Spectroscopic properties of a triarylpyrylium cation

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Abstract
The spectroscopic properties of a 2,4,6-triarylpyrylium cation are investigated both theoretically and experimentally. Theoretical calculations based on CS-INDO (closed shell intermediate neglect of differential overlap) and Cl (configuration interaction) methods show that, at room temperature, the rotation of the aryl substituents is hindered; at the equilibrium conformation, two of the aryl groups lie in the plane of the pyrylium ring while the third forms an angle of 40° with this plane. The two lower-lying singlet excited states are characterized by very different dipole moments; the corresponding electronic transitions, very close in energy (ΔE= 3190 cm–1), are polarized in the plane of the pyrylium core and are orthogonal to one another. Those transitions cannot be distinguished in absorption spectra of solutions. However, their existence is confirmed by fluorescence polarization measurements carried out on solid matrices; the spectrum of each transition is thus resolved and the difference in their energy is found to be ΔE= 2600 cm–1. The described spectroscopic properties are used in order to obtain information about the chromophore orientation in Langmuir–Blodgett films from the polarized absorption spectra. The experimental results confirm the existence of transitions close in energy and polarized along different directions.