Density Functional Study of Metal-to-Ligand Charge Transfer and Hole-Hopping in Ruthenium(II) Complexes with Alkyl-Substituted Bipyridine Ligands

Salmahaminati; Minori Abe; Indra Purnama; Jacob Yan Mulyana; Masahiko Hada

doi:10.1021/acsomega.0c01199

Density Functional Study of Metal-to-Ligand Charge Transfer and Hole-Hopping in Ruthenium(II) Complexes with Alkyl-Substituted Bipyridine Ligands

ACS Omega. 2020 Dec 28;6(1):55-64. doi: 10.1021/acsomega.0c01199. eCollection 2021 Jan 12.

Authors

Salmahaminati^{1

2}, Minori Abe¹, Indra Purnama^{3

4

5}, Jacob Yan Mulyana^{3

6}, Masahiko Hada¹

Affiliations

¹ Department of Chemistry, Tokyo Metropolitan University, 192-0364 Hachioji, Tokyo, Japan.
² Department of Chemistry, Islamic University of Indonesia, Yogyakarta, Jl. Kaliurang km 14.5, Sleman, 55584 Yogyakarta, Indonesia.
³ Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 192-0397 Hachioji, Tokyo, Japan.
⁴ Research Center for Sustainable Indonesia, Bangun Indonesia Berkelanjutan, Ltd., Perum. Alamanda II Blok F 23, Pekanbaru, Riau 28293, Indonesia.
⁵ Department of Agrotechnology, Universitas Lancang Kuning, Jalan Yos Sudarso Km. 8, Pekanbaru, Riau 28265, Indonesia.
⁶ Research Center for Hydrogen Energy-Based Society, Tokyo Metropolitan University, 192-0397 Hachioji, Tokyo, Japan.

Abstract

In this study, we present a density functional study of four ruthenium complexes by means of UV-visible spectroscopy and Marcus theory. These molecules, [Ru^II(bipyP)(bipy)₂] (P1), [Ru^II(bipyP)(dmb)₂] (P2), [Ru^II(bipyP)(dtbb)₂] (P3), and [Ru^II(bipyP)(dnb)₂] (P4), where bipyP = 2,2'-bipyridine-4,4'-diphosphonic acid, bipy = 2,2'-bipyridine, dmb = 4,4'-dimethyl-2,2'-bipyridine, dtbb = 4,4'-di-tert-butyl-2,2'-bipyridine, and dnb = 4,4'-dinonyl-2,2'-bipyridine, are photosensitizers for applications in dye-sensitized photo-electrochemical cells (DSPECs). Because of the undetermined P4 conformation in the experiment, we modeled three P4 conformers with straight (P4-straight) and bent nonyl chains (P4-bend1 and bend2). UV-vis absorption spectra by time-dependent density functional theory showed intense metal-to-ligand charge transfer to anchor bipyridine ligands (MLCT-anchoring) at 445-460 nm, which accurately reproduce experimental data. The largest light-harvesting efficiency of the MLCT-anchoring state was observed in the P4-bend1 conformer, which has the lowest P4 energy. This may relate to greater electron injection in the P4 and supports experimental results of dye-only systems (do-DSPEC). The calculated charge transfer rates agree well with the experimental trend. The largest rate was obtained for P2, which was attributed to the expansion of the highest-occupied molecular orbital toward the ancillary bipy ligands and also to the short distances between dyes on the TiO₂ surface. These results also support experimental results for P2, which was the best compound for lateral hole-hopping in a sacrificial agent-containing system (sa-DSPEC).