Effect of composition on the properties of novel optically active methacrylic copolymers containing side-chain zinc–porphyrin chromophores suitable to chiral recognition

Dyes and Pigments 106 (2014) 143e153

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Effect of composition on the properties of novel optically active methacrylic copolymers containing side-chain zinceporphyrin chromophores suitable to chiral recognition T. Benelli a, b, *, L. Angiolini a, D. Caretti a, M. Lanzi a, L. Mazzocchetti b, E. Salatelli a, b, L. Giorgini a, b a

Dipartimento di Chimica Industriale “Toso Montanari” and INSTM UdR-Bologna, University of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy Interdepartmental Center for Industrial Research on Advanced Applications in Mechanical Engineering and Materials Technology, CIRI-MAM, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 February 2014 Received in revised form 6 March 2014 Accepted 9 March 2014 Available online 14 March 2014

New chiral methacrylic copolymers, bearing in the side chain one or two L-lactic acid residues linked to a porphyrin moiety, with variable molar content of methyl methacrylate co-units, have been prepared by radical copolymerization of the corresponding monomers. The resulting macromolecules have been fully characterized both in solution and in the solid state with particular attention to their thermal, UVeVis and chiroptical properties by comparison with those of the related porphyrin containing homopolymers and monomeric models. The zinceporphyrin coordination complexes of the above functional derivatives have been studied as macromolecular hosts for the chiral recognition of optically active diamine on the basis of the amine/zinc coordination complex exhibiting circular dichroism spectra related to the absolute configuration of the diamine guest. The results are discussed in terms of diamine complexation ability by the zinceporphyrin complex which is related to copolymer composition and, consequently, to backbone flexibility and chromophore mobility. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Chiral porphyrins Polymers Chiral recognition Circular dichroism Acid responsive materials Metallated porphyrins

1. Introduction In recent years there has been a widespread interest in the study of porphyrins and their derivatives which are characterized by an intense and red-shifted Soret band, propensity to undergo p
p stacking and facile incorporation of metals. Such attributes, besides the vital role played for example in photosynthesis, make these functional materials available to a wide range of practical applications in areas such as catalytic asymmetric synthesis [1,2], nonlinear optics [3], molecular devices [4] and, more recently, for the development of organic solar cells and photocatalytic systems for solar energy conversion [5e7]. On the other hand, an extensive interest has been dedicated to the study of new methodologies aimed to determine the absolute * Corresponding author. Dipartimento di Chimica Industriale “Toso Montanari” and INSTM UdR-Bologna, University of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy. Tel.: þ39 051 2093720; fax: þ39 051 2093675. E-mail address: [email protected] (T. Benelli). http://dx.doi.org/10.1016/j.dyepig.2014.03.009 0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

configuration of chiral compounds [8e15]. In this context, a new system based on a reversible host/guest complexation mechanism using a porphyrin derivative as a host and a bidentate chiral substrate as a guest, has been developed [16e18]. In particular an achiral linker bridged bisporphyrin derivative (zinceporphyrin-tweezer) resulted capable of binding various chiral guests (such as acyclic a,u-diamines, amino alcohols, aminoacids etc.) to form 1:1 sandwiched chiral hosteguest complex through zinceamine coordination, where the chirality of the guest is transferred to the porphyrin cores. Thus, a preferential chiral twist in the chromophores arrangement is introduced, generating an excitoncoupled circular dichroism (CD) response with signs reflecting the absolute configuration of the complexed optically active compound (Circular Dichroism Exciton Chirality Method) [8,9,16e18]. The porphyrin tweezer methodology can also be applied to compounds carrying a single site of attachment, such as a-amino acids, and amino alcohols, but it is necessary to artificially introduce the second binding site through chemical derivatization [16,19]. To overcome this need, it has been recently reported the

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T. Benelli et al. / Dyes and Pigments 106 (2014) 143e153

development of a novel host system which is predisposed to adopt both P or M supramolecular helical structures, whose population can change upon interaction with chiral compounds [20]. In this context, we reported the possibility to use achiral [21] and chiral macromolecular metallo-porphyrin systems [22] for the absolute configuration assignment to optically active acyclic diamines. In particular, zinc derivatives of homopolymeric methacrylates bearing in the side chain the optically active L-lactic acid residue linked to the tetraphenylporphyrin chromophore {poly[(S)ML-TPP] and poly[(S,S)-MLL-TPP]} (Fig. 1) [22] resulted very sensitive to this application. These macromolecules, in fact, were able to complex the chiral guest by giving diastereomeric derivatives, more conveniently than using the related achiral homopolymer poly[(M-TPP] [21]. Furthermore, these polymeric systems, bearing two distinct functional groups (i.e. porphyrin and chiral group of one single absolute configuration) directly linked to the side chain, resulted of potential interest for several advanced applications and to investigate the influence of the macromolecular chirality on their peculiar properties. We report here an investigation on the effects induced on the thermal, UVeVis and chiroptical properties, as well as on the ability of chiral recognition by these materials, by progressively spacing out the porphyrinic optically active repeating units along the polymeric backbone through the insertion of an achiral co-monomer. This has been achieved by radical copolymerization of the optically active monomers 5-{4-[(S)-2-methacryloyloxypropanoyloxy]phenyl}10,15,20-triphenylporphyrin {(S)-ML-TPP} or 5-{4-[(S)-2-[(S)-2methacryloyloxypropanoyloxy]propanoyloxy]phenyl}-10,15,20triphenylporphyrin {(S,S)-MLL-TPP]} with methyl methacrylate

(MMA) in different molar amounts to give the corresponding copolymeric compounds, poly[(S)-ML-TPP-co-MMA]s (1:1, 1:3.6 and 1:29 mol/mol) and poly[(S,S)-MLL-TPP-co-MMA]s (1:1.5 and 1:31 mol/mol) (Fig. 1). All the above polymeric products have been fully characterized by FT-IR, 1H- and 13C NMR with particular attention to their spectroscopic and chiroptical properties which have been compared with those of the corresponding homopolymers and low molecular weight monomeric models [22]. Finally, the acid responsive properties and the chiral recognition ability of these compounds after metallation with Zn(OAc)2 have been checked by UVeVis and CD spectroscopy by complexation of the well known 1,2-diaminopropane enantiomers employed as acyclic diamine chiral guests. 2. Material and methods 2.1. Chemicals The porphyrin monomers 5-{4-[(S)-2-methacryloyloxypropanoyloxy]phenyl}-10,15,20-triphenylporphyrin [(S)-ML-TPP] and 5-{4-[(S)-2-[(S)-2-methacryloyloxypropanoyloxy] propanoyloxy] phenyl}-10,15,20-triphenylporphyrin [(S,S)-MLL-TPP] have been synthesized as previously reported [22]. MMA (Aldrich) was distilled at reduced pressure under nitrogen atmosphere in the presence of traces of 2,6-di-tert-butyl-p-cresol as polymerization inhibitor before use [23]. 2,20 -Azobisisobutyronitrile (AIBN) (Aldrich) was crystallized from abs. ethanol before use. Chloroform (CHCl3), and tetrahydrofuran (THF), were purified and dried according to reported procedures [23] and stored over molecular sieves (4  A) under nitrogen. All other reagents and solvents (Aldrich) were used as received. 2.1.1. General procedure for free radical polymerization All copolymerization reactions involving MMA and (S)-ML-TPP or (S,S)-MLL-TPP were carried out in glass vials using AIBN as thermal initiator (2 wt% with respect to the monomers) and dry THF as solvent (1 g of monomers in 15 ml of THF). Feeds of different molar composition, as reported in Table 1, were introduced into the vials under nitrogen atmosphere, submitted to several freeze-thaw cycles, and allowed to polymerize at 60  C for 72 h. Polymerizations were then stopped by pouring the mixtures into a large excess of methanol and the polymeric products purified by repeated precipitations in methanol. The materials were finally dried at 80  C for 4 days under high vacuum to constant weight. The conversions were determined gravimetrically and all the products characterized by FT-IR, SEC, 1H- and 13C NMR. Relevant data concerning the copolymers are reported in Table 1. As an example, the spectroscopic data for poly[(S)-ML-TPPco-MMA] 1:1 and poly[(S,S)-MLL-TPP-co-MMA] 1:1.5 are here reported:

