Solid States Fluorescence Study of <i>p</i>-Dimethylaminobenzonitrile by Co-grinding with Cyclodextrins

Abstract

The molecular status of a freeze-dried sample or a ground mixture of p-dimethylaminobenzonitrile (DMABN) with alpha-, beta-, or gamma-cyclodextrins (CDs) was examined using solid-state fluorescence measurements. A twisted intramolecular charge transfer (TICT) emission of DMABN crystals was shown at 475 nm. Emission peaks of freeze-dried samples were observed at 450, 380, and 393 nm in alpha-CD/DMABN, beta-CD/DMABN, and gamma-CD/DMABN systems, respectively. It was speculated that DMABN molecules existed as a twisted form in the cavity of alpha-CD, and as a plane structure in that of beta-CD or gamma-CD. On the other hand, fluorescence emission peaks of ground mixtures of DMABN with alpha-, beta-, or gamma-CD were observed at around 450 nm. When DMABN was ground together with microcrystalline cellulose, which cannot form an inclusion complex, only TICT emission was detected. These results suggest that the observed shift in the fluorescence peak could be due to inclusion phenomena. When the ground mixtures were crystallized under humid conditions, fluorescence emission peaks were observed at 450 nm in alpha-CD and of around 400 nm in beta- and gamma-CD systems. It is concluded that the conformation of the DMABN molecules in a crystalline CD/DMABN inclusion complex change depending on the size of the CD cavity.