Magnetic field dependence of monomer and excimer delayed fluorescence of aromatics in fluid solution

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REP@NDEfVCE OF MONOMER AND EXCfMER. DELAYED FLUORESCENCE

OFA~~MATI~INFE~~~US~L~ION

Received 10 Februaj

1976

We have extended our study of the magnetic field dependence of monomer (I&j) and acimer (f~) delayed ffuore~cence of aromatin in fluid solution. $ and ID show a monotonic decrease with increasing field strength at room temperature in ageement with theoretical predictions. At lower temperatures both I,, and f~ show an initial increase followed by a monotonic decrease. Thk marked temperatut! dependence can be attributed to a “freezing-out” of the effect of molecular motion on the field effect. At even lower temperatures the field effect curves of 1~~ and 1~ start to diverge. Two models have been prcposed to explain the mechanism of excimer formation in triplet-triplet annihilation. Qualitatively at least the divergence can be rationalized in terms of both models.

A number of studies of magnetic field effects on kipieS-triplet annihilation (TTA) of aromatics in solution have been published in this journal [I-4] . Several research groups [l-3] have used the effect as a probe for the study of the mechanism yielding excimer de. Iayed fluorescence (df). A survey of the results reported to date leads one to conciude that the field effect data so far have not contributed to a better understanding of the excimer df mechanism. Wyrsch and Labhart [If studied the intensity of monomer (lM) and exeimer (I23 delayed fluorescence of 1,2-benzanthracene @A) in ethanol (&oh) as function of field strength. Over the temperature range of -70” to about -130°C they observed a difference in field dependence ofI&f and ID_ This led to the conclusion that the generarion of monomer and dimer singIer excirezi states camzur ii& vuive a cummon spin-selective step. Tachikawa and Bard 121, on the other hand, reported an identical field depende‘nce of 1, and fU for both pyrene (P) and BA,ia-cyclohkxane at room temperature. A study by van Wfligen [3] showed that 1, apd 1~ of P in Etoh and 3-methylpentane were affected identically hy a field over a temperature range extending from room temperature to about --1OO”C. The results of the latter two studies suggesteri a common spin-se&i&e ‘step for monomer excired stare and excimer fdr-

-mafionl- In a-recent-review Stevens [5] cited the results .

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of Tachikawa and Br?rd (2] as evidence supporting the re-encounter model [6 J for excimer formation. However, he failed to provide an explanation for the data reported by Wyrscfi and Labhart [I]. The apparent discrepancy between the field effect data summarized above obviously deserves a more detailed expIoration. Another point of interest that prompted us to continue our investigation concerns the shape of the field dependence curves of lM and 1,. Ihi and ID of I? and BA decrease monotonically with increasing field strength at room temperature f2,3]. On the other hand, ihr and 1, of P f3] and BA [i] at low temperature can show an initial increase at low fields followed by a monotonic decrease at higher field values. Theoretical treatments [7,8] of the field effect on TTA in fluid solution predict a decrease in df intensity with increasing field strength. It has been suggested that the discrepancy between some of the field effect curves and theoretical predictions could be due to exchange interaction between the two triplets in the TTA transient [9]. Alternatively, the discrepancy could be due to interference by a triplet quenching process [3] _One must conclude, however, that insufficient data are available for an unambiguous interpretation of the phenomenon. Here we report the salient results of a more extensive study of field effects on TTA in ff ujd so&.&on. We will show that the results resolve some of the questions

CHEMICAL PHYSICS LETTERS

Volume 40. number 1

raised by previous publications and may Provide the _ basis fdr a better understanding of the energy transfer and excimer formation mechanism. A full report on .. and discussion of the experimental results will be published elsewhere. As far as the shape of the field dependence is concerned, measurements* on half a dozen aromatics as function of temperature and solvent established the following. (1) Below a certain temperature all field dependence curves exhibit an initia! increase (bulge) in Ibt (and 1,) followed by a monotonic decrease. A representative example is provided by the temperature dependence of IhI and ID versus fieid curves of BA in Etoh shown ! in fig. 1. (2) The effect is observed for excimer forming aromatics such as P and B as well as aromatics which do not form excimers (for instance, 9,10-diphenylanthracene). (3) The temperature at which the bulge is first observed is a function of solvent. (4) The low temperature curves of the aromatics * Experimental details have been outlined previously in ref. 131.

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Fii. 1. Field dependence of monomer and excimer delayed fluorescence of 1,2-benzanthxacene in ethanol at -2“, -133~and

-99°C.

