Ultrafast 2DIR probe of a host-guest inclusion complex: structural and dynamical constraints of nanoconfinement

Two-dimensional infrared (2DIR) spectroscopy is used to study the influence of nanoconfinement on the spectral diffusion dynamics of cyclopentadienyl manganese tricarbonyl (CpMn(CO)3, CMT) free in solution and confined in the cavity of β-cyclodextrin. Contrary to the reorientation correlation function of the solvent molecules, determined through molecular dynamics simulations, measurements in three different solvents indicate that CMT confined in β-cyclodextrin undergoes spectral diffusion that is faster than free CMT. To account for this discrepancy, we propose that spectral diffusion time scales contain a dynamical contribution that is dependent on the effective size of the conformational space presented by the solvation environment. This solvation state space size is related to the number of participating solvent molecules, which in turn is proportional to the solvent accessible surface area (SASA). We test the role of the number of participating solvent molecules using a simple Gaussian-Markov simulation and find that an increase in the number of participating solvent molecules indeed slows the time required to search the available conformational space. Finally, we test this dependence by comparing the spectral diffusion of a previously studied manganese carbonyl, dimanganese decacarbonyl (Mn2(CO)10, DMDC), to CMT and find that DMDC, which has a larger SASA, exhibits slower spectral diffusion. The experimental observations and the supporting simplistic solvation model suggest that vibrational probe molecules, such as CMT, might be able to function as sensors of conformational entropy.