Migration of electronic excitation energy in

MIGRATION OF ELECTRONIC EXCITATION ENERGY IN HETEROGENEOUS MEDIA UNDER CONDITIONS OF INHOMOGENEOUS BROADENING OF ENERGY LEVELS S. A. Bagnich and A. V. Dorokhin

UDC 535.37

The introduction of complex molecules in high concentrations into polymer matrices results in the formation of heterogeneous media with high microscopic nonuniformity. It was shown in [i, 2] that energy migration in such media can be described from the standpoint of percolation theory, which presumes that supermolecular cluster structures form in the system. The experimental results of these works, in particular, show that in many cases as the temperature decreases from room temperature to 77 K the efficiency of migration in the samples studied drops strongly. This effect was explained by the fact that the dispersion of the energy levels of complex molecules, which is associated with fluctuation of the interaction of the molecules with their environment, affects migration. If the molecules forming the system along which energy migration is possible have a different environment and therefore the electronic energy levels are different, there always exists a probability that the migration path will contain an energy well, which at the given temperature the excitation cannot overcome. The probability that an excitation falls into such a well increases as the average distance between traps increases. As the concentration of traps increases this distance increases and the effect of energy factors thereby also decreases. Thus in order for energy migration along a cluster to be efficient at a given temperature the relation kT > AE, where AE is the energy dispersion in the system, must be satisfied. If for an infinite cluster this relation is not satisfied, then the cluster decomposes into parts for which this relation is satisfied. The excitation in this case migrates only along finite clusters analogously to the situation at concentrations below the critical concentration C c. The sizes of these "effective" clusters will depend on the spread of the energy levels and the temperature. As the concentration increases these clusters will grow, and at a certain concentration C c a percolation cluster arises. Thus when the temperature is lowered the critical concentration corresponding to the formation of an infinite percolation cluster of impurity molecules increases, i.e., the interval of concentrations of impurity molecules where unquenched luminescence of the molecules can be observed expands significantly. In this work we studied the spectral characteristics of prolonged luminescence of benzaldehyde in polymethyl methacrylate at 77 K in a wide range of concentrations. The phosphorescence spectra of benzaldehyde in the absence of triplet-energy acceptors and in the presence of quenchers (molecules of l-bromonaphthalene) are presented in Figs. 1 and 2. Comparison of the phosphorescence intensities of triplet-energy donors and acceptors makes it possible to determine for different concentrations of benzaldehyde the probability P of capture of the excitation by a trap. According to [3] P can be expressed as follows: p --.

Itrap

(1)

Itrap+~limp'

where ~ = qtrap/qimp, if qtrap and qimp are the quantum luminescence intenslty of the trap (l-bromonaphthalene) tively. The quantum yields of one bromonaphthalene and [4], are equal to 0.14 and 0.49, respectively. For T =

yields and ItraD and I i ~ are the and impurity (b~nzaldeh~e), respecbenzaldehyde at 77 K, according to 293 K the quantum yields were esti-

B. I. Stepanov Institute of Physics, Academy of Sciences of the Belorussian SSR, Minsk. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 6, pp. 919-922, June, 1991. Original article submitted October 5, 1990. 0021-9037/91/5406-05455~12.50

9 1991 Plenum Publishing Corporation

545

r, I H,,./~ l,g...-2 I

I

I

.

*00

.,cOO

600 ~, nm

Fig. i. Luminescence spectra of benzaldehyde in PMMA at T = 77 K. The benzaldehyde concentration is equal to 0.05 (i) and 4 moles/liter (2); curve 3 is for pure benzaldehyde.

f~

A ..." FO0

"'---2"-"~:~ 600 A, nm

Fig. 2, Luminescence spectra of benzaldehyde in PMMA at T = 77 K in the presence of l-bromonaphthalene. The relative concentration of l-bromonaphthalene S = 2.10 -3 Curve 1 is the spectrum of a solution with a benzaldehyde concentration of 4 moles/liter. Curve 2 is the spectrum of pure benzaldehyde. The spectrum 3 corresponds to a specimen with benzaldehyde concentration C = 4 moles/liter in the absence of l-bromonaphthalene. The spectrum 4 is the difference of the spectra 1 and 3. P

I

5-

fO {, moles/liter

Fig. 3. Probability of capture of an excitation by a trap as a function of the benzaldehyde concentration in PMMA. The relative concentration of l-bromonaphthalene S = 2"10 -3 T = 293 (i) and 77 K (2). mated from the ratio of the phosphorescence lifetimes at 77 and 293 K under the assumption that the rate constant of radiative deactivation of the triplet state is constant (0.05 and 0.Ii, respectively). Figure 3 shows the probability of capture of an excitation by a trap as a function of the impurity concentration. It is clear from the figure that, in agreement with the above

