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Exploration of Energy Modulations in Novel RhB-TPEBased Bichromophoric Materials via Interactions of Cu2+ ion in Various Semi-Aqueous and Micellar Conditions Ravinder Singh, Atul Kumar Dwivedi, Ashutosh Singh, Chien-Min Lin, Reguram Arumugaperumal, Kung-Hwa Wei, and Hong-Cheu Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12768 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on February 28, 2016

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ACS Applied Materials & Interfaces

Exploration of Energy Modulations in Novel RhB-TPEBased Bichromophoric Materials via Interactions of Cu2+ ion in Various Semi-Aqueous and Micellar Conditions Ravinder Singh, Atul Kumar Dwivedi, Ashutosh Singh, Chien-Min Lin, Reguram Arumugaperumal, Kung-Hwa Wei, and Hong-Cheu Lin* Department of Materials Science & Engineering, National Chiao Tung University, Hsinchu, Taiwan KEYWORDS: bichromophore, tetraphenylethene, rhodamine B, photo-induced electron transfer, förster resonance energy transfer, chemodosimeter, and surfactant

*Author for Correspondence: Prof. Hong-Cheu Lin Department of Materials Science and Engineering National Chiao Tung University Hsinchu, Taiwan (ROC) Tel: 8863-5712121ext.55305 Fax: 8863-5724727 E-mail: [email protected]

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ABSTRACT: Novel bichromophoric materials TR-A and TR-B consisting of an entirely new combination of TPE and RhB units were developed to explore the optimum conditions of energy modulations via pH variation and Cu2+ interaction at various water contents of CH3CN. Interestingly, TR-A and TR-B at 60% and 70% water contents, respectively, favored the optimum Cu2+-mediated energy modulations from TPE to RhB and thus to achieve the brightest orange emissions of free RhB with complete disappearance of AIE from TPE. Furthermore, various micellar conditions of triton-X100, SDS, and CTAB were employed to adjust energy modulations of TR-A and TR-B at high water contents (at 80% and 90%, respectively). The incorporation of RhB into triton-X-100 micellar cavities disrupted AIE from TPE and thus none of the energy modulations from TPE to RhB occurred even in the presence of Cu2+ ion. Interestingly, the micellar conditions of anionic surfactant (SDS) favored the increased local concentration of Cu2+ ions in the vicinity of scavangable RhB and facilitating the generation of non-cyclic free RhB in-situ via bright orange emissions.


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INTRODUCTION The development of bichromophoric molecules connected through spacer units has been an area of particular interests among most synthetic and analytical chemists.1 Intensive research investigations on bichromophoric systems resulted in various useful findings and thus to be utilized in a variety of applications, such as photo-conducting polymers, photo-molecular devices,2,3 photodynamic therapies,4 drug biomolecules systems, and optical sensors.5,6 Due to all these advantages, remarkable efforts have been done in recent years to develop bichromophoric systems with increased efficiencies and rapid energy transfers upon light absorptions. In general, bichromophoric molecules separated by spacer units of controllable lengths determine the degrees of flexibilities through bond energy transfers.7 Generally, the efficiencies of energy transfers depend on the magnitudes of coupling between donors and acceptors, and the extents of coupling are influenced by the electronic structures of donors as well as the acceptors, surrounding media, and guest molecules. Moreover, the investigation of intra-electronic energy transfers (intra-EETs) based on different rigidities of bichromophoric molecules have to be considered. The construction of bichromophoric systems normally involves the utilization of isolated chromophores with known photophysical and photochemical properties. Therefore, the evolved changes of bichromophoric systems as a result of surrounding media or some guest species can be easily rationalized towards the understanding of mechanistically happenings during the intramolecular processes. In general, effective energy modulations in bichromophoric materials composed of rhodamine B (RhB) and other isolated chromophores have been studied by several research groups.8-14 All these examples demonstrated the utilization of spirolactam ring opening and closing phenomena in RhB and thus to transfer energy from various chromophores to RhB. Following this approach, Bojinov et al. reported the synthesis and investigations of Rhodamine 6G and 1,8-naphthalimide-based bichromophoric system.8 Both Förster resonance energy transfer (FRET) and photo-induced electron


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transfer (PET) mechanisms were reported to be operative in their studies. Das et al. developed a RhBbased fluorescent probe for the selective detection of Al3+, where the occurance of FRET mechanism via fluorometric and colorometric responses were investigated.9 Furthermore, Wanichacheva et al. synthesized a fluorescein and RhB-based bichromophore with FRET mechanism for highly sensitive and selective detection of Hg2+ ion.10 Huang et al. reported a novel RhB and coumarin-based bichromophore with a FRET platform to be used as a ratiometric Cu2+ probe.11 Furthermore, Gao et al. constructed another bichromophoric FRET platform by incorporating dansyl and RhB chromophores to possess a highly sensor selectivity towards Cu2+ ion.12 Zhao et al. fabricated a novel FRET ratiometric “off–on” bichromophore consisting of coumarin and rhodamine acid for the sensing of HOCl.13 Kubo et al. utilized TPE and RhB as isolated chromophoric units to construct the desired bichromophore, where TPE and RhB were retained as donor and acceptor in the bichromophore.14 However, such a combination of TPE and RhB-based chromophore and utilization of AIE energy are very rare. To the best of our knowledge, only one bichromophoric system with such a new combination was reported in the literature. In addition, one more recent report by Suk-Kyu Chang et al. demonstrated the generation of parent rhodamine from a dyad composed of rhodamine and densyl via the action of HOCl.15 At this point, our study represents the first example for the generation of parent rhodamine from a dyad consisting of both TPE and rhodamine units. Among bichromophoric systems, an isolated chromophore RhB in its spirocyclic structure possessing a colorless and non-fluorescent state may be generally stimulated into an open ring form with a pink color and a strong fluorescence in the presence of proton or metal ions.16-34 Similarly, TPE and its structural analogues were non-emissive in good solvents and become highly emissive in poor solvents due to the restriction of intramolecular rotations, which is one of the basic AIE characteristics. The utilization of photochemical and photophysical properties of TPEs in surrounding media as well as in


