Electronic and conformational features of derivatives of meso-thien-2-ylporphyrins on protonation and perbromination

Journal of Molecular Structure 1079 (2015) 486–493

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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc

Electronic and conformational features of derivatives of meso-thien-2-ylporphyrins on protonation and perbromination Rangaraj Prasath, Purushothaman Bhavana ⇑ Birla Institute of Technology and Science (BITS), Pilani, K.K. Birla Goa Campus, Goa 403726, India

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

 Electronic effects of meso-

substituents on the properties of the porphyrins.  Effect of orientation of mesosubstituents on the electronic properties of porphyrins.  Electronic properties of protonated and perbrominated thienylporphyrins.  Structural aspects of thienylporphyrins on protonation and perbromination.

a r t i c l e

i n f o

Article history: Received 12 August 2014 Received in revised form 21 September 2014 Accepted 21 September 2014 Available online 13 October 2014 Keywords: Thienylporphyrins Conformation Protonation Extended conjugation Sterics Imino hydrogen

a b s t r a c t The effect of substituents at the meso-position on the electronic and stereochemical properties of thienylporphyrins has been investigated by analyzing the spectra of series of porphyrins. The role of conformation in dictating the extent of electronic properties have been analysed both by electronic spectroscopy and 1H NMR spectroscopy. Changes in the electronic properties of the thienylporphyrins by bringing the conformational changes by protonation at the core and perbromination at the periphery of the macrocycle have been related to the properties of chlorophyll. Ó 2014 Elsevier B.V. All rights reserved.

Introduction The existence in different biological systems together with the applications in mimicking biocatalysts [1,2], in photodynamic therapy [3], in solar energy conversion and in optoelectronics

⇑ Corresponding author at: Department of Chemistry, Birla Institute of Technology and Science (BITS), Pilani, K.K. Birla Goa Campus, Zuarinagar, Goa 403 726, India. Tel.: +91 832 2580156 (O), +91 9421249883 (mobile); fax: +91 832 2557033. E-mail addresses: [email protected] (R. Prasath), juliebhavana@yahoo. co.in (P. Bhavana). http://dx.doi.org/10.1016/j.molstruc.2014.09.060 0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

[4,5] has made porphyrins always an interesting class of compounds for research. Electronic properties of porphyrins are highly tunable owing to the presence of a very electron rich central aromatic p-system and to the highly versatile structure on which various types of structurally and electronically different moieties can be appended. Though studied for decades, tuning the electronic properties of porphyrins is still attempted in research in order to achieve the extension of the p-electron cloud to the groups at the periphery. The significance of this type of work lies on the fact that molecules with extended p-electron cloud be potential candidates for nonlinear optics. Also molecules with high absor-

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bance in the visible region are vital for the solar energy conversion and as photosensitisers in photodynamic therapy. Derivatives of tetraphenylporphyrins have widely been studied as materials for these applications. In these compounds, there is a limitation in extending the ring conjugation effectively through the meso-position as the phenyl group is bulky and is nearly at an orthogonal disposition to the central porphyrin macrocycle (hereafter called core). For example, the electron withdrawing nature of aldehydic group has more effect on redox potentials when it is attached directly to the meso-position than on the phenyl ring at meso-position [6]. This limitation is generally overcome either by appending the groups of interest at the pyrrole b-position [7] or by using spacer like ethenyl at the meso-position [8–11] which will favour the conjugation between the aryl group and the core. Another

way to achieve the same is to use small aryl groups like thienyl as spacers at the meso-position that can come in conjugation with both parts of the molecule [12]. Brückner et al. [13] and Ghosh et al. [14] have reported the effect of smaller size in influencing the electronic interaction between the core and the meso-thienyl rings. Compared to the four Q bands that is generally observed for free base porphyrins, chlorophyll (which consists of peripherally modified porphyrin ring) has less number of electronic spectral bands in the same region [15]. The longest wavelength band in the visible region of chlorophyll is relatively intense. Mimicking these molecules is always an active area in research because of their crucial role in photosynthesis. A type of porphyrin that has a ‘modified’ (at the centre) porphyrin skeleton having similar electronic spec-

R1

2Th

NO 2

S N

NH

R2

R4

2Th5N

S CH3

2Th5Me

Br

2Th5Br

P

S

HN

N

Br

PBr

S H 3C

R3

2Th3Me S Compound

-R 1

-R 2

-R 3

-R 4

H2TaNThP

2Th5N

2Th5N

2Th5N

2Th5N

H2SThP (1a) H2TThP (1b) H2CThP (1c) H2TiThP (1d) H2TaThP (1e)

2Th 2Th 2Th 2Th 2Th

2Th5N 2Th5N 2Th 2Th 2Th

2Th5N 2Th 2Th5N 2Th 2Th

2Th5N 2Th5N 2Th5N 2Th5N 2Th

H2S5BrThP (2a) H2T5BrThP (2b) H2C5BrThP (2c) H2Ti5BrThP (2d) H2Ta5BrThP (2e)

