Pseudo Photochromism Induced Halochromism: Reversible Color Change for Porphyrin Derivatives and Their Molecular Recognition Towards Hydrochlorides

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Send Orders for Reprints to [email protected] Current Inorganic Chemistry, 2014, 4, 59-67 59 Pseudo Photochromism Induced Halochromism: Reversible Color Change for Porphyrin Derivatives and their Molecular Recognition Towards Hydrochlorides

Ya Hong Wu1, Lin Lin Qin1, Yan Yan1,*, Shan Ling Tong1, Yan Wang1, Jing Zhang2, Ling Ling Hu1 and Jian Yu1

1College of Light Industry & Chemical Engineering, Guangdong University of Technology, Guangzhou 510006, P.R. China; 2College of Chemical Engineering, Zhuhai Campus, Beijing Institute of Technology, Zhuhai 519085, P.R. China

Abstract: A series of meso-substituted phenyl porphyrin derivatives were selected for investigating their photochromic behavior in several aryl chloride solutions. After ultraviolet irradiation (UVI, = 360 nm), the solutions’ color changed rapidly and sensitively from pink to bright green; and their pink color can be recovered gradually when the green samples are radiated under visible light (VR, > 500 nm) or placed around diffused light at ambient conditions. Notably this photochromic behavior can be completely stimulated by a halochromic method. A porphyrin H2TMPP 3 was selected as a typical sample for revealing this chromic mechanism, and molecular configuration for the green porphyrin chromophores was proposed based on crystal assembly of H2TPP 1. Finally the sensitive molecular recognition of H2TMPP 3 towards hydrochlorides was examined. Keywords: Halochromism, molecular recognition, photochromism, ultraviolet irradiation, visible radiation.

INTRODUCTION Both porphyrin and phthalocyanine components acted as assistant chromospheres to enhance the chromic phenomena Photochromism (phototropy) had been attracting research of the covalently linked groups, but the chromism from these attention for a long time [1, 2]. Phytochromes as signal- macrocyclic molecules themselves were seldom discussed. transducing photoreceptors converted between inactive and After successful preparation of bis(Schiff base substituted active forms in response to different wavelengths of light, phenyl) porphyrins and their relative metal complexes, we and this conversion was used to synchronize plant develop- discovered occasionally that both 5, 15- and 5, 10-bis(Schiff ment to the exigencies of the light environment [3]. Crystals base substituted phenyl) porphyrins (9 and 13) in dichloro- of perfluorocyclopentene derivatives exhibited clearly re- methane solvent changed their color reversibly when they versible color change when they were irradiated alternately were irradiated alternately under ultraviolet and diffused with ultraviolet ( = 365 nm) and visible ( > 500 nm) light, along with reversible transformations in molecular structure light. Detailed research results indicated that almost all of the from open-ring into closed-ring isomers [4]. Conjugated metal-free porphyrins and even a zinc (II) porphyrin 2 porphyrins and phthalocyanines are considered as potential (Scheme 1) exhibited the photochromic property. optical limiters [5], molecular switches [6, 7] and photoinduced electron transfer (PET) [8]. The photochromic behav- EXPERIMENTAL ior of active components was remarkably enhanced by the Measurements: 1H-NMR spectra were recorded with a covalently linked porphyrin chromospheres and their photo- Varian Mercury-Plus 300 FT-NMR (300 MHz) spectrometer chromic mechanism can be easily monitored by reversibly in chloroform-d with tetramethylsilane (Me4Si) as an internal spectral change of the conjugated porphyrin moiety [9, 10]. standard. Chemical shifts () and coupling constants (J) are Macrocyclic phthalocyanine and porphyrin moieties with given in parts per million and hertz respectively. Infrared characteristically pohoinduced spectral properties provided spectroscopy (IR) spectra were performed using an Avatar switching behavior upon photochromic reactions [11-17]. 370 FT-IR spectrometer (Thermo Nicolet). The ultraviolet- “Photochromism” was simply defined as a light-induced visible (UV-Vis) spectra were measured by a Shimadzu UV- reversible change of color; “Halochromism” or “Acidi- 2450 spectrophotometer, using quartz substrates. EPR spec- chromism” was the reversible color change due to a change tra were acquired with a Bruker A200-9.5/12 spectrometer. in pH of a solution. Halochromism can occur in addition to RF powers ranged from 200 to 400 W across the 7 MHz photochromism [18]. scanned range, and microwave power ranged from 2 to 20 mW. The in situ experiments of ultraviolet irradiation (UVI) and visible radiation (VR) were carried out by using a deute- *Address correspondence to this author at the College of Light Industry & rium light source (30W, Lot Oriel Company, Germany) and Chemical Engineering, Guangdong University of Technology, Guangzhou 510006, P.R. China; Tel: +86-15014225076; Fax: +86-20-39322428; a halogen-tungsten light source (1000 W, Lot Oriel Com- Emails: [email protected] and [email protected] pany, Germany). In photochromic determinations, the dis-

