Structure-Property Relationships in Phosphole-Containing π-Conjugated Systems: A Quantum Chemical Study David Delaere, Minh Tho Nguyen,* and Luc G. Vanquickenborne Department of Chemistry, UniVersity of LeuVen, Celestijnenlaan 200F, B-3001 LeuVen, Belgium ReceiVed: June 3, 2002; In Final Form: September 13, 2002 The aim of this theoretical study is to provide an in-depth interpretation of the UV-vis absorption spectra and electrochemical data of two series of 2,5-dipyridyl- and 2,5-dithienylphosphole derivatives containing σ 3 - or σ 4 -P atoms. The geometric and electronic structures of those phosphole-containing π-conjugated systems were investigated using density functional theory (DFT). To assign the absorption peaks observed in the UV-vis absorption spectra, we computed the energies of lower-lying excited states within the adiabatic approximation of time-dependent DFT (TDDFT). All DFT calculations were performed using the B3LYP functional and the split valence plus polarization SV(P) basis set. To elucidate structure-property relationships, we studied in a systematic way the influence of different structural modifications on the electronic structure and emphasized the corresponding consequences for the electrochemical and optical properties. More specifically, we considered successively the influence of pyridyl and thienyl substituents at the R-position, flattening of the P atom, oxidation of the P atom by elemental sulfur, and fusion of a saturated six-membered ring onto the phosphole core. Introduction π-Conjugated organic oligomers and polymers are showing increasing potential as active components for a wide range of semiconducting devices, 1 including field effect transistors, 2 light- emitting diodes 3 (LEDs), and photovoltaic components. 4 The electrical and optical behavior of these materials originate from their typical geometric and electronic structure; hence, their electronic and optical properties could in principle be fine-tuned by manipulation of their chemical structure. For example, the width of the energy gap, which determines the optical properties of LEDs, can be changed upon chemical substitution. Because quantum mechanical methods are able to reliably describe such structure-property relationships, they are of great help when designing materials with enhanced (opto)electronic properties. 5 Five-membered heterocycles such as pyrrole and thiophene have widely been used as building blocks for the design of well- defined linear π-conjugated oligomers and polymers. Also, phosphole (Ph), the P analogue that possesses a unique geometric and electronic structure, has recently attracted more attention 6-8 as a promising building block for the engineering of π-conjugated systems with a chemically tunable energy gap. In contrast to pyrrole and thiophene, Ph 9 is a nonplanar and nonaromatic five-membered ring since it contains a rigid pyramidal tricoordinated phosphorus atom (σ 3 -P atom), which prevents strong cyclic π-conjugation between the phosphorus lone pair and the cis-1,3-butadiene unit. In oligomers or polymers, the Ph P atom however retains a versatile reactivity, which thus offers an interesting possibility of tuning their (opto)- electronic properties by chemical modification. In this context, Re ´au and co-workers 6-8 prepared and evaluated the photophysical and electrochemical properties of two series of 2,5-dipyridylphosphole (PyrPhPyr) and 2,5- dithienylphosphole (ThPhTh) derivatives containing σ 3 - or σ 4 -P atoms. These authors made several structural variations for tuning the Ph’s electronic properties and elucidating the structure-property relationships. For example, heteroaryl sub- stituents such as pyridyl and thienyl were introduced at the R-position (the 2,5-positions) of the Ph ring in conjunction with a variation of the size of the fused carbocycle and the substituents on phosphorus. Finally, chemical modification of the nucleophilic P atom was performed. Crystal structures, UV- vis absorption and fluorescence spectra, and electrochemical data of those compounds were abundantly reported. Nyula ´szi 8 performed ab initio calculations on the parent molecules and discussed the energetics of different possible conformers of 2,5- diheteroarylphospholes as well as the geometric and electronic structure of 2,5-dithienylheterocyclopentadienes and derivatives. Analysis of the electronic structure data was however restricted to the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies. Some structure- property relationships of these model compounds were suggested on the basis of the above-mentioned experimental and theoretical data. The ThPhTh derivatives appeared to be particularly interesting, as their optical and electrochemical properties vary over a wide range depending on the nature of the phosphorus moiety. These properties prompted them to prepare correspond- ing polymers by electrooxidation. Nevertheless, several ques- tions related to structure-property relationships were not answered yet. For example, we did not find in previous papers 6-8 an explanation for the opposite effect in the absorption spectra within the pyridyl and thienyl series when oxidizing the P atom by elemental sulfur. The aim of the present work is to provide an in-depth interpretation of the UV-vis absorption spectra and electro- chemical data of the two series of PyrPhPyr and ThPhTh derivatives obtained by Re ´au and co-workers. 6-8 To assign the absorption peaks, we computed the vertical lower-lying excited states. Furthermore, the redox potentials were related to the inherent electronic structure rather than ascribed to the electron- * To whom correspondence should be addressed. Fax: 32-16-32 79 92. E-mail: minh.nguyen@chem.kuleuven.ac.be. 838 J. Phys. Chem. A 2003, 107, 838-846 10.1021/jp026227t CCC: $25.00 © 2003 American Chemical Society Published on Web 01/18/2003