Bis(tridentate) polypyridyl transition metal complexes for DSSC and LEC applications
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Abstract
Tridentate polypyridine complexes of transition metals represent a wide range of applications in future-oriented technologies such as solar cells and emitters. The primary goal of replacing expensive and rare precious metals with inexpensive, naturally abundant iron, plays an important role in current
and future research even beyond the chemical field. The main goal of this work is the preparation and characterization of push-pull iron(II) complexes for
potential use as dye sensitizers in solarcells (DSSC) and as emitters in light-emitting cells (LEC). Electron-rich R2tpda (N, N'-di "R" -N, N'-di- (pyridin-2-yl) pyridine-2,6-diamine, R = H, Me, nPr, nHex) and a strongly electron- withdrawing Dcpp (dcpp, 2,6-bis (2-carboxypyridyl) pyridine) ligand leads to a distortion of the electron density at the metal center. The main difficulties in obtaining stable complexes are shown, as well as maintaining the properties important for application, e.g. a broad absorption in the visible region, a strong push-pull character by the ligands and a strong ligand field splitting. It is illustrated that small changes in the substitution at the ligand significantly affect the stability of the iron(II) complexes. The change in the electron density at the ligand by deprotonation demonstrates how to selectively influence the transitions occurring during excitation. The increased electron density at the donor ligand leads to an LL'CT instead of an MLCT transition as transition with lowest energy. It also shows how electronic symmetry can be reduced by partial deprotonation and offers the control above directional transitions between the ligands. The desired properties are confirmed experimentally as well as theoretical by density functional theory calculations. The remote target of obtaining a low-lying 3MLCT state for an emitting iron(II) complex could not be achieved yet. However, using the push-pull concept and a large bite angle of 90°, the influence on the ligand field splitting was significant enough to raise the energy energy of the 5MC state (5T2, (t2g)4(eg*)2) close to the energy level of the 3MLCT state (3T1, (t2g)5(eg*)1). As a result, one has come closer to achieve a 3MC as a low-energy metastable state after excitation, which is the first step on the way towards emitting iron complexes.
In addition, a Me2tpda-based cobalt redox mediator [Co(Me2tpda)2]2+/3+ is presented in combination with a cationic and neutral tridentate push-pull ruthenium (II) sensitizers [Ru(ddpd) {tpy(COOH)]2+ (Me2tpda = N, N'-dimethyl-N, N'-di-(pyridin-2-yl)pyridine-2,6-diamine, tpy (COOH)3 = 2,2'6,2 ''terpyridines -4,4 ', 4' '- tricarboxylic acid) from our group in DSSC applications. The aim of this work is to improve the performance of the dye and to compare the novel redox mediator with common systems such as [Co(bpy)3]2+/3+ (bpy = (2,2'-bipyridine) and I−/I3−. The higher short-circuit photocurrent and the higher electron recombination time of the electrolyte suggest that electron recombination on the TiO2 surface with [Co(Me2tpda)2]3+ complex proceeds slower by the higher energy π* orbitals of the Me2tpda ligands than [Co(bpy)3]3+ with far lower-energy π* orbitals of the bpy ligands. The [Co(ddpd)2]2+/3+-electrolytes achieved the best performance on this system. Finally, cyclometallated polypyridyl push-pull ruthenium(II) complexes with different donor substituents ([Ru(dbp-X) (tctpy)]2− ((H3tctpy=2,2’;6’,2’’-terpyridine-4,4’,4’’-tricarboxylic acid; dpbH=1,3-dipyridylbenzene; X= N(4-C6H4OMe)2, NPh2, N-carbazolyl) from Dr. C. Kreitner are analysed. Their performance in DSSCs were tested with different redox mediators and additives compared to the reference dye N719. Nevertheless, this also led to an increased resonance stabilization, which made the regeneration of the dyes more difficult, resulting in a poor performance of the cells with I−/I3− electrolytes compared to N719. However, under certain conditions the N-carbazolyl substituted dye with I−/I3− electrolytes shows comparable efficiencies to N719. In the presence of cobalt electrolytes, all efficiencies decrease, wherein the cabarzolyl-substituted dye outperforms N719 under these conditions in terms of efficiency and fill factor.