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Synthesis and solvent-dependent photochromic reactions of porphyrin–spiropyran hybrid compounds
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 541–547
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Synthesis and solvent-dependent photochromic reactions of porphyrin–spiropyran hybrid compounds Dae Young Hur, Tae Jong Park, Eun Ju Shin ⇑ Department of Chemistry, Sunchon National University, Suncheon, Jeonnam 540-950, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Three hydrid compounds Por–SP,
Por–SP2, Por–SP4 have been prepared. UV irradiation in CH2Cl2 and THF resulted in different solution colour. In dichloromethane, photoinduced protonation and reverse thermal deprotonation occurred. In THF, photoinduced ring-opening and reverse thermal ring-closing reaction occurred.
R2
R3 R2
R3
Keywords: Porphyrin Spiropyran Photochromic reaction Absorption Fluorescence
R4
R1=SP; R2,R3,R4=H R1,R3=SP; R2,R4=H R1,R2,R3,R4=SP
R1
N HN
Por-SP Por-SP Por-SP2 Por-SP4
Dark
in THF R3
O CO
N O
Me
O MC = C O
NH N
R1
N HN
Por-MC
N
Me
H2Por2+-SP H2Por2+-SP2 H2Por2+-SP4
R2
UV
R4
R1=SP; R2,R3,R4=H R1,R3=SP; R2,R4=H R1,R2,R3,R4=SP
NO2
Article history: Received 28 March 2013 Received in revised form 23 July 2013 Accepted 2 August 2013 Available online 9 August 2013
H2Por2+-SP
in CH2Cl2 NH N
R4
O
i n f o
R1
NHHN
Dark
SP =
a r t i c l e
NHHN
UV
NO2
R1=MC; R2,R3,R4=H R1,R3=MC; R2,R4=H R1,R2,R3,R4=MC
Por-MC Por-MC2 Por-MC4
a b s t r a c t Porphyrin(Por)–spiropyran(SP) hybrid compounds, including Por–SP dyad, Por–SP2 triad, and Por–SP4 pentad, were prepared and characterized by 1H NMR, MALDI-TOF MS and UV–Vis spectroscopies. Upon 350 nm UV irradiation of Por–SPn (n = 1, 2, 4) in dichloromethane, unusual red-shifted absorption spectra were observed with the colour change from pink into green. Probably due to the protonation of core nitrogens in porphyrin ring, their absorption maxima in dichloromethane were shifted from 418 (Soret band), 515, 550, 590, 645 (four Q bands) nm into 450 and 665 nm. Also, fluorescence maxima were also shifted from 650 and 715 nm to 692 nm. In the other hands, upon irradiation with 350 nm UV light in THF, the colour changed from pink into violet and absorption band at 590 nm increased and the fluorescence spectra showed the decrease of 650 and 715 nm bands and increase of 600–640 nm band, due to the normal ring-opening reaction of spiropyran moiety into merocyanine. In the dark, original absorption and fluorescence spectra were recovered very slowly in dichloromethane, but quickly in THF. The reversible photochromic reactions of Por–SPn (n = 1, 2, 4) in dichloromethane and THF were investigated by observing absorption and fluorescence spectral changes during UV irradiation or standing in the dark. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Porphyrin is one of most frequently employed photoactive molecules due to strong absorption in the range of sunlight, relatively long excited singlet state lifetimes, and good redox properties. Porphyrin excited singlet state generated by the light absorption undergoes energy or electron transfer to the neighboring ⇑ Corresponding author. Tel.: +82 617503635; fax: +82 617503630. E-mail address: [email protected] (E.J. Shin). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.08.005
component. A number of artificial light-harvesting antenna, photoinduced electron transfer systems, and molecular energy storage devices, optoelectronic switches including porphyrin-based multicomponent compounds have been investigated for a long time [1–11]. Spiropyran (SP), nonpolar UV-absorbing form, is well-known photochromic compound accomplishing reversible molecular structural changes to merocyanine (MC), polar VIS-absorbing form, by the photochemical ring opening on UV irradiation. In turn, MC is reversed to SP by ring closure thermally or on visible light
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irradiation. Reversible photochromic SP-MC transformation is one of subjects of active research on optical memory and switch, on the basis of significant difference in polarity, excited state energy, or molecular geometry between two forms [12–22]. Incorporating photochromic units into porphyrin-based multicomponent compounds is expected to control on–off switching of photoinduced energy or electron transfer. If the excited singlet state energy of porphyrin lies between the excited singlet state energies of two isomers of these photochromic compounds, light-controlled quenching of the excited singlet state of porphyrin could be possible. Therefore, various multicomponent systems containing both porphyrin and photochromic compound such as azobenzene [23–25], spiropyran [26], dithienylethene [27], dihydropyrene [28], or dihydroindolizine [29,30] have been reported with their light-controlled on–off switching of photoinduced processes. Photoswitched singlet energy transfer of porphyrin–spiropyran dyad has been reported. However, the photochromic reactions of porphyrin–spiropyran conjugates containing multiple photochromic components have not yet been systematically investigated. In this paper, we report how photochromic reactions are affected by solvents and the number of attached spiropyran units in porphyrin(Por)–spiropyran(SP) hybrid compounds. Experimental Materials All reagents were commercially available and used as received. The reagents were purchased from Sigma–Aldrich. Solvents were dried according to literature procedures.1 Solvents for chromatography (ethyl acetate and hexane) were reagent grade and used without further purification. TLC was performed on Merck Silica Gel 60 F254 glass plates and developed by UV light. Column chromatography was performed on Merck Silica Gel 60 (70–230 mesh, ASTM). Por–CO2H (5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin), Por–(CO2H)2 (5,15-di-(4-carboxyphenyl)-10,20-diphenylporphyrin), and SP-OH were synthesized following the reported procedure. Por–(CO2H)4 (5,10,15,20-tetra-(4-carboxyphenyl)-porphyrin) was purchased from Sigma–Aldrich. Spectroscopic measurements 1 H NMR spectra were recorded on Bruker Avance 400 NMR spectrometer. MALDI-TOF mass spectrum was recorded on an Applied Biosystems Voyager-DE’STR System 4407 mass spectrometer using 2,5-dihydroxybenzoic acid in THF as matrix. UV–Vis absorption spectra were recorded using a quartz cuvette in a Shimadzu UV-2401PC spectrophotometer. Steady-state fluorescence spectra were measured on a SLM-Aminco AB2 luminescence spectrophotometer. The excitation wavelength for the emission spectra was fixed at 360 nm or 515 nm for free base porphyrin derivatives and 550 nm for zinc porphyrin derivatives. Photoirradiation was carried out in a Rayonet RPR 100 photochemical reactor equipped with Southern Ultraviolet 3500 Å lamps using Pyrex reaction tube in dichloromethane or tetrahydrofuran solution. Reaction progress of photochromic reaction on UV irradiation and reverse thermal reaction in the dark was monitored by change of absorption and fluorescence spectra. UV–Vis absorption and fluorescence spectra were measured with 5 106 M solution.
Synthesis Por–SP: Por–CO2H (20 mg, 0.03 mmol) was dissolved in DMF/ CH2Cl2 (3/2, v/v) (5 mL) and then EDC(1-[3-(dimethylamino) propyl]-3-ethyl carbodiimide hydrochloride, 10 mg, 0.05 mmol),
and DMAP(4-(dimethylamino)pyridine, 6 mg, 0.05 mmol) were added successively. The reaction mixture was stirred at room temperature for 30 min under N2 flow. SP-OH (14 mg, 0.04 mmol) was then added and the reaction mixture was stirred at room temperature for 24 h. The solvents were removed in vacuo and then CH2Cl2 (20 mL) and H2O (20 mL) were added to the resulting residue. The CH2Cl2 layer was separated and successively washed with aqueous saturated NaCl solution and aqueous saturated NaHCO3 solution. The CH2Cl2 layer was dried with anhydrous Na2SO4 and concentrated under reduced pressure and the resulting residue was purified by silica gel column chromatography using hexane/ ethyl acetate (4/1, v/v) as the eluent to give Por–SP: (13 mg, yield 43%) as a purple solid: 1H NMR (400 MHz, CDCl3, ppm) d 8.89 (6H, t, J = 8.05 Hz, pyrrole), 8.79 (2H, d, J = 4.68 Hz, pyrrole), 8.42 (2H, d, J = 8.24 Hz, benzoate), 8.32 (2H, d, J = 8.21 Hz, benzoate), 8.10–8.08 (2H, m, benzene), 7.84–7.76 (9H, m, phenyl), 7.34–7.32 (1H, m, benzene), 7.17 (1H, d, J = 1.82 Hz, benzene), 7.06–6.93 (3H, m, benzene and –CHCH-benzene), 6.88 (1H, d, J = 9.22 Hz, benzene), 6.06 (1H, d, J = 4.26 Hz, –CHCH-benzene), 4.17–4.12 (2H, m, -CH2CH2OH), 3.86–3.65 (2H, m, -CH2CH2-OH), 1.37 (3H, s, methyl), 1.31 (3H, s, methyl), -2.88 (2H, s, pyrrole-NH); MALDI-TOF MS m/z calculated for C65H48N6O5 992.37, found 993.34 [M]+. Por–SP2: Por–(CO2H)2 (20 mg, 0.03 mmol) was dissolved in DMF/CH2Cl2 (3/3, v/v) (6 mL) and then EDC(20 mg, 0.1 mmol), and DMAP(13 mg, 0.1 mmol) were added successively. The reaction mixture was stirred at room temperature for 30 min under N2 flow. SP-OH(14 mg, 0.04 mmol) dissolved in CH2Cl2 (6 mL) was then added and the reaction mixture was stirred at room temperature for 65 h. The next steps were accomplished according to the procedure described above. The solvents were removed in vacuo and then CH2Cl2 (20 mL) and H2O (20 mL) were added to the resulting residue. The CH2Cl2 layer was separated and successively washed with aqueous saturated NaCl solution and aqueous saturated NaHCO3 solution. The CH2Cl2 layer was dried with anhydrous Na2SO4 and concentrated under reduced pressure and the resulting residue was purified by silica gel column chromatography using hexane/ethyl acetate (3/1, v/v) as the eluent to give Por–SP2 (16 mg, yield 39%) as a purple solid: 1H NMR (400 MHz, CDCl3, ppm) d 8.90 (4H, d, J = 4.28 Hz, pyrrole), 8.80 (4H, d, J = 4.87 Hz, pyrrole), 8.42 (4H, d, J = 8.18 Hz, benzoate), 8.32 (4H, d, J = 8.13 Hz, benzoate), 8.24 (4H, d, J = 8.13 Hz, phenyl), 8.12–8.00 (4H, m, benzene), 7.83–7.77 (6H, m, phenyl), 7.34–7.33 (2H, m, benzene), 7.18 (2H, d, J = 7.25 Hz, benzene), 7.04–6.93 (6H, m, benzene and –CHCH–benzene), 6.85 (2H, d, J = 9.40 Hz, benzene), 6.05 (2H, d, J = 10.35 Hz, –CHCH–benzene), 4.18–4.09 (4H, m, –CH2CH2– OH), 3.86–3.64 (4H, m, –CH2CH2–OH), 1.36 (6H, s, methyl), 1.28 (6H, s, methyl), 2.80 (2H, s, pyrrole-NH); MALDI-TOF MS m/z calculated for C86H66N8O10 1370.49, found 1393.43 [M+Na]+ . Por–SP4: Por–(CO2H)4 (20 mg, 0.025 mmol) was dissolved in DMF/CH2Cl2 (3/5, v/v) (16 mL) and then EDC(58 mg, 0.3 mmol), and DMAP(37 mg, 0.3 mmol) were added successively. The reaction mixture was stirred at room temperature for 30 min under N2 flow. SP-OH (10 mg, 0.15 mmol) dissolved in CH2Cl2 (10 mL) was then added and the reaction mixture was stirred at room temperature for 6 days. The next steps were accomplished according to the procedure described above. The solvents were removed in vacuo and then CH2Cl2 (20 mL) and H2O (20 mL) were added to the resulting residue. The CH2Cl2 layer was separated and successively washed with aqueous saturated NaCl solution and aqueous saturated NaHCO3 solution. The CH2Cl2 layer was dried with anhydrous Na2SO4 and concentrated under reduced pressure and the resulting residue was purified by silica gel column chromatography using CH2Cl2/ethyl acetate (50/1, v/v) as the eluent to give Por–(SP)4 (14 mg, yield 27%) as a purple solid: 1H NMR (400 MHz, CDCl3, ppm) d 8.84 (8H, d, J = 12.3 Hz, pyrrole), 8.42 (8H, d, J = 8.22 Hz, benzoate), 8.31 (8H, d, J = 13.38 Hz, benzoate), 8.10–8.07 (8H, m,
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(b)
Absorbance
Por Por-SP Por-SP2 Por-SP4
1.5 1.0 0.5 x10
0.0
300
400
500
600
700
Wavelength (nm)
Fluorescence Intensity
(a) 2.0
600
Por Por-SP Por-SP2 Por-SP4
650
700
750
800
Wavelength (nm)
Fig. 1. Absorption (a) and fluorescence (b) spectra of Por–SPn (n = 0, 1, 2, 4) in CH2Cl2.
