Influence of Chromophore Length and Acceptor Groups on the Optical Properties of Rigidified Merocyanine Dyes

Abstract

Three series of rigidified tri‐, penta‐ and heptamethine merocyanine dyes were synthesised. A piperidyl moiety was chosen as the electron‐donating substituent while the electron‐accepting group was varied from ketones to malononitriles and cyanoacetates. The structures of the compounds in the solid state and in solution were elucidated by X‐ray diffraction and NMR spectroscopy, respectively, while optical properties were investigated by absorption and emission spectroscopy. As a general trend, the acceptor properties decrease in the series malononitrile>cyanoacetate>ketone based on the analysis of their solvatochromic behaviour. The experimental results were further supported by calculations at the B3LYP 6‐311+G(d) level of theory.

Rigidified malononitrile- and ketone-merocyanines in rigid environments

“…The synthesis of the MCs is detailed in a previous publication [9]. In order to screen for po-lymorphic modifications, samples of MC-M and MC-K were recrystallized from ether, dichloromethane and benzene.…”

“…Organic functional dyes recently attracted much attention as novel functional materials with a wide range of applications, for example in molecular memories and switches, food storage, photosensitive pharmaceutics, photodynamic tumour pharmaceutics, dye-sensitized optical elements [1][2][3][4][5][6][7][8][9]. Current research challenges are prediction, optimization and ultimately control of their physical properties, such as optical absorption, emission, long-term stability, processability and reproducibility.…”

“…Merocyanines (MCs) are interesting organic dyes due to the complex solvatochromism of their absorption and fluorescence and their non-linear optical properties [7][8][9]. MCs are chemically designed by linking an electron-donor with electronwithdrawing functional group by a conjugated chain (Fig.…”

“…1b). Moreover, the variation of the length of the conjugated core and the type of acceptor group was correlated to the optical and other physical properties in solution [9]. We decided to bring this interesting concept a step further and explore the influence not only of a chemically rigidified chromophore unit, but of a completely rigidified chromophore environment.…”

“…We selected to study two of the rigidified MCs containing different electron-acceptors, a malononitrile and a ketone functional group, abbreviated as MC-M and MC-K, respectively. The crystal structure of MC-K was presented and briefly commented in a previous publication [9], whether the structure of MC-M (which crystallizes only as a polycrystalline bulk) was solved ab initio using X-ray powder diffraction (XRPD). Note that beside the advances in instrumentation and software, crystal structure solution of organic molecules by XRPD still appears to be a niche method.…”