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Highly mobile electrons and holes on polyfluorene chains formed by charge scavenging in pulse-irradiated trans-decalin solutions
ARTICLE IN PRESS
Radiation Physics and Chemistry 74 (2005) 234–238 www.elsevier.com/locate/radphyschem
Highly mobile electrons and holes on polyfluorene chains formed by charge scavenging in pulse-irradiated trans-decalin solutions Ferdinand C. Grozema, John M. Warman Radiation Chemistry Department, IRI, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands
Abstract The microwave conductivity of trans-decalin solutions of the conjugated polymer poly(2,7-(9,9-bis(2-ethylhexyl)fluorene), PEHF, increases dramatically over tens to hundreds of nanoseconds subsequent to 5 ns pulsed irradiation. This is attributed to the reaction of the primary solvent electrons and holes with the polymer to form even more highly mobile electrons and holes on the polymer chains. Lower limits to the one-dimensional mobilities of electrons and holes on the polymer chains of ca 0.5 cm2/Vs are derived. Evidence is found for electron transfer to the polymer from CO 2. r 2005 Elsevier Ltd. All rights reserved.
1. Introduction The electronic properties of conjugated polymers have received considerable attention in recent years because of their potential application in molecular (opto)electronic devices such as light emitting diodes, solar cells and field effect transistors. A fundamental understanding of the factors controlling charge transport in these materials is still in its early stages despite an intense research effort using many different experimental and theoretical approaches. We have shown how pulseradiolysis combined with optical or microwave conductivity detection techniques can provide information on the nature of charge carriers on isolated polymer chains which is relevant to furthering this understanding (Candeias et al., 2001, 2002; Fratiloiu et al., 2004; Grozema et al., 2002a, b, 2003; Hoofman et al., 1998; Prins et al., 2005). The results obtained are also of special interest to the radiation physics and chemistry community as they provide information on the kinetics Corresponding author.
E-mail address: [email protected] (F.C. Grozema).
of the reactions of primary radiolytic charge carriers with polymeric materials and on the subsequent decay kinetics of polymeric radical anion and cation species. In this paper we present a pulse-radiolysis timeresolved microwave conductivity study of dilute solutions of a conjugated, fluorene-based polymer in the saturated hydrocarbon solvent trans-decalin. The results illustrate the after-pulse growth in the microwave conductivity of such solutions as the primary solvent electrons and holes react with the polymer to form highly mobile, long-lived charge carriers on the isolated polymer chains.
2. Experimental The molecular structure of the highly soluble conjugated polymer poly(2,7-(9,9-bis(2-ethylhexyl)fluorene) (‘‘PEHF’’) studied in the present work is shown in Fig. 1. The number-averaged molecular weight of the polymer, M n , was 147 kD (polydispersity 1.78) which corresponds to an average of 380 monomer units per chain. The concentration of the polymer is given in
0969-806X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2005.04.017
ARTICLE IN PRESS F.C. Grozema, J.M. Warman / Radiation Physics and Chemistry 74 (2005) 234–238
235
30x10-9 25 ∆σ/Dv (Sm2/J)
n
20 15 10 5
Fig. 1. The molecular structure of the conjugated polymer, ‘‘PEHF’’, studied in the present work.
0
0
20
40
60
80x10-9
Time (s)
terms of the moles of monomer units per litre. The saturated hydrocarbon solvent trans-decalin (Fluka, purum 499%) was purified using a Fischer ‘‘Spaltrohr HMS 500’’ distillation column and was stored in a bulb under vacuum over Na/K alloy. Solutions containing in addition CO2 or NH3 were prepared on a vacuum line by transferring a known aliquot of the gas to the cell containing the degassed polymer solution. The CO2 and NH3 concentrations of 0.0061 and 0.0073 mol/L, respectively were calculated using the known volumes and Ostwald coefficients for trans-decalin of 1.24 and 1.98, respectively (Horsman-Van den Dool and Warman, 1981). The solutions were contained in a microwave cavity of internal dimensions 3:5 7:1 24 mm3 and were irradiated with a 5 ns pulse of 3 MeV electrons from a Van de Graaff accelerator. The energy deposited per unit volume per pulse, Dv , was measured accurately for each experiment and was ca. 2 104 J=m3 . The change in conductivity of the solution was monitored as the decrease in microwave power reflected by the cavity over the frequency range 26–38 GHz. This was converted to the conductivity change, Ds, using procedures which have been described in detail in previous publications (Grozema, 2003; Infelta et al., 1977; Warman and De Haas, 1991). Transient changes could be measured using a linear timebase with a response time of ca. 1 ns or on a logarithmic timebase with a response time of ca. 10 ns. In the latter case transients could be recorded from 108 to 103 s using a single accelerator pulse. The transients are presented as the dose-normalized conductivity change, Ds=Dv .
