Structural refinement of ladder-type perylenediimide dimers: a classical tale of conformational dynamics

We have synthesized and thoroughly characterized two representative ladder-type acetylene-bridged perylenediimide dimers bearing long alkyl chain solubilizing groups, bis[1-ethynyl-N,N'-bis(1-hexylheptyl)-perylene-3,4:9,10-tetracarboxylic diimide] ([PDICC]2, 1) and 1,1'-ethynyl-bis[N,N'-bis(1-hexylheptyl)-perylene-3,4:9,10-tetracarboxylic diimide] ([PDI]2CC, 2). In these dimeric PDI molecules, NMR-based structural characterization became nontrivial because severe (1)H spectral broadening and greater than expected numbers of observed (13)C resonances substantially complicated the interpretation of traditional 1-D spectra. However, rational two-dimensional NMR approaches based on both homo- and heteronuclear couplings ((1)H-(1)H COSY; (1)H-(13)C HSQC), in conjunction with high-level structural DFT calculations (GIAO/B3LYP/6-31G(d,p)/PCM, chloroform), were readily applied to these structures, producing well-defined analytical characterization, and the associated methodology is described in detail. Furthermore, on the basis of dynamic NMR experiments, both 1 and 2 were found to exist in a perylene-centered conformational dynamic equilibrium (ΔG‡ = 13-17 kcal/mol), which primarily caused the observed ambiguities in conventional 1-D spectra.