Perylene Bisimide and Naphthyl-Based Molecular Dyads: Hydrogen Bonds Driving Co-planarization and Anomalous Temperature-Response Fluorescence.

The origin of the positive temperature effect in fluorescence emission of a newly designed perylene bisimide (PBI) derivative with two naphthyl units containing ortho-methoxy group (NM) at its bay positions (PBI-2NM) was elucidated. A key point is the finding of a weak hydrogen bond (<5.0 kcal mol-1 ) between the methoxy group of the NM unit and a nearby hydrogen atom of the PBI core. It is the bonding that drives co-planarization of the different aromatic units, resulting in delocalization of the π-electrons of the compound as synthesized, inducing fluorescence quenching via intramolecular charge transfer (ICT). With increasing temperature, the co-planar structure could be distorted in part, resulting in a decreased degree of ICT, and hence leading to enhanced fluorescence emission. The unique positive temperature effect in emission induced by H-bond-driven co-planarization may pave a new avenue in designing functional molecular systems complementary to conventional methods.