J. Phys. Chem. 1994,98, 228-232
228
Photochemical Generation of Radical Cations from Thiophene Oligomers V. Wmtgens, P. Valat, and F. Gamier' Laboratoire des Matbriaux Molbculaires. C.N.R.S., U.P.R. 241, 2-8 rue H . Dunant. 94320- Thiais, France Received: May 24, 1993; In Final Form: September 10, 1993' The photoexcitation process for a series of thiophene oligomers nT, from terthiophene to sexithiophene, has been analyzed in dichloromethane solution using laser flash photolysis. The immediately formed excited triplet states 3nT*, which have been characterized by their visible absorption spectra, show a lifetime on the order of a few tens of microseconds. The corresponding radical cations nT'+ have been obtained either by electron transfer from 3nT* to an electron acceptor or by direct photogeneration from nT using high excitation energy. The absence of an oxygen effect together with triplet sensitization experiments indicate that nT'+ cations are formed directly from the singlet state 'nT*. The absorption spectra of these radical cations, which greatly vary with conjugation length n, are in agreement with literature data for nT'+ generated chemically in solution. The process of radical cation formation by photoionization from lnT* and its second-order decay mechanism are analyzed. The molecular properties of radical cations are discussed in terms of charge-transport properties of the corresponding solid-state materials.
Introduction In order to understand the electrical conductivity of *-conjugated polymers, as well as their promising optical and nonlinear optical properties,' charge carrier generationand transport within these polymers have become the subject of increased interest. In the generally accepted model, the coupling between the conjugated *-electron system and the local chain geometry leads to localized charge$, and in the general case of conjugated polymers with a nondegenerate ground state (such as polythiophenes), polarons and bipolarons are expected to form from charged injection. Due to large changes in chain geometry upon passing from the neutral aromatic molecules to the quinoid charged states, the energy levels of these states are shifted from the band edges into the gap. Since the strength of the electron-lattice coupling, which determines the degree of localization of charges, is not well known, many studies on the electronic and structural properties of these charged states have been carried out. Chemical doping has often been used to inject charges in conjugated chains, leading to the conclusion that doubly charged bipolarons are the preferential charge-storing species. However, chemical doping requires a dopant counteranionsto be associatedwith the positively charged chains, and the resulting coulombic interaction must largely contribute to the formation of the bipolaron. Therefore, photoexcitation across the gap, which does not involve any chemical species,appears to be more appropriatefor analyzing these charged molecules. Studies performed on various conjugated polymer chains have allowed the characterization of polaronic states through their subband-gap optical transitions.ss Another charge injection method, involving electrodes in device structures, has been proposed for the study of polaronic states. This has proven to be very powerful for determining the nature of the chargestoring states.6 These two last methods appear to be limited by the fact that the concentration of the detected intermediates is very low, which makes the study of short-lived species very difficult. Furthermore, extrinsic effects, such as stabilization of charge species at conformational defects of the chains, do not allow a clear description of the intrinsic electronic and kinetic properties of the states produced by the injection of charge in these polymer materials. However, the well-defined *-conjugated oligomers, which were later proposed as models for the parent conjugated polymers, appear much better designed for the analysis of key parameters (such as conjugation length) affecting the characteristics of the charged states involved in charge storage.7-9 Abstract published in Advance ACS Abstracts, December 1, 1993.
The solubility of these oligomers allows them to be studied both as materials in condensed phase and as molecules in homogeneous phase, the final objective being to describe the characteristics of the materials in terms of molecular properties.1° We report here the analysisof the fate of photoexcited short-conjugatedthiophene oligomers, from terthiophene to sexithiophene in solution, and describe the transient species associated with the photophysical process following photoexcitation.