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1.5 M3y 1976

are very similar. The height of the bulge ranges from 0 to 2% above zero field intensity, the position of the maximum ranges from 0.5 to 2 kG depending on the aromatic. (5) Light intensity variations do not affect the low temperature curves provided that triplet decay remains first order 131. We propose that the changes in field dependence @I temperature must be attributed to motional effects. Atkins and Evans [g] recently proposed that translational and rotational motion of triplets would play a dominant role in the spin dynamics affecting TTA in fluid solution. The growing in of the bulge can be interpreted in terms of a gradual “freezing-out” of the motional effects on TTA. This interpretation is supported by the observation that the low temperature curves are very similar to curves obtained for polycrystalline aromatics [lo] and aromatics in a rigid matrix [4]. According to Avakian et al. [7] theory predicts that the TTA rate constant for isotropically distributed static triplets decreases with increasing field. The experimental low temperature curves clearly are at variance with this theoretical prediction. The discrepancy might be due to an electronically induced orientational anisotropy in the annihilation rate constant. It does not seem farfetched to assume that (ignoring spin-selectivity effects) the probability of energy transfer between two triplets is a function of their relative orientation. Such an orientational selectivity could give rise to the single crystal type [ 111 field dependence curves observed by us. In view of the fact that little variation is found in position and height of the bulge it is unLikely that the field effect inversion is induced by intertriplet exchange interaction [9]. The divergence between the field dependence curves of ‘Ihf and ID is found at reduced temperature for all excimer forming aromatics studied (P, BA, 3,4-benzpyrene) in solvents with high viscosity at low temperatures (Etoh, metbylcyclohexane, n-propanol). The temperature at which divergence is first observed is not affected by solute concentration, but is a sensitive function of the solvent-solute system (BA-Etoh, % --6O”C, P-Etoh, = -90°C). invariably the field dependence of lD decreases with decreasing temperature, while the field dependence of fM .remains constant or increases slightly. A typical example has been reproduced in fig. 1. In all cases the bulge grows in at higher temperature than the divergence. 161

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CHELLICAL

PHYSICS

An important resuh of our study is that it establishes that.the divergence is not an “anomalous” feature of one system, but rather is a general phenomenon. The abservation that the fieId dependence offhi and ID is identical at h!gh temperature and diverges at low temperature could contribute to a better understanding of the energy transfer and excimer formation mechanism. A full discussion of the imphcations of the phenomenon is reserved for a future publication. Here we wi!l merely make some tentative observations. Since it is ID rather than Ihi that is affected one may conclude that the data rule out a mechanism invoking a long range energy transfer process contributing to IM at low temperature [ 12]_ A long range contribution would affect the field dependence of Iw only. We have previously [3] pointed out that the divergence can be reconciled with the re-encounter model proposed by Steven [6], even though this model involves a common spin selective step (1) for energy transfer and excimer formation

LETTERS

15 May 1976

is distinct as a result of differences ininter-triplet electronic interaction and/or transient geometry (in fact without‘this distinction there is no formd difference between the re-encounter and competitive models). At higher temperatures exsimer formation predominantly could be the result of re-encounters between singlet excited monomers and ground state molecules explaining the disappearance of the divergence. Financial support from the Research Corporation’ and the Petroleum Research Fund administered by the American Chemical Society is gratefully acknowledged_

References [l] D. Wyrseh and

H.

Labhart, Cbem. Phys. Letters

8 (1971)

217. [2]

H. Tachikawa

[3]

(1974) 568. H. van Wiliigen, Chem. Phys. Letters

and A.J. Bard, Chem. Phys

Letters

33 (1975)

26

540.

[4]

P. Avakian, R.P. Groff, A. Suna and H.N. Cripps, Chem. Phys. Letters 32 (1975) 466. [S] B. Stevens, in: Annual reports on the progress of chemistry,

To rationahze the divergence at low temperature one would have to assume that (a) the fieId dependent orientational anisotropy introduced in step (1) is retained during the lifetime of the singlet molecules and that (b) the exsimer formation step (2) has certain conformational requirements. On the other hand, divergence in field dependence of IhI and ID is expected to show up at low temperatures also if the competitive mechanism [ 133 applies %I* + ‘M, ?,M* + 3hP $& Tokhis end the energy transfer and excimer formation rea&ons must have distinguishable transients. Then the &ld affected spin dynamics in the two processes

section A, Vol. 71 (Chemical Society, London, 1974) p. 3.5. B. Stevens, Chem. Phys. Letters 3 (1969) 233. P. Avakian, R.P. Groff, R.E. Kellogg, R.E. Merrifield and A. Sum, in: Organic scintillators and iiquid scintillation counting (Academic Press, New York, 1971) p_ 499. P.W. Atkins and G.T. Evans, Moi. Phys. 29 (1975) 921. C.E. Swenberg and N.E. Geacintov, in: Organic molecular photophysics, Vol. 1, ed. J-B. Birks (Wley, New York, 1973) p. 529. L-R. Faulkner, Thesis, The University of Texas at Austin (1969). R.C. Johnson and R.E. Merrifield, Phys. Rev. B 1 (1970) 896. 1121 C.A. Parker, Spectrochim. Acta 22 (1966) 1677. [ 13 J H. Labhart and D. Wyrssh, Chem. Phys. Letters

378.

12 (1971)