3 tance between light source and sample was set as 20 cm 9.0410 ) and four Q bands (max, Q1 = 516 nm, Q1 = 4 4 (with 0.5 cm slit for UV-Vis determinations and 0.2 mm slit 3.510 ; max, Q2 = 552 nm, Q2 = 1.810 ; max, Q3 = 593 nm, 4 4 for EPR determinations). X-ray data collections and structure Q3 = 1.110 ; and max, Q4 = 648 nm, Q4 = 1.210 ). Both determinations were performed on a Bruker SMART CCD. spectral shape and absorbing position for H2TMPP 3 The data were collected using graphite-monochromatic Mo- changed rapidly due to UVI ( =360 nm, 0.0~20 s): the Soret K radiation ( = 0.71073 Å). The crystal structure was band at 419 nm and three Q bands, including Q1, Q2 and Q3 solved by direct methods and refined by full-matrix least- were disappearing fleetly; and a new Soret band was emerg- 2 square calculation on F with SHELX-97 program package ing demonstrably with red shift (max, Soret 2 = 447.5 nm, Soret 5 [19]. All non-hydrogen atoms were treated anisotropically. 2 = 8.7110 ); meanwhile the Q4 band was much enhanced Hydrogen atoms were placed in calculated positions. and widened out with strong red shift (max, Q4 = 672.5 nm, 5 R2 R3 Q4 = 1.210 ). In a series of changing spectra, an isosbestic point’s appearance at 429.5 nm indicated that a new porphy- N rin species had been formed, and its simplified spectrum N M N with one Soret and one Q band implied that its molecular N configuration had become more symmetrical which resulted from orbital degeneracy increasing and electron transition R1 R4 species decreasing. Within UVI at = 360 nm for 20 s, the sample’s color was changing rapidly from pink to green. More interestingly, when the sample was placed around diffused visible light at ambient conditions, its color was recov- ering gradually from green to pink again (Fig. 1-B). This color change was also recovered under visible light radiation

1: H TPP R = R = R = R = -H; M = 2H for bis(Schiff-base substituted) porphyrin 12: H2BBAPBPP 2 1 2 3 4 (Scheme 1) within 12 min when the green sample was being 2: ZnTPP R1= R2 = R3 = R4 = -H; M = Zn(II) radiated under > 500 nm. Therefore it is demonstrably 3: H2TMPP R1= R2 = R3 = R4 = -Me; M = 2H judged that these metal-free porphyrins, and even a zinc-

4: H2TMOPP R1= R2 = R3 = R4 = -OMe; M = 2H porphyrin complex 2: ZnTPP, possessed photochromic prop-

5: H2TAPP R1= R2 = R3 = R4 = -NH2; M = 2H erty with reversible color change, and their photochromic data for reversible color changes were given in Table 1. 6: H2TNPP R1= R2 = R3 = R4 = -NO2; M = 2H

