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Emission from evaporated anthracene films
Journal of Luminescence 15 (1977) 293—298 © North-Holland Publishing Company
EMISSION FROM EVAPORATED ANTHRACENE FILMS Yoshihiro TAKAHASHI Department of Electronic Engineering, A ichi Institute of Technology, Yakusacho, Toyoda 470—03, Japan
Kenji UCHIDA and Masao TOMURA Department ofApplied Physics, Osaka City University, Sumivoshiku, Osaka 558, Japan
Received 2 November 1976 Emission spectra, excitation spectra and decay times of fluorescence of anthracene films prepared by evaporation on to substrate cooled with liquid nitrogen were measured. The fluorescence spectra of such films show a broad structureless band. The fluorescence excitation spectra and decay times of the emission in the shorter wavelength side of the band are different from those in the longer wavelength side. The decay times of the emissions on the shorter and longer wavelength sides are about 6.0 and 190 ns, respectively, at liquid nitrogen temperature. It may be that the former emission is due to a crystalline structure and the latter emission to an amorphous structure. Next, in anthracene evaporated films containing tetracene as a dopant, the energy transfer from the host to the guest could be observed and this was attributed to the exciton diffusion through the crystalline structure.
1. Introduction Recently, various spectroscopic measurements on thin hydrocarbon films prepared by vacuum evaporation on to a cooled surface have been reported [1—31.If the temperature of the substrate is sufficiently low, structures of such evaporated films are essentially glass-like, that is, amorphous [1]. The fluorescence spectra of such films often show a broad structureless band and the peak energy of their emission is lower than that of crystals. Therefore, the fluorescent nature of such evaporated films is similar to that of the excimer emission of a pyrene crystal [4]~ In the present study the emission spectra, excitation spectra and emission decay times of anthracene thin films prepared by vacuum evaporation on to glass plates cooled at liquid nitrogen temperature were measured and these emission properties were compared with those of crystals. Furthermore, in anthracene thin films containing tetracene prepared by a similar method as above, the emission spectra and emission decay curves were measured. From these results the mechanism of the energy transfer from the host to the guest is discussed.
293
294 2.
Y. Taka/zashi ci a!.
/ Emission from
eraporated anthracene fl/ins
Experimental
An anthracene thin film was prepared by evaporation at about 10—6 torr on to a glass plate cooled with liquid nitrogen. Films doped with tetracene were obtained similarly by evaporation of anthracene mixed with tetracene. The tetracene concentration was estimated from the probability of the energy transfer obtained by the emission spectrum measurements at room temperature, assuming that the degree of crystallization was about 100% at room temperature as mentioned below. The starting materials were of the highest purity available [5]. For steady excitation of the specimen at 365 nm a monochromator of Shimazu Bausch and Lomb equipped with a 150W Xenon lamp was used and the emission passed through a monochroinator Hitachi EPU-2A and was received by a photomultiplier tube HTV 1 P28. The composite spectral sensitivity of the photomultiplier tube and the monochromator was calibrated by a standard tungsten lamp. For measurements of excitation spectra, specimens were excited by steady light passed through a monochromator Hitachi 139 which was equipped with a 500 W Xenon lam p. Emission decay curves were observed by a pulse method similar to that in ref. [6]. The half width of pulsed light was about 10 us. Pulsed light was irradiated on to a specimen at about 365 nm through a glass filter Toshiba UV DIC. Pulsed eniissions passed through a monochromator of Shimazu Bausch and Lomb and were received by a photoniultiplier tube HTV 1P28. The responses were displayed on a synchroscope Tektronix 465.
3. Results and discussions 3.1. Fluorescence of undoped anthracene evaporated films
The fluorescence spectra of the films freshly evaporated at liquid nitrogen temperature and the films warmed slowly up to higher temperature are shown in fig. I. The emission band at 85.5 K is broad and structureless. It is similar to that of the anthracene excimer emission [7]. The energy difference between the peaks of the monomer emission and excimer-like emission bands is about 4000 cm while that energy difference in solution is about 6000 cm’ [71. The blue shift of the peak in this excimer-like emission band may be due to the mixture of that with the monomer emission band. The excitation spectra of a freshly evaporated film and of a crystal at liquid nitrogen temperature are shown in fig. 2. The excitation spectrum for the 425 nm emission of the freshly evaporated film is clearly different from that for the 500 nm emission. The former spectrum is similar to that of the crystal. This difference inay be due to the different natures of the monomer and excimer emissions. On warming the freshly evaporated film, the vibrational structure like that in the monomer emission band appears in the shorter wavelength side of the
Y. Takahashi ci a!.
/ Emission from
evaporated ant hra cenc films
295
855 K
a
C I,
C
a
C
~
~
4tXD
S00 Wavelength ( nm)
600
Jig. 1. Emission spectra of an anthracene evaporated film under 365 nm excitation, 85.SK;----at2O5K;—------at296K.
