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One-dimensional migration of triplet excitations in polymers at low temperatures: polyquanylic acid—lTb3+ complex
Volume 203, number 2,3
CHEMICAL PHYSICS LETTERS
19 February 1993
One-dimensional migration of triplet excitations in polymers at low temperatures: polyquanylic acid-Tb3+ complex Yu.P. Blagoy, LA. Levitsky, Yu.V. Rubin and V.V. SIavin Institutefor Low Temperature Physicsand Engineering, Ukrainian Academy ofSciences, 310164 Kharkov, Ukraine Received 25 July 1992: in final form 1November 1992
The energy migration in the frozen polymer Poly(G) doped with Tb’+ ions-energy traps is studied at 77 K. It is shown that the decay of the Tb3+ luminescence is in good agreement with the theory of migration in low-dimensional systems. The approximation gives the upper limit of the jump rate to be 1.2~ IO’s ’, The influence of the energy disordering upon the excitation migration along the polymer chain is discussed.
1. Introduction The migration of electronic excitations in organic solids has been the subject of numerous experimental and theoretical studies [ 11. Of particular theoretical interest is the energy migration in quasi-onedimensional crystals and frozen polymers when the transfer dynamics manifest specific features of lowdimensional systems [ l-6 1. The majority of experiments were carried out on quasi-one-dimensional crystals (molecular [ 1,7] and inorganic [ 8 ] ), in which their own traps or impurity molecules acted as energy acceptors (the studies of migration in porous media belong to this class of investigations [ 91). It has been shown that the energytransfer rate in these objects is dependent on time, which is typical of low-dimensional transport. As a result, the luminescence decay of the donor and acceptor cannot be described by a single exponent or by a sum of two exponents, respectively. Meanwhile, experimental studies of energy migration in frozen polymers, especially biopolymers, by the method of luminescence decay are scarce [ lo]. This is largely explained by the fact that at low temperatures the spectra of polymers have appreciable inhomogeneous broadening (IB), that results in the overlapping of excimer and trap bands with those ones of delocalized excitations [ I I]. Therefore the decay of luminescence in “undoped” polymers is
usually nonexponential, and its analysis is therefore difficult and ambiguous. Besides, the choice of an organic molecule bound to the polymer as an energy acceptor is not always successful, since a disordered environment of molecules of the solvent and the polymer itself would often cause the dispersion of lifetimes [ 121. This may again impart an intricate character of decay curves even in the absence of energy transport. The magnification of these complications may be avoided if to detect the luminescence decay of rareearth ions bound to the macromolecule and acting as centres of trapping for the excitation migrating along the polymeric chain. It is important that such ions possess high symmetry and are less sensitive to the disordered environment as compared to organic molecules. The decay of their luminescence in the amorphous medium is monoexponential and their emission spectrum typically has intensive and relatively narrow bands, which do not overlap with spectra of many polymeric compounds [ 131. In this Letter some results on the energy migration in the polyquanylic acid (Poly (G) )-terbium ion ( Tb3+) complex are presented, major attention being concentrated on the detection and the analysis of the decay curves of Tb3’ luminescence.
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
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2. Experimental The experimental objects were the preparations of 5’-guanosinemonophosphate (GMP) supplied by the REANAL company and Poly (G ) - the polymer consisted of monomeric units of GMP (molecular weight is 1.5x lo’), synthesized at the Institute of High Molecular Compounds of the Russian Academy of Sciences. It is known that Poly(G) has a relatively rigid ordered structure in the form of a four-strand helix [ 141. Poly(G) was dialyzed against two changes of doubly distilled water. The mixture of 200/o propylene glycol and 80% NaOAc buffer (0.0 1 M) with NaCl (0.1 M) (pHc6.4) was used as a solvent. The concentration of GMP and Poly (G) (phosphate residues or monomeric units) was cG= 5 X 1Om4M in all the samples with few exceptions; the Tb3+ concentration c, varied from 1Om5 to 3 x 10e3 M. After adding Tb3+ ions to the solution, the measurements lasted for 24 h. The samples of solid solutions were obtained from corresponding liquid solutions quickly immersed into liquid nitrogen. All investigations were carried out at 77 K. The emission of Poly(G)-Tb3+ complexes was excited either with a steady-state xenon lamp or with a xenon pulse lamp (the pulse duration was from 40 to 60 us). Steady-state data were recorded using an excitation monochromator. The pulse lamp was used with a filter behind it whose band width 250-3 10 nm corresponded to the one of absorption of Poly(G), GMP and did not overlap with the most intensive bands ( ~220 nm) of the Tb3+ absorption spectrum. The light emitted from the sample was analyzed through an emission monochromator and collected on a photon counting photomultiplier. The decay curves were recorded using a multichannel analyzer based on “CAMAC” modules and computer (analogous PDP-11) with the best resolution 150 us per channel.