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the presence of guest molecules has been well established in several recent reports.35-39 The incorporation of both RhB and TPE units and thus to construct their bichromophoric system is very rare and may be of significant interest towards disclosing the hidden intra-electronic energy transfer (intraEET) in the intramolecular process. Therefore, the optimum conditions of energy modulations in the bichromophoric system composed of TPE and RhB units are firstly explored and investigated in this study. Moreover, the effective in-situ energy modulations of both bichromophoric chemodosimeters with symmetric and asymmetric structures were demonstrated through the generation of parent RhB dye, which also can be considered as the first example up to date. Therefore, as shown in Scheme 1, herein we design and construct novel symmetric and asymmetric bichromophores (i.e., TR-A and TR-B, respectively) with a new combination of RhB and TPE. For the first time, the blue AIE emission of TPE is well utilized and modulated by the optimized conditions of energy transfer in a semi-aqueous medium of water and CH3CN. However, TR-A and TRB were acted as chemodosimeters in the presence of a divalent Cu2+ metal ion, and thus to induce significant energy modulations with a complete disappearance of the blue AIE emission from TPE and a simultaneous appearance of the bright emission from RhB. The trends of the intra-electronic energy transfers (intra-EETs) in the intramolecular processes were further explored by employing various miceller conditions of triton-X-100, sodium dodecyl sulfate (SDS), and cetyltrimethylammonium bromide (CTAB).


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Scheme 1. Synthetic routes of TR-A and TR-B.

N

O

N

O

NH N

N N N

N N3

N3 N

O O

O

O O

O

NH N

N NH

N N N

O

O

CuSO4.5H2O, Sodium Ascorbate THF;H20 (8:2), RT -- 24 h

N

O

N

N3 CuSO4.5H2O, Sodium Ascorbate THF;H20 (8:2), RT -- 24 h

O N N N

N HN O

TR- A

TR- B

N

O

N


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EXPERIMENTAL SECTION Materials and Characterization. All the chemicals were obtained from the commercial sources and were of analytical pure and used without further purification. Solvents were dried by distillation over appropriate drying agents. All the anhydrous reactions were carried out following standard procedures under N2 gas atmosphere to avoid moisture and product formations were monitored by TLC plate. Plates were visualized under ultra-violet light (256 nm) and developed using I2 chamber. The column chromatography were performed on Merck silica gel 60 (230-400 mesh) under pressure using desired eluting solvents. Reported yields are isolated yields. UV-Vis spectra were recorded in different solvent conditions in a Jasco UV-600 spectrophotometer using 1 cm quartz cuvette. Fluorescence measurements were conducted with HITACHI 4000 Series Spectrophotometer. All emission and excitation spectra were corrected for the detector response and the lamp output. Melting points were determined using a Fargo MP-2D apparatus and are uncorrected. Elemental analyses were conducted on HERAEUS CHNOS RAPID elemental analyzer. Surface morphologies and particle distributions were investigated using a thermal field emission scanning electron microscope (SEM) (JEOL JSM6500 F). The SEM samples were prepared by drop-casting onto Si wafers followed by vacuum drying under room temperature. Dynamic light scattering (DLS) analyses were conducted on Malvern instruments-Zetasizer nano ZS90. All surfacants were prepared for micellar effects (with and without Cu2+ metal ion mediated) on energy modulations with concentrations above and below their critical micellar concentrations (CMCs) of 0.25, 8, and 1 mM for triton-X-100, SDS, and CTAB, respectively. NMR spectra were recorded on Bruker DRX-300 Avance series (1H: 300 MHz;

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C: 300

MHz) at a constant temperature of 298 K. Chemical shifts were reported in parts per million


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(ppm) from low to high field and referenced to residual solvent (CDCl3 δ = 7.26 ppm and δ = 77.23 ppm; DMSO-d6 δ = 2.49 ppm and δ = 39.56 ppm respectively) and tetramethylsilane SiMe4 (TMS) was used as internal reference for the 1H and

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C- NMR analyses. Coupling

constant (J) were reported in hertz (Hz). Standard abbreviations indicating multiplicity were used as follows: s = singlet, d = doublet, dd = double doublet, t = triplet, m = multiplet, br = broad. Mass spectra (FAB) were obtained on the respective mass spectrometer. General Synthetic Procedure for Bichromophores TR-A and TR-B: The synthetic entries to obtain TR-A and TR-B are shown in Scheme 1. Commercially available starting compound (4-hydroxy benzophenone) has been utilized to obtain the intermediate 1 and 2,40,41 under reflux conditions of THF in presence of Zn dust and TiCl4. Bromo-alkylation of 1 and 2, with 1,3-dibromo propane resulted intermediates 3 and 5 in a considerable yield of 90%, than the reported methods in the literature and incorporation of azide on the terminal ends of bromide, thus generated azide terminated intermediates 4,42 and 6,42 which were further reacted with alkyne terminated (RhB) based intermediate 8,43 via Cu(I) catalyzed click reaction to obtain TR-A and TR-B.42 Intermediate 7 was synthesized by following the procedures detailed in the literature.43 Detailed information on synthetic procedures and characterization of intermediates (1, 2, 3, 4, 5, 6, 7 and 8) have been provided in the supporting information of this article. The prepared azide terminated compounds (4), (6) and alkyne terminated Rhodamine derivatives (8) were charged into a cleaned 100 mL round bottom flask containing a mixture of solvents in ratio (THF: H2O, 8:2 (20 Times) and followed by evacuation by freeze-pump-thaw cycle. A separately degassed CuSO4.5H2O solution in H2O (10 Times) was added into the prepared reaction mixture containing azide and alkyne terminated intermediates. An immediate color changes were observed from maroon solution to violet. At the same time a separately