3. Results and discussion In Fig. 2 are shown microwave conductivity transients observed on nanosecond pulsed-irradiation of deaerated trans-decalin without additives and with added PEHF (ca. 1 mM monomer concentration). The dramatic
Fig. 2. The dose-normalized transient change in the microwave conductivity on 5 ns pulsed-irrradiation of degassed transdecalin with no additives [lower dashed trace] and containing ca. 1 mM (monomer concentration) PEHF [upper full trace].
difference on addition of the polymer is immediately apparent with a substantial decay for the solvent alone and a marked after-pulse growth of the conductivity for the polymer solution. These observations will be discussed separately below. 3.1. Trans-decalin alone The transient observed for the solvent alone (denoted RH in subsequent equations) is due to the formation of mobile electrons, e, and holes, RH+, RH ! RHþ þ e :
(A)
The dose-normalized end-of-pulse conductivity expected due to the formation of free ion pairs is given by Dseop =Dv ¼ eN A gfi ðm½e þ m½RHþ Þ ½Sm2 =J.
(1)
In (1), e is the elementary charge (1:60 1019 C), N A is Avogadro’s constant (6.02 1023 molecules/mol) and gfi is the yield of free ions which for trans-decalin at room temperature is 1:25 108 mol=J (corresponding to G fi ¼ 0:12 molecules per 100 eV) (Warman et al., 1977). The room-temperature mobilities of electrons and holes in trans-decalin have been determined to be m½e ¼ 13 103 cm2 =Vs and m½RHþ ¼ 9 103 cm2 =Vs (Warman, 1982; Warman et al., 1977), both of which are considerably larger than the average mobility of 0:26 103 determined for ions diffusing by molecular displacement (Warman, 1982; Warman et al., 1977). Taking the above values for the yield and mobilities gives Dseop =Dv ¼ 2:7 109 Sm2 =J. The value of Dseop =Dv found by extrapolation of the slowly decaying component of the conductivity to the end of the pulse for pure decalin in Fig. 2 is 3:3 109 Sm2 =J,
ARTICLE IN PRESS F.C. Grozema, J.M. Warman / Radiation Physics and Chemistry 74 (2005) 234–238
in reasonably good agreement with the expected value based on previous measurements. The slightly larger value found and the initial rapidly decaying spike in the transient are attributed to a contribution from geminately recombining, coulomb-correlated ion pairs, as discussed previously (Warman et al., 1977). The relatively slow decay of the conductivity over tens to hundreds of nanoseconds following the pulse is due to a combination of homogeneous charge recombination, reaction (B), and/or reaction of the primary charge carriers with spurious impurities, I, present, reactions (C) and (D), to form lower mobility molecular ions. e þ RHþ ! RH;
(B)
e þ Ie ! Ie ;
(C)
RHþ þ Ih ! RH þ Ih þ :
(D)
The homogeneous recombination of free ions in hydrocarbon liquids has been found to be diffusion controlled with a rate coefficient, kr , given by the Debye expression. kr ¼ eSm=0 r
(2) 12
with 0 the permittivity of vacuum (8:85 10 F=m) and r the relative dielectric constant (2.2 for transdecalin). Under these conditions the half-life of the conductivity is given by (Warman, 1982) t1=2 ¼ 0 r =Dseop .
(3) 9
2
From the end-of-pulse value of 3:3 10 Sm =J for Dseop =Dv and the dose of 2 104 J=m3 , a half-life towards recombination via reaction (B) of ca. 300 ns is estimated. This is considerably longer than the value of ca. 50 ns observed. It is apparent therefore that for this batch of solvent the decay of the conductivity is mainly controlled by reaction of the mobile charge carriers with spurious impurities; reactions (C) and/or (D). Rate constants for both electron and hole scavenging in trans-decalin on the order of 1011 M1 s1 have been determined so that impurity concentrations of only ca. 104 M or 10 ppm would be sufficient to explain the lifetime observed.
backbone. e þ P ! P ;
(E)
RHþ þ P ! Pþ þ RH:
(F)
The after-pulse increase in the conductivity of the polymer solution is further illustrated by the transient shown on a logarithmic timescale in Fig. 3. The growth occurs over more than 100 ns and eventually reaches a plateau which is an order of magnitude larger than the end-of-pulse value. From the observation that the conductivity of the polymer solution at long times actually considerably exceeds that of the end-of pulse value, we can conclude that the sum of the mobilities of charge carriers on the polymer chains is much larger than the value of Sm ¼ 0:021 cm2 =Vs for the primary electrons and holes in the solvent. Limits to the actual values of the mobilities on the polymer chains will be discussed in a subsequent section. 3.3. The effect of added NH3 and CO2 In order to determine the relative contributions of electrons and holes to the conductivity in the polymer solution we have carried out experiments in which a second solute was added which was capable of selectively reacting with one of the primary charge carriers thus preventing its rapid diffusion to and reaction with the polymer chains while leaving the other primary carrier unaffected. Ammonia has been found to react with holes in transdecalin with a rate constant of 5 1010 M1 s1 but is
40x10-9
30 ∆σ/Dv (Sm2/J)
236
20
10
3.2. The polymer solution The after-pulse growth of the conductivity shown in Fig. 2 for the polymer solution has been observed previously for solutions of conjugated polymers in benzene (Grozema et al., 2002a, b, 2003; Hoofman et al., 1998). As in the former work, this is attributed to the transfer of charge to the polymer, P, resulting in the formation of highly mobile, long-lived electrons and/or holes (radical ion sites) on the conjugated polymer
10-9
10-8
10-7
10-6
10-5
10-4
10-3
Time (s)
Fig. 3. Microwave conductivity transients on 5 ns pulsedirrradiation of trans-decalin containing ca. 1 mM (monomer concentration) PEHF with no second additive (upper full trace), with 6.1 mM CO2 (middle dotted trace), and with 7.3 mM NH3 (lowest dashed trace). Note the logarithmic timescale.