3. Results and discussion The emission spectra of Poly(G) and Poly(G)Tb3+ complex (A,,=290 nm) at 77 K are shown in fig. 1. As the Tb3+ concentration increases in the solution, the Poly(G) phosphorescence quenches significantly and Tb3+ luminescence appears, having 266
350
450 X, nm
550
Fig. 1. Luminescence spectra at 77 K (A,,=290 nm) of (a) Poly(G)and(b)Poly(G)-Tb’+complex:~~=2~IO-~M,(c) ~~=5xlO-~M. c~=SXIO-~M. Theband with maximum at 360 nm shows the Poly(G) fluorescence.
bands at A=490 and 545 nm corresponding to 5D4+7F, and 5Dq+7F5 transitions. Similar changes were observed in the luminescence spectrum of the GMP-Tb3+ complex. On the other hand, in the absence of Poly( G) and GMP, other things being equal, no Tb3+ luminescence is observed at A=490,545 nm. This suggests an electron excitation transfer (in the case of Poly (G) migration ) to the ‘D, level of IV+ ions. As the Tb3+ concentration in the solution rises to 10m4M, the quantum yield curve of Poly(G)-Tb3+ reaches saturation. For GMP-Tb3+ this occurs at the concentration 2x 10e3 M (fig. 2). This significant difference in the character of the quantum yield curves is indirect evidence that the triplet excitations effectively migrate in the polymer with subsequent trapping at the Tb3+ ions acting as traps. The energy migration in Poly(G) is most evident when comparing the decay curves at 1= 425 and 545 nm for the Poly(G)-Tb3+ and GMP-Tb3+ complexes (figs. 3 and 4). If the decay curves in the GMP phosphorescence and Tb3+ luminescence bands are monoexponential and independent of the ion concentration, the corresponding curves for the Poly(G)-Tb3+ complexes are complicated in character and essentially dependent on the Tb3+ concentration.
CHEMICAL PHYSICS LETTERS
Volume 203, number 2,3
19 February 1993
WI) 0
-2
-4
-6 UI.
1
0.2
t
10
2 C,-,d04,
10
M
Fig. 2. Quantum yield for complexes (0) Poly(G)-Tb3+ and (@)GMP-Tb’+at77K,~,=5xlO-~L,=545nm,1,,=290 nm.
1 2 -4
I
0.5
I
1.5
2
* r5 1. s
Fig. 3. Decay curves. Aoh= 425nm,co=5~10-~M. (1) GMP; (2) Poly(G); (3) Poly(G)-Tb3+. cm=5x 1V5 M.
This may be explained taking into account that in organic complexes of rare-earth ions the energy is transferred from the organic component of the complex to the ion at the rates z108-10’ s-l [ 151. Therefore the deviation from the monoexponentiality (the initial rising) in the decay curves of Tb3+ luminescence (A= 545 nm) is not observed in the time interval 10m3-1 s for the GMP-Tb3+ complex. For the same reason, the GMP phosphorescence in the GMP-Tb3+ complex is completely quenched and the phosphorescence at II= 425 nm comes from free GMP molecules (fig. 3, curve ( 1) ). It should be noted that a non-monoexponential decay of “pure” Poly( G) is most likely connected to the availability of its intrinsic traps similar to the ones
e
20
30
I. nis
Fig. 4. Decay curves. Aoh= 545nm.GMP-Tb’+: (l)co=5~10-~ M. Decay curves are identical at c,= 10m4M, 5x lo-“ M, 10e3 M. Poly(G)-T’b3+: (2)+,=2x 10e5 M. Decay curves are identicalatc,~5xIO-4M,10-3M,2.3~10-‘M.(3)c,=8~10-’ M,co=5~10-4M.(4)c,=5~10-4M.Decaycurvesareidenticalatc,=2x10-4M,3~10-4M.
found in Poly (A) [ 16] (the properties of the latter polymer in many respects resemble those of Poly(G)) rather than to the presence of two emitting centers corresponding to interacting and non-interacting guanine groups in the polymer chain, as it was assumed in ref. [ 171. Tb3+ binding to Poly (G) should lead to an increase in the energy-migration rate and thus to a higher degree of non-Linearity (on the In scale) of the decay curves of Poly(G) phosphorescence (fig. 3, curves (2 ) and (3) ). However, the non-controlled concentration of intrinsic traps, an unknown relationship of their radiative and nonradiative rates, their lifetimes and a probable spectral overlap with the Poly(G) bands - all this impedes an unambiguous analysis of the decay curves at /2=425 nm. Meanwhile, the luminescence decay of the Tb3+ ions bound to Poly (G) is free of the above complicating factors but is sensitive to the concentration variations, which permits the experiment to be approximated with theoretical curves. We used the theoretical model developed by Kenkre and Parris [ 18] as a basis, because it can give the most universal description of the energy migration processes in solids at any trap concentration. The trap excitation probability nT of eq. (2.16) was obtained in ref. [ 18I: 267
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PG nT(E)
=
e’+E’fi(t’)CT
1 (1)
t’+l/fT-l/r,’
