Volume 19, number 3 CHEMICAL PHYStCS LETTERS 1 April 1973

FLUORESCENCE FROM CYCLIC a$-UNSATURATED KETONES?

R.O. LOUTFY and J.M. MORRIS Division of Chemistry. National Research Council of Carrada, Ottawa. Canada KIA OR6

Received 24 January 1973

~luorescencc was detected from a cyclic cnone (bicycle [3.3.0] act-1(5)-a-2-one) for the first time. This escep tion to the generally accepted rule is attributed to an unusually large gap bet?veen the ‘(II, rr*) stare and the 3(n, x*) state.

Valuable information,has recently been derived from investigation of the emission properties of a number of constrained cyclic enones in rigid matrices at 77’K [I]. However, no reports of fluorescence from cyclic @unsaturated ketones in solution have appeared. We have examined the excited state proper- ties of bicycle [3.3.0] act-l(j)-en-‘--one I and tricycle [4.3.2.0] undec-8-en-7-one 2 under conditions similar to those of photochemical reactions.

The absorption spectra of 1 and S show that both have lowest singlet states of n, 7~~ character [ 1,3-l . Phosphorescence from both compounds has been ob- served in rigid matrices at 77’K: compound 1 has a lowest triplet state of rr, TI* character [ 11, whereas compound 2 has a lowest triplet state of n, n* character [2].

We have observed fluorescence from 1 in acetoni- trile at room temperature and in EPA at 77°K. The maximum of the emission intensity lies at 29000 cm-*; it is independent of the wavelength of excitation and its intensity is not affected by the presence of oxygen. The authenticity of the emission has been ensured by careful purification of compound 1 and by compari- son of the absorption and fluorescence excitation spectra, see fig. 1. The fluorescence quantum yield of 1 was estimated to be 1.3 X 10s3 at room tempera- ture using tryptophane as a standard (& = 0.12. [3] ). The fluorescence lifetime of 1 was found to be less than the 2 nsec limit of a TRW nanosecond spectral

t Pl+ast+mESCENCE

EXqnON SOLVENT : Em

77%

SOLENT-CH,CN 29PQK

ABSORPTION

.._ ‘.._,.

400 503 60 WAMLENVH (nml

Fig. 1. Emission, absorption and excitation spectra of_ I.

source system. We attempted to detect fluorescence from 2, but were unsuccessful, therefore we conclude that the quantum yield of fluorescence from 2 must. be less than 10-b.

A phosphorescence excitation spectrum of a de- .gassed 10s3 M solution of 1 in DPA at 77°K showed an absorption at 27060 cm-! which we assign as ab- sorption.to the 3(n, n*) state. The phosphorescence excitation spectrum of a degassed 0.1 M solution of 2 in ethano! showed a weak absorption at 26 150 CKI-~, i NRC Publication No. 13094.

i77

Volume 19, number 3 CHEMICAL PHYSICS LETTERS 1 April 1973

here the factor p will be smgller by a factor of ~10-~.

Fig. 2. Energy levels of bicycle (3.3.0) act-l-en-Zone 1 and tricyc:o (4.3.2.0tundec-8-en-7-one 2.

which we assign as absorption to the n, n* state of 2 as it could only be observed when 20% of ethyl .iodide was added to the solution. In fig. 2, the known energy levels of 1 and 2 are plotted.

From time dependent perturbation theory the rate constant for intersystem crossing may be expressed by the golden rule formula [4] :

ys + T) = (2n/h)&F, (1)

where J is an electronic matiix element, p the density of final states and F the Franck-Condon factor. In this case, the matrix elements governing the inter- system crossing will be matrix elements of the spin- orbit operator:

(2)

Il*(n,77*)WW136(n, r;*)j2 . (3)

El-Sayed has shown that matrix elements (2) are greater than those of type (3) by a factor of --510W3 [5]. The Franck-Condon factor has been shown to decrease rapidiy with increasing energy [6] so.that a large energy gap between initial and final states results in a slow crossing rate, k(S + T).

Now if we .I(n, n*) sta s

compare the energy gaps between the te and 3(n,,*) states in 1 and 2, we see

that in 1, GE = 3200 cm-:, whereas in 2, AE = 8.50 cm-l. Thus if we assume that the electronic matrix elements of type (2) are of similar magnitude in 1 and 2, then the Franck-Condon factor, F, will be much greater in 2 than 1. The Franck-Condon factor for crossing from ln, il* state to the 3n. n* state in 1 may be expected to be of similar magnitude to the F for the $n* to ??,?r* crossing in 2, but

‘Thus intersystem crossing in 2 wilI be much faster than in 1 and act as a much more efficient quencher for the fluorescence. Thus the observation of fluores- cence from 1 and the failure to observe it from 2 are in accord with current theories of radiationless transi- tions. Compound 1 represents an exception to the widely accepted rule that molecules having a lowest single; state of n; n* character are non-fluorescent [7].

No cY&unsaturated ketone has been previously ob- served to fluoresce, so the fluorescence of 1 can be attributed to the large 111, T*-~T, 7r* energy gap and resulting relatively inefficient intersystem crossing. The 3200 cm-l energy gap in 1 is large for this c!ass of compound: of the nine?een cyclic enones studied by Kearns et al. [S] , five have energy gaps between their In, II* and 37r, n* states which are larger than 3000 cm-l and we expect that careful study of these compounds would reveal weak fluorescence: we note that Kearns et al. used a phosphorimeter for their measurements and thus they would not have observed any fast emission.

This provides confirmation of El-Sayed’s calcula- tions of the relative sizes of the matrix elements (2) and (3), for although the 3n,n* state of 1 lies close to the In, IT* state it is still unable to provide a suffi- ciently efficient path for intersystem crossing to quench the fluorescence. A similar confirmation of these theoretical calculations has been provided by the recent observations of Dewey and Hadley on 9, lo- diazaphenanthrene [9].

Refereitces 111

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131 141

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R.L. Cargill, W.A. Bundy, D.M. Fond, A.B. Sears, J. Saltiel and J. Winterlc, Mol. Photochem. 3 (1971) 123, and references therein. R.L. Cargill, P. de Mayo, A.C. Miller, K.R. Neuberger, D.M. Pond, M.F. Tchir and J. Saltiel, Mol. Photochern. 1 (1969) 301. R.F. Chen, Anal. Letters 1 (1967) 35. A.S. Davydov, Quantum mechanics (Pergamon, Oxford, 1963) p. 295. M.A. El-Sayed, J. Chem. Phys. 38 (1963) 2834; Ac- counts Chem. Res. 1 (1968) 8. W. Siebrand, in: T5e triplet state, ed. A.B. Zahlan (Carlbridge Univ. Press, London, 1967). R.S. Becker, Theory and interpretation of fluorescence and phosphorescence (Wiley-Interscience, New York, 1969) ch. 12. G. Marsh. D.R. Keams and K. Schaffner, J. Am. Chem. Sot. 93 (1971) 3129. H. Dewey.and S.G. Hadley. Chem. Phys:Letters 12 (!971) 57.