Structure and Physicochemical Properties of Dibenzoporphycene

Porphycene is one of the constitutional porphyrin isomers, which was first prepared by Vogel and co-workers in 1986.¹⁾  Molecular structure of porphycene where two bipyrroles are linked via two ethylene bridges has a rectangular shape.  Over two decades, a series of spectroscopic and structural studies have shown that the structural nature of porphycenes provides remarkably different physicochemical properties from that observed in the corresponding porphyrins.²⁾  Furthermore, a lot of porphycene derivatives with peripheral substituents at the β-position of the pyrrole rings have been reported, whereas the modification of the ethylene bridges between two bipyrrole moieties has quite limited.  Our group has recently adopted benzene moieties as a linker between two bipyrrole instead of ethylene bridges and prepared benzene-fused porphycene.³⁾

     It is known that porphycene without any peripheral substituents gives characteristic Q-bands at 550–620 nm with relatively strong absorbance compared to porphyrin derivatives.  Therefore, porphycene derivatives could be useful for attractive materials such as PDT and optical devices.  Some groups further devoted their attentions to obtaining NIR absorption and introduced benzene moieties into the β-positions of pyrrole rings.  For example, Yamada et al. successfully shifted the Q-band at 590–670 nm upon the introduction of a benzene moiety at each pyrrole ring.⁴⁾  In contrast, our group found that only one or two benzene moieties fused into each ethylene bridge between two bipyrroles gave a typical bathochromic shift: Q-bands of monobenzoporphycene and dibenzoporphycene appeared at 600–730 nm and 610–1040 nm, respectively, in CH₂Cl₂. This finding suggests that the HOMO–LUMO gap is significantly reduced upon the introduction of benzene moieties in the porphycene framework, which is also supported by the redox potentials of dibenzoporphycene: E_ox = +0.32 V and E_red = –0.49 V (vs Ag/AgCl, CH₂Cl₂). 

     X-ray crystal structure analysis reveals that dibenzoporphycene shows a characteristic rectangular geometry with a flat plane.  In addition, the distance of the inner nitrogen atoms (N1–N4 and N2–N3) of dibenzoporphycene is determined to be 2.534 Å, which is remarkably shorter than that of unsubstituted porphycene (2.63 Å).¹⁾  The obtained distance suggests that the strong NH–N hydrogen bonds are formed in the core of the porphycene.  Furthermore, the characteristic hydrogen bonding is supported by ¹H NMR measurement: the chemical shifts of the inner NH for dibenzoporphycene and unsubstiuted porphycene appear at 8.83 ppm and 3.15 ppm, respectively, in CDCl₃.  In this presentation, we report the synthesis and physicochemical properties of benene-fused porphycene.

1)  E. Vogel, M. Kocher, H. Schmickler, J. Lex, Angew. Chem. Int. Ed., 25, 257–259 (1986).

2)  J. Waluk, In Handbook of Porphyrin Science; K. M. Kadish, K. M. Smith, R. Guilard, Eds.; World Scientific publishing: Singapore, 2010; Vol. 7, pp 359–435.

3)  K. Oohora, A. Ogawa, T. Fukuda, A. Onoda, J. Hasegawa, T. Hayashi, submitted for publication. 

4)  D. Kuzuhara, J. Mack, H. Yamada, T. Okujima, N. Ono, N. Kobayashi, Chem. Eur. J., 15, 10060–10069 (2009).