Traditional way of synthesizing sensitive rhodamine starts with rhodamine B, amino compound, and some alkali additive in the presence of weak base. However, for RH-PY, the stearic hindrance of pyrene slows the reactivity, and the reaction does not go to completion. Therefore, the reaction procedure was modified as follows. First, HATU and DIPEA were used to activate the carboxyl group of rhodamine B, then the activated rhodamine B was reacted with 1-aminopyrene. By introducing this method, the reaction completed, affording the target product in high yield (70%).
The structure of RH-PY was analyzed by 1H-NMR and ESI-TOF. Figure 51 shows the 1H-NMR spectrum of RH-PY. Interestingly, there are two series of peaks (a, a’ and b, b’), characteristics of rhodamine derivative. Peaks a and a’ are attributed to the -CH3 of RH-PY, and b and b’ are attributed to the -CH2 of RH-PY. Normally, rhodamine derivative shows only one characteristic -CH3 peak and one characteristic -CH2 peak. Therefore, if a and b peaks are set as the characteristic peaks of RH-PY, and a’ and b’ are assumed as by-product, then a and b consist of 12 H and 8 H, respectively.
Following this principle, the number of H in the aromatic region is 38, twice over the theoretical number of aromatic H on RH-PY. Therefore, we speculated that both a, a’ and b, b’ are attributed to RH-PY. Because of the large steric hindrance effect, RH-PY shows strong rotation effect and distortion effect. The characteristic -CH3 and -CH2 peaks of RH-PY are splitted into two series of characteristic peaks, labeled as a, a’ and b, b’. The ESI-TOF spectrum further demonstrates our speculation.
As shown in Figure S2, only one single peak at 642. 4 m/z is present in the spectrum, attributed to the mass of RH-PY, indicating that only one rhodamine derivative RH-PY is present in this system. Based on these results and analysis, RH-PY was successfully synthesized with relatively high yield by our modified method. 3. 2 Acid gas-chromic performance of RH-PY Rhodamine is well known for its pH sensitive properties. It is highly sensitive to acid in the solution sate, enabling its wide application as pH-dependent probes.
In acidic solutions, the ring-opened form is predominant, and therefore it is strongly fluorescent. Although this acid-sensitive equilibrium has been well known in solutions, its properties in the solid states are not well examined. Therefore, relative test of acid gas-sensitive property of RH-PY was proposed in this study. Figure 1. (A) Device schematic of acid-gas sensitive test, (B) Absorption variation of RH-PY under the stimuli of acid gas. Figure 1(A) shows the device for the acid-gas sensitive test. The bottle held concentrated HNO3 acid up to its 1/3 volume.
The sample was cast on a glass and then set on the bottleneck to fully cover the sample. Concentrated HNO3 is a volatile acid, and liquid HNO3 can easily vaporize. As expected, RH-PY is highly sensitive to gaseous HNO3. As shown in Figure 1(B), the absorption of initial sample is centered at 350 nm. Upon exposure to fuming concentrated HNO3 vapor, a new peak centered at 570 nm began to appear. This peak is assigned to the open-form of RH-PY, demonstrating that the ring-opening occurred upon the stimulation of gaseous HNO3.
The acid gas response of RH-PY was accomplished in 5 min, and such short time further shows the advantage of RH-PY in the application of acid gas sensitive area. Figure 2. Acid-gas test on filter paper with dual-color change at both room- and UV-light before: (A) and (D) and after: (B) and (C) the stimuli of acid gas. To show this switching more clearly, the sample was evenly distributed on a piece of filter paper. A filter paper (d = 0. 4 cm) was dipped in a CH2C12 solution of RH-PY (3. 0 mg of a probe in 10 mL of CH2C12) for 2-3 s.
Then, the filter paper was dried in the dark at room temperature. The filter paper shows a highly sensitive performance to acid gas. As shown in Figure 2, upon exposure to concentrated HNO3 acid, the apparent color changes from colorless to pink (Figures 2(A) and (C)) and fluorescence changes from blue-green to orange (Figures 2(B) and (D)). In this way, an acid-gas sensitive molecule with two different colors was readily developed in one paper. 3. 3 Mechanochromic performance of RH-PY RH-PY showed reversible color switch from bright blue to orange under UV and room light by grinding.
Upon heating the sample, the orange color was fully recovered to the initial state. The variation in the emission spectra upon grinding was 170 nm. Such mechanochromic properties with large emission variation arose from the excimer emission of pyrene, and the chemical reaction of rhodamine B from the spirolactam form to the ringopened amide significantly differs from the previously reported mechanochromic materials showing two color variation, and the mechanisms of control molecular structures or molecular orientation are described separately.
To the best of our knowledge, this is a rare example of the color change by a single molecule, attributing to the arrangement of chromophores and the mechanochemical reaction. To evaluate the mechanochromic properties of RH-PY, relative optical spectra measurements were performed. RH-PY in its initial state shows a sharp peak at 350 and 450 nm in the absorption and fluorescent spectra, respectively. Upon grinding, new peaks at 570 and 586 nm began to appear in the absorption and fluorescent spectra and gradually increased with the force added.
By heating at 70 °C for several minutes, new peaks (570 nm in the absorption spectra, 586 nm in the fluorescent spectra) fully restored to the initial state (Figure 3). The peak at 570 nm was assigned to the open form of rhodamine derivative, and the mechanochromism is attributed to the ring-opened form of RHPY. The large variation in the optical spectra (220 nm in the absorption spectra and 136 nm in the fluorescent spectra) allowed easy detection. Figure 3. (A) Absorbance and (B) Fluorescence spectra of RH-PY fluorophore powder with and without grinding.
To show this switching more clearly, the sample was smoothly distributed on a piece of filter paper, and it showed color transitions from blue-green to orange upon shearing (Figure 4). In this way, dual-color mechanochromic rhodamine derivative can be developed in one paper. Figure 4. Mechanochromic test of RH-PYon filter paper with reversible color change under both room- and UV-light upon grinding and heating. 3. 4 Photochromic performance of RH-PY Photochromic materials are attracting significant attention owing to their wide applications in bioimaging, drug delivery, self-assembly, logic gates, optical data storage, etc.
Utilizing light as external stimulation has numerous advantages, as lighttriggered intelligent materials response fast to the external stimulations and produce no by-products or contaminations during the response time under light exposure. Rhodamine is a class of dyes with long-wavelength absorption and emission, high absorption coefficient and quantum efficiency, and good photostability. However, because of the extreme short lifetime of its ring-open state, the photochromic rhodamine is rarely reported. RH-PY exhibits good photochromic performance.
The photochromic property of RH-PY was first evaluated in DCM solution. Upon irradiation by UV light (365 nm), a new peak at near-infrared region evolved and increased gradually with time. As shown in Figure 5, RH-PY initially exhibits a single peak centered at 350 nm, and within 3 min irradiation of UV light, a new peak centered at 560 nm evolves. With increasing irradiation time, this intensity of new peak increases gradually. This new peak is attributed to the open-form of RH-PY, demonstrating that the ring opening occurred in this period.
After 20 min irradiation time, this increment could not be obviously observed, as the ring-opening conversion rate reached saturation (Figure 5). Then, upon the storage in the dark, the intensity of this new peak began to decline gradually. Specially, the rate of decrease of the intensity of RH-PY is slower, and this new peak did not disappear until storage for 3 days. Compared to the reported photochromic rhodamine derivatives with only several hours open-form lifetime, RH-PY shows outstanding photochromic property, with the open-form lifetime of 3 days.