Promiscuous electrons: probing organic switch-on paramagnetism and predicting photoheterolysis through conical intersection searches
The research described herein comprises two avenues of thought: 1) switch-on paramagnetic probes and 2) the prediction and rational design of organic compounds capable of undergoing photoheterolytic cleavage. The organic probes herein switch from diamagnetic to paramagnetic and back with high radical fidelity based on the presence of non-covalent guest-host complexes or temperature. This switching is shown in a radical dimer of tethered viologens as well as a poly(propyl viologen) polymer, and the changes in magnetism are followed using electron paramagnetic resonance (EPR) and UV/Vis spectroscopy. The use of EPR is something never seen before in the study of viologen dimerization. Computational investigations into the dimerization support the idea of a pimer, as opposed to the more common sigma bond.
Shifting the idea of photoheterolytic cleavage from being governed by transition state control to conical intersection control allows for the generation of ground-state, unstable cations preferentially over the stable analogs and seemingly defies the chemical intuition for what should be formed. CASSCF conical intersections were done using GAMESS to identify the presence of nearby, productive conical intersections necessary for the conversion of the excited-state singlet to the ground state more rapidly than fluorescence, intersystem crossing, or radiationless decay, methods that would not produce bond scission. The combination of destabilization of the ground state through generation of donor-unconjugated, antiaromatic, or dicoordinate cations and the stabilization of the excited state due to diradical structures formed in the excited state allow for the increased likelihood of these conical intersections. Prediction of potential photoheterolytic cleavage is not limited to the necessity of calculation of the conical intersection. Instead, a less computationally expensive and more user-friendly computation of the TDDFT energy gaps associated with the Franck-Condon energy gap have shown to be good approximations for identifying potential nearby conical intersections and, consequently, compounds capable of photoheterolytic cleavage.