ARTICLE IN PRESS F.C. Grozema, J.M. Warman / Radiation Physics and Chemistry 74 (2005) 234–238
unreactive towards electrons (Warman, 1982; Warman et al., 1977). RHþ þ NH3 ! R þNH4 þ :
(G)
Addition of NH3 at a concentration of ca. 102 mol/L should therefore result in the conversion of solvent holes to relatively unreactive ammonium ions within a few nanoseconds. As can be seen in Fig. 3, addition of ammonia to the polymer solution does considerable decrease the afterpulse conductivity. However, a substantial growth over hundreds of nanoseconds still remains which reaches a plateau value close to half of that attained in the absence of NH3. This is attributed to the reaction of the unscavenged electrons with the polymer chains. The inverse scavenging experiment was carried out by adding ca. 102 mol/L carbon dioxide in order to selectively scavenge electrons leaving solvent holes unaffected. e þ CO2 ! CO2 :
(H)
As expected, the after-pulse growth in conductivity over the first few hundred nanoseconds was similar in magnitude to that found for the NH3 solution and close to a factor of 2 lower than in the absence of CO2. Interestingly, the CO2 scavenged solution shows evidence of a second, delayed growth in the conductivity on a timescale of microseconds. We attribute this to the relatively slow formation of P via electron transfer from CO 2. CO2 þ P ! P þ CO2 :
(I)
The relatively slow growth via reaction (I) is in accordance with the factor of ca. 40 lower mobility of the molecular ion CO 2 compared with that of the primary electrons in this solvent. From equilibrium electron attachment studies, the electron affinity of CO2 in the closely-related solvent cyclohexane has been determined to be 0.7 eV referred to vacuum (Holroyd, 2004). This is smaller than the value of 0.88 eV calculated for a single fluorene molecule based on the value of 0.96 eV measured for the electron affinity of biphenyl; (Holroyd, 2004) and the 0.08 eV difference between the reduction potential of fluorene and that of biphenyl (Mann and Barnes, 1970). Since the electron affinity of the present polymeric derivative of fluorene would be expected to be even greater than that of a single fluorene molecule, reaction (I) should be energetically favorable. The decay of the conductivity on a timescale of microseconds or longer, shown in Fig. 3, results from eventual charge recombination and/or trapping reactions of P+ and P. The nature and rate coefficients of these processes, together with the kinetics of the initial growth will be discussed in a future publication after a full kinetic analysis of the data has been carried out.
237
3.4. Mobilities of charge on the polymer chains If all of the freely diffusing electrons or holes were scavenged by the polymer chains and no recombination or trapping had occurred, then the eventual dosenormalized plateau conductivities attained after the initial growth, Dspl , would be given, for the NH3 and CO2 solutions, by, Dspl ½NH3 =Dv 2 ¼ eN A gfi ðm½e =P1D =3 þ m½NHþ 4 Þ ½Sm =J,
ð4Þ
Dspl ½CO2 =Dv ¼ eN A gfi ðm½hþ =P1D =3 þ m½CO 2 Þ
½Sm2 =J.
ð5Þ
+
In (4) and (5), m[e /P]1D and m[h /P]1D are the onedimensional (1D) mobilities of electrons and holes respectively along the polymer backbone and the factor of 3 is included to take into account the random, istotropic orientation of the polymer chains. Because of the presence of electron and/or hole scavenging impurities in the solvent, as mentioned above, only partial scavenging by the polymer can be assumed. Only lower limits to m[e/P]1D and m[h+/P]1D can therefore be derived from the present data, i.e. m½e =P1D X3fDspl ½NH3 =Dv g=eN A gfi ,
(6)
m½hþ =P1D X3fDspl ½CO2 =Dv g=eN A gfi .
(7)
In (6) and (7) the reasonable assumption is made that the mobilities of NH+ 4 and CO2 are negligibly small. + The values of m[e /P]1D and m[h /P]1D derived from the results shown in Fig. 3 are 0.44 and 0.52 cm2/Vs, respectively. This lower limit to the value of m[h+/P]1D is somewhat smaller than the value of 0.74 cm2/Vs determined previously for PEHF in benzene (Grozema et al., 2002a). Experiments are underway using a freshly purified sample of trans-decalin. Preliminary results indicate that the actual value of m[h+/P]1D could in fact be as much as a factor of 2–3 larger than the benzene value. These results together with a full kinetic fitting procedure will be presented in a future publication.
