513
J. Phys. Chem. 1993,97, 513-516
Effect of End Substitution on Electrochemical and Optical Properties of Oligothiopbenes Pedro Garcia and Jean Micbel Penuut Direction Recherche et Dheloppement, TELEMECANIQUE, 33 bis Avenue du MarCchal Joflre, 92000 Nanterre, France
Philippe Hapiot Laboratoire d’Electrochimie Mol8culaire, UniversitL Paris 7, 2 place Jussieu, 75251 Paris Cedex OS, France
Vbronique Wiatgens, Pierre Vaht, Francis Gander, d Didier Dehbouglise’*t Laboratoire des Matbiaux Molhlaires, CNRS, 2 rue Henri Dunant 94320 Thiais, France Received: August 19, 1992; In Final Form: October 13, 1992
A series of oligothiophenes (from bithiophene to sexithiophene) substituted at the ends of the chain by various electron donor or acceptor groups or atoms (methoxy, nitro, or bromo) have been prepared. Their electrochemical oxidations have been studied using microelectrodes in order to determine accurately the formal redox potential of the radical-cation generation. The stability of the latter can be deduced from the required potential scan rate. Electron donor substituents (methoxy and bromo) stabilize the radical-cation forms, and the dications can also be obtained from these substituted terthiophenes. The optical properties of the compounds have also been investigated. Only the nitro-substituted group induces a siknificant increase of the fluorescence quantum yield and of the lifetime of the excited state. For solution study, this positive effect is only limited to the short oligomeric chains for which an intramolecular charge transfer is evidenced by solvatochromic effects.
SCHEME I: General Structure of the Compomdn Studid
htrodlletioll
Oligothiophenes, short conjugated chains in which thiophene units are linked by their a positions, have recently been the focus of much attention. They are good models for studying charge storage in conducting polyniers.1 Alkyl substitution increases the solubility and the tractability of these organic materials, allowing refined *studies of longer conjugated chains to be performed? ’Fheifiuse as semiconductors in electronic devices has been p r o p ~ ~and ~ dthe , ~first all-organic transistor was based on sexithi~phene.~ Some interesting properties in nonlinear optics are also expected for these mat~rials.~ Band gap and h y p c p larizabilities of diiodoterthiophene and mono- and dinitroterthiophene have been measured, and interesting effects of this kind of substitution have been so demonstrated? Theoretical calculations of hyperpolarizabilities of conjugated chains substituted on one end by a donor group and an acceptor om on the other have also been proposed showing that good results may be obtained for polythiophenechains and that a maximum is possibly obtained for quaterthi~phene.~ Thin films of oligothiophenes are generally more ordered than the corresponding ones made from the conjugated plymer? They are good candidates for numerous applicationsin electronics and optoelectronics. So we thing that an extensive study of substituent effect on oligothiophenes may be of interest in view of these applications. Silyl substituent effects at the ends of oligothiophene chains have been fully investigated recently, and it appears to provide a very weak effect on theelcctrmxidationpotential* and a slight bathocluomic shift of the photoabsorption band.* We propose here an electrochemical and optical study of a differently end-substituted oligothiophene Series (from bithiophene to sexithiophene) where substituents are nlrro, methoxy, and bromo (Scheme I). High scan rate cyclic voltammetry using microelectrodes was performed to gain some reversibilii for the radical cations and the dications and so as to estimate the stabilizing effect of the substituents on the ionized states. The Stokes shift of the fluorescent emission, the qdntum yield of the latter radiative mode, and the lifetime h b ” i r a d’Ionique et d ‘ E & r o c W du Salk de Grcnobk ENSEElj, IblFG, BP 75, F-38402 St. Martin d’Hbtec, France. t Reant addrar:
0022-3654/58/2091-0513S04.00/0
pl‘
n = 0,1,2, or 4;R * Br, OCH3, or N02; R’= H,Br, OCH3 or NOz; R”= HexctptinonecaseforwbichR”=Br. 1nthetablm.thecompowh arc designated by their substituents and a digit followed by the letter T repreacntingthe oligomericchain length in thiopheneunits. For instance, 5-brom~5”-methox~e~hiophene is named Br-3T-OCH3 and the
trisubstituted compound Br-(Br3T)-NO2.
of the excited states are determinedin order to assess in a different way the substituent effect on the electronic properties of these conjugated structures.
