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

Facile
exciton transport within ordered assemblies of π-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 (ExV2+) chromophores is varied systematically using
a pair of o-, m-, or p-xylylene (o-, m-, or p-Xy) covalent
linkers to produce o-ExBox4+ (3.5 Å), m-ExBox4+ (5.6 Å), and p-ExBox4+ (7.0 Å), 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-ExBox4+ and m-ExBox4+ depends on the distance between the chromophores
within the cyclophanes, while in the weak interaction limit, as represented
by p-ExBox4+ (7.0 Å), the lowest excited singlet state of ExV2+ 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.