7: H2Tm-NPP R1= R2 = R3 = R4 = m-NO2; M = 2H Notably, when the green sample from ZnTPP 2 was place under diffused visible light, its color changed gradually into 8: H2BAPBPP R1= R2 = -NH2; R3 = R4 = -H; M = 2H the pink one with H2TPP’s characters, which indicated that 9: H2BMBAPBPP R1,R2=-N=CH-p-Me-Ph;R3,R4=-Ph;M=2H ZnTPP 2 was conversed into its free base H2TPP 1 again 10: H2BMOBAPBPP R1,R2=-N=CH-p-MeOPh;R3,R4,M= H (Scheme 2). 11: H BNBAPBPP R ,R =-N=CH-p-NO -Ph;R ,R =-H;M=2H 2 1 2 2 3 4 Meanwhile this photochromic property was greatly af- 12: H2BBAPBPP R1= R2 = -N=CH-Ph; R3 = R4 = -H; M=2H fected by electronic effect from the peripheral substituents. 13: Por-R1-Por R1=-N=CH-Ph-CH=N-;R3,R3,R4=-Ph;M=2H Comparing with the electronic spectra of H2TPP 1 before Scheme 1. The porphyrin derivatives prepared for chromic deter- and after UVI, the electron donating groups, such as -Me (3), minations. -OMe (4) and -NH2 (8), resulted in spectral red shift of Soret bands for their relative porphyrins (entries 1, 3, 4 and 8 in Materials: All chemicals and reagents were used as re- Table 1). But the electron withdrawing groups, such as -m- ceived from commercial sources without purification. Alkyl NO2 (7) and -p-NO2 (6), also resulted in red shift in their chloride solvents for photochromic and halochromic experi- spectral positions of the related porphyrins. Therefore the ments were purified in turns by reflux over anhydrous cal- substituted phenyls in meso-positions around porphyrin cium carbonate and sodium hydroxide, and vacuum distilla- chromospheres mainly contributed electron donating effect tion. Chemical reactions were carried out under an argon by conjugating model. Based on the absorbing positions of atmosphere. The meso-tetra (substituted phenyl) porphyrin Soret bands, the electron donating effects were ranked in derivatives 1-4, 6 and 7 were prepared according to literature order of Phenyl < p-MePhenyl < m-NO2Phenyl p- methods [20, 21]. H2TNPP 6 was reduced by SnCl2·2H2O in MeOPhenyl < p-NO2Phenyl < p-NH2Phenyl (Table 1 and in concentrated HCl solution to give high purity of meso-tetra Supporting Materials: Fig. 5). Therefore the electronic effect (p-aminophenyl) porphyrin (H2TAPP 5) in yield of 80% in these porphyrin derivatives should be considered as the [22]. Other amino and Schiff base porphyrin derivatives 8-13 contribution from the whole meso-substituted phenyls. were prepared by the reported methods [23]. Moreover the stability of porphyrin chromospheres was demonstrably impacted by the electric effect of the direct RESULTS AND DISCUSSION substituents. H2TAPP 5 with four amino groups as strong Fig. (1-A) showed the spectral change of mose-tetra (p- electron donators exhibited photochromic activity (Fig. 2). methylphenyl)porphyrin (H2TMPP, 3) in range of 350-750 During UVI within 25 s, the color change was clearly ob- nm. Before UVI, the absorbance spectrum of H2TMPP 3 served. But after 30 s’ irradiation, both Soret peaks at Soret 1 exhibited typical characters of a metal-free porphyrin mole- = 428 nm and Soret 2 = 433 nm vanished rapidly which indi- cule with a strong Soret band at max, Soret 1 = 419 nm (Soret 1 = cated that both species of H2TAPP 5 were unstable under Pseudo Photochromism Induced Halochromism Current Inorganic Chemistry, 2014, Vol. 4, No. 1 61

2.5 UV 0.0 s 1.2 UV 5.0 s UV 10.0 s UV 20s 2.0 1.0 UV 15.0 s VisD10min UV 20.0 s VisD 20min 0.8 VisD 40min 1.5 VisD 65min

A 0.6

1.0 Absorbance 0.4

0.5 0.2

0.0 0.0 400 500 600 700 400 500 600 700 800 λ / nm λ /nm (A) (B) -5 -3 Fig. (1). Reversible UV-Vis spectra of H2TMPP 3 (2.810 mol/dm ) in CH2Cl2 solution under UV irradiation (A, = 360 nm, 0.0~20 s,) followed by placing in diffuse visible-light (B, 0.0~ 65min).

Table 1. The UV-Vis spectral data for porphyrins 1-13 before and after UVI for photochromic determinationsa.