2Th5Br 2Th5Br 2Th5Br 2Th5Br 2Th5Br

2Th5N 2Th5N 2Th5Br 2Th5Br 2Th5Br

2Th5N 2Th5Br 2Th5N 2Th5Br 2Th5Br

2Th5N 2Th5N 2Th5N 2Th5N 2Th5Br

H2S5MeThP (3a) H2T5MeThP (3b) H2C5MeThP (3c) H2Ti5MeThP (3d) H2Ta5MeThP (3e)

2Th5Me 2Th5Me 2Th5Me 2Th5Me 2Th5Me

2Th5N 2Th5N 2Th5Me 2Th5Me 2Th5Me

2Th5N 2Th5Me 2Th5N 2Th5Me 2Th5Me

2Th5N 2Th5N 2Th5N 2Th5N 2Th5Me

H2S3MeThP (4a) H2T3MeThP (4b) H2C3MeThP (4c) H2Ti3MeThP (4d) H2Ta3MeThP (4e)

2Th3Me 2Th3Me 2Th3Me 2Th3Me 2Th3Me

2Th5N 2Th5N 2Th3Me 2Th3Me 2Th3Me

2Th5N 2Th3Me 2Th5N 2Th3Me 2Th3Me

2Th5N 2Th5N 2Th5N 2Th5N 2Th3Me

H2SBrPhP (5a) H2TBrPhP (5b) H2CBrPhP (5c) H2TiBrPhP (5d) H2TaBrPhP (5e)

PBr PBr PBr PBr PBr

2Th5N 2Th5N PBr PBr PBr

2Th5N PBr 2Th5N PBr PBr

2Th5N 2Th5N 2Th5N 2Th5N PBr

H2SPhP (6a) H2TPhP (6b) H2CPhP (6c) H2TiPhP (6d) H2TaPhP (6e)

P P P P P

2Th5N 2Th5N P P P

2Th5N P 2Th5N P P

2Th5N 2Th5N 2Th5N 2Th5N P

Fig. 1. Chemical structures of nitro group containing free base porphyrins.

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tral behaviour as that of chlorophyll is protonated porphyrins. The incorporation of two more protons at the centre of the ring using protic acids makes the central macrocycle generally nonplanar in nature [16–18]. The radical changes in their electronic properties (without reducing one of the double bond at the periphery as in chlorin ring of chlorophyll) are attributed mainly to the conformational change of the molecule induced by the repulsion between the four protons at the centre. In our previous reports, we have explained the synthesis of a few porphyrins that can be used for further functionalisation by taking advantage of the synthetic versatility of the groups at the periphery of the porphyrins. A few among those were nitro substituted tetrathien-2-ylporphyrins derivatives [19]. The synthesis of these compounds was simple by nitrating the different thien-2ylporphyrins using the mild and easy-to-handle nitrating agent, cupric nitrate trihydrate. It was previously observed that the site of nitration using cupric nitrate mainly depend on the metal ion at the centre of the phenylporphyrin and in the case of Cu(II) or Ni(II) ions at the centre, the nitro group gets directed towards the pyrrole b-position of tetraphenylporphyrin [20,21]. Further study on thienylporphyrins showed that the conjugation between aryl rings at the meso-position and the core plays a major role in the site of nitration [22,23]. This was evident by the formation of meso-5-nitrothien-2-ylporphyrin and meso-3-nitro-5-methylthien-2-ylporphyrin on nitrating the respective parent compound with Cu(NO3)2. To understand the electronic aspects of the heteroarylporphyrins better, porphyrins having two different meso-groups (nitroaryl and aryl) have been synthesized and investigated spectroscopically. One of the meso-substituent in all the series of porpyhyrins in this study is 5-nitrothien-2-yl ring (2Th5N, Fig. 1). Nitro group was chosen as the substituent on the thienyl ring as it can give further insight to the electrophilic nitration reactions of various types of thienylporphyrins reported previously and also due to its rich synthetic chemistry [24,25]. To assess on the role of conformation of free base form of perbrominated thienylporphyrins in deciding their electronic properties [19], spectral investigations have also been carried out on conformationally similar protonated nitrothienylporphyrins. The chemical structure of the nitro group containing free base porphyrins in the present study is shown in Fig. 1. Experimental

mixture of TFA and CDCl3 or in a mixture of deuteriated dimethylsulfoxide (DMSO) and CDCl3 in the ratio 1:15 (v/v). Synthesis of nitro group containing free base porphyrins Free base form of all the nitro group containing porphyrins were synthesized by following Lindsey’s approach [26] with minor modification. In a typical procedure, to a mixture of 3-methylthiophene-2-aldehyde (0.82 mL, 7.6 mmol) and 5-nitrothiophene-2aldehyde (1.2 g, 7.6 mmol) in 600 mL of dichloromethane, pyrrole (1 mL, 15.2 mmol) was added under N2 atmosphere. To this, BF3 etherate (0.62 mL, 5.06 mmol) was added and the reaction mixture was stirred for one hour at room temperature. At the end of the period, p-chloranil (3.73 g, 15.2 mmol) was added and the stirring was continued for another half an hour. Following this, triethylamine (0.7 mL, 5.06 mmol) was added and further stirred for one hour. The reaction mixture was then adsorbed on 200 g of silica gel (230–400 mesh) and the mixture of porphyrins (H2TaNThP, 4a, 4b, 4c, 4d and 4e; Fig. 1) was eluted using chloroform. The solution was then concentrated by rotary evaporation and the porphyrins were separated by column chromatography using chloroform as the eluent. The yield of all products is given in supporting information. The compounds were characterized by UV visible, 1H NMR and mass spectral techniques. UV visible spectral details are given in Table 1. 1H NMR and mass spectral details of all the synthesized compounds are given in supporting information.

