Excited state distortions in a charge transfer state of a donor–acceptor [2]rotaxane

The charge transfer excited state of a mechanically interlocked [2]rotaxane (R4+) with a donor 1,5-dioxynaphthalene (DNP) unit in the rod and the acceptor cyclobis(paraquat-p-phenylene) (CBPQT4+) ring component, along with the analogous non-interlocked [2]pseudorotaxane (P4+), is studied by resonance Raman spectroscopy and electronic absorption spectroscopy. Resonance Raman excitation profiles are obtained, calculated quantitatively using time-dependent theoretical methods, and interpreted with the assistance of DFT calculations. The active vibrational modes are consistent with an electron transfer from the HOMO centered on the DNP unit to the LUMO on the CBPQT4+ ring. Displacement vectors of highly distorted modes agree with the bonding changes predicted from the MO nodal pattern. Subtle changes in the frequency of some modes in the free components compared with those in R4+ are observed. The largest distortions are found for modes involving ring breathing in the DNP unit of the rod and the paraquat units of the CBPQT4+ ring. The individual mode contributions to the vibrational reorganization energy, as well as the total vibrational reorganization energy, are calculated. Very similar values of λv for R4+ and P4+ are calculated (∼2910 cm−1), indicating that the mechanical stoppers in the interlocked system do not significantly affect the excited state properties of R4+ compared with P4+.