Experiareat.l sectioll 1. Organic Synthesis. The various compounds have b a n prepared according to known organic reactions: condensation of aryl halides with arylmagnesium1° or arylzinc,” homocoupling of aryllithium compounds,” starting from 2-methoxythiophene (Lancaster), 5-bromo-2-nitrothiophene (Lancaster), and bithiophene (Aldrich). Brominations have b a n performed using N-bromosuccinimide (Aldrich) except for nitroterthiophenefor which bromine was used. Experimental procedures are fully described in a patent12and all the compounds were characterized by elemental analysis, N M R and mass spectra. 2. ElectrocbedcalExperiments All the cyclic voltammetry experiments were carried out at 20 f 1 O C using a cell equipped with a jacket allowing circulation of water from the thermostat. Oxygen was removed from all solutions by bubbling argon. Oligomer concentrations were under or c l o d to lO-) M. The counter electrode was a Pt wire and the reference electrode, an aqueous saturated calomel electrode (Eo/SCE = Eo/NHE 0.2412 V)with a salt bridge containing the supporting electrolyte. The SCE electrode was checked against the ferrocene/ferrocb
-
(6
1993 American Chemical Society
Garcia et el.
514 The Journal of Physical Chemistry, Vol. 97, No. 2, 1993
nium couple ( E O = 0.405 V/SCE) before and after each experiment. All the potentials are reported versus SCE. For slow cyclic voltammetry (u < lo00 Vd), electrochemical instrumentation consisted of a PAR Model 175 universal programmer and of a homebuilt potentiostat equipped with a positive feedback compensation device.13 The data were acquired with a 3 10 Nicolet oscilloscope. The working electrodes were a 3 mm diameter disk glassy carbon or a 1 mm diameter gold or platinum disk. In the case where microelectrodes were used, the working electrode was a gold wire (10 pm diameter) sealed in soft glass.“ The signal generator was Hewlett-Packard 3314A,and thecurves were recorded with a 450 Nicolet oscilloscopewith a minimum acquisition time of 5 ns per point. The potentiostat was the same as previously d&bed.l5 3. UV-Visible Spectroscopy Experimenb. The UV-visible absorption spectra were recorded with a Cary Model 2415 spectrophotometer. The fluorescenceand excitation spectra were obtained using a Perkin-Elmer Model MPF-44B apparatus, equipped with a DCSU-2 spectral correction unit. The fluorescencc quantum yields were measured in dichloromethane relative to quinine sulfate in aqueous sulfuric acid (@r = 0.55). The energy of the 0,O transitions (E0.0) was taken as the crossing point of both the excitation and absorption spectra. Singlet lifetimes were measured by excitation with a frequencytripled pulsed YAG laser (from B. M. Industries) of 30ps fwhm. For lifetimes greater than 1.5 ns the light was focused through a cylindrical lens onto the sample placed in front of a photodiode. The output from the photodiode was fed into a Tektronix 7912 AD digitized oscilloscopeand the data, which were stored in an Apple 11+ microcomputer, could be displayed on a HewlettPackard Model 7470A graphic plotter. Averaged decays could be analyzed directly by the microcomputer. For shorter lived fluorescent species, a TSN 506 streak camera was used with a ca. 10ps time resolution,and the fluorescencewas focustd through an optical coupling on the slit of the streak camera and the data were transferred to a multichannel analyzer which enabled the accumulation of several decay measurements. In this way, lifetimes of 50 ps or more could be analyzed in the same way as described for data stored in the digitized oscilloscope. Resdb and Iwsclrarion
1. Elect“icaI
Study. The electrooxidationof thiophene
oligomers is often an irreversible process mainly because the electrogenerated radical cations can undergo very fast a,& coupling leadingto higher oligomersor poly”. Thereactivities of these radical cations decrease when the oligomeric chain becomes longer, but oxidative homocoupling has been observed up to the sexithiophenes.”Sd As a consequence, formal redox potentials ofiadical-cation generation are difficult to determine in these cases. The substituents can be expected to stabilize the radical cation to a sufficient extent for an observation of its reduction on the experimental time scale. This is true for some of the studied oligomers, but, for others, shorter experimental times are required. High scan rate voltammetry with micro~lectrodes’~ has been proved very useful to determine the formal redox potentials of radical-cation formation for several pyrrole d~rivatives.~~ This technique permits very short measurement times, and it is then possible to observe transient intermediates following the electron transfer. The formal redox potentials of the anodic reactions of the substituted oligothiophenes are reported on the Table I together with the required scan rates for their determination: the lower limitscomespond to the beginningof the appearanceof a reversible process, and the upper limits are the lower scan rates at which completely reversible systems can be observed. As expected, electron donor substituents greatly stabilize the generated radical cations and only quite low scan rates are necessary for voltam-
i
Figwe 1. Cyclic voltammetry of a solutionof 3T-OCH3 (CO = 0.6 mM) inacetonitrileona 1 mmdiametergoldel~~e(scenrateu= 200V/s). i
E (V/SCE)
-
1A
1.0
Ftp+ 2 Cyclic voltammetry of a rolution of Br-3T-Na (CD 0.7 mM)in acetonitrile on a 10 fim diameter gold microelectrode (mrate D = 800 V/S).
