Fluorescence and host-guest energy transfer in polymeric chains

Journal of Luminescence 29 (1984) 309—319 North-Holland, Amsterdam

309

FLUORESCENCE AND HOST-GUEST ENERGY TRANSFER IN POLYMERIC CHAINS V.V. SLOBODYANIK, V.N. YASHCHUK, V, P. NAIDYONOV and V.Ya. POCHINOK 25201 7, Kiev University, Physics Department, Kiev, USSR

Received 8 February 1983

Intrachain singlet electronic excitation energy transfer from host polyvinylcarbazole (PVCa) to guest vinylbenzocarbazole (VBCa) units has been studied through measurements of the quenching effect on PVCa fluorescence and observation of the sensitized guest VBCa emission in solutions at 293 and 77K for different intramolecular concentrations of guest VBCa units. A method of taking into account intrinsic traps of singlet excitons (excimer forming sites, EFS), ordinarily present in an impurity free polymer macromolecule, has been elaborated. The comparison of the experimental data with theory shows that the one-dimensional singlet excitation energy transfer in a polymeric chain involves motion of incoherent excitons randomly hopping to a nearest neighbor along the line under conditions when capturing occurs upon the first visit of a trap and trapping centers of different origin become indistinguishable as regards their capturing capability.

1. Introduction Transfer of excitation energy in polymers in general includes two distinctive stages: transfer along a macrochain and that between different macrochains. The probabilities of both processes may significantly differ and what matters physically the mechanisms of intrachain and interchain energy transfer may be different. Experiments are needed which establish the peculiarities of each stage of the transfer separately and which determine the role of each, under conditions when both processes occur. The excitonic mechanism of electronic excitation energy transfer is claimed to be operative in polymers, both in one dimension [1—3]and in three dimensions [4—6].The quantitative description of intra- and interchain singlet and triplet excitons is still incomplete, the main parameters of the exciton motion so far being lacking. The paper presents experimental data on the quenching of fluorescence in macromolecules by intrachain traps of excitons along with the results on sensitized guest emission which permit to draw some definite conclusions concerning the one-dimensional transfer of singlet excitation in polymeric chains. —

0022-2313/84/$OiOO © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

310 V. V. Slobodyanik et a!.

/

Fluorescence and host — guest energy transfer in polymeric chains

Fig. 1. Absorption of PVCa (1) and PVCa host—VBCa guest system (2). 10” mol t~ solutions in 5. dioxane, T= 293 K, 1.5 % of VBCa in PVCa, M=10

2. Experimental Experiments were carried out with dilute ( i0~ mol t~) solutions of poly-N-vinylcarbazole(PVCa) and PVCa (host units) containing small (0.1—3%) amounts of copolymerized N-vinyl-5H-benzo[b] carbazole (VBCa, guest units) as well as with the solutions of low molecular analogue of the PVCa unit ethylcarbazole (ECa) and copolymers of N-vinylcarbazole (VCa, active units) with octylmethacrylate (OMA, inert units). Sensitized fluorescence of VBCa units due to host—guest excitation transfer has been expected from energetical considerations (the absorption spectra of both the PVCa and the PVCa—VBCa system are shown in fig. 1), its spectral position being resolved from that of PVCa emission, thus permitting separate observation of host (monomer-like) and guest VBCa fluorescence (fig. 2). Molecular masses of PVCa and PVCa—VBCa were of the order of io~.Intramolecular concentrations C~of guest VBCa units in PVCa were determined through UV analysis using the —

~io~, cn~ Fig. 2. Fluorescence of PVCa macromolecules containing 0.5 % of guest VBCa units: iO~’mol t~1 solutions in dioxane, T= 77 K, M=105, A~ = 313 nm, L =io’~photons cm2 s~.