Experimental Section The thiophene oligomers, terthiophene, 3T, quaterthiophene, 4T, and sexithiophene, 6T, were synthesized followinga previously described procedure.11 Tetracyanoethylene, TCNE, methylviologen, MV2+,and tetrapentylammoniumchloride, from Aldrich, as well as the solvents, spectrograde from Aldrich, were used without further purification. UV-visible spectra were recorded on a Varian-Cary spectrophotometer, Model 219. The laser flash photolysis experiments were performed on an apparatus which has been previously described.'* A frequencytripled (355 nm) and -quadrupled (266 nm) Nd-YAG pulsed laser, delivering pulses of 8-ns duration, was used as the excitation source. Pulse energies ranged from 5 to 20 mJ at 355 nm and from 2 to 5 mJ at 266 nm. The energy of the exciting light beam was attenuated in a controlled way by the use of optical sapphire slides with optical densities of 0.1 at 355 nm. A conventional setup was used for time-resolved spectral analysis, involving a pulsed lamp (Applied Photophysics) as analyzing light source, with crossed-beamoptical arrangement, a monochromator with 4-nm resolution, and a S20 photomultiplier coupled to a differential comparator amplifier of a digital oscilloscope. Data storage and accumulation, mathematical treatment, and decay time analysis were performed on a microcomputer. Results
Characterization of the Excited States of Tertiophene, Quaterthiophene, and Sexithiopbene. Laser excitation at 355 nm of a deoxygenated 3T solution in dichloromethane (absorption maximum at 354 nm) immediately generates a transient with an absorption maximum at 470 nm (Figure 1). Considering the literature results from Scaiano13and the observed quenching effect by oxygen and sensitizationby benzophenone, it can be suggested that this transient corresponds to the triplet state 33T'. When obtained under high excitation energy, this transient decays with complex kinetics, involving first- and second-orderterms, which
0022-3654/94/2098-0228$04.50/00 1994 American Chemical Society
Photochemical Generation of Radical Cations
400
500
600
700
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 229
800 NM
Figure 1. Triplet-triplet absorption spectra of 3T (A),4T (+), and 6T ( 0 )in deoxygenated dichloromethanesolution, recorded 4 ps after laser
excitation.
suggests a nonradiative triplet-triplet annihilation pathway. At low excitation energy and in a thoroughly deoxygenatedsolution, the lifetime of this triplet is longer than 50 ps. Similar results have been obtained with a 4T solution in dichloromethane (absorption maximum at 390 nm). The observed transient spectrum appears structured with two maxima at 560 nm and 610 nm (Figure 1). The lifetime of the corresponding triplet '4T* is on the order of 35 ps. In the same way, excitation of a 6T solution in dichloromethane (absorption maximum at 432 nm) leads to a transient triplet with an absorption maximum at 680 nm and a lifetime of about 24 ps (Figure 1). For these three compounds, the phosphorescence emission in EPA solution (a classical ether-isopentane-ethanol mixture) is too weak to allow a precise determination of the energy level of the triplet state of these thiophene oligomers. A value of 40 kcal/mol (1.74 eV) has been given for 33T* in the 1iterat~re.I~ We have previously determined the lifetime and the quantum yield of fluorescence for these three compounds in dichloromethane:Is 0.135 ns and 0.055, respectively, for 3T, 0.24 ns and 0.16 for 4T, and 1.1 ns and 0.32 for 6T. The energy level of the first excited singlet state has been evaluated as 3.12,2.82, and 2.52 eV respectively for 3T, 4T, and 6T.15 Photosensitized Formation of Radical Cations 3P+,4P+,and m+.Radical cations of conjugated thiophene oligomers, nT*+, can be obtained from the photogenerated triplet )nT* by electron transfer to an electron acceptor present in the solution, as already described by Scaiano et al. for 3T*+.16Using tetracyanoethylene (TCNE) as electron acceptor, an electron transfer occurs from 33T*, generating 3T'+ with a rate constant for electron transfer of 2.3 X 1010 M-l s-1. The radical cation 3T'+ has been characterized by its absorptionspectrum, which has an absorption maximum at 550 nm (2.25 eV) in acetonitrile. Following the same procedure, but using detection equipment extended to the far red, we observed that the absorption spectrum of the radical cation 3T'+ has a second absorption band located at 850 nm (1.46 eV). These two absorption bands decay following the same second-order kinetics pathway, which corresponds to the back electron transfer from T C N P - to 3T'+. The radical cation 4T'+ has been obtained under similar conditions by electron transfer between the photogeneratedtriplet 34T* and TCNE as the electron acceptor in dichloromethane. The addition of increasing concentrations of TCNE in the 4T solution leads to the decrease of the lifetime of the triplet 34T* with a quenching rate constant of 1.5 X 1010 M-1 s-1, and to the appearance of a new transient with an absorption maximum at 650 nm (1.91 eV) which is associated with the radical cation 4T'+. The radical cation 6T'+, formed by the same procedure with a quenching rate constant 1.9 x 1010 M-I s-l, has an absorption maximum at 780 nm (1 5 9 eV).