Fig. 1. Molecular structures and relative molar composition of the investigated polymers.

2.1.1.1. Poly[(S)-ML-TPP-co-MMA] 1:1. FT-IR (KBr): 3070 (nCeH arom.), 2959 (nCeH aliph.), 1733 (nC]O, ester), 1605 and 1509 (nC]C arom.) 1128 (nCeO, ester), 808 (dCH 1,4-disubst. arom. ring), 753 and 674 (dCH monosubst. arom. ring) cm
1 1 H NMR (CDCl3): 8.9-8.4 (8H, 2, 3, 7, 8, 12, 13, 17, 18-H TPP), 8.37.7 (m, 8H, 2H arom. metha to ester group and 6H arom. ortho to monosubst. phenyl groups), 7.7-7.0 (11H, 9H arom. metha and para to monosubst. phenyl groups and 2H arom. ortho to ester group), 5.5-5.1 (q, 1H, CHeCH3), 3.8-3.2 (s, 3H, OCH3), 2.2-1.5 (m, 7H, CH3e CH and backbone CH2), 1.5-0.7 (m, 6H, backbone CH3) ppm. 13 C NMR (CDCl3): 170.0 (CO), 150.4 (arom. a to phenyl groups), 142.5 (CeN), 140.4 (arom. a to ester group of disubst. phenyl group), 135.8 (arom. ortho to ester group of disubst. phenyl group), 134.9

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145

Table 1 Polymerization and characterization data of polymeric derivatives. Feed (S)-ML-TPP % mol f

100 50.0 25.0 3.2

Polymer MMA % mol

(S)-ML-TPP % mola

MMA % mola

Yield %b

Mn c

M w =M n c

Tgd

Tde

e 50.0 75.0 96.8

100 49.0 21.7 3.3

e 51.0 78.3 96.7

73 82 83 85

12,800 14,700 15,400 20,900

1.6 1.5 1.8 1.5

315 274 228 144

389 336 336 334

Feed (S,S)-MLL-TPP % mol f

100 50.0 3.2 a b c d e f

Polymer MMA % mol

(S,S)-MLL-TPP % mola

MMA % mola

Yield %b

Mn c

M w =M n c

Tgd

Tde

e 50.0 96.8

100 39.6 3.1

e 60.4 96.9

66 87 88

15,600 16,900 24,000

1.5 1.7 1.6

254 230 142

362 346 334

Determined by 1H NMR. Calculated as (g of polymer/g of monomer) $ 100. Determined by SEC in THF at 25  C. Determined by DSC, heating rate of 10  C/min under nitrogen atmosphere. Determined by TGA, heating rate of 20  C/min in air. Ref. [22].

(arom. ortho to monosubst. phenyl groups), 131.8 (b-TPP), 128.2 (arom. para to monosubst. phenyl groups), 127.1 (arom. metha to monosubst. phenyl groups), 120.1 and 120.7 (meso TPP), 70.5 (CH), 58.0 (backbone CH2eC), 52.5 (OCH3), 45.8 and 45.0 (backbone CH2eC), 19.3 and 17.8 (CH3eCH and backbone CH3eC) ppm. 2.1.1.2. Poly[(S,S)-MLL-TPP-co-MMA] 1:1.5. FT-IR (KBr): 3052 (nCeH arom.), 2971 (nCeH aliph.), 1729 (nC]O, ester), 1606 and 1507 (nC]C arom.) 1119 (nCeO, ester), 808 (dCH 1,4-disubst. arom. ring), 765 and 684 (dCH monosubst. arom. ring) cm
1. 1 H NMR (CDCl3): 9.0-8.6 (8H, 2, 3, 7, 8,12,13,17,18-H TPP), 8.4-7.8 (m, 8H, 2H arom. metha to ester group and 6H arom. ortho to monosubst. phenyl groups), 7.8-7.1 (11H, 9H arom. metha and para to monosubst. phenyl groups and 2H arom. ortho to ester group), 5.6-5.3 e 5.3-5.0 (2q, 2H, CHeCH3), 3.8-3.4 (s, 3H, OCH3), 2.2-1.4 (m, 10H, CH3eCH and backbone CH2), 1.4-0.7 (m, 6H, backbone CH3) ppm. 13 C NMR (CDCl3): 171.0 and 169.7 (CO), 150.6 (arom. a to phenyl groups), 142.6 (CeN), 140.7 (arom. a to ester group of disubst. phenyl group), 135.9 (arom. ortho to ester group of disubst. phenyl group), 131.7 (b TPP), 128.2 (arom. para to monosubst. phenyl groups), 127.2 (arom. metha to monosubst. phenyl groups), 120.8 and 120.1 (meso TPP), 71.1 and 69.4 (CH), 56.8 (backbone CH2eC), 52.5 (OCH3), 45.7 (backbone CH2eC), 19.2 and 17.6 (CH3eCH and backbone CH3eC) ppm. 2.1.2. General procedure for metallation Each TPP copolymeric derivative in a weight amount corresponding to 0.04 mmol of TPP co-units and Zn(OAc)2 (0.08 mmol) were dissolved in chloroform (30 mL) and MeOH (6 mL). The solution was kept at room temperature under nitrogen flow for 2 h, then washed with a NaCl saturated aq. solution, 5% aq. Na2CO3 and water, in that order. After drying the organic layer over anhydrous Na2SO4 and evaporation of the solvent under vacuum, the pure products were obtained in nearly quantitative yield. 2.1.3. Preparation of enantiomerically pure optically active diamine solutions As previously described [21], a solution of (R)-(þ)- or (S)(
)-1,2-diaminopropane dihydrochloride (0.102 mmol) in MeOH (2 mL) was allowed to react overnight with Na2CO3 (28 mmol) at room temperature. The solution, after filtration, was evaporated to dryness under high vacuum for 1 h and chloroform (5 mL) added to yield a clear solution of (R)-(þ)- or (S)-(
)-1,2-diaminopropane (0.02 mol L
1).

