PAH/PAH(CF3)n Donor/Acceptor Charge-Transfer Complexes in Solution and in Solid-State Co-Crystals

A solution, solid-state, and computational study is reported of polycyclic aromatic hydrocarbon PAH/PAH(CF3)(n) donor/acceptor (D/A) charge-transfer complexes that involve six PAH(CF3)(n) acceptors with known gas-phase electron affinities that range from 2.11(2) to 2.805(15) eV and four PAH donors, including seven CT co-crystal X-ray structures that exhibit hexagonal arrays of mixed pi-stacks with 1/1, 1/2, or 2/1 D/A stoichiometries (PAH=anthracene, azulene, coronene, perylene, pyrene, triphenylene; n=5, 6). These are the first D/A CT complexes with PAH(CF3)(n) acceptors to be studied in detail. The nine D/A combinations were chosen to allow several structural and electronic comparisons to be made, providing new insights about controlling D/A interactions and the structures of CT co-crystals. The comparisons include, among others, CT complexes of the same PAH(CF3)(n) acceptor with four PAH donors and CT complexes of the same donor with four PAH(CF3)(n) acceptors. All nine CT complexes exhibit charge-transfer bands in solution with lambda(max) between 467 and 600 nm. A plot of E(lambda(max)) versus [IE(donor)-EA(acceptor)] for the nine CT complexes studied is linear with a slope of 0.72 +/- 0.03 eV eV(-1). This plot is the first of its kind for CT complexes with structurally related donors and acceptors for which precise experimental gas-phase IEs and EAs are known. It demonstrates that conclusions based on the common assumption that the slope of a CT E(lambda(max)) versus [IE-EA] plot is unity may be incorrect in at least some cases and should be reconsidered.