where e’=t+ l/r,; E is the Laplace variable; r,, r, are the lifetimes of the host and the trap, respectively; p is the trap concentration; Cr is the direct trap rate. If there are k types of traps, the total concentration being p=Cj”,, p,, it may readily be shown that for the trap population of the j-type nri( t) with concentration pi, p in eq. ( 1) must be substituted with pi, while the function ti(e’) will be dependent on p. At 77 K the excitation transport may be considered completely incoherent, therefore, according to refs. [4,18], we assumed P(f’) =pIc’+ (1 -p) [ c’(e’t 4F) ] -‘I2 (Fis the jump rate) for a one-dimensional system with a random distribution of traps. We also take Cr+co, corresponding to fast reaction or infinitely deep traps. The decay curves of Tb3+ luminescence for the Poly(G)-Tb3+ complex differ from those for the GMP-Tb3+ complex, due to relatively fast capture of excitation by traps as regards their transport. Otherwise, the difference between the decay curves was not observed. The above fact justifies the application of the model of fast reaction when Cr~oo or C, s F. (As it follows from ref. [ 19 1, the condition CT>> F can be substituted by less strict Cr 3 0.1 F without gross error. ) The inverse Laplace transformation was performed using the method of contour integration for two cases: (i) 2,> rn > 1/F is the traditional relation for majority of crystals. Using the same parameters as in ref. [ 18 ] and assuming only one type of traps, we obtained a complete coincidence of our calculated curves and those in ref. [ 18 1, (ii) rH> rr> 1/F is the relation typical of the Poly(G)-Tb3+ (rr=l.l ms, rH=1.15 s are taken from the decay of the GMP-Tb3+ complex and GMP). In this case @j(t)=r;exp(-KTt)
+
$f
lt
KT-KH L W2&-KTt4F
where KT= I/tT, KH= l/rH, w=p/( 1 -p), r,-p,/p. The decay curves of Tb 3+ luminescence were approximated using eq. (2). The results are given in fig. 5. The following facts must be taken into account before analyzing these results, First, the relative concentration of thej-type traps ( Tb3+ ions) r,=pj/p explicitly enters into eq. (2) as a constant cofactor (pj is implicitly considered in p) and cannot therefore be obtained directly from the approximation since experimental nTj(t) values are obtained within the constant. Second, the parameters F and p in eq. (2) are not independent and nTj(p, F)=n&‘F). This was shown numerically in the range of reasonable values of p (from 10e3 to 6x 10m2) and F (from 10’ to lo3 s-l). For the case p< lo-’ (low trap concentration) these data correlate with conclusion [ 2-51, however at IO-‘
‘%
-1
-2
-3
-4
-’ >
exp( -KHZ)
s
exp( -xt),,/x(4F-x)
’ o [02(4&X)+X](KT-K,-~)dx’
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(2)
Fig. 5. Decay curves for Poly(G)-Tb’+ (A,,,=545 nm, co=5~10-4M)at(0)c~=2~10-5Mand(O)~Tb=2~10~4 M and their approximation by theoretical dependences (2) (- - -) and (-), respectively.
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CHEMICAL PHYSICS LETTERS
6 x lo-* the curves calculated by (2) differ from those in the low-concentration regime by no more than 3% to 5%. Thus, it may be considered, that y=p*F is the single variable parameter. Third, if the dimensionless parameter 6= c&co is introduced, the decay of Tb3+ luminescence can only change when 1/25<6< l/2.5. The lower threshold isamenable to trivial explanation: at these values of 6 the Poly (G)Tb3+ complexes have only one Tb3+ ion per polymer, so a further decrease in 6 should not cause changes in the decay curves. This was observed experimentally, when S was decreased to & through increasing the Poly( G) concentration in the solution from 5 x 1Om4M higher at the constant Tb3+ concentration, 2 x lo-’ M (fig. 4, curve (2) ). The upper limit of the decay curve variations at 6= l/2.5 (fig. 4, curve (4) ) is not so obvious and will be discussed below. Then two cases may be considered. (i) There are no intrinsic traps in the polymer (pj=p). Then for 6~ & and for the polymer of a known mean length S, p= 1/S may be found, which permit to determine the jump rate F. For the 4stranded Poly(G) S= 95 links, where every link consists of four GMP molecules (S was found from the average molecular weight of the polymer). If 6> &, p may be found from y resulting from the fitting and F obtained earlier for 6< A. This procedure led to E= 1,2 X lo5 s- ’ and the p values which were obtained growing monotonously with 6 from 1.05~ lo-’ to 2.3~ lo-*. The error in y, F (at 6~ &) and p (at 6> &) was * 30%, being mainly induced by the scattering of data for samples of different experimental series. (ii) The polymer has intrinsic traps (p,
19 February
1993
in the trap is almost three orders of magnitude lower than in the host. We made best fitting of experimental curves (fig. 5) with model curves for the system, when the energy-migration rate KS is independent of time. In this case trap excitation probability nr (t) is found as
x{exp[-(Ks+KH)t]-exp(-KTt)}.
