Indian Journal of Chemi stry Vol. 41A, September 2002, pp. 1860-1 863

Synthesis of single strapped 21-thia tetraphenylporphyrin systems

P Boobalan , Iti Gupta & M Rav ikanth '

Department of Chemi stry, Indian In stitute of Technology, Powai , Mumbai 400076, India

I?eceived 8 October 200 1; revised 14 March 2002

Synthesis and characterization of two single strapped 2 1-thi a tetraphenylporphyrins have been reported . The two adjacent phenyls that are present at the meso position of 2 1-thia tetraphenylporphyrin are linked by either rigid aromatic group or flexible alkyl chain . Absorption and flu orescence studi es indicated that the ri gid aromatic group induced more nonplanarity in the porphyrin ring than fl ex ible alkyl chain.

The OCCUlTence of porphyrin rings in nature as ligands in such crucial and multiple roles as those spanned by the heme proteins, chlorophyll, vitamin B 12 etc suggest that some advantages might be associated with thi s macrocyclic structure l. One of the main reasons for nature to choose porphyrin li gands for doing such diverse functions in biology is th at the porphyrin macrocycle is conformationally tlex ible and can adopt a range of nonplanar conformations needed for a variety of biological functions. The X-ray structures solved for several biomolecules support this observation I. For example, the domed porphyrins present in several heme proteins involved In oxygen transport, perox ide reduction and di sproportionating the mitochondrial electron transport chain and drug metaboli sm. Nonplanar conformational di stortion of tetrapyrrole pigments of photosynthetic reaction centres have been suggested to control the photophysical properties. For a better understanding of the biological role of nonplanar porphyrins, many laboratories have embarked on studies of syntheti c nonplanar tetrapyrrole

I . 12 N I . . h macrocyc es til recent years'. onp ananty In t e porphyrin macrocycle has been induced by steri c constraints including the strapping, capping etc, changing the central metal ion , ring reduction and ox idation, core modification and crysta l pac king effects I. Core modification of porphyrin rings by introducing thiophene, furan , selenophene, tellurophene in place of pyrrole leads to novel core modified porphyrins which exhibit interesting

R M

8r 8 r

- C*C- 2H 1 H2 - H2 8r 8r

--(CH2m- 2H ~

properties in terms of both aromatic character and their ability to stabilize metal s in unusual oxidation states 1.3. The X-ray structures solved for thiaporphyrin and its metal derivatives indicates that the three pyrroles and thiophene are not in one plane and the porphyrin is nonplanar:1a.b. Recently, we are exploring core modified porphyrin chemistry and we have prepared I ight-harvesti ng systems,la.d, B-substituted porphyrins4" and water-soluble po rphyrin s~r. This paper reports the synthesis and characterisation of single strapped 21-thia tetraphenylporphyrin systems (Structure I) . The two adjacent lIleso-phenyls of the porphyrin ring are linked with either rigid aromatic group or long fl ex ible alkyl chain. I H NMR, absorption and fluorescence studies indicates that the rigid aromatic group induces more deformation in the porphyrin ring than the fl ex ible alkyl chain.

Experimental

Synthesis of 2 J -tliiatefraphelzylpo rphy rin (1) 2,5-Bis(phenylhydroxymethyl)thiophene (500 mg,

1.51 mmol) and 2,3,5,6-tetrabromo-p-phenylene dimethylenedioxybenzaldehyde (I g. 1.51 mmol ) were di ssolved in 250 ml propionic acid. Pyrrole (0.350 ml , 5.04 mmol) was added and the reacti on mixture was then refluxed for 2 h. The reaction

----

/ ,

NOTES 186 1

mixture was cooled to room temperature and left aside for overni ght. The bl ack crude compound was washed several times with hot water and dried. The crude solid was dissolved in CH 2C1 2 and slurry was prepared with silica gel. Thi s was chromatographed over silica gel (60-120 mesh) to remove nonporphyrini c materials from mi xture of porphyri ns. The porphyrin mixture which contains at least four other porphyrins along with the desired porphyrin was subjected to silica ge l chromatography using petrol eum etheriC l-I2C1 2 (7 :3) and eluted slowly . The first band was tetrapheny l-2 1,23-dithiaporphyrin (S2TPP) which was collec ted. The required strapped 2 1-monothiatetrapheny l porphyrin , 1 was moved as second band which was co ll ec ted and evaporated. Thi s was rechromatographed on si I ica gel usi ng the same solvent mixture to afford the pure compound. (60 mg, 3.6%). 11-1 NMR (COCll , 8 in ppm) -2.90 (s, 11-1 , NI-I), 4.95 (d, 21-1 , OCI-I2), 5.95 (d, 21-1 , OCI-I2), 7048 (m, 21-1 , Ar), 7.65 (d, 2H , A r), 7.79 (m, 61-1 , A r), 8.05 (m, 21-1 , Ar), 8.29 (m, 41-1 , Ar), 8.54 (s, 21-1 , ~­py), 8.59 (d, 2H , ~-py), 8.65 (m, 21-1 , ~-py), 9.72 (s, 21-1, ~-thi ophene) . FAB-MS CS2 1-1 3 IN lS02 Br~ ca lc. av mass 1081.52, obsd. mlz 108 1 (M+). Ana l. caled : C, 57 .7 ; 1-1 , 2.89 ; N, 3.89. Found : C, 58.6; 1-1 , 2.92; N, 3.96. UV-vis O"maxinm, EI I03 dm.1 mor 1cm- l