2.2. Measurements 1

H- and 13C NMR spectra were obtained at room temperature, in 5e10% CDCl3 solutions, using a Varian MercuryPlus VX 400 (1H, 399.9; 13C, 100.6 MHz) spectrometer. Chemical shifts are given in ppm relative to tetramethylsilane (TMS). FT-IR spectra were recorded with a PerkineElmer 1750 spectrophotometer, equipped with an Epson Endeavour II data station, on sample prepared as KBr pellets. Number average molecular weights of the polymers ðMn Þ and their polydispersity indexes ðM w =Mn Þ were determined in THF solution by SEC using a HPLC Lab Flow 2000 apparatus, equipped with an injector Rheodyne 7725i, a Phenomenex Phenogel 5micron MXM column and a UVeVis detector Linear Instrument model UVIS-200, working at 254 nm. The calibration curve for the MXM column was obtained by using monodisperse polystyrene standards in the range 2700e200,000. The glass transition temperatures of the polymers (Tg) were determined by differential scanning calorimetry (DSC) on a TA Instruments DSC 2920 Modulated apparatus, adopting a temperature program consisting of three heating and two cooling ramps starting from room temperature (heating/cooling rate 10  C/min under a nitrogen atmosphere). Each sample (5e9 mgr) was heated up to only 250  C in order to avoid thermal decomposition. The initial thermal decomposition temperature (Td) was determined on the polymeric samples with a PerkineElmer TGA-7 thermogravimetric analyzer by heating the samples in air at a rate of 20  C/min. UVeVis absorption spectra in solution were recorded at 25  C in CHCl3 with a PerkinElmer Lambda 19 spectrophotometer by using cell path lengths of 0.1 and 1 cm and concentrations of porphyrin chromophore of about 2  10
5 mol L
1. Circular dichroism (CD) spectra were recorded at 25  C in chloroform solutions on a Jasco 810 A dichrograph, using the same path length and solution concentrations as for the UVeVis measurements. Dε values, expressed as L mol
1 cm
1, were calculated from the following expression: Dε ¼ [Q]/3300, where the molar ellipticity [Q] in deg cm2 dmol
1 refers, when no further specified, to one porphyrin chromophore. Measurements of chiral recognition have been performed by adding different aliquots of the optically active diamine solution (0.02 mol L
1) to a zinceporphyrin solution (2.0  10
5 mol L
1) in CHCl3. Amorphous thin films of the copolymers have been prepared by spin coating of a solution of the polymer in THF onto glass slides.

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Table 2 Main sequence lengths ðlÞ and molar fraction X(n) (%) of (S)-ML-TPP and MMA co-units having sequence length of n units in Poly[(S)-ML-TPP-co-MMA]s. Co-unit (S)-ML-TPP (% mol)

3.3 21.7 49.0

lðSÞ
ML
TPP

1.01 1.16 1.26

lMMA

29.49 2.25 1.78

X(S)-ML-TPP(n)

XMMA(n)

n¼1

2

3

4

>4

n¼1

2

3

4

>4

98.6 74.3 63.2

1.4 20.5 25.9

0.0 4.2 8.0

0.0 0.8 2.2

0.0 0.2 0.7

0.1 19.7 31.7

0.2 21.9 27.7

0.3 18.3 18.2

0.4 13.6 10.6

98.9 26.5 11.9

The films were then dried by heating above 80  C under vacuum for 12 h. The films thickness, measured by a Tencor P-10 profilometer, was in the range 150e350 nm. The films were submitted to exposure to HCl and subsequently NH3 vapors at room temperature for few seconds to completely visible color change and their CD and UVeVis spectra carried out under the same instrumental conditions as the related solutions. 3. Results and discussion 3.1. Synthesis and characterization The copolymerizations of (S)-ML-TTP or (S,S)-MLL-TTP with MMA in different molar amounts were carried out in THF solution under radical conditions in the presence of AIBN as thermal initiator. Under these conditions, the copolymers with MMA were obtained in appreciable yields (Table 1), with satisfactory average molecular weight and polydispersity values in the range 1.6e1.8, typical of free radical polymerization. However, the presence of the sterically hindering porphyrin chromophore in (S)-ML-TTP and (S,S)-MLLTTP seems to affect the polymerization extent. In consequence of its lower reactivity with respect to MMA, in fact, by decreasing the amount of this co-monomer in the feed, the number average molecular weight of the obtained derivatives increases (Table 1). Relevant data for the synthesized derivatives are reported in Table 1 and compared with the previously investigated homopolymers poly[(S)-ML-TTP] and poly[(S,S)-MLL-TTP] [22]. The occurrence of polymerization involving the methacrylic double bond was confirmed by FT-IR, showing the disappearance of the band around 1630 cm
1, which corresponds to the stretching vibration of the methacrylic double bond in the monomers, and the shift of the adjacent aliphatic estereal carbonyl stretching to higher frequencies as a result of the reduced electron delocalization caused by the reaction of the methacrylic double bond. Accordingly, in the 1 H NMR spectra of the copolymers, the resonances of methacrylic unsaturated methylene protons (around 5.60 and 6.10 ppm) are absent and the methyl resonances at about 2.0 ppm are shifted upfield. The final molar composition of the obtained copolymers (Table 1) was assessed by 1H NMR by comparing the integrated peak areas of porphyrinic protons in b position to nitrogen, located at around 8.50 ppm, to that one related to the methyl ester group of MMA co-units at 3.50 ppm. The comparison of all the aromatic resonances (exclusively given by TPP moieties) with those related to the aliphatic protons located at 2.2e1.8 ppm confirmed the assessed composition values. The results listed in Table 1 point out that the molar content of TPP co-units in the copolymers roughly reflects the feed composition, due to the long reaction time (72 h) adopted for polymerization, and the lower reactivity ratio of the porphyrinic containing monomer with respect to MMA. Actually, an approximate evaluation of the reactivity ratios performed by using the modified KelenTüdos method [24,25] on poly[(S)-ML-TPP-co-MMA]s series, gives r(S)-ML-TPP ¼ 0.233 and rMMA ¼ 0.859 with r(S)-ML-TPP,rMMA ¼ 0.200. Using well known methods reported in the literature [26,27], the mean sequence length of the co-units and the molar fractions of

