Unexpected multiple activated steps in the excited state decay of some bis(phenylethynyl)-fluorenes and -anthracenes

The temperature effect on the photophysical parameters of four acetylene-derivatives [bis(phenylethynyl)-anthracenes and -fluorenes with substituents of different electron acceptor efficiencies] has been investigated by absorption and emission spectroscopy, using stationary and pulsed (ns/fs resolution) techniques. The nature of the central nucleus (anthracene or fluorene) and the peripheral electron-withdrawing group (nitro or formyl) strongly affect the deactivation of the excited states of these push-pull molecules. In some cases the study evidenced an interesting role of two activated steps in the deactivation of the excited singlet state, namely an activated inter-system crossing to an upper triplet state of n,π* nature (previously hypothesized on the basis of TD-DFT calculations) and a sort of activated internal conversion, discussed also on the basis of maximum entropy method analysis of the fluorescence decay data. Nicely, an efficient ISC was found for the fluorene-derivatives where small energy gaps between S1 (π,π*) and Tn (n,π*) states had been calculated while no activated ISC was evidenced in the case of anthryl-derivatives where higher S1-Tn energy gaps are expected. A peculiar temperature effect for a fluorene-derivative was pointed out and also explained on the basis of quantum-mechanical calculations at the DFT level taking into account the solvation effects by means of the conductor-like polarizable continuum model CPCM. The presence of dual emission, at first evidenced by a shoulder in the emission spectrum of the fluorene-derivative featuring a peripheral formyl group in dichloromethane at low temperatures, was nicely confirmed by femtosecond up-conversion measurements at room temperature.