Spectroscopic Properties of Porphyrin-Like Photosensitizers: Insights from Theory Laurence Petit, ²,‡ Angelo Quartarolo, ² Carlo Adamo,* ,‡ and Nino Russo ² Dipartimento di Chimica and Centro di Calcolo ad Alte Prestazioni per Elaborazioni Parallele e Distribuite Centro d’Eccellenza MURSTsUniVersita ` della Calabria, I-87030 ArcaVacata di Rende, Italy, and Laboratoire d’E Ä lectrochimie et Chimie Analytique, CNRS UMR-7575, E Ä cole Nationale Supe ´ rieure de Chimie de Paris, 11 rue P. et M. Curie, F-75231 Paris Cedex 05, France ReceiVed: September 5, 2005; In Final Form: NoVember 21, 2005 Electronic absorption spectra of six porphyrin-like photosensitizers, porphyrin, chlorin, bacteriochlorin, pheophytin a, porphyrazin, and texaphyrin, have been calculated within the time-dependent DFT framework (TDDFT) in conjunction with the PBE0 hybrid functional. Energetic and orbital aspects are discussed by comparing systems together so as to assess the best molecules for photodynamic therapy applications. Excitation energies and oscillator strengths are found to be in good agreement with both experimental data and previous theoretical works. In particular, whereas significant discrepancies (0.3 eV) appear for Qx bands, results become more reliable as wavelengths decrease. To elucidate the effect of the local environment, we have taken into account solvation either with explicit water molecules interacting via hydrogen bonds with the system or with a continuum model (C-PCM). The supramolecular approach does not affect spectra, while using C-PCM improves Qx and B band values and strengthens intensities significantly. In both gaseous and aqueous phases, texaphyrin, pheophytin a, and bacteriochlorin Qx bands are found in the 600-800 nm range as expected by experimental works. These data are particularly interesting in the perspective of systematic studies of other photosensitizers and should make experimentalists’ works easier. 1. Introduction Porphyrins and their derivatives are commonly found in nature, and they play very important roles in various biological processes. 1-3 For instance, the heme involved in oxygen transport and storage is based on the complexation of iron by a porphyrin ring. Recently, there has been an increasing interest in the synthesis and characterization of porphyrin-like com- pounds (e.g., porphyrin, chlorin, bacteriochlorin, porphyrazine, pheophytin, and texaphyrin) due to their use in photodynamic therapy (PDT). 4-6 The basic principle 1 is to inject into the blood a photosensitizing agent that is going to gather into cells. This photosensitizer can be activated under irradiation, and the energy is then transferred to nearby molecules via a radiationless transition. In particular, triplet molecular oxygen ( 3 O 2 ) can be excited into the singlet state that is cytotoxic and can then destroy possible cancerous cells. The required excitation energy is quite low, about 1 eV (around 1300 nm), and can be easily obtained through several photosensitizers. Within this perspec- tive, numerous experimental works have been devoted to the synthesis of new photosensitizers designed for their absorption at high wavelengths, some of them being currently used for clinical applications. 5,6 At the same time, tetrapyrroles have been the subject of detailed theoretical studies with the aim to reproduce and rationalize their peculiar electronic characteristics. 7-27 The first theoretical studies are dated back to the late 1940s by Kuhn 21 and Simpson. 22 Semiempirical approaches were then attempted, 23-25 and the progressive development of quantum methods nowadays enables obtaining of very good agreement with experimental data for these kinds of systems. Among these techniques, the time-dependent extension of the density func- tional theory TDDFT 28 has proved its efficiency in the evalu- ation of electronic spectra for a wide range of compounds, including porphine rings. 7,18,20 Calculations of vertical excitation energies and oscillator strengths are indeed less cumbersome than highly correlated ab initio methods and provide comparable accuracy. 29 Such advantages are particularly interesting when dealing with large molecules such as tetrapyrroles. Moreover, the large number of possible candidate photosensitizers 4 makes it necessary for experimentalists to have a routine methodology to select only the most promising molecules. In this paper, we have undertaken the systematic study of the excited states of six porphyrin-type systems by using the TDDFT approach, i.e., free base porphyrin (FBP), free base chlorine (FBC, also called dihydroporphyrin), free base bacte- riochlorin (FBBC, also called tetrahydroporphyrin), pheophytin a (Pheo a), porphyrazine (Pz, also called tetraazaporphyrin) and texaphyrin (Tex1 and Tex2). Their structures are sketched in Figure 1. These systems should be sufficiently stable in aqueous solution and preserve their spectroscopic properties in biological media. Consequently, any realistic theoretical analysis should be carried out by taking into account the interactions with the local environment (i.e., solvent) in order to give meaningful insights on the elementary excitation mechanisms. Solvent effects on electronic properties have thus been taken into account either by considering a few explicit water molecules (supramo- lecular approach) around the macrocycle, or implicitly by using a continuum solvent model. The first approach is mandatory because protic solvents (like water) could be involved in H-bonding with the solute, while the reaction field of the bulk solvent could significantly modulate the spectral properties of the systems. 30,31 * Corresponding author. E-mail: carlo-adamo@enscp.fr. ² Universita ` della Calabria. ‡ ENSCP. 2398 J. Phys. Chem. B 2006, 110, 2398-2404 10.1021/jp055016w CCC: $33.50 © 2006 American Chemical Society Published on Web 01/12/2006