Abstract

Using femtosecond pump-probe spectroscopy, the dynamics following 400 nm excitation of the tetranitromethane-naphtalene charge-transfer complex in acetonitrile was studied. The complex has a broad unstructured charge-transfer-band in the uv-visible region. For the concentration used (0.38 M tetranitromethane, 0.19M naphtalene), >97% of the absorbance is due to charge-transfer absorption, and the <3% component (tetranitromethane) contribution was shown not to influence the signal. The charge-transfer excitation leads to the formation of a radical ion pair within the duration of the pulse (80 fs), as evidenced by the appearance of the spectrum of the naphtalene radical cation which has absorption from the uv to ~740 nm, with a major peak at 680 nm. The radical anion is from earlier investigations known to dissociate into NO₂ and trinitromethanide anion, and the dissociation was estimated to occur within a few picoseconds¹. On a longer timescale, the 680 nm signal decreased biexponentially, the main channel (80%, 80 ps) being due to the formation of the exciplex (naphtalene)⁺₂ as evidenced by the corresponding rise of the spectrum of the latter at 580 nm. The minor decay channel (20%, 15 ps) was tentatively attributed to reaction with the trinitromethanide anion radical, essentialy ocurring before the solvent separation of the ions. At probe wavelengths between 350-450 nm, the transient spectrum was due to several contributions including the naphtalene radical cation and the trinitromethanide anion. Assuming that only these species are absorbing in this spectral region 15 ps after the excitation, we calculated the contribution of the charge-transfer band bleach to the transient spectrum to 40 %. An excellent fit of the slow signal decay at wavelengths between 350-450 nm could be done using the lifetimes obtained from the analysis of the data in the visible region, in accord with the assignments of the decay channels. The early dynamics in this region featured a 2 ps rise which we assume to be due to the formation of the trinitromethanide anion. A corresponding fast decay could be observed in the region 420-490 nm, indicating that the transient radical anion absorbs in this region. A detailed analysis was difficult to apply in this region, since the absorption by the radical cation is dominating and the total signal is relatively small. The ~2ps rise of the signal at 780-880 nm could be assigned to the appearance of NO₂ due to dissociation. The interpretation of short-time dynamics of the signal between 700-780 nm was difficult, possibly due to vibrational cooling of the naphtalene radical cation

© 1998 IEEE