Sensors and Actuators B 178 (2013) 296–301 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o me pa ge: www.elsevier.com/locate/snb Water-soluble branched phenylene-ethynylene fluorophores with N-phenylcarbazole core Pharkphoom Auttapornpitak a , Mongkol Sukwattanasinitt b , Paitoon Rashatasakhon b,∗ a Program of Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Bangkok, Thailand b Organic Synthesis Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University and Nanotec-CU Center of Excellence on Food and Agriculture, Bangkok 10330, Thailand a r t i c l e i n f o Article history: Received 24 October 2012 Received in revised form 13 December 2012 Accepted 22 December 2012 Available online 3 January 2013 Keywords: Fluorescence Rigidity Sensor Copper Salicylate Superquenching a b s t r a c t Three new water-soluble fluorophores containing N-phenylcarbazole core are synthesized by Sono- gashira coupling of the triiodo core with arylethynes followed by ester hydrolysis or amino quaternization. In comparison with their analogs with triphenylamine core, these new fluorophores exhibit greater quantum efficiencies which could cause by the enhanced molecular rigidity. The quan- tum yield improvement is more pronounced in the systems devoid of intramolecular energy transfer (ICT) process. The fluorophore with salicylate peripheries exhibits a selective fluorescence switching-off by Cu 2+ above 4 micromolar. Detailed investigations indicated that the peripheries on the carbazole nitro- gen is responsible for fluorescent quenching, whereas those on the 3- and 6-position of carbazole function as the fluorescent signal amplifying units. The fluorescence switching-off is postulated to cause by syn- ergetic works between superquenching induced by multiple complexation of Cu 2+ with the fluorophore and quenching signal amplification fetched by efficient ICT. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Fluorescent sensors have become common detecting devices in biological, chemical, and environmental research. Newly designed fluorophores, both in polymeric and small molecular forms, have continuously been developed for uses as fluorescent signal trans- ducers in the sensors [1–7]. During the course of our research program, we have demonstrated the design and synthesis of several water-soluble dendritic and star-shaped phenylene-ethynylene fluorophores [8–15] which could respond selectively toward cer- tain metal ions [8,14] or proteins [15], constitute protein sensor array [11] and aptasensor [12] or function as FRET donor in DNA–PNA sensing system [13]. These monodispersed dendritic fluorophores have shown great potentials in sensory applications and can be developed further into a variety of analogs by fine-tuning the core or peripheral moieties [16–19]. In many cases, the detec- tions of analytes using these fluorophores were achieved in the signal turn-off or quenching mode. In order to improve the sensi- tivity of such sensing systems, fluorophores with higher quantum efficiencies are desirable. In 2008, Nijegorodov et al. [20] investi- gated the effect of molecular planarity and rigidity on photostability ∗ Corresponding author. Tel.: +66 2 2187620; fax: +66 2 2187598. E-mail address: paitoon.r@chula.ac.th (P. Rashatasakhon). and quantum yields of several polyaromatic hydrocarbons. Com- parisons between fluorophores with similar -conjugated system but different molecular rigidity revealed that those with higher rigidity usually possess higher quantum efficiencies as the result of lower degree of geometrical relaxation upon excitation. Herein, we report our study on the rigidity effect on quantum efficien- cies and other photophysical properties of the new fluorophores (1–3, Fig. 1), which have a comparable conjugated system with the previously reported triphenylamine analog 1a–3a [8]. 2. Experimental 2.1. Materials and instruments All chemicals were reagent grades purchased from Sigma–Aldrich. Organic solvents for reaction work up and chromatography were commercial grades, which were distilled prior to use. Thin layer chromatography (TLC) was performed on aluminum sheets precoated with silica gel (Merck Kiesegel 60 F 254 ) (Merck KgaA, Darmstadt, Germany). Column chromatography was performed on silica gel (Merck Kieselgel 60G) (Merck KGaA, Darmstadt, Germany). All 1 H NMR and 13 C NMR spectra were obtained on a Varian Mercury NMR spectrometer, which operated at 400 MHz for 1 H and 100 MHz for 13 C nuclei (Varian Company, CA, USA). Elemental (CHN) analyses were performed on PE 2400 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.12.079