at
broad structureless band. Therefore, a transition from an amorphous to a polycrystalline structure seenis to take place by warming the film. At room temperature the broad structureless spectrum disappears, and instead only the vibrational structure like that in the emission band of an anthracene crystal appears as shown in fig. 1. In fig. 3, the decay curves at 84 K are shown for the 430 and 600 nm emissions
/1’
/11 110
:
/_•~~ ~.‘
.
\
\\
t_:~
~50~
~
Crystal ~t
I
~ 10
‘
~
//~JA\~
A
at841(
• A
43Ormm
600nm
:
S
C
400
440
It
10
100
~vetength(rwn)
200 Tirneinsec)
Fig. 2. Excitation spectra at liquid nitrogen temperature for an anthracene film and anthracene crystal. --—-— for the 500 nm emission; ---- for the 425 nm emission; — - — - — for the 425 nm emission of an anthracene crystal. Fig. 3. Emission decay curves of an anthracene evaporated film at 84 K. sion; • for the 600 nm emission.
A
for the 430 nm emis-
296
Y. Takahashi Ct a!.
/ Emission from evaporated anthraccne Ji!rns
of a freshly evaporated film. The decay curve of the emission at 600 nut is of exponential type and its decay time is abou~190 us. This value is approximately equal to that of the emission of the anthracene excimer isolated in a matrix [81. On the other hand, the decay curve of the emission at 430 urn is non-exponential and appears as a sum of two exponential curves of which the decay times are 6 and 190 ns. The latter decay time agrees with that of the excimer-like emission at 600 nm, while the former decay time is closely equal to that of an anthracene crystal at liquid nitrogen temperature [51.However, these results differ from those of Perkampus and Stichtenoth [3]; in their results sevcral components of decay times were observed. It is difficult to explain their results from ours. The results mentioned above suggest that, at liquid nitrogen temperature, the freshly evaporated film has both crystalline and amorphous structures. Furthermore, the emission in the longer wavelength side is due to the amorphous structure and is excimer-like, while the emission in the shorter wavelength side shows the characteristics of both crystalline and amorphous structures. Assuming that the quantum efficiencies of the emissions from the crystalline and amorphous structures are equal, the degree of crystallization at liquid nitrogen temperature is estimated as about 10% of the film volume from the ratio of areas of the decay curves of both emissions. From the results shown in fig. I, the degree of crystallization seems to increase with an increase of temperature and its value at room temperature is about 100%. 3.2. Fluorescence of an anthracene evaporated film containing tetracene In order to investigate energy transfer in an anthracene evaporated film, evaporated films doped with tetracene were used. The emission spectra for freshly evapor. ated films doped with tetracene are shown in fig. 4. The emission spectra are different from that of the undoped crystal and show a little tetracene emission. Its intensity increases with an increase of the tetracene concentration. As shown in fig. 5, the tetracene emission intensity increases with increasing temperature for the 3 tetracene doped film. As mentioned before the structure of the 3.3 X 1017 cm film changes from amorphous to polycrystalline with an increase of temperature, and therefore the increase in the tetracene emission intensity may be attributed mainly to the increase in the amount of the polycrystalline structure in the film. Furthermore, as shown in fig. 6, the decay curve of the emission at 430 nm of the freshly evaporated film doped with 6.5 X 1016 cm3 tetracene is non-exponential at 83.5 K and is shown as a sum of two exponential functions of which the decay times are 4 and 190 ns. At liquid nitrogen temperature the amount of the shorter decay component of the emission due to the crystalline structure decreases with increasing tetracene concentration. If energy transfer by excitons occurs through the crystalline structure, the transfer probability can be obtained by a similar method to that in the case of the crystal [5]. The probability of the energy transfer for the 6.5 X 1016 cm3 tetracene doped film at liquid nitrogen temperature is calcul-
Y. Takahashi et al. / Emission from evaporated anthracene fl/ins IS
4lX)
297
atLN.T.
