Accepted Manuscript A new multi-addressable molecular switch based on a photochromic diarylethene with a 6-aryl[1,2-c]quinazoline unit Hongjing Jia, Shouzhi Pu, Congbin Fan, Gang Liu PII: DOI: Reference:
S1386-1425(14)01725-9 http://dx.doi.org/10.1016/j.saa.2014.11.066 SAA 13002
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
17 July 2014 31 October 2014 22 November 2014
Please cite this article as: H. Jia, S. Pu, C. Fan, G. Liu, A new multi-addressable molecular switch based on a photochromic diarylethene with a 6-aryl[1,2-c]quinazoline unit, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.11.066
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new multi-addressable molecular switch based on a photochromic diarylethene with a 6-aryl[1,2-c]quinazoline unit Hongjing Jia, Shouzhi Pu*, Congbin Fan and Gang Liu Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, PR China *Corresponding author: E-mail: [email protected] (S. Pu); Tel./Fax: +86-791-83831996.
1
Abstract A novel diarylethene with a 6-aryl[1,2-c]quinazoline unit has been synthesized via a nucleophilic reaction. Its photochromism and fluorescence exhibited multi-addressable behaviors by the stimulation of both light irradiation and acid/base. Addition of trifluoroacetic acid to the solution of the diarylethene resulted in notable absorption spectral change, and the protonated form also possessed excellent photochromic properties. Meanwhile, its emission intensity was enhanced remarkably and the emission peak redshifted with a notable color change from dark blue to bright green. The changes could be recovered to the initial state by neutralizing with triethylamine. Consequently, a logic circuit was constructed with the diarylethene by using the fluorescence intensity at 482 nm as output and acid/base as inputs. Keywords: Photochromism; Diarylethene; 6-Aryl[1,2-c]quinazoline; Molecular switch; Logic circuit.
2
Introduction In the past several decades, there has been considerable interest in the photochromic compounds due to their wide applications in various optoelectronic devices, such as molecular photochromic switches [1], fluorescence switches [2], chiral switches [3], redox switches [4], and pH switches [5]. Among all photochromic compounds, diarylethenes, discovered by Irie et al. [6] and Lehn et al. [7], are of major research interest because of their excellent thermal stability [8], remarkable fatigue resistance [9], and non-destructive feature [10]. Recently, various fluorescent molecular switches based on diarylethene derivatives have attracted a lot of attention. Tian and co-workers [11] synthesized a new photochromic terarylene BTO containing a benzo[b]thiophene-1,1-dioxide unit which provided access to multiple states triggered by metal ions, proton, and light. They also reported that a near-infrared photochromic diarylethene Iridium (III) complex exhibited excellent photochromic behavior accompanied by efficient quenching of phosphorescence emission [12]. Yi et al. [13] developed a multi-responsive fluorescence switch as a detector of metal ion transmembrane transport based on a diarylethene with terpyridine units. In our previous work, we reported a multiple responsive diarylethene with a salicylaldehyde Schiff base unit and found that its fluorescence could be simultaneously modulated by light, proton, and metal ions [14]. These achievements have contributed to a broad understanding of the multi-addressable fluorescence switching based on diarylethenes with various functional groups [15]. More potential applications in sensing, labeling, and high density information process has urged chemists to develop simple and efficient multi-addressable switching systems. It is well known that nitrogen-containing heterocycles play an important role in the field of organic chemistry. Among them, quinazoline derivatives are one of the most promising candidates for their wide applications in antagonists, DNA-biosensors, and imaging [16]. To date, their therapeutic activities including
