Photophysical studies on the benzophenones. Prompt and delayed fluorescences and self-quenching (original) (raw)

Abstract

The photophysical pathways of benzophenone and several 4,4'-disubstituted benzophenones (fluoro, chloro, methyl, methoxy, dimethylamino) in benzene a t room temperature have been investigated using time-resolved laser spectroscopy. Prompt fluorescence spectra are reported for benzophenone and 4.4'-difluoro-, 4,4'-dichloro-, 4.4'-dimethyl-and 4,4'-dimethoxybenzophenone. The relative prompt fluorescence and absolute phosphorescence quantum yields at room temperature in benzene are reported. Both thermal ( T p -4 1 ) and P type (TI + T I -SI + SO) delayed fluorescence are documented for these ketones. From variable temperature data on the thermal delayed fluorescence, the singlet-triplet splittings are estimated to be: benzophenone, 4.9 f 0.5 kcal/mol; 4,4'-difluoro-, 3.9 f 0.4 kcal/mol; 4,4'-dichloro-, 4.1 f 0.4 kcal/mol; 4,4'-dimethyl-, 4.5 i 0.5 kcal/mol; 4,4'-dimethoxy-, 5.1 f 0.5 kcal/mol. All of these ketones and also 4,4'-bis(dimethy1amino)benzophenone are shown to undergo self-quenching (TI + SO -2So(ksQ)). The ~S Q ' S vary from ca. lo5 to IO9 M-I sec-' and correlate well with up+, p = -1.7. It is proposed that self-quenching involves formation of an exciplex wherein the half-filled n orbital of T I is directed toward the a electron density of an aromatic ring of So.

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References (42)