sequences of length n may be calculated from the reactivity ratios values, as shown in Table 2, thus confirming that the mean sequence lengths of porphyrinic co-units ðlðSÞ
ML
TPP Þ in the copolymers result quite short, with lðSÞ
ML
TPP maximum of 1.26 for the copolymer with 49% of (S)-ML-TPP co-units. This behavior is probably due to their higher steric hindrance with respect to MMA co-monomer. Similar data may be expected for poly[(S,S)-MLL-TPPco-MMA]s derivatives. These results indicate that the optically active porphyrinic counits result essentially isolated in the copolymers containing the lowest amount of this monomer, thus implying that the interactions between chromophores in the side-chain should be quite modified with respect to the corresponding homopolymers. 3.2. Thermal analysis The thermal stability of all the copolymeric derivatives was determined by thermogravimetric analysis (TGA). Quite similar decomposition temperature (Td) values in the range 334e346  C were obtained for all the copolymeric derivatives, considerably high compared to the respective homopolymers [22] (Table 1), and much higher than that one reported for poly(MMA) (260  C) [28], indicative of a noticeable thermal stability of the macromolecules attributable to a remarkable presence of strong dipolar interactions in the solid state between the porphyrin moieties located in the macromolecular side-chains. In particular, the copolymer having ca. 97 mol-% of MMA co-units exhibits a higher thermal stability of about þ75  C with respect to homopolymer poly(MMA) although containing a few molar percent of porphyrin side-chain groups, thus making these materials interesting for application. DSC thermograms revealed only second-order transitions, originated by glass transitions, with no melting peaks, suggesting that these macromolecules, as expected, are substantially amorphous in the solid state (Table 1 and Fig. 2).

Fig. 2. DSC thermograms of poly[(S)-ML-TPP] and its relative copolymers with MMA.

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147

Table 3 UVeVis spectra of synthesized copolymers in chloroform solution. Sample

Soret band

c

(S)-ML-TPP Poly[(S)-ML-TPP]c Poly[(S)-ML-TPP-co-MMA] 1:1 Poly[Zn-(S)-ML-TPP-co-MMA] 1:1 Poly[(S)-ML-TPP-co-MMA] 1:3.6 Poly[Zn-(S)-ML-TPP-co-MMA] 1:3.6 Poly[(S)-ML-TPP-co-MMA] 1:29 Poly[Zn-(S)-ML-TPP-co-MMA] 1:29 (S,S)-MLL-TPPc Poly[(S,S)-MLL-TPP]c Poly[Zn-(S,S)-MLL-TPP]c Poly[(S,S)-MLL-TPP-co-MMA] 1:1.5 Poly[Zn-(S,S)-MLL-TPP-co-MMA] 1:1.5 Poly[(S,S)-MLL-TPP-co-MMA] 1:31 Poly[Zn-(S,S)-MLL-TPP-co-MMA] 1:31 a b c

1st Q band

2nd Q band

3rd Q band

4th Q band

lmaxa

εmaxb$10
3

lmaxa

εmaxb$10
3

lmaxa

εmaxb$10
3

lmaxa

εmaxb $10
3

lmaxa

εmaxb$10
3

418 418 418 422 418 422 418 423 418 418 422 418 423 418 422

475.7 294.3 355.1 313.9 413.3 377.2 460.8 417.1 477.5 304.3 285.3 409.2 365.8 460.8 411.3

515 516 515 e 515 e 515 e 515 515 e 515 e 515 e

18.5 18.0 19.1 e 20.4 e 18.2 e 19.0 17.8 e 19.8 e 19.0 e

550 551 550 551 550 552 550 553 550 550 552 550 553 550 551

7.6 7.6 8.0 17.3 8.6 18.0 7.6 18.3 7.9 7.7 15.6 8.3 15.7 7.9 16.9

590 590 590 592 590 592 590 593 590 590 593 590 592 590 592

5.5 5.5 5.8 4.9 6.2 5.1 5.5 5.9 5.7 5.5 6.3 6.0 6.3 5.7 6.4

645 646 646 e 646 e 645 e 645 646 e 645 e 645 e

3.6 3.7 3.9 e 4.2 e 3.7 e 3.8 4.2 e 4.0 e 3.8 e

Wavelength of maximum absorbance, expressed in nm. Expressed in L mol
1 cm
1 and calculated for one single porphyrin chromophore. Ref. [22].

The Tg values of poly[(S)-ML-TTP-co-MMA] 1:1 and poly[(S,S)MLL-TTP-co-MMA] 1:1.5 appear high (274 and 230  C, respectively) and quite similar to those of the corresponding homopolymers. This behavior confirms the presence of strong interactions among the side-chain porphyrin moieties enhancing the stiffness of the material, as suggested by TGA. The increasing insertion of MMA co-units rises the chain flexibility, as demonstrated by the decrease of the Tg values (Table 1 and Fig. 2). However, it can be noted that even the samples containing the smallest amount of TPP co-units (3 mol-% only) display higher Tg values (about 142e144  C) than that one reported for poly(MMA) (104  C) [29]. 3.3. UVeVis properties in solution The absorption spectra of all the synthesized copolymers in dilute CHCl3 solution (Table 3 and Fig. 3) result very similar to those ones of the corresponding homopolymers [22] and to other porphyrinic derivatives reported in the literature [21,30,31], showing an intense Soret band at 418 nm, related to p/p* transitions, and four Q bands at 515, 550, 590 and 645 nm of lower intensity. A significant hypochromism was formerly observed for the Soret band when passing from monomers (S)-ML-TPP and (S,S)-MLL-TPP to the corresponding homopolymers [22] (Table 3). Such behavior was attributed to the occurrence of intramolecular electrostatic dipolar interactions between porphyrin moieties, evidently as a consequence of the close proximity of the side-chain chromophores located along the macromolecular backbone. This is supported by the absorption data calculated for one single chromophoric porphyrin co-unit of the synthesized copolymers: by increasing the mean distance between TPP dyes through the insertion of non-chromophoric methacrylic co-units in poly[(S)-ML-TPP-co-MMA]s and poly[(S,S)-MLL-TPP-co-MMA]s, in fact, a lower amount of intramolecular dipolar interactions is involved, with consequent increase of the molar absorption coefficient εmax (Table 3). The metallation with Zn(OAc)2 of poly[(S)-ML-TPP-co-MMA]s and poly[(S,S)-MLL-TPP-co-MMA]s to give the corresponding zincmetallated derivatives poly[Zn-(S)-ML-TPP-co-MMA]s and poly [Zn-(S,S)-MLL-TPP-co-MMA]s, respectively, was confirmed by the UVeVis spectra. As expected [32e34], the zinc-derivatives display a red shift of the Soret band from 418 to 422 nm and the four Q bands collapse into two bands at around 552 and 594 nm (Fig. 3 and

Fig. 3. UVeVis (bottom) and CD (top) spectra in chloroform solution of poly[(S)-MLTPP-co-MMA] 1:1 (e) and metallated poly[Zn-(S)-ML-TPP-co-MMA] 1:1 (- - -).