4. Conclusions Subsequent to nanosecond pulsed irradiation of transdecalin solutions of the conjugated polymer poly(2,7(9,9-bis(2-ethylhexyl)fluorene), PEHF, the primary electrons and holes diffuse to and react with the polymer chains to form even higher mobility electrons and holes (radical ion sites) on the polymer backbone. This is observed, using the time-resolved microwave conductivity technique (TRMC), as a large after-pulse growth in the conductivity. By using competitive hole (NH3) and
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electron (CO2) scavengers lower limits to the 1D mobilities of electrons and holes on the polymer chains are determined to be 0.44 and 0.52 cm2/Vs, respectively. A second, delayed growth in conductivity found for the CO2 solution is attributed to electron transfer from CO 2 to the polymer. The transient conductivity of the polymer solutions eventually decays on a timescale of microseconds or more due to charge recombination and/ or trapping.
References Candeias, L.P., Padmanaban, G., Ramakrishnan, S., 2001. The effect of broken conjugation on the optical absorption spectra of the triplet states of isolated chains of poly (phenylene vinylene)s. Chem. Phys. Lett. 349, 394–398. Candeias, L.P., Grozema, F.C., Padmanaban, G., Ramakrishnan, S., Siebbeles, L.D.A., Warman, J.M., 2002. Positive charge carriers on isolated chains of MEH-PPV with broken conjugation: optical absorption and mobility. J. Phys. Chem. B 107, 1554–1558. Fratiloiu, S., Candeias, L.P., Grozema, F.C., Wildeman, J., Siebbeles, L.D.A., 2004. VIS/NIR absorption measurements of positively charged oligo(phenylenevinylene)s and comparison with time-dependent DFT calculations. J. Phys. Chem. B 108, 19967–19975. Grozema, F.C., 2003. Opto-electronic properties of conjugated molecular wires. Thesis, Delft University of Technology, ISBN 90-407-2422-9. Grozema, F.C., Candeias, L.P., Swart, M., van Duijnen, P.T., Wildeman, J., Hadziioannou, G., Siebbeles, L.D.A., Warman, J.M., 2002a. Theoretical and experimental studies of the opto-electronic properties of positively charged oligo(phenylene vinylene)s: effects of chain length and alkoxy substitution. J. Chem. Phys. 117, 11366–11378. Grozema, F.C., Siebbeles, L.D.A., Warman, J.M., Seki, S., Tagawa, S., Scherf, U., 2002b. Hole conduction along
molecular wires: sigma-bonded silicon versus pi-bondconjugated carbon. Adv. Mater. 14, 228–231. Grozema, F.C., Hoofman, R.J.O.M., Candeias, L.P., de Haas, M.P., Warman, J.M., Siebbeles, L.D.A., 2003. The formation and recombination kinetics of positively charged poly(phenylene vinylene) chains in pulse-irradiated dilute solutions. J. Phys. Chem. A 107, 5976–5986. Holroyd, R., 2004. In: Mozumder, A., Hatano, Y. (Eds.), Charged Particle and Photon Interactions with Matter. Marcel Dekker, New York, p. 175 (Chapter 8). Hoofman, R.J.O.M., de Haas, M.P., Siebbeles, L.D.A., Warman, J.M., 1998. Highly mobile electrons and holes on isolated chains of the semiconducting polymer poly(phenylenevinylene). Nature 392, 54–56. Horsman-Van den Dool, L.E.W., Warman, J.M., 1981. Interfaculty Institute Report no. 134-81-01, Delft, The Netherlands. Infelta, P.P., de Haas, M.P., Warman, J.M., 1977. The study of the transient conductivity of pulse irradiated dielectric liquids on a nanosecond timescale using microwaves. Radiat. Phys. Chem. 10, 353–365. Mann, C.K., Barnes, K.K., 1970. Electrochemical Reactions in Nonaqueous Solution. Marcel Dekker, New York, pp. 560. Prins, P., Candeias, L.P., Van Breemen, A.J.J.M., Sweelssen, J., Herwig, P.T., Schoo, H.F.M., Siebbeles, L.D.A., 2005. Electron and hole dynamics on isolated chains of a solution processible poly(thienylene vinylene) derivative in dilute solution. Adv. Mater. 17, 718–723. Warman, J.M., 1982. The microwave absorption technique for studying ion and ionic processes. In: Baxendale, J.H., Busi, F. (Eds.), The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis. D. Reidel Publishing Company, Dordrecht, pp. 129–161. Warman, J.M., De Haas, M.P., 1991. Time-resolved conductivity techniques, DC to microwave. In: Tabata, Y. (Ed.), Pulse Radiolysis. CRC Press, Boston, pp. 101–131 (Chapter 6). Warman, J.M., Infelta, P.P., de Haas, M.P., Hummel, A., 1977. Can. J. Chem. 55, 2249–2257.