benzene), 7.34–7.30 (4H, m, benzene), 7.18 (4H, d, J = 7.26 Hz, benzene), 7.04–6.93 (12H, m, benzene and –CHCH–benzene), 6.86 (4H, d, J = 9.6 Hz, benzene), 6.06 (4H, d, J = 7.65 Hz, -CHCH–benzene), 4.75–4.60 (8H, m, -CH2CH2-OH), 3.86–3.68 (8H, m, –CH2CH2– OH), 1.37 (12H, s, methyl), 1.29 (12H, s, methyl), 2.80 (2H, s, pyrrole-NH); MALDI-TOF MS m/z calculated for C128H102N12 O202126.73, found 2127.81 [M+1]+ . ZnPor–SP, ZnPor–SP2, and ZnPor–SP4: Por–SP, Por–(SP)2, or Por–(SP)4 (1.0 equiv.) was dissolved in MeOH/CH2Cl2 (1:4, v/v) and then Zn(OAc)2 (4.0 equiv.) was added at room temperature. The reaction mixture was stirred overnight at room temperature. The solvents were removed in vacuo and the resulting residue was purified by preparative TLC using hexane/ethyl acetate (3/1, v/v) as the eluent to give ZnPor–SP, ZnPor–SP2, or ZnPor–SP4 as a purple solid in quantitative yield. ZnPor–SP: UV–Vis (CH2Cl2) 270, 350, 419, 547, 587 nm; Fluorescence (CH2Cl2) 599, 646 nm; 1H NMR (400 MHz, CDCl3, ppm) d 8.99 (6H, s, pyrrole), 8.89 (2H, s, pyrrole), 8.34 (2H, s, benzoate), 8.33 (2H, s, benzoate), 8.26 (6H, s, phenyl), 8.08–8.02 (2H, m, benzene), 7.80 (9H, s, phenyl), 7.33 (1H, d, J = 2.45 Hz, benzene), 7.17 (1H, d, J = 7.15 Hz, benzene), 7.02–6.91 (3H, m, benzene and – CHCH–benzene), 6.84 (1H, d, J = 8.53 Hz, benzene), 6.02 (1H, d, J = 10 Hz, –CHCH–benzene), 4.61 (2H, s, –CH2CH2–OH), 3.81–3.65 (2H, m, –CH2CH2–OH), 1.36 (3H, s, methyl), 1.28 (3H, s, methyl). ZnPor–SP2: UV–Vis (CH2Cl2) 270, 350, 420, 548, 587 nm; Fluorescence (CH2Cl2) 600, 646 nm; 1H NMR (400 MHz, CDCl3, ppm) d 8.99 (4H, s, pyrrole), 8.89 (4H, s, pyrrole), 8.34 (4H, s, benzoate), 8.31 (4H, s, benzoate), 8.24 (4H, d, J = 4.83 Hz, phenyl), 8.08–7.97 (4H, m, benzene) 7.79–7.72 (6H, m, phenyl), 7.32–7.30 (2H, m, benzene), 7.16 (2H, d, J = 7.12 Hz, benzene), 7.02–6.89 (6H, m, benzene and –CHCH–benzene), 6.83 (2H, d, J = 8.66 Hz, benzene), 6.02 (2H, d, J = 10.29 Hz, –CHCH–benzene), 4.61 (4H, s, –CH2CH2–OH), 3.81–3.65 (4H, m, –CH2CH2–OH), 1.35 (6H, s, methyl), 1.27 (6H, s, methyl). ZnPor–SP4: UV–Vis (CH2Cl2) 270, 350, 421, 548, 587 nm: Fluorescence (CH2Cl2) 598, 649 nm; 1H NMR (400 MHz, CDCl3, ppm) d 8.93 (8H, t, J = 15.90 Hz, pyrrole), 8.38 (8H, d, J = 8.05 Hz, benzoate), 8.29 (8H, d, J = 7.84 Hz, benzoate), 8.08–8.03 (8H, m, benzene) 7.33–7.31 (4H, m, benzene), 7.17 (4H, d, J = 7.18 Hz, benzene), 7.03–6.91 (12H, m, benzene and –CHCH–benzene), 6.84 (4H, d, J = 8.69 Hz, benzene), 6.04 (4H, d, J = 10.38 Hz, –CHCH–benzene), 4.65 (8H, s, –CH2CH2–OH), 3.85–3.66 (8H, m, –CH2CH2–OH), 1.36 (12H, s, methyl), 1.28 (12H, s, methyl).
Results and discussion Synthesis Por–SP dyad, Por–SP2 triad, and Por–SP4 pentad were prepared and characterized using 1H NMR and MALDI-TOF mass spectra as
described in Experimental Section. Three zinc derivatives ZnPor– SP dyad, ZnPor–SP2 triad, and ZnPor–SP4 pentad were prepared by stirring after adding excess zinc acetate into these compounds, and characterized using 1H NMR spectra. Absorption spectra of Por–SPn and ZnPor–SPn (n = 1, 2, 4) Meso-tetraphenylporphyrin (Por) in dichloromethane shows a Soret band at 418 nm and four Q bands at 515, 550, 590, 645 nm, and fluorescence at 650 and 715 nm [31]. SP absorbs only in the ultraviolet region around 270 and 330 nm, and has no strong fluorescence [16]. Under UV irradiation, SP isomerizes to MC by cleavage of C–O bond. MC shows strong absorption at 590 nm and fluorescence at 657 nm. MC reverts thermally to SP. Irradiation of MC with visible light enhances the reversion rate. Fig. 1a shows the absorption spectra of Por–SPn (n = 0, 1, 2, 4) in methylene chloride, with SP absorption bands around 270 and 350 nm, porphyrin Soret band maximum at 418 nm, and porphyrin Q band maxima at 515, 550, 590, 645 nm. All the absorption spectra are similar, except that the absorption intensity around 270 and 350 nm increases with increasing the number of SP moiety. Por– SPn in methylene chloride have pink colour and the solution colour becomes deeper as the number of the SP moiety increases. Absorption spectra for Por–SPn are almost equal to the sum of the Por and SP spectra, indicating no significant electronic perturbations by interactions between the linked Por and SP moieties. The absorption spectra of ZnPor–SPn (n = 0, 1, 2, 4) in methylene chloride are shown in Fig. 2a. Soret band maximum of ZnPor is at around 420 nm and Q band maxima are observed at 547 and 587 nm, and SP absorption is observed around 270 and 350 nm. Absorption spectra for ZnPor–SPn are also superimposed to the sum of the ZnPor and SP spectra. Table 1 summarizes the absorption and fluorescence data of Por–SPn and ZnPor–SPn (n = 0, 1, 2, 4) before and after UV irradiation in dichloromethane. Fluorescence spectra of Por–SPn and ZnPor–SPn (n = 1, 2, 4) Figs. 1b and 2b show the fluorescence spectra of Por–SPn and ZnPor–SPn (n = 0, 1, 2, 4) in methylene chloride, respectively. Por in dichloromethane shows fluorescence at 650 and 715 nm [31]. SP has no strong fluorescence and MC shows fluorescence at 657 nm [16]. Fluorescence maxima are observed at 650 and 715 nm for Por–SPn, and at around 600 and 646 nm for ZnPor– SPn, with excitation at 420 nm, where the porphyrin absorbs strongly but the absorption of both SP and MC forms is negligible (Table 1). Fluorescence spectra for all Por–SPn or ZnPor–SPn (n = 1, 2, 4) are essentially identical to Por or ZnPor fluorescence. No significant fluorescence emission was observed, originating from spiropyran moiety. Thus, the spiropyran moiety has no effect
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(b)
Absorbance
2.0
ZnPor ZnPor-SP ZnPor-SP2 ZnPor-SP4
1.5 1.0 0.5 x10
0.0
300
400
500
600
ZnPor ZnPor-SP ZnPor-SP2 ZnPor-SP4
Fluorescence Intensity
(a)
700
550
Wavelength (nm)
600
650
700
750
Wavelength (nm)
Fig. 2. Absorption (a) and fluorescence (b) spectra of ZnPor–SPn (n = 0, 1, 2, 4) in CH2Cl2.
