Solvent and temperature control of the reaction mechanism and efficiency in the electrogenerated chemiluminescence of rubrene

Journal of Electroanalytical Chemistry, 372 (1994) 101-116

101

Solvent and temperature control of the reaction mechanism and efficiency in the electrogenerated chemiluminescence of rubrene Andrzej Kapturkiewicz Institute of Physical Chemistry of the PO&h Academy of Sciences, 44/52

Kasprzaka, 01-224 Warsaw (Poland)

(Received 28 June 1993; in revised form 21 December 1993)

Abstract Electrogenerated chemiluminescence (ECL) from the annihilation of rubrene cation and anion radicals has been studied in several organic solvents at temperatures down to - 60°C. The reactants were generated using the triple-potential-step method. Feldberg plot analysis indicates that the emission results from the excited rubrene singlet state produced directly in the electron transfer reaction (S route) and/or from the efficient triplet-triplet annihilation (TTA). In the special case of rubrene the energy of the lowest triplet is almost exactly half that of the first excited single, and the formation of two triplet states (in a single electron transfer act) leads to an efficient TTA (‘IT route). ECL efficiencies (r&r in the range 0.01-0.10) have been interpreted in terms of the Marcus theory, taking into account both static and dynamic dielectric properties of the solvent as well as both electronic and vibrational excitations of the reaction products. The rate of the triplet-triplet up-conversion (kupc = 1 x 10” s-’ within the rubrene triplet-triplet pair) has been also evaluated.

1. Introduction The understanding of the factors determining the rate of the electron transfer processes is considerable because of the ubiquity and essential role of electron transfer in many physical, chemical and biological processes. During the past three decades they have been the subject of extensive theoretical and experimental studies. Classical [l-31 and quantum-mechanical [4-81 treatments predict that the reaction rate will increase with increasingly negative Gibbs energy of the reaction, maximize for a moderately exergonic reaction (the normal Marcus region) and then decrease as the Gibbs energy of the reaction becomes more negative (the inverted Marcus region). Unequivocal evidence of such behaviour has recently been presented for both intraand intermolecular electron transfer reactions [9-B]. One of the most fascinating consequences of such behaviour is the emission of light during the reaction with very large exothermicity, i.e. the electrogenerated chemiluminescence (ECL). Chemiluminescence can be defined as the emission of light resulting from the generation of electronically 0022-0728/94/$7.00 SSDI 0022-0728(94)03297-G

excited states formed in a chemical reaction. Light emission arises at the expense of the energy released from an exothermic elementary step of a reaction. Three directions of chemiluminescence investigations seem to be the most interesting: study of the phenomenon mechanism; use of the chemical energy for obtaining excited molecules; application of chemiluminescence to the study of reaction kinetics and mechanism. Since the light emission is quantized, the reaction yielding the excitation must deliver, in one step, the energy equivalent to the shortest wavelength observed. Our understanding of the mechanism of chemiluminescence has advanced to the point that some predictions of light emission during a chemical reaction are now possible [16-B]. However, much remains to be done in identifying the fundamental physical processes that lead to electronic excitation. It is not generally possible to give a simple answer to the question: “What are the factors controlling excited state formation?” Obviously quantitative predictions are much more difficult because of the many factors involved in a mechanistic description of a chemiluminescent reaction. The heart of the matter is the actual excitation 0 1994 - Elsevier Science S.A. All rights reserved

102

A. Kapturkiewicz / Electrogenerated chemiluminescence of rubrene

step, which in most cases can be formulated as an inter- or intramolecular electron transfer. In the simplest case it may be realized in chemical reactions involving strong oxidants and reductants. Both methods for the preparation of the reactants, i.e. the common chemical, [19] and the electrochemical method 120-241, can be used, and the observations in these two types can be nicely related and understood qualitatively within the context of a model first proposed by Marcus [251. As was recently shown for the excited intramolecular charge transfer states [26], the Marcus model also allows for the quantitative interpretation of the experimental data. In this regard, chemically or electrochemically generated luminescence as a convenient chemical marker of electron transfer pathways, and measurements of their efficiencies provide direct information on the relative rates of bimolecular reactions in the normal and inverted regions. Electrochemiluminescence or ECL can be defined as the generation of light-emitting species by means of homogeneous electron transfers between precursors in solution. Such precursors are obtained as a consequence of heterogeneous electron transfer (electrode) reactions, leading to the formation of very active oxidizing and reducing agents. Electrochemiluminescence can arise from organic as well as inorganic compounds (transition metal complexes). The latter systems (the best-known example is Ru(2,2’-bipyridine),+ in acetonitrile solutions [27]) are often very simple from the mechanistic point of view, exhibiting two reaction pathways (i.e. formation of the emissive excited and ground states) [27-321. Extensive investigations of ECL processes of organic compounds have established a more complicated scheme for formation of the emissive excited state [20-241. The emitting singlet state may be formed directly upon electron transfer (the S route, which is the most important case because the experimental values of the ECL quantum yields can be compared directly with theoretical predictions), but in most of the organic ECL systems the high energies of the luminescent singlets preclude the S route and the electron transfer produces a non-emissive triplet intermediate (T route). In this case an up-conversion reaction (bimolecular triplet-triplet annihilation WI’A)) yields, with a rather low efficiency, the emitting singlet state (phosphorescence is not usually observed in liquid solutions). This is particularly true for the mixed systems, in which the radical cation and the radical anion are formed from different parent molecules. Of course, if the electron transfer reaction is energetically sufficient to populate the excited single directly, the formation of the lower-lying excited triplets must also take place. In some cases, excimer or exciplex emission has also been observed (E route) 133,341.

The aromatic hydrocarbons anthracene) are typical examples. cal reactions a neutral molecule dized to the corresponding radical R + respectively:

(e.g. 9,10-diphenylIn the electrochemiR is reduced or oxianion R- and cation

R+e-+R-

(la>

R-e-*RR+

(lb)

In the diffusion-controlled reaction these ions form an active complex for which at least three reaction pathways are possible: R++ R--+ S, + S,

(2a)

R++ R-4

S, + S,

(2b)

R++ R-+

S,+ T,

(2c)

where S, and T, are the excited singlet and triplet states of the parent molecule R (in the ground state SJ. The emissive S, state undergoes radiative deactivation (with fluorescence quantum yield &,, which is the intrinsic property of the emitting state). The nonemissive T, state may take part in the triplet-triplet annihilation. The efficiency of this up-conversion process is generally very low (typically, it does not exceed ca. 10-3-10-4), and th e efficiency of light production is much lower (T route) than for the direct formation of the excited single (S route). The above general mechanistic scheme which is normally used to explain almost all organic ECL systems must sometimes be amended. Rubrene (structure shown in Fig. 1) [35-521, which has fascinated investigators for three decades, seems to be the most obvious example. Taking into account the energies of the rubrene excited states, it becomes clear that two additional processes (as has was already been reported in the literature [35,40]) should be added to the reaction scheme (2aH2c). The energy of the lowest rubrene triplet has a value E,, = 9300 k 100 cm-’ [53,54]. The energy of rubrene S, state is Es, = 18500 cm-’ [55],

Fig. 1. Rubrene (5,6,11,12-tetraphenyl-tetracene).

