Determination of binding strengths of a host-guest complex using resonance raman scattering

The detection of analyte-binding events by receptors is drawing together the fields of Raman spectroscopy and supramolecular chemistry. This study is intended to facilitate this cohering by examining a model system in the solution phase. The resonance Raman scattering (RRS) spectra of the complexation between tetrathiafulvalene (TTF) and cyclobis(paraquat-p-phenylene) (CBPQT ⁴⁺) has been used as the model to characterize the binding event of a host-guest system. RRS spectra are generated by excitation (λ _exc = 785 nm) within the lowest-energy charge-transfer (CT) transition (λ _max= 865 nm) of the TTFCCBPQT ⁴⁺ complex. The paired binding curves from the RRS and UV-vis-NIR titration data agrees with prior work, and a AG of -5.7 ± 0.6 kcal mol ^-1 (MeCN, 298 K) was obtained for the complexation of TTF with CBPQT4+. Computations on the complex and its components reproduce the energy shifts and resonance enhancements of the Raman band intensities, providing a basis to identify the structural and vibrational changes occurring upon complexation. The changes in bond lengths coincide with partial depopulation of a TTF-based HOMO and population of a CBPQT ⁴⁺-based LUMO through CT mixing in the ground state of 0.46e ^-. The structural changes upon complexation generally lead to lower wavenumber vibrations and to changes in the normal mode descriptions.