J. Phys. Chem. 1991, 95, 5476-5488
5416
Time-Resolved Monomer and Excimer Fluorescence of 1&DI( I-pyreny1)propane at DMerent Temperatures. No Evidence for Distributions from Picosecond Laser Experiments wlth Nanosecond Time Resolution Maas A. Zachariasse,* Wolfgang Kiibnle, Uwe Leinhas, Peter Reyoden,+a d George Striker* Max-Planck-Institut fiir biophysikalische Chemie, Am Fassberg, Posrfach 2841, 0-3400 Gatringen, Germany (Received: May 29, 1990; In Final Form: February 15, 1991)
Monomer and excimer fluorescence decays of 1,3-di(1-pyreny1)propane(lPy(3)lPy) in n-heptane are measured at different temperatures by using timscomlated single-photon counting (SPC).The decays at 61 OC are tripleexponential with decay times and amplitudes independent of the number of total counts, excitation wavelength, and width of the excitation pulse. At -43 OC and lower temperatures, singlaexponential monomer decays are obtained, next to triplaexponential excimcr records. Aggregates of IPy(3)lPy are not detected. These results can be described by the three-state model, DMD, comprising one group of rapidly interconvertingexcited-state monomers (M*) and two excimers (D*). No evidence for distributed decay times is found down to a time resolution of 0.1 ns/channel and 2000 channels. In support of the DMD model, molecular mechanics calculations backed by 'HNMR measurements are presented, showing that only two monomer conformers of lPy(3)lPy are populated in the ground state. The interconversion between these two monomers in the excited state is too fast to be resolved by the present SPC measurements.
Introduction A discussion has arisenI4 whether distributions of monomer conformers have to be taken into account in the excimer formation kinetics of 1,3-di( 1-pyreny1)propane (lPy(3)lPy). In a recent paper,' Siemiarczuk and Ware presented monomer fluorescence decays of IPy(3)lPy at temperatures between 334 and 185 K (n-heptane, fluid solution) and at 77 K (frozen methylcyclohexane (MCH)-methylcyclopentane (MCP) (1:1) glass). Experimental excimer decays, which in our opinion3 are essential in a discussion of systems undergoing excimer formation, were not reported. The decay curves, measured with the time-correlated single-photon counting (SPC) technique employing a nanosecond flash lamp or a picosecond laser for excitation, were fitted with a discrete multiexponential decay function of up to four exponentials. In addition, at each except the highest temperature a distribution analysis was carried out by using the maximum entropy method (MEM).C8 Monomer decays iM(t)measured at 334 K with 3.2 X 10' counts in the peak channel, CPC,' (225 channels for a 400 11s time range) at various excitation wavelengths and two pulse widths,*" could be adequately fitted with three exponentials.
+
iM(t) = ,411e-f/71 A12e-f/r2+ A13e+3
(7'
> 72 > 73)
(1)
However, the decay times T~ and preexponential factors (amplitudes) AI, varied with excitation wavelength, having values different from our data published p r e v i ~ u s l y . ~ ~In' ~an - ~ earlier ~ publication,2 in which the exponential series method (ESM) was used,the failure1J5of a triple-exponential fit of a monomer decay collected at 334 K with up to 6 X lo5 peak counts (but with an in our view3 inadequate time range of only 150 ns in 225 channels) had been reported. These results were thought to point to a kinetic heterogeneity of the excited-state monomer species in lPy(3)lPy.l Siemiarczuk and Ware dismissed our suggestion3that a minor amount of an impurity with only one pyrenyl moiety, present from the start or emerging as a photoproduct (see the Experimental Section), could be responsible at least for the larger value and amplitude of their longest decay time.'S2J6 Our suggestion will be further substantiated here. The effect of temperature on the monomer decays of 1Py(3) 1Py was also investigated in ref 1, and compared with our monomer decays, which are single-exponentialbelow 243 K?J7the excimer remaining triple-exponential. We could explain our observations within the context of a three-state model, DMD, comprising one 'herent address: E. Merck OHG, FO PIGM/PE, M18, Darmstadt, Germany.