A. Kapturkiewicz / Electrogenerated chemiluminescence of rubrene

and hence 2E,, - E,, and it can be concluded that the rates of formation of two neighbouring triplets R++R-+T,+T,

(2d)

the direct formation of the excited single (reaction (2b)) should be similar. The formation of two geminate triplets would probably lead to an efficient ITA and hence diminish the quenching. The first excited singlet lies slightly below the second rubrene triplet (with energy E,, = 19200 + 100 cm-’ [56]. Therefore an interesting possibility that the rubrene triplets are populated according to R++ R-+

S, + T,

(2e)

cannot be a priori excluded from the mechanistic considerations. However, rate of this process is expected to be small in comparison with that of reaction (2b) or (2d). The ECL of rubrene has a rather complex mechanism, and it is not surprising that conflicting reports have arisen about this system. The energy released from the annihilation of rubrene ions is capable of producing both singlet and triplet excited states, and S and T routes can both contribute significantly to the ECL yields. The energy relationships are delicately balanced about the reaction channels generating the emitting singlet. Thus the distribution of events producing excited singlet, triplet and ground states may be very sensitive even to slight changes in the energetics of these steps. It is not surprising that the experimental data (ECL yields, ECL transients, influence of temperature, electrolyte concentration, effects of the magnetic field etc.) are susceptible to various interpretations. Moreover, almost all reported results have been obtained in solutions containing tetra-n-butylammonium perchlorate (TBAP) as the supporting electrolyte. It has been reported [.57] that additional parasitic processes take place in such solutions, leading to perturbation of the ECL efficiencies and transients (in fact the reported ECL efficiency values have shown a wide variation even under similar conditions). The mechanism of these processes is not known exactly (a plausible hypothesis for the effect is the oxidative or reductive liberation of hydrogen ions, e.g. from the solvent impurities (water)), but the phenomenon seems to have a general meaning [%I. Thus it is expected that the ECL of rubrene (as pointed out above, generally measured in TBAP solutions) may be influenced. All this usually makes the quantitative interpretation difficult and the rubrene ECL system is only qualitatively understood. Thus an ECL study in a more inert electrolyte (e.g. tetra-n-butylammonium hexafluorophosphate [58]) seems to be appropriate for a more detailed

103

understanding of the reaction mechanism (with eventual quantitative description) and for an explanation of the literature discrepancies. This was the purpose of the present work. 2. Experimental 2.1. Chemicals In most cases spectral quality solvents (all from Aldrich) were additionally purified just before use; only chlorobenzene (@Cl) and 1,Zdichlorobenzene (@Cl,) were used as received. Acetonitrile (ACN), benzonitrile (WN), 1,Zdimethoxyethane (DME) and tetrahydrofuran (THF) were distilled over CaH, (Merck). The same procedure was used for the purification of benzene (Ca) and toluene (QCH,). N-methylpyrrolidone (NMP) and N,N-dimethylformamide (DMF) were additionally dried using thermally activated molecular sieves (4A, 8-12 mesh, Jansen) and distilled under high vacuum. Commercially available benzonitrile (BN) and y-butyrolactone (BL) (Merck, for synthesis) were purified as follows. BN was distilled three times over KMnO, + &CO,, P205 and CaH, respectively. BL was distilled twice under high vacuum; before both distillations the crude product was additionally dried using thermally activated molecular sieves. Only the middle fraction was collected in all distillations. The properties of the solvents are summarized in Table 1. Tetra-n-butylammonium perchlorate (TBAP), tetran-butylammonium hexafluorophosphate (TBAPF,) and tetra-n-butylammonium hexafluoroarsenate (TBAAsF,) were synthesized as described in the literature [65]. The precipitated products were washed with water and recrystallized twice from anhydrous methanol. Before use both supporting electrolytes were dried in a vacuum (at 1OO’C) for 12 h. Tetramethylammonium hexafluorophosphate (TMPF,) and tetra-n-hexylammonium hexafluorophosphate (THAPF,) were synthesized and purified analogously to TBAPF,, using (CH,),NBr and (n-C,H,,),NBr instead of (n-C,H,), NBr. Rubrene was purchased from L. Light and used without further purification. Solutions containing 0.51.5 mM of rubrene and 0.05-0.2 M of the supporting electrolyte were used for the ECL and cyclic voltammetry experiments. The substance is only sparingly soluble in most of the organic solvents, but prolonged dissolving (with shaking) results in the required concentrations being obtained (as indicated in Table 2). The solutions studied were deaerated with pure Ar, which was presaturated by bubbling through the solvent studied.

104

A. Kapturkiewicz

/ Electrogenerated

chemiluminescence

of rubrene

TABLE 1. The solvents used and their properties (at 293 K) Solvent

Abbreviation

E

@Cl DME THF WI, BN @CN NMP DMF BL

5.6 7.2 7.6 9.9 20.3 25.5 32.0 36.7 43.3

ACN+@ ACN + @CH, ACN + THF ACN + (PCN BN+@ @+@CN

15.5 14.5 13.5 29 11 10

n

l/n2

1.380 1.407

0.387 0.373

0.46 0.57

0.8 0.7

1.380 1.528 1.470 1.427 1.436

0.476 0.389 0.432 0.464 0.462

0.52 1.24 1.80 0.92 1.74

0.6 3.2 2.5 1.3 y 3.3

1.40 1.39 1.36 1.39 1.39 1.51

0.445 0.446 0.465 0.484 0.430 0.338

- l/e

B/CP a

TL/PS

Single solvents

Chlorobenzene Dimethoxyethane Tetrahydrofuran 1,2-Dichlorobenzene Butyronitrile Benzonitrile N-Methylpyrrolidone N,N-dimethylformamide y-Butyrolactone Mixed solvents (I

: I)

Acetonitrile + benzene Acetonitrile + toluene Acetonitrile + tetrahydrofuran Acetonitrile + benzonitrile Butyronitrile + benzene Benzene + benzonitrile

E, the static dielectric constant; n, refractive index; 7, viscosity; rL, longitudinal relaxation time. Data for BL are from ref. 59 and those for other single solvents are from refs. 60-62 or from the compilation in ref. 63. The value of rL for BL is not available in the literature and was estimated using the Debye equation [64]. The values of E for the mixed solvents were estimated from the relationship between l/c and the differences between the standard redox potentials for the oxidation and reduction of rubrene. The values of n for the mixed solvents were estimated by assuming additivity of the molar refractive indices of the two components (1 and 2) of the mixture (12): n;,-1 -= tlf2 + 2

n;-1 -x1

nt +

n;2

+

1

-x2

nz + 2

where x is the molar fraction of the components. a 1 cP = 10-l Pa s.

