Preparation of Single Substituted Phenyl Porphyrins Form Meso-Tetraphenyl Porphyrin-Synthetic Example from Symmetric Porphyrin into Asymmetric Porphyrins

Two asymmetric porphyrins, 5-(4-chloromethylphenyl)-10, 15, 20-triphenyl porphyrin and 5-(4-formylphenyl)-10, 15, 20-triphenyl porphyrin, were successfully prepared by the symmetric meso-tetraphenyl porphyrin and relative molecular configurations and properties were characterized by spectral determinations. This work presented an example for synthesis of asymmetric porphyrin derivatives from the symmetric porphyrin. Both asymmetric porphyrins are reactive in molecular assembly, the concerned reactions including alkylation with Grignard reagents, etherification with alcohols, aldol condensation and Mannich reaction for modification and enhancing their functionality. In this work, the reaction conditions were improved, synthetic strategy and route were confirmed.


Introduction
Matured technique for preparation of single substituted porphyrin was based on the Adler's method by using condensation of pyrrole with different ratio of substituted benzaldehydes.The resulted asymmetric porphyrins were excellent candidates in assembly of molecular devices, such as molecular wires [1] [2], sensitive reagent in photodynamic therapy (PDT) for tumors and cancers [3] [4], dyes in dye sensitized solar cells [5] [6].In practice, the porphyrin reagents must Open Journal of Inorganic Chemistry be water-soluble and compatible with organisms, and therefore the asymmetric porphyrins must be modified to satisfy the condition in PDT.For overcoming the low yield limitation in synthesis of asymmetric porphyrins, a symmetric meso-tetraphenyl porphyrin (TPP) was selected as initiator, and an asymmetric porphyrin was successfully prepared by inducing a single substituent in para-position of one phenyl in the symmetric meso-tetraphenyl porphyrin.The resulted asymmetric product was 5-(4-chloromethylphenyl)-10, 15, 20-triphenyl porphyrin (ClMPTPP) with an active chlorine, and series of single substituted porphyrins can be derived from this asymmetric product, such as its derivative 5-(4-formylphenyl)-10, 15, 20-triphenyl porphyrin (FPTPP).According to reference method [7], the improved synthesis route was described in Scheme 1.
After cooling down and diluting with water until complete precipitation, the produced rough product in purple was separated by filtration and dried in a vacuum oven.Furthermore purification was carried out by chromatography as above description with a mix eluent of chloroform/mineral ether (v/v = 3/1), the second purple band was collected and after vacuum evaporation pure 5-(4-formylphenyl)-10, 15, 20-triphenyl porphyrin (FPTPP) in purple color was obtained in yield of 89% (0.5 g, 0.8 mmol). 1    (ClMPTPP, dash) and meso-tetraphenyl porphyrin (TPP, solid) in CH 2 Cl 2 solution.For FPTPP: 421nm (Soret), 511, 568, 611 and 652nm (Q).For ClMPTPP: 412 nm (Soret), 512, 545, 587 and 644 nm (Q).For TPP: 413 nm (Soret), 511, 545, 587 and 644 nm (Q).shapes and positions.But owing to formyl replacement, the spectrum for FPTPP displayed much differently both in shape and positions.The Soret band became more broadly, and all absorption peaks appeared with clear red-shift, excepting Q band at 511 nm, the Q band at 652 nm nearly disappeared.This broadly spectral change can enhance light absorption of porphyrin derivatives, and in assembly of photosensitized molecular devices, the formyl substituted porphyrin should be considered preferentially.The influences of substituent replacement were more effectively reflected in photoluminescence.After replacement of one H atom in TPP by chloromethyl and formyl to form ClMPTPP and FPTPP, the emission spectra for the latter two were quenched in large scale (Figure 2), although this H atom was just at the para position of one peripheral phenyl!In the inner figure, both emission spectra for ClMPTPP and FPTPP were enlarged nearly 100 times.The reason for fluorescence quenching is arranged to be explored.

Results and Discussions
The compared EPR spectra for 5-(4-chloromethylphenyl)-10, 15, 20-triphenyl porphyrin (ClMPTPP, broad line) and meso-tetraphenyl porphyrin (TPP, thin line) in solid state were showed in Figure 3.Both TPP and ClMPTPP displayed a strong radical signal around 0.35 T orientated from the unpaired π electron which was stabled by conjugated porphyrin ring; zero field splitting in porphyrin molecule resulted the asymmetric EPR signal.After substitution by chloromethyl, the EPR peak was furthermore split by H atoms in methylene (-CH 2 -Cl), the coupling result between single π electron and protons.

Conclusion
Two single substituted porphyrins, the asymmetric 5-(4-chloromethylphenyl)-10, 15, 20-triphenyl porphyrin (ClMPTPP) and 5-(4-formylphenyl)-10, 15, 20-triphenyl porphyrin (FPTPP) were successfully prepared from the symmetric meso-tetraphenyl porphyrin (TPP).The resulted asymmetric porphyrins were well characterized by spectral method.Photoluminescence experiment indicated that after chloromethyl and formyl replacement, the emission spectra were quenched in large scale for both asymmetric porphyrins.These asymmetric porphyrins can be selected as intermediates to assemble molecular blocks, such as porphyrin dimmers with bridge group of glycols, Grignard reagents, polyaldehydes, polyketones, and even polyamines.The porphyrin dendrimer can also be derived from these asymmetric porphyrins with bridging molecules possessing multiple side groups.These products would be potential molecule in photoelectric applications.

Figure 1
Figure 1 was the compared UV-Vis spectra for FPTPP, ClMPTPP and TPP.All porphyrins presented one Soret band and four Q bands.Since only a hydrogen atom in para position of one peripheral phenyl of TPP was replaced by chloromethyl to form ClMPTPP, their UV-Vis spectra displayed high similarity in

Figure 3 .
Figure 3.The compared EPR spectra in solid state for ClMPTPP (broad line) and TPP (thin line).