SQUARAINE CHEMISTRY - ABSORPTION, FLUORESCENCE EMISSION, AND PHOTOPHYSICS OF UNSYMMETRICAL SQUARAINES

The absorption, fluorescence emission, and photophysics of ethoxypheny l)-4'-[(dimethylamino)phenyl]-squaraine and its derivatives (USq1-USq1 4), a class of unsymmetrical donor-acceptor-donor (D-A-D) compounds, h ave been investigated. Similar to symmetrical and pseudo-unsymmetrical (with two different aniline rings) squaraines, USq1-USq14 also exhibi t intense absorption bands in the visible region. Their absorption max ima (lambda(max)) range from 562 to 593 nm and are blue-shifted relati ve to those of symmetrical and pseudo-unsymmetrical squaraines. The bl ue-shift is attributed to the decrease in the D-A-D CT character, aris ing from the introduction of a less electron-donating anisole ring. As it turned out, the introduction of asymmetry through the anisole ring has a significant impact on the electronic spectra. The asymmetry in USq1-USq14 enhances vibronic coupling during electronic transition, pr oducing vibrational fine structures in both absorption and fluorescenc e spectra. The vibronic coupling is particularly pronounced when the C -O group in the central four-membered ring is H-bonded, either intramo lecularly or intermolecularly with solvent molecules. USql-USq14 are s hown to form solvent complexes with solvent molecules. The multiple fl uorescences observed for these compounds, although quite complex, are shown to be the sum of the vibronic bands of the unsymmetrical squarai ne and its complex. At room temperature in solution, the phi(f) values for USq1-USq14 are a factor of greater than or equal to 30 lower than those of symmetrical and pseudo-unsymmetrical squaraines. Their lifet imes are less than or equal to 0.25 ns. There is a large temperature e ffect on both phi(f) and lifetime. At 77 K in a 2-methyltetrahydrofura n matrix, the phi(f) values approach greater than or equal to 0.5 and the fluorescence lifetimes increase to a constant value, similar to 2. 4 ns. Hydroxy unsymmetrical squaraines are an exception; they exhibit biexponential decays (similar to 2.4 ns and a subnanosecond decay) at 77 K in 2-methyltetrahydrofuran. The observation may be attributable t o a conformational effect. The large temperature effect on the photoph ysical processes indicates that there exists an efficient radiationles s decay process for these compounds. Evidence is provided that rotatio n of the C-C bond between the anisole ring and the four-membered ring is responsible for the radiationless decay. The fast decay is attribut able to the single-bond character of the rotating C-C bond in these co mpounds. Substituents, both in the aniline ring and the anisole ring, are shown to have effects on the electronic spectra. These substituent effects are discussed in terms of their effects on the D-A-D CT chara cter of the squaraine chromophore.