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Table 3). As far as the hypochromism of the Soret band is concerned, the zinc-metallated copolymers behave similarly to the related unmetallated samples, although showing a certain higher tendency to give electrostatic interactions among chromophores. 3.4. Chiroptical properties of porphyrin derivatives in solution It is known that porphyrins have a preference for an off-set stacking [35] and that in presence of enantiomerically pure optically active molecules an induced single helical handedness into these stacks, which can be analyzed by CD spectroscopy, is observed [36,37]. With the aim to study the optical activity of these materials and to reveal the presence of conformational dissymmetry, all the synthesized copolymers have been investigated by CD in chloroform solution, in the spectral region between 250 and 700 nm (Table 4). While the CD spectra of achiral porphyrin solutions in nonaggregative conditions resulted silent, interesting features have been evidenced in intrinsically chiral systems [22]. The CD spectra of homopolymers poly[(S)-ML-TPP] and poly[(S,S)-MLL-TPP], in fact, show a negative exciton splitting (Table 4) which suggests the presence of dipolar electrostatic interactions between side-chain neighboring chromophores disposed in a mutual chiral geometry of an anticlockwise prevailing screw sense [9,22] as shown in Fig. 4. As expected, copolymers poly[(S)-ML-TPP-co-MMA] 1:1 and 1:3.6 and poly[(S,S)-MLL-TPP-co-MMA] 1:1.5 display CD spectra very similar to those of the corresponding homopolymers [22], but with progressively lower CD intensity (Figs. 3 and 5); thus, it is reasonable to assume that, in this case also, the porphyrin chromophores are organized in a twisted arrangement with a prevailing anticlockwise handedness [9,22]. It is noteworthy that the exciton couplet intensity decreases with the progressive insertion of achiral MMA co-units in the macromolecules (Table 4 and Fig. 5), so as to reduce the extent of cooperative interactions between chiral porphyrinic co-units, in agreement with UVeVis spectroscopy and DSC analysis. The average length of chain sections with a single prevailing chiral arrangement and hence conformational order, results in fact decreased in the copolymers with respect to the homopolymers. Considering the mean sequence lengths of the chiral co-units and their molar fractions, reported in Table 2, it appears that even few adjacent chiral units or a low fraction of sequences with

sufficiently high length are able to produce a remarkable optical activity in the macromolecules. In fact, the copolymer poly[(S)-MLTPP-co-MMA] 1:3.6 at 21.7% molar content of (S)-ML-TPP already shows a cooperative optical activity larger than the monomeric compound, although it is constituted by 74.3% of isolated units and only the 4.2% of sequences of three repeating units. By further increasing the presence of achiral methacrylic comonomer in poly[(S)-ML-TPP-co-MMA] 1:29 and poly[(S,S)-MLLTPP-co-MMA] 1:31, the exciton couplet fades and the CD spectra become more similar to those of the low molecular weight monomers (S)-ML-TPP and (S,S)-MLL-TPP, displaying only one weak positive signal in correspondence of the intense Soret band (Table 4 and Fig. 5), thus pointing out the absence of any macromolecular conformational order. However, these materials, bearing in the side-chain both chiral groups of one single configuration and the porphyrinic chromophore, could be very interesting for their potential advanced technological applications. This functional combination, in fact, allows the polymers to display the properties typical of dissymmetric systems (optical activity, exciton splitting of chiroptical absorptions) and could be of interest in the field of the chiral nanotechnology and to investigate the amplification of chirality of polymeric materials. The metallation of these macromolecular porphyrin dyes with Zn2þ considerably affects the chromophore interactions. In fact, as reported before for the correspondent homopolymers [22], the negative couplet present in the spectra of poly[(S)-ML-TPP-coMMA] 1:1 and poly[(S,S)-MLL-TPP-co-MMA] 1:1.5 disappears and a new positive one is present (Fig. 3 and Table 4), thus indicating that also in these Zn-metallated copolymeric derivatives, the chromophores are disposed in a mutual chiral geometry, but with an opposite prevailing handedness [9]. The metallation of poly[(S)-ML-TPP-co-MMA] 1:3.6 and 1:29 and poly[(S,S)-MLL-TPP-co-MMA 1:31, instead, seems to disturb the macromolecular order and the CD spectra result silent. 3.5. UVeVis and chiroptical properties of thin films The behavior in the solid state of this particular class of polymers containing in the side chain chiral groups and porphyrin residues has not been investigated yet and could be of potential interest for several advanced applications.

Table 4 CD spectra of porphyrin derivatives in chloroform solution. Sample e

(S)-ML-TPP (S,S)-MLL-TPPe Poly[(S)-ML-TPP]e Poly[Zn-(S)-ML-TPP]e Poly[(S)-ML-TPP-co-MMA] 1:1 Poly[Zn-(S)-ML-TPP-co-MMA] 1:1 Poly[(S)-ML-TPP-co-MMA] 1:3.6 Poly[Zn-(S)-ML-TPP-co-MMA] 1:3.6 Poly[(S)-ML-TPP-co-MMA] 1:29 Poly[Zn-(S)-ML-TPP-co-MMA] 1:29 Poly[(S,S)-MLL-TPP]e Poly[Zn-(S,S)-MLL-TPP]e Poly[(S,S)-MLL-TPP-co-MMA] 1:1.5 Poly[Zn-(S,S)-MLL-TPP-co-MMA] 1:1.5 Poly[(S,S)-MLL-TPP-co-MMA] 1:31 Poly[Zn-(S,S)-MLL-TPP-co-MMA] 1:31 a b c d e

l1a

Dε1b

Dε10 c

l0d

l2a

Dε2b

Dε20 c

421 421 413 420 416 415 418 e 420 e 409 418 412 419 422 e

þ5.52 þ6.09 þ16.86
5.80 þ10.47
3.72 þ6.56 e þ3.83 e þ8.56
2.75 þ6.32 þ1.15 þ6.62 e

e e þ33.72
11.60 þ20.94
7.44 þ13.12 e þ7.66 e þ17.12
5.50 þ12.64 þ2.30 þ13.24 e

e e 421 426 424 425 423 e e e 417 424 418 424 e e

e e 432 433 431 433 426 e e e 426 429 423 428 e e

e e
14.17 þ18.37
5.50 þ8.26
2.78 e e e
7.01 þ10.13
4.60
0.96 e e

e e
28.34 þ36.74
11.00 þ16.52
5.56 e e e
14.02 þ20.26
9.20
1.92 e e

Wavelength (in nm) of maximum dichroic absorption. Dε expressed in L mol
1 cm
1 and calculated for one repeating unit in the polymer. Dε expressed in L mol
1 cm
1 and calculated for a porphyrin couple of two repeating units in the polymer. Wavelength (in nm) of the cross-over point of dichroic bands. Ref. [22].

T. Benelli et al. / Dyes and Pigments 106 (2014) 143e153

149

Fig. 4. Idealized representation of: a) transition moment of one single porphyrin chromophore, b) and c) cooperative interactions between two side-chain chiral porphyrin chromophores disposed in a mutual chiral geometry of anticlockwise handedness, d) and e) the corresponding idealized polymeric chain conformation.