3
anticancer, antiviral, antimicrobial, anti-inflammatory and anticonvulsant effects have been extensively investigated [17-21]. Their fluorescent properties have also drawn a lot of attention [22]. Introduction of a 6-aryl[1,2-c]quinazoline moiety into the photochromic diarylethene system can be expected to undergo favorable photochromism with interesting characteristics. However, to the best of our knowledge, the 6-aryl[1,2-c]quinazoline containing diarylethene derivatives have not hitherto been reported. In this work, a new asymmetrical diarylethene with a 6-aryl[1,2-c]quinazoline unit (1o) was synthesized and its multi-addressable photoswitching properties were systematically investigated. Its photochromism is shown in Scheme 1. Experimental General methods NMR spectra were recorded on a Bruker AV400 (400 MHz) spectrometer with CDCl3 as the solvent and tetramethylsilane as an internal standard. Infrared spectra (IR) were collected on a Bruker Vertex-70 spectrometer. Mass spectra were obtained on an Agilent 1100 ion trap MSD spectrometer. Elemental analysis was measured with a PE CHN 2400 analyzer. Melting point was measured on a WRS-1B melting point apparatus. Absorption spectra were monitored with an Agilent 8453 UV/Vis spectrophotometer. Fluorescence spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer. Photo-irradiation experiments were performed on an SHG-200 UV lamp, CX-21 ultraviolet fluorescence analysis cabinet, and a BMH-250 visible lamp. All solvents were of analytical grade and purified by distillation before use. Other reagents were used without further purification. Synthesis The synthetic route for the target compound 1o is shown in Scheme 2. The precursor diarylethene 2 was
4
synthesized by the reported method [23]. The target diarylethene 1o was prepared in anhydrous ethanol by a nucleophilic reaction using 2 and 2-(2-aminophenyl)-1H-benzimidazole as raw materials. A mixture of compound 2 (0.42 g, 1.0 mmol) and 2-(2-aminophenyl)-1H-benzimidazole (0.21 g, 1.0 mmol) in anhydrous ethanol (10 mL) was refluxed for 6 h until no compound 2 was detected on a TLC silica gel plate. After cooling the mixture solution to the room temperature, the crude product was concentrated by rotary evaporation. The crude product was purified by column chromatography on silica gel using petroleum ether/ ethyl acetate mixture (v/v = 6/1) as the eluent to give 0.21 g diarylethene 1o as a pale yellow solid in 35% yield. Mp.= 369-370 K; 1H NMR (CDCl3, 400 MHz), δ (ppm): 1.35 (s, 3H, -CH3), 1.84 (s, 3H, -CH3), 3.49 (s, 3H, -CH3), 4.92(s, 1H, -NH), 6.71 (s, 1H, heterocycle-H), 6.82 (s, 1H, pyrryl-H), 6.83 (s, 1H, thienyl-H), 6.97 (d, 2H, J = 7.6 Hz, phenyl-H), 7.11 (t, 2H, J = 8.0 Hz, phenyl-H), 7.19 (t, 2H, J = 8.0 Hz, phenyl-H), 7.81 (d, 1H, J = 8.0 Hz, phenyl-H), 8.18 (d, 1H, J = 8.0 Hz, phenyl-H),
13
C NMR (CDCl3, 100 MHz), δ
(ppm): 10.8, 11.7, 14.5, 33.0, 65.0, 105.6, 109.3, 112.8, 114.2, 115.7, 118.5, 119.7, 121.1, 122.9, 123.2, 124.3, 125.5, 125.9, 128.1, 128.7, 129.8, 132.1, 135.7, 140.7, 141.1, 142.7, IR (KBr, υ, cm-1): 736, 808, 904, 983, 1049, 1108, 1188, 1267, 1332, 1388, 1447, 1484, 1620, 2218, 2861, 2961, 3391. Anal. calcd. for C31H21F6N5S (%):C, 61.08; H, 3.47; N, 11.49. Found: C, 61.14; H, 3.51; N, 11.53. Results and Discussion Multi-addressable photochromic properties Fig. 1A shows the absorption spectral of diarylethene 1 induced by alternating irradiation with UV and visible light in acetonitrile (5.0 × 10-5 mol L-1) at room temperature. Its open-ring isomer 1o exhibited a sharp absorption peak at 301 nm (εmax = 2.5 × 104 mol-1·L·cm-1). Upon irradiation with 297 nm light, a new absorption band centered at 586 nm (εmax = 5.3 × 103 mol-1·L·cm-1) was observed due to the formation of the
5
