Photophysics of Squaraine Dyes: Role of Charge-Transfer in Singlet Oxygen Production and Removal

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2010-02, Vol.114 (7), p.2518-2525
• Main Authors: Salice, Patrizio, Arnbjerg, Jacob, Pedersen, Brian Wett, Toftegaard, Rasmus, Beverina, Luca, Pagani, Giorgio A, Ogilby, Peter R
• Format: Article
• Language: English
• Subjects: A: Kinetics, Spectroscopy; Cyclobutanes - chemistry; Electrochemistry; Fluorescent Dyes - chemistry; Molecular Structure; Phenols - chemistry; Photochemistry; Quantum Theory; Singlet Oxygen - chemistry; Stereoisomerism
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp911180n

The unique optical properties of squaraines render these molecules useful for applications that range from xerography to photodynamic therapy. In this regard, squaraines derived from the condensation of nitrogen-based heterocycles and squaric acid have many promising attributes. Key solution-phase photophysical properties of six such squaraines have been characterized in this study. One feature of these molecules is a pronounced absorption band in the region ∼600−720 nm that has significant spectral overlap with the fluorescence band (i.e., the Stokes shift is small). As such, effects of emission/reabsorption yield unique excitation wavelength dependent phenomena that are manifested in quantum yields of both fluorescence and sensitized singlet oxygen production. Comparatively small squaraine-sensitized yields of singlet oxygen production and, independently, large rate constants for squaraine-induced deactivation of singlet oxygen are consistent with a model in which there is appreciable intra- and intermolecular charge-transfer in the squaraine and squaraine−oxygen encounter complex, respectively. The results reported herein should be useful in the further development of these compounds for a range of oxygen-dependent applications.