0022-365419112095-5476$02.50/0
SCHEMEI: DMDModd
*ka(l) 1M *kpo
1
/
Dl
I
qTl
1
* D2
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group of rapidly interconverting monomer conformers IM* and two structurally different excimers IDl* and as indicated (1) Siemiarczuk, A.; Ware, W. R. J . Phys. Chem. 1989, 93, 7609.
(2) Siemiarczuk, A.; Ware, W. R. Chem. Phys. Lett. 1987, 140, 277. (3) Zachariasse, K. A.; Striker, G. Chem. Phys. Lrtt. 1988, 145, 251. (4) (a) James, D. R.; Ware, W. R. Chem. Phys. k i t . 1986,126,7. (b) Sicmiarczuk, A.; Wagner, B.D.; Wan, W. R. J. Phys. Chem. 1990,94,1661. ( 5 ) Livesey, A. K.; Brochon, J. C. Biophys. J. 1987, 52, 693. (6) Maximum-Entropy and Bayesian Methods in Inverse Problem, Ray
Smith, C., Grandy, W. T., Jr., Eds.; Reidel: Dordrecht, The Netherlands,
1985. (7) Maximum-Entropy and Bayesian Spectral Analysis and Estimation
Probiems; Ray Smith, C.; Erickson, G. J., Eds.; Reidel: Dordrecht, The Netherlands, 1987. (8) Maximum-Entropy and Bayesian Methods; Skilling, J., Ed.; Kluwer: Dordrecht, The Netherlands, 1989. (9) In our numbering of the decay times if,different from that in ref 1, the longest time has the lowest number. This follows the notation commonly used for double-exponential decays in excimer formation. See ref 44b. (10) The temperature 334 K had previously been chosen somewhat arbitrarily (ref 12a), as an example from the range of temperatures (30-94 'C) where the amplitudes of the three monomer decay times are found to be sufficiently large to enable the determination of all rate constants and lifetime0 involved in the kinetic DMD scheme (refs 12 and 13). However, 334 K is not the optimal temperature for determining the amplitudes of all three timea, as the relative contributions of the two longer times r2 and rI to the total f i = 1 and 2, are still rather monomer decay, Le., the ratios A f r f / x A f rfor small at 334 K (see Figures 3-6) and increase with temperature. This holds especially for the longest time r 1 (see ref 13). (1 1) It should be noted that the excitation pulse in the flash lamp expcriments of ref 1 has a full spectral bandwidth of 24 nm, w h e w in our experimenta (refs 3, 12-14) the lines from the emission spectrum of N, were selected with a monochromator (1-2-nm bandwidth, scc the Experimental Section). We will therefore not go into the possible influence (if any) of broad-band excitation, which apparently had to be used to obtain the, in our opinion, unnecessary and even self-defeating (scc text) large peak count numbers with the relatively weak deuterium flash lamp excitation. (12) (a) Zachariasse, K. A.; Busse, R.; Duveneck, 0.; Klihnle, W. 1. Photochem. 1985,28,237. (b) Zachariasse, K. A. In Photochemistry on SoNd Surfaces; Matsuura, T., Anpo, M., Eds.; Elsevier: Amsterdam, 1989; p 48. (13) Z a c h a m , K. A,; Duveneck G.; Bus~c,R. J. Am. Chem.Soc.1984, 106,1045. (14) Zachariasse, K. A. In Photochemical Processes in Orgunized Malecular Systems; Honda, K., Ed.; North-Holland: Amsterdam, 1991; p 83.
Q 1991 American Chemical Society
Time-Resolved Fluorescence of 1,3-Di( 1-pyreny1)propane in Scheme I. In this scheme, &,(l) and k,(2) are the rate constants of excimer formation, &,+(1) and &#) the rate constants of excimer dissociation, and fo(l), ~ ' ~ ( 2and ) T,, the two excimer lifetimes and the monomer lifetime. When a single-exponential monomer decay appears together with a triple-exponentialexcimer decay, this means (Scheme I) that the reaction from the two excimers back to the excited-state monomer (kd(1) and & (2)) has been reduced to negligible proportions (kd