V. V. Slobodyanik et aL

/

Fluorescence and host — guest energy transfer in polymeric chains 311

absorption data for poly-vinyl-5H-benzo[b] carbazole (PVBCa) solutions plus the hypochromism values (taken as 50% for the first electronic transition [7]). Absorption spectra were recorded on a Specord UV-VIS spectrophotometer, the fluorescence ones on an ISP-28 spectrograph, photoelectrically equipped, and a Hitachi MPF-4 spectrofluorometer. The conditions of measurements were standardized and both the reproducibility of the results (— 2%) for a single sample (under separated positioning) and that for different samples of the same concentration were determined. In the latter case the statistical accuracy for the host and guest fluorescence intensity measurements (the mean-square-root deviation for each 7—9 samples) was about 20%. The spectral resolution of host and guest emission at 77K did not introduce any additional error. The separation of the monomer-like and excimer emission in the region of the PVCa fluorescence at 293K, though, was more difficult, due to overlapping of these emissions, so this was done as previously described [8]. The correctness of the results obtained through the comparison of the relative quantum efficiencies (areas of the corresponding bands) was checked through the emission intensity measurements at the fixed wavelengths, representing host and guest fluorescence (A1 = 370 nm for PVCa and X~= 460 nm for VBCa).

3. Results and discussion Singlet excitons created through the absorption of the exciting light by monomer units of a PVCa macromolecule migrate along the macrochain and become trapped by excimer forming sites (EFS) [3,4,6]. The fluorescence of pure PVCa at 293 K, thus, includes the monomer-like (excitonic) and the excimer emission. Additional traps for excitons emerge when guest VBCa units are incorporated into a PVCa macrochain and an additional (guest) emission appears (fig. 2). Fluorescence due to direct excitation of VBCa units may be discarded because of the smallness of their concentration. Host—guest energy transfer in a polymeric chain is thus complicated by the presence of the primary traps (EFS), the concentration of which is unknown, such a situation being common for a variety of polymers. This constitutes one of the main reasons why one-dimensional energy transfer remains so far poorly studied experimentally. VBCa units in the PVCa chain play an active role. They are capable of absorbing the exciting light in the spectral region used and they efficiently capture the excitation energy. Copolymenzation of VCa with inert OMA units, which do not absorb the light (in the same region) and do not capture the excitons either, strongly reduces excimer formation when the content of OMA units becomes high [8]. An appropriate intramolecular concentration of these OMA units creates the conditions when excimer formation becomes negligible,

312 V. V. Slobodyanik et a!. / Fluorescence and host — guest energy transfer in polymeric chains

so that the fluorescence of such a copolymer may be taken for the one-hundred percent monomer-like emission J0 of PVCa macromolecules not quenched by any traps. The fluorescence intensity of an impurity free PVCa (when measured for the same number of VCa units in a solution, as for VCa—OMA copolymer), then, corresponds to conditions under which J0 is quenched through trapping of excitations by EFS of some unknown concentration Ce (emitting the excimeric emission). The incorporation of an additional concentration C~of guest VBCa units into a macrochain leads to further decrease of the monomer-like emission intensity of PVCa and appearance of VBCa emission. The concentration dependence of the effect of VBCa on PVCa fluorescence is presented in table 1, where J0 is the intensity of the monomer-like PVCa emission in the absence of trapping (in the VCa—OMA copolymer), J the intensity of the same emission for PVCa macromolecules containing excimeric or excimeric plus introduced VBCa traps (host fluorescence) and J~ the emission intensity of VBCa units (guest fluorescence). The general problem of one-dimensional random walk of particles and quasiparticles which includes also the migration of excitons in a linear chain was treated theoretically in a variety of works [9—~9}.Most of them do not take into account some important properties of real particles and quasiparticles, such as their finite lifetime even in the absence of traps or absorbing boundaries (as regards singlet excitons, due to radiative or nonradiative decay, transformation into a triplet or CT exciton, loss of energy on charge carrier production, photochemical conversion, etc.), so that the results obtained for these nondecayable particles should be used with reservations. An experimental verification of the theoretical conclusions for the one-dimensional migration, which of course has to be carried out on polymer macromolecules with intrachain traps, encounters additional problems: the necessity of taking into account the presence of excimer forming sites in an impurity free polymer, the difficulties of the incorporation of suitable traps of a known concentration into a chain, etc. This explains why few experimental works have dealt with real one-dimensional energy transfer and why the transfer of excitation to extrachain trapping centers such as impurity molecules contacting or adsorbed on a macromolecule has been preferentially studied. An emission probability in a one-dimensional random walk was theoretically treated in [10,11] and analysed in relevance to experimental results for polyvinyltoluene (PVT) with extrachain traps in [5]. However the monomer-like emission could not be registered due to efficient excimer formation in PVT, so that the test of the theory was based on the quenching of the excimer emission. Thus the present results on the quenching of monomer-like emission of macromolecules by intrachain trapping centers appear to produce data for quantitative description of one-dimensional energy transfer. In order to analyze the data of table I some remarks concerning the treatment given in [5,10,11] should be made. The authors of [10,11] consider