Radiation Physics and Chemistry 74 (2005) 234–238 www.elsevier.com/locate/radphyschem
Highly mobile electrons and holes on polyfluorene chains formed by charge scavenging in pulse-irradiated trans-decalin solutions Ferdinand C. Grozema, John M. Warman Radiation Chemistry Department, IRI, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands
Abstract The microwave conductivity of trans-decalin solutions of the conjugated polymer poly(2,7-(9,9-bis(2-ethylhexyl)fluorene), PEHF, increases dramatically over tens to hundreds of nanoseconds subsequent to 5 ns pulsed irradiation. This is attributed to the reaction of the primary solvent electrons and holes with the polymer to form even more highly mobile electrons and holes on the polymer chains. Lower limits to the one-dimensional mobilities of electrons and holes on the polymer chains of ca 0.5 cm2/Vs are derived. Evidence is found for electron transfer to the polymer from CO 2. r 2005 Elsevier Ltd. All rights reserved.
1. Introduction The electronic properties of conjugated polymers have received considerable attention in recent years because of their potential application in molecular (opto)electronic devices such as light emitting diodes, solar cells and field effect transistors. A fundamental understanding of the factors controlling charge transport in these materials is still in its early stages despite an intense research effort using many different experimental and theoretical approaches. We have shown how pulseradiolysis combined with optical or microwave conductivity detection techniques can provide information on the nature of charge carriers on isolated polymer chains which is relevant to furthering this understanding (Candeias et al., 2001, 2002; Fratiloiu et al., 2004; Grozema et al., 2002a, b, 2003; Hoofman et al., 1998; Prins et al., 2005). The results obtained are also of special interest to the radiation physics and chemistry community as they provide information on the kinetics Corresponding author.
E-mail address: [email protected] (F.C. Grozema).
of the reactions of primary radiolytic charge carriers with polymeric materials and on the subsequent decay kinetics of polymeric radical anion and cation species. In this paper we present a pulse-radiolysis timeresolved microwave conductivity study of dilute solutions of a conjugated, fluorene-based polymer in the saturated hydrocarbon solvent trans-decalin. The results illustrate the after-pulse growth in the microwave conductivity of such solutions as the primary solvent electrons and holes react with the polymer to form highly mobile, long-lived charge carriers on the isolated polymer chains.
2. Experimental The molecular structure of the highly soluble conjugated polymer poly(2,7-(9,9-bis(2-ethylhexyl)fluorene) (‘‘PEHF’’) studied in the present work is shown in Fig. 1. The number-averaged molecular weight of the polymer, M n , was 147 kD (polydispersity 1.78) which corresponds to an average of 380 monomer units per chain. The concentration of the polymer is given in
0969-806X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2005.04.017
ARTICLE IN PRESS F.C. Grozema, J.M. Warman / Radiation Physics and Chemistry 74 (2005) 234–238
235
30x10-9 25 ∆σ/Dv (Sm2/J)
n
20 15 10 5
Fig. 1. The molecular structure of the conjugated polymer, ‘‘PEHF’’, studied in the present work.
0
0
20
40
60
80x10-9
Time (s)
terms of the moles of monomer units per litre. The saturated hydrocarbon solvent trans-decalin (Fluka, purum 499%) was purified using a Fischer ‘‘Spaltrohr HMS 500’’ distillation column and was stored in a bulb under vacuum over Na/K alloy. Solutions containing in addition CO2 or NH3 were prepared on a vacuum line by transferring a known aliquot of the gas to the cell containing the degassed polymer solution. The CO2 and NH3 concentrations of 0.0061 and 0.0073 mol/L, respectively were calculated using the known volumes and Ostwald coefficients for trans-decalin of 1.24 and 1.98, respectively (Horsman-Van den Dool and Warman, 1981). The solutions were contained in a microwave cavity of internal dimensions 3:5 7:1 24 mm3 and were irradiated with a 5 ns pulse of 3 MeV electrons from a Van de Graaff accelerator. The energy deposited per unit volume per pulse, Dv , was measured accurately for each experiment and was ca. 2 104 J=m3 . The change in conductivity of the solution was monitored as the decrease in microwave power reflected by the cavity over the frequency range 26–38 GHz. This was converted to the conductivity change, Ds, using procedures which have been described in detail in previous publications (Grozema, 2003; Infelta et al., 1977; Warman and De Haas, 1991). Transient changes could be measured using a linear timebase with a response time of ca. 1 ns or on a logarithmic timebase with a response time of ca. 10 ns. In the latter case transients could be recorded from 108 to 103 s using a single accelerator pulse. The transients are presented as the dose-normalized conductivity change, Ds=Dv .
