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 ¹H spectral broadening and greater than expected numbers of observed ¹³C resonances substantially complicated the interpretation of traditional 1-D spectra. However, rational two-dimensional NMR approaches based on both homo- and heteronuclear couplings (¹H-¹H COSY; ¹H-¹³C 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.