V. V. Slobodyanik et a!.

/

Fluorescence and host

—

guest energy transfer in polymeric chains

313

Table I Host (J) and guest (J,) emission of PVCa—VBCa system * Concentration

J

of guest VBCa units in PVCa

(arb. un.)

C,

P

f/f

0

Symbols in

C, — C1 from

fig. 3(a)

fig. 3(a)

J,

(f0

—

J)

f~+ J,

j

(%)

T= 293 K 0 (VCa-OMA copolymer)

420(J0) I

P0

100

0.24

P1

0.12

96

0.23

P2

0.10

22

0.25

88

0.21

P3

0.26

40.5

8.2

0.50

80

0.19

P4

0.52

95

3.6

126

1.50

54

0.13

P5

1.55

207

1.77

230

2.75

38

0.09

P6

2.75

265

1.43

298

0 (pure PVCa)

T=77K 0 (VCa-OMA copolymer)

0

42 14.7

62 78.5

420(J~)

I

0 (pure PVCa)

340

0.81

P1’

0.12

315

0.75

P~

0.10

63

1.67

69

0.25

285

0.68

P~

0.23

122

1.11

125

0.50

245

0.58

P~

0.47

203

0.86

205

1.50

135

0.32

P

1.50

340

0.84

340

2.75

68

0.16

P~

2.80

486

0.72

486

*

10

The monomer-like emission intensities J0 and J~at 293 and 77 K are normalized to equal values.

nearest-neighbor random walk of an excitation in a linear chain of units, the first and the last of which are absolutely absorbing traps (the capture occurs upon the first visit of a trap) and then treat an infinite chain with the concentration of traps q. The results of both calculations appear to be identical. The total probability for an excitation to emit the light in a normal unit is given [11] by P

=

(1- q)Q(~,q),

(1)

314 V. V. Slobodyanik et a!.

/

Fluorescence and host — guest energy transfer in polymeric chains

where a is the emission probability for a single unit and Q(a, q)

=

1

srnh y

— _____

a=2

(1

2sinh(a q)U_ cosh ya

—

1)7

(2)

with

y=~log 1 + (2a 1—aa2)l~’2 Compared with the experiment this leads to the relationship [5] —

P=J/J

(3)

0,

where J0 and J are the intensities of the emission of normal polymer units in the absence and in the presence of traps, respectively; in other words J0 is the host (monomer-like) emission intensity in the absence of traps, J is the same in the presence of traps. In a one dimensional case an experiment should, of course, be carried out for isolated macromolecules, i.e. for diluted polymer solutions. Since in [11] (1 a) is taken as the probability of the jump to a nearest neighbor along the line, a should signify the total probability for an excitation to decay in a single unit, due to both radiative and different nonradiative (internal conversion, intersystem crossing, charge carrier photogeneration, photochemical reaction, etc.) processes, the latter ordinarily predominating for polymeric substances. Then P should be the total probability for an excitation to decay in any of the normal units. With these corrections eqs. (1)—(3) become valid and 1/a = n (for n>> 1), where n is the average number of steps in a chain, free of traps. In theory [10,11] any excitation stepping on a trapping unit cannot regenerate or hop across the trap, so the trapping centers of different origin appear to be indistinguishable. This makes the traps (EFS), primordially present in an impurity free polymer, and those, deliberately incorporated into a macrochain, identical, as regards their capturing capability. Then the four limiting expressions of modified eq.(3), given in [5], which take into account EFS (of concentration Ce) and the introduced trapping centers (of the concentration Ce), should be replaced by only two limiting formulas, featuring the quenching conditions for the total trap concentration C = Ce + C1. Hence (for the two fixed temperatures) combining eqs. (1) and (3) gives the probability for an excitation to decay in one of the normal units equal to —

