Synthesis and Optical Characterization of a Rhodamine B Spirolactam Dimer

Abstract

Amide derivatives of xanthene dyes such as rhodamine B are useful in a variety of sensing applications due to their colorimetric responses to stimuli such as acidity changes and UV light. The optical properties of these molecules can be influenced by intermolecular associations into dimeric structures, but the exact impact can be hard to predict. We have designed a covalently linked intramolecular dimer of the dye rhodamine B utilizing p-phenylenediamine to link the two dyes via amide bonds. The doubly closed spirolactam version of this dimer, RSL 2 , is isolated as a colorless solid. Under acidic conditions or UV exposure, RSL 2 solutions develop a pink color that is expected for the ring-opened form of the molecule. However, nuclear magnetic resonance (NMR) and single-crystal diffraction data show that the equilibrium still prefers the closed dimer state. Interestingly, the emission profile of RSL 2 shows solvatochromic blue fluorescence. Control studies of model compounds with similar structural motifs do not display similar blue fluorescence, indicating that this optical behavior is unique to the dimeric form. This behavior may lend itself to applications of such xanthene dimers to more sophisticated sensors beyond those with traditional binary on/off fluorescence profiles.

Structural and Optical Properties of Rhodamine Dye Aqueous Solutions and Thin Films

“…Rhodamine dyes are readily available, relatively inexpensive, and well known for having high quantum yields and strong absorption. − These properties make rhodamine dyes of great interest in applications as sensors, photovoltaics, and in biological systems. − The aggregation of these dyes leads to changes in the electronic states and spectral shifts in the absorption and fluorescence spectra, which have been extensively studied. − Characterization of the spectral changes between monomers and aggregates of the same dye is of interest in the development of sensors.…”

Spirocyclic rhodamine B benzoisothiazole derivative: a multi-stimuli fluorescent switch manifesting ethanol-responsiveness, photo responsiveness, and acidochromism

“… 42 Hence, RHBIT exhibits typical rhodamine spirolactam ring-opening behavior in the presence of acid and also under UV light illumination. 20 We have further studied the emission intensity of the newly formed photoinduced products at regular intervals for 48 h. After 48 h, the emission intensity decreased ∼4-fold and 140-fold for CHCl 3 ( λ max,emi = 588 nm) and DCM ( λ max,emi = 584 nm) solutions, respectively (Fig. S8 † ).…”

“…A similar observation was reported by Fogerty et al when they attempted to crystallize rhodamine B phenylene diamine dimer with HCl. 20 …”

“…A similar observation was reported by Fogerty et al when they attempted to crystallize rhodamine B phenylene diamine dimer with HCl. 20 3.3.4.1 Limit of detection and binding stoichiometry of RHBIT with PTSA. The sensitivity of RHBIT towards PTSA has been determined by estimating the limit of detection (LOD), as shown in Fig.…”

“… 16 Very recently Fogerty et al designed and synthesized a dual spirolactam moiety utilizing p -phenylene diamine that exhibited sensitivity to both pH and UV illumination; solvatochromic fluorescence emission was centered in the blue region of the spectrum, but an explanation of the linker chemistry that influences the open–closed equilibrium as well as solvatochromic emission in the dimer is lacking. 20 This color change is instantaneous in the case of chloroform and takes place over a few hours in the case of dichloromethane.…”