500
600
Wav~ength(m~
Fig. 4. Emission spectra at liquid nitrogen temperature for anthracene evaporated films doped 3 with tetracene under 365 nm excitation. — undoped; 4.9 x 1016 cm 2.6 X 1017cm3 —~ 4.9 X 1017 cm3.
ated from the decay times of the shorter emission decay components of undoped and doped anthracene films. It is about 8.0 X l0~s~. As this value of the probability agrees well with that of the anthracene crystal and the energy transfer in the crystal takes place by exciton diffusion [5], it may be concluded that the energy transfer to tetracene in the film occurs through the crystalline structure by the
exciton diffusion. On the other hand, the change of the decay time of the excimerlike emission cannot be observed with variation of the tetracene concentration. Hence, the probability of the energy transfer from excitons to acceptors through
Wavelength (nm)
Fig. 5. The emission spectra at various temperatures for the anthracene evaporated film doped with 3.3 X 1017 cm3 tetracene.
298
Y. Takahashi ci a!.
/ Emission from
evaporated anthracene fl/ins
atBl.5K
3tetracene
6.SxlO crst l.3Onm
~ ElI
>-5J
~190n5 0
20
tO 60 80 Time(nsec)
1111
Fig. 6. The decay curve of the 430 nm emission at 83.5 K for the fresh anthracene film doped with 6.5 x 1016 cm3 tetracene.
the amorphous structure is negligible. This result is in agreement with that obtained byArden [21.
References [1] Y. Maruyama and N. Iwasaki, Chem. Phys. Letters 24 (1974) 26. [21 W. Arden, L.M. Peter and G. Vaubel, J. Luminescence 9 (1974) 257. [31 H.H. Perkampus and F!. Stichtenoth, Z. Phys. Chem. 76 (1971) 18. [4] J.B. Birks et al., Proc. Roy. Soc. 291 (1966) 556. [51 Y. Takahashi and M. Tomura, J. Phys. Soc. Jap. 31(1971)1100. [61 M. Tomura and Y. Takahashi, J. Phys. Soc. Jap. 31(1971) 797. [71 B. Stevens, Spectrochim. Acta 18 (1962) 439. [81 J.B. Birks, Photophysics of aromatic molecules (Wiley—lnterscience, New York, 1970) ch. 7.
EMISSION FROM EVAPORATED ANTHRACENE FILMS Yoshihiro TAKAHASHI Department of Electronic Engineering, A ichi Institute of Technology, Yakusacho, Toyoda 470—03, Japan
Kenji UCHIDA and Masao TOMURA Department ofApplied Physics, Osaka City University, Sumivoshiku, Osaka 558, Japan
Received 2 November 1976 Emission spectra, excitation spectra and decay times of fluorescence of anthracene films prepared by evaporation on to substrate cooled with liquid nitrogen were measured. The fluorescence spectra of such films show a broad structureless band. The fluorescence excitation spectra and decay times of the emission in the shorter wavelength side of the band are different from those in the longer wavelength side. The decay times of the emissions on the shorter and longer wavelength sides are about 6.0 and 190 ns, respectively, at liquid nitrogen temperature. It may be that the former emission is due to a crystalline structure and the latter emission to an amorphous structure. Next, in anthracene evaporated films containing tetracene as a dopant, the energy transfer from the host to the guest could be observed and this was attributed to the exciton diffusion through the crystalline structure.
1. Introduction Recently, various spectroscopic measurements on thin hydrocarbon films prepared by vacuum evaporation on to a cooled surface have been reported [1—31.If the temperature of the substrate is sufficiently low, structures of such evaporated films are essentially glass-like, that is, amorphous [1]. The fluorescence spectra of such films often show a broad structureless band and the peak energy of their emission is lower than that of crystals. Therefore, the fluorescent nature of such evaporated films is similar to that of the excimer emission of a pyrene crystal [4]~ In the present study the emission spectra, excitation spectra and emission decay times of anthracene thin films prepared by vacuum evaporation on to glass plates cooled at liquid nitrogen temperature were measured and these emission properties were compared with those of crystals. Furthermore, in anthracene thin films containing tetracene prepared by a similar method as above, the emission spectra and emission decay curves were measured. From these results the mechanism of the energy transfer from the host to the guest is discussed.
293
294 2.