2.2. Apparatus

The same electrochemical cell (constructed specially for low temperature measurements [66]) was used for both the electrochemical and ECL studies. The working electrode (polished before each use with diamond paste) was a Pt disc with a diameter of 2 mm. The counter-electrode was a Pt ring with inner and outer diameters of 4 mm and 8 mm respectively. The separation between the electrodes was about 3 mm. Tungsten wire dipped in the solutions was used as a quasi-reference electrode (TQR). Its potential (ca. -0.1 V vs. aqueous saturated calomel electrode) was stable within the time-scale of the experiment. A quartz optical fibre (with a diameter of 4 mm) passed through the counter-electrode was used to collect the light emitted at the working electrode. The lateral face of the fibre was protected to minimize light losses and only the front face of the fibre was immersed directly in the solution. The electrochemical cell temperature was measured and stabilized by means of a temperature controller (type 650, UNIPAN, Poland) equipped with a Pt thermocouple. The end-face of the fibre was interfaced to the photon detection system through the specially devised high aperture grating monochromator [67]. The dis-

persed emission was detected by a 9816QA photomultiplier tube, which was cooled to -30°C in a FACT 50 MK III housing. The signal from the photomultiplier was passed to a Cl0 photon counter (Thorn-EMI, England). The spectral response of the light detection system was calibrated with a tungsten lamp as the standard of spectral irradiance. The “electrochemical” part of the measuring system was constructed from an EP22 potentiostat (with ZR compensation) and an EP20 function generator (both from ELPAN, Poland). The measuring system was equipped with an HM 205-2 digital storage scope (Hameg, FRG) which recorded the current and potential transients. Cyclic voltammetric curves were recorded with an Endim 622.1 x-y recorder. The triplet-potential-step sequence generated by a D4030 digitimer (Digitimer Ltd., England) was used to establish ECL reactions. The wavelength of the monochromator and the outputs from the photon counter and the digitimer were controlled by a Commodore 64-microcomputer. 2.3. Procedures Cyclic voltammetry was run prior to the ECL measurements (to evaluate the electrochemical characteris-

A. Kapturkiewicz / Electrogenerated chemiluminescence of rubrene

tics of the ECL system studied) as well as after them (to check the absence of electrode surface filming.) Application of a triple-step-potential sequence causes a very intense bright emission to appear. The ECL emission band was identical with the fluorescence of rubrene, indicating that the emission in ECL comes from the excited single state. The experiment began with the working electrode at a potential of no electroactivity (-0.5 V TQR). First, the electrode potential was changed to values at which the generation of the radical anion occurs (100-200 mV more negative than the standard reduction potential EFed) and subsequently to values corresponding to the formation of the radical cations (100-200 mV more positive than the standard oxidation potential E&j. The potential limits of the programme sequence were chosen to ensure production of the electrogenerated intermediates in the mass-transfer-controlled region and to minimize the influence of secondary electrochemical reactions. Subsequently, the electrode potential was again changed to the initial value. The system was allowed to equilibrate for a few seconds between each pulse sequence. Such a sequence of potentials was generally used because the radical anion was more stable than the cation. However, the opposite sequence of reactant generation, in which the radical cation was generated first, has also been used. In both cases light emission was observed during the second reactant generation step in the course of a triple-potential-step sequence. ECL spectra were recorded at 2.5 nm intervals from 360 to 740 nm. The ECL spectra obtained were corrected for the spectral response of the detection system and integrated to obtain the integrated photon intensities. The values of the measured integrated photon intensities were the averages of several independent measurements. For a particular solution, two or three recordings were made to check the temporal stability of the system studied. ECL yields were determined against the standard by comparison of the measured integrated photon intensities, taking into account the differences in the charges passed through the solution studied. The ECL system containing Ru(2,2’-bipyridine),(ClO,), in 0.1 M TBAP + ACN was used as the standard (with 4ec, = 0.05 [27]). The error limit of this method was found to be 10%-U%. ECL transients were also recorded using the photon-counting technique. This was done with a short gate time of the photon counter, switched within the time of the reverse pulse. ECL transient curves were constructed from the measurements for many successive pulses (typically 100). The transients obtained may differ to some extent from those recorded using the conventional technique, but the eventual differences should be negligibly small.

105

3. Results and discussion 3.1. Electrochemistry In all the solutions studied rubrene can be reduced and oxidized to the corresponding radical anion and cation. The reversibility of anion radical formation was ascertained by cyclic voltammetry. The potential-current curves had a peak current on the reverse positive sweep approximately equal to that on the forward negative sweep. This indicates that the radical anion formed during the reduction of the parent molecule is quite stable. Only in (PC1 and @Cl, solutions was the reduction process found to be complicated by background processes, probably involving reactions with the solvents. The cathodic reduction current in these solvents (containing 0.2 M TBAP as the supporting electrolyte, because TBAPF, was almost insoluble) was ca. 15%-20% greater than the corresponding current for the anodic oxidation process. This is probably due to the partial reduction of (PC1 and @Cl,. Therefore both solvents were excluded from the ECL studies. However, preliminary results indicated that intense ECL emission may also be observed in @Cl and @Cl, solutions. Similarly, the potential-current curves for rubrene oxidation had a peak current on the reverse positive sweep approximately equal to that on the forward negative sweep. This indicates that the radical cation formed during the oxidation of the parent molecule is quite stable. Only in the case of NMP and DMF solutions at room temperature were the cathodic peak currents somewhat smaller than the anodic values, indicating slightly irreversible behaviour. This instability of the rubrene cation radical is completely suppressed if the solutions studied are cooled (to 0°C or lower). At all temperatures, however, the anodic oxidation currents were found to be close (within experimental error) to the cathodic reduction currents, which clearly establishes that complications by background processes (reported in ref. 36 for DMF solutions containing TBABFJ are negligible if the supporting electrolyte is changed to TBAPF,. Examples of cyclic voltammograms are shown in Fig. 2. The electrochemical behaviour of rubrene agrees well with that expected from the previously reported results. The cyclic voltammetry experiments allow us to determine the mean lifetime t,,, for both rubrene radicals as well as the values of the standard redox potentials E& and EFed. It was found that in all cases t,,, lies in the range of seconds. Because the duration of ECL experiments was usually 100 ms, complications caused by radical instability can be excluded. Values of EKd and E& depend on the nature of the solvent and whether the redox couple ferrocene/

