Monitoring the intramolecular charge transfer process in the Z907 solar cell sensitizer: a transient Vis and IR spectroscopy and ab initio investigation

Abstract

We have analyzed the excited state dynamics of the heteroleptic [(NCS)2Ru(bpy-(COOH)2)(bpy-(C6H13)2)] Z907 solar cell sensitizer in solution and when adsorbed onto thin TiO2 films, by combining transient visible and infrared (IR) spectroscopies with ab initio Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) calculations. Upon excitation with ultra-short pulses in ethanol and dimethyl-sulphoxide solutions, the visible spectra show the appearance of a positive signal around 650 nm, within the instrumental time resolution (<100 fs), which in ethanol undergoes a red-shift in about 20 ps. Measurements in the IR indicate that, upon excitation, both the CN and CO marker bands, associated with the NCS and COOH groups, downshift in frequency, in response to intramolecular ligand + metal (Ru-NCS) to ligand' (bpy-COOH2) charge transfer (LML'CT). Vibrational cooling is observed in both solvents; in ethanol it is overtaken by the hydrogen bond dynamics. On the basis of DFT/TDDFT calculations, explicitly modeling the interaction of the NCS and COOH groups with solvent (ethanol) molecules, we rationalize the observed IR and visible spectral evolution as arising from the change in the hydrogen-bond network, which accompanies the transition to the lowest-energy triplet state. This interpretation provides a consistent explanation of what is also observed in the transient visible spectra. Transient IR measurements repeated for molecules adsorbed on TiO2 and ZrO2 films, allow us to identify the structural changes signaling the dye triplet excited state formation and evidence multiexponential electron injection rates into the semiconductor TiO2 film.

Toward Luminescent Iron Complexes: Unravelling the Photophysics by Computing Potential Energy Surfaces

“…Since the first paper in 1960 s on the luminescence of [Ru(bpy) 3 ] 2+ and the following work by Gafney and Adamson, in which the electron transfer (ET) quenching of the triplet charge transfer (CT) excited state was demonstrated, ruthenium polypyridyl complexes have dominated the scene in photovoltaics and solar fuel devices . One of the reasons behind the unsurpassed success of Ru(II)‐based photosensitizers and photocatalysts can be ascribed to their high electron injection quantum yields originating from the fast electron injection rates compared to the intrinsically long lifetimes (from tens to hundreds of nanoseconds) of their MLCT lowest excited states.…”

First Principle Modelling of Materials and Processes in Dye-Sensitized Photoanodes for Solar Energy and Solar Fuels

“…Ru(II)-polypyridyl complexes have been primarily employed as dye sensitizers [52,122,[134][135][136][137]. The unsurpassed performance of the [Ru(4,4 -COOH-2,2 -bpy) 2 (NCS) 2 ] complex, the N3 dye [134] (Figure 4), played a central role in significantly advancing the DSCs field, with solar to electric power efficiencies exceeding 11% [52,137].…”

“…A careful method validation is, thus, mandatory, including solvent effects and a careful analysis of the anchoring group acid-base chemistry. In fact, while bulk solvent effects (polar and non-polar environments) are usually quantitatively captured by implicit models [121], a realistic modeling of explicit solute-solvent interactions (i.e., specific hydrogen bonding) might be required in protic solvents [41,96,[122][123][124][125].…”

“…The HOMO-3 is still an almost pure NCS orbital and is stabilized by 0.9 eV passing from gas phase to water. The character and energy of From the computational point of view, the electronic, optical, and redox properties of N3, as well as analogous ruthenium polypyridyl dyes, have been successfully characterized at the DFT/TDDFT level of theory, employing functionals with a moderate fraction of HF exchange (e.g., B3LYP [138], PBE0 [139], M06 [140]) and resorting to implicit solvation models [51,52,122,[141][142][143][144][145][146]. The need of including the surroundings in the calculations has been widely highlighted [142,144,[147][148][149][150][151][152]: solvent usually induces a sizeable shift (even larger than 1 eV) to higher energies of the otherwise underestimated MLCT excitations, thus delivering calculated spectra in overall better agreement with the experimental ones.…”

Short- and Long-Range Solvation Effects on the Transient UV–Vis Absorption Spectra of a Ru(II)–Polypyridine Complex Disentangled by Nonequilibrium Molecular Dynamics

“…[7][8][9][10][11] Experimental research has been assisted by computational studies able to deliver atomistic insights into the solvent response dynamics and, in turn, on its impact on the electronic structure of the photo-excited solute. 6,[12][13][14][15][16][17][18][19][20][21][22] Within this framework, however, ultrafast experiments on solvated transition metal (TM) complexes are relatively few and recent, 18,19,[23][24][25][26][27][28][29] despite their ubiquitous employment in photovoltaics, optoelectronic devices and medical applications. [30][31][32][33][34][35] Most of the published work focuses on polypyridine complexes, 24,36 in particular on the prototype [Ru(bpy) 3 ] 2+ complex, where the electron localization/delocalization dynamics following the photoinduced CT has been widely studied and debated.…”