Sample tUVI /s 360 nm Soret / nm Q1/nm Q2/nm Q3/nm Q4/nm Isosbestic point / nm

1: H2TPP 0.0 417.0 514.5 549.0 589.5 645.5 432.5 25 443.0 ------658.0 2: ZnTPP 0.0 418.0 -- 548.0 -- -- 429.0 30 443.0 ------660.0

3: H2TMPP 0.0 418.5 516.0 552.0 593.0 648.0 429.5 20 447.5 ------672.5

4: H2TMOPP 0.0 421.5 518.0 554.0 594.0 651.0 433.5 30 454.5 ------693.5

5: H2TAPP 0.0 428.0 521.5 563.5 596.5 656.0 441.0 20 433.0 ------741.0

6: H2TNPP 0.0 424.0 514.5 550.5 590.0 648.5 435.0 45 452.0 -- --- 604.0 656.5

b 7: H2T-m-NPP 0.0 421.5 514.0 548.0 590.0 646.5 434.0 20 448.0 ------655.0

8: H2BAPBPP 0.0 423.0 519.0 556.5 592.5 651.0 436.5 25 462.0 ------701.0

c 9: H2BMBAPBPP 0.0 421.5 516.5 552.0 591.0 647.5 433.0 35 445.0 ------665.0

10:H2BMOBAPBPP 0.0 422.0 518.0 554.0 594.5 650.5 434.5 20 459.5 ------694.5

11: H2BNBAPBPP 0.0 421.0 516.5 552.0 591.0 647.5 434.5 45 455.0 ------677.5

12: H2BBAPBPP 0.0 421.0 516.5 592.5 647.5 647.5 432.5 20 454.0 ------670.0

d 13: Por-R1-Por 0.0 420.0 516.0 551.0 590.0 645.0 435.0 10 449.0 ------673.0 a. b Ultraviolet irradiation (UVI) was performed by using a deuterium light source (30 W, Lot Oriel Company, Germany). H2Tm-NPP = meso-tetra(m-nitrophenyl)-21, 24-2H- c porphyrin. For H2BMBAPBPP, after UVI for 35s, the green sample was completely conversed into the original pink one within 12 min by visible light radiation ( > 500 nm) from a d halogen-tungsten light source (1000 W, Lot Oriel company, Germany). Por-R1-Por = Porphyrin-Schiff base spacer-Porphrin; Schiff base spacer = -N=CH-Ph-CH=N- (R1), other meso-positions for two porphyrins in both terminals were substituted by phenyl groups (R2-R4). 62 Current Inorganic Chemistry, 2014, Vol. 4, No. 1 Wu et al.

m n 360 < l 3 C H C in

Scheme 2. The photochromic induced halochromic mechanism for ZnTPP. After UVI in CHCl3 solution, the center zinc (II) ion was released, and a green H4TPPCl2 was produced. Then it repeated the halochromic behavior of H2TPP.

UVI and through ring opening reaction their conjugated converse it was enhanced and widened out with red shift. rings were destroyed completely. This photochromic behavior was also proved by the sensitive color change of a concentrated sample during UVI (Fig. (3), 1.5 the under figure). In chromic life determinations, the reversible color change of H2TPP 6 reached more than 8 times UV 0.0s UV 5.0s without any energy attenuation (Fig. 4 in Supporting Infor- UV 15.0s mation). These compared results demonstrated that strong UV 20.0s electron-donating amino groups made H2TAPP 5 more ac- 1.0 UV 25.0s UV 30.0s tive and accelerated its dissociation under UVI; while nitro