  1. C. Lewis and W. R. Ware, Mol. Photochem., 5. 261 (1973).
  2. Anthracenes undergo two electron reductions: A. J. Fry, "Synthetic Or- ganic Electrochemistry", Harper and Row, New York, N.Y., 1972, Chapter 4.
  3. S. Wawzonek and J. H. Wagneknecht, "Polarography-1964", G. J. Hills, Ed., Why-lnterscience, New York, N.Y., 1966, p 1035.
  4. See, for instance: E. J. Bowen, Adv. Photochem., 1, 23 (1963); ref 2, Chapter 7.
  5. D. S. McClure, J. Chem. Phys.. 17, 905 (1949).
  6. R. F. Ziolo, S. Lipton, and Z. Dori, Chem. Commun., 1124 (1970).
  7. G. Briegleb, Angew. Chem., lnt. Ed. Engl,, 3, 617 (1964).
  8. R. S. Davidson and P. F. Lambeth. Chem. Commun.. 1098 (1969).
  9. It should be noted that the Weller function3 predicts two regions for charge transfer quenching.
  10. H. Knibbe and A. Weller, 2. Phys. Chem., 56, 99 (1967);
  11. H. Knibbe. K. Rollig, F. P. Schafer, and A. Weller, J. Chem. Phys., 47, 1184 (1967);
  12. E. A. Chandross and J. Ferguson, ibid., 45,397 (1966).
  13. E. J. Bowen and K. K. Rohatgi, Discuss. Faraday SOC., 14, 146 (1953);
  14. G. Kallmann-Oster, Acta Physiol. Pol., 26, 435 (1964). Recent evidence indicates that ground state complexation may be involved in this sup- pression: W. R. Ware and C. Lewis, J. Chem. Phys., 57,3546 (1972);
  15. C. Lewis and W. R. Ware, Chem. Phys. Lett., 15, 290 (1972). However, for another example where ground state complexation is not involved but an A+X-exciplex is thought to be formed, see: W. R. Ware, J. D. Holmes, and D. R. Arnold, J. Am. Chem. SOC., 96,7861 (1974).
  16. S. Ander, H. Blume. G. Heinrich. and D. SchuRe-Frohlinde. Chem. Com- mum, 745 (1968), and references cited therein. References and Notes (1) (a) Visiting Associate Professor, 1974-1975: (b) Department of Chemis- try; (c) Departments of Physics and Electrical Engineering.
  17. G. S. Hammond and N. J. Turro, Science, 142, 1541 (1963);
  18. A. A. Lamola and G. S. Hammond, J. Chem. Phys.. 43, 2129 (1965): (c) S. Dym, R. M. Hochstrasser. and M. Schafer, ibid., 48, 646 (1968); (d) P. J. Wagner and 0. S. Hammond, Adv. Photochem.. 5, 21-156 (1968); (e) P. J. Wagner, Mol. Photochem., 1, 71 (1969).
  19. J. R. Novak and M. W. Windsor, Science. 161, 1342 (1968): (b) J. D. Winefordner, Acc. Chem. Res., 2, 361 (1969): (c) P. M. Rentzepis, Science, 189, 239 (1970): (d) D. J. Bradley. M. H. R. Hutchinson. H. Koetser. T. Morrow, G. H. C. New, and M. S. Petty, Roc. R. Soc. Lon- don, Ser. A., 328, 97 (1972): (e) D. I. Schuster, T. M. Weil, and A. M. Halpern, J. Am. Chem. Soc., 94, 8248 (1972): (f) R. E. Brown, L. A. Singer, and J. H. Parks, ibid.. 94, 8584 (1972): (g) R. E. Brown, K. D. Legg, M. W. Wolf, L. A. Singer, and J. H. Parks, Anal. Chem., 46, 1690 (1974).
  20. S. Dym and R. M. Hochstrasser, J. Chem. fhys.. 51, 2458 (1969);
  21. P. M. Rentzepis and C. J. Mitschele, Anal. Chem., 42, 21A (1970).
  22. R. E. Brown, L. A. Singer, and J. H. Parks, Chem. Phys. Lett., 14, 193 (1972).
  23. a) J. Saltiei. H. C. Curtis, L. Mens, J. M. Miley. J. Winterle, and M. Wrighton. J. Am. Chem. SOC., 92, 410 (1970): (b) P. Jones and A. R . Calloway, ibid., 92, 4997 (1970);
  24. Chem. fhys. Lett., I O , 438 (1971): (c)
  25. S. A. Carlson and D. M. Hercules; J. Am. Chem. SOC., 93, 561 1 (1971), and references therein.
  26. J. 8. Birks, "Photophysics of Aromatic Molecules", Why-lnterscience, New York, N.Y., 1960, pp 378-399.
  27. J. A. Bell and H. Linschitz, J. Am. Chem. SOC., 85, 528 (1963); (b) 0. L. Chapman and G. Wampfler, ibid., 91, 5390 (1969); (c) C. A. Parker and T. A. Joyce, Trans. Faraday SOC.. 65, 2823 (1969); (d) T. H. Koch and A. H. Jones, J. Am. Chem. SOC., 92, 7503 (1970); (e) C. D. DeBoer and R. H. Schlessinger, ibid., 94, 655 (1972); (f) D. i. Schuster and M. D. Goldstein. ibid., 95, 986 (1973); (9) D. I. Schuster and T. M. Weil, ibid., 95, 4091 (1973); (h) L. A. Singer, R. E. Brown, and G. A. Davis, ibid., 95, 8638 (1973).
  28. C. A. Parker, "Photoluminescence of Solutions", Elsevier. New York, N.Y., 1968, pp 262-268.
  29. J. M. Morris and D. F. Williams, Chem. fhys. Lett., 25, 312 (1974).
  30. P. B. Merkeland D. R. Kearns, J. Chem. fhys., 58, 398 (1973).
  31. The fluorescence "purity" of these ultrafast gated spectra depends on exactly where in time the sampling gate is established relative to trigger- ing of the laser. Since it is impossible to exactly assess this, the tech- nique we employ is to move the 10 nsec gate forward and back slightly (several nsec) near the trigger point and compare the recorded spectra. Spectra contaminated with si nificant phosphorescence show a slight bulge in intensity near 4500 8. We estimate that our best spectra to date have less than 10% phosphorescence mixed in.
  32. Recent Hvckei and CNDOI2 calculations suggest similar equilibrium geometries for the ground and n . r * excited states of benzophenone: R. Hoffman and J. R. Swenson, J. fhys. Chem., 74,415 (1970).
  33. The estimated limiting lifetimes, rL, from the Stern-Volmer extrapola- tions may be slightly lower than the actual limiting lifetimes. In a few systems studied, the Stern-Volmer plots show some downward depar- ture from linearity at low concentrations. This effect has been noted for benzophenone in other aprotic solvents, and also with benzil in cycio- hexane. This phenomenon is being investigated further.
  34. H. C. Brown and Y. Okamoto, J. Am. Chem. SOC.. 80, 4979 (1958).
  35. J. B. Guttenplan and S. G. Cohen, J. Am. Chem. SOC., 94, 4040 (1972).
  36. J. B. Birks, ref 7, p 420.
  37. J. B. Birks. ref 7, p 327.
  38. See S. P. McGlynn. T. Azumi. and M. Komoshita. "Molecular Spectros- copy of the Triplet State", Prentice-Hall, Englewood Cliffs, N.J.. 1969, p 284, for a discussion of the effect of charge transfer interaction on spin-orbit coupling.
  39. G. Favaro, Chem. Phys. Lett., 21, 401 (1973), measured the phospho- rescence decay rates for a number of substituted benzophenones in benzene at room temperature. He observed a correlation of the kd's with the Hammen u substituent constant with p = 1.34. Our limiting room temperature phosphorescence lifetime data in Table I show a sim- ilar trend. Thus, the substkuents are operating on the benzophenone triplets (acceptors) in an inverse manner as they operate on the ground state r donors (substiiuted benzophenones or benzenes). These com- peting substituent effects may be the cause of the lesser slope ob- served in our self-quenching study in comparison with the benzophe- none/substituted benzenes study of Gunenplan and Cohen." In our study, the substituents are operating oppositely on the acceptor and the donor with the latter dominating.
  40. See N. J. Turro, J. C. Dalton, G. Farrington, M. Niemczyk. and D. M. Pond, J. Am. Chem. Soc.. 92, 6978 (1970), and references therein for a study of aikanone fluorescence quenching by electron-rich and elec- trondeficient alkenes which indicates modes of reaction similar to the structures shown in 1 and 2, respectively.
  41. We thank Professor P. J. Wagner for pointing out to us this interpreta- tion.
  42. We are using a bp+ for CeHSCO-of 0.6 based on the value of 0.56 re- ported for CH3CO-: C. G. Swain and E. C. Lupton. Jr., J. Am. Chem. SOC.. 90, 4328 (1968).