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

Year: 2015

Authors: Azzaroli N., Lobello M.G., Lapini A., Iagatti A., Bussotti L., Di Donato M., Calogero G., Pastore M., De Angelis F., Foggi P.

Autors Affiliation: European Laboratory for Non Linear Spectroscopy (LENS), Università di Firenze, via Nello Carrara 1, Sesto Fiorentino, Florence, 50019, Italy; Computational Laboratory for Hybrid Organic Photovoltaics (CLHYO), Istituto CNR di Scienze e Tecnologie Molecolari, Via Elce di Sotto 8, Perugia, I-06123, Italy; Dipartimento di Chimica Ugo Schiff, Universita’ di Firenze, via della Lastruccia 13, Sesto Fiorentino, FI 50019, Italy; INO-CNR, Istituto Nazionale di Ottica, Consiglio Nazionale Delle Ricerche, Largo Fermi 6, Florence, 50125, Italy; CNR-IPCF, Istituto per i Processi Chimico-Fisici, Via F. Stagno D’Alcontres 37, Messina, 98158, Italy; Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Via Elce di Sotto 8, Perugia, 06123, Italy

Abstract: We have analyzed the excited state dynamics of the heteroleptic [(NCS)(2)Ru(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. Journal/Review: PHYSICAL CHEMISTRY CHEMICAL PHYSICS (PRINT)

Volume: 17 (33)      Pages from: 21594  to: 21604

More Information: The authors gratefully acknowledge the financial support from the Italian MIUR: program FIRB “Futuro in Ricerca 2010” grant RBFR10Y5VW to M.D.D. and RBFR109ZHQ supporting A.L., project CNR-EFOR L. 191/2009 art. 2 comma 44. The research leading to these results has also received funding from LASERLAB-EUROPE (grant agreement no. 284464, EC’s Seventh Framework Programme). MGL, FDA and MP thank MIUR-PRIN 2010-2011 Project No. 20104XET32 “DSSCX”.
KeyWords: Interfacial electron-tranfer; Nanocrystalline TI02; Ultrafast Dynamics; Injection dynamics; Dye; State; Complex; Films; Light; Efficiency
DOI: 10.1039/c5cp03314d

Citations: 10
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