperature

(77 K) measurements

The diamond anvil cell was immersed in liquid nitrogen to measure absorption and luminescence spectra at 77 K under high pressure. Liquid nitrogen was also used as the pressure transmitting medium. Details of this equipment and the experimental technique have been given elsewhere [7]. The optical arrangement for the measurements of absorption and luminescence spectra was similar to that used for room-temperature measurements, except for the use of a brighter monochromator (Narumi RM 23(11)). 3. Experimental results

measurements

The crystal structure of ol-perylene is dimeric and each lattice point is shared with two planeparallel molecules. The powder material obtained from a commercial source was purified and single crystals were vapor-grown in forms of flakes with well developed ab face [6]. To measure the luminescence under hydrostatic pressure, a diamond anvil cell was used with 80% aqueous solution of glyceraldehyde as the pressure-transmitting medium. The specimen was excited by 476.5 nm (20986 cm-‘) light obtained from an Ar ion laser in nearly normal incidence, and the luminescence emitted from the specimen was detected through a monochromator (Spex 1403) in backward scattering geometry. The energy of the exciting light corresponds to the high-energy wing of the strong O-O absorption band. This condition was favorable for minimizing the reabsorption effect on the luminescence spectrum. The absorption spectra under high pressure were also measured using the same diamond anvil cell. The light beam was obtained from a 40 W W-lamp.

Fig. 1 shows polarized luminescence spectra of a-perylene measured under various pressures at room temperature. The spectra were corrected for the instrumental factor. To confirm the reversibility of the spectra against repeated application of pressure, the spectra were measured again on the WAVELENGTH

( nm 1

i

!

!41oal

I

i

.J

&

18000

III_‘_

24000

.A__

18000

12000

WAVENUMBER (cm-l )

Fig.

1. Pressure spectra

dependence in a-perylene

of the polarized luminescence at room temperature.

A. Matsui

et al. / a-petylene

same specimen after the pressure was reduced to ambient pressure. The lineshape and the peak energy of the luminescence band were almost the same before and after the application of pressure up to 32.1 kbar. Several sharp structures observed in the low-frequency region in the spectra at 0, 2.7, and 4.5 kbar are attributed to the R, and R, luminescence lines of ruby in the cell for monitoring pressure. At high pressures, vibronic structures denoted YI and Yz were observed. They shifted to the red as the pressure was increased. Fig. 2 shows polarized absorption spectra under high pressure and at room temperature. The first singlet absorption band is labeled as AO-0, or AO-0, according to the polarization along the crystallographic (I- or b-axis, respectively. The peak positions of the absorption and luminescence bands are plotted in fig. 3 as a function of pressure. We have shown only the b-axis polarized spectra, because the lowest exciton state in a-perylene is allowed for b-axis polarization. The AO-0, and AO-1, absorption peaks shift linearly to the red with pressure. The cross on the ordinate indicates the energy position of the luminescence band F observed previously [S] without using the diamond anvil cell. This band is not seen in the corresponding spectra shown on top of fig. 1 probably due to the strain in the

WAVELENGTH

3 90.. 490 ..“,.

-. ,_

( nm

1

YO ‘5F’ 1 AO-0,

//a

WAVENUMBER

( cm-’ )

Fig. 2. Pressure dependence of the polarized absorption in cr-perylene measured at room temperature.

spectra

under

high pressure

a

123

ST 16000

+-+ i

15000

a 1

82 PRESSURE

( kbar)

Fig. 3. Peak position of the b-polarized absorption and luminescence spectra under high pressure at room temperature.

crystal when it is mounted in the diamond anvil cell. This band becomes observable in the spectra at high pressure as seen in fig. 1. The band F is shifted from the O-O, band by about - 600 cm-’ at 0 kbar (referred to fig. 3). Unpolarized luminescence spectra at 77 K are shown in fig. 4 for various pressures. The 0 kbar spectrum on top is composed of a single broad band (ST) due to the radiative annihilation of self-trapped excitons. At high pressures, there appear vibronic luminescence bands (Y,, YI, Y,) indicated by open circles. A weak luminescence band (F) marked by open squares appear at high pressures. Polarized luminescence spectra at 21.6 and 27.6 kbar are shown in fig. 5. Solid curves represent the b-axis polarized spectra and dot-dashed ones represent the u-axis polarized spectra. The band F is predominantly polarized along the b-axis, while the band Y shows comparable intensities for either polarization.

