Mechanism of Photobase Generators based on DBU by Transient Absorption Measurements

Photobase generators are key molecules in the development of various photofunctional materials, which induce no degradation or corrosion of substrates, in contrast to photoacid generators when they remain in materials. However no photobase generators with strong basicity and good photobase generation efficiency have been developed for practical use up to now. Both 1,8-Diazabicyclo-[5.4.0]undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) are strong organic bases and so photochemical generation of DBN or DBU is an attractive technique for photopolymers. We have investigated the mechanism of photochemistry of DBU and DBN with photo-removable protecting groups by transient absorption measurements. Nitrobenzyl capped DBU (NB-DBU) was converted to a new chemical species without removal of the capped group after photoexcitation, but electron transfer from NB-DBU to thioxanthone (TX) as a sensitizer occurred to accelerate the decomposition of NB-DBU. In contrast to NB-DBN, energy transfer via electron exchange between TX and benzoyl-protected DBN (BZ-DBN) was the key step in the photoinduced decomposition of BZ-DBN upon photoirradiation, as determined by the increase in the photosensitized emission. Thus, elucidation of the photochemical reaction mechanisms of the capped bases should contribute to designing novel highperformance photobase generators.