) 431 ( 111.0), 513 ( 11.0), 545 (2.9), 6 16 ( 104), 677 ( 1. 8). Fluorescence O\.cx = 5 15 nm, Am"x) 681, 753 (q) = 0.0089). UV-vi s (Am"xinm, EII03 dm3mol- 1cm- l

) for 11-132+ 457(74.0), 439(sh) (50.0), 551 (3.80), 603 (3.70), 669 (2 .80).

Syllthesis of 21-thiatet raphellylporphy rill (2 ) 2,5-Bi s(pheny lhydroxymeth y l)thiophene ( 100 mg, 0.337 mmol), 2,2 ' ( I ,6-hexy Idioxy -benzaldehyde) ( 135 mg, 00404 mmol), and pyrrole (0.062 ml, 0.895 mmol ) were condensed in 25 ml propionic acid fo llowed by puri ficati on under si mi lar col umn chromatographic conditions mentioned for the synthesis of 1 y iclded the required 2 (10 mg, 4%). 11-1 NMR (COCI3, 8 in ppm) - 2.63 (s, 11-1 , NI-I ), 1.25 (s, 81-1 , CI-I2), 3.78 (m, 4H, OCI-I2), 7042 (m, 21-1 , Ar), 7.78 (m, 81-1, Ar), 8. 13 (m, 21-1 , -Ar), 8.25 (m, 41-1, Ar), 8.65 (m, 41-1 , ~-py) , 8.83 (d, 21-1, ~-py), 9.71 (s, 21-1 , ~­th iophene). FAB-MS Cso 1-l3<) N3S02 calc. av mass 745.73, obsd. mlz 746 (M+). Anal. caled: C, 80.5; 1-1 , 5.27; N , 5.63. Found : C, 8 1.6; 1-1 , 5.92; N, 5.71 . UV­vis (Alllaxinm, Ell 03 dm3mol- 1 em- I) 430 ( 124.0), 516 ( 14.2), 550 (3 .9), 615 (2.3 ), 676 (3 .7). Fluorescence (Aex = 515 nm, Amax) 680, 752 (</> = 0.0124).

H&O 8r 8r Hyo r o- c t}-c-o~ + 3 o~

+ 1 '" I H;'}---( H, V N Sr Br H

Reflux 2 h

~-!J + Three N4 Porphynns

o 1

STPPH I : 'hca Gel Column Chromatography

l

Sc hcme I- Synlhelic scheme for the prcparalion o f I .

Results and discussion The single strapped 2 1-monothiaporphyrins, 1 and

2 were prepared as outlined in Scheme I. The required precursors thiophene diols and dialdehydes6 were prepared by the literature procedure. Condensation of one equivalent of thi ophene diol, one equi valent of dialdehyde and three equi valents of pyrrole in propionic ac id resulted in the formation of mi xture of porphyrins. The porphyrin mi xture was expected to contain four other porphyrin s along with the desired porphyrin 1 or 2. The TLC analys is showed an indication of formation of only two major compounds and two minor compounds. S2TPP was the maj or component along w ith the desired porphyrin and two N~ porphyrin s were formed in tracc amounts. Aftcr doing the filtrati on co lumn on silica gel with C1-I2C1 2

to remove the nonporphyrinic impurities , the mi xtu re of porphyrin s were loaded on silica gel co lumn and eluted slowly with pet etherldichloromethane mi xture (7:3). The desired porphyrins 1 and 2 were moved as a second band. Both 1 and 2 were characterized by 11-1 NMR, FAB mass, absorption and emission spectroscopies. In 11-1 NMR, both 1 and 2 showed a singlet for thiophene protons as in parent STPPI-I . However, the splitting of pyrrole signals was vari ed in

