This journal is c The Royal Society of Chemistry 2012 Chem. Commun. Cite this: DOI: 10.1039/c2cc33218c Utilising tetraphenylethene as a dual activator for intramolecular charge transfer and aggregation induced emissionw Guo-Feng Zhang, Matthew P. Aldred,* Wen-Liang Gong, Chong Li and Ming-Qiang Zhu* Received 4th May 2012, Accepted 15th June 2012 DOI: 10.1039/c2cc33218c We report a simple design and synthesis of a donor–acceptor tetraphenylethene–naphthalimide (TPE–NI) dyad, in which TPE acts both as an electron-donor for intramolecular charge transfer (ICT) and activator for aggregation induced emission (AIE). Strong solvent-dependent photoluminescence covering almost the whole visible spectrum and AIE in its nanoparticle state compared to its solution state are demonstrated. In recent years there has been widespread research interest in 1,8-naphthalimide (NI) primarily concerned with its optical properties. NI exhibits excellent photostability, high fluorescence quantum yields and electron accepting abilities. Due to the latter property NI has been mostly used in sensor-based research. 1 Most sensory materials based on NI function by taking advantage of photoinduced electron transfer (PET) and/or ICT and accordingly the fluorescence intensity can be altered upon binding with a guest. PET sensors typically use the structural configuration; fluorophore-spacer-receptor, in which the receptor can be an amine, and fluorescence quenching (‘‘off’’ state) is observed by electron transfer of the lone-pairs on the nitrogen to the fluorophore. 2 In the presence of a guest molecule the ‘‘on’’ state is observed by interaction with these lone electrons that are no longer able to transfer electrons to the NI, therefore preventing PET. ICT sensors are donor–acceptor (D–A) based fluorophores with electron donating groups, such as amines or carbazoles, directly conjugated to the NI core. 3 After excitation the molecule undergoes D–A ICT, which increases its dipole moment and as a result the molecule becomes sensitive to its environment. Opposite to ICT, PET is a through-space transfer of electrons that does not require conjugation. It is worth noting that many NI-based sensors utilise a combination of both PET and ICT in one molecule for their detection mechanism. Besides sensory applications NI derivatives have also shown promise in bio-related research areas, 4 organic light emitting diodes (OLEDs) 5 and dye-sensitised solar cells. 6 TPE has recently become a material with great prospects due to its AIE properties. 7 Additionally it exhibits bright blue fluorescence with high fluorescence quantum yields of approximately 50% in the solid state. TPE and TPE-based chromophores exhibit quenching in solution, contrary to its solid (or ‘‘aggregated’’) state, which shows strong luminescence. This is opposite to aggregation caused quenching (ACQ), which demon- strates larger fluorescence quantum efficiencies in solution compared to its solid state. Restriction of intramolecular rotation (RIR) in the solid state has been used to explain the AIE phenomenon. 8 TPE can easily be modified to yield water soluble fluorophores that can be applied to biological systems and used as bio-sensors and in imaging. Also, many TPE-based fluoro- phores exhibit quantitative photoluminescence quantum efficiencies (PLQE) in the solid state and exhibit high glass transition temperatures. Therefore, TPE-based materials have found applica- tions in two major areas; sensory 9 and OLED 10 applications. Fluorophores that exhibit both AIE and ICT behaviour possess a conventional electron donating group, such as triarylamine 11 and carbazole, 12 conjugated to an electron accepting group, such as cyano-substituted styrylbenzene, 12 borondipyromethene (BODIPY), 13 2-(4H-pyran-4-ylidene)malononitrile (DCM) 14 and cyano-substituted TPE. 15 Our aim was to use TPE as both the electron-donor for ICT and activator for AIE, attached to an electron-acceptor (NI), hopefully with a synergistic effect, and investigate its optical properties. Here, we report the synthesis of a TPE-based fluorophore (Scheme 1) containing naphthalimide (TPE–NI) that exhibits strong solvent-dependent fluorescence and AIE behaviour. From PL measurements we observe large red-shifts (180 nm) and fluorescence quenching in polar solvents compared to non-polar solvents because of ICT. Additionally, the PLQE in its nanoparticle state (90% water–THF) is enhanced to 20.4%, compared to 7.1% in THF solution. We attribute the ICT behaviour to the electron-donating ability of TPE and the AIE behaviour to the RIR of the TPE rotors. The aim of this paper is to demonstrate the dual ability of TPE to induce AIE behaviour and behave as an electron donor in a way that can potentially improve the fluorescence of donor–acceptor based materials in aqueous media. Naphthalimide-based D–A materials generally exhibit high fluorescence quantum yields. 3d However, the PLQE of TPE–NI in solution is relatively low (Table 1) suggesting a strong influence of the internal rotations/vibrations in the TPE group. The PLQE in chlorinated solvents (CHCl 3 and CH 2 Cl 2 ) are Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, Hubei, China 430074. E-mail: mqzhu@hust.edu.cn, mpaldred@hotmail.com; Fax: +86 27 87793419; Tel: +86 27 87793419 w Electronic supplementary information (ESI) available: Full instru- mentation details, experimental procedures/characterisation, other optical spectra, molecular simulation and Lippert-Mataga plots. See DOI: 10.1039/c2cc33218c ChemComm Dynamic Article Links www.rsc.org/chemcomm COMMUNICATION Downloaded by Huazhong University of Science & Technology on 02 July 2012 Published on 15 June 2012 on http://pubs.rsc.org | doi:10.1039/C2CC33218C View Online / Journal Homepage