2,4,6-Trimethylbenzoyldiphenylphosphine oxide (TPO) analog: a non-cytotoxic type-I photoinitiator for free radical photopolymerization†

2,4,6-Trimethylbenzoyldiphenylphosphine oxide (TPO) is a highly efficient and widely used photoinitiator, but it is currently facing significant concerns regarding cytotoxicity. This work investigates a type-I photoinitiator designed as a safer alternative to TPO. In particular, a TPO analog was synthesized, namely 6-(2,4,6-trimethylbenzoyl)-(6H)-dibenz[c,e][1,2]oxaphosphorin 6-oxide (TDOPO), incorporating 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) motif. This compound presents promising characteristics as a type I photoinitiator. It can be synthesized efficiently in just two steps under mild conditions with a reduced environmental impact using commercially available chemicals, without need for extensive purification procedures. Notably, TDOPO exhibits absorption close to the visible spectrum, suggesting its potential for use in visible light curing applications. Comparative studies reveal that TDOPO exhibits strong photoinitiation ability for the free radical photopolymerization of acrylate in both thin and thick samples. The photochemical mechanism studies indicate its capability to generate three distinct types of free radicals: the 2,4,6-trimethylbenzoyl, an oxygen-centered and a phosphorus-centered free radical. In addition, TDOPO shows no cytotoxicity in a 20-hour assay, a result far superior to that of TPO. These findings underscore the potential of TDOPO as an effective and safer alternative to TPO in photopolymerization processes.