p-Phenylenecarbenonitrene and its halogen derivatives: how does resonance interaction between a nitrene and a carbene center affect the overall electronic configuration?

A series of para-conjugatively coupled phenylenecarbenonitrenes [(4-nitrenophenyl)methylene (3a), (4-nitrenophenyl)fluoromethylene (3b), (4-nitrenophenyl)chloromethylene (3c), and (4-nitrenophenyl)bromomethylene (3d)] were generated in argon matrix at low temperature (10 or 13 K) and characterized by IR and UV/vis spectroscopy. Density functional theory (B3LYP/6-31G(d)) and ab initio (MCSCF, CASPT2) methods were used to study the ground- and some low-lying excited states of 3a-d. The experimental and computational data suggest that 3a-d have singlet ground states (S0) and can be thought of as quinonoidal biradicals. In all cases, the lowest triplet (T1) and quintet (Q1) states lie about 2 kcal mol(-1) and 28-29 kcal mol(-1), respectively, higher in energy than S0. On the other hand the substituent is found to have a significant effect on the relative energy of the second excited triplet (T2) state. This state tends to become relatively more stable as the ability of the substituent to enforce a closed-shell configuration at the carbene subunit increases. Interestingly, the energy difference between the T2 and S0 states in 3a-d is found to depend linearly on the S-T gap of the corresponding phenylcarbenes 7a-d. This relationship is helpful in predicting when a substituted p-phenylenecarbenonitrene may have a triplet ground state instead of a singlet one.