Novel donor/acceptor setup for dye sensitized solar cells based on an aliphatic bridge. Computational design and experimental essay.

DIEGO DUSSO; E. LAURA MOYANO; PRISCILA A. LANZA-CASTRONUOVO; ALEJANDRO R. PARISE; CRISTINA L. RAMIREZ; CARLOS A. CHESTA

Villa Carlos Paz

Congreso; XIII Elafot.; 2017

Dye sensitized solar cells (DSSCs) are usually based on a rod molecule connected to a semiconductor or nanoparticle. The rod organic dye bears a donor (D) and an acceptor (A) moieties connected through a conjugated  system bridge (D--A).[1] Upon photoexcitation, the charge separation drawn in the lowest singlet (S1) state (or in the relaxed S1* state) is harnessed to store electrons into the semiconductor. An efficient charge separation in the S1 state is the first requirement for a working DSSC, appart from many other factors which could enhace or deplete the efficiency, the fast charge recombination (back electron transfer) being one of the most relevant. Recently,[2] a Tröger base bridge (hereafter -T-), a strained heterocycle, has been proposed as an efficient but also selective bridge, capable of mediating the charge separation. Thus the setup D -T- A is here tested as a viable DSSC model dye, with the hope of taking advantages of its unusual charge separation properties.[2] In the present study the absorption and emission spectra of a pool of known ´conventional´ dyads (D--A) were calculated using time-dependent DFT (TDDFT) approaches in order to find the best functional and solvation model for further prediction of novel (D -T- A) dyads. From the pool of novel dyads (D -T- A), four of the simplest ones are picked for detailed analyses. The evaluation was focused on the absorption wavelenght, on the existance of a lowest singlet with the maximum charge separation character (CT states) which would be able to be experimentally characterized by its fluorescence and on the appropriate redox potentials at the donor and acceptor edges. Compound 1a (Figure. 1) is the simplest synthetic model and corresponds to the smaller D/A redox gap. Its parent 1b lacks of the main donor group and could be taken as control. The absoprtion and emission spectra were simulated in solvents of increasing polarity from cyclohexane to acetonitrile and experimentally measured in the case of 1a/b, they showing a good agreement between the experimental and calculated Stokes´ shifts. From both the analysis of the experimental shifts and the calculated electron densities, the S1 dipole moment was found to correspond to a CT state. However, the differences between 1a and 1b were rather mild, which indicates that the nitrogen of the bridge is the main donor of the systems and thus, although clear ( of 12 Debyes) the charge is not completely separated from on edge to the other. Relying on the good agreement between the energies and properties of the calculated excited states and the photophysical experimental data, several structures, 2a among them, were proposed and they are currently being synthetized. By increasing the D/A gap, 2a showed more than twice the dipole change observed in the 1a parent (or in its own control 2b). On the other hand, the analysis of the transition densities indicates that the charge is completely separated in the emitting state, in contrast to the partial separation shown on Fig. 1for 1a. Based on the the properties of the relaxed S1* state of 2a, a set of other dyads were envisaged with a complete charge separation on their emitting states and at different absorption and emission wavelenghts, which could be considered as proof of principles models for these novel DSSCs.