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degassed sodium ascorbate solution in H2O (10 Times) was added at room temperature slowly and further degassed by freeze-pump-thaw cycle and allowed to stir for 12 h at room temperature. Reaction was monitored by TLC. After completion, reaction mixture was evaporated under reduced pressure and thus resulting the remained aqueous solution, which was then further subjected to the addition of DCM (200 mL) and 17% aqueous ammonia solution (200 mL) and stirred at room temperature for 30 min. to remove the excess Cu as [Cu(NH3)3]+ salts. DCM layer was separated and washed with H2O (2 x 100 mL) followed by washing with brine solution (2 x 100 mL) and dried over MgSO4, and then filtered and evaporated under reduced pressure to get the crude residue. Crude residue was subjected to column chromatography (silica gel, hexane/ethyl acetate) for further purification. A light grey colored pure target compounds (TR-A) and (TR-B) were recovered after crystallization in DCM/pentane solvent mixture. Synthesis of Target Compound TR-B: ((2-(4-(3-azidopropoxy)phenylethene-1,1,2-triyl) tribenzene) (4) (0.25 g, 1 eq.), 3’,6’-bis(diethylamino)-2-(prop-2-yn-1-ylamino) spiro[isoindoline-1,9'xanthen]-3one (8) (0.30 g, 1.05 eq.), (THF: H2O, 16:4 mL), CuSO4.5H2O (0.15 g, 1 eq.) in H2O (1.5 mL), sodium ascorbate (0.25 g, 2 eq.) solution in H2O (2.5 mL), Crude residue was purified by column chromatography (silica gel, hexane/ethyl acetate: 3/7 to 1/9). A light grey color compound (TR-B) was recovered after crystallization with DCM/pentane (0.40 g) in yield 75%. HRMS (ESI) (m/z): (M+H) Calcd for C60H60N7O3: 926.4752, Found: 926.4766. Element analysis calcd for TR-B (C60H59N7O3): C = 77.81, H = 6.42, N = 10.59, found: C = 77.79, H = 6.55, N = 10.51. 1

H NMR (300 MHz, DMSO-d6) δ (ppm): 7.76 (1H, m, J = 6.0 Hz), 7.49-7.56 (4H, m), 7.08 (9H, m, J =

6.0 Hz), 6.95 (6H, m, J = 6.0 Hz), 6.82 (2H, d, J = 6.0 Hz), 6.63 (2H,d, J = 6.0 Hz), 6.26-6.32 (6H, m),


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5.24 (1H, t, J = 6.0 Hz), 4.33 (2H, t, J = 6.0 Hz), 3.73-3.81(4H, dd), 3.28 (8H, q, J = 6.0 Hz), 2.11 (2H, m), 1.05 (12H, t, J = 6.0 Hz). 13

C NMR (300 MHz, DMSO-d6) δ (ppm): 165.7, 156.8, 153.0, 151.6, 148.1, 143.9, 143.4, 140.1, 139.6

135.5, 132.8, 131.9, 130.6, 129.7, 128.2, 127.7, 126.4, 123.7, 122.9, 122.2, 113.6, 107.6, 105.6, 97.2, 64.6, 64.1, 46.2, 45.4, 43.6, 29.4, 12.4. Synthesis of Target Compound TR-A: 1,2-bis(4-(3-azidopropoxy)phenyl)-1,2-diphenylethene) (6) (0.25 g, 1 eq.), 3’,6’-bis(diethylamino)-2-(prop-2-yn-1-ylamino)spiro[isoindoline-1,9'xanthen]-3-one (8) (0.47 g, 2.05 eq.), (THF: H2O, 15:5 mL), CuSO4.5H2O (0.23 g, 1 eq.) in H2O (2.3 mL), sodium ascorbate (0.37 g, 4 eq.) solution in H2O (3.7 mL). Crude residue was purified by column chromatography (silica gel, hexane/ethyl acetate: 6/4 to 1/9). A light grey color compound (TR-A) was recovered after crystallization with DCM/pentane (0.42 g) in yield 60%. HRMS (ESI) (m/z): (M+H) Calcd for: C94H99N14O6 = 1519.7867, found: 1519.7930. Element analysis calcd for TR-A (C94H98N14O6): C = 74.28, H = 6.50, N = 12.90, found: C = 73.67, H = 6.44, N = 12.62. 1

H NMR (300 MHz, DMSO-d6) δ (ppm): 7.77 (2H, m, J = 6.0 Hz), 7.50-7.56 (8H, m), 7.08 (6H, m),

6.98 (4H, m, J = 12.0 Hz), 6.80 (4H, d, J = 6.0 Hz), 6.63 (4H, d, J = 6.0 Hz), 6.31 (12H, m, J = 6.0 Hz), 5.24 (2H), 4.34 (4H), 3.75-3.81 (8H, dd), 3.28 (16H, q, J = 6.0 Hz), 2.12 (4H, m), 1.05 (24H). 13

C NMR (300 MHz, DMSO-d6) δ (ppm): 166.5, 157.5, 153.9, 152.4, 149.0, 144.6, 140.1, 136.6, 133.7,

132.8, 131.6, 130.6, 129.1, 128.6, 127.1, 124.6, 123.8, 123.1, 114.5, 108.5, 106.5, 98.1, 65.5, 65.0, 47.1, 46.3, 44.5, 30.3, 13.3.


