## 2 Materials and Methods  
### Chemistry  
All reagents and solvents were purchased from commercial suppliers and used without further purification. Dichloromethane was dried as described in the literature [44] and used freshly distilled. 3-Hexyl-2-methylbenzothiazol-3-ium iodide (**2**) [45], 3,4-dibutoxy-3-cyclobuten-1,2-dione (**4**) [46], 3-butoxy-4-[(3-hexylbenzothiazol-2(3\(H\))-ylidene)methyl]-cyclobut-3-en-1,2-dione (**5**) [47], 3-[(3-hexyl-benzothiazol-2(3\(H\))-ylidene)methyl]-4-hydroxycyclobut-3-en-1,2-dione (**6**) [47], 6-iodo-1-hexyl-quinaldine (**8**) [48] and 6-iodo-1-hexylquinaldinium iodide (**9**) [49] was prepared according to the corresponding literature procedures. Reactions were monitored by thin-layer chromatography (TLC) on aluminium plates with 0.25 mm of silica gel (TLC Silicagel 60 F\({}_{254}\), Merck,Darmstadt, Germany), which were, whenever necessary, visualized by UV detection. Purification by column chromatography was carried out on silica gel 60 (70-230 mesh) using a mixture of ethyl acetate and petroleum ether (1:1) or CH\({}_{2}\)Cl\({}_{2}\) as eluent. Melting points (m.p.) were recorded on a hot plate binocular microscope apparatus (URA Technic, Porto, Portugal) and are uncorrected. Infrared (IR) spectra were obtained using KBr pellets with an IRAffinity-1S FTIR spectrophotometer (Shimadzu, Duisburg, Germany) using Shimadzu LabSolutions IR software; \(\upsilon_{\max}\) in cm\({}^{-1}\). The intensity of the band was described as s (strong), m (medium) and w (weak). Vis/NIR absorption spectra were measured on a Lambda 25 instrument (Perkin Elmer, Waltham, USA) using PerkinElmer UV WinLab Data processor and Viewer software; \(\lambda_{\max}\) in nm. These spectra were obtained from acetonitrile (ACN), acetone (ACT), dichloromethane (DCM), DMEM, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), 1,4-dioxane (DXN), ethanol (EtOH), methanol (MeOH) and tetrahydrofuran (THF). \({}^{1}\)H- and \({}^{13}\)C-NMR spectra were recorded at 298.15 K, unless otherwise stated, on an Avance III 400 NMR spectrometer (Bruker, Bremen, Germany) operating at 9.4 T, observing \({}^{1}\)H at 400.13 MHz and \({}^{13}\)C at 100.63 MHz or on a Bruker Avance III 600 NMR spectrometer operating at 14.09 T, observing \({}^{1}\)H at 600.13 and \({}^{13}\)C at 150.91 MHz. Solutions were prepared in CDCl\({}_{3}\) or DMSO-\(d_{6}\). Proton peak splittings are expressed as s (singlet), br s (broad singlet), d (doublet), t (triplet), br t (broad triplet), qt (quintet), br qt (broad quintet) or m (multiplet). The assignments of the carbons were made based on DEPT 90 and DEPT 135 spectra. The \({}^{1}\)H-NMR and \({}^{13}\)C-NMR spectra of all synthesized dyes are presented as supplementary material. High resolution electrospray ionization time-of-flight mass spectra (HRESI-TOFMS) were carried out on a microTOF (focus) spectrometer Bruker Daltonics (Bremen, Germany, located at University of Vigo). 1.1 Synthesis of 2-[(1-hexyl-6-iodoquinolin-2(1\(H\))-ylidene)methyl]-4-[(3-hexylbenzothazol-3-ium-2-yl)methylene)- 3-oxocyclobut-1-en-1-olate (**10**)  
A mixture of monosubstituted intermediate **6** (0.80 g, 2.43 mmol) and quaternary ammonium salt **9** (1.17 g, 2.43 mmol) was heated under reflux for 3h in \(n\)-butanol/pyridine (10%) (120 mL). After cooling at r.t. CH\({}_{2}\)Cl\({}_{2}\) was added, and the mixture was washed with cold water. The organic layer was dried over anhydrous Na\({}_{2}\)SO\({}_{4}\) and the solvent removed under reduced pressure. The resulting solid was recrystallized from Et\({}_{2}\)O/CH\({}_{2}\)Cl\({}_{2}\)/MeOH. Yield: 53%. Golden brown crystals. M.p. 143-144 \({}^{\circ}\)C. Vis \(\lambda_{\max}\) (DMSO): 726 nm, log \(\varepsilon\) = 5.26. Vis \(\lambda_{\max}\) (DMEM): 640 nm. Vis \(\lambda_{\max}\) (ACN): 714 nm, log \(\varepsilon\) = 5.25. Vis \(\lambda_{\max}\) (ACT): 720 nm, log \(\varepsilon\) = 5.12. Vis \(\lambda_{\max}\) (DCM): 723 nm, log \(\varepsilon\) = 5.16. Vis \(\lambda_{\max}\) (DMF): 725 nm, log \(\varepsilon\) = 5.14.