Visible-Light Initiated Free Radical Polymerization by Homomolecular Triplet-Triplet Annihilation

Polymerization reactions initiated by ultraviolet light are ubiquitous in scores of industrial applications but would markedly benefit from visible light activation to overcome stability, energy consumption, and sample geometry limitations. The current work leverages visible-light driven homomolecular triplet-triplet annihilation (TTA) in zinc(II) meso-tetraphenylporphyrin (ZnTPP) to initiate facile free-radical polymerization in trimethylolpropane triacrylate (TMPTA) monomers through ultrafast electron transfer quenching. Selective Q-band (S1) excitation of ZnTPP in the green or yellow (𝜆ex = 514.5 or 552 nm) sensitizes TTA occurring between two 3ZnTPP* energized chromophores, ultimately generating the highly reducing S2 excited state on one ZnTPP molecule (Ered = -2.13 V vs SCE). Subsequently, this S2 state engages in electron transfer with TMPTA, thereby initiating the radical polymerization process. Estimation of the free energy of the reaction combined with electrochemistry strongly suggest that electron transfer is only plausible from the S2 excited state of ZnTPP. Consistent with this hypothesis, the S1 and T1 excited states of ZnTPP showed no evidence of static or dynamic quenching by TMPTA. The bimolecular electron transfer was then verified by concentration-dependent dynamic fluorescence quenching of the ZnTPP S2 excited state using optically gated fluorescence upconversion measurements. In acetonitrile, this electron transfer was concentration-dependent, with time constants ranging between 2.57 ps to 1.04 ps, with TMPTA concentrations from 0 to 3.47 M, respectively. The dynamic bimolecular quenching rate constant (9.5 × 1011 M-1s-1, in toluene) was in the same order of magnitude with that obtained from static fluorescence measurements (3.2 × 1011 M-1s-1, in toluene). FT-IR spectroscopy confirmed visible light-initiated polymerization through monitoring consumption of the olefins in TMPTA as a function of photolysis time. EPR experiments, in conjunction with trapping radical species with 2-methyl-2-nitrosopropane (t-BuNO), identified that acrylate radicals were indeed formed through green light activation. Furthermore, we demonstrate that acrylate polymers of various macroscopic shapes as well as micron-scale objects can be produced using numerous low power visible light sources in the presence of only ZnTPP and TMPTA. These well-defined images represent the general applicability of homomolecular TTA mediated photoinitiated polymerization in both micro- and macrofabrication applications.