The Effect of Heavy Atoms on Photoinduced Electron Injection from Nonthermalized and Thermalized Donor States of MII–Polypyridyl (M=Ru/Os) Complexes to Nanoparticulate TiO2 Surfaces: An Ultrafast Time-Resolved Absorption Study

✍ Scribed by Sandeep Verma; Prasenjit Kar; Amitava Das; Dipak K. Palit; Hirendra N. Ghosh

• Publisher: John Wiley and Sons
• Year: 2010
• Tongue: English
• Weight: 390 KB
• Volume: 16
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/chem.200901937

✦ Synopsis

Abstract

We have synthesized ruthenium(II)– and osmium(II)–polypyridyl complexes ([M(bpy)~2~L]^2+^, in which M=Os^II^ or Ru^II^, bpy=2,2′‐bipyridyl, and L=4‐(2,2′‐bipyridinyl‐4‐yl)benzene‐1,2‐diol) and studied the interfacial electron‐transfer process on a TiO~2~ nanoparticle surface using femtosecond transient‐absorption spectroscopy. Ruthenium(II)‐ and osmium(II)‐based dyes have a similar molecular structure; nevertheless, we have observed quite different interfacial electron‐transfer dynamics (both forward and backward). In the case of the Ru^II^/TiO~2~ system, single‐exponential electron injection takes place from photoexcited nonthermalized metal‐to‐ligand charge transfer (MLCT) states. However, in the case of the Os^II^/TiO~2~ system, electron injection takes place biexponentially from both nonthermalized and thermalized MLCT states (mainly ^3^MLCT states). Larger spin–orbit coupling for the heavier transition‐metal osmium, relative to that of ruthenium, accounts for the more efficient population of the ^3^MLCT states in the Os^II^‐based dye during the electron‐injection process that yields biexponential dynamics. Our results tend to suggest that appropriately designed Os^II^–polypyridyl dye can be a better sensitizer molecule relative to its Ru^II^ analogue not only due to much broader absorption in the visible region of the solar‐emission spectrum, but also on account of slower charge recombination.