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RESULTS AND DISCUSSION Design and Synthesis. The design strategy for the construction of desired bichromophoric system TR-A and TR-B (Chart 1), composed of an xanthene dye RhB and an AIE activator TPE,43-48 involves the effective utilization of AIE energy from TPE, with the simultaneous bright orange emission from opened spirolactum RhB. Since TPE act as an efficient donor49,50,51 and RhB as an acceptor, and moreover excellent conformational switching properties associated with RhB were supposed to be offered by a significant energy modulation between TPE and RhB.

Chart 1. Chemical structures of TR-A and TR-B. Spectral Results (AIE Emissions) in Semi-Aqueous Media. TR-A and TR-B in CH3CN with the concentration of 4 µM were found to be highly soluble (with both TPE and RhB units in their molecular forms). Figure S1 depicted typical absorption spectra of TR-A and TR-B in CH3CN medium with two prominent bands centered at 272 and 314 nm, where the band centered at 272 nm was related to RhB. However, the band centered at 314 nm was considered to be originated from π-π transitions of TPE and RhB,52,53 which was supposed to be overlapped by the indistinguishable π-π transitions. Since the TPE unit is well known to have aggregation induced emissions as a result of the involvement through restricting free rotations of phenyl rings in aqueous media. Therefore, the AIE behavior was monitored by introducing water in CH3CN solutions of TR-A and TR-B. As a result of increasing water amounts,


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significant UV-Vis and PL spectral changes in TR-A and TR-B are summarized and depicted in Figure 1 and S2, respectively.

Figure 1. (a) UV-vis spectra of TR-A with increasing water fractions: 0, 50, 60, 70, 80, and 90%. (b) PL spectra of TR-A with increasing water fractions: 0, 10, 20, 30, 40, 50, 60, 70, 80, and 90%, Inset: photoimages of TR-A under UV lamp in the absence and presence (80%) of water. (c) The maximum PL intensities of TR-A and TR-B with increasing water fractions: 0, 10, 20, 30, 40, 50, 60, 70, 80, and 90%, with an excitation wavelength at λex = 380 nm. Both UV-vis and PL spectra of TR-A in CH3CN at various water contents (0-90%, vol.) are shown in Figures 1(a) and 1(b), respectively. As observed in Figure 1(b), the PL spectra of TR-A in CH3CN at lower water contents (0-50%) all show similar weak fluorescence. However, the corresponding absorption bands of TR-A centered at 272 nm and 314 nm in Figure 1(a) exhibited hyperchromic shifts but no spectral shifts in their peak positions. As the water fraction increased from 60% to 80%, the PL peak intensities (centered at 480 nm) of TR-A in Figure 1(b) were significantly enhanced towards bright blue (Quantum yield = 29% for TR-A at 60% water content by using quinine hemisulphate standard) as shown in the inset of Figure 1(b). According to this result, the fluorescence emission of TR-A due to the formation of self-assembled intermolecular π-π stacking from TPE started from a minimum water content of 60% to a maximum PL emission at 80%, which increased 20-fold PL


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intensity in contrast to 60% water content. However, further increasing the water content to 90% the PL intensity was slightly reduced. By increasing the water content to 60%, the related absorption bands of TR-A (in CH3CN) at 272 nm and 314 nm were slightly red-shifted to 288 nm and 332 nm, respectively, with an unstructured broad spectral pattern. At 80% water content, the absorption bands of TR-A shifted back to 320 and 277 nm. Interestingly, more structured and resolved absorption patterns of TR-A were observed at 80% water content in comparison with the red-shifted pattern of 60% water content. Followed by the same experimental conditions, similar spectral changes in TR-B were visualized with increasing water contents (see Figure S2). However, as shown in Figure 1(c) TR-B exhibited less PL emission (Quantum yield = 11% for TR-B at 70% water content by using quinine hemisulphate standard) as compared to TR-A, where more water consumptions (70-90%) are required to show the AIE behavior of TPE due to the lower restriction of asymmetrical TPE units in TR-B in contrast to TRA (with more restrictions of symmetric TPE units). Aggregation States in Semi-Aqueous Media: In order to realize the AIE behavior of TPE, the morphological patterns of TR-A at similar water contents (i.e., 0%, 50%, 60%, 80%, and 90%) were further investigated by scanning electron microscopy (SEM) studies, which are shown in Figures 2(a)2(e), respectively. As shown in Figures 2(a)-2(b), the morphological images of TR-A in CH3CN at water contents of 0% and 50% exhibited a uniform pattern with little aggregation (a few nano-spheres formed in 50%), which explained the lack of AIE behavior for TPE at 0%-50% water contents in Figure 1(c). As water contents reached 50%, 60%, and 80%, TR-A were aggregated into nano-sphere patterns with more amounts and larger sizes sequentially in Figures 2(b)-2(d), which verified the AIE behavior of TPE induced by the enhanced aggregation of TR-A at water contents > 50% in Figure 1(c). As shown in Figure 1(b), the maximum PL emission of TR-A at 80% water content was due to the existence of the largest aggregates as compared at 50% and 60% water contents, which was further confirmed by later


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dynamic light scattering (DLS) experiments. However, the PL emission of TR-A at 90% water content was reduced over the saturated aggregation of TR-A at 80% water content, which was also verified in the morphological test of Figure 2(e) with smaller aggregation sizes in contrast to Figure 2(d). Likewise, TR-B at water contents of 0%, 60%, 70%, and 90% also exhibited similar behaviors of nano-sphere assemblies in Figures S3(a)-S3(d), respectively, where the aggregation behavior of morphological images also matched the AIE trend in Figure 1(c). Accordingly, rather than 50% and 60% in TR-A, the largest aggregation enhancement happened in TR-B between water contents 60% and 70% was also confirmed to observe the AIE thresholds of water contents in Figure 1(c).

