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LUMINESCENCE OF DOPEDAROMATIC CRYSTALS
cluded that the general lines of the photoconductivity and its relations with the decay of excitons are not far
from being understood. No experimental data are yet available for or against a diffusion of excitons.
Luminescence of Doped Aromatic Crystals’
by A. Schmillen Physikalisches Institut der Unirersitat Giessen, Germany
(Received October 5 , 1964)
The transfer constant, a, for the radiationless transfer of excitation energy from a photoexcited host crystal to foreign molecules in the lattice with a lower excitation level is given hy the equation T ~ / T ( c ) = 1 ac, where T O and T are the fluorescence decay times of the host without and with the impurity, and c is the concentration of the impurity, which has been determined for several systems. A phase fluorometer was used to determine decay times from the phase angles of the directly excited fluorescence of the host and of the impurity in the lattice and of the fluorescence of the impurity excited by transfer. For 2,3-diniethylsee. and a is 1.05 X lo5. naphthalene as host and anthracene as solute, r0 is 69.7 X This value of a is similar to the value 0.87 X lo5 for 2,3-dimethylnaphthalene-perylene, although the lowest excitation levels of perylene are considerably below those of anthracene, suggesting perhaps that overlap of host emission with the foreign ~noleculeabsorption may not be a dominating factor in transfer. For fluorene-anthracene, r0 is 7.3 X sec. and a is 8.5 X lo3,although these values may be subject to some systematic experimental error. I n fluorene-pyrene, two anonialies prevent the determination of a. At low concentrations of pyrene, only the fluorescence of anthracene appeared, although the anthracene concentration was less than I t is suggested that a pyrene-anthracene coniplex, with strong mutual coupling, would account for the observations. Also, at high concentrations of pyrene, the excited dimer or “exciiner” fluorescence appeared.
+
One of the main problem on the luniinescence of molecular crystals is the transfer of excitation energy froni the host lattice to foreign niolecules having a lower-lying excitation level. To the nuiiierous experimental and theoretical investigations treating this problem, this paper adds a further modest contribution. Without restriction to a defined energy transfer model, let us suppose that the radiationless transition of excitation energy from the host lattice to the foreign niolecules is a process competing with the luminescence emission of the host material. Further, we may assuine that the transfer probability is proportional to the concentration of foreign niolecules.
Then the following relation should hold between the decay times T and r0 of the host luminescence (with and without foreign molecules) and the concentration C of foreign molecules
where a is a characteristic constant of the transfer. If we use for the decay time ineasureiiients a phase fluoroineter with a periodically varying excitation intensity, the three nieasurable phase angles & (the (1) Presented t o t h e International Conference on Photosensitization in Solids, Chicago, Ill., J u n e 22-24, 1964.
Volume 69. S u m b e r S
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1
cp
i
1 FIuorerE-An f'hrocene
100
90
-
70-
-t
60
30
50-
I
10
I
!-
\
15
-5
-4
-3
-2
log c
Figure 1 , Phase angles of the system 2,3-dimethylnaphthaleneanthracene: curve 1, A,, = 3130-A. 2,3-dimethylnaphthalene fluorescence; c y v e 2, k,, = 3 130-A. anthracene fluorescence; 4, kex = 3660-A. anthracene fluorescence.
Figure 2. Phase angJes of the system fluorene-an thracene; curve 1, ,A, = 3130-8. fluorene fluorescence, curye 2, A,, = 3130-A anthracene fluorescence; A, ,A, = 3660-d anthracene fliiorescence.
phase angle of the host lattice fluorescence), 4a (the phase angle of the foreign molecule fluorescence directly excited by longer wave length), and &* (the phase angle of the foreign niolecule fluorescence excited by transfer froni the host lattice) should be related by
ficulties in separating the two fluorescence coniponent,s from each other and froin the exciting radiation. If we compare the value of the transfer constant a of the systeni 2,3-diniethylriaphthalene-anthracene with that of the system 2,3-diniethylnaphthaleneperylene ( a = 87,000) we find them to be very siniilar although anthracene and perylene have lowest excitation levels which differ in energy. (The eiiiission of perylene lies in the spectral range 22,000-18,000 cni.-' and that of anthracene in the range 26,000-22,000 cni. - l . ) This result niay indicate that the overlapping of the host lattice emission and the foreign molecule absorption is riot the dominating factor of the energy transfer. In the saiiie representation, the results for the system fluorene-anthracene are plotted in Figure 2. The agreement between measured points and the curve calculated by using the parameters n = 73.7 X sec. and a = 8.5 X lo3 is not as convincing as in the first example. The constant a is fitted to the measured points of the indirectly excited anthracene fluorescence (curve 2). However, at the highest concentration the phase angle of the host lattice fluorescence gives too high a value, which may be caused by superposition of
Thus, if we measure the three phase angles, we are able to prove the validity of eq. 1 and to deterniine the transfer constant a. Recently, we carried out such measurements on doped single crystals of aromatic hydrocarbons having well-defined concentrations of foreign niolecules. I n addition to the already published results on 2,3diniethylnaphthalene-perylene and dibenzyl-tetracene, Figure 1 shows data for the system 2,3-diniethylnaphthaleIie-anthracene.2 The solid curve 1 is calculated by using eq. l with the values T~ = 69.7 X l o p 9sec. and a = 1.05 X 105. Curve 2 is derived froin curve 1 by addition of the dashed line for the phase angles of t h r directly excited anthracene molecules (Atx = 3660 A). The circles and crosses are the nieasured values of &* or qia, respectively. The measured points fit the solid curves calculated according to the initial suppositiotis satisfactorily, considering the difT h e Joiirnal of Physical Chemistry
( 2 ) A . Bchinillen and J. Kohlinaniisperger, Z. .\hfiO:forsch., 18a, 627 (1963).