Table 1 Absorption and fluorescence spectral data of Por–SPn (n = 0, 1, 2, 4) before and after UV irradiation in CH2Cl2. Compound
Por Por–SP Por–SP2 Por–SP4
ka (nm)
kf (nm)
Before UV irradiation
After UV irradiation
Before UV irradiation
After UV irradiation
418, 270, 268, 268,
446, 274, 268, 268,
653, 650, 651, 651,
690 692 692 693
515, 550, 590, 645 350, 418, 515, 550, 590, 645 350, 418, 515, 550, 590, 645 350, 418, 515, 550, 590, 645
610, 662 350, 448, 610, 664 350, 450, 612, 665 350, 452, 612, 665
(b) Abs at 450 nm
Absorbance
2.0 1.5 1.0
1.5 1.0 0.5 0.0
0
10
20
30
40
50
60
T (min)
0.5 0.0
300
400
500
600
700
Wavelength (nm)
Fluorescence intencity
(a)
716 715 715 715
600
650
700
750
800
Wavelength (nm)
Fig. 3. Absorption (a) and fluorescence (b) spectral changes of Por–SP2 with irradiation time (0–60 min) at 350 nm in CH2Cl2.
on the fluorescence properties of the porphyrin moiety in Por–SPn or ZnPor–SPn (n = 1, 2, 4). Unfortunately, all ZnPor–SPn (n = 1, 2, 4) in dichloromethane precipitates completely after 350 nm irradiation for 180 min (Table 1). In this study, photoinduced reactions were investigated with focusing on Por–SPn, but not ZnPor–SPn. Photoinduced conversion and thermal reversion of Por–SPn (n = 1, 2, 4) in dichloromethane
Photoinduced conversion and colour change of Por–SPn (n = 1, 2, 4) in dichloromethane A colourless solution of SP in dichloromethane absorbs only in the ultraviolet region around 270 and 330 nm, and shows no fluorescence [16]. Upon UV irradiation, the colourless SP switches to the purple MC. The absorption of SP in dichloromethane has decreased in the 330 nm region and new absorption at 590 nm and emission at 647 nm have appeared as converting to MC form [12–19]. The purple MC switches completely to the colourless SP, when the solution is stored in the dark. In order to investigate whether porphyrin-linked spiropyran compounds conduct a similar photochromic reaction of colourless
SP to purple MC like as simple spiropyran, a dichloromethane solution of Por–SPn (n = 1, 2, 4) was irradiated at 350 nm, where the spiropyran moiety absorbs, and the absorption spectra were measured with irradiation time. Fig. 3a shows the absorption spectral changes of Por–SP2 in dichloromethane with irradiation time on irradiation with 350 nm light. During 350 nm irradiation of Por–SPn (n = 1, 2, 4), the porphyrin Soret absorption band at 418 nm porphyrin Q bands at 515, 550, 590, 645 nm were decreased and new absorption bands at 450 nm and 665 nm were enhanced (Fig. 3a and Table 1). The solution colour changes from pink into green. Upon UV irradiation of Por–SPn, new absorption band of photoreaction product develops at 665 nm and does not match merocyanine absorption observed normally at 590 nm [16,30,31]. Also, the porphyrin Soret absorption at 418 nm was replaced by new band at 450 nm. These absorption spectral changes are unexpected from the photochemical ring-opening reaction from colourless closed SP form to purple open planar MC form. In other words, the photochromic reactions from Por–SPn to Por–MCn did not occur. Absorption at 450 and 665 nm matches that of diprotonated porphyrin H2Por2+ [31], but not MC. In fact, similar changes were reported when neutral porphyrin Por was transformed into diprotonated porphyrin H2Por2+ with
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HCl addition [31–33]. Addition of acid to free base porphyrin changes from a Soret band at 418 nm and the four Q bands at 516, 550, 590, and 646 nm into red-shifted Soret band at 446 nm and the two visible Q bands at 610 and 660 nm, characterized as the acid dication H2Por2+ [34,35]. In the neutral free base porphyrin Por, the phenyl rings lie almost perpendicular to the porphyrin plane, whereas in the highly distorted structure of the dication H2Por2+ they rotate towards a more parallel position. It has been reported that the overlap of p orbitals from the porphyrin and phenyl rings in dication H2Por2+ results in red-shifted spectra in comparison with that of neutral Por [31]. Also, it has been reported that irradiation of Por in chlorocarbon solvent leads to the formation of diprotonated porphyrin H2Por2+ through HCl production from porphyrin-catalyzed photodecomposition of chlorocarbon solvent [34,35]. Even sonication of chlorinated solvents produced diprotonated porphyrin by HCl generated from sonodegradation of chlorinated solvents [36–38]. The red shift of absorption bands under UV irradiation of Por– SPn (n = 1, 2, 4) from 418, 515, 550, 590, 645 nm to 450, 610, 665 nm (Table 1) might be attributed to the formation of diprotonated H2Por2+–SPn, due to protonation of the core nitrogen atoms in porphyrin moiety through porphyrin-catalyzed photodecomposition of dichloromethane solvent followed by HCl formation, in the same way as UV irradiation of Por in dichloromethane. Plots of absorbance at around 450 nm absorption maximum with UV irradiation time were inserted into Fig. 3a. Absorbance at 450 nm increased linearly with irradiation time until the completion of the reaction, related to the formation of corresponding H2Por2+–SPn. Pink colour of Por–SPn in methylene chloride gradually turned into green colour of H2Por2+–SPn with irradiation time. As shown in Table 2, it takes 50, 60, and 70 min for complete conversion of Por–SP, Por–SP2 (Fig. 3a), and Por–SP4, respectively, supported by the disappearance of 418 nm band and no more enhancement of 450 and 665 nm bands. Protonation of Por by UV irradiation in dichloromethane is completed in 40 min. As the number of SP moiety linked with porphyrin moiety increases, reaction rate decreases and it takes longer time (50 ? 60 ? 70 min) for the complete photoinduced conversion from Por–SPn to H2Por2+–SPn. It is inferred that peripheral SP moieties interfere with the protonation of the core nitrogen atoms in central porphyrin ring. The fluorescence spectra with 360 nm excitation wavelength were measured after 350 nm irradiation for Por–SPn (n = 1, 2, 4) in dichloromethane. The fluorescence intensity at 650 and 715 nm, originating from Por moiety, is reduced with irradiation time and new fluorescence band arises at 692 nm (Fig. 3b), longer wavelength than merocyanine fluorescence at 657 nm [12–19]. Photoproducts certainly are not merocyanine derivatives Por– MCn. These fluorescence spectral changes are also ascribed to the formation of corresponding protonated porphyrin derivatives H2Por2+–SPn and resultant distortion of porphyrin structure by nitrogen protonation. As the number of SP linked with porphyrin moiety increases, fluorescence intensity at 692 nm of corresponding H2Por2+–SPn becomes smaller. More peripheral SP moieties lead to less efficient fluorescence from protonated porphyrin.
Thermal reversion of photoproduct of Por–SPn (n = 1, 2, 4) in dichloromethane The absorption spectral changes resulting from 350 nm irradiation of Por–SPn (n = 1, 2, 4) turn back with time when stored in the dark after UV irradiation is turned off, and the original spectrum is slowly recovered in the dark (Fig. 4a). The solution colour changes from green into pink. Absorption bands at 450 and 665 nm are diminished and the porphyrin Soret and Q bands at 418 nm and 515, 550, 590, 645 nm are enhanced. It is attributed to the deprotonation of the porphyrin ring, i.e., the thermal reversion from H2Por2+–SPn to Por–SPn by expelling HCl from the solution. In the dark, dissipation of H2Por2+–SPn seems to be exponential, but do not follow simple first order kinetics. As the number of SP moiety increases, the reaction rate increases and it takes shorter time for the complete thermal reversion. 920 min (H2Por2+–SP), 730 min (H2Por2+–SP2, Fig. 4a), or 390 min (H2Por2+–SP4) are required for the restoration to the original spectrum of Por–SPn (Table 2). After Por–SPn (n = 1, 2, 4) was irradiated with 350 nm lamps during enough time for the complete photoconversion to H2Por2+–SPn (n = 1, 2, 4), the fluorescence spectral changes were measured with the incubation time in the dark. While fluorescence at 692 nm originating from H2Por2+ moiety becomes weakened, the fluorescence intensity at 650 and 715 nm originating from Por moiety increases with dark incubation time (Fig. 4b). These fluorescence spectral changes could be also ascribed to the deprotonation of the H2Por2+–SPn and the recovery of normal neutral Por–SPn. Photoinduced conversion and thermal reversion of Por–SPn (n = 1, 2, 4) in tetrahydrofuran
Photoinduced conversion and colour change of Por–SPn (n = 1, 2, 4) in tetrahydrofuran By irradiation in dichloromethane, unfortunately, we observed the protonation of porphyrin from Por–SPn into H2Por2+–SPn, rather than normal photochromic reaction from Por–SPn into Por–MCn. Therefore, same experiments with UV irradiation were repeated in tetrahydrofuran. Fig. 5a shows the absorption spectral changes of Por–SP2 in tetrahydrofuran with irradiation time upon irradiation with 350 nm light. During 350 nm irradiation of Por–SPn (n = 1, 2, 4), the Soret and Q absorption of porphyrin at 418 nm and 515, 550, 590, 645 nm remain unchanged, but absorption at 590 nm corresponding to merocyanine absorption is enhanced (Fig. 5a and Table 3). In addition, the solution colour changes from pink into violet. Although the porphyrin was linked covalently, normal SP-MC photochromic ring opening reactions from Por–SPn to Por–MCn (n = 1, 2, 4) took place in tetrahydrofuran, in contrast with the situation in dichloromethane. On irradiation at 350 nm, absorbance at 590 nm of Por–SPn increases linearly with irradiation time, as inserted in Fig. 5a, due to the photochemical ring opening reaction of SP moiety into merocyanine (MC). Pink colour of Por–SPn in tetrahydrofuran gradually
Table 2 Conversion time for photochemical forward and thermal (dark) reverse reactions of SP and Por–SPn (n = 1, 2, 4) in CH2Cl2 and THF. Compound
In CH2Cl2 (pink ¢ green) Por–SPn ¢ H2Por2+–SPn Conversion time (min)
In THF (pink ¢ purple) Por–SPn ¢ Por–MCn Conversion time, s (k, s1)
UV
dark
UV
dark
SP Por–SP Por–SP2 Por–SP4
N.A. 50 60 70
N.A. 920 730 390
20 25 30 35
80 (5.8 102) 170 (2.0 102) 180 (1.9 102) 290 (1.3 102)
546
D.Y. Hur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 541–547
(a)
(b) Fluorescence Intensity
Absorbance
2.0 1.5 1.0 0.5 0.0
300
400
500
600
700
600
650
Wavelength (nm)
700
750
800
Wavelength (nm)
Fig. 4. Absorption (a) and fluorescence (b) spectral changes of H2Por2+–SP2 with dark incubation time (0–730 min) in CH2Cl2.
(a)
(b)
1.5
0.3 0.2 0.1 0.0
1.0
Fluorescence Intensity
Abs at 590 nm
Absorbance
2.0
0
10
20
30
40
T (sec)
0.5 0.0 300
400
500
600
700
550
600
Wavelength (nm)
650
700
750
800
Wavelength (nm)
Fig. 5. Absorption (a) and fluorescence (b) spectral changes of Por–SP2 with irradiation time (0–40 s) at 350 nm in THF.