400 500 600 700 800 NM Figure 2. Absorption spectra of 3T'+ (A), 4T'+ (+), and 6T'+ ( 0 ) generatedfrom TT, QT, and ST dichloromethanesolutions, respectively, by high-energy excitation, recorded 100 ws after laser excitation.
e]
A O.D.
En
E Energy Figure 3. Variation of absorption of 3T'+ (A) and 4T'+ (+) followed respectively at 550 and 650 nm versus excitation energy.
Direct Photogeneration of Radical Cations nP+. When the light excitation energy directed to a thoroughly deoxygenated dichloromethane solution of oligomer nT is increased by a factor 10 compared to the preceding experiments, transient spectra of their triplet excited state 3nT* are observed in the microsecond time scale. Their decay kinetics are faster, due to a very efficient triplet-triplet annihilation process under those conditionsof light excitation. On a time scaleof some 20 ps,when triplet absorptions are no longer observed, other long-lived transients can be detected in the medium which show absorption characteristics similar to those of the radical cations nT'+ previously obtained by electron transfer between the triplet )nT* and a strong electron acceptor. The spectra of the transient obtained under these conditions of direct photoexcitationin dichloromethane solutionare presented in Figure 2. When performing these experiments in aerated dichloromethane solutions, a fast monoexponentialdecay of the triplet 'nT* is observed, as expected from its quenching by oxygen. However, the transient radical cations nT'+ are still formed, and their concentrations, as given by the optical density variation at the absorptionmaximum, are not modified. These results indicate that the triplet state %T* is not involved in the formation of radical cation nT'+ under high-energy photoexcitation. An increase in optical density associated with the radical cations nT'+ is also observed when the light excitationenergy is increased, as shown in Figure 3, a straight line being obtained at low energy. Table 1 shows thevariation of optical density at 550,650, and 780 nm after excitation of 3T, 4T, and 6T dichloromethane solutions of the same initial optical density at 355 nm. Molar extinction coefficient values have been given in the literature for 3T'+ in acetonitrile as s = 29 OOO M-1 cm-1 16 and for 6T'+ in dichloromethane as e = 43 000 M-1 s-1.17 The increase in the
Wintgens et al.
230 The Journal of Physical Chemistry, VoL 98, No. 1, 1994
TABLE 2 0 bcal Density Variation Measured at Different Text) of a Mchloromethane Solution of 3T, Wavelengths Excited at 355 nm, or 3T and Benzophenone, Excited at 266 nm
(%e
l-4
cn Z
W
0 -I
< u
AOD
AOD
AOD
0.50
0.023
0
w
c
a 0
;Lx= 355 nm
Bzo
+ 3T
;Lx= 266 nm
500 600 700 800 NM Figure 4. Transient spectra of 3T dichloromethane solution obtained a t high-excitation energy and recorded after 20 p (O),200 ps (A),and 700 300
PS
400
(+I.