106

A. Kapturkiewicz / Electrogenerated chemiluminescence of rubrene

ferricinium is used as an internal reference standard [68]. The difference E& - EFed decreases with increasing solvent polarity, indicating that the energies of solvation of the anion R- and cation R+ are larger than that of the neutral molecule R. An approximately linear relationship has been found between Ezx - EFed and l/e, where E is the static dielectric constant of the solvent, (Fig. 3 and Table 2). However, the effect is rather small and can be simply attributed to nonspecific solvent-solute interactions and interpreted in terms of the familiar Born theory [69] describing the solvation energy AGsolV of the monovalent ion with radius ri as follows:

E = 9.9). No differences were observed in E& - EKd when the supporting electrolyte was changed from TBAPF, to TMAPF, or THAPF, (and to TBAP or

TQE N

where NA, e, and E, are the Avogadro constant, the elementary charge and the permittivity of vacuum respectively. Consequently, the difference E&-EFe, can be simply related to l/e: E;, - EK,, = const + -;S(&+$)f

(a)

3t i a

(4)

where F is the Faraday constant and r_ and r+ represent the radii of ions R- and R+ respectively. The solid line in Fig. 3 corresponds to the theoretical slope calculated according to eqn. (4) with r_= I+= r = 0.70 nm. The above calculation was performed assuming the same shape for both rubrene radicals as well as a similar charge redistribution (deduced from electron spin resonance (ESR) spectra [70]). A comparable value (0.69 nm) can be evaluated from the rubrene diffusion coefficients D (e.g. D = 4.8 X lop6 cm2 s-l in DMF solutions [42], calculated from the cyclic voltammetry peak currents using the Randles-Sevcik equation) and the Stokes-Einstein equation (relation between D and 7). A somewhat smaller value of 0.55 nm is obtained from the molar volumes of rubrene [71]. With decreasing temperature, the standard redox potentials are shifted to more positive EFed or more negative EI, values. This indicates that there are negative solvation entropies for the ion radicals studied, but the effects are rather small and can be attributed to the solvent dielectric permittivity. The observed differences Ez, - EFed may be fully related to changes in l/e, as is shown in Fig. 3 (with DME and DMF as examples). This supports the previous conclusion that both radicals are relatively weakly solvated. The results also indicate that the association between both the ion radicals and the ions of the supporting electrolyte is negligibly small (e.g. Ez, - Ezed is nearly the same in DME at -40°C with E = 9.8 and in @Cl, at 20°C with

4.5

(b)

. t

I

Ia

ia

(c) Fig. 2. Cyclic voltammogram of (a> 0.5 mM rubrene in 0.1 M TBAPF, + BN, (b) 1.0 mM rubrene in 0.1 M TBAPF, + DMF and Cc) 1.5 mM rubrene in 0.2 M TBAPF, + DME. Pt electrode; T = 223 K; scan rate, 50 mV SC’.

107

A. Kapturkiewicz / Electrogenerated chemiluminescence of rubrene

Gibbs energies AG,, (production of the ground state) of reaction (2a) can be calculated as follows:

(E:,-

Eyed)

2.70.

N AG,, =F(%

-EL)

-w,(d)

(5a)

where F is the Faraday constant and w,(d) is the Coulomb interaction energy between two oppositely charged ions required to bring the reactants together at the most probable separation distance d at which the electron transfer takes place, preferably within a small range of distances 6r [72-741. In a similar way AG,, and AG,, for reactions (2b) and (2~) can be calculated using the corresponding energies of both (singlet and triplet) excited rubrene states:

2.30

l/& ,

0.00

0.05

0.10

0.15

0.20

Fig. 3. Difference E& - EFe,, between the standard redox potentials fit to the Born equation of rubrene as a function of l/e: temperature data for DME with r = 0.70 nm; 0 data at 293 K, and DMF.

TBAAsF, respectively). Electrolyte changes also had no effect.

-w,(d)

+J%r

(5b)

AGr, =F(EL

-w,(d)

+&I

(5c)

-J%,)

Equations (2a), (2b) and (2~) are not exactly correct because of the mixture of Gibbs energy and energy terms. The entropic contribution AS needed to avoid this inconsistency can be evaluated from the following equation:

concentration

AS = - 6AGs,/6T

3.2. Energetics of rubrene ion annihilation As noted earlier the energy released during the annihilation of rubrene ions is similar to that needed for the reaction channel generating the emitting singlet state. The electrochemical data obtained enable this to be discussed in more detail. In the present case, the

TABLE 2. Effect of the solvent on the electrochemical Solvent

AGs, = F( EL -KJ

= -8[F(EFe,

-E&l

- wr(d)]P

(6)

It should be noted that the correction for the Coulombit interaction energy terms leads to much smaller values of AS compared with those calculated for the isolated ions (at the reaction distance formally equal to

and ECL properties of rubrene (at 293 K)

crubrene/mM

Electrolyte

l/e

1.5 1.5 1.5 1.5 0.5 1.5 1.5 1.0 0.5

0.2 M TBAP 0.2 M TBAPF, 0.2 M TBAPF, 0.2 M TBAP 0.1 M TBAPF, 0.1 M TBAPF, 0.1 M TBAPF, 0.1 M TBAPF, 0.1 M TBAPF,

0.179 0.139 0.132 0.101 0.049 0.039 0.031 0.027 0.023

1.0 1.0 1.0 1.0 1.5 1.5

0.1 M TBAPF,

AGsJeV

4 ccl

- 0.02 - 0.03

0.041 0.044

- 0.02 - 0.02 - 0.01 0.01 0.03

0.025 0.053 0.032 0.033 0.028

0.00 0.00 0.00

0.031 0.025 0.020 0.020 0.030 0.050

Single solvents @Cl

DME THF @Cl, BN @CN NMP DMF BL

2.59 2.53 2.54 2.48 2.39 2.37 2.35 2.32 2.28

Mixed solvents (1: I)

ACN/@ ACN/QCH, ACN/THF ACN/@CN BN/@ @/WIN

0.1 M 0.1 M 0.1 M 0.1 M 0.1 M

TBAPF, TBAPF, TBAPF, TBAPF, TBAPF,

2.39 2.40 2.41 2.33 2.44 2.46

0.02 -0.01 - 0.01

Ei, and EL, standard redox potentials of rubrene oxidation and reduction; AG,,, Gibbs energy for reaction (2b); decl, ECL emission efficiency. The following 4ec, values have been reported in the literature (in TBAP solutions): THF, 0.038 [44]; DME, 0.041 [44]; DMF, 0.0025 [44], 0.0026 [38], 0.0067 [42], 0.010 [37]; @CN, 0.0077 [44], 0.0062 [38], 0.019 [411, 0.087 1431;ACN/@, 0.015 [371.

