Effect of alkoxy side chain length on the solid-state fluorescence behaviour of bisazomethine dyes possessing dipropylamino terminal group

A study was conducted on the optical properties of alkoxy-substituted bisazomethine dyes possessing a dipropylamino terminal group in solution and the solid state. All of the derivatives had similar optical properties in chloroform solution, showing weak fluorescence quantum yield of 5%. In the solid state, the fluorescence behaviour was found to be dependent on the length of the alkoxy side chain. The solid-state fluorescence intensity of the derivatives with the short alkoxy side chains (methoxy, propoxy, and butoxy) was rather low compared to that of the other derivatives in the near-infrared region. However, in the case of the derivatives with the ethoxy side chain the red solid-state fluorescence quantum efficiency improved to 13% at around 700 nm. On the other hand, the derivatives with the middle (pentyloxy to octyloxy) to long (dodecyloxy, hexadecyloxy, and eicosanoxy) alkoxy side chains emitted at around 680 nm, producing enhanced red fluorescence compared to that of the chloroform solution. In particular, the derivative with the hexyloxy side chain showed the highest fluorescence quantum efficiency of over 20% in the solid state. To determine the influence of the alkoxy side chain length on the observed solid-state fluorescence behaviour, X-ray structure analysis was carried out focusing on the molecular arrangement of bisazomethine dyes in crystals. The crystal structures were analysed based on the molecular arrangements of their geometrical characteristics. The result indicated that their solid-state fluorescence depends on the relative positional relationship of the neighbouring molecules and the intermolecular distance resulting from the alkoxy side chain length. (C) 2016 Elsevier Ltd. All rights reserved.