I48 J. Phys. Chem. zyxwvu 1994,98, zyxwvu 748-751 Electronic Structure of Ethynylthiophenes Igor Novak,’ Siu Choon Ng,’ Jiye Fang, Cbup Yew Mok, and Hsing Hua Huang Department of Chemistry, National University of Singapore, Singapore 051 1, Singapore Received: August 30, 1993@ He I and He 11 photoelectron spectra of all isomeric mono- and diethynylthiophenes were recorded. The electronic structure was analyzed and spectra assigned on the basis of empirical considerations and semiempirical MO calculations. The spectra indicate various interactions taking place between thiophene u and ethynyl u orbitals. The relationship between deduced electronic structure and formation of novel polymer materials is discussed. Introduction Conjugated polymers (CP), i.e., thosewithdelocalized u states, have been studied extensively over the last 2 decades primarily because they are a basis of new materials with technological applications in the fields of electrochemical energy storage (rechargeable batteries), electrochromic devices (displays), pho- tovoltaic cells, and various sensors. The applicability is a direct consequence of their physicochemical properties in particular nonlinear optical response and highly anisotropic electrical conductivity.’ Two main types of CPs can be distinguished: CPs with degenerate ground state (e.g. polyacetylene) and CPs with nondegenerate ground state (e.g. polythiophene).2 The mech- anism of electrical conduction is different in the two types being based on solitons in the former and on polarons (or bipolarons) in the latter. Charge carrying excitations (resembling geometrical defects) introduced by doping with suitable electron donors or acceptors are responsible for electrical conductivity. Doping process introduces geometrical and electronic structure changes into the polymer backbone creating new self-localized energy states in the forbidden energy gap between valence and conduction bands. The above mentioned processes are rather complex and remain unclear in detail. The electronic structure of the polymer ground state must play an important role. It is dificult to study the electronic structure of a polymer sample (both experimentally and theoretically) on the account of its wide distribution in molecular sizes, conformational, and isomeric flexibilities. In- stead, a carefullyprepared monomer or oligomer “buildingblocks” are selected and the real polymer properties inferred by extrap- olation or ana10gy.~ We follow the same approach in this work by attempting to analyze the electronic structure of ethynylthiophenes, “building blocks” for a polymer consistingof ( - t h i o p h e n s - ) repeating units. This new polymer would be a combination of the two CP types previously mentioned. Relevance of such “building block” studies can be illustrated on the example of polythiophene and its ’building block” bithiophene. UV photoelectronspectroscopy (UPS) studies have shown3s4 that substituted bithiopheneshave low inter-ring torsional energy barrier and are prone to significant electronic structure variations as a result. Polythiophenes have the advantage of processability (solubility in organic solvents) but at the cost of poorly defined electronic and molecular structures. A secondary interest in the electronic structure of ethynylthiophenes stems from possible but yet unexplored interactions between thiophene ring and substituents with u moiety. e Abstract published zyxwvutsrqp in Aduance ACS Abstracts, December 15, 1993. Experimental Section Ultraviolet PhotoelectronSpectroscopy (UPS). The He I and He I1UPS spectra were recorded on a UPG-200 Leybold-Heraeus spectrometer using Ar+ 2P3/z and zPl/z calibration lines and achieving resolution of 25-30 meV. The sampleinlet temperatures were in the range 2&30 OC. The ionization energies (Ei) quoted in Figures 1-3 and Table 1 refer to adiabatic or vertical energies, whichever could be measured more accurately. Strong band overlap and pronounced vibrational fine structure made in some cases accurate measurement of adiabatic Ei preferable to an unreliable estimate of vertical Eis. Narrow range scans (4 eV wide, not presented in Figures 1-3) were used for accurate measurements of vibrational fine structures. Vibrational normal mode assignments given in Table 1 are based on thiophene molecular frequenciesS and ethyne ion frequencies.6 UV spectra were also recorded (in hexane and methanol) and the following values measured for lowest energy band maxima, which corre- spond to u-r* electronic transitions (compound, X/nm (E,/eV): 2-, 256 (4.8); 3-, 250 (5.0); 2,5-, 290 (4.3); 2,4-, 260 (4.8); 3,4-, 222 (5.6); 2,3-, 274 (4.5). Theoretical Calculations. MO eigenvalues and eigenvectors were calculated at the semiempirical level (AM1 and PM3 Hamiltonians) with MOPAC 6.0 package. Experimental bond lengths for thiophene and ethyne moieties were used as initial guess after which all geometries were fully optimized. Synthesis. The syntheticroute to ethynylthiophenesis depicted in Scheme 1. The regioselective ethynyl cross-coupling procedures of Sarkar et al.’ and Neenan et a1.8 were used. Compounds la, lb, and 2a were prepared according to the first procedure, while higher yields were obtained with the latter methodology for compounds 2b-2d. Compounds 2b-2d are hitherto unknown. All compounds were characterized by 300-MHz ‘H NMR, mass spectrometry and C,H elemental analysis. Results and Discussion The electronic structure of substituted thiophenes has been studied previously using The main effort was directed toward spectral assignmentsand unraveling of many interactions between substituent and the five-member ring u orbitals. The picture which emerged from analysis indicated that considerable electronicstructure variationsoccur dependingon the substituent’s position zyxwv (CY or zyxwv /3) and type. Some substituents had ?F orbitals which could be expected to interact with ring ?r systems (-CN, -COCH3, -NO2 groups) but their spectral bands were either not sharp or were embedded within a cluster of u ionizations making monitoring of such u-?F interactions difficult. In order to get a better insight into ring-substituent T-T interactions we have recorded spectra of two isomeric ethynyl and four isomeric diethynylthiophenes. 0022-3654/94/2098-0748$04.50/0 0 1994 American Chemical Society