Notes Bull. Korean Chem. Soc. 2013, Vol. 34, No. 10 3125 http://dx.doi.org/10.5012/bkcs.2013.34.10.3125
Solvent-dependent Photoreactions of Porphyrin-Spiropyran Dyad:
Ring-opening or Protonation
Dae Young Hur and Eun Ju Shin*
Department of Chemistry, Sunchon National University, Suncheon, Jeonnam 540-742, Korea. *E-mail: [email protected] Received July 5, 2013, Accepted July 17, 2013
Key Words : Porphyrin, Diprotonated porphyrin, Spiropyran, Merocyanine, Aggregation
Porphyrin (Por) has a strong absorption characteristics in the range of sunlight due to a chemical structure of high conjugation and rigidity, good redox properties, and well- established synthetic methods. Accordingly, porphyrin-based multicomponent compounds have been investigated in a variety of applications such as artificial light-harvesting antenna, molecular energy storage devices, solar cells, opto- electronic switches, and photodynamic therapy.1-5
Spiropyran (SP) is a typical photochromic molecule performing the reversible interconversion between colorless SP and colored merocyanine (MC). UV-absorbing SP is turned into VIS-absorbing MC by the photochemical ring opening reaction on UV irradiation. Reversely, MC is trans- formed to SP by the thermal or visible light-induced ring closing reaction. Reversible SP-MC transformation is one of subjects of active research on optical memory and switch in a point of view that the polarity, the excited state energy, or molecular geometry of SP and MC are significantly different.6-8 It is expected that the combination of photoactive Por and photochromic SP could lead to an interesting light-controll- able molecular device to control on-off switching of photo- induced processes.
In this work, Por-SP dyad linked by ester linkage was prepared by the esterification reaction between carboxylic acid-functionalized porphyrin and hydroxy-functionalized spiropyran and its photoinduced transformation was investi- gated in tetrahydrofuran and dichloromethane. For Por-SP in tetrahydrofuran, the porphyrin Soret-band maximum is at 416 nm, the porphyrin Q-band maxima are at 515, 550, 590, 645 nm and absorption of spiropyran appears at 350 nm.
On irradiation at 350 nm, the absorption spectral changes of Por-SP in tetrahydrofuran were measured with the irradiation time in 5-seccond intervals for 40 sec (Figure 1).
Porphyrin characteristic bands at 416 (Soret band), 515, 550, 645 (Q bands) nm remain unchanged, but only the absor- bance in 590 nm region corresponding to merocyanine absorption has increased as spiropyran moiety carries out the photochemical ring opening reaction to merocyanine. In tetrahydrofuran, Por-SP is photochemically transformed into Por-MC. These are the same results as reported previously for Porphyrin-Spiropyran dyad linked by methylene link- age.9 As shown in inset of Figure 1, complete conversion from Por-SP into Por-MC takes ~25 sec. Solution colour changes from red to violet.
Figure 2 shows absorption spectral changes of Por-SP with UV irradiation time in dichloromethane. In contrast with the case in tetrahydrofuran, on 350 nm irradiation of Por-SP in dichloromethane, porphyrin characteristic bands at 416 (Soret band), 515, 550, 590, 645 (Q bands) nm have decreased and new absorption bands at 448 and 664 nm have increased after long time irradiation.
These spectral features are similar to those of diprotonated H2Por2+, formed by the addition of acid to free base Por where two pyrrole imine nitrogens in the core accept two protons.10,11 In the free base Por, four phenyl rings are not conjugated with porphyrin ring because they lie almost perpendicular to the porphyrin plane, whereas in the highly distorted structure of the diprotonated H2Por2+ they rotate towards a more parallel position and have more highly con- Figure 1. Absorption spectral changes of Por-SP with UV irradia- tion time (0-40 sec, 5 sec interval) in tetrahydrofuran (Inset: Plot of A590nmvs. irradiation time).
3126 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 10 Notes
jugated structure. The overlap of π orbitals from porphyrin and phenyl rings in diprotonated H2Por2+ results in red- shifted spectrum relative to that of free base Por.12,13
Without acid addition, only irradiation14,15 or sonication16-18 of Por in chlorocarbon solvents such as chloroform, di- chloroethane, and dichlorobenzene also generates diproton- ated H2Por2+ by HCl production from the porphyrin-cata- lyzed photodecomposition or sonodegradation of chloro- carbon solvents.
The red shift of absorption bands at 416, 515, 550, 590, 645 nm to 448, 664 nm under UV irradiation of Por-SP (Figure 2) might be attributed to the formation of diproton- ated H2Por2+-SP, due to the protonation of the core nitrogen atoms in porphyrin moiety through porphyrin-catalyzed photodecomposition of dichloromethane solvent followed by HCl formation.