(3)
This approximation was extremely unsatisfactory and showed that the model of three-dimensional isotropic migration, when Ks=const., is inadequate for low-dimensional systems. This agrees with theoretical concepts [l-6] postulating that decreasing dimensionality (d-c 3 ) should lead to the time dependence of the energy-migration rate. The value of F turns out to be unexpectedly low when compared with the jump rate of triplet excitations in molecular crystals, z 1OS- 10’’s- I. On the other hand, F=2.5x lo3 s-’ was obtained for Poly(A) at 77 K [20], though the theoretical estimates are =109 s-’ [21]. Most probably, the presence of energy disordering among the triplet levels of the polymer may be the main factor responsible for the slow migration. Under this condition the applicability of the resonance migration model is limited since the probability of a jump between two molecules is not symmetric and depends on the energy gap between them [ 22 1. The probability of a jump “upwards” will differ from that of a jump “downwards” by a factor of exp( -AE/ kT). Thus, the energy migration would occur simultaneously with relaxation of excitations to the bottom of the continuum of levels. It may therefore be spoken only about the average jump rate, which is likely to be lower than the corresponding value under the energy resonance. A theoretical model was proposed [ 22 1, which describes the migration of excitations under the condition of energy disordering. However, this can hardly be used for approximating experimental data since there is no analysis of onedimensional systems. Spatial disordering between polymer molecules can be the other mechanism resulting in the small value F (this effect was studied in ref. [23] ). But since, from all the polynucleotides Poly(G) has the most 269
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rigid structure, due to four H-bond helices, we helieve that partial stacking breaking would not induce the significant decrease of the exchange interaction. Some discrepancy between theoq and experiment is, to a certain extent, due to the fact that the poly mer length is finite, unlike an infinite one-dimensional chain; besides, there is distribution over the polymer lengths and over the number of ions bound to the polymer. So the energy migration in the polymer can be described most adequately at IB by some model constructed using the Monte Carlo method, which the authors are planning to do the next work. The upper limit at 6= l/2.5 in 6 dependence of the decay curves is interesting biophysically. This leads to the conclusion that a further increase in 6 (more than l/2.5) does not form new traps - Tb” ions bound to the polymer. Otherwise, the decay curve could have gone on changing up to a complete coincidence with the exponent (when each link of the polymer would have been bound to Tb3+ ion - the case similar to the GMP-Tb3+ complex). As the complex Poly(G)-Tb3+ is formed, the Tb3+ ions interact not only with the phosphate groups of the polymer, but also with the bases mainly at the sites of disrupted hydrogen bonds, where the helix is damaged. This finding was confirmed by experiments on Poly (G) and other polynucleotides, DNA, RNA [24] and may be taken as fully ascertained. It is likely that only the Tb 3+ ions bound to the bases (at the helix centre) are the energy traps; the ions bound to the phosphate groups do not participate in trapping excitations since the phosphate residues are at the outer surface of the polymer far from the chromophores (bases). It is therefore reasonable to assume that the existence of the limiting trap concentration at 62 I /2,5 is due to the screening effect of the ions bound to the phosphate groups preventing a further Tb3+ binding to the bases. To conclude, it should be noted that the use of rareearth ions bound to the polynucleotides as “luminescence time” probes is essentially advantageous as compared to the traditional approaches (stationary excitations, polymer phosphorescence decay) to understanding the nature of triplet excitation migration in these polymers, It also offers possibilities of extending studies to more complex biological objects such as DNA and RNA.
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Acknowledgement The authors wish to thank Dr. V.A. Karachevtsev for helpful comments and discussions. References [ 1] M.D. Fayer, in: Spectroscopy and excitation dynamics of condensed molecular systems, eds. V.M. Agranovich and R.M. Hochstrasser (North-Holland, Amsterdam, 1983) p. 185. B.Y.Balagurovand V.G. Vaks,Soviet Phys. JETP 38 (1974) 968. 1B. Movaghar, G.W. Sauer and D. Wurtz, J. Stat. Phys. 27 (1982) 473. V.M. Kenkre, in: Exciton dynamics in molecular crystals and aggregates, ed. G. Hohler (Springer, Berlin, 1982) p. 1. J.K. Anlauf, Phys. Rev. Letters 52 ( 1984) 1845. A.I. Onipko, Phys. Letters A 101 ( 1984) 383. [ 71A.A. Avdeenko, V.V. Eremenko and V.A. Karachevtsev, Soviet Phys. JETP 67 (1988) 1677. [8] W.J. Rodriguez, M.F. Herman and G.L. McPherson, Phys. Rev. B 39 (1989) 13; R. Knochenmuss and H.U. Giidel,J. Chem. Phys. 86 (1987) 1104. [9] R. Kopelman, Phil. Mag. B 56 (1987) 717. [lo] S.E. Webber and P.E. Avots-Avotins, Macromolecules 12 (1979) 708. [ 111J. Guillet, Polymer photophysics and photochemistry (Cambridge Univ. Press, Cambridge, 1985). [ 121I.A. Levitsky and Yu.V. Rubin, Russian I. Chem. Phys., in press; Z. Salmon and H. BBssler,Chem. Phys. 100 (1985) 393. [13 ES. Richardson, Chem. Rev. 82 (1982) 541. [14 S.B. Zimmermann, G.H. Cohen and D.R. Davies, J. Mol. Biol. 92 (1975) 181. [15 V.L. Ermolaev, E.N. Bodunov, E.B. Sveshnikova and T.A. Shakhverdev, Radiativeless transfer of electron excitation energy, Leningrad ( 197I ) (in Russian). [16 K. Helen, in: Electrical, optical and magnetic properties of nucleic acids and components, Vol. 1, ed. J. Duchesne (Academic Press, New York, 1973) p. 130. [17 V. Kleinwlchter, Collection Czech. Chem. Commun. 37 (1972) 2333. (1983) 3221. [18 V.M.KenkreandR.E.Panis,Phys.Rev.B27 [19 G.L. McPherson, M.F. Herman and W.J. Rodriguez, Chem. Phys. Letters 144 (1988) 541. [20] J. Eisinger and R.G. Shulman, Proc. Natl. Acad. Sci. US 55 (1966) 1387. [21] B. Sommer and J. Jortner, J. Chem. Phys. 49 (1968) 3919. [22] B. Movaghar, M. Grunewald, B. Reis, D. Wurtz and H. Blssler, Phys. Rev. B 33 ( 1986) 5545. [23] H. Katayama, S. Maruyama, S. Ito, Y. Tsujii, A. Tsuchida and M. Yamamoto, J. Phys. Chem. 95 ( 1991) 3480. 1241D.S. Gross and H. Simpkins, J. Biol. Chem. 256 (1981) 9593; D.P. Ringer, B.A. Howell and D.E. Kizer, Ann. Biochem. 103 (1980) 3397.
