PAPER www.rsc.org/dalton | Dalton Transactions A highly sensitive viscosity probe based on ferrocene-BODIPY dyads† Xiaodong Yin, a,b Yongjun Li, a Yulan Zhu,* c Xiao Jing, c Yuliang Li* a and Daoben Zhu* a Received 14th April 2010, Accepted 20th July 2010 DOI: 10.1039/c0dt00309c Two novel ferrocene-BODIPY molecules FcEB and FcB have been synthesized and characterized. An “interlock” packing mode was demonstrated in both FcEB and FcB by analysis of X-ray diffraction data. FcEB exhibits a molecular rotor property with a significant enhancement of quantum yield in viscous solvent. The result shows that the emission of FcEB increased dramatically as the viscosity of solution increased and also confirms that the structure–optical properties can be tuned by the structure design of molecule and introduction of different functional group. Our studies reveal that the system has great potential for developing viscosity probes in biological systems. Introduction Recently, fluorescent methods based on molecule rotors for mea- suring bulk macroscopic viscosity have been well developed with the range of viscosity measurements up to 1000 cP (centipoise). 1–4 However, materials suitable for measurement of local viscosity in microenvironments like living cells are still rare. 5–6 Viscosity is one of the major parameters in biological systems. Changes in viscosity link to the variation of diffusion and bimolecular reaction rates, which leads to disease and malfunction at the cellular level. 7–9 As we know, the viscosity of cellular cytoplasm and blood plasma are only 1–2 cP, 10–12 for which most of the probes, that have been reported, are not sensitive enough. The goal is to produce a new organic material that has distinct properties and high sensitivity to biological systems. This may include either new structures or characteristic chemical properties that can be exploited for high efficiency probing on biological systems. However, design of higher performance materials for a range of future applications for monitoring local viscosity in a low-viscous environment and providing a diagnostic tool for the detection of diseases remains a challenge. During the past decade, scientists have developed techniques for synthesizing and characterizing many new materials based on BODIPY molecule for testing the viscosity-sensitive properties of BODIPY derivatives. 6,13–14 Understanding the structure–function relationships that relate specifically to organic functional materials is possible to lead to new design concept for producing benign, high performance new molecular systems. In this work, we design- and synthesized a typical D-p-A system of BODIPY-ethynyl- Ferrocene molecule FcEB, with a long rigid bridge (Fig. 1), and a Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Organic Solids, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China b Graduate University of Chinese Academy of Sciences, Beijing, 100190, P. R. China. E-mail: ylli@iccas.ac.cn; Fax: (+86) 10 8261 6576 c Jiangsu Key Laboratory for Chemistry of Low-dimensional Materials, Huaiyin Normal University, Huai’an, Jiangsu, 223300, P. R. China †Electronic supplementary information (ESI) available: Crystal packing of FcEB and FcB (Fig. S1), photo-physical properties of FcEB, FcB and ethynyl bodipy (Fig. S2, S3, S4, Table S1), literature viscosity of different solvents (Table S2), spectro-electrochemistry data (Fig. S5), and distribution mode of FcEB in THF–H 2 O system (Fig. S6). CCDC reference numbers 772783 and 772784. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0dt00309c Fig. 1 Structure of BODIPY and FcEB dyads. the BODIPY moiety substituted on 1,7-position of FcEB. The new FcEB molecule with highly fluorescent quantum yield is expected to be advantageous for application in viscosity measurement of biological system. Results and discussion Synthesis and crystal structure As shown in Scheme 1, compound 1 15,16 was connected with ethynylferrocene 17 by Sonogashira cross-coupling to obtain FcEB as a deep-red needle-shaped crystal with a moderate yield of 30%. In order to investigate the influence of the bridge connecting the donor and acceptor, compound FcB was synthesized as an orange crystal according to the typical synthesis route of BODIPY with a yield of 40%. Ethynyl BODIPY was synthesized as a reference compound following the literature. 18 Scheme 1 Synthesis of FcEB, FcB and ethynyl bodipy. Single crystals of FcEB and FcB were gained by slow evap- oration of a mixture of hexane and dichloromethane, and This journal is © The Royal Society of Chemistry 2010 Dalton Trans., 2010, 39, 9929–9935 | 9929 Downloaded by Institute of Chemistry, CAS on 21 October 2010 Published on 22 September 2010 on http://pubs.rsc.org | doi:10.1039/C0DT00309C View Online