A groups with strong electron-withdrawing effect made H2TNPP 6 stable enough during repeated UVI. Before and 0.5 after UVI, both spectra of H2TNPP 6 were enhanced. These results indicated that UVI enhanced - electron transition for both species of H2TNPP 6, meanwhile accelerated molecular conversion from an asymmetric to a symmetric con- 0.0 figuration. The reverse color change for H2TNPP 6 was also observed (Fig. 3 in Supporting Information). 300 400 500 600 700 800 For investigating the photochromism in detail, H TMPP λ /nm 2 3 was selected as a candidate in the following experiments. Fig. (2). The spectral change of H2TAPP 5 under UVI. The photo- Several organic solvents, including chloroform, tetrachloro- chromic behavior was also observed, but both species of H2TAPP methane, 1,2-dichloroethane, benzene, toluene, nitrobenzene, were not stable enough, and after 30 s’ irradiation, they were almost acetic acid, propanoic acid, methanol, ethanol, ether, ace- decomposed completely. tone, hexane, cyclohexane, ethyl acetate, dimethyl forma- mide (DMF) and tetrahydrofuran (THF) were also selected On the contrary, H2TNPP 6 with four nitro phenyls as for observing the photochromic behavior of H2TMPP 3. strong electron acceptors exhibited more stable photochro- Determination results indicated that in chloroform, tetra- mic activity (Fig. (3), the upper figure). During UVI, the chloromethane and 1, 2-dichloroethane solutions, the color Soret band at Soret 1 = 424 nm was enhancing within 0-15 s; changes were observed before and after UV irradiation; then it was dropping rapidly along with a new peak emerging while in solutions of benzene, toluene, nitrobenzene, acetic at Soret 2 = 452 nm, and Q1 and Q2 were disappearing while acid, and propanoic acid, under UVI no color change for Q3 and Q4 were enhanced and widen out with clearly red H2TMPP 3 was observed, but their electronic spectra were shift, and finally Q3 and Q4 bands moved from 590.0 and clearly enhanced. Since small solubility of H2TMPP 3 in the 648.5 nm to 604.0 and 656.5 nm respectively. Notably dur- rest solvents, the experiment data for neither color change ing UVI, the Q3 band of H2TPP 6 did not disappear and in nor enhanced spectra were obtained. Pseudo Photochromism Induced Halochromism Current Inorganic Chemistry, 2014, Vol. 4, No. 1 63

TNPP UV 0.0s 1.0 UV 5.0s UV 10.0s UV 15.0s UV 20.0s

A UV 25.0s UV 30.0s

0.5 500 600 700 UV 40.0s UV 45.0s

0.0 400 500 600 700 λ / nm

-5 -3 Fig. (3). Upper: Visible spectrum change of H2TNPP 6 (0.510 mol/dm ) in CH2Cl2 solution under UV irradiation ( = 360 nm, 0.0~30 s). Under: Absorbing spectrum change for H2TNPP (6, 0.01 mol/L in dichloromethane) vs. tUVI ( = 360 nm). The pink color is enhanced (within 30 s’ UVI), followed by color change from pink, yellow to bright green gradually, and then the green color was also enhanced. The yellow color was come form the mixed color of pink and green.

-2 Fig. (4). EPR spectral change of H2TMPP 3 (1.010 mol/L in dichloromathne) vs. tUVI ( = 360 nm, 0.0~240 min), the EPR signal intensity was weakening along the time of UVI was described in the inner figure. 64 Current Inorganic Chemistry, 2014, Vol. 4, No. 1 Wu et al.

EPR determinations were also employed to examine the color change and the spectral enhancement of porphyrin derivatives under UVI was also observed. The EPR spectral 1.5 in base

changes vs. time under UVI were shown in Fig. (4). Before 418.5 nm 447.5 nm 447.5 in acid UVI, H2TMPP 3 gave a sharp EPR signal with radical char- acter at 0.354 T; under UVI, its spectral intensity was being enhanced within 15 min; and then it began to decline within 1.0 15-240 min, along with color changing from pink to bright green. In fact both electronic spectra before and after color A change were all enhanced under UVI, and this phenomena 0.5 was easy to be observed in electronic spectra of H2TNPP 6 Fig. (3). Therefore UVI resulted in enhancement of the con- 672.5 nm