Results and discussion Synthesis All the porphyrins in the study are synthesized by following the procedure by Lindsey et al. [26] using the mixture of aldehydes (1:1 ratio). p-Chloranil was used as the oxidizing agent. Cis and trans compounds were found to form in higher yield compared to the other meso-‘tri + mono’ aryl and nitroaryl- and, tetra aryl/ nitroaryl-derivatives. Compounds were separated by column chromatography and characterized by UV visible, 1H NMR and mass spectral techniques. All the compounds showed well defined Soret and four Q bands of etio pattern (Table 1).

Materials

Electronic spectroscopy

Pyrrole, p-chloranil, trifluoroacetic acid (TFA) and various thiophene-carboxaldehydes were procured from Sigma–Aldrich and were used as received. Other required chemicals were procured from sd fine chemicals, India. Details of the synthesis of perbrominated thienylporphyrins is reported in literature [19].

UV visible spectroscopy is widely used to study the electronic properties of the porphyrins as they show a very attractive and unique absorption spectrum, which consists of Soret (B) band and visible bands (generally 4 in free base and 2 in metallo derivatives). The visible bands are also called Q bands. Gouterman’s four orbital approach explain the presence of B and Q bands, their relative intensity, multiplicity and also the spectral change on altering the porphyrin skeleton [27]. The shift in the spectral bands (both in position as well as in intensity) on substitution as well as on conformational changes can be explained by four orbital model. In this model, various spectral aspects were explained based on the one electron transition between the accidentally degenerate HOMOs (initially) and LUMOs. In the present work, the extent of shift in the electronic spectral bands on systematically varying the groups at the meso-position of the porphyrin has been investigated. Investigation was done on both the electronic effect (inductive and resonance) of substituents and the effect due to the conformational changes on the spectral properties of various free base porphyrins (different series) and of their protonated forms. Analysis of spectral changes in porphyrin series in their neutral form is made initially followed by a

Measurements 1 H NMR spectra were recorded on a Bruker AVANCE III 500 MHz spectrometer using tetramethylsilane as the internal standard. Optical absorption spectra were recorded on a JASCO V-570 model UV visible/NIR spectrophotometer using quartz cells of 1 cm path length. Electrospray mass spectra were recorded on a THERMO Finnigan LCQ Advantage max ion trap mass spectrometer. Protonated porphyrins were prepared by treating the free base porphyrin with TFA in the ratio 1:2. Further increase in the concentration of the acid in the mixture did not bring any further change in the electronic spectrum of the protonated form. For recording the NMR spectra, samples were prepared by dissolving porphyrin in CDCl3. For the study of the difference in the conformation of free base and protonated porphyrins, spectra were also recorded in

489

R. Prasath, P. Bhavana / Journal of Molecular Structure 1079 (2015) 486–493 Table 1 UV visible absorption spectral dataa (in nm) of various porphyrins in CH2Cl2.

a

Series

Porphyrin

Soret band

Q bands

fwhm

1a 1b 1c 1d 1e

H2TaNThP H2SThP H2TThP H2CThP H2TiThP H2TaThP

432 431 428 429 426 426

(5.50) (5.42) (5.43) (5.43) (5.51) (5.59)

522 523 523 523 522 523

(4.45), (4.34), (4.30), (4.35), (4.30), (4.30),

560 562 563 562 562 560

(4.12), (4.04), (4.05), (4.08), (4.02), (4.02),

596 596 596 596 596 597

(4.05), (3.95), (3.90), (3.99), (3.90), (3.88),

656 658 658 658 658 661

(3.46) (3.39) (3.55) (3.54) (3.48) (3.86)

30.73 29.45 26.11 26.77 21.46 16.56

2a 2b 2c 2d 2e

H2S5BrThP H2T5BrThP H2C5BrThP H2Ti5BrThP H2Ta5BrThP

432 431 431 430 429

(5.38) (5.50) (5.28) (5.47) (5.55)

523 523 524 524 524

(4.34), (4.37), (4.15), (4.27), (4.23),

561 563 563 564 562

(4.06), (4.08), (3.86), (3.97), (3.91),

597 596 597 597 597

(3.99), (3.97), (3.77), (3.85), (3.76),

657 658 658 658 658

(3.52) (3.46) (3.20) (3.31) (3.22)

