Photochemistry of squaraine dyes: excited states and reduced and oxidized forms of 4-(4-acetyl-3,5-dimethylpyrrolium-2-ylidene)-2-(4-acetyl-3,5-dimethylpyrrol-2-yl)-3-oxocyclobut-1-en-1-olate
Abstract
Singlet and triplet excited states of 4-(4-acetyl-3,5-dimethylpyrrolium-2-ylidene)-2-(4-acetyl-3,5-dimethylpyrrol-2-yl)-3-oxocyclobut-1-en-1-one (SQ) have been investigated by picosecond and nanosecond laser flash photolysis. Both singlet and triplet excited states of SQ have distinct absorption bands in the visible region. The fluorescence yields and singlet lifetimes were higher in CH2Cl2(ϕf= 0.08, τs= 222 ps) than in polar solvents such as CH3CN (ϕf= 0.04, τs < 100 ps). The triplet excited states of the dye were characterized by both direct excitation (ϕT= 0.02 in CH2Cl2) and T–T sensitization models. The oxidized (SQ.+) and reduced (SQ.–) forms of the dye exhibited absorption maxima at 505 nm (in CH2Cl2) and 435 nm (in acetonitrile). The results of the photoelectrochemical reduction of SQ in colloidal TiO2 suspension are also presented.