Y. Taka/zashi ci a!.
/ Emission from
eraporated anthracene fl/ins
Experimental
An anthracene thin film was prepared by evaporation at about 10—6 torr on to a glass plate cooled with liquid nitrogen. Films doped with tetracene were obtained similarly by evaporation of anthracene mixed with tetracene. The tetracene concentration was estimated from the probability of the energy transfer obtained by the emission spectrum measurements at room temperature, assuming that the degree of crystallization was about 100% at room temperature as mentioned below. The starting materials were of the highest purity available [5]. For steady excitation of the specimen at 365 nm a monochromator of Shimazu Bausch and Lomb equipped with a 150W Xenon lamp was used and the emission passed through a monochroinator Hitachi EPU-2A and was received by a photomultiplier tube HTV 1 P28. The composite spectral sensitivity of the photomultiplier tube and the monochromator was calibrated by a standard tungsten lamp. For measurements of excitation spectra, specimens were excited by steady light passed through a monochromator Hitachi 139 which was equipped with a 500 W Xenon lam p. Emission decay curves were observed by a pulse method similar to that in ref. [6]. The half width of pulsed light was about 10 us. Pulsed light was irradiated on to a specimen at about 365 nm through a glass filter Toshiba UV DIC. Pulsed eniissions passed through a monochromator of Shimazu Bausch and Lomb and were received by a photoniultiplier tube HTV 1P28. The responses were displayed on a synchroscope Tektronix 465.
3. Results and discussions 3.1. Fluorescence of undoped anthracene evaporated films
The fluorescence spectra of the films freshly evaporated at liquid nitrogen temperature and the films warmed slowly up to higher temperature are shown in fig. I. The emission band at 85.5 K is broad and structureless. It is similar to that of the anthracene excimer emission [7]. The energy difference between the peaks of the monomer emission and excimer-like emission bands is about 4000 cm while that energy difference in solution is about 6000 cm’ [71. The blue shift of the peak in this excimer-like emission band may be due to the mixture of that with the monomer emission band. The excitation spectra of a freshly evaporated film and of a crystal at liquid nitrogen temperature are shown in fig. 2. The excitation spectrum for the 425 nm emission of the freshly evaporated film is clearly different from that for the 500 nm emission. The former spectrum is similar to that of the crystal. This difference inay be due to the different natures of the monomer and excimer emissions. On warming the freshly evaporated film, the vibrational structure like that in the monomer emission band appears in the shorter wavelength side of the
Y. Takahashi ci a!.
/ Emission from
evaporated ant hra cenc films
295
855 K
a
C I,
C
a
C
~
~
4tXD
S00 Wavelength ( nm)
600
Jig. 1. Emission spectra of an anthracene evaporated film under 365 nm excitation, 85.SK;----at2O5K;—------at296K.
at
broad structureless band. Therefore, a transition from an amorphous to a polycrystalline structure seenis to take place by warming the film. At room temperature the broad structureless spectrum disappears, and instead only the vibrational structure like that in the emission band of an anthracene crystal appears as shown in fig. 1. In fig. 3, the decay curves at 84 K are shown for the 430 and 600 nm emissions
/1’
/11 110
:
/_•~~ ~.‘
.
\
\\
t_:~
~50~
~
Crystal ~t
I
~ 10
‘
~
//~JA\~
A
at841(
• A
43Ormm
600nm
:
S
C
400
440
It
10
100
~vetength(rwn)
200 Tirneinsec)
Fig. 2. Excitation spectra at liquid nitrogen temperature for an anthracene film and anthracene crystal. --—-— for the 500 nm emission; ---- for the 425 nm emission; — - — - — for the 425 nm emission of an anthracene crystal. Fig. 3. Emission decay curves of an anthracene evaporated film at 84 K. sion; • for the 600 nm emission.