TABLE 1: Optical Density Variation (AOD), Concentration (c), and Quantum Yield of Formation (&). of 3T'+, 4T'+, and W +in Mchloromethane, Analyzed Respectively at 550, 650, and 780 nm 3T'+ AOD c (106 M) *form
0.122 4.2 0.02
4T'+ 0.384 11.5 0.05
6T'+ 0.094 2.2
0.01
extinctioncoefficient with conjugation length, which has already received some theoretical support,'* allows us to roughly interpolate the value of c for 4T*+to be 33 500 M-l s-I. Even if these c values have been used in a first approximation, the obtained data allow us to evaluate the concentrations of radical cations nT'+ and their formation quantum yields to be 2%, 5%, and 1% for TT*+,QT'+,and ST'+ (Table l), respectively. The yield of formation of the radical cations nT'+ appears to be solvent dependent. In the case of 3T, no signal associated with 3T'+ is obtained when using benzene, cyclohexane, diethyl ether, or ethyl acetate as solvents. A weaksignal obtained in acetonitrile increases upon addition of water. In chlorinated solvents such as 1,2-dichloroethane,dichloromethane,or chloroform,the signal becomes large and the relative yields of formation of 3T'+ reach 0.43, 0.77, and 1, respectively. Once formed, the radicalcationsnT'+ decay in dichloromethane without any modification of their visible absorption spectrum, as shown in thecaseof 3T'+ (Figure4), which rules out the formation of any species absorbing between 400 and 870 nm. This decay of 3T'+, analyzed at its absorption maxima of 550 and 850 nm, strictly follows second-orderkinetics, whether in the presence or absence of oxygen. Using 29 000 M-l s-1 for the molar extinction coefficient of 3T'+, a value of 2 X 108 M-1 s-1 is obtained for the secondsrder decay rate constant of this radical cation. However, in the bleaching spectral range (320-390 nm), the optical density variation is not proportional to the decrease of absorption at 550 and 850 nm, suggesting that the decay of 3T'+ leads to the formation of a new species absorbing in the near-UV range, the structure of which will be discussed later. Transients Obtained by Triplet Sensitization. In order to more precisely determine the contribution of the triplet state 3nT* in the formation of the radical cation nT'+. experiments under sensitized triplet 3nT* formation have been performed. In the case of 3T, the triplet 33T* has been obtained through energy transfer from benzophenone. This compound has a large absorption at 266 nm, where the absorption of 3T is low, and a yield of unity for the intersystem crossing process to its triplet state whose energy is 69 kcal/mol. For reference, deoxygenated dichloromethane solutions of 3T have been excited at 355 nm, and the decay of the obtained
transients analyzed at 465 nm for the triplet 33T*, and at 550 and 850 nm for the radical cation 3T'+. Variations of optical densities are listed in Table 2. Those correspondingto the radical cation were recorded 100 ps after excitation in order to eliminate any uncertainty due to residual triplet absorption. For an optical density variation of 0.5 at 465 nm, associated with triplet 33T*, optical density variations of 0.026 and 0.007 are observed at 550 and 850 nm, respectively, associatedwith the radical cation 3T'+. Benzophenone was added to an identical 3T solution until its concentration was 1.5 X 1W M. This sensitizer absorbs more than 90% of photons at the exciting wavelength of 266 nm. The benzophenone triplet formed by laser excitation is quenched by 3T, and the growth of the signal due to 33T* (monitored at 465 nm) can be observed on a 2-ps time scale. This efficient energy transfer from the benzophenone triplet allows population of the triplet 33T* level, the concentration of which can be controlled by varying the excitation energy at 266 nm. When adjusting this excitationenergy to obtain the same initial concentrationof triplet 33T* (AOD = 0.5 at 465 nm) from this sensitization process as from the preceding direct excitation, one observes that the signal due to the radical cation 3T'+ at 850 nm is no longer detected under these conditions. The signal observed at 550 nm is due to the ketyl radical, formed by the photoreduction of benzophenone in dichloromethane. The lack of the observation of the radical cation 3T'+ and of the radical anion of the benzophenone confirms that the quenching of the benzophenone triplet by 3T is mainly due to an energy-transfer process and not to an electron-transfer process. This experiment also shows that the radical cation 3T'+ is not formed through the triplet state 33T*but directly from the excited singlet state 'nT*.
