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Influence of molecular geometry on the absorption spectra of p-quaterphenyl crystals and phase transition
JOURNAL OF
LUMINESCENC ELSEVIER
Journal
of Luminescence
72-74 (1997) 501-502
Influence of molecular geometry on the absorption spectra of p-quaterphenyl crystals and phase transition Kenji Uchida”3 *, Tohru Kakei”, Yoshihiro Takahashib aFukui Institute of Technology, 3-6-1, Gakuen. Fukui, Japan bAichi Institute of Technolog. YakusaQo, Tqvota 470-03, Japan
Abstract
The fluorescence and the absorption spectra of the p-quaterphenyl crystals have been measured at 300-14 K. We found the Gaussian bands in the absorption edge region and its peak shift with the temperature below 233 K is explained by the displacive-type phase transition. Keywords:
Spectroscopy; Phase transition; p-quaterphenyl
The molecule of p-quaterphenyl (QP) in solution has a non-planar ground state (SO) and planar first singlet excited state (Si). However, in the crystal, it has a planar ground state with the torsional oscillation of the central phenyl rings at room temperature [l]. On cooling, the crystals undergo a continuous phase transition at T, = 233 K, below which the molecule is twisted. Then, the lattice sites are not displaced in the course of the phase transition. Related substances, i.e., p-terphenyl undergoes nearly continuous order-disorder phase transition and biphenyl crystals do the continuous displacive-type phase transition. In the present paper we report the temperature dependence of the fluorescence and the absorption spectra of QP crystals, and a interpretation of the change of the electronic spectra is attempted and the nature of the phase transition is discussed. Fig. 1 shows the temperature dependence of the fluorescence spectrum. In the fluorescence *Corresponding author. [email protected].
Fax:
+ 81-776-29-7891;
e-mail:
0022-2313/97/$17.00 [; 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(97)00067-7
spectrum at 300 K, vibrational bands are observed at 407 nm and around 432 nm. On cooling the crystals, additional bands appear in the higherenergy region of these spectra. Appearance of such bands is probably attributed to the decrease of the reabsorption effect of the higher-energy side of the emission [ 11. We cannot assign the 366 nm emission peaks as the o-0 transition in the fluorescence spectrum at 14K, because the reabsorption effect due to the overlap between the absorption and the emission spectrum occurs in the higher-energy side of this band. So, taking account of the vibrational modes observed in Raman spectra, we extrapolate the position of the 0-O transition to 363 nm; the vibrational structures were found to correspond to two Raman active modes 1275 and 1603 cm-‘. Fig. 2 shows the temperature dependence of b-polarized weak absorption edge spectra of QP crystals. These weak spectra are obtained by subtracting the strong background absorption, which was found to consist of two Gaussian bands. These peaks are located at 362 and 346 nm at room
502
K. Uehida et al. / Journal qf’luminescence
400 WAVELENGTH (nm) Fig. 1. The temperature p-quaterphenyl crystals.
dependence
500
of fluorescence
spectra
of
72-74 (1997) 501-502
temperature, and the energy difference between these peaks agrees approximately with one of the energies of the vibration modes observed in the emission spectrum. Therefore, it seems that the weak absorption spectrum has the relation of a mirror image with the emission spectrum. These results probably indicate that the ground state of the weak absorption band has the same geometry as that of the excited state which is planar. This is consistent with the results of the X-ray diffraction study of QP crystals at room temperature [2]. The peaks of these bands continuously shift to higher energies when the temperature decreases from about 240-14K and its energy shift is about 600 cm- ’ at 14 K. This peak shift is explained by the change of the conformation of the ground state molecules due to the torsional oscillation of central phenyl rings below T, = 233 K. The phenyl rings are in a double-well potential as a function of torsional angle 0 [l]. If this phase transition belongs to the order-disorder type, the peak of the weak absorption bands do not shift below T,. If this transition belongs to the displacive type, the 0 increases continuously by decreasing the temperature below T,. Then, the absorption bands shift to higher energy on decreasing the temperature. Therefore, our results suggest that the displacive phase transition occurs for QP crystals. The amplitude of the peak shift is the sum of the barrier height (AE) of the double well potential in the ground state and the potential difference (A.&,) between planar at room temperature and the twisted geometry at 14K in the excited state molecule. As AE,, is estimated as 400 cm-’ [3], the barrier height is obtained as 200 cm- ‘. This value is similar to that of p-terphenyl crystal [2]. In conclusion, we assigned the 0-O transition of the fluorescence spectrum as 363 nm. Furthermore, we observed the weak bands in the low-energy region of the absorption spectra. Its peak shift with the temperature is explained by the displacive-type phase transition. References
WAVELENGTH [nm] [I] K. Saito et al., J. Chem. Thermodyn. Fig. 2. The b-polarized weak absorption crystals at various temperatures.
edge spectra
of QT
17 (1985) 539. [2] P.J. Baudour et al., Acta Crystallogr. B 33 (1977) 1773. [3] G. Swiatkowski et al., J. Lumin. 37 (1987) 183.