108

infinity). The appropriate formed using wr( d) = -N,ei/4rrc,ed

A. Kapturkiewicz / Electrogenerated chemiluminescence of rubrene

calculations

have been per-

(7)

with d = 0.9 nm. The reaction distance of 0.4-0.6 nm 17.51is typical of electron transfer reactions involving aromatic systems. In the case of rubrene, however, the phenyl rings are almost perpendicular to the tetracene kernel, which must lead to an increase in d, and the larger value of 0.8-1.0 nm seems to be more appropriate. The AS values obtained with the intermediate value of d = 0.9 nm are negligibly small, leading to the conclusion that eqns. (.5a), (5b) and (5~) can be used directly for the estimation of the reaction driving forces. The values AG,, obtained for the direct formation of the excited singlet (reaction (2b)) at 293 K are shown in Table 2. We can conclude that this particular reaction channel (as well as the formation of two triplets (reaction (2d))) is accessible in all the solutions studied. Only in BL and ACN + QCN solutions is the formation of the excited rubrene singlet somewhat endothermic. The formation of the second triplet (reaction (2e)) is ca. 0.10-0.15 eV more endothermic and can be neglected, at least to a first approximation. The exothermicities AG,, = - 2.30 eV of reaction (2a) and AG,, = - 1.15 eV of reaction (2c) are almost constant in all the solvents studied. Moreover, the driving forces of the reactions, evaluated as described, are almost constant (+O.OOS eV) in the temperature range studied (213-293 K). The observed temperature changes of the difference Eyed - E& in the standard redox potentials are compensated by the changes in w,(d) because both quantities depend on T (through l/e) in a similar way. These considerations allow us to conclude that the changes in the energetics of rubrene ion annihilations are not responsible for the differences in the measured ECL efficiencies &,. The correlation between &,, and the solvent polarity parameter (the Pekar factor l/n2 - l/e) has been found and is shown in Fig. 4. The Pekar factor describes the influence of the medium on the outer (solvent) reorganization energy. It strongly suggests that the Marcus theory may be useful in the quantitative description of the electrochemiluminescence of rubrene. 3.3. ECL transients The energetics of the electron transfer reaction provide only a rough indication of the possible ECL mechanism. In order to decide which reaction pathway is dominant, a more definitive analysis must be applied to differentiate between the S and T routes. The most useful diagnosis is based on the relationship between the ECL intensities Z(t,> and the square root of the

0.08

4eel

0.06

0.04

0.02 ACNIOCN

_.__

0.30

0.35

0.40

0.45

0.50

Fig. 4. ECL efficiency &,_, as a function of the solvent polarity parameter (the Pekar factor l/n2 -l/e).

ratio between the times of duration of the forward and reverse potential steps (in the triple-potential-step technique). The appropriate relationship (usually called a Feldberg plot) can be derived after an analytical treatment of diffusion (to and from the electrode) and the overall electron transfer rate 176-781. In the case of the simple S route the appropriate plots according to I(tr)

=aJff/t,--b

(8)

are linear with a slope-to-intercept ratio a/b = 0.959 [78]. In eqn. (8) t, is the duration of the first (forward) step of the triple-step sequence and t, is the duration of the second (reverse) step. In the cases of the “pure” T route (treated as a superposition of two bimolecular reactions) and the “mixed” ST route the plots of the ECL intensities I(t,) vs. (tf/t,>‘/2 are non-linear [78]. This criterion can be used to distinguish the S route from the T or ST routes in ECL experiments. However, similar deviations from linearity are also caused by reactant instability (an analytical treatment incorporating their pseudo-first-order decay rate constant has been presented 1791). The more unstable the electrogenerated species, the larger are the deviations from linearity observed. Therefore it is very important to exclude radical instability when using the Feldberg plot to distinguish between the S and T routes. Other parasitic processes also lead to deviations from the linearity of Feldberg plots [57,58]. Therefore the analysis may be impeded and should be performed very carefully. Generally speaking, this analysis may only be conclusive in the case of a “pure” S-route with linear Feldberg plots. In the case of rubrene both radical ions are relatively stable; thus possible complications caused by their instabilities can be excluded. Their mean life-

A. Kapturkiewicz / Electrogenerated chemiluminescence of rubrene

times are much longer than the times of the ECL experiment (100 ms or less) or the times required for annihilation of the diffusion-controlled ions. For all the solutions studied the ECL transients measured have been the same for both sequences of reactant generation (within the experimental error). The Feldberg plot analysis indicates that the observed ECL emission decays are characteristic of the S route. The experimental slope-to-intercept ratios (a/b = 0.95-0.97) agree very well with the theoretical prediction. At shorter times t, the picture was the same. Some representative examples are shown in Fig. 5. Some deviations from the linearity in the Feldberg plots have been found with increasing time in the triple-potential-step experiments. This can probably be attributed to the presence of parasitic processes, as described elsewhere [57,58], or to radical cation insta-

Light intensity

‘I

I

0.2

0.0

2

0.4

3

4

5

6

1.0

0.8

0.6

Light intensity

r3

DME

0

7,i\:.

0

0

0.2

0.4

I

2

0.6

3

4

0.8

5

6

1.0

(b) Fig. 5. ECL decay curves and plots of intensities U = 293 K, tf = KKI ms): (a) anion radicals generated radicals generated first.

vs. (t,/t,)‘/* first: (b) cation

109

bilities (for DMF or NMP solutions). However, in ethereal solvents (DME and THF) linear Feldberg plots were observed for experiment times up to 500 ms. At lower temperatures, S-route behaviour was observed over a broader range of t,, which seems to be understandable. Our results are in only partial agreement with those reported in literature [356,36,38]. At low temperatures (in DMF solutions containing TBAP) the picture is congruent in both electrolytes, but an important difference (lack of the Feldberg plot linearity) occurs at room temperature. It is clear that perchlorate (or tetrafluoroborate) ions interfere with the production of the emissive excited state. Since the rubrene electrochemistry is not modified by the supporting electrolyte, this indicates that some additional processes must occur during the electrogeneration of the reactant, which also lead to reduction of the measured ECL efficiencies (see Table 2). The scarcity of experimental data precludes a more detailed discussion. However, the 4 ec, efficiences for the ECL system presented in Table 2 seem to be unaffected by any additional processes and they may be used directly for the discussion of the reaction mechanism. The &cl values obtained are fairly large, which in turn is in accord with the behaviour of ECL transients. Such large values suggest that the emitting rubrene singlet is formed directly by electron transfer between R- and Rf (reaction (2b)) or in the triplet-triplet up-conversion within the triplet pair (reaction (2d)l. This last, formally bimolecular, reaction channel can be treated as a pseudo-first-order process (primary ITA) if the up-conversion takes place before the triplet formed is separated. Of course, the rubrene molecules in the T, state (not up-converted in primary TTA and produced directly in reaction (2~)) may also take part in the triplet-triplet annihilation (real bimolecular secondary TTA). Such a process should lead to a much less efficient formation of the excited rubrene singlet. In view of the linearity of the Feldberg plots, it can be argued that the amount of light emitted in this way is negligible small. The &ec, values, at least an order of magnitude larger than those found for ECL processes occuring according to a “pure” T route [34], agree well with the above conclusion. 3.4. ECL efficiencies as a function of temperature The analysis of the results from the EC1 transient studies suggest that two reaction routes may be operative in the formation of the emissive rubrene singlet. For a deeper insight into the reaction mechanism measurements of the temperature dependence of 4ec, have been performed with the results shown in Fig. 6. The most important feature of the data in Fig. 6 is that 4,,,