Discussion These results, determined in the homogeneous phase, provide insight into the molecular properties of thiophene oligomers. The transient absorption spectra of their excited triplet states 33T*, )4T*, and 36T* (Figure 1) show a shift toward lower energy as the conjugation length increases, which, togetherwith the presence of vibrational structure in the fluorescence spectrum, argue for a planar geometry for both the excited singlet and triplet states of these thiophene oligomers. The spectra of the three radical cations nT'+ (Figure 2) agree with comparablespectra previously published in the literature, although obtained under different experimental conditions. The spectrum of 3T*+observed in the present work parallels the one obtained by Scaiano through electron transfer between the excited triplet 33T* and electron acceptor TCNE or MV2+ 16 but has been extended into the far red region which has a band at 850 nm (1.46 eV). In our experimentalconditions, no ground-statecharge-transfercomplex has been observed. Relatively stable radical cations 4T'+ and 6T*+have been generated from the correspondingoligomers 4T and 6T by chemical oxidation with Fe3+either in solution or in a zeolite. These have also been characterized in the literature by visible and near IR spectroscopy which show absorption maxima at 645 nm (1.92 eV) and 1065 nm (1.16 eV) for 4T'+ and 780 nm (1.59 eV) and 1473 nm (0.84 eV) for 6T'+.9J9 Similarly,thevisible and near infrared absorption spectraobtained here for 4T'+, whether generated by electron transfer or by direct excitation, and for 6T'+, obtained by direct excitation, agree with literature results on radical cationsobtainedby chemical oxidation
Photochemical Generation of Radical Cations of 4T and 6T in various environments. It must be pointed out that this latter method of chemical oxidation of thiophene oligomersdid not allow the spectral characterization of the radical cation 3T'+, as the reactivity of this species is much too high to beobserved by conventional spectroscopy. This 3T'+ leads directly to the dimer 6T by homocoupling and, by further oxidation, to the radical cation 6T'+.9 Various methods to stabilize the radical cations derived from short conjugated thiophene oligomers have been proposed, including chemical substitution on the a carbon atoms by alkyl groups20or silane groups,Z1which prevent the fast homocoupling at the a positions, or incorporation of the oligomer in a ~eo1ite.I~ Although these substituentor environmental effects, observed in these last works, may be thought to modify the intrinsic properties of 3T'+, they have allowed precise spectroscopic characterization of the radical cations. The presence of a transition from which the energy varies between 2.14 and 2.38 eV is confirmed. They also showed the existence of a second transition at lower energy (1.4G1.49 eV), which is expected from theoretical considerations for this radical cation.22 Comparable values of transition energies have been also obtained for radical cationsderived from thiopheneoligomerssubstituted at both their CY and their w ends by alkyl groups.20 The low-energy transition in thiophene oligomer radical cations, 1.49 eV, 1.19 eV, and 0.86 eV for 3T*+,4T*+and 6T*+,respectively, is of particular interest, as it represents the transition between the highest fully occupied molecular orbital (HOMO) and the single occupied molecular orbital (SOMO). This gives theenergy level of the radicalcations. Key questions in more fully understanding the role played by these radical cations in the solid-state propertiesof these materials concern the mechanism of their generationas well as the pathways for their evolution. The different oligomers show comparable behavior, but due to the very low solubility of 4T and 6T, the discussion will be mainly focused on the more soluble 3T. The photochemically induced formation of a radical cation can proceed through either a biphotonic or a monophotonic process, occurring with the excited singlet or triplet states. The analysis of the dependence of the yield of radical cation 3T'+ formation on the energy of the excitation pulse shows a linear variation, which allowsonetoconclude that mainly monophotonic processes are involved in this photochemicalreaction (Figure 3). The deviation from linearity of about 16%, as observed in the case of 4T'+, suggests that some participation of biphotonic processes may occur, but to a very limited extent. An increase by a factor of 6.25 would be expected from a purely biphotonic process. The participation of the medium in the production of radical cation has also been proposed in the literature.'& In the case of oxygenated acetonitrile solution of 3T, formation of 3T'+ has been observed and tentatively attributed to the quenching of the triplet state 3T* by oxygen (with the associated formation of superoxide ion 0 2 . 9 ; the lack of the observation of 3T'+ in a deoxygenated acetonitrile solution was probably due to the low concentration of 3T*+and to the strong signal of the long-lived '3T* overlapping the weak one of 3T'+. Under the present experimental conditions, however, the yield of formationof radical cation nT'+ appears independent of the presence of oxygen. In aerated dichloromethanesolutions, the excited triplet state 33T* is effectively quenched by oxygen, as shown by the decrease of its lifetime by a factor of 30. The very short lifetime of the singlet excited state precludes quenching of '3T* by oxygen, and no decrease in the yield of formation of 3T'+ is observed in aerated solutions. Furthermore, in deoxygenatedsolutions, the observed decay of the excited triplet state follows second-order kinetics, which indicates triplet-triplet annihilation, but with no modification of the yield of radial cation production. A kineticanalysis carried out at different wavelengths did not indicate a direct buildupof the radical cation 3T*+from the triplet 33T*, although it must be remembered that the very strong signal due to the triplet could mask the weaker ones of the radical cation, making the observation of a variation of 3T'+ within the decay of 33T* difficult.