closed-ring isomer 1c, accompanied with an evident color change from colorless to blue. When arrived at the photostationary state (PSS), its photoconversion ratio from its open-ring to the closed-ring isomers was determined to be 67% by HPLC analysis. Reversely, the blue colored solution was completely bleached upon irradiation with visible light (λ > 500 nm), indicating that 1c returned to the initial state of 1o. The cyclization and cycloreversion quantum yields of 1 were determined as 0.17 and 0.088, respectively, using 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene as the reference [24]. Compared to precursor 2 [23], the absorption maximum of 1c was blue-shifted by 80 nm. This may be attributed to the lower -conjugation of 6-aryl[1,2-c]quinazoline unit than that of the formyl group. The multiple photoswitching behavior of 1 was investigated by the stimulation of acid/base and light. Fig. 1 shows the spectral and color changes of 1 in acetonitrile (5.0 × 10-5 mol L-1). Addition of TFA (20.0 μL, 1.3 × 10-1 mol L-1) to 1o in acetonitrile resulted in a decrease at 322 nm (εmax = 1.43 × 104 mol-1 L cm-1) and a concomitant increase of a new band at 382 nm (εmax = 6.39 × 103 mol-1 L cm-1) due to the formation of the protonated 1o' (Fig. 1B). By plotting the absorbance intensity ratio at 322 nm to that at 382 nm (A322 nm / A382 nm)
as a function of TFA concentration, the relationship curve was obtained (the inset of Fig. 1B). It could be
clearly seen that the intensity ratio decreased gradually with the increment of TFA concentration from 0 to 25 equivalents, followed by a plateau with further titration. The protonated 1o' could return to the initial state of 1o by neutralizing with TEA (40 μL, 1.0 × 10-1 mol L-1). 1o' also underwent photoisomerization upon alternating irradiation with UV and visible light (Fig. 1C). Upon irradiation with 297 nm light, the absorption band at 306 nm (εmax = 2.77 × 104 mol-1 L cm-1) decreased and a new absorption band centered at 594 nm (εmax = 3.11 × 104 mol-1 L cm-1) emerged due to the formation of the protonated closed-ring isomer 1c'. This process was accompanied by a color change from colorless to cyan. It should be noted here that a reversible transformation between the colored 1c and 1c' could be controlled by the stimulation of TFA/TEA
6
(Fig. 1D). The photos of color change by photoirradiation and TFA/TEA are shown in Fig. 1E. The results indicated that the multi-addressable photochromism of 1 could be efficiently modulated by the stimulation of both acid/base and light. The fatigue resistance of 1 (repeat cycles between 1o and 1c) and 1' (repeat cycles between 1o' and 1c') were tested at room temperature (Fig. 2). In acetonitrile, 1 and 1' were irradiated alternatively with 297 nm and visible light (λ > 500 nm), respectively. The irradiation time was long enough for coloration to reach the photostationary state. As shown in Fig. 2, the coloration and decoloration cycles of 1 and 1' could repeat more than 100 cycles with the degradation of 12% for 1c and 5% for 1c'. The results indicated that the fatigue-resistance of 1 could be evidently enhanced after being protonated by TFA. Mulit-addressable fluorescent properties Fig. 3 shows the fluorescence changes of 1o upon alternating irradiation with UV and visible light in acetonitrile (2.0 × 10-5 mol L-1) at room temperature. The emission peak of 1o was observed at 426 nm when excited at 291 nm in acetonitrile. Upon irradiation with 297 nm UV light, the emission intensity at 426 nm decreased gradually along with the photoisomerization. When arrived at the photostationary state, its emission intensity was quenched to ca. 79%, indicating that 1 was a weak fluorescence switch upon photoirradiation. The residual fluorescence in the photostationary state may be attributed to the incomplete cyclization reaction and the existence of residual open-ring isomers with parallel conformation [25]. Back irradiation of the appropriate wavelength of visible light (λ > 500 nm) regenerated the open-ring isomer 1o and recovered the original emission spectra.