110

A. Kapturkiewicz

0.10

I

0.08

/ Electrogenerated

4ccl Dm

0.06

0.04

DMF

0.02

BN % BL

T/K

_.__200

220

240

260

280

300

Fig. 6. ECL efficiency #J_, as a function of the temperature

T.

initially increases as the temperature is reduced, goes through a maximum and then decreases at still lower temperatures. Similar results (increase of 4ec,) have been reported [35,37], but the temperature range applied was not so wide. Such behaviour seems to be a general rule. A preliminary study of (PCN solutions indicates that in this solvent ECL efficiency increases slightly with the reduction of temperature towards the melting point of the solvent (ca. - 150. In THF solutions the picture resembles that presented for DME. The trivial explanation that the rubrene fluorescence quantum yield & increases at lower temperatures can be excluded because &, is already almost unity at room temperature [80]. The fact that the position of the temperature TmaXof the 4ec, maximum depends on the solvent seems to play a key role in the understanding of the reaction mechanism. For instance, in NMP or BL solutions the +ec, maxima are located at higher temperatures compared with those for DME, DMF or BN. No correlation between T,,, and the driving forces of reactions has been found. However, there is an obvious correlation between T,,,,, and the solvent viscosity 7. This implies an important role for the solvent viscosity in the reaction mechanism. An explanation taking into account the decrease in the rate of triplet quenching (e.g. by an ion radical 1371) can be excluded from the considerations. Such a mechanism requires the importance of the T route, which seems to be dubious in view of the analysis ,of ECL transients. The hypothesis proposed in ref. 35, i.e. reduction of the rate of T, state population (reaction (2e)), can also be excluded, since for energetic reasons such a reaction channel does not seem to be operative. The simplest explanation (as will be discussed later) is given by the assumption of the important contribution

chemiluminescence

of rubrene

of primary triplet-triplet annihilation within a triplet pair (produced directly in reaction (2d)) to the formation of the emissive rubrene S, state. The dissociation of the triplet-triplet pair into separate products is a diffusion-controlled process. Thus the efficiency of the primary TTA will be governed by the ratio of the to the separation rate kdiff, up-conversion rate k,, and will increase with medium viscosity. The reaction mechanism of S, state formation in rubrene ECL seems to be a superposition of two parallel processes. The excited single is produced directly in the electron transfer reaction 6 route) and/or in the efficient triplet-triplet annihilation within the germinate triplet pair (IT route), with the branching ratio between the two routes governed by temperature. Thus in the rubrene ECL system 4ec, is the sum of efficiencies for the separate reaction pathways and can be expressed as follows: &cl = 4J04s1+ 4&T&r*

= 4% + &-r&rrA

(9)

where &i and brr are the fractions of electron transfer events that produce excited singlets (via reaction (2b)) and triplet pairs (via reaction (2d)) respectively, and &,-rA is the efficiency with which a contact triplet pair is converted to excited singlet. The fact that the fluorescence efficiency 4, of rubrene is close to unity simplifies the analysis of the experimental data. Both and &-r are given, according to Marcus model 4 [2?1, by the ratio of the rate constants for electron transfer processes producing the excited and the ground state products respectively. Thus, to a first approximation, &i is given by 4si = ks1/(&Xl+

3krif

94-r + 41)

(10)

where k,,, k,,, k, and k,, are the rate constants of processes (2a), (24 (2d) and (2b) respectively. The statistical factors from the spin multiplicity (of the given reaction product) are also introduced in eqn. (10) [813. In a similar way, the efficiency &-r is given by 4r-r =

9k,/(ks,

+ 3k,,

+ 9k,

+ ks,)

(11)

The factor 9 attached to the rate k, may not be exactly correct. Nine encounter pair spin states (which constitute the sublevels of encounter pairs with quintet, triplet and singlet multiplicities) are formally possible for two interacting triplets. However, two of the quintet sublevels (with the spin quantum numbers f2) may not be directly accessible because of the spin conservation rule [821. If this is the case, the factor 9 should be replaced by a factor 7. Using eqn. (11) with the factor 9 may lead to some overestimation of &r.. Triplet-triplet annihilation may also produce triplets, singlets or quintets. With the usual assumption that the formation of rubrene in the excited quintet state is

A. Kapturkiewicz

/ Electrogenerated

energy forbidden, the spin statistics predict an average yield +r-rITAof l/4 singlets per triplet pair. However, such a large efficiency cannot be obtained in most cases because some of the triplet-triplet pair will be able to dissociate into separated reactants before the up-conversion takes place. 3.5. Rates of the electron transfer processes As discussed above, the experimental values of the excited state yields are attributed to competition between the parallel electron transfer processes. The electron transfer rate generally depends on the exothermicity of the reaction and, according to the golden rule model, is given as a function of the electronic coupling matrix element V and of the reorganization energies (inner Ai and outer A,) for the high frequency (inner molecular vibration) and low frequency (mainly solvent) motions involved. In a simplified treatment (presented by Jortner and Bixon [83]) incorporating both high frequency modes (important in the inverted Marcus region) and low frequency modes (important in the case of relatively large values of VI, the reaction rate k,, is expressed as follows: k,, =

;v2

d

sj

1

4rA,RT

cj=O

e-S

j!

chemiluminescence

111

of rubrene

strongly influence the rates of electron transfer processes. Equation (12) describes the transition from the non-adiabatic limit to the solvent-dynamics-controlled adiabatic limit. The latter may be realized for some channels depending on the V and S values. These values may be different for all electron transfer processes contributing significantly to the overall reaction rate (and determining &i and &-,.). Appropriate values of V and S terms are not available, but some conclusions can be drawn by taking into account the literature data for similar systems. Relatively large values of V are characteristic of aromatic hydrocarbons, for example values of V in the range 0.05-0.10 eV were found for the electron exchange between anthracene and the anthracene radical anion [84,85]. In the alternate hydrocarbon approximation V values in this range are expected for electron exchange involving other hydrocarbons (between the parent molecule and the corresponding radical anion or the corresponding radical cation). This is also true for electron transfer reactions involving the excited states. Thus, from the geometric mean approximation, relatively large values of I/ are expected for reactions in the rubrene ECL system (also taking into account that in this particular case the reaction takes place at a large distance because of the steric effects). The rates of reactions (2b) and (2d), both occurring in the normal Marcus region (Fig. 71, can be expressed as follows:

(12) where S = A,/hv, is the electron-vibration coupling constant equal to the inner reorganization energy expressed in units of vibrational quanta, vi is the mean vibrational frequency of the reactant and product bonds, R and h have their usual meanings, and 7L is the longitudinal relaxation time of the given solvent. Each summand in eqn. (12) represents the rate for a single contribution to the total rate from a reactant (0) + (j = 0, 1, 2,. . .I non-radiative vibronic transition. For electron transfer with small to moderate exothermicity (- AG < A,), the electron transfer reaction product is principally formed in the vibrational ground state (j = 0). However, in the case of the inverted Marcus region channels with j < -(A, + AG)/hvi may be accessible. Vibrational excitation of the high frequency modes accompanying the electron transfer lowers the energy gap in the inverted Marcus region, which results in a decrease in the effective activation energy and in the relative enhancement of the reaction rate. It is clear that both static (through A,) and dynamic (through T,_) dielectric solvent properties may

(13)

R++R-

R+ + R-

So+% -5 + -4 SO+%

So+-6

so+so

reaction

coordinate

-

enera

Fig. 7. Reaction co-ordinate diagrams for (a) a single and (b) mixed rubrene ECL systems. Potential energy curves are presented in the zero-order approximation (without removing the degeneracy at the crossing points).

A. Kapturkiewicz/ Electrogeneratedchemiluminescenceof rubrene

112

Since in most cases AG,, = AG,-r = 0 (corresponding to isoenergetic resonance electron transfer), eqn. (13) can be simplified further:

rubrene lows:

ECL efficiency +ec, can be expressed as fol-

( 17a) k Sl = k,=

$&Te”p(

-2%)

Neglect of small AG,, or AG, terms may lead to some overestimation (or underestimation) of appropriate k,, rates. However, in the view of data from Table 2, such over- or underestimation cannot exceed a factor of 1.5 in the rate. Reactions (2a) and (2~) are so exergonic that both lie in the inverted Marcus region (Fig. 7). In both cases vibronic excitation in the reaction products can take p!ace (vibronic sublevels are indicated in Fig. 7 by dotted parabolas), leading to an increase in the electron transfer rates. Depending on the I/ and S values, the electron transfer rate constitutes a superposition of a solvent-dynamics-controlled contribution and a nonadiabatic contribution [83]. A detailed discussion of this problem has recently been presented [86], with the main conclusion that the electron transfer rate achieves its maximum value over a wide range of the reaction exothermicities (for AG up to ca. - 1.5 eV for ACN solutions with the parameters V= 0.10 eV, S = 1 and T,_= 0.2 ps) and drops sharply for AG < - 1.5 eV. In the solvents used in this work the solvent dynamics limitations should be more pronounced (cf. values of 7L in Table l), particularly at low temperatures. Simple calculations (with V= 0.02-0.05 eV and S = 0.250.50) indicate that conclusions from ref. 86 can also be applied in the case of electron transfer reactions in the rubrene ECL system. The less exergonic formation of the rubrene T, state is expected to occur at the maximum accessible rate: (15) The more exergonic formation of the ground state is inhibited and is expected to occur at the rate k,, < k,,. In view of these conclusions and the fact that the ECL efficiency does not exceed a few per cent, it is probable that the overall rate of ion annihilation during rubrene ECL is mostly determined by the formation of the T, state (according to reaction (2~) with rate k,,). Consequently, eqns. (10) and (11) can be simplified to 49, = k&3&-1

(16a)

4m = 34-r/k,,

( 16b)

and to a first approximation

the overall value of the

In the two limiting cases (no triplet-triplet up-conversion and full triplet up-conversion) eqn. (17) can be further simplified to

w-9 ( 174 In the intermediate cases (rate of triplet-triplet upconversion comparable with the rate of the separation) the final expression is rather more complicated:

J exp( - &) k;,h

UPC

or l-

kupr:kdi.j]

“‘(

- -ii&)

( 18b) In view of eqns. (18a) and (18b) the temperature dependence on $pec,becomes understandable. Reduction of the temperature decreases the formation efficiency of both the S, state and the contact triplet-triplet pair. This is related to the exponential term in eqn. (18). However, reduction of the temperature (and of k,,,,) increases the efficiency of triplet-triplet upconversion. This is formally related to the pre-exponential factor. The two effects act in the oposite directions, leading to the appearance of a maximum on +ec, vs. temperature. The fact that T,, depends on the solvent viscosity can also be rationalized. At low temperatures k,, c k,, and eqn. (18b) can be simplified to

Quantitative analysis (according to eqn. (18~) and described below) of the relation between T and +ecl also allows more detailed discussion of the parameters A, and kupc. 3.6. Kinetic analysis of the rubrene ECL efficiencies The experimental results for the ECL efficiency (at low temperatures below T,,,) can be linearized as ln(4,,,) vs. l/T. The intercept values (in the range

113

A. Kapturkiewicz/ Electrogeneratedchemiluminescenceof rubrene TABLE 3. Results of the kinetic analysis of the temperature

dependence

of the ECL efficiency

Solvent

Temperature range/K

Statistical factor

&/eV

Statistical factor at 293 K

DME DMF BN BL

213-233 213-253 213-233 213-273

0.95 0.97 0.88 1.05

0.049 0.059 0.062 0.080

0.30 0.36 0.33 0.65

The following values of the ratio of the pre-exponential factors are predicted from the theoretical considerations: 13/12 = 1.08 for the case of full triplet-triplet pair up-conversion (11/12 = 0.92 if the population of the two quintet sublevels of the encounter pair with spin quantum number f 2 is neglected); l/3 = 0.33 for the case of no triplet-triplet up-conversion (encounter pair fully dissociative).

