A combined theoretical and experimental investigation on the influence of the bromine substitution pattern on the photophysics of conjugated organic chromophores.

A large series of structurally related two-photon photosensitizers with heavy atom substitution were synthesized and evaluated through a combined spectroscopic (steady-state and time resolved), photophysical and computational study. Our aim was to identify some relevant parameters related to their excited state dynamics including photo-induced singlet oxygen generation. Although these dynamics result from the interplay of many factors, we show that the triplet excited state generation kinetics can generally be correlated with the calculated values of both the spin-orbit coupling and the energy gap between S1 and T1 states, which themselves mostly depend on the positioning of the heavy atoms along the π-conjugated structure rather than their number.

Photophysical, amplified spontaneous emission and charge transport properties of oligofluorene derivatives in thin films.

We investigate the photophysical and amplified spontaneous emission properties of a series of monodisperse solution-processable oligofluorenes functionalized with hexyl chains at the C9 position of each fluorene unit. Thin films of these oligofluorenes are then used in organic field-effect transistors and their charge transport properties are examined. We have particularly focused our attention on the influence of oligofluorene length on the absorption and steady-state fluorescence spectra, on the HOMO/LUMO energy levels, on the photoluminescence lifetime and quantum yield as well as on the amplified spontaneous emission properties and the charge carrier mobilities. Differential scanning calorimetry and X-ray diffraction measurements demonstrate that, among all oligofluorene derivatives used in this study, only the structure and morphology of the pentafluorene film is significantly modified by a thermal treatment above the glass transition temperature, resulting in a 9 nm blue-shift of the fluorescence spectrum without significant changes in the photoluminescence quantum yield and in the amplified spontaneous emission threshold. In parallel, hole field-effect mobility is significantly increased from 8.6 × 10(-7) to 3.8 × 10(-5) cm(2) V(-1) s(-1) upon thermal treatment, due to an increase of crystallinity. This study provides useful insights into the morphological control of oligofluorene thin films and how it affects their photophysical and charge transport properties. Moreover, we provide evidence that, because of the low threshold, the tunability of the amplified spontaneous emission and the photostability of the films, these oligofluorenes are promising candidates for organic solid-state laser applications.