29.30 25.75 26.32 22.05 17.80

3a 3b 3c 3d 3e

H2S5MeThP H2T5MeThP H2C5MeThP H2Ti5MeThP H2Ta5MeThP

432 433 434 432 431

(5.52) (5.58) (5.61) (5.40) (5.50)

522 524 525 524 526

(4.44), (4.41), (4.49), (4.15), (4.14),

560 566 565 567 567

(4.05), (4.15), (4.22), (3.90), (3.98),

596 597 596 596 597

(4.00), 658 (2.96) (3.97), 661 (3.38) (4.12), 659 (3.38) (3.68), 661 (3.60) (3.77), 662 (3.49)

29.20 28.40 28.35 21.68 18.49

4a 4b 4c 4d 4e

H2S3MeThP H2T3MeThP H2C3MeThP H2Ti3MeThP H2Ta3MeThP

431 429 429 427 425

(5.52) (5.66) (5.50) (5.55) (5.42)

523 522 523 522 520

(4.45), (4.55), (4.39), (4.38), (4.20),

561 562 561 560 556

(4.08), 596 (4.22), 596 (4.03), 596 (3.98), 596 (3.80), 596

(4.01), (4.10), (3.96), (3.92), (3.76),

656 656 656 655 661

(3.16) (3.54) (3.12) (3.22) (3.69)

29.45 26.43 26.86 21.17 15.87

5a 5b 5c 5d 5e

H2SBrPhP H2TBrPhP H2CBrPhP H2TiBrPhP H2TaBrPhP

428 423 424 420 419

(5.36) (5.42) (5.49) (5.55) (5.52)

521 518 519 517 515

(4.31), (4.25), (4.40), (4.33), (4.19),

559 557 557 554 551

(4.03), (4.08), (4.07), (4.03), (3.81),

594 593 592 591 590

(3.92), (3.84), (3.99), (3.84), (3.66),

652 651 650 648 646

(3.53) (3.75) (3.52) (3.60) (3.44)

29.10 24.37 25.03 18.20 12.78

6a 6b 6c 6d 6e

H2SPhP H2TPhP H2CPhP H2TiPhP H2TaPhP

428 422 423 419 417

(5.62) (5.65) (5.64) (5.66) (5.58)

521 518 520 517 515

(4.56), (4.50), (4.55), (4.45), (4.19),

560 558 558 554 550

(4.28), (4.38), (4.25), (4.18), (3.83),

594 593 592 591 591

(4.17), (4.11), (4.16), (4.02), (3.68),

652 651 650 647 647

(3.76) (4.10) (3.76) (3.85) (3.61)

30.84 25.75 26.98 18.28 14.22

The values in parenthesis refer to loge values, e in dm3 mol1 cm1.

discussion on the conformational aspects due to the protonation at the porphyrin core. Comparison of UV visible spectral properties of free base porphyrins In the following discussion, members of each series are abbreviated after the group other than 2Th5N group at the meso-position. One of the substituents at the meso-position(s) in all series is 2Th5N group. Electronic spectral measurements were carried out in dichloromethane and the details are summarized in Table 1. In series 6, meso-positions are occupied by P and 2Th5N groups. As the number of P groups increases in the series, it is observed that the absorption maximum of B band decreases and for H2TaPhP (commonly known as H2TPP in literature) the value is 417 nm. As the number of P groups increases, the conjugation of p-electrons between the core and the P groups decreases and so a blue-shift is observed. The requirement of coplanarity of the core with the groups present at the meso-position in bringing the red-shift of electronic spectral bands is further clear on analysing the absorption spectral maxima of molecules in series 5. In this series, PBr group at the meso-position cannot come effectively in conjugation with the core like the phenyl group in series 6. But the other group, i.e., 2Th5N group can easily come in conjugation with the core. So as expected and as in series 6, there is a blue-shift in the B band on increasing the number of PBr groups. Considerable difference in the spectral values of series 5 and 6 is not observed. The Soret band positions of the members of series 1 show a blue-shift on increasing the 2Th groups. As the number of 2Th5N group increases, there is an enhanced conjugation due to the increase in the number of electron withdrawing (mesomeric, -M) nitro groups. The full width at half maximum (fwhm) also increases with increase in the number of nitro groups on the molecule. There is a blue-shift in the Soret band position on increasing

