The Dynamics of Carbene Solvation: An Ultrafast Study of p-Biphenylyltrifluoromethylcarbene

Abstract

Ultrafast photolysis (lambda(ex) = 308 nm) of p-biphenylyltrifluoromethyl diazomethane (BpCN2CF3) releases singlet p-biphenylyltrifluoromethylcarbene (BpCCF3) which absorbs strongly at 385 nm in cyclohexane, immediately after the 300 fs laser pulse. The initial absorption maximum shifts to longer wavelengths in coordinating solvents (nitrile, ether, and alcohol). In low viscosity coordinating solvents, the initial absorption maximum further red shifts between 2 and 10 ps after the laser pulse. Similar effects are observed upon ultrafast photolysis of 2-fluorenyltrifluoromethyl diazomethane (FlCN2CF3) and therefore cannot be associated with torsional motion around the two phenyl rings of the biphenyl compound. Instead, the effect is attributed to the dynamics of solvation of the singlet carbene. The time constant of solvation in normal alcohols lengthens with solvent viscosity in a linear manner. Furthermore, the time constants of the red shift in methanol-O-d (16 ps), ethanol-O-d (26 ps), 2-propanol-OD (40 ps), and 2,2,2-trifluoroethanol-O-d (14 ps) are longer than those recorded in methanol (9.6 ps, KIE = 1.7), ethanol (14.3 ps, KIE = 1.8), 2-propanol (28 ps, KIE = 1.4), and 2,2,2-trifluoroethanol (4.4 ps, KIE = 3.2), which indicates that the solvent reorganization involves formation of hydrogen bonds. The kinetic data are consistent with motion of the solvent to achieve a specific interaction with the carbene, with the creation of a new hydrogen bond. The solvated carbene reacts with the solvent over tens, hundreds, and thousands of ps, depending upon the solvent.