A. Mutsui et al. / a-peqk=ne

124

under high premm

WAVELENGTH ST

450

425

( nm 1

500

525

In

.k

3.2

.

. 1’ *--

1.9 0

15.1kbar

31.9kbar

00

24

20000

16000

WAVENUMBER

( cm-l 1

.1

22000

Fig. 4. Unpolarized

luminescence spectra pressures.

at 77 K for various Fig. 6. Pressure

Unpolarized absorption presures at 77 K are shown splitting in cw-perylene is so components are observable spectra. The lowest band,

spectra under various in fig. 6. The Davydov large that the splitting even in unpolarized marked AO-0, corre-

dependence

20000

19000

( cm-l 1

of the absorption K.

spectrum

at 77

sponds to the b-axis polarized O-O band and the second one corresponds to the u-axis polarized O-O band. The energy separation between these

23000 04 Perylene

21000

WAVENUMBER

-

0 -Perylene

y

77 K

fir

2:

00

WAVENUMBER Fig. 5. Polarized

14

18000

luminescence spectra sures.

( cm-l

1

at 77 K for two pres-

0

IO

20

PRESSURE

30

40

50

(kbar)

Fig. 7. Pressure dependence of the absorption and luminescence band maxima. Data are obtained from figs. 5 and 6.

A. Matsui et al. / cr-pe$ene

under high pressure

125

PERYLENE MOLECULE

MOLECULE 0

00

Y 0

CRYSTALlspu-\

0:

a-PERYLENE

“e d-Perylene

.!...

1

77 K

D=D,-Dg (A)

Pressure Fig. 8. Integrated

Fig. 9. (A) Schematic energy levels of free molecule and crystal. (B) Energy level correlation in a free molecule and a crystal of a-perylene.

1kbar)

luminescence intensities and ST at 77 K.

of the bands

F, Y,

bands gives the Davydov splitting of about 720 cm-’ for the O-O absorption band. The peak positions of the absorption and luminescence bands at 77 K are plotted as a function of the pressure in fig. 7. The symbols are the same as used in figs. 4-6. The luminescence band F and the luminescence bands (Y,,, Yi, YZ) shift linearly to the red with pressure, but the rate of shift is a little smaller for the band Y than the band F. The significance of this difference is discussed below. Fig. 8 shows the pressure dependence of the integrated intensities of the luminescence bands F, Y, and ST at 77 K. The luminescence band F becomes observable when the pressure is raised above 15 kbar. As the intensity of the luminescence band ST decreases with pressure, the intensity of the band Y increases as if the latter band is going to replace the former.

4. Discussions

where E,, represents the excitation energy of the lowest excited state in a free molecule, D is the site-shift energy, which is given by the difference between the site-shift energies for the excited state (0,) and the ground state (D,), and is usually negative. B is one half of the exciton bandwidth. The relationship among those parameters is illustrated in fig. 9A. In fig. 9B, the schematic energy levels in cu-perylene crystal are shown. The lowest singlet molecular excitation level ‘&” splits into four levels ‘A “’ ‘BU’‘A g, ‘Bg in the crystal. The lowest energy excitation indicated by the dashed vertical line is the transition from the ground state ‘As to one of the four split states ‘As which is located a little below the ‘A, state. This transition, which is originally forbidden, becomes allowed by the mixing of the ‘A, state into the ‘A, state. Due to this mixing effect, the transition b&omes weakly observable for light polarized along the crystallographic b-axis [9]. 4.2. Free-exciton state

4.1. Electronic states in free molecule and crystal

Generally, the energy of the lowest exciton E,, is expressed as E,, = Em,,, + D - B,

(B)

(3)

The characteristic polarization of a luminescence band reflects the symmetry of the excitedstate wavefunction, and is useful for the identification of the electronic transition responsible for the luminescence. The free-exciton (FE) luminescence