3. Results and discussion In Fig. 2 are shown microwave conductivity transients observed on nanosecond pulsed-irradiation of deaerated trans-decalin without additives and with added PEHF (ca. 1 mM monomer concentration). The dramatic
Fig. 2. The dose-normalized transient change in the microwave conductivity on 5 ns pulsed-irrradiation of degassed transdecalin with no additives [lower dashed trace] and containing ca. 1 mM (monomer concentration) PEHF [upper full trace].
difference on addition of the polymer is immediately apparent with a substantial decay for the solvent alone and a marked after-pulse growth of the conductivity for the polymer solution. These observations will be discussed separately below. 3.1. Trans-decalin alone The transient observed for the solvent alone (denoted RH in subsequent equations) is due to the formation of mobile electrons, e, and holes, RH+, RH ! RHþ þ e :
(A)
The dose-normalized end-of-pulse conductivity expected due to the formation of free ion pairs is given by Dseop =Dv ¼ eN A gfi ðm½e þ m½RHþ Þ ½Sm2 =J.
(1)
In (1), e is the elementary charge (1:60 1019 C), N A is Avogadro’s constant (6.02 1023 molecules/mol) and gfi is the yield of free ions which for trans-decalin at room temperature is 1:25 108 mol=J (corresponding to G fi ¼ 0:12 molecules per 100 eV) (Warman et al., 1977). The room-temperature mobilities of electrons and holes in trans-decalin have been determined to be m½e ¼ 13 103 cm2 =Vs and m½RHþ ¼ 9 103 cm2 =Vs (Warman, 1982; Warman et al., 1977), both of which are considerably larger than the average mobility of 0:26 103 determined for ions diffusing by molecular displacement (Warman, 1982; Warman et al., 1977). Taking the above values for the yield and mobilities gives Dseop =Dv ¼ 2:7 109 Sm2 =J. The value of Dseop =Dv found by extrapolation of the slowly decaying component of the conductivity to the end of the pulse for pure decalin in Fig. 2 is 3:3 109 Sm2 =J,
ARTICLE IN PRESS F.C. Grozema, J.M. Warman / Radiation Physics and Chemistry 74 (2005) 234–238
in reasonably good agreement with the expected value based on previous measurements. The slightly larger value found and the initial rapidly decaying spike in the transient are attributed to a contribution from geminately recombining, coulomb-correlated ion pairs, as discussed previously (Warman et al., 1977). The relatively slow decay of the conductivity over tens to hundreds of nanoseconds following the pulse is due to a combination of homogeneous charge recombination, reaction (B), and/or reaction of the primary charge carriers with spurious impurities, I, present, reactions (C) and (D), to form lower mobility molecular ions. e þ RHþ ! RH;
(B)
e þ Ie ! Ie ;
(C)
RHþ þ Ih ! RH þ Ih þ :
(D)
The homogeneous recombination of free ions in hydrocarbon liquids has been found to be diffusion controlled with a rate coefficient, kr , given by the Debye expression. kr ¼ eSm=0 r
(2) 12
with 0 the permittivity of vacuum (8:85 10 F=m) and r the relative dielectric constant (2.2 for transdecalin). Under these conditions the half-life of the conductivity is given by (Warman, 1982) t1=2 ¼ 0 r =Dseop .
(3) 9
2
From the end-of-pulse value of 3:3 10 Sm =J for Dseop =Dv and the dose of 2 104 J=m3 , a half-life towards recombination via reaction (B) of ca. 300 ns is estimated. This is considerably longer than the value of ca. 50 ns observed. It is apparent therefore that for this batch of solvent the decay of the conductivity is mainly controlled by reaction of the mobile charge carriers with spurious impurities; reactions (C) and/or (D). Rate constants for both electron and hole scavenging in trans-decalin on the order of 1011 M1 s1 have been determined so that impurity concentrations of only ca. 104 M or 10 ppm would be sufficient to explain the lifetime observed.
backbone. e þ P ! P ;
(E)
RHþ þ P ! Pþ þ RH:
(F)
The after-pulse increase in the conductivity of the polymer solution is further illustrated by the transient shown on a logarithmic timescale in Fig. 3. The growth occurs over more than 100 ns and eventually reaches a plateau which is an order of magnitude larger than the end-of-pulse value. From the observation that the conductivity of the polymer solution at long times actually considerably exceeds that of the end-of pulse value, we can conclude that the sum of the mobilities of charge carriers on the polymer chains is much larger than the value of Sm ¼ 0:021 cm2 =Vs for the primary electrons and holes in the solvent. Limits to the actual values of the mobilities on the polymer chains will be discussed in a subsequent section. 3.3. The effect of added NH3 and CO2 In order to determine the relative contributions of electrons and holes to the conductivity in the polymer solution we have carried out experiments in which a second solute was added which was capable of selectively reacting with one of the primary charge carriers thus preventing its rapid diffusion to and reaction with the polymer chains while leaving the other primary carrier unaffected. Ammonia has been found to react with holes in transdecalin with a rate constant of 5 1010 M1 s1 but is
40x10-9
30 ∆σ/Dv (Sm2/J)
236
20
10
3.2. The polymer solution The after-pulse growth of the conductivity shown in Fig. 2 for the polymer solution has been observed previously for solutions of conjugated polymers in benzene (Grozema et al., 2002a, b, 2003; Hoofman et al., 1998). As in the former work, this is attributed to the transfer of charge to the polymer, P, resulting in the formation of highly mobile, long-lived electrons and/or holes (radical ion sites) on the conjugated polymer
10-9
10-8
10-7
10-6
10-5
10-4
10-3
Time (s)
Fig. 3. Microwave conductivity transients on 5 ns pulsedirrradiation of trans-decalin containing ca. 1 mM (monomer concentration) PEHF with no second additive (upper full trace), with 6.1 mM CO2 (middle dotted trace), and with 7.3 mM NH3 (lowest dashed trace). Note the logarithmic timescale.
