Effect of Chlorin Structure on Theoretical Electronic Absorption Spectra and on the Energy Released by Porphyrin-Based Photosensitizers

Abstract

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Marcela Palma^†, Gloria I. Crdenas-Jirn*^† and M. Isabel Menndez Rodrguez*^‡

Laboratorio de Qumica Terica, Departamento de Ciencias del Ambiente, Facultad de Qumica y Biologa, Universidad de Santiago de Chile, USACH Casilla 40, Correo 33, Santiago, Chile, and Departamento de Qumica Fsica y Analtica, Facultad de Qumica. Universidad de Oviedo, C/Julin Clavera 8, 33006 Oviedo, Asturias, Spain

J. Phys. Chem. A, 2008, 112 (51), pp 13574–13583

DOI: 10.1021/jp804350n

Publication Date (Web): November 24, 2008

Copyright © 2008 American Chemical Society

†

Universidad de Santiago de Chile.

,

* Authors to whom correspondence should be addressed: e-mail gloria.cardenas@usach.cl (G.I.C.-J.) or isabel@uniovi.es (M.I.M.R.).

,

‡

Universidad de Oviedo.

Abstract

In this work eight porphyrins (p) and eight chlorins (c) are theoretically characterized [BLYP/6-31G(d)] in their singlet and triplet states. Nine of them (1_p, 1_c, 2_p, 3_p, 4_p, 5_p, 6_c, 7_c, and 8_c) have already been synthesized and are in trial use in photodynamic therapy (PDT). The seven remaining were built up as chlorins analogous to porphyrins 2_p−5_p and porphyrins analogous to chlorins 6_c−8_c. The aim is to investigate the effect of the chlorin structure on the Q-band of electronic spectra at BLYP/6-31G(d) (gas phase, methanol solution) and at BHANDHLYP/6-31+G(d) (methanol solution), and on the triplet → singlet energy emission, as these two factors determine the quality of a good photosensitizer. It is found that meso substituents lead to greater geometry distortions than β-substituents in both porphyrins and chlorins and in both singlet and triplet states. In methanol solution, chlorin-like structures with β substitution present significantly red-shifted Q-bands in comparison with their porphyrin analogues, so they would be better photosensitizers than porphyrins. Concerning to the triplet → singlet energy emission calculated in methanol solution, three porphyrins (4_p, 6_p, and 8_p) and all the studied substituted chlorins could be useful to generate active ¹O₂. 4_c would be the best photosensitizer, as it absorbs the largest wavelength in the therapeutic window (approximately 690 nm) and releases the amount of energy closest to the required one (1.22 eV).

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Re 636 nm spectral poperties

Absorption Spectra of Formylchlorins

The absorption spectra of the formylchlorins (in toluene at room temperature) are summarized in Table 2. Three classes are noted: zinc chlorins bearing a 10-mesityl group (ZnC-M¹⁰ series); zinc chlorins bearing a 5-p-tolyl group and a 10-mesityl group (ZnC-T⁵M¹⁰ series); and various free base chlorins (FbC series). Selected absorption spectra of the ZnC-M¹⁰ series are shown in Figure 1 (see Supporting Information for additional spectra).

Table 2 Spectral Properties of Chlorins.a

Figure 1 Absorption spectra (normalized) in toluene at room temperature of ZnC-M10 (trace a, λQy 606 nm),17ZnC-M10F13 (trace b, λQy 634 nm), ZnC-F5P10 (trace c, λQy 650 nm), and ZnC-F3M10F13 (trace d, λQy 667 nm).

The long-wavelength absorption band can now be tuned over the range of 606 – 667 nm.

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