Photochemistry of squaraine dyes:  excited triplet state and redox properties of crown ether squaraines

Abstract

The excited state and redox properties of three crown-ether derivatives of squaraines have been studied. Direct laser excitation of these dyes leads to the formation of excited triplets, dye radical cations and anions, via a biphotonic photoionization mechanism. All these transient species absorb strongly in the visible region. The triplet states of these dyes generated via triplet-triplet energy transfer from ³(1-pyrenecarboxaldehyde)^* and ³C₆₀^* photosensitizers also produce dye radical cations and anions via a self-quenching process. The bimolecular rate constants for the self-quenching process are in the range of (0.5-1.9) × 10⁹ M^-1 s^-1. The redox properties of these dyes have been investigated by electrochemical, spectroelectrochemical, and radiolytic methods. They exhibit two reversible oxidations in the potential range of 0.7-1.2 V vs SCE. The spectral features of electrochemically generated one electron oxidation product (λ_max = 680 nm) match those of dye radical cations produced by photochemical and radiolytic methods.