Figure 2. Morphological images of (a) TR-A in CH3CN (b) TR-A at 50% water content (c) TR-A at 60% water content (d) TR-A at 80% water content (e) TR-A at 90% water content.


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The aggregation behaviors of TR-A and TR-B in CH3CN solutions at various water contents were further verified by dynamic light scattering (DLS) experiments and illustrated in Figure S4. As shown in Figures S4(a)-(d), TR-A in CH3CN at water contents of 0%, 50%, 60%, and 80% revealed the aggregation mean sizes of 189.8, 200.4, 500.5, and 541.8 nm, respectively. According to the DLS data, we can conclude that the aggregation of TR-A in semi-aqueous solutions started to occur at 60% water content, which perfectly matched with the morphological results of Figures 2(a)-2(d) and again explained the aggregation emission behavior of TR-A in Figure 1(c). In addition, TR-B in CH3CN also exhibited the similar aggregation behavior (possessing the aggregation mean sizes of 173.1, 181.2, 273.1, and 293.8 nm at water contents of 0%, 60%, 70%, and 90%, respectively) but started to aggregate at water contents higher than 70%, which are also compatible with the morphological results of Figures S3(a)-S3(d). Overall, the observations in DLS experiments are in good agreement with the results of photophysical and microscopy studies. pH Effects on Absorbed Energy Modulations. The spirolactam ring opening and closing behavior of RhB could be obtained by the alterations of various pH conditions and thus to examine the absorbed energy modulations from TPE to RhB. Different pH conditions in TR-A at the fixed 60% water content in CH3CN were obtained by the addition of HCL and NaOH alternatively, where the pH effects on the PL spectral changes of TR-A are described in Figure 3.


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Figure 3. (a) Spirocyclic conformational changes in RhB under acidic and basic conditions. (b) Various PL spectra of TR-A with different pH conditions from 2 to11. Inset: photoimages of TR-A under UV lamp in acidic (pH = 2) and basic (pH = 8) conditions, with an excitation wavelength at λex = 380 nm. (c) The maximum PL intensities of TR-A with different pH values from 2 to11. The PL spectra of TR-A at 60% water content in various pH conditions from acidic (pH = 2) to basic (pH = 11) are shown in Figure 3(b). As observed in Figure 3(b), the PL spectra of TR-A in the acidic conditions of pH 2-3 exhibited a less intensive emission band (centered at 580 nm) and the AIE emission band (centered at 480 nm) from TPE disappeared simultaneously, which was related to the open spirolactam ring of RhB under stronger acidic conditions, and thus as expected a non-cyclic RhB was obtained. According to this result, the emission band corresponding to the non-cyclic RhB at 580 nm was found to be less intensive as compared to the parent RhB in open spirolactam form, due to the occurrence of effective photo-induced electron transfer (PET) mechanism from N atom of –NH– instead of –NH– (adjacent to the carbonyl group) to RhB through the conjugation of RhB with a non-cyclic


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form. However, upon increasing pH from 3 to 5, a significant decrease in the emission band at 580 nm and a simultaneous increase in the emission band at 480 nm were observed. Then, the cyclic spirolactam ring in RhB was completely recovered on pH conditions of 6 and 7 (or even further to all basic conditions), where the emission band at 580 nm was fully vanished. In general, the trends of both less intensive RhB emission band (centered at 580 nm) and the AIE emission band (centered at 480 nm) are demonstrated in Figure 3(c), where non-cyclic RhB emission band gave the highest intensity at pH=2 and the AIE emission band (with cyclic spirolactam ring in RhB) possessed the highest intensity at pH=8. Followed by the similar experimental conditions, TR-B was found to have similar results of photoluminescence spectral changes. However, as shown in the inset figures of Figures 3 and S5, due to the strong PL emission of AIE (i.e., the cyclic spirolactam ring in RhB) with respect to the weak PL emission of the open spiro cyclic emission in RhB, TR-A exhibited a larger PL contrast than TR-B. Therefore, the PL spectral analysis under various pH conditions provided us an evidence of the energy transfer between chromophoric TPE and RhB units connected by a triazole appended spacer. Metal Ion Mediated Energy Transfer in Organic Solvent Media (without AIE). Compared with the high AIE of TPE, the lower fluorescence of noncyclic RhB needs further investigations towards exploring the effective energy modulations from TPE to RhB. However, the less fluorescence behavior of non-cyclic RhB in TR-A and TR-B were due to the occurrence of PET mechanism in the spirolactam ring. Therefore, the metal ion mediated ring opening of the spirolactam ring with the simultaneous disruption of effective PET from N donor atom attached to the conjugation of spirolactam ring were supposed to occur. Following this hypothesis, metal ions as effective scavengers of sensitive covalent bonds were choosen to interact with TR-A and TR-B. Various alkali as well as transition metal ions, such as Fe2+, Ni2+, Cu2+, Al3+, Fe3+, Pb2+, Hg2+, Ag+, Co2+, Zn2+, Ca2+, Na+, Ca+, Cu+, Li+, Mn2+, Mg2+, Ba2+, Eu3+ and Sn2+, were included to monitor the opening capability of cyclic spirolactam ring in RhB.


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Firstly, the interactions of TR-A and TR-B towards various metal ions were examined in CH3CN by maintaining a concentrations of 4 µM. Upon the addition of various metal ions (0 to 10 eq.) into TR-A solutions, the corresponding spectral changes of UV/vis and PL spectra are summarized in Figure 4.

