Electronic structures of azulene-fused porphyrins as seen by magnetic circular dichroism and TD-DFT calculations Katsunori Nakai a , Kei Kurotobi b , Atsuhiro Osuka b, * , Masanobu Uchiyama c , Nagao Kobayashi a, * a Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan b Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan c The Institute of Physical and Chemical Research, Riken, Wako-shi, Saitama 351-0198, Japan Received 28 June 2007; received in revised form 29 July 2007; accepted 20 September 2007 Available online 28 November 2007 Abstract A combination of magnetic circular dichroism (MCD), electronic absorption spectroscopy and time-dependent density functional the- ory (TD-DFT) calculations has been used to investigate the electronic structure of azulene-fused p-expanded porphyrins based primarily on the spectral properties of absorption bands in the near infrared region. From MCD experiments, it was suggested that in the case of a mono-azulene-fused porphyrin DHOMO ’ DLUMO (where DHOMO is the magnitude of the energy gap between the HOMO and HOMO–1 and DLUMO is the magnitude of the energy gap between the LUMO and LUMO+1), while in the case of an oppositely- di-azulene-fused porphyrin, DHOMO < DLUMO. Since Faraday A terms are observed for both the Soret and Q bands in the MCD spectrum of tetra-azulene-fused porphyrin the corresponding excited states are clearly accidentally degenerate despite the C 2 molecular symmetry. Transition dipole moment analysis clearly demonstrates that the electronic absorption spectrum of tetra-azulene-fused por- phyrin has out-of-plane electronic transitions slightly to the blue of the main Q and Soret bands. Comparison with distorted porphyrins and phthalocyanines strongly suggests that these out-of-plane transitions appear as intense Gaussian-shaped Faraday B terms in the MCD spectra. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Porphyrin; Azulene; Magnetic circular dichroism; TD-DFT 1. Introduction Photodynamic therapy (PDT) is currently being studied intensively in the diagnosis, management, and treatment of various neoplasms, based on the selective uptake of a pho- tosensitizer into the malignant tissues followed by local irradiation with either visible or near-infrared light [1–4]. The first photosensitizer in commercial use was Photo- frin TM , which is a complex mixture of fractions derived from hematoporphyrin derivatives. Although some promising results have been obtained with Photofrin TM , it has a num- ber of key disadvantages. These include the small size of its extinction coefficient at 630 nm, the wavelength of radi- ation normally used, as well as a tendency to accumulate in the skin and remain there for a prolonged period of time. This has led to an examination of many possible alternative photosensitizers. It is believed that, upon irradiation, the PDT process involves the photochemical generation of sin- glet oxygen and the subsequent oxidation of tissue based on direct attack of biological substrates. Recently, two- photon absorption (TPA) has attracted interest as an alter- native to the conventional one-photon absorption (OPA) PDT approach. A key advantage is that light absorption by the skin is avoided [5–7], since there is simultaneous absorption of two photons in the near-infrared region 0162-0134/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jinorgbio.2007.09.012 * Corresponding authors. Fax: +81 75 753 3970 (A.O), fax: +81 22 795 7719 (N.K). E-mail addresses: osuka@kuchem.kyoto-u.ac.jp (A. Osuka), nagaok@ mail.tains.tohoku.ac.jp (N. Kobayashi). www.elsevier.com/locate/jinorgbio Available online at www.sciencedirect.com Journal of Inorganic Biochemistry 102 (2008) 466–471 JOURNAL OF Inorganic Biochemistry