126

A. Mutsui et al. / a-petylene

is usually polarized along the specific crystallographic direction according to the symmetry of the relevant wavefunction. Contrary, the luminescence band due to the self-trapped exciton (STE) is mostly unpolarized [lo], because the STE state is composed of a mixture of various excited states due to lattice deformation at the self-trapped lattice site. As seen in the spectra shown in fig. 1 (room temperature) and in fig. 5 (77 K), the luminescence band F is predominantly polarized along the b-axis. From the discussion in section 4.1, this polarization can be accounted for by attributing the band F to the optical transition from the FE state to the ground state. From figs. 3 and 7, the luminescence band F is located at energy a little lower than the lowest allowed absorption band AO-0 corresponding to the ‘A, + ‘A, transition. This is reasonable because the final state for the AO-0 transition is the ‘A, state, while the FE luminescence originates from the ‘As state, which is located below the ‘A, state (cf. fig. 9B). To support this interpretation, there should be no difference in energy between the luminescence band F and the weak absorption band F (indicated by a cross in fig. 3) at 0 kbar. However, in the spectra shown in figs. 2 and 6, the absorption band associated with this iAs + ‘A, transition is not observed at 0 kbar due to the strain in the specimen mounted on the diamond anvil cell, as described before. 4.3. Self-trapped-exciton state We next examine the behavior of exciton spectra at 77 K. A remarkable pressure dependence is found in the STE luminescence band (marked ST) as seen in fig. 7. It shifts to the blue by +550 cm-’ when pressure is raised to about 10 kbar, suggesting that the energy of the STE state increases with pressure. On the other hand, the energy of the FE state is reduced by about 580 cm-’ up to 10 kbar. Because of the opposite shift of the STE energy to the FE energy, the depth E STE of the STE state relative to the FE state becomes shallower with pressure. The value for E STE has been estimated to be about -650 cm-’ at ambient pressure [8]. From the result men-

under high pressure

tioned above, the self-trap depth at 10 kbar is considered to become + 480 cm- ‘. The reversal of sign for Es,, implies that the energy orders of the FE and STE states are interchanged at high pressures. A rough estimation indicates that the crossing of the two states takes place at about 6 kbar. In other words, Es, = 0 at about 6 kbar. The fact that the self-trap depth is positive ( + 480 cm- ‘) at 10 kbar at 77 K is clearly demonstrated in the feature of luminescence spectra. As seen in fig. 8, the integrated intensity of the STE luminescence decreases abruptly at about 12 kbar, and at the same time the intensities of the Y luminescence and the FE luminescence increase with pressure. Therefore, the energy of the STE is considered to be lifted above the two states under pressures above 13 kbar. A similar feature is also seen in the spectra at room temperature. In fig. 3 the band F (the FE luminescence band) shifts to lower frequency with pressure at a rate of - 58 cm-‘/kbar, whereas the band ST (the STE luminescence band) blue-shifts with a rate of +240 cm-‘/kbar. From these results, the STE state at 4 kbar is supposed to be located about +540 cm-’ higher than the FE state. The blue-shift of the STE luminescence band under pressure is an indication of the reduction of E,, by pressure as discussed before. On the other hand, the exciton bandwidth 2B is expected to increase with pressure. Referring to eq. (2), one finds that both a reduction of E,, and an increment in B lead to an increase in EsrE or even to the change of its sign from negative to a positive one. Such a change in Es, may occur in a molecular crystal in which the intermolecular distance is fairly small. In cw-perylene, the intermolecular distance between molecules is quite small (3.86 A) even at ambient pressure [4]. In this case, a reduction in the intermolecular distance between two plane-parallel molecules under pressure makes the crystal “tighter” due to an increased repulsive force between molecules upon compression. Consequently, an additional local lattice shrinkage produced by the formation of a self-trapped exciton will requ-ire a larger energy under high pressure than at ambient pressure. This may explain why the STE energy increases with pressure.