0X3-1.05 (Table 3)) allow us to conclude that at low temperatures the contact triplet-triplet pair is fully converted to the emissive S, state of rubrene, with the efficiency predicted by simple spin statistics (factor of l/4). They also indicate that possible quenching processes (of triplets introduced into up-conversion) are negligible and that the up-conversion rate k,,, is large compared with the separation rate kdiff. Thus, according to Eqn. (17c), the slopes obtained can be simply attributed to the A,/4 value. The observed sequence for DME, DMF and BN solutions (Table 3) agrees well with that expected from the values of the Pekar factor l/n* - l/c. Only in BL is the quantity found to be somewhat larger. This can be explained by taking into account the fact that in this solvent only reactions (2b) and (2d) are somewhat endothermic. If it is assumed that A,/4 is constant over the whole temperature range, the ratio x of the statistical factors (at a given temperature T) can be simply calculated from the following relation (using the A,/4 values evaluated as above):

kdi,

calculated

from the standard Debye equation The slope of the solid line corresponds to a value of 1 x 10” s-l, which agrees well with the triplet-triplet up-conversion rates found for aromatic hydrocarbons [87]. The rate k,, estimated in this way seems to be constant in all the solvents as well as over the whole temperature range studied; possible variations are less than the measurement accuracy. The discussion presented above strongly suggests that triplet-triplet up-conversion is marginal at room temperature and in the low viscosity solvents (with 77< 1 cP, which is satisfied for nearly all the ECL systems studied). Thus the ECL efficiencies at 293 K allow us to evaluate the outer reorganization energies A,,. This was done assuming that the ratio of the pre-exponential factors is exactly equal to l/3. The results obtained are shown in Fig. 9, with the solid line through the origin of the l/n* - l/e and A, coordinates. The A, values obtained are relatively small but are comparable with those found for electron self-exchange between tetracene and the tetracene radical anion [@I. In terms of the Marcus theory, the outer (kdi, =

8RT/3q).

x = &cl exp( A./4W

(19) DME, DMF and BN at room temperature lie in the range 0.30-0.36 (Table 3), which is in agreement with the value of l/3 expected for the case of a fully dissociative triplet-triplet encounter pair. The larger x value obtained in very viscous BL (1.74 CP at 293 K) suggests that in this particular case the triplet-triplet up-conversion takes place at room temperature. The statistical factors evaluated as described above depend on temperature changing from near unity (at low temperatures) to l/3 (at room temperature), indicating that T dependence on bet, can be described in the terms of eqns. (18aH18c). According to eqn. (1%~) the “effective” x value can be approximately described as follows: 13 "=12-T

3

kcliff

kUp, i-1

(20)

Figure 8 shows the relationship between the x values evaluated from eqn. (19) and the rate of separation

Ratio

ofthe statistical_factors

0.8

0

DME

0

DMF

A BL v BN

0.6

V.”

0.0

0.3

0.6

0.9

I.2

1.5

Fig. 8. Ratio of the pre-exponential factors as a function of temperature: ~ fit with triplet-triplet up-conversion rate kupc= 1 x 1O’O s-1.

A. Kapturkiewicz / Electrogenerated chemiluminescence of rubrene

114

o’30ho

0.3

and only the bimolecular triplet-triplet up-conversion can produce light. However, the ECL efficiency is rather low, much lower than expected from the amount of the triplet generated. As argued elsewhere [37,42], the maximum efficiency of an ECL system, in which the light is produced via “pure” T route, should reach about 0.06. The observed values for the mixed system are lower by about two orders of magnitude. It should be noted that the maximum value of 0.06 can be achieved only in the case of a uniform distribution of rubrene molecules in the triplet state. Of course, such a requirement is not satisfied under the conditions of ECL reactions, particularly if the triple-step-potential technique is used for the generation of reactants.

IeV

0.35

0.4

0.45

0.5

Fig. 9. Outer reorganization energy as a function of the Pekar factor l/n2 -l/e: corresponds to the reaction distance d = 0.95 nm.

reorganization

energy can be expressed as follows: (21)

Using the sum 1/2r_+ 1/2r+ from the solvent dependence of the difference in the redox potentials and from the slope of the solid line in Fig. 9, the value of d = 0.95 nm (in agreement with the value assumed for the reaction distance in the energetic considerations) is obtained. The discussion presented above implies that the efficiency of the T, state formation (reaction (2~)) is very large, probably close to unity, raising the question of why bimolecular up-conversion of these triplets (on the T route) is not observed in these experiments. The answer is simple, if the conditions of the experiment are considered. The case of the triple-step-potential can be treated as a diffusion-controlled reaction between two separated layers containing oppositely charged ions. Their annihilation occurs at the separation plane, forming a very thin layer containing the reaction products. The concentration profile (e.g. for the T, state) is such that diffusion out of this layer is much more pronounced compared with the formation of the bimolecular active complex needed for effective triplet-triplet up-conversion. The free triplets obtained from the separation of the directly formed triplet-triplet pair (reaction (2d)) behave similarly. In the mixed system (e.g. rubrene radical anion and N, N, N’,N’-tetramethyl-p-phenylenediamine radical cation or rubrene radical cation and p-benzoquinone radical anion [35,40,45]) only the formation of the T, state is possible because the energy released in ion annihilation is less than that required for reactions (2b) or (2d) (cf. Fig. 7)

4. Conclusions The results presented here indicate that the electrochemiluminescence of rubrene can be interpreted in terms of the Marcus theory. The reaction mechanism discussed is not the only one possible. However, it seems to be the most probable from the kinetic point of view. The available experimental data do not permit us to choose any other option. The present work should be treated as a semiquantitative approach until the role of other factors can be established. Measurements at high temperatures may give a decisive answer. From the kinetic model presented in this work we can predict that the ECL efficiency should pass through a minimum and increase if the temperature becomes higher than room temperature owing to the increase in the rate of formation of excited rubrene singlets. However, such experiments seem to be rather difficult because of the problems caused by the radical ion instability and the more pronounced parasitic processes. Preliminary studies gave inconclusive results. More detailed investigations of the influence of the magnetic fields (unfortunately it was not possible with our present instrumentation) appear to be required. Preliminary results reported in the literature [35,40,45] are not in conflict with the reaction mechanism proposed in this work and can be explained by assuming that triplet-triplet annihilation (within a triplet encounter pair) is being altered in the magnetic field. In a low viscosity solvent (e.g. DME or THF) such an alteration should be relatively small because the separation of triplet pairs is very fast. In very viscous media (e.g. DMF at low temperatures) such an alteration cannot increase the efficiency of the triplet-triplet up-conversion because this process is already sufficiently fast. In the case of intermediate viscosities (e.g. DMF or QCN at room temperatures) application of the magnetic field should lead to the enhancement of the ECL efficiency because both the up-conversion and separa-

A. Kapturkiewicz / Electrogenerated chemiluminescence of rubrene

tion processes have similar rates. In fact, such behaviour has been described in the literature, additionally supporting the proposed reaction mechanism. The measurements of the combined influence of temperature and magnetic field in DME or THF solutions should provide a decisive answer. It is expected that the effect of the magnetic field on the efficiency of rubrene ECL in these solvents will be strongest for the moderately negative temperatures (ca. - 20°C). This prediction arises from the temperature-induced changes in the branching ratio between the S and IT routes caused by changes in DME or THF viscosity. Acknowledgements

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