Reflectance and photophysical properties of rhodamine 6G/2-(4-methyl-2-oxo-2H-chromen-7-yloxy) acetic acid as cold hybrid colorant

Rhodamine 6G (Rh6G) is modified by ethylenediamine to obtain rhodamine with amine functional groups (Rh6G-NH2). Rh6G-NH2 as an initial core is used to bond coumarin derivatives. Synthesized fluorescent colorants are specified using Fourier transform infrared spectroscopy (FT-IR), proton and carbon nuclear magnetic resonance (1H NMR and 13C NMR), X-ray diffraction (XRD), and field emission scanning electron microscopy (FE-SEM) to analyze the structure of the fluorescent pigments. Fluorescence microscopy, fluorescence spectrophotometer, and UV–visible–NIR reflectance spectra are used to demonstrate the optical properties. UV–Vis–NIR reflectance spectra showed that synthesized colorants were transparent in NIR region. Also, photophysical properties of 2-(4-methyl-2-oxo-2H-chromen-7-yloxy) acetic acid (MOHCYAA), Rh6G-NH2, and hybrid 2-(4-methyl-2-oxo-2H-chromen-7-yloxy) acetic acid/rhodamine 6G (HMR) were investigated. Type of solvent had a strong effect on quantum yield. Rh6G-NH2 (ϕs = 0.66) and HMR (ϕs = 0.72) displayed the maximum quantum yield in ethanol due to good interaction with ethanol and the formation of ring-opened amide form of rhodamine group. Finally, Rh6G-NH2 and HMR displayed the maximum quantum yield in ethanol due to good interaction of structure with ethanol and the formation of ring-opened amide form of rhodamine group in compound.

and industrial fields [18][19][20] . Among all the systems studied in early years, fluorescent probes were chosen as a good candidate to be used as pH detection system on account of high sensitivity, selectivity, and potential use in many fields [21][22][23] . However, due to the unique properties of rhodamine-based pH probes, they have got considerable attention as dual-switch pH sensors and cold pigments [24][25][26] . Amani et al. 27 investigated the photophysical and reflective properties of perylene-3,4,9,10-tetracarboxyl diimide (PTCDI)/rhodamine 6G hybrid for use in cold colors. Fluorescence quantum yield of PTCDI-Rh6G hybrid was investigated in different solvents and the highest efficiency was obtained 0.27 in DMF solvent. They found that synthesized fluorescent dyes were classified as transparent and adsorbent dyes in the NIR region.
Heretofore, the effect of rhodamine 6G on the structure of dye and hybridization with coumarin has not been studied in scientific sources. This has a significant impact on the cold paint industry. The purpose of this work is synthesis and investigation of optical and NIR reflectance properties of hybrid rhodamine 6G-coumarin dye. Rh6G-NH 2 is prepared by modification of rhodamine 6G (Rh6G) by ethylenediamine. Then, amine-functionalized rhodamine is used as core and 2-(4-methyl-2-oxo-2H-chromen-7-yloxy) acetic acid is reacted to amines via amidation reaction to prepare rhodamine 6G-2-(4-Methyl-2-oxo-2H-chromen-7-yloxy) acetic acid hybrid (HMR). Finally, NIR reflectance and photophysical behaviors under different conditions are investigated.

Results and discussion
Synthesis of fluorescence hybrid dye. Hybrid dye was designed with rhodamine 6G covered with coumarin derivative. FT-IR, 1 H NMR, 13 C NMR, XRD, UV-vis-NIR, FS, FM, TGA, DLS, and FE-SEM analyses were used to evaluate and confirm in each step. The FT-IR and 1 H NMR spectra of different pigments including COUM, MOHCYAA, Rh6G, Rh6G-NH 2 , and HMR pigments are depicted in Figs. 1 and 2, respectively and the most important peaks were mentioned in "Experimental methods" section. Also, 13 C NMR spectra are shown in Fig. S1. To proof Rh6G modification process, different pigments were analyzed by TGA. Rh6G-NH 2 and HMR thermograms are shown in Fig. 1. The degradation temperature (T d,max ) and weight loss of pigments were obtained 345 °C and 69.0% for Rh6G-NH 2 , 352 °C and 64.5% for HMR. X-ray diffraction patterns were collected to confirm the crystal structure of samples and investigate how crystallinity is affected by different reactions 30 . XRD patterns of all samples are shown in Fig. 1 The crystallinity of pigments was obtained 50.9, 79.2, 55.6, 48.5, and 64.8% for Rh6G, COUM, MOHCYAA, Rh6G-NH 2 , and HMR, respectively. Figure 3 shows FE-SEM images and DLS results of Rh6G-NH 2 and HMR. The pigment structure of Rh6G-NH 2 is small aggregates resembling flake particles with small sections. After reaction of Rh6G-NH 2 with MOH-CYAA, almost spherical particles were observed. DLS was performed to investigate the changes in size of samples after each step. To this end, a 1 mg/mL solution of Rh6G-NH 2 and HMR were analyzed at 25 °C. Z-average particle size of Rh6G-NH 2 and HMR were reported 887.5 and 1810 nm, respectively. The PDI values for Rh6G-NH 2 and HMR were 0.37 and 0.55, respectively.