P=J/Jo=(1—Ce—Ct)Q(a, Ce+Cj),

for293K

(4)

P=J/Jo—~(1—Ce’—Ct)Q(a,Ce’+Ct),

for77K

(5)

since the effective concentrations Ce and Ce’ of excimer forming sites at 293 and 77 K differ. Eqs. (4) and (5) give the theoretical formulas to be checked through

V. V. Slobodyanik et al.

/

Fluorescence and host — guest energy transfer in polymeric chains 315

experimental data on the host fluorescence intensities J0, J and J0’, J’ measured at 293 and 77K, respectively. The data on the excimer J~and guest J1 fluorescence intensities, within that same theoretical model, should fit eqs. (6)—(9) which are derived in the Appendix: (Jo~1)/1e/$e+(/3/$e)Ct/Ce

(6)

(~1o)//$t+($/I3t)Ce/Ct for293K

(7)

(J~ J’)/J~=

(8)

—

(J~ J’)/J’ —

$‘/f3~

+ (/3’//3~)C~/c

= $‘/f~~’ +

(/3’/f31’)ç/C,

for 77K.

(9)

Here /3, $e’ /3~ and /3’, /3~’, f~ denote the quantum yields of monomerlike, excimer and trap emission at direct excitation for 293 and 77K, respectively. In eqs. (4) and (5) the value of Q is determined by eq. (2) with Ce + C~and Ce’ + C1, substituted for q, respectively. Since excimer formation strongly reduces upon lowering the temperature down to 77K [20,21], C~ Ce. Eqs. (4), (5) and (6)—(9) permit independent verification of the theory through the experimental data on host and guest fluorescence, taken separately. In the first case all the values P1 (see table 1) measured at a given temperature, are marked on a theoretical curve P(C) calculated for a given value of n (n = 1/a) (C is the total concentration of trapping centers) and the succession of intervals on abscissa C,. C1 (i> 1) is compared with the succession of known concentrations C1. If C, C1 values turn out to be consecutively equal (see table 1) to known concentrations C~of guest units in a macrochain then such a theoretical curve is needed and C1 gives the wanted value of the concentration of excimer forming sites in an impurity free polymer macromolecule. In fig. 3(a) the solid line shows the proper theoretical curve (calculated for a = 1/3500) (experimental points F, for 293 K from table 1 are indicated with circles) which gives for 293 K n = 3500 and Ce = C1 = 2.25%. The intensities of both host and guest fluorescence alter upon going from 293 to 77 K, nevertheless, if the character of the excitation migration does not change in this temperature interval, then the values P1 for 77 K (see table 1) should lie in the analogous manner on the same curve P(C). Since Ce’ ~ Ce and P~= P0 = 1, all the succession of points P,’ is expected to be left shifted relative to that of P1. This is confirmed experimentally, so that the data for F,’ at 77 K (triangles in fig. 3(a)) again fit the same theoretical dependence P(C = q) (n = 3500), which gives Ce’ = C~= 0.22% for T= 77 K. Now the data on guest fluorescence can be analyzed independently. Eqs. (6) and (8) present linear dependences on the guest concentration C1, the segments cut-off on the ordinate and abscissa being equal to $/$e, Ce and $‘/$e’, C~, respectively; eqs. (7) and (9) express linear dependences on the inverse of C1, the segments cut-off on the axes being equal to $//3~,1/Ce and $‘/$~, 1/Ce’, respectively. This linearity is verified experimentally (fig. 4) for the data on the ‘~

—

—

316 V. V. Slobodyanik et al.

/

Fluorescence and host

—

guest energy transfer in polymeric chains

p YOPPO

YET6

Fig. 3. (a) Quenching of host PVCa fluorescence by excimer forming traps (concentration C~)and guest VBCa traps (concentration C,) at 293 (circles) and 77 K (triangles) P = J/JØ, C = C~+ C,. The solid line is the theoretical dependence P(C) for n = 3500. (b) The probability of the decay of singlet excitations in traps P, at 293 (circles) and 77 K (triangles) through data on sensitized guest emission. The solid lines represent theoretical dependences P,(C) for n = 3000 and 3500.

guest fluorescence intensity J~,presented in table 1, which gives the set of values: C~= 2.5 %, Ce’ = 0.2 %, /3//3~= 0.75, $‘/$~‘= 0.65. The relative intensity of the excimer emission, due to the smallness of the quantum yield ~ is low in doped samples which makes the data on J,~for

1/(C~9

Fig. 4. Sensitized guest VBCa emission in coordinates (J0 — ~ versus 1/C, at 293 (1) and 77 K (2). Segments cut-off on the ordinate are equal to (;/$y1 and (/3/$~)_1, respectively; those cut-off on the abscissa are l/C~and l/C,.