7
As mentioned previously, the spectral absorption of 1 could be effectively modulated by the stimulation of TFA/TEA. It is interesting to observe that its fluorescence could be also modulated by acid/base stimulation. As shown in Fig. 4A, the emission intensity was remarkably enhanced by 2.7 fold when TFA (80 L, 1.0 × 10-2 mol L-1) was added to the acetonitrile solution containing 1o. The fluorescence quantum yield of 1o' was determined as 0.36 by using anthracene (Φ = 0.27 in acetonitrile) as the reference [26]. The fluorescence quantum yield of 1o' was 12 times larger than that of 1o (0.03). Meanwhile, the emission peak was redshifted from 426 nm to 482 nm, accompanied with a notable color change from dark blue to bright green due to the formation of the protonated 1o. The fluorescence spectrum of 1o could return to the initial state 1o by neutralizing with TEA (15 L, 1.0 × 10-1 mol L-1). Similar to 1o, the emission intensity of 1o could also be effectively modulated by photoirradiation (Fig. 4B). Upon irradiation with 297 nm light, the emission intensity of 1o' was quenched to ca. 69% when arrived at the photostationary state. The reverse irradiation of appropriate wavelength visible light recovered its original emission intensity. Compared to 1o, both the fluorescence quantum yield and the fluorescent modulation efficiency of 1o' was significantly improved by intramolecular charge transfer (ICT) [27]. In addition, the reversible modulation of the fluorescence between 1o and 1o' by additions of TFA and TEA is shown in Fig. S1. The switchable fluorescence cycles could be repeated at least 10 times with only 10% degradation. To further confirm the intramolecular charge transfer (ICT) of 1, its fluorescence properties were investigated in different organic solvents, such as in hexane, THF, acetonitrile, and N,N-dimethylformamide (DMF). As shown in Fig. S2, the emission spectra of 1o exhibited a positive solvent effect in various organic solvents. The emission peak of 1o showed a notable bathochromic shift along with the increase of solvent polarity (from 414 nm in hexane to 431 nm in DMF) (Fig. S2A). Similarly, upon the addition of TFA, the
8
emission spectra of 1o' also shifted to a longer wavelength along with the increase of solvent polarity (Fig. S2B). Compared to that in hexane (468 nm), its emission peak in DMF (496 nm) showed a marked redshift ( = 28 nm). Based on the previous suggestion for explaining the solvent-dependent fluorescence [27], the observed phenomena may be ascribed to the generation of a significant ICT state upon excitation of 1o and 1o'. Construction of the logic circuit Molecular and supramolecular logic gates are potential candidates for computation at nanoscale [28]. There are many reported examples of logic circuit based on diarylethene derivatives [29]. According to the experiment described above, the proton has a more significant influence on the fluorescence intensity modulation of 1o than light. Hence, we constructed a new simple INHIBIT logic circuit based on 1o by utilizing TFA and TEA as the input signals, and fluorescence intensity as an output signal. 1o is regarded as the initial state, and its fluorescence changes can be combined as a molecular switch with two inducing inputs: In1 (TFA) and In2 (TEA). The emission intensity of 1o at 426 nm is regarded as the original value. The output signal can be regarded as “on” when the relative emission intensity at 482 nm is larger than 2.7 times of the original value, otherwise as “off”. Hence, we can use the binary digits (0 and 1) instead of the two levels (off and on). That means 1o can read a string of two inputs and write one output. For example, the input strings “1” and “0” correspond to the “on” and “off” states for In1 and In2. Under these conditions, 1o was turned to 1o by TFA stimulus and the emission intensity is enhanced. Thus, the output digit is “1” and the output signal is “on”. All the possible strings of the two inputs are listed in Table 1 and the combinational logic circuits equivalent to the truth table is shown in Fig. 5. The results demonstrated that the diarylethenes could be one of the promising candidates for applications in molecular switches [30].