ARTICLE IN PRESS F.C. Grozema, J.M. Warman / Radiation Physics and Chemistry 74 (2005) 234–238
unreactive towards electrons (Warman, 1982; Warman et al., 1977). RHþ þ NH3 ! R þNH4 þ :
(G)
Addition of NH3 at a concentration of ca. 102 mol/L should therefore result in the conversion of solvent holes to relatively unreactive ammonium ions within a few nanoseconds. As can be seen in Fig. 3, addition of ammonia to the polymer solution does considerable decrease the afterpulse conductivity. However, a substantial growth over hundreds of nanoseconds still remains which reaches a plateau value close to half of that attained in the absence of NH3. This is attributed to the reaction of the unscavenged electrons with the polymer chains. The inverse scavenging experiment was carried out by adding ca. 102 mol/L carbon dioxide in order to selectively scavenge electrons leaving solvent holes unaffected. e þ CO2 ! CO2 :
(H)
As expected, the after-pulse growth in conductivity over the first few hundred nanoseconds was similar in magnitude to that found for the NH3 solution and close to a factor of 2 lower than in the absence of CO2. Interestingly, the CO2 scavenged solution shows evidence of a second, delayed growth in the conductivity on a timescale of microseconds. We attribute this to the relatively slow formation of P via electron transfer from CO 2. CO2 þ P ! P þ CO2 :
(I)
The relatively slow growth via reaction (I) is in accordance with the factor of ca. 40 lower mobility of the molecular ion CO 2 compared with that of the primary electrons in this solvent. From equilibrium electron attachment studies, the electron affinity of CO2 in the closely-related solvent cyclohexane has been determined to be 0.7 eV referred to vacuum (Holroyd, 2004). This is smaller than the value of 0.88 eV calculated for a single fluorene molecule based on the value of 0.96 eV measured for the electron affinity of biphenyl; (Holroyd, 2004) and the 0.08 eV difference between the reduction potential of fluorene and that of biphenyl (Mann and Barnes, 1970). Since the electron affinity of the present polymeric derivative of fluorene would be expected to be even greater than that of a single fluorene molecule, reaction (I) should be energetically favorable. The decay of the conductivity on a timescale of microseconds or longer, shown in Fig. 3, results from eventual charge recombination and/or trapping reactions of P+ and P. The nature and rate coefficients of these processes, together with the kinetics of the initial growth will be discussed in a future publication after a full kinetic analysis of the data has been carried out.
237
3.4. Mobilities of charge on the polymer chains If all of the freely diffusing electrons or holes were scavenged by the polymer chains and no recombination or trapping had occurred, then the eventual dosenormalized plateau conductivities attained after the initial growth, Dspl , would be given, for the NH3 and CO2 solutions, by, Dspl ½NH3 =Dv 2 ¼ eN A gfi ðm½e =P1D =3 þ m½NHþ 4 Þ ½Sm =J,
ð4Þ
Dspl ½CO2 =Dv ¼ eN A gfi ðm½hþ =P1D =3 þ m½CO 2 Þ
½Sm2 =J.
ð5Þ
+
In (4) and (5), m[e /P]1D and m[h /P]1D are the onedimensional (1D) mobilities of electrons and holes respectively along the polymer backbone and the factor of 3 is included to take into account the random, istotropic orientation of the polymer chains. Because of the presence of electron and/or hole scavenging impurities in the solvent, as mentioned above, only partial scavenging by the polymer can be assumed. Only lower limits to m[e/P]1D and m[h+/P]1D can therefore be derived from the present data, i.e. m½e =P1D X3fDspl ½NH3 =Dv g=eN A gfi ,
(6)
m½hþ =P1D X3fDspl ½CO2 =Dv g=eN A gfi .
(7)
In (6) and (7) the reasonable assumption is made that the mobilities of NH+ 4 and CO2 are negligibly small. + The values of m[e /P]1D and m[h /P]1D derived from the results shown in Fig. 3 are 0.44 and 0.52 cm2/Vs, respectively. This lower limit to the value of m[h+/P]1D is somewhat smaller than the value of 0.74 cm2/Vs determined previously for PEHF in benzene (Grozema et al., 2002a). Experiments are underway using a freshly purified sample of trans-decalin. Preliminary results indicate that the actual value of m[h+/P]1D could in fact be as much as a factor of 2–3 larger than the benzene value. These results together with a full kinetic fitting procedure will be presented in a future publication.
