Origin of the Red Shift for the Lowest Singlet π → π* Charge-Transfer Absorption of p-Nitroaniline in Supercritical CO2.

Abstract

The origin of the unusual solvatochromic shift of p-nitroaniline (PNA) in supercritical carbon dioxide (SCCO2) is theoretically investigated on the basis of experimental data. Ab initio quantum chemistry calculations have been employed to unveil the interaction of CO2 with this archetypical molecule. It is demonstrated that the nitro group of PNA works as an electron-donating site binding to the electron-deficient carbon atom of CO2, most probably via a Lewis acid-base interaction. Moreover, a cooperative C-H···O hydrogen bond seems to act as an additional stabilizing source during the solvation process of PNA in SCCO2. To support the influence of solute-solvent specific interactions on the lowest singlet π → π* charge-transfer excitation, we perform a sequential Monte Carlo time-dependent density functional theory simulation to evaluate the excited states of PNA in SCCO2 (T = 315 K, ρ = 0.81 g/cm(3)). A critical assessment of this simulation, compared to calculations carried out within the polarized continuum model, gives strong evidence that our proposed complexes are important in describing the solvatochromic shift of PNA in SCCO2. The calculated red shift from the gas phase accounts for 66% to 80% (depending on the degree of complexation) of the experimental data. Finally, these results also alleviate possible failures commonly attributed to long-range corrected functionals in reproducing the solvatochromism of PNA.