Table 3 Absorption and fluorescence spectral data of Por–SPn (n = 0, 1, 2, 4) before and after UV irradiation in THF. Compound
ka (nm)
kf (nm)
Before UV irradiation Por Por–SP Por–SP2 Por–SP4
414, 268, 266, 266,
512, 368, 362, 348,
546, 416, 416, 418,
590, 512, 512, 513,
645 546, 590, 645 546, 590, 645 547, 590, 645
After UV irradiation
Before UV irradiation
After UV irradiation
268, 372, 416, 512, 546, 590 ("), 645 268, 372, 418, 514, 546, 590 ("), 645 268, 374, 418, 518, 590 (")
651, 650, 651, 651,
692 692 693
turned into violet colour of Por–MCn with irradiation time. As the number of MC in tetrahydrofuran solution of Por–MCn increases, absorbance at 590 nm increases and violet colour becomes deeper. Photochromic conversion reactions of Por–SP, Por–SP2 (Fig. 5a), and Por–SP4 into Por–MC, Por–MC2, and Por–MC4 in tetrahydrofuran are much faster than the photoinduced protonation in dichloromethane and are completed for 25, 30, and 35 s (Table 2), respectively, judging from no more enhancement of 590 nm bands. The photochemical ring opening reaction of simple spiropyran into merocyanine in tetrahydrofuran occurs within 20 s, faster than those of Por–SPn (n = 1, 2, or 4). Any absorption spectral changes are not observed on UV irradiation of Por in tetrahydrofuran. As the number of SP moiety linked with porphyrin moiety increases, reaction becomes slower and it takes longer time (25 ? 30 ? 35 s) for the complete photoconversion from Por–SPn to Por–MCn. The fluorescence spectra with 360 nm excitation wavelength were also measured after the complete conversion of Por–SPn into Por–MCn (n = 1, 2, 4) by 350 nm irradiation in tetrahydrofuran. The fluorescence intensity at 650 and 715 nm, originating from Por moiety, is reduced with irradiation time and new fluorescence band arises at 600–640 nm (Fig. 5b), due to merocyanine
717 715 715 715
fluorescence at around 657 nm [12–19]. These fluorescence spectral changes matched to the formation of corresponding Por–MCn. Thermal reversion of photoproduct of Por–SPn (n = 1, 2, 4) in tetrahydrofuran The absorption spectral changes by 350 nm irradiation of Por– SPn (n = 1, 2, 4) in tetrahydrofuran turn back to the original spectra with time in the dark after UV irradiation is turned off. As shown in Fig. 6, absorption at 590 nm decreases and the original spectrum of Por–SPn is rapidly recovered in the dark. The solution colour changes from violet into pink. It is attributed to the thermal ring closure of the merocyanine moiety to spiropyran, i.e., the reversion from Por–MCn to Por–SPn. In the dark, dissipation of Por–MCn in tetrahydrofuran seems to be exponential (Fig. 6). In dilute solution, it is supposed that thermal reversion reaction follows pseudo-first-order kinetics. With standing in the dark the solution of Por–MCn generated from irradiation at 350 nm of Por–SPn, absorbances at 590 nm decreases exponentially following the first order kinetics (inset in Fig. 6).
At ¼ kt ln A0
D.Y. Hur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 541–547 0.0
ln (A/A0)
Abs at 590 nm
0.5 0.4
547
mechanism under UV irradiation of Por–SP in dichloromethane and tetrahydrofuran.
-0.5
Appendix A. Supplementary material
-1.0
0.3 -1.5 0
0.2
50
100
Time (sec)
0.1 0.0
0
50
100
150
200
250
300
Time (sec) Fig. 6. Plot of absorbance at 590 nm of (a) MC (solid line), (b) Por–MC (dotted line), (c) Por–MC2 (dashed line), or (d) Por–MC4 (dash-dotted solid line).
where A0 is initial absorbance at 590 nm immediately after complete conversion of Por–SPn into Por–MCn by UV irradiation, At is absorbance at 590 nm at any time t during stored in the dark after UV irradiation, and k is the reaction rate constant for thermal reversion reaction from Por–MCn into Por–SPn. As shown in Table 2, the rate constants k for the thermal reversion reaction of MC, Por–MC, Por–MC2, and Por–MC4 in tetrahydrofuran were estimated to be 0.058 s1, 0.020 s1, 0.019 s1, and 0.013 s1, respectively. As the number of MC moiety increases, the reaction rate constant decreases. Thermal reversion reaction of MC without porphyrin moiety is faster than any porphyrin–merocyanine conjugates, i.e., Por–MCn (n = 1, 2, 4). It takes 80 s (MC), 170 s (Por–MC), 180 s (Por–MC2), or 290 s (Por–MC4) for the restoration to the original spectrum of SP or Por–SPn (Table 2 and Fig. 6). The thermal ring closing of merocyanine itself occurs very fast within 80 s. As the number of MC moiety linked with porphyrin moiety increases, it takes longer time (170 ? 180 ? 290 s) for the complete thermal reversion.
Conclusions Porphyrin–spiropyran hybrid compounds, Por–SP dyad, Por–SP2 triad, and Por–SP4 pentad were prepared. On irradiation with 350 nm light in dicloromethane, Por–SPn (n = 1, 2, 4) was converted into protonated porphyrin derivatives H2Por2+–SPn (n = 1, 2, 4), with the colour change from pink into green and the red shift of absorption bands and the fluorescence spectral changes. As standing in the dark, protonated H2Por2+–SPn reverted very slowly into Por–SPn, with the colour change from green into pink. As the number of the attached SP increases, forward photochemical protonation reaction rate decreases, while reverse thermal deprotonation reaction rate increases. On the other hand, upon UV excitation in tetrahydrofuran, Por–SPn conducts very fast ring-opening reaction to yield Por–MCn with colour changes from pink into violet. With standing in the dark, colour was returned from violet into pink, as Por–MCn reverted rapidly to Por–SPn, by the thermal back ring-closing reaction. As the number of the attached SP increases, both forward ring-opening reaction rate and reverse ring-closing reaction rate decreases. Por–SPn (n = 1, 2, 4) is noble molecules displaying solventdependent reversible photochromic reactions; photochemical protonation/thermal deprotonation reaction between pink Por–SPn and green H2Por2+–SPn in dichloromethane, and the photochemical ring-opening/thermal ring-closing reaction between pink Por–SPn and purple Por–MCn in tetrahydrofuran. Further studies are under investigation for the detailed photoinduced conversion reaction
Chemical structures, photoreaction scheme, absorption and fluorescence spectral changes of Por–SP and Por–SP4 during photolysis and dark incubation. Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.saa.2013.08.005. References [1] M.R. Wasielewski, Chem. Rev. 92 (1992) 435–461; M.R. Wasielewski, J. Org. Chem. 71 (2006) 5051–5066. [2] R. Lomoth, A. Magnuson, M. Sjödin, P. Huang, S. Styring, L. Hammarström, Photosynth. Res. 87 (2006) 25–40. [3] J.H. Alstrum-Acevedo, M.K. Brennaman, T.J. Meyer, Inorg. Chem. 44 (2005) 6802–6827. [4] M.S. Choi, T. Yamazaki, I. Yamazaki, T. Aida, Angew. Chem., Int. Ed. 43 (2003) 150–158. [5] H. Imahori, Y. Mori, Y. Matano, J. Photochem. Photobiol. C 4 (2003) 51–83. [6] H. Imahori, Org. Biomol. Chem. 2 (2004) 1425–1433. [7] I. Willner, B. Willner, Top. Curr. Chem. 159 (1991) 153–218. [8] K.M. Kadish, K.M. Smith, R. Guilard (Eds.), The Porphyrin Handbook, Academic Press, New York, 2000. [9] F. D’Souza, P.M. Smith, M.E. Zandler, A.L. McCarty, M. Itou, Y. Araki, O. Ito, J. Am. Chem. Soc. 126 (2004) 7898–7907. [10] D. Gust, T.A. Moore, A.L. Moore, Acc. Chem. Res. 26 (1993) 198–205; D. Gust, T.A. Moore, A.L. Moore, Acc. Chem. Res. 34 (2001) 40–48. [11] D.M. Guldi, Chem. Soc. Rev. 31 (2002) 22–36. [12] A.K. Chibisov, H. Görner, J. Phys. Chem. A 101 (1997) 4305–4312. [13] G. Buntinx, O. Poizat, S. Foley, M. Sliwa, S. Aloise, V. Lokshin, A. Samat, Dyes Pigm. 89 (2011) 305–312. [14] V.I. Minkin, Chem. Rev. 104 (2004) 2751–2776. [15] G. Berkovic, V. Krongauz, V. Weiss, Chem. Rev. 100 (2000) 1741–1754. [16] F.M. Raymo, S. Giordani, J. Am. Chem. Soc. 123 (2001) 4651–4652. [17] F.M. Raymo, M. Tomasulo, Chem. Eur. J. 12 (2006) 3186–3193. [18] R. Rosario, D. Gust, M. Hayes, J. Springer, A.A. Garcia, Langmuir 19 (2003) 8801–8806. [19] X. Song, J. Zhou, Y. Li, Y. Tang, J. Photochem. Photobiol. A 92 (1995) 99–103. [20] L. Zhu, W. Wu, M.Q. Zhu, J.J. Han, J.K. Hurst, A.D.Q. Li, J. Am. Chem. Soc. 129 (2007) 3524–3526. [21] S. Nagashima, M. Murata, H. Nishihara, Angew. Chem., Int. Ed. 45 (2006) 4298– 4301. [22] K.Y. Tomizaki, H. Mihara, J. Am. Chem. Soc. 129 (2007) 8345–8352. [23] S. Venkataramani, U. Jana, M. Dommaschk, F.D. Sönnichsen, F. Tuczek, R. Herges, Science 331 (2011) 445–448. [24] T. Yamamura, A. Momotake, T. Arai, Tetrahedron Lett. 45 (2004) 9219–9223. [25] Z. Chen, C. Zhong, Z. Zhang, Z. Li, L. Niu, Y. Bin, F. Zhang, J. Phys. Chem. B 112 (2008) 7387–7394. [26] J.L. Bahr, G. Kodis, L. de la Garza, S. Lin, A.L. Moore, T.A. Moore, D. Gust, J. Am. Chem. Soc. 123 (2001) 7124–7133. [27] P.A. Liddell, G. Kodis, A.L. Moore, T.A. Moore, D. Gust, J. Am. Chem. Soc. 124 (2002) 7668–7669. [28] P.A. Liddell, G. Kodis, J. Andreasson, L. de la Garza, S. Bandyopadhyay, R.H. Mitchell, T.A. Moore, A.L. Moore, D. Gust, J. Am. Chem. Soc. 126 (2004) 4803– 4811. [29] Y. Terazono, G. Kodis, J. Andreasson, G. Jeong, A. Brune, T. Hartmann, H. Durr, A.L. Moore, T.A. Moore, D. Gust, J. Phys. Chem. B 108 (2004) 1812–1814. [30] S.D. Straight, J. Andreasson, G. Kodis, A.L. Moore, T.A. Moore, D. Gust, J. Am. Chem. Soc. 127 (2005) 2717–2724. [31] K. Kalyanasundaram, Photochemistry of Polypyridine and Porphyrin Complexes, Academic Press, London, 1992. p. 427. [32] B.K. Manna, S.C. Bera, K.K. Rohatgi-Mukherjee, Spectrochim. Acta A 51 (1995) 1051–1060. [33] Y. Zhang, M.X. Li, M.Y. Lü, R.H. Yang, F. Liu, K.A. Li, J. Phys. Chem. A 109 (2005) 7442–7448. ˇ oz, A.S. Cohen, L.M. Nguyen, T.A. MeIntosh, P.E. Hoggard, Photochem. [34] Z. Mun Photobiol. Sci. 7 (2008) 337–343. [35] R. Ddic, A. Molnár, A. Svoboda, J. Hála, J. Porphyrins Phthalocyanines 14 (2010) 962–967. [36] E.M.N. Mhuircheartaigh, W.J. Blau, M. Prato, S. Giordani, Carbon 45 (2007) 2665–2671. [37] E.M.N. Mhuircheartaigh, S. Giordani, W.J. Blau, J. Phys. Chem. B 110 (2006) 23136–23141. [38] E.M.N. Mhuircheartaigh, W.J. Blau, M. Prato, S. Giordani, Phys. Stat. Sol. B 244 (2007) 4227–4230.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis and solvent-dependent photochromic reactions of porphyrin–spiropyran hybrid compounds Dae Young Hur, Tae Jong Park, Eun Ju Shin ⇑ Department of Chemistry, Sunchon National University, Suncheon, Jeonnam 540-950, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Three hydrid compounds Por–SP,
Por–SP2, Por–SP4 have been prepared. UV irradiation in CH2Cl2 and THF resulted in different solution colour. In dichloromethane, photoinduced protonation and reverse thermal deprotonation occurred. In THF, photoinduced ring-opening and reverse thermal ring-closing reaction occurred.