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 231 Finally, the discussion of the results obtained by triplet sensitization allows one to rule out the hypothesis of formation of 3T'+ from 33T*. In fact, the triplet energy of benzophenone, 69 kcal/mol (2.99 eV), appears compatible with the sensitized formation of the '3T* triplet, for which an energy of 40kcal/mol has been given.14 However, despite this very efficient sensitized production of a large concentration of triplet 33T*, no formation of radical cation 3T'+ could be observed, as shown by the absence of its correspondingabsorption spectrum. One can thus conclude that in dichloromethane solution, the radical cation 3T'+ must be formed directly from the excited singlet state SI,through a monophotonic process which appears compatible with theobserved short lifetime of 130 ps.
- hu
+
3T '3T* 3T" eTwo opposite contributions can be thought to govern the efficiency of this photochemical production of radical cations. On one hand, the increase in the conjugation length of the thiophene oligomer nT, from 3T to 6T, leads to a decrease of its electrochemicaloxidation potential, along with a decrease of the excited singlet state energy level. On theother hand, an activation barrier must exist for the conversion of excited singlet state 1nT* into radical cation n P + , which can concurrently increase as the conjugation length n increases. The experimentally observed increase in the yield of radical cation by a factor of 2.7, when passing from 3T to 4T, and decrease by a factor of 2 when passing from 4T to 6T, shows that these two contributions are involved in the photochemical formation of radical cations nT'+. Monophotonic photochemical ionization of easily oxidizable chemical compounds has already been described in the literature, but not such as for N,N,N',N'-tetramethyl-pphenylenediamine,23 for this class of conjugatedderivatives. The yield of radical cation formation has been shown to depend on the solvent used, but solvent polarity does not seem to be the pertinent criterion. In fact, whereas the yield for 3T'+ is large in chloroform, no radical cation formation is observed in less polar solvents, such as cyclohexane,or in more polar solvents such as ethyl acetate. This reaction appears to require chlorinated solvents, which are known to capture electrons through the following rea~tion:~' RCl+e--R'+ClThe observed rate constants for this reaction in butyl chloride, dichloromethane, and chloroform are 4.5 X lo8 M-l s-l, 6.3 X 109 M-1 s-I, and 3 X 10'0 M-1 s4,2' respectively. In the case of 3T'+, assuming the same value of 29 000 M-I cm-l for the molar extinctioncoefficient in these three solvents, yields of 0.096,1.796, and 2.2% are respectively obtained for 3T*+ formation. The observed increase in yield appears to be directly proportional to the ability of the solvent to capture the electron produced in the photoionization step of 3T. In the case of nonchlorinatedsolvents, recombination between the radical cation and the electron may be much faster than the solvation of the electron, which would explain the absence of radical cation formation in those media within the experimental detection limit. Adding water, known for its electron-solvatingproperties, increasesthe yield of radical cation in acetonitrile,which argues for this interpretationof solvent effect on the formation of nT'+. The decay mechanism of the radical cation nT'+ formed by photoionization from the excited singlet state must now be considered. The experimentalresults show that the decay follows second-order kinetics, with a rate constant of 2 X 108 M-I s-I for 3T*+ in dichloromethane. These second-order kinetics can be explained by an annihilation mechanism between two radical cations nT'+ or by the reaction between the radical cation and another species,photochemically generated in equal concentration as nT+, such as the radical CH2CI' produced by the reaction of dichloromethane with the electron. However, in an oxygenated medium, the CH2CI*radical reacts very rapidly with oxygen, as the rate of this type of quenching reaction is about 5 X lo9 M-'
Wintgens et al.