the number of 2Th5Br groups at the meso-position in series 2. But compared to series 1, the extent of hypsochromic shift is less (H2TaNThP ? 2e, 3 nm). This less magnitude in the shift in the Soret band compared to that in series 1 can be due to the combined I (inductive) and +M (mesomeric) effect of the bromo groups on the thien-2-yl ring(s). It is known that the groups with electron withdrawing effect (I) shifts the absorption spectral bands hypsochromically. By comparing series 2 and 5, the effect of substituents on the aryl groups with different size on the electronic properties of the porphyrins is analysed. When the number of bromoaryl group decreases, the effect is higher in series 5 than that in series 2. The conjugation is not much affected in series 2 as all the groups at the meso-position are derivatives of thien-2-yl groups. Also the +M effect of bromo substituent(s) is considerably felt in this series. Effect of the size of the meso-aryl group is also clear on comparing the series 1 and 6. On going down the series, the number of unsubstituted aryl groups (2Th and P, respectively) increases (keeping 2Th5N as the other meso-substituent(s)) and the effect on the electronic properties is prominent in series 6. The difference in absorption maximum of Soret band of H2TaNThP and tetra-(6e) phenyl compounds is 15 nm whereas the same between H2TaNThP and tetra-(1e) thien-2-yl compounds is only 6 nm. The reason for the lesser effect in series 1 than in series 6 can be similar to that between series 2 and 5. Effect due to the orientation of the groups at the meso-position of the porphyrin on the electronic properties is investigated by comparing two ‘isomeric’ series 3 and 4. The difference between the analogous members of these series is in the position of the methyl group(s). There is an interesting trend in the shift in Soret band position on decreasing the number of 2Th5N groups on the

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molecules. In contrast to the series 1 and 2, as the number of 2Th5N groups on the molecule decreases, first Soret band got shifted to red region and then blue shifted in series 3. So, in effect, there is only one nanometer blue shift on moving from H2TaNThP to H2Ta5MeThP (3e) in the series (Fig. 2). As the nitro group decreases, the Soret band get blue shifted due to the lesser electron delocalization. This result shows that resonance effect of nitro has a prominent role over the inductive effect and mesomeric (through no-bond resonance) effect of the methyl group in dictating the shift of Soret band. Though methyl group(s) is present, the conformation has the major role in deciding the energy of the molecular orbitals of the members in series 4. The effect of electron donating inductive nature of methyl group(s) on the energy of molecular orbitals of porphyrin is expected to be higher in series 4 than in series 3 based on the lesser number of intervening bonds of methyl group with the core. The position of the methyl group in series 4 makes the corresponding thien-2-yl group of the molecule perpendicular/nearly perpendicular to the p-system at the centre. There is no drastic influence of 2Th3Me groups in dictating the electronic properties of the compounds. The observed blue shift on moving down the series may be due to the decrease in the number of 2Th5N groups. The initial increase in the Soret absorption in compounds 3a to 3b (or 3c) shows the considerable effect of 2Th5Me group on the properties of porphyrin. This effect is absent in series 4 only due to the completely off plane orientation of the 2Th3Me ring with the core which in turn shows the importance of conformation in influencing the electronic properties of porphyrins. The absence of resonance effect from methyl group due to the orientation of 2Th3Me ring is clear on comparing series 1 and 4. Decrease in absorption wavelength of Soret band due to decreasing number of groups other than 2Th5N groups is more series 4 than in 1 (though the difference is very less). Though an electron donating methyl group (no-bond resonance) is present, its effect on the orientation of the 2Th3Me group brings out the difference in the absorption maxima between the members H2TaNThP and 4e more than that between the analogous members in series 1. This trend confirmed the importance of resonance in the electronic spectrum of porphyrins. The UV visible spectra of H2Ta3MeThP were found to be same when recorded at 22 and 50 °C. This indicates that even at 50 °C, the 2Th3Me group is unable to overcome the rotational barrier (Fig. 3). Comparison of UV visible spectral properties of protonated porphyrins In general, there is a high red-shift in the electronic spectral bands of porphyrins on protonation of the core. In the case of para substituted tetraphenylporphyrin, red-shifted Soret band and blue-shifted Q bands were observed on protonation, but on correcting the data by considering the structural and electronic factors

Fig. 3. Optical absorption spectra of H2Ta3MeThP at 22 °C and 50 °C recorded in CHCl3.

of centre porphine, it is found that Q bands also undergo a bathochromic shift [16]. The red-shift of bands on protonation is ascribed to the nonplanarity of the core which in turn allows the aryl groups at the meso-position to come in plane with the core. The nonplanarity of the core affect the energy level of a1u orbital as well as the eg orbitals and this results in the red-shift of the Soret band. Shift in Q bands (which arises due to the a2u to eg transition) is accounted by the electronic effect of the substituents at the meso-position. In order to shed further light on the effect of extended conjugation due to the coplanarity/near planarity of the meso-substituents on the spectral properties (conformational aspects), members of series 1 and 6 were protonated using TFA and their electronic spectral shifts were monitored (Fig. 4, Table 2). In series 6, one of the groups at the meso-position, P, is highly offplane with the core due to steric reason. On increasing the amount of TFA in the solution of free base porphyrin, there was a reduction in the intensity of its Soret band. Intensity of the Soret band of protonated form increased gradually

R1

NH

2Th

HN R2

R4

P

S

NH HN

NO 2

2Th5N

S R3

Wavelength (nm)

433

series 3

Compound

431 429

series 4

427 425

0

1

2

3

4

Number of 3 - or 5- methylthien-2-yl groups at meso posion Fig. 2. Plot of the position of Soret band maximum with the number of 3- or 5methylthien-2-yl rings in series 3 and series 4.