Photophysical properties.
A bichromophoric light-harvesting system was designed, including a coumarin donor and a rhodamine 6G receptor. Due to the pH-sensitive nature of rhodamine 6G, we expect new fluorescence signals. Behavior of Rh6G-NH 2 and HMR pigments depends on pH. In alkaline solution, rhodamine 6G derivatives are colorless in the form of spirolactam closed form. In acidic environment, the rhodamine 6G spirolactam ring is opened and the energy of coumarin in HMR pigment is transferred to rhodamine 6G and emits a yellow-green fluorescence signal (Scheme 2).
Photophysical properties of pigments in H 2 O, DMF, and ethanol were investigated and results are summarized in Table 1. MOHCYAA, Rh6G-NH 2 , and HMR pigments were used as model pigments to evaluate the photophysical properties. Using Eq. (2) 32 , the fluorescence quantum efficiency of pigments was calculated.
In this equation, Φ r is the quantum yield of the standard pigment, m s is the slope of the linear fit for the integrated fluorescence intensity of the fluorescent pigment as a function of absorbance, and η s and η r are the refractive index of the fluorescent pigment and the standard solutions, respectively. Figures S2, S3, 4  www.nature.com/scientificreports/ not significantly affected by solvent. Observed deflections in different spectra can be related to slight differences in the solubility of Rh6G and coumarin molecules in individual solvents. Figure 8 shows the fluorescence images of COUM, MOHCYAA, Rh6G, Rh6G-NH 2 , and HMR. Owing to the red and yellow emission of rhodamine6G derivatives and the blue emission of coumarin derivatives, synthesized hybrid samples were evaluated by using two and three filters for Rh6G-NH 2 and HMR, respectively. Stability of the fluorescence emission of Rh6G-NH 2 in red and green and HMR in red, green, and blue fluorescence filters is quite evident. Figure 9 shows the UV-vis-NIR spectra of Rh6G-NH 2 and HMR in the wavelengths of 250-2500 nm. Reflection of pigments in the visible region is slightly different due to differences in their color 33 . The colors absorb the most ultraviolet light, which is in line with the organic nature of these colors and causes a similar reflection in the ultraviolet region. In addition, colors created different reflections on the white and black backgrounds. The reflection rate on the black and white substrates was < 20% and > 70%, respectively. Therefore, Rh6G-NH 2 and HMR are classified as transparent pigments in NIR area 34 . Furthermore, integrals of Rh6G-NH 2 and HMR pigments in different regions of UV-Vis-NIR curves were investigated and the data are summarized in Table 2. Rh6G-NH 2 and HMR had reflection of 95.4 and 95.4% in range of 700-1000 nm, respectively, and showed very good transparency.

Conclusions
Rh6G was modified by EDA to obtain Rh6G-NH 2 . Rh6G-NH 2 as an initial core was used to bond coumarin derivatives. 1 H NMR, FT-IR, XRD, TGA, FE-SEM, visible ultraviolet, Fluorescence spectrophotometer, DLS, and UV-Vis-NIR reflectance were used to confirm the success of various processes. Photophysical properties of MOHCYAA, Rh6G-NH 2 , and HMR were investigated. Type of solvent had a strong effect on quantum yield. Rh6G-NH 2 (ϕ s = 0.66) and HMR (ϕ s = 0.72) displayed the maximum quantum yield in ethanol due to good interaction with ethanol and the formation of ring-opened amide form of rhodamine group. UV-Vis-NIR reflectance spectra showed that Rh6G-NH 2 and HMR had a reflectance of 95.4% and 95.4% in 700-1000 range, respectively. As a result, Rh6G-NH 2 and HMR showed good transparency. www.nature.com/scientificreports/   www.nature.com/scientificreports/   www.nature.com/scientificreports/

Data availability
The datasets generated and/or analyzed during the current study are not publicly available at this time as the data form part of an ongoing study. However, the datasets are available from the corresponding author (Mehdi Salami-Kalajahi, m.salami@sut.ac.ir) on reasonable request.