Phorophemisrry and
Pkorobiology, 1975. Vol. 22, pp. 149-150.
Pergamon Press. Printed
In
Great Brltaln
RESEARCH NOTE
QUENCHING OF SINGLET EXCITATIONS BY TRIPLET EXCITATIONS* ROBERTS. KNOXand VINITAJ. GHOSH Institute for Fundamental Studies and Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, U.S.A. (Received 15 April; accepted 30 April 1975)
state (acceptor) is closer than RosT to the molecule in the excited singlet state (donor), transfer of the excitation energy is more likely than radiation. A more useful parameter when intrinsic monomolecular quenching is present is
Recently, it was shown (Rahman and Knox, 1973) that an adaptation of the Forster theory predicts very large singletriplet exciton bimolecular quenching rate constants in the case of three important but widely differing molecules (anthracene, chlorophyll a, and rhodamine 6G). The theory was mainly designed to reinterpret certain solid state data, and this research note is presented with the thought that photochemists and photobiologists might find the results to be of interest, especially in view of the large quenching probabilities involved. We also report an extension of the calculations to all-trans retinal. The transfer of excitation from the first excited singlet state to the triplet may be represented by
RoST
where So and S, represent ground- and first-excitedsinglet state molecules, and To and T, represent molecules in the lowest triplet state and a higher triplet state, respectively. The second step, Eq. 2, is not essential to our consideration except that in order for Eq. 1 to represent singlet quenching, the triplet T, should not cross over to an S , . If the latter were to occur, the process would constitute triplet quenching by singlets. Following the detailed reasoning given by Rahman and Knox, we apply resonance transfer theory (Forster, 1948). The rate of an individual transfer process 1 is written as
(3) where z, is the natural radiative lifetime of the singlet (no internal quencher present), R is the distance between two molecules, and Ros' is computed in the usual way from the overlap of the singlet emission and the triplet absorption spectra. The physical interpretation of RosT is that if the molecule in the triplet
* Supported in part by the National Science Foundation (Grant DID 71-04010-AO2).
= 4F116
R,ST
where dF is the unimolecular fluorescence yield. The interpretation of EoST is that, if the molecules are closer than EosT,transfer is more likely than all of the unimolecular decay processes which normally compete with it. Because of the central importance of all-transretinal in the visual process (see, e.g. Thomas, 1965) it seemed useful to extend our calculations to this molecule. While the singlet may be heavily quenched at normal temperatures (Waddell et d.,1973), it has a non-vanishing quantum yield at low temperatures (Moore and Song, 1973; Waddell et al., 1973) and therefore can be a viable donor. The singlet fluorescence spectrum of ull-tramretinal dissolved in 3-methylpentane at 77K was obtained from Evans et al. (1967). The triplet absorption spectrum at 23°C in methanol and methylcyclohexane was obtained from Fisher and Weiss (1974) (see also Dawson and Abrahamson. 1962). Comparing the singlet absorption maximum in methanol and methylcyclohexane (Dawson and Abrahamson, 1962) and that in 3-methylpentane (Evans rt al., 1967), we see that the singlet maximum is the same in 3-methylpentane and methanol but shifted towards lower wavelengths by 10 nm in methycyclohexane. We assume that a similar shift would be introduced in the triplet absorption spectrum. From the Harvard work (1959), we see that as the temperature falls from room temperature to liquid nitrogen temperature the singlet absorption peak shifts to higher wavelengths and the molar decadic extinction coefficient increases. Since we have used the room temperature triplet absorption spectrum in our calculation, we can say that our computed values for RoSTand RosTare likely to be lower limits of these values at 77K. The unimolecular fluorescence yield 4fiat 77K was taken to be -0.02 (Moore and Song 1973). Unfortunately, 4p
I49
150
ROBERTS. KNOXand VINITAJ. GHOSH
depends strongly on exciting wavelength and on solWe conclude that singlet-triplet fusion will be an vent (Christensen and Kohler, 1974) and it is difficult efficient quenching mechanism in all-trans-retinal at to determine the best effective value to use in this high excitation levels and at low temperatures. In a Sense the triplet carries a sphere of destructive incalculation. Using the triplet absorption spectrum with meth- fluence with it, with a radius of 21 A. We wish to emphasize that the dynamical interacanol at the solvent, the values of RosT and EoSTwere computed to be RoST= 47 A and RoST= 25 A. Using tion between singlet and triplet excitations is by no triplet absorption data with methylcyclohexane as the means a new concept. It has been introduced often solvent, the values of RoSTand EoSTwere found to as a mechanism for energy conversion between two be 40 A and 20.8 A, respectively. These more conser- specific and generally different molecules (e.g. Bennett vative values, along with the previous results for and Kellogg, 1967). Our findings are that in four imchlorophyll a, anthracene and rhodamine 6G (Rah- portant cases, interactions within the host matrix man and Knox, 1973) are summarized in Table I. itself will be non-negligible in the determination of exciton kinetics at relatively high densities. Studies Table 1. Values of RoST(A),& and BoST(A)for all-tians- of this type are of increasing importance in the charretinal, chlorophyll a, anthracene and rhodamine 6G. acterization of condensed aromatic systems (see, e.g. Values in the first column are discussed and referenced Swenberg and Geacintov, 1973). The relatively large in the present text; those in the other three columns in 'sphere of influence' of the chlorophyll triplet is 'also Rahman and Knox (1973). of direct interest in assessing models of photosynthetic energy conversion in which triplets interact with triplets (Stacy et al., 1970) or singlets (Fong 1974, 1975) at reaction centers. Acknowledgements-The authors acknowledge helpful conversations with Ms. T. S . Rahman.