TABLEI: EM”kdDafafortheRuUaK!atkmd -Dicrtiaa ccwrrtionr of the E a d - s p k t i u CH~O-ZT-OCHI Br-4T-OCHs 3T-OCH3 Br-3T-OCH3 3T-Br Br-3T-Br Br-2T-Br NOr(fTBr)-Br N*3T-Br
**
0.781 0.01 0.815 0.01 0.874 t 0.005 0.903 0.01 1.137 0.01 1.174 f 0.01 1.442 0.005 1.445 0.005 1.469 0.01
**
~~
~~
0.14.5 0.97 f 0.02 1.126 0.01 1.131 0.012
**
xo.05 20-50
0.14.5
4ooo-8o0o 0.05-0.2 5-50 1040 SO04000
metric study of 5,S’-dimethoxybithiophene, 5,5”-dibromotcrthiophene, 5-bromo-5”-methoxytcrthiophene, and 5-brom0-5’~~methoxyquaterthiophene (corresponding to lifetimes from 0.5 to 5 8). For the latter two, the reversible formation of the dication can also be observed as in the case of 5-methoxytcrthiophene(a typical voltammogram is presented in Figure 1). The donor substituentsstabilizethe dication even forshort oligomericohah. It is noteworthy that, for umubstitutcd oligothiophencs, 5 4 trotcrthiophene, and all the dinitro compounds, no reversibility was attained even at high scan rates. However, in all cues, bromo and methoxy substituents stabilize the generated radical cations even when the substitution reeults in a increase of the redox potential e.g., 5,5’-dibromoterthiophene, 5-bromo-5’t-methoxyterthiophene, and 5-bromo-5”-nitroterthiophene;for thia latter case see the voltammogram of Figure 2 showing complete reversibility despite high value of almost 1.5 V/SCE and short lifetime of about 300 w. Substitution of a m n d bromine atom at the B position of the latter compound (3,S-dibromo-5”nitroterthiophene arising from a side reaction during bromine attack of 5-nitroterthiophene) even induacl a greater stability for
Properties of Oligothiophenes
The Journal of Physical Chemistry, Vol. 97, No. 2, I993 515
---
1.4
3T-NO2
- Br-3T.B
Em
1.2
,,,,,----.*
”
‘.......
Eo(V) i
*....
0.8 0.6
-1
50
-0.6
-0.2
0.2
0.6
F
1 40
bcp Fig" 3. Formal redox potentials of radical-cation generation for endsubstituted terthiophenes, relative to the Hammet constant of the substituents. The Hammet constants are additive for disubstituted compounds. (A) 3T-OCH3; (B) B r - 3 T a H 3 ; (C) 3T-Br; (D) Br3T-Br; (E) NOp3T-Br.
30
AS(” 20 10
TABLE Ik opticrlData of tbe S M e d Compounds in Dichlorameth”
0
absorpn emission Lux
oligomer 2T Br-T-OCH, CHIO-ZT-OCH, Br-ZT-Br Br-3T-OCH3 3T Br-3T-Br CH30-3T CH3WT-OCH3 4T Br-4T-OCH3 CH@-6T-NOz 6T CH@-6T4CH, GNdT-NOz Br-3T-NOz Br-(BrJT)-NOz 02N-3T
(nm) 302 258 332 321 375 354 362 368 405 390 403 452 432 442 465 430 430
440
Lux (nm) 361 320 400 370,390 450 411,433 430,448 454 469,495 454,484 516 531,564 512,548 528,562 623 590 600 610
A h a x EO,O (nm) (ev)
59 62 68 69 75 79 86 86 90 94 104 112 116 120 150 160 170 170
3.68 4.10 3.37 3.50 2.99 3.11 3.02 2.99 2.74 2.82 2.64 2.47 2.52 2.47 2.29 2.43 2.40 2.36
@f
n
Tt
(ns)
1.8