Figure 4. (a) PL intensity diagrams of TR-A in CH3CN (RhB emission only) towards various metal ions. (b) UV-Vis spectra of TR-A in CH3CN solution with and without Cu2+ metal ion. (c) PL spectra of TRA in CH3CN with and without Cu2+ metal ion, Insets: photoimages of TR-A by naked eye observation and PL emission, respectively, in the absence and presence of Cu2+ metal ion, with a PL excitation wavelength at λex = 380 nm. As shown in Figure 4(a), upon the addition of most alkali and most transition metal ions (even more than 10 eq.), TR-A in CH3CN (without AIE from TPE due to no water contents) could illustrate the strongest PL emission towards Cu2+ metal ion. The UV-Vis spectra of TR-A solution with Cu2+ (10 eq.) in Figure 4(b) exhibited a new absorption band at 554 nm along with some other absorption bands centered at 258, 307, 353, and 516 nm, where the intense band at 554 nm along with shoulders at 353 and 516 nm indicated the Cu2+ mediated opened spirocyclic RhB in TR-A. In addition, the PL emission band centered at 580 nm was also observed as Cu2+ metal ion was added into TR-A solution (see Figure 4(c)). As shown in the insets of Figures 4(b) and 4(c) the addition of Cu2+ metal ion into TR-A solution resulted in a color change (from colorless to purple) by naked eye observation and a prominent PL


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emission (from the quenched state to the orange emission). However, AIE from TPE in CH3CN did not exist either in the absence or presence of Cu2+ metal ion. Therefore, the disappearance of AIE in TR-A would induce less efficient energy transfer from TPE to RhB (i.e., less emission of RhB in TR-A.). Following the similar pattern, TR-B also exhibited an enhanced emission from RhB in the presence of Cu2+ metal ion over other various metal ions in CH3CN solution medium as shown in Figure S6. However, PL emission intensity from TR-B was found to be less intense in contrast to TR-A under similar conditions. Overall, the photo-physical properties of TR-A and TR-B by UV-vis (naked-eye observations) and PL measurements were highly sensitive towards Cu2+ metal ion over other alkali and transition metal ions. Moreover, the occurrence of FRET between the chromophoric unit TPE (donor) and RhB (acceptor) in TR-A is demonstrated in Figure 5(a). As shown in Figure 5(b), a significant 50% spectral overlap between AIE emission of TPE and absorption of non-cyclic RhB in TR-A was observed and

Figure 5. (a) Diagrammatic demonstration for the occurrence of FRET between the chromophoric unit TPE (donor) and RhB (acceptor) in TR-A. (b) Spectral overlaps between AIE emission of TPE and absorption of non-cyclic RhB in TR-A.


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thus the required conditions for plausible energy modulations to induce the effective FRET in TR-A was visualized. In the meanwhile, we also examined the absorbed energy modulation between the chromophoric unit TPE (donor) and RhB (acceptor) in TR-B. Furthermore, the interactions of TR-A and TR-B with Cu2+ metal ion were monitored by performing electro-spray ionization massspectrometry (ESI-MS) studies in CH3CN medium as shown in Figure 6. The mass spectra of TR-A and TR-B (in CH3CN) in the presence of Cu2+ are shown in Figures 6(b) and 6(c), respectively, where the molecular-ion peak verified the formation of isolated [rhodamine B] [MW: found 443.3; calcd. 443.24]. Therefore, the mass spectrometry provided the evidence of TR-A and TR-B to be selectively chemodosimetric in the presence of Cu2+ metal ion over the other metal ions and thus to have the PL emission at 580 nm in CH3CN, which may be correlated to the isolated RhB in CH3CN medium.

Figure 6. (a) Pictorial diagram depicting designed bichromophores to be chemodosimetric in the presence of Cu2+ metal ion (10 eq.). Mass spectra of (b) TR-A (c) TR-B in the presence of Cu2+ metal ion (10 eq.) in CH3CN solvent medium.

Metal Ion Mediated Energy Transfer in Semi-Aqueous Media (with AIE). Since very less energy modulation from TPE (without AIE) to RhB in the presence of Cu2+ metal ion was observed in Figure


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4(c), further investigations are required to explore the effective energy modulations from TPE to RhB by incorporating various water contents to utilize the AIE emissions from TPE. Therefore, the energy modulations of TR-A and TR-B from TPE (with AIE) to RhB in the presence of Cu2+ metal ion were examined by increasing water fractions from 0 to 80% in semi-aqueous CH3CN (4 µM). Upon increasing the water content of TR-A solutions with Cu2+ (10 eq.), the corresponding spectral changes of UV/vis and PL spectra are summarized in Figure 7. As observed in Figure 7(a), the UV-vis spectral changes of TR-A in the presence of Cu2+ (10 eq.) exhibited a similar pattern (from purple to pink by increasing water fractions from 0 to 70%) with an intense band at 554 nm, which is related to the isolated free form of non-cyclic RhB. Similar to Figure 6 (in the presence of Cu2+ without water), the mass spectrometry in Figure S7 also provided the evidence of TR-A even at 60% water content to be selectively chemodosimetric in the presence of Cu2+, where the molecular-ion peak verified the formation of isolated [rhodamine B] [MW: found 443.3; calcd. 443.24]. However, over 80% water content the enhanced aggregation of TPE induced the less interaction of TR-A with Cu2+ and thus to

Figure 7. Upon the addition of Cu2+ (10 eq.) (a) UV-vis spectral changes (b) PL spectral changes of TRA by increasing water fractions: 0, 10, 20, 30, 40, 50, 60, 70, and 80%, Insets: photoimages of TR-A in the presence of Cu2+ (at various water contents of 0, 60%, and 80%) by naked eye observation and PL emission, respectively, with a PL excitation wavelength at λex = 380 nm.


