Probing Distance Dependent Charge-Transfer Character in Excimers of Extended Viologen Cyclophanes Using Femtosecond Vibrational Spectroscopy

Facile exciton transport within ordered assemblies of pi-stacked chromophores is essential for developing molecular photonic and electronic materials. Excimer states having variable charge transfer (CT) character are frequently implicated as promoting or inhibiting exciton mobility in such systems. However, determining the degree of CT character in excimers as a function of their structure has proven challenging. Herein, we report on a series of cyclophanes in which the interplanar distance between two phenyl-extended viologen (ExV(2+)) chromophores is varied systematically using a pair of o-, m-, or p-xylylene (o-, m-, or p-Xy) covalent linkers to produce o-ExBox(4+) (3.5 angstrom), m-ExBox(4+) (5.6 angstrom), and p-ExBox(4+) (7.0 angstrom), respectively. The cyclophane structures are characterized using NMR spectroscopy in solution and single-crystal X-ray diffraction in the solid state. Femtosecond transient mid-IR and stimulated Raman spectroscopies show that the CT contribution to the excimer states formed in o-ExBox(4+) and m-ExBox(4+) depends on the distance between the chromophores within the cyclophanes, while in the weak interaction limit, as represented by p-ExBox(4+) (7.0 angstrom), the lowest excited singlet state of ExV(2+) exclusively photo-oxidizes the p-Xy spacer to give the p-Xy(+.)-ExV(+.) ion pair. Moreover, the vibrational spectra of the excimer state show that it assumes a geometry that is intermediate between that of the locally excited and CT states, approximately reflecting the degree of CT character.