jugated electron transition, and this spectral enhancement 516.0 nm 552.0 nm 648.0 nm 593.0 nm was directly reflected in both EPR and visible spectral de- 0.0 terminations. 400 500 600 700 800 IR and 1H-NMR determinations were also employed to examine the chromic mechanism and spectral enhanced ef- λ / nm -5 fects. IR determinations for H2TMPP 3 indicated that only Fig. (5). The electronic spectra of H2TMPP 3 in CH2Cl2 (1.010 -1 C-H stretch vibration around 2920 cm from the para- mol/L) with acid (HCl, dash line) and base (triethylanmine or pyri- methyphenyls were intensively enhanced after UVI (see Fig. dine, solid line). 7 in Supporting Information). Therefore the improving con- jugation under UVI resulted in strong electron transition, and In a single crystal core, two molecules of hydrochloride this enhancement effect was directly reflected in both visible were linked by the hydrogen bonding (HB) interactions of N- and EPR spectra (Fig. 3 Upper and Fig. 4). HCl in both sides of H2TPP 1 molecule; and a water mole- 1 cule was also linked by H2TPP molecule at peripheral posi- Before UVI, the H-NMR’s data of H2TMPP 3 were in tion through a C-HO HB interaction, and finally one accordance with the reported values [24]. But H2TMPP was easily decomposed after UVI, and the 1H-NMR’s chemical H4TPPCl2H2O core was produced. This configuration change supplied a model for porphyrin derivatives to recog- shift for NPyrrole-H around = -2.79 ppm was not observed. The formation of hydrogen bonding intensively affects its nize hydrogen chloride molecules, and the halochromic be- chemical shift. This influence is also reflected in the sample havior of the porphyrin derivatives can also be better ex- concentration and solvent effect. Therefore the 1H-NMR plained by this molecular model. signal for H in hydrogen bonding was observed. The same photochromic and halochromic behavior indi- For explaining this photochromic behavior, a chemically cated that after UVI or adding hydrogen chloride, the por- simulated system was designed. Interestingly, this photo- phyrin derivatives formed the same molecular configuration chromic mechanism can be reproduced reversibly by adding of H4TMPPCl2. Hydrogen chloride producing mechanism acid or base in the same system (Fig. 5). The photochromic should be investigated. behavior of the examined porphyrin derivatives was just re- Dichloromethane photodegradation led to the formation peated by halochromic method, and their photochromic spec- of a number of long-lived intermediate species including CO, tra were completely repeated by adding concentrated acids HCl, phosgene, chloroform, and CCl4 in the presence of (HF, HCl, HBr, HNO3, HClO4, H2SO4, H3PO4, and even an oxygen over TiO2 catalysts [26, 27]. In fact under UVI, di- organic acid, formic acid) or bases (triethylanmine or pyri- chloromethane, chloroform, 1, 2-dichloroethane and even dine). Experiment results indicated that these porphyrin de- CCl4 were also decomposed in the presence of oxygen with rivatives possessed functionality for molecular recognition trace water to form hydrogen chloride and the related com- towards the common inorganic acids. Meanwhile they pounds, and the produced HCl molecule being recognized by seemed with dual chromic mechanism, including both pho- the free base porphyrin derivatives led to sharp color change tochromic and halochromic ones. But no photochromic phe- in the above solutions. When the pure solvents including nomenon was observed when H2Porphyrins were added in dichloromethane, chloroform, 1, 2-dichloroethane and even acetic and propanoic acids, although their halochromic phe- CCl4 were irradiated under UV within enough time, followed nomena were also reversibly repeated when acid (HCl) or by adding the H2TMPP 3, the obtained solutions were di- organic bases (triethylanmine or pyridine) were added into rectly observed in bright green color owing to the their solutions. These evidences indicated that weakly or- H4TMPPCl2 core’s formation. Meanwhile these H4TMPPCl2 ganic acids can not be recognized by the free base porphyrin cores can be gradually decomposed when they were radiated derivatives. Just the steric effect from acetic and propanoic under directly visible light ( > 500 nm), or placed around acids with great hindrance prevented porphyrin derivatives to diffused visible light, or were rapidly decomposed after drip- recognize themselves. The porphyrin derivatives can recog- ping organic bases (triethylanmine or pyridine) or a mixed nize the selected inorganic acids, associating with clearly solution of water-methanol (wMeOH = 50%), and their solu- color change. Self-assembly of H2TPP 1 provided strong tion colors were recovered from green into pink, and the evidences to prove the change in molecular configurations single H2TMPP 3 molecule was obtained again. (Fig. 6) [25]. Pseudo Photochromism Induced Halochromism Current Inorganic Chemistry, 2014, Vol. 4, No. 1 65

Fig. (6). The molecular configuration of H4TPPCl2 after self-assembly of H2TPP with HCl by hydrothermal method.