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show the transparent state in the photo image inset of Figure 7(a). Interestingly, the PL spectra of TR-A at 60% water content (in the presence of Cu2+) in Figure 7(b) revealed the strongest orange emission (at 580 nm) from RhB with the largest energy modulation from TPE in contrast to the weakest orange emission at 0% water content (in the presence of Cu2+) from RhB without energy modulation from TPE. Moreover, increasing water contents up to 70-80% in Figure 7(b) the intense AIE of TR-A was recovered due to the inactive energy modulation of TPE towards RhB with Cu2+ metal ion, which suggested that larger aggregates of TPE in TR-A at a higher water content are inactivate towards the Cu2+ mediated scavenging effect. Under the similar experimental conditions in Figure S8, TR-B at 70% water content also exhibited the same pattern with less bright orange emission intensity from RhB and even at higher water contents (80-90%) had failed energy modulation towards RhB with Cu2+ metal ion. Therefore, both TR-A and TR-B at 60% and 70% water contents, respectively, possessed some specific aggregated transition state that favored Cu2+ mediated scavenging via effective disruption of PET effects, and thus the evolution of bright orange emission from noncyclic isolated RhB through the complete utilization of AIE emission energy from TPE was observed. Thus, both bichromophoric systems were established as chemodosimeteric in the presence of Cu2+ metal ion, so Cu2+ metal ion played a significant role towards modulation of the absorbed radiation from donor to acceptor. The mechanism of efficient energy modulations from TPE to RhB were further investigated by scanning electron microscopy (SEM) studies of TR-A at 50% and 60% water contents in the presence of Cu2+, which are shown in Figures S9(a) and S9(b), respectively. Importantly, compared with those SEM images without Cu2+ in Figures 2(b) and 2(c), respectively, the morphological images of TR-A at 50% and 60% water contents with Cu2+ exhibited very different morphological phenomena to have smallersized aggregates of TPE with rough surfaces. This new aggregation behavior of TPE with Cu2+ (induced by water) clarified the energy modulations from TPE to RhB even without AIE emissions of TPE


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observed in Figure 7(b). Therefore, the most efficient energy modulation from TPE to RhB (i.e., with the strongest orange PL emission) in TR-A at 60% water content with Cu2+ has been further verified by this morphological exploration. Micellar effects (with and without Cu2+ metal ion mediated) on energy modulations. Since no energy modulations from TPE to RhB in the presence of Cu2+ were observed in TR-A and TR-B solutions at 80% and 90% water contents, respectively, this phenomenon required further investigations to recover their maximum effective energy modulations. Since self-assemblies of surfactants into their supramolecular structures are well known to modulate photo-physical properties of most chromophores,54,55 these investigations could be proceeded under various micellar conditions, including neutral, cationic and anionic media, under the hypothesis of being more informative and towards understanding the significant role of AIE in prominent energy modulations. Therefore, the maximum aggregated states of TR-A and TR-B solutions were employed in the micellar conditions with a neutral surfactant triton-X-100. Since triton-X-100 micelles are known to have incorporation capabilities of RhB dyes into their micellar cavities in many reports,56,57 efficient incorporations of terminal RhB of TR-A and TR-B solutions into the micellar cavities of triton-X-100 and thus to induce significant energy modulations from TPE to RhB were observed. To prove this hypothesis, the PL measurements of TR-A (4 µM) solutions at 80% water content in various triton-X-100 concentrations, including 0, 0.125, 0.25, 0.5, 1, 1.5, 2, 2.5, 5, 7.5, and 10 mM, were performed with and without Cu2+ metal ion (10 eq.) and shown in Figures 8(b), and 8(c), respectively.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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Figure 8. (a) Systematic pictorial diagrams of TR-A at 80% water content with the incorporation of RhB into the micellar cavities of triton-X-100, and then the energy modulation disabled upon the addition of Cu2+ (10 eq.). (b) PL spectra of TR-A at 80% water content in various triton-X-100 concentrations without Cu2+. (c) PL spectra of TR-A at 80% water content in various triton-X-100 concentrations with Cu2+ (10 eq.). (d) Systematic pictorial diagrams of TR-A at 80% water content into the micellar conditions of SDS in the absence and presence of Cu2+ (10 eq.). (e) PL spectra of TR-A at 80% water content in various SDS concentrations without Cu2+. (f) PL spectra of TR-A at 80% water content in various SDS concentrations with Cu2+, with a PL excitation wavelength at λex = 380 nm. The PL spectra of TR-A at water content 80% in the micellar solution of triton-X-100 with and without Cu2+ are shown in Figures 8(b) and 8(c), respectively. As observed in Figure 8(b), the AIE emission from TPE (centered at 480 nm) of TR-A at water content 80% decreased from the maximum to the lowest intensity effectively upon the addition of triton-X-100 from 0 to 10 mM, where the significant decrease in AIE emission may be attributed to the removal of aggregation among the TPE molecules in TR-A. Therefore, it revealed the possibility of effective incorporation of RhB terminal ends into the micellar cavities of triton-X-100 and thus to render the whole bichromophoric system TR-A into the disassembled molecular form suggested in Figure 8a. Moreover, such a decrement of AIE was not accompanied to the simultaneous increase in the emission from RhB in the absence and presence of Cu2+ metal ion (see Figures 8(b) and 8(c), respectively). Thus, the effective involvement of AIE towards emission enhancement in RhB into TR-A was completely evident. Moreover, to understand the alterations and nature of energy modulations under non-neutral micellar media, similar experimental conditions of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were utilized as cationic and anionic surfactants, respectively.