REFERENCES
Bennett. R., and R. E. Kellogg (1967) In Progress in Reaction Kinetics (Edited by G . Porter) p. 215 Pergamon Press, London. Christensen, R. L., and B. E. Kohler (1974) Photochem. Photobiol. 19. 401-410. Dawson, W., and E. W. Abrahamson (1962) J. Chem. Phys. 66. 2542-2547. Evans, T. R.,A. F. Toth and P. A. Leermakers (1967) J . Am. Chem. SOC. 89, 5061-5062. Fisher, M. M., and K. Weiss (1974) Photochem. Photobiol. 20. 423432. Fong, F. K. (1974) J . Theoret. Bid. 46. 407420. Fong, F. K. (1975) Appl. Phys. 6. 151-166. Forster, Th. (1948) Ann. Phys. (Germany) 2. 55. Forster, Th. (1965) In Modern Quantum Chemistry, Part 111 Action of Light and Organic Crystals (Edited by 0. Sinanoglu), p. 135. Academic Press, New York. Harvard University, Staff of the Biological Laboratories (1959) Nature 184. 614624. Moore, T. A., and P. S . Song (1973) Nature (New Biol) 242. 30 32. Rahman. T. S.. and R. S. Knox (1973) Phys. Stor. Sol. (h) 58. 715 720. Photobiol. 14. 197-219. Stacy, W. T., T. Mar, C . E. Swenberg, and Govindjee (1971) Pl~~tochrin. Swenberg, C. E., and N. E. Geacintov (1973) In Organic Molecular Photophysics: Vol. 1; Exciton Interactions in Organic Solids (Edited by J. B. Birks). p. 489. Wiley, New York. Thomas, J. B. (1965) Primary Photoprocesses in Bioloyy, pp. 163 ff. North Holland, Amsterdam. Waddell, W. H., A. M. Schaffer, and R. S. Becker (1973) J . Am. Chem. SOC.95. 8223-8227.
Pkorobiology, 1975. Vol. 22, pp. 149-150.
Pergamon Press. Printed
In
Great Brltaln
RESEARCH NOTE
QUENCHING OF SINGLET EXCITATIONS BY TRIPLET EXCITATIONS* ROBERTS. KNOXand VINITAJ. GHOSH Institute for Fundamental Studies and Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, U.S.A. (Received 15 April; accepted 30 April 1975)
state (acceptor) is closer than RosT to the molecule in the excited singlet state (donor), transfer of the excitation energy is more likely than radiation. A more useful parameter when intrinsic monomolecular quenching is present is
Recently, it was shown (Rahman and Knox, 1973) that an adaptation of the Forster theory predicts very large singletriplet exciton bimolecular quenching rate constants in the case of three important but widely differing molecules (anthracene, chlorophyll a, and rhodamine 6G). The theory was mainly designed to reinterpret certain solid state data, and this research note is presented with the thought that photochemists and photobiologists might find the results to be of interest, especially in view of the large quenching probabilities involved. We also report an extension of the calculations to all-trans retinal. The transfer of excitation from the first excited singlet state to the triplet may be represented by
RoST
where So and S, represent ground- and first-excitedsinglet state molecules, and To and T, represent molecules in the lowest triplet state and a higher triplet state, respectively. The second step, Eq. 2, is not essential to our consideration except that in order for Eq. 1 to represent singlet quenching, the triplet T, should not cross over to an S , . If the latter were to occur, the process would constitute triplet quenching by singlets. Following the detailed reasoning given by Rahman and Knox, we apply resonance transfer theory (Forster, 1948). The rate of an individual transfer process 1 is written as
(3) where z, is the natural radiative lifetime of the singlet (no internal quencher present), R is the distance between two molecules, and Ros' is computed in the usual way from the overlap of the singlet emission and the triplet absorption spectra. The physical interpretation of RosT is that if the molecule in the triplet
* Supported in part by the National Science Foundation (Grant DID 71-04010-AO2).
