The effect on MLCT excited states of electronic delocalization in the acceptor ligand

Abstract

1991 photochemistry, radiation chemistry, chemoluminescence photochemistry, radiation chemistry, chemoluminescence O 0160 -094Effect on MLCT Excited States of Electronic Delocalization in the Acceptor Ligand.-Metal to ligand charge transfer (MLCT) excited states of (Ru(bpy))2+ ( bpy: 2,2'-bipyridyl) and related complexes are utilized as sensitizers of photochemical electron transfer. To circumvent limitations of these complexes (decrease of lifetimes of the excited states with increase of the wavelength of the light absorbed), the ligand-bridged complex ( III) is prepared which does not undergo trans/cis isomerization even after an extended photolysis period. It has an absorption feature at 500 nm in addition to a dπ → π* transition at 460 nm; ( III)* emits at 750 nm. From the experimental results (including cyclic voltammetry) it is concluded that the lowest lying excited state in ( III)* is a MLCT state in which the excited electron is on the (II) ligand. The lifetime of (III)* (1.31 ± 0.05 µs; MeCN, 298 K) is surprisingly long as is that of some analogous complexes having (II) as a ligand. So a new strategy is opened for the design of MLCT based photosensitizers operating much further in the red while maintaining acceptable lifetimes.

Models for the Carbonyl-ene Cyclization Reaction: Open and Closed Transition States

“…[2] The bulky monocyclic [3] methylaluminum bis(4-bromo-2,6-di-tert-butylphenoxide) (MABR) also gives a clean reaction but with the opposite stereochemical preference, as defined by Yamamoto and co-workers. [4] As in Yamamotos work, the configuration of products 2 and 3 was confirmed by reduction; in our case OH-directed stereospecific addition of H 2 with Crabtrees catalyst [5] gave 4 and 5, respectively (Scheme 1).…”

Fine-Tuning the Electronic Properties of Binuclear Bis(terpyridyl)ruthenium(II) Complexes

“…[3] The protracted triplet lifetime can be explained within the framework of the energy-gap law in terms of residence of the promoted electron in an extended p* orbital that encompasses much of the bridging ligand. [4] Enlargement of the LUMO lowers the triplet energy, and thereby curtails mixing with higher-energy, metal-centered excited states. [5] In attempting to exploit this effect further we have found that the acetylene residue must be attached directly to the coordinated terpyridine ligand while the polyacetylenic bridge cannot comprise more than four ethynylene groups.…”

Electron Delocalization in Ruthenium(II) and Osmium(II) 2,2‘-Bipyridyl Complexes Formed from Ethynyl-Bridged Ditopic Ligands

“…[1][2][3][4][5][6][7][8][9] Such prolonged triplet lifetimes have been recorded both in fluid solution at ambient temperature and in frozen glasses and for complexes formed from either 2,2′-bipyridine 1-4 or 2,2′:6′,2′′-terpyridine ligands. [5][6][7][8] Closely-related effects have also been observed 2,9 for the corresponding binuclear osmium(II) and rhenium(I) polypyridine complexes.…”