3504
J . Phys. Chem. 1993,97, 3504-3509
Electronic and Geometrical Structure of Methylthiophenes and Selected Dimethyl-2,2‘- bithiophenes Giuseppe Distefano’ and Maurizio Dal Colle Dipartimento di Chimica, Universith di Ferrara. via Borsari, 46, 441 00 Ferrara, Italy
Derek Jones, Massimo Zambianchi, and Laura Favaretto ICoCEA, Consiglio Narionale delle Ricerche, via della Chimica, 8, 40064 Ozrano Emilia, Bologna, Italy
Albert0 Modelli Dipartimento di Chimicu ‘G. Ciamician”, Universith di Bologna, via Selmi, 2, 401 27 Bologna, Italy Received: November 1 I , 1992
The ionization energy and the attachment energy values of 4,4’-, 3,4’-, and 3,3’-dimethyL2,2’-bithiopheneshave been recorded by means of photoelectron H e I and electron transmission spectroscopy, respectively. The data obtained, together with those previously reported for thiophene, 2- and 3-methylthiophene, and 2,2’-bithiophene, are discussed with the help of a b initio 3-21G* and semiempirical calculations. Ab initio data closely reproduce ionization energy trends, while experimental values for virtual molecular orbitals (MO’s) compare better with M I N D 0 / 3 results. At the ab inito 3-21G* level, gaseous 2,2’-bithiophene and its 4,4’ and 3,4’derivatives have two stable rotamers (anti, 4 = 140-150O; syn, 4 = 40-50°), while in 3,3’-dimethyL2,2’-bithiophene the two rings are nearly orthogonal to each other. Conformational changes cause variations in the electronic structure clearly visible in the ionization and attachment energy spectra.
introduction The electronic structures of five-membered heterocyclic compounds and their dimers are of great interest since these species are the building blocks for the synthesis of organic polymers with promising technological applications. Polythiophenes are among the most studied materials because of their chemical stability and high electrical conductivity in the oxidized (doped) state. The insertion of a 3-alkyl side chain to the thiophene ring can give rise to polymers with improved properties. For example, electrical conductivity (2000 S cm-l) several times higher than that of polythiophene has been reported for poly(3-methylthiophene).6 In additional, substituted polythiophenes can show enhanced. proce~sability,~-~ may be soluble in common solvents, or exhibit solvatochromism and thermochromism. The latter two properties are associated with the rotation of neighboring thiophene rings. Rotation leads to a decrease of r-conjugation, which in turn causes electronic structure changes reponsible for the color changes.I@-lt In the neutral substituted polymers, the intrachain sulfur alkylgroup steric interaction forces the backbone out of planarity reducing r-conjugation as indicated by the energy of the T* A interband transition,13-15whose variation is less than expected on the basis of the substituent electronic effect. However, in the absence of strong steric interactions, crystal packing (if present) and the electronic energy associated with the increased *-electron conjugation favor the appearance of more planar conformations.’6 In the doped substituted polymers a reduction of conductivity with respect to unsubstituted polythiophene prepared under the same conditions has not been observed for small substituents.6J7-20 According to the model proposed by Br6das and co-workers,21-24 the presence of the charge on the polymer chain leads to a local geometry relaxation from an aromatic structure toward a quinoid structure. This planarization upon doping, which increases the mobility of charge carriers, has received abundant experimental and theoretical support.25-*8 The available information, therefore, indicates that a chain formed by thiophene rings has a very mobile environmentally dependent conformation. The coupling of the electronic properties
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to geometrical changes is of special importance for this class of compounds.1i-21The departure of the backbone from planarity is critically influenced by the type and the regiochemistry of sub~titution.5.