ABSTRACT

1. R L i PENTACENE R ANTHRACENE (ii) (i) (i) (ii) (i) (ii) (i) (ii) (i) (ii) 8 hr 7 min 4 hr 0 min FORMATION AND STABILITY STUDIES OF PERSISTENT ACENES Irvinder Kaur and Glen P. Miller University of New Hampshire OUR APPROACH TO PERSISTENT PENTACENES The most reactive rings on pentacene are those indicated with arrows. We have prepared several pentacene derivatives in which these sites exhibit varying degrees of steric hindrance ABSTRACT A convenient, facile synthesis of a series of stabilized acenes that incorporate several different aryl substituents located at different positions along the acene backbone is described. These structurally modified acenes show vastly improved solubility (processability) and greater photooxidative stability. WHY SHOULD STRUCTURALLY MODIFIED PENTACENES RESIST PHOTO-OXIDATION? MECHANISTIC INSIGHTS… Photo-oxidation of pentacene is believed to require two approaches by oxygen. First, triplet oxygen is converted to singlet oxygen via photoexcited pentacene. Second, singlet oxygen cycloadds across the acene in a [4+2] fashion. Sterically hindered pentacenes can gain kinetic stability by slowing either or both of these steps. Phenyl rings of the modified acene lie nearly perpendicular to the plane of the acene unit. The alkyl groups are expected to lie above and below the most reactive sites and therefore impede the approach of oxygen. RESULTS 5,7,12,14-Tetrasubstituted Pentacenes UV-Vis Data: IMPROVED PHOTOOXIDATIVE STABILITY A C • INTRODUCTION • During the last few years, the development of organic thin film transistors (OTFTs) has attracted much interest because organic molecules can be deposited over large surface areas and are compatible with flexible plastic substrates[1]. • Pentacene molecules have been deposited on a wide variety of materials to build flexible electronics. • Acenes including pentacene has been utilized in photovoltaics, organic light emitting diodes (OLEDs) and other electronic applications. • Problems encountered include a lack of solubility and photo-oxidative instability. 6,13-Disubstituted Pentacenes B D E Comparison of several different structurally modified pentacene Change in UV-Vis spectrum of structurally modified pentacene upon exposure to light in ambient atmosphere Absorbance at different maxof a single structurally modified pentacene with time PICTORIAL EVIDENCE SOLUBILITY STABILITY SYNTHETIC DETAILS 6,13-Disubstituted Pentacenes 2 • CONCLUSIONS • Modified pentacenes show enhanced photo-oxidative stability and improved solubility • Increasing the alkyl chain length at the ortho position of the phenyl substituents increases the kinetic stability of the corresponding acene • 6,13-disubstituted pentacenes show greater photooxidative stability than 5,7,12,14- tetrasubstituted pentacenes. Thus, the center-most rings are most prone to oxidation and should be the focus of further synthetic efforts to prepare persistent acenes. 5,7,12,14-Tetrasubstituted Pentacenes 2 REFERENCES [1] J. Briggs and G. Miller Comptes Rendus Chimie, 2006, 9, 916. [2] W. Dehaen et al Synlett, 2005, 2, 217. This work was supported under the Nanoscale Science and Engineering Centers Program of the National Science Foundation (Award # NSF-0425826)