LUMINESCENC ELSEVIER
Journal
of Luminescence
72-74 (1997) 501-502
Influence of molecular geometry on the absorption spectra of p-quaterphenyl crystals and phase transition Kenji Uchida”3 *, Tohru Kakei”, Yoshihiro Takahashib aFukui Institute of Technology, 3-6-1, Gakuen. Fukui, Japan bAichi Institute of Technolog. YakusaQo, Tqvota 470-03, Japan
Abstract
The fluorescence and the absorption spectra of the p-quaterphenyl crystals have been measured at 300-14 K. We found the Gaussian bands in the absorption edge region and its peak shift with the temperature below 233 K is explained by the displacive-type phase transition. Keywords:
Spectroscopy; Phase transition; p-quaterphenyl
The molecule of p-quaterphenyl (QP) in solution has a non-planar ground state (SO) and planar first singlet excited state (Si). However, in the crystal, it has a planar ground state with the torsional oscillation of the central phenyl rings at room temperature [l]. On cooling, the crystals undergo a continuous phase transition at T, = 233 K, below which the molecule is twisted. Then, the lattice sites are not displaced in the course of the phase transition. Related substances, i.e., p-terphenyl undergoes nearly continuous order-disorder phase transition and biphenyl crystals do the continuous displacive-type phase transition. In the present paper we report the temperature dependence of the fluorescence and the absorption spectra of QP crystals, and a interpretation of the change of the electronic spectra is attempted and the nature of the phase transition is discussed. Fig. 1 shows the temperature dependence of the fluorescence spectrum. In the fluorescence *Corresponding author. [email protected].
Fax:
+ 81-776-29-7891;
e-mail:
0022-2313/97/$17.00 [; 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(97)00067-7
spectrum at 300 K, vibrational bands are observed at 407 nm and around 432 nm. On cooling the crystals, additional bands appear in the higherenergy region of these spectra. Appearance of such bands is probably attributed to the decrease of the reabsorption effect of the higher-energy side of the emission [ 11. We cannot assign the 366 nm emission peaks as the o-0 transition in the fluorescence spectrum at 14K, because the reabsorption effect due to the overlap between the absorption and the emission spectrum occurs in the higher-energy side of this band. So, taking account of the vibrational modes observed in Raman spectra, we extrapolate the position of the 0-O transition to 363 nm; the vibrational structures were found to correspond to two Raman active modes 1275 and 1603 cm-‘. Fig. 2 shows the temperature dependence of b-polarized weak absorption edge spectra of QP crystals. These weak spectra are obtained by subtracting the strong background absorption, which was found to consist of two Gaussian bands. These peaks are located at 362 and 346 nm at room
502
K. Uehida et al. / Journal qf’luminescence
400 WAVELENGTH (nm) Fig. 1. The temperature p-quaterphenyl crystals.
dependence
500
of fluorescence
spectra
of
72-74 (1997) 501-502
temperature, and the energy difference between these peaks agrees approximately with one of the energies of the vibration modes observed in the emission spectrum. Therefore, it seems that the weak absorption spectrum has the relation of a mirror image with the emission spectrum. These results probably indicate that the ground state of the weak absorption band has the same geometry as that of the excited state which is planar. This is consistent with the results of the X-ray diffraction study of QP crystals at room temperature [2]. The peaks of these bands continuously shift to higher energies when the temperature decreases from about 240-14K and its energy shift is about 600 cm- ’ at 14 K. This peak shift is explained by the change of the conformation of the ground state molecules due to the torsional oscillation of central phenyl rings below T, = 233 K. The phenyl rings are in a double-well potential as a function of torsional angle 0 [l]. If this phase transition belongs to the order-disorder type, the peak of the weak absorption bands do not shift below T,. If this transition belongs to the displacive type, the 0 increases continuously by decreasing the temperature below T,. Then, the absorption bands shift to higher energy on decreasing the temperature. Therefore, our results suggest that the displacive phase transition occurs for QP crystals. The amplitude of the peak shift is the sum of the barrier height (AE) of the double well potential in the ground state and the potential difference (A.&,) between planar at room temperature and the twisted geometry at 14K in the excited state molecule. As AE,, is estimated as 400 cm-’ [3], the barrier height is obtained as 200 cm- ‘. This value is similar to that of p-terphenyl crystal [2]. In conclusion, we assigned the 0-O transition of the fluorescence spectrum as 363 nm. Furthermore, we observed the weak bands in the low-energy region of the absorption spectra. Its peak shift with the temperature is explained by the displacive-type phase transition. References
WAVELENGTH [nm] [I] K. Saito et al., J. Chem. Thermodyn. Fig. 2. The b-polarized weak absorption crystals at various temperatures.
edge spectra
of QT
17 (1985) 539. [2] P.J. Baudour et al., Acta Crystallogr. B 33 (1977) 1773. [3] G. Swiatkowski et al., J. Lumin. 37 (1987) 183.