Therefore chromic mechanism of this reversible color [H TMPPCl ] 13 K = 4 2 = 3.8410 change for the porphyrin derivatives was judged as one of 1 2 [H 2TMPP][HCl] pseudo photochromism induced halochromism: several alkyl -7 chlorides were photodegraded to generate hydrochloride and when HCl was added more than 2.410 mol/L, more wa- the related compounds, followed by the sensitive molecular ter was taken into the system. Therefore, in the second range recognition by the pink H2Porphyrin chromophores to form the ionization of HCl cannot be ignored, the chemical equi- the green H4Porphyrin ones. This sensitive and sharp color librium for the second range was supposed to dependant on change was significant in halodynamic research on molecu- the reaction (2). lar recognition and would be applied potentially in photody- The amounts of ionization for HCl can be estimated us- namic therapy. ing A1 or A2, and the equilibrium constant for reaction For examining the molecular recognition towards hydro- (2) was calculated by the following equation: chlorides with the porphyrin derivatives, H2TMPP 3 was + [H 4TMPPCl2 ][H ][Cl ] 9 selected as a typical example in the followed quantitative K2 = = 7.8010 [H TMPP]{[HCl] [H + ]}3 analysis. Using visible spectral detection system, the absorb- 2 ency of H TMPP 3 conversing to H TMPPCl in dichloro- 2 4 2 Where [H2TMPP] and [H4TMPPCl2] were calculated by methane solutions with different concentrations of hydro- + A1 = S1bc(H2TMPP) and A2 = S2bc(H4TMPPCl2), [H ] = chloride was measured, and the results showed that in two A2 + - linearly specified ranges the absorption rule of S1 and S2 [Cl] = 2c(H4TMPPCl2) = 2 , or [H ] = [Cl ] = b agreed with Lambert-Beer Law (Fig. 7). In the first and sec- S2 ond linear ranges, the standard constant for this halochromic A 13 9 1 conversion was calculated as K = 3.8410 and 7.8010 . 2c(H2TMPP) = 2 : 1 b The halochromic processes were described as Equilibria (1) S1 and (2). + [H 4TMPPCl2 ][H ][Cl ] K2 = + 3 = (1) [H 2TMPP]{[HCl] [H ]} (2) 0.706 1.0325 0.706 2 ( ) 9 -7 8.71 105 8.71 105 = 7.8010 In the first range, before cHCl reached 2.410 mol/L, the 0.706 1.0325 0.706 halochromism was just dependant on the reaction (1). (1.0 105 )[(4.0 105 ]3 8.71 105 8.71 105 According to Lambert-Beer Law, A1 = S1bc(H2TMPP); This competing ionization resulted in descending for this A2 = S2bc(H4TMPPCl2), the concentrations for H2TMPP 9 conversion (Equilibriums (1) + (2), K2 = 7.8010 ). Any- and H4TMPPCl2 with definite amounts of hydrochloride were calculated; and the average equilibrium constant for way the haolchromism of H2TMPP 3 at both linear ranges reaction (1) was deduced from the following equation: was sensitive enough for molecular recognition towards hydrochlorides. 66 Current Inorganic Chemistry, 2014, Vol. 4, No. 1 Wu et al.

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2 2 2 2 2 1/2 160 °C for 36 h. After cooling down to room temperature, several R1 = ||Fo|–|Fc||/|Fo|. wR2 = [w(Fo – Fc ) /w(Fo ) ] .CCDC: purple crystals in prism shape suitable for X-ray analysis were fil- 700701. trated in 44% yield. Calc. for H4TPPCl2·H2O: C, 74.68; H, 4.53; N, [26] Torimoto, T.; Okawa, Y.; Takeda, N.; Yoneyama, H.; Effect of 7.39 %. Found: C, 74.82; H, 4.28; N, 7.43%. Crystal data: Empiri- activated carbon content in TiO2-1oaded activated carbon on pho- cal formula C44H34Cl2N4O; Formula weight 705.65; Temperature todegradetion behaviors of dichloromethane. J. Photochem. Photo- 294(2)K; Monoclinic system; P2(1)/c space group; a = 21.743(4) bio. A, Chem., 1997, 103, 153-157. Å, b = 8.3273(14) Å, c = 20.276(3) Å, = 90°, = 103.124(3)°, = [27] Borisch, J.; Pilkenton, S.; Miller, M.L.; Raftery, D.; Francisco, J.S. 3 3 90°; V = 3575.3(10) Å ; Z = 4, Dc = 1.311 mg/m ; μMoK = 0.223 TiO2 Photocatalytic Degradation of Dichloromethane: An FTIR -1 mm ; 17955 reflections collected; 6271 reflections unique; Rint = and Solid-State NMR Study. J. Phys. Chem. B, 2004, 108, 5640- 2 0.0383; GOF on F = 1.014; R1 [I > 2(I)] = 0.0592, wR2 = 0.1527. 5646.

Received: September 11, 2013 Revised: December 18, 2013 Accepted: December 22, 2013