LUMINESCENCE OF DOPED AROMATIC CRYSTALS
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30,000
26,000
22,000
18,000
Wave number, om. -1.
26,000
Figure 5. Luminescence of tbe system fluorene-pyrene by excitation with X = 3130 A. for different pyrene concentrations.
18,000
22,000
Wave number, om. -1.
Figure 3. Lumineecence of the system fluorene-pyrene by excitation with A = 3660 A. for different pyrene concentrations.
30,000
26,000
22,000
18,000
Wave number, om. -1.
Figure 4.
the not fully separated anthracene fluorescence. However, the adaption of the transfer constant to the @a* values of indirectly excited anthracene (curve 2) may not be correct in this case, because eventually a direct excitation of anthracene a t the highest concentration contributes much shorter values to the measured phase angles. I n every case a should not be greater than 8.5 X loa. A third investigated system, fluorene-pyrene, does not allow a n interpretation in that manner, because two remarkable effects hinder the determination of the phase angle of the directly excited pyrene fluorescence. I n the spectra of the directly excited foreign molecule fluorescence (excitation with 3660 A.), Figure 3, a t the two lowest concentrations used the expected pyrene fluorescence does not appear. In-
I
I 26,000
,
I 22,000
I
I
18.000
14,000
Wave number, cm. -1.
Figure 6. Luminescence of the system fluorene-pyrene by excitation with X = 3660 b. for different pyrene concentrations.
stead, an anthracene fluorescence shows up, the intensity of which increases by addition of pyrene. Only a t higher pyrene concentrations does the pyrene emiesion arise and the anthracene fluorescence disappear. Anthracene is an impurity in the technical fluorene product, but we reduced its concentration by purification to values snialler than The increase of the anthracene fluorescence by addition of small amounts of pyrene could be caused by an energetical coupling between these two foreign molecules. Perhaps the niolecules are located a t neighboring sites in t h e lattice. (anthracene-pyrene complex centers). Then one could understand the disappearance of the anthracene fluorescence at higher pyrene concentrations, since Volume 69, r u m b e r 3
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10-6
I
I 30.0110
I
I 26,000
22,000
18,000
14,000
IVave number, cm. - 1 .
Figure 7 . Luminescence of a single crystal of 2-methylnaphthalene-10-3 pyrene: X e x = 3130 8.; . . . . . ., Xex = 3660 A.
after the occupation of all anthracene neighboring sites with pyrene molecules, the absorption and emission would result more and more from pyrene centers without anthracene. This explanation is supported by the fact that the anthracene fluorescence in the same crystals does not appoear by exciting with short wave lengths (Aex = 3130 A.) absorbed from the host lattice. I n this case the energy is absorbed in a very thin layer, Figures 4 and 5 . The hypothetical anthracene-pyrene complex centers do not seem to participate in the energy transfer. However, this interesting question needs further investigation. I n Figure (3, at the highest concentrations, the excimer fluorescence of pyrene arises as indicated by an increase of the fluorescence intensity a t 20,000 cm.-l.
The Joz~rnalof Ph?~/sicalChemistry
10-6 10 -4 10 - 2 Concentration, mole fraction.
10 - 2
Figure 8 . Phase angles of the systemofluorene-pyrene: X, fluorene fluorescence (tex = 3130 A , ) ; 0 , pyrene fluorescence (Aex = 3130 A , ) ; 0, pyrene fluorescence (Aex = 3660 A,).
This emission arises preferably by direct excitation. The effect is especially impressive for the system 2methylnaphthalene-pyrene ( Figure 7 . The intensity and decay time of the excinier fluorescence increase reversibly with cooling to liquid nitrogen temperature. I n accordance with earlier measurements on polycrystalline powders, the maximum of the excinier fluorescence in the single crystal 2-methylnaphthalene-pyrene lies a t 20,000 c m -I, whereas that of pure pyrene crystals lies at 21,500 cni.-'. Therefore, we can definitely conclude that this emission is not due to precipitated pyrene. The decay time diagram (Figure 8) shows immediately the difficulties caused by the gradual change from the anthracene fluorescence over the monomer pyrene fluorescence to the excinier emission with increasing concentration.