R2
R3 R2
R3
Keywords: Porphyrin Spiropyran Photochromic reaction Absorption Fluorescence
R4
R1=SP; R2,R3,R4=H R1,R3=SP; R2,R4=H R1,R2,R3,R4=SP
R1
N HN
Por-SP Por-SP Por-SP2 Por-SP4
Dark
in THF R3
O CO
N O
Me
O MC = C O
NH N
R1
N HN
Por-MC
N
Me
H2Por2+-SP H2Por2+-SP2 H2Por2+-SP4
R2
UV
R4
R1=SP; R2,R3,R4=H R1,R3=SP; R2,R4=H R1,R2,R3,R4=SP
NO2
Article history: Received 28 March 2013 Received in revised form 23 July 2013 Accepted 2 August 2013 Available online 9 August 2013
H2Por2+-SP
in CH2Cl2 NH N
R4
O
i n f o
R1
NHHN
Dark
SP =
a r t i c l e
NHHN
UV
NO2
R1=MC; R2,R3,R4=H R1,R3=MC; R2,R4=H R1,R2,R3,R4=MC
Por-MC Por-MC2 Por-MC4
a b s t r a c t Porphyrin(Por)–spiropyran(SP) hybrid compounds, including Por–SP dyad, Por–SP2 triad, and Por–SP4 pentad, were prepared and characterized by 1H NMR, MALDI-TOF MS and UV–Vis spectroscopies. Upon 350 nm UV irradiation of Por–SPn (n = 1, 2, 4) in dichloromethane, unusual red-shifted absorption spectra were observed with the colour change from pink into green. Probably due to the protonation of core nitrogens in porphyrin ring, their absorption maxima in dichloromethane were shifted from 418 (Soret band), 515, 550, 590, 645 (four Q bands) nm into 450 and 665 nm. Also, fluorescence maxima were also shifted from 650 and 715 nm to 692 nm. In the other hands, upon irradiation with 350 nm UV light in THF, the colour changed from pink into violet and absorption band at 590 nm increased and the fluorescence spectra showed the decrease of 650 and 715 nm bands and increase of 600–640 nm band, due to the normal ring-opening reaction of spiropyran moiety into merocyanine. In the dark, original absorption and fluorescence spectra were recovered very slowly in dichloromethane, but quickly in THF. The reversible photochromic reactions of Por–SPn (n = 1, 2, 4) in dichloromethane and THF were investigated by observing absorption and fluorescence spectral changes during UV irradiation or standing in the dark. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Porphyrin is one of most frequently employed photoactive molecules due to strong absorption in the range of sunlight, relatively long excited singlet state lifetimes, and good redox properties. Porphyrin excited singlet state generated by the light absorption undergoes energy or electron transfer to the neighboring ⇑ Corresponding author. Tel.: +82 617503635; fax: +82 617503630. E-mail address: [email protected] (E.J. Shin). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.08.005
component. A number of artificial light-harvesting antenna, photoinduced electron transfer systems, and molecular energy storage devices, optoelectronic switches including porphyrin-based multicomponent compounds have been investigated for a long time [1–11]. Spiropyran (SP), nonpolar UV-absorbing form, is well-known photochromic compound accomplishing reversible molecular structural changes to merocyanine (MC), polar VIS-absorbing form, by the photochemical ring opening on UV irradiation. In turn, MC is reversed to SP by ring closure thermally or on visible light
542
D.Y. Hur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 541–547
irradiation. Reversible photochromic SP-MC transformation is one of subjects of active research on optical memory and switch, on the basis of significant difference in polarity, excited state energy, or molecular geometry between two forms [12–22]. Incorporating photochromic units into porphyrin-based multicomponent compounds is expected to control on–off switching of photoinduced energy or electron transfer. If the excited singlet state energy of porphyrin lies between the excited singlet state energies of two isomers of these photochromic compounds, light-controlled quenching of the excited singlet state of porphyrin could be possible. Therefore, various multicomponent systems containing both porphyrin and photochromic compound such as azobenzene [23–25], spiropyran [26], dithienylethene [27], dihydropyrene [28], or dihydroindolizine [29,30] have been reported with their light-controlled on–off switching of photoinduced processes. Photoswitched singlet energy transfer of porphyrin–spiropyran dyad has been reported. However, the photochromic reactions of porphyrin–spiropyran conjugates containing multiple photochromic components have not yet been systematically investigated. In this paper, we report how photochromic reactions are affected by solvents and the number of attached spiropyran units in porphyrin(Por)–spiropyran(SP) hybrid compounds. Experimental Materials All reagents were commercially available and used as received. The reagents were purchased from Sigma–Aldrich. Solvents were dried according to literature procedures.1 Solvents for chromatography (ethyl acetate and hexane) were reagent grade and used without further purification. TLC was performed on Merck Silica Gel 60 F254 glass plates and developed by UV light. Column chromatography was performed on Merck Silica Gel 60 (70–230 mesh, ASTM). Por–CO2H (5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin), Por–(CO2H)2 (5,15-di-(4-carboxyphenyl)-10,20-diphenylporphyrin), and SP-OH were synthesized following the reported procedure. Por–(CO2H)4 (5,10,15,20-tetra-(4-carboxyphenyl)-porphyrin) was purchased from Sigma–Aldrich. Spectroscopic measurements 1 H NMR spectra were recorded on Bruker Avance 400 NMR spectrometer. MALDI-TOF mass spectrum was recorded on an Applied Biosystems Voyager-DE’STR System 4407 mass spectrometer using 2,5-dihydroxybenzoic acid in THF as matrix. UV–Vis absorption spectra were recorded using a quartz cuvette in a Shimadzu UV-2401PC spectrophotometer. Steady-state fluorescence spectra were measured on a SLM-Aminco AB2 luminescence spectrophotometer. The excitation wavelength for the emission spectra was fixed at 360 nm or 515 nm for free base porphyrin derivatives and 550 nm for zinc porphyrin derivatives. Photoirradiation was carried out in a Rayonet RPR 100 photochemical reactor equipped with Southern Ultraviolet 3500 Å lamps using Pyrex reaction tube in dichloromethane or tetrahydrofuran solution. Reaction progress of photochromic reaction on UV irradiation and reverse thermal reaction in the dark was monitored by change of absorption and fluorescence spectra. UV–Vis absorption and fluorescence spectra were measured with 5 106 M solution.
Synthesis Por–SP: Por–CO2H (20 mg, 0.03 mmol) was dissolved in DMF/ CH2Cl2 (3/2, v/v) (5 mL) and then EDC(1-[3-(dimethylamino) propyl]-3-ethyl carbodiimide hydrochloride, 10 mg, 0.05 mmol),
and DMAP(4-(dimethylamino)pyridine, 6 mg, 0.05 mmol) were added successively. The reaction mixture was stirred at room temperature for 30 min under N2 flow. SP-OH (14 mg, 0.04 mmol) was then added and the reaction mixture was stirred at room temperature for 24 h. The solvents were removed in vacuo and then CH2Cl2 (20 mL) and H2O (20 mL) were added to the resulting residue. The CH2Cl2 layer was separated and successively washed with aqueous saturated NaCl solution and aqueous saturated NaHCO3 solution. The CH2Cl2 layer was dried with anhydrous Na2SO4 and concentrated under reduced pressure and the resulting residue was purified by silica gel column chromatography using hexane/ ethyl acetate (4/1, v/v) as the eluent to give Por–SP: (13 mg, yield 43%) as a purple solid: 1H NMR (400 MHz, CDCl3, ppm) d 8.89 (6H, t, J = 8.05 Hz, pyrrole), 8.79 (2H, d, J = 4.68 Hz, pyrrole), 8.42 (2H, d, J = 8.24 Hz, benzoate), 8.32 (2H, d, J = 8.21 Hz, benzoate), 8.10–8.08 (2H, m, benzene), 7.84–7.76 (9H, m, phenyl), 7.34–7.32 (1H, m, benzene), 7.17 (1H, d, J = 1.82 Hz, benzene), 7.06–6.93 (3H, m, benzene and –CHCH-benzene), 6.88 (1H, d, J = 9.22 Hz, benzene), 6.06 (1H, d, J = 4.26 Hz, –CHCH-benzene), 4.17–4.12 (2H, m, -CH2CH2OH), 3.86–3.65 (2H, m, -CH2CH2-OH), 1.37 (3H, s, methyl), 1.31 (3H, s, methyl), -2.88 (2H, s, pyrrole-NH); MALDI-TOF MS m/z calculated for C65H48N6O5 992.37, found 993.34 [M]+. Por–SP2: Por–(CO2H)2 (20 mg, 0.03 mmol) was dissolved in DMF/CH2Cl2 (3/3, v/v) (6 mL) and then EDC(20 mg, 0.1 mmol), and DMAP(13 mg, 0.1 mmol) were added successively. The reaction mixture was stirred at room temperature for 30 min under N2 flow. SP-OH(14 mg, 0.04 mmol) dissolved in CH2Cl2 (6 mL) was then added and the reaction mixture was stirred at room temperature for 65 h. The next steps were accomplished according to the procedure described above. The solvents were removed in vacuo and then CH2Cl2 (20 mL) and H2O (20 mL) were added to the resulting residue. The CH2Cl2 layer was separated and successively washed with aqueous saturated NaCl solution and aqueous saturated NaHCO3 solution. The CH2Cl2 layer was dried with anhydrous Na2SO4 and concentrated under reduced pressure and the resulting residue was purified by silica gel column chromatography using hexane/ethyl acetate (3/1, v/v) as the eluent to give Por–SP2 (16 mg, yield 39%) as a purple solid: 1H NMR (400 MHz, CDCl3, ppm) d 8.90 (4H, d, J = 4.28 Hz, pyrrole), 8.80 (4H, d, J = 4.87 Hz, pyrrole), 8.42 (4H, d, J = 8.18 Hz, benzoate), 8.32 (4H, d, J = 8.13 Hz, benzoate), 8.24 (4H, d, J = 8.13 Hz, phenyl), 8.12–8.00 (4H, m, benzene), 7.83–7.77 (6H, m, phenyl), 7.34–7.33 (2H, m, benzene), 7.18 (2H, d, J = 7.25 Hz, benzene), 7.04–6.93 (6H, m, benzene and –CHCH–benzene), 6.85 (2H, d, J = 9.40 Hz, benzene), 6.05 (2H, d, J = 10.35 Hz, –CHCH–benzene), 4.18–4.09 (4H, m, –CH2CH2– OH), 3.86–3.64 (4H, m, –CH2CH2–OH), 1.36 (6H, s, methyl), 1.28 (6H, s, methyl), 2.80 (2H, s, pyrrole-NH); MALDI-TOF MS m/z calculated for C86H66N8O10 1370.49, found 1393.43 [M+Na]+ . Por–SP4: Por–(CO2H)4 (20 mg, 0.025 mmol) was dissolved in DMF/CH2Cl2 (3/5, v/v) (16 mL) and then EDC(58 mg, 0.3 mmol), and DMAP(37 mg, 0.3 mmol) were added successively. The reaction mixture was stirred at room temperature for 30 min under N2 flow. SP-OH (10 mg, 0.15 mmol) dissolved in CH2Cl2 (10 mL) was then added and the reaction mixture was stirred at room temperature for 6 days. The next steps were accomplished according to the procedure described above. The solvents were removed in vacuo and then CH2Cl2 (20 mL) and H2O (20 mL) were added to the resulting residue. The CH2Cl2 layer was separated and successively washed with aqueous saturated NaCl solution and aqueous saturated NaHCO3 solution. The CH2Cl2 layer was dried with anhydrous Na2SO4 and concentrated under reduced pressure and the resulting residue was purified by silica gel column chromatography using CH2Cl2/ethyl acetate (50/1, v/v) as the eluent to give Por–(SP)4 (14 mg, yield 27%) as a purple solid: 1H NMR (400 MHz, CDCl3, ppm) d 8.84 (8H, d, J = 12.3 Hz, pyrrole), 8.42 (8H, d, J = 8.22 Hz, benzoate), 8.31 (8H, d, J = 13.38 Hz, benzoate), 8.10–8.07 (8H, m,
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(b)
Absorbance
Por Por-SP Por-SP2 Por-SP4
1.5 1.0 0.5 x10
0.0
300
400
500
600
700
Wavelength (nm)
Fluorescence Intensity
(a) 2.0
600
Por Por-SP Por-SP2 Por-SP4
650
700
750
800
Wavelength (nm)
Fig. 1. Absorption (a) and fluorescence (b) spectra of Por–SPn (n = 0, 1, 2, 4) in CH2Cl2.