V V. Slobodyanik et al.

/

Fluorescence and host—guest energy transfer in polymeric chains 317

different C1 not reliable. However, for pure PVCa (C1 = 0) (J0 J)/Je = = F, so using the experimental values for F = 7.5 at 293 K and F 8 at 77 K we derive /~/$e= 7.5 and $‘/$e’ 8 for these two temperatures. Since the total probability for an excitation to decay in one of the guest units (see the Appendix) is equal to —

1,

(10)

Pt{[(/3e//3)Ce/(Ce+Ct)+($t/$)C1/(Ce+Ct)1J/(Je~1~1}

and P 1

1 P, so using the derived values (first Ce = 2.5%, /3//~~= 0.75, = 7.5 and then Ce’ = 0.2%, /3’//3~= 0.65, /~‘//~e’ 8) we obtain through eq. (10) experimental points for P~at 293 and 77 K which fit the theoretical dependences P1(C) calculated for n = 3000 and 3500, respectively (circles and triangles in fig. 3(b)). In the second calculation the use of the approximate value for $‘/$e’ does not lead to any significant error since in this case (at 77 K) the term with the above value in the sum of eq. (10) may be omitted due to its relative smallness. It should be noted that the manner of plotting the points onto the graph in fig. 3(b) differs from that used in fig. 3(a): here the knowledge of Ce(C~~) value gives the complete set of C = Ce + C1 (C’ = Ce’ + C1) values and thus of P1 (P1’) values, scattered around the theoretical curve. Usual scattering of points around the theoretical curve is missing in fig. 3(a), the experimental inaccuracy being materialized this time, due to a specific manner of plotting described above, in the deviations of the found from fig. 3(a) C1 and Ce values (for 293 and 77 K). On the whole the nearness of Ce, Ce’ and n values found from figs. 4 and 3(b) to those obtained from fig. 3(a) leads to the conclusion that all the data on the guest fluorescence independently corroborate the theoretical model of the host—guest energy transfer deduced from the host fluorescence quenching experiments. =

—

4. Conclusions The consistency of the experiment with the theory observed for a wide range of experimental data (high and low temperatures, efficient and weak excimer formation, results of the quenching of host fluorescence and sensitization of guest emission) suggests that the general picture of one-dimensional excitation migration underlying the theory [10,11] is rational. This picture of incoherent exciton motion involves a nearest-neighbor random walk of an excitation in a linear chain under conditions when the traps of different origin are indistinguishable, as regards their capturing capability, and the capture of an excitation occurs on the first visit of a trap. Such an indistinguishability of traps of different type may be a specific feature of one-dimensional energy

318 V. V. Slobodyanik et a!.

/

Fluorescence and host — guest energy transfer in polymeric chains

transfer or reflect the behavior of traps sufficiently deep in energy (— 1000 cm’ for excimer forming sites [6], 2000 cm’ for nitrated units in PVCa [22], 3700 cm~ for guest VBCa units studied in this work). The main quantitative characteristic of the exciton motion under these conditions is obtained as the average number n of steps of an excitation in a macromolecule, free of traps, and its value as determined from measurements on the host PVCa—guest VBCa system is found to be 3500 ±500. This value should be compared to n = 3000 ±300 determined from the quenching of PVCa fluorescence by nitrated units reported in [22]. The values of Ce = 2.3 ±0.3 %, the concentration of excimer forming sites at 293 K, and Ce’ = 0.20 % ±0.05 % at 77 K are near to those obtained in [22] (3 and 0.25 %, respectively) and to ç = 4—7 % at 293 K found in [3] from photochemical data, and reflect high efficiency of excimer formation in PVCa at elevated temperatures. Lowering the temperature and going to a rigid solution strongly reduces excimer formation and decreases the effective concentration of EFS. Analogously, the transition from the solution to a solid state of a polymer also reduces Ce: the value reported for polymer films give for iO~ for PVCa at 293 K [4] and 6 x iO~ for PVT at 293 K [5], but those were calculated on the basis of a three-dimensional isotropic energy migration. The predominance of the excimer emission resulting from the transition to the solid state, then, is provided by the rise of interchain energy transfer in addition to intrachain energy transfer. —