CHEMICAL PHYSICS LETTERS
19 February 1993
One-dimensional migration of triplet excitations in polymers at low temperatures: polyquanylic acid-Tb3+ complex Yu.P. Blagoy, LA. Levitsky, Yu.V. Rubin and V.V. SIavin Institutefor Low Temperature Physicsand Engineering, Ukrainian Academy ofSciences, 310164 Kharkov, Ukraine Received 25 July 1992: in final form 1November 1992
The energy migration in the frozen polymer Poly(G) doped with Tb’+ ions-energy traps is studied at 77 K. It is shown that the decay of the Tb3+ luminescence is in good agreement with the theory of migration in low-dimensional systems. The approximation gives the upper limit of the jump rate to be 1.2~ IO’s ’, The influence of the energy disordering upon the excitation migration along the polymer chain is discussed.
1. Introduction The migration of electronic excitations in organic solids has been the subject of numerous experimental and theoretical studies [ 11. Of particular theoretical interest is the energy migration in quasi-onedimensional crystals and frozen polymers when the transfer dynamics manifest specific features of lowdimensional systems [ l-6 1. The majority of experiments were carried out on quasi-one-dimensional crystals (molecular [ 1,7] and inorganic [ 8 ] ), in which their own traps or impurity molecules acted as energy acceptors (the studies of migration in porous media belong to this class of investigations [ 91). It has been shown that the energytransfer rate in these objects is dependent on time, which is typical of low-dimensional transport. As a result, the luminescence decay of the donor and acceptor cannot be described by a single exponent or by a sum of two exponents, respectively. Meanwhile, experimental studies of energy migration in frozen polymers, especially biopolymers, by the method of luminescence decay are scarce [ lo]. This is largely explained by the fact that at low temperatures the spectra of polymers have appreciable inhomogeneous broadening (IB), that results in the overlapping of excimer and trap bands with those ones of delocalized excitations [ I I]. Therefore the decay of luminescence in “undoped” polymers is
usually nonexponential, and its analysis is therefore difficult and ambiguous. Besides, the choice of an organic molecule bound to the polymer as an energy acceptor is not always successful, since a disordered environment of molecules of the solvent and the polymer itself would often cause the dispersion of lifetimes [ 121. This may again impart an intricate character of decay curves even in the absence of energy transport. The magnification of these complications may be avoided if to detect the luminescence decay of rareearth ions bound to the macromolecule and acting as centres of trapping for the excitation migrating along the polymeric chain. It is important that such ions possess high symmetry and are less sensitive to the disordered environment as compared to organic molecules. The decay of their luminescence in the amorphous medium is monoexponential and their emission spectrum typically has intensive and relatively narrow bands, which do not overlap with spectra of many polymeric compounds [ 131. In this Letter some results on the energy migration in the polyquanylic acid (Poly (G) )-terbium ion ( Tb3+) complex are presented, major attention being concentrated on the detection and the analysis of the decay curves of Tb3’ luminescence.
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
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2. Experimental The experimental objects were the preparations of 5’-guanosinemonophosphate (GMP) supplied by the REANAL company and Poly (G ) - the polymer consisted of monomeric units of GMP (molecular weight is 1.5x lo’), synthesized at the Institute of High Molecular Compounds of the Russian Academy of Sciences. It is known that Poly(G) has a relatively rigid ordered structure in the form of a four-strand helix [ 141. Poly(G) was dialyzed against two changes of doubly distilled water. The mixture of 200/o propylene glycol and 80% NaOAc buffer (0.0 1 M) with NaCl (0.1 M) (pHc6.4) was used as a solvent. The concentration of GMP and Poly (G) (phosphate residues or monomeric units) was cG= 5 X 1Om4M in all the samples with few exceptions; the Tb3+ concentration c, varied from 1Om5 to 3 x 10e3 M. After adding Tb3+ ions to the solution, the measurements lasted for 24 h. The samples of solid solutions were obtained from corresponding liquid solutions quickly immersed into liquid nitrogen. All investigations were carried out at 77 K. The emission of Poly(G)-Tb3+ complexes was excited either with a steady-state xenon lamp or with a xenon pulse lamp (the pulse duration was from 40 to 60 us). Steady-state data were recorded using an excitation monochromator. The pulse lamp was used with a filter behind it whose band width 250-3 10 nm corresponded to the one of absorption of Poly(G), GMP and did not overlap with the most intensive bands ( ~220 nm) of the Tb3+ absorption spectrum. The light emitted from the sample was analyzed through an emission monochromator and collected on a photon counting photomultiplier. The decay curves were recorded using a multichannel analyzer based on “CAMAC” modules and computer (analogous PDP-11) with the best resolution 150 us per channel.