1862 INDI AN J CHEM, SEC A, SEPTEMBER 2002

both 1 and 2 compared to STPPH. All three pyrrole signal s in STPPH appeared as doublets and the pyrrole which was opposite to thi ophene ex peri enced more down field shift compared to the other two pyrroles7

. In 1, the pyrrole which was opposi te to thi ophene experienced more upfi eld shift indicating that the 2,3,5,6- tetrabromo xy lene strap inducing the nonplanarity in the porphyrin ring which resu lts in the shi ft of pyrrole protons due to change in the ring curren t. The other two py rroles al so showed little upfield shift and complex multiplet unlike the parent STPPH. However, the hexy l strap in 2 didn 't induce much structural change in the porphyrin as evidenced in no shift of pyrrole protons compared to STPPH . However, unlike doubl ets for pyrroles in STPPH , the pyrroles in 2 appeared as complex multipl et. The NH proton of 1 also experienced up field shift compared to STPPH and 2 indicating that the 2,3,5,6-tetrabromo p-xylene strap altering the structure of the porphyrin. The bridging -OCH2 protons in 1 appeared as two doublets unlike the singlet observed in corresponding dialdehyde suggesting that they are inequi valent in 1. The - OCH2 protons also ex peri enced shi fts due to ring current effect of the porphyrin ri ng. Thus, the' H NMR studies indicates that the structure of 1 is more altered due to rigid aromatic st rap than 2. The FAS mass showed molec ul ar ion peak at 108 1 for 1 and 746 for 2 confirming the products.

The absorption spectra of 1 and its dication, 1 H,2+ was recorded in toluene. It is seen that both 1 and 2 show four Q-bands and one Soret band with no shifts in the peak pos itions W.r.t. STPPH. However, the extinction coeffi cients of all bands were drasti ca lly reduced compared to that of STPPH supporti ng the change of structure in the porphyrin ring caused by the bridging groups. The change was more pronounced with 2,3,5,6-tetrabromo p-xy lyl strap indicating more nonplanarity in the porphyrin ring of 1 than in 2. The addition of trifluoroacet ic acid to the porphyrin in toluene results in the formation of dication, 1H/+ in which all three nitrogens have protons. The addition of protons to the inner nitrogens results in the steric crowding in the core. Due to crowding in the porphyrin core, the pyrro les prefers to tilt in such way that the pyrroles are almost in pl ane with phenyl groups. Due to thi s, the resonance interaction between meso-pheny l and porphyrin group increases which refl ects in red shifts in the absorpti on bands. This is the case of dicati on of STPPH which show only one Q-band which was red shifted co mpared to STPPH. However, the dication, 1H32+

showed three Q-bands which were blue shifted W.r. t 1 indicating that the meso-phenyl groups were not in plane with porphyrin ring. This is due to the strap which prevents the ro tati on of lIIeso-phenyl groups into the plane of the porphyrin rin"". The emi ss ion spectra of 1 and 2 recorded in toluene (68 1 and 750 nm) didn ' t show any shift compared to STPPH . However, the quantum yields measured for 1 (<I> =

0.0089) and 2 (<I> = 0.0124) are lower than STPPH (<I> = 0.0 168) indicating th at the straps causes some deformati on in the porphyrin ring. The reducti on in quantum yield was much lower for 1 than 2 suggesting that the 2,3,5,6-tetrabrorno p-xy ly l group causing more deformation. An add iti onal heavy halogen effect in 1 due to the presence of bromines on bridging phenyl also contributes to the low quantum yield of 1 than 2 and STPPH.

It can be concluded that two eli fferent si ngl e strapped 21-monothiaporphyrins were sy nthes ized for the first time. The st raps were varied from flexibl e hexy l chain to ri g id 2,3,5,6-tetrabro mo p-xy lyl groups. The ri gid aromatic bridging group caused more deformat ion In the porphyrin ring as evidenced in 'H NMR, absorption and flu orescence data. Efforts are being made sy nthes ize of dou bly strapped core mod ified porphyrin s, to study the effect of deformation in the porphyrin ring on ground and and exc ited properties of core modifi ed porphyrins.

Acknowledgement Financial ass istance from the Council of Scientific

ancllndustri al Research is gratefull y acknowledged.

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NOTES 1863

5 Pandian R P, Reddy D, Chidambaram N & Chandrashekar T K, Proc III ilia II Acad Sci, (Chelll Sci) 102 ( 1990) 307.

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Ravikanth M, Reddy D, Misra A & Chandrashekar T K, J chelll Soc Daltoll TraIlS, ( 1993) 11 37.

7 Pandian R P & Chandrashekar T K, J chem Soc Dalton Trails, (1993) 11 9.