A
for the 430 nm emis-
296
Y. Takahashi Ct a!.
/ Emission from evaporated anthraccne Ji!rns
of a freshly evaporated film. The decay curve of the emission at 600 nut is of exponential type and its decay time is abou~190 us. This value is approximately equal to that of the emission of the anthracene excimer isolated in a matrix [81. On the other hand, the decay curve of the emission at 430 urn is non-exponential and appears as a sum of two exponential curves of which the decay times are 6 and 190 ns. The latter decay time agrees with that of the excimer-like emission at 600 nm, while the former decay time is closely equal to that of an anthracene crystal at liquid nitrogen temperature [51.However, these results differ from those of Perkampus and Stichtenoth [3]; in their results sevcral components of decay times were observed. It is difficult to explain their results from ours. The results mentioned above suggest that, at liquid nitrogen temperature, the freshly evaporated film has both crystalline and amorphous structures. Furthermore, the emission in the longer wavelength side is due to the amorphous structure and is excimer-like, while the emission in the shorter wavelength side shows the characteristics of both crystalline and amorphous structures. Assuming that the quantum efficiencies of the emissions from the crystalline and amorphous structures are equal, the degree of crystallization at liquid nitrogen temperature is estimated as about 10% of the film volume from the ratio of areas of the decay curves of both emissions. From the results shown in fig. I, the degree of crystallization seems to increase with an increase of temperature and its value at room temperature is about 100%. 3.2. Fluorescence of an anthracene evaporated film containing tetracene In order to investigate energy transfer in an anthracene evaporated film, evaporated films doped with tetracene were used. The emission spectra for freshly evapor. ated films doped with tetracene are shown in fig. 4. The emission spectra are different from that of the undoped crystal and show a little tetracene emission. Its intensity increases with an increase of the tetracene concentration. As shown in fig. 5, the tetracene emission intensity increases with increasing temperature for the 3 tetracene doped film. As mentioned before the structure of the 3.3 X 1017 cm film changes from amorphous to polycrystalline with an increase of temperature, and therefore the increase in the tetracene emission intensity may be attributed mainly to the increase in the amount of the polycrystalline structure in the film. Furthermore, as shown in fig. 6, the decay curve of the emission at 430 nm of the freshly evaporated film doped with 6.5 X 1016 cm3 tetracene is non-exponential at 83.5 K and is shown as a sum of two exponential functions of which the decay times are 4 and 190 ns. At liquid nitrogen temperature the amount of the shorter decay component of the emission due to the crystalline structure decreases with increasing tetracene concentration. If energy transfer by excitons occurs through the crystalline structure, the transfer probability can be obtained by a similar method to that in the case of the crystal [5]. The probability of the energy transfer for the 6.5 X 1016 cm3 tetracene doped film at liquid nitrogen temperature is calcul-
Y. Takahashi et al. / Emission from evaporated anthracene fl/ins IS
4lX)
297
atLN.T.
500
600
Wav~ength(m~
Fig. 4. Emission spectra at liquid nitrogen temperature for anthracene evaporated films doped 3 with tetracene under 365 nm excitation. — undoped; 4.9 x 1016 cm 2.6 X 1017cm3 —~ 4.9 X 1017 cm3.
ated from the decay times of the shorter emission decay components of undoped and doped anthracene films. It is about 8.0 X l0~s~. As this value of the probability agrees well with that of the anthracene crystal and the energy transfer in the crystal takes place by exciton diffusion [5], it may be concluded that the energy transfer to tetracene in the film occurs through the crystalline structure by the
exciton diffusion. On the other hand, the change of the decay time of the excimerlike emission cannot be observed with variation of the tetracene concentration. Hence, the probability of the energy transfer from excitons to acceptors through
Wavelength (nm)
Fig. 5. The emission spectra at various temperatures for the anthracene evaporated film doped with 3.3 X 1017 cm3 tetracene.
298
Y. Takahashi ci a!.
/ Emission from
evaporated anthracene fl/ins
atBl.5K
3tetracene
6.SxlO crst l.3Onm
~ ElI
>-5J
~190n5 0
20
tO 60 80 Time(nsec)
1111
Fig. 6. The decay curve of the 430 nm emission at 83.5 K for the fresh anthracene film doped with 6.5 x 1016 cm3 tetracene.
the amorphous structure is negligible. This result is in agreement with that obtained byArden [21.
References [1] Y. Maruyama and N. Iwasaki, Chem. Phys. Letters 24 (1974) 26. [21 W. Arden, L.M. Peter and G. Vaubel, J. Luminescence 9 (1974) 257. [31 H.H. Perkampus and F!. Stichtenoth, Z. Phys. Chem. 76 (1971) 18. [4] J.B. Birks et al., Proc. Roy. Soc. 291 (1966) 556. [51 Y. Takahashi and M. Tomura, J. Phys. Soc. Jap. 31(1971)1100. [61 M. Tomura and Y. Takahashi, J. Phys. Soc. Jap. 31(1971) 797. [71 B. Stevens, Spectrochim. Acta 18 (1962) 439. [81 J.B. Birks, Photophysics of aromatic molecules (Wiley—lnterscience, New York, 1970) ch. 7.