Compounds which are capable of generating acid on photolysis are disclosed, and the uses of these compounds, especially for deprotecting the termini of nucleic acid molecules or peptides during synthesis of arrays. The compounds described herein may be employed in the detritylation of 5'-O-dimethoxytrityl (DMT) protected nucleotides by photolysing the compounds to generate an acid capable of removing the DMT group allowing oligonucleotide arrays to be synthesised using readily available 5'-O-DMT-nucleoside-3'-O-phosphoramidite monomers conventionally used in solid phase nucleic acid synthesis. A method of avoiding the effects of stray light in projection lithography techniques is also disclosed.