~~-15 The knowledge of the structure-properties relationships in polythiophene substitution is still rather rudimentary.5 To gajn a deeper insight into the influence of ring substituents on the polymer properties, we present here a gas-phase analysis of the effects induced by methyl groups on the electronicand geometrical properties of thiophene (TH) and 2,2’-bithiophene (2TH). The studied molecules are 3-methylthiophene (3MTH), 4,4’- 3,4’and 3,3’-dimethyL2,2’-bithiophene(2TH44,2TH34, and 2TH33) together with TH, 2TH, and 2-methylthiophene (2MTH) for comparison purposes. In particular, ionization eenrgy and electron affinity values have been determined by means of ultraviolet photoelectron spectroscopy and electron transmission spectroscopy (UPS and ETS, respectively), these data being compared with theoretical (abinitio3-21GS and MIND0/3) r a n d A* molecular orbital (MO) energies, and the most stable conformations computed at the ab initio 3-21G* level. Experimental Section Ultraviolet P b o t o d e c t r o o S p e e h p y . The UPS spectra were obtained with a Perkin-Elmer PS 18 photoelectron spectrometer connected to a Datalab DL4000 signal analysis system. The bands, calibrated against rare-gas lines, were located using the position of their maxima, which were taken as corresponding to the vertical ionization energies (IEs). The accuracy of the IE values was estimated to be better than h0.05 eV (except for shoulders). Electron Transmission Spectroscopy. ETS allows determination of the energy (attachment energy, AE) at which electrons are temporarily captured by an atomic or molecular species in the gasous phase. These AE values are, to a first approximation, the negative of the vertical electron affinities (EA’S) of the capturing species. Our apparatus (see ref 28 for more details) is in the format devised by Sanche and S c h ~ l z .The ~ ~derivative ~ of the transmitted current as a function of the incident electron 0 1993 American Chemical Society
Methylthiophenes and DimethyL2,2’-bithiophenes
The Journal of Physical Chemistry, Vol. 97, No. 14, 1993 3505
energy is recorded. ET spectra were obtained by using the apparatus in the ‘high-rejection” mode, which yields a signal that can be related to the nearly total electron scattering cross section.30b The AE’s reported correspond to thevertical midpoints between the minima and the maxima of the differentiated signal. The energy scales were calibrated by using the ( ls12s2)2Sanion state of He. The estimated accuracy is i0.05 or io.1 eV according to the number of decimal places reported. Only AE values related to negative EA’S can be measured with this technique. Due to the finite width of the electron beam, which occurs at zero energy in the ET spectrum, the lower limit for AE values is about 0.2 eV. Theoretical Calculations. The geometrical parameters have beencomputedat theabinitio3-2lGS levelusing theGAUSSIAN 90 series of programs.)’ TH was assumed to belong to the Czc symmetry group, while for the methyl derivatives and 2TH33 all parameters, except planarity of the rings, were allowed to vary independently in the optimization procedure. Calculations on 2TH and 2TH44 were performed at selected values of the angle 4 (the rotational angle between the rings). For 2TH, bond lengths and bond angles optimized for the trans planar conformer (4 = 180’) were used. Bond lengths and bond angles of 2TH44 were taken from those of 2TH and corrected according to the small perturbations caused by 3-methyl substitution on the geometry of the monomer. A combination of spectroscopic and theoretical information was used (see below) to estimate the most probable 4 values of 2TH34. r - and r*-MO energies were also computed at the semiempirical ( M I N D 0 / 3 and, in some cases, MNDO) level for comparison with the experimental IE and AE values. The calculations were performed at the Cineca Computing Center of Bologna, Italy.
Results and Discussion The UPS and ET spectra of 2TH44,2TH34, and 2TH33 are reported in Figure la,b, respectively. The spectra of 2TH, 2MTH, 3MTH, and TH have been previously r e p ~ r t e d , ~ -but ’ ~ not thoroughly analyzed. All of the IE and AE values quoted have been obtained in our laboratory. A selection of the computed geometrical parameters are presented in Figure 2 and Table I (see Scheme I for atom numbering). Correlation diagrams of experimental IE and AE values of the various compounds, these data, and the corresponding computed M O energies, as well as the variation of total or orbital energies on changing 4, are reported in Figures 3-7. Thiophene and Methylthiophenes. The structural parameters of TH, 2MTH, and 3MTH computed a t the 3-21G* level as shown in Figure 2. The optimized data obtained for TH are satisfactorily related to those obtained from a microwave ~ t u d y . 3 ~ Methyl substitution causes only small variations of bond lengths (
