Rational design of substituted N-alkoxypyridine-2(1H)thiones with increased stability against daylight

N-alkoxypyridine-2(1H)thiones serve as valuable photochemical alkoxyl radical precursors in photobiological studies, but due to a broad absorption band at about 360 nm (pi --> pi* excitation), these molecules decompose readily when exposed to daylight. The goal of the present work is to propose N-alkoxypyridine-2(1H)thiones which due to a blue shift of this band become more stable with respect to daylight and consequently are easier to handle. The shift of the pi --> pi* excitation toward shorter wave length shall be achieved by substituents introduced at the pyridine heterocycle. To study the substituent effects, excitations to the first to singlet states were calculated applying the CASPT2 approach and time dependent density functional theory (TD-DFT). The study indeed showed that electron rich substituents (like the methoxylgroup) at the positions 3, 4, and 6 of the pyridinethione heterocycle yield the desired hypsochromic shift. A free rotation of the substituent, however, is expected to quench these effects. Fluorine atoms, employed to model the influence of electron withdrawing substituents, induce also a blue shift for a substitution at the 3, 4, and 6 positions. For the multiply fluorinated molecule N-methoxy-3,4,6-trifluorinepyridine-2(1H)thione a blue shift of even 24 nm is predicted. Substituents that can conjugate with the pi electrons of the heterocycle (NO2 served as a model) only induce strong bathochromic shifts on the pi --> pi* excitation energy and therefore are not able to eliminate the daylight sensitivity of the precursor molecules.