-R 1

-R 2

-R 3

-R 4

H4TaNThP

2Th5N

2Th5N

2Th5N

2Th5N

H4SThP (7a) H4TThP (7b) H4CThP (7c) H4TiThP (7d) H4TaThP (7e)

2Th 2Th 2Th 2Th 2Th

2Th5N 2Th5N 2Th 2Th 2Th

2Th5N 2Th 2Th5N 2Th 2Th

2Th5N 2Th5N 2Th5N 2Th5N 2Th

H4SPhP (8a) H4TPhP (8b) H4CPhP (8c) H4TiPhP (8d) H4TaPhP (8e)

P P P P P

2Th5N 2Th5N P P P

2Th5N P 2Th5N P P

2Th5N 2Th5N 2Th5N 2Th5N P

Fig. 4. Chemical structures of protonated form of substituted meso-thien-2ylporphyrins.

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R. Prasath, P. Bhavana / Journal of Molecular Structure 1079 (2015) 486–493 Table 2 UV visible absorption spectral dataa (in nm) of various protonated porphyrins in CH2Cl2.

a

Porphyrin

Soret band

Q band

fwhm

7a 7b 7c 7d 7e

H4TaNThP H4SThP H4TThP H4CThP H4TiThP H4TaThP

474 473 470 470 465 459

(5.72) (5.59) (5.66) (5.64) (5.62) (5.55)

704 712 719 718 719 720

(5.01) (4.90) (4.98) (4.96) (4.94) (4.83)

31.56 30.30 29.40 29.39 25.84 20.77

8a 8b 8c 8d 8e

H4SPhP H4TPhP H4CPhP H4TiPhP H4TaPhP

466 457 458 447 437

(5.71) (5.73) (5.69) (5.70) (5.69)

692 688 679 667 654

(4.99) (4.92) (4.96) (4.95) (4.79)

32.05 27.54 32.28 26.93 20.66

Series 1 and 7

6 and 8

Groups at meso-posi ons 2Th

P

Shi in nm

2Th5N

Q

S

48 → 59

42 → 33

Q

S

2Th5N

48 → 7

42 → 20

Scheme 1. Difference in the position of Soret and of Qx(0, 0) bands for the extreme members in the respective free base and protonated series (": increasing; ;: decreasing).

The values in parenthesis refer to loge values, e in dm3 mol1 cm1.

(Fig. 5). Increasing the concentration of TFA beyond 2-fold did not bring any noticeable change in the electronic spectrum of the compounds. The difference in the positions of Soret as well as in Qx(0, 0) band of extreme members of each series on protonation were compared (difference in the Soret band positions of H4TaNThP and H2TaNThP is compared with that of 1e and 7e, for example) in order to quantify the extent of nonplanarity induced in different molecules. On protonation, Soret and Q bands of all compounds has shifted to the red region. As the number of 2Th5N group decreases, in series 1 and its corresponding protonated analogue, series 7, the difference in the positions of Soret band decreases [42 nm (H4TaNThP–H2TaNThP) ? 33 nm (7e–1e)] where as the difference in the positions of lowest energy electronic band increases [48 nm (H4TaNThP–H2TaNThP) ? 59 nm (7e–1e)] (Scheme 1, Table 3). But for series 6 and its protonated series 8, the difference in the positions of both Soret band [42 nm (H4TaNThP–H2TaNThP) ? 20 nm (8e–6e)] and lowest energy electronic band [48 nm (H4TaNThP–H2TaNThP) ? 7 nm (8e–6e)] decreases. It is known that on protonation, the core become highly nonplanar and this affects the energy of the a1u orbital that has electronic contribution from a and b carbons of the pyrrole. The transition responsible for the Soret band is a1u to eg orbitals. Comparing the trends in the shift of Soret band on protonation given above, it is understood that the effect of induced nonplanarity by protonation is higher in H2TaNThP (42 nm) than in H2TaThP (33 nm)

Fig. 5. Absorption changes of H2SThP with increasing concentration of TFA.

Table 3 UV visible absorption spectral shift (in nm) of protonated porphyrins from the respective free base. Porphyrin

