Ultrafast Formation of Trinitromethanide (C(NO2)3-) by Photoinduced Dissociative Electron Transfer and Subsequent Ion Pair Coupling Reaction in Acetonitrile and Dichloromethane

Abstract

Tetranitromethane-naphthalene charge-transfer complexes in acetonitrile and dichloromethane were excited at 400 nm, and the subsequent dynamics were studied using femtosecond pump−probe spectroscopy. The excitation produces a radical ion pair consisting of a naphthalene radical cation (N.+) and a tetranitromethanide radical anion. We show that the latter dissociates into NO2 and trinitromethanide (T-) within 200 fs in both solvents and that the following dynamics up to ∼1 ns, which we model using a simple kinetic scheme, are dominated by reactions involving N.+ and T-. The main products after 1 ns result from ion-pair coupling (the trinitromethylnaphthalenyl radical, TN·) or reaction of N.+ with ground state naphthalene (naphthalene dimer cation). The former is the dominating product in dichloromethane, whereas the latter dominates in acetonitrile. This difference is explained mainly by different rates of ion-pair separation in the two solvents. (It is faster in acetonitrile, the more polar solvent.) The product TN· was found to have a strong absorption band around 340 nm. On shorter time scales (<30 ps), spectral evolution around 350 nm in acetonitrile is interpreted in terms of vibrational relaxation of T-. In dichloromethane, we observed a 5 ps decay in the visible region, which we attribute to the coupling between N.+ and vibrationally excited T-.