An In-Depth Mechanistic Investigation of the Radical Initiation Behavior of Monoacylgermanes

Five para-substituted monoacyltrimethylgermane derivatives, i.e., p-fluorobenzoyltrimethylgermane (pFBG, lambda(max) = 405 nm), p-methoxybenzoyltrimethylgermane (pMBG, lambda(max) = 397 nm), benzoyltrimethylgermane (pHBG, lambda(max) = 409 nm), p-cyanobenzoyltrimethylgermane (pCBG, lambda(max) = 425 nm), and p-nitrobenzoyltrimethylgermane (pNBG, lambda(max) = 429 nm) are investigated via a combination of pulsed laser polymerization with subsequent electrospray ionization and mass spectrometry (PLP-ESI-MS) as well as femtosecond transient absorption spectroscopy. The relative initiation efficiencies of the initiating benzoyl radical fragments of pFBG, pMBG, and pHBG are determined using PLP-ESI-MS. The para-substituted derivatives with the electron-donating groups, pFBG and pMBG, display a factor 1.5 and 1.3, respectively, superior overall initiation efficiency compared to the unsubstituted pHBG. In contrast, the derivatives pCBG and pNBG carrying electron-withdrawing groups display only weak initiation behavior at a factor 4 higher total energy of similar to 112 J (similar to 28 J for typical PLP experiments with pMBG, pFBG, and pHBG at similar to 320 J and 90 000 pulses). The differences in the initiation efficiencies are representative for two classes of monoacyltrimethylgermane initiators, i.e., efficient initiators and weak initiators, each distinct in their specific radical cleavage mechanism. The efficient initiators pMBG, pFBG, and pHBG show an ultrafast intersystem crossing within 2-4 ps after pulse irradiation and subsequent formation of benzoyl and trimethylgermyl radical fragments. In contrast, the weak initiators pCBG and pNBG relax to the ground state after photoexcitation via a dominating ultrafast internal conversion (IC) within 13 and 2 ps, respectively, disallowing effective initiation under typical PLP conditions (similar to 320 J/pulse with 90 000 pulses resulting in similar to 28 J total energy per sample). pCBG features weak initiation behavior additionally forming methyl and p-cyanobenzoyldimethylgermyl radicals at a factor 4 higher total energy of similar to 112 J. Consistent with a considerably faster IC relaxation, pNBG features a factor 10 weaker monomer conversion than pCBG.