4. Conclusions Subsequent to nanosecond pulsed irradiation of transdecalin solutions of the conjugated polymer poly(2,7(9,9-bis(2-ethylhexyl)fluorene), PEHF, the primary electrons and holes diffuse to and react with the polymer chains to form even higher mobility electrons and holes (radical ion sites) on the polymer backbone. This is observed, using the time-resolved microwave conductivity technique (TRMC), as a large after-pulse growth in the conductivity. By using competitive hole (NH3) and
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electron (CO2) scavengers lower limits to the 1D mobilities of electrons and holes on the polymer chains are determined to be 0.44 and 0.52 cm2/Vs, respectively. A second, delayed growth in conductivity found for the CO2 solution is attributed to electron transfer from CO 2 to the polymer. The transient conductivity of the polymer solutions eventually decays on a timescale of microseconds or more due to charge recombination and/ or trapping.
References Candeias, L.P., Padmanaban, G., Ramakrishnan, S., 2001. The effect of broken conjugation on the optical absorption spectra of the triplet states of isolated chains of poly (phenylene vinylene)s. Chem. Phys. Lett. 349, 394–398. Candeias, L.P., Grozema, F.C., Padmanaban, G., Ramakrishnan, S., Siebbeles, L.D.A., Warman, J.M., 2002. Positive charge carriers on isolated chains of MEH-PPV with broken conjugation: optical absorption and mobility. J. Phys. Chem. B 107, 1554–1558. Fratiloiu, S., Candeias, L.P., Grozema, F.C., Wildeman, J., Siebbeles, L.D.A., 2004. VIS/NIR absorption measurements of positively charged oligo(phenylenevinylene)s and comparison with time-dependent DFT calculations. J. Phys. Chem. B 108, 19967–19975. Grozema, F.C., 2003. Opto-electronic properties of conjugated molecular wires. Thesis, Delft University of Technology, ISBN 90-407-2422-9. Grozema, F.C., Candeias, L.P., Swart, M., van Duijnen, P.T., Wildeman, J., Hadziioannou, G., Siebbeles, L.D.A., Warman, J.M., 2002a. Theoretical and experimental studies of the opto-electronic properties of positively charged oligo(phenylene vinylene)s: effects of chain length and alkoxy substitution. J. Chem. Phys. 117, 11366–11378. Grozema, F.C., Siebbeles, L.D.A., Warman, J.M., Seki, S., Tagawa, S., Scherf, U., 2002b. Hole conduction along
molecular wires: sigma-bonded silicon versus pi-bondconjugated carbon. Adv. Mater. 14, 228–231. Grozema, F.C., Hoofman, R.J.O.M., Candeias, L.P., de Haas, M.P., Warman, J.M., Siebbeles, L.D.A., 2003. The formation and recombination kinetics of positively charged poly(phenylene vinylene) chains in pulse-irradiated dilute solutions. J. Phys. Chem. A 107, 5976–5986. Holroyd, R., 2004. In: Mozumder, A., Hatano, Y. (Eds.), Charged Particle and Photon Interactions with Matter. Marcel Dekker, New York, p. 175 (Chapter 8). Hoofman, R.J.O.M., de Haas, M.P., Siebbeles, L.D.A., Warman, J.M., 1998. Highly mobile electrons and holes on isolated chains of the semiconducting polymer poly(phenylenevinylene). Nature 392, 54–56. Horsman-Van den Dool, L.E.W., Warman, J.M., 1981. Interfaculty Institute Report no. 134-81-01, Delft, The Netherlands. Infelta, P.P., de Haas, M.P., Warman, J.M., 1977. The study of the transient conductivity of pulse irradiated dielectric liquids on a nanosecond timescale using microwaves. Radiat. Phys. Chem. 10, 353–365. Mann, C.K., Barnes, K.K., 1970. Electrochemical Reactions in Nonaqueous Solution. Marcel Dekker, New York, pp. 560. Prins, P., Candeias, L.P., Van Breemen, A.J.J.M., Sweelssen, J., Herwig, P.T., Schoo, H.F.M., Siebbeles, L.D.A., 2005. Electron and hole dynamics on isolated chains of a solution processible poly(thienylene vinylene) derivative in dilute solution. Adv. Mater. 17, 718–723. Warman, J.M., 1982. The microwave absorption technique for studying ion and ionic processes. In: Baxendale, J.H., Busi, F. (Eds.), The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis. D. Reidel Publishing Company, Dordrecht, pp. 129–161. Warman, J.M., De Haas, M.P., 1991. Time-resolved conductivity techniques, DC to microwave. In: Tabata, Y. (Ed.), Pulse Radiolysis. CRC Press, Boston, pp. 101–131 (Chapter 6). Warman, J.M., Infelta, P.P., de Haas, M.P., Hummel, A., 1977. Can. J. Chem. 55, 2249–2257.