The amorphous homo- and copolymers in the solid state exhibit strong absorptions in the visible region centered at about 420 nm (Soret band) and at 515, 550, 590 and 645 nm (Q bands) assigned to the same electronic transitions previously observed in solution (Fig. 3 and Table 3). As an example the electronic spectrum of a native thin film of poly[(S)-ML-TPP-co-MMA] 1:1 (130 nm thick) on glass slide is shown in Fig. 6 (bottom). The CD spectrum of this amorphous thin film, as represented in Fig. 6 (top), is quite similar to that shown by the same sample in solution (Fig. 3 and Table 4): all the copolymers displaying in diluted solution an exciton couplet related to the Soret band show analogous behavior in the solid state. The close similarity between the CD spectra of the native polymeric films and those of the polymers in solution suggests that the macromolecules maintain chiral conformations of one prevailing helical handedness also in the solid amorphous state, at least for chain segments. In the solid state, too, it appears that separating the porphyrinic chromophores by insertion of non-chromophoric MMA co-units in the macromolecular chain, lowers the possibility of intramolecular interactions, giving rise to a behavior resembling that one of the monomers in solution, where the lack of structural restraints originates a random distribution of the chromophores and prevents the occurrence of chiral dipolar interactions. Recently, we have observed that achiral macromolecular methacrylic porphyrin poly[M-TPP] [21], as well as the analogous

chiral derivatives poly[(S)-ML-TPP] and poly[(S,S)-MLL-TPP] [22], resulted promising for application as acid responsive organic materials. A similar behavior in response to pH changes is shown in dilute solution, as expected, also by the optically active copolymers reported here. In fact, the addition of increasing amounts of trifluoroacetic acid (TFA) to solutions of the copolymeric derivatives alters significantly their electronic spectra, turning immediately the reddish solution to a brilliant green. The same behavior is shown also in the solid state: exposure of thin films of polymeric compounds of (S)-ML-TPP and (S,S)-MLLTPP to a saturated HCl atmosphere generates an analogous color change of the materials from reddish to brilliant green. The process is completely reversible: after exposure to NH3 vapors the color turns identical to the original one. The UVeVis spectra of the copolymers, before and after exposure to HCl gas, are qualitatively identical to those shown by the homopolymers in solution. In particular, as reported in Fig. 6 (bottom) for poly[(S)-ML-TPP-co-MMA] 1:1, the intensity of the Soret band at around 420 nm decreases after exposure to HCl, and a new band centered about 440 nm, attributed to the formation of a protonated species, grows. At the same time, the intensity of the first three Q bands decreases, but that one at about 650 nm increases. It is noteworthy that pH changes caused variations not only on the UVeVis spectra of these materials, but also on their chiroptical properties. The CD spectra of protonated poly[(S)-ML-TPP], poly

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T. Benelli et al. / Dyes and Pigments 106 (2014) 143e153

Fig. 5. CD spectra in chloroform solution of poly[(S)-ML-TPP] (
), poly[(S)-ML-TPP-coMMA] 1:1 (-$-$), poly[(S)-ML-TPP-co-MMA] 1:3.6 ($$$), and poly[(S)-ML-TPP-coMMA] 1:29 (- - -).

[(S,S)-MLL-TPP], poly[(S)-ML-TPP-co-MMA] 1:1 and 1:3.6 and poly [(S,S)-MLL-TPP-co-MMA] 1:1.5 display in fact, similarly to the related solutions, a positive exciton couplet (two dichroic bands of opposite sign with a cross over point at around 440 nm), in correspondence to the UV maximum wavelength of the Soret band. The close similarity between CD spectra of the polymeric thin film (expressed as ellipticity) before and after addition of HCl, suggests that the macromolecules maintain similar chiral conformations also after exhaustive protonation of the porphyrin rings, without substantially affecting the dipoleedipole chiral interactions between side-chain chromophores. The reversibility of the protonationedeprotonation reaction of thin films, which is related to fatigue resistance properties after several exposure cycles to HCl and NH3 vapors, seems promising for their application as acid responsive organic materials. For example, after several exposure steps with acid and alkaline vapors alternatively acting on the films, the values of ellipticity (Q) are quite similar. Analogous result are obtained monitoring the color change in the UVeVis spectra. Thus, several cycles could be performed without any significant change of behavior. 3.6. Chiral recognition ability of Zneporphyrin complexes It was recently demonstrated that optically active homopolymeric systems bearing in the side chain Zn-TPP chromophores display significant ability to chiral recognition towards chiral diamines [14]. The complexation of the chiral guest, in fact, produces two different diastereoisomeric derivatives with consequent variation of their CD spectra. In the present study, the optically active porphyrinic repeating units have been progressively spaced out along the polymeric backbone through the insertion of the achiral MMA co-monomer, with the aim to study the effect of increasing the mean distance between chromophores on their complexation ability. To this purpose solutions of zinc-metallated copolymeric derivatives containing known amounts of (R)-(þ)- or (S)-(
)-1,2-

Fig. 6. CD (top) and UVeVis (down) spectra of a 130 nm thick film of poly[(S)-ML-TPPco-MMA] 1:1 on glass slide as prepared (e) and after exposure to HCl (- - -) vapors.

diaminopropane were prepared and submitted to UVeVis and CD spectra in solution. The experimental absorption data have been referred to two porphyrin moieties (Table 4), in order to allow a comparison with those reported in the literature for the so-called porphyrin tweezers [16]. The formation of the host/guest complex via coordination of the amine nitrogens to the zinc atoms linked to the side chain porphyrin moieties is revealed by the color change of the solutions, which turns from reddish to teal blue, and is confirmed by UVeVis and CD spectroscopy (Fig. 7). As reported in Fig. 7, in fact, upon addition of diamine to the intrinsically chiral zinceporphyrin polymer, the Soret band and the Q bands show a slight red shift (from 422 to 424 nm and from 552 and 594 nm to 563 and 605 nm, respectively) while a new band rises at 640 nm. Variations also occur in the CD spectra: by adding increasing amounts of (S)-(
)-1,2-diaminopropane (30e50 eqs), the positive CD exciton couplet originally present in the spectra of poly[Zn-(S)ML-TPP-co-MMA] 1:1 and 1:3.6, enhances in intensity, the crossover point undergoes a red shift (from 422 to 429 nm) and a new positive band arises at 370 nm (Fig. 8). In addition, and for the first

T. Benelli et al. / Dyes and Pigments 106 (2014) 143e153

Fig. 7. UV spectra in chloroform solution of poly[Zn-(S)-ML-TPP-co-MMA] 1:1 before (e) and after addition of 50 eqs of (R)-(þ)- (- - -) or (S)-(
)-1,2-diaminopropane ($$$).