3. Results and discussion The emission spectra of Poly(G) and Poly(G)Tb3+ complex (A,,=290 nm) at 77 K are shown in fig. 1. As the Tb3+ concentration increases in the solution, the Poly(G) phosphorescence quenches significantly and Tb3+ luminescence appears, having 266
350
450 X, nm
550
Fig. 1. Luminescence spectra at 77 K (A,,=290 nm) of (a) Poly(G)and(b)Poly(G)-Tb’+complex:~~=2~IO-~M,(c) ~~=5xlO-~M. c~=SXIO-~M. Theband with maximum at 360 nm shows the Poly(G) fluorescence.
bands at A=490 and 545 nm corresponding to 5D4+7F, and 5Dq+7F5 transitions. Similar changes were observed in the luminescence spectrum of the GMP-Tb3+ complex. On the other hand, in the absence of Poly( G) and GMP, other things being equal, no Tb3+ luminescence is observed at A=490,545 nm. This suggests an electron excitation transfer (in the case of Poly (G) migration ) to the ‘D, level of IV+ ions. As the Tb3+ concentration in the solution rises to 10m4M, the quantum yield curve of Poly(G)-Tb3+ reaches saturation. For GMP-Tb3+ this occurs at the concentration 2x 10e3 M (fig. 2). This significant difference in the character of the quantum yield curves is indirect evidence that the triplet excitations effectively migrate in the polymer with subsequent trapping at the Tb3+ ions acting as traps. The energy migration in Poly(G) is most evident when comparing the decay curves at 1= 425 and 545 nm for the Poly(G)-Tb3+ and GMP-Tb3+ complexes (figs. 3 and 4). If the decay curves in the GMP phosphorescence and Tb3+ luminescence bands are monoexponential and independent of the ion concentration, the corresponding curves for the Poly(G)-Tb3+ complexes are complicated in character and essentially dependent on the Tb3+ concentration.
CHEMICAL PHYSICS LETTERS
Volume 203, number 2,3
19 February 1993
WI) 0
-2
-4
-6 UI.
1
0.2
t
10
2 C,-,d04,
10
M
Fig. 2. Quantum yield for complexes (0) Poly(G)-Tb3+ and (@)GMP-Tb’+at77K,~,=5xlO-~L,=545nm,1,,=290 nm.
1 2 -4
I
0.5
I
1.5
2
* r5 1. s
Fig. 3. Decay curves. Aoh= 425nm,co=5~10-~M. (1) GMP; (2) Poly(G); (3) Poly(G)-Tb3+. cm=5x 1V5 M.
This may be explained taking into account that in organic complexes of rare-earth ions the energy is transferred from the organic component of the complex to the ion at the rates z108-10’ s-l [ 151. Therefore the deviation from the monoexponentiality (the initial rising) in the decay curves of Tb3+ luminescence (A= 545 nm) is not observed in the time interval 10m3-1 s for the GMP-Tb3+ complex. For the same reason, the GMP phosphorescence in the GMP-Tb3+ complex is completely quenched and the phosphorescence at II= 425 nm comes from free GMP molecules (fig. 3, curve ( 1) ). It should be noted that a non-monoexponential decay of “pure” Poly( G) is most likely connected to the availability of its intrinsic traps similar to the ones
e
20
30
I. nis
Fig. 4. Decay curves. Aoh= 545nm.GMP-Tb’+: (l)co=5~10-~ M. Decay curves are identical at c,= 10m4M, 5x lo-“ M, 10e3 M. Poly(G)-T’b3+: (2)+,=2x 10e5 M. Decay curves are identicalatc,~5xIO-4M,10-3M,2.3~10-‘M.(3)c,=8~10-’ M,co=5~10-4M.(4)c,=5~10-4M.Decaycurvesareidenticalatc,=2x10-4M,3~10-4M.
found in Poly (A) [ 16] (the properties of the latter polymer in many respects resemble those of Poly(G)) rather than to the presence of two emitting centers corresponding to interacting and non-interacting guanine groups in the polymer chain, as it was assumed in ref. [ 171. Tb3+ binding to Poly (G) should lead to an increase in the energy-migration rate and thus to a higher degree of non-Linearity (on the In scale) of the decay curves of Poly(G) phosphorescence (fig. 3, curves (2 ) and (3) ). However, the non-controlled concentration of intrinsic traps, an unknown relationship of their radiative and nonradiative rates, their lifetimes and a probable spectral overlap with the Poly(G) bands - all this impedes an unambiguous analysis of the decay curves at /2=425 nm. Meanwhile, the luminescence decay of the Tb3+ ions bound to Poly (G) is free of the above complicating factors but is sensitive to the concentration variations, which permits the experiment to be approximated with theoretical curves. We used the theoretical model developed by Kenkre and Parris [ 18] as a basis, because it can give the most universal description of the energy migration processes in solids at any trap concentration. The trap excitation probability nT of eq. (2.16) was obtained in ref. [ 18I: 267
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PG nT(E)
=
e’+E’fi(t’)CT
1 (1)
t’+l/fT-l/r,’