benzene), 7.34–7.30 (4H, m, benzene), 7.18 (4H, d, J = 7.26 Hz, benzene), 7.04–6.93 (12H, m, benzene and –CHCH–benzene), 6.86 (4H, d, J = 9.6 Hz, benzene), 6.06 (4H, d, J = 7.65 Hz, -CHCH–benzene), 4.75–4.60 (8H, m, -CH2CH2-OH), 3.86–3.68 (8H, m, –CH2CH2– OH), 1.37 (12H, s, methyl), 1.29 (12H, s, methyl), 2.80 (2H, s, pyrrole-NH); MALDI-TOF MS m/z calculated for C128H102N12 O202126.73, found 2127.81 [M+1]+ . ZnPor–SP, ZnPor–SP2, and ZnPor–SP4: Por–SP, Por–(SP)2, or Por–(SP)4 (1.0 equiv.) was dissolved in MeOH/CH2Cl2 (1:4, v/v) and then Zn(OAc)2 (4.0 equiv.) was added at room temperature. The reaction mixture was stirred overnight at room temperature. The solvents were removed in vacuo and the resulting residue was purified by preparative TLC using hexane/ethyl acetate (3/1, v/v) as the eluent to give ZnPor–SP, ZnPor–SP2, or ZnPor–SP4 as a purple solid in quantitative yield. ZnPor–SP: UV–Vis (CH2Cl2) 270, 350, 419, 547, 587 nm; Fluorescence (CH2Cl2) 599, 646 nm; 1H NMR (400 MHz, CDCl3, ppm) d 8.99 (6H, s, pyrrole), 8.89 (2H, s, pyrrole), 8.34 (2H, s, benzoate), 8.33 (2H, s, benzoate), 8.26 (6H, s, phenyl), 8.08–8.02 (2H, m, benzene), 7.80 (9H, s, phenyl), 7.33 (1H, d, J = 2.45 Hz, benzene), 7.17 (1H, d, J = 7.15 Hz, benzene), 7.02–6.91 (3H, m, benzene and – CHCH–benzene), 6.84 (1H, d, J = 8.53 Hz, benzene), 6.02 (1H, d, J = 10 Hz, –CHCH–benzene), 4.61 (2H, s, –CH2CH2–OH), 3.81–3.65 (2H, m, –CH2CH2–OH), 1.36 (3H, s, methyl), 1.28 (3H, s, methyl). ZnPor–SP2: UV–Vis (CH2Cl2) 270, 350, 420, 548, 587 nm; Fluorescence (CH2Cl2) 600, 646 nm; 1H NMR (400 MHz, CDCl3, ppm) d 8.99 (4H, s, pyrrole), 8.89 (4H, s, pyrrole), 8.34 (4H, s, benzoate), 8.31 (4H, s, benzoate), 8.24 (4H, d, J = 4.83 Hz, phenyl), 8.08–7.97 (4H, m, benzene) 7.79–7.72 (6H, m, phenyl), 7.32–7.30 (2H, m, benzene), 7.16 (2H, d, J = 7.12 Hz, benzene), 7.02–6.89 (6H, m, benzene and –CHCH–benzene), 6.83 (2H, d, J = 8.66 Hz, benzene), 6.02 (2H, d, J = 10.29 Hz, –CHCH–benzene), 4.61 (4H, s, –CH2CH2–OH), 3.81–3.65 (4H, m, –CH2CH2–OH), 1.35 (6H, s, methyl), 1.27 (6H, s, methyl). ZnPor–SP4: UV–Vis (CH2Cl2) 270, 350, 421, 548, 587 nm: Fluorescence (CH2Cl2) 598, 649 nm; 1H NMR (400 MHz, CDCl3, ppm) d 8.93 (8H, t, J = 15.90 Hz, pyrrole), 8.38 (8H, d, J = 8.05 Hz, benzoate), 8.29 (8H, d, J = 7.84 Hz, benzoate), 8.08–8.03 (8H, m, benzene) 7.33–7.31 (4H, m, benzene), 7.17 (4H, d, J = 7.18 Hz, benzene), 7.03–6.91 (12H, m, benzene and –CHCH–benzene), 6.84 (4H, d, J = 8.69 Hz, benzene), 6.04 (4H, d, J = 10.38 Hz, –CHCH–benzene), 4.65 (8H, s, –CH2CH2–OH), 3.85–3.66 (8H, m, –CH2CH2–OH), 1.36 (12H, s, methyl), 1.28 (12H, s, methyl).
Results and discussion Synthesis Por–SP dyad, Por–SP2 triad, and Por–SP4 pentad were prepared and characterized using 1H NMR and MALDI-TOF mass spectra as
described in Experimental Section. Three zinc derivatives ZnPor– SP dyad, ZnPor–SP2 triad, and ZnPor–SP4 pentad were prepared by stirring after adding excess zinc acetate into these compounds, and characterized using 1H NMR spectra. Absorption spectra of Por–SPn and ZnPor–SPn (n = 1, 2, 4) Meso-tetraphenylporphyrin (Por) in dichloromethane shows a Soret band at 418 nm and four Q bands at 515, 550, 590, 645 nm, and fluorescence at 650 and 715 nm [31]. SP absorbs only in the ultraviolet region around 270 and 330 nm, and has no strong fluorescence [16]. Under UV irradiation, SP isomerizes to MC by cleavage of C–O bond. MC shows strong absorption at 590 nm and fluorescence at 657 nm. MC reverts thermally to SP. Irradiation of MC with visible light enhances the reversion rate. Fig. 1a shows the absorption spectra of Por–SPn (n = 0, 1, 2, 4) in methylene chloride, with SP absorption bands around 270 and 350 nm, porphyrin Soret band maximum at 418 nm, and porphyrin Q band maxima at 515, 550, 590, 645 nm. All the absorption spectra are similar, except that the absorption intensity around 270 and 350 nm increases with increasing the number of SP moiety. Por– SPn in methylene chloride have pink colour and the solution colour becomes deeper as the number of the SP moiety increases. Absorption spectra for Por–SPn are almost equal to the sum of the Por and SP spectra, indicating no significant electronic perturbations by interactions between the linked Por and SP moieties. The absorption spectra of ZnPor–SPn (n = 0, 1, 2, 4) in methylene chloride are shown in Fig. 2a. Soret band maximum of ZnPor is at around 420 nm and Q band maxima are observed at 547 and 587 nm, and SP absorption is observed around 270 and 350 nm. Absorption spectra for ZnPor–SPn are also superimposed to the sum of the ZnPor and SP spectra. Table 1 summarizes the absorption and fluorescence data of Por–SPn and ZnPor–SPn (n = 0, 1, 2, 4) before and after UV irradiation in dichloromethane. Fluorescence spectra of Por–SPn and ZnPor–SPn (n = 1, 2, 4) Figs. 1b and 2b show the fluorescence spectra of Por–SPn and ZnPor–SPn (n = 0, 1, 2, 4) in methylene chloride, respectively. Por in dichloromethane shows fluorescence at 650 and 715 nm [31]. SP has no strong fluorescence and MC shows fluorescence at 657 nm [16]. Fluorescence maxima are observed at 650 and 715 nm for Por–SPn, and at around 600 and 646 nm for ZnPor– SPn, with excitation at 420 nm, where the porphyrin absorbs strongly but the absorption of both SP and MC forms is negligible (Table 1). Fluorescence spectra for all Por–SPn or ZnPor–SPn (n = 1, 2, 4) are essentially identical to Por or ZnPor fluorescence. No significant fluorescence emission was observed, originating from spiropyran moiety. Thus, the spiropyran moiety has no effect
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(b)
Absorbance
2.0
ZnPor ZnPor-SP ZnPor-SP2 ZnPor-SP4
1.5 1.0 0.5 x10
0.0
300
400
500
600
ZnPor ZnPor-SP ZnPor-SP2 ZnPor-SP4
Fluorescence Intensity
(a)
700
550
Wavelength (nm)
600
650
700
750
Wavelength (nm)
Fig. 2. Absorption (a) and fluorescence (b) spectra of ZnPor–SPn (n = 0, 1, 2, 4) in CH2Cl2.
