Article pubs.acs.org/JACS
Probing Distance Dependent Charge-Transfer Character in Excimers of Extended Viologen Cyclophanes Using Femtosecond Vibrational Spectroscopy Yilei Wu, Jiawang Zhou, Brian T. Phelan, Catherine M. Mauck, J. Fraser Stoddart, Ryan M. Young,* and Michael R. Wasielewski* Department of Chemistry, Argonne-Northwestern Solar Energy Research Center, and Institute for Sustainability and Energy at Northwestern, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States S Supporting Information *
ABSTRACT: 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 pExBox4+ (7.0 Å), respectively. The cyclophane structures are characterized using NMR spectroscopy in solution and singlecrystal 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.
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electron transfer processes23 and singlet exciton fission,24,25 as well as serving as probes of molecular structure.26−28 Formally, the excimer state |Ex⟩ in a molecular dimer is a superposition of the locally excited (LE) or Frenkel exciton states |M1*M2⟩ and |M1M*2 ⟩ of each monomer (M) as well as the two CT (also referred to as charge-resonance, CR) states |M+1 M−2 ⟩ and |M−1 M+2 ⟩, such that21,29−32
INTRODUCTION Understanding molecular charge transfer and transport is essential to advancing fields ranging from organic electronics to solar energy conversion.1−11 Covalent multichromophore systems have been used extensively to investigate the through-bond and through-space electronic interactions responsible for charge transport. Among these systems, cyclophanes containing two aromatic units provide a structurally well-defined platform to characterize π−π chargetransfer (CT) interactions between them. Most of these studies have focused on π-stacked systems with interchromophore distances typically
