We examine the wavelength-dependent efficiency of a set
of oxime
ester based photoinitiators via photopolymerization action plots in
methyl methacrylate (MMA), assessing the polymer yield wavelength-by-wavelength
after irradiation between 325 and 460 nm with a constant number of
photons at each wavelength. We systematically vary the structural
elements within three carbazole-based oxime esters, i.e., 1, 1-(9-dodecyl-6-nitro-9H-carbazol-3-yl)ethan-1-one O-acetyl oxime, 2, 1-(9-dodecyl-6-nitro-9H-carbazol-3-yl)ethan-1-one O-(4-methoxybenzoyl)
oxime, and 3, 1-(9-dodecyl-6-nitro-9H-carbazol-3-yl)ethan-1-one O-(4-nitrobenzoyl) oxime,
changing their substitution pattern on the carboxyl group from alkyl
to two substituted aromatic functionalities. The resulting photopolymerization
action plots are strongly mismatched with the extinction spectra of
each oxime ester photoinitiator by close to 75 nm to the red edge
of the main absorption maximum. The strongly red-shifted reactivity
confirms thatat least under the examined conditionsextinction
spectra constitute no valid guide for predicting maximum photopolymerization
yields. We subsequently examine the wavelength-resolved dependence
of the initiator decay in two solvents in the absence of MMA yet identical
initiator concentrations by following the photofragmentation reaction
via 1H NMR spectroscopy. In both examined solvents, i.e.,
dimethyl sulfoxide (DMSO) and methyl isobutyrate (MIB), the obtained
photoinitiator decay action plots also display a significant mismatch
between the extinction spectrum and the wavelength-resolved photochemical
action. The maximum reactivity in MIB and DMSO is red-shifted by close
to 50 and 65 nm, respectively, and the photochemical action plot in
DMSO exhibits a secondary local maximum. Notably, the decay action
plots also do not display a reactivity maximum at the absorption maximum.
We submit that the red-shifted maximum of the initiator decay is correlated
with the enhanced photopolymerization activity on the red-edge of
the absorption spectrum.