Table 1 Absorption and fluorescence spectral data of Por–SPn (n = 0, 1, 2, 4) before and after UV irradiation in CH2Cl2. Compound
Por Por–SP Por–SP2 Por–SP4
ka (nm)
kf (nm)
Before UV irradiation
After UV irradiation
Before UV irradiation
After UV irradiation
418, 270, 268, 268,
446, 274, 268, 268,
653, 650, 651, 651,
690 692 692 693
515, 550, 590, 645 350, 418, 515, 550, 590, 645 350, 418, 515, 550, 590, 645 350, 418, 515, 550, 590, 645
610, 662 350, 448, 610, 664 350, 450, 612, 665 350, 452, 612, 665
(b) Abs at 450 nm
Absorbance
2.0 1.5 1.0
1.5 1.0 0.5 0.0
0
10
20
30
40
50
60
T (min)
0.5 0.0
300
400
500
600
700
Wavelength (nm)
Fluorescence intencity
(a)
716 715 715 715
600
650
700
750
800
Wavelength (nm)
Fig. 3. Absorption (a) and fluorescence (b) spectral changes of Por–SP2 with irradiation time (0–60 min) at 350 nm in CH2Cl2.
on the fluorescence properties of the porphyrin moiety in Por–SPn or ZnPor–SPn (n = 1, 2, 4). Unfortunately, all ZnPor–SPn (n = 1, 2, 4) in dichloromethane precipitates completely after 350 nm irradiation for 180 min (Table 1). In this study, photoinduced reactions were investigated with focusing on Por–SPn, but not ZnPor–SPn. Photoinduced conversion and thermal reversion of Por–SPn (n = 1, 2, 4) in dichloromethane
Photoinduced conversion and colour change of Por–SPn (n = 1, 2, 4) in dichloromethane A colourless solution of SP in dichloromethane absorbs only in the ultraviolet region around 270 and 330 nm, and shows no fluorescence [16]. Upon UV irradiation, the colourless SP switches to the purple MC. The absorption of SP in dichloromethane has decreased in the 330 nm region and new absorption at 590 nm and emission at 647 nm have appeared as converting to MC form [12–19]. The purple MC switches completely to the colourless SP, when the solution is stored in the dark. In order to investigate whether porphyrin-linked spiropyran compounds conduct a similar photochromic reaction of colourless
SP to purple MC like as simple spiropyran, a dichloromethane solution of Por–SPn (n = 1, 2, 4) was irradiated at 350 nm, where the spiropyran moiety absorbs, and the absorption spectra were measured with irradiation time. Fig. 3a shows the absorption spectral changes of Por–SP2 in dichloromethane with irradiation time on irradiation with 350 nm light. During 350 nm irradiation of Por–SPn (n = 1, 2, 4), the porphyrin Soret absorption band at 418 nm porphyrin Q bands at 515, 550, 590, 645 nm were decreased and new absorption bands at 450 nm and 665 nm were enhanced (Fig. 3a and Table 1). The solution colour changes from pink into green. Upon UV irradiation of Por–SPn, new absorption band of photoreaction product develops at 665 nm and does not match merocyanine absorption observed normally at 590 nm [16,30,31]. Also, the porphyrin Soret absorption at 418 nm was replaced by new band at 450 nm. These absorption spectral changes are unexpected from the photochemical ring-opening reaction from colourless closed SP form to purple open planar MC form. In other words, the photochromic reactions from Por–SPn to Por–MCn did not occur. Absorption at 450 and 665 nm matches that of diprotonated porphyrin H2Por2+ [31], but not MC. In fact, similar changes were reported when neutral porphyrin Por was transformed into diprotonated porphyrin H2Por2+ with
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HCl addition [31–33]. Addition of acid to free base porphyrin changes from a Soret band at 418 nm and the four Q bands at 516, 550, 590, and 646 nm into red-shifted Soret band at 446 nm and the two visible Q bands at 610 and 660 nm, characterized as the acid dication H2Por2+ [34,35]. In the neutral free base porphyrin Por, the phenyl rings lie almost perpendicular to the porphyrin plane, whereas in the highly distorted structure of the dication H2Por2+ they rotate towards a more parallel position. It has been reported that the overlap of p orbitals from the porphyrin and phenyl rings in dication H2Por2+ results in red-shifted spectra in comparison with that of neutral Por [31]. Also, it has been reported that irradiation of Por in chlorocarbon solvent leads to the formation of diprotonated porphyrin H2Por2+ through HCl production from porphyrin-catalyzed photodecomposition of chlorocarbon solvent [34,35]. Even sonication of chlorinated solvents produced diprotonated porphyrin by HCl generated from sonodegradation of chlorinated solvents [36–38]. The red shift of absorption bands under UV irradiation of Por– SPn (n = 1, 2, 4) from 418, 515, 550, 590, 645 nm to 450, 610, 665 nm (Table 1) might be attributed to the formation of diprotonated H2Por2+–SPn, due to protonation of the core nitrogen atoms in porphyrin moiety through porphyrin-catalyzed photodecomposition of dichloromethane solvent followed by HCl formation, in the same way as UV irradiation of Por in dichloromethane. Plots of absorbance at around 450 nm absorption maximum with UV irradiation time were inserted into Fig. 3a. Absorbance at 450 nm increased linearly with irradiation time until the completion of the reaction, related to the formation of corresponding H2Por2+–SPn. Pink colour of Por–SPn in methylene chloride gradually turned into green colour of H2Por2+–SPn with irradiation time. As shown in Table 2, it takes 50, 60, and 70 min for complete conversion of Por–SP, Por–SP2 (Fig. 3a), and Por–SP4, respectively, supported by the disappearance of 418 nm band and no more enhancement of 450 and 665 nm bands. Protonation of Por by UV irradiation in dichloromethane is completed in 40 min. As the number of SP moiety linked with porphyrin moiety increases, reaction rate decreases and it takes longer time (50 ? 60 ? 70 min) for the complete photoinduced conversion from Por–SPn to H2Por2+–SPn. It is inferred that peripheral SP moieties interfere with the protonation of the core nitrogen atoms in central porphyrin ring. The fluorescence spectra with 360 nm excitation wavelength were measured after 350 nm irradiation for Por–SPn (n = 1, 2, 4) in dichloromethane. The fluorescence intensity at 650 and 715 nm, originating from Por moiety, is reduced with irradiation time and new fluorescence band arises at 692 nm (Fig. 3b), longer wavelength than merocyanine fluorescence at 657 nm [12–19]. Photoproducts certainly are not merocyanine derivatives Por– MCn. These fluorescence spectral changes are also ascribed to the formation of corresponding protonated porphyrin derivatives H2Por2+–SPn and resultant distortion of porphyrin structure by nitrogen protonation. As the number of SP linked with porphyrin moiety increases, fluorescence intensity at 692 nm of corresponding H2Por2+–SPn becomes smaller. More peripheral SP moieties lead to less efficient fluorescence from protonated porphyrin.
Thermal reversion of photoproduct of Por–SPn (n = 1, 2, 4) in dichloromethane The absorption spectral changes resulting from 350 nm irradiation of Por–SPn (n = 1, 2, 4) turn back with time when stored in the dark after UV irradiation is turned off, and the original spectrum is slowly recovered in the dark (Fig. 4a). The solution colour changes from green into pink. Absorption bands at 450 and 665 nm are diminished and the porphyrin Soret and Q bands at 418 nm and 515, 550, 590, 645 nm are enhanced. It is attributed to the deprotonation of the porphyrin ring, i.e., the thermal reversion from H2Por2+–SPn to Por–SPn by expelling HCl from the solution. In the dark, dissipation of H2Por2+–SPn seems to be exponential, but do not follow simple first order kinetics. As the number of SP moiety increases, the reaction rate increases and it takes shorter time for the complete thermal reversion. 920 min (H2Por2+–SP), 730 min (H2Por2+–SP2, Fig. 4a), or 390 min (H2Por2+–SP4) are required for the restoration to the original spectrum of Por–SPn (Table 2). After Por–SPn (n = 1, 2, 4) was irradiated with 350 nm lamps during enough time for the complete photoconversion to H2Por2+–SPn (n = 1, 2, 4), the fluorescence spectral changes were measured with the incubation time in the dark. While fluorescence at 692 nm originating from H2Por2+ moiety becomes weakened, the fluorescence intensity at 650 and 715 nm originating from Por moiety increases with dark incubation time (Fig. 4b). These fluorescence spectral changes could be also ascribed to the deprotonation of the H2Por2+–SPn and the recovery of normal neutral Por–SPn. Photoinduced conversion and thermal reversion of Por–SPn (n = 1, 2, 4) in tetrahydrofuran
Photoinduced conversion and colour change of Por–SPn (n = 1, 2, 4) in tetrahydrofuran By irradiation in dichloromethane, unfortunately, we observed the protonation of porphyrin from Por–SPn into H2Por2+–SPn, rather than normal photochromic reaction from Por–SPn into Por–MCn. Therefore, same experiments with UV irradiation were repeated in tetrahydrofuran. Fig. 5a shows the absorption spectral changes of Por–SP2 in tetrahydrofuran with irradiation time upon irradiation with 350 nm light. During 350 nm irradiation of Por–SPn (n = 1, 2, 4), the Soret and Q absorption of porphyrin at 418 nm and 515, 550, 590, 645 nm remain unchanged, but absorption at 590 nm corresponding to merocyanine absorption is enhanced (Fig. 5a and Table 3). In addition, the solution colour changes from pink into violet. Although the porphyrin was linked covalently, normal SP-MC photochromic ring opening reactions from Por–SPn to Por–MCn (n = 1, 2, 4) took place in tetrahydrofuran, in contrast with the situation in dichloromethane. On irradiation at 350 nm, absorbance at 590 nm of Por–SPn increases linearly with irradiation time, as inserted in Fig. 5a, due to the photochemical ring opening reaction of SP moiety into merocyanine (MC). Pink colour of Por–SPn in tetrahydrofuran gradually
Table 2 Conversion time for photochemical forward and thermal (dark) reverse reactions of SP and Por–SPn (n = 1, 2, 4) in CH2Cl2 and THF. Compound
In CH2Cl2 (pink ¢ green) Por–SPn ¢ H2Por2+–SPn Conversion time (min)
In THF (pink ¢ purple) Por–SPn ¢ Por–MCn Conversion time, s (k, s1)
UV
dark
UV
dark
SP Por–SP Por–SP2 Por–SP4
N.A. 50 60 70
N.A. 920 730 390
20 25 30 35
80 (5.8 102) 170 (2.0 102) 180 (1.9 102) 290 (1.3 102)
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(a)
(b) Fluorescence Intensity
Absorbance
2.0 1.5 1.0 0.5 0.0
300
400
500
600
700
600
650
Wavelength (nm)
700
750
800
Wavelength (nm)
Fig. 4. Absorption (a) and fluorescence (b) spectral changes of H2Por2+–SP2 with dark incubation time (0–730 min) in CH2Cl2.
(a)
(b)
1.5
0.3 0.2 0.1 0.0
1.0
Fluorescence Intensity
Abs at 590 nm
Absorbance
2.0
0
10
20
30
40
T (sec)
0.5 0.0 300
400
500
600
700
550
600
Wavelength (nm)
650
700
750
800
Wavelength (nm)
Fig. 5. Absorption (a) and fluorescence (b) spectral changes of Por–SP2 with irradiation time (0–40 s) at 350 nm in THF.