127

A. Mrrtsui et al. / a-perylene under high pressure

In the above discussion we have not taken account of the possible effect of pressure upon the ground state. This effect, if any, may result in an additional decrease of the EsTE value. However, the essential point of discussion as mentioned above would not be altered much by the effect. At room temperature, another weak, broad luminescence band ST’ appears at about 25 kbar on the low-energy side of the ST band and is greatly shifted (see figs. 1 and 3). It shifts to the red with increasing pressure, in contrast to the blue-shift in the STE luminescence band. Therefore, this band may be attributed to different origin from the STE luminescence band, which is not clear at present. 4.5. Y state There are several vibronic luminescence bands (Y,, Yi, Y,) in the spectra shown in fig. 5 (77 K) and in fig. 1 (room temperature). Those vibronic bands do not show appreciable polarization. We attribute these bands to so-called Y luminescence bands which have been observed at ambient pressure below about 60 K [ll]. The following discussion may explain why these Y luminescence bands are observable under high pressure at temperatures above 60 K. They are observed between the FE luminescence band and the STE luminescence band, indicating that the zero-phonon state associated with these vibronic bands is probably located between the FE state and the STE state. In this case, excitons in the Y state must be in thermal equilibrium with both free and self-trapped excitons, because, as mentioned before, free excitons are in thermal equilibrium with self-trapped excitons. However, there is no observable Y luminescence band at room temperature at ambient pressure, though it is observed when the luminescence is time resolved [12]. This seems to indicate that the Y state does not have a well developed dip on the potential energy curve. We then consider the situation at 77 K and at 13 kbar. As discussed before, under 13 kbar, the STE state is located above the FE state. In this situation the lowest exciton state is either the FE state or the Y state. The shift of Y luminescence is

- 500 cm-’ for pressure up to 15 kbar, while that of AO-0, absorption band is -900 cm-‘, giving a relative increase in energy of the Y state to +400 cm-‘. Despite this increase in the Y-state energy, the Y state is probably located below the FE state, because the intense Y luminescence is observed at 13 kbar, while there is no observable FE luminescence (cf. fig. 8). Finally, we give a brief comment on the behavior of excitons in the Y state. The origin of the Y luminescence has been discussed by several groups, mostly attributing it to lattice defects [5,8,13-151. Recently, Walker et al. [16] suggested that Y luminescence is intrinsic of a-perylene and that the STE (so-called excimer) is produced only via the Y state. They proposed an interpretation that excitons in the Y state may be related with the precursor for the excimer. Their interpretation seems to be consistent with the results of present observation at 0 kbar, because the Y state at 0 kbar is located between the FE and the STE states. However, it has been demonstrated in this paper, that the Y state is lowered below the STE state at high pressures. It is not conceivable that a precursor state is located below the final state. Therefore their interpretation seems to need further examination. In conclusion, we have found the exciton-phonon interaction g decreases with pressure in 01perylene. This change in g is in contrast with that in anthracene where g increases with pressure. The relative energy of the Y state varies with pressure, and turns out to be the most stable exciton state at high pressures. Acknowledgement We are grateful to Dr. H. Nishimura and Dr. G.J. Sloan for important comments on this work. To perform this work, Cambridge crystallographic data files were used through computer facilities of Osaka and Kyoto Universities.

References [l] K. Mizuno 2431.

and A. Matsui,

J. Phys. Sot. Japan

55 (1986)

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[21 P.F. Jones and M. Nicol, J. Chem. Phys. 48 (1968) 5440. and R. Jankowiak, J. Luminescence 22 [31 D. Kalinowski (1981) 397. and J. Trotter, Proc. Roy. Sot. A 279 [41 A. Camerman (1966) 129. J. Kinder and M.E. Michel-Beyerle, [51 E. von Freydorf, Chem. Phys. 27 (1978) 199. 161 M. Iemura and A. Matui, Memoirs Konan Univ. Sci. Ser. 27 (1981) 7. Y. Ohno, S. Endo, K. Cho and S. Narita, [71 M. Kobayashi, Physic 117B, 118B (1983) 272. A. Matsui and M. Iemura, J. Phys. Sot. 181 H. Nishimura, Japan 51 (1982) 1341. [91 A. Matsui, K. Mizuno and M. Iemura, J. Phys. Sot. Japan 51 (1982) 1871.

J. Phys. Sot. [lOI A. Matsui, K. Mizuno and H. Nishimura, Japan 53 (1984) 2818. [ill J. Tanaka, Bull. Chem. Sot. Japan 36 (1963) 1237. [121 K.A. Nelson, D.D. Dlott and M.D. Fayer, Chem. Phys. Letters 64 (1979) 88. [131 A. Inoue, K. Yoshihara, T. Kasuya and S. Nagakura, Bull. Chem. Sot. Japan 45 (1972) 720. [141 N.Y.C. Chu and D. Keams, Mol. Cryst. Liquid Cryst. 16 (1972) 61. [151 J. Tanaka, T. Kishi, and M. Tanaka. Bull. Chem. Sot. Japan 47 (1974) 2376. [161 B. Walker, H. Port, and H.C. Wolf, Chem. Phys. 92 (1985) 177.