ELSEVIER Synthetic Metals 96 (1998) 177-189 SL," ImTIRI TIII Comparison of geometries and electronic structures of polyacetylene, polyborole, polycyclopentadiene, polypyrrole, polyfuran, polysilole, polyphosphole, polythiophene, polyselenophene and polytellurophene U. Salzner a,1, J.B. Lagowski b,., P.G. Pickup a,2, R.A. Poirier a,3 Department of Chemistry, Memorial University of Ne~foundland, St. John's, Nfld., AIB 3X7, Canada uDepartment of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John's, Nfld., AIB 3X7, Canada Received I9 September 1997;received in revised form i8 May 1998; accepted22 May 1998 Abstract Geometries of monomers through hexamers of cylopentadiene, pyrrole, furan, silole, phosphole, thiophene, selenophene and tellurophene, and monomers through nonamers of borole were optimized employing density functional theory with a slightly modified B3P86 hybrid functional. Bandgaps and bandwidths were obtained by extrapolating the appropriate energy levels of trimers through hexamers (hexamers through nonamers for borole) to infinity. Bandgaps increase with increasing ~-donor strengths of the heteroatom. In general, second period heteroatoms lead to larger bandgaps than their higher period analogs. Polyborole is predicted to have a very small or no energy gap between the occupied and the unoccupied w-levels. Due to its electron deficient nature polyborole differs significantly from the other polymers. It has a quinoid structure and a large electron affinity. The bandgaps of heterocycles with weak donors (CH2, Sill2 and PH) are close to that of polyacetylene. For polyphosphole this is due to the pyramidal geometry at the phosphorous which prevents interaction of the phosphorus lone pair with the or-system. The bandgap of polypyrrole is the largest of all polymers studied. This can be attributed to the large w-donor strength of nitrogen. Polythiophene has the third largest bandgap. The valence bandwidths differ considerably for the various polymers since the avoided crossing between the flat HOMO- 1 band and the wide HOMO band occurs at different positions. The widths of the wide HOMO bands are similar for all systems studied. All of the polymers studied have strongly delocalized or-systems. © 1998 Elsevier Science S.A. All rights reserved. Ke),,vords: Electronic structures; Geometries; Polyborole;Polycyclopentadiene; Polyfuran;Polysilole;Polyphosphole; Polyselenophene;Polytellurophene 1. Introduction Due to their chemical stability, their high conductivity upon doping, and their non-linear optical properties, poly- pyrrole (PPy) and polythiophene (PTh) are among the most widely studied conjugated organic polymers, experimentally [1-4] and theoretically [5-9]. Since PPy and PTh have relatively large bandgaps, 2.85 [ 10] and 2.0 eV, [ 11 ] respec- tively, the neutral forms are insulators. Intensive research has * Corresponding author. Tel.: + 1 709 737 2667; fax: + 1 709 737 8937; e-mail:jolantal@smaug.physics.mun.ca 1 Tel.: + 1 709 737 2151; fax: + 1 709 737 3702; e-mail: uli@smaug. physics.mun.ca. Present address: Departmentof Chemistry,Bilkent Univer- sity, 06533 Bilkent, Ankara, Turkey. Tel.: + 90 312 266 4000, ext. 2122; fax: +90 312 266 4579; e-marl: salzner@fen.bilent.edu.tr 2 Tel.: + 1709 737 8657;fax: + 1709 737 3702;e-marl:ppickup@morgan. ucs.mun.ca 3 Tel.: + 1 709 737 8609;fax: + 1709 737 3702;e-mail:rpoirier@morgan. ucs.mun.ca been dedicated to chemical modification of these systems with the aim to decrease the bandgaps [4]. Less is known about polycyclopentadiene (PCp) and poly- furan (PFu) and higher period analogs of PTh, polyseleno- phene (PSe) and polytellurophene (PTe). In the case of PFu this is caused by the experimental difficulties involved with synthesis and with doping [ 12,13]. However, high quality PFu has been produced. The bandgap was determined to be 2.35 eV [ 12] and conductivity upon doping of 102 S/cm was reported [ 14]. This has to be compared to maximal conduc- tivities of 500 S/cm for PPy [3] and 2000 S/cm for PTh [4]. The bandgap of polyselenophene (PSe) was found to be the same as that of PTh, 2.0 eV [ 15] but the conductivity was only 3.7 × 10- 2. Polytellurophene was generated readily at potentials lower than those required for thiophene, but the conductivity after doping was low, 7.6 × 10 .6 S/cm [ 16]. This shows that conceptually similar systems can differ vastly in their electrical properties, chemical stability and ease 0379-6779/98/$ - see front matter © 1998Elsevier Science S.A. All rights reserved. PII80379-6779(98)00084-8