Two Closely Related Organic Charge-Transfer Complexes Based on Tetrathiafulvalene and 9H-fluorenone Derivatives. Competition between Hydrogen Bonding and Stacking Interactions

Two 1:1 charge-transfer organic complexes were formed using tetrathiafulvalene as a donor and a 9H-fluorenone derivative as acceptor: 4,5,7-trinitro-9H-fluoren-9-one-2-carboxylic acid (complex 1) or 4,5,7-trinitro-9H-fluoren-9-one-2-carboxylic acid methyl ester (complex 2). Both systems crystallize with alternated donor and acceptor stacks. However, the crystal structure of 1 is influenced by classical hydrogen bonds involving carboxylic acid groups, which force to arrange acceptors as centrosymmetric dimers in the crystal, via R-2(2) (8) ring motifs, while such a restriction is no longer present in the case of 2, affording thus a different crystal structure. This main difference is reflected in stacking interactions, and, in turn, in the degree of charge transfer observed in the complexes. The degree of charge transfer, estimated using Raman spectroscopy, is delta(1) = 0.07 for 1 and delta(2) = 0.14 for 2. It thus seems that, at least for the studied complexes, hydrogen bonding is an unfavorable factor for charge transfer.