time, three negative CD signals in the spectral region connected to the Q bands at 532, 592 and 639 nm are observed (Fig. 8). Conversely, upon addition of increasing amounts of (R)-(þ)-1,2diaminopropane (30e50 eqs), the positive CD couplet connected to the Soret band vanishes and a negative new one appears (Fig. 8). At the same time, a positive band arises at 370 nm and, in correspondence of the Q bands, three positive dichroic bands appear at 534, 590 and 639 nm (Fig. 8). The dependence of the observed UVeVis and chiroptical properties on sample concentration has also been investigated in the range 10
4e10
6 mol L
1, based on porphyrin repeating unit, by using variable cell pathlength. In these conditions, the UVeVis and CD spectra result, within the limit of experimental errors, practically unchanged, thus confirming that the observed spectroscopic behavior is related to individual polymeric chains and only originated by intra-chain interactions between chromophores. Similar behavior is shown by poly[Zn-(S)-ML-TPP-co-MMA] 1:29 with the only difference that, in this case, the CD spectrum is initially silent, due to the absence of chiral interactions between

151

chromophores in dilute solution. However, in the presence of (S)(
)- or (R)-(þ)-1,2-diaminopropane, respectively, a positive or a negative CD couplet does appear in the spectra, thus allowing the correlation with the absolute configuration of the diamine compound. Again, dichroic signals of opposite sign appear in correspondence of the Q bands and at 370 nm. The poly[Zn-(S,S)-MLL-TPP-co-MMA] copolymers also give qualitatively similar dichroic effects, although with a reduction of intensity, when compared to poly[Zn-(S)-ML-TPP-co-MMA]s. It therefore appears that, despite the decreased content of porphyrinic chromophores and consequent increase of their mean mutual distance with respect to the related homopolymers, these copolymeric materials result nevertheless capable to complex chiral diamines in solution. The intensity variation of the exciton couplet related to the Soret band as a function of the molar fraction of MMA (fMMA) in poly[Zn(S)-ML-TPP-co-MMA]s copolymers is visually represented in Fig. 9. Passing from the homopolymer poly[Zn-(S)-ML-TPP] to copolymers poly[Zn-(S)-ML-TPP-co-MMA] 1:1 and 1:3.6, the couplet intensity enhances in the presence of 30 equivalents of (R)-(þ)-1,2diaminopropane (Fig. 9(a)). This behavior could be attributed to the increased flexibility of the polymer backbone and to the consequent improved availability of side-chain porphyrin chromophores in solution (as demonstrated by UV analysis) which makes the diamine complexation easier with respect to the homopolymeric sample [38]. Spacing out further the zinceporphyrin moieties, as in poly[Zn(S)-ML-TPP-co-MMA] 1:29, the formation of the 1:1 sandwiched chiral hosteguest complex appears of reduced extent, because it involves folding of the backbone. Consequently, the CD couplet intensity fades, still allowing, however, to observe a chiroptical effect not much lower than in the homopolymer. The addition of (S)-(
)-1,2-diaminopropane to the copolymers, as expected, gives mirrored results with respect to those obtained with the dextrorotatory enantiomer, as shown in Fig. 9(b). Thus, the method results sensitive for the absolute configuration assignment to chiral diamine, requiring only few micrograms of the guest compound, as well as a copolymer with a very low content of Zneporphyrin functional moieties. In addition, being the amine complexation reversible, it allows to repeatedly use the polymeric Zneporphyrin system for further chiral recognition. Work is in progress to assess the applicability of this system to other classes of chiral compounds. 4. Conclusions

Fig. 8. CD spectra in chloroform solution of poly[Zn-(S)-ML-TPP-co-MMA] 1:1 before (e) and after addition of 50 eqs of (R)-(þ)- (- - -) or (S)-(
)-1,2-diaminopropane ($$$). For better comprehension, the parts marked with “10” are magnified ten times.

New methacrylic copolymers, bearing in the side chain optically-active L-lactic acid residues linked to a porphyrin moiety, containing variable content of methyl methacrylate co-units (MMA), have been obtained by radical copolymerization of the corresponding monomers. The presence of a chiral group of one single configuration interposed between the polymeric backbone and the porphyrinic chromophore provides these materials with the further possibility to assume a conformational dissymmetry, as revealed by their CD spectra. The presence of an exciton couplet, in fact, even if reduced in amplitude with respect to the related homopolymers, suggests that these macromolecules assume, in solution, chiral conformations of one prevailing helical handedness, at least for chain sections. These macromolecules maintain their chiral properties also as thin films in the solid state. In addition, they result promising for application as acid responsive organic materials. The zinc derivatives of these porphyrin copolymers result capable to complex the (S)-(
) and (R)-(þ)-1,2-diaminopropane enantiomers, as demonstrated by spectroscopic analysis. In particular,

152

T. Benelli et al. / Dyes and Pigments 106 (2014) 143e153

Fig. 9. Maximum intensities of the bisignate dichroic bands of the excitonic couplet related to the Soret band of poly[Zn-(S)-ML-TPP], poly[Zn-(S)-ML-TPP-co-MMA] 1:1, 1:3.6 and 1:29 vs the molar fraction of MMA (fMMA) in the copolymers after the addition of 30 eqs of (R)-(þ)- (a) and (S)-(
)-1,2-diaminopropane (b). Black (C) and white (B) circles represent the bisignate signals located at higher and lower wavelength, respectively.

positive or negative CD couplets are observed in the spectral region connected to porphyrin Soret band in the presence of the negative or the positive diamine enantiomer, respectively. The appearance of well distinct dichroic signals in the porphyrin Q bands spectral region with sign related to the added diamine enantiomer is also observed for the first time. Interestingly, the diamine complexation may take place even when a very low content of Zneporphyrin moieties is present in the copolymers. The method appears very sensitive and convenient, being the material ease to synthesize, and thermally stable and requiring only few micrograms of the guest compound. Acknowledgments Financial support from Consortium INSTM and PRIN (grant number 2010XLLNM3) 2010e2011 is gratefully acknowledged. References [1] Gross Z, Ini S. Remarkable effects of metal, solvent, and oxidant on metalloporphyrin-catalyzed enantioselective epoxidation of olefins. J Org Chem 1997;62:5514e21. [2] Collman JP, Zhang X, Lee VJ, Uffelman ES, Brauman JI. Regioselective and enantioselective epoxidation catalyzed by metalloporphyrins. Science 1993;261:1404e11. [3] Senge MO, Fazekas M, Notaras EGA, Blau WJ, Zawadzka M, Locos OB, et al. Nonlinear optical properties of porphyrins. Adv Mater 2007;19:2737e74. [4] Krivokapic A, Anderson HL, Bourhill G, Ives R, Clark S, McEwan KJ. Meso-tetraalkynyl porphyrins for optical limiting e a survey of group III and IV metal complexes. Adv Mater 2001;13:652e6. [5] Sun X, Li D, Chen G, Zhang J. A series of new porphyrin dyads: the synthesis and photophysical properties. Dyes Pigment 2006;71:118e22. [6] Ferreira JAB, Serra VV, Sánchez-Coronilla A, Pires SMG, Faustino MAF, Silva AMS, et al. The near-mid-IR HOMOeLUMO gap in amide linked porphyrinerhodamine dyads. Chem Commun 2013;49:8809e11. [7] Angiolini L, Benelli T, Cocchi V, Lanzi M, Salatelli E. Side chain porphyrin moiety linked to polymer-fullerene composite solar cell. React Funct Pol 2013;73:1198e206. [8] Huang X, Nakanishi K, Berova N. Porphyrins and metalloporphyrins: versatile circular dichroic reporter groups for structural studies. Chirality 2000;12:237e55. [9] Berova N, Nakanishi K. Exciton chirality method: principles and applications. In: Berova N, Nakanishi K, Woody RW, editors. Circular dichroism. Principles and applications. New York: John Wiley & Sons; 2000. pp. 337e82. [10] Takeuchi M, Imada T, Shinkai S. Highly selective and sensitive “sugar tweezer” designed from a boronic-acid-appended m-oxobis[porphinatoiron(III)]. J Am Chem Soc 1996;118:10658e9. [11] Mizutani T, Kurahashi T, Mukakami T, Matsumi N, Ogoshi H. An artificial receptor for dimethyl aspartate. J Org Chem Soc 1996;61:539e48. [12] Konishi K, Yahara K, Toshishige H, Aida T, Inoue S. A novel anion-binding chiral receptor based on a metalloporphyrin with molecular asymmetry. Highly enantioselective recognition of amino acid derivatives. J Am Chem Soc 1994;116:1337e44.