where e’=t+ l/r,; E is the Laplace variable; r,, r, are the lifetimes of the host and the trap, respectively; p is the trap concentration; Cr is the direct trap rate. If there are k types of traps, the total concentration being p=Cj”,, p,, it may readily be shown that for the trap population of the j-type nri( t) with concentration pi, p in eq. ( 1) must be substituted with pi, while the function ti(e’) will be dependent on p. At 77 K the excitation transport may be considered completely incoherent, therefore, according to refs. [4,18], we assumed P(f’) =pIc’+ (1 -p) [ c’(e’t 4F) ] -‘I2 (Fis the jump rate) for a one-dimensional system with a random distribution of traps. We also take Cr+co, corresponding to fast reaction or infinitely deep traps. The decay curves of Tb3+ luminescence for the Poly(G)-Tb3+ complex differ from those for the GMP-Tb3+ complex, due to relatively fast capture of excitation by traps as regards their transport. Otherwise, the difference between the decay curves was not observed. The above fact justifies the application of the model of fast reaction when Cr~oo or C, s F. (As it follows from ref. [ 19 1, the condition CT>> F can be substituted by less strict Cr 3 0.1 F without gross error. ) The inverse Laplace transformation was performed using the method of contour integration for two cases: (i) 2,> rn > 1/F is the traditional relation for majority of crystals. Using the same parameters as in ref. [ 18 ] and assuming only one type of traps, we obtained a complete coincidence of our calculated curves and those in ref. [ 18 1, (ii) rH> rr> 1/F is the relation typical of the Poly(G)-Tb3+ (rr=l.l ms, rH=1.15 s are taken from the decay of the GMP-Tb3+ complex and GMP). In this case @j(t)=r;exp(-KTt)
+
$f
lt
KT-KH L W2&-KTt4F
where KT= I/tT, KH= l/rH, w=p/( 1 -p), r,-p,/p. The decay curves of Tb 3+ luminescence were approximated using eq. (2). The results are given in fig. 5. The following facts must be taken into account before analyzing these results, First, the relative concentration of thej-type traps ( Tb3+ ions) r,=pj/p explicitly enters into eq. (2) as a constant cofactor (pj is implicitly considered in p) and cannot therefore be obtained directly from the approximation since experimental nTj(t) values are obtained within the constant. Second, the parameters F and p in eq. (2) are not independent and nTj(p, F)=n&‘F). This was shown numerically in the range of reasonable values of p (from 10e3 to 6x 10m2) and F (from 10’ to lo3 s-l). For the case p< lo-’ (low trap concentration) these data correlate with conclusion [ 2-51, however at IO-‘
‘%
-1
-2
-3
-4
-’ >
exp( -KHZ)
s
exp( -xt),,/x(4F-x)
’ o [02(4&X)+X](KT-K,-~)dx’
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(2)
Fig. 5. Decay curves for Poly(G)-Tb’+ (A,,,=545 nm, co=5~10-4M)at(0)c~=2~10-5Mand(O)~Tb=2~10~4 M and their approximation by theoretical dependences (2) (- - -) and (-), respectively.
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6 x lo-* the curves calculated by (2) differ from those in the low-concentration regime by no more than 3% to 5%. Thus, it may be considered, that y=p*F is the single variable parameter. Third, if the dimensionless parameter 6= c&co is introduced, the decay of Tb3+ luminescence can only change when 1/25<6< l/2.5. The lower threshold isamenable to trivial explanation: at these values of 6 the Poly (G)Tb3+ complexes have only one Tb3+ ion per polymer, so a further decrease in 6 should not cause changes in the decay curves. This was observed experimentally, when S was decreased to & through increasing the Poly( G) concentration in the solution from 5 x 1Om4M higher at the constant Tb3+ concentration, 2 x lo-’ M (fig. 4, curve (2) ). The upper limit of the decay curve variations at 6= l/2.5 (fig. 4, curve (4) ) is not so obvious and will be discussed below. Then two cases may be considered. (i) There are no intrinsic traps in the polymer (pj=p). Then for 6~ & and for the polymer of a known mean length S, p= 1/S may be found, which permit to determine the jump rate F. For the 4stranded Poly(G) S= 95 links, where every link consists of four GMP molecules (S was found from the average molecular weight of the polymer). If 6> &, p may be found from y resulting from the fitting and F obtained earlier for 6< A. This procedure led to E= 1,2 X lo5 s- ’ and the p values which were obtained growing monotonously with 6 from 1.05~ lo-’ to 2.3~ lo-*. The error in y, F (at 6~ &) and p (at 6> &) was * 30%, being mainly induced by the scattering of data for samples of different experimental series. (ii) The polymer has intrinsic traps (p,
19 February
1993
in the trap is almost three orders of magnitude lower than in the host. We made best fitting of experimental curves (fig. 5) with model curves for the system, when the energy-migration rate KS is independent of time. In this case trap excitation probability nr (t) is found as
x{exp[-(Ks+KH)t]-exp(-KTt)}.
