Chem. Mater. 1996, 8, 161-166
161
Chain Amplified Photoacid Generation from Vicinal Dibromides. A General Strategy for the Efficient Generation of Hydrogen Bromide across the Ultraviolet and Visible Spectrum J. C. Scaiano* and Mo´nica Barra* Department of Chemistry, University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada
Roger Sinta* Shipley Company Research and Development Laboratories, 455 Forest Street, Marlborough, Massachusetts 01752-3092 Received July 20, 1995. Revised Manuscript Received September 12, 1995X
Vicinal dibromides are efficient HBr photogenerators that have found application in several acid-hardened photoresists. In this report we describe the photodecomposition of vicinal dibromides as a general chain reaction for HBr generation; a reaction that propagates in the presence of many substrates (i.e., alcohols, amines, sulfides) having the correct hydrogendonor and reducing properties. In addition, we show that entry into the chain propagation steps of these reactions can be photoinduced by numerous initiation processes (e.g., hydrogen abstraction, fragmentation, electron transfer). Thus, provided a suitable photoinitiator is identified, it is possible to not only amplify the acid generation process but also use vicinal dibromides as photoacid generators across the UV and visible spectrum (and possibly the near infrared).
Introduction
Scheme 1
Vicinal dibromides are efficient photochemical sources of hydrogen bromide. These photoacid generators (PAGs) have found application in several acid-hardened advanced negative resists. The mechanism of acid generation has been the subject of recent attention in our laboratory1,2 and elsewhere.3,4 In essence, most simple dibromides undergo rapid photoinduced C-Br bond cleavage with a quantum yield of 1. In nonstabilized systems, the β-bromo-substituted alkyl radical formed undergoes rapid cleavage to yield a second bromine atom and the corresponding alkene. Thus, reactions 1 and 2 in Scheme 1 combined lead to bromine atom formation with quantum yields approaching 2.2,5 We have recently shown that in the presence of alcohols containing the HCOH functionality, it is possible to trigger a chain reaction that effectively amplifies the acid-generation process.5 The reaction involves reduction of the dibromide by the ketyl radical produced by hydrogen abstraction by the bromine atom, reactions 3 and 4 in Scheme 1. Thus, vicinal dibromides are effective bromine atom sources, and it is simply the tendency of the bromine atom to abstract hydrogen that makes them effective PAGs. β-Bromoalkyl radicals have lifetimes of
