Photoreactivity of Norrish Type Photoinitiators for 3D Laser Printing via First Principles Calculations

In the rapidly evolving field of 3D laser nanoprinting, achieving high resolution and high printing speed relies heavily on the effective use and design of sensitive photoinitiators. However, their photoreactivity is hitherto not well described. This study investigates the photochemical and photophysical characteristics of a high-performing Norrish Type II photoinitiator (2E,6E)-2,6-bis(4-(dibutylamino)benzylidene)-4-methylcyclohexanone, known as BBK, as well as commonly employed Norrish Type I photoinitiators: Irgacure 651, Irgacure 369. Using quantum mechanical calculations, multiphoton absorption, the formation of the excited states, and the radical formation mechanisms most probably involved in the initiation of free radical polymerization are examined. Their relation to the observations during 3D printing experiments is discussed, aiming to uncover the molecular foundations behind varying performances of photoinitiators. Bond dissociation energies and energy barriers for bond cleavage of Irgacure photoinitiators are demonstrated to confirm radical formation in the lowest triplet state, whereas this pathway is shown to be less probable for BBK. The radical polymerization initiation upon absorption from the triplet manifold of BBK and reactions with pentaerythritol triacrylate (PETA) monomers are described. Deactivation pathway via reversible intersystem crossing, as well as the photoactivation characteristics, are compared with relation to the 7-diethylamino-3-thenoylcoumarin (DETC) photoinitiator.