(3)
This approximation was extremely unsatisfactory and showed that the model of three-dimensional isotropic migration, when Ks=const., is inadequate for low-dimensional systems. This agrees with theoretical concepts [l-6] postulating that decreasing dimensionality (d-c 3 ) should lead to the time dependence of the energy-migration rate. The value of F turns out to be unexpectedly low when compared with the jump rate of triplet excitations in molecular crystals, z 1OS- 10’’s- I. On the other hand, F=2.5x lo3 s-’ was obtained for Poly(A) at 77 K [20], though the theoretical estimates are =109 s-’ [21]. Most probably, the presence of energy disordering among the triplet levels of the polymer may be the main factor responsible for the slow migration. Under this condition the applicability of the resonance migration model is limited since the probability of a jump between two molecules is not symmetric and depends on the energy gap between them [ 22 1. The probability of a jump “upwards” will differ from that of a jump “downwards” by a factor of exp( -AE/ kT). Thus, the energy migration would occur simultaneously with relaxation of excitations to the bottom of the continuum of levels. It may therefore be spoken only about the average jump rate, which is likely to be lower than the corresponding value under the energy resonance. A theoretical model was proposed [ 22 1, which describes the migration of excitations under the condition of energy disordering. However, this can hardly be used for approximating experimental data since there is no analysis of onedimensional systems. Spatial disordering between polymer molecules can be the other mechanism resulting in the small value F (this effect was studied in ref. [23] ). But since, from all the polynucleotides Poly(G) has the most 269
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rigid structure, due to four H-bond helices, we helieve that partial stacking breaking would not induce the significant decrease of the exchange interaction. Some discrepancy between theoq and experiment is, to a certain extent, due to the fact that the poly mer length is finite, unlike an infinite one-dimensional chain; besides, there is distribution over the polymer lengths and over the number of ions bound to the polymer. So the energy migration in the polymer can be described most adequately at IB by some model constructed using the Monte Carlo method, which the authors are planning to do the next work. The upper limit at 6= l/2.5 in 6 dependence of the decay curves is interesting biophysically. This leads to the conclusion that a further increase in 6 (more than l/2.5) does not form new traps - Tb” ions bound to the polymer. Otherwise, the decay curve could have gone on changing up to a complete coincidence with the exponent (when each link of the polymer would have been bound to Tb3+ ion - the case similar to the GMP-Tb3+ complex). As the complex Poly(G)-Tb3+ is formed, the Tb3+ ions interact not only with the phosphate groups of the polymer, but also with the bases mainly at the sites of disrupted hydrogen bonds, where the helix is damaged. This finding was confirmed by experiments on Poly (G) and other polynucleotides, DNA, RNA [24] and may be taken as fully ascertained. It is likely that only the Tb 3+ ions bound to the bases (at the helix centre) are the energy traps; the ions bound to the phosphate groups do not participate in trapping excitations since the phosphate residues are at the outer surface of the polymer far from the chromophores (bases). It is therefore reasonable to assume that the existence of the limiting trap concentration at 62 I /2,5 is due to the screening effect of the ions bound to the phosphate groups preventing a further Tb3+ binding to the bases. To conclude, it should be noted that the use of rareearth ions bound to the polynucleotides as “luminescence time” probes is essentially advantageous as compared to the traditional approaches (stationary excitations, polymer phosphorescence decay) to understanding the nature of triplet excitation migration in these polymers, It also offers possibilities of extending studies to more complex biological objects such as DNA and RNA.
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Acknowledgement The authors wish to thank Dr. V.A. Karachevtsev for helpful comments and discussions. References [ 1] M.D. Fayer, in: Spectroscopy and excitation dynamics of condensed molecular systems, eds. V.M. Agranovich and R.M. Hochstrasser (North-Holland, Amsterdam, 1983) p. 185. B.Y.Balagurovand V.G. Vaks,Soviet Phys. JETP 38 (1974) 968. 1B. Movaghar, G.W. Sauer and D. Wurtz, J. Stat. Phys. 27 (1982) 473. V.M. Kenkre, in: Exciton dynamics in molecular crystals and aggregates, ed. G. Hohler (Springer, Berlin, 1982) p. 1. J.K. Anlauf, Phys. Rev. Letters 52 ( 1984) 1845. A.I. Onipko, Phys. Letters A 101 ( 1984) 383. [ 71A.A. Avdeenko, V.V. Eremenko and V.A. Karachevtsev, Soviet Phys. JETP 67 (1988) 1677. [8] W.J. Rodriguez, M.F. Herman and G.L. McPherson, Phys. Rev. B 39 (1989) 13; R. Knochenmuss and H.U. Giidel,J. Chem. Phys. 86 (1987) 1104. [9] R. Kopelman, Phil. Mag. B 56 (1987) 717. [lo] S.E. Webber and P.E. Avots-Avotins, Macromolecules 12 (1979) 708. [ 111J. Guillet, Polymer photophysics and photochemistry (Cambridge Univ. Press, Cambridge, 1985). [ 121I.A. Levitsky and Yu.V. Rubin, Russian I. Chem. Phys., in press; Z. Salmon and H. BBssler,Chem. Phys. 100 (1985) 393. [13 ES. Richardson, Chem. Rev. 82 (1982) 541. [14 S.B. Zimmermann, G.H. Cohen and D.R. Davies, J. Mol. Biol. 92 (1975) 181. [15 V.L. Ermolaev, E.N. Bodunov, E.B. Sveshnikova and T.A. Shakhverdev, Radiativeless transfer of electron excitation energy, Leningrad ( 197I ) (in Russian). [16 K. Helen, in: Electrical, optical and magnetic properties of nucleic acids and components, Vol. 1, ed. J. Duchesne (Academic Press, New York, 1973) p. 130. [17 V. Kleinwlchter, Collection Czech. Chem. Commun. 37 (1972) 2333. (1983) 3221. [18 V.M.KenkreandR.E.Panis,Phys.Rev.B27 [19 G.L. McPherson, M.F. Herman and W.J. Rodriguez, Chem. Phys. Letters 144 (1988) 541. [20] J. Eisinger and R.G. Shulman, Proc. Natl. Acad. Sci. US 55 (1966) 1387. [21] B. Sommer and J. Jortner, J. Chem. Phys. 49 (1968) 3919. [22] B. Movaghar, M. Grunewald, B. Reis, D. Wurtz and H. Blssler, Phys. Rev. B 33 ( 1986) 5545. [23] H. Katayama, S. Maruyama, S. Ito, Y. Tsujii, A. Tsuchida and M. Yamamoto, J. Phys. Chem. 95 ( 1991) 3480. 1241D.S. Gross and H. Simpkins, J. Biol. Chem. 256 (1981) 9593; D.P. Ringer, B.A. Howell and D.E. Kizer, Ann. Biochem. 103 (1980) 3397.