= 4F116
R,ST
where dF is the unimolecular fluorescence yield. The interpretation of EoST is that, if the molecules are closer than EosT,transfer is more likely than all of the unimolecular decay processes which normally compete with it. Because of the central importance of all-transretinal in the visual process (see, e.g. Thomas, 1965) it seemed useful to extend our calculations to this molecule. While the singlet may be heavily quenched at normal temperatures (Waddell et d.,1973), it has a non-vanishing quantum yield at low temperatures (Moore and Song, 1973; Waddell et al., 1973) and therefore can be a viable donor. The singlet fluorescence spectrum of ull-tramretinal dissolved in 3-methylpentane at 77K was obtained from Evans et al. (1967). The triplet absorption spectrum at 23°C in methanol and methylcyclohexane was obtained from Fisher and Weiss (1974) (see also Dawson and Abrahamson. 1962). Comparing the singlet absorption maximum in methanol and methylcyclohexane (Dawson and Abrahamson, 1962) and that in 3-methylpentane (Evans rt al., 1967), we see that the singlet maximum is the same in 3-methylpentane and methanol but shifted towards lower wavelengths by 10 nm in methycyclohexane. We assume that a similar shift would be introduced in the triplet absorption spectrum. From the Harvard work (1959), we see that as the temperature falls from room temperature to liquid nitrogen temperature the singlet absorption peak shifts to higher wavelengths and the molar decadic extinction coefficient increases. Since we have used the room temperature triplet absorption spectrum in our calculation, we can say that our computed values for RoSTand RosTare likely to be lower limits of these values at 77K. The unimolecular fluorescence yield 4fiat 77K was taken to be -0.02 (Moore and Song 1973). Unfortunately, 4p
I49
150
ROBERTS. KNOXand VINITAJ. GHOSH
depends strongly on exciting wavelength and on solWe conclude that singlet-triplet fusion will be an vent (Christensen and Kohler, 1974) and it is difficult efficient quenching mechanism in all-trans-retinal at to determine the best effective value to use in this high excitation levels and at low temperatures. In a Sense the triplet carries a sphere of destructive incalculation. Using the triplet absorption spectrum with meth- fluence with it, with a radius of 21 A. We wish to emphasize that the dynamical interacanol at the solvent, the values of RosT and EoSTwere computed to be RoST= 47 A and RoST= 25 A. Using tion between singlet and triplet excitations is by no triplet absorption data with methylcyclohexane as the means a new concept. It has been introduced often solvent, the values of RoSTand EoSTwere found to as a mechanism for energy conversion between two be 40 A and 20.8 A, respectively. These more conser- specific and generally different molecules (e.g. Bennett vative values, along with the previous results for and Kellogg, 1967). Our findings are that in four imchlorophyll a, anthracene and rhodamine 6G (Rah- portant cases, interactions within the host matrix man and Knox, 1973) are summarized in Table I. itself will be non-negligible in the determination of exciton kinetics at relatively high densities. Studies Table 1. Values of RoST(A),& and BoST(A)for all-tians- of this type are of increasing importance in the charretinal, chlorophyll a, anthracene and rhodamine 6G. acterization of condensed aromatic systems (see, e.g. Values in the first column are discussed and referenced Swenberg and Geacintov, 1973). The relatively large in the present text; those in the other three columns in 'sphere of influence' of the chlorophyll triplet is 'also Rahman and Knox (1973). of direct interest in assessing models of photosynthetic energy conversion in which triplets interact with triplets (Stacy et al., 1970) or singlets (Fong 1974, 1975) at reaction centers. Acknowledgements-The authors acknowledge helpful conversations with Ms. T. S . Rahman.
REFERENCES
Bennett. R., and R. E. Kellogg (1967) In Progress in Reaction Kinetics (Edited by G . Porter) p. 215 Pergamon Press, London. Christensen, R. L., and B. E. Kohler (1974) Photochem. Photobiol. 19. 401-410. Dawson, W., and E. W. Abrahamson (1962) J. Chem. Phys. 66. 2542-2547. Evans, T. R.,A. F. Toth and P. A. Leermakers (1967) J . Am. Chem. SOC. 89, 5061-5062. Fisher, M. M., and K. Weiss (1974) Photochem. Photobiol. 20. 423432. Fong, F. K. (1974) J . Theoret. Bid. 46. 407420. Fong, F. K. (1975) Appl. Phys. 6. 151-166. Forster, Th. (1948) Ann. Phys. (Germany) 2. 55. Forster, Th. (1965) In Modern Quantum Chemistry, Part 111 Action of Light and Organic Crystals (Edited by 0. Sinanoglu), p. 135. Academic Press, New York. Harvard University, Staff of the Biological Laboratories (1959) Nature 184. 614624. Moore, T. A., and P. S . Song (1973) Nature (New Biol) 242. 30 32. Rahman. T. S.. and R. S. Knox (1973) Phys. Stor. Sol. (h) 58. 715 720. Photobiol. 14. 197-219. Stacy, W. T., T. Mar, C . E. Swenberg, and Govindjee (1971) Pl~~tochrin. Swenberg, C. E., and N. E. Geacintov (1973) In Organic Molecular Photophysics: Vol. 1; Exciton Interactions in Organic Solids (Edited by J. B. Birks). p. 489. Wiley, New York. Thomas, J. B. (1965) Primary Photoprocesses in Bioloyy, pp. 163 ff. North Holland, Amsterdam. Waddell, W. H., A. M. Schaffer, and R. S. Becker (1973) J . Am. Chem. SOC.95. 8223-8227.
