Cyanobuta-1,3-dienes as electron acceptors for photoinduced charge separation systems

The electrochemical and photophysical properties of three multicomponent systems (1-3) featuring a Zn(II) porphyrin linked to aniline-substituted pentacyano- (PCBD) or tetra-cyanobuta-1,3-dienes (TCBD), with and without a spacer, are presented. [1] By means of steady-state and time-resolved spectroscopy, photoinduced intramolecular energy and electron transfer processes are evidenced, upon excitation of the porphyrin unit. These results indicate that the strongly electron-accepting cyanobuta-1,3-dienes might become promising alternatives to quinone-, perylenediimide-, and fullerene-derived acceptors in multicomponent systems featuring photoinduced electron transfer. Unfortunately, these electron acceptors are not luminescent and their lowest singlet excited state (S1) decays to S0 within few picoseconds, preventing a simple and straightforward photophysical characterization of any system featuring cyanobuta-1,3-dienes as electron acceptors. This is also a significant drawback if cyanobuta-1,3-dienes are to be considered as active materials in photovoltaic devices where generation of charge-separated states must compete with internal deactivation processes. By means of CASSCF calculations we were able to locate an accessible S1/S0 conical intersection for these acceptor molecules, which is responsible for their observed fast non-radiative deactivations. [2] Further studies are currently ongoing to determine viable strategies to chemically modify these acceptors and avoid fast non-radiative deactivations, so as to grant photophysical and electrochemical properties that can make these strong electron donors attractive for optoelectonic applications. [1] F. Tancini, F. Monti et al., Chem. Eur. J. 2014, 20, 202-216. [2] CISSAS, ISCRA project (class C2) at CINECA: http://www.hpc.cineca.it/services/iscra