S1386-1425(14)01725-9 http://dx.doi.org/10.1016/j.saa.2014.11.066 SAA 13002
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
17 July 2014 31 October 2014 22 November 2014
Please cite this article as: H. Jia, S. Pu, C. Fan, G. Liu, A new multi-addressable molecular switch based on a photochromic diarylethene with a 6-aryl[1,2-c]quinazoline unit, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.11.066
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new multi-addressable molecular switch based on a photochromic diarylethene with a 6-aryl[1,2-c]quinazoline unit Hongjing Jia, Shouzhi Pu*, Congbin Fan and Gang Liu Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, PR China *Corresponding author: E-mail: [email protected] (S. Pu); Tel./Fax: +86-791-83831996.
1
Abstract A novel diarylethene with a 6-aryl[1,2-c]quinazoline unit has been synthesized via a nucleophilic reaction. Its photochromism and fluorescence exhibited multi-addressable behaviors by the stimulation of both light irradiation and acid/base. Addition of trifluoroacetic acid to the solution of the diarylethene resulted in notable absorption spectral change, and the protonated form also possessed excellent photochromic properties. Meanwhile, its emission intensity was enhanced remarkably and the emission peak redshifted with a notable color change from dark blue to bright green. The changes could be recovered to the initial state by neutralizing with triethylamine. Consequently, a logic circuit was constructed with the diarylethene by using the fluorescence intensity at 482 nm as output and acid/base as inputs. Keywords: Photochromism; Diarylethene; 6-Aryl[1,2-c]quinazoline; Molecular switch; Logic circuit.
2
Introduction In the past several decades, there has been considerable interest in the photochromic compounds due to their wide applications in various optoelectronic devices, such as molecular photochromic switches [1], fluorescence switches [2], chiral switches [3], redox switches [4], and pH switches [5]. Among all photochromic compounds, diarylethenes, discovered by Irie et al. [6] and Lehn et al. [7], are of major research interest because of their excellent thermal stability [8], remarkable fatigue resistance [9], and non-destructive feature [10]. Recently, various fluorescent molecular switches based on diarylethene derivatives have attracted a lot of attention. Tian and co-workers [11] synthesized a new photochromic terarylene BTO containing a benzo[b]thiophene-1,1-dioxide unit which provided access to multiple states triggered by metal ions, proton, and light. They also reported that a near-infrared photochromic diarylethene Iridium (III) complex exhibited excellent photochromic behavior accompanied by efficient quenching of phosphorescence emission [12]. Yi et al. [13] developed a multi-responsive fluorescence switch as a detector of metal ion transmembrane transport based on a diarylethene with terpyridine units. In our previous work, we reported a multiple responsive diarylethene with a salicylaldehyde Schiff base unit and found that its fluorescence could be simultaneously modulated by light, proton, and metal ions [14]. These achievements have contributed to a broad understanding of the multi-addressable fluorescence switching based on diarylethenes with various functional groups [15]. More potential applications in sensing, labeling, and high density information process has urged chemists to develop simple and efficient multi-addressable switching systems. It is well known that nitrogen-containing heterocycles play an important role in the field of organic chemistry. Among them, quinazoline derivatives are one of the most promising candidates for their wide applications in antagonists, DNA-biosensors, and imaging [16]. To date, their therapeutic activities including
3