5. Appendix Designating the probability that the random walk of an excitation terminates by decay in a host unit or in a trap as P and F1, in the absence of any regeneration of an exciton from the trap or hopping across the trap, we obtain the monomer-like host, excimer and guest emission intensities, respectively, as J=R/3P,

(Al)

JeR$ePtCe/(Ce+ ç), = R/91P1ç/(C~+ C1),

(A2) (A3)

where R is some constant, Ce, C~are the concentrations of excimer forming sites and guest traps, /3, $~, /3, are the quantum yields of emission from a host unit, excimer and guest trap at their direct excitation, respectively. Taking into account that the emission intensity of the host with no traps J0 = R/3, P = J/J0, P~= 1 F, and combining eqs. (A1)—(A3), we find the expressions (A4)—(A6) given in eqs. (6)—(10) of the text: (J0 —J)/J~= 1/13: +(l//3)(Ct/Ce), (A4) —

(AS)

V. V. Slobodyanik et a!.

/

Fluorescence and host — guest energy transfer in polymeric chains 319

(A6) Here $:=$e/13,$~=13t/13,Ce*=Ce/(Ce+Ct),Ct*=Ct/(Ce+Ct).

Acknowledgments The authors are grateful to V.P.. Vorob’yov for the opportunity to fulfil some of the measurements and to S.Ya. Shevchenko for computer calculations of the theoretical curves.

References [1] N.F. Pasch and S.E. Webber, Macromolecules 11(1978) 727. [2] I. McInally, R.F. Reid, H. Rutherford and I. Soutar, Eur. Polym. J. 15 (1979) 723. [31AN. Faidysh, V.V. Slobodyanik and V.N. Yashchuk, J. Lumin. 21(1979) 85. 141 W. Klopffer, J. Chem. Phys. 50 (1969) 2337. [5] R.C. Powell, J. Chem. Phys. 55 (1971) 1871. [6) W. Klopffer, Annals N.Y. Acad. Sci. 366 (1981) 373. [71A. Itaya, K. Okamoto and S. Kusabayashi, Bull. Chem. Soc. Jpn 51(1978) 79. [8] V.V. Slobodyanik, V.P. Naidyonov, V.Ya. Pochinok and V.N. Yashchuk, Chem. Phys. Lett. 81(1981) 582. [91S. Chandrasekhar, Rev. Mod. Phys. 15 (1943) 1. [10] H.B. Rosenstock, SIAM J. AppI. Math. 9 (1961) 169. [Ill N.Levinson, SIAM J. AppI. Math. 10 (1962) 442. [12] R. Bersohn and I. Isenberg, J. Chem. Phys. 40 (1964) 3175. [13] E.W. Montroll, J. Math. Phys. 10 (1969) 753. [14] E.W. Montroll, J. Phys. Soc. Jpn Suppl. 26 (1969) 6. [15] G.H. Weiss and R.J. Rubin, J. Stat. Phys. 14 (1976) 333. [16] K.E. Shuler, Physica 95A (1979) 12. [17] B. Movaghar and G.W. Sauer, J. Phys. C 13 (1980) 4933. [181 A. Blumen and G. Zumofen, Chem. Phys.Lett. 70 (1980) 387. [19) M. Muthukumar and R.I. Cukier, J. Stat. Phys. 26 (1981) 453. [20] C. David, M. Piens and G. Geuskens, Eur. Polym. J. 8 (1972) 1291. [21] AN. Faidysh, V.V. Slobodyanik and V.N. Yashchuk, Izv. ANSSSR (ser. fiz.) 42 (1978) 318. [22] V.V. Slobodyanik, AN. Faidysh, V.N. Yashchuk and L.N. Fyodorova, Zhurnal Prikladnoi Spektroskopii (USSR) 36 (1982) 309.