Photophysical and electrochemical characterization of new optoelectronic materials

V. SPINELLA; H. MONTEJANO; K-T. WONG; L. OTERO; F. FUNGO; C. CHESTA

Los Cocos, Argentina

Congreso; 9thLatin American Conference oh Physical Organic Chemistry; 2007

Conferencia Latinoamericana de Fisicoquímica Orgánica

Optoelectronic is the area of science that studies and develops new materials (and devices) able to work as electro-optic transducers. The electro-optic transduction phenomenon relies on the generation of luminescent excited states from the annihilation of radical ion pairs, charged species formerly (electro)generated in the bulk of the solid material (a) as it is shown bellow:Thus, the efficiency of the transduction depends on an important number of concomitant processes, such as the capacity of charge transport of the organic material (b), the thermodynamic feasibility of annihilation reaction (c) and the emission efficiency of the generated excited states (d). The two last processes are closely related to the redox and photophysical properties of the D/A pair; and therefore, a study of these D/A properties can be used to predict the capacity of a new material to work as electro-optic transducer.We report here a characterization of compounds L1 and L2, which were recently synthesized for optoelectronic applications. The oxidation and reduction potentials of L1 and L2 were measured in acetonitrile using the cyclic voltammetry technique. The free energy change for process (b) was estimated to be ~ -2.61 eV and ~ -2.64 eV, for L1 and L2, respectively. Both compounds showed a strong absorption around 400 nm (~10-8 x104 M-1cm-1). The emission spectra exhibited a remarkable red shift with increasing solvent polarity. The values of obtained from typical Lippert-Mataga plots were ~1.15 eV, which indicate a large internal charge transfer (ICT) character of the singlet excited state. Fluorescence quantum yields () were measured in a series of solvents covering a large range of solvent polarity () using both, quinine bisulphate and eosin as fluorescence standards. L2 showed the largest (0.90 in c-hexane and 0.21 in acetonitrile). Fluorescence lifetimes were determined using the TCSPC technique. The t   (~ 4-2 ns) showed a slight decrease with increasing polarity of the medium. Approximated values for the singlet excited states energies () were obtained from the absorption and emission spectra in the different media.  values measured in c-hexane were ~2.85 eV and ~2.90 eV for  L1 and L2, respectively.  Steady state photolysis of the compounds in c-hexane and acetonitrile did not show evidences of trans / cis isomerization of the azo moiety. Laser flash photolysis (µsec time scale) and phosphorescence experiments (EPA at 77 K) suggested the existence of very short lived triplet excited state which energy must be lower that 2.2 eV. The information obtained in the experiments described above was used for constructing relative energy diagrams which involve the different redox and excited state in the transduction process. Interestingly, it can be concluded that these spiro-fluorene compounds should behave as fairly poor electro-optic transducers. Changes in the molecular structures leading to an improvement of their performance as transducers are also discussed.