anticancer, antiviral, antimicrobial, anti-inflammatory and anticonvulsant effects have been extensively investigated [17-21]. Their fluorescent properties have also drawn a lot of attention [22]. Introduction of a 6-aryl[1,2-c]quinazoline moiety into the photochromic diarylethene system can be expected to undergo favorable photochromism with interesting characteristics. However, to the best of our knowledge, the 6-aryl[1,2-c]quinazoline containing diarylethene derivatives have not hitherto been reported. In this work, a new asymmetrical diarylethene with a 6-aryl[1,2-c]quinazoline unit (1o) was synthesized and its multi-addressable photoswitching properties were systematically investigated. Its photochromism is shown in Scheme 1. Experimental General methods NMR spectra were recorded on a Bruker AV400 (400 MHz) spectrometer with CDCl3 as the solvent and tetramethylsilane as an internal standard. Infrared spectra (IR) were collected on a Bruker Vertex-70 spectrometer. Mass spectra were obtained on an Agilent 1100 ion trap MSD spectrometer. Elemental analysis was measured with a PE CHN 2400 analyzer. Melting point was measured on a WRS-1B melting point apparatus. Absorption spectra were monitored with an Agilent 8453 UV/Vis spectrophotometer. Fluorescence spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer. Photo-irradiation experiments were performed on an SHG-200 UV lamp, CX-21 ultraviolet fluorescence analysis cabinet, and a BMH-250 visible lamp. All solvents were of analytical grade and purified by distillation before use. Other reagents were used without further purification. Synthesis The synthetic route for the target compound 1o is shown in Scheme 2. The precursor diarylethene 2 was
4
synthesized by the reported method [23]. The target diarylethene 1o was prepared in anhydrous ethanol by a nucleophilic reaction using 2 and 2-(2-aminophenyl)-1H-benzimidazole as raw materials. A mixture of compound 2 (0.42 g, 1.0 mmol) and 2-(2-aminophenyl)-1H-benzimidazole (0.21 g, 1.0 mmol) in anhydrous ethanol (10 mL) was refluxed for 6 h until no compound 2 was detected on a TLC silica gel plate. After cooling the mixture solution to the room temperature, the crude product was concentrated by rotary evaporation. The crude product was purified by column chromatography on silica gel using petroleum ether/ ethyl acetate mixture (v/v = 6/1) as the eluent to give 0.21 g diarylethene 1o as a pale yellow solid in 35% yield. Mp.= 369-370 K; 1H NMR (CDCl3, 400 MHz), δ (ppm): 1.35 (s, 3H, -CH3), 1.84 (s, 3H, -CH3), 3.49 (s, 3H, -CH3), 4.92(s, 1H, -NH), 6.71 (s, 1H, heterocycle-H), 6.82 (s, 1H, pyrryl-H), 6.83 (s, 1H, thienyl-H), 6.97 (d, 2H, J = 7.6 Hz, phenyl-H), 7.11 (t, 2H, J = 8.0 Hz, phenyl-H), 7.19 (t, 2H, J = 8.0 Hz, phenyl-H), 7.81 (d, 1H, J = 8.0 Hz, phenyl-H), 8.18 (d, 1H, J = 8.0 Hz, phenyl-H),
13
C NMR (CDCl3, 100 MHz), δ
(ppm): 10.8, 11.7, 14.5, 33.0, 65.0, 105.6, 109.3, 112.8, 114.2, 115.7, 118.5, 119.7, 121.1, 122.9, 123.2, 124.3, 125.5, 125.9, 128.1, 128.7, 129.8, 132.1, 135.7, 140.7, 141.1, 142.7, IR (KBr, υ, cm-1): 736, 808, 904, 983, 1049, 1108, 1188, 1267, 1332, 1388, 1447, 1484, 1620, 2218, 2861, 2961, 3391. Anal. calcd. for C31H21F6N5S (%):C, 61.08; H, 3.47; N, 11.49. Found: C, 61.14; H, 3.51; N, 11.53. Results and Discussion Multi-addressable photochromic properties Fig. 1A shows the absorption spectral of diarylethene 1 induced by alternating irradiation with UV and visible light in acetonitrile (5.0 × 10-5 mol L-1) at room temperature. Its open-ring isomer 1o exhibited a sharp absorption peak at 301 nm (εmax = 2.5 × 104 mol-1·L·cm-1). Upon irradiation with 297 nm light, a new absorption band centered at 586 nm (εmax = 5.3 × 103 mol-1·L·cm-1) was observed due to the formation of the
5