Table 3 Absorption and fluorescence spectral data of Por–SPn (n = 0, 1, 2, 4) before and after UV irradiation in THF. Compound
ka (nm)
kf (nm)
Before UV irradiation Por Por–SP Por–SP2 Por–SP4
414, 268, 266, 266,
512, 368, 362, 348,
546, 416, 416, 418,
590, 512, 512, 513,
645 546, 590, 645 546, 590, 645 547, 590, 645
After UV irradiation
Before UV irradiation
After UV irradiation
268, 372, 416, 512, 546, 590 ("), 645 268, 372, 418, 514, 546, 590 ("), 645 268, 374, 418, 518, 590 (")
651, 650, 651, 651,
692 692 693
turned into violet colour of Por–MCn with irradiation time. As the number of MC in tetrahydrofuran solution of Por–MCn increases, absorbance at 590 nm increases and violet colour becomes deeper. Photochromic conversion reactions of Por–SP, Por–SP2 (Fig. 5a), and Por–SP4 into Por–MC, Por–MC2, and Por–MC4 in tetrahydrofuran are much faster than the photoinduced protonation in dichloromethane and are completed for 25, 30, and 35 s (Table 2), respectively, judging from no more enhancement of 590 nm bands. The photochemical ring opening reaction of simple spiropyran into merocyanine in tetrahydrofuran occurs within 20 s, faster than those of Por–SPn (n = 1, 2, or 4). Any absorption spectral changes are not observed on UV irradiation of Por in tetrahydrofuran. As the number of SP moiety linked with porphyrin moiety increases, reaction becomes slower and it takes longer time (25 ? 30 ? 35 s) for the complete photoconversion from Por–SPn to Por–MCn. The fluorescence spectra with 360 nm excitation wavelength were also measured after the complete conversion of Por–SPn into Por–MCn (n = 1, 2, 4) by 350 nm irradiation in tetrahydrofuran. The fluorescence intensity at 650 and 715 nm, originating from Por moiety, is reduced with irradiation time and new fluorescence band arises at 600–640 nm (Fig. 5b), due to merocyanine
717 715 715 715
fluorescence at around 657 nm [12–19]. These fluorescence spectral changes matched to the formation of corresponding Por–MCn. Thermal reversion of photoproduct of Por–SPn (n = 1, 2, 4) in tetrahydrofuran The absorption spectral changes by 350 nm irradiation of Por– SPn (n = 1, 2, 4) in tetrahydrofuran turn back to the original spectra with time in the dark after UV irradiation is turned off. As shown in Fig. 6, absorption at 590 nm decreases and the original spectrum of Por–SPn is rapidly recovered in the dark. The solution colour changes from violet into pink. It is attributed to the thermal ring closure of the merocyanine moiety to spiropyran, i.e., the reversion from Por–MCn to Por–SPn. In the dark, dissipation of Por–MCn in tetrahydrofuran seems to be exponential (Fig. 6). In dilute solution, it is supposed that thermal reversion reaction follows pseudo-first-order kinetics. With standing in the dark the solution of Por–MCn generated from irradiation at 350 nm of Por–SPn, absorbances at 590 nm decreases exponentially following the first order kinetics (inset in Fig. 6).
At ¼ kt ln A0
D.Y. Hur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 117 (2014) 541–547 0.0
ln (A/A0)
Abs at 590 nm
0.5 0.4
547
mechanism under UV irradiation of Por–SP in dichloromethane and tetrahydrofuran.
-0.5
Appendix A. Supplementary material
-1.0
0.3 -1.5 0
0.2
50
100
Time (sec)
0.1 0.0
0
50
100
150
200
250
300
Time (sec) Fig. 6. Plot of absorbance at 590 nm of (a) MC (solid line), (b) Por–MC (dotted line), (c) Por–MC2 (dashed line), or (d) Por–MC4 (dash-dotted solid line).
where A0 is initial absorbance at 590 nm immediately after complete conversion of Por–SPn into Por–MCn by UV irradiation, At is absorbance at 590 nm at any time t during stored in the dark after UV irradiation, and k is the reaction rate constant for thermal reversion reaction from Por–MCn into Por–SPn. As shown in Table 2, the rate constants k for the thermal reversion reaction of MC, Por–MC, Por–MC2, and Por–MC4 in tetrahydrofuran were estimated to be 0.058 s1, 0.020 s1, 0.019 s1, and 0.013 s1, respectively. As the number of MC moiety increases, the reaction rate constant decreases. Thermal reversion reaction of MC without porphyrin moiety is faster than any porphyrin–merocyanine conjugates, i.e., Por–MCn (n = 1, 2, 4). It takes 80 s (MC), 170 s (Por–MC), 180 s (Por–MC2), or 290 s (Por–MC4) for the restoration to the original spectrum of SP or Por–SPn (Table 2 and Fig. 6). The thermal ring closing of merocyanine itself occurs very fast within 80 s. As the number of MC moiety linked with porphyrin moiety increases, it takes longer time (170 ? 180 ? 290 s) for the complete thermal reversion.
Conclusions Porphyrin–spiropyran hybrid compounds, Por–SP dyad, Por–SP2 triad, and Por–SP4 pentad were prepared. On irradiation with 350 nm light in dicloromethane, Por–SPn (n = 1, 2, 4) was converted into protonated porphyrin derivatives H2Por2+–SPn (n = 1, 2, 4), with the colour change from pink into green and the red shift of absorption bands and the fluorescence spectral changes. As standing in the dark, protonated H2Por2+–SPn reverted very slowly into Por–SPn, with the colour change from green into pink. As the number of the attached SP increases, forward photochemical protonation reaction rate decreases, while reverse thermal deprotonation reaction rate increases. On the other hand, upon UV excitation in tetrahydrofuran, Por–SPn conducts very fast ring-opening reaction to yield Por–MCn with colour changes from pink into violet. With standing in the dark, colour was returned from violet into pink, as Por–MCn reverted rapidly to Por–SPn, by the thermal back ring-closing reaction. As the number of the attached SP increases, both forward ring-opening reaction rate and reverse ring-closing reaction rate decreases. Por–SPn (n = 1, 2, 4) is noble molecules displaying solventdependent reversible photochromic reactions; photochemical protonation/thermal deprotonation reaction between pink Por–SPn and green H2Por2+–SPn in dichloromethane, and the photochemical ring-opening/thermal ring-closing reaction between pink Por–SPn and purple Por–MCn in tetrahydrofuran. Further studies are under investigation for the detailed photoinduced conversion reaction
Chemical structures, photoreaction scheme, absorption and fluorescence spectral changes of Por–SP and Por–SP4 during photolysis and dark incubation. Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.saa.2013.08.005. References [1] M.R. Wasielewski, Chem. Rev. 92 (1992) 435–461; M.R. Wasielewski, J. Org. Chem. 71 (2006) 5051–5066. [2] R. Lomoth, A. Magnuson, M. Sjödin, P. Huang, S. Styring, L. Hammarström, Photosynth. Res. 87 (2006) 25–40. [3] J.H. Alstrum-Acevedo, M.K. Brennaman, T.J. Meyer, Inorg. Chem. 44 (2005) 6802–6827. [4] M.S. Choi, T. Yamazaki, I. Yamazaki, T. Aida, Angew. Chem., Int. Ed. 43 (2003) 150–158. [5] H. Imahori, Y. Mori, Y. Matano, J. Photochem. Photobiol. C 4 (2003) 51–83. [6] H. Imahori, Org. Biomol. Chem. 2 (2004) 1425–1433. [7] I. Willner, B. Willner, Top. Curr. Chem. 159 (1991) 153–218. [8] K.M. Kadish, K.M. Smith, R. Guilard (Eds.), The Porphyrin Handbook, Academic Press, New York, 2000. [9] F. D’Souza, P.M. Smith, M.E. Zandler, A.L. McCarty, M. Itou, Y. Araki, O. Ito, J. Am. Chem. Soc. 126 (2004) 7898–7907. [10] D. Gust, T.A. Moore, A.L. Moore, Acc. Chem. Res. 26 (1993) 198–205; D. Gust, T.A. Moore, A.L. Moore, Acc. Chem. Res. 34 (2001) 40–48. [11] D.M. Guldi, Chem. Soc. Rev. 31 (2002) 22–36. [12] A.K. Chibisov, H. Görner, J. Phys. Chem. A 101 (1997) 4305–4312. [13] G. Buntinx, O. Poizat, S. Foley, M. Sliwa, S. Aloise, V. Lokshin, A. Samat, Dyes Pigm. 89 (2011) 305–312. [14] V.I. Minkin, Chem. Rev. 104 (2004) 2751–2776. [15] G. Berkovic, V. Krongauz, V. Weiss, Chem. Rev. 100 (2000) 1741–1754. [16] F.M. Raymo, S. Giordani, J. Am. Chem. Soc. 123 (2001) 4651–4652. [17] F.M. Raymo, M. Tomasulo, Chem. Eur. J. 12 (2006) 3186–3193. [18] R. Rosario, D. Gust, M. Hayes, J. Springer, A.A. Garcia, Langmuir 19 (2003) 8801–8806. [19] X. Song, J. Zhou, Y. Li, Y. Tang, J. Photochem. Photobiol. A 92 (1995) 99–103. [20] L. Zhu, W. Wu, M.Q. Zhu, J.J. Han, J.K. Hurst, A.D.Q. Li, J. Am. Chem. Soc. 129 (2007) 3524–3526. [21] S. Nagashima, M. Murata, H. Nishihara, Angew. Chem., Int. Ed. 45 (2006) 4298– 4301. [22] K.Y. Tomizaki, H. Mihara, J. Am. Chem. Soc. 129 (2007) 8345–8352. [23] S. Venkataramani, U. Jana, M. Dommaschk, F.D. Sönnichsen, F. Tuczek, R. Herges, Science 331 (2011) 445–448. [24] T. Yamamura, A. Momotake, T. Arai, Tetrahedron Lett. 45 (2004) 9219–9223. [25] Z. Chen, C. Zhong, Z. Zhang, Z. Li, L. Niu, Y. Bin, F. Zhang, J. Phys. Chem. B 112 (2008) 7387–7394. [26] J.L. Bahr, G. Kodis, L. de la Garza, S. Lin, A.L. Moore, T.A. Moore, D. Gust, J. Am. Chem. Soc. 123 (2001) 7124–7133. [27] P.A. Liddell, G. Kodis, A.L. Moore, T.A. Moore, D. Gust, J. Am. Chem. Soc. 124 (2002) 7668–7669. [28] P.A. Liddell, G. Kodis, J. Andreasson, L. de la Garza, S. Bandyopadhyay, R.H. Mitchell, T.A. Moore, A.L. Moore, D. Gust, J. Am. Chem. Soc. 126 (2004) 4803– 4811. [29] Y. Terazono, G. Kodis, J. Andreasson, G. Jeong, A. Brune, T. Hartmann, H. Durr, A.L. Moore, T.A. Moore, D. Gust, J. Phys. Chem. B 108 (2004) 1812–1814. [30] S.D. Straight, J. Andreasson, G. Kodis, A.L. Moore, T.A. Moore, D. Gust, J. Am. Chem. Soc. 127 (2005) 2717–2724. [31] K. Kalyanasundaram, Photochemistry of Polypyridine and Porphyrin Complexes, Academic Press, London, 1992. p. 427. [32] B.K. Manna, S.C. Bera, K.K. Rohatgi-Mukherjee, Spectrochim. Acta A 51 (1995) 1051–1060. [33] Y. Zhang, M.X. Li, M.Y. Lü, R.H. Yang, F. Liu, K.A. Li, J. Phys. Chem. A 109 (2005) 7442–7448. ˇ oz, A.S. Cohen, L.M. Nguyen, T.A. MeIntosh, P.E. Hoggard, Photochem. [34] Z. Mun Photobiol. Sci. 7 (2008) 337–343. [35] R. Ddic, A. Molnár, A. Svoboda, J. Hála, J. Porphyrins Phthalocyanines 14 (2010) 962–967. [36] E.M.N. Mhuircheartaigh, W.J. Blau, M. Prato, S. Giordani, Carbon 45 (2007) 2665–2671. [37] E.M.N. Mhuircheartaigh, S. Giordani, W.J. Blau, J. Phys. Chem. B 110 (2006) 23136–23141. [38] E.M.N. Mhuircheartaigh, W.J. Blau, M. Prato, S. Giordani, Phys. Stat. Sol. B 244 (2007) 4227–4230.