232 The Journal of Physical Chemistry, Vol. 98, No. 1. 1994 5-1.26 Owing to the large excess in oxygen as compared to radical cation in the analyzed solutions (a factor larger than lo3),the participation of CHzCI' can thus be ruled out. Another species generated in similar concentrations as 3T'+ is the anion C1-, which can be proposed as quenching species for 3T'+. Furthermore, quenching experiments carried out on the radical cation 3T'+ by the addition of tetrapentylammonium chloride led to a rate constant of 7 X 105 M-I s-l, much lower than theobserved secondorder rate constant of the decay. The most probable pathway for the radical cation nT'+ decay thus appears to consist of a bimolecular radical cation-radical cation annihilation. The spectral analysis of the reacting medium, underdone up to the millisecond time range, shows that the fast disappearance of 3T*+absorption a t 550 nm is not compensated by any variation of absorption at long wavelengths and, on the contrary, that after 3T'+ has fully decayed, a remaining absorption band exists in the bleaching spectral range in the near UV, around 360 nm. Different species could be considered as resulting from this bimolecular annihilation process. As recently proposed by L. L. Miller et al.27in the case of chemical or photochemical generation of oligothiophene radical cations nT*+,a *-dimerization can occur between thesespecies, following an equilibrium reaction governed by the radical cation concentration and solvent polarity. This *-dimer (nT+)2has been duly characterized, absorption bands being expected around 520 and 840 nm for a dimer issued from terthiophene deri~atives.2~The absence of such transitions in the observed spectrum (Figure 4), together with the very low concentrations of species produced by this photoionization process, argues for the absence of such a dimerization process. As a second hypothesis, the chemical coupling of these 3T'+ radical cations could be envisioned to lead directly to the hexamer sexithiophene, 6T, following a pathway which has been proposed for the chemical1IJ9 or electrochemical28 polymerization process of thiophene, involving first a deprotonation of 3T'+ followed by the coupling of the resulting radicals 3T' into the neutral state of 6T. In the case of a very fast deprotonation process, under the experimental conditions used here, the transient absorbing a t 550 nm should be attributed to the radical 3T' which should show the same absorption spectrum as the radical cation 3T*+. Furthermore, in an aerated reacting medium, this radical 3T' should be rapidly quenched by oxygen, with a first-order kinetic decay. The contradiction between these conclusions and experimental observations precludes this hypothesis. Finally, this bimolecular interaction between two radical cations 3T'+ could be thought to lead to a transient (I), which has already been proposed in the pathway from 3T'+ to 6T.19927928This species should possess spectral properties close to those of its unconjugated terthiophene subunits and absorb in the near UV range.
The experimentally observed absorption only in the near UV range argues for this last hypothesis. As already shown in a previous work, it must also be pointed out that this reactivity of oligothiophene radical cations nT*+ appears largely to decrease as conjugation length n increases, the species 6T'+ showing a long term stability." These characterizations of the homogeneous-phase properties of radical cations associated with conjugaged oligothiophenes could be of high potential interest for analyzing the structural factors which control the charge-transport properties of conjugated materials in the solid state. As a matter of fact, well-definedconjugated oligomers are interesting models for understanding the effect of conjugation length on the electrical properties of these materials, through the description of the characteristics of their charged states, the polaronic type radical cation, and the bipolaronic type dication. Although great care must be taken when homogeneous-phase properties are compared with solid-state ones, the relatively easy determination of spectroscopic and kinetic data for oligothiophenes
and their related charged species in solution appears to be a powerful way for describing, at least relatively, the evolution of these species. In this regard, it has been shown from electrical characterizations that the chargetransport efficiency, as expressed by the carrier mobility value, increases by many orders of magnitude upon varying the conjugation length, being unmeasurable for 3T and increasing to l t 7 , lt5, and 1 t 2 cm2 V-I s-', successively, for 4T, 5T, and 6T.29 The charged storing species associated with charge stransport in these solid-state oligomers has been shown to be the radical cation nT*+,30and it appears tempting to relate this increase to the concomitant increase in stability of these radical cations, as observed in the homogeneous phase. Further work under way, toward a precise description of the energetic levels associated with these charged species, should allow a quantitative interpretation of the effect of conjugation length on charge transport in conjugated materials.
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