closed-ring isomer 1c, accompanied with an evident color change from colorless to blue. When arrived at the photostationary state (PSS), its photoconversion ratio from its open-ring to the closed-ring isomers was determined to be 67% by HPLC analysis. Reversely, the blue colored solution was completely bleached upon irradiation with visible light (λ > 500 nm), indicating that 1c returned to the initial state of 1o. The cyclization and cycloreversion quantum yields of 1 were determined as 0.17 and 0.088, respectively, using 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene as the reference [24]. Compared to precursor 2 [23], the absorption maximum of 1c was blue-shifted by 80 nm. This may be attributed to the lower -conjugation of 6-aryl[1,2-c]quinazoline unit than that of the formyl group. The multiple photoswitching behavior of 1 was investigated by the stimulation of acid/base and light. Fig. 1 shows the spectral and color changes of 1 in acetonitrile (5.0 × 10-5 mol L-1). Addition of TFA (20.0 μL, 1.3 × 10-1 mol L-1) to 1o in acetonitrile resulted in a decrease at 322 nm (εmax = 1.43 × 104 mol-1 L cm-1) and a concomitant increase of a new band at 382 nm (εmax = 6.39 × 103 mol-1 L cm-1) due to the formation of the protonated 1o' (Fig. 1B). By plotting the absorbance intensity ratio at 322 nm to that at 382 nm (A322 nm / A382 nm)
as a function of TFA concentration, the relationship curve was obtained (the inset of Fig. 1B). It could be
clearly seen that the intensity ratio decreased gradually with the increment of TFA concentration from 0 to 25 equivalents, followed by a plateau with further titration. The protonated 1o' could return to the initial state of 1o by neutralizing with TEA (40 μL, 1.0 × 10-1 mol L-1). 1o' also underwent photoisomerization upon alternating irradiation with UV and visible light (Fig. 1C). Upon irradiation with 297 nm light, the absorption band at 306 nm (εmax = 2.77 × 104 mol-1 L cm-1) decreased and a new absorption band centered at 594 nm (εmax = 3.11 × 104 mol-1 L cm-1) emerged due to the formation of the protonated closed-ring isomer 1c'. This process was accompanied by a color change from colorless to cyan. It should be noted here that a reversible transformation between the colored 1c and 1c' could be controlled by the stimulation of TFA/TEA
6
(Fig. 1D). The photos of color change by photoirradiation and TFA/TEA are shown in Fig. 1E. The results indicated that the multi-addressable photochromism of 1 could be efficiently modulated by the stimulation of both acid/base and light. The fatigue resistance of 1 (repeat cycles between 1o and 1c) and 1' (repeat cycles between 1o' and 1c') were tested at room temperature (Fig. 2). In acetonitrile, 1 and 1' were irradiated alternatively with 297 nm and visible light (λ > 500 nm), respectively. The irradiation time was long enough for coloration to reach the photostationary state. As shown in Fig. 2, the coloration and decoloration cycles of 1 and 1' could repeat more than 100 cycles with the degradation of 12% for 1c and 5% for 1c'. The results indicated that the fatigue-resistance of 1 could be evidently enhanced after being protonated by TFA. Mulit-addressable fluorescent properties Fig. 3 shows the fluorescence changes of 1o upon alternating irradiation with UV and visible light in acetonitrile (2.0 × 10-5 mol L-1) at room temperature. The emission peak of 1o was observed at 426 nm when excited at 291 nm in acetonitrile. Upon irradiation with 297 nm UV light, the emission intensity at 426 nm decreased gradually along with the photoisomerization. When arrived at the photostationary state, its emission intensity was quenched to ca. 79%, indicating that 1 was a weak fluorescence switch upon photoirradiation. The residual fluorescence in the photostationary state may be attributed to the incomplete cyclization reaction and the existence of residual open-ring isomers with parallel conformation [25]. Back irradiation of the appropriate wavelength of visible light (λ > 500 nm) regenerated the open-ring isomer 1o and recovered the original emission spectra.