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The ionic surfactant concentrations of TR-A were chosen below and higher than their critical micellar concentration (CMC) values, including 0, 4, 8, 12, 16, 24, 32, 40, and 48 mM of SDS as well as 0, 0.125, 0.25, 0.5, 1, 1.5, 2, 2.5, 5, 7.5, 10, and 20 mM of CTAB. The PL spectral changes are shown in Figures 8(d), 8(e) and S11, which were subjected to both micellar conditions in the absence and presence of Cu2+ metal ion (10 eq.). Upon increasing the concentration of SDS from 0 to 48 mM in Figure 8(e), the AIE emissions of TR-A also decreased gradually. More importantly, in the presence of Cu2+ in Figure 8(f) TR-A enhanced the RhB emissions significantly upon the addition of SDS with the complete disappearance of AIE emissions. Therefore, we believed electrostatic interactions between the anionic surfactant (SDS) and metal cation (Cu2+) played an important role towards scavenging of the cyclic spirolactum ring of RhB.58 Moreover, the supramolecular self-assemblies of SDS dramatically increased the local concentration of Cu2+ ion in the vicinity of scavenging RhB site. Therefore, the SDS micellar conditions facilitated effective energy modulations from TPE to RhB with complete disappearance of AIE emissions through the occurrence of facile energy transfer to the opened spirocyclic form of RhB. Thus, a free RhB into the micellar solution was possibly generated by the specific interaction of RhB with Cu2+ metal ion. However, Figure S11(a) without Cu2+ depicts the decreased AIE emissions as moving towards the higher concentrations of CTAB from 0 to 20 mM, which were lower than those with the micellar media of SDS. Such decreases in AIE emissions might be induced by the interactions between TR-A and CTAB molecules. In the presence of Cu2+ (10 eq.) in Figure S11(b), the further decrement in AIE emissions and a slight increase in RhB emissions were observed simultaneously, which might be considered to be less pronounced than those with the micellar media of SDS.


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Followed by similar experimental conditions, TR-B at water content 90% was also subjected to investigations under these micellar conditions of triton-X-100, SDS, and CTAB as shown in Figures S10, S13, and S14, respectively. Therefore, TR-B presented similar behaviour as those of TR-A only with less pronounced surfactant effects of SDS on energy modulations. Therefore, the addition of surfactant SDS with proper ionic types in the presence of Cu2+ might enhance the recoveries of their maximum energy modulations of TR-A and TR-B at 80% and 90% water contents, respectively, from TPE to RhB, which were observed in TR-A and TR-B at 60% and 70% water contents, respectively. Furthermore, the inner filter effect was also excluded from our system due to the completely mismatched absorption of all surfactants with the AIE emission of TPE as shown in Figure S12.

Dynamic light scattering (DLS) experiments were further performed to confirm the effective incorporation of TR-A and TR-B into the micellar cavities of triton-X-100. The measured patterns of DLS for triton-X-100 at 80% water content without and with TR-A are shown in Figures S15(a), and S15(b), respectively. The micellar solution of triton-X-100 at 80% water content in Figure S15(a) exhibited micellar assemblies with a mean size of 67.3 nm without TRA. However, as shown in Figure S15(b) the addition of TR-A altered the micellar mean size to 435.5 nm. Similarly, triton-X-100 at 90% water content exhibited micellar structures with a mean size of 79.2 nm and the addition of TR-B enhanced the micellar mean size to 309.1 nm (see Figures S16(a), and S16(b), respectively). According to both results, TR-A and TR-B were successfully incorporated into the micellar cavities of triton-X-100 and were simultaneously lost AIE emissions from TPE units. However, the micellar mean sizes of DLS derived from SDS and CTAB were found to be slightly decreased in the presence of TR-A and TR-B. (see Figure S17).


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Therefore, it suggested that TR-A and TR-B were captured only into the micellar cavities of triton-X-100 rather than CTAB and SDS. The reduced micellar sizes of CTAB and SDS might be attributed to the molecular interactions and therefore reduced AIE emissions, which were visualized in our previous experiments.

CONCLUSION Two bichromophoric materials TR-A and TR-B with a new combination of TPE and RhB chromophoric units were successfully synthesized and characterized as chemodosimeters. Efficient energy modulation conditions from TPE to RhB were well optimized in semi-aqueous media at various water contents. The Cu2+-mediated enhanced energy modulations have been established at lower water contents of TR-A and TR-B (60% and 70%, respectively), resulting bright orange emissions of free RhB with complete disruption of AIE from TPE. However, the inactive energy modulations at high water contents of TR-A and TR-B (80% and 90%, respectively) with bright blue emissions of AIE from TPE were well explored in micellar conditions of triton-X-100, SDS, and CTAB in the absence and presence of Cu2+. The disruption of AIE from TPE into micellar medium of triton-X-100 was attributed to the effective incorporation of RhB chromophoric end into micellar cavity. Therefore, the absence of AIE exhibited none of the energy modulations from TPE to RhB even in the presence of Cu2+, which demonstrates the necessity of AIE to derive significant energy modulations in TR-A and TR-B. Interestingly, supramolecular self-assemblies by the micellar medium of SDS enhanced the local concentration of Cu2+ into the vicinity of scavenging RhB site and thus to induce bright orange emissions from free non-cyclic RhB with complete disruption of AIE from TPE in both TR-A and TR-B. Accordingly, the designed bichromophoric materials become valuable by efficient


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energy modulations


under semi-aqueous and

micellar conditions through naked-eye

observations.

ASSOCIATED CONTENT Supporting Information. Synthetic procedure and characterization, DLS, UV-vis and PL spectra, 1H & 13

C NMR, Elementary analysis and HRMS (ESI), and Mass data. This material is available free of

charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACKNOWLEDGMENT The financial supports of this project are provided by the Ministry of Science and Technology (MOST) in Taiwan through MOST 103-2113-M-009-018-MY3 and MOST 103-2221-E-009-215-MY3.

ABBREVIATIONS TPE, tetraphenylethene RhB, rhodamine B AIE, Aggregation induced emission


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SDS, sodium dodecyl sulfate CTAB, Cetyl trimethylammonium bromide

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Exploration of Energy Modulations in Novel RhB ... - ACS Publications

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