[13] Crossley MJ, Mackay LG, Try AC. Enantioselective recognition of histidine and lysine esters by porphyrin chiral clefts and detection of amino acid conformations in the bound state. J Chem Soc Chem Commun; 1995:1925e8. [14] Arimori S, Takeuchi M, Shinkai S. Sugar-controlled aggregate formation in boronic acid-appended porphyrin amphiphiles. J Am Chem Soc 1996;118: 245e6. [15] Imai H, Nakagawa S, Kyuno E. Recognition of axial ligands by a zinc porphyrin host on the basis of nonpolar interligand interaction. J Am Chem Soc 1992;114:6719e23. [16] Huang X, Rickman H, Borhan B, Berova N, Nakanishi K. Zinc porphyrin tweezer in host
guest complexation: determination of absolute configurations of diamines, amino acids, and amino alcohols by circular dichroism. J Am Chem Soc 1998;120:6185e6. [17] Kurtàn T, Nesna N, Li Y-Q, Huang X, Nakanishi K, Berova N. Chiral recognition by CD-sensitive dimeric zinc porphyrin host. 1. Chiroptical protocol for absolute configurational assignments of monoalcohols and primary monoamines. J Am Chem Soc 2001;123:5962e73. [18] Berova N, Pescitelli G, Petrovic AG, Proni G. Probing molecular chirality by CD-sensitive dimeric metalloporphyrin hosts. Chem Commun; 2009: 5958e80. [19] Proni G, Pescitelli G, Huang XF, Nakanishi K, Berova N. Magnesium tetraarylporphyrin tweezer: a CD-sensitive host for absolute configurational assignments of a-chiral carboxylic acids. J Am Chem Soc 2003;125:12914e27. [20] Anyika M, Acho R, Borhan B, Gholami H, Ashtekar KD. Point to axial chirality transfer e a new probe for “sensing” the absolute configurations of mono amines. J Am Chem Soc 2014;136:550e3. [21] Angiolini L, Benelli T, Giorgini L. Polymethacrylic zinc porphyrin: a new approach to chiral recognition. React Funct Pol 2011;71:204e9. [22] Angiolini L, Benelli T, Giorgini L. Novel optically active methacrylic polymers containing side-chain porphyrin moieties for chiral recognition. Polymer 2011;52:2747e56. [23] Perrin DD, Amarego WLF, Perrin DR. Purification of laboratory chemicals. Oxford: Pergamon Press; 1966. [24] Kelen T, Tüdos FJ. Analysis of the linear methods for determining copolymerizatiom reactivity ratios I. A new improved linear graphical method. Macromol Sci Part A Chem 1975;9:1e27. [25] Rao SP, Ponratman S, Kapur SL, Iyer PK. Kelen-tüdös method applied to the analysis of high-conversion copolymerization data. J Polym Sci Polym Lett Ed 1976;14:513e6. [26] Mayo FR, Walling C. Copolymerization. Chem Rev 1950;46:191e287. [27] Chiellini E, Solaro R, Galli G, Ledwith A. Optically active vinyl polymers containing fluorescent groups. 8. Synthesis and properties of copolymers of n-vinylcarbazole and (-)-menthyl acrylate and (-)-menthyl methacrylate. Macromolecules 1980;13:1654e60. [28] Peat IR, Reynolds WF. Application of 13C magnetic resonance spectroscopy to the determination of the tacticity of stereoregular poly (methyl methacrylate). Tetrahedron Lett; 1972:1359e62. [29] Di Marzo GA, Dowell F. Theoretical prediction of the specific heat of polymer glasses. J Appl Phys 1979;50:6061e6. [30] Gouterman M. Study of the effects of substitution on the absorption spectra of porphin. J Chem Phys 1959;30:1139e61. [31] Corwin AH, Chivvis AB, Poor RW, Whitten DG, Baker EW. Porphyrin studies. XXXVII. The interpretation of porphyrin and metalloporphyrin spectra. J Am Chem Soc 1968;90:6577e83. [32] Liao MS, Scheiner S. Electronic structure and bonding in metal porphyrins, metal ¼ Fe, Co, Ni, Cu, Zn. J Chem Phys 2002;117:205e19. [33] Wang R, Hoffman BM. Systematic trends in metalloporphyrin optical spectra. J Am Chem Soc 1984;106:4235e40.

T. Benelli et al. / Dyes and Pigments 106 (2014) 143e153 [34] Shelnutt JA, Ortiz V. Substituent effects on the electronic structure of metalloporphyrins: a quantitative analysis in terms of four-orbital-model parameters. J Phys Chem 1985;89:4733e9. [35] Hunter CA, Sanders JK. The nature of p
p interactions. J Am Chem Soc 1990;112:5525e34. [36] de Witte PAJ, Castriciano M, Cornelissen JJLM, Monsù Scoalro L, Nolte RJM, Rowan AE. Helical polymer-anchored porphyrin nanorods. Chem Eur J 2003;9:1775e81.

153

[37] van Hameren R, van Buul AM, Castriciano MA, Micali N, Schön P, Speller S, et al. Supramolecular porphyrin polymers in solution and at the solideliquid interface. Nano Lett 2008;8:253e9. [38] Matile S, Berova N, Nakanishi K, Fleischhauer J, Woody RW. Structural studies by exciton coupled circular dichroism over a large distance: porphyrin derivatives of steroids, dimeric steroids, and brevetoxin B. J Am Chem Soc 1996;118:5198e206.