Soret band

Lowest energy band

7a 7b 7c 7d 7e

H4TaNThP H4SThP H4TThP H4CThP H4TiThP H4TaThP

42 42 42 41 39 33

48 54 61 60 61 59

8a 8b 8c 8d 8e

H4SPhP H4TPhP H4CPhP H4TiPhP H4TaPhP

38 35 35 28 20

40 37 29 20 7

which in turn is more than that of H2TaPhP (20 nm). So the trend in extent of induced nonplanarity on protonation, based solely on the shift in Soret band, looks like H2TaNThP > H2TaThP > H2TaPhP. A comparison of lowest energy electronic band on protonation also has been done owing to the possibility in explaining the interaction between TFA/trifluoroacetate and imino hydrogens (at the centre of the molecule), and also to shed light on the electronic influences felt at the meso-carbons due to the substituents resulting from the induced nonplanarity in the molecule. The shift of lowest energy band on protonation is in the order H2TaThP > H2TaNThP > H2TaPhP. In nonplanar protonated porphyrins, since the hydrogen on all the four pyrrole nitrogens can bind to the TFA/trifluoroacetate, the a2u orbital which has electronic contribution from pyrrole nitrogen will get stabilized and so a blue shift is generally expected for lowest energy electronic band. Observed red shift shows, therefore, that the effect due to the nonplanarity of the core and in turn the coplanarity/near planarity of meso-aryl ring with the mean porphyrin plane together decide the position of the lowest energy electronic band of protonated porphyrins. Even in the protonated state, the phenyl rings of H2TaPhP are not in conjugation to the extent that (nitro)thienyl rings of H2TaNThP and H2TaThP are with their respective core. Also since the induced nonplanarity is less, the interaction of the imino hydrogens with the TFA/trifluoroacetate is less in H2TaPhP. Both these result in a relatively very less difference (only 7 nm) of lowest energy electronic band. A difference of 5 nm between the Qx(0, 0) band of H2TaNThP and H2TaThP in free base form itself was observed due to both I and M effect of nitro group (Table 1). Comparing with trend of shift in Soret band, difference in the shift of lowest energy band on protonation indicates that as more saddled the structure in H4TaNThP (saddle conformation is observed in most of the protonated porphyrin), more electron density is donated to TFA/trifluoroacetate. Even when nitro groups are present to extent the conjugation

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R. Prasath, P. Bhavana / Journal of Molecular Structure 1079 (2015) 486–493

L H

HN

N N

NH

2L H+

N

N N N

H

H

N

H L Scheme 2. Representation of imino hydrogen interacting with two TFA/trifluoroacetate in nonplanar protonated forms (L = TFA or trifluoroacetate).

effectively in the molecule, in effect, only 48 nm shift is observed after overcoming the effect due to the stabilization of a2u orbital (Scheme 2). However, the 59 nm shift observed in the case of H2TaThP can be attributed to the in-plane orientation of the meso2Th ring. The stabilization of a2u in this molecule due to interaction with TFA/trifluoroacetate will be less as this molecule is less nonplanar than H2TaNThP. Comparison of 1H NMR spectral properties of porphyrins 1 H NMR spectral data was also analysed in order to delineate the effect of various thien-2-yl groups at the meso-position in extending the conjugation in porphyrin. Cases where there is no interference by the group on the thieny-2-yl ring (2Th, 2Th5N, 2Th5Me and 2Th5Br) in dictating the orientation of the group at the meso-position with respect to core, signal corresponding to pyrrole b-proton is singlet and appeared at 9.07 ppm or downfield to that. In H2Ta3MeThP, methyl groups make the pyrrole b-proton signal multiplet in nature due to the possible atrop isomers (unable to separate by column chromatography). Also, related to the above mentioned molecules, the signal of the pyrrole b-proton is relatively upfielded (8.91–9.09 ppm). This is due to the lesser electronic conjugation between the meso-groups with the core and in turn the relatively less ring current (anisotropy). The almost similar chemical shift value for pyrrole b-protons (8.89–8.93 ppm) in the 1H NMR spectrum recorded at 50 °C showed that the 2Th3Me groups in H2Ta3MeThP could not overcome the rotational barrier. This is further reflected in the position of signal for imino hydrogens which appeared at 2.62 ppm and 2.60 ppm in spectra recorded at 22 °C and 50 °C respectively. It is known that in cases where there is enhanced ring current as a result of extended conjugation, generally the imino hydrogen signal get upfielded and pyrrole b-protons get downfielded [28,29]. The enhanced nonplanarity of H2TaNThP compared to H2TaThP on protonation is also investigated by 1H NMR spectroscopy. Spectra were recorded in CDCl3 in presence of DMSO (deuteriaed) and TFA separately. DMSO is a highly coordinating solvent and can coordinate with the imino hydrogens of the core. The chemical shift of imino hydrogen signal in sample containing DMSO with respect to that in CDCl3 alone is almost same for both H2TaThP (2.64 ppm in CDCl3, 2.65 ppm in mixture of CDCl3 and DMSO) and H2TaNThP (2.82 ppm in CDCl3, 2.76 ppm in mixture of CDCl3 and DMSO). The nitro group on the H2TaNThP is not influencing the conformation of the core of the molecule and so the imino hydrogens behave almost similar to that in H2TaThP. Coordination of imino hydrogens in the planar form with oxygen of the DMSO is not to a great extent, but is similar in both the molecules. But the chemical shift values of imino hydrogens are very different for the samples which contain TFA. On protonation using TFA (as