546

Fig. 4. Phosphorescence spectra of diacetyl in PMMA at T = 77 K. The diacetyl concentration is equal to 0.05 (i) and 4 moles/liter (2); curve 3 is the spectrum of pure diacetyl. TABLE i. Spectral Characteristics in PMMA at T = 77 K

of Diacetyl and Benzaldehyde

Spectral characteristic v 0-0

max

Compound

' ~'V

o * --

v 0-0

,

max

cm- I cm_ I

cm-i

'

AV

o-*

v 0-0

,

max

cm -I ]cm-i

'

AVe--

cm 4

o IvO--O ,

max

cm-i

I

' AV

o-o I

cm -I

,

vO--O tlrtax

'

cm-I

[

Aye--e

]cm-I

C, moles/liter

0.05 Diacetyl Benzaldehyde

I

0.5

1120119084 I

l

4

870 118976 772 19418~1216I 19342t 24907/ --J248761 1077 24691 942 124213 901

194L8I 785

24039

--

discussion, the phase transition corresponding to the formation of an infinite percolation cluster along which energy is transported to traps, shifts into the region of higher impurity concentrations as the temperature decreases. The experimental point at C - i0 moles/ liter corresponds to pure glass-like benzaldehyde. It should be noted that unlike the "supertransfer" regime, which is realized for mixed crystals [5, 6], in the amorphous state with the trap concentration employed its luminescence spectrum is observed even for the pure substance (Fig. 2), as a result of which P # i. The observed directed energy transfer in a system of inhomogeneously broadened centers is usually called spectral diffusion. It is obvious that such a process should cause the spectra to shift into the long-wavelength side. However spectral diffusion is, in this case, not the only process that changes the spectrum as the concentration increases. Since the triplet-energy transfer is, as a rule, associated with exchange-resonance processes, it occurs at distances at which aggregation of molecules is possible. The gradual replacement of matrix structures in the environment of a molecule by molecules of the same kind can result in a shift of the energy level in one or another direction. The magnitude and direction of the shift are determined by the difference between the "guest"--matrix and "guest"--"guest" interactions; this difference is manifested in the relative arrangement of the spectra corresponding to cases of a weakly diluted solution and the pure substance. For the benzaldehyde molecule this difference is quite significant (see Fig. i). At the same time for the diacetyl molecule, which is related to the benzaldehyde molecule, no difference is observed in the position of the maximum of the 0--0 phosphorescence band ~(~~176 when the solid solution of diacetyl in PMMA is replaced by amorphous pure diacetyl (Fig. 4). In discussing the different mechanisms of transformation of spectra from dilute solutions to a pure amorphous state, it may be noted that spectral diffusion results not only in a shift of the spectra but also in narrowing of the spectra within the inhomogeneously broadened luminescence band. This fact makes it possible to determine in each specific case which mechanism prevails in the formation of the spectra. As shown above, the processes by which the nearest molecular environment of diacetyl change have no effect on the structure of the energy levels of diacetyl as its concentration increases. At the same time, as one

547

can see from Table i, in the entire range of concentrations accessible for measurements as the concentration increases shifts comparable in magnitude and narrowing of the 0 - 0 band are observed. This behavior of the spectra indicates that spectral diffusion plays an important role in this case. In the case of benzaldehyde analogous effects are observed up to concentrations of 2 moles/liter. At the same time, as the concentration increases from 2 to 4 moles/liter the spectral band shifts almost by 500 cm -1 and narrows by -40 cm -I In this case the formation of the spectral structure is probably mainly affected by the aggregation of benzaldehyde molecules. LITERATURE CITED 1.

2. 3. 4. 5. 6.

S. A. Bagnich and A. V. Dorokhin in: Modern Problems in Spectroscopy, Laser Physics, and Plasma Physics [in Russian], Minsk (1990), pp. 11-14. S. A. Bagnich and A. V. Dorokhin in: Lasers and Optical Nonlinearity [in Russian], Minsk (1989), pp. 169-172. R. Kopelman, E. M. Monberg, and F. W. Ochs, Chem. Phys. Lett., 19, No. 3, 413-427

(1977). V. L. Ermolaev, Usp. Fiz. Nauk, 80, No. I , 3-40 (1963). D. C. Ahlgren and R. Kopelman, Chem. Phys. Lett., 77, No. i, 135-138 (1981). C. Borczyskowski and T. Kirski, Ber. Bunsenges. Phys. Chem., 93, No. ii, 1373-1377

(1989).

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