Fabio Rizzo

Polar fluorenes and spirobifluorenes: fluorescence and fluorescence anisotropy spectra.

An extensive spectroscopic study is presented of two asymmetrically substituted fluorene dyes and of the related spiro dimers in both liquid and glassy solvents. Essential-state models are developed to accurately reproduce linear absorption and fluorescence spectra and their complex dependence on solvent polarity. The same models are exploited to quantitatively calculate fluorescence excitation and anisotropy spectra in rigid matrixes. Impressive red-edge effects observed for spiro dimers in glassy polar solvents are accurately reproduced and understood. Interchromophore interactions in the spiro dimers are very small, leading to marginal effects in absorption and fluorescence spectra, but they effectively promote energy transfer between the two chromophores, as best evidenced comparing anisotropy spectra of the substituted fluorenes and of corresponding spiro dimers dissolved in glassy solvent matrixes.

From Blue to Green: Fine-Tuning of Photoluminescence and Electrochemiluminescence in Bifunctional Organic Dyes

We describe the synthesis, computational analysis, photophysics, electrochemistry and electrochemiluminescence (ECL) of a series of compounds formed of two triphenylamines linked by a fluorene or spirobifluorene bridge. The phenylamine moieties were modified at the para-position of the two external rings by electron-withdrawing or electron-donating substituents. These modifications allowed for fine-tuning of the photoluminescence (PL) and ECL emission from blue to green, with an overall wavelength span of 73 (PL) and 67 (ECL) nm, respectively. For all compounds, we observed a very high PL quantum yield (79-89%) and formation of stable radical ions. The ECL properties were investigated by direct annihilation of the electrogenerated radical anion and radical cation. The radical-ion annihilation process is very efficient and causes an intense greenish-blue ECL emission, easily observable even by naked eye, with quantum yield higher than the standard 9,10-diphenylanthracene. The ECL spectra show one single band that almost matches the PL band. Because the energy of the annihilation reaction is higher than that required to form the singlet excited state, the S-route is considered the favored pathway followed by the ECL process in these molecules. All these features point to this type of molecular system as promising for ECL applications.