indicated by electronic spectroscopy and supported by shift in imino hydrogen signals of both the protonated molecules in the 1 H NMR spectra), the conformational changes of the molecules seems to be considerably different. Relatively enhanced conformational change (induced nonplanarity of the core) makes the coordination of all the Hs (of NH) of H4TaNThP with the TFA/ trifluroacetate moiety more effective compared to that in H4TaThP. This resulted in the considerable downfield shift of NH signals of H4TaNThP (signals at d 0.44 ppm and 0.69 ppm for 2H each due to the two different interacting species TFA and trifluoroacetate). The difference of the position of these two signals from the signal in DMSO containing sample is 3.20 and 3.45 ppm, respectively and that only in CDCl3 is 3.26 and 3.51 ppm, respectively. In the 1 H NMR spectrum of H4TaThP, a broad signal appeared for NH near to the signal of the reference, tetramethylsilane. Here the exact integration of the signal was not possible, and the difference of the position of this signal from that in DMSO containing sample is approximately 2.71 ppm. These differences in the values clearly indicate that the nonplanarity induced due to the protonation of the core is higher in H2TaNThP. The 1H NMR spectral difference of H2TaNThP and H2Ta3MeThP in presence of TFA gives clear indication on the difference in the planarity of these porphyrins on protonation. The difference in the chemical shift of imino proton signal of H2Ta3MeThP is 2.13 ppm (d at 2.62 ppm in CDCl3, 0.49 ppm in TFA containing CDCl3 solution of the sample) on protonation. This lower shift of H2Ta3MeThP on protonation compared that of H2TaNThP shows that though getting protonated, the extent of deformation is less in H2Ta3MeThP compared to that in H2TaNThP. The presence of methyl group at the third position of the thien-2-yl rings hinders the core from being highly nonplanar. This can be attributed to the repulsion between methyl group and b-H of the pyrrole ring [30–33]. The imino protons of less deformed core has relatively upfielded signal (compared to that of H4TaNThP). Modifying the basic skeleton by reduction (as in chlorin) and protonating the core are the two ways to alter the electronic properties of porphyrins considerably. Highly altered electronic properties can also be obtained for thienylporphyrins by perbromination at the periphery (pyrrole b-position) [19]. By these methods, the core is neither getting reduced at the double bond (Cb@Cb) nor getting protonated at the core. Porphyrins (free base) with

Fig. 6. Optical absorption spectra of 5,10,15,20-tetrakis(5-bromothien-2-yl)porphyrin (solid line) and 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(5-bromothien-2-yl)porphyrin (dashed line) recorded in CH2Cl2.

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R. Prasath, P. Bhavana / Journal of Molecular Structure 1079 (2015) 486–493 Table 4 UV visible absorption spectral dataa (in nm) of perbrominated tetrahalothien-2-ylporphyrins in CH2Cl2. Porphyrin b

H25ClThOBP H25BrThOBPc H2OBPd H2TaPhPe a b c d e

Soret bands (nm)

Q bands (nm)

366 365 369 417

654 654 571 515

(4.61), 484 (5.46) (4.41), 484 (5.22) (4.37), 469 (5.23) (5.58)

(4.42), (4.19), (3.88), (4.19),

760 761 627 550

(4.14) (3.93) (4.07), 746 (3.85) (3.83), 591 (3.68), 647 (3.61)

The values in parenthesis refer to loge values, e in dm3 mol1 cm1. 2,3,7,8,12,13,17,18-Octabromo-5,10,15,20-tetrakis(5-chlorothien-2-yl)porphyrin. 2,3,7,8,12,13,17,18-Octabromo-5,10,15,20-tetrakis(5-bromothien-2-yl)porphyrin. 2,3,7,8,12,13,17,18-Octabromo-5,10,15,20-tetraphenylporphyrin. 5,10,15,20-Tetraphenylporphyrin (commonly known as H2TPP in literature).

considerably modified electronic spectrum (like in protonated cases) by bromination can be obtained if the groups at the mesoposition are smaller group like thienyl (compared to phenyl) (Fig. 6). The reduced number of Q bands of these perbrominated porphyrin shows that it behaves more like chlorins (Table 4). The intensity gain of higher wavelength band on perbromination support this fact and indicates that it will be one of the methods to achieve porphyrins that can be used for further functionalisation to get molecules with high absorption in a higher wavelength (without reducing the double bond or protonating the core).

Conclusions The 2Th ring at the meso-position has high influence in dictating the electronic properties of free base as well as the protonated form of porphyrin compared to that of P groups. Electronic and 1 H NMR spectral study showed that the 2Th3Me group in H2Ta3MeThP is perpendicular to the core and it is not coming in conjugation effectively even in the protonated form. The rotational barrier for 2Th3Me group in this molecule is so high that even at 50 °C, these groups are not in effective conjugation with the remaining part of the molecule. The similar electronic behaviour of perbrominated thien-2-ylporphyrin and protonated thien-2ylporphyrin with that of chlorophyll related systems shows that the careful selection of groups of appropriate size at the meso-position of the porphyrin and their drastically altered conformation by functionalisation at the peripheral b-position can lead to new molecules which are potential candidates for biomimetic and optoelectronic studies.

Acknowledgements PB gratefully acknowledges Council of Scientific and Industrial Research (CSIR), India for research grant (02(0076)/12/EMR-II). RP acknowledges the Senior Research Fellowship received from CSIR (09/919(0014)/2012-EMR-I).

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2014. 09.060.

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