However, complete conversion of Por-SP to H2Por2+-SP took too long about 55 min. To study the detailed reaction process, absorption spectral changes have been traced in short time intervals during irradiation. As shown in Figure 2(a), up to 1 min. irradiation, only 590 nm band increases with maintaining the absorption characteristics of porphyrin.
For 1-9 min. irradiation, no significant spectral changes are observed (Figure 2(b)). After more than 9 min. of exposure to light, new bands at 448, 664 nm has arisen, while the
absorption at 416, 515, 550, 590, 645 nm has decreased (Figure 2(c)). Red Por-SP in dichloromethane changes initially to violet Por-MC and finally turned into green H2Por2+-SP (Figure 2).
Figure 3 shows absorbance changes of Por-SP at various wavelengths with irradiation time in dichloromethane. Inferr- ing from the absorbance changes at 590 nm (filled circle), a merocyanine form, Por-MC, increases sharply for initial 1 min. of irradiation time and then is nearly unchanged up to 9 min. and then decreases after 9 min. and completely dis- appears after 55 min. Absorbance of Por moiety at 416 nm (filled triangle) decreases from 9 min. to 55 min. of irradia- tion time. Absorbances at 448 nm (open triangle) and 664 nm (open circle), related to the formation of H2Por2+-SP, start to increase rapidly after the solution is irradiated for 9 min. and become flat after 55 min. From these observations, it has been suggested that H2Por2+-SP is not formed directly by the photolysis of Por-SP in dichloromethane, but via zwitterionic Por-MC formed by initial conversion of Por-SP to Por-MC.
Figure 4 shows TEM images of Por-SP before irradiation and H2Por2+-SP obtained after UV irradiation. No aggre- gation is observed for Por-SP before irradiation. But, some aggregation is observed after irradiation, due to the formation of ionic H2Por2+-SP. The average size of aggregates estimates Figure 2. Photochromic reactions (upper) and absorption spectral changes (lower) of Por-SP with UV irradiation time ((a) 0-1 min (10 sec interval), (b) 1-9 min (1, 2, 3, 4, 5, 7, 9 min), (c) 9-70 min (9, 11, 13, 15, 25, 40, 55, 70 min) in dichloromethane.
Notes Bull. Korean Chem. Soc. 2013, Vol. 34, No. 10 3127
about 50 nm. Porphyrins, especially diprotonated porphyrins favor aggregation due to their structural characteristics.19-21 Red-shifted Soret band indicates the formation of edge-to- edge aligned J-aggregate, while blue-shift indicates the formation of face-to-face aligned H-aggregate. However, absorption spectral feature of green solution in Figure 2 are different to that of J- or H-aggregate of protonated porphy- rin.19-21
In summary, Por-SP dyad conducts solvent-dependent photoreactions, ring-opening reaction in THF (red to violet) or protonation in CH2Cl2 (red to green). In CH2Cl2, as irradiated in a very short time, Por-SP dyad changes from red into violet at first, due to the photochromic ring-opening reaction to merocyanine derivative Por-MC, as in THF.
Further irradiation alters the solution color finally into green, due to the formation of diprotonated porphyrin H2Por2+-SP with HCl formed by porphyrin-catalyzed photodecomposi- tion of CH2Cl2. Some aggregates are observed in TEM
image for H2Por2+-SP.
Experimental
Materials. The reagents were purchased from Sigma- Aldrich. TLC was performed on Merck Silica Gel 60 F254 glass plates and developed by UV light. Column chromato- graphy was performed on Merck Silica Gel 60 (70-230 mesh, ASTM). Por-CO2H (5-(4-carboxyphenyl)-10,15, 20- triphenylporphyrin)22,23 and SP-OH (2-(3',3'-dimethyl-6- nitro-3'H-spiro[chromene-2,2'-indol]-1'-yl)-ethanol)24 were prepared following the reported procedure.
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-(di- methylamino)propyl]-3-ethylcarbodiimide, 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 anhydr- ous 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) δ 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, -CH2CH2-OH), 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]+.
Methods. UV-vis absorption spectra were measured with 5 × 10−6 M solution using a quartz cuvette in a Shimadzu UV-2401PC spectrophotometer. Irradiation 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. Trans- mission electron microscope (TEM) images were obtained with FEI TECNAI 20. 1H 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-dihydr- oxybenzoic acid in THF as matrix.
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Figure 4. TEM images of (a) Por-SP and (b) H2Por2+-SP.
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