7
As mentioned previously, the spectral absorption of 1 could be effectively modulated by the stimulation of TFA/TEA. It is interesting to observe that its fluorescence could be also modulated by acid/base stimulation. As shown in Fig. 4A, the emission intensity was remarkably enhanced by 2.7 fold when TFA (80 L, 1.0 × 10-2 mol L-1) was added to the acetonitrile solution containing 1o. The fluorescence quantum yield of 1o' was determined as 0.36 by using anthracene (Φ = 0.27 in acetonitrile) as the reference [26]. The fluorescence quantum yield of 1o' was 12 times larger than that of 1o (0.03). Meanwhile, the emission peak was redshifted from 426 nm to 482 nm, accompanied with a notable color change from dark blue to bright green due to the formation of the protonated 1o. The fluorescence spectrum of 1o could return to the initial state 1o by neutralizing with TEA (15 L, 1.0 × 10-1 mol L-1). Similar to 1o, the emission intensity of 1o could also be effectively modulated by photoirradiation (Fig. 4B). Upon irradiation with 297 nm light, the emission intensity of 1o' was quenched to ca. 69% when arrived at the photostationary state. The reverse irradiation of appropriate wavelength visible light recovered its original emission intensity. Compared to 1o, both the fluorescence quantum yield and the fluorescent modulation efficiency of 1o' was significantly improved by intramolecular charge transfer (ICT) [27]. In addition, the reversible modulation of the fluorescence between 1o and 1o' by additions of TFA and TEA is shown in Fig. S1. The switchable fluorescence cycles could be repeated at least 10 times with only 10% degradation. To further confirm the intramolecular charge transfer (ICT) of 1, its fluorescence properties were investigated in different organic solvents, such as in hexane, THF, acetonitrile, and N,N-dimethylformamide (DMF). As shown in Fig. S2, the emission spectra of 1o exhibited a positive solvent effect in various organic solvents. The emission peak of 1o showed a notable bathochromic shift along with the increase of solvent polarity (from 414 nm in hexane to 431 nm in DMF) (Fig. S2A). Similarly, upon the addition of TFA, the
8
emission spectra of 1o' also shifted to a longer wavelength along with the increase of solvent polarity (Fig. S2B). Compared to that in hexane (468 nm), its emission peak in DMF (496 nm) showed a marked redshift ( = 28 nm). Based on the previous suggestion for explaining the solvent-dependent fluorescence [27], the observed phenomena may be ascribed to the generation of a significant ICT state upon excitation of 1o and 1o'. Construction of the logic circuit Molecular and supramolecular logic gates are potential candidates for computation at nanoscale [28]. There are many reported examples of logic circuit based on diarylethene derivatives [29]. According to the experiment described above, the proton has a more significant influence on the fluorescence intensity modulation of 1o than light. Hence, we constructed a new simple INHIBIT logic circuit based on 1o by utilizing TFA and TEA as the input signals, and fluorescence intensity as an output signal. 1o is regarded as the initial state, and its fluorescence changes can be combined as a molecular switch with two inducing inputs: In1 (TFA) and In2 (TEA). The emission intensity of 1o at 426 nm is regarded as the original value. The output signal can be regarded as “on” when the relative emission intensity at 482 nm is larger than 2.7 times of the original value, otherwise as “off”. Hence, we can use the binary digits (0 and 1) instead of the two levels (off and on). That means 1o can read a string of two inputs and write one output. For example, the input strings “1” and “0” correspond to the “on” and “off” states for In1 and In2. Under these conditions, 1o was turned to 1o by TFA stimulus and the emission intensity is enhanced. Thus, the output digit is “1” and the output signal is “on”. All the possible strings of the two inputs are listed in Table 1 and the combinational logic circuits equivalent to the truth table is shown in Fig. 5. The results demonstrated that the diarylethenes could be one of the promising candidates for applications in molecular switches [30].
![Multi-functional ion-sensor based on a photochromic diarylethene with a 1H-imidazo [4,5-f][1,10] phenanthroline unit.](/assets/img/hold.png)
