The past several decades have seen heightened interest in molecular devices such as molecular switches, organic light emitting diodes (OLED's), molecular wires, and numerous other molecularly engineered devices. Many of these devices are organic-based materials that exploit E-Z isomerization of carbon-carbon, nitrogen-nitrogen, and/or carbon-nitrogen double bonds. As intriguing as the current literature is on these compounds, the possibility of substituting heavier main group elements, such as phosphorus, into these potentially useful systems has not been much explored. Heavier atoms may introduce new properties that currently are not available with the lighter atom cousins.
High-level computational methods, including time-dependent density functional theory (TD-DFT) and complete active space ab initio methods with (CASPT2) and without (CASSCF) perturbation theory, have been used to study the E-Z isomerization of aryl-diphosphenes (Ar-P=P-Ar) and aryl-phosphaalkenes (Ar-P=C(H)-Ar). Both the thermal and photo-excited processes (energies, geometries, transitions) have been explored for three proposed isomerization pathways. The pathways examined include (1) rotation about the central double bond (C-P=E-C, E = P or C(H)), (2) in-the-plane inversion about the E=P-C bond angle, and (3) dissociation of the phosphorus-phosphorus and phosphorus-carbon double bonds for diphosphenes and phosphaalkenes, respectively.
In both, diphosphenes and phosphaalkenes, rotation dominates as the preferred thermal (ground state) pathway. The calculations are also consistent with rotation as the primary mechanism for photo-induced isomerization by excitation to the low-lying electronic states. The photoisomerization of diphosphenes shows strong similarities to recently proposed models of the analogous process in azobenzene; namely, it involves a doubly excited phantom state. In contrast, phosphaalkenes parallel stilbene’s classic rotation photoisomerization pathway. These results provide invaluable insight for synthetic chemists who are attempting to understand, design, and fabricate photoactive polymeric materials.